Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H43: Focus Session: Physics of Thermoelectric Materials and Phenomena II |
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Sponsoring Units: FIAP DMP Chair: Qiang Li, Brookhaven National Laboratory Room: Colorado Convention Center 506 |
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H43.00001: Solid State Thermionic Energy Conversion Invited Speaker: An overview of the activities at the Thermionic Energy Conversion Center will be given. This is a consortium of twelve research groups that are working to optimize thermoelectric properties of embedded metallic nanoparticles and multilayers. Hot electron filtering using heterostructure barriers is used to break the trade off between high Seebeck coefficient and high electrical conductivity. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity without significant effect on electrical conductivity. The implication of the superlattice transport on the electronic thermal conductivity and Lorenz number will also be discussed. Cross-plane and in-plane thermoelectric properties are characterized in a wide temperature range. The effective ZT of the thin film is measured using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n- and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. Potential for energy conversion efficiency exceeding 20{\%} and high power density $>$1W/cm2 will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 8:48AM |
H43.00002: High temperature cross-plane Seebeck coefficient measurement of ErAs:InGaAs/InGaAlAs superlattice Zhixi Bian, Mona Zebarjadi, Ali Shakouri, Gehong Zeng, John Bowers The 3$\omega $ technique is used to measure the Seebeck coefficient across 2.4 micron superlattices made of 80$\times $((InGaAs)$_{0.6}$(lnAlAs)$_{0.4}$ -10nm / InGaAs-20 nm) films lattice matched to InP substrate. ErAs nanoparticles are randomly distributed inside the 20 nm InGaAs layer. We characterized 4 samples with different doping concentrations (from 2$\times $10$^{18}$ cm$^{-3}$ to 10$^{19}$cm$^{-3})$ in a temperature range of 300K to 600 K. A significant increase in the cross plane Seebeck coefficient compared to the in plane one is observed. Comparison with DC measurement shows that the 3$\omega $ method is more accurate especially at high temperatures. Theoretical analysis based on the solution of the coupled Schr\"{o}dinger and Poisson equations, together with modified Boltzmann transport equation is used to explain the experimental results. [Preview Abstract] |
Tuesday, March 6, 2007 8:48AM - 9:00AM |
H43.00003: Electronic Properties of Layered Cobaltates. Qing Jie, Qiang Li Electronic properties of layered cobaltates are strikingly different from those in conventional metals, that result in water-induced superconductivity and large thermoelectric power factor. Here we report a transport study of thermoelectric power and electrical conductivity as a function of temperature and magnetic field in several cobaltate single crystals and thin films: Ca$_{3}$Co$_{4}$O$_{9}$ and highly doped Na$_{x}$CoO$_{2}$. There are three temperature regions where the electronic transport properties of these cobaltates exhibit distinctive behaviors. At low temperatures, the transport property is strongly influenced by the magnetism. At the elevated temperatures, it shows the feature of a correlated metal. At high temperatures, it has weak temperature dependence. These results will be compared with the infrared studies. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H43.00004: Crystal Structure of Misfit Thermoelectric Compound [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ by Electron Diffraction and High Resolution Electron Microscopy P. Oleynikov, V. Volkov, Q. Jie, Q. Li, Y. Zhu Layered cobaltates are of great interest from a physics point of view, as shown by their thermoelectric and magnetoresistance properties. In order to understand the origin of physical properties of layered cobaltates and, in particular, of misfit cobalt oxide [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ with high thermoelectric power, an accurate determination of the crystal structure is required. Ambiguities of the structure analysis of this compound performed by X-ray methods stimulated us to re-examine its structure by the electron diffraction patterns (DP) and high resolution electron microscopy (HREM) methods. Single crystal [Ca$_{2}$CoO$_{3}$]$_{0.62}$CoO$_{2}$ grown under SrCl$_{2}$-flux has a misfit lattice structure with rock-salt type [Ca$_{2}$CoO$_{3}$] slabs and interpenetrating CdI$_{2}$-type [CoO$_{2}$] layers stacked along $c$-axis and incommensuration along $b$-axis. The nearest commensurate structure with ratio of sublattice parameters $b_{1}$/$b_{2}$ = 5:8 closely matches the 0.62 composition index. Analysis of HREM images and the presence of lines with diffuse scattering and weak spots on the ($h$0$l)$ DP lead to the tripled $a$ unit cell parameter, which was significantly different from the average structure determined by X-ray and neutrons. Work supported by the U.S. DOE, BES (DE-AC02-98CH10886). [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H43.00005: Infrared Studies of Charge Dynamics in Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals Jiufeng Tu, Zhijun Xu, Weidong Si, Qiang Li In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out infrared reflectivity studies of Ca$_{3}$Co$_{4}$O$_{9}$ thin films and single crystals as a function of frequency and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. Several important features have been observed: (1) the overall reflectivity is low as a result of a small carrier density in this system (the plasma frequency around 100 meV); (2) several phonon features are observed and some exhibit evidence of strong electron-phonon coupling; (3) a gap-like structure is seen in the low frequency region below 100 K that could be responsible to the insulating behavior observed in transport measurements at low temperatures. Further infrared studies will be performed in magnetic field (both parallel and perpendicular to the CoO$_{2}$ layers). A good understanding of our infrared results should shed light on the origin of high thermo-power in these 2D-layered cobaltates. