Alloys and Compounds for Thermoelectric and Solar Cell Applications V: Session III
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

Tuesday 8:30 AM
February 28, 2017
Room: 21
Location: San Diego Convention Ctr

Session Chair: Albert Wu, National Central University; Soon-jik Hong, Kongju National University

8:30 AM  Invited
Enhanced Thermoelectric Figure of Merit in Bi-Sb-Te based Composites with Dispersed ZrO2 Nanoparticles: Babu Madavali1; Chul-Hee Lee1; Hyo-Seob Kim2; Kap-Ho Lee3; Soon-Jik Hong1; 1Kongju National University and Institute for Rare Metals; 2Ames Laboratory; 3Chungnam National University
    Thermoelectric generators (TEGs) are much fascinating devices in recent years due to their high potential in waste heat recovery and energy harvesting applications. In this research, p-type BiSbTe/(x-wt%)ZrO2(2, 4, and 6) nanocomposites (NC) were fabricated by ball milling, and spark plasma sintering. The phase and existence of ZrO2nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The Seebeck coefficient was significantly improved, whereas the total thermal conductivity was considerably decreased for the NCs due to the scattering of carriers at newly formed BiSbTe/ZrO2 interfaces. The thermoelectric transport properties indicate that the nanocomposite structures improve the thermoelectric figure of merit of 1.34 for the 2 wt% added BiSbTe/ZrO2 nanocomposite bulks. The hardness of the nanocomposites was significantly improved (≥ 27%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism.

8:50 AM  Invited
Bismuth Telluride Based Compounds with High-density Current Stressing: Dopant Migration, Structural Evolution and Transport Property Modulation: Yao-Hsiang Chen1; Cheng-Tang Li1; Chien-Neng Liao1; 1National Tsing Hua University
    A large electric current can be introduced through compacted metallic or ceramic powders to enhance solid-state sintering during a hot-press process. One of the current-assisted sintering processes, known as spark plasma sintering (SPS), has been frequently used to manufacture thermoelectric materials. Although the SPS’ed thermoelectric materials demonstrated many spectacular properties, the interactions between thermoelectric materials and electric current are still not fully understood. A slight process variation may cause a huge difference in transport properties of the SPS’ed thermoelectric materials. The thermoelectric transport properties are very sensitive to charged lattice defects, micro- and nano-structures and secondary phases formed during the fabrication. In this study, we intend to investigate how a high-density electric current interacts with charged lattice defects and induces microstructural evolutions in bismuth telluride based compounds. Atomic migration of constituent elements and dopant species driven by electric current is delineated by microstructural inspection and transport property measurements.

9:10 AM  
Fabrication of BiSbTe-based Thermoelectric Materials Using Water Atomization and Spark-plasma Sintering Techniques: ChulHee Lee1; EunBeen Kim1; KapHo Lee2; P Dharmaiah1; M Babu1; SoonJik Hong1; 1Kongju National University; 2Chungnam National University
    The field of renewable energy technology has been attracted a lot of interest due to depletion of fossil fuels, energy crisis and global warming. Thermoelectric technology is one of the best renewable energy technologies, which can convert waste heat to electrical energy or electrical to heat in eco-friendly manner. Thermoelectric material (TE) properties are directly influenced by preparation techniques. However, the complexity and cost for fabrication of TE materials have considerable high with conventional methods. Hence, we are focused on fabrication of BiSbTe materials by using water atomization which is simple and easy process control. Subsequently, as-prepared powders was consolidated at different temperatures (300, 350, 400 and 450 oC) by well known advanced technique such as spark plasma sintering (SPS). The crystal structure and cross-sectional microstructures were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and EBSD techniques. Thermoelectric and mechanical properties of sintered bulk samples were investigated systematically.

