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 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 Ca3SiOs(s) -> Ca2Si04(s) + CaO(s) 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 - dissociation AB -» A + B Ca3SiOs(s) -> Ca2Si04(s) + CaO(s) 3 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 4 Direct Reactions of Solids Other Examples Oxides BaC03 + Ti02-► BaTi03 + BaTi2Os + 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 » 473 K » YBa2Cu307x air 02 1130 K Tl203+2BaO + 3CaO + 4CuO -► Tl2Ba2Ca3Cu4012 5 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 6 Direct Reactions of Solids Chlorides 3 CsCl + 2 ScCl3 -► Cs3Sc2Cl9 6 NH4C1 + 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 HC1 Aluminosilicates NaA102 + Si02 -► NaAlSi04 Chalcogenides 1400 K Pb + Mo + S -► PbMo6S8 Chevrel phases (MxMo6X8, M = RE, Sn, Pb, Cu, X = S, Se, Te) 7 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 8 Milling Planetary ball mill r* Movement of dbdi Cenodftgal force Rotation speed: up to 400 rpm Milling jars: alumina, YSZ, tungsten carbide, agate Planetary ball mill Rotation and counter-wise spining Milling Compaction - Pressing m 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 mini Calcination Maximum temperature: 1450 °C or 1600 °C Vacuum Furnace Furnace-tube diameter: up to 75 mm in vacuum or Ar, N2, 02 atmosphere Maximum temperature: 1200 °C Chamber Dimensions: 150x200x250 mm3 12 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) 13 Direct Reactions of Solids Experimental Considerations 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 ^ 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 Working Temp., K Pyrex CaF2 770 1420 Thermal Shock Resistance GOOD FAIR W m"1 K"1 Thermal Coefficient of Other Conductivity, Linear Properties Expansion xlO6, K_1 113 32 Permeable to air athighT 24 Si02 SÍ3N4 1530 VERY GOOD 1.38-2.67 1770 FAIR 10-33 0 4-06 Permeable to air at high T, 6.4 devitrification above 1670K Pt 1950 VERY GOOD 73 911 Plastic at high T BN 1970 Vitreous C 2070 A120, A1N BeO Zr02 Ir 2170 2270 2570 2570 2600 VERY GOOD 5.02 GOOD 4.19-8.37 FAIR FAIR GOOD GOOD VERY GOOD 35-39 50- 170 230 1.97 148 0.2-3 2-3.5 8 5.7 8.4 4.5 6.8 Oxidizes in air above 970 K Oxidizes in air above 900 K Reacts with metals above 1800 K Reacts with metals above 1800 K MgO Th02 2870 3070 FAIR FAIR 37.7 4.19 25 6 High vapor pressure Reacts with C above 2290 K Direct Reactions of Solids §^ 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 ^ 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 ^ Controlled atmosphere (oxidizing, reducing, inert) or vacuum. Unstable oxidation states, preferential component volatilization if T too high, composition dependent atmosphere (02, NH3, H2S,...) 16 Reaction Paths between Two Solids gas phase diffusion volume diffusion interface diffusion surface diffusion 17 Direct Reactions of Solids Model reaction, well studied: MgO + A1203 MgAl204 Spinel (ccp O2-, Mg2+ 1/8 Td, Al3+ 1/2 Oh) 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) Interfacial growth rates 3 : 1 Linear dependence of interface thickness x2 versus t Easily monitored rates with colored product at interface, T and t NiO + A1203 -> NiAl204 MgO + Fe203 -> MgFe204 18 The Spinel Structure: (A)[B2]04 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 19 The spinel structure: MgAl204 (A)[B2]04 The spinel structure: MgAl204 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 c 'S V A1203 MgO c Phase 2: growth of nuclei MgO MgA12°J ai,o 2^3 •4-► X 22 Direct Reactions of Solids # Structural differences between reactants and products, major structural reorganization in forming product spinel 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 ^'Making and breaking many strong bonds (mainly ionic), high temperature process as D(Mg2+) and D(A13+) large for small highly charged cations #'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 #'Nucleation of product spinel at interface, ions diffuse across thickening interface, oxide ion reorganization at nucleation site 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 interfacial 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 Reaction Mechanism interface I interface II A1203 MgAl204 MgO 2 Al 3+ 3Mg 2+ interface I interface II A1203 MgAl204 MgO 2 Al 3+ 2 2Mg2+ q2. 