Extruded and Pressureless‐Sintered Al₂O₃–SiC_wComposite Rods: Fabrication, Structure, Electrical Behavior, and Elastic Modulus

Abstract

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 drypressing. Pressureless sintering resulted in anisotropic shrinkage and the rods having lower density (D), dc conductivity (r dc ), and SiC w -SiC w connection quality compared to fully dense hot-pressed disks. Porosity (P = 1 À 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 r dc for the rods compared to hot-pressed disks. The r dc 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.R. Cutler-contributing editor Manuscript No. 29560.

Processing of Alumina and Corresponding Composites

“…For microwave applications, a relatively large number of sizeable ceramic rods (w30 cm long and 8-14 mm wide) are needed and the mechanical requirements are much lower than required in the case of cutting tools. Therefore, extrusion followed by pressureless sintering has been proposed as the adequate processing route (Bertram, Gerhardt, & Schultz, 2011).…”

Assessment of Homogeneity of Extruded Alumina-SiC Composite Rods Used in Microwave Heating Applications by Impedance Spectroscopy

“…The reason the 20% concentration of whiskers was chosen was because this would set the samples in a region well beyond the percolation threshold for extruded alumina-SiC w ceramic composite rods. 4 This buffer would provide a cushion of security making sure the differences in conductivity before and after the percolation threshold do not become an additional variable when investigating the homogeneity of the rods themselves. The magnitude of the impedance and phase angle plotted versus log frequency for the eight rods is presented in Figure 2.…”

“…[1][2][3][4][5] Silicon carbide whiskers are 0.5 μm in diameter and can have aspect ratios as high as 20. The different processing methods therefore result in different whisker alignment, and consequently they have different electrical responses.…”

“…6 One of the previous papers demonstrated that the long rods behave differently than thin sections cut from them. 3,4 Detailed impedance measurements of the full length rods and thinly sliced samples were studied in detail. In the large rods, it was found that the Schottky contact electrode effect 2 was buried under the large magnitude of the bulk impedance, whereas in the thin slices, the bulk and electrode contributions were more comparable.…”

“…SiC w shown in the alumina matrix in parallel and perpendicular directions with respect to the extrusion direction4 …”