9:30 AM  
Enhanced Thermoelectric Properties of Sb2Te3 Nanoplates Incorporated Bi0.5Sb1.5Te3 Composites: Peyala Dharmaiah1; Chul Lee1; Dongwon Shin1; Jar-Myung Koo1; Soon-Jik Hong1; 1Kongju National University
    In this work, Sb2Te3 nanoplates incorporated in Bi0.5Sb1.5Te3 (BST) matrix were prepared by mixing various ratios ( BST/xSb2Te3, x=0, 12 and 20 wt%) using low energy ball milling and spark plasma sintering, and their thermoelectric properties were investigated. The existences of nanoplates in the matrix were investigated using SEM analysis. The results indicate that the electrical conductivity significantly increased (38%) and less reduction in Seebeck coefficient with the addition of Sb2Te3 nanoplates due to the increased carrier concentration. Simultaneously, the thermal conductivity (κ) decreased with the addition of nanoplates, which is 9% lower than BST matrix owing to enhanced phonon scattering of nanoplates. As a result, the highest ZT value obtained was 1.28 at 350 K for the incorporation of 12wt% Sb2Te3 nanoplates in Bi0.5Sb1.5Te3 matrix. This result enables that the overall thermoelectric performance of BST composites can be effectively improved with the addition of Sb2Te3 nanoplates.

9:50 AM Break

10:10 AM  Invited
Interfacial Reactions at the Joints of PbTe Thermoelectric Modules: Sinn-wen Chen1; Jen-chieh Wang1; Ling-chieh Chen1; 1National Tsing Hua University
    PbTe-based alloys are important mid-temperature thermoelectric materials. Their application temperatures are too high for Sn-based solders. Ag-Ge eutectic alloy, Ag-24.5at%Ge, and Ag-Sb eutectic alloy, Ag-41.0at%Sb, are good candidates of braze for the PbTe-based thermoelectric modules. Ni and Co are potential barrier layers. The interfacial reactions at the Ag-Ge/Ni/PbTe, Ag-Ge/Co/PbTe, Ag-Sb/Ni/PbTe and Ag-Sb/Co/PbTe are examined. Two reaction phases, CoGe and β-Co5Ge3, are formed in the Ag-Ge/Co couples reacted at both 400oC and 750oC. Only the ε-Ni5Ge3 phase is formed in the Ag-Ge/Ni couples reacted at both 400oC and 750oC. The reaction phases are different in the Ag-Sb/Ni couples reacted at 350oC and 550oC, and they are NiSb/NiSb2 and Ni3Sb/Ni5Sb2/NiSb, respectively. Only the CoSb phase is observed in the Ag-Sb/Co couples reacted at 350oC, and both CoSb and CoSb3 phases are observed in the couples reacted at 550oC. All these results reveal the reactions with Ni are of higher reaction rates.

10:30 AM  Invited
Evaluation of Cobalt Diffusion Barrier for Low and Medium Temperature Thermoelectric Module: Albert T. Wu1; Hsien-Chien Hsieh1; Chun-Hsien Wang1; 1National Central University
    In thermoelectric (TE) module, Ni films are usually selected as the diffusion barrier. For low temperature Bi2Te3 TE materials, Ni reacts with Te and forms NixTey intermetallic compound (IMC). For medium temperature PbTe TE materials, Ni dissolves quickly at the working temperature. The mechanical strength becomes weak and degrades the reliability of the joints. Co is an effective diffusion barrier in electronic devices. Compare to Ni-based UBM, the joints with Co-based UBM has several advantages, such as low contact angle, high mechanical strength and low intermetallic compound (IMC) hardness. In this study, Co film was selected as the diffusion barrier for the joints between solders and TE materials. The morphology and the composition of the IMC at the interfaces is reported in this paper. The bonding strength is used to evaluate the effectiveness of Co as the diffusion barrier for TE modules.

