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Inorganic Materials Chemistry

and Functional Materials

Helmer Fjellvåg and Anja Olafsen Sjåstad

Lectures at CUTN spring 2016

Material Synthesis

2

Materials

Natural materials (wood, bone, leather, cellulose, minerals, etc..)

Metals

Semiconductors

Non-metallic inorganic materials (oxides, ceramics, glass)

Organic Polymers

Composite materials (polymer-, ceramic-, metal matrix)

Inorganic Materials Synthesis

Bulk powders

Nanoparticles

Thin films - coatings

Metals-alloys

Surfaces

Single crystals


Solid State Reactions Nanoparticle synthesis

Solids from the gas phase


Solid State Reactions

Our text book has extended the definition

to any reaction involving a solid:

•Solid/solid

•Solid/gas (Reaction, decomposition)

•Intercalation

•(Solid/liquid)

Solid state reaction: a reaction between two or more solids.

Solid State Reactions

7

- Direct reaction between two or more solids to form the final product. In

principle no decomposition is involved.

Ceramic Method – ‘Shake and Bake’

- Solids do not react with solids at room temperature even if

thermodynamics is favorable; i.e. high temperatures needed.

- Solid-solid reactions are simple to perform, starting materials

are often readily available at low cost and reactions are ‘clean’;

i.e. do not involve other elements (beneficial for the industry.)

- Disadvantages include the need for high temperatures, the

possibility of inhomogeneity, contamination from containers etc.

8

Ceramic Method – Synthesis of YBCO

ABO3

YBCO = YBa2Cu3O7x

9

YBCO = YBa2Cu3O7x

Oxidation step


- Direct reaction between Y2O3, BaO2, CuO

(Reaction between three solid components)

- Grind to obtain large surface area (see next slide)

- Press into pellets (contact)

- Heat in alumina boat, temperature profile:

10

Surface area:

Surface area is increased by crushing/milling

1 cm cube: surface area 6 cm2

1 mm cubes: surface area 60 cm2

10 m cubes: surface area 6000 cm2

10 nm cubes: surface area 6000000 cm2 (600m2)

Solid State Reactions – General Aspects

11

- Other precursors may be used; e.g. BaCO3, which may be decomposed to

fine grained BaO during the reaction.

(oxalates, nitrates, cyanides etc. also work fine). Why such precursors?


Keywords:

- Create more surface area of component by “chemical grinding”

- Decomposition of the precursor must be performed in a controlled manner in

order to avoid violent decomposition (i.e. choosing an appropriate

temperature). Why?

Keywords:

- Splashing in furnace alternate composition, contamination of furnace

- Other reasons why e.g. BaCO3 or Ba-oxalate is a better precursor than BaO

in synthesis of YBCO by the ceramic method?

Keywords:

- BaO very basic oxide, i.e. react with CO2 and H2O air – alter composition

12

- Will chloride salts be suited precursors? E.g. BaCl2?


Note:

Many other ways of synthesizing YBCO:

Precursor methods

Sol/gel

Flux

Many melt-texturing methods

including partially melting/fluxing

- Regrinding – Why?

Keywords:

- Chlorides not form volatile products – i.e. reminds in the powder

Keywords:

- Reduce diffusion distances – see next slide

13

• Nucleation at the interface (depends on the

degree of structural rearrangement involved)

• Formation of product layer, two reaction

interfaces

• Diffusion/counter diffusion through the

product layer - growth

Longer diffusion paths, slower reaction rate.

Simple case: lattice diffusion through a planar

layer (rate law 1/x):


Ctktxxkdt

dx )ln()(1

A(s) + B(s) C(s)

x = thickness of layer

t = time

14


How to increase rate of reaction, dx/dt?

1. Increase area of contact between reactants

2. Increase rate of nucleation

3. Increase rate of diffusion – ions through various phases

15



1. Increase area of contact between reactants

(cross-reference to earlier slide)

- Small particles by grinding, ball milling, freeze drying etc.

16



2. Increase rate of nucleation

Nucleation may be facilitated by structural similarity between the reacting

solids, or products.

For compounds produced by solid state reaction, often a structural

orientational relationship between the reactant, nuclei and product is seen.

(In the reaction between MgO and Al2O3 to form MgAl2O4 (spinel), MgO

and spinel have similar oxide ion arrangements. Spinel nuclei may then

easily form at the MgO surfaces (see below)).

