Direct Reactions of Solids "HEAT-AND-BEAT" or "SHAKE-AND-BAKE" Solid state reactions At least one of the reactants and one of the products are solid Reactions in a lattice of atoms Atomic mobility High temperatures No mobility without defects - perfect crystal = no chemistry Reactions on the interphase between phases Microstructure Diffusion controls the reaction rate 1 ^m^ ^ Point Defects w €1 ô 0 ®'>-*#L Schottky imperfection • ':-V^Ä U 0 | C|:: B i* % Frenkel imperfection *> / y- ■: 0; 1 |1 oWRi Schottky-imperfection: vacancy, missing ions moved to the surface Frenkel-imperfection: vacancy, missing ions on interstitial positions Reaction Types Solid - solid synthesis - addition A + B -^ AB MgO(s) + Al203(s) -> MgAl204(s) MgO(s) + Si02(s) -> MgSi03(s) or Mg2Si04(s) Solid - solid synthesis - exchange, metathesis AB + C -^ AC + B CaC03(s) + Si02(s) -> CaSi03(s) + C02(g) Ge(s) + 2 Mo03(s) -> Ge02(s) + 2 Mo02(s) Solid - solid synthesis - exchange and addition PbS04 + Zr02 + K2C03 -> K2S04 + PbZr03 + C02 Solid - solid synthesis - dissociation AB -^ A + B Ca3Si05(s) -> Ca2Si04(s) + CaO(s) Reaction r Solid - solid synthesis - addition MgO(s) + Al203(s) -> MgAl204(s) MgO(s) + Si02(s) -> MgSi03(s) or Mg2Si04(s) Solid - solid synthesis - exchange, metathesis CaC03(s) + Si02(s) -> CaSi03(s) + C02(g) Ge(s) + 2 Mo03(s) -> Ge02(s) + 2 Mo02(s) Solid - solid synthesis - dissociation Ca3Si05(s) -> Ca2Si04(s) + CaO(s) A + B -> AB AB + C -> AC + B AB -^ A + B Reaction Types Solid - gas synthesis A + B -> AB 2 Fe304(s) + 1/2 02(g) -> 3 Fe203(s) 2 SiCl4(g) + 4 H2(g) + Mo(s) -> MoSi2(s) + 8 HCl(g) High temperature corrosion of metals in air Solid - gas dissociation AB -» A + B CaC03(s) -► CaO(s) + C02(g) Al4Si4O10(OH)8(s) -+ Al4(Si4O10)O4(s) + 4 H20(g) Kaolinite Metakaolinite 5 Direct Reactions of Solids Other Examples Oxides BaC03 + Ti02-------► BaTi03 + BaTi205 + C02 873 K UF6 + H2 + 2H20 —► U02 (powder) + 6 HF dust = radiological hazard, milling, sintering to U02 pellets YBCO 123 Superconductor (1987) Y203 + BaC03 + CuO 1223 K » 473K » YBa2Cu307x air 02 1130 K Tl203+2BaO + 3CaO + 4CuO ---------► Tl2Ba2Ca3Cu4012 Direct Reactions of Solids Other classes than oxides Pnictides HOOK Na3E + ME + E ------► Na2M3E4 M = Eu, Sr, E = P, As Metals UF4 + 2 Ca ------* U + 2 CaF2 Manhattan Project Direct Reactions of Solids Chlorides 3 CsCl + 2 ScCl3 ____► Cs3Sc2Cl9 6 NH4CI + Y203 -------► 2 YCI3 + 3 H20 + 6 NH3 6 NH4CI + Y ---------► (NH4)3YC16 + 1.5 H2 + 3 NH3 4 NH4CI + 3 NH4Re04 --------► 3 Re + 12 H20 + 3.5 N2 + 4 HCl Aluminosilicates NaA102 + Si02 -------► NaAlSi04 Chalcogenides 1400 K Pb + Mo + S ---------► PbMo6S8 Chevrel phases (MxMo6X8, M = RE, Sn, Pb, Cu, X = S, Se, Te) 8 Direct Reactions of Solids Powder Mixing Method Precise weighing for exact stoichiometry Mixing (components, dopants, additives) Milling or grinding (ball mill, mortar) Compaction (pelleting, organic binders) Calcination @ high temperature (> 1000 °C) Firing/grinding cycles 9 Direct Reactions of Solids Planetary ball mill Planetary ball mill „ , .. Rotation and counter-wise spining Horizontal section ť fe Rotation speed: up to 400 rpm Milling jars: alumina, YSZ, tungsten carbide, agate Atritor mill n Compaction - Pressing Hydraulic Uniaxial Press Maximum pressure: 120 MPa Warm Isostatic Press Max. pressure: 400 MPa Max. temperature: 80 °C Volume: 2,5 1 Hot press Max. temperature: 1250 °C Max. pressure: 100 MPa Max. diameter: 25 mrfli2 Calcination Tube Furnace in air and in