Study On Preparation And Thermoelectric Properties Of Sic-based Materials

Thermoelectric material is a kind of new function materials which could converse between thermal and electric energy. It could be widely used in thermoelectric generation and thermoelectric refrigeration fields. SiC have many advantages such as good chemical stability, high hardness and high temperature oxidation resistance et al. It would become the most promising high-temperature thermoelectric material as one of the most important wide band-gap semiconductors.In this paper, SiC was selected as matrix phase of thermoelectric material with Ni or B4C as adulterant. The compound powders were mixed through ball milling, sol-gel method and coated method respectively. Then the condensed SiC-based thermoelectric materials were prepared respectively by conventional sintering and hot pressed sintering method.The phase composition, microstructure and thermal behavior were studied by XRD, SEM and DTA-TG. The physical properties and the performance of thermoelectric were investigated. The testing temperature was from 100 to 600℃.The mainly results were shown as the followings:(1)SiC powders were mixed with nickel powders and sintering aids by ball milling. After dry pressing, the greens were sintered at 1500℃through conventional sintering method to form n-type semiconductor. As the amount of Ni increasing, the density, hardness and electrical conductivity of the samples increased, with the Seebeck coefficient decreased.The electrical conductivity of the samples increased with the sintering temperature increasing. The Seebeck coefficient first increased and then decreased with the sintering temperature increasing. Maximum power factor of the samples is 0.02×10-6 Wm-1K-2 at about 500℃.(2) SiO2 was coated on B4C powders by sol-gel method. Tetraethylorthosilicate (TEOS)and sugar were used as the source material for SiO2 and C respectively. The SiC-B4C composites were sintered through reactive hot processing at 1500℃and 1600℃.N-type semiconductor character was indicated with the continuous structure of SiC phase forming. Lath-shaped SiC were detected, and then were connected with the sintering temperature increasing. As the amount of B4C increasing, the hardness increased, the density first increased and then decreased.As the testing temperature was above 400℃, the electric conductivity first decreased and then increased, the Seebeck coefficient first increased and then decreased. This indicated that the carrier concentration and mobility change. Maximum hardness is about 28.13 GPa for the sample with 50 wt.% B4C sintered at 1600℃.Maximum power factor of the samples is 3.84×10-4 Wm-1K-2 at about 440℃.(3)Polyvinyl alcohol (PVA) was coated on B4C powders by liquid coating technology. Si powders were added to above mixed powders, then SiC-B4C composites were prepared through reactive hot processing at 1400℃.P-type semiconductor character was indicated as the continuous structure of B4C phase forming. The fine dispersion SiC particles were scattered among the B4C particles. As the amount of B4C increasing, the hardness increased, the density first increased and then decreased. As the sintering temperature increasing, the Seebeck coefficient first increased and then decreased. The electric conductivity also had descending phenomenon. Maximum power factor of the samples reached to 0.13×10-4 Wm-1K-2 at about 500℃.