Thermo-mechanical behavior of beta-eucryptite and eucryptite based composites

Abstract

Beta-eucryptite (LiAlSiO4) has received widespread attention, both industrially and academically because of its low negative average coefficient of thermal expansion (CTE) and one dimensional Li-ion conductivity. Beta-eucryptite is also known to undergo a reversible pressure-induced phase transformation under compression at ~0.8 GPa to a metastable polymorph, epsilon-eucryptite. The present work evaluates the thermal and mechanical behavior of beta-eucryptite and eucryptite composites with several different experiments. In the first set of experiments, nanoindentation was used to determine the activation volume of the pressure-induced phase transformation in polycrystalline and single crystal beta-eucryptite. It is shown that the phase transformation is a thermally activated event. It is the first time that nanoindentation has been used to find the activation volume in a pressure-induced phase transformation. Results suggest that the nucleation event that marks the onset of the phase transformation is approximately the size of the silica and alumina tetrahedra comprising the beta-eucryptite structure. The effect of Zn doping on the phase transformation was also examined using in situ diamond anvil cell-Raman spectroscopy and nanoindentation experiments. The present work is the first to employ both techniques in a comparative manner to investigate pressure-induced phase transformation in the same material. This study demonstrates that, though the data obtained from the two techniques are complementary, important differences including the loading rate and the state of stress between the two techniques must be noted when understanding data from the two experiments. The next set of experiments examined the effect of Zn doping on the thermal expansion of beta-eucryptite to understand the structure-property relationship in the material. The results revealed that doping significantly increases the bulk CTE - negative for pure beta-eucryptite to slightly positive for Zn-doped beta-eucryptite in the range from 25°C to 1000°C, and also lowers the tendency to microcracking. No difference in grain size was observed between the pure and doped samples, and all the ceramics exhibited high relative densities. It is believed that an intrinsic modification in the beta-eucryptite structure is responsible for the observed behavior. In the final study, the potential of transformation toughening was examined by fabricating composites of 10 mol % yttria stabilized zirconia reinforced with varying amounts (0 to 20 vol. %) of beta-eucryptite (LiAlSiO4) with the intent of stabilizing in situ the high-pressure eucryptite polytype, epsilon-eucryptite, so that the reverse transformation to beta-eucryptite under a crack tip stress field may impart toughening by volume expansion. The results indicate that the change in toughness with volume fraction of beta-eucryptite is best explained by considering a transformation toughening mechanism in conjunction with the toughening decrement predicted due to matrix tensile stresses which arise due to the thermal expansion mismatch.