Preparations And Properties Of Hot-Pressed Bi₂Te₃ Based Nano-Composite Thermoelectric Materials

Thermoelectric (TE) materials are functional materials which can convert energy between electricity and heat. The devices made by TE materials are applicable to refrigeration and power generation with the advantages in low-weight, non-noise, non-pollution, non-motion part and non-maintenance.In the present work, Bi2Te3 based thermoelectric materials were prepared by hot uniaxial pressing (HUP) the powders from the commercial zone-melted (ZM) ingots. Bi2Te3 based nano-composites were prepared by HUP from 90wt% zone-melted alloy powders and 10wt% hydrothermally synthesized nanopowders. TE modules were made using ZM ingots and various HUP materials. The properties and microstructures of the materials have been investigated with mechanical, electrical and thermal measurements as well as XRD and SEM analyses. The performance of the TE modules were also investigated.It was found that relative densities over 97% of the pristine ZM blocks can be obtained by HUP with 50MPa / 350℃ / 30min from the milled powders. All HUP samples have a shear strength above 21MPa, which is about four times higher than that of a ZM Bi2Te3 based alloy along the (001) cleavage plane. It was found that the electric properties of the HUP samples are lower than those of the corresponding ZM samples, which is considered being originated from the change of the effective carrier density during the HUP process. It was also found that the temperature corresponding tO3the best TE properties decreases for the p-type HUP samples while increases for the n-type HUP samples in comparison to the ZM samples. TE property anisotropy was also found with a higher figure of merit along the base plane than that vertical to the base plane of the HUP samples.Microscopic Seebeck coefficient distribution analyses show a difference on Seebeck coefficient up to 40-50 V-K"1 at various measuring locations on the same sample, and there is no visible relationship between the microscopic Seebeck coefficient distribution and the local materials features, such as surface morphology, alloying and doping element distributions. For the directionally solidified sample, periodic variation of the Seebeck coefficient was found experimentally, which has the feature originated from the disturbance by external field during solidification of the material.An improvement of about 15% on power factors can be achieved by adding 15wt% hydrothermally synthesized BijTe3 nanopowders in the nano-composite samples. The thermal conductivity of nano-composites is only about 40% of that of the ZM sample with the same chemical composition. The dimensionless figures of merit, ZT, of the nano-composite samples are comparable to that of a ZM sample near the room temperature, but are about twice higher than that of ZM sample above 400K.