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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 29 (2009) 893–897

The unification of paste rheologies for the co-extrusionof solid oxide fuel cells

Jonathan Powell a,∗, Stuart Blackburn b,c

a Department of Metallurgy and Materials, The University of Birmingham, UKb Department of Chemical Engineering, The University of Birmingham, UK

c Interdisciplinary Research Centre in Materials Processing, The University of Birmingham, UK

Available online 20 September 2008

bstract

urrent methods used to manufacture solid oxide fuel cells (SOFCs) involve multiple steps, applying successive layers to produce laminatedomponents associated with repeated sintering steps. This work aims to cut the number of processing steps involved by co-extruding a completeOFC tube from pastes in a single step, which is then co-sintered. Several aspects must be overcome in order to co-extrude such a fuel cell. Crucially

he co-extrusion process requires that the rheological properties of the materials are well matched. This prevents the formation of flow defects andllows for the continuous operation of the co-extrusion process. This work has investigated the manipulation of the rheological parameters throughesign of the paste formulation as well as the manufacturing process.

Two pastes, one made from a fine yttria stabilised zirconia (YSZ) and the other from a combination of a fine YSZ, a coarser nickel oxide and

ctivated carbon, have been characterised using the Benbow/Bridgwater model and their parameters have been unified through adjustment of theirolids loading. Using packing theory and the assumption that pastes will only flow when the powder void space has been filled with a slight excessf liquid to overcome inter-particulate friction, an intermediate paste has been designed to possess similar rheological properties.

2008 Elsevier Ltd. All rights reserved.

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eywords: Co-extrusion; Solid oxide fuel cell; Paste rheology

. Introduction

Solid oxide fuel cells (SOFCs) are considered to offer aighly efficient, environmentally friendly solution to the grow-ng problem of providing sustainable power generation for theuture. Fuel cells electrochemically convert the chemical poten-ial energy contained in fuels directly into electrical energy. Theyork in a similar manner to batteries, except that fuel cells areesigned to be continuously replenished with the reactants ashey are consumed, and their electrodes are catalytic and areherefore not used up. Fuel cells are typically designed to run onydrogen and oxygen, but can be run on other fuels.

There are a variety of fuel cells available, but of particularnterest here are the SOFCs, which are in essence a combination

f functional ceramics.

The SOFC uses a non-porous ceramic material as the elec-rolyte, which is typically zirconia that only permits the transfer

∗ Corresponding author. Tel.: +44 121 4143432; fax: +44 121 4143441.E-mail address: jdapowell@hotmail.com (J. Powell).

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955-2219/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.jeurceramsoc.2008.07.038

f oxygen ions. The anode and the cathode are usually made oforous ceramic and cermet materials, through which electrons,ases and ions can travel. The most common materials used forhe anode and cathode material are NiO/YSZ cermets and stron-ium doped lanthanum manganite respectively. However manyther materials, such as gandolinia doped ceria (electrolyte) oranthanum nickel ferrite (cathode), may be considered whereo-processing is being proposed (Fig. 1).

SOFCs come in two main forms: a planar design where theuel cells are stacked together or as tubular structures. Cur-ent planar manufacturing methods involve adding layers onto arefabricated anode or cathode support. This allows for the man-facture of thin electrolyte layers, helping to improve the powerensity of the fuel cell. However these processes require manyrocessing and sintering steps, increasing the production costsf the fuel cell. Self-supporting SOFCs use much thicker elec-rolytes, but co-processing potentially allows manufacture to be

arried out in fewer steps, as the electrolyte can be manufacturednd sintered with the anode and/or the cathode.

Co-sintering presents its own problems due to thermo-hysical incompatibilities between the materials. During

894 J. Powell, S. Blackburn / Journal of the European Ceramic Society 29 (2009) 893–897

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Fig. 1. Schematic of a self-supporting SOFC and its p

ooling, residual stresses in the SOFC structure can causeracking due to differences in thermal expansion and thermalxpansion rates. It is therefore important to match materials, sohat these stresses are minimized. One method of doing this iso use a functionally graded design, where the two materials areonnected by intermediate layers made with graduated compo-itions changing from one layer to the next. As well as reducinghe thermal and residual stresses this structure also increases theumber of triple phase sites, where the electrons, gas and ionseet to electrochemically react.The manufacture of a multilayered self-supported tubular

