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) + Al2O3(s) — MgAl2O4(s) MgO(s) + SiO2(s) — MgSiO3(s) or Mg2SiO4(s) Solid - solid synthesis - exchange, metathesis AB + C — AC + B CaCO3(s) + SiO2(s) — CaSiO3(s) + CO2(g) Ge(s) + 2 MoO3(s) — GeO2(s) + 2 MoO2(s) Solid - solid synthesis - exchange and addition PbSO4 + ZrO2 + K2CO3 — K2SO4 + PbZrO3 + CO2 Solid - solid synthesis - dissociation AB — A + B Ca3SiO5(s) — Ca2SiO4(s) + CaO(s) 2 Reaction Types Solid - solid synthesis - addition MgO(s) + Al2O3(s) MgAl2O4(s) MgO(s) + SiO2(s) — MgSiO3(s) or Mg2SiO4(s) Solid - solid synthesis - exchange, metathesis CaCO3(s) + SiO2(s) — CaSiO3(s) + CO2(g) Ge(s) + 2 MoO3(s) — GeO2(s) + 2 MoO2(s) Solid - solid synthesis - dissociation Ca3SiO5(s) — Ca2SiO4(s) + CaO(s) A + B — AB AB + C — AC + B AB — A + B 3 Reaction Types Solid - gas synthesis A + B AB 2 Fe3O4(s) + 1/2 O2(g) — 3 Fe2O3(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 CaCO3(s) — CaO(s) + CO2(g) Al4Si4O10(OH)8(s) — Al4(Si4O10)O4(s) + 4 H2O(g) Kaolinite Metakaolinite 4 Direct Reactions of Solids Other Examples Oxides BaCO3 + TiO2-* BaTiO3 + BaTi2O5 + CO2 873 K UF6 + H2 + 2 H2O —► UO2 (powder) + 6 HF dust = radiological hazard, milling, sintering to UO2 pellets YBCO 123 Superconductor (1987) .. ,A , _ , „ _ 1223 K 473 K ^ ~ Y2O3 + BaCO3 + CuO_^ _^ YBa2Cu3O7-x air O2 1130 K Tl2O3 + 2BaO + 3CaO + 4CuO ► Tl2Ba2Ca3Cu4O12 5 Direct Reactions of Solids Other classes than oxides Pnictides 1100 K 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 NH4Cl + Y2O3 ► 2 YCl3 + 3 H2O + 6 NH3 6 NH4Cl + Y -► (NH4)3YCl6 + 1.5 H2 + 3 NH3 4 NH4Cl + 3 NH4ReO4 3 Re + 12 H2O + 3.5 N2 + 4 HCl Aluminosilicates NaAlO2 + SiO2 -► NaAlSiO4 Chalcogenides 1400 K Pb + Mo + S -► PbMo6S8 7 Chevrel phases (MxMo6X8, M = RE, Sn, Pb, Cu, X = S, Se, Te) 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 Planetary ball mill Rotation and counter-wise spining Rotation speed: up to 400 rpm Milling jars: alumina, YSZ, tungsten carbide, agate 9 Milling Compaction - Pressing Hydraulic Uniaxial Press Warm Isostatic Press Hot press Maximum pressure: 120 MPa Max. pressure: 400 MPa ^ tem^atU^ l250 °C Max. temperature: 80 oC Max.pressure: 100 MPa Volume: 2,5 l Max. diameter: 25 mmi1 lation Tube Furnace in air and in controled atmosphere Maximum temperature: 1450 oC or 1600 °C Furnace-tube diameter: up to 75 mm Vacuum Furnace in vacuum or Ar, N2,O2 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 (Na2O, PbO, ...) uptake of ambient gas (O2 in superconductors) 13 Direct Reactions of Solids Experimental Considerations tf* Reagents Drying, fine grain powders for maximum SA, surface activation (Mo + H2), in situ decomposition (CO3 -, OH-, O2 -, C2O4 -) for intimate mixing, precursor reagents, homogenization, organic solvents, grinding, ball milling, ultrasonication tf* 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 14 Properties of Common Container Materials Material Maximum Working Temp., K Pyrex CaF2 770 1420 Thermal Shock Resistance GOOD FAIR Thermal Coefficient of Other Conductivity, Linear W m-1 K-1 Expansion xl06, K-1 1.13 3.2 24 Properties Permeable to air at high T SiO2 Si3N4 Pt 1530 VERY GOOD 1.38 - 2.67 0.4 - 0.6 1770 1950 FAIR VERY GOOD 10 - 33 73 6.4 9.11 Permeable to air at high T, devitrification above 1670 K Plastic at high T BN 1970 VERY GOOD Vitreous C 2070 Al2O3 AlN 2170 2270 5.02 GOOD 4.19 - 8.37 FAIR FAIR 35 - 39 50 - 170 0.2-3 2-3.5 8 5.7 Oxidizes in air above 970 K Oxidizes in air above 900 K Reacts with metals above 1800 K BeO ZrO2 2570 2570 GOOD GOOD 230 1.97 8.4 4.5 Reacts with metals above 1800 K Ir MgO ThO2 2600 VERY GOOD 2870 3070 FAIR FAIR 148 37.7 4.19 6.8 25 6 High vapor pressure Reacts with C above 2290 K 15 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 4* 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 4* Controlled atmosphere (oxidizing, reducing, inert) or vacuum. Unstable oxidation states, preferential component volatilization if T too high, composition dependent atmosphere (O2, NH3, H2S, ...) 