氧化鋯含量對莫來石/氧化鋯複合材料氧擴散與導電性之影響

Abstract

本研究利用三種不同方法測量莫來石/氧化鋯複合材料之氧擴散與電導。經由18O-16O同位素交換反應並利用二次離子質譜儀(SIMS)可測得莫來石/氧化鋯複合材料的氧擴散係數；使用交流阻抗(AC impedance)量測莫來石/氧化鋯複合材料的電導；在氧分壓20.2到2.02 kPa範圍內，利用電導鬆弛法(conductivity relaxation method)可得到多孔莫來石/氧化鋯複合材料的氧擴散係數及表面交換係數。 同位素交換反應研究顯示，在溫度從1000°C到1350°C之間，氧化鋯含量0 vol%∼80 vol%之莫來石/氧化鋯複合材料其氧擴散係數為10-21∼10-10 m2/s。在氧化鋯含量30 vol%與40 vol%之間，氧擴散係數有著極大的變化。由於在高氧化鋯含量複合材料中氧化鋯相互連結形成一個快速路徑，而氧離子可經由此路徑移動。因此高氧化鋯含量複合材有較高的氧擴散係數。氧擴散之活化能隨著氧化鋯含量增加而有降低的趨勢。 在莫來石/氧化鋯複合材料的電導研究中，純氧化鋯與莫來石/氧化鋯複合材料的阻抗圖譜皆顯示兩個半圓，而此兩半圓分別是由材料中的晶粒及晶界所貢獻。純莫來石陶瓷的阻抗圖譜顯示一個半圓，而此半圓是由莫來石之晶粒所貢獻。莫來石/氧化鋯複合材料的電導隨著氧化鋯含量增加而增加。在交流頻率1 MHz測量下的導電度符合Lichtenecker’s rule的預測，而在交流頻率1 kHz測量下的導電度則符合混合方程式(general mixing equation)。 在多孔莫來石/氧化鋯複合材料的研究中，高氧化鋯含量複合材料的表面交換係數趨近一常數。而低氧化鋯含量複合材的氧擴散與表面交換係數則隨著氧化鋯含量增加而增加。多孔莫來石/氧化鋯複合材料的表面交換係數存在一臨界體積分率在氧化鋯含量約40 vol%左右。研究結果顯示低氧化鋯含量複合材的氧擴散係數與氧分壓無關，此是因為材料的氧擴散與其氧空孔息息相關，但氧空孔濃度不隨著氧分壓而變動。另外，高氧化鋯含量複合材料的表面交換係數則隨著氧分壓增加而減少。最後，經推測氧表面交換反應的速率決定步驟(rate-limiting step)應為電荷轉移過程(charge-transfer process)。

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.