Woo Chul Kim

Atomic substitution in alloys can efficiently scatter phonons, thereby reducing the thermal conductivity in crystalline solids to the "alloy limit." Using In0.53Ga0.47As containing ErAs nanoparticles, we demonstrate thermal conductivity reduction by almost a factor of 2 below the alloy limit and a corresponding increase in the thermoelectric figure of merit by a factor of 2. A theoretical model suggests that while point defects in alloys efficiently scatter short-wavelength phonons, the ErAs nan

We discuss representative strategies of phonon engineering by categorizing them into the methods affecting each component of thermal conductivity.

An approximate analytical solution is proposed to estimate the phonon scattering cross section of polydispersed spherical nanoparticles. Using perturbation of the Hamiltonian due to differences in mass and bond stiffness between a host medium and a spherical nanoparticle, an analytical solution is obtained for the scattering cross section in the Rayleigh limit when the size parameter approaches zero. In the geometrical scattering limit, when the size parameter approaches infinity, the van de Hul

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 obse

Magnetite (Fe3O4) is believed to be half metal, providing 100% spin-polarized conduction electrons. The half-metallic nature of magnetic electrodes for tunneling junction devices is expected to induce a large magnetoresistance. We investigated the structural and chemical properties of interfaces in ferromagnet–insulator–ferromagnet (Fe3O4/MgO/Fe) tunnel junctions. Al/Ag/Fe3O4/MgO multilayers for magnetic tunnel junction have been fabricated on α-Al2O3 (001) and MgO (100) substrates by a molecula

This paper describes the manufacturing method and properties of a superalloy as a gas turbine blade material and a thermal barrier coating to protect it. The development process of superalloy and characteristics of each casting method were introduced. In particular, the single crystal superalloys were analyzed for creep and tensile properties with temperature according to chemical composition. In addition, the theories of creep life prediction models were summarized and comparative analysis was

We studied the cross-plane lattice and electronic thermal conductivities of superlattices made of InGaAlAs and InGaAs films, with the latter containing embedded ErAs nanoparticles (denoted as ErAs:InGaAs). Measurements of total thermal conductivity at four doping levels and a theoretical analysis were used to estimate the cross-plane electronic thermal conductivity of the superlattices. The results show that the lattice and electronic thermal conductivities have marginal dependence on doping lev

Thermal conductivity of a crystalline solid at high temperature is dominated by the Umklapp process because the number of high frequency phonons increases with temperature. It is challenging to reduce the thermal conductivity of crystalline solids at high temperature although it is widely known that, by increasing the atomic defect concentration, thermal conductivity of crystalline solids can be reduced at low temperature. By increasing the concentration of ErAs nanoparticles in In 0.53Ga 0.47As

The thermal conductivity of individual single-crystalline Bi nanowires grown by the on-film formation of nanowires (ON-OFF) has been investigated. We observed that the thermal conductivity of single-crystalline Bi nanowires is highly anisotropic. Thermal conductivity of nanowires (diameter ∼100 nm) in the off-axis [102] and [110] directions exhibits a difference of ∼7.0 W/m·K. The thermal conductivity in both growth directions is diameter-dependent, which indicates that thermal transport through