Showing papers by "Zhifeng Ren published in 2011"

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

Abstract: The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. A highly efficient solar to electric energy conversion device based on nanostructured thermoelectric materials and high solar concentration is now demonstrated. The results show potential for cost effective solar thermoelectric generation.

Thermoelectric Property Studies on Cu-Doped n-type CuxBi2Te2.7Se0.3 Nanocomposites

Abstract: Combining high energy ball-milling and hot-pressing, significant enhancements of the thermoelectric figure-of-merit (ZT) have been reported for p-type Bi0.4Sb1.6Te3 nanocomposites. However, applying the same technique to n-type Bi2Te2.7Se0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n-type Bi2Te2.7Se0.3. Even though texturing was effective in improving peak ZT of Bi2Te2.7Se0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi2Te2.7Se0.3 to make Cu0.01Bi2Te2.7Se0.3 samples. A peak ZT value of 0.99 was achieved in Cu0.01Bi2Te2.7Se0.3 samples without texturing. With texturing by re-pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.

Power factor enhancement by modulation doping in bulk nanocomposites.

Abstract: We introduce the concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit. Modulation-doped samples are made of two types of nanograins (a two-phase composite), where dopants are incorporated only into one type. By band engineering, charge carriers could be separated from their parent grains and moved into undoped grains, which would result in enhanced mobility of the carriers in comparison to uniform doping due to a reduction of ionized impurity scattering. The electrical conductivity of the two-phase composite can exceed that of the individual components, leading to a higher power factor. We here demonstrate the concept via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.

Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers

Abstract: Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

Abstract: An enhancement in the dimensionless thermoelectric fi gure-of-merit ( ZT ) of an n-type half-Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 ° C‐700 ° C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.

Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires

Abstract: Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

Physics and applications of aligned carbon nanotubes

Abstract: Ever since the discovery of carbon nanotubes (CNTs) by Iijima in 1991, there have been extensive research efforts on their synthesis, physics, electronics, chemistry, and applications due to the fact that CNTs were predicted to have extraordinary physical, mechanical, chemical, optical, and electronic properties. Among the various forms of CNTs, single-walled and multi-walled, random and aligned, semiconducting and metallic, aligned CNTs are especially important since fundamental physics studies and many important applications will not be possible without alignment. Even though there have been significant endeavors on growing CNTs in an aligned configuration since their discovery, little success had been realized before our first report on growing individually aligned CNTs on various substrates by plasma-enhanced chemical vapor deposition (PECVD) [Science 282 (1998) 1105–1108]. Our report spearheaded a new field on growth, characterization, physics, and applications of aligned CNTs. Up to now, there have ...

Colloidal synthesis of Cu2CdSnSe4 nanocrystals and hot-pressing to enhance the thermoelectric figure-of-merit.

Abstract: We report a solution-based synthesis of monodispersed Cu2CdSnSe4 nanocrystals and a study on the thermoelectric properties of these wide-band-gap dense materials compacted from nanocrystals for the first time. With the help of copper dopants and selenium vacancies generated during wet-chemistry synthesis, a large increment in the power factor is observed, and the dimensionless figure-of-merit ZT reaches a peak value of 0.65 at 450 °C.

Modeling of concentrating solar thermoelectric generators

Abstract: The conversion of solar power into electricity is dominated by non-concentrating photovoltaics and concentrating solar thermal systems. Recently, it has been shown that solar thermoelectric generators (STEGs) are a viable alternative in the non-concentrating regime. This paper addresses the possibility of STEGs being used as the power block in concentrating solar power systems. STEG power blocks have no moving parts, they are scalable, and they eliminate the need for an external traditional thermomechanical generator, such as a steam turbine or Stirling engine. Using existing skutterudite and bismuth telluride materials, concentrating STEGs can have efficiencies exceeding 10% based on a geometric optical concentration ratio of 45.

