ZT Materials Research Articles

Decomposition behavior and thermoelectric properties of copper selenide—graphite composites

Cu2Se is a high ZT material consisting of cheap and earth abundant elements. Its high thermoelectric performance is mainly based on very low thermal conductivity associated with mobile atoms. This comes with the drawback of a critical voltage, over which the material decomposes to elemental copper and a copper-deficient Cu2−xSe phase. Composites of copper selenide with graphite do not enhance the critical voltage significantly as copper selenide is not completely encapsulated by graphite. Thermoelectric properties of pellets of such composites obtained by mixing and pressing Cu2Se powder and graphite flakes are comparable to pristine Cu2Se, but thermal stability increases remarkably. Samples with different grain sizes show that the reproducibility of measurements is worse for larger grains, which in addition lead to inferior thermoelectric performance. Higher graphite contents increase both thermal and electrical conductivity, and an optimum of the thermoelectric figure of merit ZT is observed for fine-grained samples with 4 wt. % graphite.

Iterative design of a high zT thermoelectric material

Designing a high efficiency thermoelectric material for thermal to electric energy conversion means simultaneously optimizing multiple properties of the material. Although it might seem straightforward to maximize the electrical power and minimize thermal losses, the convoluted relationship between these properties makes optimization complex, requiring a more sophisticated algorithm. The Accelerated Insertion of Materials (AIM) methodology developed to engineer the mechanical properties of complex multiphase steel alloys provides a framework for optimization that can be applied to engineer the thermal and electrical transport properties of a multiphase thermoelectric material. The AIM methodology can be utilized in creating a high figure of merit (zT) material by considering the effects of each structural parameter, such as grain size and grain boundary properties, precipitate volume fraction, and doping and defect concentration of the matrix phase on the zT of the material using a variety of analytical models. The combination of these models provides a way to accelerate the design of high zT materials.

Effect of Calcination Temperature on the Photocatalytic Activity of Zn2Ti3O8 Materials for Phenol Photodegradation

Zinc titanate (Zn2Ti3O8) is a bimetal oxide material that is especially attractive as a photocatalyst. In the preparation of the Zn2Ti3O8, the calcination temperature is a crucial parameter. Hence, in the present work, we aimed to synthesize the Zn2Ti3O8 materials from zinc(II) nitrate and titanium(IV) isopropoxide as precursors by using a sol-gel method and followed by calcination at 700, 900, and 1100 °C to give ZT-700, ZT-900, and ZT-100 materials, respectively. The ZT materials were characterized using Fourier transform infrared (FTIR), diffuse reflectance ultraviolet-visible (DR UV-vis), and fluorescence spectroscopies. It was confirmed that the ZT materials contained O−Ti−O, Zn−O−Ti, Zn−O, Ti−O−Ti, and Ti−O functional groups as shown from their FTIR spectra. Similar fluorescence properties were only observed on the ZT-700 and ZT-900. From the bandgap energy analysis, ZT-700 and ZT-900 contained spinel and cubic Zn2Ti3O8 (spl-Zn2Ti3O8 and c-Zn2Ti3O8) crystal phases), while ZT-1100 contained c-Zn2TiO4 and TiO2 rutile crystal phases. The kinetic analysis of photocatalytic phenol degradation showed that both ZT-700 and ZT-900 materials exhibited high photocatalytic activity with the reaction rate constants of 0.0353 and 0.0355 h−1, respectively. These values were higher than that of the ZT-1100 (0.0206 h−1). This study demonstrated that calcination at 700 and 900 °C resulted in the formation of the spl-Zn2Ti3O8 and c-Zn2Ti3O8 phases, which were effective as the photocatalyst, but the formation of c-Zn2TiO4 and rutile TiO2 at calcination of 1100 °C deteriorated the photocatalytic activity. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Novel optimization perspectives for thermoelectric properties based on Rashba spin splitting: a mini review.

