Coloration of mica glass-ceramic for use in dental CAD/CAM system
Introduction
Glass-ceramics have been used commercially as synthetic tooth filling materials and in dental prostheses [1], [2]. Two most common fabrication techniques of the glass-ceramic restorations are casting via the classic lost wax process and the rather recent CAD/CAM technique [3], [4]. Grossman and Johnson Janet [2] describe a family of tetrasilicic fluormica glass-ceramics, which is eminently suitable for use in dental CAD/CAM systems, because of its good strength and translucency. Machinability in this glass-ceramic is associated with cleavage of mica grains. Despite these advantages, coloration of mica glass-ceramic remains a problem [5].
In conventional dental ceramics, such as dental porcelain, a ceramic pigment or combination of several pigments is incorporated into the composition. The crystals of the pigments remain sufficiently intact during the firing process to impart the desired color in the finished product. Ceramic pigments consist mostly of refractory crystals containing color centers frequently produced by the ions of such transition metals as Co, Cr, Fe, Mn, Ni, and V. The actual color provided by those pigments, however, is governed to a large extent by the crystalline structure of the host compound into which the transition metal ions are incorporated [6].
In order to produce acceptable colors for use in dental glass-ceramics, colorants are needed which produce yellow to yellow-red in the crystallized product. Except uranium, no single transition metal or rare earth metal ion has been identified which yields a clear yellow effect. However, government regulations regarding radioactivity restrict the use of uranium. However, methods for combining certain pairs of colorants ions have been described. It has long been known that the combination of Ce and Ti oxides can give yellow colors in glazes and glasses. In 1919, Taylor disclosed the use of a combination of CeO2 and V2O5 in soda lime silica glass to produce a very pale yellow color [6]. In 1959, Weyl [7] reported the use of CeO2, which by itself produced no color in glass, but which in combination with TiO2 imparted strong yellow colors. That yellow color can be modified by incorporating the rare earth metal oxide Er2O3. The combination of CeO2 and TiO2 works well in visible light, but fluorescence under long wave ultraviolet illumination does not take place because the presence of TiO2 strongly absorbs ultraviolet radiation. In 1979, Smyth and Lee-You [8] disclosed the use of Tb2O3 to modify the color of CeO2 fluorescence in conventional dental porcelains. In 1995, Grossman [6] reported a color package of mica glass-ceramic comprising 0.05–0.5% V2O5, 0.25–5%CeO2, and 0.1–1% Tb2O3 which could give fluorescence. Besides these colorant reports, George, Bowen and Zhang [9], [10], [11], [12] also reported such factors affecting the coloration results as heat treatment process and amount of nucleation agent.
In this study, coloration effects and factors affecting coloration results are investigated.
Section snippets
Experimental
The proportions of basic ingredients used in preparing the melt consisted essentially, expressed in terms of parts by weight on the oxide basis, except for fluorine which is reported on an elemental basis, of 55–65% SiO2, 14–19% MgO, 4–9% F, 8–18% K2O, 0–2% Al2O3, 0–7% ZrO2. The ingredients were melted at about 1500 °C for about 2 h to obtain a homogenous liquid. The resulting melts thereafter poured into steel molds to yield glass cylinders with 23±0.2 mm in diameter, and then transferred
Coloration results
In the field of glass-ceramics, it is quite common for the color produced in the precursor or parent glass to be different from the color developed in the crystallized glass-ceramic as a result of heat treating the parent glass as shown in Table 1. This change is brought about through the alteration in the amorphous structure, which occurs to the composition of the residual glass as the components comprising the crystals, which are removed therefrom. There may also be some incorporation of the
Conclusion
The coloration of mica glass-ceramic, which exhibits a yellow coloration both in visible light and under ultraviolet radiation, is produced through a color package comprising 2–5% CeO2, 0.1% V2O5. The preferred heat treatment is 650 °C for 1 h, plus 1000 °C for 3 or 4 h, and the ratio of Mg2+/2K+ in composition should be in the range of 0.8–1.6.
Acknowledgements
State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University supports this research. The author thanks Prof. Jiemo Tian of Institute of Nuclear Energy Technology, Tsinghua University, for his critical reading of this paper.
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