Preparation, Structure And Thermoelectric Properties Of Nanocrystalline Bi₂te₃Bulks

In order to solve the nowadays energy and pollution problems facing human beingcommunity, searching and developing sustainable, low-cost, and environmentally friendlynew energy sources have become one of the most important research focuses.Thermoelectric materials (TMs) have attracted a lot of research interests because TMdevices have advantages of high reliability, lightweight, small size, and no moving part inpower generation and solid-state refrigeration. However, the low conversion efficiency hasset a severe barrier for large-scale commercial applications since the Seebeck effect wasdiscovered in1822. In recent years, low-dimensional TMs have been found promising andexhibit significantly enhanced ZT. However, the high complexity and cost for fabricatinglow-dimensional materials highly limit the applicability of these materials. Therefore,synthesis of TMs in bulk phase with comparably high ZT values using more convenientprocesses scalable to massive production at lower cost is therefore appealing. In thisdissertation, nanocrystalline Bi₂Te₃bulks produced with ambient pressure and highpressure sintering technique are investigated, and good thermoelectric properties aredemonstrated.Thermoelectric Bi₂Te₃nanopowders were synthesized via mechanical alloying ofbismuth and tellurium as starting materials. The reaction from elemental constituents tonanocrystalline Bi₂Te₃compound during mechanical alloying were investigated withX-ray diffraction, field emission scanning electron microscopy, transmission electronmicroscopy, energy dispersive spectroscopy, and differential thermal analysis. Themicrostructure of the ball-milling product was characterized. Results showed that thenanocrystalline Bi₂Te₃compound was formed after40h of milling. Differential thermalanalysis showed that the synthesized phase was stable under melting point.Nanocrystalline Bi₂Te₃with15nm mean-grain size and lamellar structure was obtainedafter100h of milling.Originally, nanocrystalline Bi₂Te₃bulks were fabricated via a nanostructuring methodthat included steps of ball milling, cold pressing, and ambient pressure sintering. In thetemperature range of325-525K, nanocrystalline Bi₂Te₃bulks exhibit good thermoelectric properties with a peak ZT of0.94. The experimental results suggest that the partiallycoherent boundaries allow the maintenance of low grain growth rate and low resistivity.The enhancement of ZT is attributed to a large reduction in the phonon thermalconductivity and a strong suppression of the bipolar effect at high temperatureUsing Bi₂Te₃nanopowders prepared from ball milling of elemental Bi and Te,nanocrystalline Bi₂Te₃bulks were fabricated with high pressure sintering technique undervariable pressures. The structural and thermoelectric properties were characterized,revealing a strong correlation with the sintering pressure. The nanocrystalline Bi₂Te₃bulkfabricated under2GPa exhibits good thermoelectric properties with ZT over0.8in thetemperature range from300to460K and a peak ZT of1.03occurring at403K, which canbe attributed to the small thermal conductivity from enhanced phonon scattering by grainboundaries and defects, as well as to the good electrical property comparable to that of thezone melting material.The defects of the nanocrystalline Bi₂Te₃bulks from high pressure sintering wereinvestigated using positron annihilation technology combined with first-principlescalculations. The positron annihilation lifetime and coincident Doppler broadening spectrameasurements indicate the presence of structural vacancies in the nanocrystalline Bi₂Te₃bulks. The calculation results of the defect formation energy show that the TeBiantisitedefect has the lowest energy and is responsible for the n-type conduction thenanocrystalline Bi₂Te₃bulks. Antisite defects lead to a flat-band defect level at the bandedge, enhancing the density of the states near the Fermi level. The application of pressurefurther induces the peaks of the density of states to shift toward the Fermi level, which isfavorable to the electrical transport properties of intrinsic semiconductors.Bi_0.5Sb_1.5Te₃nanopowders were synthesized via mechanical alloying of Bi, Sb and Teas starting materials, the mean grain size is10nm. Nanocrystalline Bi_0.5Sb_1.5Te₃bulkswere fabricated with high pressure sintering technique. The thermal conductivity wassignificantly reduced and the ZT was remarkably enhanced. The ZT was over1.1in thetemperature range from300to480K and a peak ZT of1.3occurs at360K. High pressuresintering approach thus provides an alternative and effective way for thermoelectricnanocrystalline bulk fabrication.