Gas Phase Reactions Heating: furnace, laser, plasma, flame, arc Gas-Metal Rxn Ti + N2 180° K ► TiN 3 Si + 2 N2 --------► SÍ3N4 W + CH4 -----------►WC + H2 mp2720°C WC dissolved in Co = cemented carbides Ti + CH4 -----------►TiC + H2 mp2940°C cementite steel + H2/CO + CH4 + NH3 --------► Fe3C + nitrides Gas Phase Reactions Gas-Gas Rxn homogeneous nucleation from supersaturated vapor (nano) Flame hydrolysis volatile compounds are passed through an oxygen-hydrogen stationary flame: o2 -►Si02 + HC1 fumed silica reagent bp/°C product SiCl4 57 Si02 AICI3 180 (subl.) A1203 TiCl4 137 Ti02 Cr02Cl2 117 Cr203 Fe(CO)5 103 Fe203 GeCl4 84 Ge02 Ni(CO)4 42 MO SnCl4 114 Sn02 ZrCl4 331 (subl.) Zr02 VOCI3 127 v2o5 Gas Phase Reactions 4* V *V S*V Agglomerates - Aggregates Molten Spherical Primary Particles I Reactants Flame Reaction Zone Burner Tube SiCl4 + H20 -> OSiCl2 + 2 HCl OSiCl2 + H20 -> SiClOOH + HCl SiClOOH-» Si(X +HC1 ««-#-*-«*- A-48-«?-«- « « 4l » &-$»-« Gas Phase Reactions Spray Flame Supporting Flame * * • Sheath gas i imi i CHJO, I—n Liquid precursor 0, Gas Phase Reactions Calcium phosphate nanoparticles Ca/P molar ratios 1.43 to 1.67 synthesized by simultaneous combustion of Ca(OAc)2 + OP(OnBu)3 in a flame spray reactor Fluoro-apatite and zinc or magnesium doped calcium phosphates adding trifluoroacetic acid or metal carboxylates into the fuel. Nanoparticle morphology At a molar ratio of Ca/P < 1.5 promoted the formation of dicalcium pyrophosphate (Ca2P207). Phase pure tricalcium phosphate obtained with a precursor Ca/P ratio of 1.52 after subsequent calcination at 900 °C micropores and the facile substitution of both anions and cations possible application as a biomaterial. 6 Gas Phase Reactions High-power C02 lasers 3SiH4 + 4NH3 ---------► Si3N4+12H2 HN(SiMe3)2 + NH3 --------► Si3N4 + SiC DC-Ar Plasma 1300 K TiCl4 + NH3 ---------► TiN + HCl Tarnishing of Metal Surfaces oxide, hydroxide layers Arc Graphite ------------► C60 7 Vapor Phase Transport Syntheses Sealed glass tube reactors Solid reactant(s) A + gaseous transporting agent B Temperature gradient furnace AT ~ 50 °C Equilibrium established A(s) + B(g) <-» AB(g) Equilibrium constant K A + B react at T2 Gaseous transport by AB(g) AB(g) decomposes back to A(s) at Tl5 crystals of pure A Temperature dependent K Equilibrium concentration of AB(g) changes with T Different at T2 and Tt Concentration gradient of AB(g) = driving force for gaseous ai usion traces of a transporting agent B (e.g. I2) Tl 8 Vapor Phase Transport Syntheses Whether Tl < T2 or Tl > T2 depends on the thermochemical balance of the reaction ! Transport can proceed from higher to lower or from lower to higher temperature Example: Pt(s) + 02(g) <-> Pt02(g) Endothermic reaction, Pt02 forms at hot end, diffuses to cool end, deposits well formed Pt crystals, observed in furnaces containing Pt heating elements Chemical vapor transport, T2 > Tl5 provides concentration gradient and thermodynamic driving force for gaseous diffusion of vapor phase transport agent AB(g) Uses of VPT • synthesis of new solid state materials • growth of single crystals • purification of solids 