Right sizes of nano- and microstructures for high-performance and rigid bulk thermoelectrics

In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to similar to 2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2 similar to 6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to similar to 0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano-and microstructures of the material

Mat-like flexible thermoelectric system based on rigid inorganic bulk materials

This paper reports on a mat-like flexible thermoelectric system (FTES) based on rigid inorganic bulk materials, i.e. Bi–Te compounds. Inorganic bulk materials exhibit higher thermoelectric performance and can create a larger temperature drop due to their considerable height compared with organics and printable inorganics, meaning the FTES can produce an impressive power output. We show that the FTES, wherein both a thermoelectric module and a heat sink are integrated, is flexible enough to be adapted to any irregularly shaped surface. In the FTES, p- and n-type legs composed of a thermoelectric module are placed inside holders, which are connected to one another using flexible wires. Powered by a portable battery, the FTES was used to refrigerate human skin. As a result, a temperature drop of approximately 4 K was experimentally demonstrated, which humans felt as ‘cold’ or ‘very cold’, based on analysis. This indicates the feasibility of using the proposed FTES to control the temperature of the human body, even when using a portable battery. This was also applied to body heat harvesting. The FTES generated approximately 88 µW of power, which is sufficient to operate most wearable and/or implantable sensors. Our analysis based on human thermoregulatory modeling indicates that both refrigeration and power generation capacity can be further enhanced by improving the thermal contact between the FTES and human skin. The FTES shows potential for wearable refrigeration and body heat harvesting.</p

More than half reduction in price per watt of thermoelectric device without increasing the thermoelectric figure of merit of materials

In a power generation system, the price per watt ($/W) is an important parameter to be considered for checking the feasibility for practical implementation. In this paper, we experimentally demonstrate that$/W of a thermoelectric device can be reduced to around 60%. The conventional approach to reducing $/W in thermoelectrics is to enhance the thermoelectric figure of merit (zT) of the thermoelectric materials used, which can increase the power output (W). We propose that$/W can be reduced by lowering the material consumption ($) with a slight sacrifice in power output by changing the device architecture. A simple calculation suggests that zT ∼ 6 is needed for such a reduction in$/W based on the conventional approach. This method can be accompanied by a search for high-zT material so that further reduction in $/W can be achieved with efficient thermoelectric materials.</p

Ultralow Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in SnTe:Ga Materials

International audienceUltralow thermal conductivity is of great interest in a variety of fields, including thermoelectric energy conversion. We report, for the first time, experimental evidence that Ga-doping in SnTe may lower the lattice thermal conduction slightly below the theoretical amorphous minimum at high temperature. Such an effect is justified by the spontaneous formation of nanoprecipitates we characterized as GaTe. Remarkably, the introduction of Ga (2-10%) in SnTe also improves the electronic transport properties by activating several hole pockets in the multivalley valence band. Experimental results are supported by density functional theory calculations. The thermoelectric figure of merit, ZT, reaches similar to 1 at 873 K in Sn0.96Ga0.07Te, which corresponds to an similar to 80% improvement with respect to pure SnTe

High Thermoelectric Performance of a Heterogeneous PbTe Nanocomposite

In this paper, we propose a heterogeneous material for bulk thermoelectrics. By varying the quenching time of Na doped PbTe, followed by hot pressing, we synthesized heterogeneous nanocomposites, a mixture of nanodot nanocomposites and nanograined nanocomposites. It is well-known that by putting excess amounts of Na (i.e., exceeding the solubility limit) into PbTe, nanodots with sizes as small as a few nanometers can be formed. Nanograined regions with an average grain size of ca. 10 nm are observed only in materials synthesized with an extremely low quenching rate, which was achieved by using a quenching media of iced salt water and cold water. Dimensionless thermoelectric figures of merit, <i>zT</i>, of those heterogeneous nanocomposites exhibited a <i>zT</i> around 2.0 at 773 K, which is a 25% increase compared to <i>zT</i> of a homogeneous nanodot nanocomposite with the largest quenching time in our experiment, i.e. furnace cooled. The power factor increase is 5%, and the thermal conductivity reduction is 15%; thus, <i>zT</i> increase mainly comes from the thermal conductivity reduction