Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V35: Focus Session: Thermoelectric Phenomena in Nanostructured Materials |
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Sponsoring Units: DMP FIAP Chair: Chris Vineis, MIT Lincoln Laboratory Room: Morial Convention Center 227 |
Thursday, March 13, 2008 11:15AM - 11:51AM |
V35.00001: Thermoelectric Properties of Complex Zintl Phases Invited Speaker: Complex Zintl phases make ideal thermoelectric materials because they can exhibit the necessary ``electron-crystal, phonon-glass'' properties required for high thermoelectric efficiency. Complex crystal structures can lead to high thermoelectric figure of merit (\textit{zT}) by having extraordinarily low lattice thermal conductivity. A recent example is the discovery that Yb$_{14}$MnSb$_{11}$, a complex Zintl compound, has twice the \textit{zT} as the SiGe based material currently in use at NASA. The high temperature (300K - 1300K) electronic properties of Yb$_{14}$MnSb$_{11}$ can be understood using models for heavily doped semiconductors. The free hole concentration, confirmed by Hall effect measurements, is set by the electron counting rules of Zintl and the valence of the transition metal (Mn$^{+2})$. Substitution of nonmagnetic Zn$^{+2}$ for the magnetic Mn$^{+2}$ reduces the spin-disorder scattering and leads to increased \textit{zT} (10{\%}). The reduction of spin-disorder scattering is consistent with the picture of Yb$_{14}$MnSb$_{11}$ as an underscreened Kondo lattice as derived from low temperature measurements. The hole concentration can be reduced by the substitution of Al$^{+3}$ for Mn$^{+2}$, which leads to an increase in the Seebeck coefficient and electrical resistivity consistent with models for degenerate semiconductors. This leads to further improvements (about 25{\%}) in \textit{zT} and a reduction in the temperature where the \textit{zT} peaks. The peak in \textit{zT} is due to the onset of minority carrier conduction and can be correlated with reduction in Seebeck coefficient, increase in electrical conductivity and increase in thermal conductivity due to bipolar thermal conduction. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V35.00002: A theoretical study of thermoelectric metamaterial Jianwei Cai, Gerald Mahan An artificial crystalline material with nanoparticles connected by conducting linkers is proposed for thermoelectric application. Such materials can be synthesized with chemical approaches. The advantage of such materials for thermoelectric application will be discussed. A simple theoretical model for such materials is proposed. Basic theoretical results about the band and properties of the materials based on perturbation theory will be presented. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V35.00003: High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. B. Poudel, Q. Hao, Y. Ma, A. Minnich, A. Muto, Y.C. Lan, B. Yu, X. Yan, D.Z. Wang, D. Vashaee, X.Y. Chen, M.S. Dresselhaus, G. Chen, Z.F. Ren Bismuth Telluride and its alloys are best thermoelectric materials for near room temperature applications like refrigeration and waste heat recovery. We have been pursuing an approach of random nanostructures in bulk to improve ZT of these materials. Here we report that ZT values of these random nanostructured materials were improved significantly over the state-of-the-art values. Experimental data coupled with microstructure studies and modeling shows that the ZT improvement mainly comes from a lower thermal conductivity because of the increased phonon scattering by defects and grain boundaries. Significantly improved power generation and cooling data produced from these samples confirmed the high ZT values. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V35.00004: The Effect of Bi/Sb ratio on Maximum ZT in Nanostructured p-type Bi$_{x}$Sb$_{2-x}$Te$_{3}$ Alloys Yi Ma, Bo Yu, Bed Poudel, Yucheng Lan, Jian Yang, Zhifeng Ren, Qing Hao, Gang Chen Alloy nanoparticles Bi$_{x}$Sb$_{2-x}$Te$_{3}$ (x = 0.2, 0.3, 0.4, 0.5 and 0.6) were synthesized from bismuth, antimony, and tellurium and compacted into nanostructured bulk samples with full density. The thermoelectric properties were measured in the temperature range of 25 to 250$^{\circ}$C. We showed that a maximum dimensionless figure-of-merit (ZT) reached 1.27 at about 100$^{\circ}$C for Bi$_{0.4}$Sb$_{1.6}$Te$_{3}$(x = 0.4), a significant improvement over the state-of-the-art p-type Bi$_{2}$Te$_{3}$ alloys. It is demonstrated that the enhanced ZT mainly comes from a reduced thermal conductivity due to the increased phonon scattering due to the nano size of the grains. Also, the ZT peaks shift from low temperature to high temperature with the decreasing of x, which indicates the less Bi content would benefit the potential power generation application while the more Bi content materials could be used as a potential cooling device. This new route of enhancing ZT value can be applied to other materials. