Bed Poudel

High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.

The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100°C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250°C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86°, 106°, and 119°C with hot-side temperatures set at 50°, 100°, and 150°C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m(-2)) conditions. The efficiency is 7-8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.

Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

Experimental Studies on Anisotropic Thermoelectric Properties and Structures of n-Type Bi2Te2.7Se0.3

The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi(2)Te(3)-based n-type single crystals is about 0.85 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 1.65 W m(-1) K(-1) even though the power factor is 47 x 10(-4) W m(-1) K(-2). In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT. Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 0.85 to 1.04 at about 125 degrees C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity. Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller.

Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

Structure study of bulk nanograined thermoelectric bismuth antimony telluride.

The microstructures of bulk nanograined p-type bismuth antimony telluride with a thermoelectric dimensionless figure-of-merit ZT = 1.4 are investigated using transmission electron microscopy. It is found that the bulk material contains both nano- and microsized grains. Between the nanograins, bismuth-rich interface regions with a 4 nm thickness were detected. In addition, nanoprecipitates as well as other defects are also found to be embedded in the nanograins. The high ZT is attributed to the slight increase in the electrical conductivity, and to the large decrease of the thermal conductivity.

Thermoelectric property studies on bulk TiOx with x from 1 to 2

100% dense bulk TiOx samples have been made by first oxidizing TiO and then direct current induced hot press. It was found that the Seebeck coefficients are negative at room temperature and change to positive at higher temperature for 1<x<1.25, positive at room temperature and change to negative at higher temperature for 1.25<x<5∕3, and always negative up to 800°C for 5∕3<x<2. The highest achieved dimensionless figure of merit is about 0.2–0.23 for 1.83<x<1.9, about eight times lower than the reported value.

Low-dimensional phonon specific heat of titanium dioxide nanotubes

The specific heat of multiwalled titanium dioxide (anatase phase) nanotubes has been measured between 1.5 and 95K. Bulk anatase and rutile were also measured. The nanotube specific heat approaches that of bulk anatase at high temperatures. Below about 50K the nanotube specific heat begins to show large enhancements compared to bulk. Using an isotropic elastic continuum model, this can be understood qualitatively as a transition to low-dimensional behavior. Below about 3K there is a second transition and the nanotube specific heat becomes nearly constant, exceeding bulk anatase by an order of magnitude or more at 1.5K.

Synthesis of gram-scale germanium nanocrystals by a low-temperature inverse micelle solvothermal route

We report a simple low-temperature inverse micelle solvothermal route for the synthesis of a large quantity of single-crystalline germanium nanocrystals. X-ray diffraction measurement indicates that the as-prepared nanocrystals are composed of pure Ge with a cubic structure. The morphology, size, chemical composition, crystallinity, and structural features of the as-prepared nanocrystals were characterized by transmission electron microscopy and energy-dispersive x-ray spectroscopy.

Effects of plasma surface modification on interfacial behaviors and mechanical properties of carbon nanotube-Al2O3 nanocomposites

The effects of plasma surface modification on interfacial behaviors in carbon nanotube (CNT) reinforced alumina (Al2O3) nanocomposites were studied. A unique plasma polymerization method was used to modify the surfaces of CNTs and Al2O3 nanoparticles. The CNT-Al2O3 nanocomposites were processed by both ambient pressure and hot-press sintering. The electron microscopy results showed ultrathin polymer coating on the surfaces of CNTs and Al2O3 nanoparticles. A distinctive stress-strain curve difference related to the structural interfaces and plasma coating was observed from the nanocomposites. The mechanical performance and thermal stability of CNT-Al2O3 nanocomposites were found to be significantly enhanced by the plasma-polymerized coating.