▪ Abstract Recently there has been a resurgence of research efforts related to the investigation of new and novel materials for small-scale thermoelectric refrigeration and power generation applications. These materials need to couple and optimize a variety of properties in order to exhibit the necessary figure of merit, i.e. the numerical expression that is commonly used to compare one potential thermoelectric material with another. The figure of merit is related to the coefficient of performance or efficiency of a particular device made from a material. The best thermoelectric material should possess thermal properties similar to that of a glass and electrical properties similar to that of a perfect single-crystal material, i.e. a poor thermal conductor and a good electrical conductor. Skutterudites are materials that appear to have the potential to fulfill such criteria. These materials exhibit many types of interesting properties. For example, skutterudites are members of a family of compounds we call...

SKUTTERUDITES : A phonon-glass-electron crystal approach to advanced thermoelectric energy conversion applications

A magnetotransport study of zirconium pentatelluride now reveals evidence for a chiral magnetic effect, a striking macroscopic manifestation of the quantum and relativistic nature of Weyl semimetals The chiral magnetic effect is the generation of an electric current induced by chirality imbalance in the presence of a magnetic field It is a macroscopic manifestation of the quantum anomaly1,2 in relativistic field theory of chiral fermions (massless spin 1/2 particles with a definite projection of spin on momentum)—a remarkable phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea The recent discovery3,4,5,6 of Dirac semimetals with chiral quasiparticles opens a fascinating possibility to study this phenomenon in condensed matter experiments Here we report on the measurement of magnetotransport in zirconium pentatelluride, ZrTe5, that provides strong evidence for the chiral magnetic effect Our angle-resolved photoemission spectroscopy experiments show that this material’s electronic structure is consistent with a three-dimensional Dirac semimetal We observe a large negative magnetoresistance when the magnetic field is parallel with the current The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect The observed phenomenon stems from the effective transmutation of a Dirac semimetal into a Weyl semimetal induced by parallel electric and magnetic fields that represent a topologically non-trivial gauge field background We expect that the chiral magnetic effect may emerge in a wide class of materials that are near the transition between the trivial and topological insulators

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Chiral magnetic effect in ZrTe 5

Thermoelectric (Peltier) heat pumps are capable of refrigerating solid or fluid objects, and unlike conventional vapor compressor systems, they can be miniaturized without loss of efficiency. More efficient thermoelectric materials need to be identified, especially for low-temperature applications in electronics and devices. The material CsBi(4)Te(6) has been synthesized and its properties have been studied. When doped appropriately, it exhibits a high thermoelectric figure of merit below room temperature (ZT(max) approximately 0.8 at 225 kelvin). At cryogenic temperatures, the thermoelectric properties of CsBi(4)Te(6) appear to match or exceed those of Bi(2-x)Sb(x)Te(3-y)Se(y) alloys.

https://science.sciencemag.org/content/sci/287/5455/1024.full.pdf

CsBi4Te6: A High-Performance Thermoelectric Material for Low-Temperature Applications

This review highlights the recent progress made in the area of thermoelectric (TE) applications of conducting polymers and related composites. Several examples of such materials and their TE properties are discussed. TE properties of new poly(2,7-carbazole) derivatives are highlighted. References are also made to carbon nanotube/polymer composites and their improved electrical and TE performance. Studies on polymer/inorganic materials composites have also taken a step forward and have shown very promising TE properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

https://onlinelibrary.wiley.com/doi/pdf/10.1002/polb.22206

Conducting polymers: Efficient thermoelectric materials

The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced. To understand the anomalous electronic transport properties of ZrTe5 remains an elusive puzzle. Here, Zhang et al. report direct electronic evidence to the origin of the resistivity anomaly and temperature induced Lifshitz transition in ZrTe5, indicating it being a weak topological insulator.

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Electronic evidence of temperature-induced Lifshitz transition and topological nature in ZrTe5.

Temperature dependent high-resolution angle-resolved photoelectron spectroscopy has been performed on the quasi-two-dimensional compound ZrTe5, a metal at low temperatures ( K) that exhibits a maximum resistivity at a temperature (Tc), concomitant with a sign change of the thermopower. A semiconducting gap has been observed in the photoemission spectra, where the valence band maximum shifts upward from 82 meV (75 K) to 40 meV (170 K) as a function of temperature. The band shifts are accompanied by small band distortions. Based on the photoemission experiments, in conjunction with the metallic character of ZrTe5 at low temperatures, we have modelled the thermopower of ZrTe5 by treating it as a metal at low temperatures and a semiconductor at elevated temperatures.

https://iopscience.iop.org/article/10.1088/0953-8984/16/30/L02/pdf

Observation of a semimetal–semiconductor phase transition in the intermetallic ZrTe5

The thermoelectric properties (resistivity and thermopower) of single crystals of the low dimensional pentatelluride materials, HfTe5  and ZrTe5, have been measured as a function of temperature from 10 K<T<320 K. The effect of small amounts of Ti substitutional doping (M1−xTixTe5, where M=Hf, Zr) on the thermoelectric properties is reported here. A resistive transition occurs in the pentatellurides, as evidenced by a peak in the resistivity, TP≈80 K for HfTe5 and TP≈145 K for ZrTe5. Both parent materials exhibit a large positive (p-type) thermopower near room temperature which undergoes a change to negative (n-type) below the peak temperature. The thermal conductivity is relatively low (≈5 W/m K) for the MTe5 materials. The Ti substitution affects the electronic properties strongly, producing a substantial shift in the peak temperature while the large values of thermopower remain essentially unaffected. These results warrant further investigation of these materials as candidates for low temperature thermo...

