Oxidation Of Silicon Carbide Research Articles

The Oxidation of Silicon Carbide at 1150° to 1400°C and at 9 × 10 − 3 to 5 × 10 − 1 Torr Oxygen Pressure

A kinetic study was made of the oxidation of high‐purity silicon carbide using a dynamic‐type reaction system. Two types of oxidation behavior were found. Passive oxidation occurred for conditions where silica films were formed on the surface. Active oxidation occurred for conditions where volatile silicon monoxide was formed. The transition conditions between the two types of oxidation were studied as a function of oxygen pressure and temperature. The transition temperatures and pressures were related to thermochemical conditions for the reaction of silicon carbide with silica to form silicon monoxide and carbon monoxide.

Effects of Water Vapor on Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of water vapor. The diffusion‐rate constant was found to vary with the logarithm of the partial pressure of water vapor according to the theory of thin‐film oxidation as proposed by Engell and Hauffe. The products of oxidation were cristobalite and tridymite, depending on the temperature. The diffusing species appeared to be the same in the presence of partial pressures of water vapor and in the presence of partial pressures of oxygen.

原料の選択が半炭化ケイ素さやの耐久性に及ぼす影響

A heat-resistant semi silicon carbide sagger body could be produced by using black silicon carbide. Experiments were made to examine the effects of kinds of aggregate, combining clays and other additives on thermal shock resistance and oxidation of silicon carbide.The results obtained are as follows:(1) On thermal shock resistance, “Kibushi” clay of high plasticity both as an aggregate and as combining clay gives good effects. Burned “Kibushi” clay used as an aggregate gives better effects at S.K. 14 than S.K. 9, but fused “Kibushi” clay such as corhart is not effective as an aggregate.(2) Effects of other additives on oxidation of black silicon carbide by short-time heating are investigated through D.T.A., weight increase of black silicon carbide after burning and X-ray analysis. Copper carbonate and manganese fluoride increase the oxidation of silicon carbide, and borax which shows lower melting point prevents the oxidation. These results can be explained by the partial pressure of oxygen and their eutectic points with silica.(3) Borax as an additive gives better effects not only on the above physical properties but also on preventing the oxidation of silicon carbide.

Oxidation of silicon carbide in the temperature range 1200 to 1500°C.

Oxidation of silicon carbide in the temperature range 1200 to 1500°C.

145. Oxidation of silicon carbide in the temperature range 1200 to 1500°.: Note by Robert F. Adamskym, J. Phys. Chem., 63, 305–307, Feb. 59

145. Oxidation of silicon carbide in the temperature range 1200 to 1500°.: Note by Robert F. Adamskym, J. Phys. Chem., 63, 305–307, Feb. 59

Effects of Oxygen Partial Pressure on the Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of oxygen. The diffusion rate constant was found to vary with the logarithm of the partial pressure of oxygen according to the theory of oxidation of thin films as proposed by Engell and Hauffe. An alternative explanation based on the change of free energy with surface coverage was also found to fit the data.

