Oxide-based Thermoelectric Materials Research Articles

Enhancing thermoelectric properties of CuNiO thin films through post-annealing: Effect on Seebeck coefficient and power generation

The need for the best thermoelectric power generation to extract low-grade heat energy from the human body has generated considerable interest. However, developing oxide-based thermoelectric materials with both high stability and Seebeck coefficient remains challenging. In this work, we have prepared CuNiO thin film with an economical and simple thermal evaporation method and post-annealed at different temperatures from 400 to 550 °C. XRD and SEM have determined the crystal structure and surface morphology of the CuNiO thin film. The electrical conductivity and charge carrier concentration increase with post-annealing temperature due to the formation of the secondary phases. The Seebeck coefficient has decreased from 65.1 to 37.9 µV/K with post-annealing temperature due to improved charge carrier density, the electronic band structure of the material and secondary phases. The maximum power factor has been achieved in the unannealed sample.

Oxide Materials for Thermoelectric Conversion.

Thermoelectric technology has emerged as a prominent area of research in the past few decades for harnessing waste heat and improving the efficiency of next-generation renewable energy technologies. There has been rapid progress in the development of high-performance thermoelectric materials, as measured by the dimensionless figure of merit (ZT = S2 · σ · κ-1). Several heavy-metal-based thermoelectric materials with commercial-level performance (ZT = 1) have so far been proposed. However, the extensive application of these materials still faces challenges due to their low thermal/chemical stability, high toxicity, and limited abundance in the Earth's crust. In contrast, oxide-based thermoelectric materials, such as ZnO, SrTiO3, layered cobalt oxides, etc., have attracted growing interest as they can overcome the limitations of their heavy-metal-based counterparts. In this review, we summarize the recent research progress and introduce improvement strategies in oxide-based thermoelectric materials. This will provide an overview of their development history and design schemes, ultimately aiding in enhancing the overall performance of oxide-based thermoelectric materials.

Ba1/3CoO2: A Thermoelectric Oxide Showing a Reliable ZT of ∼0.55 at 600 °C in Air.

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba1/3CoO2. We fabricated Ba1/3CoO2 epitaxial films by the reactive solid-phase epitaxy method (Na3/4CoO2) followed by ion exchange (Na+ → Ba2+) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba1/3CoO2 epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m–1 K–2 and the thermal conductivity gradually decreased to ∼1.9 W m–1 K–1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.

Regulating Multiscale Defects to Enhance the Thermoelectric Performance of Ca0.87Ag0.1Dy0.03MnO3 Ceramics.

Achieving high thermoelectric properties of CaMnO3 ceramics is significant for its applications at high temperature. Herein, Ca0.87Ag0.1Dy0.03MnO3 ceramics with plate-like template seeds additives were prepared by using a solid-state reaction method. The multiscale defects, including grain boundaries, oxygen defects, and Ag nanoprecipitations, which were regulated by the different sintering atmospheres, were beneficial for electron transport and phonon scattering. The grain boundaries as coherent interfaces could act as an alternative phonon scattering source. Oxygen vacancies coupled with Ag nanoprecipitations were verified by geometric phase analysis and annular bright-field analysis. The decrement in oxygen vacancies concentration strongly depended on the enriched oxygen environment, which could reduce electrical resistivities. Compared to the sample sintered at Ar atmosphere, a 17.5 times increment in power factor and a 20.1% reduction of the total thermal conductivity were obtained for the sample sintered at O2 atmosphere. As a result, the maximum ZT value of 0.22 was obtained at 500 °C. It is an effective way for improving the thermoelectric performance of oxide-based thermoelectric materials.

Double perovskite Pr2CoFeO6 thermoelectric oxide: Roles of Sr-doping and Micro/nanostructuring

Owing to their excellent thermal stability, non-toxicity, and low cost, oxide-based thermoelectric materials have considerably expanded research interests and industrial application. Here, we, for the first time, report a new-type Pr2CoFeO6 oxide-based thermoelectric materials from both theoretical and experimental aspects. Our first-principles calculation results indicate that Pr2CoFeO6 is a p-type semiconductor with narrow bandgap. The experimental thermoelectric evaluation shows that pristine Pr2CoFeO6, synthesized by a combination of sol–gel method and conventional sintering, has a peak figure of merit, ZT of 0.015 at 773 K with a high positive Seebeck coefficient of 250 μV K−1 and very low thermal conductivity of 0.7 W m−1 K−1 at this temperature. Further Sr2+ doping on Pr-sites (Pr3+) enhances the carrier concentration from 4.03 × 1014 cm−3 to 5.22 × 1017 cm−3, contributing to an improved power factor up to 46 μW m−1 K−2. Besides, Sr-doping induces point defects in the matrix and further suppresses the thermal conductivity to 0.58 W m−1 K−1, leading to a promising ZT up to 0.05 at 673 K in Pr1.8Sr0.2CoFeO6, which is significantly improved by 233% compared to pristine Pr2CoFeO6. We also predict that a high ZT of > 0.2 can be achieved by the optimization of carrier density, band engineering, and energy filtering, which is comparable to many other oxide-based thermoelectric materials.