controled atmosphere Maximum temperature: 1450 °C or 1600 °C Furnace-tube diameter: up to 75 mm Vacuum Furnace in vacuum or Ar, N2, 02 atmosphere Maximum temperature: 1200 °C Chamber Dimensions: 150x200x250 mm3 13 Direct Reactions of Solids Advantages simple equipment low cost and easily accessible starting materials well studied Disadvantages impurities from grinding (Fe, Cr,...) broad particle size distribution some phases unstable @ high T, decomposition formation of undesirable phases slow formation, diffusion, long reaction times large grain size poor chemical homogeneity: poor mixing of large crystallites (milling lower limit ~ 100 nm) volatility of some components (Na20, PbO,...) uptake of ambient gas (02 in superconductors) 14 Direct Reactions of Solids Experimental Considerations é^ Reagents Drying, fine grain powders for maximum SA, surface activation (Mo + H2), in situ decomposition (C032~, OH", 022~, C2042~) for intimate mixing, precursor reagents, homogenization, organic solvents, grinding, ball milling, ultrasonication Š* Container Materials Chemically inert crucibles, boats, ampoules (open, sealed, welded) Noble metals: Au, Ag, Pt, Ni, Rh, Ir, Nb, Ta, Mo, W Refractories: alumina, zirconia, silica, BN, graphite Reactivities with containers at high temperatures needs to be carefully evaluated for each system, pelleting minimizes contact with container, sacrificial pellet Properties of Common Container Materials Material Maximum Thermal Working Shock Thermal Coefficient of Other Conductivity, Linear Properties Temp., K Resistance Wm^K"1 Expansion xlO6,^1 Pyrex 770 GOOD 1.13 3.2 Permeable to air at high T CaF2 1420 FAIR - 24 - Si02 1530 VERY GOOD 1.38-2.67 0.4 - 0.6 Permeable to air at high T, devitrification above 1670 K SÍ3N4 1770 FAIR 10-33 6.4 - Pt BN A1203 A1N BeO Zr02 Ir MgO Th02 1950 VERY GOOD 73 1970 VERY GOOD 5.02 Vitreous C 2070 2170 2270 2570 2570 2600 2870 3070 GOOD 4.19-8.37 FAIR FAIR GOOD GOOD VERY GOOD FAIR FAIR 35-39 50- 170 230 1.97 148 37.7 4.19 9 \\ Plastic at high T 0.2-3 Oxidizes in air above 970 K 2-3.5 Oxidizes in air above 900 K 8 Reacts with metals above 1800 K 5.7 - 8.4 Reacts with metals above 1800 K 4.5 - 6.8 25 High vapor pressure 6 Reacts with C above 2290 K Direct Reactions of Solids #Ŕ Heating Program Slow or fast heating, cooling, holding at a set point temperature Tammann's rule: Tr > 2/3 Tm Furnaces, RF, microwave, lasers, ion or electron beam éF Prior decomposition Initial cycle at lower temperature to prevent spillage or volatilization, frequent cycles of heating, cooling, grinding, boost SA. Overcoming sintering, grain growth, fresh surfaces. Pelleting, hot pressing, enhanced contact area increases rate and extent of reaction é* Controlled atmosphere (oxidizing, reducing, inert) or vacuum. Unstable oxidation states, preferential component volatilization if T too high, composition dependent atmosphere (02, NH3, H2S,...) 17 Reaction Paths between Two Solids gas phase diffusion volume diffusion interface diffusion surface diffusion 18 Direct Reactions of Solids Model reaction, well studied: MgO + A1203 -> MgAl204 Spinel (cep O2", Mg2+ 1/8 Td, Al3+ 1/2 On) Single crystals of precursors, interfaces between reactant grains On reaction, new reactant-product MgO/MgAl204 and Al203/MgAl204 interfaces are formed Free energy negative, favors reaction but extremely slow at normal temperatures (several days at 1500 °C) Interracial growth rates 3 : 1 Linear dependence of interface thickness x2 versus t Easily monitored rates with colored product at interface, T and t MO + A1203 -> NiAl204 MgO + Fe203 -> MgFe204 19 The Spinel Structure: AB204 fee array of O2" ions, A occupies 1/8 of the tetrahedral and B 1/2 of the octahedral holes -» normal spinel: AB204 Co304, GeNi204, WNa204 -» inverse spinel: B[AB]04 Fe304: Fe3+[Fe2+Fe3+]04, TiMg204, NiLi2F4 -» basis structure for several magnetic materials 20 The spinel structure: MgAl204 fc.v V.V:V-V-V V ■ ;-;\.'v-y'.: v -.4 ,\v.v«v;%yX'.