Rama Venkatasubramanian, RTI International
Takao Mori, National Institute for Materials Science
Christopher Dames, University of California, Berkeley
Harald Boettner, Fraunhofer-Institut fuuml;r Physikalische Messtechnik IPM
Symposium Support Aldrich Materials Science
H2: Superlattices and Thin Films
Tuesday PM, April 02, 2013
Moscone West, Level 2, Room 2006
2:30 AM - *H2.01
3D Superlattice Ceramics of Strontium Titanate (STO) for Thermoelectrics
Kunihito Koumoto 1
1Nagoya University Nagoya JapanShow Abstract
We have demonstrated a quantum confinement effect giving rise to two dimensional electron gas (2DEG) in a 2D superlattice, STO/STO:Nb, which could generate giant thermopower while keeping high electrical conductivity. Then, a “synergistic nanostructuring” concept incorporating 2DEG grain boundaries as well as nanosizing of grains has been applied to our STO material and 3D superlattice ceramics was designed and proposed. This 3D superlattice ceramics was verified by numerical simulation to be capable of showing ZT>0.8 @300K. We, then, have attempted to develop a new process to realize 3D superlattice ceramics through hydrothermal synthesis of La-STO nanocubes, Nb attachment to nanocube surfaces, self-assembly of these nanocubes into 3D superlattices, followed by sintering in a reducing atmosphere.
3:00 AM - H2.02
Enhanced Power Factor in Strained Silicon Nanomesh Thin Film
Xiao Guo 1 Bingyuan Huang 1 Duck Hyun Lee 1 Anish Tuteja 1 Peter Green 1 Akram Boukai 1
1University of Michigan Ann Arbor USAShow Abstract
The thermoelectric figure of merit is determined by ZT=S2T/ρκ, in which S2/ρ is the power factor, S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the temperature. It has been known that tensile strained n-type silicon exhibits splitting of the six-fold degenerate conduction band , which leads to decreased inter-valley scattering and increased electron mobility . Silicon nanomesh materials also result in a decrease of the thermal conductivity due to increased phonon scattering. Thus, ZT could be potentially increased by using n-type tensile strained silicon nanomesh thin films.
Here we report a unique method of patterning nanomesh features with self-assembled block copolymers on tensile strained SOI that yields an enhacement of the power factor over unstrained silicon. The nanoscale features are obtained by transferring the self-assembled block copolymer pattern to the underlying silicon device layer by reactive ion etching. Seebeck coefficient and electrical resistivity measurements are performed on the tensile strained silicon nanomesh devices in an evacuated cryostat over a wide temperature range.
 C. Euaruksakul, et al. Influence of Strain on the Conduction Band Structure of Strained Silicon Nanomembranes. Phys. Rev 101, 147403 (2008)
 F. Schäffler. High-mobility Si and Ge structures. Semicond. Sci. Technol 12, 1515-1549 (1997)
 R. Ruiz, et al. Density multiplication and improved lithography by directed block copolymer assembly. Science 321, 936-939 (2008)
3:15 AM - H2.03
High-temperature Stability and Thermoelectric Properties of NaxCoO2 Thin Films and Superlattices
Peter Brinks 1 Guus Rijnders 1 Mark Huijben 1
1University of Twente Enschede NetherlandsShow Abstract
Oxide materials have attracted much attention as potential thermoelectric materials, especially since the discovery of a high thermoelectric power factor of 50 mu;W/K2cm in NaxCoO2 single crystals. However, the overall thermoelectric performance remains limited, because of the relatively high thermal conductivity of NaxCoO2 and other cobaltites. Significant reduction of the thermal conductivity of NaxCoO2 remains a challenge and no successful attempts have been reported. We present a study that aims at suppressing the thermal conductivity in NaxCoO2 by confinement in thin films and superlattices.
Previously it is shown that single-phase NaxCoO2 thin films can be deposited by pulsed laser deposition and that chemical stability can be achieved by deposition of an amorphous AlOx capping layer.  These thin films of NaxCoO2 are shown to have thermoelectric potential based on their room temperature properties, which are approaching the properties of single crystals. [1, 2]
We demonstrate structural and chemical stability of these capped NaxCoO2 thin films up to approximately 800K, in addition to the previously reported room temperature stability . Furthermore, we present high-temperature thermoelectric properties of various NaxCoO2 capped thin films, focusing on the effect of layer thickness on the properties to demonstrate the effect of confining NaxCoO2 in thin films. These high-temperature measurements reveal the thermoelectric potential of NaxCoO2 thin films.
Additionally, we present the growth and characterization of thermoelectric superlattices, with NaxCoO2 layers as thermoelectric building blocks. Various insulating materials are used as barrier layer and their effect on the thermoelectric properties of these cobaltite superlattices is shown. Additionally, the effect of the layer thickness within these samples is shown and the high-temperature thermoelectric properties of these NaxCoO2 based superlattices are presented.
 I. Terasaki, Y. Sasago and K. Uchinokura, Phys. Rev. B, 1997, 56, R12685-R12687
 P. Brinks, H. Heijmerikx, T.A. Hendriks, G. Rijnders, M. Huijben, RSC Advances, 2012, 2, 6023-6027
3:30 AM - H2.04
Thermoelectric Properties of Electrically Gated Silicon Nanowires - Towards Volume Inversion and Optimal Power Factor
Benjamin Michael Curtin 1 John E. Bowers 1
1University of California Santa Barbara USAShow Abstract
Since the observation that nanostructured single-crystalline silicon can achieve high zT through large reductions in thermal conductivity, considerable effort has been placed into understanding the viability of silicon as a thermoelectric material. As the thermal conductivity of nanostructured Si approaches the amorphous limit, further increases in zT will need to be made with strategies that improve power factor. In optimally doped n-type Si, the high density of ionized impurities strongly scatters charge carriers, which results in a relatively low electron mobility. This presents a unique opportunity for improving the power factor of Si nanowires (NWs), where an electrical gate-all-around structure can modulate a conductive channel inside an intrinsic NW. Similar to field-effect transistors, the electrical gate is used to induce mobile charge within the Si NW while maintaining high mobility due to the absence of ionized impurities. In this work, we model the thermoelectric properties of gated Si NWs with the 1-D Boltzmann transport equation (BTE) and by self-consistently solving the Poisson and Schrödinger equations for various NW geometries and gate biases. We first validated our BTE solver and scattering rates with n-type bulk Si and found good agreement between simulated and published optimal power factor, which was ~4 x 10-3 W/m-K2 at 300 K for a doping density of ~5 x 1019 cm-3. For gated Si NWs, we find that the room temperature power factor approaches 1 x 10-2 W/m-K2 for Si NWs with diameters < 10 nm and positive gate bias. As the NW diameter increases, the gated power factor decreases below the optimal n-type bulk Si value because the conductive area relative to the cross-sectional area of the NW decreases. We are currently implementing surface roughness scattering in these calculations, which will likely reduce the power factor of both gated and highly doped Si NWs with small diameters. A discussion of gated Si NW thermoelectrics will be presented along with an analysis of our modeling results.
We are also processing gated Si NW devices on silicon-on-insulator (SOI) substrates to experimentally determine power factor enhancement. Si NWs with diameters of ~25 nm are fabricated using electron-beam lithography to define the NWs, which are then etched into SOI substrates and thermally oxidized to both reduce their diameter and grow a thin gate oxide. An aluminum gate is then deposited around the Si NWs, and Ti/Pt electrodes, heaters, and thermometers are patterned on top of the NWs for Seebeck and electrical conductivity measurements. Process development is currently underway and recent progress will be presented.
4:15 AM - *H2.05
Hierarchical Length-scale Influence in Bulk Nanostructured Thermoelectrics
Vinayak P. Dravid 1
1Northwestern University Evanston USAShow Abstract
There are exciting emerging prospects for understanding and tailoring microstructure of bulk thermoelectrics with due attention to the hierarchical length-scale influence across atomic-, nano- and micro-meter dimensions. Utilizing this core idea, we have recently demonstrated that nanostructured PbTe- and PbSe-based bulk thermoelectric systems exhibit high figure of merit (ZT) that reduce thermal conductivity through multiple means, and does not appreciably compromise the power factor. In the presentation, we will present this intricate but tractable relationship between various microstructural attributes (point, line and interfacial defects) and lattice thermal conductivity in important nanostructured thermoelectric systems based on PbTe and PbSe matrices. Strategies for improvement in power factor through interfacial mediation and dual-nanostructuring will be discussed. The need for overarching approach will be emphasized for understanding the hierarchical length-scale influence to enable specific design strategies for the next generation thermoelectric materials.
