Grain boundary mobility transition in alumina

Alumina is a classical model system to study the mass transport, sintering, and microstructural evolution of ceramics. It is known to have strong crystal anisotropy and abnormal grain growth, that is, abnormally accelerated grain boundary mobility of a single or some grain boundaries leading to fast growth of some grains in consumption of their neighboring grains. Here, we reported another phenomenon in pressurelessly sintered fine-grain alumina at relatively low temperatures. Statistically averaged grain boundary mobilities of alumina between 1300 degrees C and 1500 degrees C were systematically calculated in the normal parabolic growth regime and sample microstructure with weak anisotropy. While the higher temperature data at 1400-1500 degrees C with a reasonably activation energy of 4.9 eV agree well with extrapolated ones from Dillon and Harmer, the lower temperature data at 1300-1400 degrees C show rapid slowdown with a very large apparent activation energy of 9.0 eV. Previously, such grain boundary mobility transition was only reported in cubic yttria-stabilized zirconia and body-centered cubic tungsten, both with high symmetric and low anisotropy. The new report of similar phenomenon in high-anisotropy alumina system suggests a general feature of grain boundary mobility quenching at low temperatures and a sweet processing window to sinter fine ceramics with extrinsically controlled microstructure, properties, and reliability.