Strategies for Manipulating Phonon Transport in Solids

Thermal transport is one of the most important physical phenomena in both scientific and engineering fields. As the modulation of electrical properties such as electrical conductivity has driven the era of semiconductor, thermal properties will be also important in understanding physics in solids and thermally tunable materials in applications. We reviewed up-to-date strategies to manipulate thermal transport in solids in the respect of the phonon transport that have diffusive or ballistic behaviors. We hope this review can bring meaningful insights to the researchers in the field of phonon transport in solids.

ACS Nano 15, 2182 (2021)

Effect of Phonon Confinement on the Thermal Conductivity of In0.53Ga0.47As Nanofilms

Understanding thermal transport in solids has been based on the phonon behavior. From this point of view, designing nano-scale materials can modulate their thermal conductivities. For example, nanofilm with nanometer-scale thickness has lower thermal conductivity then that of the same bulk material because the mean free path of phonon is effectively reduced by its thickness. In0.53Ga0.47As is a good candidate to analyze the thickness effect to the thermal conductivity since it is already very low thermal conductivity by the alloy scattering. In-plane thermal conductivities of In0.53Ga0.47As with various thicknesses are measured by T-bridge method, and also evaluated by the ab-initio calculation. The results show that such thin nanofilms with thickness below 20 nm have the phonon confinement effect where the phonon dispersion relation changes by the reduction of the interatomic force due to its thin structure.

J. Appl. Phys. 123, 245103 (2018)

Self-charging Wearables for Continuous Health Monitoring

Remarkable advances in wearable electronics have brought numerous multi-functionalities in health monitoring, but demanded more power, necessitating larger batteries and frequent recharging. Replacement or recharging of batteries, however, poses undesirable downtime in health monitoring. Thermoelectrics are promising in sustainably supplying power by converting body heat but wearable thermoelectrics have not been capable of producing power large or stable enough for the continuous operation of commercial health monitoring sensors. Here synergistic integration of a wearable thermoelectric generator (WTEG) and an emerging Li-S battery has delivered power sustainably and continuously, overcoming the biggest hurdle in utilizing thermoelectrics for wearable electronics in practice. The major drawback of low thermoelectric output voltage for charging batteries has been greatly alleviated with the high-performance Li-S battery whose charging voltage is only a half those of Li-ion batteries. The WTEG continuously produces large power up to 378 µW, operating a commercial glucose sensor (64 µW) and storing the remainder in the Li-S batteries for providing a stable voltage of 2 V even under large fluctuations in power supply and demand. This work demonstrates feasibility of operating a commercial glucose sensor only with body heat for the first time, to our best knowledge, engendering sustainable operation of wearables without interruption and tedious recharging/replacement of batteries.

Nano Energy 79, 105419 (2021)

Radiative Cooling Materials and Establishing Evaluation Method

Radiative cooling is a way of cooling with radiative heat transfer, using low temperature (3K) space as a free heat sink. With high reflectivity in the UV/NIR range and high emissivity in the IR range (especially in 8~13 µm), radiative cooling material can cool itself below the ambient temperature. Since it does not require any external energy input, it is expected to reduce global energy consumption. Our research is mainly focused on measuring the radiative cooling material’s cooling power without any interference from the atmosphere, which will vary from region to region. Moreover, not only do we focus on the material’s ability to cool but also its ability to refrigerate an isolated control volume from the ambient air.


Battery Thermal Management

The biggest challenge that lies in front of the world is global warming and climate change. These crises urge scientific community all over the world to look out for alternative energy sources that are cheap, affordable, and environmentally friendly. The world is transforming, and changes are inevitable. To minimize green- house effect, battery electric vehicles have appeared in the market as a potential replacement to vehicles with internal combustion engine. Lithium- ion batteries have emerged in the market as an alternative electrical energy storage device because of their high energy density, high power density, low self – discharge rate. It surpasses other electrical energy device in the market. Because of these salient features, it is widely used in EVs and HEVs industry to power vehicles. However, heat generation in Lithium-ion battery is inevitable. As charge/discharge C- rate increases, heat generation in the battery is significantly increases which can cause thermal runway in the battery pack and ignite fire in EVs. Our lab focuses on designing novel Battery Thermal Management Systems for controlling temperature of Lithium-ion batteries in the battery pack within the operating range and maintain temperature uniformity in the battery packs.


