Showing papers by "Zhifeng Ren published in 2010"

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

Abstract: Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

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

Abstract: The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi2Te3-based n-type single crystals is about 085 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 165 W m−1 K−1 even though the power factor is 47 × 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 085 to 104 at about 125 °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

A molecular-imprint nanosensor for ultrasensitive detection of proteins

Abstract: Carbon nanotube tips containing imprints within a non-conducting polymer coating can detect proteins with high sensitivity, offering a label-free alternative to sensors based on biomolecule recognition.

Effects of nanoscale porosity on thermoelectric properties of SiGe

Abstract: The recent achievement of the high thermoelectric figure of merit in nanograined materials is attributed to the successful optimization of the consolidation process. Despite a thermal conductivity reduction, it has been experimentally observed that the porous nanograined materials have lower thermoelectric figure of merit than their bulk counterpart due to significant reduction in the electrical conductivity. In this paper, nanoscale porosity effects on electron and phonon transport are modeled to predict and explain thermoelectricproperties in porous nanograined materials.Electron scattering at the pores is treated quantum mechanically while phonon transport is treated using a classical picture. The modeling results show that the charge carriers are scattered more severely in nanograined materials than the macroscale porous materials, due to a higher number density of scattering sites. Porous nanograined materials have enhanced Seebeck coefficient due to energy filtering effect and low thermal conductivity, which are favorable for thermoelectric applications. However, the benefit is not large enough to overcome the deficit in the electrical conductivity, so that a high sample density is necessary for nanograined SiGe.

Efficient nanocoax-based solar cells

Abstract: Despite requiring thick layers of rela-tively costly material, the vast majority of today’s solar pho-tovoltaic (PV) cells employ crystalline media, due to their superior energy conversion efficiency compared to non-crystalline, “thin film” cells [1]. The dominant material, crystalline silicon (c-Si), has weak optical absorption, and so must be relatively thick (~200 μm) to efficiently collect light. However, with charge carrier (electron and hole) mean free paths comparable to this distance, high power conver-sion efficiency η can still be achieved (η ~ 25% for single-junction cells) [2]. Noncrystalline materials such as amor-phous silicon (a-Si), on the other hand, are strongly absorb-ing, such that thin films (under 1 μm) suffice for efficient light collection. However, mean free paths in a-Si are sig-nificantly shorter (~100 nm) than in c-Si, such that thin film efficiency (η < 10%) severely lags its crystalline counterpart [3, 4]. Both types of solar cells are therefore compromised by a coupling of the optical and electronic length scales: crystalline in terms of cost, thin film in terms of efficiency. This “thick–thin” paradox is difficult to resolve in the con-ventional, planar solar cell configuration, where photons and electrons travel essentially in the same direction, i.e. normal to the cell surface. Here we propose to resolve this problem by employing a cell structure based on a coaxial cable.

Multiferroicity — The Coupling Between Magnetic and Polarization Orders

Abstract: Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Theoretical studies on the thermoelectric figure of merit of nanograined bulk silicon

Abstract: In this paper, we investigate the phonon transport in silicon nanocomposites using Monte Carlo simulations considering frequency-dependent phonon mean free paths, and combine the phonon modeling with electron modeling to predict the thermoelectric figure of merit (ZT) of silicon nanocomposites. The model shows that while grain interface scattering of phonons is negligible for large grain sizes around 200 nm, ZT can reach 1.0 at 1173 K if the grain size can be reduced to 10 nm. Our results show the potential of obtaining a high ZT in bulk silicon by the nanocomposite approach.

Experimental Studies on Anisotropic Thermoelectric Properties and Structures of

Abstract: The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi2Te3-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 × 10 -4 Wm -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 °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.

Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite.

Abstract: Graphene nanosheets have been synthesized from commercial expandable graphite by heating in a microwave oven and dispersing in ethanol by ultrasonication. Scanning and transmission electron microscopy and electron energy-loss spectroscopy and atomic force microscope showed that the nanosheets were about 2 nm in thickness and 10 microm in diameter. The field emission of the graphene sheets has been investigated. An emission current density of 1 mA/cm2 has been achieved at an electric field of 3.7 V/microm with a turn-on field of 1.7 V/microm at 0.01 mA/cm2. The annealing of the samples at 400 degrees C in vacuum greatly improved the field emission performance.

Thermoelectric system and method of operating same

Abstract: An apparatus includes an evacuated enclosure which comprises a tubular member extending along a longitudinal axis, a radiation absorber disposed in the enclosure and having a front surface and a back surface, the front surface being adapted for exposure to solar radiation so as to generate heat, at least one thermoelectric converter disposed in the enclosure and thermally coupled to the absorber, the converter having a high-temperature end to receive at least a portion of the generated heat, such that a temperature differential is achieved across the at least one thermoelectric converter, a support structure disposed in the enclosure coupled to a low-temperature end of the thermoelectric converter, where the support structure removes heat from a low-temperature end of the thermoelectric converter, and a heat conducting element extending between the support structure and the evacuated enclosure and adapted to transfer heat from the support structure to the enclosure. The absorber, the at least one thermoelectric converter, and the support structure are arranged as a planar unit located within the tubular member.