The energy problem has recently become increasingly more serious, therefore the rational use of heat energy and conversion into electrical energy is particularly important. The thermoelectric (TE) field is closely related to human life, as heat from automobiles, heat dissipation from high-power electrical appliances, or other electrical products that produce a lot of heat, can all be transformed with TE materials. The search for TE materials with an excellent performance and effective TE optimization strategies (STs) has attracted significant attention owing to the fact that thermal energy can be directly converted into electric energy. In contrast to the common TE-optimized STs, such as constructing point defects or reducing dimensionality, spin-related optimization STs have emerged from previous published research, such as the spin Seebeck effect or the Rashba effect, in which the Rashba effect shows an effective method to break through the bottleneck of ZT optimization. In this review, typical high ZT materials, common traditional optimized STs, Rashba-type TE materials and their corresponding ZT values are comprehensively discussed. The TE performance of Rashba-type materials is analysed, such as BiTeX (X = I, Br), GeTe, BiSbSeTe2, and the BiSb monolayer. Moreover, the TE optimization mechanisms (band engineering, phonon engineering, and Rashba spin-split engineering) are summarised. Finally, the development and challenges of Rashba spin-split combined with TE in breaking the bottleneck in ZT optimization are highlighted.

Strategy to replace animal-derived ECM by a modular and highly defined matrix.

Many extracellular matrices (ECM) used for modern cell culture are derived from animals. An alternative approach is the recombinant production of individual matrix protein components. A further development of this strategy uses a constant core protein polymer that is modifiable with functional domains of various ECM proteins. This way, a single, highly defined ECM system could be used for a large variety of cell types. Self-assembling protein domains from human muscle sarcomeres, termed here ZT material (ZT), have been shown to be suitable for this modular approach of generating ECMs. We explored in a proof-of-concept study, whether ZT, modified with the fibronectin 10 domain (ZTFn10) is able to substitute bovine serum-derived fibronectin as coating for neural crest cell (NCC)-based toxicity testing. Human NCC were generated from pluripotent stem cells and used in the automated version of a NCC migration assay (cMINC). ZTFn10, but not the unmodified core material (ZT), allowed for a high migration activity. The classical cMINC setup, with bovine fibronectin coating, was used as positive control, and detailed analysis of NCC migration by time-lapse recording indicated that the novel ECM fully matched the bioactivity of the traditional ECM. A final set of experiments showed that various positive controls of the cMINC assay (PCB180, LiCl, cytochalasin D) showed nearly identical inhibition curves on the traditional and the novel ECM. Thus, the cMINC, and possibly other bioassays, can be performed with a ZT-based ECM instead of traditional animal-derived protein coatings.

Strategy to replace animal-derived ECM by a modular and highly defined matrix_suppl2

Many extracellular matrices (ECM) used for modern cell culture are derived from animals. An alternative approach is the recombinant production of individual matrix protein components. A further development of this strategy uses a constant core protein polymer that is modifiable with functional domains of various ECM proteins. This way, a single, highlydefined ECM system could be used for a large variety of cell types. Self-assembling protein domains from human muscle sarcomeres, termed ZT material (ZT), have been shown to be suitable for this modular approach of generating ECMs. We explored in a proof-of-concept study whether ZT modified with the fibronectin 10 domain (ZTFn10) is able to substitute bovine serum-derived fibronectin as a coating for neural crest cell (NCC)-based toxicity testing. Human NCCs were generated from pluripotent stem cells and used in the automated version of an NCC migration assay (cMINC). ZTFn10, but not the unmodified core material (ZT), allowed high migration activity. The classical cMINC setup, with bovine fibronectin coating, was used as positive control, and detailed analysis of NCC migration by time-lapse recording indicated that the novel ECM fully matched the bioactivity of the traditional ECM. A final set of experiments showed that various positive controls of the cMINC assay (PCB180, LiCl, cytochalasin D) showed nearly identical inhibition curves on the traditional and the novel ECM. Thus, the cMINC, and possibly other bioassays, can be performed with a ZT-based ECM instead of traditional animal-derived protein coatings.

Strategy to replace animal-derived ECM by a modular and highly defined matrix_suppl5

Many extracellular matrices (ECM) used for modern cell culture are derived from animals. An alternative approach is the recombinant production of individual matrix protein components. A further development of this strategy uses a constant core protein polymer that is modifiable with functional domains of various ECM proteins. This way, a single, highlydefined ECM system could be used for a large variety of cell types. Self-assembling protein domains from human muscle sarcomeres, termed ZT material (ZT), have been shown to be suitable for this modular approach of generating ECMs. We explored in a proof-of-concept study whether ZT modified with the fibronectin 10 domain (ZTFn10) is able to substitute bovine serum-derived fibronectin as a coating for neural crest cell (NCC)-based toxicity testing. Human NCCs were generated from pluripotent stem cells and used in the automated version of an NCC migration assay (cMINC). ZTFn10, but not the unmodified core material (ZT), allowed high migration activity. The classical cMINC setup, with bovine fibronectin coating, was used as positive control, and detailed analysis of NCC migration by time-lapse recording indicated that the novel ECM fully matched the bioactivity of the traditional ECM. A final set of experiments showed that various positive controls of the cMINC assay (PCB180, LiCl, cytochalasin D) showed nearly identical inhibition curves on the traditional and the novel ECM. Thus, the cMINC, and possibly other bioassays, can be performed with a ZT-based ECM instead of traditional animal-derived protein coatings.

