M.M. Godkhindi

Distribution of silica in rice husks and its effect on the formation of silicon carbide

Abstract An investigation into the distribution of silica in raw rice husk and its effect on the formation of SiC particles and whiskers is carried out by a detailed study of raw rice husks, burn rice husks and converted rice husks through XPS, XRD and SEM analyses. The present study has revealed that silica is present all over the rice husks. However, the protuberances and hairs on the outer epidermis and the inner epidermis adjacent to the rice kernel are shown to be rich in silica. The XPS and XRD results indicate that silicon in raw rice husks is bonded to organic material. The SEM analysis of the converted rice husks shows that the part of the rice husks rich in carbon (organic materials) tends to form a SiC skeleton structure. It appears that the part of the rice husk rich in silica is the main souirce of SiO for the formation of SiC whiskers through gaseous reaction between SiO and CO.

Effect of Si3N4 additions on the formation of SiC whiskers from rice husks

Abstract Attempts have been made to quantitatively correlate the effect of silicon nitride additions on the conversion of rice husk black ash to silicon carbide (particles and whiskers) during vacuum pyrolysis in the temperature range of 1200 to 1400°C. Addition of Si3N4 was found to increase the yield of SiC whiskers and to reduce the amounts of SiC particles and excess carbon. At 1400°C the mixtures with 50 and 60 wt% of Si3N4 were found to yield a mass of SiC(w) without any free carbon and with about 5 wt% of unreacted SiO2.

Carbothermal synthesis of zirconium diboride (ZrB2) whiskers

Abstract In the present investigation, carbothermal synthesis technique is used for the production of ZrB2 whiskers. The synthesis is carried out by heating the mixture of ZrB2, H3BO3 and carbon black over the temperature range of 1300 to 1700°C in argon atmosphere. Different elements such as Ni, Co and Fe are tested for their role as catalysts. The synthesised powder contains ZrB2 as the major phase with minor phases of ZrC and B4C. Whisker yield is found to be low at lower temperatures (<1500°C) and then increases with the pyrolysis temperature. The Ni addition seems more effective as a catalyst than Co or Fe. Shorter length whiskers are found in the case of Co catalyst, whereas use of Fe catalyst shows whisker with rod shape and a special birds nest type whiskers. The electron microscope study of whisker reveals the presence of various defects.

Formation of SiC whiskers from compacts of raw rice husks

The formation of SiC whiskers from compacts of raw rice husks without coking and catalyst has been studied. A pyrolysis temperature of 1600°C has yielded a considerable quantity of SiC whiskers. The formation of spherical particles of silica is observed. Whisker formation occurs by reaction between SiO(g) and CO(g).

Maximisation of SiC whisker yield during the pyrolysis of burnt rice husks

Studies have been made on the conversion of burnt rice husks to SiC over a wide temperature range. Four distinct processes (crystallization of silica and carbon, formation of SiC whiskers and particles) have been identified as occurring during the pyrolysis of burnt rice husks. The dependence of relative rates of these four reactions on the temperature has been ascertained through detailed X-ray diffraction, scanning electron microscopy, chemical analysis and macro structural study. It was also revealed that a multistep pyrolysis to 1310°C yields a higher SiC(w) than the single step pyrolysis. The yield of SiC(w) from burnt rice husks has been found to be lower than that from raw rice husks. A correlation between the amount of SiC(w) formed and the appearance of pyrolysed rice husks has been identified.

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa2Cu3Ox (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO3, Y2O3 and CuO (which on calcination at 1173 K gives YBa2Cu3Ox) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol−1 in argon, 1593 kJ mor−1 in air and 2142 kJ mol−1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol−1, and a second one above 1173 K withQ=383.7 kJ mol−1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.

Studies on the formation of SiC whiskers from pulverized rice husk ashes

Abstract Conversion of rice husk precursors (raw husks, pulverized black ash and mixtures of white ash and carbon black) to silicon carbide through vacuum pyrolysis over a wide temperature range, has been studied. The products of the pyrolysis (SiC whiskers and particles) were analyzed with the help of XRD, SEM and chemical analysis. The present study indicates that a temperature range of 1250–1370°C, carbonaceous substrate, sufficient void space, controlled release of SiO vapour, and the presence of CO are favourable conditions that tend to maximize the formation of SiC whiskers. Under similar experimental conditions, the formation of SiC (w) from rice husk precursors has been observed to increase in the following order: the lowest from the pulverized black ash, intermediate from the white ash and carbon black mixtures, and the highest from the raw rice husks.

Sintering mechanisms of attrition milled titanium nano powder

Detailed sintering studies have been carried out on attrition milled nanocrystalline titanium powder through isothermal dilatometry over a temperature range of 300–1250 °C along with microstructural and x-ray diffraction studies. The sintering behavior of attrition milled nanocrystalline titanium appears to be characterized by: (i) very low activation energies, (ii) high shrinkage anisotropy, (iii) very rapid grain growth in the beta range, and (iv) two kinds of densification processes, namely, intra-agglomerate and inter-agglomerate. Analysis of the kinetic data through sintering diagram approach indicates the operation of particle sliding and grain boundary rotation, type of mechanism in addition to the grain-boundary diffusion, and lattice diffusion as the dominant mass transport mechanisms.

Electrical discharge machining studies on reactive sintered FeAl

AbstractbdElectrical discharge machining (EDM) studies on reactive sintered FeAl were carried out with different process parameters. The metal removal rate and tool removal rate were found to increase with the applied pulse on-time. The surface roughness of machined surface also changed with the applied pulse on-time. XRD analysis of machined surface of sintered FeAl showed the formation of Fe3C phase during the EDM process. The debris analysis was used to identify the material removal mechanism occurring during the EDM of sintered FeAl.

Modelling the powder?binder separation in injection stage of PIM

In this paper, a non-isothermal multiphase flow numerical model has been developed based on Eulerian approach to simulate the powder-binder separation phenomenon in the injection stage of Powder Injection Moulding (PIM) and the simulated results are validated with experiments. The model takes into account of the non Newtonian viscosity of the powder and binder phases. Simulation as well as experiments have been carried out for a standard tensile test specimen with gating and ejection systems. A temperature correction method has been introduced during solidification taking into account of the latent heat release of binder. It can be seen from the simulated results that injection temperature has larger effect on phase segregation than injection speed due to larger effect of temperature on viscosity.