Microstructure of polycrystalline lanthanum silicate oxyapatite formed by solid–gas reactive diffusion

Abstract

The c-axis-aligned polycrystalline lanthanum silicate oxyapatite (LSO) was prepared by the reactive diffusion between random grain oriented La₂SiO₅ polycrystal and [SiO + 1/2O₂] gases at 1873 K for 10 h. Based on the grain sizes, aspect ratios, elongation directions, and c-axis orientations for the constituent LSO crystals, the polycrystalline microtexture was classified into three regions denoted by I, II, and III. Region I was located on the innermost side of the pellet sample, region III was situated on the surface, and region II was positioned between the regions I and III. The region I with the layer thickness of approximately 140 µm was composed of relatively large crystal grains with their elongation directions along the c-axes. The individual crystal grains were aligned almost along their c-axes, with their a-axis directions being randomly oriented around the grain alignment direction. Although this region exhibited the highest orientation compared to the other two regions, there were also significantly lower orientation grains present, accounting for approximately 6.1 % of the total. If the formation of these grains that reduce the degree of orientation can be prevented, a higher degree of orientation can be achieved for the region I. On the other hand, there were much smaller crystal grains in the regions II and III. The LSO polycrystal with the region II showed the lowest orientation degree among those of the three regions. The region III consisted of grains with a relatively high degree of c-axis orientation and elongated along the c-axis. The differences in microtexture between the three regions would be attributed to the distinct growth behaviors of the constituent LSO crystal grains.