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Material Synthesis Inorganic Materials Chemistry and Functional Materials Helmer Fjellvåg and Anja Olafsen Sjåstad Lectures at CUTN spring 2016 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) 2 Bulk powders Metals-alloys Nanoparticles Inorganic Materials Synthesis Surfaces Thin films - coatings Single crystals Inorganic Materials Synthesis Solid State Reactions Nanoparticle synthesis Solids from the gas phase Inorganic Materials Synthesis Solid State Reactions Solid State Reactions Solid state reaction: a reaction between two or more solids. Our text book has extended the definition to any reaction involving a solid: •Solid/solid •Solid/gas (Reaction, decomposition) •Intercalation •(Solid/liquid) Ceramic Method – ‘Shake and Bake’ - Direct reaction between two or more solids to form the final product. In principle no decomposition is involved. - 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. 7 Ceramic Method – Synthesis of YBCO YBCO = YBa2Cu3O7x ABO3 8 Ceramic Method – Synthesis of YBCO YBCO = YBa2Cu3O7x - 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: Oxidation step 9 Solid State Reactions – General Aspects 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) 10 Ceramic Method – Synthesis of YBCO - 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 11 Ceramic Method – Synthesis of YBCO - Will chloride salts be suited precursors? E.g. BaCl2? Keywords: - Chlorides not form volatile products – i.e. reminds in the powder - Regrinding – Why? Keywords: - Reduce diffusion distances – see next slide Note: Many other ways of synthesizing YBCO: Precursor methods Sol/gel Flux Many melt-texturing methods including partially melting/fluxing 12 Solid State Reactions – General Aspects A(s) + B(s) C(s) • 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): dx k x 1 dt x(t ) k ln(t ) C x = thickness of layer t = time 13 Solid State Reactions – General Aspects 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 14 - Solid State Reactions – General Aspects How to increase rate of reaction, dx/dt? 1. Increase area of contact between reactants (cross-reference to earlier slide) Small particles by grinding, ball milling, freeze drying etc. 15 Solid State Reactions – General Aspects How to increase rate of reaction, dx/dt? 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)). 16 Solid State Reactions – General Aspects Structural similarity through - Epitactic and Topotactic Reactions 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. 17 Solid State Reactions – General Aspects Example: Production of the MgAl2O4 spinel from MgO and Al2O3 MgO and MgAl2O4 have O atoms in cubic close packing Al2O3 crystallizes in a completely different structure (Rhombohedral) Al2O3 MgO MgAl2O4 Spinel nuclei form on the MgO surface - structural orientational relationship as well as similar interatomic distances (topotactic reaction) 18 Solid State Reactions – General Aspects Topotactic Reduction Oxygen getter Zr, NbO,.. LaNiO3-x LaNiO3-x’ + O2(g) Zr + O2(g) ZrO2 LaNiO3.00 LaNiO2.75 O2 Sample LaNiO2.50 LaNiO2.00 19 Solid State Reactions – General Aspects How to increase rate of reaction, dx/dt? 3. Increase rate of diffusion – ions through various phases - Diffusion is a thermally activated process, following an Arrhenius behavior - 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 20 Solid State Reactions – General Aspects 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 21 Will not be discussed at lecture Solid State Reactions – General Aspects Synthesis of BaTiO3 – nucleation of an intermediate phase 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. 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) At least three stages are involved in formation of BaTiO3 from BaO and TiO2. • 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. 22 Will not be discussed at lecture Solid State Reactions – General Aspects 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 23 Chemical Mixing in Solution Decrease diffusion lengths by using intimately mixing of cations. Solid precursors containing the desired cations. Precursors for Ceramic Synthesis 1. Precursors via co-precipitation 2. Precursors via solid solutions and compounds 24 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. 