High-temperature x-ray diffraction measurement of sanidine thermal expansion

Dental porcelains that are designed to be fused to PFM (porcelain-fused-to- metal) alloys are formulated by their manufacturers to be closely matched i n thermal expansion to these alloys. The high thermal expansion of the mine ral leucite has been exploited to regulate porcelain expansion. Leucite, ho wever, has been observed to convert to the sanidine polymorph of feldspar d uring certain heat treatments within the normal firing range of dental porc elain. The effects of this conversion on porcelain thermal expansion and po rcelain-metal thermal compatibility have been uncertain, due to the paucity of published data on the thermal expansion of sanidine. The purpose of thi s study was to measure the thermal expansion of sanidine by high-temperatur e x-ray diffraction over the temperature range in which thermal mismatch st resses can develop in porcelain-fused-to-metal restorations, i.e., from roo m temperature to 700 degrees C. The lattice parameters a, b, c, and beta we re determined from the d-spacings and hkl values of multiple reflections by means of a least-squares iteration. The dependence of each lattice paramet er on temperature was determined via analysis of variance, and the coeffici ent of thermal expansion, alpha was obtained from this analysis. The lattic e parameters of sanidine at room temperature were determined to be: a = 0.8 524 +/- 0.0015 nm, b = 1.3020 +/- 0.0004 nm, c = 0.7165 +/- 0.0002 nm and b eta = 116.02 degrees +/- 0.01 degrees (mean +/- 95% confidence interval). T he linear thermal expansion coefficient, alpha over the range from room tem perature to 700 degrees C was determined to be 4.1x10(-6) K-1 +/- 0.6x10(6) K-1 (mean +/- 95% confidence interval). Because the coefficient of thermal expansion for sanidine is substantially lower than that of leucite (the ef fective linear thermal coefficient of thermal expansion of leucite over the range of 25 degrees to 700 degrees C is 28X10(6) K-1), the conversion of l eucite to sanidine during porcelain heat treatments would produce a detrime ntal lowering of the porcelain thermal expansion.