Session Chair: David Joseph Singh, University of Missouri Session Chair: FuKe Wang, Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR)
Phonon Engineering in TE materials
1:30pm - 2:00pm Invited
Phonon Engineering of Silicon-based Nanostructured Materials for Thermoelectrics
1The University of Tokyo, Japan; 2National Institute of Materials Science, Japan
Motivated by the progress in nanostructuring and the need for lowing the cost, there are increasing number of works reported to develop silicon-based nanostructured thermoelectric materials. Silicon is one of the most abundant species, and its compatibility with the existing silicon technology would reduce the process cost for module integration. The simplest type of nanostructured materials is a bulk nanocrystalline structure, which typically consist of closed-pack grains with average size of tens to hundreds of nanometers. Yet, there are various controllable parameters, such as average grain size, dispersion of the size distribution, and physical/chemical structures at the interfaces. There are also more advanced nanostructured materials that involve more complexity in terms of geometry and composition (including voids). With the recent advances to analyze phonon transport properties, it has become possible to predict the effect of the controllable parameters on the thermal conductivity, which has enhanced the designablility of thermoelectric materials. In this talk, we will introduce some of our recent studies aiming at engineering phonon transport in silicon-based nanostructured materials by selective and multiple scattering of phonon particles, and interference and resonance of phonon waves. The studies use calculation, measurement, synthesis, and informatics techniques in multiscale-fashion and often in combination.
2:00pm - 2:30pm Invited
Thermal Conductivity of Thermoelectric Materials
John TSE1, Niall ENGLISH2, Yong XUE3, Shou-wang YANG3
1University of Saskatchewan, Canada; 2University College Dublin, Ireland; 3Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), Singapore
An efficient method for the direct calculations of thermal conductivity of solids based on the Einstein diffusion equation of the heat momenta computed from the trajectories of finite temperature molecular dynamics has been implemented. This method takes into account of thermal and anharmonic effects in a straightforward manner. The accuracy of the method is checked against the measured lattice thermal conductivities of potential thermoelectric materials, Ba-doped Si clathrate, Te doped CoSb3 and Group-IV selenides. The agreements are consistently good. We also compare results of the highly anharmonic SnSe against lattice dynamic calculations.
2:30pm - 2:45pm Oral
Seeking New Thermoelectric Chalcogenides with Effective Phonon Scattering
Laboratoire de Cristallographie et Sciences des Matériaux (CRISMAT), Centre National de la Recherche Scientifique (CNRS), France
In the quest of finding new materials for thermoelectric applications, the question of “how selecting promising materials?” is raised. Since low thermal conductivity (κ) is one of the components that can lead to good thermoelectric materials, ideal candidates would be materials fulfilling this specification. Many studies showed that low dimensional structures associated to disorder are conditions needed to obtain low lattice thermal conductivities (κL). It was then chosen to focus our research on finding new phases inside families that present low dimensional features. The successfully synthesized quasi one-dimensional pseudo-hollandite Ba0.5Cr5Se8  and the known compound TlIn5Se8  show thermal conductivities as low as 0.8 and 0.45 W.m-1.K-1, respectively. Additionally to these studies, a new layered compound of the MnPSe3 family was characterized. A transmission electron microscopy in HREM mode analysis shows a high rate of stacking fault inside the material leading to low thermal conductivity of 0.35 W.m-1.K-1.
 Lefèvre, R.; Berthebaud, D.; Perez, O.; Pelloquin, D.; Hébert, S.; Gascoin, F. Polar Transition-Metal Chalcogenide: Structure and Propertiesof the New Pseudo-Hollandite Ba0.5Cr5Se8. Chem. Mater. 2015, 27 (20), 7110–7118.
 Lefèvre, R.; Berthebaud, D.; Perez, O.; Pelloquin, D.; Boudin, S.; Gascoin, F. Ultra low thermal conductivity of TlIn5Se8 and structure of the new complex chalcogenide Tl0.98In13.12Se16.7Te2.3. J. Solid State Chem. Submitted
2:45pm - 3:00pm Oral
Low Thermal Conductivity of Single-Crystalline Porous Silicon Nanowires
Yunshan ZHAO1, Lina YANG2, Lingyu KONG1, Mui Hoon NAI1, Jing WU3, Yi LIU1, Baowen LI4, John T L THONG1, Kedar HIPPALGAONKAR3
1National University of Singapore, Singapore; 2California Institute of Technology, United States; 3Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; 4University of Colorado, United States
Porous structures provide a novel way to reduce the thermal conductivity of nano-materials by not only enhancing phonon scattering from the boundaries of the pores and decreasing the phonon mean free path but also by presenting a larger surface-to-volume ratio. Especially in nanostructures, it remains a great challenge to experimentally deduce the porosity and to measure its effect on thermal transport due to lack of suitable techniques. Here we measure the porosity and thermal conductivity of electrolessly-etched single-crystalline silicon nanowires by means of an electron-beam heating technique. Such porous silicon nanowires with quantitatively measured porosity exhibit extremely low thermal conductivity, even lower than that of amorphous silicon, can be unequivocally attributed to two things: (a) reduction of group velocity due to increased surface-to-volume ratio and (b) diffusive scattering at the pore boundaries.
To verify our hypothesis of the effect of phonon softening on thermal transport we have performed new experiments on the Young’s Modulus of single porous silicon nanowires. We provide, for the first time, a one-to-one correlation between the measured thermal conductivity and Young’s Modulus & porosity of exactly the same nanowire. Further, the smallest crystallite size (4.3 nm) among all measured crystalline nanostructures is measured by HRTEM. Moreover, within the limit of diffusive phonon transport, an effective thermal transport model is then used to explain the dependence of the experimentally obtained thermal conductivity on the nano-structure size, while accounting for the experimental change in group velocity as well as reduced mean free path. Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. To corroborate the experimental observation, Young’s Modulus vs Porosity of porous silicon nanowires is now simulated theoretically in our revised study. Our work is an important experimental and theoretical advance in the field of nanoscale thermal transport.
3:00pm - 3:15pm Oral
Investigation of Optimal Annealing Temperature for Enhanced Thermoelectric Properties of MOCVD Grown ZnO
Government College University, Pakistan
In this study, we have demonstrated the optimization of annealing temperature for enhanced thermoelectric properties of ZnO. Thin films of ZnO were grown on sapphire substrate using MOCVD technique. The grown films were annealed in oxygen environment at 600oC – 1000oC, keeping a step of 100oC for one hour. Room temperature Seebeck measurements revealed that Seebeck coefficient of un-annealed sample was 152 µV/K having carrier concentration ND ~ 1.46 × 1018 cm-3. The Seebeck coefficient of annealed films increased from 212 to 415 µV/K up to 900 0C and then decreased at 1000 0C. The power factor was calculated and found increasing trend with annealing temperature. This observation was explained by the theory of Johanson and Lark-Horovitz that thermoelectric properties enhanced by improving the structre of ZnO thin films. The Hall measurements and PL data strongly justified the proposed argument.