Thermal plasma synthesis of SiC from rice hull (husk)
Recently the synthesis of ultrafine powders has drawn considerable attention because these powders can have physical and chemical properties superior to those of bulk specimens. In particular, emphasis is on the preparation of ultrafine ceramic powders for their applications in the development of sintered products having higher density and lower sintering temperature. One such industrially important ceramic material is SiC. In many cases ultrafine SiC has been prepared by thermal plasma processes [1]. However, these processes require one or more reactive/corrosive gases. This letter describes a new method to overcome such problems. For this purpose thermally pretreated rice hull was used as the solid precursor. Rice hull, an agricultural waste product, was first used by Cutler [2] as a starting material for the production of silicon carbide. Since the rice hull route promises to be most economical, much attention has recently been paid to it [3-6]. Rice hull consists of silica in hydrated amorphous form, and cellulose which yields carbon when thermally decomposed. The relative abundance of silica with very high surface area in close proximity to active carbon in rice hull makes it amenable for SiC formation during the pyrolysis process. The possible reaction was given by Lee and Cutler [7] as
Synthesis of ultrafine SiC from rice hulls (husks): A plasma process
Heat-treated rice hulls have been used as precursor material for synthesis of ultrafne SiC in a RF plasma reactor. Rice hulls containing finely distributed silica and active carbon act as a source for SiC formation. The plasma-synthesized powder contained ultrafnc β-SiC with excess carbon and some unreacted silica. Post-treatment processes such as oxidation and acid (HF) treatment appear to be effective in removing the excess carbon and silica.
Control of reflective cracking
The first is 'fatigue' cracking of the layer. This type of cracking is the result of the application of loads on the pavement and is controlled by appropriate thickness design which 'ill not be discussed in detail here. Thickness design includes not only the design strength and thickness of the stabilised, but other design elements such as delaying reflection cracking, or limiting stresses in the stabilised layer, by adopting a minimum thickness of overlying material. This may include an unbound layer utilising the 'upside down' pavement approach. The actual thickness required of the overlying material varies but for example PIARC (Ref.1) recently reported when the thickness of coated materials exceeds 20 cm the emergence of shrinkage cracks at the surface is generally uncommon; when it is 15 cm thick, the cracks emerge at the surface, but do not shorten the life of the surface course, especially if they are sealed. This experience has been gained 'with legal axle loads of 130 kN, considerably greater than the Australian legal axle load.