17

Epitactic: structural similarity restricted to the surface / interface

between two crystals (common arrangement of ions at the interface)

Topotactic: Structural similarity through the crystal. Reaction is

controlled by the crystal structure of a reactant.

Structural similarity through - Epitactic and Topotactic

Reactions


MgO and MgAl2O4 have O atoms in cubic close packing

Al2O3 crystallizes in a completely different structure (Rhombohedral)

Spinel nuclei form on the MgO surface - structural orientational relationship

as well as similar interatomic distances (topotactic reaction)

18


MgO

Example: Production of the MgAl2O4 spinel from MgO and Al2O3

Al2O3 MgAl2O4

19

Topotactic Reduction


O2

LaNiO3.00

LaNiO2.75

LaNiO2.50 LaNiO2.00

LaNiO3-x LaNiO3-x’ + O2(g)

Zr + O2(g) ZrO2

Sample

Oxygen getter

Zr, NbO,..

20

- High temperatures, as a rule of thumb, at 2/3 of the melting

temperature (of one component) the diffusion is sufficient to achieve solid

state reactions.



- Reduction of diffusion distances by incorporating the cations in the

same solid precursor or perform a co-precipitation reaction – calls for

more advanced methods

3. Increase rate of diffusion – ions through various phases

- Diffusion is a thermally activated process, following an Arrhenius

behavior

21

Nucleation limited reactions may be described by the Avrami-Erofeyev

equation:

x(t) = 1-exp(-ktn) {or x(t) = 1-exp(-(kt)n)}

n is a real number, usually between 1 and 3 (n>4 is often interpreted as autocatalytic

nucleation)

For n > 1 the curve is sigmoidal

Note: This is more empirical for most solid state reactions. It is difficult to talk about

reaction order and activation energy for these reactions.


x = thickness of layer

t = time

Will not be discussed at lecture

22

BaTiO3 - electroceramic (thermistors, capacitors, optoelectronic devices, DRAMs)

Formation of BaTiO3 by reacting BaCO3 and TiO2 is an example of a seemingly simple reaction is

more complex than expected.

• BaO react with the surface of TiO2, forming nuclei and a surface layer of BaTiO3.

• Reaction between BaO and BaTiO3 to form Ba2TiO4. NB! This is a necessary phase for

increasing migration of Ba2+ ions.

• Ba2+ ions from the Ba-rich phase migrate into the TiO2 phase and form BaTiO3.


Recipie – ‘Shake and bake’

BaCO3 is decomposed to reactive BaO: (Rock salt type structure, ccp of the oxide anions, Ba2+ in

octahedral sites),

TiO2 (Rutile type structure, hcp of oxide ions, Ti4+ in half of the octahedral sites)

Synthesis of BaTiO3 – nucleation of an intermediate phase

At least three stages are involved in formation of BaTiO3 from BaO and TiO2.


23

Reactions between two solids:

Reaction rates depends on:

• Area of contact between the reacting solids, i.e. surface area and “density”

• The rate of nucleation

• Rates of diffusion of ions (and other species)

Disadvantages, e.g.:

•Nucleation and diffusion related problems (high temperature)

•Formation of undesired phases (reaction paths) (e.g. Ba2TiO4)

•Homogeneous distribution, especially for dopants, is difficult

•Difficult to monitor the reaction directly, in-situ…??

•Separation of phases after synthesis is difficult

•Reaction with containers/crucibles

•Volatility of one or more of the components


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Chemical Mixing in Solution

1. Precursors via co-precipitation

2. Precursors via solid solutions and compounds

Precursors for Ceramic Synthesis

Decrease diffusion lengths by using intimately mixing of cations.

Solid precursors containing the desired cations.

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Precursors for Ceramic Synthesis

Precursor via co-precipitation:

Salts of different metals are precipitated together (as low solubility solids).

Either a solid solution or an intimate mixture of two salts.

Precursor via solid solutions and compounds:

Cations in targeted composition are incorporated in a stoichiometric solid.

Precursor converted to targeted compound via thermal decomposition at

relatively low temperature – frequently in more steps.

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- Soluble salts of the desired cations are dissolved in solvent (usually in water)

- Trigging co-precipitation by heating (evaporation of solvent) or by addition of a

precipitating agent (forming insoluble salts)

- Hydroxides, carbonates, oxalates, formates, citrates… (What about nitrates?)