1/2 O, 1/2 O, Thermodynamic and kinetic factors Direct Reactions of Solids si General kinetic expression __ - k(r)f{a) Reaction rate dt Rate constant -Reaction order 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 Pt = the value of a property at time t — t_Q_ P0 = the value of a property at the beginning T) T) Pe = the value of a property at the end ~ 0 a -Q_\ e.g. Pt = mass loss, x,...... 26 Direct Reactions of Solids da = k(T)f{a) da jk(T)dt dt J f(a) g(a) =J 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 a = P -P p -P a = 0-l MgO AUO 2W3 X 27 Mechanism model Diffusion controlled One-dimensional a2 Two-dimensional a + (1 - a) In (1 - a) Three-dimensional, Jander [1 — (1 — a)1/3]2/3 Three-dimensional, Ginstling (1 _ 2/3a) - (1 - a)2 3 Three-dimensional, Carter (1 + oc)2 3 + (1 - a)2 3 Growth controlled General [1 - (1 - a)ln] 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 — a)1/2 Spherical boundary \ — (1 — a)1/3 Avrami Plot a=l-exp[-(kt)n] Conversion is 50% Complete Incubation Time k = rate constant n = exponent T is the time required for 50% conversion .1e2 .1e3 .1e4 .1 e5 t, Time (s) 2 Direct Reactions of Solids Perform the measurements in a range of temperatures T use Arrhenius equation to evaluate the activation energy k(T) = k0 exp(-Ea/RT) 0.8H 0.6- 0.4h 0.2 135 °C 120 °C 80 °C Time, s .1 e2 .1e3 t .1 e4 .1 e5 Cation Diffusion in LaCoO La203 1 c 2 \ A 1 3 2 Co3* Marker experiments CoO DCo » DLa Rate-determining step: Diffusion of Co cations LaCoO 31 Growth Kinetics of LaCoO 12000 8000 \- 4000 h I I In air 1673 K J. - - - A 1573 K / 1478 K . -*-,--*- x2 = 1370 K Parabolic rate law valid = diffusion controlled process Growth Kinetics of LaCo03 -9.5 CM -10.0 ■ w -10.5 - E J* g> -11.0 -11.5 - -12.0 5.5 6.0 6.5 7.0 104/T (K_1) 7.5 8.0 Dcrfr» 1370 CllltfLeSQKlO-" U78 (1.49*0. W)ilO-11 l.40ilO-11 \m (ioimiejiio-11 \m 1.33kl0 u EA = (250 ± 10) kJ mol1 33 FACTORS INFLUENCING REACTIONS OF SOLIDS CONTACT AREA 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 DIFFUSION RATE ^Surface area of reactants ^Particle size ^Pelleting, pressing, precursors ^Diffusion rates of atoms, ions, molecules in solids ^Reaction temperature, pressure, atmosphere ^Diffusion length, particle size ^Defect concentration, defect type ^Reaction mechanism 34 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] 47cr2 SA = A/m = - = 3000/rp [m2/g] 4/3jcr3.p 35 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/dparticle Thus particle sizes and surface area inextricably connected and obviously x oc d and SA particle size affect the interfacial thickness Direct Reactions of Solids These relations suggest some strategies for rate enhancement in direct reactions: ■^Hot pressing densiflcation of particles High pressure squeezing of reactive powders into pellets (700 atm) Pressed pellets still 20-40% porous. Hot pressing improves densiflcation ■^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 DIRECT REACTION OF SOLIDS DIFFUSION RATE Fick'slaw J = -D(dc/dx) J = flux of diffusing species, #/cm2s (dc/dx) = concentration gradient, #/cm4 C D = diffusion coefficient, cm2/s, for good reaction rates > 10"12 ^ ~ ^a> exP V 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) Nucleation Homogeneous nucleation Liquid melt to crystalline solid Cluster formation _ AH, AT v m AGV = AGV = driving force for solidification (negative) Tf below the equilibrium melting temperature, Tm AT = undercooling, AHV = enthalpy of solidification (negative) Small clusters of crystallized solid form in a melt because of the random motion of atoms within the liquid Driving force is opposed by the increase in energy due to the creation of a new solid-liquid interface Ysl = the solid/liquid interfacial energy >r¥l rrp (AGN+AGD) Nucleation rate n Nucleation rate n / /at i at ^ Liquid to solid fl — fl^ exp - V kT j AGN = thermodynamic barrier to nucleation AGD = kinetic barrier to diffusion across the liquid/nucleus interface Assume, that solid phase nucleates as spherical clusters of radius r AGN = the net (excess) free energy change for a single nucleus AGN = AGS + 4/37ir3AGv AGS = 47ir2ySL surface free energy change positive 4/371 r3AGv volume free energy change negative, 1 to s lowers the energy Nucleation Retarding energy AGS = surface free-energy change = 4nr2y = total free-energy change Radius of particle, r 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 &GV = volume free-energy change AGN = 47ir2ySL + 4/37ir3AGv 41 Critical Radius r* The critical radius r* = the radius at which AGN is maximum AG, AH, AT The energy barrier to homogeneous nucleation 3 ag; ah;atj The temperature-dependence r* = l/AT AG*r = 1/AT2 42 Nucleation a sub-critical cluster unstable for r < r* the cluster re-dissolves < DC E 3 £ § s B Retarding energy A(?s = surface free-energy change = 4wr2y a nucleus stable for r >r* the stable nucleus continues to gro^ AGj. = total free-energy change Radius of particle, r kGv — volume free-energy change 43 Heterogeneous Nucleation Nuclei can form at preferential sites: flask wall, impurities, catalysts,..... The energy barrier to nucleation, AG*, is substantially reduced The critical nucleus size, r* is the same for both heterogeneous and homogeneous nucleation 44 Heterogeneous Nucleation a solid cluster forming on a wall: • the newly created interfaces (i.e. solid-liquid and solid-wall) • the destroyed interface (liquid-wall) 45 Heterogeneous Nucleation Liquid cosO = Ywl Yws Ysl -2y SL _ = n ham 9 = wetting angle Shape factor S(0) (2 + cos0)(l-cos0): 46 Wetting Angle Force equilibrium Tos =rgl^e+Ys l Snldcr I'd cosO = Tos - Ysl Ygl Non-wetting Wetting gas Liquid Solid The Interface Liquid $ 0 47 Heterogeneous Nucleation The critical radius r* is the same for both homogeneous and heterogeneous nucleation The volume of a critical nucleus and AG* can be significantly smaller for heterogeneous nucleation due to the shape factor, depending on the wetting angle, 6 4 Direct Reactions of Solids Solidification AG = 4/3 7i r3 AGV +4 n r2 ySL - Volume free energy + surface energy One solid phase changing to another AG = 4/3 7i r3 AGV +4 n r2 ySL + 4/3 n r3 8 - Volume energy + surface energy + strain energy - the new solid does not take up the same volume as the old solid - a misfit strain energy term, AGS = V 8 -2y N3* = 16 wy yap = the a/(3 interfacial energy Ratio of j8/ů lattice param Nucleation Transformation from liquid to solid phase requires: •Nucleation of new phase •Growth of new phase Nucleation depends on: •driving force toward equilibrium - cooling of a melt increases as we move to lower temperatures •diffusion of atoms into clusters increases at higher temperatures diffusion Combination of these two terms (multiplication) determines the total nucleation rate 50 Nucleation rate / Nucleation rate [m~3 s_1] I = P n* n* = the steady-state population of critical nuclei (m~3) f a ^* \ n0 = the number of potential nucleation sites per unit volume AG* = the critical free energy of nucleation n - n0 exp AG P = the rate at which atoms join critical nuclei (s_1), thereby making them stable, a diffusion-dependent term &=o) exp(-g/kT) co = temperature independent term incorporating vibrational frequency and the area to which atoms can join the critical nucleus 51 Q = an activation energy for atomic migration Nucleation rate / the steady-state population of critical nuclei ( Nucleation 53 Nucleation vs. Growth Equilibrium transformation temperature Rate 54 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. 55 Nucleation vs. Crystal Growth 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 Temperature > 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 57 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 Direct Reactions of Solids Topotactic reactions More specific, require interfacial 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. - k Psevda-Sflid-State mmm 59 Direct Reactions of Solids Surface structure and reactivity Nucleation depends on actual surface structure of reacting phases. Different Miller index faces exposed, atom arrangements different, different surface structures, implies distinct surface reactivities. Face-Centred Cubic Body-Centred Cubic 60 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 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) 63 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 65 DIRECT REACTION OF SOLIDS SHS reactions: ^ heterogeneous ^ exothermic, high temperatures, Tf = 1500 - 3000 °C ^ high thermal gradients ^ redox ^ frontal mode, reaction wave velocity u = 1 -10 mm.s metastable phases State of the substance in the reaction front: solid (Tf < Tm, p < p0) „solid flame" liquid, melt (Tf > Tm) gaseous Thermite reaction Zr + Fe203 Ti + C Ti + B * ZrlxFex02 + Fe ► TiC - TiB Self-Propagating Metathesis Grinding of components in a glove box addition of NaCl, KC1 or NH4C1 as a heat sink, S.S. vessel, ignition by a resistively heated wire, reaction time 1 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 + HC1 CrCl3 + Li3N + NH4CI -► Cr + Cr2N + c-CrN Crl3 + Li3N -► Cr2N Crl3 + Li3N + NH4CI -► c-CrN MoCl5 + Li3N -► explosive MoCl5 + Ca3N2 + NH4CI —► cubic y-Mo2N 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 Combustion Synthesis Examples ZnO(90%) - Bi203- Sb203 Non-Ohmic behavior I = (U/C)a C, a = constants, a = 50 Voltage stabilization, surge absorption 10 (+Sbz03. CrjOa) grain boundary phase Bi segregate phas^ \y. \ (=20A) pu*..' \ J^X varistor voltage 100 200 300 voltage (V) 70 Self-Propagating Metathesis Combustion Synthesis Examples LiN03 + NH4VO3 + (NH4)2Mo04 + glycine -> LiVMo06 mixing 1:1:1 in aqueous solution, drying at 90 °C combustion at 250 °C calcination to LiVMo06 cathode material for Li-ion Combustion Synthesis Yttrium Iron Garnet (YIG) Y3Fe5012 Y(N03)3 6H20 Fe(N03)3 9H20 citric acid monohydrate Solution in water Y:Fe = 3:5 The solution evaporated at 85 °C stirrired until viscous gel Increasing the temperature up to 250 °C f ignition of the gel MN/CA ratio controls the size l.i 2 MN/CA DIRECT REACTION OF SOLIDS Carbothermal Reduction Acheson 2000 K Si02 + 3 C-► 2 CO + SiC AH = 478 kJ 3SK)2 + 6C + 2N2 -► 6CO + Si3N4 C + Si02 «—► SiO(g) + CO Si02 + CQ4—► SiO + co2 C + C02 «—* 2 CO 2 C + SiO SiC + CO 75 DIRECT REACTION OF SOLIDS Carbothermal Reduction Borides 1300 K Ti02 + B203 + 5 C -► 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 + 9 C -► Al4C3 + 6CO 2 B203 + 7 C 1820 S B4C + 6 CO 970 K W03 + 4 C-► WC + 3 CO Nitrides 1970 K A1203 + N2 + 3C -► 2A1N + 3CO 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 77 Fusion-Crystallization from Glass £ a f Glass Glass_— - ___' i * ! 1 I 1 A Crystals C 7* 1 1 1 1 1 1 * m Temperature 78 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 -►Li2Si2Os Li2Si205 forms as a melt. Hold at 1100°C for 2-3 hrs. Homogeneous, rapid cooling, fast viscosity increase, quenches transparent glass Li2Si2Os, glass 500-700°C, Tg ~ 450°C from DSC -> Li2Si2Os, crystals in 2-3 hrs., principle of crystallizing a glass above its glass transition 79 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 p-spodumene Vacuum tube components Li2O/SiO2/Al2O3(<10%) nucl. P2Os Li-disilicate, quartz Missile radomes MgO/Si02/Al203 nucl. Ti02 cordierite, cristobalite 80 Cements 5600 BC - the floor of a villa in Serbia, a red lime binder (calcium oxide). Lime obtained by burning gypsum, limestone or chalk 2589-2566 BC - Egypt, the Great Pyramid of Cheops, gypsum-derived binders 800 BC the Greeks, 300 BC the Romans, limestone-derived cements became widespread Vitruvius, De Architectura the Appian Way, the Coliseum, the Pantheon cements based on a mixture of natural and synthetic aluminosilicates with lime - pozzolan 1756 John Smeaton, lighthouse, a pozzolanic binder from lime, volcanic ash and copper slag, able to withstand the harsh coastal environment 1824 Joseph Aspdin, Leeds, England, developed and patented Portland cement. Portland cement - made by heating at 1450°C chalk, shale, and clay or limestone in a kiln to form a partially fused mixture - clinker, which is then finely ground with gypsum 81 Cements Hydraulic cements - materials that set and harden by reacting with water, produce an adhesive matrix, combined with other materials, are used to form structural composite materials. Non-hydraulic cements - lime and gypsum plasters, set by drying out, must be kept dry, gain strength slowly by absorption of C02 to form calcium carbonate through carbonatation Concrete - a mixture of cement (binding agent) and water with aggregate (varying amounts of coarse and fine sand and stone). Consumption of concrete - 2.5 tonnes per person per year. Mortar - used to bind bricks together, made from cement but with finer grade of added materials. Portland cement Component Tricalcium silicate B-dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite C3S C2S C2A C4AF Formula Ca2(Al/Fe)05 Ca3Si05 Ca2Si04 Ca3Al206 Phase Alite Belite Aluminate Ferrite wt% 50-70 15-30 5-10 5-15 82 Chemical Cement Nomenclature s SÍ02 c A M203 F T TÍ02 M K K20 N H H20 Č C02 Š S03 CaO Fe2(ľ)3 MgO Na20