project final report publishable summary

SSEEC – Final summary report

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PROJECT FINAL REPORT

Publishable summary Grant Agreement number: 214864

Project acronym: SSEEC

Project title: Solid State Energy Efficient Cooling

Funding Scheme: NMP Collaborative Project (Small)

Period covered: from 1/10/2008 to 30/9/2011

Name of the scientific representative of the project's co-ordinator, Title and Organisation:

Karl G. Sandeman

Lecturer in Physics and Royal Society University Research Fellow

Imperial College London,

United Kingdom

Tel: +44 207 594 7861

Fax: +44 207 594 2077

E-mail: [email protected]

Project website address: http://www.sseec.eu


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!www.sseec.eu

1. EXECUTIVE SUMMARY 3 2. PROJECT CONTEXT AND OBJECTIVES (4 PAGES MAX.) 4 2.1 CONTEXT 4 2.2 OBJECTIVES 5 2.3 STRUCTURE 6 3. MAIN S&T RESULTS AND FOREGROUNDS (25 PAGES MAX.) 7 3.1 WP1: SYNTHESIS OF SINGLE PHASE LA-FE-SI AND CO-MN-SI MATERIALS 7

3.1.1 Highlights of La-Fe-Si activities 8 3.1.2 Highlights of Co-Mn-Si activities 11 3.1.3 Conclusions of WP1 13

3.2 WP2: ANISOTROPIC MATERIALS INCLUDING INTRINSIC AND ARTIFICIAL SPIN REORIENTATION 13 3.2.1 Highlights of hexaferrite and Nd-Co activities 14 3.2.2 Conclusions of WP2 17

3.3 WP3: ADVANCED CHARACTERISATION 18 3.3.1 Guide to refrigerant performance 18 3.3.2 Round robin exercise: the need for accurate measurement comparison 20 3.3.3 Development of specialised characterisation tools 20 3.3.4 Conclusions of WP3 21

3.4 WP4: THEORETICAL MODELLING 22 3.4.1 Thermodynamic modelling of La-Fe-Si and Co-Mn-Si (WP1 materials) 22 3.4.2 Exchange coupling in nano-structured refrigerants 24 3.4.3 Conclusions of WP4 26

3.5 WP5: PROTOTYPING 26 3.5.1 Application specification 27 3.5.2 Refrigerant characterisation (heat capacity and shape of refrigerant plates) 28 3.5.3 Delivery of three prototypes 29 3.5.4 Conclusions of WP5 (NW to edit) 31

4. POTENTIAL IMPACT AND MAIN DISSEMINATION ACTIVITIES (10P MAX.) 33 4.1 POTENTIAL IMPACT 33

4.1.1 Magnetic cooling: the future 33 4.1.2 Magnetic phase transition research 35

4.2 MAIN DISSEMINATION / EXPLOITATION OF RESULTS 35 5. PROJECT WEBSITE AND CONTACT DETAILS 37

SSEEC partners at the final project meeting in London, September 2011


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1. Executive Summary SSEEC (Solid State Energy Efficient Cooling) has been a 36-month project dedicated to the development of magnetic refrigerant materials and the technology required to integrate them in an end-user application: a heat pump. The aim has been to lower the economic barrier to entry of magnetic cooling as a high efficiency cooling technology. Conventional cooling relies on the expansion and compression of a volatile liquid, typically an HFC. It therefore harnesses the latent heat that is evolved during a gas/liquid transformation, and is driven by changes in applied pressure. However, the volatility of HFCs is one factor that drives research into solid state refrigerants. A recent study predicted that HFCs could account for between 28 and 45 percent (CO2-equivalent basis) of projected global CO2 emissions by 20501. Magnetic cooling is a solid state cooling method that promises high system efficiency. It utilises the temperature change that occurs when a magnetic material is driven through a change of state by an applied magnetic field. Magnetic refrigerants have magnetic phase transitions around room temperature, just as HFCs evaporate readily at room temperature. Our project has been the first of its kind to examine all stages in the production of a magnetic cooling application solution, from the synthesis, fabrication and shaping of refrigerants, to measurement of their physical properties using the latest bespoke characterisation tools and the integration of refrigerants in cooling engines designed specifically according to models that optimise cost and efficiency. This technology chain has been iterated with three prototype cooling engines, the ultimate being an integrated heat pump system. We have produced a roadmap for magnetic cooling technology, addressing the domestic refrigeration sector, and also suggested further necessary steps in exploring: material processing; fundamental magnetism; standardisation of refrigerant characterisation and more unusual magnetic cooling effects in the future. Our major achievements in SSEEC are now listed. We have successfully: Single phase magnetic refrigerants and novel cooling mechanisms

• Produced single phase refrigerants in small thicknesses suitable for heat exchange, with tuned transition temperatures

• Developed new refrigerant synthesis processes to address refrigerant machinability • Investigated the fundamental magnetism of metamagnets that improve our understanding of novel

magnetic cooling mechanisms based on magnetic order-order transitions. Novel cooling mechanisms

• Investigated and syntheised micro- and nano-scale refrigerants that harness anisotropy rather than the degree of magnetic ordering

• Synthesised single crystal anisotropic refrigerants and textured polycrystalline materials. Advanced characterisation

• Provided new plots to provide a simple means by which to compare refrigerant performance. • Conducted round-robin activities that highlight the importance of sample preparation and

comparative measurement techniques. These have also verified the accuracy of our methods. • Developed advanced characterisation tools across the partnership which have been integrated into

our materials development work, enabling greater flow of ideas and materials between partners. Theoretical modelling

• Modelled the static, thermodyamic properties of MCE materials with continuous phase transitions • Predicted of the effect of dynamics in sharp first order transitions, defining a range for the increase in

magnetic field requirement, depending on transformation kinetics • Established new magnetic cooling mechanisms due to “artificial spin reorientation” in a hard

ferromagnetic multilayer system Prototyping

• Constructed 3 prototype magnetic cooling engines using Co-doped La-Fe-Si • Met temperature span targets set for prototypes I and II • Integrated with heat exchangers to build a heat pump in prototype III • Developed a technology roadmap for magnetic cooling (see Section 4.1)

We hope that our results will provide the springboard for European development of magnetic cooling as a high efficiency, green and cost effective form of cooling.

1 “The large contribution of projected HFC emissions to future climate forcing”, G.J.M. Velders et al, Proc. Natl. Acad. Sci. USA. 106 10949 (2009) doi:10.1073/pnas.0902817106.


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2. Project context and objectives

2.1 Context The ultimate goal of our project was to lower the economic barrier to entry of magnetic cooling as a high efficiency cooling technology. Our efforts in this direction were centred around the development of a heat pump based on a highly efficient magnetic refrigeration cycle. The devices feature a magnetic refrigerant being magnetised and demagnetised by a permanent magnet.

Magnetic cooling: basic formalism Our project has been the first of its kind to integrate all elements in the production of a magnetic cooling engine. Magnetic refrigerants have been synthesised, shaped, characterised, modelled and three prototype cooling engines have been built using the optimal materials that we have produced. For the description of the scientific and technological highlights that follow, it is useful to provide a brief overview of the evaluation of magnetic refrigerant materials and the nature of the magnetic cooling cycle used in prototypes. How we evaluate magnetic refrigerants In what follows we use the term “magnetic refrigerant” and “magnetocaloric” interchangeably. This terminology is because a magnetic refrigerant relies upon the magnetocaloric effect (MCE), the change of temperature that a material undergoes when exposed to a change in applied magnetic field. In non-magnetic materials it is very small (hundredths of a Kelvin) but in materials that have a magnetic phase transition it can be as large as 3-4 K in a 1 Tesla field – a field easily achievable with a permanent magnet. The applied field can trigger the change of magnetic phase, and with it bring about a release or uptake of heat by/from the material. The amount of heat exchanged with the surroundings can be sufficient to build a cooling engine; in the best MCE materials, around 1 kW cooling power per kilogram of refrigerant might be expected. What we require of a magnetic refrigerant Magnetic refrigerants therefore need to have the following properties:

1. A phase transition temperature that is tunable over the range of temperature required, so that the useful MCE response of the refrigerant is large over the required application temperature range. The phase transition has to be able to be triggered by an applied field.

2. Large heat release/uptake or temperature change when the field is applied (depending on the conditions of applying the field)

3. Non toxic, inexpensive, and shapeable These conditions imply a set of more complicated requirements for magnetic properties, heat capacity and even thermal conductivity in application. However, the primary concern is that, when the magnetic field is applied condition 2 is satisfied. This means that:

• When the magnetic field is applied isothermally (at a constant temperature) a large heat is released or taken in by the material (~ 10-20 J kg-1K-1). We quantify this in terms of isothermal entropy change, ∆S.

• When the magnetic field is applied adiabaticaly (at constant entropy, or under conditions of no heat exchange with the material) a large temperature change is observed (~ 2 Kelvin in a 1 Tesla field change). We call this quantity the adiabatic temperature change, ∆Tad.

Figure 2.1: A schematic magnetic cooling cycle


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How to make a magnetic cooling engine: the thermodynamic cycle If a material satisfies the above requirements it can potentially be used in a magnetic cooling device. A schematic of such a device is shown in Figure 2.1. The magnetocaloric material is magnetised, resulting in a temperature change (e.g. an increase as shown in the Figure). Then heat can be removed by a secondary heat exchange fluid, and released by a radiator, such as exists at the back of a domestic refrigerator. On removal of the applied magnetic field, the material temperature changes, but now it is lower than the original temperature since heat was already removed to the radiator in the previous step. Therefore heat can be put into the material from the object(s) to be cooled, using another heat exchange fluid. This finally raises the temperature of the material back to the starting value, or close to it. The cycle is analagous to that of conventional gas compression, with the application and removal of a magnetic field corresponding to the compression and evaporation strokes respectively. How to build a temperature span: the regnerator If the change of temperature of a material on application of a 1 Tesla field is 2 Kelvin, how do we build a device that can span the 30-40 K that might exist between the hot end and cold end of an application? The answer lies in the use of a regenerator. This component allows different parts of a refrigerant block or stack to operate around different starting temperatures, thus meaning that a temperature gradient can be built along the length of the refrigerant. Since the MCE response of a magnetic refrigerant is not limited to a single temperature, but is instead spread over a range of temperatures around the phase transition temperature, heat can be exchanged with the refrigerant over a range of temperature along the length of an individual composition. If a wider temperature range of response is required, different material compositions are added. Composition typically shifts the transition temperature, and with it, the window of response of the magnetocaloric material. In light of the above we often display plots of either isothermal entropy change (∆S) or adiabatic temperature change (∆Tad) as a function of temperature for several compositions in a series. These plots inform how composition affects the functionality of a material via how the MCE response varies over a temperature range. In summary, magnetic refrigerants and permanent magnets are the two essential materials components of a magnetic cooling engine. The SSEEC project was structured not only to innovate in fundamental materials research, producing new magnetic refrigerant nano-architectures, but also to pull through from those materials fundamentals to delivery of a technology where considerable improvements in energy efficiency in the cooling and heat pumping markets can be offered. In order to achieve these objectives, the consortium was made of four materials research institutions, an SME capable of developing prototype cooling engines, a medium scale materials manufacturer that is able to design and produce magnetocaloric materials as well as permanent magnets and a major systems end-user that provides industrially-guided feedback on system design and performance.

2.2 Objectives Our project was supported under NMP-2007-2.2-3 Advanced material architectures for energy conversion. As outlined above, the magnetic cooling engine is a high efficiency energy converter for the conversion of mechanical work into cooling power, and represents the only solid state cooling technology capable of achieving a system efficiency greater than that provided by conventional gas compression technology. Although already used in research to achieve milliKelvin temperatures, economic viability of magnetic cooling engines for future room temperature applications will rely on two key materials factors:

(1) The availability of low cost, low hysteresis magnetic refrigerants, and (2) The ability of these refrigerants to provide substantial cooling power in magnetising fields low

enough to be provided by permanent, rather than superconducting- or electro-magnets; and two cooling engine factors:

(3) Optimisation of heat exchange with the refrigerant, and (4) Minimisation of refrigerant and permanent magnet volume

Before SSEEC started, prototype magnetic cooling engines were limited by all four factors, and relied on expensive refrigerants in a large and heavy cooling engine design. We aimed for SSEEC to produce refrigerant material architectures with new compositions, new morphologies and new underlying physical mechanisms (addressing items 1 and 2 above), whilst reducing the necessary volume of refrigerant and thereby both the cost of refrigerant and permanent magnet components in a cooling engine


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(item 4). Optimisation of heat exchange with the refrigerant (item 3) was to be an integral component of the prototype design. Refrigerant costs would also be reduced by improved synthesis processes. We thus aimed to lower the economic barrier to entry for high efficiency magnetic cooling. Our materials methodology harnessed the combined expertise of the partners in materials synthesis, modeling and fundamental characterisation to produce a series of improved single phase refrigerant materials with a focus guided by items (3) and (4) above. Importantly, we also set out to explore a completely new avenue of magnetic refrigerant research – anisotropic nano-composite refrigerants - in an effort to widen the range of magnetic refrigerant materials open to the research and development community. The family of single-phase materials was to be implemented in a series of prototype heat pumps by our industrial partners. We specifically addressed item (4) above by exploring of prototype operation at high cycle frequency – a world first. In summary, our scientific objectives were therefore:

a) To synthesise, characterise and optimise a family of improved single phase magnetic refrigerant materials

b) To model and develop a family of new anisotropic nano-composite refrigerants; c) To design and produce low cost permanent magnet arrays; d) To integrate the materials from (a), (b) and (c) into viable prototype heat pumps and air

conditioners operating at high cycle frequency. By the project completion date we assessed the material architectures developed within the project and integrated them with systems components such as the heat exchangers, regenerators and magnetic arrays required to produce an efficient heat pump. We also created a roadmap for the future deployment of magnetic cooling technology.