16 Reaction Paths between Two Solids Direct Reactions of Solids Model reaction, well studied: MgO + Al2O3 — MgAl2O4 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/MgAl2O4 and Al2O3/MgAl2O4 interfaces are formed Free energy negative, favors reaction but extremely slow at normal temperatures (several days at 1500 oC) 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 + Al2O3 — NiAl2O4 MgO + Fe2O3 — MgFe2O4 18 The Spinel Structure: AB204 fcc array of O2- ions, A occupies 1/8 of the tetrahedral and B 1/2 of the octahedral holes normal spinel: AB2O4 Co3O4, GeNi2O4, WNa2O4 — inverse spinel: B[AB]O4 Fe3O4: Fe3+[Fe2+Fe3+]O4, TiMg2O4, NiLi2F4 — basis structure for several magnetic materials 19 The spinel structure: MgAl204 20 The spinel structure: MgAl204 21 Direct Reactions of Solids Model for a classical solid-solid reaction (below melting point !): Planar interface between two crystals MgO + Al2O3 MgAl2O4 (Spinel) Phase 1: nucleation Phase 2: growth of nuclei MgAl2O4 Al2O3 <-► 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 Al2O3 hcp O2-, Al3+ in 2/3 Oh sites MgAl2O4 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(Al3+) 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 ln 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 2Al3+ MgO/MgAl2O4 Interface: 2Al3+ -3Mg2+ + 4MgO 1MgAl2O4 MgAl2O4/Al2O3 Interface: 2+ 3+ 3Mg -2Al + 4Al2O3 — 3MgAl2O4 Overall Reaction: 4MgO + 4Al2O3 — 4MgAl2O4 the Kirkendall Effect : RHS/LHS growth rate of interface = 3/1 24 Reaction Mechanism interface I interface II Al2O3 2 Al 3+ MgAl2O4 MgO 3 Mg 2+ interface I interface II Al2O3 2 Al °2-»- 2 e" 3+ MgAl2O4 MgO 2+ 2 Mg 2" O 25 1/2 O2 1/2 O2 Thermodynamic and kinetic factors Direct Reactions of Solids General kinetic expression _— k (t )f (a) Reaction rate dt Rate constant - da Reaction order I_— g (a) — I k (T )dt J f (a) J 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 a — t_— P0 = the value of a property at the beginning P _ P Pe = the value of a property at the end e " 0 a — 0 _ 1 e.g. Pt = mass loss, x,...... 26 Direct Reactions of Solids da = k (T )f(a) da J k (T )dt dt v v 7 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 a = P - P 11 10 P - P 1 e 1 0 a = 0 -1 MgO x2 = kt Al2O3 x 27 Mechanism model g(a) Diffusion controlled One-dimensional Two-dimensional Three-dimensional, Jander Three-dimensional, Ginstling Three-dimensional, Carter Growth controlled General First order, n = 1 Nucleation controlled Power law Nucleation-Growth controlled Avrami Erofeev Planar boundary Spherical boundary a2 a + (1 - a) ln (1 - a) [1 - (1 - a)1/3]2/3 (1 - 2/3a) - (1 - a)2/3 (1 + a)2/3 + (1 - a)2/3 [1 - (1 - a)1-n] [- ln (1 - a)] a1/n [- ln (1 - a)]1/2 [- ln (1 - a)]1/3 1 - (1 - a)1/2 1 - (1 - a)1/3 28 Avrami Plot H 0.8- 0.6- 0.4- 0.2- 0 a =1- exp(-ctn) Conversion is 50% Complete T is the time required for 50% conversion L .1e2 .1e3 .1 e4 1e5 Time (s) 29 Direct Reactions of Solids Perform the measurements in a range of temperatures T use Arrhenius equation to evaluate the activation energy E La2O3 Cation Diffusion in LaCoO3 Marker experiments 1 2 1 3 Co3* CoO DCo >> DLa Rate-determining step: Diffusion of Co cations LaCoO3 31 Growth Kinetics of LaCoO3 12000 8000 - 4000 - x2 = kt 1370 K Parabolic rate law valid = diffusion controlled process 32 Growth Kinetics of LaCoO3 9.5 i-1-.-.