Effect of zirconia content on electrical conductivities of mullite/zirconia composites measured by impedance spectroscop
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Kuo-Chuang Chiu Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan (Received 8 November 2007; accepted 18 April 2008)
Electrical conductivities of various mullite/zirconia composites, as well as monolithic mullite and zirconia, were measured using AC impedance spectroscopy from 100 Hz to 10 MHz at temperatures ranging from 150 to 1300 °C. The impedance spectra of monolithic zirconia and mullite/zirconia composites showed two semicircles because of the contributions from grains and grain boundaries, while those of monolithic mullite had one semicircle due to the predominant contribution from grains. This indicates that the conductivities of the mullite/zirconia composites increased with zirconia content. The activation energies of electrical conduction in mullite and zirconia were about 65 and 79 kJ/mol, respectively, and those of mullite/zirconia composites were between 65 and 79 kJ/mol. While the conductivities of various composites at 1 MHz were fitted by Lichtenecker’s rule, the general mixing equation could be applied to the conductivities measured at 1 kHz.
I. INTRODUCTION
Although mullite has good high-temperature strength and chemical stability, its low toughness limits its use in advanced structural applications. The mechanical properties of PSZ/mullite, SiCw/mullite, and SiCw/PSZ/ mullite composites have been studied by many investigators.1–5 It is well known that partially stabilized zirconia (PSZ) and/or SiC whiskers can significantly improve the mechanical properties of mullite via mechanisms such as phase transformation toughening, whisker bridging, and crack deflection. As far as electric properties are concerned, mullite is a good insulator at room temperature and can be considered a semiconductor at high temperatures.6,7 Gerhardt and Ruh8 reported that the incorporation of SiC whiskers significantly affected the electrical properties of mullite matrix composites because SiC has a large dielectric constant (42 for SiC and 6.5 for mullite at 1 MHz)9 and a small electric resistivity (5 × 105 ohm-cm for SiC and 1 × 1013 ohm-cm for mullite at room temperature)10,11 with respect to mullite. Previous studies have also been conducted on other composites.12,13 Gerhardt12 showed the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0263 J. Mater. Res., Vol. 23, No. 8, Aug 2008
influence of volume fraction, size, and shape of reinforcing agents on the electrical properties of ceramic matrix and polymer matrix composites. Runyan et al.13 studied the electrical conductivities of BN–B4C and BN–SiC composites under different orientations of the applied electrical field, and the results were fit by the McLachlan equation or general effective medium equation. It is believed that the dramatic change in electrical properties with respect to the volume fraction is attributable to the formation of interconnected channels of the good conducting phase within the composites. For electrochemical applications, mullite and
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