Example from text book: Preparation of the spinel ZnFe2O4

1) Zinc and iron oxalate are dissolved in water in correct ratio

2) Heated to evaporate water precipitate as fine powder, solid solution?

3) Heat to decompose:

Fe2(C2O4)3(s) + Zn(C2O4) (s) ZnFe2O4 (s) + 4CO(g) + 4CO2(g)

Precursor Formation – Co-precipitation

Bigger grains of Fe2(C2O4)3 and Zn(C2O4) converted to finer grains or a

solid solution of Fe-Zn-Ox better mixing of atoms

Keywords: nitrate may act oxidizing during decomposition; high solubility

27

Target:

Formation of mixed oxides: M1xM’xO, (M, M’ = Ca, Mg, Mn, Fe, Co, Zn, Cd)

Carbonate Precursors Forming a Stoichiometric Solid Solution

Calcite, CaCO3

M1xM’xCO3 M1xM’xO + CO2(g)

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Prepare a slightly acidic solution of

the targeted cations (e.g. nitrates) in

desired ratio

Add ammonium carbonate,

(NH4)2CO3 to precipitate

Filter and wash the precipitate

Calcine in appropiate atmosphere

for 0.5 – 150 hours at 800-1000 °C

Co-precipitation depends on:

•Similar solubility of the metal salts

•Similar precipitation rate

•Formation of solid solution

Successful preparation by calcination

may depend on similar

decomposition temperatures for the

metal precursors

•Structural similarity and compatibility

of the oxides

Carbonate Precursors – Preparation Method

M1xM’xCO3 M1xM’xO + CO2(g)

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Low decomposition temperature

may allow formation of oxides

preserving a high oxidation state.

Formation of CaMnO3 by

standard methods requires

1300°C, days of heating and

repeated crushing/heating.

Using a carbonate precursor

CaMnO3 may be prepared at

900°C for 30 minutes.

May also be used for preparation

of low temperature phases.

Note change in oxidation state for

Mn in this example

Carbonate Precursor Method - Example

CaMnO3

Ca0.5Mn0.5CO3 + O2(g) Ca0.5Mn0.5O3 + CO2(g)

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Other Oxides by the

Carbonate Precursor Method

Note:

Startig with cations with oxidation state +II; Final product may have cations

with other oxidation states – choice of atmosphere during calcination.

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Hydroxides Ln1xMx(OH)3 (Ln = La, Nd, M = Al, Cr, Fe, Co, Ni) Ln1xMxO3

La1xyM’xM’’y(OH)3 (M’ = Ni, M’’ = Co, Cu) La1xyM’xM’’yO3

LaFe0.5Co0.5(CN)65H2O LaFe0.5Co0.5O3

Co-precipitation Stoichiometric Solid Solutions

Nitrates

Ba0.5Pb0.5(NO3)2 BaPbO3

Note:

Change in oxidation state of

Pb from +II to +IV

Cyanides

32

NiFe2O4 may be prepared from: Ni3Fe6O3(OH)(OAc)1712py

200-300 ºC to burn off the organic part

Heating in air at 1000 ºC for 2-3 days

Stoichiometric Solid Solutions


33

Some time the thermodynamically stable material is not the material with the

targeted properties

Metastable compound may be difficult to prepare using classical high

temperature routes; then soft chemistry (Chimie douce = soft chemistry)

methods is a good alternative.

Preparation of metastable phases:

1. Synthesis under conditions where the material is thermodynamically

stable followed by quenching to ambient conditions (Diamond, glasses).

2. Preparation of thermodynamically stable compounds, and transformation

of these to a metastable compound by a low temperature, soft chemistry,

method (Reduced nickelates).

3. Synthesis under non-equilibrium conditions; kinetic control of product

formation (Zeolities).

Formation of Metastable Solids

34

Formation of Metastable Solids – Example

Graphite and Diamond

Thermodynamically only small differences in stability between the two

modifications at ambient conditions. Difficult to activate phase transformation

from diamond to grafite at ambient conditions due to nucleation issues;

structurally very different. Will not be discussed at lecture

35

Formation of Metastable Solids – Example

O2

LaNiO3.00

LaNiO2.75

LaNiO2.50

LaNiO2.00

Metastable/stable? 2LaNiO2.5 La2O3 + 2NiO II III III II

36

Combustion Synthesis

1) Applicable only for highly exothermic reactions – fuels may be added

2) Activation from external energy source, sufficient heat is released

to maintain reaction self sustaining (electrical arc, spark, chemical reaction, etc.)