Zinc oxide-based direct thermoelectric gas sensor for the detection of volatile organic compounds in air

Using thermoelectricity as a platform for sensor design and fabrication commercially and historically precedes its power generation aspect, but recent advancements in science and engineering of materials have failed to elevate this platform as the researchers mainly pursue higher figure of merit (zT) materials. It has been shown that thermoelectricity in polycrystalline semiconductors is partly related to the grain boundary effects, and the profound contribution of grain boundaries to the Seebeck voltage (SV) generation has been demonstrated. It immediately follows that all grain boundary-related effects should be detectable via SV measurement, as well. Here, we report the variations of the SV generated at the presence of a constant temperature gradient along a ZnO pellet with respect to the concentration of the various volatile organic compounds (VOCs) in the surrounding atmosphere. The presence of VOCs reversibly reduces the SV generated along the sample. Similar to the case of chemoresistivity, the recorded chemo-thermoelectric responses obey the power law; providing another reason for the common origin of these two apparently distinct phenomena.

Development of integrated two-stage thermoelectric generators for large temperature difference

Multi-stage thermoelectric (TE) modules can withstand a large temperature difference and can be used to obtain a high conversion efficiency. In this study, two-stage PbTe/Bi2Te3 TE modules were developed with an enhanced efficiency through a comprehensive study of device structure design, module fabrication, and performance evaluation. PbTe-based AgPbmSbTem+2 (abbreviated as LAST) is a typically high ZT material, while the corresponding TE module was rarely reported so far. How to utilize LAST to fabricate high efficiency TE modules therefore remains a central problem. Finite element simulation indicates that the temperature stability of the two-stage module for LAST is better than that of two-segmented module. Compared to Cu, Ni, and Ni-Fe alloys, Co-Fe alloy is an effective metallization layer for PbTe due to its low contact resistance and thin diffusion layer. By sintering a slice of Cu on TE legs, pure tinfoil can be used as a common welding method for mid-temperature TE modules. A maximum efficiency (ηmax) of 9.5% was achieved in the range of 303 to 923 K in an optimized PbTe/Bi2Te3 based two-stage module, which was almost twice that of a commercial TE module.

The intrinsic low lattice thermal conductivity in the rock salt SnSe

The rock-salt IV-VI compounds usually have low thermal conductivity, while the orthorhombic Tin Selenide (SnSe) was reported to show extremely low lattice thermal conductivity (κL) and thus achieves a record high zT value. To understand why SnSe has low thermal conductivity and how important role played by the orthorhombic structure, we studied κL for both forms of SnSe, the rock salt and the orthorhombic, based on first-principles calculations and the Boltzmann transport equation. We found an even smaller value of κL for the rock salt structure, which establishes a possibility for even larger zT materials. Detailed analyses were performed on thermal properties for both structures. We attributed the low κL of the rock salt SnSe to its large Grüneisen parameters, which stem from the strongly anharmonic phonons. Our findings indicate the indecisive role that the orthorhombic structure is playing in achieving low κL for SnSe and provide fresh insight in understanding the thermal properties of SnSe.

Defect modulation on CaZn1−xAg1−ySb (0 < x < 1; 0 < y <1) Zintl phases and enhanced thermoelectric properties with high zT plateaus

New Zintl phases CaZn1−xAg1−ySb (0 < x < 1; 0 < y < 1) show tunable thermoelectric properties with a broad high zT plateau (zT > 1.0).

