Pulverization of rice husks and the changes of husk densities

Well-dried rice husks were pulverized using a rotating knife cutter, and then classified using sieve separation technique. Most of the milled rice husks, which were screened by a 40-mesh sieve at the bottom of the cutter, were found to be in the range of −50/+100 mesh. Morphology of the milled rice husks drastically changed with size from a flake-like shape at +70 mesh to a dust-like shape at −325 mesh. Tap density of unmilled raw rice husks was about 0.1 while that of milled rice husks was over 0.4. True densities of milled rice husks were higher than 1.4, and increased with decreasing milled husk sizes. Compared to bulky raw rice husks, the pulverized rice husks can be handled as a powdery material for further industrial processing. This revised version was published online in November 2006 with corrections to the Cover Date.     

Coarsening of tungsten grains in liquid nickel-tungsten matrix

In sintered W-Ni alloys with 1,7, and 30 wt pct Ni the tungsten grain growth in liquid matrix at 1540°C was investigated. The observed grain size distributions and growth rate are compared with the theoretical predictions of Wagner, Lifshitz and Slyozov, Lay, and Ardell. In the 70 pct W-30 pct Ni alloy the tungsten particles settled to the bottom of the specimens immediately upon melting of the matrix, but the spherical grain shape is maintained during the initial stage of annealing. In these specimens the linear intercept distribution of the grains agrees with the prediction of the LSW (Lifshitz, Slyozov, and Wagner) theory for the reaction controlled growth mechanism. On the other hand the growth rate appears to follow the t^1/3 law predicted for the diffusion controlled mechanism. These results are consistent with Lay and Ardell's theory in which the concentration gradient around grains is inversely proportional to the average grain size in the limit of small matrix fraction. In the alloys with 1 and 7 pct Ni a meaningful comparison of the observed linear intercept distribution of the grains with theoretical predictions is difficult because of grain contact flattening due to densification. The grain growth is larger with less matrix fraction in the specimens and this result provides an evidence for the diffusion controlled grain growth during the liquid phase sintering of this alloy. Formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science, Seoul, Korea. On leave at the Max-Planck-Institut für Metallforschung in Stuttgart, West Germany.     

Electrical Properties of Cubic, Stabilized, Single ZrO2–Gd2O3 Crystals

Electrical properties of single-crystal, cubic ZrO₂ solid solutions containing 10.0, 12.5, and 15.0 mol% Gd₂O₃ were investigated. Electrical conductivities were measured by a two-terminal ac technique up to 831°C in air, and the observed data were compared with those reported for polycrystalline materials. Electrical conductivity and activation energy increased, and preexponential factor decreased with Gd₂O₃ content.     

Effect of Sintering Temperature on the Densification of Al2O3

Alumina powder compacts sintered at various temperatures were isostatically hot-pressed. The specimens sintered to the closed-pore state can be densified further by hot isostatic pressing. If the open pores are eliminated during sintering, sintering at a low temperature is desirable to achieve a full densification after hot-pressing. Sintering at high temperatures causes pores to be trapped inside the grains; these pores are difficult to eliminate by subsequent hot-pressing.