4:45 AM - H2.06
Interfacial Polarization Enhanced Thermoelectric Properties in III-Nitride Heterostructures and Superlattices
Alexander Sztein 1 John E Bowers 1 2 Steven P Denbaars 1 2 Shuji Nakamura 1 2
1University of California, Santa Barbara Santa Barbara USA2University of California, Santa Barbara Santa Barbara USAShow Abstract
Despite the great promise offered by thermoelectric devices for solid-state electricity generation, heating, and cooling, there has been only limited progress in improving the efficiency of thermoelectric devices. Improvements in device efficiencies and material ZT have been difficult due to the correlations that link the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity. Many recent thermoelectric strategies focus on methods of breaking these relationships in order to improve ZT. A new strategy is demonstrated in this study which uses the exceptionally large interfacial polarization charges present in III-Nitride materials to confine electrons to high mobility channels near interfaces, thus selectively reducing electron scattering and breaking the classic relationships between electrical conductivity, Seebeck coefficient, and thermal conductivity and allowing improvements in ZT. Previous reports have shown ZT < 1 for III-Nitride materials due either to high thermal conductivity in binary GaN, or low power factors (S2*σ) in ternary III-Nitrides. This new strategy allows a combination of the low thermal conductivities present in ternary materials with the high power factors of binary III-Nitrides for improved ZT.
Enhanced power factors are demonstrated in metal organic chemical vapor deposition (MOCVD) grown thin film GaN/InAlN heterostructures, GaN/AlN/InAlN heterostructures and GaN/AlN/AlGaN superlattices. GaN/InAlN samples with InAlN thicknesses of 34 nm have shown power factors as high as 2×10-4 W/mK2, which is three times higher than InAlN alone. The inclusion of a 1 nm AlN interlayer greatly improves electron mobility while leaving the Seebeck coefficient unchanged and results in power factors as high as 8.4×10-4 W/mK2, which is an order of magnitude improvement over bulk InAlN. Finally, high mobility GaN/AlN/AlGaN (7 nm/1 nm/2 nm) superlattices are shown to display power factors as high as 2.3×10-3 W/mK2, which is even higher than GaN without alloying or nanostructures. These power factor enhancements are due to the spatial confinement of electrons on the GaN side of interfaces caused by the interfacial polarization charges present in this material system. This confinement largely isolates the electrons from both alloy and ionized impurity scattering centers, resulting in improved electron mobilities with relatively unchanged Seebeck coefficients and thus realizing power factor improvements. Enhanced power factors have been demonstrated to extend through temperatures as high as 815 K. In-plane thermal conductivity measurements of GaN/AlN/AlGaN superlattices are currently underway.
5:00 AM - H2.07
Plasma Treated Flexible CNT Sheet with Improved Thermoelectric Properties
Weiyun Zhao 1 Hui Teng Tan 1 Qingyu Yan 1
1Nanyang Technological University Singapore SingaporeShow Abstract
Although theoretical calculation indicated that the thermoelectric figure of merit, ZT, of carbon nanotubes (CNTs) could reach >2, typical experimentally reported ZT values of CNTs are in the range of 0.001 to 0.01, which is too low for thermal energy conversion applications. Herein, we modified flexible CNT sheets via plasma irradiation for thermoelectric applications. The ZT values of the CNT sheets could be significantly enhanced after enough plasma irradiation. A maximum ZT value of plasma treated CNT sheet is around 40 times than the pristine one. The improved thermoelectric properties were mainly due to the great increased Seebeck coefficient and reduced thermal conductivity. It was found that the plasma irradiation have the effects on tuning carrier concentration and change electrical conduction behaviour. In an attempt to further enhance the power factor of the system and shift the Seebeck coefficient peak position, plasma treated CNTs can be decorated by some dopants to further tuning the carrier concentration and modify the electronic structure. Such improvement makes the plasma treated CNT sheets promising as a new type of thermoelectric materials for certain niche applications as they are easily processed, mechanically flexible and durable, and chemically stable.
5:15 AM - H2.08
Silicon Nanostructures for Thermoelectric Applications
Hartmut S. Leipner 1 Peter Werner 2 Nadine Geyer 2 Katrin Bertram 1 Markus Trutschel 1 Bodo Fuhrmann 1 Aleksandr Tonkikh 2
1Martin Luther University Halle Germany2Max Planck Institut for Microstructure Physics Halle GermanyShow Abstract
Si-Ge superlattices are expected to have an increased figure of merit due to the two- dimensional structure. A high ZT can be related to a decrease in the cross-plane thermal conductivity as a result of the phonon scattering at the interfaces. Sim-Gen superlattices with stacks of m Si and n Ge layers of different thicknesses and doping levels were grown by molecular beam epitaxy on (001) or (111) Si substrates. The structures were characterized by cross-section transmission electron microscopy. The lithographic preparation and measurement schemes for the cross plane transport properties in dependence of temperature are outlined, and the results are presented.
In a further approach, we prepared Si and Si-Ge nanopillars. They were produced in a top-down fabrication scheme by metal-assisted chemical wet etching . The understanding of the etching mechanism  is the prerequisite of the precise fabrication of nanopillars with hexagonal symmetry and adjustable diameters ranging from about 10 nm to several micrometers over large areas. High area densities and the control of diameter, length, position, and the internal structure of the nanowires are possible. Well-defined nanopillars with diameters below 10 nm exhibit important size-dependent quantum effects, which account for the decoupling of the electrical conductivity, the Seebeck coefficient, and the thermal conductivity. Furthermore, Si-Ge superlattice nanopillars are promising candidates to reduce further the thermal conductivity.
In a third approach, we investigate nanocrystalline silicon particles embedded in an oxide matrix of SiO2 as effective thermoelectric hybrid materials. These quantum dots are formed in a phase-separation process in thin films deposited by chemical or physical vapor deposition. Alternatively, a solid-state transformation is used in a quartz-aluminum system to form Si nanoparticles in an Al2O3 layer. The formation of the nanocrystals can be tuned by rapid thermal annealing with respect to the uniformity in size, distribution, and surface structure. In order to maximize the thermoelectric power factor, a high doping level of the particles is required. With the low thermal conductivity of the amorphous matrix, a figure of merit close to 1 may be achieved at room temperature.
 N. Geyer, Z. Huang, B. Fuhrmann, S. Grimm, M. Reiche, T.-K. Nguyen-Duc, J. de Boor, H. S. Leipner, P. Werner, U. Gösele, Nano Lett. 2009, 9, 3106-3110.
 N. Geyer, B. Fuhrmann, Z. Huang, J. de Boor, H. S. Leipner, P. Werner, J. Phys. Chem. C 2012, 116 (2012) 13446-13451.
H1: Novel Materials and New Approaches I
Tuesday AM, April 02, 2013
Moscone West, Level 2, Room 2006
9:00 AM - *H1.01
Some Strategies for Enhancing ZT
Mildred Dresselhaus 1
1MIT Cambridge USAShow Abstract
The thermoelectric field has advanced significantly since its renaissance in 1992 with the concept of nano-thermoelectricity. Once this concept gained favor by the research community, interest in this research field increased as progress was made in increasing ZT through the introduction of nanostructures to allow some independent control of the electrical and thermal conductivity. Twenty years have passed since the Hicks-Dresselhaus papers were published and now new concepts for increasing ZT have been introduced and some have even been demonstrated. A review of the status of progress will be given.