Effect of selenium deficiency on the thermoelectric properties of n -type In 4 Se 3 − x compounds

Abstract: Thermoelectric properties of dense bulk polycrystalline In4Se3-x (x = 0, 0.25, 0.5, 0.65, and 0.8) compounds are investigated. A peak dimensionless thermoelectric figure of merit (ZT) of about 1 is achieved for x = 0.65 and 0.8. The peak ZT is about 50% higher than the previously reported highest value for polycrystalline In4Se3-x} compounds. Our In4Se3-x samples were prepared by ball milling and hot pressing. We show that it is possible to effectively control the electrical conductivity and thermal conductivity by controlling selenium (Se) deficiency x. The ZT enhancement is mainly attributed to the thermal conductivity reduction due to the increased phonon scattering by Se deficiency, defects, and nanoscale inclusions in the ball-milled and hot-pressed dense bulk In4Se3-x samples.

Dramatic thermal conductivity reduction by nanostructures for large increase in thermoelectric figure-of-merit of FeSb2

Abstract: In this report, thermal conductivity reduction by more than three orders of magnitude over its single crystal counterpart for the strongly correlated system FeSb2 through a nanostructure approach was presented, leading to a significant increase of thermoelectric figure-of-merit (ZT). For the samples processed with the optimal parameters, the thermal conductivity reached 0.34 Wm−1 K−1 at 50 K, leading to a ZT peak of about 0.013, compared to 0.005 for single crystal FeSb2, an increase of about 160%. This work suggests that nanostructure method is effective and can be possibly extended to other strongly correlated low temperature thermoelectric materials, paving the way for future cryogenic temperature cooling applications.

High thermoelectric figure-of-merit in kondo insulator nanowires at low temperatures.

Abstract: We predict a large thermoelectric figure-of-merit in Kondo insulator nanowires at low temperatures. The high ZT values are due to the Kondo effect for electrons and boundary scattering on phonons. We simulated the electron properties of the bulk Kondo insulators within the framework of dynamical mean field theory and found that electrons have short mean free path. In nanowire structures, electron transport is hardly affected by the boundary scattering due to their small intrinsic mean free paths while phonons are strongly scattered due to classical size effect. The results suggest that the nanostructures of Kondo insulators can be designed for high performance thermoelectric cooling devices at low temperatures.

Multiferroic phase diagram of Y partially substituted Dy1−xYxMnO3

Abstract: The effect of nonmagnetic Y partial substitution at the Dy site in Dy(1-x)Y(x)MnO(3) up to x=0.2 on magnetism, specific heat, and ferroelectricity is investigated, which resulted in a preliminary multiferroic phase diagram. It is revealed that the Y partial substitution suppresses the Dy-spin ordering point (T(Dy)) and ferroelectric ordering point (T(FE)) but enhances the Mn-spin ordering point (T(N)). The interaction between the spins of Dy and Mn is remarkably affected by Y substitution. The measured electrical polarization depends on the Y substitution in a complex way because the ferroelectricity is sensitive to the interaction between the spins of Dy and Mn. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3536506]

Nanocoax solar cells based on aligned multiwalled carbon nanotube arrays

Abstract: We fabricated and studied solar cells based on a distributed nanocoax architecture by depositing amorphous silicon as photovoltaic medium on arrays of aligned multiwalled carbon nanotubes. These inexpensive cells demonstrate an initial efficiency of 6.1% that can be further enhanced by increasing the nanocoax density per unit area and improving the amorphous silicon quality.

Transmission electron microscopy study of Pb-depleted disks in PbTe-based alloys

Abstract: density of around 9 � 10 17 cm � 3 , resulting in lattice strain fields (3–20 nm) on both sides of the disks along their normal directions. Moreover, such disks were also observed in Pb-rich Pb1.3Te, Pb-deficient PbTe1.3, and thallium (Tl)-doped Tl0.01Pb0.99Te and Tl0.02Pb0.98Te crystals. Because of theeffectsofdiffractioncontrastimagingbytransmissionelectronmicroscopyandorientationsofthe crystals, these native lattice strain fields were incorrectly recognized as precipitates or nanoinclusions in PbTe-based materials. This discovery provides new insight into the formation mechanism of the precipitates or nanoinclusions in PbTe-based materials.

