Investigation of thermal parameters and crystallisation in a ternary CaO-Na2O-P2O5-based glass system

This paper presents the results of a study of the thermal properties of a range of phosphate-based glasses in the system CaO-Na(2)O-P(2)O(5). The glasses had a fixed P(2)O(5) content of 45 mol% and the CaO:Na(2)O ratio was varied. The glasses were produced and ground and their thermal properties measured using a Setaram differential thermal analyser (DTA). Initial data showed that for the high Na(2)O containing glass. NaPO(3) forms and a second unidentified phase that is probably calcium rich. In the middle compositional region. multiple phases precipitated out and were identified as NaPO(3) and Na(4)Ca(PO(3))(6). For the high CaO content glasses. Na(4)Ca(PO(3))(6) was the only phase formed. Further studies were carried out to examine whether the phases were bulk or surface nucleating. by grinding the samples to different particle sizes. From this data, it is evident that the NaPO(3) forms via a bulk nucleation mechanism and that Na(4)Ca(PO(3))(6) forms via surface nucleation. The factors controlling this precipitation process are discussed and ionic radius as a controlling factor is hypothesised. (C) 2001 Elsevier Science Ltd. All rights reserved.