3) Heating rates (103 to 106 K/s) – pseudo adiabatic; all produced

energy used to heat sample

4) Applicable process for many technological important solid materials

(carbides, silicides, nitrides, hydrides, alloys, etc.)

5) Good reasons for performing a combustion synthesis are:

- Less energy demanding (use energy produced)

- Short reaction times

- Small investments in processing equipment

- One pot synthesis

- Possible formation of non-equilibrium phases (fast cooling)

6) Control reaction rates by

- Addition of diluents; Increase particle size of reactants

37

Combustion Synthesis

Self propagating mode

Simultaneous combustion mode

Classification Combustion Synthesis

Synthesis from the elements

Thermite-type reactions

Solid state metathesis

38

Self propagating mode(A):

Self propagating high-temperature synthesis (SHS). Combustion is initiated in a point,

and propagate rapidly through the reaction mixture. (combustion wave).

Simultaneous combustion mode(B): When the entire mixture has been heated to the

ignition temperature (Tig), reaction takes place simultaneously throughout the reactant

mixture (thermal explosion).


39

SHS reactions may be characterized by an adiabatic combustion

temperature (Tad)*.

Rule-of-thumb:

If Tad < 1200˚C combustion do not occur

If Tad > 2200˚C self-propagating reactions occur

If Tad is between 1200 and 2200˚C, self propagation may occur e.g. by

preheating.

Example: Ti + Al TiAl (Tad = 1245˚C, Tig = 640˚C)

Self-propagating if heated above 100˚C

Combustion Synthesis – SHS

*Tad: Temperature system is heated to if assumed all energy released is

consumed to heat the system without loosing energy to its surroundings

40

SHS

41

Synthesis from the elements: Carbides, silicides, borides, nitrides, oxides,

hydrides.

E.g. Ti + C TiC (20 kg in 60-90 s, + cooling 1.5 – 2 h)

Thermite-type reactions: Extension of Goldschmith process (reduction of an ore

using a metal). Mg and Al often used. MgO may be leached by hydrochloric acid.

Either reduction of an oxide to the element or reduction followed by reaction with

another element.

3Fe3O4 + 8 Al 9Fe + 4 Al2O3 (Thermite reaction) (see next few slides)

SiO2 + C + 2Mg SiC + 2 MgO

TiO2 + B2O3 + 5Mg TiB2 + 5MgO

Solid state metathesis (SSM): Rapid, low-temperature-initiated solid-state

exchange reactions. E.g.:

MnCl2 + Li2Fe2O4 MnFe2O4 + 2LiCl


See later slides – Si3N4

42

3 Fe3O4 + 8 Al 9 Fe + 4 Al2O3

H reac = 849 kJ/mol

Thermite reactions

43

3 Fe3O4 + 8 Al 9 Fe + 4 Al2O3

H reac = 849 kJ/mol

Thermite reactions

44

Ceramic lining of steel pipes

Thermite reaction inside spinning pipes

(centrifugal thermite reaction)


45

Solid-State Metathesis (SSM)

ZrCl4 + 4/3Li3N ZrN + 4LiCl + 1/6N2

- Rapid, low-temperature inisiated solid state exchange reaction

- Produces very fine scale crystallites; 10-100 nm


46

Solid-State Metathesis (SSM)


47

Solid-Gas Reactions – Use Si3N4 as example

- By combining solid and gas reactants some of the problems

by poor contact between reactants are overcome.

- Equilibrums shifted to product formation, le Chatelier’s principle*

A + B(g) = AB(s)

*Note: Le Châtelier's principle states that if a dynamic equilibrium is disturbed by

changing the conditions, the position of equilibrium shifts to counteract the change to

re-establish an equilibrium.

48

Solid-Gas Reactions – Use Si3N4 as example

- Both modifications are hexagonal build up by corner sharing SiN4-tetrahedra

with covalent bonding

- α-Si3N4 low temperature modification (< 1650˚C) has the best sintering

behavior

α-Si3N4 -Si3N4 = 1650 oC

Technical important routes to produce Si3N4

- Carbothermal nitration of SiO2

- Ammonolysis (NH3) of SiCl4 (g/l) or SiH4 (PV industry; Si solar cells)

- Nitridation of silicon powder

- Combustion

49

The nitridation reaction:

3Si + 2N2(g) Si3N4 H = 760 kJ/mol

Very exothermic, but kinetics is slow below 1100 ˚C.