2.3 Structure Having outlined the basic principles of MCE materials and their use, we move in the next section to a presentation of research highlights, organised in work package (WP) order. For clarity, the WPs were:

WP1: Synthesis of single phase La-Fe-Si and Co-Mn-Si materials WP2: Exploiting magnetic anisotropy energy (intrinsic and artificial spin reorientation) WP3: Advanced characterisation of materials WP4: Theoretical modelling WP5: Prototyping (the production of three prototypes labelled I, II and III) WP6: Management

Full details are given in the technical annexe accompanying this report. A key feature of the project was the inter-relation of project elements. Material compositions identified and synthesised in WP1 were optimised through advanced characterisation and feedback in WP3. The same materials were trialled in prototypes I and II in WP5, with magnetocaloric and mechanical performance data being relayed back to WP1. In a similar way, materials synthesised to exploit magnetic anisotropy energy (MAE) in WP2 were put through advanced characterisation in WP3 while new nano-architectures harnessing the MAE were postulated through theoretical modelling in WP4. That same modelling also helped to shape the choice of single phase vs. multi phase studies in WP2. Theoretical modelling in WP4 also underpinned our understanding of the dynamics of first order magnetic phase transitions throughout the project.


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3. Main S&T results and foregrounds

3.1 WP1: Synthesis of single phase La-Fe-Si and Co-Mn-Si materials Objectives • To establish economically viable routes to fabrication of optimised single phase magnetic refrigerant

materials with workable entropy changes, low hysteresis and tunable critical temperatures • Elimination of significant volume of parasitic second phases that dilute useful properties within the

working volume of material • Routine characterisation of these materials in terms of structure (XRD,SEM) and magnetisation M(T,H). Summary of activities Our efforts here have focussed on a better understanding of two families of single-phase alloys, LaFeSi and CoMnSi, that have magnetic field-induced phase transitions around room temperature. The LaFeSi material system is one that promises to be cost effective and deliver significant cooling power. Before we started SSEEC it had not been used in an end user device. Our project examined all steps of material synthesis, fabrication and shaping required to bring about this goal. We have deployed LaFeSi in all of our prototype cooling engines. The activities of the two principle partners in this effort, IFW and VAC, are detailed in section 3.1.1. We synthesised this material by two different routes – powder metallurgy and melt-spinning – and examined the dependence of its magnetic properties on synthesis conditions, machining, and microstructure. In addition we examined the material’s thermal and structural properties. The importance of thermal properties to magnetic cooling is relatively obvious; structural properties perhaps less so. However, in many potential magnetic refrigerants there is a change of volume associated with the magnetic field-induced change of refrigerant entropy. In a solid such changes of volume can lead to irreversibilities, cracking, and even in difficulties in machining the material if the associated magnetic transition is around room temperature, which it has to be for our applications. A variety of synthesis routes and conditions have been tested on the La-Fe-Si system. This system contains an abrupt, “first order” magnetic transition that can be tuned to room temperature by the addition of cobalt (which reduces the first order nature) or by the absorption of hydrogen (which maintains the first order nature and leads to a larger MCE). In the first 18 months Co-doped materials with smooth, continuous magnetic transitions have been studied from the point of view of applications. These have the advantage of good mechanical and machining properties, but are not the materials in the La-Fe-Si with the highest MCEs. In the second 18 months, the focus shifted to the role of hydrogenation. Fully hydrogenated La-Fe-Si has a Curie temperature well above room temperature. Partially hydrogenated material can have a Curie temperature in the useful range for room temperature applications (-20˚C to 40˚C) but can be unstable with respect to either hydrogen redistribution within the material or to dehydrogenation at elevated temperatures. The partners, led in this effort by IFW and VAC built an understanding of the mechanisms of dehydrogenation, and of phase (in)stability in the ternary (La,Fe,Si) phase diagram. Importantly, substitutants were explored that allow the first order nature and high MCE of fully hydrogenated material to be harnessed at room temperature, by suppressing the Curie temperature of the non-hydrogenated starting compound by an appropriate amount. Material machinability was explored so that structural changes did not interfere with the end product. Two new processes were developed and used by VAC: (i) the thermal decomposition and recombination (TDR) process for the production of plates of Co-doped material and (ii) a solid hydrogenation process (SH) used to hydrogenate materials in their desired shape, by avoiding cracking during hydrogenation. We have thereby examined link between the magnitude of structural changes and magnetothermal properties. We also successfully made materials with various Curie temperatures, thus extending the temperature range of the magnetocaloric effect compared to a material of a single composition. The delivery of single phase, cost-effective materials to span the temperature ranges of all three prototypes has thus been achieved, although the mechanical tolerances of these materials are a problem that is addressed in section 3.5.2 below. A range of analyses of these materials was


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undertaken, to establish the relation between magnetic, structural, compositional and magnetocaloric properties, and to explore the opportunities for optimisation of the latter. Our second material family, based on Co-Mn-Si is used as a tool for gaining knowledge about the mechanisms of or driving forces behind “metamagnetic” changes of state. These changes are less frequently observed in magnetic materials but can have large entropic effects associated with them. In this system, rather than a magnetic order-disorder (Curie), a transition between different types of magnetic order is studied. While this system shows less commercial promise, it has given rise to a number of fundamental insights in the metamagnetism of Mn-based alloys of the same (Pnma) space group and has resulted in three high profile publications. This work, performed mainly by Cambridge and Imperial, is detailed in section 3.1.2.

3.1.1 Highlights of La-Fe-Si activities The highlights presented in this section are arranged under three headings:

• Preparation of single phase material with Curie transition tuned by Co, H and Mn • Stability of hydrogenated materials and its improvement with carbon • Machinability of materials improved by two new processes

Preparation of single phase material with Curie transition tuned by Co, H and Mn La(Fe,Si)13 is an interesting MCE material because its magnetic transition – a Curie transition, Tc, at which temperature the material magnetically disorders – is sharp, or “first order”. This means that, on application of a magnetic field, a large coordinated jump in entropy is observed as macroscopic magnetic ordering is induced. Without the addition of other elements, the Curie temperature, and therefore the temperature range of maximum MCE is about 190 K. In order to optimise the material for use as a room temperature magnetic refrigerant, its Curie temperature must be raised. The two most common ways are either to use Co-substitution for Fe, or by introducing interstitial hydrogen. Whereas the former is associated with a significant decrease of the isothermal entropy change the latter yields alloys of the general composition LaFe13-xSixHz which, for z≈1.6, have a Curie temperature of about 350 K and a large isothermal entropy change of about 20 J/kgK for a magnetic field change of 1.5 Tesla. In principle it is possible to vary the hydrogen content, z, and obtain any Curie temperature between 190 K and 350 K while retaining the large isothermal entropy change.

Figure 3.1.1: Left: Volume fraction of constituent phases as a function of annealing temperature for bulk alloys (closed symbols determined by SEM images, open symbols determined by Rietveld refinement). Right: Phase concentration in melt spun ribbons as a function of annealing temperature. Annealing time for all samples was 2 hours. Before considering the action of either method of Curie temperature tuning in detail, we first present results aimed at isolating single phase La-Fe-Si. Normally this material is metastable, forming from the melt by a peritectic reaction, and contains a parasitic α-Fe second phase, which does not contribute to the MCE. IFW has compared the structure and properties of melt-spun materials with those produced by conventional means (e.g. arc-melting and induction melting). The annealing temperature is of crucial importance for maximising the amount of 1:13 phase in LaFe13-xSix. However, little knowledge about the ternary phase diagram of La(Fe,Si)13 is available. The first aim was to optimize the annealing temperature for La(Fe,Si)13 bulk alloys with a first-order Curie transition.


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Figure 3.1.1 shows that in both bulk alloys and melt-spun ribbons the percentage of the parasitic α-Fe phase is decreased by increasing the annealing temperature up to 1373 K. The result for melt-spun materials is particularly interesting as the annealing time, and hence process cost, can be reduced from several days to 2 hours. A maximum amount of the 1:13 phase of ~ 90 vol.% can be achieved by annealing at 1373 K.

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Figure 3.1.2: Magnetic entropy change as a function of temperature of: La(Fe0.915CoxSi0.085)13 (left) and five LaFe11.74-yMnySi1.26H1.53 alloys with different y (right) for a magnetic field change of 1.6 T. The entropy change is higher than that seen in gadolinium (Gd, left plot only). Figure 3.1.2 shows the influence of Co and Mn substitutions on the Curie temperature and the entropy change of Co-substituted and hydrogenated, Mn-substituted La-Fe-Si alloys produced by VAC. It is clear that for both alloy types the entropy change is higher than that seen in gadolinium (Gd, left plot only) and that the Curie temperature can be tuned precisely across the relevant room temperature range. The combined Mn/H tuning in the series in the right hand plot of Figure 3.1.2 is important because the magnetocaloric effect is even higher than in the case of Co-substitution alone. These alloys are also fully, rather than partially hydrogenated, which is preferable for reasons now explained. Stability of hydrogenated materials and its improvement with carbon (IFW) As explained above, one way to tune the Curie temperature of LaFe13-xSix alloys is to partially hydrogenate them and obtain transition temperatures between 190 K and 350 K. While this route is attractive because of the large entropy change of such alloys it has been found that magnetocaloric properties of partially hydrogenated LaFe13-xSixHz are not stable. Partially hydrogenated LaFe13-xSixHz alloys exhibit a phase separation effect by which a material with one well defined Curie temperature degrades into a material with two broader transitions if stored at the Curie temperature2. Details of the mechanism of this “peak splitting” are unknown but it is very likely that a partial hydrogen concentration causes the problem. This assumption is reasonable because alloys which are fully saturated with hydrogen do not exhibit “peak splitting”.3

Figure 3.1.3: Data for LaFe11.6Si1.4CX (x = 0.0 - 0.4) alloys annealed at 1323 K for seven days. Left: temperature dependence of magnetization; middle: entropy change in a 2 Tesla field change; and right: temperature dependence of heat flow using a 5 K/min temperature sweep-rate. 2 A. Barcza et al. IEEE Trans. Magn. vol. 47, no. 10 3391 (2011). 3 Fully saturated with hydrogen (Hsat) indicates the maximum hydrogen content that can be achieved by the SH process described here.


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We studied the use of Carbon (C) in combination with H to improve the thermal stability of our alloys. Based on microscopy and magnetisation data (below) we took the most promising C-containing samples – namely x = 0.1 and 0.2 with seven days annealing – and fully hydrided them. The results of DSC measurements are shown in Fig. 3.1.3 for x = 0, 0.1 and 0.2. The endothermic peak shows that desorption of H starts at 460 K for the carbon-free alloy (black curve). In the carbon-containing samples, the desorption temperature is increased to 500 K (x = 0.1) and 540 K (x = 0.2). This shows that C stabilizes the H in LaFe11.6Si1.4CxHy alloys and is especially important as the materials should be stable over extended periods of time; even small losses in H content translate to large decrease in the working temperature as set by Tc. Machinability of materials and its improvement via two new processes (VAC) One characteristic of the magnetic phase transition in La-Fe-Si is the large magnetovolume effect associated with the change of the magnetic state. This effect leads to a large strain in the material around its Curie temperature as indicated by the red line in Fig. 3.1.4. During machining of La-Fe-Co-Si, local heating leads to a change of the magnetic state in a small volume of the material and the material cracks. In order to be able to machine La-Fe-Co-Si with Curie temperatures around room temperature a new process was developed. For this process the instability of the magnetocalorically active 1:13 phase was utilized. If La-Fe-Co-Si is annealed at temperatures below about 1000°C the 1:13 phase decomposes into α-Fe and La-rich phases. These phases do not exhibit anomalous thermal expansion and hence machining of the material is possible in this magnetocalorically inactive state (see Fig. 3.1.4). In this state the material contains up to about 75% α-Fe. After machining in the decomposed state the α-Fe reacts with the La-rich phases to form the desired 1:13 phase again. This homogenization process is conducted at about 1050°C. The material is then rapidly cooled to room temperature to preserve the magnetocalorically active state. The whole process is called Thermal Decomposition and Recombination (TDR). We also developed a way to avoid cracking during hydrogenation. The key is the temperature at which hydrogen is offered to react with the alloy (the starting temperature). A second important factor is the cooling from the hydrogenation temperature to room temperature. Tests that were carried out show that disintegration can be avoided if hydrogenation of parts is conducted by first heating to 500°C under an inert atmosphere, e.g. argon. Then the inert gas has to be replaced with hydrogen followed by a dwell at 500°C for about one hour. The parts have to be cooled down to room temperature slowly (about eight to ten hours). This process is called the solid hydrogenation process (SH). Pictures of plates (12 x 6 x 0.6 mm) that were hydrogenated at different starting temperatures are shown in Figure 3.1.5. From there it is seen that if the starting temperature is below 400°C the product of the hydrogenation is powder. The lower the starting temperature the finer the powder which is received after hydrogenation.

Figure 3.1.5 Photographs of plates of La-Fe-Co-Si before hydrogenation (a) and after hydrogenation (b) – (f). The starting temperature for the hydrogenation is 100°C (b), 200°C (c), 300°C (d), 400°C (e), and 500°C (f).

(a) (b) (c) (d) (e) (f)

Figure 3.1.4: In order to be able to machine parts made of sintered La-Fe-So-Si the materials has to undergo the TDR process as described in the text.

