Woo Hyun Nam

High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite

2 mol% Al-doped ZnO nanoparticles were consolidated into a ZnO nanocomposite with ZnAl2O4 nanoprecipitates by spark plasma sintering and its high-temperature charge transport and thermoelectric properties were investigated up to 1073 K. The carrier concentration in the nanocomposite was not dependent on the temperature, while the Hall mobility showed positive temperature-dependence due to grain boundary scattering. The negative Seebeck coefficient of the nanocomposite was linearly proportional to the temperature, and the density of the state effective mass (md*) was evaluated to be 0.33me by using the Pisarenko relation. Drastic reduction of thermal conductivity (k < 2 W m−1 K−1) was achieved in the nanocomposite, and the maximum ZT of 0.34 was obtained at 1073 K.

Structurally Nanocrystalline-Electrically Single Crystalline ZnO-Reduced Graphene Oxide Composites

ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.

Preparation and visible-light photocatalysis of hollow rock-salt TiO1−xNx nanoparticles

A simple method for the preparation of hollow rock-salt TiO1−xNx nanoparticles and their visible-light photocatalytic activities are presented. By thermal annealing of TiO2 nanoparticles under NH3/N2 gas flow at 800 °C, a rock-salt TiO phase was achieved far below its transition temperature to a monoclinic TiO phase (1250 °C). The composition of the nanoparticles was revealed to be Ti0.7(O0.67N0.33)1 by detailed structural and compositional characterizations, and their structural stability was critically determined by the formation of vacancies and the incorporation of N. The hollow rock-salt TiO1−xNx nanoparticles presented efficient visible-light photocatalytic activity, and their origins were discussed.

Sintering behaviour and microstructures of nanostructured ZnO–ZnS core–shell powder by spark plasma sintering

Densification of nanostructured ZnO–ZnS core–shell powder was carried out by spark plasma sintering to produce a bulk ZnO–ZnS composite. By adjusting the sintering temperature, we could fabricate a bulk ZnO–ZnS composite without destroying the original core–shell structure of the powder. X-ray diffraction and transmission electron microscopy were used to characterize the microstructural properties of the core–shell powder and its sintering behaviour. During spark plasma sintering, phase transition from a sphalerite to a wurtzite structure was observed in the ZnS shell and the crystallographic orientation of the ZnS shell was affected by the ZnO core.

Effect of Interface Control Using Multiwalled Carbon Nanotubes on the Thermoelectric Properties of TiO 2 Nanocomposites

We report the effect of interface control using multiwalled carbon nanotubes (MWCNT) on thermoelectric properties of TiO2. By consolidating TiO2 nanoparticles with MWCNT (0.5, 1, 2, 4, and 8 wt%) using spark plasma sintering, we prepared interface-controlled TiO2-MWCNT nanocomposites, where TiO2 grains were surrounded with a MWCNT network. Simultaneous control of charge and thermal transport was successfully achieved by interface control using MWCNTs. The electrical conductivity increased monotonically as the MWCNT content was increased. As determined in our previous report, the charge transport mechanism in the nanocomposites is based on percolation and hopping models. The formation of new interfaces at the grain boundaries using the MWCNT network led to additional phonon scattering in the nanocomposites, and the thermal conductivities decreased monotonically with increasing MWCNT content. However, the incorporation of the MWCNT did not lead to a significant increase in power factor due to a reduction in the Seebeck coefficient. Consequently, the highest ZT value of 4.6 × 10 was obtained from the TiO2-0.5 wt% MWCNT nanocomposite at 1073 K. Our results introduce a strategy for the independent control of electron and phonon transport based on interface control using carbon nanomaterials in hybrid thermoelectric materials. (Received May 8, 2018; Accepted May 23, 2018)

Effect of High-Energy Ball Milling on Thermoelectric Transport Properties in CoSb 3 Skutterudite

In this study, we investigate the effect of high-energy ball milling on thermoelectric transport properties in double-filled CoSb3 skutterudite (In0.2Yb0.1Co4Sb12). In0.2Yb0.1Co4Sb12 powders are milled using high-energy ball milling for different periods of time (0, 5, 10, and 20 min), and the milled powders are consolidated into bulk samples by spark plasma sintering. Microstructure analysis shows that the high-energy ball milled bulk samples are composed of nanoand micro-grains. Because the filling fractions are reduced in the bulk samples due to the kinetic energy of the high-energy ball milling, the carrier concentration of the bulk samples decreases with the ball milling time. Furthermore, the mobility of the bulk samples also decreases with the ball milling time due to enhanced grain boundary scattering of electrons. Reduction of electrical conductivity by ball milling has a decisive effect on thermoelectric transport in the bulk samples, power factor decreases with the ball milling time.