1) The reaction rate depends on the particle

size and purity of the starting material.

Semiconductor grade Si may be used. The

process is catalyzed by iron.

2) Due to the exothermic nature of the

reaction, the reaction rate must be carefully

controlled to keep the temperature low enough

(1250-1350 ˚C) to avoid sintering, melting and

formation of the b-phase.

3) Limit N2 access (e.g. diluting with 20-

40%H2) or dilute Si with Si3N4).

Solid-Gas Reactions – Si3N4

50

Combustion Mode, and Self Sustaining

Solid-Gas Reactions – Si3N4

Si3N4 may be prepared in a combustion mode, but high gas pressures must be

used to obtain self-sustaining reactions (avoid depletion of gas at the reaction

front). At high pressures the reaction is self propagating, and temperatures

above 1700 ˚C are reached resulting in > 95% -Si3N4.

Co-current Counter-current

Oxides, hydrides, nitrides, oxynitrides may be formed by heating in air

(oxygen), hydrogen, nitrogen or ammonia.

Gas flow in self-sustaining gas/solid reactions


51

Decomposition and Dehydration Reactions

- Reactions involving transport of gases away from the reaction.

- Carbonates, oxalates, nitrates, hydroxides etc. Used for formation of

metal oxides.

- 2AlOOH -Al2O3 + H2O

- CaCO3 CaO + CO2

- Thermal decomposition is also controlled by nucleation and growth, e.g.

surfaces, grain boundaries, dislocations.

Topochemically controlled reactions: The reactivity is controlled by

crystal structure rather than by chemistry.

Reactions my occur within the material without separation of a new phase.

52

Formation of Metastable WO3 Termodynamically stable phase WO3 with a ReO3 type structure. (monoclinic)

(corner sharing octahedra, perovskite type arrangement without the large A-cations)

Metastable hexagonal WO3 may be obtained from WO3⅓H2O by heating at 300 °C.

Structural elements in the hydrate: planes built from 6-rings of WO6 octahedra stacked

with a shift of ½ ring diameter.

In the metastable WO3 the rings are stacked on top of each other.



LaNiO2.50 53

LaNiO3-x LaNiO3-x’ + O2(g)


O2

LaNiO3.00

LaNiO2.75

LaNiO2.00

54

Intercalation and Deintercalation

Intercalation:

Insertion of species (atoms, molecules, ions) into a crystalline host material.

The host material (and guest) may undergo perturbations in their geometrical,

chemical or electronic environment.

Two general systems:

Three dimensional lattices: with parallel channels or interconnected

channels (e.g. zeolites). Uptake is restricted by channel size.

Low dimensional host lattices:

Layered materials (2D) or materials built from chain structures (1D). Spacing

between layers or chains may change to accommodate guest species.

In this course we restrict ourselves to layered materials.

Chemical bonding - Layered Materials

Layered Materials

MoS2 Graphite (and h-BN)

Metal halides

MoCl2 – CrCl3

Layered double hydroxides

Oxyhalides – FeOCl

III-VI layered

Semiconductors – GaSe

Layered silicates

Perovskite related materials

56

Layered Materials

Neutral layers: van der Waals interactions, e.g. graphite, oxychlorides

(FeOCl) and metal disulfides

(Also compounds with neutral layers held together by covalent bonding)

Negatively charged layers separated by mobile (exchangeable) cations,

e.g. clays, transition metal phosphates

Positively charged layers separated by anions, e.g. layered double

hydroxides, MII1-xM

IIIx(OH)2(A

n-)x/nmH2O

What are layered double hydroxides?

Brucite - Mg(OH)2

Charge neutral layers

Ca(OH)2, Mn(OH)2,Ni(OH)2,

(Fe,Ni)(OH)2

P3m1

MII1-x M

IIIx(OH)2(A

n-)x/n˙mH2O

Cations:

MII: Mg, Mn, Ni, Fe, Co, Cu, Zn, Ca

MIII: Al, Mn, Fe, Co, Cr, Ga

Anions (charge balancing):

CO32–, NO3

–, Cl–, organic anions

Positively

charged layers

Hydrotalcite

group

Huge flexibility in tuning chemical

composition of the

LDHs and nanoflakes

through synthesis

Synthesis of LDHs

InPro

H

e

“Nucleation”-promoted

(Co-precipitation)

“Crystal growth”-promoted

(e.g. Urea-method)

“Crystal growth”-promoted:

Good crystallinety and big crystals

Poor cation control

“Nucleation”-promoted:

Poor crystallinety and small crystals

Good cation control

5 10 15 20 25

2(o)

5 10 15 20 25

2()

59

How to intercalate target species in interlayer

gallery?

