The phase equilibrium of binary MgO- and CaO-phosphate glasses

Abstract

In previous investigations on phosphate glasses containing oxides of magnesium and calcium, a region of very stable glasses was found [1-3] having interesting applications for bioactive glass ceramic materials [4] and glass-to-metal seals [5]. Physical and mechanical properties have also been discussed with respect to composition [6, 7]. The properties which exhibit an anomalous behaviour can be best interpreted by considering the structure of glasses which are dominated by the P O M g crosslink density involving the Mg 2+ in the glass [8]. Also, the coordination number of Mg 2+ is 4~ in contrast with 6 for Ca 2+, Sr 2+ and Ba 2+ [9]. However, to date, no attempt has been made to relate these properties with the thermal behaviour of the glass. This is important since the data can be used to interpret and forecast the properties of these materials. In ~he present study, calcium and magnesium oxides have been used to form the binary system of phosphates with the aim of building up their phase equilibrium diagram. For both systems, the thermal variations and properties are examined and discussed with respect to the compositional changes. Glasses of composition ( 1 x)MO-xP205 (M = Mg, Ca) were prepared using a conventional melt quenching technique. Laboratory grade analar with purity beyond 99.5% of MgO, CaCO3 and P205 oxides were used as starting materials. A calculated tool% of oxides was well mixed in an alumina crucible before being heated in an electric furnace for 1 h at 1200 °C in air. The melts were periodically stirred using an alumina rod to ensure homogenity. After the required viscocity was obtained, the melts were quenched between two brass plates, followed by annealing at 250 °C for 1 h and then allowed to cool to room temperature at an initial cooling rate of 5 °Cmin -~ . About 10 mg of a relatively fine powder sample was subjected to differential thermal analysis (DTA) using a Perkin Elmer DTA 7. The powder was put in an alumina sample holder with an alumina powder being used as a reference. A heating rate of 10°Cmin -1 was used throughout the experiments. For the c6mpositions which did not form glass a thermal analysis was conducted directly on a proportional well-mixed mixture of oxides and P205 powder. This is necessary since the data can be used as a compliment to form the phase equilibrium diagram. An X-ray phase analysis was also conducted on several preheated samples using a Phillips High Angle Goniometer PW 1319 with Cukc~ radiation and a nickel filter. The sample density was measured by an Archimedes method using toulene as immersion liquid. A series of glasses were made successfully and Table I shows the glass formation range which has been obtained. It is found that glass containing MgO can be formed only in the range 0.4 ~< x ~< 0.8, and glasses containing CaO, in the range 0.4 ~< x ~< 0.7. For higher x values, the melts do not form glass while for lower x values, the melts totally crystallized even during the quenching process. Figure 1 shows the DTA traces of glasses in the MgO P205 system and several endothermic and exothermic effects are observed on the thermograms. The DTA traces of glasses in the CaO-P205 system are shown in Fig. 2. There are two types of endothermic effect; one which change insignificantly with composition (at 700 °C to 730 °C for MgOP205 and 470 °C to 500 °C for CaO-P2Os) and corresponds to the temperature of peritectic deco~nposition of phosphates, and the other which changes more significantly with variation of composition and corresponds to the liquidus temperature. The exothermic effect which corresponds to the solid state reaction or crystallization of phosphates can also be seen around 280-495 -45 °C for MgO-P20~ (due to the formation of MgP206) and 280467 + 5 °C for the CaO-PzOs system (presumably due to the formation of Ca3(PO4)2.