-1-1 12.0 1-'-'-'-'-1 5.5 6.0 6.5 7.0 7.5 8.0 104/T(K"1) . EA = (250 ± 10) kJ mol-1 33 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 ■JfcNucleation of product phase within the reactant with similar crystal structure •JfcEpitactic and topotactic reactions ^Surface structure and reactivity of different crystal planes/faces 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] 4nr2 SA = A/m = - = 3000/rp [m2/g] 4/3nr3.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 6.10-4 109 103 0.6 1018 10-6 600 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 1/Acontact and furthermore Acontact « 1/dparticle Thus particle sizes and surface area inextricably connected and obviously x oc d and SA particle size affect the interfacial thickness 36 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"/d 37 DIRECT REACTION OF SOLIDS DIFFUSION RATE Fick's law J = - D(dc/dx) J = flux of diffusing species, #/cm s (dc/dx) = concentration gradient, #/cm 4 f n \ - D D Q D = diffusion coefficient, cm2/s, for good reaction rates > 10-12 D = Do exp n^ V Rt j 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) 38 Nucleation Homogeneous nucleation Liquid melt to crystalline solid Cluster formation . „ A//T,AT agv =--- AGv = driving force for solidification (negative) 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 39 Nucleation rate n Nucleation rate n Liquid to solid n = n0 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 + 4/3nr3AGV AGs = 4nr2YsL surface free energy change 4/3nr3AGy volume free energy change positive negative, l to s lowers the energy AG Nucleation 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 = 4nr2YSL + 4/3nr3AGV 41 Critical Radius r* The critical radius r* = the radius at which AGN is maximum AG; AH.AT The energy barrier to homogeneous nucleation 3AGf AH2 AT2 The temperature-dependence r* = 1/AT AG*r = 1/AT2 42 Nucleation a sub-critical cluster unstable for r < r* the cluster re-dissolves a nucleus stable for r >r* the stable nucleus continues to grow 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 cos <9 = Ywl - Yws Ysl 6 = wetting angle Shape factor S(6) 46 Wetting Angle Force equilibrium rGS = ygl cos <9 + ysl cos <9 = Vgl 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 n r3 AGv +4 n r2 ySL - Volume free energy + surface energy One solid phase changing to another AG = 4/3 n r3 AGv +4 n r2 ySL + 4/3 n r3 s - 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 s Yaß = the a/ß interfacial energy Ratio of jS/ů lattice param B 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 Combination of these two terms (multiplication) determines the total nucleation rate 50 Nucleation rate I Nucleation rate [m-3 s-1] I = ß n* n* = the steady-state population of critical nuclei (m-3) ^ ag * ^ n0 = the number of potential n = n0 exp kT j nucleation sites per unit volume AG* = the critical free energy of nucleation P = the rate at which atoms join critical nuclei (s-1), thereby making them stable, a diffusion-dependent term o = 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 I n* = the steady-state population of critical nuclei (m-3) 52 Nucleation 53 Nucleation vs. Growth Equilibrium transformation temperature Rate 54 Nucleation vs. Crystal Growth (solution or melt) Undercooling-coolingbelow themelting 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 r I . 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, Al2O3, MgAl2O4 example MgO (rock salt) and MgAl2O4 (spinel) similar ccp O2-but distinct to hcp O2- in Al2O3 phase Spinel nuclei, matching of structure at MgO interface Oxide arrangement essentially continuous across MgO/MgAl2O4 interface Bottom line: structural similarity of reactants and products promotes nucleation and growth of one phase within another Lattice of oxide anions, mobile Mg and Al 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 58 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. 