Thermoelectric Properties of Crystallized Vanadate Glasses Prepared by Using Microwave Irradiation
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Thermoelectric Properties of Crystallized Vanadate Glasses Prepared by Using Microwave Irradiation Takuya Aoyagi, Tadashi Fujieda, Yuichi Sawai, Motoyuki Miyata, Takashi Naito, and Hiroki Yamamoto Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi-shi, Ibaraki-ken 319-1292, Japan ABSTRACT This study examined the crystallization of vanadate glasses by using microwave irradiation. A second aim was comparing the thermoelectric properties of crystallized glasses when using microwave irradiation to conventional heating. V2O5-P2O5-Fe2O3-CuO glasses were prepared by using the melt quenching method. These glasses were irradiated by 2.45-GHz microwaves and heated in an electric furnace. MxV2O5 (M= Cu, Fe x=0.26-055) crystals were selectively precipitated by using the microwave irradiation. The crystal growth was also promoted by it. As a result, precipitation crystals formed a fiber-like structure. The electrical conductivity of the microwave irradiated glass was 6.3×101 S/m at room temperature, which was three times higher than the value of conventionally-heated glass. The Seebeck coefficient of the microwave irradiated glass was -127 ȝV/K at room temperature, which was two times higher than that of conventionally-heated glass. This caused the power factor to be improved about 12 times. These results show that microwave irradiation is a potential candidate for obtaining conductive crystallized vanadate glasses. INTRODUCTION Microwave heating caused by interaction between microwaves and materials has some benefit over conventional heating [1-5]. Numerous observations have been reported in literature on the enhanced mass transport and solid-state reaction rate during microwave heating or processing of ceramics. Crystallization of glass by microwave irradiation is interesting for selective heating, which is a feature of microwave heating. The selective heating for the crystallization of glass could possibly be used to control precipitation crystals and microstructures. However, only limited studies have been carried out on the crystallization of glasses by using microwave irradiation [6, 7]. This results from the weak interaction with microwaves on general silicate glasses. The strength of microwave-material interactions is determined by the dimension of the electric and magnetic losses in the materials [1, 8]. Thus, the glasses that can be heated using microwaves are limited; for example, ion conductive glass and semiconductor glass. Vanadate glasses have semiconductor properties due to the hopping conduction between V4+ and V5+ ions [9, 10]. The electrical conductivity of these glasses is reported to be from 10-6 to10-4 S/m. Therefore, vanadate glasses can be heated and crystallized using microwave irradiation. In this paper, we examined the crystallization of vanadate glasses by using
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microwave irradiation and made a comparison between the thermoelectric properties of crystallized glasses after microwave irradiation and conventional heating. EXPERIMENT Glass was prepared by using a melt quenching met
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