- Direct reaction with host

- Ion-exchange - Exfoliation/Delamination

followed by

reconstruction (or flocculation)

Incorporated directly

during synthesis

See more details p. 51-57

Delamination of layered double hydroxides

Delamination - Exfoliation:

Weaken interlayer forces:

- External force (shear; ultrasonication)

- Soft chemistry

- Stabilizing nanoflakes in suspension

Nanocomposite

Present work:

- Solvent: Formamide

- Interlayer anion: NO3

- Ultrasonication

MII1-x M

IIIx(OH)2(NO3)x˙mH2O

Delamination of layered double hydroxides

MII1-x M

IIIx(OH)2(NO3)x˙mH2O – delamination - mechanism

nitrate

A

B

Formamide

62

Electronic Properties - Intercalation

• Insulator host lattices: (e.g. zeolites, layered aluminosilicates,

metal phosphates, layered double hydroxides): The basic physical

properties (of the host lattice) are not changes by intercalation.

Catalysts, catalyst supports, adsorbants, ion exchangers.

• Host lattices with redox properties: May reversibly uptake

electrons from or donate electrons (less often) to the guest. E.g.

graphite, metal dichalcogenides, metal oxyhalides… Strong changes

in the physical properties, e.g. electric conductivity. Interesting as

electrode materials for lithium batteries, electrochromic systems,

sensor materials…

64


Rechargeable (secondary) lithium batteries

65

Intercalation in Graphite

Single-atom thick graphite layers.

Interlayer distance 3.35Å (0.335nm)

Intercalation of:

Lithium (Li): 3.71Å

Potassium (K): 5.35Å

AsF5: 8.15Å

KHg: 10.22 Å (three layers of K and Hg)


Direct Synthesis

Important for Li-ion batteries

66

Classic example. TiS2: Li + TiS2 ↔ Li+[TiS2]-

Ti4+ reduced to Ti3+ and Li oxidized to Li+

The layers become charged (negative) and the Van der Waals

forces are replaced by Coulomb interactions. The interlayer spacing

is expanded in LiTiS2 compared to TiS2.

LixCoO2 have a similar mechanism.


Important for Li-ion batteries

LixCoO2 analogue

67

Rechargeable (secondary) lithium batteries Reversible lithium intercalation/deintercalation is the heart of lithium battery

electrode technology. In order to operate at ambient temperatures, a highly mobile

ion is needed which is able to penetrate into solids. Apart from protons, lithium is

the obvious choice. Lithium also have suitable redox properties.

Both electrodes in lithium batteries use reversible lithium insertion. Both are usually

layered intercalation materials. (e.g. graphite and LiCoO2)

Discharge: The difference in chemical potential for Li at the two electrodes results

in a flow of Li+ cations through the electrolyte from the cathode to the anode.

Discharge delivers energy

Charge: Reverse process, cost energy.

Max LiC6


68

Mechanistic Aspects – Staging Staging in intercalation During intercalation, interactions between layers are broken, and are replaced by

interactions between layers and guests.

Staging has the effect of decreasing the difficulty of breaking interlayer interactions.

Some layers are partly or completely filled, while others are empty. This is often

obtained in an ordered fashion.

The order of the staging is given as the number of layers between successive filled or

partially filled layers.

Illustrated by the easy of intercalation in MX2: Disulfides easily intercalated at milder

conditions than the diselenides, while the ditellurides have not been reported to

intercalate.

69

V.A. Sethuraman et al. / Journal of Power Sources 195 (2010) 3655–3660

Staging in graphite anode

70

TEM: Mixed staging in C- FeCl2

71

Carbothermal Reduction

• Commercially used for producing non-oxide ceramic powders

72

SiC (carborundum)

• Abrasive: Cutting, grinding, lapping. In resin or ceramic matrix: Grinding

wheels, whetstones…

• Deoxidizer: in cast iron and steel to remove oxygen, for carburization

and siliconization

• Refractory material: linings in furnaces and kilns

• Electric heating elements: operation in oxidizing atmospheres up to

1500˚C.

