Browsing by Author "Ko, Hong-Da"

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Effect of zirconia content on electrical conductivities of mullite/zirconia composites measured by impedance spectroscopy
(2008-08-01)
Ko, Hong-Da
;
Lin, Chien-Cheng
;
Chin, Kuo-Chuang
;
材料科學與工程學系
;
Department of Materials Science and Engineering
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 degrees 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 I MHz were fitted by Lichtenecker's rule, the general mixing equation could be applied to the conductivities measured at 1 kHz.
Oxygen Diffusivities and Surface Exchange Coefficients in Porous Mullite/Zirconia Composites Measured by the Conductivity Relaxation Method
(2010-01-01)
Ko, Hong-Da
;
Lin, Chien-Cheng
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Oxygen diffusivities and surface exchange coefficients in various porous mullite/zirconia composites were measured at oxygen partial pressures ranging from 20.2 to 2.02 kPa using the conductivity relaxation method. However, the oxygen diffusivities in porous high-zirconia composites could not be determined because of the predominant surface exchange reaction. Oxygen diffusivities and surface exchange coefficients in low-zirconia composites increased with the zirconia content, while the surface exchange coefficients in high-zirconia composites were approximately constant. A percolation threshold of the surface exchange coefficients occured at similar to 40 vol% zirconia for porous zirconia/mullite composites. The oxygen diffusivities in porous low-zirconia composites were independent of the oxygen partial pressure, implying that oxygen diffusion in these composites was related to the migration of oxygen vacancies, whose concentration was independent of the oxygen partial pressure. The surface exchange coefficients of high-zirconia composites decreased with increasing oxygen partial pressure. Finally, it was inferred that the rate-limiting step for oxygen surface exchange could be the charge-transfer process.
Oxygen diffusivities in mullite/zirconia composites measured by O-18/O-16 isotope exchange and secondary ion mass spectrometry
(2008-02-01)
Ko, Hong-Da
;
Lin, Chien-Cheng
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Oxygen diffusivities in mullite/zirconia composites were measured by O-18/O-16 isotope exchange and secondary ion mass spectrometry. They exhibited a wide range of values from 10(-21) to 10(-10) m(2)/S at temperatures between 1000 and 1350 C in the composites with 0 to 80 vol% zirconia. At a fixed temperature, oxygen diffusivities m high-zirconia composites were larger by at least eight orders of magnitude than those in low-zirconia composites. The percolation threshold occurred between 30 and 40 vol% zirconia, where oxygen diffusivities dramatically changed. There was a clear tendency of the activation energies of oxygen diffusion in composites to decrease with increasing zirconia contents. The large oxygen diffusivities in the high-zirconia composites were attributed to the interconnected channels of zirconia from the microstructural aspect.