H3: Poster Session: Nanoscale Thermoelectrics
Tuesday PM, April 02, 2013
Marriott Marquis, Yerba Buena Level, Salons 7-8-9
9:00 AM - H3.01
Thermoelectric Properties of an alpha-Rhombohedral Boron Related Compound with Sulfur
Oksana Sologub 1 Yoshitaka Matsushita 1 Takao Mori 1 2
1National Institute for Materials Science (NIMS) Tsukuba Japan2University of Tsukuba Tsukuba JapanShow Abstract
There has been a high activity to find viable thermoelectric (TE) materials. One need exists to develop materials which can function at high temperature, T, for applications utilizing high temperature waste heat, such as focused solar power, RTG, etc. Boron-rich compounds are attractive materials for their stability, typically exhibiting melting points above 2200 K, and they have been found to possess intrinsic low thermal conductivity, κ, even for single crystals . Novel borides promising for TE were found, such as p-type REB44Si2 , and REB17CN, REB22C2N, and REB28.5C4, the long awaited n-type counterparts to p-type boron carbide , which is one of the few previously commercialized TE materials . In a further recent striking development, YAlB14 was found to be able to be controlled and strong p and n characteristics were freely controlled in a compound with the same crystal structure and same composing elements .
In this work, a series of icosahedral boron rich solids, with nominal compositions B6S1-x (0.37 lE;x lE;0.40) have been synthesized from the reaction of amorphous boron and excess of sulfur at 1473 -1573 K in a BN crucible using a RF furnace. Rietveld analysis of intensity data collected with synchrotron and conventional X-ray sources revealed a rhombohedral structure with the alpha-rhombohedral boron framework of B12 icosahedra and a narrow filling fraction range for sulfur atoms in the octahedral voids. Although obtained samples had high resistivities due to low densities, B6S1-x exhibited p-type semiconducting behavior similar to boron carbide with large Seebeck coefficients reaching a maximum of ~220 mu;VK-1 at the highest measured temperature 810 K.
 T. Mori, “Higher Borides” in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 38, (North-Holland, Amsterdam, 2008) pp. 105-173 (2008).
 T. Mori et al., J. Appl. Phys. 97 (2005) 093703, Dalton Trans. 39 (2010) 1027 (Hot Article).
 T. Mori et al., J. Solid State Chem. 179 (2006) 2908, J. Appl. Phys. 101 (2007) 093714.
 C. Wood et al., Phys. Rev. B 29 (1984) 4582.
 S. Maruyama et al., Appl. Phys. Lett. 101 (2012) 152101.
9:00 AM - H3.02
Mg2Si Composites for High Temperature Thermoelectric Applications
Julia V. Zaikina 1 Susan M. Kauzlarich 1
1University of California at Davis Davis USAShow Abstract
Thermoelectric devices directly convert heat into electric energy and thus can be used for the waste heat recovery in automotive engines. Efficient thermoelectric material should combine high electrical conductivity and Seebeck coefficient with low thermal conductivity, thus making figure-of-merit zT reasonably high. Other requirements for the material include high stability at the temperature range of the exhaust manifold of automobile engines, low cost and light weight. Magnesium silicide Mg2Si a promising material, since it is stable at high temperatures and consists of earth abundant, light weight elements. Electrical conductivity of pristine Mg2Si can be tuned by changing the carrier concentration with dopants to either p- or n-type. However thermal conductivity of Mg2Si remains fairly high (above 40 mW/cmK). Nanostructuring of Mg2Si was recently suggested as a new approach to reduce thermal conductivity and improve zT. It was shown that thermal conductivity of Mg2Si can be reduced by introducing Si nanoparticles, which act as phonon scattering centers. This approach can be further extended by introducing Ge in the form of nanoparticles. Mg2Si composites with embedded Ge nanoparticles were prepared by two approaches. In the first one, Ge nanoparticles were produced by solution assisted method developed in our group. In the second one, Ge was ball milled together with Si in order to obtain micron size powders of Si1-xGex. Mg2Si nanocomposites with different concentrations of Ge were successfully synthesized utilizing reaction of MgH2 with Si/Ge and densified by means of spark plasma sintering (SPS) technique. The presence of hydride provides the reducing atmosphere and minimizes magnesium oxide impurity according to powder X-ray diffraction of the products. Microprobe analysis showed that Ge is distributed over both phases, Mg2Si and Si, thus leading to the Mg2Si1-xGex and Si1-xGex phases. The addition of Ge either in the form of nanoparticles prepared by solution assisted method or by alloying of Si with Ge leads to the considerable lowering of thermal conductivity. Further optimization of the electrical properties of the Mg2Si/Ge nanocomposites by introducing different concentrations of dopants is currently underway. The results of the thermoelectric performance optimization of Mg2Si nanocomposites will be discussed.
9:00 AM - H3.03
Thermal Diffusivity Measurements of Thermoelectric Nanocomposite Using Infrared Thermography
Lalat Indu Giri 1 Manish Sharma 1 Suneet Tuli 1
1Indian Institute of Technology Delhi New Delhi IndiaShow Abstract
The paper reports thermal diffusivity measurements of porous anodic alumina (AAO) templates and bismuth telluride nanowires embedded in AAO matrix. The technique combines infrared (IR) lock-in thermography and a simple data evaluation procedure for a fast noncontact measurement of thermal diffusivity. AAO templates are extensively used for electrodeposition of bismuth telluride nanowires for thermoelectric applications etc. Thermal characterization of individual thermoelectric nanowire is a big challenge. So one way of estimating the value is to perform measurements on empty AAO templates and nanowires embedded template matrix. The reported methods of thermal diffusivity measurements for AAO templates are the 3omega; method and the photothermoelectric technique which are both contact methods. Till date Infrared thermography based study of AAO samples are not reported.
Our study reports application of a noncontact process similar to the Armstrong IR Thermography method in measuring the thermal diffusivity of AAO templates. The study includes measurement of perpendicular channel thermal diffusivity values of 20nm, 100nm and 200nm pore AAO templates. Also included is the thermal diffusivity of bismuth telluride nanowires embedded AAO matix. The technique is based on the non contact IR Lock-in thermography [18,19] detection of oscillating temperature distribution, i. e., thermal wave, produced by the absorption of an intensity modulated optical laser beam. The sample is subjected to a periodic heat flux by a semiconductor laser and an infrared image sequence is recorded as a function of time with an infrared camera. In our work, we have used both reflective and transmissive geometries, and we give a detailed comparison of the benefits and disadvantages of both. We describe the equipment needed as well as the mathematical modelling of the dynamic equations of heat propagation through the thermoelectric nanocomposite material. In the end, we present results to show that our technique can be used to measure thermal diffusivity of thermoelectric nanowires to a high degree of accuracy. The technique lends itself to use for probing any three-dimensional conductiing material at the micron scale.
9:00 AM - H3.04
Modelling of Thermal Properties in Silicon Nanostructures
Emigdio Chavez 1 2 John Cuffe 1 Francesc Alsina 1 Juan Sebastian Reparaz 1 Clivia Sotomayor Torres 1 2 3
1Catalan Institute of Nanotechnology (ICN) Barcelona Spain2Universitat Autonoma de Barcelona Barcelona Spain3Catalan Institution for Research and Advanced Studies (ICREA) Barcelona SpainShow Abstract
Ultra-thin silicon membranes and nanowires have been the subject of extensive studies due to their low dimensionality that leads to enhanced thermo-electric properties and improved figure of merit, ZT. This increase of ZT is attributed to the decrease of the thermal conductivity, which is predicted to be related in part to the modification of the acoustic dispersion relation due to periodicity, e.g. phononic crystal, or spatial confinement of the phonon modes, e.g. nanowires.
We investigate the acoustic phonon dispersion in ultra-thin free-standing silicon membranes and nanowires based on the elastic continuum model. The thermal properties are calculated comparing both, the modified dispersion relation and Debye dispersion relation approximation. In addition, we explicitly calculate the relative contribution of each scattering processes to the total relaxation time.
The theoretical predictions are compared with other reported theoretical and experimental results as well as with our measurements.
