Power/Performance Bits: May 17

Shrinking perovskites; cooling with graphene.


Shrinking perovskites

Researchers from Imperial College London, Oxford University, Diamond Light Source, Pohang University of Science and Technology in Korea, and Rutgers University have discovered a material that can be chemically tailored to either expand or contract in a precise way and over a wide temperature range. This could lead to new composite materials that do not expand when heated, improving the stability of many essential components in electronic equipment.

The contraction, known as negative thermal expansion (NTE), is caused by certain vibrations of atoms within the material, causing them to become closer together on average. The size of the effect depends on the proportions of two elements in the material, strontium and calcium. Changing the amount of these two elements allows the researchers to tune the amount of NTE.

Crystal structure of the material. (Source: Imperial College)

Crystal structure of the material. (Source: Imperial College)

The new materials can be designed to contract more or less under heating, and so can be matched to the expansion of any other material to cancel it out, and create components that do not change shape when heated.

“In many materials with NTE the effect only lasts in a narrow range of temperature as other types of atomic vibrations dominate and begin to cause expansion. The atoms in our material are able to keep vibrating in a way that sustains the NTE effect,” said Imperial PhD student Chris Ablitt. “Knowing this could help us make small adjustments to materials that display NTE only at low temperatures and potentially sustain NTE at the operating temperatures of electronic devices.”

Dr. Arash Mostofi and Dr. Nick Bristowe from Imperial’s Departments of Materials and Physics said: “The discovery of how to control thermal expansion in these materials is very exciting. Our understanding of the processes underlying the effect means that we can search for it in related materials in the perovskite family or in other classes of materials with wide applications.”

Cooling with graphene

Researchers at Chalmers University of Technology developed an efficient way of cooling electronics by using functionalized graphene nanoflakes.

“Essentially, we have found a golden key with which to achieve efficient heat transport in electronics and other power devices by using graphene nanoflake-based film. This can open up potential uses of this kind of film in broad areas, and we are getting closer to pilot-scale production based on this discovery,” said Johan Liu, Professor of Electronics Production at Chalmers.

Functionalization mediates heat transport of graphene nanoflakes. (Source: Johan Liu, Philip Krantz, Krantz Nanoart)

Functionalization mediates heat transport of graphene nanoflakes. (Source: Johan Liu, Philip Krantz, Krantz Nanoart)

The researchers studied the heat transfer enhancement of the film with different functionalized amino-based and azide-based silane molecules and found that the heat transfer efficiency of the film can be improved by over 76% by introducing functionalization molecules compared to a reference system without the functional layer. This is mainly because the contact resistance was drastically reduced by introducing the functionalization molecules. The results suggested potential thermal management solutions for electronic devices.

“This is the first time that such systematic research has been done. The present work is much more extensive than previously published results from several involved partners, and it covers more functionalization molecules and also more extensive direct evidence of the thermal contact resistance measurement,” said Liu.