Yuzuru Miyazaki, Professor at the Department of Applied Physics, Graduate School of Engineering, at Tohoku University enlightens us on his research on exploring novel energy-harvesting materials
In this interview with Yuzuru Miyazaki, Full Professor at the Department of Applied Physics, Graduate School of Engineering, at Tohoku University in Japan, we learn about exploring novel energy-harvesting materials, such as thermoelectric materials, cathode materials for secondary batteries and photovoltaic materials, amongst a number of other exciting areas of research in the field.
The focus of this compelling interview includes Yuzuru Miyazaki’s research, his work on high-quality structure analyses using neutron and X-ray diffraction, and the challenges around thin-film thermionic multilayers and organic thermoelectric materials, amongst other areas.
Can you introduce your work when it comes to exploring novel energy-harvesting materials, such as thermoelectric materials, cathode materials for secondary batteries and photovoltaic materials?
I studied basic materials science during my bachelor course. Just before my graduation, a scientific fever of cuprate superconductors occurred. I was, therefore, deeply motivated by this positive impact and began to study pursue novel cuprates during my Master and PhD courses at Tohoku University.
Fortunately, I was able to find several cuprates which possess interesting structural units. Some of them actually exhibited superconductivity. From this successful experience, I decided to explore novel functional materials which exhibit special properties, in particular, generating electricity based on a knowledge of solid state chemistry. At that time, the 21st Century was approaching and the energy crisis was one of the critical issues in academic research.
Thermoelectric (TE) materials can generate electricity from the temperature difference by means of the Seebeck effect. TE materials themselves do not generate noises or vibrations and do not emit toxic gases and, therefore, they can be regarded as clean energy sources. However, apart from bismuth telluride-based and lead telluride-based materials, both comprise quite rare and toxic elements, indeed, there are quite a few compounds that can be used as potential TE materials utilising the waste heat of our surroundings. In our research, we focused on oxide materials as they are stable at high temperatures and the constituents are abundant.
My experience on oxides (cuprates) was also quite helpful when it came to studying oxide TE materials. We discovered several cobalt oxide-based TE materials, which are quite stable even at a high temperature up to 1000 K. The crystal structure of them is quite similar to that used as a cathode of the lithium-ion battery. So, we have also begun to explore cathode materials for secondary batteries in layered cobalt oxides that do not contain lithium. We are also studying photovoltaic materials in relation to layered oxide materials.
What is the current focus or themes around your research?
Our main focus when it comes to our research is the waste heat recovery from automobile engines. Currently, the waste heat of around 46 peta calories is emitted annually from automobiles in Japan. The temperature range here is 600-1000 K. If we can recover electricity from such waste heat by the efficiency of ~10%, a number of large thermal power plants can be scrapped and CO2 emissions can, therefore, be greatly reduced.
Higher manganese silicide, HMS (MnSi~1.7) is one of the potential TE materials, to be used at around 800 K. HMS consists of naturally abundant elements and is chemically stable up to 1000 K. Typical samples exhibit the dimensionless figure-of-merit, zT~ 0.3 around 800 K, which needs much improvement. As the zT value roughly corresponds to the thermal-to-electric conversion efficiency, zT~ 0.6 at 800 K is at the very least, necessary. To practically generate electricity, tens or hundreds of p- and n-type TE materials (legs) should be joined electrically in series and thermally, in parallel. We call such a device a pi-type TE module. Electrons (holes) are major conducting carriers for n- (p-) type TE materials. As the HMS is a p-type material, a corresponding potential n-type TE material is necessary. The first choice should be another silicide, Mg2Si-based material. Currently, we are financially supported by the NEDO/TherMAT project, Japan.
Tell us about your work when it comes to high-quality structure analyses using neutron and X-ray diffraction, combined with first-principles calculations?
Generally, the functions of materials are highly dependent on their electronic structure. The electronic structure is derived from the crystal structure of materials. Hence, a deep knowledge of the precise arrangement of atoms is critical to understand their properties. Such an arrangement can be determined from diffraction experiments of X-ray, neutron and/or synchrotron X-rays. We generally use the first-principles calculation to elucidate electronic structures and, hence, the precise arrangement of atoms in a material is crucial to predict its properties. Unfortunately, many researchers today do not care about having a deep understanding of the crystal structures.
How many novel materials have been discovered based on your guiding principles?
It depends on how we define the difference. If we define the difference as the distinctly different crystal structures, the number should be around 20. Some examples are strontium cuprate oxycarbonate Sr2CuO2CO3 and its relatives Sr2(Y,Ca)Cu2Ox(CO3) and Sr2(Y,Ce)2Cu2Ox(CO3), and so on. The latter two cuprates are based on Sr2CuO2CO3 but another structural unit of either (Y,Ca)Cu or (Y,Ce)2Cu is inserted. If we expand the definition to include the same crystal structure but different components, the number could be more than 40. They are like P2-type CaxCoO2, SrxCoO2 and BaxCoO2, all derived from the solid-state ion-exchange from the layered cobaltate NaxCoO2.
What challenges are there concerning challenges on thin-film thermionic multilayers and organic thermoelectric materials?
We have quit these studies but instead, we have started to fabricate new types of TE modules, tilted-multilayer TE modules, which can exclude the problem of electrodes. The electric current flows parallel to the temperature gradient in conventional pi-type TE modules and, hence, electrodes are necessary in between every p- and n-legs.
However, the new type TE module utilises the off-diagonal Seebeck effect, which generates electricity perpendicular to the temperature gradient. In such a module, multilayers of TE materials and metals are simply tilted to an angle of 30-50 degrees to the temperature gradient and electrodes are only necessary at the uppermost and the opposite sides of the multilayer. Currently, we are financially supported by The Japan Society for the Promotion of Science (JSPS) project in our country.
What are your research priorities for the future?
We should expand other fields of energy-relating materials. One possibility could be to explore potential cathode materials for calcium-ion batteries to make use of our accumulated experience. We have discovered several unopened layered materials suitable for the diffusion of calcium-ions and hope that they will exhibit superb electrochemical performance.
Is there anything you would like to add?
In five years’ time, our university will construct an ultimate synchrotron radiation facility, named SLiT-J. This facility will herald a new era when it comes to investigating the static and dynamic crystal structures of matter with a high degree of accuracy. We are, therefore, very keen to commit to SLiT-J, so that we can bring much insight when it comes to producing superb energy harnessing materials.
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Department of Applied Physics, Graduate School of Engineering, Tohoku University
Tel: +81 22 795 7970
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