OFC in a single manufacturing step also poses its own diffi-ulties. The co-extrusion of multiple pastes requires fine tuningnd unification of the paste rheologies in order to prevent theormation of flow defects and to allow the use of continuousxtrusion. This, therefore, requires the formulation of a num-er of pastes containing powders with differing particle sizes,ize distributions, morphology and surface chemistry, as well asore size, distribution and shape. Previous works by Liang andlackburn1,2 have looked at the co-extrusion of multilayered

ubes.This paper examines the matching of two paste rheologies,

hich will form the YSZ electrolyte and the NiO/YSZ anode80/20, w/w). Using packing theory and the measured rheolog-cal parameters, the required liquid content of a third paste has

een predicted in order to obtain three pastes of similar rhe-logy. A methodology is proposed for the prediction of pasteormulations for the formation of a complete anode–electrodetructure.

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sed use in a domestic combined heat and power unit.

. Experimental procedure

.1. Materials

The electrolyte paste was prepared using an yttria stabi-ized zirconia powder HSY8 (Daiichi Kigenso Kagaku Kogyoo., Japan), which has an average particle size of 0.5 �m.he anode pastes were made up as mixtures of HSY8, blackickel oxide (−325 mesh, Cerac, USA), and activated carbonNorit, UK), which acted as a pore forming agent. A proprietaryrganic binder (FMG, UK) was fixed throughout the experi-ents.

.2. Paste preparation

The powders were mixed thoroughly in the dry state prior toetting. The liquid was added to the powder and the materialas then transferred to one of two mixers, a twin roll mill and a

-blade kneader (Werner and Pfleiderer, D).The material was mixed for a total of 15 minutes on the twin

oll mill. The paste was finally removed as a sheet and rollednto rods ready for laying up into the extruder. This method of

ixing is limited in terms of sample size and is open to thetmosphere limiting the mixing period due to loss of binder.he twin roll mill can only mix pastes over a narrow range of

olid loadings. When the solids loading is too low, the paste isot sheared effectively and cannot be removed from the mill. Ifhe solids loading is too high, the paste becomes unworkable inater processes.

uropean Ceramic Society 29 (2009) 893–897 895

3

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hBtstthe intercept of the plotted line on the x axis can, by conversion,give an approximation of the void volume in-between the packedparticles at maximum random packing.

J. Powell, S. Blackburn / Journal of the E

In the z-blade kneader the material was mixed for a totalof 4.5 h inside a small sealed unit, kept cool using a supply ofwater through a jacket. Material was removed from the sides ofthe container and the blades intermittently to ensure thoroughmixing. A much wider range in solids loadings could be madeusing this mixer, making it possible to produce pastes with awider range of rheologies.

All pastes were left to equilibrate for half an hour after mixingand before extrusion, therefore eliminating any time dependentvariation between pastes.

The initial anode pastes were made up with 30% carbon, 56%NiO and 14% HSY8 by weight. The carbon was added to giveporosity to the anode. Each paste was manufactured using thesame binder system formulation. The electrolyte and the anodepastes were made to a selection of solids loadings, working tothe limits of the mixing and the extrusion process.

2.3. Paste characterization

The paste rheological parameters were evaluated by capillaryrheometry using a ram extruder driven by a compression loadframe. The 20.3-mm diameter barrel was fitted with interchange-able dies of 6.0 mm diameter, and lengths 8.0, 24.0 and 47.3 mm.The die entry angle was 90◦. The pastes were extruded by adjust-ing cross head speed, giving extrudate velocities ranging from5.000 to 0.125 mm s−1. The data was then used to characterisedthe pastes by applying the Benbow/Bridgwater six parameterequation3–5

P=P1 + P2=2(σ0 + αVm) ln

(D0

D

)+ 4

(L

D

)(τ0 + βVn)

(1)

where P is the total pressure drop and P1 and P2 are the pressuredrops due to convergent flow and plug flow respectively. D0 isthe barrel diameter, D is the die diameter, L is the die length andV is the extrudate velocity. The convergent flow yield stress isrepresented by σ0 and the plug flow initial wall shear stress isrepresented by τ0. β and α are the velocity factors, that representthe velocity dependency of the bulk and the wall shear stressrespectively and are related to viscosity in liquid shear flow. V,the extrudate velocity, can therefore be related to the shear rate.m and n are the velocity exponents relating to bulk and wall shearstress respectively. For the most part these are not reported inthis text and fixed at a nominal value but are evident from thecurvature of the theoretical fits in Fig. 2.

