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A representation of the crystal lattice of the thermoelectric compound Mg3Sb2 (magnesium atoms in orange, antimony in blue). An electric current is generated when heat passes through the material, propelled by phonon waves. Credit: ORNL / Jill Hemman
The landing of NASA’s Perseverance rover was another step forward not only for space exploration, but also for the technology that powers the craft during its multi-year mission to Mars, a thermoelectric generator that transforms the heat into electricity.
In search of the next leap in thermoelectric technologies, researchers at Duke University and Michigan State University have gained fundamental new knowledge about two magnesium-based materials (Mg3Sb2 and mg3With a2) which have the potential to significantly outperform traditional thermoelectric designs and would also be more environmentally friendly and less expensive to manufacture. Contrary to the prevailing scientific wisdom regarding the use of heavy elements, researchers have shown that replacing atoms of heavier elements such as calcium and ytterbium with lighter magnesium atoms actually multiplied by three performance of magnesium-based materials.
In their research, published in the journal Scientists progress, the team used neutron and x-ray scattering experiments at the Department of Energy’s (DOE) Oak Ridge National Laboratories (ORNL) and Argonne, as well as supercomputer simulations at National Energy Research Scientific Computing Center (NERSC). Research at the atomic scale has revealed the origin and mechanism of the ability of materials to convert thermal energy at room temperature into electricity. The results point to possible new avenues for improving thermoelectric applications such as the Perseverance rover and a myriad of other power generation devices and technologies.
Thermoelectric materials essentially create voltage from a temperature difference between the hot and cold sides of the material. By converting thermal energy into electricity, or vice versa, thermoelectric devices can be used for refrigeration or the generation of electricity from exhaust heat.
“Traditional thermoelectric materials rely on heavy elements such as lead, bismuth and tellurium, which are not environmentally friendly elements, and they are also not very abundant, so they tend to be expensive,” he said. declared Olivier Delaire, associate professor at Duc. “Magnesium, on the other hand, is lighter and more abundant, making it an ideal material for transportation and space flight applications, for example. “
“These magnesium materials, however, have remarkably low thermoelectric conductivity despite low mass density. These properties could potentially open the door to the design of new types of thermoelectrics that do not rely on heavy materials containing toxic elements,” Delaire explained.
The magnesium materials the team studied belong to a larger class of metal compounds called Zintls. The atomic structure, or arrangement of atoms, in Zintl compounds is such that it is relatively easy to experiment with and substitute different elements in the material, for example by replacing a heavy element with a light element to obtain optimal performance and functionality.
“In chemical studies, exploring the possibilities of new materials often involves substituting one element for another just to see what happens. Usually we replace them with chemically similar elements in the periodic table, and one of the big ones advantages of using Zintls is that we can experiment with a lot of different elements and different combinations, ”said the first author of the article, Jingxuan Ding, a graduate student researcher of the Delaire Group at Duke. ‘expected magnesium to be the best compound, but when our collaborators in the State of Michigan replaced it in the ingredients of the materials, we were surprised to find that it actually was, so the next step was to find out why. “
Atoms in a material are not static or stationary; they vibrate with amplitudes which increase with higher temperatures. Collective vibrations create a ripple effect, called a phonon, which looks like sets of waves on the surface of a pond. These waves carry heat through a material, which is why measuring phonon vibrations is important in determining the thermal conductivity of a material.
Neutrons are particularly suitable for the study of quantum phenomena such as phonons, because neutrons have no charge and can interact with nuclei. Delaire compared neutron interactions to the plucking of a guitar string in that they can transfer energy to atoms to excite vibrations and gain hidden information about the atoms inside a material.
The team used the Wide Angular Range Chopper Spectrometer, or ARCS, at ORNL’s Spallation Neutron Source (SNS) to measure phonon vibrations. The data they acquired allowed them to trace the favorable low thermal conductivity of the materials to a special magnesium bond that disrupts the path of phononic waves through the material causing them to interfere with each other.
“Neutrons are one of the best ways to measure atomic vibrations like the ones we are studying in these materials,” Ding said. “ARCS can detect a wide range of frequencies and wavelengths that help us measure the phonon waves found in the material, which is exactly what we need to better understand how these low thermal conductivity materials work. . ”
The neutron scattering measurements provided the research team with a broad study of the internal dynamics of Zintl magnesium materials that helped guide and refine computer simulations and subsequent X-ray experiments led by Ding. These have been used to build a comprehensive understanding of the origins of thermal conductivity of materials.
X-ray experiments complementary to the Argonne Advanced Photon Source (APS) were used to zoom in on specific phonon modes in crystal samples too small for neutron measurements. Neutron and X-ray measurements were consistent with supercomputer simulations performed at NERSC.
In addition to Ding and Delaire, the article’s co-authors are Tyson Lanigan-Atkins, Mario Calderón-Cueva, Arnab Banerjee, Douglas L. Abernathy, Ayman Said and Alexandra Zevalkink.
“Thermoelectric is essential in applications like the Mars Perseverance rover which require simpler, lighter and more reliable designs instead of the bulky motors with moving parts which are traditionally used to generate electricity from heat.” , said Delaire. “These magnesium-based materials are a great breakthrough in the field that could offer significantly higher energy efficiency and a lot of potential for more advanced thermoelectric applications.”
“Floppy” atomic dynamics help transform heat into electricity
Jingxuan Ding et al, Soft anharmonic phonons and ultra-low thermal conductivity in Mg3 (Sb, Bi) 2 thermoelectrics, Scientists progress (2021). DOI: 10.1126 / sciadv.abg1449
Provided by the Oak Ridge National Laboratory
Quote: The buzz about thermoelectricity is intensifying with promising new magnesium-based materials (2021, July 23) retrieved on July 23, 2021 from https://phys.org/news/2021-07-thermoelectrics-magnesium-based- materials.html
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