High performance (ZT>1) n-type oxide thermoelectric composites from earth abundant materials

Oxide-based thermoelectric materials have the advantages of high-temperature stability, low toxicity and low processing cost comparted to other metal-based systems. However, achieving ZT > 1 remain elusive especially for n-type bulk oxide thermoelectrics. Here, we report the experimental demonstration of ZT > 1 in n-type oxides by synthesizing composites of Nb-doped SrTiO3 (STN) with natural graphite. Introduction of conductive graphite inclusions in STN matrix has led to a surge in electrical conductivity due to 21-times increase in weighted mobility (μw) of electrons resulting in high thermoelectric power factor ~5400 µWmK2, which is ~16-times higher than that of pure STN. Furthermore, we could restrain the increase in thermal conductivity by attaining enhanced Umklapp scattering along with phonon-glass-like temperature-independent phonon mean-free-path above Debye temperature. We have achieved the maximum ZT ~1.42 in STN+ 0.5 wt% G composite, which is 15-times enhancement compared to STN. Our proposed way of designing rare-earth-free composites with graphite can potentially open up the possibility of fabricating novel high-temperature thermoelectric generators.

Oxychalcogenides as Thermoelectric Materials: An Overview

Thermoelectric materials, which can convert heat into electricity and vice versa, have essential applications in power generation, thermocouples, sensors, and cooling. In the past decade, a lot of research has been devoted to developing various oxide-based thermoelectric materials, for mid- to high-temperature applications, ensuring robustness, long lifetimes, and low production costs. Among these oxide materials, one popular class is oxychalcogenides. A comprehensive discussion on the structural, electronic, and thermoelectric properties of both p-type and n-type oxychalcogenides is presented in this review article. The alternatively stacked conducting and insulating layers in these oxychalcogenides combined with a unique bonding network lead to interesting electronic properties and intrinsically low thermal conductivity. The article focuses primarily on Bi-based oxychalcogenides which have shown relatively good thermoelectric performance in the mid- to high-temperature range, with an elaborate discussion on n-type compositions. Several approaches such as chemical doping, modulation doping, introducing vacancies, band structure engineering, and so forth are summarized, vastly improving the zT in these materials and enabling their potential viability for thermoelectric applications. Finally, we discuss a few strategies as a future direction to further enhance the thermoelectric performance in these oxychalcogenide materials.

SrTiO3-based thermoelectrics: Progress and challenges

Thermoelectrics, enabling the direct energy conversion between electricity and heat, provide alternatives for conventional power generation and refrigeration with considerable efficiency. As one of the most promising oxide thermoelectric candidates, SrTiO3 shows an intrinsically high Seebeck coefficient and high stability at high temperatures. This review aims to summarize the progress of SrTiO3-based thermoelectrics from crystal and band structures to the strategies for tuning electrical and phonon transport. The article also highlights recent advances in the field of two-dimensional electronic gas system in SrTiO3 and suggests potential methods for further improving their thermoelectric performance. This review fills the gap of an overview of the progress and challenges associated with the development of SrTiO3-based thermoelectrics and paves a way to design high-performance oxide-based thermoelectric materials and devices.

Enhancement of thermoelectric properties of La-doped SrTiO3 bulk by introducing nanoscale porosity.

Electron-doped SrTiO3 is a well-known n-type thermoelectric material, although the figure of merit of SrTiO3 is still inferior compared with p-type metal oxide-based thermoelectric materials due to its high lattice thermal conductivity. In this study, we have used a different amount of the non-ionic surfactant F127 during sample preparation to introduce nanoscale porosities into bulk samples of La-doped SrTiO3. It has been observed that the porosities introduced into the bulk sample significantly improve the Seebeck coefficient and reduce the thermal conductivity by the charge carrier and phonon scattering respectively. Therefore, there is an overall enhancement in the power factor (PF) followed by a dimensionless figure of merit (zT) over a wide scale of temperature. The sample 20 at% La-doped SrTiO3 with 600 mg of F127 surfactant (SLTO 600F127) shows the maximum PF of 1.14 mW m−1 K−2 at 647 K which is 35% higher than the sample without porosity (SLTO 0F127), and the same sample (SLTO 600F127) shows the maximum value of zT is 0.32 at 968 K with an average enhancement of 62% in zT in comparison with the sample without porosity (SLTO 0F127).