9:00 AM - H3.05
Phonon Normal Mode Analysis Based on Anharmonic Phonon Eigenvectors
Tianli Feng 1 Bo Qiu 1 Xiulin Ruan 1
1Purdue University West Lafayette USAShow Abstract
A scheme of normal mode analysis based on anharmonic phonon eigenvectors to study the thermal properties is proposed. Compared to the traditional normal mode analysis, our scheme has several advantages of more convenience, stronger capability, and higher accuracy. As an application, the phonon properties such as the phonon dispersion, relaxation time, mean free path and thermal conductivity of bulk argon and bulk germanium at different temperatures have been investigated using this scheme based on classical molecular dynamics (MD). We found the dependence of phonon relaxation time to frequency omega; and temperature T for different temperatures and different modes vary, respectively, from ~omega;^(-1.3) to ~omega;^(-1.8) and ~T^(-0.8) to ~T^(-1.8) for argon, and from ~omega;^(-0.6) to ~omega;^(-2.8) and ~T^(-0.4) to ~T^(-2.5) for germanium. The predicted thermal conductivities are in reasonable agreement with these obtained from classical Green-Kubo method and more accurate than other Boltzmann Transport Equation based MD method in high temperature range. The effective mean free path for argon and germanium at 20K and room temperature (300K) are approximately 2.5nm and 39nm after correction, respectively. The most promising application of our scheme is to calculate the thermal properties of materials based on First principle MD or Tight Binding MD with atomic potential unknown, which should produce a bright prospect and a fairly competitive area in the future.
9:00 AM - H3.07
Colossal Thermoelectric Power Factor in K7/8RhO2
Nirpendra Singh 1 2 Yasir Saeed 1 Udo Schwingenschlogl 1
1King Abdullah University of Science and Technology Thuwal Saudi Arabia2KAUST Thuwal Saudi ArabiaShow Abstract
The thermoelectric properties of the layered oxides KxRhO2 (x = 1/2 and 7/8) are investigated by means of the electronic structure, as determined by ab inito calculations and Boltzmann transport theory. In general, the electronic structure of KxRhO2 is similar to NaxCoO2, but with strongly enhanced transport. K7/8RhO2 exceeds the ultrahigh power factor of Na0.88CoO2 reported previously by more than 50%. The roles of the cation concentration and the lattice parameters in the transport properties in this class of compounds are explained.
9:00 AM - H3.08
Pyroelectric Nanogenerators as Self-powered Sensors
Ya Yang 1 Zhong Lin Wang 1
1Georgia Institute of Technology Atlanta USAShow Abstract
Micro/nanosensors have attracted much attention due to their potential applications in detecting the micro/nano-objects such as a particle, cell, DNA, and so on. Micro/nanomaterials are important for fabricating these small scale sensors. The self-powered nanotechnology is based on driving a nanodevice by harvesting energy from its working environment instead of a conventional battery or any other energy storage/supply system. There is an urgent need to develop nanotechnology that harvests energy from the environment to power these sensors. It means that we can use the pyroelectric nanogenerator as a self-powered temperature sensor that automatically detects temperature without using a battery as the power source. This approach can greatly enhance the adaptability and mobility of such devices. Although some temperature sensors have been reported, there are few studies about using a single PZT micro/nanowire pyroelectric nanogenerator as a self-powered temperature sensor. Here, a single PZT micro/nanowire pyroelectric nanogenerator was fabricated, which was used as a self-powered temperature sensor. The output voltage of the sensor was found to linearly increase with an increasing rate of change in temperature of the detected heat sources. The response time and reset time of the fabricated sensor are about 0.9 and 3 s, respectively. It can be used to detect the minimum temperature change of about 0.4 K at room temperature. We also demonstrated that the temperature sensor can be used to detect the temperature of the finger surfaces and light a LCD under a heated temperature of 473 K. The self-powered temperature sensors developed here have potential applications in temperature measurements in environmental sciences, safety monitoring, medical diagnostics, and more.
 Ya Yang, Jong Hoon Jung, Byung Kil Yun, Fang Zhang, Ken C. Pradel, Wenxi Guo, and Zhong Lin Wang. Flexible pyroelectric nanogenerators using a composite structure of lead-free KNbO3 nanowires. Advanced Materials. 2012, 24, 5357-5362.
 Ya Yang, Yusheng Zhou, Jyh Ming Wu, and Zhong Lin Wang. Single micro/nanowire pyroelectric nanogenerators as self-powered temperature sensors. ACS Nano. 2012, 6, 8456-8461.
9:00 AM - H3.09
Phonon Transport in an Initially Twisted Nanowire for Thermoelectric Applications
Monrudee Liangruksa 1
1National Science and Technology Development Agency Klong Luang ThailandShow Abstract
Phonon transport in a low dimensional nanostructure has played a key role in determining the lattice thermal conductivity. In most semiconductors and insulators where the electron conduction is relatively low, phonons are the dominant energy carriers. Thus the quantitative understanding of their interactions has become crucial for micro/nanoscale applications. It normally requires a material with the reduced phonon thermal conductivity to enhance the performance of thermoelectric materials. The incorporation of structure complexity and phonon engineering can be employed to design a novel material with the favorable thermal conductivity. In addition, the stress field in a confined structure could be a parameter for future phonon engineering since varying mechanical stress can alter the velocity of phonons. However, it still lacks of knowledge on the influence of phonon tailoring by mechanical stresses. We consider an initially twisted nanowire made of a conductive polymers and investigate the influence of mechanical stress due to torsion on the phonon transport using the continuum approach. Phonon dispersion relation, phonon group velocity, and lattice thermal conductivity are computed to provide a complete picture of the transport mechanism. This study can suggest a phonon engineering approach to tune the conductivity of nanomaterials.
9:00 AM - H3.11
Nanostructured Bulk Zinc Oxide for Thermoelectrics
Markus Engenhorst 1 Devendraprakash Gautam 1 Carolin Schilling 1 Markus Winterer 1 Gabi Schierning 1 Roland Schmechel 1
1University of Duisburg-Essen Duisburg GermanyShow Abstract
In search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our approach starts with very small (<10 nm) aluminum-doped ZnO nanoparticles prepared by chemical vapor synthesis in a hot wall reactor. The nominal doping concentration was varied between 0.5 % and 8 %. In order to obtain bulk nanostructured solid bodies, the powders were compacted in a current-activated pressure-assisted densification (CAPAD) process at 900 °C. Electrical transport coefficients and thermal conductivity were characterized in an Ulvac ZEM-3 system and in a Netzsch LFA 457 laserflash apparatus, respectively.
Very high electrical conductivities with values considerably above 105 S/m can be observed over a broad temperature range, independent of the doping concentration. The high conductivity values indicate a highly efficient doping mechanism during the synthesis and densification, but the high charge carrier concentration adversely affects the Seebeck coefficient. Using modified synthesis conditions, the Seebeck coefficient can be increased to minus;110 µV/K at 700 °C resulting in a power factor of 4×10-4 W/m/K2.
Microstructural analysis of the sintered samples clearly shows that addition of aluminum beyond its solubility limit in ZnO hinders grain growth and leads to the formation of ZnAl2O4 precipitates. This is confirmed by the thermal conductivity values which strongly decrease with increasing aluminum content. The sample with the highest figure of merit zT=0.09 at 700 °C shows a low thermal conductivity of 4 W/m/K.
9:00 AM - H3.12
Fabrication and Characterization of Nanostructured Bulk Skutterudites
Muhammet Toprak 1 2 Mohsin Saleemi 1 Mohsen Y Tafti 1
1KTH Royal Institute of Technology Kista-Stockholm Sweden2Yildirim Beyazit University Ankara TurkeyShow Abstract
Latest nanotechnology concepts applied in TE research have opened many new avenues to improve the zT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Therefore, demands for enhanced zT of bulk NS materials have expanded the research on large scale production of TE materials. Bulk NS TEs, on the other hand, are normally fabricated using a bulk process rather than a nano-fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices.
We developed fabrication strategies for bulk nanostructured skutterudite materials based on CoSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS CoSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk bulk quantities TE nanopowder fabrication.
This work has been funded by EC-FP7 program under NEXTEC project and in part by the Swedish Foundation of Strategic Research - SSF.