10:50 AM  
Investigation of Defects in CZT Single Crystals: Bengisu Yasar1; Merve Kabukcuoglu1; Yasin Ergunt1; Mehmet Parlak1; Rasit Turan1; Eren Kalay1; 1METU
    The II-IV compound semiconductor CZT single crystal is a highly prominent material for the fabrication of various industrial applications such as gamma ray/X-ray detectors, electro-optic modulators and solar cells. Often, CZT single crystals suffer from various crystallographic defects emerged during crystal growth and post-growth cooling processes. In the scope of this research, the nano scale defects were investigated by using HRTEM. Single crystalline CZT crystals were grown using a three-zone vertical Bridgman system. Single crystals oriented along {111} and {211} crystals were rotated with the help of Laue Back Reflection method and cut with a diamond cutter. Tellurium precipitates, twins and stacking faults with the help of quantitative scanning transmission electron microscopy (QSTEM) simulation package and compare them with the experimentally observed HRTEM images. APT was used for the investigation of tellurium segregation. The quantitative interpretations of HRTEM along with corresponding simulations will be presented and discussed in details.

11:10 AM  
Scalable Synthesis of Silicon-implanted CZTS Nanoparticles for Catalysis and Thermoelectrics: Stephen Exarhos1; Edgar Palmes1; Alejandro Alvarez1; Lorenzo Mangolini1; 1University of California, Riverside
    By its nature, CZTS is a very adaptable material system. It is relatively straightforward to alloy into the material primarily as a method of band gap control. By optimizing the band gap and band alignment of alloyed CZTS-like nanoparticles, we intend to improve the catalytic quality of CZTS-based heterostructures. Using aerosol spray pyrolysis, we process a solution with copper-, tin-, and zinc-diethyldithiocarbamate precursors to form phase-pure, surface-ligand-free, kesterite CZTS nanoparticles with a size distribution average of ~ 20 nm. By adding hydrogen-terminated silicon nanoparticles — synthesized in-house by a non-thermal plasma process — to the precursor solution, we can implant silicon into the material (making CZTSiS), and in turn widen the band gap. We present studies characterizing CZTS and CZTSiS nanoparticles for potential use as a photocatalytic heterostructure material, and also preliminary studies for their application as a bulk-nanostructured thermoelectric.

11:30 AM  
Thermoelectric Behaviour of Polyvinyl Acetate /CNT Composites: Hussein Badr1; Mostafa Youssef1; Mohamed Gamal1; Hebatullah Abd-elsalam1; Mirna Mohamed1; Iman El Mahallawi1; Ahmed Abdel-rehim2; 1Cairo University; 2British University in Egypt
    In this work an organic polymer/ carbon nanotubes is introduced for low cost, versatile, nontoxic, light weight and high performance thermoelectric materials. Organic polymers have low thermal conductivity which is desirable for thermoelectric field. Composite films of polyvinyle Acetate and multi wall carbon nanotubes (MWCNT) are demonstrated as a new candidate material for thermoelectric application. MWCNT wt. percentage ranging from 1 to 70 wt.% were manufactured and tested for thermopower and electrical conductivity. Dioctyl sodium sulfosuccinate is used as the wetting agent in ratios of 2, 25 and 100% of CNT weight content. A study of the effect of the dispersant to CNT ratio on the electrical conductivity and thermopower of the composite was also carried out. The composite thermoelectric properties were measured as a function of CNT concentration at room temperature.

11:50 AM  Invited
Nanostructure and Phonon Engineering in Oxide Thermoelectric Materials: Michitaka Ohtaki1; 1Kyushu University
    Metal oxides include a large number of material families with an overwhelmingly wide variety of physical properties: from insulators to superconductors with the highest Tc; from dielectrics via ferroelectrics to metallic conductors; from antimagnets to ferromagnets; from transparent amorphous glasses to perfectly light-absorbing materials. The variety of the properties of oxides is far vaster than those of elemental metals and metallic compounds, as well as than those of covalent semiconductors. If so, why oxides do not include some good thermoelectrics? Actually, metal oxides are a prospective new member in practical thermoelectrics because of their minimal environmental impacts and superior durability at high temperature in air. In this paper, a new strategy for high-performance oxide thermoelectric materials will be depicted with focusing on nanostructure and phonon engineering to showcase how to overcome their inherent disadvantages that had kept oxides far away from consideration. Current issues and future prospects will be discussed.