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H43.00006: Thermoelectric properties of epitaxial Li$_{x}$CoO$_{2}$ thin films . Zhigang Ma, A. Venimadhav, Qi Li, X. X. Xi, H. P. Sun , Xiaoqing Pan We have studied the thermoelectric properties of layered cobaltate Li$_{x}$CoO$_{2}$ since the similar compound Na$_{x}$CoO$_{2}$ has shown exceptionally high thermoelectric power. Both \textit{in situ} epitaxial grown and topotaxial Li$_{x}$CoO$_{2}$ films have been achieved. Epitaxial films were grown by pulsed-laser deposition technique and topotaxial films were prepared by converting an epitaxial Co$_{3}$O$_{4}$ film to Li$_{x}$CoO$_{2 }$by annealing in Li vapor. X-ray diffraction analysis showed the films are $c$-axis oriented. For topotaxial Li$_{x}$CoO$_{2 }$the largest thermoelectric power of the samples is found to be around 380 $\mu $V/K at room temperature, while\textit{ in situ} films show thermopower of 100 $\mu $V/K. Both show semiconducting behaviors. The difference on the thermopower of the two types of samples will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H43.00007: Thermal Conductivity Minima in Superlattices and Localization-like Phenomena Rama Venkatasubramanian It is becoming clear in many 2-dimensional superlattice (SL) material systems that there exists a minimum lattice thermal conductivity for an optimal SL period. These have been first observed and reported in the Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$, PbTe/PbTeSe and Si/Ge SL systems by us in RTI. These minima become evident when the electronic thermal conductivity, using Lorentz parameter, is subtracted from the total thermal conductivity to monitor the lattice thermal conductivity as a function of SL period. The basis for the numerical value of Lorentz parameter, observed from many facets of material and device characteristics, will be presented. Such a lattice thermal conductivity minimum has also been recently observed in other SL material systems. Recently, a similar behavior has also been observed in the thermal conductivity of superlattices embedded with an ordered array of nanoparticles. We will explore the commonality of these results in terms of a localization-like behavior for phonons. The arguments for the complex relationship between the SL period and the low-frequency cut-off wavelength, traceable to a cut-off frequency originating from diffusive transport of a temperature wave, will be presented. The physics behind what triggers the localization-like phenomena of phonons in such nanostructures will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H43.00008: Si/Ge Superlattice Structures for Thermoelectric Power Generation James Caylor, Rama Venkatasubramanian Research conducted at RTI into the use of thin films, in particular superlattice materials, for thermoelectric power generation has focused on three materials families: Bi$_{2}$Te$_{3}$, PbTe, and Si/Ge. The Bi$_{2}$Te$_{3}$-based superlattice materials have already produced record ZT (thermoelectric figure-of-merit) and devices using these low temperature materials ($\sim $200\r{ }C) have surpassed bulk performance during power generation. RTI has also developed the growth of Si/Ge superlattice films as well as their fabrication into power generation devices applications at higher temperatures ($\sim $500\r{ }C). Results presented will include confirmation of superlattice structure via X-ray diffraction, showing well formed satellite peaks indicative of a high-quality superlattice. In addition, data will be presented that shows the lowering of thermal conductivity by the superlattice structure with nearly a 5x reduction in lattice thermal conductivity compared to the alloy film. Initial thin-film power device results showing $>$2{\%} efficiency and 2x improvement of ZT over SiGe alloys, at $\Delta $T of 378K and T$_{mean}$ of 484K, will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H43.00009: Ultra-low thermal conductivity in disordered, layered tungsten diselenide Catalin Chiritescu, David Cahill, Nguyen Ngoc, David Johnson, Arun Bodapati, Pawel Keblinski, Paul Zschack Ultra low thermal conductivity of tungsten diselenide (WSe$_{2})$ thin films is achieved by controlling order and disorder of two dimensional WSe$_{2}$ sheets. We prepared highly textured nanocrystalline WSe$_{2}$ films by modulated elemental reactant (MER) method. Synchrotron X-ray diffraction shows the WSe$_{2}$ sheets are well aligned with the Si (100) substrate and the films have completely random crystalline orientation in the a-b plane. The cross-plane thermal conductivity of thin films of WSe$_{2}$ is as small as 0.05 W/m-K at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe$_{2}$ and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. Molecular dynamics simulation on model structures suggests that the ultra-low thermal conductivity in layered, disordered crystals is a general phenomenon and not restricted to WSe$_{2}$. Ion irradiation of the samples disrupted the layered structure and the crystallinity of the WSe$_{2}$ sheets and lead to an increase with a factor of 3 in thermal conductivity. We attribute the ultra-low thermal conductivity to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe$_{2}$ sheets. