“Sandwiched” copper nanosprings mediate between materials that respond differently to heat

A fundamental problem occurs when two different materials are bonded or soldered together, for example in a computer microchip or in power electronics: when exposed to heat, because the two materials expand at different rates, one could break or de-bond from the other. This can have catastrophic consequences. University of Illinois researchers in the Department of Aerospace Engineering in collaboration with General Electric (GE) Research developed and tested a type of thin film made of copper nanosprings, sandwiched between two different materials, to conduct the heat and at the same time accommodate the relative expansion or contraction of the two materials.

“There are other materials such as polymers which are quite compliant but do not conduct heat well, which is essential in this application,” said Professor Ioannis Chasiotis, who led the research at U of I. “If the solder or glue between two materials is stronger than either one, the material on either side could break or separate from the adhesive. So, just using a very strong bond isn’t the best solution when two different materials are subjected to extreme heat or cold.”

Chasiotis, along with graduate students Dimitrios Antartis, Ryan Mott, post-doctoral scholar Debashish Das and David Shaddock from GE Research hypothesized that a film made from copper nanosprings just 10 microns high could provide a thermally conductive interface between two different materials and allow them to expand quite freely. It worked very well, better than other more “exotic” solutions such as carbon nanotubes, for example.

When exposed to temperatures ranging from negative 40 degrees Celsius to 85 degrees Celsius (up to 185 degrees Fahrenheit), the copper nanospring film has the compliance of soft polymers but up to 100 times higher thermal conductivity than any other material with similar elasticity.

“The nanospring layer accommodates the materials on either side rather than preventing one or the other from expanding or contracting,” Chasiotis said. “We bond a lot of things together in life. But a thin continuous layer of adhesive or solder can’t adjust to large differences in expansion or contraction when sandwiched between different materials exposed to extreme heat or cold. The beauty of a nanospring film is that it is much thinner than a typical layer of solder yet is very compliant because it is made of discrete nanosprings. The copper nanosprings pass heat through while allowing the materials above and below to expand without de-bonding.”

In this project, the mechanical behavior was investigated at Chasiotis’ labs at U of I and the thermal measurements were conducted by GE. With the support by the Air Force Office of Scientific Research, Chasiotis’ lab acquired the facility to fabricate large area films of copper nanosprings by using a method called Glancing Angle Deposition, or GLAD.

Proving that the structure of nanosprings works well as an interface between two materials was just the first step. Next, the researchers created a model of the precise geometry of GLAD nanosprings that could be used for other purposes and applications.

“Such a model could be used to design films of nanosprings with specific properties and for particular applications,” Chasiotis said. “This would allow users to change the number of coils and other variables, such as the number of turns of a spring, its diameter, and the distance between springs in a film, to make good predictions of the mechanical properties of the nanospring film. To develop our design approach, we used nanosprings made from silicon. With this first model for the nanospring geometry versus mechanical behavior, we can start designing functional interphases that bring in multiple functionalities in applications such as onboard electronics on aircraft or spacecraft where heat needs to flow between dissimilar materials.”

The study, “Cu Nanospring Films for Advanced Nanothermal Interfaces,” appeared in Advanced Engineering Materials. It was co-authored by Dimitrios A. Antartis, Ryan N. Mott, Debashish Das, David Shaddock (GE Research), and Ioannis Chasiotis.

The study to understand the nanospring geometry, structure and properties relations entitled “Silicon nanosprings fabricated by glancing angle deposition for ultra-compliant films and interfaces,” appeared in Materials & Design. It was co-authored by Dimitrios A. Antartis, Ryan N. Mott, and Ioannis Chasiotis.

Antartis received a doctorate in aerospace engineering from the U of I and now works in Portland, Oregon for Intel. Mott received a master’s degree in aerospace engineering from Illinois and currently works for Boeing in St. Louis, Missouri.

The research was supported by the Air Force Office of Scientific Research, and DARPA.