Raghavarapu Venkata Krishnarao

STUDIES ON THE FORMATION OF BLACK PARTICLES IN RICE HUSK SILICA ASH

Abstract The formation of black (carbon fixed) particles in rice husk silica ash has been studied in detail. The formation of black particles in the silica obtained by calcination of raw untreated rice husks, is higher than that in acid treated rice husks. The tendency to form black particles increases with increase in the heating rate and the temperature of calcination of the untreated rice husks. Potassium has been shown to cause the formation of black particles in rice husk silica. By treating with 3.0 N HCl acid the formation of black particles can be avoided. There is no effect of heating rate on the formation of black particles in silica from acid treated rice husks. Treatment with HCl acid was found to decrease the oxidation (burning) of carbon at lower temperature (400°C ).

Conversion of raw rice husks to SiC by pyrolysis in nitrogen atmosphere

Abstract Raw rice husks without precoking were directly pyrolysed in a graphite resistance heating furnace at 1100–1400 °C in nitrogen atmosphere. The furnace chamber pressure was varied from atmospheric pressure (14.7 Psi or 101.3 KPa) to 24 Psi (165 KPa). Considerable quantities of SiC whiskers were formed at 1200–1400 °C. Small quantities of Si 2 N 2 O were also formed at 1100 and 1200 °C. No Si 3 N 4 was formed. The increase in chamber pressure has resulted in (i) an increase in the crystallization of silica and carbon in rice husks, (ii) decrease in the formation of SiC whiskers. Pyrolysis in nitrogen atmosphere resulted in the formation of good quality (needle type) SiC whiskers.

Effect of acid treatment on the formation of SiC whiskers from raw rice husks

Abstract Raw rice husks have been treated by boiling for 1 h in 5 N hydrochloric acid. The washed and dried acid-treated raw rice husks (TRRH) and untreated raw rice husks (RRH) were directly pyrolysed (without precoking) in argon atmosphere at different temperatures between 1050 and 1600 °C. Silica obtained from TRRH has a lower level of impurities than that obtained from RRH. Acid treatment has been found to decrease the degree of crystallization of silica and carbon in rice husks. The whisker formation has been decreased in TRRH. As the carbon loss due to the formation of CO was high, the SiC content in the pyrolysed TRRH was higher than that in pyrolysed RRH.

Synthesis of TiC Whiskers through Carbothermal Reduction of TiO2

TiC whiskers were produced through carbothermal reduction of TiO2 in the presence of potassium (K2CO3) and nickel (NiCl2). The effect of potassium, nickel, and heating rate on the formation of whiskers was studied. Potassium was found to be an essential constituent for whisker formation. Nickel acts as a catalyst for TiC whisker formation only in the presence of potassium. The yield of whiskers was maximum at 1000–1200°C. At higher temperatures, formation of particulates of TiC was the dominant process. An increase in K2CO3 concentration during fast heating and decrease in K2CO3 concentration during slow heating was found to be beneficial in increasing the formation of TiC whiskers. A vapor–liquid–solid growth mechanism of whisker formation was explained.

Studies on the formation of MoSi2SiC by reaction of Si3N4 with molybdenum and carbon

Abstract Si 3 N 4 powder was reacted with Mo powder and carbon black at 1200–1600°C in an argon atmosphere. The effects of carbon content and pressure on the formation of molybdenum silicides were studied. MoSi 2 and SiC formed at all temperatures. With a low initial carbon content or on conducting the reaction under a positive pressure of argon, the formation of Mo 5 Si 3 and Mo 3 Si can be suppressed. However, at higher temperatures (1500 and 1600°C), the formation of large quantities of SiC, Mo 5 Si 3 , and Mo 3 Si was observed. Vapor—liquid—solid whisker type morphology was observed at lower temperatures. At higher temperatures various types (hexagonal, trigonal, polyhedral) of SiC particulate formed. The tendency of the reaction product to agglomerate was found to increase with a decrease in the carbon content.

Formation of MoSi2-SiC composite powder by reacting Si3N4 with molybdenum and carbon

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