Monday 8:30 AM

February 27, 2017

Room: 21

Location: San Diego Convention Ctr

Conversion of waste heat into electrical energy via thermoelectric technology requires highly efficient thermoelectric materials. Developing thermoelectric materials with high efficiency is becoming an important area of research to develop alternative energy technologies to reduce our dependence on fossil fuels and reduce emissions of greenhouse gas. In this presentation, an overview of various TE phenomena, materials developed so far, several key parameters involved in thermoelectric research with their current applications will be discussed. Potential thermoelectric such as half-Heusler, Zintl phase Mg3Sb2 compound, SiGe, Mg2Si and Cu2Se will be presented in terms of challenges in their processing, optimization of TE properties and their applications. Fundamental understanding, unresolved issues related to carrier transport and highlight the present research resolving these issues for emerging a next generation of efficient bulk nanostructured materials will also be the part of this presentation.

In the last decade, first-principles approaches to thermal transport have emerged as powerful tools to obtain predictive estimates of the thermal conductivity of crystalline semiconductors. User-friendly solvers and interfaces to first-principles codes have been developed and released to the public, helping popularize those techniques. However, this is only the first step on the way to modeling thermal transport in real semiconductors of technological interest. The conductivity of the single crystal is an upper bound to the value found in actual samples, where crystallographic defects and interfaces affect phonon scattering. The components needed to analyze such multiscale problems are far less developed, with most studies still relying on parametric relations with limited predictive power. Improving the situation is a key objective of H2020 project ALMA (www.almabte.eu), which aims to develop a software framework for thermal conductivity calculations in more complex systems. This talk will be devoted to presenting the capabilities of the code and discussing the simulation techniques implemented therein. Examples of applications will include the cross-plane thermal conductivity of crystalline and solid-solution thin films, phonon scattering by several types of localized defects, and optimization of III-V superlattice profiles.

At the National Institute of Standards and Technology (NIST), we have developed a combinatorial film deposition facility and a suite of high throughput instrumentation to perform combinatorial characterization of higher efficiency thermoelectric materials. These capabilities include a Pulsed Laser Deposition (PLD) apparatus for combinatorial thin film synthesis, an X-ray diffractometer for phase characterization, and a suite of tools for screening functional properties such as the Seebeck coefficient, electrical resistance, and thermal effusivity of combinatorial films. The local Seebeck coefficient and resistance are measured via custom-built automated apparatus at and above room temperatures. Thermal effusivity is measured using a frequency domain thermoreflectance technique. Our talk will discuss the applications using these tools to investigate thermoelectric materials, including combinatorial composition-spread films of various systems, and also conventional films, single crystals and other bulk materials.

The application of data science approaches in materials science opens up new avenues for accelerated discovery and design, as also recognized by MGI. In this talk, I will describe our ongoing work employing state-of-the-art data analytics on publicly available materials databases, to understand the PSPP linkages in thermoelectric materials. The thermoelectric performance figure of merit (ZT) is a function of several other properties, like electrical and thermal conductivities, Seebeck coefficient, etc. We have developed data-driven predictive models for some of these properties, and deployed them in a user-friendly online web-tool, which can make very fast predictions of these properties for a given material composition, structure, and processing parameters. Such a tool is expected to be a very useful resource for the materials science researchers and practitioners to assist in their search for new and improved quality thermoelectric materials. The tool is available at http://info.eecs.northwestern.edu/ThermoEl

We performed density functional theory-based calculations to investigate the structural, electrical and thermal properties of transition-metal dichalcogenides (TMDs). Two-dimensional TMDs are of broadening research interest due to their novel physical, electrical, and thermoelectric properties. Having the chemical formula MX2, where M is a transition metal and X is a chalcogen, there are many possible combinations to consider for materials-by-design exploration. We investigated the structure, electronic, and phonon properties of eighteen different TMD MX2 materials compositions as a benchmark to explore the impact of various elements. Our results identified key factors, including atomic weight, radius, oxidation state and interfacial lattice mismatching, to optimize MX2 compositions for desired thermoelectric performance. We also added substitutional dopants and created heterojunctions in the TMD materials to enhance their thermoelectric properties. We will present a new computational screening approach to draw correlation among the physical properties of constituent transition metals, substitutional dopants, heterojunctions, and resultant 2D-TMDs.