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At a starting temperature of 500°C only little hydrogen is taken up by the magnetocaloric material. Since this temperature is relatively high hydrogen can diffuse easily in the material and generate a homogenous concentration throughout the specimen. The slow cooling guarantees the gradual and homogenous increase of the hydrogen concentration until the alloy is fully saturated with hydrogen at room temperature. If hydrogenation is carried at lower temperatures, a competition between diffiusion (slow) and hydrogenation (fast) takes place, resulting in a hydrogen gradient through the sample. The resultant strain leads to disintegration of the parts.

3.1.2 Highlights of Co-Mn-Si activities The CoMnSi system offers an alternative possibility for exploring magnetocaloric effects. As outlined in section 3.1.1, CoMnSi is antiferromagnetic, rather than ferromagnetic, and a magnetic field induces a “metamagnetic” transition to a state of high magnetisation. All thermal and entropic features are of opposite sign, as compared to those of a ferromagnet such as La-Fe-Si at its Curie transition. For this reason, metamagnetic systems are known as “inverse” MCE materials. Nevertheless, the potential for application of metamagnets and the conditions on their use in magnetic cooling systems are the same as for ferromagnets. We wish to obtain a large (inverse) MCE in a small field, using as much of the material in phase transition process as possible (no second phase). Hysteresis, too, should be kept to a minimum. CoMnSi has an interesting feature in this regard; it can exhibit a continuous field-induced phase transition in low fields, or a first order, step-like phase transition at large field strengths, all in a single sample. The material therefore offers a so-called tricritical point, where the onset of first order behaviour, and associated large MCE is brought about with minimal hysteresis4. Our goals have been to:

1. Identify whether suitable alloying could bring the tricritical point in CoMnSi to room temperature. In the stoichiometric compound it occurs at a field of 3 Tesla, at 280 K.

2. Establish the link between sample synthesis and phase transition temperature. Literature values of the metamagnetic transition temperature in small fields vary by 200 degrees.

Figure 3.1.6: Temperature-dependent separation of nearest-neighbour manganese atoms in CoMnSi (a) and Co0.95Ni0.05MnSi (b), as measured using neutron diffraction at ISIS. These separations, named d1 and d2, are shown in (c), together with the approximate spiral helimagnetic state used in DFT calculations (see text). The Cambridge/Imperial team benefited from excellent access to the high resolution powder diffactometer

4 The first order transition in La-Fe-Si can also become 2nd order, but in high fields (rather than in low fields in CoMnSi). In La-Fe-Si, the hysteresis is intrinsically small, which is another advantage for application.


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(“HRPD”) and the GEneral Materials Diffractometer (“GEM”) at the ISIS neutron diffraction facility. The former is used for detailed, zero-field structural measurements, including atomic coordinates. The latter is capable of taking unit cell data in an applied magnetic field of up to 7 Tesla. During Months 1-12 we used a total of 6 days of beamtime on HRPD to examine the thermal expansion of CoMnSi in detail. Literature reports from the 1970s of the thermal expansion of this material suggested negative thermal expansion in at least one direction, but suffered from large data scatter. However, HRPD is one of the highest resolution instruments of its kind in the world and is capable of 1x10-4 fractional resolution of lattice dimensions.We found that in CoMnSi and all 3 of the other doped materials that we studied, there is large negative lattice expansion along the a-axis and, most significantly, a crossover in nearest neighbour Mn-Mn distances within the low temperature antiferromagnetic state. These two Mn-Mn distances, named d1 and d2 henceforth, change by about 2%, which is the largest such change in atomic separations seen in a metallic magnet. Data from neutron diffraction on HRPD for CoMnSi and Co0.95Ni0.05MnSi is shown in Figure 3.1.6. The data tell us that as the antiferromagnetic state changes with temperature, there is a very large feedback to the atomic positions. It also means that the crossover in d1 and d2 probably underpins the second-to-first-order behaviour with increasing magnetic field. We expect that a sufficiently large magnetic field couples the Néel transition of the helical antiferromagnet (at 380 K in zero field) to the lower temperature structural feature in d1 and d2 shown above, and thereby provides a tricritical point, and a first order magnetoelastic phase transition with enhanced MCE. We wanted to understand the link between structure and tricriticality. We therefore set about ab-initio calculations using Density Functional Theory (DFT) in order to calculate the magnetic ground state of CoMnSi and to help predict the features of this compound that give rise to metamagnetism. The analysis proceeded to examine the difference between the electronic densities of states (DOS) of CoMnSi in a ferromagnetic state (high magnetization) and in the closest commensurate version of the antiferromagnetic groundstate that we could simulate (shown in Figure 3.1.6(c)). A significant difference was seen in the DOS at the Fermi level, and this seems to drive formation of the antiferromagnetic state in zero field. However, we noticed that the CoMnSi structure sits in an unusual position relative to other Mn-based materials of the same Pnma space group; the distances d1 and d2 are of much more comparable size, compared to, say those in CoMnP (a ferromagnet) or CoMnGe (also a ferromagnet). We therefore set about a theoretical analysis of the Pnma prototype structure, starting from MnP, a well-known system that exhibits a variety of temperature-dependent and field-dependent magnetic structures, including non-collinear order. We have found that in MnP and in Pnma ternaries, the nearest-neighbour Mn-Mn inter-atomic distances (“d1”,”d2”) are most important and determine the nature of ferromagnetic (FM) or antiferromagnetic (AFM) state seen.

Figure 3.1.7 Left: Total energy, E of non-magnetic (NM), ferromagnetic (FM) and 3 different antiferromagnetic (AFM) solutions as a function of critical d1 next neighbour Mn-Mn interatomic distance in MnP. AFM configurations become stable where E(AFM)-E(FM}<0, ie when at least one line drops below the zero level of energy difference. This occurs, for two AFM solutions, for 2.96 Å < d1 < 3.36 Å. Ferromagnetism is again stable at the highest d1 values. The vertical dashed line labelled “Exp. lattice” represents the strain free Mn-separation values. Right: CoMnGe1-xPx alloys synthesised by design: compositions chosen to bring about d1 in the range that facilitates metamagnetism, which is seen from the rise in M(T) for intermediate compositions. In particular we compared 12 ternary XMnZ-type Pnma structures from the literature and found an excellent agreement between our simple model and the experimentally observed magnetic states. As an example of this, Figure 3.1.7 shows the calculated effect of an isotropic expansion of the MnP structure on its magnetic properties. An expansion of 5% makes a non-collinear antiferromagnetic state more stable than the usual


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ferromagnetic (FM) ground state. We examined the possibility of 3 different AFM states, labelled AFM1, AFM2 and AFM3. Interestingly, the two that did yield an energy minimum were those that had antiferromagnetic coupling between Mn atoms separated by d1. We believe that this is an essential ingredient to the metamagnetism of such alloys.Obviously the MnP binary, or other similar compounds, cannot be simply expanded by about 5% in all three dimensions to reach the borderline between FM and AFM stability as is evident in low fields in CoMnSi. However, the above result is a key to metamagnetism – the field-driven transition between AFM and high magnetisation, or FM-like states. The only relevant approach to extend the lattice is instead to apply chemical pressure. An example of ternary alloying of the system is FeMnP. Interestingly enough, if pure iron is added to the ferromagnetic MnP binary, the alloy shows non-collinear AFM properties experimentally. The reason for that we found, that the d1=3.05 Å and d2=3.12 Å inter-Mn distances are pushed higher into the AFM regime in Figure 2. From this analysis we have been able to predict new metamagnetic alloys, such as CoMn(Ge,P) which have been synthesised during Months 18-36 (see right of Figure 3.1.7).

3.1.3 Conclusions of WP1 In summary, the main results of work in WP1 are:

• single phase La-Fe-Si (LFS) materials produced with small thicknesses • Production of LFS specimens with tuned compositions and working temperatures • Two new synthesis processes: thermal decomposition and reaction, “TDR” and solid hydrogenation

“SH”, particular to this material system have been developed. • Optimal TDR and SH conditions have been identified • Carbon has been shown to stabilise hydrogenated La-Fe-Si materials. • Machinability has been addressed through TDR and SH syntheses as well as via other processes (see

detailed annexe) • Neutron investigation of Mn-based metamagnets, discovery of giant magnetoelastic coupling and

theoretical extrapolation of metamagnetism in other Mn-containing materials

3.2 WP2: Anisotropic materials including intrinsic and artificial spin reorientation Objectives • To set up a wet-chemistry route suitable for laboratory scale synthesis of nanoparticles of hexagonal

barium ferrites and derived compounds. • To investigate the effect of calcination temperature and other production parameters on the phase

composition, mean particle size and magnetic properties of the above. • To produce anisotropic polycrystalline W-type hexaferrites by pressing the synthesized powder in applied

magnetic field with subsequent conventional sintering (VAC). • To grow W-type hexaferrite single crystals from a mixture of oxides by the flux method (CNRS). • To fabricate aligned NdCo5 intermetallic compacts using the following route: alloy preparation → rapid

solidification → ball milling → spark plasma sintering (CNRS and IFW). • To confirm the presence of crystallographic anisotropy and spin reorientation transitions in the obtained

materials (CNRS) and to measure the magnetocaloric effect in isothermal and adiabatic conditions as a function of temperature and magnetic field (INRIM and IFW).

Summary of activities This part of the project was devoted to the development of a less-explored source of generating magnetocaloric effects. In the materials studied in WP2 the magnetocaloric effect is associated with changes in magnetic anisotropy rather than in the degree of magnetic order. The most common of these are so-called “spin reorientation” transitions (SRT) at which the ordering direction of the magnetisation changes. This is due to a strong temperature dependence of the magnetocrystalline anisotropy constants


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(symbol: K ) which are the properties of a magnet that determine the orientation of the magnetisation relative to the crystalline axes, and how easy it is to change that orientation. Much of WP2 was devoted to two material families. The first were hexagonal ferrites, which are natural candidates to study an anisotropy-led magnetocaloric effect, due to the presence of numerous spin reorientation transitions that are composition dependent. Early work by Naiden and co-workers5 had examined the change in orientation of magnetisation at the spin reorientation transtion and the possibility of a large adiabatic temperature change, but there was little calorimetric data. We set about making nanocrystalline samples, with the option of exchange-coupling these to other phases, and also single crystals for identification of the intrinsic entropy change available at the spin reorientation transition(s). The second family were rare-earth Nd-Co intermetallics. During the project, our IFW partner observed a large adiabatic temperature change at the spin reorientation transition in NdCo5. Textured polycrystalline compounds were developed by CNRS and IFW using a multi-stage process of solidification, milling and sintering. Comprehensive characterisation of both families included measurements of magnetisation, crystallite size (by laser light scattering), SEM of sub-micron particles (for the hexaferrites), differential scanning calorimetry, transmission electron microscopy and Peltier cell calorimetry (by INRIM).

3.2.1 Highlights of hexaferrite and Nd-Co activities The highlights presented in this section are arranged under three headings:

• Preparation of M-ferrite and sub-micron W-ferrite • Magnetically oriented polycrystalline W-ferrite, magnetic anisotropy and SRT • Single crystal W-ferrite, magnetic anisotropy and calorimetry around the SRT • Fabrication of aligned NdCo5 intermetallic compacts

Preparation of M-ferrite and sub-micron W-ferrite The production of metallic nanoparticles by cryogenic melting technique is a well-established procedure in CNRS-ICMPE. Our first goal was to implement the capacity for bottom-up synthesis of nanostructured ferrites. M-type hexagonal ferrite BaFe12O19 presents easy axis anisotropy at all temperatures below Curie point. And is easier to produce and require lower temperature of synthesis. However, W-ferrites are reported to display better magnetocaloric properies6. W-type hexagonal ferrites with the general formula BaM2Fe16O27 (where M = Mg, Fe, Co, Ni, Zn, …) are also known to undergo spin reorientation due to changes in the anisotropy configuration. In the substituted BaCoxZn2-xFe16O27 compound with x = 0.7-0.8 the transitions between easy plane (EP), easy cone (EC) and easy axis (EA) magnetization occur in the vicinity of room temperature7. For both hexaferrite types EP ↔ EC is expected to be a first order (sharp) transition whereas EC ↔ EA is generally continuous. To prepare reference and substituted barium hexaferrite powders, a sol-gel technique was implemented. Compared to other methods of nanoparticle production based on mixing components in liquid state, finer particles can be obtained, due to the more homogeneous precursor which requires a lower calcination temperature for single final phase formation. M-ferrites of BaFe12O19 and BaCoxTixFe12-2xO19 (x = 1) compositions were synthesized at temperatures from 600°C to 1000°C to find the optimal production conditions. W-ferrites have much higher temperature of formation. To find optimal conditions of synthesis, variations of the production route were examined and the calcination temperature was adjusted. W-ferrite becomes the major phase at 1300°C while at lower calcination temperatures the final product consists of a mixture of W-ferrite, M-ferrite and S-ferrite (CoFe2O4 with spinel structure). At 1300°C, when W-ferrite phase rapidly grows and becomes dominant, the apparent size of crystallites significantly increases.

5 E.P. Naiden et al., Phys. Stat. Sol. A 120 209 (1990) 6 S.M. Zhilyakov et al., Russ. Phys. J. 36 944 (1993). 7 ibid.; E.P. Naiden et al., Phys. Stat. Sol. A 120 209 (1990); A. Paoluzi et al., J. Appl. Phys. 63 5074 (1988).


