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

Thursday 8:30 AM
March 2, 2017
Room: 22
Location: San Diego Convention Ctr

Session Chair: Franck Gascoin, Laboratoire CRISMAT; Yang-yuan Chen, Academia Sinica

8:30 AM  Invited
Diamond-Like and "Diamond-Unlike" Ternary Copper Based Semiconductors for Thermoelectrics: Donald Morelli1; 1Michigan State University
    Ternary diamond-like semiconductors can be built-up from simple group IV, group III-V, and group II-VI diamond and zincblende structure elements and compounds by replacing single cation elements with two elements of total electron valence equal to that of the replaced atom. This process leads to the formation of new classes of semiconductors including I-III-VI2 compounds, I2-IV-VI3 compounds, and I3-V-VI4 compounds. Examples include CuGaSe2, Cu2SnSe3, and Cu3SbSe4, respectively. With increasing complexity of the unit cell the electronic band gap narrows and the lattice thermal conductivity decreases, two effects favorable for thermoelectricity. Several compounds in these families possess thermoelectric figure of merit in excess of unity. By altering the composition of these diamond-like structures, more new compounds with different bonding types and crystal structures can be formed. These “diamond-unlike” compounds possess unique features that are scientifically interesting and that provide for potential new pathways to higher figure of merit.

8:50 AM  Invited
Intrinsic Thermoelectric Properties of SnSe Single Crystals and Its Associates: Yang-Yuan Chen1; P.C. Wei1; 1Institute of Physics, Academia Sinica
    SnSe single crystal has been reported to exhibit a high figure of merit, ZT = 2.6 at 930 K, this makes the practical application of thermoelectric materials realized. Unfortunately, no similar result on single crystal SnSe has been reported since its publication two years ago. To study its intrinsic properties, a series of associated SnSe single crystals of pristine, deficiency, and element doping has been fabricate for extensive study. The pristine high-quality SnSe single crystals grown by Bridgman method show a higher mass density and thermal diffusivity (~0.48 mm2/s as compared with 0.22 mm2/s reported) than earlier report, the peak ZT at 850 K along c axis is about 1. For extra Se doped SnSe1+x (x=0.005-0.01), the thermal conductivity of SnSe1+x is higher than the pristine SnSe crystal. To examine the effect of deficiency of Se, SnSe1+x single crystals with x=-0.055 and -0.01 are will be reported as well.

9:10 AM  
Engineering High-zT In-doped GeTe: The Phase Equilibria and Thermoelectric Properties: Jie-Ru Deng1; Hsin-jay Wu1; 1Department of Materials and Optoelectronic science, National Sun Yat-sen University
    Thermoelectric (TE) materials, which can convert waste heat into precious electricity, has been viewed as an effective solution for the worldwide energy issue. The simple binary compound, GeTe, has long been considered as a promising thermoelectric materials in the temperature range of 200℃-500℃. Phase diagram provides basic yet essential information that probes the thermodynamically phase stability behaviors of complicated materials systems. With an aid of phase diagram and microstructural evolution, the TE properties can be optimized. We aim to determine the ternary phase diagram of Ge-Te-In system. The efforts of this study include: (1) determining the liquidus projection by water-quenched alloys, (2) constructing the 350˚C isothermal section by thermally-equilibrated alloys, and (3) measuring the thermoelectric property of In-doped GeTe alloys. Metallographic observations upon the quenched or thermally-equilibrated alloys are conducted using SEM while the phase identifications and compositional analysis are carried out by XRD and EDS, respectively.

9:30 AM  
Thermoelectric Properties of PbTe-based Materials Fabricated by a Melt Spinning Method: Preeyakarn Eaksuwanchai1; Ken Kurosaki1; Michihiro Ohta2; Priyanka Jood2; Yuji Ohishi1; Hiroaki Muta1; Shinsuke Yamanaka1; 1Osaka University; 2AIST
    PbTe-based materials are well known as high performance thermoelectric materials because they have large Seebeck coefficient (S) and moderately low lattice thermal conductivity (κlat). However, due to the bipolar conduction, the S decreases and the κlat increases at high temperatures, which greatly decreases the zT. Here, we prepared MTe-PbTe nanocomposites, one example of M is Mg, by a melt-spinning method to suppress the bipolar conduction based on the minority carrier scattering concept. The size and the distribution of MTe in the PbTe matrix were controlled by changing mainly the cooling rate of the melt spinning. The microstructure and the thermoelectric properties of the MTe-PbTe nanocomposites will be presented.

9:50 AM  
Thermoelectric Properties of Amorphous Half-Heusler Thin Films Synthesized by Magnetron Sputtering: Liangliang Li1; 1Tsinghua University
    Half-Heusler (HH) compounds such as ZrNiSn and FeVSb are potential thermoelectric materials due to low cost and non-toxicity. However, the thermal conductivity, κ, of HH compounds is large and reduces the thermoelectric figure of merit, ZT. An amorphous microstructure may be helpful to reduce the κ. In this work, we synthesized amorphous Zr-Ni-Sn (J Electronic Materials, 44, 1957-1962, 2015) and Fe-V-Sb films by magnetron sputtering. The HH films were sputtered using self-made targets and the sputtering parameters such as pressure and power were optimized. Then, the microstructure of the HH films was studied by HRTEM and XRD and an amorphous structure was identified. The Seebeck coefficient, electrical conductivity, and κ were measured. The amorphous structure greatly reduced the κ to 1-2 W/(mK), which was one order less than that of bulk HH compounds. The power factors of the films were 2-3 mWK-2m-1. Therefore, amorphous HH films are promising thermoelectric materials.

