Features of Solidification of the 2Bi2O3•3GeO2 Melt under Different Cooling Conditions

The influence of heat treatment of the melt (composition 40 mol % Bi2O3 and 60 mol % GeO2), temperature of the beginning of its cooling and cooling rate on the phase composition, macro- and microstructure of the solid phases formed during crystallization were studied. The role of platinum in the formation of a metastable state of the melt and, as a consequence, the formation of metastable phases during crystallization is also considered. Using X-ray phase analysis, optical and scanning electron microscopy, it was found that the main factors determining the formation of the phase composition of crystallization products on the composition of 40 : 60 mol % (Bi2O3 : GeO2) are the preliminary heat treatment of the melt, which provides the most complete implementation of the metastable equilibrium in the melt (thermodynamic factor), and its interaction with solid platinum oxides on the walls and bottom of the crucible (kinetic factor). It is shown that at slow cooling rates (with a furnace) the melt crystallizes with the formation of a mixture of phases consisting of metastable Bi2GeO5 and eutectoid (Bi2GeO5 + alpha-GeO2). In this case, the decomposition of this metastable mixture is possible with a partial or complete transition to a stable state with the formation of Bi4Ge3O12. It was found that high-speed cooling of the melt is accompanied by the formation of either glass (quenching with a crucible in water) or a mixture of metastable Bi2GeO5 and an amorphous component.

Automated summary

The influence of heat treatment, cooling temperature, and cooling rate on the phase composition and structure of solid phases during crystallization was studied. The role of platinum in the formation of metastable states was also considered. It was found that the heat treatment and interaction with platinum oxides are crucial factors in determining the phase composition. The cooling rate affects the formation of different phases or glass.