DETERMINATION OF THE THERMAL-EXPANSION OF KAPPA-AL2O3 BY HIGH-TEMPERATURE XRD

In this work, the thermal expansion of kappa-Al2O3 has been characteri sed using high temperature X-ray diffraction of kappa-Al2O3 powder obt ained from crushed chemical vapour deposited kappa-Al2O3 coatings. The lattice parameters, volume, thermal linear and volume expansion coeff icients have been determined in the range 23 to 840 degrees C. It was found that kappa-Al2O3 expands anisotropically, the expansion being ab out equal along the b- and c-axis, while it is significantly lower alo ng the a-axis. Compared with alpha-Al2O3, the expansion is slightly lo wer along c(kappa) than along c(alpha), similar along b(kappa) and a(a lpha), while it is significantly lower along a(kappa) than along a(a)l pha. The expansion data imply that all aluminium ions in kappa-Al2O3 a re not ordered in a hexagonal mesh as in alpha-Al2O3, but are ordered in lines along the a-axis.