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Phase formation of non- and unidirectional solidification of Ba8GaxSn30 solutions
Ba8Ga16Sn30 clathrate compound
vertical solution Bridgman method
ternary phase diagram
In this study, the vertical Ga solution Bridgman method was used for attempt to prepare the P-type Ba8Ga16Sn30 clathrate. First, the Ga flux ratio of solutions was adjusted, and the liquidus temperature of each solution ratio was measured by the cooling curve method. As the proportion of Ga ratio increases, the liquidus temperature decreases. From the metallographic result of the crystal prepared by the non-directional solidification, the γ phase Ba(Ga/Sn)4 in the sample was surrounded by Ba8Ga16Sn30 clathrate. It reveals that a peritectic reaction occurs. A crystal shape of lameller growth was observed therefrom. The proportion of γ phase in the samples increases with the increases of Ga ratio. The measured phase change temperature and composition ratio from EDX analysis can be plotted in the Ba-Ga-Sn ternary phase diagram. The α-β phase transition of Ba8Ga16Sn30 was confirmed by the XRD reasults. In the liquidus temperature measurement experiment, the γ phase formation temperature, the Ba8Ga16Sn30 generation temperature, and the α-β phase change temperature were measured. From the results of first stage, the Ba8-Ga20-Sn30 solution with the least γ phase was used to prepare samples with larger crystals of P-type Ba8Ga16Sn30 clathrate by the vertical solution Bridgman method. From the metallographic result of the samples prepared by the unidirectional solidification, although the addition of the excess Ga solution results in a decrease of the grain size, the γ phase was successfully separated from Ba8Ga16Sn30. The sample prepared under the condition of larger temperature gradient has a smaller grain size. The unidirectional solidification under the condition of smaller temperature gradient can separate the γ phase more effectively and the sample has a larger grain size.
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