Bimodal Size Distribution and Shape Anisotropy in Ball-Milled Nano-Sized α-Al2O3

Abstract

α-Al2O3 prepared by combustion technique was ball-billed in a planetary mill in toluene medium at 300 rpm in ZrO2 pot with a ball to powder ratio 10:1 for 1, 5, 10, 15, and 20 hours. X-ray diffraction pattern from the milled materials showed super-Lorentzian peak shapes for α-Al2O3 peak profiles. The super-Lorentzian peak shape has been attributed to the bimodal size distribution of the α-Al2O3 particles. Rietveld analysis using two different phase fractions of α-Al2O3 with different microstructural features yielded a low goodness-of-fit of the x-ray data indicating the suitability of the assumed model. The phase fraction of α-Al2O3 particles with smaller size increases with the milling time. Further the particles shapes were observed to be cylindrical in this case with the cylinder axis along the crystallographic c-axis. The cylinder diameter and the length were obtained to be 86 Å and 140 Å respectively after 20 hrs of milling. The α-Al2O3 particles of larger size are isotropic. It is, thus, proposed that milling induces bimodal size distribution in the initial hours of milling.