Je-Hyeong Bahk

Flexible thermoelectric materials and device optimization for wearable energy harvesting

In this paper, we review recent advances in the development of flexible thermoelectric materials and devices for wearable human body-heat energy harvesting applications. We identify various emerging applications such as specialized medical sensors where wearable thermoelectric generators can have advantages over other energy sources. To meet the performance requirements for these applications, we provide detailed design guidelines regarding the properties of the material, device dimensions, and gap fillers by performing realistic device simulations with important parasitic losses taken into account. For this, we review recently emerging flexible thermoelectric materials suited for wearable applications, such as polymer-based materials and screen-printed paste-type inorganic materials. A few examples among these materials are selected for thermoelectric device simulations in order to find optimal design parameters for wearable applications. Finally we discuss the feasibility of scalable and cost-effective manufacturing of thermoelectric energy harvesting devices with desired dimensions.

Effect of nanoparticle scattering on thermoelectric power factor

The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.

High efficiency semimetal/semiconductor nanocomposite thermoelectric materials

Rare-earth impurities in III–V semiconductors are known to self-assemble into semimetallic nanoparticles which have been shown to reduce lattice thermal conductivity without harming electronic properties. Here, we show that adjusting the band alignment between ErAs and In0.53Ga0.47−XAlXAs allows energy-dependent scattering of carriers that can be used to increase thermoelectric power factor. Films of various Al concentrations were grown by molecular beam epitaxy, and thermoelectric properties were characterized. We observe concurrent increases in electrical conductivity and Seebeck coefficient with increasing temperatures, demonstrating energy-dependent scattering. We report the first simultaneous power factor enhancement and thermal conductivity reduction in a nanoparticle-based system, resulting in a high figure of merit, ZT=1.33 at 800 K.

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

Significance PbTe is known to be a promising thermoelectric material for waste heat recovery, so it has been the subject of extensive research involving new approaches. It is important to note that the performances of these developed materials can depend on the material synthesis conditions. We investigated three different routes of synthesizing 2% Na-doped PbTe and found that its thermoelectric figure of merit, zT, can be enhanced to ∼2.0 at 773 K. Also, the mechanical hardness of the sample synthesized by this condition was nearly double than that of the other samples. Our study shows that the size of nano- and microstructures can vary significantly by the choice of synthesis methods, which can explain the variation in zTs and mechanical hardness. 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 ∼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∼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 ∼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.

Enhancing the thermoelectric figure of merit through the reduction of bipolar thermal conductivity with heterostructure barriers

In this paper, we present theoretically that the thermoelectric figure of merit for a semiconductor material with a small band gap can be significantly enhanced near the intrinsic doping regime at high temperatures via the suppression of bipolar thermal conductivity when the minority carriers are selectively blocked by heterostructure barriers. This scheme is particularly effective in nanostructured materials where the lattice thermal conductivity is lowered by increased phonon scatterings at the boundaries, so that the electronic thermal conductivity including the bipolar term is limiting the figure of merit zT. We show that zT can be enhanced to above 3 for p-type PbTe, and above 2 for n-type PbTe at 900 K with minority carrier blocking, when the lattice thermal conductivity is as low as 0.3 W/m K.

Thermoelectric power generator module of 16×16 Bi2Te3 and 0.6% ErAs:(InGaAs)1−x(InAlAs)x segmented elements

We report the fabrication and characterization of thermoelectric power generator modules of 16×16 segmented elements consisting of 0.8 mm thick Bi2Te3 and 50 μm thick ErAs:(InGaAs)1−x(InAlAs)x with 0.6% ErAs by volume. An output power up to 6.3 W was measured when the heat source temperature was at 610 K. The thermoelectric properties of (InGaAs)1−x(InAlAs)x were characterized from 300 up to 830 K. The finite element modeling shows that the performance of the generator modules can further be enhanced by improving the thermoelectric properties of the element materials, and reducing the electrical and thermal parasitic losses.

Evaluating Broader Impacts of Nanoscale Thermal Transport Research

The past two decades have witnessed the emergence and rapid growth of the research field of nanoscale thermal transport. Much of the work in this field has been fundamental studies that have explored the mechanisms of heat transport in nanoscale films, wires, particles, interfaces, and channels. However, in recent years there has been an increasing emphasis on utilizing the fundamental knowledge gained toward understanding and improving device and system performances. In this opinion article, an attempt is made to provide an evaluation of the existing and potential impacts of the basic research efforts in this field on the developments of the heat transfer discipline, workforce, and a number of technologies, including heat-assisted magnetic recording, phase change memories, thermal management of microelectronics, thermoelectric energy conversion, thermal energy storage, building and vehicle heating and cooling, manufacturing, and biomedical devices. The goal is to identify successful examples, significant challenges, and potential opportunities where thermal science research in nanoscale has been or will be a game changer.

Hot carrier filtering in solution processed heterostructures: a paradigm for improving thermoelectric efficiency.

An approach based on a solution-based synthesis that produces a thermally stable Ag/oxide/S₂ Te₃ -Te metal-semiconductor heterostructure is described. With this approach, a figure of merit of zT = 1.0 at 460 K is achieved, a record for a heterostructured material made using wet chemistry. Combining experiments and theory shows that the large increase in the materials Seebeck coefficient results from hot carrier filtering.

Structure and Thermoelectric Properties of Spark Plasma Sintered Ultrathin PbTe Nanowires

Solution-synthesized thermoelectric nanostructured materials have the potential to have lower cost and higher performance than materials synthesized by solid-state methods. Herein we present the synthesis of ultrathin PbTe nanowires, which are compressed by spark plasma sintering at various temperatures in the range of 405-500 °C. The resulting discs possess grains with sizes of 5-30 μm as well as grains with sizes on the order of the original 12 nm diameter PbTe nanowires. This micro- and nanostructure leads to a significantly reduced thermal conductivity compared to bulk PbTe. Careful electron transport analysis shows suppressed electrical conductivity due to increased short-range and ionized defect scatterings, while the Seebeck coefficient remains comparable to the bulk value. The PbTe nanowire samples are found unintentionally p-type doped to hole concentrations of 2.16-2.59 × 10(18) cm(-3). The maximum figure of merit achieved in the unintentionally doped spark plasma sintered PbTe nanowires is 0.33 at 350 K, which is among the highest reported for unintentionally doped PbTe at low temperatures.

ErAs:(InGaAs)1−x(InAlAs)x alloy power generator modules

We report a wafer scale approach for the fabrication of 400 element power generator modules composed of 200 n-type ErAs:(InGaAs)0.8(InAlAs)0.2 and 200 p-type ErAs:InGaAs alloy thermoelectric elements. The thermoelectric properties of the materials were characterized. Two sets of generator modules with the element thicknesses of 20 and 10μm, respectively, were fabricated. The 20μm module had an output power density of 2.5W∕cm2 and 3.5V open circuit voltages, and the 10μm generator modules had an output power density of 1.12W∕cm2 and open circuit voltage of 2.1V. The performance of thermoelectric generator modules can further be improved by increasing the thicknesses of the elements and reducing the electrical and thermal parasitic resistances.