A series of pastes which varied in solids loading were testedboth for the electrolyte paste and the end member anode paste.The relationship between the paste rheological parameters andthe solids loading could then be analysed. From this informationit was possible to predict the required solids loadings for pasteswith intermediate compositions, made up from components inthe electrolyte and the anode pastes that possess similar rheolog-

ical parameters. For the purposes of this study a six parametermodel was employed to characterise the paste flow behaviour.However only the yield stress and velocity factors were used tomatch the paste rheologies.

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Fig. 2. Benbow/Bridgwater model and experimental extrusion data.

. Results and discussion

The paste rheological parameters were obtained using theenbow/Bridgwater fit. The parameters were put back into thequation and the fitted extrusion pressures plotted with the exper-mental data, as shown in Fig. 2. The relationship offers anccurate representation of the experimental data, and fitted thexperimental data with typical R2 values of 0.99. A plot of extru-ion pressure versus L/D can also indicate how well a paste isixed and for most pastes this plot gave a linear relationship,

howing that the paste is not changing along the die land as its extruded. However for low liquid contents the data tended toeviate from this linear relationship, indicating that the pastesere not being adequately mixed, or that they were more sen-

itive to localized loss of solvent giving an inhomogeneousaste.

Data for the bulk yield stress (σ0) is represented in Fig. 3. Itas been shown previously by Benbow et al.3 and Blackburn andöhm,6 that there is an approximate linear relationship between

he reciprocal parameters and the liquid content. It has also beenhown that pastes are unextrudable when containing less liquidhan that required to fill the inter-particulate voids. Therefore

ig. 3. Relationship between the reciprocal convergent flow yield stress and theaste liquid content.

896 J. Powell, S. Blackburn / Journal of the European Ceramic Society 29 (2009) 893–897

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Table 1Rhelogical parameters and liquid content of the rheologically unified pastes

Anode paste z-Blade-mixed electrolyte paste

Liquid (vol%) 17.74 11.70σ0 1.01 0.67α

β

τ

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ig. 4. Relationship between the reciprocal convergent velocity factor and theaste liquid content.

The method by which the Benbow/Bridgwater parametersre sequentially calculated causes a decrease in the accuracy ofome parameters. As σ0 and α are the first to be calculated theyive the most consistent results, which can be seen in Figs. 3–6.t was possible to fit straight lines to all the plots of 1/α and 1/σ0sing least squares regression, however the parameters 1/β and/τ did not always follow a linear relationship with the liquid

0ontent of the paste.

In the case of the NiO–C–ZrO2 pastes the 1/τ0 data implieshat for the range of solid loadings analysed, the initial wall shear

ig. 5. Relationship between the reciprocal wall shear yield stress and the pasteiquid content.

ig. 6. Relationship between the reciprocal wall shear velocity factor and theaste liquid content.

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Rσ

α

β

τ

23.30 23.305.33 10.56

0 0.11 0.11

tress decreases with increasing solids loading. The carbon usedn these formulations has a very high surface area and consider-ble work is required to force the liquid phase to fully wet theystem. This occurs most readily in the pastes with lower liquidontents where better mixing is achieved. In the opposite sense,t is possible that the higher pressures required for extrusion atow liquid contents, are insufficient to release the liquid tied upn the carbon after mixing and lubricate the die walls.

Data has not been presented here for a z-blade mixed anodeaste, as that mixer was unable to produce a paste with theseaterials; this is probably due to insufficient shear stress being

pplied to the system.The electrolyte pastes produced using the two different mix-

rs show different rheological behaviour, with the z-blade mixedaste showing a stronger rheological dependence on the solidsoading. However the data generated shows, that the minimumiquid content measured from the Benbow/Bridgwater parame-ers can be accurately measured when mixed by either method.

he implication is, that provided sufficient torque can be gener-ted, the two mixers performance is similar, when the free liquidver and above that required to fill the inter-particulate voids ist a minimum, and the expected extrusion pressures are high.

able 2enbow/Bridgwater parameters and RBV values for the intermediate paste

olids mass fractionZrO2 0.54NiO 0.40Carbon 0.06

BV 1.730 0.81

26.2813.15

0 0.13

Fig. 7. RBV values for intermediate compositions.

J. Powell, S. Blackburn / Journal of the Europe

Fig. 8. Composition diagram. Open circles are specimen formulations. Closedctc

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ircles predicted formulations required for co-extrusion. Curved lines are con-ours of equal RBV. A is the electrolyte composition. B is the outer anodeomposition.

