The higher TEC in this glass as compared to other glasses can be associated with the higher ionic radii of La3+. After heat treatment when it turns into glass ceramic the TEC value increases, which is due to the formation of crystalline phases which improve the TEC of this glass. BaY glass does not show much variation in TEC with heat treatment as very limited amount of crystallization takes place in this glass which indicates that glass matrix has undergone very little structural rearrangement. The formation of hexacelsian phase, which has a relatively higher TEC value of ∼8 × 10−6 K−1[28] greatly improves the TEC of BaAl glass after heat treatment. The change in TEC after heat treatment of this glass is observed around 10%. In case of BaCr the as prepared glass was already in crystalline form and its TEC has low value of 6.8 × 10−6 K−1. However, after heat treatment for 10 h the crystallization of BaSi2O5 in the glass matrix increased the TEC value (7.6 × 10−6 K−1). With further increase in volume fraction of BaSi2O5 phase due to 100 h heat treatment the TEC value again increased to 8.1 × 10−6 K−1. In this sample, formation of BaSi2O5 phase might be having high TEC which leads to higher % change in TEC. It might also be attributed that crystalline phase (BaSi2O5) do not exhibit the intermediate cations which may be responsible for low change in TEC due to small ionic radii as compared to the Ba2+ (1.49 Å).
Table 2.