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.

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)