Thermophysical Properties of Nickel-based Superalloy CM247LC

Nickel-based superalloys, which are used as gas turbine blade materials, have been produced using various methods for use in high-temperature and high-pressure environments. CM247LC is used in directional solidification (DS) to minimize the growth of grain boundaries in a direction perpendicular to the centrifugal force, to reduce stress during turbine operation. In this study, thermal conductivity was derived by measuring the thermal diffusivity and the specific heat as functions of temperature to study the thermal properties according to the directionality of DS CM247LC. Because the grain size was more than 300 μm, it was confirmed that the thermal conductivity of the superalloy with a mean free path of 1 Å was not affected by the grain boundaries.

Trans. Korean Soc. Mech. Eng. B 44, 619 (2020)

Spacer-Inserted Thermoelectric Devices and its Reduced $/W in Power Generation

 While the thermoelectric generator is clearly advantageous when it generates electricity from the low-quality heat, most of the thermoelectric materials are too expensive to compete against conventional energy conversion devices. To solve these problems, we invented the spacer-inserted thermoelectric device (SITED). This structure is made of thermoelectric material (TEM) and thermally low-conductive dielectric material called a spacer that surrounds the TEM. Instead of the TEM, the spacer can sustain a large temperature difference from a low-quality heat so that the performance in power generation per its material consumption can be innovatively enhanced resulting in low $/W.

Applied Energy 139, 205 (2015)

Right Sizes of Nano- and Microstructures for High-performance and Rigid Bulk Thermoelectrics

Na is very well known p-type dopant for PbTe system. Because of this, PbTe has a Multi-band structure and high power factor caused by its heavy effective mass. Our research investigated the 3 different route for synthesis and found the size distribution and mean spacing of Na rich precipitations could be different by these 3 method. (1. QH, 2. QAH, 3. AH; each means, Quenching Hot press, Quenching Annealing Hot press, Annealing Hot press). The highest thermoelectric figure of merit,  zT,  is 2.0 at 773K of QH samples, which is caused by strong phonon scattering by smallest nano-precipitation size (2~6 nm).

PNAS 111, 10949 (2014)

Effect of Na Percipitation in NaxPb1-xTe0.85Se0.15 for Thermoelectric Energy Conversion

Thermoelectric nanocomposite was suggested for increasing thermoelectric conversion efficiency due to the reduction of thermal conductivity. Recent research attempts were made to enhance thermoelectric efficiency by not only reducing thermal conductivity but also increasing power factor. NaxPb1-xTe0.85Se0.15 alloy is representative research that increasing power factor by band engineering. We found that excess Na in PbTeSe system made special microstructures and scattered phonons. In addition, high temperature region over 660K, microstructures start to dissolve and penetrate inside the matrix. Dissolved Na atom supply additional carriers so that electrical conductivity increases after 660K. As a result, thermoelectric figure of merit reach 1.7 at 773K

Journal of Electronic Materials 43, 353 (2014)

Nanostructured PbTe for Thermoelectric Energy Conversion Device

Alkali element Na can control the interaction of light- and heavy-hole valence bands. The adjustment of band structure can lead to improvement in electrical properties, especially at higher temperatures. Based on this theory, we have successfully synthesized Na doped PbTe (Na: 2mol%, 2.25mol%) through the melting-quenching method. The pure phase of PbTe which os obtained from XRD data is compared with the microstructure of samples which have abundant nano-scaled precipitates surrounding the main grain. Electrical properties are optimized, and thermal conductivity is reduced. These results contribute to a larger figure of merit zT, 1.29 and 1.41, obtained for 2% and 2.25% respectively.

Journal of Chemistry A 1, 11269 (2013)

PbTe based Thermoelectric Nanodot Nanocomposite for Waste Recovery

 Thermoelectric nanocomposite which can lead to thermal conductivity reduction was suggested as a way to increase thermoelectric conversion efficiency. Many studies have reported nanodot nanocomposites which exist as a secondary phase nanodot inside a host crystalline matrix. It is hard to control their intrinsic nanostructure. On the other hand, it is possible to control the nanodot’s size distribution, distance between nanodots, and shape in extrinsic nanodot nanocomposites. Moreover, extrinsic nanodot nanocomposites can be applied to any thermoelectric material. We synthesize the extrinsic nanodot nanocomposite with PbTe fabricated through the ball-mill and hot-press method. By calculating the limit of thermoelectric efficiency of nanostructures, we expect that this can lead to thermoelectric figure of merit (zT) up to ~3.