10:10 AM Break

10:30 AM  Invited
Exploratory Research of New Polar Chalcogenides: Robin Lefèvre1; Stefan Maier1; David Berthebaud1; Franck Gascoin1; 1CRISMAT Laboratory
    As John Corbett wrote “It is always difficult to predict the unimaginable”, hence the endless need for exploratory research. Closely associated to this research, the serendipitous discovery of interesting and first-class physical properties, including thermoelectric properties, is also at stake. We have thus embarked in the systematic exploration of different intermetallic systems. In order to “force the chance”, we do not have any restriction on the choice of the elements used, thus we consider that toxicity or cost are irrelevant issues at this stage. Rather, we focused on required structure – properties relationships in order to maximize the probability to obtain interesting transport properties. Features that fit that bill are numerous, but essentially, complex structure and a clear separation between the anionic and cationic sub-networks are amongst our prerequisites. In this contribution, the first result of our systematic exploration will show that in certain systems (selenides and tellurides), thermal and electronic transport properties are promising and that the richness of these systems probably still hides good thermoelectric materials.

10:50 AM  Invited
Theoretical and Experimental Investigation of the Electronic Structure and Thermoelectric Properties of the Fe2VAl Heusler Compound: Subrahmanyam Bandaru1; Florence Rouessac1; Philippe Jund1; 1ICGM-Montpellier University
    Efficient and eco-friendly thermoelectric devices are needed to recover energy losses. Heusler-type Fe2VAl compounds attained a great attention due to their semiconducting nature over a wide temperature range up to 1200 K. First principles calculations are performed to study the electronic structure of the full Heusler compound Fe2VAl and the formation energy of the intrinsic point defects such as vacancies, antisites and interstitials is analyzed as well. The electronic transport properties of Fe2VAl and the most favorable defects are analyzed based on the Boltzmann transport theory within the constant relaxation time approach. The lattice thermal conductivity is also being analyzed by using the sets of second- and third-order interatomic force constants which can be calculated from ab intio methods. Pure Fe2VAl compounds in parallel have been synthesized by the combination of mechanical alloying and spark plasma sintering.The transport properties obtained from the experiments are compared with the calculated values.

11:10 AM  
Thermoelectric Properties of MnTe- and MnTe2- based Materials: Quansheng Guo1; Takao Mori1; 1NIMS
    Magnetic semiconductors, such as chalcopyrite CuFeS2, have received considerable attention due to their excellent thermoelectric properties.1,2 Here we report on the modifications of p- type semiconductor hexagonal MnTe with high Néel temperature (TN = 310 K).3 Mn1–xCrxTe (x = 0, 0.04, 0.08) were prepared by melting, quenching and consolidation through spark plasma sintering (SPS). Their phase compositions were checked by X-ray diffraction technique and transport properties investigated. The electrical and thermal properties of various doped MnTe2 compounds were also characterized. This work is supported by CREST, JST. 1 Angewandte Chemie International Edition 54 2015 12909. 2 Applied Physics Express 6 2013 043001. 3 Physics Review B 73 2006 104403.

11:30 AM  
Thermoelectric Performance of Undoped and Ag Doped Mg2Sn Alloys: Rameshkumar Varma1; Sitarama Kada1; Matthew Barnett1; 1Deakin University
    Mg2Sn alloys are potential candidates for thermoelectric applications. In this research, undoped Mg2Sn and silver doped alloys are produced using radio frequency (RF) induction casting method with varying magnesium concentrations to account for the vaporisation of magnesium. The main aim of the research is to evaluate the thermoelectric performance of the cast alloys as a function of magnesium concentration. The thermoelectric properties are measured as a function of temperature during heating and cooling. Preliminary investigations showed that the electrical conductivity of the alloys increased with increase in the magnesium, and relatively high Seebeck coefficient was achieved in the Ag doped alloy and with high magnesium concentration. However, it was observed that the high value of Seebeck coefficient is sensitive to the local microstructure variations such as the segregation of Sn in Mg phase and the large dendritic structure observed in the cast alloys.

11:50 AM  
The Impact of Various Wafer Cleans on Surface Recombination in Crystalline Silicon: Haider Ali1; Kristopher Davis1; Winston Schoenfeld1; 1University of Central Florida
    Recombination losses occur at the silicon surface leading to lower cell efficiencies in crystalline silicon solar cells. To overcome this, the silicon surface is often passivated by the deposition of silicon nitride (SiNx) or an aluminum oxide film with SiNx as capping layer (Al2O3/SiNx stack). Further, proper surface preparation and cleaning of Si wafers prior to deposition also plays an important role in minimizing surface recombination. In the present work, the effect of various cleans based on different combinations of HCl, HF, HNO3, and ozonated deionized water (DIO3) on surface passivation quality of boron-diffused and undiffused {100} n-type Cz Si wafers was studied. It was observed that for SiNx passivated Si, carrier lifetime was strongly influenced by cleaning variations and that a DIO3-last treatment resulted in higher lifetimes. Moreover, DIO3+HF+HCl→HF→DIO3 and HNO3→HF→HNO3 cleans emerged as potential low-cost alternatives to HCl/HF clean in the photovoltaics industry.

12:10 PM Concluding Comments