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 in thermal expansion to these alloys. The high thermal expansion of the mineral leucite has been exploited to regulate porcelain expansion. Leucite, however, has been observed to convert to the sanidine polymorph of feldspar during certain heat treatments within the normal firing range of dental porcelain. The effects of this conversion on porcelain thermal expansion and porcelain-metal thermal compatibility have been uncertain, due to the paucity of published data on the thermal expansion of sanidine. The purpose of this study was to measure the thermal expansion of sanidine by high-temperature x-ray diffraction over the temperature range in which thermal mismatch stresses can develop in porcelain-fused-to-metal restorations, i.e., from room temperature to 700°C. The lattice parameters a, b, c, and β were determined from the d-spacings and hkl values of multiple reflections by means of a least-squares iteration. The dependence of each lattice parameter on temperature was determined via analysis of variance, and the coefficient of thermal expansion, α, was obtained from this analysis. The lattice parameters of sanidine at room temperature were determined to be: a = 0.8524 ± 0.0015 nm, b = 1.3020 ± 0.0004 nm, c = 0.7165 ± 0.0002 nm, and β = 116.02° ± 0.01° (mean ± 95% confidence interval). The linear thermal expansion coefficient, α, over the range from room temperature to 700°C was determined to be 4.1 × 10^-6 K^-1 ± 0.6 × 10⁶ K^-1 (mean ± 95% confidence interval). Because the coefficient of thermal expansion for sanidine is substantially lower than that of leucite (the effective linear thermal coefficient of thermal expansion of leucite over the range of 25° to 700°C is 28 × 10^-6 K^-1), the conversion of leucite to sanidine during porcelain heat treatments would produce a detrimental lowering of the porcelain thermal expansion.