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		NiO
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'-«?-«-
«j <j 49-£-
*'-»-•
Gas Phase Reactions
Spray Flame
Supporting Flame
* *
\
•
Sheath gas
i imi i
1    ľ     t
CHJO,       —O,
Liquid        0S
precursor
Y203 Particles by Flame Aerosol Process
alumina filter
sampling tube
nanop articles
Gas Temperature
vacuum
syn in e s i s flame
thermocouple

oxidant
stream
From furnace V particle
f Amorphous -*. f yttrium oxide
Further cooling
mm
heater
7
furnace
m.
m
hyd
rogen
fuel gas in
precursor solution
ultrasonic
transducer
oxidant si ream in
Flow Direction In Synthesis Flame
oxygen
itfsŕ-Fsvíi      .  droplets ■Wit****   *
precursor
precursor
solulion in
waler bain
Y(N03)
3^3
6
Particle Size Control
-f----1-----ř—P----1----P----P----P----r----1—i----p----r----p----p----1-----1-----f----f-----p-
Particle size control by precursor concentration Higher concentration = larger size
■0.65 M -0.ŮMM
^■»dp^.yi   i
500           1000              1500
Partical Diameter (rim)
2000
500 nm
7
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.
8
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
9
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)
T2     ^------^      -----------£m---------------►          v^k     Tl
10
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
11
Vapor Phase Transport Syntheses
Thermodynamics of VPT
Reversible equilibrium needed: AG0 = -RTlnK^ = AH0 - TAS°
& 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) *> 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!
13
Applications of VPT Methods
é* Purification of Metals: Van Arkel 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 * exothermic: transport from cold to hot
W-filament (ca. 1500 K)
Ti-crystals
Ti-powder (ca. 800 K)
14
Applications of VPT Methods
é* Double Transport Involving Opposing Exothermic-Endothermic Reactions
Endothermic:
W02(s) + I2(g) (Ti 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
15
Applications of VPT Methods é* Vapor Phase Transport for Synthesis
A(s) + B(g) (TO o (T2) AB(g)
AB(g) + C(s) (T2) o (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)
16
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)
17
Applications of VPT Methods é* 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