Effect of Vanadium Oxide on the Oxidation of Silicon Carbide

This paper presents the results of the investigations on the effect of vanadium oxide on the oxidation of pure silicon oxide powder, since the former has proved to be most influential material for such action (J. Ceram. Assoc., Japan, 67 [6] 189 (1959)).V2O5 melts at 660°C and in order to investigate the wettability of the molten oxide against silicon carbide the change of contact angle by temperature as well as the penetrability of the molten liquid into the powder were measured.Through the measurement of the change of weight and by X-ray analysis of mixed powder heated in argon atmosphere the temperature dependency of the chemical reaction and its details were studied. From the results obtained the rate of reaction at 950° and 1150°C in oxygen stream was measured. Furthmore, through the measurement of the penetrability V2O5 into thedry-pressed silica powder, and of the pyrometric cone equivalent temperature of the mixture the author has tried to approach to the mechanism of chemical reaction.The results are summarised as follows:1) Large wettability of SiC with the melt from V2O5 in air as well as in argon atmosphere was confirmed. At the temperatures higher than the melting point the contact angle decreased rapidly approaching soon to practical zero value. No bubble at the interface was observed. Also, the melt denetrated rapidly into the pressed mass of the fine powder of SiC. Especially, the rate of penetration of the melt into the pressed silica powder was to be very high between the temperatures of 950° and 1150°C.2) V2O5 reacts with SiC in three steps in the atmosphere of argon, namely at 640°-670°C, 750-920°C and 1020°-1180°C-. It was confirmed that V2O5 was reduced to V2O4 in the second step, and to V2O3 in the third step. The temperature of the completion of the third step changed by the nature of V2O5, but not by the kind of SiC used. On the other hand, as V2O3 was oxidized to V2O5 through V2O4 at the temperatures as low as 500°C, and it may be concluded that the coexistence of O2 is to accelerate the rate of oxidation of SiC by V2O5 which naturaly will be influenced strongly by temperature.3) When V2O5+SiC mixture was heated to 900°C in argon and then O2 was introduced oxidation proceed very slowly at the begining, fast gradually, and again slowed down with time. This may be interpreted by assuming the fact that when SiO2, the reaction product of the oxidation of SiC, accumulate over ten per cent the migration of V+5 became difficult owing to the solidification of the melt. In this stage V2O4 is formed at the interface.4) When the same experiment was carried out at a higher temperature, 1150°C, two types of the reaction were observed, namelya) When all V2O5 was reduced to V2O3 the reaction rate was possible to be expressed by a second order equation, i.e. very high at the initial stage falling gradually with time.b) When most part was reduced to V2O4 the reaction represented by the equation.logΔr=kt+c, where Δr is the amount of the oxidized SiC measured by the thickness from the surface, t is the time, and k, c are constants.Taking a serious view of the roll played by SiO2 at the interface between the oxide film and the surface of SiC crystal the author discussed the mechanism of the oxidation of SiC with the mobility of V+5 in the system SiO2-V2O5 and the cone equivalent temperature.

Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was measured in an atmosphere of dry oxygen between 900° and 1600°C. The rate was studied by using a thermogravimetric apparatus and was found to be diffusion controlled. The products of oxidation were amorphous silica and cristobalite, depending on the temperature. The effect of surface area was determined, and a correlation between the various sizes was made with the aid of an equation derived on the assumptions that (1) the reaction was diffusion controlled, (2) the particles were essentially spherical, and (3) the interfacial area was constantly changing. The presence of water vapor in the gaseous atmosphere was found to be extremely critical.

Oxidation of Silicon Carbide in the Temperature Range 1200 to 1500°

Oxidation of Silicon Carbide in the Temperature Range 1200 to 1500°

Effects of Impurities on the Oxidation of Silicon-Carbide Powders

As a part of the studies on the oxidation of silicon-carbide powders the oxidazability of Si and β-SiC, both found in the market products of silican carbide, as well as the influence of Fe2O3, Al2O3 and MgO on the oxidation of pure green silicon carbide were investigated. The results obtained are summarized as follows:(1) Although it was confirmed that metallic silicon was oxidized much easier than α-SiC, the small volume increase accompanied by the reaction would play no more roll than to reduce the SiC content as long as the silicon carbide is used as refractories.(2) β-SiC is fine powder showing a very slow oxidation compared favourably with α-SiC in this respect. Consequently, it showed no trend to become porous, since no volume increase occured even though it was exposed to oxidizing atmosphare for a long time. The contamination of β-SiC in raw material is not only harmless but rather favourable.(3) It was confirmed that Fe2O3 did not give any serious influence on the oxidation of SiC at a temperature lower than 1200°C, and that there was no appreciable volume increase even in contact with water vapour. At about 1400°C the reaction became violent, which however, did not continue long, showing that Fe2O3 is not so harmful an admixture as it is universally believed.(4) At all temperatures Al2O3 held back more or less the oxidation of SiC. It was observed that a violent oxidation occured at 1150°C under the coexistence of water vapour, accompanying large volume and porocity increase. It was thus confirmed that Al2O3 is an impurity to which we should use due precaution against its contamination.(5) MgO is an impurity accerelating the oxidation of SiC at the temperatures higher than 700°C, it is one of the most undesirable impurities.(6) The influence of oxides on the oxidation of SiC is governed:(a) by the stability discriminated by the free energy of the formation of oxides, (b) by the location of the melting points in the binary system [metal oxide]-SiO2, (c) by the reactivity of metal oxide with SiO2, (d) by the properties of the reaction products.Referring to the influence of the addition of Pb3O4, V2O5, and Li2CO3 on the oxidation of SiC such conditions as above were explained satisfactory.(7) The prolonged oxidation of SiC is governed mainly by the nature of the oxide film covering the crystal surface. The fact that even the pure green silicon carbide behaved like an ordinary black silicon carbide when mixed with Al2O3 or MgO may be attributed to the nature of accerelating the crystallization of amorphous silica to quartz and cristobaite.