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, Mg or O - hexagonal arrangement 61 Direct Reactions of Solids Cubic (rocksalt) MgO crystal: different netplan.es 62 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 + NaNO2 5 NaN3 + NaNO3 > 2 Na2O + 5 N2 >* 3 Na2O + 8 N2 9 NaN3 + 3 NaNO2 + 2 ZnO -► 2 Na6ZnO4 + 15 N2 8 NaN3 + 4 NaNO2 + Co3O4 -► 3 Na4CoO4 + 14 N2 2 NaN3 + 4 CuO -► 2 NaCu2O2 + 3 N2 64 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: tf* heterogeneous tf* exothermic, high temperatures, Tf = 1500 - 3000 °C tf* high thermal gradients tf* redox tf* frontal mode, reaction wave velocity u = 1 - 10 mm.s-1 tf* 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 + Fe2O3 —► Zr1-xFexO2 + Fe Ti + C —► TiC Ti + B ► TiB 66 Self-Propagating Metathesis Grinding of components in a glove box addition of NaCl, KCl or NH4Cl 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 67 DIRECT REACTION OF SOLIDS Self-Propagating Metathesis Silicon production Na2SiF6 + 4 Na -► 6 NaF + Si Hard materials production TaCl5 + Li3N + NaN3 + NH4Cl -► c-TaN + LiCl + NaCl + N2 + HCl CrCl3 + Li3N + NH4Cl -► Cr + Cr2N + c-CrN CrI3 + Li3N ► Cr2N CrI3 + Li3N + NH4Cl ^ c-CrN MoCl5 + Li3N -► explosive MoCl5 + Ca3N2 + NH4Cl —► cubic y-Mo2N 68 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(NO3)2.6H2O + CO(NH2)2 ► ZnO + N2 + CO2 + H2O 69 Combustion Synthesis Examples ZnO(90%) - Bi2O3 - Sb2O3 Non-Ohmic behavior I = (U/C)a C, a = constants, a = 50 Voltage stabilization, surge absorption 10 70 Reaction front propagation: glycine-iron nitrate J 4 L =0s t=Ls r 1 t=3s Self-Propagating Metathesis 72 Combustion Synthesis Examples LiNO3 + NH4VO3 + (NH4)2MoO4 + glycine LiVMoO6 mixing 1:1:1 in aqueous solution, drying at 90 °C combustion at 250 °C calcination to LiVMoO6 cathode material for Li-ion Combustion Synthesis Yttrium Iron Garnet (YIG) Y3Fe5O12 Y(NO3)3*6H2O Fe(NO3)3*9H2O 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 ignition of the gel MN/CA ratio controls the size H i i i i i i I 'J 0.5 1 1.5 2 2.5 3 3.5 MN/CA DIRECT REACTION OF SOLIDS Carbothermal Reduction Acheson 2000 K SiO2 + 3 C-► 2 CO + SiC AH = 478 kJ 3 SiO2 + 6 C + 2 N2 -► 6 CO + Si3N4 C + SiO2 * ► SiO(g) + CO SiO2 + CO * SiO + CO2 C + CO2 4 * 2 CO 2 C + SiO * * SiC + CO 75 DIRECT REACTION OF SOLIDS Carbothermal Reduction Borides 1300 K TiO2 + B2O3 + 5 C -► 5 CO + TiB2 2300 K_ 2 TiO2 + B4C + 3 C 4 CO + 2 TiB2 1820 K Al2O3 + 12 B2O3 + 39 C -+ 2 AlB12 + 39 CO Carbides 2220 K 2 Al2O3 + 9 C -► Al4C3 + 6 CO 2 B2O3 + 7 C 1 820 K ► B4C + 6 CO 970 K WO3 + 4 C-► WC + 3 CO Nitrides 1970 K Al2O3 + N2 + 3 C -► 2 AlN + 3 CO 1820 K 2 liO2 + N2 + 4 C -► 2 liN + 4 CO 76 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 DIRECT REACTION OF SOLIDS Fusion-Crystallization from Glass Fusion-Crystallization from Glass Production of window glass Abrasive grains 2100 K Al2O3 + MgO -► melt, solidify, crush, size Crystallizing an inorganic glass, lithium disilicate 1300 K, Pt crucible Li2O + 2SiO2 + Al2O3 -► Li2Si2O5 Li2Si2O5 forms as a melt. Hold at 1100oC for 2-3 hrs. Homogeneous, rapid cooling, fast viscosity increase, quenches transparent glass Li2Si2O5, glass 500-700oC, Tg ~ 450oC from DSC Li2Si2O5, crystals in 2-3 hrs., principle of crystallizing a glass above its glass transition 78 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. TiO2 (3-spodumene Vacuum tube components Li2O/SiO2/Al2O3(<10%) nucl. P2O5 Li-disilicate, quartz Missile radomes MgO/SiO2/Al2O3 nucl. TiO2 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 CO2 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)O5 Ca3SiO5 Ca2SiO4 Ca3Al2O6 Aluminate Ferrite Phase Alite Belite wt% 50-70 15-30 5-10 5-15 82 Chemical Cement Nomenclature s A T K H Č š SiO2 AkO3 H2O CO2 sO3 C CaO F Fe2O3 M MgO N Na2O