Carbothermal Reduction,The Acheson Process

Acheson furnace - 1896 Niagara

• Still same design in use today; Current furnaces 12-18 m long

• Extremely energy demanding – 1kg SiC requires 12 kWh

• Pure SiC is transparent

• N2 contaminations green

• Carbon contaminations black

73


Overall reaction: SiO2 + 3C SiC + 2CO(g)

SiO2: Sand

Quartzite

Quartz

C: Petrolium coke

Carbon blac

Coke

Graphite..

Assumed to be a solid

state reaction.

Particle sizes 5-10 mm

makes this less likely

74

C(s) + SiO2(s) SiO(g) + CO(g)

SiO2(s) + CO SiO(g) + CO2(g)

C(s) + CO2(g) 2CO(g)

2C(s) + SiO(g) SiC(s) + CO(g)


Overall reaction: SiO2 + 3C SiC + 2CO(g)

- Extremely fast kinetics indicative not a solid-solid reaction

- Via non stable gas specie – SiO*

*B2O3 reduced via B2O2; Al2O3 via Al2O

A small taste on silicon chemistry for

solar cells

Wafers

Casting and cutting

Systems

Power plants

Chemical processing - II

Super Pure Si for PV (purity 99.9999 – 99.999999%)

Surface treatment

Cells

Assembly

Modules

Metallurgical Grade Si (purity 98.5 – 99.9%)

Chemical processing - I

Raw Materials for Metallurgical Grade Si

SiO2 and Carbon

From sand to electricity

How is metallurgical grade silicon produced?

Carbothermic reduction (1900-2100 oC)

SiO2(l) + 2C(s) Si(l) + 2CO(g)

Raw Materials for Metallurgical Grade Si

SiO2 and Carbon

Metallurgical Grade Si (purity 98 – 99%)

Chemical processing - I

Metallurgical grade silicon = MG-Si

Purity > 98% (Fe, Al, Ca, C, Mg, Ti, Mn, Ni, Cu, V, B, P)

(Impurities are controlled by raw materials and type of electrode system

used used in the carbothermic reduction)

SiO2(l) + 2C(s) Si(l) + 2CO(g)

Core of furnace (hot zone):

2SiO2(l) + SiC(s) 3SiO(g) + CO(g)

SiO(g) + SiC(s) 2Si(l) + CO(g)

Outer part of the furnace (T < 1900 oC):

SiO(g) + 2C(s) SiC(s) + CO(g)

2SiO(g) Si(l) + SiO2(s)

Important actors

Norway

France

China

South-Africa

America

Brazil

1900-2100 oC

How is metallurgical grade silicon produced?

79

Applications for MG-Si

1) Additive alloys (500.000 ton/year)

- Alloys of Al

2) Production of silicones

(400.000 ton/year)

3) Synthetic SiO2 (optic fibers; 100.000 ton/year)

SiCl4(g) + 2H2(g) + O2(g) SiO2 + 4HCl(g)

4) Si for electronic and solar cell industry (50.000 + 200.000 ton/year)

- Including silane and chlorosilane production

REC SILICON - Moses Lake, USA

Silane production unit – approx. 9000 ton SiH4 gas per year

From MG-Si SiH4 (g) Super Pure Si REC Silicon Silane Process

REC Silicon Silane Prosess

H2 MG-Si SiCl4

Fluidised bed

Separator

HSiCl3 SiCl4

H2SiCl2 SiCl4

SiH4 HSiCl3

Destillations

*SiH4

Si + 3SiCl4(g)+ 2 H2(g) 4HSiCl3(g)

2HSiCl3 H2SiCl2+ SiCl4

2H2SiCl2 HSiCl3+ SiH4



REC silane pyrolysis units; SiH4 Si + 2H2

REC silane pyrolysis units SiH4 Si + 2H2



Working in the laboratory…..

..and out in the field…..

Wafer Production

1: Poly-Si in

crucibles 5: Ingot cut in

blocks

2: Melting in

furnace 6: Handling

3: Solidification

Bottom up 7: Sawing

4: Polycrystalline

Si formation 8: Wafere

From Si to wafers

Wafer produced

Areal: 154×154 (mm2)

Solar Cell Production

1: Wafer 5: Screen printing

2: Etching 6: Soldering

3: Doping 7: Module

production

4: Anti reflecting

coating 8: Distribution

Process steps from wafer to module

Tedlar

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