Researchers decode thermal conductivity with light

Revolutionary science is often the result of true collaboration, with researchers in a variety of fields, viewpoints and experiences coming together in a unique way. One such effort by Clemson University researchers led to a discovery that could change the way thermoelectric science advances.

Graduate Research Assistant Prakash Parajuli; research assistant professor Sriparna Bhattacharya; and founding director of the Clemson Nanomaterials Institute (CNI) Apparao Rao (all CNI members in the College of Science Department of Physics and Astronomy) worked with an international team of scientists to examine a highly efficient thermoelectric material in a new way, using light.

Their research was published in the journal Advanced science and is titled “High ZT and its origin in Sb doped GeTe single crystals”.

“Thermoelectric materials convert thermal energy into useful electrical energy; therefore, there is a lot of interest in materials that can convert it more efficiently,” said Parajuli.

Bhattacharya explained that the key to measuring progress in the field is the figure of merit, referred to as zT, which is heavily dependent on the property of thermoelectric materials. “Many thermoelectric materials show a zT of 1-1.5, which also depends on the temperature of the thermoelectric material. Only recently have materials with zT of 2 or higher been reported.”

“This begs the question, how many other similar materials can we find, and what is the fundamental science that is new here through which it is possible to obtain a zT greater than 2?” Rao added. “Basic research is the seed from which applied research grows and to stay at the forefront of the thermoelectric sector we have collaborated with the team of Professor Yang Yuan Chen at the Sinica Academy, Taiwan.”

Chen and Rao’s teams focused on germanium telluride (GeTe), a unique crystalline material.

“GeTe is interesting, but simple GeTe without doping does not exhibit exciting properties,” Bhattacharya said. “But once you add some antimony, it shows good electronic properties, as well as very low thermal conductivity.”

While others have reported GeTe-based materials with high zT, these were polycrystalline materials. Polycrystals have boundaries between the many small crystals of which they are formed. Although such boundaries favorably hinder heat transfer, they mask the origin of the fundamental processes leading to elevated zT.

“Here, we had pure, doped GeTe single crystals whose thermoelectric properties have not been reported,” Bhattacharya said. ‘Therefore, we were able to evaluate the intrinsic properties of these materials that would otherwise be difficult to decipher in the presence of competing processes. This may be the first antimony doped GeTe crystal that has shown these unique properties, primarily conductivity.

This low thermal conductivity was a surprise, as the simple crystalline structure of the material should allow heat to flow easily through the crystal.

“Electrons carry heat and electricity, so if you block electrons, you have no electricity,” Parajuli said. “So, the key is to block the flow of heat from quantized lattice vibrations known as phonons, while allowing electrons to flow.”

Doping GeTe with the right amount of antimony can maximize the electron flow and minimize the heat flow. This study found that the presence of 8 antimony atoms for every 100 GeTe gives rise to a new set of phonons, which effectively reduce heat flux, both experimentally and theoretically confirmed.

The team, along with collaborators who cultured the crystals, performed electronic and heat transport measurements as well as density functional theory calculations to find this mechanism in two ways: first, through modeling, using thermal conductivity data; second, through Raman spectroscopy, which probes phonons within a material.

“This is a whole new point of view for thermoelectric research,” Rao said. “We are sort of pioneers in this way: decoding the thermal conductivity in the thermoelectric with light. What we discovered using light agreed well with what we found through heat transport measurements. Future research in the thermoelectric sector should use light: it is a very powerful non-destructive method for clarifying heat transport in the thermoelectric. Shine the light on the sample and gather information. You are not destroying the sample. “

Rao said that the employees’ wide range of skills was the key to their success. The group included Fengjiao Liu, a former PhD. CNI student; Rahul Rao, research physicist at the Air Force Research Laboratory, Wright-Patterson Air Force Base; and Oliver Rancu, a high school student at Governor’s School for Science and Mathematics of South Carolina who worked with the team through Clemson’s Summer Program for Research Interns (SPRI). Due to the pandemic, the team worked with Rancu via Zoom, guiding him through some of Parajuli’s calculations using an alternate Matlab code.

“I am very grateful for the opportunity to work with the CNI team members this summer,” said Rancu, who hails from Anderson, South Carolina. “I learned a lot about both physics and the research experience in general. It was truly priceless and this research publication is just another addition to an already fantastic experience.”

“I was very impressed with Oliver,” added Parajuli. “He quickly understood the necessary framework for the theory.”