25 Precursor Formation – Co-precipitation - 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?) Keywords: nitrate may act oxidizing during decomposition; high solubility 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) 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 26 Carbonate Precursors Forming a Stoichiometric Solid Solution Target: Formation of mixed oxides: M1xM’xO, (M, M’ = Ca, Mg, Mn, Fe, Co, Zn, Cd) M1xM’xCO3 M1xM’xO + CO2(g) Calcite, CaCO3 27 Carbonate Precursors – Preparation Method 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 M1xM’xCO3 M1xM’xO + CO2(g) 28 Carbonate Precursor Method - Example Ca0.5Mn0.5CO3 + O2(g) Ca0.5Mn0.5O3 + CO2(g) CaMnO3 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 29 Mn in this example 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. 30 Co-precipitation Stoichiometric Solid Solutions 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 Nitrates Ba0.5Pb0.5(NO3)2 BaPbO3 Note: Change in oxidation state of Pb from +II to +IV Cyanides LaFe0.5Co0.5(CN)65H2O LaFe0.5Co0.5O3 31 Stoichiometric Solid Solutions 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 32 Will not be discussed at lecture Formation of Metastable Solids 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). 33 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; 34 structurally very different. Will not be discussed at lecture Formation of Metastable Solids – Example Metastable/stable? II III II III 2LaNiO2.5 La2O3 + 2NiO LaNiO3.00 LaNiO2.75 O2 LaNiO2.50 LaNiO2.00 35 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 36 Combustion Synthesis Classification Combustion Synthesis Self propagating mode Synthesis from the elements Simultaneous combustion mode Thermite-type reactions Solid state metathesis 37 Classification Combustion Synthesis 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 38 mixture (thermal explosion). Combustion Synthesis – SHS 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 *Tad: Temperature system is heated to if assumed all energy released is 39 consumed to heat the system without loosing energy to its surroundings SHS 40 Classification Combustion Synthesis 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) See later slides – Si3N4 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 41 Thermite reactions 3 Fe3O4 + 8 Al 9 Fe + 4 Al2O3 H reac = 849 kJ/mol 42 Thermite reactions 3 Fe3O4 + 8 Al 9 Fe + 4 Al2O3 H reac = 849 kJ/mol 43 Ceramic lining of steel pipes Thermite reaction inside spinning pipes (centrifugal thermite reaction) 44 Will not be discussed at lecture Solid-State Metathesis (SSM) - Rapid, low-temperature inisiated solid state exchange reaction - Produces very fine scale crystallites; 10-100 nm ZrCl4 + 4/3Li3N ZrN + 4LiCl + 1/6N2 45 Will not be discussed at lecture Solid-State Metathesis (SSM) 46 Will not be discussed at lecture 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. 47 Solid-Gas Reactions – Use Si3N4 as example 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 α-Si3N4 1650 oC = -Si3N4 - Both modifications are hexagonal build up by corner sharing SiN4-tetrahedra with covalent bonding - 48 α-Si3N4 low temperature modification (< 1650˚C) has the best sintering behavior Solid-Gas Reactions – Si3N4 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 2040%H2) or dilute Si with Si3N4). 49 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 Gas flow in self-sustaining gas/solid reactions Oxides, hydrides, nitrides, oxynitrides may be formed by heating in air (oxygen), hydrogen, nitrogen or ammonia. 50 Will not be discussed at lecture 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. 51 Decomposition and Dehydration Reactions 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. 52 Will not be discussed at lecture Decomposition and Dehydration Reactions LaNiO3-x LaNiO3-x’ + O2(g) LaNiO3.00 LaNiO2.75 O2 LaNiO2.50 LaNiO2.00 53 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. 