Table 1 shows the Benbow/Bridgwater paste parameters andhe liquid content of the selected anode and electrolyte paste for-

ulations, which provide the most closely matched rheologicalarameters. It was found that the closest fit could be achieved,f the electrolyte paste was z-blade mixed, as this provided aroader range in rheological parameters from which to selectmatch. The range of possible paste parameter values in the

win roll milled pastes was extremely limited particularly for α

nd β and effective matching was not possible. The parametersere matched in the sequence α, σ0, τ0 and β to attain the best

ompromises in fitting.From the data presented it is possible to calculate the for-

ulation of any intermediate composition. The intercept of theeciprocal parameters on the x axis can be used to calculate theoid fraction of the constituent solids. This leads to the packingraction (1-void fraction), that can be converted to the relativeulk volume (RBV) for ease of graphical representation. RBVs the reciprocal of the packing fraction. In this case the RBVf the intermediate compositions was calculated using Karlssonnd Spring model.7 Fig. 7 shows the curve obtained from thist. Reverse calculation from the RBV plot (Fig. 7) allows theolume of liquid required to fill the voids of the composition toe calculated. This, when combined with the average volumef liquid over and above that required to just fill the voids androvide the required paste properties (from Figs. 3–6) allowsntermediate paste formulations to be derived.

A paste containing 60% ZrO2 and 40% NiO by volumeafter firing) was formulated following the above procedure andrepared following the standard mixing procedure, then rhe-

logically tested. Its derived parameters are given in Table 2.he parameters are closely matched to the electrolyte and anodeastes and the predicted formulation was therefore appropriatend effective. These pastes are well matched for co-extrusion.

6

7

an Ceramic Society 29 (2009) 893–897 897

The procedure outlined above assumes that the intermediateompositions have a progressively decreasing addition of theore forming agent towards the final electrolyte compositionhich contains no pore forming agent. This may not yield theost ideal structure for optimisation of the fuel cell kinetics in

peration, thus a method of calculating the required paste for-ulations must be considered. The data presented in Fig. 7 is

epresented by the line AB in Fig. 8. The preferred compositionould lie on the line ACB. Within the ZrO2–NiO–C space a con-

our plot of RBV derived from 1/σ0 = 0 can be created using therinciples outlined for the bimodal plot shown in Fig. 7. Fromhis data a direct make up of matched pastes should be possible.or any of the proposed compositions shown in Fig. 8, a pro-edure of reverse calculation could be performed, as describedbove for the preparation of the intermediate paste formulation,resented in Table 2. Current work is evaluating the success ofhese predictions.

. Conclusion

It has been possible to unify the rheology of three pastes.his has been achieved by analysing the relationship between

he rheological parameters and the solids loading of the two endember compositions and using that data to predict interme-

iate packing behaviour. Knowledge of the packing behaviourllowed calculation of the intermediate composition formula-ions, reducing the number of experiments required to obtain

atched rheology. This work has also shown that the method ofixing the paste plays a significant role in the manipulation of

he paste rheology.

cknowledgements

Thanks go to Dr. T. Button, Mr. G. Dolman and Mr. C. Meggsor their advice on paste formulation and extensive use of theirquipment. Thanks also to Mr. P. McGuire for his help withodeling of the extrusion. The EPSRC is acknowledged for thenancial support for this work.

eferences

. Liang, Z. and Blackburn, S., Co-extrusion of multi-layered tubes. Br. Ceram.Proc., 1998, 58, 113–124.

. Liang, Z. and Blackburn, S., Design and characterisation of a co-extruder toproduce trilayer ceramic tubes semi-continuously. J. Eur. Ceram. Soc., 1998,21, 113–124.

. Benbow, J. J., Oxley, E. W. and Bridgwater, J., The extrusion mechanics ofpastes: the influence of paste formulation on extrusion parameters. Chem.Eng. Sci., 1987, 42, 2151–2163.

. Benbow, J. J., Lawson, T. A., Oxley, E. W. and Bridgwater, J., Predic-tion of paste extrusion pressures. Am. Ceram. Soc. Bull., 1989, 68(10),1821–1824.

. Benbow, J. J. and Bridgwater, J., Paste Flow and Extrusion. Oxford Series of

Advanced Manufacturing. Clarendon Press, Oxford, 1993, pp. 7–11.

. Blackburn, S. and Böhm, H., The influence of powder packing on pasteextrusion behaviour. Trans. IChemE. A, 1993, 71, 250–256.

. Karlsson, K. and Spring, L., Packing of irregular particles. J. Mater. Sci.,1970, 5, 340–344.