Results
3.1. Thermal analysis
DTA studies were performed on as-quenched amorphous glass samples. Fig. 1(a) shows DTA traces of BaPF glasses with different CaF2 contents. All the samples exhibited an endotherm corresponding to their glass transition temperature and two exotherms indicating their crystallization temperatures followed by another endotherm corresponding to the re-melting of glasses. From DTA traces, the values of glass transition temperature, Tg (onset of glass transition region), onset crystallization temperature, Tx (onset of first crystallization peak) and melting temperature, Tm (minimum of endotherm corresponding to the re-melting of glasses) were obtained as shown in Fig. 1(b) and tabulated in Table 1. Both glass transition temperature and onset crystallization temperature of BaPF glasses increased with the increase in CaF2 content. However, the glasses with higher CaF2 content depict lower melting temperature.
The quantity Tx − Tg, which indicates the thermal stability of glasses against crystallization, and Hruby's parameter (Kgl) specifying the glass forming ability given by (Tx − Tg) / (Tm − Tx) were calculated from the obtained thermal parameters and given in Table 1. It can be observed that both Tx − Tg and Hruby's parameter (Kgl) increased with the increase in CaF2 content in the glass batch, suggesting that incorporation of more CaF2 content into the barium phosphate glass network leads to better thermal stability against crystallization and improvement in glass forming ability [7], [11] and [12]. In Fig. 1(a), it can be noted that the area of exothermic peak (Tp2), which is proportional to the heat of crystallization decreases sharply above 4 mol% of the CaF2 content. This result further confirms that the addition of CaF2 above 4 mol% into the barium phosphate glass batch results in substantial improvement in thermal stability against crystallization [13].
Fig. 2 shows the X-ray diffraction patterns of BaPF0 and BaPF10 glasses heat treated at 600 °C for 1 h.