Experimental and theoretical studies are made to account for the formation of segregated networks in SiC/Si3N4 particulate composite. It was observed in the present experimental work that fine SiC particles are uniformly dispersed along boundaries of Si3N4 matrix grains. The proposed theoretical model takes into account the volume fraction of dispersed particles necessary to form a percolating network as a function of the size ratio of SiC particle to Si3N4 grain. Threshold particle loading for percolation is estimated by applying a modified two-dimensional bond percolation model and then electrical resistivity of the composite by using percolation and general effective media equations. Both predicted values of threshold particle loading and electrical resistivity were found to agree closely with the present experimental values.
SUMMARYThe performance of a liquid metal MHD generator is investigated with a two-dimensional numerical simulation. The effects of the electrode length, the position of current lead connection, and the insertion of an insulator on the performance are examined taking account of the current flow in the electrode. There exists an optimal electrode length for a given distribution of applied magnetic flux density. For a short electrode, the efficiency decreases because the power output becomes small. For a long electrode, on the other hand, the efficiency also decreases owing to the leakage current from the upstream and downstream edges of the electrode. An optimal current lead position was revealed. This fact is ascribed to the distributions of induced magnetic field and the current flow in the electrode. It was found that insertion of the insulator is effective for improving the performance, by which the eddy current can be reduced.
The internal friction of a silicon nitride with a sintering aid of yttrium oxide was measured from room temperature to 1400 °C. A mechanical loss peak was typically observed at 1000 °C on heating the as-sintered specimen; however, it disappeared on cooling. Also, the peak was not observed upon heating a specimen that had been heated above 1400 °C. When we carried out thermal cycling, raising terminal temperatures from 1050 to 1400 °C, the integrated intensity of the peak gradually decreased due to the progressive crystallization of a grain-boundary amorphous phase.
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