Papers by Keyword: Glass Crystallization

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Authors: Carlos M. Queiroz, Maria Helena F.V. Fernandes, Jorge R. Frade
Abstract: Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)20(SiO2)35(MgO)38.354(K2O)6.646, developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800°C and 860°C, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)7 crystallizes at about 860°C, after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca5(PO4)3O) and tricalcium phosphate (Ca3(PO4)2). The conductivity of bulk glass samples decreases at 800°C, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860°C.
Authors: Tomasz Czeppe, Elena Vassileva, Jan Dutkiewicz
Authors: Carlos M. Queiroz, Maria Helena F.V. Fernandes, Jorge R. Frade
Abstract: Impedance spectroscopy was used to monitor the non-isothermal crystallisation of a phosphate phase in bulk samples of a glass with the molar composition 0.45SiO2–0.36MgO– 0.09K2O–0.1(3CaO⋅P2O5), intended for biomedical applications. The K+ alkali ions are probably the main charge carriers in glass of this composition [1]. Impedance spectra were obtained as a function of temperature, following non-isothermal heat treatments at 2°Cmin-1. It is shown that suitable analysis of impedance spectra can be used to evaluate the crystallization peak of a orthophosphate phase, Ca9MgK(PO4)7. Results obtained by this method are comparable to those obtained by differential thermal analysis (DTA) of the corresponding glass frits. Impedance spectroscopy is thus suitable to study the crystallization of bulk samples, and can be used both with variable temperature and under isothermal conditions.
Authors: Pawel Stoch
Abstract: Vitrification has been recognized as the best method of the neutralization and immobilization of the radioactive and toxic wastes. Toxic elements are introduced into the structure of chemically durable glasses (waste glasses). Radioactive 137Cs is a dangerous contaminant of hospital and laboratory wastes. Incineration of these waste followed by vitrification of the ashes by co-melting them with borosilicate glass is a prospective method of immobilization of 137Cs for safety waste storage. The influence of partial substitution of Na by Cs and introduction of CaO as the main component of the hospital waste incineration ash on the structure, crystallization ability, and physical properties of SiO2-B2O3 -Al2O3-Na2O glass was studied. It was established that due to crystallochemical differences between Cs+ and Na+ as the glass structure modifiers, in the presence of even a relatively small amount of Cs2O (5 mass %) the degree of polymerization of the glass network and its stability increases which means, simultaneously the increase of the chemical durability of the glass. On the other hand above glass transformation temperature (Tg), cesium acts as the strong depolymeriser of network, increasing the tendency for crystallization of the basic glass. It becomes incorporated into the structure of the newly formed crystal phases such as the chemically resistant aluminosilicates (pollucite, nepheline).
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