New research provides a better understanding into how crystal dislocations, which are a common type of defect in materials, can influence heat and electrical transport via crystals, at a microscopic, quantum mechanical level.
Dislocations in crystals are areas where the neatly arranged 3D structure of a crystal lattice - whose array of atoms repeats with precisely the same spacing - is disturbed. The effect looks as if a knife had sliced through the crystal and then the pieces were stuck back together, askew from their original positions.
These dislocations have a powerful effect on phonons, the modes of lattice vibration that play a part in the electrical and thermal properties of the crystals via which they move. But an exact understanding of the mechanism of the dislocation-phonon interaction has been obscure and controversial, which has slowed advancement toward using dislocations to modify the thermal properties of materials.
MIT researchers have been able to learn crucial details about how those interactions function, which could help future endeavors to build thermoelectric devices and other electronic systems. The findings can be found in the Nano Letters journal, in a paper co-authored by postdoc Mingda Li, Department of Mechanical Engineering head Professor Gang Chen, the late Institute Professor Emerita Mildred Dresselhaus, and five others.
Li describes dislocations as "atomic irregularities in a regular crystal". They are highly common defects in crystals, and they impact, for instance, how heat disperses via a silicon microchip or how well current flows via a silicon solar panel.
There have been two competing strategies to clarifying phonon-dislocation interactions, Li explains, and a few other questions regarding them have stayed unanswered. The MIT team has prepared a new mathematical approach to analyze such systems, using a new quasiparticle they developed called a "dislon," which is a quantized version of a dislocation, which seems to solve these very old mysteries.
People have tried to learn how the dislocations change the material properties -- the electrical and thermal properties. Before now, there were many empirical models, which need fitting parameters to be complete. There was a long debate about the nature of phonon scattering in dislocations.
Mingda Li, MIT
Li says that the new theory has a different starting point, as it is based on meticulous quantum field theory. It seems to resolve many issues, including a contest between two views known as the static and dynamic scattering methods, displaying they are just two extreme cases within this new structure. Also, while both of these methods fail to explain behavior at the nanoscale, the new method functions well at such scales.
The findings could influence the search for better thermoelectric materials, which can change heat to electricity. These are used for providing heaters for car seats, or producing power from waste heat. Thermoelectric systems can also provide cooling, for cold-drink chests, for instance.
Chen, who is the Carl Richard Soderberg Professor of Power Engineering, associates the new discoveries to Li's initiative.
I didn't put that much hope in it. It's a pretty complex problem: how dislocations affect these very important properties. ... I was very surprised when he came back with this new theory. He started from basic principles and derived a quantum description for it.
Gang Chen, Professor, MIT
Li and his team have made “a breakthrough by being able to account for the long-range nature of the dislocation strain field, by treating it as a new quantum mechanical object called the dislon,” says Jeffery Snyder, a professor at Northwestern University, who was not connected to this work. “Combining this with the quantum mechanical treatment of the dislon-electron interaction could lead to new strategies to optimize materials by using metallurgical approaches to engineer the structure, type, and location of dislocations within a material.”
Dislocations have profound effects on properties of materials, but until now the long-range nature of the strain field has prevented direct calculations of dislocation effects. The quantization developed in this paper goes a long way to solving these problems. I expect that this new formalism will lead to greatly improved understanding of the effects of dislocations on the electrical and thermal properties of materials. This work is a major step forward.
David J. Singh,Professor, University of Missouri
The research team also included Zhiwei Ding, Jiawei Zhou, and Professor Hong Liu at MIT, and Qingping Meng and Yimei Zhu at Brookhaven National Laboratory. The work was supported by S3TEC, the Energy Frontier Research Center funded by the U.S. Department of Energy’s Office of Basic Energy Sciences, and the Defense Advanced Research Projects Agency of the U.S. Department of Defense.