Experimental investigations on a novel thermoelectric material (Na x CoO2)

Thermoelectric (TE) devices can be used reversely as a heat pump for refrigeration (heat management) and mostly oxide materials are used in these devices. Metal oxide-based TE materials possess several advantages compared to metal chalcogenides. These materials can be employed, since they are environment-friendly and cost-effective. TE performance can be easily tuned and this can be achieved through composition and structure. In the present work, Na0.77CoO2/nano Co3O4 composites are chosen. Although Na x CoO2 possesses superior TE performance to other oxide-based materials, its application has been limited due to its instability in air. It decomposes into the insulating Co(OH)2 and/or CoCO3 by absorbing moisture and/or CO2 from the ambient environment. Upon heating, both Co(OH)2 and CoCO3 decompose further into the insulating Co3O4 and CoO. Therefore, it is important to explore the chemical stability of Na x CoO2. This study analyses the reasons for the chemical instability of Na x CoO2 without impairing its TE properties. Na0.77 CoO2 was prepared by the solid-state synthesis method and the single phase was observed in XRD. Again XRD was taken after 15 days, to analyse the stability of the prepared sample. The peaks corresponding to Na0.77CoO2 disappeared. The reasons for this instability were analysed and possible solutions are suggested.

Lattice vibration modes of the layered material BiCuSeO and first principles study of its thermoelectric properties

BiCuSeO has recently been shown to be one of the best oxide-based thermoelectric materials. The electrical properties of this material have been widely studied; however, the reasons for its intrinsically low thermal conductivity have only been briefly discussed. In this paper, we calculated the band structure and the electrical properties of BiCuSeO. The phonon spectrum, mode Grüneisen parameters and the thermal properties were also investigated. Additionally, we proposed a new method for illustrating the interlayer interactions in this material. For the first time, using first principles calculations, we provide direct evidence of the structural in-layer and interlayer off-phase vibration modes, which contribute to the anharmonic vibrations and structural scattering of phonons and result in an intrinsic low lattice thermal conductivity for BiCuSeO.

Thermal Conductivity of A-Site Cation-Deficient La-Substituted SrTiO3 Produced by Spark Plasma Sintering

Abstract During the last decade oxide-based thermoelectric materials have received increased attention due to their high stability and thermal robustness at high temperatures as well as the availability and nontoxic nature of a number of promising candidates. In the present study we are investigating the thermoelectric properties of an n-type La-substituted SrTiO3 with the specific composition (La0.12Sr0.88)0.05TiO3. Nanosized powder precursors were spark plasma sintered (SPS) for 5 minutes between 900 and 1,200°C, resulting in densities between 73 and 98% and crystallite sizes between 40 nm and 1 μm. The formation of a rutile phase (TiO2) was observed in samples sintered at 1,150 and 1,200°C; at lower temperatures only single-phase cubic perovskite was observed. There was no clear evidence that the presence of rutile affected the thermal conductivity (κ). A significant reduction in κ was observed both with increasing porosity and reducing crystallite size, showing a minimum at ~700°C corresponding to ~1.3 W m−1 K−1.

Study of lattice vibration and thermal conductivity of BiCuSeO from first-principles calculations

ABSTRACTThe BiCuSeO has been proved to be one of the best oxide-based thermoelectric materials in recent years. Its electric properties have been widely studied, yet the lattice thermal conductivity was only discussed roughly. Our investigation suggests that the anharmonic vibration and the interlayer-interaction plays the crucial role in reducing the intrinsic lattice thermal conductivity. The thermal conductivity has been calculated based on quasi-harmonic approximation and detailed contribution have been discussed. The calculated data have good agreement with the experimental data.

Electrical and Thermal Transport Behavior in Zn-Doped BiCuSeO Oxyselenides

Oxide-based thermoelectric materials such as BiCuSeO have been researched for several years to optimize their thermoelectric performance. In this article, we present a series of Zn-doped BiCuSeO ceramics produced by fine processing. The results indicate that Zn doping can effectively increase the power factor through Cu vacancies along with a suppressed thermal conductivity through increased phonon scattering at ZnSe impurity phase. Consequently, a higher ZT value of 0.65 at 873 K is obtained for 10% Zn doping, being about three times larger than that of the pure sample.

Post-calcination, a novel method to synthesize cobalt oxide-based thermoelectric materials

Cobalt oxide-based Ca3Co4O9 ceramics are known for their good thermoelectric (TE) performance and chemical stability at high temperatures (<900°C); however, the crystalline phase of these ceramics is unstable beyond 926°C due to thermal decomposition. This low limit (926°C) on the sintering temperature produces a very low-density material with poor TE properties. Here, a novel method (post-calcination) to synthesize high-density and single-phase Ca3Co4O9 ceramics is investigated by high-temperature X-ray diffraction analysis. The post-calcination method, which includes cooling and reheating of the phase decomposed Ca3Co4O9 ceramics, synthesizes high-density (relative density=92%) and single-phase Ca3Co4O9 ceramics via a solid-state reaction at a high sintering temperature of 1200°C, and improved TE properties (power factor=0.245mWK−2m−1 at 800°C) are observed. The post-calcination method is expected to be applicable to Ca3Co4O9 and other materials that are difficult to obtain in high-density form due to thermal decomposition. Furthermore, this technique improves the sinterability and production efficiency without using complicated processes.