Session V11: Thermoelectrics: McGroddy Prize and Novel Materials

James C. McGroddy Prize for New Materials Talk: From discovery to design of new materials

The design and discovery of new materials and their crystal growth is critical for continued scientific and technological progress far into the future. It is also a fundamental goal of condense matter science. We have been developing the chemistry of novel chalcogenide and intermetallic materials which define a remarkably broad set of structurally diverse compounds, associated with a wide range of physical properties and impacting a variety of physics and materials science issues. In contrast to solid-state methods, materials syntheses in liquid fluxes permit crystallization at lower temperatures due to facile diffusion and possible chemical reactions with the flux itself. These reactions can produce a wide range of materials, often metastable such as oxides, chalcogenides and intermetallics, but typically the formation paths are obscure or poorly understood. In this talk I will describe how we observe, understand, and engineer the formation of compounds from inorganic melts and an approach we call “panoramic synthesis”. I will also highlight some of our recent results on the discovery of remarkable materials and crystal structures and how they can be leveraged for achieving unusual or enhanced properties of interest in a variety of fields such as thermoelectrics, $\gamma$ ray detection, superconductivity, topological properties, nonlinear optics, etc.   

Thermoelectric Properties of Cd Based Zintl Phase Compounds

Zintl phase compounds can be described as covalently-bonded anion substructures surrounded by highly electro-positive cations exhibiting essential features for thermoelectric applications. By combining first principles electronic structure and Boltzmann transport theory, here we report excellent thermoelectric properties of CdSb and ACd$_2$Sb$_2$ (where, A = Ca, Ba, Sr). The electronic structure shows heavy and light bands near the band edges, which lead to large power factor resulting in good thermoelectric performance. We also calculate lattice thermal conductivity by solving Boltzmann Transport equation using an iterative method. The large Gr\"{u}neisen parameters and low phonon group velocity indicate strong anharmonicity in these compounds, which results in low lattice thermal conductivity. The low thermal conductivity and the excellent transport properties lead to a high ZT value of 1.9 in CaCd$_2$Sb$_2$ and BaCd$_2$Sb$_2$ at moderate p and n-type doping. These results indicate that well optimized Cd based Zintl phase compounds have a potential to match the performance of conventional thermoelectric materials.   

Impact of electron doping on structure and dynamics of synthetic tetrahedrite

Microscopic control of lattice thermal conductivity is critical to the development of thermoelectric materials.~ One route to this control is manipulation of anharmonic lattice dynamics with chemical doping.~ Tetrahedrite compounds, which display an intrinsic lattice anharmonicity, are promising candidates for thermoelectric application, and here we present results of neutron scattering studies on synthetic Cu$_{12-x} $Zn$_x$Sb$_4$S$_{13}$, ($x = 0,~2$).~ The undoped compound exhibits a structural phase transition associated with a metal-insulator transition near 88~K. Doping with Zn results in the stabilization of the structure, and enhancement of a low energy vibrational mode associated with incoherent oscillations of Cu. The low energy mode is localized, and exhibits pronounced softening with temperature.~ The ability to tune the location of this low-energy mode with doping may provide a means to enhance the phonon-phonon scattering that leads to low thermal conductivity in these materials, and a corresponding enhancement of the thermoelectric properties.   

Spectroscopic evidence for temperature dependent relative movement of light and heavy hole valence bands of PbQ (Q=Te,Se,S)

We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) studies of the electronic structures of PbTe, PbSe and PbS. Our ARPES measurements provide direct evidences for the light hole upper valence bands (UVBs) and the so-called heavy hole lower valence bands (LVBs), and an unusual temperature dependent relative movement between their band maxima leading to a monotonic decrease in the energy separation between LVBs and UVBs with increase in temperature. This enables convergence of these valence bands and consequently an effective increase in the valley degeneracy in PbQ at higher temperatures, which has long been believed to be the driving factor behind their extraordinary thermoelectric performance.   

$p \times n$-Transverse Thermoelectrics: Single leg thermoelectrics with scalable integration and cryogenic promise

Under the $p \times n$–type transverse thermoelectric paradigm electrons dominate conduction in one direction and holes dominate perpendicularly, allowing electrical current to drive transverse heat flow [1]. Bulk anisotropic crystals, superlattices, and nanowire arrays have all been previously proposed as viable $p \times n$ materials. This talk will describe the general philosophy behind $p \times n$-type transverse thermoelectrics as well as the tensor equations that define their anisotropic Seebeck effect. The advantages of single-leg thermoelectric devices -- available only to transverse thermoelectrics -- are detailed. Various device geometries are discussed which take advantage of the single-leg nature, in particular to provide advantages for cryogenic thermoelectric cooling and integrated thermal management. \newline [1] Chuanle Zhou, S. Birner, Yang Tang, K. Heinselman, and M. Grayson, Phys. Rev. Lett. 110, 227701 (2013).   

Copper Selenide Nanocrystals as a High Performance, Solution Processed Thermoelectric Material

Nano-structuring a thermoelectric material often results in enhanced performance due to a decrease in the materials’ thermal conductivity. Traditional nano-structuring techniques involve ball milling a bulk material followed by spark plasma sintering, a very energy intensive process. In this talk, we will describe the development of a self-assembled, high-performing, nano-structured thin film based on copper selenide nanocrystals. Mild thermal annealing of these films results in concurrent increases in the Seebeck coefficient and electrical conductivity. We are able to achieve power factors at room temperature that are as high as the best spark plasma sintered materials. These solution-processed films have potential applications as conformal, flexible materials for thermoelectric power generation.   

