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X-ray revealed why thermoelectric material was damaged by heat

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A very powerful X-ray and some inventive chemists at Aarhus University answered a question from TEGnology ApS about why the company's thermoelectric materials had such a short lifespan. Collaboration through LINX resulted in a new method of analysis to benefit all materials research.

Thermoelectric material in modules: A power brick from TEGnology, which can convert heat into electricity. The small module is intended for applications with low power consumption, such as wireless temperature sensors or thermostats. Graphic: TEGnology

Thermoelectric materials can convert heat into electricity, and this has great potential, because over half of all the global energy produced ends up as waste heat. Universities and companies all over the world are therefore researching into developing efficient and inexpensive materials to contribute to the green energy systems of the future.

But it's not easy. The material must have high electrical conductivity, low thermal conductivity and, of course, a chemical composition that enables it to convert heat energy into electrical energy.

And clearly the material must be able to withstand the high temperatures in the environments in which it will be used.

Tolerating high temperatures turned out to be exactly the problem facing an otherwise promising material, zinc antimonide (Zn4Sb3), which in 2012 became the basis for the company TEGnology ApS in Hedensted, Denmark. The company develops and sells thermoelectric materials and components that can effectively convert heat into electricity. Unfortunately, it turned out that the material broke up at high temperatures.

Researchers to the rescue

A picture of the sample positioned in the new setup. The sample is mounted between two towers, each of which can have its own temperature. 2x2 probes are placed on top of the sample to measure the electrical resistance of the sample and Seebeck coefficient. In the background is the outlet for the incoming radiation. When the beam hits the sample, it is scattered and the scattered radiation is collected by a detector. In this image, the detector is placed behind the camera. Photo: Lasse Rabøl Jørgensen

To find the cause, TEGnology needed to look inside the zinc antimonide while the material was in use at high temperatures.

Among other things, this requires a very powerful X-ray, so TEGnology contacted the social partnership LINX (Linking Industry to Neutrons and X-rays, see "Facts about LINX"), which put the company in contact with the Center for Materials Crystallography at the Department of Chemistry.

The centre has access to the world's most powerful synchrotron-based X-ray sources. However, in order to find out why the material broke up, researchers had to develop a method to mimic the conditions it is exposed to when it is used in industry. In other words, the zinc antimonide samples had to be warmed up to 400-500 degrees centigrade at one end, and kept cold at the other end, while;they were exposed to intense X-ray radiation.

A new instrument for everything

In short, they successfully developed an instrument that could simultaneously

  • • heat up the sample, either completely or at one end to create a heat gradient
  • • measure electrical resistance in the sample
  • • measure the ability of the sample to convert a temperature difference into electricity (Seebeck coefficient)
  • • place the sample between the end of a beam in a synchrotron and a detector that can register the spread of the radiation after it has hit the sample. In this case, they used the Petra III synchrotron in Hamburg.

… after which, using two different methods of analysis – powder diffraction and PDF (Pair Distribution Function) – the researchers could look at both the crystalline and amorphous components and get a complete picture of the whole sample.

Migrant ions

And what did the image show?

That the electrical potential created by the material during the warming makes zinc ions migrate. This makes the zinc-deficient materials in the sample change, and the sample breaks.

"We haven’t used the tool to observe something that we haven’t been able to explain before. We can look at very small changes, small expansions, and movements of atoms that you can't see otherwise," explains PhD student Lasse Rabøl Jørgensen, who helped develop the instrument.

The tool can be used for anything related to ion conduction in materials research, for example to analyse ferro electrical materials, piezo electrical materials and electrolytes in batteries.

 

The study of zinc antimonide resulted in an article in the scientific journal IUCrJ.

Collaboration means everything

Since then, the research group has examined other samples from TEGnology in connection development of new systems by the company.

The head of technology at TEGnology and co-owner, Hao Yin, is enthusiastic about the collaboration.

"This has meant everything for our future. Only one thing can secure development for a small start-up company like TEGnology: namely the technology. When we talk to customers, investors and partners, the collaboration means that we have access to state-of-the-art research, development and results. We’re a hi-tech company, based on a patent that is now ten years old, and when new technology becomes old technology, we need to keep up with high-speed research and development. We get this at Aarhus University through LINX," says Hao Yin.