Tristan Day

High thermoelectric performance in non-toxic earth-abundant copper sulfide

A new type of high performance thermoelectric material Cu_(2-x)S composed of non-toxic and earth-abundant elements Cu and S is reported. Cu_(2-x)S surprisingly has lower thermal conductivity and more strikingly reduced specific heat compared to the heavier Cu_(2)Se, leading to an increased zT to 1.7.

Thermoelectric properties of p-type polycrystalline SnSe doped with Ag

Many IV–VI semiconductors tend to be good thermoelectric materials, these include all Pb chalcogenides as well as Pb-free SnTe: all of which crystallize in a NaCl cubic structure. Another group of IV–VI compounds form layered orthorhombic structures. SnSe is one of these compounds, whose transport properties as a polycrystalline thermoelectric material have rarely been studied. Here we present our study of p-type polycrystalline SnSe doped with Ag, prepared by melting and hot pressing. SnSe has anisotropic properties with hysteresis observed in resistivity between 300 and 650 K regardless of doping. Ag is not an ideal dopant but is able to increase the carrier density significantly, as a result a peak zT of 0.6 was observed at 750 K. Transport properties of doped SnSe can be explained with a single parabolic band model, which suggests promising potential for this compound together with its challenges.

Chalcopyrite CuGaTe2: A High‐Efficiency Bulk Thermoelectric Material

CuGaTe(2) with a chalcopyrite structure demonstrates promising thermoelectric properties. The maximum figure of merit ZT is 1.4 at 950 K. CuGaTe(2) and related chalcopyrites are a new class of high-efficiency bulk thermoelectric material for high-temperature applications.

Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu2Zn1–xFexGeSe4

Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu(2)Zn(1-x)Fe(x)GeSe(4). Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials.

Thermoelectric transport in Cu7PSe6 with high copper ionic mobility.

Building on the good thermoelectric performances of binary superionic compounds like Cu2Se, Ag2Se and Cu2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu7PSe6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure-property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu7PSe6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes.

Enhanced Thermoelectric Properties in Bulk Nanowire Heterostructure-Based Nanocomposites through Minority Carrier Blocking

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Phase transition enhanced thermoelectric figure-of-merit in copper chalcogenides

While thermoelectric materials can be used for solid state cooling, waste heat recovery, and solar electricity generation, low values of the thermoelectric figure of merit, zT, have led to an efficiency too low for widespread use. Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of zT in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu 2Se over a broad (360–410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of zT (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the zT of Cu 1.97 Ag .03Se (almost 1.0 at 400 K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high zT and new green applications for thermoelectrics.

Chemical Stability of (Ag,Cu)2Se: a Historical Overview

Recent work on Cu2−xSe has caused strong interest in this material due to its high reported peak zT (1.5) and the reduction of thermal conductivity through the mechanism of liquid-like suppression of heat capacity. In the 1960s, 3M patented Cu1.97Ag0.03Se as “TPM-217.” Over the following decade it was tested and developed by the 3M Corporation, at the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory, Teledyne Energy Systems, and the General Atomics Corporation for use as a next-generation thermoelectric material. During these tests, extreme problems with material loss through Se vaporization and chemical reactions between the material and the device contacts were found. These problems were especially severe while operating under conditions of high

Composition Modulation of Ag2Te Nanowires for Tunable Electrical and Thermal Properties

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