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a) b)

Fig. 3.2.1. TEM images of hexaferrite powders: (a) BaCo2Fe16O27 precursor (mixture of ferrites) after calcination at 900°C for 2 h; (b) W-type BaCoxZn2-xFe16O27 (x = 0.75) ferrite synthesized during calcination for 2 h at 1275°C. Direct observations of W-ferrite particles were performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All powders calcined at 900°C (whether they have M-type or W-type nominal compositions) consist of aggregates of clean and almost round particles not exceeding ~ 100 nm, an example is shown in Fig. 3.2.1(a). The shape of crystallites reflects their hexagonal crystal structure. Powders calcined at temperatures high enough to form W-ferrite exhibit different, plate-like morphology as shown in Fig. 3.2.1(b). High-resolution TEM images from thin particles show that they are basically not single crystals. This morphology is observed not only for 1300°C when W-ferrite is almost single phase, but also for lower temperatures when W-ferrite appears in the precursor. The average particle size of BaCoxZn2-xFe16O27 (x = 0.75) powder estimated by laser light scattering analysis is ~ 700 nm, which agrees with TEM observations. Magnetically oriented polycrystalline W-ferrite, magnetic anisotropy and SRT Textured polycrystalline W-type BaCoxZn1–xFe16O27 (x = 0.7) hexaferrites have been prepared using powder magnetic alignment as follows:

• pressing of hexaferrite powder in simultaneously applied magnetic field; • sintering of the crystallographically oriented compact at high temperature.

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H parallel to alignment axis H perpendicular to alignment axis Figure 3.2.2: Adiabatic temperature change in the anisotropic polycrystalline W-type hexaferrite as a function of temperature and magnetic field applied in orthogonal directions.

The powders were synthesized at CNRS using a sol-gel precursor method and studied by XRD, TEM, SEM, VSM. The production parameters were chosen so that to ensure the high purity (negligible amount of secondary phases), good magnetic properties (crystalline structure is well formed) and a limited (sub-micron)


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mean particle size. In particular, the precursor (a mixture of nanostructured metal oxides) was calcined at 1300 °C. Composition BaCoxZn1–xFe16O27 (x = 0.7) gives W-type hexaferrite structure with a SRT close to room temperature. Industrial-grade pressing equipment available in VAC was used to fabricate magnetically aligned solid samples of approximately 9 × 9 × 4 mm size. The magnetic field acts to align the particles along their easy magnetisation direction, and therefore yield a sample with bulk alignment of its constituent particles. Sintering was performed in a dilatometer, to observe the completion of subsequent shrinkage. All pressing conditions and sintering conditions were optimised. X-ray diffraction revealed a preferred orientation of the polycrystalline compacts, as expected if the magnetic orientation was successful. Magnetisation curves also confirmed anisotropy in the compacts, which would be absent (averaged out) if the particles’ relative orientation was random. The last stage was to confirm the presence of a magnetocaloric effect, using the direct method at IFW. Figure 3.2.2 shows the variation in adiabatic temperature change with applied field, at several temperatures. Data taken with the magnetic field applied in different directions relative to the alignment axis of the compact are shown. The MCE is small (<0.2 K) but is interesting in that it has two components: one that is orientation dependent and the other orientation independent. The results are in good correspondence with the direct entropy change measurements on single crystals (Figure 3.2.3 below shows similar behaviour of magnetic entropy where temperatures are lower due to different composition). The MCE is driven by a SRT from the low entropy easy plane state to the high entropy easy axis state. Since the orientation of crystallites in sintered material is always incomplete, single crystals perform better (but are less suitable for practical applications). Single crystal W-ferrite, magnetic anisotropy and calorimetry around the SRT Single crystals of BaCoxZn1–xFe16O27 (x = 0.62 and x = 0.7) compositions were grown using a flux method. The starting mixture was prepared from BaCO3, CoO, ZnO, Fe2O3 and Na2CO3 powders so that estimated hexaferrite to flux (NaFeO2) mass ratio was 2:1. XRD control of single crystals was performed at CNRS. X-ray diffraction confirmed the single crystalline nature of the samples, and their W-type hexaferrite phase composition. Magnetisation measurements (CNRS) of W-type BaCoxZn1–xFe16O27 (x = 0.62) hexaferrite single crystals are presented show typical easy axis anisotropy behaviour at room temperature. The crystal orientations determined from X-ray diffraction and magnetometry match. DSC data seems to confirm that easy plane → easy cone → easy axis spin reorientation in W-type hexaferrites proceeds through two continuous (not first order) phase transitions.

Figure 3.2.3: Entropy change ∆s measured at different fixed temperatures by applying and removing a field of 1 T parallel (left) or perpendicular (right) to the c axis.

To establish the MCE more precisely we performed careful entropy change measurements on an x=0.62 sample of mass m = 31.81 mg and of lateral size of a few millimetres. Isothermal entropy change was measured using INRIM’s bespoke calorimetric set-up (see section 3.3.3). In Figure 3.2.3 the entropy change, measured by applying the magnetic field parallel or perpendicular to the c axis, is shown in a range of temperatures spanning from 120 K to 300 K. In both cases we observe two different classes of ∆s(Ha) curves – either linear, or non-monotonic. From Figure 3.2.3 we see that ∆sk is positive under application of


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Ha parallel to c for temperatures below about 200 K. This corresponds to a transition from a low entropy easy plane configuration toward a high entropy easy axis one. However, ∆sk is negative when Ha is perpendicular to c and the temperature is above 200 K. This corresponds to a transition from easy axis (high entropy) to easy plane (low entropy). From our data we are also able to calculate an anisotropy field K(T), plotted against temperature, obtained from H* after subtraction of the demagnetising field. The data, the first in our knowledge obtained by direct calorimetry, allow to identify: the temperature interval where the transition takes place, the maximum entropy change associated with SRT, and an unexpected observations that the entropy change can be described using a single anisotropy constant K model where K changes sign at the transition temperature. Fabrication of aligned NdCo5 intermetallic compacts The purpose of this work was similar to that on polycrystalline hexaferrite; to take a material with known SRT (and larger associated magnetocaloric effect than the hexaferrite) and make an oriented compact from polycrystalline powder. Such a material should exhibit a spin reorientation MCE, since it consists of aligned, rather than randomised particles, but is much simpler to produce than a single crystal. Spark plasma sintering (SPS) is a “flash” technique of sintering where heat is internally generated by a pulsed current directly passing through the die as well as the powder compact (where a plasma can appear in the gaps between particles). Simultaneously an external pressure is applied. The process is very fast and ensures full densification of nanosized powders without significant coarsening. For us it is also important that the uniaxial pressure can induce some texture in the final product, especially when the starting material is amorphous or nano-structured.8 To increase even more the texture strength, a hot deformation implemented in the same SPS setup (with modified die) can be applied to already sintered sample (hot deformation after hot pressing). NdCo5 (CNRS) and NdCo7.7 (IFW) powders were prepared by rapid solidification and mechanical milling of arc-melted alloys, with shrinking under annealing being monitored as in the case of hexaferrites, in a dilatometer. At all stages (melt-spun flakes, milled powder, sintered sample) the phase compositions of Nd-Co compounds were controlled by XRD. X-ray diffraction was used to monitor the evolution of mean crystallite size (determined by Rietveld refinement) when the nanosized powder produced by high energy ball milling is treated by SPS. Analysis of XRD spectra taken from different sides of sintered samples indicates a slight preferred orientation. Thus, we have produced NdCo5 intermetallic compounds with SRT using a sophisticated procedure comprising rapid solidification, ball milling and spark plasma sintering.

3.2.2 Conclusions of WP2 • Oriented polycrystalline W-type hexaferrites have been obtained by powder pressing in magnetic

field followed by conventional sintering. XRD and VSM confirm crystal texture and anisotropy of magnetic properties.

• W-type hexaferrite single crystals have been synthesized using a flux method. XRD and VSM confirm correct crystal structure and orientation.

• Spark plasma sintering is proposed as a technique to produce textured NdCo5 and related compounds without application of magnetic field.

• Isothermal entropy change and adiabatic temperature change resulted from SRT have been measured in W-type hexaferrite single crystals and anisotropic polycrystalline samples, respectively, as a function of temperature and magnetic field.

• A simple anisotropy model has been proposed for the description of SRT-driven MCE in hard hexaferrites.

8 T. Saito, T. Takeuchi, H. Kageyama, J. Appl. Phys. 97 (2005) 10H103.


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3.3 WP3: Advanced Characterisation Objectives • Standardised methods of measurement and definition of a guide to refrigerant performance • Characterise a set of refrigerants for prototype I • Characterise a set of refrigerants with increased temperature range of operation (for prototype II) • Develop new techniques of measurement as required for comprehensive characterisation of

thermodynamic and/or other relevant refrigerant properties Summary of activities Work package 3 lies at the heart of the SSEEC project as it provides information to WP5 prototype design, modelling of performance WP4, characterisation of materials WP1 and WP2 and fundamental study of materials in WP3. The aim of the work package is to unify measurement method across the partners (using round robin samples), provide advanced characterisation and report on refrigerant capacity of all the materials produced in the program. In the original program we called the measure of material quality a “figure of merit”, but “guide to materials performance” is more accurate. There is no single numeric that can be used to define the quality of the material, and we have recognised it to be far more valuable to plot comparative graphs from which the best material performance can be identified. These graphs are described here. Two round-robin activities were used to compare and contrast measurement activities. The first used Gd as a benchmark as well as La-Fe-Si materials supplied by IFW and VAC. A satisfactory comparison of entropy change (∆S) and temperature change (∆Tad) was obtained. A more detailed study in the second 18 months, presented here, used hydrogenated La-Fe-Si materials from VAC to motivate further comparison of measurement techniques, consistent sample preparation and measurement of MCE materials in the future. We hope that this work will contribute to an ongoing initiative by the International Institute of Refrigeration (IIR) in setting standards for the measurement of MCE materials. Several partners are involved in the latter effort by the IIR’s working group. Lastly we list several developments by partners in the measurement of key quantities for MCE materials. These measurements often contribute to results described throughout the entire report, and to the round-robin activities detailed above. Here we present some details of advances in the measurement of entropy change (∆S, calorimetry) and temperature change (∆Tad) as well as magnetic/thermal imaging, and some key results that have been generated. These techniques will last well beyond the life of the project and will contribute greatly to the visibility of our research and its impact on related areas of science.

3.3.1 Guide to refrigerant performance At month 18, a report on the notion of a “Figure of Merit” was delivered. This found that, rather than a single figure of merit9, a graphical comparison of material performance is most instructive. In particular, the cooling engine analysis in WP5 has highlighted the importance of the adiabatic temperature change ΔTad. A plot of this quantity against peak isothermal entropy change in a fixed field of 2 Tesla (chosen due to the availability of literature data) was chosen as the clearest single plot with which to compare materials, although it is acknowledged that other “dimensions” to this plot are important, for example operable temperature span of the material. But the graphical approach, similar to that of an Ashby Map in materials engineering, is recommended as a general route. The details below are taken from report D3.1 and include a further insight into the maximum size of ΔTad that we should be able to find theoretically. Interestingly it is larger than yet found in any material.

9 The dimensionless form, ZT is used for thermoelectrics. However, the efficiency of a thermoelectric device is not just determined by this material parameter. Indeed, it is known that in such a case, the operating temperature range is key, and research in that field seeks to optimise this quantity, whilst maintaining thermodynamic compatibility of the materials that make up the device. Taking a similar line, we have used the results of modelling work by Camfridge in WP5 to determine the parameter or set of parameters that is most relevant to the efficiency of a magnetic cooling engine.


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In the first 12 months, Camfridge’s modelling of the magnetic regenerator identified and quantified the three principal sources of thermodynamic loss that occur. These are: (a) heat flow between the solid refrigerant and the liquid heat exchange; (b) viscous flow entropy generation in the liquid heat exchanger and (c) thermal backflow in the refrigerant bed. In WP5, the analysis of these losses has resulted in a recommendation for the maximum power of the prototype cooling system. Here we use the results of the cooling power analysis to enable a device-led figure of merit to be established. The cooling power of a device based on a magnetic regenerator scales as adiabatic temperature change, ∆Tad squared. However, much of the materials research literature has focussed on obtaining and comparing values of the field-induced isothermal entropy change, ∆S. This is probably because of the relative ease with which magnetisation measurements can be made, from which (using a Maxwell relation under the correct circumstances) the isothermal entropy change can be obtained. In addition there is no doubt an appeal to measuring the size of the entropy change analogous to the latent heat of vaporisation currently utilised in gas compression devices. A comparison of literature materials There are few straightforward comparisons of ∆Tad for different materials in the literature. The most significant recent reviews10 focus on entropy change. Indeed a plot of isothermal entropy change vs. Curie temperature for a large range of material systems and compositions reveals no clear features (Figure 3.3.1, left).

Figure 3.3.1: Left: A comparison of isothermal entropy change, plotted vs Curie temperature, for various material systems [Brück11 and references therein]. Symbols are: Pentagons: La-Fe-Si materials ; Hexagons: Gd-Si-Ge materials; Circles and squares: MnFe(P,As,Si,Ge) materials apart from 1: DyCo2 , 2: Mn3GaC, 3: Gd, 4: MnAs, 5: MnAsSb 6: LaFe11.2Co0.7Si1.1 , 7: LaFe11.8Si1.2 (melt-spun), 22 and 23: Ni-Mn-Ga materials Right: Our favoured comparison of magnetocaloric materials in terms of peak ∆Tad vs peak ∆S for an applied field change of 2 Tesla. Data sources are given in published Viewpoint article12. Experimental data are diamonds with compositional variations in magnetocaloric prroprties given by the error bars. However, if we instead plot of ∆Tad vs ∆S for materials with magnetocaloric effects in the room temperature range (270 K to 320 K) there are noteworthy differences between materials.13 In such a plot, the ideal material will occupy the upper right area of the figure, with high values of both ∆Tad and ∆S. As can be seen in Figure 3.3.1 (right), current materials are scattered across the plot and several clues as to how to

10 B.F. Yu et al., International Journal of Refrigeration 26 622–636 (2003); E. Brück, J. Phys. D: Appl. Phys. 38 R381–R391(2005); A.M. Tishin, Journal of Magnetism and Magnetic Materials 316 351–357 (2007); M. Liu and B. Yu, J. Cent. South Univ. Technol. 16 1-12 (2009); B.G. Shen et al., Advanced Materials 21 4545 (2009); M-H. Phan and S-C. Yu, Journal of Magnetism and Magnetic Materials 308 325 (2007). 11 E. Brück, J. Phys. D: Appl. Phys. 38 R381–R391(2005) 12 K.G. Sandeman, Scripta Materialia 67 566-571 (2012). 13 It is worth noting that there is a lack of direct ∆Tad data compared to that available for ∆S. In some cases, similar material compositions will have been examined using one technique, but not another. This reduces the data set available to construct Figure 3.3.4.