* / . ftp* III?, wm . ^T r\ % j—— i» c. ^ ■ t ■ ' ■ ■ J : ■;: fn ík / J^ FXfvi DM 7!ff:j5;t.,í;< : ^^ /!|x^sf . / YV Ľ / ¥;S\. —< v^_ > li \i SD 21 The spinel structure: MgAl204 Direct Reactions of Solids Why is nucleation, mass transport so difficult? MgO ccp O2, Mg2+ in Oh sites A1203 hep O2, Al3+ in 2/3 Oh sites MgAl204 ccp O2, Mg2+ 1/8 Td, Al3+ 1/2 Oh é* Structural differences between reactants and products, major structural reorganization in forming product spinel é* Making and breaking many strong bonds (mainly ionic), high temperature process as D(Mg2+) and D(A13+) large for small highly charged cations é* Long range counter-diffusion of Mg2+ and Al3+ cations across interface, usually RDS (= rate determining step), requires ionic conductivity, substitutional or interstitial hopping of cations from site to site to effect mass transport é* Nucleation of product spinel at interface, ions diffuse across thickening interface, oxide ion reorganization at nucleation site é* Decreasing rate as spinel product layer thickens Parabolic rate law: dx/dt = k/x x2 = kt 23 Direct Reactions of Solids Kinetics: Linear x2 vs. t plots observed In k vs. 1/T experiments provide Arrhenius activation energy Ea for the solid-state reaction Reaction mechanism requires charge balance to be maintained in the solid state interracial reaction: 3Mg2+ diffuse in opposite way to 2A13+ MgO/MgAl204 Interface: 2A13+ -3Mg2+ + 4MgO -> lMgAl204 MgAl204/Al203 Interface: 3Mg2+ -2A13+ + 4A1203 -> 3MgAl204 Overall Reaction: 4MgO + 4Al203-> 4MgAl204 the Kirkendall Effect: RHS/LHS growth rate of interface = 3/1 24 Direct Reactions of Solids Model for a classical solid-solid reaction (below melting point!): Planar interface between two crystals MgO + A1203 -> MgAl204 (Spinel) Phase 1: nucleation Phase 2: growth of nuclei ^ MgO 0 c c A1203 MgO 0 MgAl204 A1,0 2^3 X 25 Reaction Mechanism interface I interface II A1203 2A13+ -------^ MgAl204 MgO ■*-------------- ----3Mg interface I interface II A1203 2 Al 3+ o2^2 MgAl204 MgO 2Mg2+ 02. 1/2 O, 1/2 O, 26 Direct Reactions of Solids Thermodynamic and kinetic factors doc / \ / \ — = KT)Aa) General kinetic expression at g(ct) =J k(T) dt g(a) = k(T) t J -J7~\ = g\a) = J k\Tpt a - the molar fraction of the reacted product at a time t k(T) - the rate constant of the process Experimentally evaluate a at different t Fit data into a g(a) = k(T) t expression to obtain k(T) and the type of mechanism model P -P jy _ t______0_ Pt = the value of a property at time t q t) P0 = the value of a property at the beginning 1q -'q Pe = the value of a property at the end & — v 1 e.g. Pt = mass loss, x. 27 Direct Reactions of Solids da dt = k{T)f{a) í da jk(T)dt g(a) =i k(T) dt g(a) = k(T) t Decreasing reaction rate as spinel product layer (x) thickens Here a = x Parabolic rate law: dx/dt = k/x x2 = kt MgO A1203 X 28 Mechanism model g(ot) Diffusion controlled One-dimensional ot2 Two-dimensional a + (1 - a) In (1 - a) Three-dimensional, Jander [1 — (\ — ot)1/3]2/3 Three-dimensional, Ginstling i\ _ 2/3ct) — (1 — a)2/3 Three-dimensional, Carter i\ + oc)2/3 + (1 — a)2/3 Growth controlled General [1 - (1 - a)1""] First order, n = 1 [- In (1 - a)] Nucleation controlled Power law a1/n Nucleation-Growth controlled Avrami [- In (1 - a)]1/2 Erofeev [- In (1 - a)]1/3 Planar boundary 1 - (1 - a)1/2 Spherical boundary 1 - (1 - a)1/3 l-\ T3 °^ -> O cá ?h 0.6- Ö O • 1—I +-> o S 0-4- a =1- exp( Conversion is 50% Complete 0.2- Incubation Time 0' .1e2 ni Plot T is the time required for 0% conversion .1e3 .1e4 .1e5 Time (s) 30 Direct Reactions of Solids Perform the measurements in a range of temperatures T use Arrhenius equation to evaluate the activation energy E k(T) = k„ exp(-Ea/RT) Direct Reactions of Solids Perform the measurements in a range of temperatures T use Arrhenius equation to evaluate the activation energy E k(T) = k0 exp(-Ea/RT) T3 a GA U H ö o +-> o 32 FACTORS INFLUENCING REACTIONS OF SOLIDS CONTACT AREA ^Surface area of reactants ^Particle size ^Pelleting, pressing, precursors DIFFUSION RATE ^Diffusion rates of atoms, ions, molecules in solids ^Reaction temperature, pressure, atmosphere ^Diffusion length, particle size ^Defect concentration, defect type ^Reaction mechanism NUCLEATION RATE ^Nucleation of product phase within the reactant with similar crystal structure ^Epitactic and topotactic reactions ^Surface structure and reactivity of different crystal planes/faces 33 Direct Reactions of Solids KEY FACTORS IN SOLID STATE SYNTHESIS CONTACT AREA and surface area (SA) of reacting solids control: ■^ Rates of diffusion of ions through various phases, reactants and products ■^Rate of nucleation of the product phase Reaction rate is greatly influenced by the SA of precursors as contact area depends roughly on SA of the particles Surface Area (SA) of Precursors spherical particles, radius r [nm], density p [g/cm3] 47ir2 SA = A/m = --------------- = 3000/rp [m2/g] 4/371 r3, p 34 Direct Reactions of Solids Consider 1 g of a material, density 1.0 g/cm3, cubic crystallites number of cubes edge length, cm SA, m2/g 1 1 6.104 109 103 0.6 1018 106 600 Contact area not in reaction rate expression for product layer thickness versus time: dx/dt = k/x But for a constant product volume x oc l/Acontact and furthermore Acontact oc l/dparticie Thus particle sizes and surface area inextricably connected and obviously x oc d and SA particle size affect the interfacial thickness 35 Direct Reactions of Solids These relations suggest some strategies for rate enhancement in direct reactions: ■^Hot pressing densification of particles High pressure squeezing of reactive powders into pellets (700 atm) Pressed pellets still 20-40% porous. Hot pressing improves densification ■^Atomic mixing in composite precursor compounds ■^Coated particle mixed component reagents, corona/core precursors ■^Decreasing particle size, nanocrystalline precursors Aimed to increase interfacial reaction area A and decrease interface thickness x, minimizes diffusion length scales dx/dt = k/x = k'A = k'Vd 36 DIRECT REACTION OF SOLIDS DIFFUSION RATE Fick's law J = - D(dc/dx) J = flux of diffusing species, #/cm2s (dc/dx) = concentration gradient, #/cm4 D = diffusion coefficient, cm2/s, for good reaction rates > 10"12 D increases with temperature, rapidly as you approach the melting point Tammann's rule: Extensive reaction will not occur until the temperature reaches at least 2/3 ot the melting point of one or more of the