Thermal expansion of delta-yttrium disilicate

Crystals of delta-Y2Si2O7 (space group P12(1)/c1) were examined using high-temperature powder X-ray diffractometry to determine their unit-cell dimensions from 296 to 1473 K. The lattice deformation induced by thermal expansion was investigated using matrix algebra analysis to determine the directions and magnitudes of the principal distortions (lambda(i), i = 1, 2, and 3). The directions of lambda(1) and lambda(3) were defined by the acute angle lambda(1) <^> c, which linearly decreased from 5(2)degrees to -5.5(3)degrees with increased temperature from 504 to 1473 K. The lambda(2)-axis invariably coincided with the crystallographic b-axis. The magnitudes of lambda(1) and lambda(2) steadily increased to, respectively, 1.0061(1) and 1.0068(1) during heating to 1473 K, while lambda(3) remained almost constant for the entire temperature range. The mean principal distortion, lambda(m) (= (lambda(1) + lambda(2) + lambda(3))/3), steadily increased to 1.0044(1) with increased temperature to 1473 K. The coefficient of mean linear thermal expansion (alpha) was derived from the mean principal strain (lambda(m) - 1) as alpha = (lambda(m) - 1)/DeltaT. The temperature dependence was determined to be alpha = 2.03 X 10(3) + 1.36(T - 296) (10(-9) K-1). Provided that the rule-of-mixtures holds for the Y2Si2O7/Y2SiO5 composites as protective coating on SiC substrates, the volume fractions of 0.72-0.77 (70-75 mass%) would be necessary for the Y2Si2O7 component to match the alpha-values of both materials.