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compare popular materials emerge. MnAs scores very poorly, as its ∆Tad is very low below 2.5 Tesla, due to the large transition hysteresis. The second order ferromagnets, Gd and La(Fe,Co)Si appear to the left of the plot, their entropy change values limited by the lack of sharpness of their phase transition. The first order materials occupy the right hand side of the plot, with Fe-Rh a notable exception in that its ∆Tad is remarkably high. The theoretical limit of ∆Tad has been explored as part of this WP and is described in the accompanying technical annexe.

3.3.2 Round robin exercise: the need for accurate measurement comparison As the MCE research field grows the community is faced with a specific problem related to the reliability of measurement of the key physical properties M, ∆S, Cp and ΔTad. In order to address this problem we have conducted a second round robin exercise, passing a series of La(Fe,Si,Mn)Hδ samples between four partners (denoted here as IC, IFW, INRIM, and VAC), equipped with different commercial and bespoke measurement facilities in order to examine variability of measurement and analysis methods and the resulting measurements. We found that absolute agreement between measurements is a challenge, primarily due to variation in sample shape and measurement protocols used in different laboratories. Most notably commercial instrumentation can lead to the greatest discrepancies. The work highlights the need for a well characterised set of standards in order to unify methods of measurement across the community working in this field. As an example of the data that we produced, Figure 3.3.2 shows heat capacity data – key data in determining the heat evolved at a phase transition. The top panel shows data taken by INRIM (on their bespoke calorimeter, see section 3.3.3) in different magnetic fields. The lower panel shows data taken by three different partners, including INRIM, on the same sample in zero field. The sample, labelled MCP1011, has the composition LaFe11.384Mn0.356Si1.26H1.52 (see also right hand panel of Figure 3.1.2). We see that in addition to a thermal offset to temperature calibration (between IC’s commercial “PPMS” and INRIM’s bespoke system) there is a large reduction of the heat capacity peak found at the Curie transition (~286 K) in the case of the IFW measurement. Two sources of such a reduction are (i) the pressing process used to produce a pellet for the IFW measurement; and (ii) a difference in the temperature rise used, ΔTrise, when making the measurement in the commercial system. These results and others produced serve to demonstrate the need for full documentation of measurement protocol and careful scrutiny of experimental data. They have been drawn together for publication by the consortium in the International Journal of Refrigeration14.

14 K. Morrison et al., Int. J. Refrigeration, 35 1528-1536 (2012).

3.3.3 Development of specialised characterisation tools A number of advanced characterisation tools have been developed across the partner nodes, now described. Thermal Conductivity (IFW) Although not written into the original Technical Annex, the measurement of thermal conductivity was set up at IFW and samples from WP1 were studied. Combined thermal and magnetisation measurement (Imperial) A probe developed at Imperial combines thermometry and magnetometry. M(H) and sample temperature measurements were performed using a VSM in combination with a Pt100 platinum resistance thermometer

Figure 3.3.2: Heat capacity data for the MCP1011 sample: (a) INRIM data taken in various fixed external fields, and (b) heat capacity in zero field: a comparison of heat capacity taken in three laboratory environments using different methods.


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(sensitivity ±0.01 K) mounted on an alumina block stuck to the free surface of the plate using thin layer of GE varnish. The other surface of the sample plate was attached to the polymer sample holder. All samples were oriented with respect to the field direction to minimise demagnetizing effects. The probe fits into a cryostat with up to 8 Tesla magnetic field. The probe allows the study of temperature rise during the magnetic M-H loop cycle and demonstrates a break down in the isothermal conditions that depends on the sweep rate of the magnetic field (for some samples, sample size dependent). This is shown in Figure 3.3.3 below. Direct ∆T in pulsed magnetic field (IFW) An ultra sensitive ultra fast ∆T measurement system has been developed at IFW allowing a proper adiabatic measure of the temperature rise in pulsed high magnetic field. In this way dynamic trends can be studied. It was found that under adiabatic conditions, the hysteresis is not field sweep rate dependent. The system has been used to examine many of the materials produced by the partnership; results are referred to throughout this report. Direct Calorimetry over extended temperature range (INRIM) In order to be able to perform measurements of the magnetocaloric effect of LaFeCoSi samples with low Co content, the temperature range of the existing setup had to be extended to lower temperatures. To this aim a second, low temperature, calorimeter was developed. With this setup can be measured: a) the isothermal entropy change due to the magnetic field and b) the specific heat as a function of temperature in isofield. The temperature can be changed in the range from 77K to 300K. The measurement is based on the evaluation of heat flux from the sample to a thermal bath measured by Peltier cells. The magnetic field is applied by an electromagnet (Hmax=1.8 T). Thermal imaging (VAC) Vacuumschmelze developed an apparatus for fast, direct measurement of the temperature change of a magnetocaloric material as a function of applied magnetic field. In this apparatus a circular magnet system with four different field strengths (0.4, 0.8, 1.2 and 1.6 T) along the arcs of the circle is rotated over a sample of a size of about 10 x 7 x 1 mm3. The temperature of the sample is continuously monitored using a contactless infrared thermometer. The development of this device was performed outside the budget of the SSEEC project but it has been used within the project.

3.3.4 Conclusions of WP3 • Plots of ∆S vs. ∆Tad provide a simple means by which to compare refrigerant performance. • Two round-robin activities have highlighted the importance of sample preparation and comparative

measurement techniques. The have also verified the accuracy of our methods. • A number of advanced characterisation tools have been developed across the partnership and have

been integrated into our materials development work, enabling greater flow of ideas and materials between partners.

Figure 3.3.3: (a) M(H) loops for a sample Co-doped La-Fe-Si taken at 209 K; the narrow inner loop is at 0.02 T/min (black curve); the middle loop is at 0.2 T/min (red); and the wide outer loop is at 0.7 T/min (green). The critical fields HC1 to HC4 are indicated for the 0.7 T/min curve. Inset to (a) shows temperature dependence of HC1(T). (b) Sample temperature ∆T relative to the bath T.


22

3.4 WP4: Theoretical modelling Objectives • to formulate a micromagnetic model of exchange coupling in nano-composites • to formulate a thermodynamic model of La-Fe-Si materials • to write a report on the modeling of magnetic cooling cycles using La-Fe-Si data • to formulate a kinetic-thermodynamic model of magnetocaloric materials that have a first order phase

transition Summary of activities Modelling has been essential to SSEEC. Our work aimed to take the first truly integrated approach to magnetic cooling for an end-user application. This entails characterisation of materials, integration of those into prototype cooling engines, and integration of those engines into an end-user (air conditioning) system. The modelling required for the engineering of the prototype is contained in WP5. WP4, on the other hand, deals primarily with the magneto-thermodynamics of both single phase, hysteretic (“real”) materials and of nano-scale exchange-coupled systems, that are also less explored experimentally. Theoretical work helps to give us invaluable feedback on experimental development, reducing the time that would otherwise be required to make multiple measurements or a full phase space of physical samples. There are examples of this signposting for experimental work, for example in relation to the activities in WP2 on nano-scale anisotropic materials.

3.4.1 Thermodynamic modelling of La-Fe-Si and Co-Mn-Si (WP1 materials) A key requirement for magnetic refrigeration development is the ability to model the response of a refrigerant when it is subjected to a magnetic field change. When such a materials has a second order transition the usual approach of thermodynamic equilibrium can be applied, while when the transition is first order the presence of hysteresis15 implies the use of hysteresis model.16 One of the objectives of the project is the application of the hysteresis developed by the INRIM group17,18,19 to the description of the materials of the project.

Figure 3.4.1: Solid lines - M vs. T of VAC-MPS1049 sample measured under different magnetic fields. Dots - Model output obtained using the parameters obtained fitting the magnetization loops, introducing the effect of a non perfect thermal contact of the sample with its thermal bath. 15 V. Provenzano, A. J. Shapiro, and R. D. Shull, Nature 429, 853 (2004). 16 G. Bertotti, V. Basso, M. LoBue, and A. Magni, Thermodynamics, hysteresis, and micromagnetics, in Science of Hysteresis vol.II, p.1 (G. Bertotti and I. Mayergoyz eds.) Elsevier, 2006. 17 V. Basso, M. LoBue, C. P. Sasso, G. Bertotti, J. Appl. Phys., 99, 08K907 (2006). 18 V. Basso, C.P. Sasso, M. Lo Bue, J. Magn. Magn. Mater. 316, pp. 262-268 (2007). 19 V. Basso, C.P. Sasso, G. Bertotti, M. LoBue, L. Morellon, C. Magen, Refrigeration Science and technology proceedings (A. Poredos and A. Sarlah, eds.) IIR-IIF, N.2007-1, pp. 263-270 (2007) ISBN: 978-2-913149-56.


23

Modelling thermodynamic refrigerant properties As a test material we selected the La1(FeCoSi)13 alloy family, used in all 3 prototypes, as prepared by VAC. Previous experiments20 and theories21 had focused on the size and origin of the available entropy, rather than the impact of hysteresis on this quantity. The particular alloy selected for theoretical investigation has a first order transition at around 206 K. The basics of the model used are described elsewhere.18,19 The starting idea is that in a first order transition two phases coexist and the resulting hysteresis is the consequence of the small energy barriers that hinder the transformation of one phase into the other. In the model this is done assuming an expression for the Gibbs free energy of each 'pure' phase and an independent description of the energy barriers. For a transition without discontinuities the energy barrier becomes a distribution of bistable units in which both the coercivity and the mean value are distributed. The resulting distribution takes the name of Preisach distribution because the resulting hysteresis scheme is equivalent to the Preisach model of hysteresis. The Preisach distribution was determined by comparison of the model with the experimental magnetization loops that have been measured under strictly static and isothermal conditions. Without changing any of the parameters of the model determined from purely static measurements, M(T)H curves measured by VAC and the entropy curves measured by INRIM under constant magnetic field, s(T)H, have been fitted changing the values for the thermal resistance R only, see Fig. 3.4.1 and Fig. 3.4.2. This demonstrates the accuracy of our thermodynamic modelling.

Thermodynamic cycles INRIM also considered how to compute the thermodynamic cycle of the cross section of an active magnetic regenerator (AMR), the refrigeration device that uses a fluid to exchange heat between the magnetic material and the thermal reservoirs – see Section 2.1. We take a given section of the AMR and we suppose a perfect thermal contact between the material and the fluid and no thermal diffusion along the length. A realistic thermodynamic cycle of two adiabats and two iso-field transformations has been successfully computed, and shows little hysteresis in the case of Co-substituted (non first order) refrigerant materials. In AMR transformations the body is in contact with the fluid and the total entropy (body plus fluid) is then the sum of the two contributions. The example has been computed by solving the equation for adiabatic transformations as given elsewhere.19,22 The computation have been performed in static conditions. From the model, the role of hysteresis of LaFeCoSi is very small, but the eventual presence of intrinsic kinetic effects remains a critical issue the use of magneto-caloric materials with first order transition. Prediction of refrigerant parameters based on the kinetics of transformation of pure materials In order to reach a desired temperature difference in a refrigerator based on a magnetocaloric refrigerant, an AMR cycle has to be run cyclically. It is therefore of crucial importance determine the maximum cycling frequency in order to achieve the maximum cooling power. The upper limit for the operation frequency of a magnetic refrigeration depends on both the intrinsic properties of the magnetic refrigerant and on the extrinsic effects related to the heat flow with the exchange fluid. Among the intrinsic properties there is the characteristic time for the material to achieve the entropy change. In materials with a first order phase transition, this time constant may be large as the transformation kinetics is governed by the energy barriers between the two phases. In this activity we have investigated the problem of the transformation kinetics from the theoretical viewpoint.

20 A. Fujita, K. Fukamichi, J.-T. Wang, and Y. Kawazoe, Phys. Rev. B 68, 104431 (2003); A. Fujita, K. Fukamichi, M. Yamada, and T. Goto, Phys. Rev. B 73, 104420 (2006). 21 M. D. Kuzmin and M. Richter, Phys. Rev. B 76, 092401 (2007). 22 V. Basso, C.P. Sasso, M. LoBue, and G. Bertotti, Int. J. Refr. 29, 1358-1365 (2006).

Figure 3.4.2: Solid lines – Entropy curves of VAC-MPS1049 sample as obtained by integrating specific heat measured at INRIM by the DSC operating in magnetic field. Measurements have been performed without magnetic field and under 1 T. Dashed lines – Model output.


