The team’s purified tin selenide in polycrystalline form could lead to affordable devices that convert heat waste into electricity. (Image Credit: Northwestern University)


Scientists from Northwestern University and Seoul National University recently found a way to convert heat into electricity through a purified tin selenide in polycrystalline form, an inexpensive solution. The thermoelectric material outperforms the single-crystal form, making it the most efficient system to date. Their work, achieving a high conversion rate, builds on previous studies that had an oxidation problem, resulting in performance degradation.  This new approach could pave the way toward next-gen car engines, industrial furnaces, and energy-generating devices.


Thermoelectric devices rely on the internal material, which is situated in the center, to perform well. While one side of the device remains hot, the other stays cold. Heat passes through the material, causing electrical charges to transfer from one side to the other. Electricity stops flowing as soon as the heat warms up the cold side. However, some of that heat converts to electricity, leaving the device through wires. 


The material must have very low thermal conductivity and high electrical conductivity to efficiently convert heat into electricity. It also needs to be stable since the temperature could reach 400-500 degrees Celsius. Those issues make it difficult to develop thermoelectric devices.


Mercouri Kanatzidis, Northwestern University materials scientist and his team discovered a crystal of tin selenide that was the most efficient at converting heat into electricity. However, the single crystal material could not be mass-produced due to its fragile quality and tendency to flake.


Instead, the team used processed tin and selenium powders to form grains of polycrystalline selenide. These inexpensive grains can be heated and compressed into 3 to 5 centimeter-long ingots, which can be used to create devices. The ingots are more robust, and the boundaries between each grain can slow the heat distribution. After testing the material, the team discovered that it had high thermal conductivity. 


“We realized something diabolical was happening,” Kanatzidis said. “The expectation was that tin selenide in polycrystalline form would not have high thermal conductivity, but it did. We had a problem.”


Then, in 2016, the team discovered that extremely thin skin of tin oxide formed around individual grains of polycrystalline tin selenide before being processed into ingots. This skin provided a passageway for the heat to travel from grain to grain through the material. Now, the team devised a technique that used heat to move oxygen away from each grain.


This resulted in thermal conductivity lower than a single-crystal tin selenide, making it the most efficient to date. Waste heat conversion in thermoelectric devices is measured through its figure of merit, also known as ZT. A higher number means it has a better conversion rate. The single-crystal tin selenide had a ZT of 2.2 to 2.6 at 913 Kelvin. Meanwhile, the purified tin selenide in polycrystalline form has a ZT of 3.1 at 783 Kelvin.


“This opens the door for new devices to be built from polycrystalline tin selenide pellets and their applications explored,” Kanatzidis said.


The polycrystalline tin selenide contains sodium atoms, producing a p-type material with positive charge conducting capabilities. Before producing devices, the team also needs an n-type that can conduct negative charges. The team is in the process of developing an n-type polycrystalline version. Once both n-type and p-type versions are combined, the team could develop the next-gen ultra-efficient thermoelectric generators. These could then be deployed in vehicle exhaust pipes, water heaters, and industrial furnaces.


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