Thermal behavior and the properties of BaO–B₂O₃–P₂O₅ glasses

Abstract

Borophosphate glasses of the BaO–B₂O₃–P₂O₅ ternary system were prepared and studied within the large glass-forming region of the system. Fourteen investigated glass samples of barium borophosphate glasses were discussed for a comparison of the compositional trends in their properties in four compositional series such as A: (100 − x)Ba(PO₃)₂–xB₂O₃, B: 40BaO–yB₂O₃–(60 − y)P₂O₅, C: (50 − z)BaO–zB₂O₃–50P₂O₅, and D: (60 − w)BaO–wB₂O₃–40P₂O₅. Changes in glass density and molar volume primarily depend on the BaO content in the glasses. The thermal properties were studied with a differential thermal analysis, hot-stage microscopy, and dilatometry. Glass transition temperature increases with a rising B₂O₃ content most rapidly in the glass series A and B. It reaches its maximum in these series for the glasses containing 20–30 mol%, where the glass network represents a 3D structure interlinked with P–O–B, P–O–P, and B–O–B bridges. The thermal expansion coefficient decreases, in contrast, with the increasing B₂O₃ content in these ternary glasses as well as their chemical durability. The crystallization of these glasses, in the samples with a low B₂O₃ content, results in the formation of barium metaphosphate BaP₂O₆. Certain crystallized samples also contain a small amount of BPO₄, and diffraction lines of ternary compounds BaBPO₅ and Ba₃B(PO₄)₃ were revealed in a number of samples. A new compound of the composition Ba₅B₂P₈O₂₈ was identified in the crystallized glass 50BaO–10B₂O₃–40P₂O₅.