24

To estimate the rate of a refrigerator at 100 Hz we consider that 1/4 of the period is for the application of the magnetic field. With a maximum field of 1T then the maximum rate is 400 Ts-1. The increase of coercivity due to thermal activation is

10

d (nm) zf (Jkg�1) Tf (K) µ0Hf (T) gc (Jkg�1) �c (s) µ0Hmin (Ts�1)0.1 3.7·105 3.1·104 6.7·103 9.2·106 0.63 1.1·104

1 3.7·102 3.1·101 6.7 9.2·103 0.63 113 13.6 1.1 0.25 3.4·102 0.72 0.3410 0.37 3.1·10�2 6.7·10�3 10.6 28 2.4·10�4

100 3.7·10�4 3.1·10�5 6.7·10�6 1.4 � ⇤ � 0

From the table one sees that the most reasonable values are obtained by linear volumes in the range d � 10 nm. Toestimate the rate of a refrigerator at 100 Hz we consider that 1/4 of the period is for the application of the magneticfield. With a maximum field of 1T then the maximum rate is µ0H ⇥ 400 Ts�1. The increase of coercivity due tothermal activation is

�Hc = Hf ln

�Hfast

Hslow

⇥(46)

by using d = 10 nm, µ0Hfast = 400 Ts�1 and µ0Hslow = 1 · 10�3 Ts�1 we have µ0�Hc = 0.08 T, an increase ofcoercivity which is completely negligible in a practical application. However these e⇥ects are extremely sensible to dand by using d = 3 nm we have µ0�Hc = 3.2 T: a large increase of magnetic field that would require a decrease tothe maximum operating frequency.

[Avrami-1939] M. Avrami, Kinetics of Phase Change. I, Journal of Chemical Physics 7 p.1103 (1939).[Basso-2000] V. Basso, C. Beatrice, M. LoBue, P. Tiberto, and G. Bertotti, Connection between hysteresis and thermal relaxation

in magnetic materials, Phys. Rev B 61 p. 1278 (2000).[Basso-2007] V. Basso, C.P. Sasso, M. Lo Bue, Thermodynamic aspects of first order phase transformations with hysteresis in

magnetic materials, J. Magn. Magn. Mater. 316, pp. 262-268 (2007).[Bertotti-2006] G. Bertotti, V. Basso, M. LoBue, and A. Magni, Thermodynamics, hysteresis, and micromagnetics, in Science

of Hysteresis vol.II, p.1 (G. Bertotti and I. Mayergoyz eds.) Elsevier, 2006.[Brown-1996] S. D. Brown, R. Street, R. W. Chantrell, P. W. Haycock, and K. OGrady, Time dependence and mechanisms of

magnetization reversal in Tb-Fe-Co films, J. Appl. Phys. 79, 2594 (1996).[Estrin-1989] Y. Estrin, P. G. McCormick, and R. Street, A phenomenological model of magnetisation kinetics, J. Phys.:

Condens. Matter 1, p. 4845 (1989).[Gardiner-1985] C. W. Gardiner, Handbook of Stochastic Methods, Springer- Verlag, Berlin (1985).[Kuzmin-2007c] M. Kuz’min, Factors limiting the operation frequency of magnetic refrigerators, Appl. Phys. Lett. 90, 251916

(2007).[Planes-1992] A. Planes and J. Ortin, Study of thermoelastic growth during martensitic transformations, J. Appl. Phys. 71,

p.950 (1992).[Raquet-1996] B. Raquet, R. Mamy, J.C. Ousset, Magnetization reversal dynamics in ultrathin magnetic layers, Phys. Rev. B

54 p.4128 (1996)[Risken-1996] H. Risken, The Fokker-Planck Equation, 2nd ed. Springer-Verlag, Berlin (1996).[Street-1949] R. Street and J. Wolley, A study of magnetic viscosity, Proc. Phys. Soc., London, Sect. A 62, p.562 (1949).

By using realistic parameters we have µ0Hc = 0.08 T, an increase of coercivity which is completely negligible in a practical application. However these effects are extremely sensitive to the transition lengthscale; changing this from 10 nm to d = 3 nm results in µ0Hc = 3.2 T: a large increase of magnetic field that would require a decrease to the maximum operating frequency. Magneto-elastic effects In parallel with the hysteresis models which are essentially phenomenological in nature, we have investigated the microscopic origin of a first order phase transformation by analyzing the coupling between magnetism and structure. For this purpose one has to develop microscopic models of the magnetic and of the structural constituents. These models have been already studied in the MCE literature but have yielded discrepancies when compared with experimental data. A proper reconsideration was needed. The point of our work has been to describe the properties of the structural lattice in the simplest possible way, but taking into account the compatibility with the thermodynamic requirements constituted by the Maxwell relations. The model we have investigated is the Bean-Rodbell model23 which predicts magnetic first order transitions because of the dependence of the ferromagnetic exchange interaction on the interatomic distances. This fact creates an interaction between the magnetism and the structural lattice which may also correspond to an enhanced magnetocaloric effect. In our work we have discovered that the MCE is not always increased by the lattice, and that this occurs only for certain combinations of structural and magnetic parameters.

3.4.2 Exchange coupling in nano-structured refrigerants We have investigated the possibility to enhance the MCE by considering a composite material with a nanoscale magnetic (“exchange”) coupling between a two phases. There are two examples:

1. A spin reorientation (SRT) phase of known MCE, with a soft ferromagnetic phase of high magnetic moment but with no relevant MCE. The aim is to discover whether the second ferromagnetic phase can enhance the properties of another MCE material.

2. Two hard ferromagnets, with perpendicular easy axes. The aim is to examine the possibility for generating a MCE from two phases that alone have no significant MCE. The mechanism of MCE in the composite should be an “artificial” spin reorientation, as we expect a spontaneous change of magnetisation direction of one or both phases as a function of temperature.

The first route led to a rationalisation of WP2, as the second phase was found theoretically only to reduce the available entropy change. However, the second example led to the possibility of new MCEs due to artificial spin reorientation and should be the subject of experimental investigation beyond the life of SSEEC. Composite of SRT material and soft ferromagnet Here the main MCE phase is the Co-Zn substituted W-type barium ferrite, see Section 3.2.1. With increasing temperature, this material displays a change in the orientation of the magnetization from easy plane to easy axis at T=230 K. The theoretical analysis (based on the theory originally developed by the group of Asti24) reveals that there is an entropy change associated with the reorientation of the spins and its magnitude is related to the temperature dependence of the anisotropy constants. It is then immediately clear that the best material for MCE is the one with the highest change of anisotropy. We have computed the MCE around the spin reorientation transition. This is done by taking an expansion for the anisotropy energy at the third order of the anisotropy constants25 and taking systematically the global minimum of the free energy. We have checked against existing literature26 and found some discrepancies. The value of the MCE is rather low in comparison to most MCE materials, and compared to earlier claims. We have therefore considered to enhance the effect by considering anisotropic nanocomposite materials.

23 C. P. Bean and D. S. Rodbell, Phys. Rev. 126, 104 (1962). 24 G. Asti, F. Bolzoni, F. Licci and M.Canali, IEEE Trans. Magn. 14, pp. 883-885 (1978). 25 H. Horner, C.M. Varma, Phys. Rev. Lett. 20, 845 (1968); G. Asti and F. Bolzoni, J. Magn. Magn. Mat. 20, 29 (1980). 26 E.P. Naiden and S.M.Zhilyakov, Phys. Solid State 39, 967-968 (1997).


25

Intrinsic parameters can be artificially set by a proper manipulation of the nanostructure of the composite material. For example the saturation magnetization of a compound can be increased by introducing a material with a high saturation magnetization such as iron. By using the intrinsic parameters of the ferrite under investigation and of iron we derive a required grain size of 10 nm, a number which is promising for the possible practical realization of such a nanostructure because many current techniques for the preparation of nanosized grains can realize particles with controlled size in that range. From the viewpoint of the gain in magnetocaloric performances for the chosen CoZn W-ferrite material the anisotropic nano-composite does not represent an enhancement. In fact, the effect of increasing the saturation magnetization is mainly that of increasing the temperature range and decreasing the entropy change. In the case of the ferrite the entropy change is relatively low but the temperature interval is potentially wide. Therefore in order to exploit the potential of the exchange coupling for magnetic cooling one has to look for materials with a spin reorientation characterized by higher and narrower entropy peak. Therefore with a very high dK/dT. These material can be found in the rare-earth transition-metal compounds (e.g. Nd-Co) which were investigated in parallel in WP2. Composite of two hard ferromagnets In this subtask we considered how to exploit the MCE associated with the temperature dependence of the magnetocrystalline anisotropy energy K1(T). In the expression of the entropy of a magnetic system one finds: i) one term proportional to dMs/dT, which describes the contribution associated with ordering or disordering of magnetic spins, and ii) one term proportional to dK1/dT , which is an additional contribution related to the magnetic anisotropy energy. The microscopic origin of the term proportional to dK1/dT is the anisotropic magnetization mechanism27: i.e. the spin system is more disordered (high entropy state) when the magnetization points along an hard direction. The interest in the anisotropy degree of freedom is in the fact that the entropy change can be achieved by a rotating field rather then by an alternating one, with possible simplification in refrigeration devices.28 We have examined the magnetocaloric effect associated with a nanocomposite in which two magnetic materials A and B have both uniaxial anisotropy, but orthogonal orientations of their easy axis. The exchange-coupled composite may give rise to an artificial reorientation of the magnetization and the associated MCE may be possibly maximized. To simplify the problem we take a multilayer with alternating A and B layers: we consider material A with an easy axis directed along z and material B with an easy axis directed along y perpendicular to z. This is depicted in Figure 3.4.3. If one has the A material with dKA/dT > 0 and the B material with dKB/dT > 0 (i.e. dK1B/dT < 0) the two contributions will sum up and the entropy change between (||) and () will be maximised. In order to determine the conditions for exchange coupling between A and B we have studied a multilayer problem by minimizing the micromagnetic energy of the composite.

3

T

K

KA

KB

KA+KB

FIG. 1: Sketch of anisotropy constants as a function of temperature for layers A and B giving a maximum entropy change atthe transition temperature (KA + KB = 0)

III. STUDY OF THE MULTILAYER STRUCTURE

The exchange coupling has been between studied in several contexts and for di�erent magnetic materials. Gonzaleset al.[Gonzales-1993] and Navarro et al. [Navarro-1993] discuss exchange coupling in hard-soft composites. Theproblem of the exchange coupling in a magnetic multilayer structure in micromagnetic theory has been studied byAsti et al. [Asti-2004] who studied the nucleation field of hard-soft multilayer. They used the integral of the Euler-Lagrange equation (see Appendix) but made a fourth order power expansion around � = 0 to determine the switchingfield. Alvarez-Prado et al. [Alvarez-Prado-2007] solved numerically the micromagnetic equation for the case of abilayer with orthogonal anisotropy axis. Dubuget et al. [Dubuget-2009] solved numerically the integral equation forthe bilayer. Here we solve numerically the problem under rotating magnetic field in order to determine the typicalthicknesses for exchange coupling.

A. Multilayer problem

Here we pose the problem of exchange coupling between two layers A and B with di�erent magnetic properties.The problem of the minimization of the micromagnetic energy by di�erent methods is summarized in Appendix A.We consider a composite system composed by an infinite sequence of alternating layers A and B (shown in Fig.2).We limit to the case in which the magnetic field is in the (y � z) plane (Hz = H⇥ and Hy = H�) and magnetizationchanges in space only along x. We have mx = 0 and my = sin �, mz = cos �. We let the limits of the system in the(y � z) plane to go to infinity and in this way the configurations are stray field free (HM = 0). The thickness of thelayer A is dA, the thickness of the layer B is dB . The sum is 2d = dA + dB . We place the point xA in the middle oflayer A, the point xB in the middle of layer B and the point x0 at the conjunction of the two layers. Any solutionmust have a period 2d in space and the points xA and xB are extrema of the solution:

⇥�

⇥x

��xA,xB

= 0 (11)

For the multilayer geometry described before we have the e�ective field:

Heff =2A

µ0Ms

⌅�m

⇥⇥�

⇥x

⇤2

+dmd�

⇥2�

⇥x2

⇧� 1

µ0Ms

⇥fAN

⇥m+ Ha (12)

laying in the (y � z) plane, the boundary condition is

4

z

y

x

A B

d

x0xA xB

θ

FIG. 2: Illustration of the angle of the magnetization �(x) in the multilayer. xA and xB are the middle points of the layers Aand B respectively.

AA⇤⇥

⇤x

��x�0

= AB⇤⇥

⇤x

��x+0

(13)

and the dynamic equation

Heff⇥ = �G⇤m⇤⇥

⇤⇥

⇤t(14)

that for the amplitude is

Heff⇥ = �G⇤⇥

⇤t(15)

To obtain the amplitude of Heff⇥ we multiply Eq.(12) by dm/d⇥ = cos ⇥ j � sin ⇥ k which is perpendicular tom = sin ⇥ j + cos ⇥ k, then

Heff⇥ =2A

µ0Ms

⇤2⇥

⇤x2� 1

µ0Ms

⇤fAN

⇤⇥�Haz sin ⇥ + Hay cos ⇥ (16)

B. Perpendicular anisotropies

For the uniaxial magnetic anisotropies the e�ective field Heff⇥ = Heffi is

Heffi =2Ai

µ0Msi

⇤2⇥

⇤x2� 2Ki

µ0Msi

sin ⇥ cos ⇥ � (Haz sin ⇥ �Hay cos ⇥) (17)

By defining the anisotropy field

Figure 3.4.3: Left: Sketch of anisotropy constants as a function of temperature for layers A and B giving a maximum entropy change at the transition temperature (KA + KB = 0). Right: Illustration of the angle of the magnetization θ(x) in the multilayer. xA and xB are the middle points of the layers A and B respectively. Exchange coupling has been studied in several contexts and for different magnetic materials.29,30 Here we

27 E.R. Callen and H.B. Callen, J. Phys. Chem. Solids 16, 310 (1960). 28 M. D. Kuz’min, A. M. Tishin, J. Phys. D: Appl. Phys. 24, 2039 (1991). 29 G. Asti, M. Solzi, M. Ghidini, and F. M. Neri, Phys. Rev. B 69, 174401 (2004). 30 J. M. Gonzalez, F. Cebollada, A. Hernando, J. Appl. Phys. 73 p. 6943 (1993); I. Navarro, E. Pulido, P. Crespo, A. Hernando, J. Appl. Phys. 73 p.6525 (1993); L. M. Alvarez-Prado, S. M. Valvidares, J. I. Martin, and J. M. Alameda, Phys.


