Alloys and Compounds for Thermoelectric and Solar Cell Applications V: Session II
Sponsored by: TMS Functional Materials Division, TMS: Alloy Phases Committee
Program Organizers: Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, Ensicaen University of Caen; Soon-jik Hong, Kongju National University; Philippe Jund, Université de Montpellier; Lan Li, Boise State University; Takao Mori, National Institute for Materials Science; Ce-Wen Nan, Tsinghua University; Hsin-jay Wu, National Sun Yat-Sen University

Monday 2:00 PM
February 27, 2017
Room: 21
Location: San Diego Convention Ctr

Session Chair: Lan Li, Boise State University; Takao Mori, National Institute for Materials Science

2:00 PM  Invited
Bottom-up Nanostructuring for Thermoelectrics: Takao Mori1; 1National Institute for Materials Science (NIMS)
    Efforts worldwide to find viable thermoelectrics (TE) are intensifying [1]. We have achieved selective scattering of phonons and thereby critical enhancement of TE properties through nanostructuring: both a) mechanical and b) bottom-up methods. For example, previously, quick, inexpensive, bottom-up wet processes were found to fabricate nanosheets of telluride thermoelectric materials leading to enhanced ZT [2]. We have also recently discovered a bottom-up nanostructuring method leading to a 100% enhancement (i.e. ZT≈1.6) in“empty” rare earth-free skutterudites [3], and details and possibilities to apply to other materials will be presented. This work is supported by CREST, JST. [1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, (Springer, Heidelberg, 2013). [2] C. Nethravathi et al., J. Mat. Chem. A, 2, 985 (2014). [3] A. U. Khan et al., submitted.

2:20 PM  
A Facile Route for Ge Addition to Nanostructured Fe-Si Alloys Towards Improved Thermoelectric Properties: Naiming Liu1; Wade Jensen1; Long Chen1; Brian Donovan1; Patrick Hopkins1; Jerrold Floro1; 1University of Virginia
    Synthesis of semiconducting β-FeSi2 bulk material often involves prolonged annealing to eliminate the intermediate ε-FeSi phase. This inevitably results in severe grain coarsening, degrading thermoelectric properties. Our approach is based on the eutectoid transformation α-FeSi2 → β-FeSi2 + Si. By mixing cast α with Ge via ball milling, we find that the Si product phase alloys with Ge during spark plasma sintering. A well-sintered β-FeSi2/Si1-xGex nanocomposite with consistent composition has been obtained at temperatures below 900℃ in just a few minutes, with no post-annealing. Its hierarchical structure is promising for reducing lattice thermal conductivity. We achieved facile tuning of the Ge concentration in the diamond cubic phase, which should enable engineering of the band structure across the heterointerfaces to enhance electrical transport. In addition, modulation doping may be achieved by preferentially doping β-FeSi2, taking advantage of the high mobility of undoped SiGe. Support by the II-VI Foundation is gratefully acknowledged.

2:40 PM  
Silicon Carbide Particles as Nanoinclusions for Improved Thermoelectrics: Devin Coleman1; Sabah Bux2; Lorenzo Mangolini1; 1University of California, Riverside; 2Jet Propulsion Laboratory
    The synthesis of beta-phase silicon carbide nanoparticles exhibiting a hollow core-shell morphology is demonstrated by means of a two-step non-thermal plasma process. These particles are mixed with silicon nanopowders produced by means of high energy ball milling, and densified via hot press into bulk samples for silicon-based thermoelectric materials. While the silicon nanoparticles exhibit grain growth during the densification process, the isolated silicon carbide nanoparticles retain their initial size in the bulk material. Moreover, the morphology of initial silicon carbide nanoparticles can be controlled by tuning of the plasma process to produce ~10nm solid silicon carbide particles, or ~25nm hollow silicon carbide nanoshells with a ~10nm void. The introduction of hollow silicon carbide particles into the silicon matrix offers the opportunity to controllably insert nanovoid inclusions into a bulk material, and elucidate their effect on thermal conductivity.

3:00 PM  Invited
Enhancement of Thermoelectric of PbTe Bulks Visa Heterogeneous Nanostructure: Hongchao Wang1; Junphil Hwang2; Chunlei Wang1; Woochul Kim2; 1Shandong University; 2Yonsei University
    In here, we will show the nanostructured effect on the thermoelectric properties of PbTe alloys. Firstly, the relationship between nano grain size and thermoelectric properties is shown. It is found that thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to ∼2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Secondly, by varying the quenching time of Na doped PbTe, followed by hot pressing, heterogeneous nanocomposites, a mixture of nanodot nanocomposites and nanograined nanocomposites, have been synthesized. It is found that figures of merit, zT, of those heterogeneous nanocomposites exhibited a zT in 2.0-2.2 at 773 K, which is a 25% increase compared to zT of a homogeneous nanodot nanocomposite. Therefore, the heterogeneous nanostructure should be a better way to enhance the thermoelectric performance of bulk PbTe.