9:00 AM - H3.14
Anomalous Interatomic Force Constant in Rocksalt-like Materials with Narrow Band Gap
Sangyeop Lee 1 Keivan Esfarjani 2 Tengfei Luo 3 Gang Chen 1
1Massachusetts Institute of Technology Cambridge USA2Rutgers University New Brunswick USA3University of Notre Dame Notre Dame USAShow Abstract
Materials having rocksalt-like structures and narrow band gaps have great importance for thermoelectrics and phase change materials. For example, PbTe, Bi2Te3, and Bi-Sb alloy are the best thermoelectric materials from sub-room temperature to high temperature. In addition, pseudo binary solutions of GeTe and Sb2Te3 are considered the most promising candidate for phase change materials due to their large optical contrast and fast phase transition. For both thermoelectrics and phase change materials, it is essential to have better understanding on lattice dynamics to reduce thermal conductivity and enhance phase transition characteristics. In this presentation, we will show using first principles that fourth neighbors in the rocksalt-like structures (PbTe, Bi2Te3, Bi, and Sb) commonly exhibit stronger interatomic force constants than second and third neighbors even though the atomic distance is longer. The reasons of this anomalous behavior are collinear bonding in rocksalt-like structures and large electronic polarizability. The large electronic polarizability in PbTe and Bi2Te3 can be understood within the context of the resonant bonding theory. In Bi and Sb, the large electronic polarizability is due to their semi-metallic behavior. This observation will lead to the better understanding on lattice thermal transport and phase change characteristics.
Acknowledgment: This work is support by AFOSR MURI through OSU (S.L. and G.C.) and S3TEC, a DOE EFRC (K.E. and T.L.).
9:00 AM - H3.15
First principle Studies of electronic and Thermoelectric Properties of AlxZn1-xO and CuxZn1-xO
Shiyun Xiong 1 Oleksiy Pokropivny 1 Jose Ordonez 1 Pietro Cortona 2 Sebastian Volz 1
1Ecole Centrale Paris CHamp;#194;TENAY-MALABRY Cedex France2Ecole Centrale Paris CHamp;#194;TENAY-MALABRY Cedex FranceShow Abstract
ZnO is a semiconductor with large Seebeck coefficient (S) while very low electrical conductivity (σ). It is found σ can be improved without too much decrease of S by doping with metallic elements and this made it to be a possible thermoelectric candidate. In this paper, we are trying to improve thermoelectric performance of ZnO with the doping of Al and Cu. We studied electronic and thermoelectric properties of ZnO, AlxZn1-xO and CuxZn1-xO (x=0.05-0.2) by using the density functional theory and the Boltzmann semi-classical equations. It is found that pure ZnO has a high Seebeck coefficient while the electrical conductivity remains very low. With the doping of aluminum, σ increases dramatically (by three orders of magnitude at most), while still presenting a large Seebeck coefficient (80-140 mu;V/K). Those trends lead to a significant increase of the power factor. However, Cu doping does not help a lot to increase the power factor as the Seebeck coefficient decreases substantially even though the electrical conductivity increases. Current improvements of the power factor for AlxZn1-xO are still not satisfactory for targeting applications and further studies should be performed.
9:00 AM - H3.16
Piezoelectric and Thermal Conductivity Coefficients of Bulk ZnO Using MD Simulations
Wassim Mohammad Fa'ez Kassem 1 Juliana Jaramillo-Fernandez 1 2 Emmanuel Ollier 2 Yann Chalopin 1 Sebastian Volz 1
1Ecole Centrale Paris, CNRS-UPR 288 Champ;#226;tenay-Malabry France2CEA Grenoble FranceShow Abstract
We study the effect of piezoelectric stress on the thermal conductivity in dielectric material from the point of view of a bulk crystal. Equilibrium MD simulations are used to calculate the piezoelectric coefficients and thermal conductivity of a perfect ZnO crystal, and the connection between the two is made initially through the classical picture of phonons and kinetic theory. The model used for ZnO in the study is a core only one, i.e. the effect of electron shell delocalization on the polarization is not considered. We report the calculation of piezoelectric coefficients such as e33 and e31 using fluctuation-dissipation theorem and of the thermal conductivity using Green-Kubo formulation i.e. equilibrium molecular dynamics, and compare our results with those published in other papers. As such, the effect of the induced strain on the bulk thermal conductivity is studied in hopes of locating a suitable avenue for optimization.
9:00 AM - H3.20
Enhanced ZT due to Energy Filtering Effect in Thermoelectric Nano-composites Based on Facile Solution Phase Synthesis of Dumbbell-like PbTe-Ag2Te-PbTe Heterostructures
Haoran Yang 1 Je-Hyeong Bahk 2 Haiyu Fang 1 Genqiang Zhang 1 Ali Shakouri 2 3 Yue Wu 1 2
1Purdue University West Lafayette USA2Purdue University West Lafayette USA3Purdue University West Lafayette USAShow Abstract
Abstract: Improving energy efficiency by converting waste heat into electricity based on thermoelectric is of great interest, and the key to the fulfillment of thermoelectric waste heat recovery lies in the design and fabrication of high-performance thermoelectric materials. Here, we present a pioneering design and synthesis of dumbbell-like PbTe-Ag2Te-PbTe heterostructures and demonstrate the fabrication of these nano building blocks into “nano-bulk” thermoelectric composites. The research is motivated by the previous experimental observation of the high thermoelectric figure of merit (ZT) in thin film superlattice structures and nano-inclusions-in-bulk structures as well as the theoretical prediction of significantly reduced thermal conductivity due to phonon scattering at nanoscale surface/interface and enhanced power factor due to energy filtering. Experimental studies showed 35% increase of ZT in our nano-bulk composite comparing to ZT in the bulk counterparts primarily due to the enhancement in Seebeck coefficient. Theoretical modeling showed that the exceptionally high Seebeck coefficient approaching 400 mu;V/K at 400 K could be explained by energy filtering effect. Our synthesis approach of the dumbbell-like heterostructures can be readily modified for a serial of telluride cation heterostructures such as PbTe-Bi2Te3-PbTe, Bi2Te3-Ag2Te-Bi2Te3, etc, which facilitates the design of nano-composites with optimal band alignments in order to achieve even higher ZT. More importantly, unlike the other nanotechnology-based thermoelectric research, our synthesis approach based on wet chemistry is facile and scalable, which enables the fabrication of nanostructured bulk materials for the real production of thermoelectric modules with superior performance compared to the state of the art.
9:00 AM - H3.21
Cage-breathing Lattice Dynamics of Pnicogen-ring-confined Skutterudites for Low Lattice Conductivity
Hyoungchul Kim 1 Hang Chi 2 John C. Thomas 3 Anton Van der Ven 3 Ctirad Uher 2 Massoud Kaviany 1
1Univ. of Michigan Ann Arbor USA2Univ. of Michigan Ann Arbor USA3Univ. of Michigan Ann Arbor USAShow Abstract
Recently it has been shown that filling the cages in CoSb3 skutterudite crystal lowers lattice conductivity. Since the vibration modes of pnicogen rings in the skuttterudites dominate the heat transport, we show configuring the pnicogen rings also reduces lattice conductivity. We present an analysis of pnicogen ring configuration role in reducing of lattice conductivity double-substituted skutterudites CoSb3-x-yGexTey. Based on ab-initio lattice dynamics calculations, the substituted Ge atoms form the softest bonds acting as a pseudo-rattler with distinct mode-flattening feature and acting as a local phonon softener (comparable to common rattler in filled skutterudites). We also show the collective modes of this lattice configuration induce breathing mode in the cage which is highly correlated with the reduction in lattice conductivity. The lattice conductivity predicted with non-equilibrium ab-initio molecular dynamics is in good agreement with the experimental results and the point-defect scattering model. Our lattice dynamics and thermal transport analyses can guide design of high thermoelectric figure-of-merit skutterudites.