26

have solved the problem numerically under rotating magnetic field in order to determine the typical thicknesses for exchange coupling. The anisotropy constant and the saturation magnetization are taken to be the same for both layers and we are interested to the behavior under rotating field. For our work we can determine the thickness of the layer needed to achieve the desired coupling. A typical thickness d=2lw is sufficient to reduce the magnetic field necessary for the rotation to 0.1 times the anisotropy field, where lw is the exchange length. Using this model, we have been able to select materials that would maximize the MCE of this new kind of spin reorientation, and the thickness of those materials required to obtain zero anisotropy at room temperature. As a result we might expect to find this new form of MCE in future experimental studies, beyond SSEEC.

3.4.3 Conclusions of WP4 • Successful modelling of the static, thermodyamic properties of MCE materials with continuous phase

transitions • Prediction of the effect of dynamics in first order transitions, defining a range for the increase in

magnetic field requirement, depending on transformation kinetics • Thermodynamic cycle modelling • Determination of the MCE of a composite of spin reorientation material and a soft ferromagnet

(reduced entropy change, enhanced temperature range) • Established new MCE due to “artificial spin reorientation” in a hard ferromagnetic multilayer system

3.5 WP5: Prototyping Objectives • Specification of end-user application (power, temperature span etc.) • Refrigerant characterisation (heat capacity and shape of refrigerant plates) • Delivery of 3 prototypes

o a zero-load, 15 K span cooling engine (prototype I) o 35 K span, multi-refrigerant system (prototype II) o an integrated heat pump (prototype III) span and field as in prototype II with complete

integration with heat pump/air conditioner application; heat exchangers, control systems and environment

Summary of activities WP5 is the technology stream of SSEEC, aiming to exploit the properties of magnetocaloric materials to deliver a cooling system capable of powering a domestic heat pump appliance. The resulting cooling system must be capable of being cost-competitive with current gas compressor technology. In this regard the single most important system variable is frequency of operation, as highlighted in our initial proposal; maximisation of operating frequency minimizes the amount (and thus the cost) of refrigerant and permanent magnetic material required for a given quantity of cooling power. It is therefore an objective for the technology delivered in this work package to operate at the maximum possible frequency. At the same time the goal is to deliver systems with greater than 40% exergy efficiency, using refrigerants that are lower cost (than Gadolinium), have low hysteresis (i.e. no intrinsic refrigerant losses), are not environmentally toxic and are capable of being produced in scale. The goals of WP5 have been achieved through the design and development of three prototypes systems (the three deliverables and milestones of this work package), of ever increasing scale over the duration of the project. WP5 was structured to progressively developed three prototype systems – the first demonstrating that La-Fe-Si-Co is a viable low-cost non-gadolinium containing refrigerant, the second to extend the operating temperature range of La-Fe-Si-Co-based refrigerant systems by using a multi-material approach, and the

Rev. 76, 214419 (2007); V. Dubuget, A. Thiaville, F. Duverger, S. Dubourg, O. Acher, and A. Adenot-Engelvin, Phys. Rev. B 80 134412 (2009).


27

third focusing on a fully integrated magnetically based heat pump system driven by the requirement of Clivet, defined within the first 6 months of the project. Within this progressive framework, several additional elements were to be demonstrated:

• Increase in production of LaFeSiCo by VAC from gram to kg quantities • Progressively increase the operating frequency of prototype systems from 1Hz to 10Hz and above

From the outset a key challenge was identified as obtaining the required geometries from the new materials. The consortium researched all aspects of magnetically driven heating and cooling systems - the active magnetocaloric materials, their shaping, the design of the magnetic regenerator, heat exchangers and the integration of all components into a final application. The first 24 months of SSEEC established the potential performance of LaFeSiCo as a magnetic refrigerant, and demonstrated that the operating span of a LaFeSiCo-based cooling system could be extended to 35K by using a cascade of differently doped materials with varying Curie temperatures along the length of the regenerator. The final prototype demonstrates that magnetic cooling can be a technically viable alternative to the gas compressor by meeting the size and the weight requirements for certain applications; indeed follow-on work from prototype III indicates it can also be a viable commercial competitor, albeit not for heat pumps but rather for smaller appliances.

3.5.1 Application specification The specific application selected by Clivet within the first 6 months of the project was the Elfo air-air exchanger, a system for air renewal and purification in buildings. A key element of the Elfo system is a heat pump technology that actively recovers heat from extracted interior air which is used to pre-heat fresh filtered replacement air from the external environment. The application is shown in Figure 3.5.1. Table 3.5.1 below summarises Clivet’s requirements for the magnetic cooler (prototype 3) required for the Elfo system. As part of the WP5 effort, a comprehensive cost-benefit analysis of magnetic cooling at different levels of cooling power was undertaken, and reported at the 12 month point. The results of this analysis, which motivate future work in the domestic cooling sector, are given in Section 4.1.1.

Figure 3.5.1: Clivet’s Elfo system; the compressor sits inside the box pumping heat from air flowing through one duct to

the other. (Source: Clivet)

Specification Clivet (Current) Magnetic Target

Cost: €/kW 13 Open Weight: kg/kW 10 Overall weight @10 kg

Volume: dm3 / kW 35 Comparable to gas compressor Efficiency (W/W) 40%-50% >50%

Operating Temperature Range (Span) (K)

30 K 30 K

Cooling Power (kW) >1 Open Table 3.5.1: Clivet specifications for prototype 3


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3.5.2 Refrigerant characterisation (heat capacity and shape of refrigerant plates) Heat capacity Heat capacity is the critical measurement for designing and optimising the shape for active magnetic regenerators; from this single measurement most of the relevant thermodynamic properties of the refrigerant materials can be derived. A new piece of equipment (developed by Özcan and Burdett at Cambridge) has facilitated the measurement of heat capacity and entropy of first and second order magnetic materials in a magnetic field using the Quantum Design PPMS system – this unique piece of kit has now been adopted by Quantum Design and is available from them. Shaping of Co-substituted LaFeSi The simplest parallel plate refrigerant geometry was selected as it seemed the easiest structure to fabricate in LaFeSiCo using the techniques available within the consortium. For a material such as LaFeSiCo the “ideal” plate geometry for 10 Hz operation required plates with flatness tolerance of maximum 3%. This was according to the ideal geometry specified by Camfridge to VAC near the beginning of the project. Co-substituted LaFeSi has a complex metallurgy making this brittle material hard to cut. However, with the development and refinement of the TDR (thermal decomposition and recombination) process in the first 18 months of the project, VAC successfully solved the material science issues associated with cutting the LaFeSiCo alloys (see section 3.1.1). This technique allows the shaping of material through subsequent cutting using wire EDM – VAC’s cutting technique of choice. Regular geometries can thus be created. The tolerance requirement is complicated by two factors. Wire EDM – performed on an industrial scale – can easily generate rough surfaces and the stresses in the plates can create bowing of the cut plates during the recombination process. The issue of bowing (figure 3.5.2, left) of the plates during the final recombination heat treatment was solved by using a bespoke La-Fe-Si support structure. Using this technique sufficiently flat plates can be produced (figure 3.5.2, right).

Figure 3.5.2: Left: Bow caused by unsupported heat treatment; Right: VAC’s solution yields no bow in the plate.

Wire EDM cutting of the material provided complicated and challenging throughout the project. Figure 3.5.3 clearly shows the required tolerances are achievable. However, the higher throughput machines available tended to deliver material more like figure 3.5.4. The tolerances achievable are roughly twice as large as the target tolerance.

Figure 3.5.3: Ideally cut material – completely flat (Image: Camfridge)

Figure 3.5.4: Non-ideal material - showing waviness (Image: Camfridge)

The limitations encountered in the strength and shaping of the material and the resulting reduced specification limits the maximum operating frequency to 5Hz (rather than 10Hz). This, coupled with the difficulties in achieving large scale high-quality production, severely limited the achievable cooling powers for prototype 3. The material processing limitations have prompted further collaboration to develop higher tolerance, better quality and less expensive processing techniques.


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3.5.3 Delivery of three prototypes The final deliverable, a fully integrated system, using the highest possible operating frequencies required careful design from the outset. The operating frequency focus was critical to achieving size, weight and power targets of an end-user application. If 1 kg of refrigerant can deliver 100 J of cooling per cycle, then when cycling at 10 Hz a total of 1 kW of cooling could be achieved. Thus operating frequency is a key driver to scaling cooling power upwards, or the size, weight or cost of a magnetic system downwards. The challenge with increasing operating frequency is to do so without increasing losses. Extensive regenerator modelling was undertaken by Camfridge, to identify the loss mechanisms and ways to minimise their effect. Two critical loss mechanisms identified were pressure losses (which increase the input work to the system, lowering the overall efficiency), and heat exchange losses (associated with the dynamic transfer of heat between the exchange fluid and the refrigerant) which limits the useable cooling power. Pressure losses can be minimised by using refrigerants shaped in regular geometries to ensure near laminar exchange liquid flow; the alternative - a random alignment of refrigerant such as through a powder bed - massively increases pressure losses as operating frequency is increased and severely limits the potential efficiency achievable at high frequencies. Regular shaped geometries were thus selected. Heat exchange losses can also be reduced through shaping. Narrow flow channels in the direction of heat transfer (which is transverse to the flow direction) minimise these losses. Such a requirement translates into achieving tight tolerances on shaped parts; this makes them harder to fabricate. Failure to achieve tight tolerances leads to hard to assemble regenerators and non-reproducible results. Finally, the overall frequency performance of the system can be increased by reducing the length scale of the refrigerant; effectively reducing the total cyclic quantity of heat being transferred per unit volume of exchange fluid. This necessitates small refrigerant length scales.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

-20 -10 0 10 20 30 40 50 60 70T [°C]

−Δ

S m [J

/(kgK

)]

MPS/1110MPS/1111MPS/1112MPS/1113MPS/1114MPS/1116MPS/1117MPS/1118target

Figure 3.5.5: Left: A set of tuned VAC-LaFeSiCo refrigerants; Right: Multi-material Regenerator (materials from VAC) delivering 34.7K span (source: Camfridge)


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0

1

2

3

4

5

6

7

8

9

10

-20 0 20 40 60temperature (°C)

-ΔS m

(J kg

-1 K

-1)

ZMP-1064ZMP-1065ZMP-1066ZMP-1067ZMP-1068

ΔH = 16kOe

Figure 3.5.6: Materials produced for prototype 3 (Source: VAC).

Operating span Prototype 1 yielded a 15 K zero-load span using a single Co-substituted La-Fe-Si material thereby satisfying the target associated with the relevant deliverable. Prototype 2 demonstrated that an operating span of 35 K is achievable; see Figure 3.5.5 (right, again achieving the target set out in the project plan). For prototype 3, VAC produced bulk materials with more finely tuned Curie temperatures (Fig. 3.5.6). These materials have even more accurate Curie temperatures than those produced for prototype 2, thus enhancing operating span. Final prototype (prototype III) Figure 3.5.7 (top) shows for the first time (a world first) the magnetic cooling engine (designed by Camfridge) that is the same size and comparable weight to a gas compressor. In terms of absolute size and weight the design for prototype 3 meets Clivet’s requirements. The magnetic cooling engine is to be plugged into a pair of characterised heat exchangers simulating the ducts in the Elfo system (Figure 3.5.7, left). In associated work, Camfridge has developed a novel magnetically flow control system that:

• Easily integrates the magnetic cooling engine with Clivet heat exchangers (figure 3.5.7) • Fully supports regenerative AMR cycles • Minimises dead volume • Eliminates the need for sliding seals • Avoids fluid mixing in heat exchangers • Scales to an arbitrary number of regenerator pairs • Enables simple integration with appliances • Utilizes a single hot and cold exchanger • Requires a single pump to drive the system • Does not need secondary loops • Provides control mechanisms for defrosts and temperature control


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Figure 3.5.7: Top: Comparison of gas compressor (left) with the Camfridge’s Prototype 3 (right); Left: Clivet Elfo test

system integrated with the magnetic cooling engine; Right close up of cooling engine (Images: Camfridge)

Cooling power and efficiency The biggest challenge for the prototype 3 is the cooling power. The only way to achieve a kW of cooling within weight and volume constrained design is to increase operating frequency. However, as discussed earlier, the maximum frequency of operation is limited by material processing (shorter length scales and better flatness31) and the result achieved enables 5 Hz operation. Prototype 3 should achieve at least ~100 W cooling power over a 35K operating range, below the Clivet application target but achieving the original research proposal target in terms of operating temperature range and driving field. We note that this power range is also more in line with the cost-benefit optimum presented in Section 4.1.1. Without final system testing, the efficiency of the system has not been determined. The design specification is 50% efficiency, and all aspects of design have been directed to achieving this.

