Self-powdering phenomenon of β′-RE₂(MoO₄)₃ formed in crystallization of glasses and its mechanism (RE: Gd, Sm, Dy)

抄録

Some glasses in the RE₂O₃–MoO₃–B₂O₃ system (RE: Gd, Sm, Dy) were prepared by using a conventional melt quenching method, and the self-powdering phenomenon of β′-Gd₂(MoO₄)₃ crystals, i.e., the breaking into small pieces during the crystal growth, was examined using polarized optical microscope observations in a heating stage. In Gd₂O₃–MoO₃–B₂O₃ glasses with Gd₂O₃/MoO₃ = 1/3 and with different B₂O₃ contents (15–30 mol %) such as 20Gd₂O₃–60MoO₃–20B₂O₃, β′-Gd₂(MoO₄)₃ crystals were formed and the self-powdering phenomenon was observed irrespective of B₂O₃ content. The crack formation radiating out from the center part was clearly observed in the inside of crystals (diameter >40 µm) formed at 570°C in the glasses with Gd₂O₃ or Dy₂O₃. But, any clear crack formation was not observed in the glass with Sm₂O₃. It was found that the base glasses have larger densities at room temperature compared with β′-RE₂(MoO₄)₃ crystals formed, which is a unique feature in the RE₂O₃–MoO₃–B₂O₃ system compared with the crystallization of other glass systems. Such differences in the density might induce the accumulation of extremely large stresses in the inside of crystals, eventually causing the breaking of crystals.