Development of Si-Based High Efficiency Thermoelectric Materials

Impact Objectives • Investigate potential non-toxic, abundant bases with thermoelectric properties • Develop high-efficiency silicon-based thermoelectric materials Generating electricity from waste heat Associate Professor Ken Kurosaki describes the potential of thermoelectric materials to improve energy efficiency by turning waste heat into an inexpensive source of electrical power nuclear materials and TE material converts semiconductor materials such as silicon, it to usable electricity. The performance is to alter the microstructure at the of a TE material is determined by the nanoscale level. This helps scatter and material’s figure of merit called zT, where block heat phonons, thereby lowering a higher zT correlates to a higher-efficiency thermal conductivity. We are also focusing TE module built from that material. Much on improving the Seebeck coefficient, research effort has been concentrated which is a measure of how much voltage is Professor Ken Kurosaki Sora-at Tanusilp on trying to achieve higher efficiency, but produced by the material in response to the only small improvements are being made. temperature difference across the material. Can you tell us a bit about high efficiency Our goal is to try new strategies involving thermoelectric materials? nanostructuring combined with mixed What is your background and how did valence states, to improve the performance you become involved in thermoelectric Thermoelectric materials can convert heat of TE materials and thus the efficiency of TE materials? to electricity and vice versa and thus can be modules. utilised as power generation or solid-state I started out in nuclear engineering, cooling modules. They have many potential Why are you concentrating on silicon particularly in the characterisation of uses and may greatly enhance fuel and compounds? nuclear fuels. Nuclear fuels need a high energy efficiency. They also have the ability thermal conductivity because that property to generate electricity from waste heat, and The most commonly-used TE materials are improves the efficiency of nuclear power our energy conversions result in a high bismuth telluride (Bi2Te 3) and lead telluride generation and reduces the temperature percentage of waste heat. For instance, the (PbTe). The problem with these is that they of the fuels. TE materials need low thermal thermal efficiency of automobile engines are either toxic, as is the case with lead, or conductivity to create temperature gradient is in the range of 25 to 30 percent, while rare elements like tellurium. This makes as large as possible. However, both are the rest is lost through exhaust gases. their preparation both dangerous and solid state materials and the methods Also, thermoelectric materials can be used expensive. Various other compounds have used for synthesis and characterisation in reverse. By passing an electric current achieved reasonable efficiencies at different are very similar. Therefore, there are many through such a material, a temperature temperatures. For instance, my team has commonalities and my skills were readily gradient is formed, resulting in a cooling concentrated on thallium compounds, transferable to this area. For the last decade effect. which have shown great promise. However, or so, I have worked in both fields, with thallium is also poisonous to humans half my time spent on each. TE is a field Why is important to research and optimise and other mammals and is therefore only that greatly interests my PhD students and thermoelectric materials? suitable for specialist applications. I enjoy teaching and working with these young scientists. We have strong links with The problem today is that the performance On the other hand, silicon is both abundant Thailand and many of my PhD students are of thermoelectric (TE) materials is not on earth and non-toxic. However, without from there and return home fully-trained high, which means that they are only used modification, silicon and its compounds are to begin their own projects in the field of where there is no alternative, for instance only low-efficient TEs. A much-researched TEs. in spacecraft, where heat is generated by means of improving TE performance in www.impact.pub 21 Silicon-based thermoelectrics A team at Osaka University has pioneered the use of silicon-based compounds in creating efficient non-toxic thermoelectric materials that exhibit good thermoelectric properties SustainableThermoelectric (TE) materials are receiving Kurosaki says: ‘If haircarewe can create TE modules achieved at high temperatures. Kurosaki much attention at present, owing to their based on silicon – a material that is widely says, ‘The ultimate aspiration of researchers potential to convert waste heat to electricity. available and non-toxic – it opens up the is to achieve a zT of around 1.5 at room Associate Professor Ken Kurosaki’s team potential for TEs to be used in a wide range temperature, using materials that are both at Osaka University has many years’ of energy-saving applications.’ widely available and non-hazardous.’ experience in this area, having achieved performance improvements through THERMOELECTRICS EXPLAINED The most common TE materials used today doping - the addition of small quantities TE materials are semiconductors that are lead telluride and bismuth telluride. of a different element to compounds create an electrical current when there Lead and bismuth are heavy metals that are - and the creation of new compounds. is a temperature difference across them. toxic, while tellurium is both rare and toxic Kurosaki believes a major application for Kurosaki explains: ‘A TE module consists of to humans. Kurosaki’s team has achieved TE materials, ‘is in the conversion of waste pairs of n-type and p-type semiconductor good TE performances over recent years heat from the exhausts of automobiles to thermocouples that are joined electrically by creating thallium-based compounds electricity, which would greatly improve in series and thermally in parallel.’ and doping PbTe with small amounts of fuel efficiency.’ There are many other areas Semiconductors are materials that are thallium. Like the classical TE materials, where such materials could be used to not natural electrical conductors but thallium is a toxic heavy metal readily minimise energy use and they are viewed as can be induced to conduct under certain absorbed by the human body. However, being a key component of a lower emissions conditions. N-type semiconductors have research into these different compounds future. spare electrons that are electrical carriers, means that Kurosaki and his team are whereas in p-type semiconductors, the highly skilled in the preparation and Since the efficiency of TE energy conversion ‘holes’ are the majority electrical carriers. characterisation of TE materials and ideally is currently limited to several percent, they When n- and p-type semiconductors are placed to extend the performance of these are only used in specialist applications joined in a thermocouple and exposed to materials and develop novel compounds. such as spacecraft and some pacemakers. a heat source on one side, electricity is The goal of Kurosaki’s team is to push generated via the Seebeck effect. SILICON-BASED COMPOUNDS efficiencies closer to 15 percent, a level at Silicon is an abundant semiconductor which TE materials become cost effective. The performance of a TE material is material that is affordable, readily-available Traditional TE materials also contain described by a zT ‘dimensionless figure and non-toxic, but on its own does not toxic and rare elements such as lead and of merit.’ The zT is calculated using show a particularly strong TE effect. Silicon tellurium, further limiting their use, since an equation that includes the Seebeck compounds, called silicides, are being manufacture and handling becomes coefficient as well as electrical and thermal studied by different research groups. dangerous, highly skilled and expensive. conductivity and absolute temperature. Kurosaki says that to improve the TE The Seebeck coefficient is a measure performance in silicon-based compounds, of the induced voltage in response to a ‘Scientists have concentrated on reducing temperature difference across the material. the thermal conductivity. Well-known For a material to score a high zT, both methods include nanostructuring, the Seebeck coefficient and the electrical which causes heat-carrying phonons conductivity should be maximised, and to be scattered.’ This avenue has been thermal conductivity minimised. The zT followed by Osaka University, but Kurosaki varies according to temperature, and there believes, ‘There is a limit to how much is an optimal temperature at which each the TE properties can be improved using TE material operates most efficiently. The nanostructuring.’ Therefore, he is focusing most efficient materials today have a zT on both reducing thermal conductivity and Energy flow for a typical automobile of around one, but this is generally only increasing the Seebeck coefficient. 22 www.impact.pub If we can create thermoelectric (TE) modules Project Insights based on silicon, a material that is widely available COLLABORATION A part of this project was a collaboration with Hitachi. The project was supported and non-toxic, it opens up the potential for TE in part by JST, PRESTO , Grant Number materials to be used in a wide range of energy- JPMJPR15R1. CONTACT DETAILS saving applications Professor Ken Kurosaki Project Leader T: +81 66879 7905 A system based on ytterbium silicide

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