Heusler and half-Heusler alloys are expected to provide a promising base for well performing thermoelectric materials and a rather robust characteristics for applications. While the figure of merit of half-Heusler alloys reached already values well above one, the performance of full Heusler alloys is still lacking such values mostly because of the very large thermal conductivity. In this contribution we show how ab-initio calculations regarding the electronic structure help to select proper starting materials like Fe2VAl and how the effect doping and substitutions modifies the electronic structure and thus the electronic transport in the system and, accordingly, how thermal conductivity decreases due to scattering of the heat carrying phonons on point defects. Additionally, we demonstrate how a substantial reduction of grain sizes by ball milling and severe plastic deformation positively influences thermal conductivity and thus improves the thermoelectric figure of merit. Work supported by the CDL for Thermoelectricity

Recent experimental characterizations on thermoelectric materials revealed novel phenomena that require in-depth theoretical investigations to understand the underlying mechanisms. Conventional theoretical approaches have been shown to fail in reproducing some of the anomalous experimental results, stimulating the development of new methods. The effects of temperature, pressure and dopants on the lattice dynamics and electronic band structures of thermoelectric compounds are investigated based on newly developed first-principles approaches. Insight into the thermal and electrical transport properties are obtained from the electronic and atomistic scale simulations. Anharmonic lattice dynamics related to the intrinsically low thermal conductivity are studied from phonon interactions. Electronic band structures of thermoelectric materials are discussed based on the unfolding technique. Boltzmann transport theory is combined with density functional theory to predict the transport properties under pressure.

Within the last few years it has been possible to compute the lattice thermal conductivity of bulk materials using ab initio methods. The interactions between the phonons are obtained from density functional theory and this information is incorporated into the Boltzmann to obtain the thermal conductivity. The good accuracy obtained from those calculations allows trying to use them to find new materials. We present several strategies that we used performing such a search.The first method we used is datamining. We screened the entire Material Project library to find materials with ultra low thermal conductivity. The second method is based on polymorphism and was used to study Zn-Sb compounds. Finally we conclude showing how ab initio calculations can be combined with Monte Carlo simulations to describe thermal conduction at the micron scale.

The ability of first-principles computational approaches to provide access to relevant properties is critical for accelerating the pace at which materials design and discovery occurs. Yet, in many instances the complexity of the properties of interest renders direct calculations difficult to attain within the required accuracy. A practical way of dealing with this challenge is the development of physically motivated proxies and/or semi-empirical models that combine first-principles based methods with experimental knowledge. In this way computationally tractable approaches that are able to guide identification of novel functional materials can be achieved. In this talk, I will discuss recent work in developing and applying such computational approaches to predict electric and thermal transport properties of semiconductors to advance the search for new thermoelectric materials. Our recent developments provide quantitative prediction and enable high-throughput calculations and identification of new candidate materials. Results of these efforts are open and available via TEDesignLab (http://tedesignlab.org).

Sulfides as CZTS are candidates of thermoelectric materials. For example, Cu-S system sulfides are interesting material. Cu2S is positive semiconductor, however, it turns on negative semiconductor by adding Fe as Cu5FeS4 (so called bornite). Other sulfide based on metals were also useful materials. For sorting the other Cu-S system and/or other sulfides, first principle simulation were conducted for several sulfides as screening for synthesizing the materials. Based on the results of first principle simulation, seebeck coefficient was calculated by using simulation packages. On the other hand, sulfides were also synthesized by vacuum atmosphere experimentally. As a result, calculated seebeck coefficient was consistent with that of the experimentally synthesized materials in some sulfides. However, the results did not match among the some samples. These results depended on purity of sulfides which was determined by synthesizing conditions.