Flame Synthesis of High Purity, Nanosized Crystalline Silicon Carbide Powder
- PDF / 1,075,839 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 106 Downloads / 244 Views
167 Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 Materials Research Society
(1)
the reactants should be exactly in the ratio [SiH 4 ]/[C 2H 2]=2 (i.e., Si/C=I) to avoid excess Si or C in the products. Based on thermodynamic calculations using the ISP code [10], an adiabatic flame temperature of 2045 K is predicted for Reaction 1. The calculations also predict that there is a finite range of ratios over which pure SiC is produced: [SiH 4 ]/[C 2H 2 ] = 1.986 to 2.001. The lines, solid and dashed, in Fig. 1 represent the thermodynamic equilibrium product mole fractions. The corridor where pure SiC is predicted (Fig. Ia), is the result of two factors. Some SiC decomposes at high temperatures producing free Si with the excess carbon forming methane and acetylene. At lower [SiH4 ]/[C 2 H2 ] ratios (i.e., carbon rich) methane and acetylene, as well as graphite, are formed.
10° . ._._. .... i
... C
.....
Figure 1.Comparison of theoretical
S10.1experimental
0 Sand
id
product yields from flames with different silane/acetylene
-
10-&
mole ratios. Lines are thermodynamic
1-2
predictions for adiabatic flames, and symbols are experimental product mole fractions.
o10-3 10-4 . . . . 1.8 1.9
2.0
2.1
2.2
SILANE/ACETYLENE (a) Condensed Phase Products
S10_O o S 10-1 1 0 -i
x
Hydrogen
z
10-2
Methane
3x•--" '
Sl0.
Acetylene
,
o 1010-6
1.8
1.9
2.0
2.1
2.2
SILANE/ACETYLENE
(b) Gas Phase Products EXPERIMENTAL Mixtures of silane/acetylene were ignited in a constant volume ignition bomb (vol = 9.3 liters; P = 110 kPa) to confirm the thermodynamic predictions. Silane (>99.95% pure) was used directly from the tank. Acetylene was passed through an activated charcoal filter to remove acetone, the major impurity. Purity was periodically checked using gas chromatography. The mixtures were prepared by partial pressure to an accuracy of± 0.2 kPa. After mixing, a high voltage spark was used to ignite the mixture. The chamber pressure was recorded as a function of time to estimate flame speeds. After recording the final chamber pressure, the combustion product gases were sampled and characterized by gas chromatography using an HP-5830 Gas Chromatograph with calibrated thermal conductivity detection. Temperature programming allowed clean separation and quantification of hydrogen, methane, all C2 hydrocarbons, and silane on a 3.6 meter 80/100 mesh
168
Porapak N column. Performing a mass balance on the bomb gases (before and after ignition), provides, by difference, the elemental composition of the solid products. Ceramic powders resulting from ignition of various reactant mixtures were characterized both physically and chemically. Wet chemical analysis by the ANSI method was performed. IR spectroscopy and TEM provided additional chemical insight. Powders were also characterized as to surface area (BET), particle density (He pycnometry), and particle size (TEM). RESULTS Silane/acetylene mixtures with mole fraction ratios ranging from 1.7 to 2.2 resulted in final to initial gas mole ratios of 1.65±0.3, consistent wit
Data Loading...
