The liquid-liquid phase separation process in the stable immiscibility regions of binary borate melts was directly observed by the hot-thermocouple method and was investigated using phase-separated glasses obtained by the hot-thermocouple and crucible methods. Furthermore, to describe the degree of progress of the phase separation state quantitatively, the phase separation coefficient was defined, and factors influencing the phase separation process were discussed by means of Stokes' law. The liquid-liquid phase separation process progresses owing to the coarsening of particles by connection and coalescence and the traveling process of coarser particles to the interface of the two-liquid phase. The coarsening process of particles was controlled by the volume fraction Vf of particles and the interfacial tension γA/B between the particles and the dispersed phase, and the growth rate α of particles was decided by the cohesive coefficient Vf · γA/B. The traveling process of particles was examined using Stokes' law, and the phase separation coefficient Ω representing the degree of progress of the phase separation state was estimated by using the progressive coefficient of the phase separation (UH and Uc) defined by Stokes' law.
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