Thermal and chemical properties of a glass in the SiO2-CaO-F system for dental applications

The crystallization behaviour of a glass in the SiO2-CaO-F system was analy zed using differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Three crystalline phases wer e detected according to ICDD patterns. The first phase formed at 583 degree s C was identified as CaF2. The morphology was spherulitic with a diameter of approximately 100 nm. The second phase was formed at 664 degrees C. It w as identified as calcium fluoride silicate 'Ca2SiO2F2' (ICDD 35-0002). SEM investigation showed that the crystals were spherulitic with a diameter sma ller than 100 nm. The crystals were precipitated in the volume of the glass and homogeneously distributed. As a third phase, cristobalite crystallized at 895 degrees C. The simultaneous release of calcium and fluorine ions from the vitreous gla ss in lactate buffer solution at pH 4.0, simulating an acidic oral environm ent, was investigated using X-ray photoelectron spectroscopy (XPS). The rel ease of calcium and fluorine ions is of special interest for dental applica tions. The atomic ratios of the components Si, Ca and F at the glass surfac e after different leaching periods were determined. In order to investigate the leaching process, concentration profiles were measured using ion beam sputtering with Ar+-ions. The dependence of the atomic ratios of Si, Ca and F on the sputter time was determined in order to measure the depth of the leaching layers. Most probably, the release of calcium and fluoride was con trolled by a surface layer rich in calcium and flourine ions which dissolve d with increasing leaching time. After 2 min leaching, a fluoride-rich surf ace layer measuring approximately 10 nm was detected. The atomic ratios of Si, Ca and F were different from the bulk composition ratios in a surface r eaction layer of 800 nm thickness. After 30 min leaching time, a calcium- a nd fluoride-rich surface layer approximately 50 nm thick was formed. The bu lk composition was reached at a depth of approximately 500 nm. The main com ponent in the surface layer, after 12 days leaching in acidic environment, was silicon.