3:20 PM Break

3:40 PM  Invited
Phononic Crystal Nanopatterning in Si and SiGe Thin Films for Thermoelectric Application: Masahiro Nomura1; 1University of Tokyo
    We investigate thermal phonon transport in Si and SiGe thin films with phononic crystal nanostructures at room temperature. Systematic measurement on thermal conductivity and electrical conductivity clarified how the thermoelectric material performance was increased by nanopatterning. We found that the information on thermal mean free path spectrum is important to improve the thermoelectric property effectively. For a polycrystalline Si thin film, phononic crystal patterning with a period of 300 nm reduced the thermal conductivity by 75% while the electrical conductivity decreased by 20%, which results in three-fold increase in the figure of merit of thermoelectric material. For a SiGe thin film, we observed that polycrystallization of an amorphous SiGe lengthened thermal phonon mean free path and that nanopatterning has more impact on thermal conductivity in a poly-SiGe thin film.

4:00 PM  
Nanostructure of Si/transition Metal Silicide Composite Prepared by a Melt Spinning Method: Yuji Ohishi1; Tomoki Ebata1; Jun Xie1; Hiroaki Muta1; Ken Kurosaki1; Shinsuke Yamanaka1; 1Osaka University
    Several previous studies have demonstrated that formation of nanostructures in thermoelectric materials drastically reduced the thermal conductivity and enhance the thermoelectric figure of merit. In recent years, our group have shown that nanometer-sized composite structure of Si and transition metal (TM) silicide is generated by a rapid solidification of the melt alloys. In this study, the relationship between the type of the TM and the resultant nanostructure was systematically studied. Si/TM silicide composites that had a composition ratio of Si:TM = 19:1 were synthesized by a melt spinning method. The nanostructures of the melt spun samples were observed by scanning electron microscope. The thermal diffusivity was measured with an AC calorimetric method and the thermal conductivity was evaluated. The relationship between the type of TM, the formed nanostructure and the thermal conductivity will be discussed.

4:20 PM  Cancelled
Incorporation of HfO2 Nanoprecipitates: Way to Improve Half-Heusler Thermoelectric Material: Alizée Visconti1; Guillaume Bernard-Granger2; Christelle Navone1; 1CEA Grenoble; 2CEA Marcoule
    Recently, Ti1-x-yZrxHfyNiSn1-zSbz n-type half-Heusler alloys are attracting strong interest as thermoelectric materials. Earlier investigations show the formation of HfO2 nanoprecipitates at the material’s grain boundary, which reduce the lattice-thermal conductivity contribution by scattering lattice vibrations. Based on the fact that Hf is much more expensive than other elements used in this material, lowering as much as possible the Hf concentration is becoming an important challenge. In this work, we used a Ti1-xZrxNiSn1-zSbz raw powder synthesized by levitation melting and ball milling under argon atmosphere mixed with 0, 1, 1.5 and 2 %wt of an HfO2 nanopowder. Then the mixtures were sintered by SPS for 5 min at 1140 °C. The microstructure and thermoelectric properties were investigated and compared. A link is established between both. The Zr0.5Ti0.5NiSn0.994Sb0.006 formulation, including 1.5 wt% of HfO2, leads to a better ZT than the Zr0.25Hf0.25Ti0.5NiSn0.994Sb0.006 one with a consequent reduction of the Hf content used.

4:40 PM  
Microstructure and Thermoelectric Properties of Silicon and Metal Silicides Nanocomposites Synthesized by a Melt Spinning Method: Ken Kurosaki1; Sora-at Tanusilp1; Yuji Ohishi1; Hiroaki Muta1; Shinsuke Yamanaka1; 1Osaka University
    Since conventional thermoelectric (TE) materials such as Bi2Te3 and PbTe contain highly-toxic and rare elements, high-performance TE materials made from non-toxic and Earth-abundant elements need to be developed. Si and metal silicides nanocomposites would be a good candidate of such materials. Generally, metal silicides dispersed into the Si matrix in nanoscale scatter heat carrying phonons effectively, leading to the enhancement in the TE performance of Si. In the present study, the nanocomposites were synthesized by a melt spinning method, in which the size and the distribution of the metal silicides were controlled by changing the cooling rate. The microstructure and the TE properties of nanocomposites composed of Si and various metal silicides (e.g. VSi2, Mg2Si, NiSi2, …) will be presented. This work was supported in part by JSPS KAKENHI Grant Number 252890 and JST, PRESTO. Additional support was provided in 2014 by the Program for Creating Future Wisdom, Osaka University.

5:00 PM  Invited
Binary Titanium Alloys as Templates for Co-doping Titanium Oxide Photocatalysts: J. Shang1; Zhengchao Xu2; Qi Li2; 1University of Illinois; 2Institute of Metal Research
    Doping is widely used to control the bandgap and photocatalytic efficiency of semiconductor photocatalysts in energy and environment applications. While wet chemistry methods have been extensively used to synthesize TiO2 photocatalysts with either cationic or anionic dopants because of their simplicity and versatility, cationic and anionic co-doping of TiO2 remains a challenge. Here we explore the synthesis of TiO2 photocatalysts co-doped with transition metal elements and with N and C, in nanotube arrays by templating on binary titanium alloys with Nb, Mo, and Ni. Results indicated that Nb, Mo and Ni were doped into TiO2 crystal lattice by substituting Ti4+ with Nb5+, Mo6+ and Ni2+, respectively. Doing with Nb5+ and Mo6+ did not influence the crystal structure of TiO2, but Ni2+ doping promoted phase transformation of TiO2 from anatase to rutile. Further anionic doping by N and C resulted in significant improvement of the visible-light photocatalytic efficiency of TiO2.