Abstract
Yttrium silicate (Y2SiO5) and yttrium disilicate (Y2Si2O7) are promising materials for oxidation protection coatings for SiC composites. Crystals of X1- Y2SiO5, X2- Y2SiO5 and δ-type Y2Si2O7 were examined by high temperature powder X-ray diffractometry to determine the changes in unit-cell dimensions with temperature up to 1473K. The lattice deformations of the three phases induced by thermal expansion were investigated by matrix algebra analysis to determine the directions and magnitudes of the principal distortions. The lattice change of X1-RE2SiO5 (RE = Y, and Yb-La), which was induced by the substitution of RE ions, showed a striking resemblance with the lattice deformation of X1- Y2SiO5, which was caused by the thermal expansion. Because the lattice change of the former must be caused by the isotropic expansion of the RE sites, the anisotropic thermal expansion of the latter would be essentially attributable to the isotropic thermal expansion of the YO9 and YO7 polyhedron.
