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The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures. DOI: 10.1103/PhysRevLett.117.276601 Thermoelectric (TE) materials and the thermoelectric effect are an interesting alternative energy source, harvesting waste heat from power production and other thermal engines. Despite their vast potential impact, only few materials are used in practice: most thermoelectric materials are highly toxic, expensive, and the devices present too low efficiencies to compete with other forms of power generation in industry. The main concern in this field is to discover or design thermoelectric materials that deal with these issues. The efficiency of a TE material is quantified by the thermoelectric figure of merit zT ¼ S 2 σT=ðκ el þ κ l Þ, which is the ratio of the electrical conductivity (σ), multiplied by the Seebeck coefficient (S) squared and the absolute temperature (T), over the thermal conductivity, which has both ionic (κ l ) and electronic (κ el ) contributions. The recent demonstration of zT ¼ 2.6 in monocrystalline tin selenide [1] or zT ¼ 1.34 in device form [2] has given a new breath to the field of thermoelectrics. By more than doubling the efficiency record for intrinsic bulk systems, SnSe has shown that economically competitive, nontoxic TE devices are within reach. The microscopic mechanism responsible for the performance is, however, not fully established, in particular due to sublimation effects in the high-T phase. Bulk SnSe is a narrowband-gap semiconductor that undergoes a phase transition spanning the temperature range from 600 to 807 K, from a Pnma low-temperature phase as illustrated in Fig. 1 (space group 62) to a Cmcm high-temperature phase (space group 63) [3]. Both are distorted phases of rocksalt Fm3m (the isoelectronic structure of PbTe and SnTe). Exceptional values of zT are obtained for two main reasons: the intrinsically low thermal conductivity (in both phases) and the strong enhancement of the carrier concentration and conductivity in the Cmcm phase. This intricate interplay opens perspectives for many other layered or heterostructure materials, and calls for a profound understanding of the mecha...

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In this work we identify p-type half-Heusler thermoelectrics using high-throughput techniques. We have scanned a large database of potential candidates and report NbCoSn and TaCoSn as new, attractive, previously unexplored p-type half-Heuslers.

We explore a material design strategy to optimize the thermoelectric power factor. The approach is based on screening the band structure changes upon a controlled volume change. The methodology is applied to the binary silicides and germanides. We first confirm the effect in antifluorite Mg2Si and Mg2Ge where an increased power factor by alloying with Mg2Sn is experimentally established. Within a high-throughput formalism we identify six previously unreported binaries that exhibit an improvement in their transport properties with volume. Among these, hexagonal MoSi2 and orthorhombic Ca2Si and Ca2Ge have the highest increment in zT with volume. We then perform super-cell calculations on special quasi-random structures to investigate the possibility of obtaining thermodynamically stable alloy systems which would produce the necessary volume changes. We find that for Ca2Si and Ca2Ge the solid solutions with the isostructural Ca2Sn readily forms even at low temperatures. PACS numbers:Despite their importance, the discovery of new materials are often based on trial and error. High-throughput (HT) computational screening 1,2 is an important step towards identifying materials with desired properties in a more systematic way. Thermoelectric (TE) materials are attractive for such computational searches because continuous development of computational methodology means that all parts of the TE figure of merit, zT can in principle be calculated from first principles.3-6 In practice computational HT searches for new TE materials have focused on parts of the zT = S 2 σT /κ, where S is the Seebeck coefficient, σ the electrical and κ the thermal conductivity.7-11 Despite this, there are now a few works where computational screening has led to high performance TE materials that could be experimentally realized. 12-15Beyond screening known compounds, there still exists great challenges designing new materials with specific properties. This is especially the case for electronic structure dependent properties, which have highly non-Schematic illustration of volumetric band alignment for a n-type material. VBM stands for the Valence Band Minimum.trivial dependencies on the atomic structure. 16 For TE materials one strategy for designing new alloys with optimized properties is by a controlled volume change. We label this procedure volumetric band-structure alignment (VBA). The idea is illustrated in Fig. 1, where the energy dependence of two bands vary differently upon a change of volume. Thereby a scenario can occur when the band edges are aligned, as schematically illustrated in the midpanel of Fig. 1. How this optimizes the TE power factor, P F = S 2 σ, can be understood by considering the generalized transport coefficients,where f is the Fermi-distribution and σ(ε) the transport distribution. The Seebeck and electric conductivity are given as S = L (1) /qT L (0) and σ = L (0) , respectively. For two channels (labelled and ) conducting in parallel, L (α) is given as the sum of the contributions from each channel, so that the PF is...

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Abstract Extensive efforts have been made in the last decade for the development of natural fibre composites. This development paved the way for engineers and researchers to come up with natural fibre composites (NFCs) that exhibit better mechanical properties. The present review is based on the mechanical properties of jute, abaca, coconut, kenaf, sisal, and bamboo fibre-reinforced composites. Before selecting any NFC for a particular application, it becomes necessary to understand its compatibility for the same, which can be decided by knowing its mechanical properties such as tensile, flexural, and impact strengths. This review paper emphasises on the factors influencing the mechanical properties of NFCs like the type of matrix and fibre, interfacial adhesion, and compatibility between matrix and fibre. Efforts are also made to unveil the research gaps from the past literatures, as a result of which it is inferred that there is very limited work published in the field of vibration incorporating potential fillers such as red mud and fly ash with NFCs.