A three-year project on the development of thermoelectric materials with high efficiency and especially on the study of their long-term operational stability was evaluated as excellent by the Grant Agency of the Czech Republic (GACR). The MU Faculty of Science team was led by Pavel Brož, who undertook the study alongside a team from the Czech Academy of Sciences (CAS), led by Jiří Buršík, and colleagues from the University of Vienna. Congratulations!
Thermoelectric (TE) materials are able to convert thermal energy into electrical energy (known as the Seebeck effect) and vice versa (the Peltier effect). The degree to which this conversion takes place is controlled by a material’s TE efficiency, which depends on both the composition of the material and its crystal arrangement, and also its microstructure, which may be controlled by the way the material is prepared. To date, the thermal and phase stability of such materials has been little studied, despite this being a necessary condition for ensuring long-term operability at elevated temperatures.
The high application potential of TE materials is obvious. Environmental pollution has become a global problem in recent decades, with energy, automotive, chemical and other industrial sectors making a significant contribution. One application for TE materials could involve the recovery of heat produced during different thermal processes (waste heat) and converting it into useful work. While heat exchangers have been in use for some time, waste heat could be converted directly into electricity using TE materials. The Seebeck effect (discovered in 1821) has already been put into use in cars, particularly at those sites where there is significant heating (e.g. engine, exhaust pipes), to at least partially recover useful electrical energy. This process also allows space probes to operate far away from the sun, where it is no longer possible to use solar panels. In such cases, the heat from the probe’s nuclear reactor heats a TE module, which then supplies the probe with electricity. TE materials also work in the opposite direction, and this effect (discovered by Peltier in 1835) is commonly used in small refrigeration plants. These TE refrigeration devices have an advantage over conventional refrigerators in that they have no mechanical parts. “We can see for ourselves that large liquid nitrogen vessels for cooling analytical equipment are gradually disappearing from our electron microscopes and being replaced by noteless and maintenance-free Peltier cells”, say the project developers.
The disadvantage of current TE materials is their limited efficiency, with levels generally not exceeding 20%. The material potential for TE applications is determined by the ZT factor, which includes the Seebeck coefficient (voltage-temperature difference ratio) and the material’s electrical and thermal conductivity. The first step on the way to production and use of technically usable materials is the preparation of materials with a high value of ZT, which is controlled not only by the material’s structure but also the method used to prepare it (compact or nanostructured form, existence of internal defects, etc.). Second important step is preparation of materials which are structurally stable at long-term operating temperatures.
The aim of the project highlighted here was to study the long-term thermal and structural stability of two categories of advanced TE materials. The first category comprised skutterudites, based on cobalt and antimony either doped with iron and didymium (a mixture of neodymium and praseodymium) producing p-type The materials were prepared and physically characterised at the laboratories of the University of Vienna, in the group supervised by Profs. Rogl and Bauer. Based on previous orientation measurements, a combination of thermal analysis and Knudsen effusion mass spectrometry has been shown to be most effective for this type of research. While the first method mainly provides information on phase transformations in the material, the second allows for the monitoring of the evaporation characteristics of volatile elements (especially abundant antimony) within the material at very low pressures, and thus the kinetics of the process responsible for destabilising the material’s primary structure and loss of TE properties. The Knudsen effusion method is based on monitoring the effusion of components from the investigated system into a vacuum through a small orifice in the lid of the Knudsen cell (Knudsen, 1909). Mass spectrometry is then used to detect the gasified components after their ionisation and to determine component vapour pressures and component time losses. This method has the advantage of monitoring events in situ, i.e. information is obtainable at any time without the need to interrupt the measurement process. The ‘laboratory of thermal analysis and Knudsen effusion mass spectrometry’, where the measurements were performed, is the only workplace in the Czech Republic using this effusion technique. Both the primary structure and the resulting structure after measurement using these methods were then investigated using analytical electron microscopy. The teams then compared these measurements with available information on phase diagrams of the systems examined. Based on this comparison, the teams undertook further investigations or refined data in the phase diagrams where such information was missing.
All the materials investigated displayed sufficient long-term thermal and phase stability; however, half-Heusler alloys proved the most stable and p-type skutterudites least stable. These findings were shown to be in good agreement with the temperature stability of the corresponding primary thermoelectric phases. The half-Heusler phases displayed highest decomposition temperatures, while skutterudite phase with iron and didymium displayed the lowest. An important outcome of the project has been the methodology developed for assessing the long-term thermal and phase stability of materials, whether TE materials that the project focused on, or other material types, and their potential use in practice. Using these methods, the teams were also able to demonstrate that material nanostructuring caused by severe plastic deformation increases the ZT factor, particularly in optimally doped materials, and were able to describe the microstructural mechanisms of this phenomenon. As part of the project outputs, the teams, which also included several Masaryk University students, produced thirteen publications in prestigious peer-reviewed foreign journals.
Pavel Brož is the head of the Laboratory of Thermal Analysis and Knudsen Effusion Mass Spectrometry at Department of Chemistry, at the Masaryk University’s Faculty of Science in Brno, a workplace that has also formed part of the Central European Institute of Technology (CEITEC) at Masaryk University in previous years. Associate prof. Brož specialises in physical chemistry, which he has been interested in since his entering university, and materials chemistry. In addition to his teaching activities, Associate prof. Brož undertakes studies on thermodynamics, kinetics of phase transformations, phase equilibria, thermal analysis, Knudsen effusion and mass spectrometry. He is engaged in research on creep-resistant alloys, nickel and aluminium superalloys, lead-free solders, metal and alloy nanoparticles and, most recently, TE materials and alloys that form TE units. This focus is reflected in the professional guidance he gives his students. In addition to foreign contacts obtained within the framework of international COST projects, he is now most actively cooperating with the research group of Profs. Rogl and Bauer from the University of Vienna. He is also actively involved in coordinating student stays abroad.
Jiří Buršík is a senior researcher at the Institute of Physics of Materials of the Czech Academy of Sciences. RNDr. Buršík specialises on the microstructure and properties of a range of materials, including creep-resistant steels, nickel alloys and superalloys, magnesium alloys, lead-free solders, carbon nanotubes, metal and alloy nanoparticles, quasicrystals, materials for photonics and optoelectronics, magnetic Heusler phases and, most recently, skutterudite and half-Heusler phase TE materials. Much of his work relies on a combination of experimental methods utilising analytical electron microscopy, high-resolution transmission electron microscopy and electron diffraction. He presently lectures on electron microscopy at Masaryk University’s Faculty of Science.
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