Department of Restorative Sciences: Theses and Dissertations
The Effect of Leucite Crystallization and Thermal History on Thermal Expansion Measurements of Dental PorcelainsObjectives. Measurement of thermal expansion in glassy materials is complicated by thermal history effects. The purpose of this research was to determine whether the occurrence of structural relaxation in glassy materials, such as dental porcelains, an changes in porcelain leucite content could interfere with the accurate measurement of the coefficient of thermal expansion during the thermal expansion measurement itself. Methods. In a randomized design, thermal expansion specimens were fabricated using six commercial body porcelains and the leucite-containing Component No. 1 frit (Weinstein et al. patent, 1962), and subjected to one of the following heat treatments: a single heating run at 3°C/min in a conventional dilatometer followed by air quenching; three successive low-rate heating and cooling thermal expansion runs at 3°C/min in a conventional dilatometer; or three successive high-rate heating and cooling thermal expansion runs at 600°C/min in a laser dilatometer. The remaining specimens were left untreated and served as controls. Potential changes in porcelain leucite content were monitored via quantitative X-ray diffraction. Thermal expansion data for each run over a temperature range of 25-500°C and the leucite content of all specimens were subjected to repeated measures analysis o f variance. Results. The thermal expansion coefficient measured on first slow heating was significantly lower than the values for succeeding low-rate heating and cooling runs in all materials (p < 0.001). The high-rate thermal expansion coefficient obtained on first heating was not significantly different from the values of succeeding heat and cool runs in all materials (p > 0.0S). No significant effect of dilatometer thermal treatments on leucite content (p > 0.05) was shown for all materials studied using both dilatometers. Significance. The crystallization of additional amounts of leucite during thermal expansion runs can be ruled out as a possible interference in the determination of the thermal expansion coefficient of dental porcelain. Conventional dilatometer measurements exhibited structural relaxation during the first heating run, as evidenced by the significant difference between the first heating and subsequent runs, while the laser dilatometer measurements were not affected by this thermal history effect. Therefore, high-rate dilatometry provides a more accurate thermal expansion measurement that is free of interference from structural relaxation and additional leucite crystallization.