Physico-Chemical development of oxide-based ceramics for thermoelectric energy harvesting

The thermoelectric effect describes the conversion of heat into electrical energy and it is the pivotal element for the utilization of waste heat via Energy Harvesting. Especially high-temperature applications in industrially relevant areas such as basic materials production or transportation hold enormous potential for unused energy. Proper thermoelectric materials for these high-temperature applications are mandatory to ensure high conversion efficiency and high electrical power output. Oxides are auspicious candidates for this task as they exhibit high thermal stability under air and are less toxic than most alternatives. Therefore, it is expedient to investigate the electrical performance and the energy conversion efficiency of these promising high-temperature materials more intensively. The currently best performing oxide material is Ca3Co4O9 (CCO), so improving its thermoelectric properties was the main research focus of this thesis. Novel composite materials based on CCO in combination with other oxides such as La2NiO4 (LNO), Na2Ca2Nb4O13 (NCNO), NaxCoO2 (NCO) and Bi2Ca2Co2O9 (BCCO) as well as the oxyselenide BiCuSeO (BCSO) were synthesized and thoroughly investigated regarding synergistic effects. Close attention was given to the microstructure and elemental composition, which were investigated by various methods such as electron microscopy as well as electron or X-ray diffraction. Further analyses regarding the thermoelectric performance were made by measuring the Seebeck coefficient, the electrical conductivity and the thermal conductivity. All investigated materials exhibit layered crystal structures and hence anisotropic transport properties. Preferred crystal orientations within the composite ceramics were observed due to uniaxial pressing during processing. In case of LNO or NCNO, the orientation was influenced by using large plate-like particles synthesized by molten-flux synthesis. Sintering resulted in advantageous reactions between the mixing partners. Regarding LNO or NCNO, heavily doped perovskite phases La(Co,Ni)O3 and Ca(Nb,Co)O3 were ascertained with beneficial impact on the thermoelectric properties. Especially the power factor was positively influenced, either by an enhanced electrical conductivity or by a higher Seebeck coefficient. Approaches in the nanodimension were realized by a triple-phase ceramic including NCO and BCCO as two-dimensional nanostructures within a CCO matrix or by using BCSO nanosheets as a mixing partner. In both cases, the electrical conductivity and the Seebeck coefficient were increased simultaneously as a result of doping and emerged phases. While the large particles could increase either the average or even the maximum figure-of-merit compared to CCO, the nanodimensional strategy led to a significantly enhanced power factor.Der thermoelektrische Effekt beschreibt die Umwandlung von Wärme in elektrische Energie und ist das zentrale Element für die Ausnutzung von Verlustwärme durch "Energy Harvesting". Besonders Hochtemperatur-Anwendungen in industriell relevanten Sektoren, wie der Herstellung von Basismaterialien oder des Transportwesens, umfassen große Potentiale von ungenutzter Energie. Um eine hohe Effizienz beim Umwandeln der Energieformen und eine hohe elektrische Leistungsabgabe zu erreichen, sind für diese Hochtemperatur-Anwendungen geeignete thermoelektrische Materialien erforderlich. Oxide sind vielversprechende Kandidaten, da sie eine hohe Temperaturstabilität und eine geringere Toxizität als die meisten Alternativen aufweisen. Es ist daher zielführend, die elektrische Leistungsfähigkeit und die Effizienz der Energieumwandlung dieser vielversprechenden Hochtemperatur-Materialien intensiver zu untersuchen. Momentan stellt Ca3Co4O9 (CCO) das leistungsstärkste oxidische Material dar, weshalb die Verbesserung seiner thermoelektrischen Eigenschaften den Hauptaspekt der vorliegenden Dissertation darstellt. Hierzu wurden neuartige Kompositmaterialien basierend auf CCO in Kombination mit anderen Oxiden wie La2NiO4 (LNO), Na2Ca2Nb4O13 (NCNO), NaxCoO2 (NCO) und Bi2Ca2Co2O9 (BCCO) oder dem Oxyselenid BiCuSeO (BCSO) hergestellt und ausgiebig hinsichtlich synergistischer Effekte untersucht. Ein besonderes Augenmerk lag auf der Mikrostruktur und der elementaren Zusammensetzung, welche mittels zahlreicher Analysemethoden wie etwa der Elektronenmikroskopie oder der Elektronen- und Röntgen-Beugung untersucht wurden. Weitere Analysen bezüglich des thermoelektrischen Verhaltens erfolgten durch Messung des Seebeck-Koeffizienten, der elektrischen sowie der thermischen Leitfähigkeit. Alle untersuchten Materialien weisen geschichtete Kristallstrukturen und damit anisotrope Transporteigenschaften auf. Durch das uniaxiale Pressen während der Prozessierung konnten innerhalb der Komposit-Keramiken bevorzugte Kristall-Orientierungen erzeugt werden. Im Falle von LNO und NCNO wurde die Orientierung durch den Einsatz großer plättchenförmiger Partikel, welche durch Schmelzfluss-Synthese gewonnen wurden, beeinflusst. Das Sintern resultierte in vorteilhaften Reaktionen zwischen den jeweilig gemischten Komponenten. Unter Einsatz von LNO oder NCNO konnten die hochdotierten Perowskit-Phasen La(Co,Ni)O3 und Ca(Nb,Co)O3 mit vorteilhaften Auswirkungen für die thermoelektrischen Eigenschaften identifiziert werden. Insbesondere der Leistungsfaktor wurde positiv beeinflusst, entweder durch eine erhöhte elektrische Leitfähigkeit oder durch einen größeren Seebeck-Koeffizienten. Eine auf Nanodimensionalität beruhende Strategie wurde durch eine dreiphasige Keramik mit NCO und BCCO als zweidimensionale Nanostrukturen innerhalb einer CCO Matrix oder durch den Einsatz von BCSO als Mischungspartner realisiert. In beiden Fällen konnten mittels Dotierung und neu entstandener Phasen die elektrische Leitfähigkeit und der Seebeck Koeffizient simultan erhöht werden. Während die großen Partikel den Durchschnittswert oder sogar den Maximalwert der Gütezahl gegenüber CCO erhöhen konnten, führte der nanodimensionale Ansatz zu einer signifikanten Erhöhung des Leistungsfaktors

