Pressure-Induced Nano-Crystallization of Y 2 O 3
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Pressure-Induced Nano-Crystallization of Y2O3 Stuart Deutsch, Jafar F. Al-Sharab, Bernard H. Kear and Stephen D. Tse Department of Materials Science & Engineering, Rutgers University 607 Taylor Rd. Piscataway, NJ 08854 ABSTRACT A reversible-phase transformation process to convert coarse-grained polycrystalline cubic-Y2O3 directly into the nanocrystalline state is being developed. The process involves a forward cubic-to-monoclinic phase transition under high pressure and a backward transformation from monoclinic-to-cubic under a lower pressure. The process has been used to reduce the grain size of fully dense cubic-Y2O3 from 300 μm to 0.1 μm. A surface modification effect, comprising a columnar-grained structure, has also been observed. Preliminary work indicates that the surface structure is modified, apparently formed by interaction between the graphite heater and sample. INTRODUCTION Over the past decade, there has been growing interest in the fabrication of nanocrystalline ceramics for structural and functional applications. Typical fabrication processes involve pressure- or field-assisted sintering of nanopowder compacts. However, it is difficult to achieve complete densification without causing significant grain coarsening due to rapid grain-boundary migration during the final stages of sintering, when growth-limiting nanopores start to disappear. Cubic-Y2O3 displays excellent optical transmittance in the mid-IR range1, representing a potential replacement for aluminum oxide (Al2O3) in key applications. However, there is a need to increase the hardness of Y2O3 without compromising optical transmittance, which can be achieved by reducing the grain size of the material to nanoscale dimensions2. Taking a new approach, we are developing a reversible phase transformation (RPT) process for the direct conversion of bulk polycrystalline Y2O3 into the nanocrystalline state. The cubic-to-monoclinic and monoclinic-to-cubic Y2O3 bulk phase transitions are being investigated in order to optimize RPT processing parameters. A better understanding of the transformation kinetics will allow for better control of grain size and microstructural uniformity, resulting in improved physical properties of the final nano-grained cubic-Y2O3 product. DISCUSSION In initial tests, disc-shaped samples (4 mm x 4 mm) of c-Y2O3 (300-μm grain size) were subjected to 8 GPa/1000°C for 1, 15, 60 and 240 minutes3. Irrespective of holding time, the effect was to transform the material into a known monoclinic phase, accompanied by a 6% reduction in volume. Most importantly, the cubic-monoclinic phase transformation was accompanied by a major reduction in grain size, typically from 300 μm to 100 nm. The effect may be attributed to prolific nucleation of nano-grains of the m-Y2O3 phase under high pressure, with limited grain growth due to the reduction in bulk diffusion. Re-processing of the newly formed nano-grained m-Y2O3 material at 1 GPa/1000°C converted the sample back to the cY2O3 phase, with a final grain size cubic conversion. CONCLUSIONS In thi
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