New material offers ecofriendly solution to converting waste heat into energy

Credit: Unsplash/CC0 public domainNASA's 2020 Mars Rover Perseverance is powered by a thermoelectric device. This converts heat into useful electricity.The radioactive decay plutonium of plutonium is Mars' heat source. The device's conversion efficiency for Mars is 4-5%. This is good enough to power Perseverance, its operations, but not enough for applications on Earth.Scientists from Seoul National University in Korea and Northwestern University have now demonstrated a thermoelectric material that is highly efficient in device development. The polycrystalline form of purified tin-selende outperforms single-crystal forms in converting heat into electricity. It is the most efficient thermoelectric system ever recorded. After identifying an oxidation issue that had affected performance in previous studies, the researchers were able achieve a high conversion rate.It is possible to develop the polycrystalline tin-selenide for solid-state thermoelectric devices in many industries. This could lead to huge energy savings. The key goal is to capture industrial waste heat, such as that from automobiles, power plants and brick- and glass-making factories and convert it into electricity. Waste heat accounts for more than 65% of global energy production from fossil fuels.Mercouri Kanatzidis from Northwestern, a chemist with a specialization in the development of new materials, said that although thermoelectric devices are still in use, they are only used in niche applications. These devices are not as popular as solar cells and it is difficult to make good ones. We are working on a material that will be cost-effective and high in performance, and help thermoelectric devices become more widely used.Kanatzidis is the Charles E. and Emma H. Morrison Professor of Chemistry at the Weinberg College of Arts and Sciences. He is also a co-author of the study. He is also a co-author of the study with ArgonneNational Laborator.Nature Materials will publish Aug. 2 details about the thermoelectric material, including its record-breaking performance.The paper's co-corresponding author is In Chung, Seoul National University. Vinayak Dravid is the Abraham Harris Professor of Materials Science and Engineering of Northwestern's McCormick School of Engineering. He is one of the senior authors of this study. Dravid has been a long-standing collaborator of Kanatzidis.Kanatzidis says that thermoelectric devices are well-defined. But what makes them work is the thermoelectric material. The device has one side that is hot and the opposite side that is cold. In the middle is the thermoelectric material. The material is heated by heat, and some heat is converted into electricity. This electrical energy leaves the device via wires.To be effective at waste heat conversion, the material must have a very low thermal conductivity and good electrical conductivity. The material must be stable at extremely high temperatures, as the heat source can reach 400-500 degrees Celsius. These and other challenges make thermoelectric devices harder to produce than solar cell."Something terrible was happening"Kanatzidis's team discovered a material that could convert waste heat into useful electricity. It was the crystal form of the chemical compound Tin selenide. Despite being an important discovery, it is not practical to mass produce the single-crystal form due to its fragility and tendency flake.The researchers needed to study tin selenide in its polycrystalline form. This is stronger and easier to shape for specific applications. They discovered that the material's thermal conductivity was higher than the desired low level in single-crystal forms. This was a surprise.Kanatzidis stated that they realized something was wrong. "The expectation was that Tin Selenide in Polycrystalline Form would have low thermal conductivity. But it did. "We had a problem."The researchers found a skin made of oxidized zinc on the material. The conductive skin was heated, which increased the thermal conductivity. This is not desirable in a thermoelectric device.A solution is found, opening doorsThe Korean team discovered that oxidation was caused by both the process and the starting materials. They then devised a method to remove oxygen. They were able to produce tin-selenide pellets without oxygen which they tested.As expected, the true thermal conductivity for the polycrystalline form was measured. Its ability to convert heat into electricity, making it the most efficient thermoelectric device ever measured.It is the "figure of merit" that measures the efficiency of thermoelectrics waste heat conversion. This number is called ZT. Higher numbers indicate a higher conversion rate. The ZT for single-crystal tinselende was previously found to be between 2.2 and 2.6 at 913 Kelvin. Researchers found that the ZT of purified tin-selenide in polycrystalline form was approximately 3.1 at 783 Kelvin. Its thermal conductivity was extremely low, much lower than single-crystals.Kanatzidis stated that this opens up the possibility of new devices being built using polycrystalline tin-selende pellets, and their potential applications.Northwestern holds the intellectual property rights to the tin-selenide material. The potential applications for the thermoelectric material are the automotive industry (a large amount of gasoline's energy is lost through a vehicle's tailpipe), the heavy manufacturing industries (such glass and brick making, refineries and coal- and gas fired power plants), and areas where large combustion engines run continuously (such a large ship or tanker).The paper's title is "Polycrystalline Se with a thermoelectric figure greater than the single crystal."Continue reading Development of a new thermoelectric material with record high conversion efficiencyMore information: Polycrystalline snSe has a thermoelectric value of merit that is greater than one crystal, Nature Materials (2021). Journal information: Nature Materials Polycrystalline SnSe with a thermoelectric figure of merit greater than the single crystal,(2021). DOI: 10.1038/s41563-021-01064-6