9:00 AM - H3.22
Disassociated Electrical Conductivity and Thermal Conductivity by Dissimilar Nanocrystals Building Blocks
Ding Weng 1 Qiangfeng Xiao 1 Xinfeng Tang 2 Javier Garay 3 Yunfeng Lu 1
1UCLA Los Angeles USA2Wuhan University of Technology Wuhan China3University of California, Riverside Riverside USAShow Abstract
Phonon-glass electron-crystal materials were firstly named by G. A. Slack, which require material properties like a large effective mass, a high mobility for carriers and a low lattice thermal conductivity. However, actually a high mobility needs a low mobility effective mass but a high density of stated demands a large density of stated effective mass. Therefore to achieve ideal phonon-glass electron-crystal property in a single compound bulk device requires judicious selection of alloy components or complex superlattice structure. Here we introduce a facile solution to achieve disassociated Electrical Conductivity and Thermal Conductivity by building bulk device from PbTe and TiO2 nanocrystals. Nanocrystals were prepared from one step solvothermal synthesis following by homogeneous mixing and ligands removing process. Utilizing spark plasma sintering process, PbTe and TiO2 nanocomposites were sintered into densified tablet with 1 inch in diameter which have shown almost 90% of theoretical density. And transport properties were tested that electrical conductivity and thermal conductivity has been reduced to 1/30 and 1/4, respectively. After sintering, PbTe nanocrystal forged into a network-like structure and supplied thermoelectric property for as-prepared bulk device, while the interfaces at PbTe-TiO2 nanocrystals and TiO2-TiO2 nanocrystals scattered transportation of phonon, therefore reduced thermal conductivity. We believe that this is a facile solution to prepare economical and practical phonon-glass electron-crystal materials in real application and further improving electrical conductivity is able to achieve advanced ZT.
9:00 AM - H3.24
Detailed Theoretical Investigation and Comparison of the Thermal Conductivities of n- and p-type Bi2Te3 Based Alloys
Ovgu Ceyda Yelgel 1 Gyaneshwar P. Srivastava 1
1University of Exeter Exeter United KingdomShow Abstract
We present a detailed theoretical investigation and comparison of the thermal conductivities of n-type (Bi2Te3)0.85(Bi2Se3)0.15 single crystal doped with 0.1 wt.% CuBr and p-type (Bi2Te3)0.20(Sb2Te3)0.80 single crystal doped with 3 wt.% Te. The thermal conductivity contributions from carriers (electrons or holes) (κc), electron-hole pairs (κbp), phonons (κph) are computed to explain available experimental results [1,2]. For the theoretical calculations of κc and κbp, Wiedemann-Franz law  and Price&’s theory  are employed, respectively. The phonon thermal conductivity (κph) is calculated by using the Debye model within the single-mode relaxation-time approximation. Boundary, alloy, isotope, electron-phonon (or hole-phonon) and phonon-phonon interactions are included rigorously. For anharmonic phonon interaction we restrict ourselves to only three-phonon scattering events by following Srivastava&’s scheme . The frequency dependence of phonon thermal conductivity is also studied for the p-type alloy and compared with the results for the n-type alloy [6,7]. The calculated total thermal conductivities (κtotal = κc+ κbp + κph) of n- and p-type Bi2Te3 based alloys successfully explain the experimental results obtained by Hyun et al.  and Li et al. . The value of κtotal is found to be 3.15 W/(K.m) at 380 K for the n-type alloy and 1.145 W/(K.m) at 400 K for the p-type alloy. From the theoretical calculations, it is found that the most dominant effect to have a smaller value of κtotal for the p-type alloy throughout the temperature range 290 K le; T le; 500 K is the bipolar contribution on thermal conductivity. Narrower indirect band gap of the p-type alloy leads to eight times smaller value of κbp compared with the result of the n-type alloy. Moreover, nearly four times smaller value of κph for the p-type alloy is obtained since it has larger mass defect and anharmonic scatterings. These results indicate that using p-type Bi2Te3 based alloy rather than n-type alloy is likely to give rise to a higher value of thermoelectric figure of merit (ZT) for Bi2Te3 based alloys.
 D. B. Hyun et al., J.Mat.Sci. 33, 5595 (1998).
 D. Li et al., Intermet.19, 2002 (2011).
 C. Kittel, ‘Introduction to Solid State Physics&’ (John Wiley and Sons Inc, USA, 2005).
 P. J. Price, Phil. Mag. 46, 1252 (1955).
 G. P. Srivastava, ‘The Physics of Phonons&’ (Taylor and Francis Group, New York, 1990).
 Ö. C. Yelgel, G. P. Srivastava, Phys. Rev. B 85, 125207 (2012).
 Ö. C. Yelgel, G. P. Srivastava, Mater. Res. Soc. Symp. Proc. 1404, mrsf11-1404-w03-02 (2012). 449
9:00 AM - H3.26
High Purity Crystals of Higher Manganese Silicides Grown by Chemical Vapor Transport
Steven N Girard 1 Song Jin 1
1UW Madison Madison USAShow Abstract
Higher manganese silicides (HMS), MnSi1.73, are attractive thermoelectric materials with a reported maximum ZT ~0.6 at 800 K, close to the ZT values of pure PbTe or SiGe, but without expensive or toxic elements. However, HMS are notoriously difficult to synthesize in pure form without significant MnSi or Si impurities. Stoichiometric HMS (nominal composition MnSi1.73) undergo peritectic solidification from the liquid to solid phases, resulting in the initial formation of metallic MnSi lamellae that self-assemble along the  axis of HMS. It has been suggested that these MnSi striations deleteriously affect the thermoelectric transport. Conversely, Si-rich HMS (nominal MnSi1.98) solidify by a eutectic transformation, resulting in a considerable concentration of embedded Si particles. Herein we report the synthesis of extremely high purity HMS crystals by chemical vapor transport. By reacting HMS powders and differing metal halides in fused silica ampoules kept at static vacuum over a temperature gradient, crystals of impurity-free HMS crystals between 0.1 - 1 mm in size can be produced. SEM/EDS, HRTEM, microprobe, ICP, and synchrotron PXRD analysis all confirm these HMS crystals to be extremely pure with little MnSi impurity present. Based on these findings, the potential impact that embedded MnSi impurities may have on the thermoelectric properties of HMS will be discussed.
H1: Novel Materials and New Approaches I
Tuesday AM, April 02, 2013
Moscone West, Level 2, Room 2006
9:30 AM - *H1.02
Thermoelectric Transport in Topological Insulators
Oleg A Tretiakov 1 2
1Texas Aamp;M University College Station USA2Tohoku University Sendai JapanShow Abstract
Topological insulators (TI) are novel quantum materials with insulating bulk and topologically protected metallic conducting surfaces with Dirac-like band structure. I will talk about a new proposal on how to increase thermoelectric efficiency originating from the topological properties of the band structure imparted by the strong spin-orbit interaction in TIs. The thermoelectric properties of 3D TIs with many holes (or pores) propagating through the bulk will be discussed. I will show that at high density of these holes the thermoelectric efficiency, called ZT, can be large due to high ratio of perfectly conducting surface states to the bulk and the suppressed phonon thermal conductivity. These large values of ZT, much higher than unity, make this system an ideal candidate for applications in heat management of nanodevices, especially at low temperatures. Then I will discuss how to extend this idea to the entire class of TIs and wider range of temperatures. The proposal is based on the fact that the dislocations in certain 3D TIs have topologically protected 1D conducting channels. We predict that at high densities of the dislocations ZT can be dominated by these 1D states which can reduce the thermal conductivity on one hand and increase the conductivity and thermopower on the other. I will show that in principle this system can have very high ZT of order 10.
10:00 AM - H1.03
Thermoelectric Effects in Phase-change Memory Cells
Azer Faraclas 1 Gokan Bakan 1 Nicholas E Williams 1 Ali Gokirmak 1 Helena Silva 1
1University of Connecticut Storrs USAShow Abstract
Phase change memory (PCM) is a recently commercialized high-speed and nonvolatile memory , but considerable improvements in energy efficiency, endurance, and density are still required before widespread adoption . A nanoscale volume of a phase-change material (Ge2Sb5Te5, or GST, is the most commonly used) is rapidly and repeatedly switched between the high-resistance amorphous state and the low-resistance crystalline states by self-heating above melting or crystallization temperatures. The state can be detected with a relatively small read pulse, yielding a 0 or 1 for binary logic. It has been shown that thermoelectric transport can have a large impact on PCM device operation due to the large current densities and temperature gradients involved . In this study, 2D rotationally symmetric PCM mushroom cells are simulated using COMSOL Multiphysics to study the thermoelectric effects and its implications for PCM structures. In these simulations, a 100 nm thick layer of GST is sandwiched between a wide TiN top contact and a 10 nm wide TiN heater along with a series nFET access device. The material parameters of the active materials are modeled with full temperature dependency from 300 to 1000 K, including electrical resistivity, thermal conductivity, and Seebeck coefficient; the thermoelectric terms are included in the current continuity and heat equations . Thermal boundary resistance between GST and TiN is accounted for. The separate effects of Thomson heat, resulting from a Seebeck gradient along a current carrying homogeneous material, and Peltier heat, resulting from a Seebeck difference at an interface between two materials, are analyzed. Simulation results show favorable thermoelectric heating for one polarity.