reactants. Factors influencing cation diffusion rates: ■^Charge, mass and temperature ■^Interstitial versus substitutional diffusion ■^Number and types of defects in reactant and product phases All types of defects enhance diffusion of ions (intrinsic or extrinsic, vacancies, interstitials, lines, planes, dislocations, grain boundaries) Diffusion Diffusion coefficients show an exponential temperature dependence (Arrhenius type): D^: D for T^ oo, Q: activation energy of diffusion R: the gas constant f r, \ D = D„ exp Q v RTj The logarithmic representation of D verus 1/T is linear the slope corresponds to the activation energy Q and the intercept to D 38 Diffusion C in Fe Ag in Ag 10" 40 -10 ______100 =!=F: 300 800 °C i—r-n 750 1000 K t 10-6 10-7 10-e 10-9 KT« KT11 .Ober-. flache Korng Ag in Ag renzen Volumen 20 10 16 % 12-10"4 K 8 -*— MT Direct Reactionsjjf Solids f (AGN+AGD)^ NUCLEATION RATE n = nQ exp v kT j Nucleation rate n Nucleation rate =const. x exp{- (AGN + AGD)/kT} AGN = thermodynamic barrier to nucleation AGD = kinetic barrier to diffusion across the liquid/nucleus interface AGN = AGS + 4/37ir3AGv AGS = 47ir2y surface free energy change positive 40 4/37ir3AGv volume free energy change negative, 1 to s lowers energy Nucleation r AG NUCLEATION RATE o •a a» c -C Š2 c c 4; £ Driving energy Retarding energy AGs = surface free-energy change = 4rrr2> AGr — total free-energy change Radius of particle, r ■ AGV ~ volume free-energy change = 4 rrr3 AG. n r: radius of spheric seed r*: critical radius (r>r* seed grows by itself) AGN: total free energy change AGS: surface free energy change AGV: volume free energy change AGN = 47ir2y + 4/37ir3AGv 41 Direct Reactions of Solids Solidification AG = 4/3 ti r3 AGV +4 % r2 y - Volume free energy + surface energy One solid phase changing to another AG = 4/3 7i r3 AGV +4 % r2 y + 4/3 7C r3 s - Volume energy + surface energy + strain energy - the new solid does not take up the same volume as the old solid Nucleation vs. Crystal Growth (solution or melt) Undercooling - cooling below the melting point relations between undercooling, nucleation rate and growth rate of the nuclei large undercooling: many small nuclei (spontaneous nucleation) growth rate small - high viscosity, slow diffusion small undercooling: few (evtl. small) nuclei growth rate high - fast diffusion close to the m.p. 43 Nucleation vs Rate of nucleation Rate of growth Ta = small undercooling, slow cooling rate Fast growth, slow nucleation = Few coarse crystals Tb = larger undercooling, rapid cooling rate Rapid nucleation, slow growth = many fine-grained crystals Tc = very rapid cooling Nearly no nucleation = glass Crystal Growth DIRECT REACTION OF SOLIDS NUCLEATION RATE Nucleation requires structural similarity of reactants and products less reorganization energy = faster nucleation of product phase within reactants MgO, A1203, MgAl204 example MgO (rock salt) and MgAl204 (spinel) similar ccp O2" but distinct to hep O2" in A1203 phase Spinel nuclei, matching of structure at MgO interface Oxide arrangement essentially continuous across MgO/MgAl204 interface Bottom line: structural similarity of reactants and products promotes nucleation and growth of one phase within another Lattice of oxide anions, mobile Mg2+ and Al3+ cations Topotactic and epitactic reactions Orientation effects in the bulk and surface regions of solids Implies structural relationships between reagent and product Topotaxy occurs in bulk, 1-, 2- or 3-D