Preparation And Properties Of Bi₂Te₃ Based Bulk Nanosized Thermoelectric Materials

Thermoelectric (TE) materials can realize coupling and conversion between heat and electric energy. The devices made of TE materials are applicable for refrigeration and power generation with advantages of low weight, no noise, no pollution and no moving parts. Bi₂Te₃ alloys with good chemical stability and the highest dimensionless figure of merit close to unity are known as the best TE materials near room temperature.In the present work, Bi₂Te₃ based bulk TE materials were prepared by hot uniaxial pressing (HUP). The effect of hot-pressing parameters on microstructure and TE properties of bulk materials was investigated for process optimization. Addition of homogeneous nanopowders was also attempted to further improve TE properties of materials. Hydrothermal synthesis of Bio.5Sb1.5Te3 ternary compounds was studied by XRD, SEM and electric property measurement. Abstracts are as follows:Bi₂Te₃ nanopowders hydrothermally synthesized with Bi(NO₃)₃ and Te powders as precursors, NaBH₄ as reductive, and NaOH as alkaline regulator were hot pressed into bulk. The effects of both hot-pressing time and temperature on microstructure and electric property of materials were investigated. The XRD pattern showed that bulk materials were of single phase of Bi₂Te₃. Dimension measurement indicated that density would increase with the increase of either time or temperature. When relative density was below 80%, the electric conductivity and Seebeck coefficient were both quite low, so was power factor. The sample hot pressed at 250℃ for 30 min had the best performance.Bulk materials were obtained from nano- and micropowders on the same hot pressing conditions, of which the electric transport properties were compared. Results indicated that power factor of bulk nanomaterial with high electric conductivity and low absolute value of Seebeck coefficient was more than bulk micromaterial by 15%.Comparison of electric transport property was done between mixed powders and powders ground (GP) from Bi₂Te₃ based zone-melted ingot. It was found that power factor of n-type nanocomposite was 19% higher than that of GP while power factor of p-type nanocomposite was a little lower. Power factor of the p-type sample blended with micropowders and nanocapsules was much low due to the n-type conduction of blended powders. Power factor of the n-type sample blended with micropowders was 28% higher than that of the n-type GP, while power factor of the n-type sample blended with nanocapsules was slightly lower than that of the n-type GP.The impact of adding order, reaction temperature and reaction medium on product synthesis was investigated and the growth mechanism of nanoparticles was presented. Also, Bi_0.5Sb_1.5Te₃ was prepared by vacuum melting method for comparison of electric transport property with that prepared by hot press.BiSbTe₃ nanopowders were hydrothermally synthesized with BiCl₃, SbCl₃ and Te powders as precursors. Due to low relative density the sample hot pressed had a small valueof power factor. Hydrothermal synthesis was also applied to preparation of Bi₂(Te, Se)₃ with BiCl₃, Te powders and Se powders as precursors. The XRD result showed that the product was the phase of Bi₄Te₃ which did not correspond to the designed.