1 Heating: furnace, laser, plasma, flame, arc Gas-Metal Rxn Ti + N2 TiN 3 Si + 2 N2 Si3N4 W + CH4 WC + H2 mp 2720 °C WC dissolved in Co = cemented carbides Ti + CH4 TiC + H2 mp 2940 °C cementite steel + H2/CO + CH4 + NH3 Fe3C + nitrides 1800 K Gas Phase Reactions 2 Gas-Gas Rxn homogeneous nucleation from supersaturated vapor (nano) Flame hydrolysis volatile compounds are passed through an oxygen-hydrogen stationary flame: SiCl4 + H2 + O2 SiO2 + HCl fumed silica reagent bp/°C product SiCl4 57 SiO2 AlCl3 180 (subl.) Al2O3 TiCl4 137 TiO2 CrO2Cl2 117 Cr2O3 Fe(CO)5 103 Fe2O3 GeCl4 84 GeO2 Ni(CO)4 42 NiO SnCl4 114 SnO2 ZrCl4 331 (subl.) ZrO2 VOCl3 127 V2O5 Gas Phase Reactions 3 SiCl4 + H2O OSiCl2 + 2 HCl OSiCl2 + H2O SiClOOH + HCl SiClOOH SiO2 + HCl Gas Phase Reactions 4 Gas Phase Reactions 5 6 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 (Ca2P2O7). 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. Gas Phase Reactions 7 High-power CO2 lasers 3 SiH4 + 4 NH3 Si3N4 + 12 H2 HN(SiMe3)2 + NH3 Si3N4 + SiC DC-Ar Plasma TiCl4 + NH3 TiN + HCl Tarnishing of Metal Surfaces oxide, hydroxide layers Arc Graphite C60 1300 K Gas Phase Reactions 8 Sealed glass tube reactors Solid reactant(s) A + gaseous transporting agent B Temperature gradient furnace T ~ 50 o 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 T1, crystals of pure A Temperature dependent K Equilibrium concentration of AB(g) changes with T Different at T2 and T1 Concentration gradient of AB(g) = driving force for gaseous diffusion T1T2 traces of a transporting agent B (e.g. I2) A AB Vapor Phase Transport Syntheses 9 Example: Pt(s) + O2(g) PtO2(g) Endothermic reaction, PtO2 forms at hot end, diffuses to cool end, deposits well formed Pt crystals, observed in furnaces containing Pt heating elements Chemical vapor transport, T2 > T1, provides concentration gradient and thermodynamic driving force for gaseous diffusion of vapor phase transport agent AB(g) Uses of VPT ˇ synthesis of new solid state materials ˇ growth of single crystals ˇ purification of solids Whether T1 < T2 or T1 > T2 depends on the thermochemical balance of the reaction ! Transport can proceed from higher to lower or from lower to higher temperature Vapor Phase Transport Syntheses 10 Thermodynamics of VPT Reversible equilibrium needed: Go = -RTlnKequ = Ho - TSo Exothermic Ho < 0 Smaller T implies larger Kequ AB forms at cooler end, decomposes at hotter end of reactor W + 3Cl2 WCl6 400/1400 (exo) Ni + 4CO Ni(CO)4 50/190 (exo) Endothermic Ho > 0 Larger T implies larger Kequ AB forms at hotter end, decomposes at cooler end of reactor 2Al + AlCl3 3 AlCl 1000/600 (endo) 4Al + Al2S3 3Al2S 1000/900 (endo) Vapor Phase Transport Syntheses 11 Estimation of the thermochemical balance (H) of a transport reaction: e.g.: ZnS(s) + I2(gas) ZnI2(gas) + S(g) H = ?? Zn(s) + I2(g) ZnI2(gas) H = - 88 kJ mol-1 ZnS(s) Zn(s) + S(g) H = +201 kJ mol-1 ---------------------------------------------------------------- ZnS(s) + I2(gas) ZnI2(gas) + S(g) H = +113 kJ mol-1 endothrmic reaction, transport from hot to cold! Vapor Phase Transport Syntheses 12 Applications of VPT Methods Purification of Metals: Van Arkel Method Cr(s) + I2(g) (T2) (T1) CrI2(g) Exothermic, CrI2(g) forms at T1, 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 TiI4 H = -376 kJ mol-1 exothermic: transport from cold to hot W-filament (ca. 1500 K) Ti-powder (ca. 800 K) I2 Ti-crystals 13 Double Transport Involving Opposing Exothermic-Endothermic Reactions Endothermic: WO2(s) + I2(g) (T1 800o C) (T2 1000o C) WO2I2(g) Exothermic: W(s) + 2H2O(g) + 3I2(g) (T2 1000o C) (T1 800o C) WO2I2(g) + 4HI(g) The antithetical nature of these two reactions allows W/WO2 mixtures to be separated at different ends of the gradient reactor using H2O/I2 as the transporting VP reagents Applications of VPT Methods 14 Applications of VPT Methods Vapor Phase Transport for Synthesis A(s) + B(g) (T1) (T2) AB(g) AB(g) + C(s) (T2) (T1) AC(s) + B(g) Concept: couple VPT with subsequent reaction to give overall reaction: A(s) + C(s) (T2) (T1) AC(s) Examples: Direct reaction sluggish even at high T SnO2(s) + 2CaO(s) Ca2SnO4(s) Useful phosphor, greatly speeded up with CO as VPT agent: SnO2(s) + CO(g) SnO(g) + CO2(g) SnO(g) + CO2(g) + 2CaO(s) Ca2SnO4(s) + CO(g) 15 Direct Reaction: Cr2O3(s) + NiO(s) NiCr2O4(s) Greatly enhanced rate with O2 Cr2O3(s) + 3/2O2 2CrO3(g) 2CrO3(g) + NiO(s) NiCr2O4(s) + 3/2O2(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) (T1 700o C) (T2 800o C) 2AlI3(g) + 3/2S2(g) Applications of VPT Methods 16 Applications of VPT Methods Vapor Phase Transport for Synthesis Zn(s) + S(s) ZnS(s) passivation prevents reaction to completion Endothermic: ZnS(s) + I2(g) (T1 800o C) (T2 900o C) ZnI2(g) + 1/2S2(g) VPT Synthesis of ZnWO4: A Real Phosphor Host Crystal for Ag+ , Cu+ , Mn2+ WO3(s) + 2Cl2(g) (T1 980o C) (T2 1060o C) WO2Cl2(g) + Cl2O(g) WO2Cl2(g) + Cl2O(g) + ZnO(s) (T2 1060o C) ZnWO4(s) + Cl2(g) Growing Epitaxial GaAs Films by VPT Using Convenient Starting Materials GaAs(s) + HCl(g) GaCl(g) + 1/2H2(g) + 1/4As4(g) AsCl3(g) + Ga(s) + 3/2H2 GaAs(s) + 3HCl(g) Serves to establish initial equilibrium