3.5.4 Conclusions of WP5 WP5 has achieved most of its goals

• 3 prototype magnetic cooling engines have been constructed using Co-doped La-Fe-Si • Temperature span targets have been met in prototypes I and II • End user integration has been performed in prototype III • A technology roadmap that best fits the characteristics of magnetic cooling has been developed

(see Section 4.1.1)

The SSEEC project has rapidly advanced the development of magnetic cooling technology. Materials, produced for the first time to specification from a production process, have been integrated into a series of prototype systems targeted at achieving technical and commercial specifications for the technology. The relationship between Camfridge and Vacuumschmelze is continuing, and is focused now on the development of cooling engine for domestic appliances (see section 4.1). As a result of SSEEC, an increasing number of domestic fridge manufacturers (Whirlpool, Indesit and Arcelik) are interested in the technology.

31 All of which is achievable, but advanced powder processing techniques are required; these being outside the scope of the SSEEC project.


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Although the testing of prototype 3 is not fully complete at the time of writing this report, it can be demonstrated that this work, now redirected towards domestic appliances, continues to move forward rapidly. In particular Camfridge, with Vacuumschmelze material, expects to showcase a prototype domestic appliance within the next few weeks, directly exploiting many of the development within the SSEEC project. The SSEEC project has led to a far better commercialisation roadmap that seeks to reduce costs even further by reducing, even eliminating entirely, rare-earth materials. This will allow the cost of magnetic cooling to compete directly with gas compressor based technologies used in the next generation of high-efficiency domestic appliances. Although SSEEC has demonstrated immense development in the processing of refrigerant materials, the project has also highlighted that this is the key area for further development work. Modelling work, begun within SSEEC, is further refining the material shaping requirements. A detailed refrigerant processing roadmap has been developed, allowing the identification of key partners whose involvement would create a more complete industrial supply chain containing the processing techniques needed for even better, cheaper and more scalable refrigerant processing.


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4. Potential impact and main dissemination activities

4.1 Potential impact Today, approximately 1/3 of the energy consumed in EU countries is destined to comfort in buildings (heating, air cooling and air exchange). As far as energy consumption of buildings is concerned, EC initiatives have mainly focused on the development of renewable energies (with associated long times and elevate set-up costs) and to the reduction of energy requirements (building energy efficiency certification). Nevertheless in the short term, significant results can be achieved by the adoption of low energy consumption systems. Focussing on cooling engines, there is considerable legislative pressure on manufacturers. Firstly, as a result of the 1998 Kyoto protocol on Climate Change, popular conventional refrigerants such as HFCs are classed as greenhouse gases. In a European context, the implementation of the European Union's WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restrictions on the Use of Certain Hazardous Substances) Directives requires cooling engine manufacturers to examine their current systems and look for alternatives with a lower environmental impact over the complete lifetime of the product. The net effect is that solid state cooling technology is attracting interest from all sectors. However, the research and development teams required for a paradigm shift away from gas compression do not exist in any refrigeration, heat pump or air conditioning company, and until SSEEC the viability of magnetic cooling engines was at least partly limited by the refrigerant materials available. It was therefore imperative to bring together the necessary steps in the evolution of a magnetic cooling engine, starting from the goal-driven synthesis and characterisation of refrigerant materials. Our work represents, to our knowledge, the first such integrated approach. Whereas our end use is focused here on heat pumps for buildings, the potential impact of our magnetic refrigerant materials is even wider; namely in the refrigeration sector. A comparison of the rate of innovation in the refrigeration sector in Europe with that in the Far East reveals a worrying trend, but one which could be reversed by this proposal. Significantly, industry leaders in domestic refrigeration informed us that, whereas the number of patents filed in Europe in the refrigeration sector plateaued 8 years ago (and is now falling), the number filed was still rising in the Far East when SSEEC began. We hope that our project has served to reduce the gap between Far Eastern and European innovation in this area by researching the materials necessary for energy efficient magnetic cooling.

4.1.1 Magnetic cooling: the future The WP5 team considered concurrently what sectors would most benefit from switching to a magnetic cooling engine, in order to achieve the maximum possible synergy of the partners' skill sets, and to maximise the project's benefits for society beyond its 3-year life. At the 12-month point, a careful system-wide cost and efficiency analysis revealed the benefit of magnetic cooling at low powers (< 500 Watt). This led to a re-evaluation of the long-term road map for the technology.

Thermodynamic

Ideal

Current Solution (Butane)

Magnetic A+++ Solution (no

vacuum panels) Target Cooling Power (W) 30 30 30 External Temperature (˚C) 25 40 35 Internal Temperature (˚C) 5 -15 0 Cooling Engine Power (W) 30 150 30 Technology Efficiency (%) 100% 43% 50%

Running Time (%) 100% 20% 100% Carnot COP 14 2 4

Relative Efficiency 100% 14% 28% Energy Consumption (W) 2.2 15 7.5

Table 4.1.1: Implication of magnetic refrigeration on efficiency of low power applications


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Figure 4.1.1: Efficiency versus cooling power for gas compressor.

An analysis of the efficiency shows that the efficiency of low power gas compressors falls dramatically – by 50%. This means there is a significant opportunity to improve lower power cooling appliances – such as the domestic fridge. The analysis shown in Table 4.1.1 shows a magnetic solution embedded into, for example an A+ cabinet can easily meet the A+++ specification as defined in EC C(2010) 6481 final. This can be achieved without the use of expensive vacuum panels in the insulation, as is preferable. Coupling the avoidance of vacuum panels with the observation that the cost per watt of a gas compressor also rises at lower power we can determine that a magnetic solution can competitively enter the market. Indeed, this result has enabled Camfridge to collaborate with leading domestic fridge manufacturers – Whirpool (#1 in the world), Arcelik (#1 in UK and #3/4 in Europe) and Indesit (#3/4 in Europe). This exciting result has profound implications for the potential societal impact of magnetic cooling. Heat pumps are a small, albeit growing market, and they are already rather efficient – operating at 50% of Carnot. The domestic fridge, is a far larger market, only achieves 13-15% of Carnot efficiency (depending on the model – see table 4.1.1). Magnetic cooling has the potential to reduce the electricity consumption of domestic fridge by 50%. Currently the total electricity consumption of domestic fridge is about 5-6% of Europe’s total electricity production. This translates into significant CO2 savings. In addition the adoption of magnetic cooling will have a significant impact on the reduction of greenhouse gas emissions through the solid nature of the refrigerant. A recent UN Environment Programme (Unep) report32 projected that the global warming potential of HFCs in 2050 could be comparable with current emissions from the global transport sector reinforcing the need for alternative refrigeration technologies. The key challenge for magnetic cooling is the shaping of refrigerant materials. Powder processing routes look the most promising and should be the topic of future projects. Improved tolerances will allow a further operating frequency increase – to 10Hz and beyond. First order materials – rather than the Co-substituted LaFeSi materials used in all 3 prototypes here - will allow magnetic field reductions, further shrinking size, weight and cost. The Mn/H –doped La-Fe-Si systems developed in WP1 will be ideal for this purpose and should be the topic of future collaborative research. Competing in the kW range is potentially difficult – these systems are already efficient, very compact and relatively light weight. However, in the low power regime, magnetic cooling looks not only comparatively very efficient (100% improvement), but by eliminating vacuum panels the technology can also be cost competitive.

32 “HFCs: A Critical Link in Protecting Climate and the Ozone Layer”, http://www.unep.org/dewa/Portals/67/pdf/HFC_report.pdf


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4.1.2 Magnetic phase transition research Our work has investigated continuous and first order phase transitions from many perspectives. We have aimed to advance: theoretical understanding, modelling of static and dynamic phenomena, tailoring of materials from theoretical work, bespoke experimental probes for the measurement of key materials properties, synthesis of micro-scale and nano-scale materials and single phase metastable alloys, shaping of refrigerant plates and non-destructive hydrogenation. Our work on refrigerants will be of relevance to a number of communities in physics, physical metallurgy, materials science and chemistry. The control of functional materials is a burgeoning research field, and the simultaneous control of morphology, crystal structure and function is of interest in fields ranging from multiferroicity to novel forms of device memory. The partners have sought to disseminate research results in a wide selection of forums and have established research symposia at international meetings in order to raise the profile of magnetic cooling research and engage with researchers in this field and in related ones. We therefore anticipate that our new experimental tools (see Section 3.3.3) will be used for the examination of a number of materials systems in the future, both in MCE research and in other fields. Our dissemination activities and the questions of shaping raised above have already sparked new collaborations and efforts towards further investigation of key material properties that will bring magnetic cooling as a high efficiency energy conversion technology closer to fruition.

4.2 Main dissemination / exploitation of results We have exploited many different forms of dissemination, a list of which is given in section A, “Use and dissemination of foreground” of our SESAM submission accompanying this document. A total of 146 separate acts of dissemination were carried out, ranging from journal publications, oral and poster presentations at conferences, invited seminars at various institutions across the world, media interviews, magazine articles and articles in the popular press. These help to demonstrate that magnetic cooling is receiving increased attention within the physics, materials science and engineering research communities and from refrigerations industries and the general public. A summary under relevant headings is now presented. Journal articles During the course of the project (Oct 2008 – Sept 2011) we have published 16 peer-reviewed journal articles in high impact journals such as Physics Review Letters, Acta Materialia and Applied Physics Letters. Many of these involve co-authorship by two or more project partners, highlighting the degree of interaction within our consortium. Since the conclusion of the project another 8 peer-reviewed journal articles have followed, including one in Nature Materials. The total of 24 journal articles thus far will continue to grow in the coming months. Conference presentations The visibility of magnetocaloric materials and magnetic cooling research has greatly increased during the project, in no small way due to the activities of the consortium partners. Members of the consortium gave over 50 invited or contributed presentations at national or international conferences. Symposia on, or featuring, magnetocalorics and magnetic cooling were organised either wholly or in part at several conferences by Oliver Gutfleisch (IFW Dresden) and Karl Sandeman (SSEEC coordinator, Imperial), thus adding to the opportunities for dissemination of our results and interaction with other researchers. These symposia were held at Euromat 2009 (Glasgow, UK), MRS Fall 2010 (Boston, USA), TMS 2011 (San Diego, USA) and Euromat 2011 (Montpellier, France). Invited presentations During the course of the project a total of 18 invited presentations were given by partners at local meetings, workshops or as seminars at research institutions. These invitations were worldwide; seminars were given across Europe, the US, Canada, Australia and in Japan. The coordinator was invited to speak about SSEEC at the inaugural ICYRAM meeting in Singapore, after the project’s conclusion. Media interviews and articles in the popular press The application of magnetic cooling to different market sectors continues to attract interest from general media and industry-led journals. Camfridge gave 3 BBC interviews during the project and featured in


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articles in New Scientist, The Daily Telegraph and 3 industry-led magazines. The coordinator wrote an invited article on gas-free refrigeration featuring the work of SSEEC for the launch edition of Magnetics Technology International in 2011. IEEE Distinguished Lecturer Series 2011 Oliver Gutfleisch (IFW Dresden) was one of the 2011 IEEE Magnetics Society’s distinguished lecturers, and gave 19 lectures across Europe, USA, China and Japan on the subject of “Magnetic materials in sustainable energy”, including the work of SSEEC, during the period January-September 2011. This dissemination activity continued after the project with a further 16 lectures across Europe and the USA during Oct-Dec 2011, yielding a total of 35 lectures delivered worldwide.

Figure 4.2.1: Some examples of dissemination by SSEEC partners


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5. Project website and contact details The project website is www.sseec.eu and www.sseec.org. Both route to the same set of webpages. The website has attracted over 2200 visitors since tracking statistics were first collected in January 2009. 70% of visitors are from Europe, 22% from the USA and 7% from Asia. The website also acted as a central resource for partners to share key information, via a private area. Contact details for the 7 consortium partners are given below. 1. Imperial College (coordinator) Dr. Karl Sandeman EXSS Group, Department of Physics, Imperial College London Prince Consort Road London SW7 2AZ United Kingdom Email: [email protected] Telephone: +44 207 594 7861 Fax: +44 207 594 2077 2. IFW Dresden Dr. Oliver Gutfleisch33 Institut für Materialwissenschaft FG Funktionale Materialien Technische Universität Darmstadt Petersenstr. 23 64287 Darmstadt Germany Email: [email protected] Tel: +49 6151 16-75559 Fax: +49 6151 16-72559 3. CNRS-SATIE Dr. Martino Lo Bue ENS de Cachan 61, Avenue du Président Wilson 94235 Cachan France Email: [email protected] Tel: +33 1 47 40 74 89 Fax: +33 1 47 40 21 99 4. INRIM Dr. Vittorio Basso Istituto Nazionale di Ricerca Metrologica Strada delle Cacce 91 10135, Torino Italy Email: [email protected] Tel: +39 011 3919 842 Fax: +39 011 3919 834 33 Prof. Gutfleisch moved to TU Darmstadt after the SSEEC project ended. His new (non-IFW) contact details are given.

5. Clivet S.p.A. Neviano dal Degan Clivet S.p.A. Via Camp Lonc, 25 Z.I.Villapaiera Feltre 32032 Italy Email: [email protected] Tel: +39 0424 503625 Fax: +39 0439 313382 6. Vacuumschmelze GmbH & Co. KG Dr. Matthias Katter Vacuumschmelze GmbH & Co. KG Grüner Weg 37 D-63450 Hanau Germany Email: [email protected] Tel: +49 6181 38-2083 Fax: +49 6181 38-82083 7. Camfridge Ltd. Dr. Neil Wilson Copley Hill Business Park Lower Court 1 Babraham Road Babraham Cambridge CB22 3GN United Kingdom Email: [email protected] Tel: +44 7903 502329

project final report publishable summary

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