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In this study, the n-type Ba8Ga16Sn30 (BGS) type-VIII clathrate was prepared by the horizontal solution Bridgeman method. A sol-gel method is employed to prepare SiO2 layer. The sol was coated on the surface of BGS crystals by dip-coating and it followed by calcining at 300˚C in argon to form SiO2 layer. The appearances before and after the oxidation and the weight increase percentage per unit area after oxidation of BGS crystals with SiO2 layers via coating for 2, 4, 5, 7, and 10 times and oxidized at 250˚C in air for the same time were compared. The results showed that the BGS crystals with SiO2 layer via coating for 5 times exhibited the best antioxidant effect. Subsequently, the BGS crystals with SiO2 layer via coating for 5 times were subjected to oxidation treatment in air at 250˚C for 2, 4, 8, and 16 hours. It is confirmed from XRD that the samples were still the type-VIII clathrate after oxidation. The interfaces between the BGS crystal, oxide layer, and SiO2 layer were observed by SEM, and the compositions of the different regions were analyzed by EDS. In the samples after 2 and 4 hours oxidation, the three interfaces were still observed. But in the samples after 8 and 16 hours oxidation, the SiO2 layer was difficult to be observed. After oxidation, the samples showed a decrease in Ga content, an increase in Sn content, and a decreased Ga/Ba ratio. Before and after oxidation, the thermoelectric and carrier properties of SiO2-coated samples were measured. The samples oxidized at 250˚C in air for 2 and 8 hours showed no significant changes in the zT value. The sample oxidized for 4 hours showed an increase in the zT value from 0.81 to 0.97. However, the zT value of the sample oxidized for 16 hours decreased from 1.20 to 0.58.
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