Pure Al2O3 powder compacts, which were sintered at 1400C after adding 100ppm[mol] of SiO2, exhibited grain boundaries that were flat; even across the triple junctions. Transmission electron microscopic observations showed that these flat grain boundaries were parallel to the basal planes of the grains on one side. These flat grain boundaries had to be singular.

Singular Grain Boundaries in Alumina Doped with Silica. M.J.Kim, S.M.Kim, D.Y.Yoon: Journal of the American Ceramic Society, 2004, 87[3], 455-9