54 Chemical bonding - Layered Materials Layered double hydroxides Graphite (and h-BN) MoS2 Layered silicates Oxyhalides – FeOCl Layered Materials Perovskite related materials Metal halides MoCl2 – CrCl3 III-VI layered Semiconductors – GaSe 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-xMIIIx(OH)2(An-)x/nmH2O 56 What are layered double hydroxides? MII1-x MIIIx(OH)2(An-)x/n˙mH2O Positively charged layers Hydrotalcite group 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 Brucite - Mg(OH)2 Charge neutral layers Ca(OH)2, Mn(OH)2,Ni(OH)2, (Fe,Ni)(OH)2 P3m1 Huge flexibility in tuning chemical composition of the LDHs and nanoflakes through synthesis Synthesis of LDHs “Crystal growth”-promoted “Nucleation”-promoted (e.g. Urea-method) (Co-precipitation) 5 10 15 2( ) 20 25 5 10 15 20 o 2( ) “Crystal growth”-promoted: Good crystallinety and big crystals Poor cation control “Nucleation”-promoted: Poor crystallinety and small crystals Good cation control H e InPro 25 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 59 See more details p. 51-57 Delamination of layered double hydroxides Delamination - Exfoliation: Weaken interlayer forces: - External force (shear; ultrasonication) - Soft chemistry MII1-x MIIIx(OH)2(NO3)x˙mH2O - Stabilizing nanoflakes in suspension Present work: - Solvent: Formamide - Interlayer anion: NO3 - Ultrasonication Nanocomposite Delamination of layered double hydroxides MII1-x MIIIx(OH)2(NO3)x˙mH2O – delamination - mechanism B nitrate A Formamide 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… 62 Rechargeable (secondary) lithium batteries 64 Will not be discussed at lecture Intercalation in Graphite Important for Li-ion batteries 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 Will not be discussed at lecture 65 Classic example. TiS2: Li + TiS2 ↔ Li+[TiS2]Important for Li-ion batteries LixCoO2 analogue 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. Will not be discussed at lecture 66 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 Will not be discussed at lecture 67 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 68 intercalate. Staging in graphite anode 69 V.A. Sethuraman et al. / Journal of Power Sources 195 (2010) 3655–3660 TEM: Mixed staging in C- FeCl2 70 Carbothermal Reduction • Commercially used for producing non-oxide ceramic powders 71 Carbothermal Reduction,The Acheson Process 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. 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 72 Carbothermal Reduction,The Acheson Process 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 73 Carbothermal Reduction,The Acheson Process Overall reaction: SiO2 + 3C SiC + 2CO(g) - Extremely fast kinetics indicative not a solid-solid reaction - Via non stable gas specie – SiO* 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) 74 *B2O3 reduced via B2O2; Al2O3 via Al2O A small taste on silicon chemistry for solar cells Raw Materials for Metallurgical Grade Si From sand to electricity SiO2 and Carbon Metallurgical Grade Si (purity 98.5 – 99.9%) Super Pure Si for PV (purity 99.9999 – 99.999999%) Chemical processing - I Chemical processing - II Wafers Cells Modules Systems Casting and cutting Surface treatment Assembly Power plants 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) How is metallurgical grade silicon produced? 1900-2100 oC 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 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 79 From MG-Si SiH4 (g) Super Pure Si REC Silicon Silane Process REC SILICON - Moses Lake, USA Silane production unit – approx. 9000 ton SiH4 gas per year REC Silicon Silane Prosess H2 MG-Si SiCl4 Fluidised bed Separator HSiCl3 H2SiCl2 SiCl4 Si + 3SiCl4(g)+ 2 H2(g) 4HSiCl3(g) SiCl4 2HSiCl3 H2SiCl2+ SiCl4 SiH4 HSiCl3 2H2SiCl2 HSiCl3+ SiH4 Destillations *SiH4 REC silane pyrolysis units; SiH4 Si + 2H2 Super Pure Si for PV (purity 99.9999 – 99.999999%) Chemical processing - II REC silane pyrolysis units SiH4 Si + 2H2 Super Pure Si for PV (purity 99.9999 – 99.999999%) Chemical processing - II Working in the laboratory….. ..and out in the field….. Wafer Production Super Pure Si for PV (purity 99.9999 – 99.999999%) Chemical processing - II From Si to wafers 1: 2: 3: 4: Poly-Si in crucibles Melting in furnace Solidification Bottom up Polycrystalline Si formation 5: 6: 7: 8: Ingot cut in blocks Handling Sawing Wafere Wafer produced Areal: 154×154 (mm2) Solar Cell Production Process steps from wafer to module 1: 2: 3: 4: Wafer Etching Doping Anti reflecting coating 5: 6: 7: 8: Screen printing Soldering Module production Distribution 92 Tedlar