Effect of Ti substitution on the thermoelectric properties of the pentatelluride materials M1−xTixTe5 (M=Hf, Zr)

We have measured the resistivity and thermopower of single crystals as well as polycrystalline pressed powders of the low-dimensional pentatelluride materials: HfTe5 and ZrTe5. We have performed these measurements as a function of temperature between 5K and 320K. In the single crystals there is a peak in the resistivity for both materials at a peak temperature, Tp where Tp≈ 80K for HfTe5 and Tp ≈ 145K for ZrTe5. Both materials exhibit a large p-type thermopower around room temperature which undergoes a change to n-type below the peak. This data is similar to behavior observed previously in these materials. We have also synthesized pressed powders of polycrystalline pentatelluride materials, HfTe5 and ZrTe5. We have measured the resistivity and thermopower of these polycrystalline materials as a function of temperature between 5K and 320K. For the polycrystalline material, the room temperature thermopower for each of these materials is relatively high, +95 µV/K and +65 µV/K for HfTe5 and ZrTe5 respectively. These values compare closely to thermopower values for single crystals of these materials. At 77 K, the thermopower is +55 µV/K for HfTe5 and +35 µV/K for ZrTe5. In fact, the thermopower for the polycrystals decreases monotonically with temperature to T ≈ 5K, thus exhibiting p-type behavior over the entire range of temperature. As expected, the resistivity for the polycrystals is higher than the single crystal material, with values of 430 mΩ-cm and 24 mΩ-cm for Hfre5 and ZrTe5 respectively, compared to single crystal values of 0.35 mΩ-cm (HfTe5) and 1.0 mΩ-cm (ZrTe5). We have found that the peak in the resistivity evident in both single crystal materials is absent in these polycrystalline materials. We will discuss these materials in relation to their potential as candidates for thermoelectric applications.

Electrical Transport Properties of the Pentatelluride Materials HFTE5 and ZRTE5

The focus of our research revolves around a search for new thermoelectric materials with improved performance over present materials. We have performed preliminary experiments to access the potential of quasicrystalline materials for thermoelectric applications. The investigation of known classes of quasicrystals and quasicrystal approximants is important for potential thermoelectric materials as well as a search for entirely new phases of these materials. The advantage of quasicrystals is that their electrical conductivity ( and possibly their thermoelectric power) can be manipulated through variation of their composition, synthesis or annealing conditions without sacrificing their usually low thermal conductivity. Quasicrystalline materials exhibit characteristics which are suggestive of Slack's idea of a phonon-glass, electron-crystal material for thermoelectric applications. We will present some of our results of resistivity, thermopower, and thermal conductivity on measurements to show the potential of these materials. A more thorough investigation of this class of materials and a systematic search for new quasicrystalline and related compounds might very well yield new materials for thermoelectric devices.

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Potential of quasicrystals and quasicrystal approximants for new and improved thermoelectric materials

The thermopower and resistance of single crystal pentatellurides in the series Hf/sub 1-X/Zr/sub X/Te/sub 5/(x=0, .25, .50, and 1.0) have been measured as a function of temperature from 10 K to 300 K. The results show that HfTe/sub 5/ and ZrTe/sub 5/ contain broad resistance peaks at a temperature, T/sub p/, of 76 K and 147 K respectively, which are in agreement with previous measurements. Both compounds possess relatively large p-type thermopower (/spl sim/+100 /spl mu/V/K) which decreases rapidly through zero at a temperature T/sub 0/ before changing to an equally large n-type thermopower (/spl sim/-100 /spl mu/V/K) at a temperature T<T/sub 0/. Through isoelectronic substitution of Zr for Hf (Hf/sub 1-X/Zr/sub X/Te/sub 5/), systematic shifts are observed in both T/sub p/ and T/sub 0/ as the Zr concentration increases. These ternary compounds retain the large p- and n-type thermopowers.

M. L. Wilson

Papers

Observation of a semimetal–semiconductor phase transition in the intermetallic ZrTe5

Effect of Ti substitution on the thermoelectric properties of the pentatelluride materials M1−xTixTe5 (M=Hf, Zr)

Electrical Transport Properties of the Pentatelluride Materials HFTE5 and ZRTE5

Potential of quasicrystals and quasicrystal approximants for new and improved thermoelectric materials

Effect of isoelectronic substitution of thermopower and resistivity of Hf/sub 1-X/Zr/sub X/Te/sub 5/