9 Vapor Phase Transport Syntheses Thermodynamics of VPT Reversible equilibrium needed: AG0 = -RTlnK^ = AH0 - TAS° t* Exothermic AH0 < 0 Smaller T implies larger K^ AB forms at cooler end, decomposes at hotter end of reactor W + 3C12 <-> WC16 400/1400 (exo) Ni + 4CO <-> Ni(CO)4 50/190 (exo) ^Endothermic AH0 > 0 Larger T implies larger Kequ AB forms at hotter end, decomposes at cooler end of reactor 2Al + AlCl3<-> 3A1C1 1000/600 (endo) 4ai + ai2s3 <->3Ai2s 1000/900 (endo) van't Hoff equation Vapor Phase Transport Syntheses Estimation of the thermochemical balance (AH) of a transport reaction: e.g.: ZnS(s) + I2(gas) o Znl2(gas) + S(g) AH = ?? Zn(s) + I2(g)<-> Znl2(gas) AH = - 88 kJ mol1 ZnS(s) *> Zn(s) + S(g) AH = +201 kJ moľ1 I ZnS(s) + I2(gas) ^ Znl2(gas) + S(g) AH = +113 kJ moľ1 endothermic reaction, transport from hot to cold! 11 Applications of VPT Methods é^ Purification of Metals: Van Ar kel Method Cr(s) + I2(g) (T2) o (T,) Crl2(g) Exothermic, Crl2(g) forms at Tl5 pure Cr(s) deposited at T2 Useful for Ti, Hf, V, Nb, Cu, Ta, Fe, Th Removes metals from carbide, nitride, oxide impurities Ti + 2I2 <-» Til4 AH = -376 kJ mol"1 exothermic: transport from cold to hot W-filament (ca. 1500 K) Ti-crystals Ti-powder (ca. 800 K) 12 Applications of VPT Methods é^ Double Transport Involving Opposing Exothermic-Endothermic Reactions Endothermic: W02(s) + I2(g) (Tj 800°C) *> (T21000°C) W02I2(g) Exothermic: W(s) + 2H20(g) + 3I2(g) (T21000°C) <-> (Tt 800°C) W02I2(g) + 4HI(g) The antithetical nature of these two reactions allows W/W02 mixtures to be separated at different ends of the gradient reactor using H2O/T2 as the transporting VP reagents 13 Applications of VPT Methods é^ Vapor Phase Transport for Synthesis A(s) + B(g) (TO o (T2) AB(g) AB(g) + C(s) (T2) <-* (TO AC(s) + B(g) Concept: couple VPT with subsequent reaction to give overall reaction: A(s) + C(s) (T2) ^ (TO AC(s) Examples: Direct reaction sluggish even at high T Sn02(s) + 2CaO(s) -> Ca2Sn04(s) Useful phosphor, greatly speeded up with CO as VPT agent: Sn02(s) + CO(g) <-> SnO(g) + C02(g) SnO(g) + C02(g) + 2CaO(s) <-> Ca2Sn04(s) + CO(g) 14 Applications of VPT Methods Direct Reaction: Cr203(s) + NiO(s) -> NiCr204(s) Greatly enhanced rate with 02 Cr203(s) + 3/2O2 <-> 2Cr03(g) 2Cr03(g) + NiO(s) <-> NiCr204(s) + 3/202(g) Overcoming Passivation Through VPT Al(s) + 3S(s) —» Al2S3(s) passivating skin stops reaction In presence of cleansing VPT agent I2: Endothermic: Al2S3(s) + 3I2(g) (T! 700°C) <-* (T2 800°C) 2AlI3(g) + 3/2S2(g) 15 Applications of VPT Methods éř Vapor Phase Transport for Synthesis Zn(s) + S(s) -> ZnS(s) passivation prevents reaction to completion Endothermic: ZnS(s) + I2(g) (Ti 800°C) <-> (T2 900°C) Znl2(g) + l/2S2(g) VPT Synthesis of ZnW04: A Real Phosphor Host Crystal for Ag+, Cu+, Mn2+ W03(s) + 2Cl2(g) (Ti 980°C) <-» (T21060°C) W02Cl2(g) + Cl20(g) W02Cl2(g) + Cl20(g) + ZnO(s) (T21060°C) <-> ZnW04(s) + Cl2(g) Growing Epitaxial GaAs Films by VPT Using Convenient Starting Materials GaAs(s) + HCl(g) <-> GaCl(g) + l/2H2(g) + l/4As4(g) AsCl3(g) + Ga(s) + 3/2H2 <-> GaAs(s) + 3HCl(g) Serves to establish initial equilibrium