N- and P-type Nano Bismuth(antimony)tellurium(selenium) Alloy Materials with Enhanced Thermoelectric Figure of Merit and Device Performance

To make thermoelectric device technology competitive in energy harvesting and power generation, it requires both high performance n- and p-type materials through nano-engineering. In contrast to p-type materials, the development of n-type Bi2Te3-based materials has been rather lackluster and consequently limits the device performance.;This work presents both n-type Bi2Te2.7Se 0.3 and p-type Bi0.4Sb1.6Te3 bulk nanocomposite materials with significantly enhanced figures of merit (ZT) through an optimized high-pressure consolidation process of mechanical alloyed powders that maintains a high concentration of nanoscale structures. With electron microscopy, our materials possess a wide distribution of grain sizes with 5 to 20 nm precipitates dispersed throughout. The nanoscale structuring leads to increased Seebeck coefficients and reduced lattice thermal conductivities while maintaining good electrical conductivities over a wide range of temperature.;The combination of these improvements results in a significantly enhanced ZT between 25 °C and 175 °C, with peak ZT of ∼2.5 at ∼100 °C for the best n-type and ∼3 at 50 to 100 °C for the best p-type. Both types of material have peak ZT at similar temperature range around 100 °C which is ideal for coupling them into the thermoelectric device.;Incorporation of these nano-materials into early heat-to-electricity conversion devices is shown to result in a efficiency of 7.9% compared to ∼5.6% in state-of-the-art commercially available devices using non-nano materials, representing about 41% improvement in device efficiency. Furthermore, the conversion efficiencies are 52% better than commercial devices at commonly available lower temperatures of energy harvesting around 50 °C. Thus this work demonstrates an important transition of materials to device technology for real world power generation and cooling applications by waste heat recovery and solar thermal energy.;An optimal hot pressing pressure to obtain the best Seebeck coefficient was found for Bi(Se)Te material, about 2 GPa. This phenomenon could be contributed by the pressure-driven changes in the density of antisite defects and electronic band structure at elevated temperatures. Microstructure analysis as a function of hot pressing pressure are presented and discussed.;From the discoveries of thermoelectric phenomena to their definitions by modern physics, conventional thermoelectric materials have been revived by nano engineering such as quantum confinement and energy filtering effect introduced by low-dimensional structure. Their roles are discussed via mathematically formulated thermoelectric properties to better understand their inter-relationships and the challenge and approach in the ZT enhancement.