Oxygen diffusivities in mullite/zirconia composites measured by 18 O/ 16 O isotope exchange and secondary ion mass spect
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Oxygen diffusivities in mullite/zirconia composites were measured by 18O/16O isotope exchange and secondary ion mass spectrometry. They 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% zirconia. At a fixed temperature, oxygen diffusivities in 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.
I. INTRODUCTION
Mullite-matrix composites are good candidates for high-temperature structural applications among ceramicmatrix composites, because of their low thermal expansion coefficient, excellent strength, and creep resistance at high temperatures, and good thermal shock resistance. Previous studies have reported that partially stabilized zirconia (PSZ) and silicon carbide (SiC) effectively improved the mechanical properties of mullite-matrix composites.1–4 However, the oxidation of the SiC constituent would deteriorate the mechanical properties of such composites under high-temperature oxidizing environments. Moreover, the oxidation resistance of SiC-reinforced ceramic-matrix composites could be seriously degraded because of the incorporation of PSZ into the matrix. Several investigations on the oxidation behavior of mullite/PSZ/SiC composites have been carried out.5–10 In a pioneering study, Lin et al.10 first phenomenologically proposed two distinct oxidation modes, designated as mode I and mode II, of mullite/PSZ/SiC composites at elevated temperatures: mode I was based on the assumption that oxygen diffusion in the silica layer was much faster than in the mullite/zirconia matrix, and mode II operated otherwise. However, oxygen diffusivities in
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0073 J. Mater. Res., Vol. 23, No. 2, Feb 2008
various mullite/PSZ composites have not been available to date. All the measurements of oxygen diffusivities in previous studies were conducted on monolithic ceramics.11–13 Using secondary ion mass spectrometry, Fielitz et al.11 measured oxygen diffusivities in 2/1-mullite and reported that oxygen diffusivities ranged from 5 × 10−20 to 9 × 10−18 m2/s at temperatures ranging from 1250 to 1525 °C. Ikuma et al.12 determined oxygen diffusivities in single crystal mullite to be between 1 × 10−21 and 8 × 10−20 m2/s at 1100 to 1300 °C. Kim et al.13 measured oxygen diffusivities in 2.8 mol% Y2O3–ZrO2 to be 3.5 × 10−11 m2/s at 1000 °C using Raman spectroscopy combined with the 18O–16O isotope exchange technique. In this study, oxygen diffusivities in mullite/zirconia composites with various zi
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