 J. Rice, Micron Announces Availability of Phase Change Memory for Mobile Devices: First PCM Solution in the World in Volume Production. 2012. Available:http://investors.micron.com/releasedetail.cfm?ReleaseID=692563.
 H. -. P. Wong, S. Raoux, S. B. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi and K. E. Goodson, "Phase Change Memory," Proc. IEEE, vol. 98, pp. 2201-2227, 2010.
 A. Padilla, G. W. Burr, C. T. Rettner, T. Topuria, P. M. Rice, B. Jackson, K. Virwani, A. J. Kellock, D. Dupouy and A. Debunne, "Voltage polarity effects in Ge2Sb2Te5-based phase change memory devices," J. Appl. Phys., vol. 110, pp. 054501, 2011.
 L. D. Landau, E. M. Lifshitz and L. P. Pitaevskii, Electrodynamics of Continuous Media. 2nd ed., Burlington, MA: Butterworth-Heinemann, 1984.
10:15 AM - H1.04
Advanced Thermionic Energy Converters for Concentrated Solar Power
Kunal Sahasrabuddhe 1 4 Karl Littau 2 4 Nicholas Melosh 2 3 4
1Stanford University Stanford USA2Stanford University Stanford USA3Stanford University Stanford USA4Stanford University Stanford USAShow Abstract
Thermionic energy converters (TECs), which convert heat to electricity via thermionic emission from a hot cathode in a vacuum diode, are promising devices for solar energy conversion in concentrator systems owing to their high theoretical conversion efficiencies. While TECs have been investigated for nearly a century, conversion efficiencies have been severely limited by the effects of space charge in the gap between the cathode and anode. We present a path to higher energy conversion efficiencies from TECs through the reduction of the inter-electrode gap and the collector work function. We describe the design and construction of a novel, easy-to-assemble thermionic converter with micro-scale gaps using size selected ceramic microbeads. We test this architecture under concentrated solar illumination to characterize performance in real-world conditions. We demonstrate that the space charge effect can be minimized considerably using micron-scale gaps.
10:30 AM - H1.05
Examination of the Interaction between Single-crystalline Nanostructures and Phonons at Their Characteristic Length Scale
Jongwoo Lim 1 4 Kedar Hippalgaonkar 2 4 Hungta Wang 3 Peidong Yang 1 4 Sean C Andrews 2 4 Arun Majumdar 1 4
1Univ California Berkeley Berkeley USA2University of California, Berkeley Berkeley USA3University of Alabama Tuscaloosa USA4Lawrence Berkeley National Laborabory Berkeley USAShow Abstract
It has been recently reported that thermal conductivity decreases greatly without damaging electrical properties in single crystalline silicon nanostructures due to their nanoscopic features : surface roughness and nano-hole arrays. There have been many attempts to understand the mechanism of suppressing the phonon transport in these nanostructures in terms of thermal phonon wavelength and mean free path (mfp) as well as the phonon dispersion relation. However, the lack of the experimental evidence has made difficult to reach a meaningful conclusion. In this report, silicon nanowires were controllably roughened and the surface roughness near the characteristic length scale of thermal phonon wavelength is shown to interfere with phonon transport. Furthermore, silicon membranes with periodic nano-holes, where both electrical and thermal properties can be measured simultaneously, were prepared to study decoupling of electron and phonon transport associated with their mfp.
First, roughened vapor-liquid-solid (VLS) grown nanowires were thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information was extracted from the surface profile of a specific nanowire, the thermal conductivity of the same nanowire was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1-100nm length scale range. We also emphasize the importance of the ‘a_p&’ parameter to characterize the roughness and show that this roughness parameter captures the physics of phonon scattering better than conventionally used roughness parameters: the root-mean-square and the correlation length,
Secondly, a platform to simultaneously measure thermal conductivity, electrical conductivity, and the Seebeck coefficient for a silicon membrane with nano-hole arrays (pitch distance: 50 ~ 70 nm) has been developed to extract ZT directly from a single device. Using this measurement platform, we have found that phonon transport was more sensitive to the neck width in the narrow range from 20nm to 30nm than electron transport, causing optimized ZT. This result has identified the characteristic mfp and boundary scattering cross-section of phonons in silicon.
10:45 AM - H1.06
A New Thermoelectric Concept Using Large Area PN Junctions
Ruben Chavez 1 Andre Becker 1 Victor Kessler 1 Markus Engenhorst 1 Nils Petermann 2 Gabi Schierning 1 Roland Schmechel 1
1University of Duisburg-Essen Duisburg Germany2University of Duisburg-Essen Duisburg GermanyShow Abstract
A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines phonon absorption in a space charge region with the Seebeck effect by applying a temperature gradient parallel to the PN junction. The PN junction is produced by stacking n-type and p-type nanoparticles powder prior to a densification process. The nanoparticulate nature of the densified PN junction enhances the thermoelectric properties and increases the intraband traps density which we propose is beneficial for transport across the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator.
11:30 AM - H1.07
Chalcogenide Glasses and Glass-ceramics in the Cu-As-Te System: Toward New Thermoelectric Materials?
Jean-Baptiste Vaney 4 3 Gaelle Delaizir 1 Eric Alleno 2 Andrea Piarristeguy 3 Judith Monnier 2 Claude Godart 2 Elsa Lopes 5 Antonio Pereira Goncalves 5 Annie Pradel 3 Bertrand Lenoir 4
1CNRS Limoges France2CNRS Paris France3CNRS Montpellier France4CNRS Nancy France5ITN Lisbon PortugalShow Abstract
We report on the thermoelectric properties at room temperature for glass and glass-ceramics with different crystalline fraction in the Cu-As-Te system. Glass-ceramics including metastable β-As2Te3 crystalline phase have been obtained through Spark Plasma Sintering (SPS) experiments from amorphous powder. Preliminary results show that the thermoelectric performances increase as the crystallization proceeds. The highest power factor of 260µW.K-2.m-1 at T=300K has been found for the highest crystalline fraction leading to a maximum dimensionless thermoelectric figure of merit ZT~0.1.
11:45 AM - H1.08
Isotope Defect Engineering of Silicon for Thermoelectric Applications
Hartmut Anton Bracht 1 Soizic Eon 1 Rafael Frieling 1 Daniel Issenmann 2 Anton Plech 2 Erwin Peiner 3 John Lundsgaard Hansen 4 Arne Nylandsted Larsen 4 Joel W. Ager 5 Eugene E. Haller 5
1University of Muenster Muenster Germany2Karlsruhe Institute of Technology Eggenstein-Leopoldshafen Germany3Technische Universitamp;#228;t Braunschweig Braunschweig Germany4University of Aarhus Aarhus Denmark5Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
The thermal conductivity of silicon (Si) is too high to be well suited for thermoelectric applications. Recently we demonstrated that the thermal conductivity of Si can be effectively reduced by alternating layers of highly enriched 28Si and 29Si . A 20 bilayer structure of (28Si/29Si)20 yields a reduction of almost a factor of three compared to the thermal conductivity of natural Si. This strategy to reduce the thermal conductivity of Si without affecting the electronic properties is very desirable for the optimization of thermoelectric materials. In order to investigate the potential of our approach, we prepared alternating epitaxial layers of 28Si and 30Si on Si wafer substrates by means of molecular beam epitaxy. Isotope multilayer structures with both periodic and aperiodic sequences in layer thickness were grown. Moreover, isotopically modulated Si nanopillars were prepared by reactive ion etching. The thermal conductivity of the multilayers and nanopillars was investigated with time-resolved X-ray scattering (TRXS). For these measurements the isotope structures and appropriate reference samples were covered by a 2 nm thick chromium layer followed by a 28 nm gold film. In pump-and-probe experiments conducted at the beamline ID09B of the European Sychrotron Radiation Facility (ESRF) in Grenoble (France) the cooling of the Au layer after pulse-heating with a femtosecond laser was followed by measuring the Au lattice constant with X-ray pulses synchronized with the laser beam. The Au lattice expansion is a direct measure for the temperature of the Au layer . The effective thermal conductivity of the isotopically modulated structures is deduced from numerical simulations of the heat transport problem. To understand the nature of the phonon scattering process, the effective thermal conductivity of the isotope structures is compared to predictions on phonon transport in isotopically modulated structures deduced from solutions of the Boltzmann transport equation and from molecular dynamics simulations.