Flexible Thermoelectric Composite Films of Polypyrrole Nanotubes Coated Paper

Flexible thermoelectric composite films of polypyrrole (PPy) nanotubes coated paper were fabricated by an in-situ polymerization procedure using methyl orange as a template and paper as the substrate for the first time. Both the electrical conductivity and Seebeck coefficient of the polypyrrole nanotubes coated paper composite films have been enhanced (from ~0.048 S/cm to ~0.068 S/cm and from ~5.34 μV/K to ~8.21 μV/K for the average value for three measurements, respectively) as the temperature increased from ~300 K to ~370 K, which lead to the same trend of the power factor. The thermal conductivity of the polypyrrole nanotubes coated composite films was very low (~0.1275 W·m−1·K−1 at ~300 K), and a highest ZT (material’s dimensionless figure of merit (S2σT/κ)) value of 3.2 × 10−7 was obtained at ~370 K.

Chemical Potential Tuning and Enhancement of Thermoelectric Properties in Indium Selenides.

Researchers have long been searching for the materials to enhance thermoelectric performance in terms of nano scale approach in order to realize phonon-glass-electron-crystal and quantum confinement effects. Peierls distortion can be a pathway to enhance thermoelectric figure-of-merit ZT by employing natural nano-wire-like electronic and thermal transport. The phonon-softening known as Kohn anomaly, and Peierls lattice distortion decrease phonon energy and increase phonon scattering, respectively, and, as a result, they lower thermal conductivity. The quasi-one-dimensional electrical transport from anisotropic band structure ensures high Seebeck coefficient in Indium Selenide. The routes for high ZT materials development of In4Se3−δ are discussed from quasi-one-dimensional property and electronic band structure calculation to materials synthesis, crystal growth, and their thermoelectric properties investigations. The thermoelectric properties of In4Se3−δ can be enhanced by electron doping, as suggested from the Boltzmann transport calculation. Regarding the enhancement of chemical potential, the chlorine doped In4Se3−δCl0.03 compound exhibits high ZT over a wide temperature range and shows state-of-the-art thermoelectric performance of ZT = 1.53 at 450 °C as an n-type material. It was proven that multiple elements doping can enhance chemical potential further. Here, we discuss the recent progress on the enhancement of thermoelectric properties in Indium Selenides by increasing chemical potential.

Exploring the implications of the Lorentz number approaches one in organic thermoelectrics

Organic thermoelectrics offer an alternative to their inorganic counterpart in the pursuit for high ZT materials. While a few PEDOT-related organics has shown significant promise, there is nonetheless a lack of general understanding on the transport mechanism with respect to the carriers and the phonons. This work examined the implication of the Lorentz number approaching one in de-doped PEDOT–PSS (Poly (3,4-ethylene dioxythiophene): Poly(styenesulphonate)) and compared to related properties found in oxidized PEDOT:Tos (poly(3,4-ethylenedioxythiophene-tosylate)). In our analysis, we assumed that the carriers were tightly-coupled to the phonons and their transport properties were basically metallic. Using data reported in the open literature, we computed the Lorentz numbers of the two and showed that there were major differences. For instance, the Lorentz number of oxidized PEDOT:Tos was larger than that of a metal until the organic turned metallic while the Lorentz number of de-doped PEDOT:PSS which persistently was smaller than one. Potentially, the latter would imply a drop in the phonon relaxation time. We attributed the decrease to the formation of a porous structure which reduced the effectiveness in heat transfer.

Sb and Se Substitution in CsBi4Te6: The Semiconductors CsM4Q6 (M = Bi, Sb; Q = Te, Se), Cs2Bi10Q15, and CsBi5Q8

The solid solutions of CsBi4Te6, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi4-xSbxTe6 and CsBi4Te6-ySey. The introduction of Sb and Se in the lattice of CsBi4Te6 is possible but only to a limited extent. The Sb and Se atoms substituted are not uniformly distributed over all crystallographic sites but display particular site preferences. The structure of new Sb/Bi solid solutions retains the original framework of CsBi4Te6 composed of NaCl-type Bi/Te slabs interconnected by characteristic Bi–Bi bonds and Cs atoms located in the interlayer space. A structurally modified phase in Se/Te solid solutions was found from the reactions targeted for 0.2 < y < 2.4 with the formula of CsBi5Te7.5-ySey (or Cs2Bi10Q15, (Q = Se, Te)). The new structure is constructed by the same structural motif with an extended Bi/Te slab (29 A) compared to that in CsBi4Te6 (23 A). The CsBi5Te7.5-ySey possesses Bi/Te slabs that extend by an additional “Bi2Te3” unit compared t...

