Oxidation of SiC powder by high-temperature, high-pressure H 2 O
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INTRODUCTION Silicon carbide, as well as silicon nitride, has attracted great attention in recent years because of its potential application as one of high-temperature structural ceramics. Oxidation resistance is one of the most important properties in these applications. As for silicon carbide powders, the oxidation behavior, investigated for many samples in various oxidizing atmospheres, indicates a parabolic kinetics.1"7 The reaction is believed to be as follows: SiC
CO2
Suzuki3 and Jorgensen etal.1 studied the effects of water vapor on the oxidation of silicon carbide powders. Singhal8 also examined the effects of water vapor on the oxidation of hot-pressed silicon carbide. They argued that the oxidation rate was accelerated when water vapor was added to oxygen. However, the reaction of SiC by only H 2 O has not been studied yet. We have studied the reactions between SiC powder and H 2 O under hydrothermal conditions where almost only high-temperature, high-pressure H 2 O exists. Furthermore, we will discuss the effect of H 2 O on the oxidation of SiC in comparison with a similar study on Si3N4 reported previously.9
400° to 800 °C, and time for 0 to 72 h. After the treatment the pressure vessel was quenched, then the sample tube was dried and weighed to calculate the oxidation ratio of SiC. Solid products were examined by x-ray diffractometry (XRD) and scanning electron microscopy (SEM). The oxidation ratio was also checked by XRD through the decrease of the SiC phase. Gaseous products in the vessel were analyzed by a mass spectrometer. III. RESULTS Under 100 MPa H 2 O, the oxidation of SiC was not observed at 400 °C but proceeded above 500 °C as a function of time (Fig. 1). Weight gain and appearance of a halo in x-ray diffraction indicated that amorphous silica was formed by the oxidation of SiC as the first step at every temperature above 500 °C. Cristobalite and tridymite crystallized above 700 °C after 8 h where SiC was almost completely changed into amorphous silica. Scanning electron micrographs showed that the surfaces of the SiC particles were covered with amorphous
II. EXPERIMENTAL PROCEDURE The starting material was SiC powder (Starck A10, average grain size 0.65 /um, 94% a form). The sample powder was placed in an Au tube (2.7 mm i.d., 0.15 mm thick, and 35 mm long) of which the top was flattened and bent by pressing to prevent the escape of solid materials. The tube was heated in a test-tube-type pressure vessel under pressures of 10 or 100 MPa of redistilled water. In the treatment the temperature was varied from 100
J. Mater. Res. 1 (1), Jan/Feb 1986
http://journals.cambridge.org
3
24 (h)
Tine
700 *C
500 •c
•
600•c
o 0
800-C
Sol id 11ne
Equation (2)
Crystalline S1O2
FIG. 1. Isothermal oxidation of SiC powder under 100 MPa H 2 O.
0003-6951 /86/010100-04$01.75
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© 1986 Materials Research Society
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Yoshimura, Kase, and Somiya: Oxidation of SiC powder
01 3
24
Time
700 "C
500-C
72 ( h )
S o l i d l i n e : Equation (
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