PORTLAND, Ore.—Nanoscaling critical dimensions of advanced piezoelectric and thermoelectric materials is inching them toward commercial energy harvesting applications, according to separate research groups at Northwestern University and Boston College.
The first research group, led by Northwestern University professor Horacio Espinosa, claims that the piezoelectric coefficient can be boosted by 20- to 100-times by narrowing energy harvesting nanowires to under 2.4 nanometers. Likewise, the second research group, led by Boston College researcher Xiao Yan, claims that thermoelectric materials can harvest heat from automobile exhaust 60 to 90 percent better by hot pressing them from 5 to 10 nanometer grains.
The piezoelectric researchers at Northwestern, including doctoral candidate Ravi Agrawal, showed that by narrowing to under 2.4 nanometers gallium nitride (GaN, a III-V semiconductor) and zinc oxide (ZnO, a II-VI semiconductor) nanowires, their coefficient for generating energy from motion is boosted 20 times and 100 times, respectively. They also used computational density function theory (DFT) to calculate the range of performance down to .6 nanometers.
"Constructing nanogenerators, sensors and other devices from smaller nanowires will greatly improve their output and sensitivity,” said Espinosa.
Piezoelectric nanowires undergo coupled electromechanical testing with a MEMS device.
Thermoelectrics boosted, too
Likewise the separate researcher team at Boston College, with collaborators at MIT, Clemson University and the University of Virginia, claimed that the thermoelectric coefficient—called its figure-of-merit—can be boosted by 60-90 percent by virtue of first ball milling the theromoelectric material into 10 nanometer grains, and then hot pressing them together. As a result p-type half-Heusler thermoelectric semiconductors could potentially harvest heat energy anywhere it is being wasted today, and turn it into electricity.
"Half-Heuslers can potentially be widely used in the car exhausting systems, in which the 'free' exhaust waste heat can be recovered and transferred back into electricity," said Yan. "Our work paves the way for half-Heuslers to become competitive candidates used in car exhaust system and other power generation systems."
Yan, worked with Boston College professor Zhifeng Ren and MIT’s professor Gang Chen, who claim that the milling and pressing methods used are commercially viable.
"This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner," Ren said. "Our method is low cost and can be scaled for mass production."
Also on the team were Boston College researchers Giri Joshi, Weishu Liu, Yucheng Lan and Hui Wang, MIT’s Sangyeop Lee, University of Virginia professor Joe Poons and professor Terry Tritt.