 H. Bracht, N. Wehmeier, S. Eon, A. Plech, D. Issenmann, J. Lundsgaard Hansen, A. Nylandsted Larsen, J.W. Ager III, and E.E. Haller, Appl. Phys, Lett. 101, 064103 (2012).
 D. Issenmann, N. Wehmeier, S. Eon, H. Bracht, G. Buth, S. Ibrahimkutty, and A. Plech, Thin Solid Films (2012), accepted.
12:00 PM - H1.09
Excellent p- and n-type Control in a High Temperature Thermoelectric Boride
Satofumi Maruyama 1 2 Yuzuru Miyazaki 2 Kei Hayashi 2 Tsuyoshi Kajitani 2 Takao Mori 1 3
1National Institute of Materials Science Tsukuba Japan2Tohoku University Sendai Japan3University of Tsukuba Tsukuba JapanShow Abstract
Boron-rich compounds possess atomic networks which are formed through the covalent bonding of boron. Through incorporation of rare earth or other metal atoms and third elements like C, N and Si in the framework, network structures and physical properties can be controlled to some degree . In addition to this, these compounds generally have attractive properties such as thermal and chemical stability, high hardness and the framework is composed of abundant elements. We focused on YAlB14  and YB25  which have boron icosahedra clusters forming similar network structures. Korsukova reported a single crystal sample of YAlB14 which was prepared using high temperature molten Al flux method, with a refined composition of Y0.62Al0.71B14. However, details on the thermoelectric properties haven&’t been fully investigated, and only a small range of composition for YAlB14 has been reported. Through synthesis processes, we were successful in synthesizing YxAlyB14 (x sim; 0.57) with different Al occupancy y (0.41 le; y le; 0.63) and found striking thermoelectric properties .
Positive Seebeck coefficients were obtained for Al-poor samples which were shifted in the negative direction with increase of y. Maximum Seebeck coefficient values were approximately 400 mu;VK-1 at 850 K and -200 mu;VK-1 at 1000 K, for p-type and n-type, respectively. Although ZT values are still not high due to the poor electrical resistivity which can be partly attributed to non-optimal densities of the samples, excellent control of p-n characteristics were achieved in a system with the same crystal structure and consisting of the same elements.
 T. Mori, Handbook on the Physics and Chemistry of Rare-earths, Vol. 38, ed. K. A. Gschneidner Jr., J. -C. Bunzli and V. Pecharsky, North-Holland, Amsterdam (2008) 105.
 M. M. Korsukova, T. Lundstrom and L.-E. Tergenius, J. Alloy Comp. 187 (1992) 39.
 T. Mori, F.X. Zhang and T. Tanaka, J. Phys.: Condens. Matter. 13 (2002) L423.
 S. Maruyama, Y. Miyazaki, K. Hayashi, T. Kajitani and T. Mori, Appl. Phys. Lett. 101 (2012) 152101.
12:15 PM - H1.10
Disordered Porous Silicon as High-efficiency Thermoelectric Material
Giuseppe Romano 1 Jeffrey Grossman 1
1MIT Cambridge USAShow Abstract
Thanks to their ability to significantly suppress phonon transport, nanostructured materials offer a unique platform for designing high-efficiency thermoelectric devices. One of the key challenges behind tuning the thermal conductivity in some semiconductor materials is that the phonon mean free paths span many orders of magnitude, revealing that engineering heat transport is intrinsically a multi-scale problem. To this end, we introduce a new computational technique based on the coupling of the Fourier model with the frequency-dependent Boltzmann transport equation (FDBTE). The model, which is based on the Discontinuous Galerkin discretization, ensures energy conservation and it is computationally affordable. We apply the developed method to compute heat transport across porous-Si with arbitrary pore arrangements, a situation for which the standard FDBTE would be very computationally expensive, and find, in agreement with recent nanoscale heat measurements, that classical size effects occur even at the microscale. We demonstrate that pores disorder is a crucial degree of freedom for optimizing ZT, predicting improvements in ZT of up to 5-10 times that found for the case of aligned pores. Although the presented method has been applied to porous-Silicon, it can be readily applied to any nanostructured material.
12:30 PM - H1.11
Tuning of Band Gap in Type-I Clathrate Ba8NixZnyGe46-x-y-zSnz for ZT~1
Peter Franz Rogl 1 Matthias Falmbigl 1 Andriy Grytsiv 1 Ernst Bauer 2 Esmaeil Royanian 2 Patrick Heinrich 2
1University of Vienna Wien Austria2Vienna University of Technology Wien AustriaShow Abstract
A detailed investigation of the influence of the Sn-content at different Ni/Zn-ratios on the thermoelectric behaviour of the quinary type I clathrate Ba8NixZnyGe46-x-y-zSnz is presented. Varying the Ni-content, the band gap can be tuned in a wide range and hence the maximum in the thermoelectric figure of merit, ZT, can be shifted in a large temperature range from 530 to 830 K. Whereas for Ba8Zn7.78Ge38.22 a maximum ZT=0.4 is observed at 800 K, Ni-doping rises the figure of merit to ZT=0.65 and additions of tin yield ZT=0.8 for Ba8.2Zn7.66Ge36.35Sn1.79. The highest ZT-value of 0.9 at 830 K was observed for Ni/Sn doped Ba8Ni0.22Zn7.22Ge37.03Sn1.53. This is one of the highest ZT-values achieved for clathrate type-I polycrystalline bulk samples.
12:45 PM - H1.12
Low-thermal-conductivity Oxide Thermoelectrics Based on Coherently Intergrown Cobaltates
Bostjan Jancar 1 Damjan Vengust 1 Andreja Sestan 1 Vid Bobnar 2 Zdravko Kutnjak 2 Danilo Suvorov 1
1Jozef Stefan Institute Ljubljana Slovenia2Jozef Stefan Institute Ljubljana SloveniaShow Abstract
The research of oxides as possible thermoelectric materials was triggered by the discovery that metallic layered cobaltate NaxCoO2 exhibits a large Seebeck coefficient combined with a high electrical conductivity and a low thermal conductivity which was attributed to its layered crystal structure consisting of two dimensional sheets of edge sharing CoO6 octahedra intercalated by Na ions. The highest reported zT values of NaxCoO2 are sim;1.0 for single crystalline and sim;0.8 for polycrystalline material at the temperatures in the vicinity of 800oC. With such properties it was considered to be a good candidate for the high-temperature p-type thermoelectric material. However the chemistry of layered sodium cobaltates is governed by the high mobility of interlayer sodium, which reacts with atmospheric moisture and carbon dioxide. Furthermore layered crystal structure of NaxCo2O4 enables intercalation of molecules such as water, which can lead to exfoliation and thus degradation of the material. Because of this the focus of the research turned to a semiconducting misfit-layered cobaltate Ca3Co4O9 the structrure of which consist of triple Ca2CoO3 layers and single layers of CoO2 analogous to CoO6 sheets of NaxCoO2 compounds. The highest zT reported for this structural type was sim;0.6. We found that the sheets of octahedrally coordinated Co ions, which are the common structural element of NaxCoO2 and Ca3Co4O9 phases allow spontaneous intergrowth of the two structures leading to significant improvement of environmental stability. Furthermore coherent intergrowth of the two structural types results in effective texturing in polycrystalline material with the preferred grain growth aligned in-plane with common CoO6 layers thus allowing high electrical conductivity. The nanostructured integrowths also result in a significant reduction of thermal conductivity, which was at 700oC measured to be sim;0.3 W/mK for “out-of plane” and sim;0.6 W/mK for “in-plane” direction. With the measured power factor of sim;6.5*10-4 W/mK2 the calculated “in-plane” zT of intergrowth structure material with nominal composition Ca2.2Na0.8Co4O9 is sim;1.0 at 700oC.