Epitaxy occurs at interfaces, 2-D DIRECT REACTION OF SOLIDS Epitactic reactions require 2-D structural similarity, lattice matching within 15% to tolerate oriented nucleation otherwise mismatch over large contact area, strained interface, missing atoms Example: MgO/BaO, both rock salt lattices, cannot be lattice matched over large contact area Lattice matched crystalline growth Best with less than 0.1% lattice mismatch. Causes elastic strain at interface Slight atom displacement from equilibrium position. Strain energy reduced by misfit-dislocation Creates dangling bonds, localized electronic states, carrier scattering by defects, luminescence quenching, killer traps, generally reduces efficacy of electronic and optical devices, can be visualized by HR- TEM imaging 46 Direct Reactions of Solids Topotactic reactions More specific, require interracial and bulk crystalline structural similarity, lattice matching Topotaxy: involves lock-and-key ideas of self-assembly, molecule recognition, host-guest inclusion, clearly requires available space or creates space in the process of adsorption, injection, intercalation etc. 47 Direct Reactions of Solids Surface structure and reactivity Nucleation depends on actual surface structure of reacting phases. Face-Centred Cubic Body-Centred Cubic (111) (110) Different Miller index faces exposed, atom arrangements different, different surface structures, implies distinct surface reactivities. (100) (100) (110) (111) 48 Direct Reactions of Solids Example: MgO (rock salt) {100} MgO alternating Mg2+, O2" at corners of square grid {111} MgO, Mg2+ or O2" hexagonal arrangement Direct Reactions of Solids 50 Direct Reactions of Solids Different crystal habits possible, depends on rate of growth of different faces, octahedral, cubooctahedral, cubic possible and variants in between CRYSTAL GROWTH Most prominent surfaces, slower growth Growth rate of specific surfaces controls morphology Depends on area of a face, structure of exposed face, accessibility of a face, adsorption at surface sites, surface defects Play major role in reactivity, nucleation, crystal growth, materials properties (electronic, optical, magnetic, charge-transport, mechanical, thermal, acoustical etc) 51 DIRECT REACTION OF SOLIDS Azide Method 3 NaN3 + NaN02 ____► 2 Na20 + 5 N2 5 NaN3 + NaN03 -------► 3 Na20 + 8 N2 9 NaN3 + 3 NaN02 + 2 ZnO -----► 2 Na6Zn04 + 15 N2 8 NaN3 + 4 NaN02 + Co304 -----► 3 Na4Co04 + 14 N2 2 NaN3 + 4 CuO --------------------*2 NaCu202 + 3 N2 Self-Sustained High-Temperature Synthesis (SHS) Mixing Metal powders (Ti, Zr, Cr, Mo, W,....) + other reactants Pressing into pellets Ignition by energy pulse (W wire) S.S. reactor, under Ar Exothermic reaction Byproduct removal 53 DIRECT REACTION OF SOLIDS SHS reactions: é* heterogeneous é* exothermic, high temperatures, Tf = 1500 - 3000 °C é* high thermal gradients é* redox é* frontal mode, reaction wave velocity u = 1 -10 mm.s é* metastable phases State of the substance in the reaction front: solid (Tf < Tm, p < po) „solid flame" liquid, melt (Tf > Tm) gaseous Thermite reaction Zr + Fe203 —► ZrlxFex02 + Fe Ti + C —► TiC Ti + B —► TiB Self-Propagating Metathesis Grinding of components in a glove box addition of NaCl, KCl or NH4C1 as a heat sink, S.S. vessel, ignition by a resistively heated wire, reaction time 1 s, washing products with MeOH, water, drying 3 ZrCl4 + 4 Na3P ---------► 3 c-ZrP + 12 NaCl + P 3 HfCl4 + 4 Li3P ---------► 3 c-HfP + 12 LiCl + P c-ZrP and c-HfP