Nanocomposite Materials for Thermoelectric Energy Conversion: A Brief Survey of Recent Patents

The past few decades have witnessed an increasing demand for renewable energy technology, including that of direct thermal to electrical energy conversion via thermoelectricity. Central to high efficient thermoelectric (TE) energy conversion is a high figure of merit, ZT, of the TE material, which requires high electrical conductivity, high thermopower and low thermal conductivity. For simple bulk materials it is hard to simultaneously satisfy these criteria because these physical quantities are inter-dependent: optimizing one quantity often adversely affects the others. Over the past decade, the pursuit of higher ZT materials has culminated into a new paradigm, namely, nanocomposite thermoelectric materials (NcTMs). A NcTM is typically multi-phased, and the characteristic length scale of at least one constituent is on the order of nanometers. The resulting classical and quantum size effects arising from the nanophase(s) and at the interfaces help decouple the inter-dependence of those TE properties, leading to outstanding TE performance. In this paper, we present a brief survey of relevant patent disclosures for NcTMs, focusing on the preparation methods and the peculiar micromorphologies. Keywords: Renewable energy, energy conversion, heat management, thermoelectrics, nanocomposite, micro-morphology, interface, self-assembly

Enhancement of the thermoelectric properties in nanoscale and nanostructured materials

Thermoelectric materials can be used for solid state power generation and heating/cooling applications. The figure of merit of thermoelectric materials, ZT, which determines their efficiency in a thermoelectric device, remains low for most conventional bulk materials. Nanoscale and nanostructured thermoelectric materials are promising for increasing ZT relative to the bulk. This review introduces the theory behind thermoelectric materials and details the predicted and demonstrated enhancements of ZT in nanoscale and nanostructured thermoelectric materials. We discuss thin films and superlattices, nanowires and nanotubes, and nanocomposites, providing a ZT comparison among various families of nanocomposite materials. We provide some perspectives regarding the origin of enhanced ZT in nanoscale and nanostructured materials and suggest some promising and fruitful research directions for achieving high ZT materials for practical applications.

High temperature Seebeck coefficient metrology

We present an overview of the challenges and practices of thermoelectric metrology on bulk materials at high temperature (300 to 1300 K). The Seebeck coefficient, when combined with thermal and electrical conductivity, is an essential property measurement for evaluating the potential performance of novel thermoelectric materials. However, there is some question as to which measurement technique(s) provides the most accurate determination of the Seebeck coefficient at high temperature. This has led to the implementation of nonideal practices that have further complicated the confirmation of reported high ZT materials. To ensure meaningful interlaboratory comparison of data, thermoelectric measurements must be reliable, accurate, and consistent. This article will summarize and compare the relevant measurement techniques and apparatus designs required to effectively manage uncertainty, while also providing a reference resource of previous advances in high temperature thermoelectric metrology.

Zintl phases for thermoelectric devices

By converting waste heat into electricity and improving the efficiency of refrigeration systems, thermoelectric devices could play a significant role in solving today's energy problems. Increasing the thermoelectric efficiency (as measured by the thermoelectric material's figure-of-merit, zT) is critical to the development of this technology. Complex Zintl phases, in particular, make ideal candidates for thermoelectric materials because the necessary "electron-crystal, phonon-glass" properties can be engineered with an understanding of the Zintl chemistry. A recent example is the discovery that Yb(14)MnSb(11), a transition metal Zintl compound, has twice the zT as the material currently in use at NASA. This perspective outlines a strategy to discover new high zT materials in Zintl phases, and presents results pointing towards the success of this approach.

Approaches to thermal isolation with thin-film superlattice thermoelectric materials

ABSTRACTWe propose the use of hetero-structured high ZT materials in thermally isolated thermoelectric couples for the purpose of improving overall device efficiency. Use of high ZT material incorporated in convective flow devices will yield a two-fold improvement in COP over isothermal TE devices. The analytical approach presented in this paper requires optimization of TE element aspect ratio according to available ΔT at location in the device. We discuss the limitations on thermoelectric element characteristics and geometry to achieve high COP. We also utilize moderate temperature differentials to meet the target of high COP. If space requirements are restrictive, or materials processing is limited, the materials described herein provide the unique ability to alter Seebeck coefficient, thermal conductivity, and electrical such that the required aspect ratio is suitable. Improvement in Seebeck coefficient and electrical resistivity independently increase COP by as much as 25% or more when applied to thermally isolated device considered.