Influence of Clay Binders on the Oxidation of Sintered Silicon Carbide Bodies

In order to investigate some preventive measures against the oxidation of silicon carbide refractories the sintered specimens of fine SiC powders bonded by Kibushi clay, a popular binder in Japan, were heated at 1175°-1450°C for 50-100 hours in the current of dry oxygen to measure the weight increase, apparent porosity, length and the expansion up to 500°C. The results were compared with those obtained with the bondless bodies formed by adding a small amount of Syton 2X, a silica gel preparation. They weres ubjected to oxydation under the same conditions (as before).Based on the following experimental results, and on some additional considerations the author advanced a tentative plan of improving the characteristics of clay bonded refractories.(1) The bondless body used in this experiment was liable to oxidation with a large volume change. But, at the temperatures higher than 1400°C a small amount of impurity promoted remarkably the lowering of apparent porosity during the process of oxydation, so that the oxydation itself was held back to some degree.(2) Sintered body with 15-30% Kibushi clay as binder showed only a small volume change after long hour's heating at 1300°-1350°C or higher, because of the slow rate of oxidation. At a lower temperature (1175°C) the crystallization of the oxidation product, SiO2, and the influence of cristobalite liberated from the clay were corelative to show a larger expansion than bondless bodies. At higher temperatures (above 1400°C) a melt of low viscosity came out from the body making the surface more porous. The author concluded that the increase of clay substance gave practically no merit of preventing the oxidation of SiC. Taking into other important points the amount of clay should be as low as possible.(3) Kibushei clay used as bond failed to prevent the oxidation at the temperatures lower than about 1230°C. The bodies of low porosity produced by maturing sufficiently at a temperature higher than 1450°C the material added by the components such as Na2O and K2O, which effectively glassify SiO2, may be expected to improve the defects at least to some degree.

Effects of Bonds in the System K<sub>2</sub>O-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> on the Oxidation of Silicon-Carbide Bodies

The principal purpose of this investigation was to make clear the oxidation behavior at 1200° to 1400°C of silicon carbide bodies containing 15% bonds in the system K2O-Al2O3-SiO2.The oxidation resistance was determined using a small plate specimen 14mm in diameter and about 8mm thick. For most of work the specimen was oxidized at 1200° and 1400°C in a flowing atmosphere with controlled amounts of oxygen and water vapor. The weight gain, volume change, and porosity of the specimen were determined. The results obtained are summarized as follows:(1) The bonds containing large amounts of K2O and Al2O3 retarded the oxidation of silicon carbide. At 1200°C K2O was more effective on reduction of the oxidation than Al2O3, but at 1400°C the effect of Al2O3 on the oxidation resistance became somewhat high.(2) Silicon carbide was oxidized very strongly by the mixture of water vapor and oxygen when heated above 1200°C. It was observed that at such a high temperature the specimen swelled and cracked and permitted oxidation.(3) The presence of Al2O3, K2O in the bodies and water vapor in the atmosphere caused the formation of cracks, which had also been found in ceramic bonded silicon carbid refractories. The mechanism of formation of cracks was explained on the basis of reactions in the system K2O-Al2O3-SiO2-H2O.(4) After heating the specimen in air at 1400°C for 10 hrs the rate of oxidation by water vapor at 1200°C diminished considerably.

Oxidation Behavior of Silicon Carbide

The oxidation of purified green silicon carbide in controlled atmospheres was studied by weight‐gain measurement and by observation of the surface reaction products, including optical measurement of the thickness of the oxide surface film. The rate of oxidation was much greater for silicon carbide in contact with fluid silicate glasses than for silicon carbide alone. In a vacuum of 0.1 mm., oxidation proceeded with loss of weight, because of the formation of volatile SiO2, and at a greater rate than at atmospheric pressure. It is postulated that the rate‐controlling process in the normal oxidation of silicon carbide is the formation of solid SiO2 on the surface.