| Budget Amount *help |
¥3,200,000 (Direct Cost: ¥3,200,000)
Fiscal Year 1999: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1998: ¥2,600,000 (Direct Cost: ¥2,600,000)
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| Research Abstract |
X-ray diffractometry (XRD) was used to analyze the crystal phases of three commercial dentin and incisal porcelains prepared by normal cooling, quenching in water, and controlled slow annealing after normal firing. The results suggest that the cooling condition has little effect on the microstructural changes of leucite crystals in the dental porcelains, since the leucite crystals were formed at a higher temperature range than the firing - and cooling- temperature range and were stabilized by incorporation of additives into the lattice.
The leucite crystals having modified compositions from stoichiometric one were prepared by the incorporation of Na, Rb, Cs, Ca, Ba, and Sb substituted for 5 -50 at% of K in the leucite. The lattice parameters of a- and c-axes of these tetragonal leucite were derived from the diffraction angles of (400) and (004) peaks due to tetragonal leucite, respectively. It is concluded that the crystal lattice of leucite was deformed by the incorporation of cations
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substituted for K and their variations mainly depend upon the kind of cation, namely ionic size and substitution site in the lattice.
Thermal expansion coefficients were determined using a dilatometer on commercial dental porcelains, stoichiometric tetragonal leucite, cesium added cubic leucite, three kinds of glasses, and mixtures of leucite and glass. Thermal expansion coefficients of leucite were also derived from the lattice parameter changes using high temperature X-ray diffractometry. The results suggested that the glass surrounded around leucite particles is required to have a high glass transformation temperature than 650℃ because of the remains of high stress in the surrounded matrix glass.
The influences of cubic leucite having various contents, particle sizes, and firing conditions on various properties (biaxial flexure strength, Vicker's hardness, thermal expansion, and abrasion of human enamel) of dental porcelains were investigated. FE-SEM and XRD analyzes revealed that cubic leucite was changed to tetragonal leucite on the boundary between cublic leucite particle and glass matrix by firing. There were no significant differences in the biaxial flexure strengths of the experimental porcelains containing different content of cubic leucite (p<0.05). It is concluded that the flexure strength of feldspathic dental porcelains was independent on the dispersion of cubic leucite, but depended on the dispersion of tetragonal leucite. Vicker's hardness values of the fired specimens containing cubic leucite increased with the particle size of cubic leucite. Thermal expansion coefficient of the fired specimens containing cubic leucite increased with their contents. Contents of cubic leucite had little effect on abrasion rate. Less
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