Extruded and Pressureless-Sintered Al_2O_3-SiC_w Composite Rods: Fabrication, Structure, Electrical Behavior, and Elastic Modulus

摘要

Ceramic composite rods of alumina filled with varying volume fractions of silicon carbide whiskers (SiC_w) were made by extrusion and pressureless-sintering. The electrical response was measured over a wide range of frequency (1 mHz-1.8 GHz) and was compared to that of disks made by dry-pressing and pressureless-sintering the same starting powder blends. Extrusion resulted in increased whisker alignment and electrical conductivity along the processing direction compared to dry-pressing. Pressureless sintering resulted in anisotropic shrinkage and the rods having lower density (D), dc conductivity (σ_(dc)), and SiC_w-SiC_w connection quality compared to fully dense hot-pressed disks. Porosity (P = 1 - D) increased with SiC_w loading and was linearly related to the rod elastic modulus, which was determined via sonic resonance. SiC_w percolation in the extruded rods required greater SiC_w loading, took place over a wider compositional range, and resulted in significantly decreased σ_(dc) for the rods compared to hot-pressed disks. The σ_(de) of percolated rods having composition near the percolation threshold was sensitive to small changes in porosity and was correlated to the elastic modulus. The weak percolation transition was correlated to a gradual evolution of the complex permittivity spectra. As SiC_w content was increased, such spectra exhibited dc-conductivity tails in the loss which grew in magnitude, and dielectric relaxations which grew in strength, shifted to lower frequency, spread out over wider frequency ranges, and seemed to evolve toward power-law behavior for the dielectric constant.