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H43.00010: Micro- and nanomachined tools for measuring in-plane thermal conductivity of thermoelectric thin films R. Rahman, R. Sultan, F. Baset, B. L. Zink Many of the potential next-generation thermoelectric materials being studied are either thin films or nanostructures that are expected to have anisotropic properties. For example, the thermal conductivity of a layered thin film in the plane of the film, $k_{\parallel}$, is likely to be different from that perpendicular to the layers, $k_{\perp}$. Techniques such as the $3\omega$ method and picosecond thermoreflectance allow accurate measurements of $k_{\perp}$ at temperatures relevant to thermoelectrics, but measuring $k_{\parallel}$ is often difficult. In this talk we discuss our efforts to design and demonstrate accurate measurements of $k_{\parallel}$ of thin films from $77-475$ K using micro- and nanomachined thermal isolation platforms. Using thin-film structures to support the thin-film sample reduces background contributions, and careful control of the geometry keeps radiation errors small. We will also discuss plans for a next-generation device that will simultaneously measure thermal conductivity, thermopower, and electrical conductivity of a thin-film or nanostructure, allowing determination of the thermoelectric figure-of-merit, $ZT$. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H43.00011: Figure of merit for thermoelectric power generation estimated from enhanced mobility in [100] oriented \textit{$\beta $}-FeSi$_{2}$ thin film Hirofumi Kakemoto, Hajime Shibata, Satoshi Wada, Takaaki Tsurumi \textit{$\beta $}-FeSi$_{2}$ has been attracted to be applied to the thermoelectric device, for instance, the Seebeck coefficient shows the maximum value about 500$^{o}$C that it is good for thermoelectric power generation. However low figure of merit (Z) has been reported about 5x10$^{-4}$ K$^{-1}$. The Z is represented as $m$*$^{2/3}$(\textit{$\mu $}/\textit{$\kappa $}$_{ph})$, where $m$*, \textit{$\mu $} and \textit{$\kappa $}$_{ph}$ are effective mass, mobility and thermal conductivity, respectively. Although \textit{$\kappa $}$_{ph}$ is good for thermoelectric power, low \textit{$\mu $} has been reported as polaronic-conduction in \textit{$\beta $}-FeSi$_{2}$ crystal. In 3D electron density distribution of \textit{$\beta $}-FeSi$_{2}$ crystal, Si layer in the crystal shows covalent bonding network with Si atoms, and it suggests the new possibility for enhancement of \textit{$\mu $}. In this report, the objective is to exhibit the possibility for enhancement of Z in order to control the crystallographic orientation of \textit{$\beta $}-FeSi$_{2}$ crystal by means of film formation. \textit{$\beta $}-FeSi$_{2}$ thin film was prepared on Si(100) substrate using molecular beam epitaxy method. The crystallographic orientation of sample showed about 80{\%} of [100] direction from x-ray diffraction pattern. The transport properties were investigated using Hall measurement with van der Pauw electrode configuration. The resistivity and \textit{$\mu $} were also measured, and they were compared with \textit{$\beta $}-FeSi$_{2}$ polycrystal. In addition, enhancement of Z was estimated using above formula. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H43.00012: High-temperature ZT of InGaAlAs Thin Films with Embedded ErAs Nanoparticles Rajeev Singh, Zhixi Bian, Younes Ezzahri, Ali Shakouri, Gehong Zeng, John Bowers, Joshua Zide, Art Gossard We have measured the thermoelectric (TE) figure-of-merit (ZT) of InGaAlAs thin films with embedded ErAs nanoparticles over a wide temperature range (300K - 650K). This material system is currently being explored for use in power generation applications such as waste heat recovery. A novel high-speed measurement system was developed to measure the ZT of thin films of thicknesses on the order of 1um with a transient thermal signal resolution of 200ns at temperatures up to 900K. In order to resolve the intrinsic ZT of thin-film materials, TE devices were fabricated to minimize electrical and thermal parasitics and differential measurement was employed on TE devices of varying film thicknesses. The improvement in ZT of the material with ErAs nanoparticles embedded in the semiconductor matrix is verified throughout the temperature range. The increase in TE ZT is found to be mainly due to the reduction in material thermal conductivity due to phonon scattering by the ErAs nanoparticles. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H43.00013: Thermal Conductance Measurements of Aromatic and Aliphatic Self-Assembled Monolayers Robert Wang, Sung-Yeon Jang, R. A. Segalman, Arun Majumdar Thermal conductance measurements of solid-solid junctions separated by an interfacial organic self-assembled monolayer (SAM) suggest that molecular heterostructures are a promising new class of thermoelectric materials. Au-SAM-GaAs junctions were fabricated by nanotransfer printing and their thermal conductance was measured by the 3$\omega $ technique. SAMS investigated in this study consisted of quaterphenyldithiol and alkanedithiols of varying length. This study explores two key aspects of thermal transport in molecules: 1) the effect of aromatic versus aliphatic molecular structure 2) the effect of molecular length in the aliphatic molecule. Measurements of thermal conductance using the 3$\omega $ method were very robust to processing history and no thermal dependence on alkane chain length was observed. [Preview Abstract] |
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