NMR and specific heat study of atomic dynamics and spin-orbit behavior in Cu$_{\mathrm{2-x}}$Ag$_{\mathrm{y}}$Te

We report studies of Cu$_{\mathrm{2}}$Te and Cu$_{\mathrm{2-x}}$Ag$_{\mathrm{y}}$Te, promising candidates for thermoelectric and photovoltaic applications. Cu and Te NMR show that above a well-defined 200 K onset, Cu$_{\mathrm{2}}$Te exhibits Cu-ion hopping, leading to the higher-temperature superionic motion. In Cu$_{\mathrm{1.98}}$Ag$_{\mathrm{0.2}}$Te the onset increases to 250 K. In the low-temperature static phase the properties are nearly identical. Aside from Korringa terms there are large diamagnetic contributions for all nuclei, comparable to those for other systems with very large spin-orbit and/or inverted band configurations. Thus the system may be a topologically interesting system like the similar phase Ag$_{\mathrm{2}}$Te. Results will be compared to DFT calculations of NMR shifts. The low-temperature spectra also indicate two distinct local environments for Cu sites, one corresponding to high symmetry such as characterizes the high-temperature cubic phase, and one with much more asymmetry. In addition, specific heat results are consistent with about 50{\%} of the Cu ions being weakly bound on Einstein-oscillator sites. We tentatively connect these results to reported local inhomogeneity due to vacancy condensation in similar systems.   

Thermoelectric Properties of CuAgSe doped with Co, Cr.

Thermoelectric materials represent one way that reliable cooling below the boiling point of nitrogen can be realized. Current materials do not exhibit sufficiently high efficiencies at cryogenic temperatures, but significant progress is being made. One material that has generated significant interest recently is CuAgSe. It has been demonstrated (Ishiwata et al., Nature Mater. 2013) that doping CuAgSe with 10{\%} Ni at the Cu sites increases the material's thermoelectric figure of merit (ZT) at 100 K from 0.02 to 0.10. This is intriguing not just because of the dramatic effect that the Ni doping produces, but also because CuAgSe is a semimetal and semimetals are not usually able to exhibit the kind of asymmetric carrier activation necessary for strong thermoelectric performance. In order to further investigate the unusual nature of thermoelectricity in CuAgSe and its strong dependence on chemical composition, we have synthesized and measured the thermoelectric properties of a series of CuAgSe samples doped with Co and Cr. Temperature-dependent magnetic and thermoelectric transport properties of CuAgSe as a function of Co and Cr doping will be discussed.   

Low Temperature Thermoelectric Characterization of Ag$_{\mathrm{2}}$Se.

Previous work on Ag$_{\mathrm{2}}$Se showed that this n-type material could have a dimensionless thermoelectric figure of merit (zT) \textasciitilde 1 at room temperature, due to its high mobility and low thermal conductivity. However, the results from the initial reports have not yet been reproduced. In this talk, I will summarize our efforts to replicate the aforementioned thermoelectric performance, and also discuss the experimental setup that we utilized in order to perform the low-temperature thermoelectric characterization of this material.   

Molecular-Dynamics (MD) Simulations of Copper Diffusion in Copper Chalcogenides

It was recently discovered that copper chalcogenides Cu2S and Cu2Se are viable candidates for thermoelectric materials with high figure of merit (ZT) values at temperatures around 1,000 K [1,2]. And the possible reason for the high ZT is the low thermal conductivity arising from liquid-like Cu atoms in those phases. In this work, we perform first-principles molecular dynamics simulations to study the motion of Cu atoms in the high-temperature phases of Cu2S and Cu2Se and confirm the liquid nature of Cu atoms. To get a better understanding of the diffusion patterns of the systems, we have examined all the three phases of Cu2S (monoclinic, hexagonal and cubic phases with increasing temperature). Starting from the hexagonal phase the Cu atoms show a disordered/liquid-like feature with a jump diffusion pattern. We find that the diffusion is faster in x-y directions than in the z direction. A more isotropic diffusion pattern is found for high-temperature cubic phase with a much larger diffusion coefficient.1] Y. He et al. Adv. Mater. 26, 3974 (2014) 2] H. L. Liu et al. Nat. Mater. 11, 422 (2012)   

Impact of Lone-Pair Electrons on Thermal Conductivity in CuSbS2 Compound

Compounds with intrinsically low lattice thermal conductivity are of practical importance for thermoelectric energy conversion. Recent studies suggest that s2 lone pair orbital electrons are a key contributing factor to the anomalously low lattice thermal conductivity of chalcogenide compounds that contain a nominally trivalent group VA element. CuSbS2 has an orthorhombic structure with space group Pnma. The pyramidal SbS5 units are separated by CuS4 tetrahedron so that the base of the square pyramidal units are aligned to face one another, thus directing the Sb lone pair electron density into the void separating the SbS5 units. Different from tetrahedrite, all the Cu atoms are bonded in the CuS4 tetrahedron. So, it has a perfect structure to study the influence of electron lone pair on thermal conductivity without the impact from trigonal coordinated Cu. In this work, the trivalent transition metal atom Fe and IIIA atom Ga without lone-pair electrons were chosen to substitute Sb in CuSbS2. The changes in the bonding environment by foreign atoms and their influences on the thermal properties have been studied and correlated.