Anisotropic Grain Boundary Mobility in Undoped and Doped Alumina

Abstract

The grain boundary mobility of polycrystalline alumina (α‐Al2O3), and the effective grain boundary mobility of the basal (0001) plane as it grew into polycrystalline alumina, was determined for undoped alumina, alumina doped with 23 ppm MgO, and alumina doped with 13 ppm CaO at 1600°C. Doping with MgO at a level below the solubility limit decreased the grain‐boundary mobility from 2.7 × 10−15 to 1.5 × 10−15 m2/s, and doping with CaO at a level below the solubility limit increased the mobility to 3.5 × 10−15 m2/s. For the undoped samples at 1600°C, the activation energy for the average grain boundary mobility was 372 ± 39 kJ/mol. The mobility of the (0001) plane growing into alumina doped with MgO at a level below the solubility limit decreased to 1.1 × 10−15 m2/s compared with the mobility of the (0001) plane growing into undoped alumina (2.5 × 10−15 m2/s), and the mobility of the (0001) plane growing into alumina doped with CaO (below the solubility limit) increased to 3.2 × 10−15 m2/s. The activation energy for the mobility of the (0001) plane was 483 ± 76 kJ/mol. Although a measured Ca excess of 2.6 Ca/nm2 at the boundary between the (0001) plane and CaO‐doped alumina is correlated with an increased mobility, the platelike morphology of CaO‐doped polycrystalline alumina is associated with an increased mobility of nonbasal planes.

Cation grain‐boundary diffusivity in SiO2‐ and MgO‐doped Al2O3

“…This variation in diffusivity is small considering the relatively large variation in impurities between samples made from different source powders and tested by different groups. 26 Al grain-boundary diffusion in Y 2 O 3 -doped alumina is consistent with reported values for Cr 3+ chemical tracer measurements in similar undoped and Y 2 O 3 -doped materials. However, the results have been rationalized in the context of aliovalent charge compensating impurities that tend to balance one another in the undoped samples.…”

“…Those values are only slightly lower than our Cr 2 O 3 chemical tracer values and are reasonably consistent with values measured indirectly from permeability studies, 20 when extrapolated to higher temperatures. 5,26 The experimental results 27 provide new insights into how average Al 2 O 3 grain-boundary diffusivity is affected by aliovalent dopants. The reproducibility is unanticipated given Al 2 O 3 's reputation as an impurity sensitive highly extrinsic material and the fact that the samples were prepared from powders of different purity, in different labs, and tested using different methods.…”

“…4,8 Key findings include the observation that for nominally undoped samples, O 2À (oxygen anion) diffusivity falls within a range of 1 order of magnitude. Recent measurements of 26 Al tracer diffusion have been made in Y 2 O 3 -doped Al 2 O 3 . These impurities might be expected to dominate the O 2À vacancy concentration.…”

Growth of single crystalline seeds into polycrystalline strontium titanate: Anisotropy of the mobility, intrinsic drag effects and kinetic shape of grain boundaries

“…Experiments in this geometry have been used for more than three decades in measuring growth rate anisotropy in ceramics, e.g. Alumina [22][23][24][25][26][27][28][29], Barium titanate [30][31][32][33] and Strontium titanate [34][35][36]. In some of these previous experiments and in this paper, measurements of the mean migration distance of different single crystal orientations into the polycrystalline matrix and of the matrix grain size as a function of time, temperature, and single crystal surface orientation are obtained.…”

Restraining of calcium contamination in near‐net shape alumina ceramics during slip casting

“…Therefore, Ca ions could contaminate the green body and thus cause abnormal grain growth (especially in the case of alumina). Akiva et al demonstrated that the grain boundary mobility of alumina doped with 13 ppm CaO increased in comparison to the undoped alumina. The morphology of CaO‐doped polycrystalline alumina was plate‐like.…”