Studying effect of MoO₃ on elastic and crystallization behavior of lithium diborate glasses

Abstract

The effect of MoO₃ addition on the crystallization characteristics of 2Al₂O₃–23Li₂O–(75 − x) B₂O₃ glass (where x MoO₃ = 0, 10, 20, and 40 mol %) has been investigated. The compositional dependence of the glass transition (T _g), and crystallization (T _c) temperatures was determined by the differential thermal analysis (DTA). It was found that both the T _g and T _c decrease with increasing MoO₃ content. The amorphous nature of the as-quenched glass and crystallinity of the produced glass–ceramics were confirmed by X-ray powder diffraction (XRD) analysis. Glass–ceramics embedded with diomignite (lithium diborate, Li₂B₄O₇) were produced from all investigated glasses by heat-treating the as-quenched glasses at the appropriate temperatures obtained from the DTA traces. Addition of MoO₃ to the glass composition at 10% MoO₃, causes the formation of lithium molybdenum oxide (Li₄MoO₅) crystalline phase in addition to the diomignite phase. Increasing MoO₃ content to 20% causes a phase transformation of lithium molybdenum oxide from the (Li₄MoO₅) to the (Li₂MoO₄) phase and the formation of another lithium borate (Li₄B₂O₅) phase in addition to the diomignite. Further increase of MoO₃ content to 40% results in another phase transformation to the lithium aluminum molybdenum oxide [LiAl(MoO₄)₂], and, in this case, the molybdenum content was excess enough to crystallize the molybdate (MoO₃) itself. Scanning electron microscopy (SEM) was used to characterize the morphology and microstructure of the formed solid solution phases. The values of the T _g decrease with increasing the MoO₃ content. The ultrasonic wave velocities and elastic moduli were determined using the pulse-echo method. Both velocities (v _L and v _T) were increased as the MoO₃ content, this increase can be attributed to the higher bond strength of Mo–O (607 kJ mol⁻¹) than that of B–O (392 kJ mol⁻¹).