Nanostructured materials and their application in thermoelectric energy harvesting

Abstract

Thermoelectric materials can, reversibly and without any moving parts, convert thermal energy into electrical energy by coupling an entropic and an electrical flux with each other. This ability is becoming increasingly important in concerns of a growing energy demand of the world, sustainability and climate change. The material class of thermoelectric oxides can play an important role in waste heat harvesting from industrial processes and power plants. On the basis of advantageous properties of oxides in the high-temperature range under oxidizing atmosphere, an application in electrical power generation from infinite heat sources seems promising. The thermoelectric research is a highly interdisciplinary field of study, which consists of solid state chemistry and solid state physics, when synthesizing new materials and measuring their properties. In addition, thermoelectric research is also a matter of process and electrical engineering, when p- and n-type materials are assembled to a thermoelectric generator and its power characteristic is estimated. Material classes like alloys, tellurides, half-Heusler or Zintl phases such as Bi2Te3, PbTe-PbS, SiGe, SnSe, FeNbSb and Yb14MnSb11, possess significantly improved thermoelectric properties, especially in the low- to mid-temperature range under inert or reducing atmosphere. But these material classes show inferior stability in the high-temperature range under oxidizing conditions, are toxic, or consist of expensive and rare elements. Concerning the thermoelectric figure-of-merit zT, oxides can not compete with other material classes. For this reason, research on oxides should focus on improving the thermoelectric power factor in the high-temperature range in air. Furthermore, the field of application of materials and conditions under which these are applied should be critically discussed. For the scenario of application of a limited heat source and if a maximized conversion efficiency should be obtained, a high-zT material is desirable. However, for the case of an infinite heat source and under the assumption of waste heat recovery at high-temperatures, a stable high-power factor material is superior for power generation. For this reason, on the one hand an unusual approach of manufacturing porous ceramics was carried out to decrease the heat conductivity and therefore, to improve the zT value. On the other hand, co-doping and compositionally alloyed nanostructures were used to develop high-power materials. The applicability of materials for power generation can be evidently estimated in a so-called Ioffe plot, which displays the thermoelectric power factor as a function of the electrical conductivity. The influence and possibility to maximize the electrical power output at high temperatures from infinite heat sources were recently theoretically described. A maximized power factor and a moderate heat conductivity are the most important parameters to improve power generation under these conditions. In the present work, different oxides were synthesized and investigated in terms of their thermoelectric properties. Furthermore, synthesis, growing mechanism and structure were investigated using X-ray diffraction (XRD) methods, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The influence of different processing techniques on micro-, nanostructure and thermoelectric properties were investigated using SEM, TEM, energy dispersive X-ray spectroscopy (EDXS) and thermoelectric property measurements. The stability in air of different materials were analyzed by thermogravimetry, XRD and cycle testing of thermoelectric properties. The behavior of different materials in terms of their coefficient of thermal expansion was investigated using dilatometry. The primarily investigated phases are p-type Ca3Co4O9 (CCO) and n-type In2O3 semiconducting oxides. The crystal structure and phase formation of CCO is very complex and subsequent ceramic processing is intricate. However, highly porous CCO ceramics possess significantly reduced values of the heat conductivity and a subsequently enhanced figure-of-merit zT of 0.4, which is the highest reported for pure CCO ceramics. A new thermoelectric triple-phase Ca3Co4O9-NaxCoO2-Bi2Ca2Co2O9 (CCO-NCO-BCCO) nanocomposite with enhanced thermoelectric properties was developed and characterized. The anisotropic thermoelectric properties were es- timated. The highest power factor, electrical conductivity of 6.5 µW·cm-1·K-2, 116 S·cm-1 (Ioffe plot) and figure-of-merit zT of 0.35 for CCO-NCO-BCCO nanocomposites, respectively, were obtained perpendicular to the pressing direction at 1073 K in air. This zT value is the highest obtained for a p-type thermoelectric oxide in a study, which reports anisotropic transport properties perpendicular and parallel to the pressing direction of the ceramic. The thermoelectric properties could be simultaneously enhanced in a co-doped nanocomposite with all-scale hierarchical architecture of three misfit layered oxides, which grew semi-coherently on each other. By interdiffusion and incorporation in a CCO matrix, the thermally less stable phases NCO and BCCO could be stabilized and utilized at high temperatures. The different synthesized, investigated and developed p- and n-type materials were used and assembled in different designs of thermoelectric generators, which were characterized in terms of proof of principle and maximizing the electrical power output and power density. The first generator was constructed using a conventional chess-board design and showed an enhanced electrical power output and power density at high temperatures, when electrically highly conducting indium oxide phases were used. A finite-element simulation tool was developed to predict power characteristics of thermoelectric generators and to exhibit different fluxes within the devices. In addition, the importance of minimizing contact resistances to improve the electrical power output of a thermoelectric generator was illustrated. By spark plasma sintering of Ca3Co4O9 and CaMnO3 powders an all-oxide thermoelectric generator was developed. A complex p-p-n junction formed in-situ at high temperatures, which reduces the contact resistance (ohmic behavior) and boosts the electrical voltage and thereby the electrical power output of the generator by utilizing a transversal thermoelectric effect at the interfaces of the p-p-n junction. This all-oxide generator could be reported for the first time and is comparable in terms of electrical power output and electrical power density to most conventional oxide-based generators. The p-p-n junction at the interface between p- and n-type materials abstain from metallic connectors and is hence applicable to even higher temperatures compared to conventional thermoelectric generators. The impact of high-power and high-zT materials on the electrical power output and electrical power density of thermoelectric generators was comprehensively studied. A p-type triple-phase CCO-NCO-BCCO nanocomposite was processed using different sintering techniques and the power factor and electrical conductivity (Ioffe plot) were improved to reach 8.2 µW·cm-1·K-2 and 143 S·cm-1 , respectively, at 1073 K in air. The developed p- and n-type materials were used to build three different prototypes of thermoelectric generators. The characterization of the prototypes confirmed the theoretical postulations, a high thermoelectric power factor and a moderate heat conductivity is much more important than a high zT value, to harvest waste heat at high temperatures from infinite heat sources. The best generator provided an electrical power output and electrical power density of 22.7 mW and 113.5 mW·cm-2 , respectively, when a hot-side temperature of 1073 K and a temperature difference of 251 K were applied. Even though a comparable low temperature difference was applied, the obtained power density is the highest reported from oxide-based generators.