Thermal Expansion of δ‐Yttrium Disilicate

Crystals of δ‐Y 2 Si 2 O 7 (space group P 12 1 / c 1) 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 (Λ i , i = 1,2, and 3). The directions of Λ 1 and Λ 3 were defined by the acute angle Λ 1 c , which linearly decreased from 5(2)° to —5.5(3)° with increased temperature from 504 to 1473 K. The Λ 2 ‐axis invariably coincided with the crystallographic b ‐axis. The magnitudes of Λ 1 and Λ 2 steadily increased to, respectively, 1.0061(1) and 1.0068(1) during heating to 1473 K, while Λ 3 remained almost constant for the entire temperature range. The mean principal distortion, Λ m (= (Λ 1 +Λ 2 +Λ 3 )/3), steadily increased to 1.0044(1) with increased temperature to 1473 K. The coefficient of mean linear thermal expansion (α) was derived from the mean principal strain (Λ m ‐ 1) as α= (Λ m ‐ 1)/Δ T. The temperature dependence was determined to be α= 2.03 times 10 3 + 1.36( T ‐ 296) (10 ‐9 K ‐1 ). Provided that the rule‐of‐mixtures holds for the Y 2 Si 2 O 7 /Y 2 SiO 5 composites as protective coating on SiC substrates, the volume fractions of 0.72‐0.77 (70–75 mass%) would be necessary for the Y 2 Si 2 O 7 component to match the α‐values of both materials.