Effect of Zirconia Content on Oxygen Diffusivities and Electrical Conductivities in Mullite/Zirconia Composites

• Main Authors: Ko, Hong-Da, 柯宏達
• Other Authors: Lin, Chien-Cheng
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/97785798551739608969

博士 === 國立交通大學 === 材料科學與工程系所 === 98 === Oxygen diffusivities and electrical conductivities in various mullite/PSZ composites were measured by 18O/16O isotope exchange method using secondary ion mass spectrometry and AC impedance spectroscopy, respectively. Additionally, oxygen diffusivities and surface exchange coefficients in various porous mullite/PSZ composites were measured at oxygen partial pressures ranging from 20.2 to 2.02 kPa using the conductivity relaxation method. Oxygen diffusivities in mullite/PSZ composites exhibited a wide range of values from 10-21 to 10-10 m2/s at temperatures between 1000 and 1350°C in the composites with 0 to 80 vol% PSZ. The percolation threshold occurred between 30 and 40 vol% PSZ, where oxygen diffusivities dramatically changed. There was a clear tendency of the activation energies of oxygen diffusion in composites to decrease with increasing PSZ contents. The large oxygen diffusivities in the high-PSZ composites were attributed to the interconnected channels of PSZ from the microstructural aspect. For the measurement of electrical conductivities in mullite/PSZ composites, the impedance spectra of monolithic PSZ and mullite/PSZ 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/PSZ composites increased with PSZ content. 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. For the measurement of oxygen diffusivities and surface exchange coefficients in porous mullite/PSZ composites, oxygen diffusivities and surface exchange coefficients in low-PSZ composites increased with PSZ content, while the surface exchange coefficients in high-PSZ composites were approximately constant. A percolation threshold of the surface exchange coefficients took place at ~40 vol% PSZ for porous mullite/PSZ composites. The oxygen diffusivities in porous low-PSZ composites were independent of the oxygen partial pressure, implying that oxygen diffusion in these composites was related to the migration of oxygen vacancies, of which the concentration was independent of the oxygen partial pressure. The surface exchange coefficients of high-PSZ composites decreased with increasing oxygen partial pressure. Finally, it was found that the rate-limiting step for oxygen surface exchange could be the charge-transfer process.