Effect of filler mass and binding on thermal conductivity of fully filled skutterudites

Abstract: The relations between the thermal conductivity of cagelike structures and their crystal parameters are investigated using a two-dimensional toy model. The model consists of host atoms on a rectangular lattice with fillers at the center of each rectangle. The effect of mass and size of the filler on thermal conductivity is investigated using equilibrium molecular-dynamics simulations. We show that the thermal conductivity decreases with increasing atomic displacement parameter while it has local minima versus the filler to host mass ratio. Similar trends were observed in experiments on filled skutterudites. The trends are explained by analyzing the effect of the filler on the phonon dispersion and relaxation times of the host material.

Effect of filler mass and binding on thermal conductivity of fully filled skutterudites

Abstract: The relations between the thermal conductivity of cagelike structures and their crystal parameters are investigated using a two-dimensional toy model. The model consists of host atoms on a rectangular lattice with fillers at the center of each rectangle. The effect of mass and size of the filler on thermal conductivity is investigated using equilibrium molecular-dynamics simulations. We show that the thermal conductivity decreases with increasing atomic displacement parameter while it has local minima versus the filler to host mass ratio. Similar trends were observed in experiments on filled skutterudites. The trends are explained by analyzing the effect of the filler on the phonon dispersion and relaxation times of the host material.

Percolation and polaritonic effects in periodic planar nanostructures evolving from holes to islands

Abstract: We study interaction of the electromagnetic radiation with a series of thin film periodic nanostructures evolving from holes to islands. We show, through model calculations, simulations, and experiments, that the responses of these structures evolve accordingly, with two topologically distinct spectral types for holes and islands. We find also, that the response at the transitional pattern is singular. We show that the corresponding effective dielectric function follows the critical behavior predicted by the percolation theory and thus the hole-to-island structural evolution in this series is a topological analog of the percolation problem, with the percolation threshold at the transitional pattern.

Na2SO4 Monocrystal Nanowires—Aspect Ratio Control and Electron Beam Radiolysis

Abstract: This paper presents the synthesis of water-dissolvable Na2SO4 nanowires and nanorods by a simple chemical reaction between CuSO4 and NaBH4 in ethylene glycol. By adjusting the pH and the monomer concentration, the aspect ratio and size of the Na2SO4 nanowires could be tuned. Na2SO4 nanorods, nanowhiskers, nanowires, and submicrorods were obtained. Optimal chemical potential is believed to be the dominant driving force for the growth of Na2SO4 nanowires during the synthesis. We also demonstrated the Na2SO4 nanotubes obtained by the electron beam radiolysis of Na2SO4 nanowires. The mechanism of selective radiolysis is also investigated.

Nanoscopically thin photovoltaic junction solar cells

Abstract: Nanoscopically thin photovoltaic junction solar cells are disclosed herein. In an embodiment, there is provided a photovoltaic film 100 that includes a p-doped region 102, an n-doped region 106, and an intrinsic region 104 positioned between the p-doped region 102 and the n-doped region 106, wherein an overall thickness of the photovoltaic film is between about 15 nm to about 30 nm so as to extract hot carriers excited across a band gap, wherein the extracted hot carriers are capable of resulting in an open circuit voltage, Voc, of the photovoltaic film that increases with optical frequency, and wherein the extracted hot carriers are capable of resulting in a total short-circuit current density, Jsc, between about 4 mA/cm 2 and about 8 mA/cm 2 .

Grids for Applications in High-Temperature High-Resolution Transmission Electron Microscopy

Abstract: New TEM grids coated with ultrathin amorphous Al 2 O 3 films have been developed using atomic layer deposition technique. The amorphous Al 2 O 3 films can withstand temperatures over 600 ∘ C in air and 900 ∘ C in vacuum when the thickness of the Al 2 O 3 film is 2 nm, and up to 1000 ∘ C in air when the thickness is 25 nm, which makes heating TEM grids with nanoparticles up to 1000 ∘ C in air and immediate TEM observation without interrupting the nanoparticles possible. Such coated TEM grids are very much desired for applications in high-temperature high-resolution transmission electron microscopy.

Grids for Applications in High-Temperature High-Resolution Transmission Electron Microscopy

Abstract: New TEM grids coated with ultrathin amorphous Al2O3 films have been developed using atomic layer deposition technique. The amorphous Al2O3 films can withstand temperatures over 600◦C in air and 900◦C in vacuum when the thickness of the Al2O3 film is 2 nm, and up to 1000◦C in air when the thickness is 25 nm, which makes heating TEM grids with nanoparticles up to 1000◦C in air and immediate TEM observation without interrupting the nanoparticles possible. Such coated TEM grids are very much desired for applications in high-temperature high-resolution transmission electron microscopy.