Metal deposition using seed layers

Abstract: Methods of forming a conductive metal layers on substrates are disclosed which employ a seed layer to enhance bonding, especially to smooth, low-roughness or hydrophobic substrates. In one aspect of the invention, the seed layer can be formed by applying nanoparticles onto a surface of the substrate; and the metallization is achieved by electroplating an electrically conducting metal onto the seed layer, whereby the nanoparticles serve as nucleation sites for metal deposition. In another approach, the seed layer can be formed by a self-assembling linker material, such as a sulfur-containing silane material.

Nanoparticle-enabled selective electrodeposition.

Abstract: Electroplating is a common process used in a wide range of industries. For example, electroplated copper and copper-based alloys are used in ultralarge-scale integration devices requiring multilevel metallization. [ 1 , 2 ] Electroplating has more recently been used in optoelectronic components, such as transparent thin-fi lm transistors, fl at panel displays, light-emitting diodes, photovoltaic cells, and electrochromic windows, where substrates are typically semiconductors (GaAs) or transparent conductive oxides (TCOs, e.g., zinc oxide, indium tin oxide, and fl uorine-doped tin oxide). [ 3 , 4 ] However, metallization directly on glass and other low-roughness ceramics is diffi cult because the smooth interface does not provide opportunities to interlock at the interface between the substrate and the materials to be plated. [ 5 ] Accordingly, prior to electroplating, etching is commonly used to increase surface roughness, followed by sputtering a thin adhesive layer (such as titanium) to improve adhesion. For electroplating metallization, the key is the nucleation process, which is determined by the formation energy, excess energy, and internal strain energy. [ 6–9 ] In general, a smooth and hydrophobic semiconductor surface, such as silicon, gallium arsenide, or transparent conductive glass, has low surface energy and poor wettability, leading to a relatively high excess energy for electroplating nucleation. As a consequence, scattered and irregular grains grow on a small number of nucleation sites, causing poor interface adhesion and large surface roughness. Strain energy, originating from the different atomic arrangement of the two adjacent layers, increases with increasing fi lm thickness and can sometimes cause the fi lm to spontaneously peel off. In many applications, it is relatively common to electroplate metal over an entire surface of a base conductor even though only small areas of the metal are needed on the surface. The use of electroplating in this context typically consists of a patterning process followed by a metallization process. Photolithography is the most popular method to create such patterns where a photoresist is used as the patterning layer. [ 10 , 11 ] The metallization process typically consists of sputtering, electroplating, and chemical mechanical polishing. Metal is fi rst sputtered onto the patterned regions, which improves both the adhesion and electrical conductivity of the primary structure,

Half-Heusler Alloys with Enhanced Figure of Merit and Methods of Making

Abstract: Thermoelectric materials and methods of making thermoelectric materials having a nanometer mean grain size less than 1 micron. The method includes combining and arc melting constituent elements of the thermoelectric material to form a liquid alloy of the thermoelectric material and casting the liquid alloy of the thermoelectric material to form a solid casting of the thermoelectric material. The method also includes ball milling the solid casting of the thermoelectric material into nanometer mean size particles and sintering the nanometer size particles to form the thermoelectric material having nanometer scale mean grain size.

Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3

Abstract: We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La(1-x)Ca(x)MnO(3). Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La(5/8)Ca(3/8)MnO(3). The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.

Growth of aligned carbon nanotubes on ALD-Al2O3 coated silicon and quartz substrates

Abstract: The smooth surface of the amorphous Al2O3 film on either silicon or quartz, coated by atomic layer deposition (ALD), was changed to rough surface by annealing in either air or hydrogen at high temperature (745°C) due to the formation of nanosized pinholes and micrometre pimples during the crystallisation of the amorphous Al2O3. The rough surface makes the growth of long carbon nanotubes (CNTs) by chemical vapour deposition impossible. Nevertheless, we were able to develop new catalyst recipes for successful growth of vertically aligned CNTs on ALD-Al2O3 coated silicon and quartz substrates. The lengths of the CNTs reached 90 µm on silicon substrates and 180 µm on quartz substrates. Furthermore, it is observed that the adhesion of CNTs on silicon substrates is much stronger than that on quartz substrates.

Thermoelectric energy conversion using nanostructured materials

Abstract: King Fahd University of Petroleum and Minerals (Center for Clean Water and Clean Energy at MIT and KFUPM )

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.