High power factor vs. high zT-A review of thermoelectric materials for high-temperature application

Energy harvesting with thermoelectric materials has been investigated with increasing attention over recent decades. However, the vast number of various material classes makes it difficult to maintain an overview of the best candidates. Thus, we revitalize Ioffe plots as a useful tool for making the thermoelectric properties of a material obvious and easily comparable. These plots enable us to consider not only the efficiency of the material by the figure of merit zT but also the power factor and entropy conductivity as separate parameters. This is especially important for high-temperature applications, where a critical look at the impact of the power factor and thermal conductivity is mandatory. Thus, this review focuses on material classes for high-temperature applications and emphasizes the best candidates within the material classes of oxides, oxyselenides, Zintl phases, half-Heusler compounds, and SiGe alloys. An overall comparison between these material classes with respect to either a high efficiency or a high power output is discussed

Geometry Optimization of Thermoelectric Modules: Deviation of Optimum Power Output and Conversion Efficiency

Besides the material research in the field of thermoelectrics, the way from a material to a functional thermoelectric (TE) module comes alongside additional challenges. Thus, comprehension and optimization of the properties and the design of a TE module are important tasks. In this work, different geometry optimization strategies to reach maximum power output or maximum conversion efficiency are applied and the resulting performances of various modules and respective materials are analyzed. A Bi2Te3-based module, a half-Heusler-based module, and an oxide-based module are characterized via FEM simulations. By this, a deviation of optimum power output and optimum conversion efficiency in dependence of the diversity of thermoelectric materials is found. Additionally, for all modules, the respective fluxes of entropy and charge as well as the corresponding fluxes of thermal and electrical energy within the thermolegs are shown. The full understanding and enhancement of the performance of a TE module may be further improve