hard and chemically inert materials, hexagonal to cubic transitions: ZrP 1425 °C, HfP 1600 °C DIRECT REACTION OF SOLIDS Self-Propagating Metathesis Silicon production Na2SiF6 + 4 Na -------------► 6 NaF + Si Hard materials production TaCl5 + Li3N + NaN3 + NH4C1 -> c-TaN + LiCl + NaCl + N2 + HCl CrCl3 + Li3N + NH4CI -----► Cr + Cr2N + c-CrN Crl3 + Li3N ----------------► Cr2N Crl3 + Li3N + NH4CI -------► c-CrN MoCl5 + Li3N -------► explosive MoCl5 + Ca3N2 + NH4CI —► cubic y-Mo2N 56 Combustion Synthesis Oxidizing reagents (metal nitrates) mixed with fuel (urea, glycine) by melting or in solution drying combustion ignited at 300-500 °C exothermic self-propagating non-explosive reaction (excess of fuel) reaction time 1 min, flame temperature 1000 °C product dry foam, crumbles to a fine powder. Zn(N03)2.6H20 + CO(NH2)2 ---------► ZnO + N2 + C02 + H20 57 Combustion Synthesis Examples ZnO(90%) - Bi203- Sb203 Non-Ohmic behavior I = (U/C)a C, a = constants, a = 50 Voltage stabilization, surge absorption 10 I— (+Sbř03. CrjOa) grain boundary _ _______L^ phase H Zn0 graii H í 5 \ i + CoO) 1 J 1 jj \ V fi^~~—< / § \ ^s--~. ^f \y M \/ ^\ 7 H \. ^V* j/ ö " ' LiVMo06 mixing 1:1:1 in aqueous solution, drying at 90 °C combustion at 250 °C calcination to LiVMo06 cathode material for Li-ion DIRECT REACTION OF SOLIDS Carbothermal Reduction Acheson 2000 K Si02 + 3 C--------------► 2 CO + SiC AH = 478 kJ 3Si02 + 6C + 2N2 -----------► 6CO + SÍ3N4 C + Si02 «—► SiO(g) + CO Si02 + CO 4—► SiO + co2 C + C02 ^=* 2 CO 2 C + SiO <=^ SiC + CO 62 DIRECT REACTION OF SOLIDS Carbothermal Reduction Borides 1300 K Ti02 + B203 + 5C ------------► 5CO + TiB2 2300 K 2 Ti02 + B4C + 3 C ---------------► 4 CO + 2 TiB2 1820 K A1203 + 12 B203 + 39 C ____► 2 A1B12 + 39 CO Carbides 2220 K 2A1203 + 9C -------► Al4C3 + 6CO 2 B203 + 7 C 1820S B4C + 6 CO 970 K W03 + 4 C-------► WC + 3 CO Nitrides 1970 K A1203 + N2 + 3C -------► 2A1N + 3CO 1 820 K 2 Ti02 + N2 + 4 C -------► 2 TiN + 4 CO DIRECT REACTION OF SOLIDS Fusion-Crystallization from Glass Fusion-Crystallization from Glass Mixing powders Melting to glass: single phase, homogeneous (T, C), amorphous Temperature limits: mp of reagents volatility of reagents Nucleation agent Homogeneous nucleation, few crystal seeds Slow transport of precursors to seed Lowest possible crystallization temperature Crystallizing a glass above its glass transition Metastable phases accessible, often impossible to prepare by other methods 64 DIRECT REACTION OF SOLIDS Fusion-Crystallization from Glass Fusion-Crystallization from Glass Production of window glass Abrasive grains 2100 K A1203 + MgO ---------► melt, solidify, crush, size Crystallizing an inorganic glass, lithium disilicate 1300 K, Pt crucible Li20 + 2Si02 + A1203 ---------------------►I^^Os Li2Si205 forms as a melt. Hold at 1100°C for 2-3 hrs. Homogeneous, rapid cooling, fast viscosity increase, quenches transparent glass Li2Si205, glass 500-700°C, Tg ~ 450°C from DSC -> Li2Si205, crystals in 2-3 hrs., principle of crystallizing a glass above its glass transition 65 Fusion-Crystallization from Glass 66 DIRECT REACTION OF SOLIDS Fusion-Crystallization from Glass Fusion-Crystallization from Glass Glass Ceramics polyxtalline materials made by controlled xtallization of glasses Cooking utensils Li2O/SiO2/Al2O3(>10%) nucl. Ti02 ß-spodumene Vacuum tube components Li2O/SiO2/Al2O3(<10%) nucl. P205 Li-disilicate, quartz Missile radomes MgO/Si02/Al203 nucl. Ti02 cordierite, cristobalite 67