Prismatic single crystals of α-Si
3N
4 were prepared on a graphite susceptors by vapor phase reaction from SiCl
4, N
2 and H
2 gaseous mixture. Growth conditions were chosen as follows; SiCl
4=2.7×10
-4mol/min, H
2=1000cc/min, N
2=1000cc/min, total pressure=1atm, susceptor temperature=1600°C, duration=3-6h.
1) Oxygen and water in a gaseous mixture were analysed from electromotive force of an oxygen concentration cell and dew point. The amounts of oxygen and water supplied in the reaction vessel were 8.9×10
-7mol per min and 4.46×10
-8mol per min at room temperature, respectively.
2) Equilibrium oxygen partial pressure at elevated temperature was calculated using the data of raw gas analysis, growth condition and JANAF thermochemical data. The results calculated in the system were as follows;
Po
2=3.5×10
-26atm at 1300°K
Po
2=6.7×10
-20atm at 1900°K
The values were very much lower than those of equilibrium state governed by the reduction of H
2 or CO which had been used in the conventional processes.
3) Calculation of equilibrium oxygen partial pressure in the system indicated the following results;
1. The effect of hydrogen-nitrogen mole ratio were very small.
2. Oxygen as an impurity in raw gas mixture affected
Po
2 distinctly at low temperature.
3. Water as an impurity in raw gas mixture affected
Po
2 distinctly at high temperature.
4) Oxygen content and structural data of α-Si
3N
4 crystals prepared by this method under the above mentioned conditions were compared with those already reported. It contained less oxygen (0.05-0.09wt%) than any other α-Si
3N
4 crystals ever prepared.
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