Reaction Sintering of Ca3Co4O9 with BiCuSeO Nanosheets for High-Temperature Thermoelectric Composites

Ceramic composites composed of oxide materials have been synthesized by reaction sintering of Ca3Co4O9 with BiCuSeO nanosheets. In situ x-ray diffraction and thermogravimetric analyses of the compound powders were conducted to understand the phase transformations during heating up to 1173 K. Further thermogravimetric analyses investigated the thermal stability of the composites and the completion of reaction sintering. The microstructure of the formed phases after reaction sintering and the composition of the composites were investigated for varying mixtures. Depending on the amount of BiCuSeO used, the phases present and their composition differed, having a significant impact on the thermoelectric properties. The increase of the electrical conductivity at a simultaneously high Seebeck coefficient resulted in a large power factor of 5.4 μW cm−1 K−2, more than twice that of pristine Ca3Co4O9

Combination of Laser and Thermal Sintering of Thermoelectric Ca3Co4O9 Films

The manufacturing technology of thermoelectric materials is laborious and expensive often including complex and time-intensive preparation steps. In this work, a laser sintering process of the oxide-based thermoelectric material Ca3Co4O9 is investigated. Samples based on spray-coated Ca3Co4O9 were prepared and subsequently sintered under various laser parameters and investigated in terms of the microstructure and thermoelectric properties. Here, the combination of laser sintering and subsequent thermal sintering proved to be a promising concept for the preparation of thermoelectric films. Laser sintering can thus make a great contribution in improving the processing of thermoelectric materials, especially when films are applied that cannot be sintered under pressure. © 2021 The Authors. Chemie Ingenieur Technik published by Wiley-VCH Gmb

Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

The thermoelectric properties of CaMnO3-δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO-MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3-δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3-δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3-δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3-δ to the interface between CaMnO3-δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3-δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm-1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m-1K-1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3-δ based composites

High Power Factor vs. High zT—A Review of Thermoelectric Materials for High-Temperature Application

Energy harvesting with thermoelectric materials has been investigated with increasing attention over recent decades. However, the vast number of various material classes makes it difficult to maintain an overview of the best candidates. Thus, we revitalize Ioffe plots as a useful tool for making the thermoelectric properties of a material obvious and easily comparable. These plots enable us to consider not only the efficiency of the material by the figure of merit zT but also the power factor and entropy conductivity as separate parameters. This is especially important for high-temperature applications, where a critical look at the impact of the power factor and thermal conductivity is mandatory. Thus, this review focuses on material classes for high-temperature applications and emphasizes the best candidates within the material classes of oxides, oxyselenides, Zintl phases, half-Heusler compounds, and SiGe alloys. An overall comparison between these material classes with respect to either a high efficiency or a high power output is discussed

A comprehensive study on improved power materials for high-temperature thermoelectric generators

Dense Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO-NCO-BCCO) nanocomposites were produced from sol-gel derived Ca2.25Na0.3Bi0.35Tb0.1Co4O9 powder by four methods: Hot-pressing (HP), spark plasma sintering (SPS) and pressureless sintering in air or O2 atmosphere. Nanocomposites from HP and SPS revealed nanosized grains and showed a thermoelectric power factor of 4.8 and 6.6 μW cm−1 K−2, respectively, at 1073 K in air. A dense 2D nanocomposite with structures on multiple length scales and enhanced thermoelectric properties was obtained from pressureless sintering in O2 atmosphere. The resulting 2D nanocomposite enabled the simultaneous increase in isothermal electrical conductivity σ and Seebeck coefficient α, and showed a thermoelectric power factor of 8.2 μW cm−1 K−2 at 1073 K in air. The impact of materials with enhanced electrical conductivity and power factor on the electrical power output of thermoelectric generators was verified in prototypes. A high electrical power output and power density of 22.7 mW and 113.5 mW cm−2, respectively, were obtained, when a hot-side temperature of 1073 K and a temperature difference of 251 K were applied. Different p- and n-type materials were used to verify the effect of the thermoelectric figure-of-merit and power factor on the performance of thermoelectric generators