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Posted: Nov 17, 2016
Science corrected: Why findings of a popular thermoelectricity study couldn't be replicated
(Nanowerk News) A team of physicists in Clemson University’s College of Science and Academia Sinica in Taiwan has determined why other scientists have been unable to replicate a highly influential thermoelectricity study published in a prestigious, peer-reviewed journal.
In the April 2014 issue of the journal Nature ("Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals"), a group of scientists described an emerging crystalline material made of tin selenide that provided the highest efficiency ever recorded for thermoelectricity, the process of capturing wasted energy which is released as heat and making it available again as electricity. The paper has been viewed 45,000 times and its findings have been referenced in 600 subsequent studies, according to Google Scholar.
A thermoelectricity (TE) module captures waste energy, released as heat, converts it to electricity and returns it to a device. (Image: Thomas Masservy, Clemson University)
The 2014 article stated that tin selenide is a good electrical conductor but a very poor heat conductor.
“If it were true, basically, they would have found a crown jewel,” said Apparao Rao, the Robert A. Bowen professor of Physics and the director of the Clemson Nanomaterials Institute.
Thermoelectricity could provide enormous monetary and environmental savings because it is sustainable; instead of requiring fuel it continually captures wasted heat energy and puts it to use. And there’s a lot of wasted energy; about 70 percent in most machines, including cars.
“When your laptop gets hot, energy is released as waste heat because it doesn’t use all the supplied electricity. Machines have limited efficiency,” according to Ramakrishna Podila, assistant professor of physics and astronomy at Clemson who co-authored the paper solving the mystery.
But, so far, the perfect material for capturing and creating thermoelectricity has proven elusive.
Heat and electrical current can flow through any material when heat is applied to one side. But to efficiently harness thermoelectricity, the material has to trap heat on one side while letting the current flow. The difference in temperature, from one side to the other, generates energy.
Imagine cookware. Expensive pots and pans are copper or they have copper cores. Copper is a great heat-conducting material: it quickly and evenly spreads heat so food cooks evenly. Copper makes for good cookware, but poor thermoelectric material.
In an ideal thermoelectric material, current-carrying electrons should flow unimpeded from the hot side to the cold side, but heat-carrying phonons, which are atomic vibrations, must be blocked, either by large atoms or defects where the material is of lower density.
Rao; Podila; Sirparna Bhattacharya, a research assistant professor in astronomy and physics; and Jian He, an associate professor in physics and astronomy at Clemson and a thermoelectrics expert, performed their own study on tin selenide in collaboration with Academia Sinica’s Institute of Physics in Taipei.
Right away, Bhattacharya noticed a problem. “The most puzzling thing was that when we measured our own tin-selenide material, we observed the same electrical flow as reported in the 2014 article, but the heat carried by the phonons was relatively higher,” Bhattacharya said.
The original research group “made tin-selenide crystal that was not fully dense,” Bhattacharya said. Ideally, a crystalline material matches its “theoretical density,” meaning it’s as dense as it can be expected to get.
“Instead of reaching 100 percent theoretical density, it reached 89 percent. A 10 percent difference might not seem like much,” she said, but it can have a huge implication on the electron and phonon flow.
The Clemson-Taiwan collaborators are now focusing on their own assessment of thermoelectricity in tin-selenide. They expect to publish soon.