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V35.00005: XAS study of Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ Superlattices and Sb$_{1.5}$Bi$_{0.5}$Te$_{3}$ Alloy Film Azzam Mansour, Rama Venkatasubramanian Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices are being developed for high performance thermoelectric devices. We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Sb in 20/40 {\AA} and 10/50 {\AA} Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices, a Sb$_{1.5}$Bi$_{0.5}$Te$_{3}$ alloy film, and a Sb$_{2}$Te$_{3}$ reference film. The films were grown on GaAs using a novel low temperature metal-organic chemical vapor deposition method. Initially, we evaluated the local structure parameters of Sb in the superlattices and the alloy relative to those of the reference Sb$_{2}$Te$_{3}$ film. Using temperature dependent Sb K-edge XAS measurements, we were able to separate structural disorder from thermal disorder for the Sb-Te pair. The temperature dependence of thermal disorder was analyzed using the Einstein and Debye models for lattice vibrations. A comparison of the results for the superlattices, the alloy and the reference film will be made. The goal is to identify key structural factors that may contribute to the enhanced value of the figure of merit ``ZT'' and, hence, the improved conversion efficiency for the superlattices relative to the alloys. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V35.00006: Thermoelectric properties of PbTe/PbSe mesomaterials Feng Chen, Yaqi Wang, Yuyi Xue, C. W. Chu, Jun Zhang, Jiye Fang, Chunhu Tan, Zhigang Lin, Bob Liu Ball milled PbTe mixed with PbSe nano-wires (PTSW) or with PbSe nano-crystals (PTSC) are sintered under high pressure. Different sintering conditions are tested to preserve the mesostructures. Thermoelectric properties (resistivity, Seebeck coefficient and thermal conductivity) are measured at various temperatures. Pure ball milled PbTe are also sintered and measured for comparison. In this talk, we will present these data and compare with various PbTe data from the literature. Our results show that this mesostructure approach is promising and the sintering condition is the key factor for further improvement. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V35.00007: Transport Properties of Lead Chalcogenide Nanocomposites George Nolas, Joshua Martin, Stevce Stefanoski, Li Wang, Lidong Chen Lead chalcogenide dimensional nanocomposites were prepared by densifying nanocrystals, synthesized in high purity - high product yield employing an alkaline aqueous solution-phase reaction, by Spark Plasma Sintering. Transport properties were evaluated through temperature dependent resistivity, Hall, thermopower, and thermal conductivity measurements. The physical properties of these materials are strongly sensitivity to stoichiometry, doping, and porosity. The transport properties of the nanocomposites were compared to that of bulk polycrystalline materials with similar compositions and carrier concentrations. An enhancement of the room temperature thermoelectric properties of up to 30{\%} over that of the bulk was realized. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V35.00008: Nanostructured thermoelectric skutterudite Co$_{1-x}$Ni$_{x}$Sb$_{3}$ alloys Qinyu He, Xiaowei Wang, Jian Yang, Yucheng Lan, Xiao Yan, Bo Yu, Yi Ma, Bed Poudel, Giri Joshi, Dezhi Wang, Zhifeng Ren, Qing hao, Gang Chen Nanostructured Ni-doped skutterudites Co$_{1-x}$Ni$_{x}$Sb$_{3}$ (with x ranging from 0.01 to 0.09) have been prepared by ball-milling and direct-current induced hot press. It was found that the thermal conductivity was much reduced due to strong electrons-phonons scaterring from Ni-doping and grain boundary phonons scattering from nano-structures in the samples. A maximum dimensionless figure-of-merit of 0.71 has been obtained in Co$_{0.91}$Ni$_{0.09}$Sb$_{3}$ at 525 \r{ }C. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V35.00009: Thermal conductivity reduction by interface roughness in AlN$_{x}$-GaN$_{y}$ superlattices. Yee Kan Koh, Yu Cao, David Cahill, Debdeep Jena A reduction of cross-plane thermal conductivity $\Lambda $ by a factor of two is achieved in AlN$_{4 nm}$-GaN$_{52 nm}$ superlattices by varying the plasma power during growth. This reduction is attributed to interface roughness, introducing a new parameter to be considered in the design and fabrication of superlattices for thermoelectric applications. Thermal conductivity of AlN$_{x}$-GaN$_{y}$ superlattices, x $\sim $ 4 nm and 2 $<$ y $<$ 1000 nm, with rough interfaces is then measured by time-domain thermoreflectance. $\Lambda $ decreases monotonically as the GaN thickness y decreases, $\Lambda $ = 6.35 W m$^{-1}$ K$^{-1}$ at y = 2.2 nm. We observe no minimum thermal conductivity as a function of period for these rough superlattices. A continuum model incorporating the effects of interface roughness indicates that diffuse scattering is dominant when y $>$ 20 nm, and significant coherent transmission occurs when y $<$ 20 nm. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V35.00010: High dimensionless figure-of-merit in nanostructured p-type bulk SiGe alloys Giri Joshi, Hohyun Lee, Xiaowei Wang, Goahua Zhu, Yucheng Lan, Dezhi Wang, Bed Poudel, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon Germanium (SiGe) alloys have been used for high temperature power generation in thermoelectric generators that provided the onboard electrical power to several US space vehicles. Since their performance is related to dimensionless figure-of-merit (ZT), material scientists have focused their attention on possible improvements in ZT of SiGe alloys through an increase in power factor and decrease in thermal conductivity. We have been pursuing an approach of random nanostructures to reduce the thermal conductivity based on the understanding that the reduction of thermal conductivity is primarily responsible for ZT enhancement in superlatices. We have observed 100{\%} improvement in ZT, compared to the state-of-the-art values, in p-type SiGe nanostructured bulk materials, which comes mainly from decrease in thermal conductivity due to the increase in phonon scattering by defects and grain boundaries in random nanostructures. These bulk materials also possess superior mechanical properties making them more suitable for fabrication and integration into systems which were made by hot pressing of nanopowders prepared by using high energy ball milling. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V35.00011: Nanotructured high thermoelectric figure-of-merit in n-type bulk SiGe alloys Xiaowei Wang, Hohyun Lee, Gaohua Zhu, Yucheng Lan, Dezhi Wang, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon germanium alloys (SiGe) have been the exclusice choice for radiosotope thermoelectric generators (RTGs) because of its reliability and high operating temperatures. In this paper, by using mechanical alloying method, nano-sized phosphorous-doped (n-type) SiGe alloy powders were produced, followed by direct current induced hot press technique, bulk samples were obtained with $\sim $100{\%} density. The thermoelectric properties, including electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured in the temperature range from 25 to 900 $^{\circ}$C. A maximum ZT of around 1.3 at 900$^{\circ}$C was obtained under certain doping concentration and hot press conditions, mainly due to thermal conductivity reduction from nano-structured SiGe grains of 20 nm. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V35.00012: Thermoelectric properties of n-type nano bulk Si Gaohua Zhu, Hohyun Lee, Xiaowei Wang, Giri Joshi, Yucheng Lan, Jian Yang, Dezhi Wang, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Nano Si has been noticed as a promising substitute material for SiGe. We have observed a 150 to 350 {\%} increase in ZT values in the heavily doped n-type nano Si over the single crystal Si, which mainly comes from the significantly lower thermal conductivity due to the nano size of grains achieved by mechanical alloying and hot press. The key to get higher ZT value is to optimize the doping concentration to achieve high power factor and avoid grain growth during hot-pressing to achieve nano particle size in the final bulk form. So far our research on nano bulk Si has shown promising ZT values close to 1, comparable to that of the traditional SiGe alloys. Silicon is lighter, more refractory, and has better thermal stability than Si$_{0.8}$Ge$_{0.2}$, and also it is much cheaper than Ge. Pure Si may have advantage over SiGe alloy in mass application of power generation systems. [Preview Abstract] |
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