Crystal Defects in Ni-Based Alloys can Advance Thermoelectric Technology

The imminent environmental crisis can be prevented if people find sustainable and efficient means to avoid wastage.

In this context, the recycling of waste heat produced from technological devices and industrial processes into electricity is one area that has immense scope for improvement. In this field, thermoelectric materials are at the heart of studies because they enable the generation of clean power at a minimal cost.

If thermoelectric materials had to be utilized in highly different fields, like steelworks and transportation, they should be able to work in both low- and high-temperature conditions.

As such, “half-Heusler Ni-based alloys” are now attracting a great deal of interest because of their excellent durability, mechanical strength, and thermoelectric efficiency.

Although considerable efforts have been made to interpret and enhance these unusual alloys, investigators have found it quite challenging to explain why a high-conversion efficiency is exhibited by half-Heusler Ni-based alloys.

Some scientists have hypothesized that defects in the crystal structure of the material boost its thermal conductivity and, consequently, its conversion efficiency. But the crystal structure around the flaws, including their particular contributions, is not known.

In a new study that appeared in the Scientific Reports journal, a research team from Turkey and Japan—headed by Associate Professor Hidetoshi Miyazaki from Nagoya Institute of Technology in Japan—has now tried to resolve this problem.

The researchers’ study integrated experimental and theoretical analyses in the form of X‑ray absorption fine structure (XAFS) spectra on NiZrSn alloys and large-scale crystal structure simulations.

With the help of these methods, the researchers initially estimated the structural effects of an additional Ni atom (defect) on the arrangement of NiZrSn crystals. Then , using different kinds of XAFS measurements, they validated the hypothetical predictions.

In our theoretical framework, we assumed crystal lattice distortions to be a consequence of atomic defects to perform first-principles band structure calculations. XAFS made it possible to obtain detailed information on the local crystal structure around atomic defects by comparing the experimental and theoretical spectra of the crystal structure.

Dr Hidetoshi Miyazaki, Associate Professor, Nagoya Institute of Technology

Such observations enabled the team to precisely measure the strain caused by Ni defects in neighboring atoms. The researchers also studied the mechanisms through which such modifications lead to a higher thermal conductivity (and thus conversion efficiency).

The study results will be fundamental in developing thermoelectric technologies.

We expect that our results will contribute to the development of a strategy centered around controlling the strain around defective atoms, which in turn will allow us to engineer new and better thermoelectric materials.

Dr Hidetoshi Miyazaki, Associate Professor, Nagoya Institute of Technology

It is hoped that this will significantly advance the thermoelectric conversion technology and speed up the transition to a less decarbonized and wasteful society—one in which surplus heat is not merely discarded but rather recovered as an energy source.

In conclusion, Dr Miyazaki emphasized that the methods used to visualize fine modifications in strain in crystalline structures can be quickly modified to suit other kinds of material, like those meant for catalysts and spintronic applications.

Undoubtedly, there is much to gain from exploring the intricate details in materials science, and researchers can be confident that this analysis represents a step in the right direction toward a better future.

The study was partly funded by the Japan Society for the Promotion of Science’s (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) (C) (17K06771, 18K04748, 20K05100, and 20K05060).

Journal Reference:

Miyazaki, H., et al. (2020) Probing local distortion around structural defects in half-Heusler thermoelectric NiZrSn alloy. Scientific Reports.


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