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
Y2O3 Particles by Flame Aerosol Process
oxygen
hydrogen
Y(NO3)3
7
Particle Size Control
Particle size control by precursor concentration
Higher concentration = larger size
8
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 TCP - Ca3(PO4)2
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
9
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
10
Sealed glass tube reactors
Solid reactant(s) A + gaseous transporting agent B
Temperature gradient furnace ΔT ~ 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
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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
• synthesis of new solid state materials
• growth of single crystals
• 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
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Thermodynamics of VPT
Reversible equilibrium needed: ΔGo
= -RTlnKequ = ΔHo
- TΔSo
Exothermic ΔHo
< 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 ΔHo
> 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
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
Δ
==−
21
0
1
2
12
11
lnlnln
TTR
H
K
K
KK
van’t Hoff equation
13
Estimation of the thermochemical balance (ΔH) of a transport reaction:
e.g.:
ZnS(s) + I2(gas) ↔ ZnI2(gas) + S(g) ΔH = ??
Zn(s) + I2(g) ↔ ZnI2(gas) ΔH = - 88 kJ mol-1
ZnS(s) ↔ Zn(s) + S(g) ΔH = +201 kJ mol-1
----------------------------------------------------------------
∑ ZnS(s) + I2(gas) ↔ ZnI2(gas) + S(g) ΔH = +113 kJ mol-1
endothermic reaction, transport from hot to cold!
Vapor Phase Transport Syntheses
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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 ΔH = -376 kJ mol-1
exothermic: transport from cold to hot
W-filament (ca. 1500 K)
Ti-powder (ca. 800 K)
I2
Ti-crystals
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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
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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)
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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
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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
19
Laser-induced homogeneous pyrolysis, LIHP
C2H4 + hν → C2H4
*
Excitation energy transferred to
vibrational-translational modes
⇒ T increases
20
Sensitizer
SF6
948 cm-1
Isopropanol
958 cm-1
laser wavelength
10.60 ± 0.05 μm
21
Reaction Zone
Overlap between
the vertical reactant gas stream
and the horizontal laser beam
away from the chamber walls
nucleation of nanoparticles
less contamination
narrow size distribution
22
23
Iron-oxide Nanoparticles by Laser-induced
Pyrolysis
2 Fe(CO)5 + 3 N2O → Fe2O3 + 10 CO + 3 N2