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- ItemCrystallization, microstructure and properties of selected glasses and glass-ceramics in the SiO2-Li2O-ZnO-K2O-P2O5 system(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Clausbruch, Sascha Cramer von; Schweiger, Marcel; Höland, Wolfram; Rheinberger, VolkerThe crystallization behavior of selected glasses in the multicomponent SiO2-Li2O-ZnO-K2O-P2O5 system was investigated using high-temperature X-ray diffraction and differential scanning calorimetry. Proceeding from a dental model glass with the molar composition 63.2 SiO2, 29.1 Li2O, 3.3 ZnO, 2.9 K2O and 1.5 P2O5, 20 glasses were melted, varying the concentrations of the single components systematically. Glass compositions mainly influenced the formations and dissolutions of metastable and stable phases in the temperature range of 480 to 1100 °C. In the majority of the glasses hthium disilicate (Li2Si2O5) precipitated as the main crystal phase during heating, while lithium metasilicate, lithium orthophosphate, cristobalite and quartz crystallized as secondary phases. After a one-step heat treatment microstructures of glass-ceramics and morphologies of Li2Si2O5 crystals were characterized by scanning electron microscopy. With few exceptions glass-ceramics showed a finegrained microstructure with interlocking, rodshaped Li2Si2O5 crystals with 0.5 to 10 µm in length. The corresponding glass-ceramics were processed by a hot-pressing technique to test samples for mechanical and optical measurements. Concentration variations of the base glasses caused bending strengths between 224 and 675 MPa. Within a comparison of translucency according to dental standard BS 5612 contrast ratios between 0.3 and 0.8 were determined.
- ItemMicrostructure formation and surface properties of a rhenanite-type glass-ceramic containing 6.0 wt% P2O5(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2005) Höland, Marlies; Dommann, Alex; Höland, Wolfram; Apel, Elke; Rheinberger, VolkerThe aim of this report was to characterize the microstructure formation of a rhenanite, NaCaPO4, glass-ceramic and to determine its surface properties. The composition of the material was (in wt%) 58.0 SiO2, 6.0 P2O5, 22.8 Na2O, 12.9 CaO, and 0.3 F. Nucleation and crystallization of the monolithic base glass was carried out at temperatures between 650 and 1000°C for one hour. To characterize the different microstructures of the glass-ceramics using scanning electron microscopy (SEM), a special sample with phosphoric acid was prepared. The control of microstructure formation resulted in the precipitation of isolated α-rhenanite crystals with a crystal size of 40 nm up to 1 μm and a crystal number density of 20 to more than 2000 particles per 100 μm2. The α-phase transformed into the β-phase at approximately 690 °C. The surface properties were determined by soaking the specimens in simulated body fluid for up to 10 d. In the process, the glass-ceramics exhibited a high surface activity. Morphological investigations after soaking showed ball-shaped hydroxyapatite-type crystals consisting of two half shells. The authors concluded that rhenanite crystals acted as heterogeneous nuclei for hydroxyapatite on the surface of the bioactive glass-ceramics. Therefore, this material was assumed to have the ability to bond to living bone.
- ItemEffect of ZnO on the crystallization, microstructure, and properties of glass-ceramics in the SiO2-Li2O-ZnO-K2O-P2O5 system(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Clausbruch, Sascha Cramer von; Schweiger, Marcel; Höland, Wolfram; Rheinberger, VolkerSix glasses in the SiO2-Li2O-ZnO-K2O-P2O5 system were prepared. Starting from a dental glass-ceramic model composition the ZnO content was varied from 0.0 to 8.3 mol% with the molar ratios of the other components being invariant. Crystallization processes of these glasses were investigated using high-temperature X-ray diffraction and differential scanning calorimetry. Crystal phases occurred in the temperature range of 500 to 1050 °C, while the main precipitated phase was lithium disilicate (Li2Si2O5). Lithium metasilicate, lithium orthophosphate, and cristobalite were secondary phases. Morphology and microstructure of the glass-ceramics were studied using scanning electron microscopy. Li2Si2O5 crystals in glass-ceramics had elongated shapes along the c-axis, crystal lengths ranging from 1 to 5 µm and aspect rados between 2 and 5. The lath-shaped Li2Si2O5 crystals were randomly oriented in the volume of the glass-ceramics and were responsible for the good mechanical properties. With increasing ZnO content a decrease of flexural strengths from 520 to 450 MPa was observed. It was concluded that the ZnO variation mainly influenced the translucency of the glass-ceramics. Relating to dental standard contrast ratios between 0.23 and 0.74, method limits of 0 and 1 provided, were determined.
- ItemGlass development and controlled crystallization in the SiO₂ - Li₂O - ZrO₂ - P₂O₅ System(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Höland, Wolfram; Frank, Martin; Schweiger, Marcel; Wegner, Susanne; Rheinberger, VolkerThe objective of this investigation was to analyze glass formation and crystaUization in t h e SiO₂ - Li₂O- ZrO₂ - P₂O₅ System for the development of high-strength glass-ceramics produced with glass powder. In the course of this study, it was found that the base glass or the semicrystalline glass-ceramic could be pressed to form glass-ceramic final products. The analysis of glass formation demonstrated that in a relatively large composition range, up to almost 30 wt%, ZrO₂ could be incorporated into the base glass and increased the glass forming tendency. Other additives were also tested. The presence of ZrO₂ crystals, ZrSiO₄ crystals and lithium phosphate crystals as well as other secondary phases in t he microstructure following heat treatment at approximately 1000 °C was estabhshed in SEM and XRD examinations. This glass-ceramic possesses fiexural strengths of 280 MPa. Translucency was observed in 0.5 mm thick layers. Growth rates of the main crystal phases were determined with cast monolithic samples in a semiquantitative kinetic evaluation of phase development. In contrast to the kinetics of mica crystal growth processes in machinable glass-ceramics, no time lags were observed.