May 4, 2011 — A team of researchers at MIT has found a way to manipulate both the thermal and electrical conductivity of materials by changing the external conditions. The technique can change electrical conductivity by factors of well over 100, and heat conductivity by more than threefold.

Researchers used percolated composite materials and manipulated them by controlling their temperature.

Image. An artistic rendering of the suspension as it freezes shows graphite flakes clumping together to form a connected network (dark spiky shapes at center), as they are pushed into place by the crystals that form as the liquid hexadecane surrounding them begins to freeze. SOURCE: Jonathan Tong.

The researchers suspended tiny flakes of one material in a liquid that, like water, forms crystals as it solidifies. For their initial experiments, they used flakes of graphite suspended in liquid hexadecane, but they showed the generality of their process by demonstrating the control of conductivity in other combinations of materials as well. The liquid used in this research has a melting point close to room temperature but the principle should be applicable for high-temperature use as well.

The process works because when the liquid freezes, the pressure of its forming crystal structure pushes the floating particles into closer contact, increasing their electrical and thermal conductance. When it melts, that pressure is relieved and the conductivity goes down. In their experiments, the researchers used a suspension that contained just 0.2% graphite flakes by volume. Particles remain suspended indefinitely in the liquid, as was shown by examining a container of the mixture three months after mixing.

By selecting different fluids and different materials suspended within that liquid, the critical temperature at which the change takes place can be adjusted at will, said Gang Chen, MIT’s Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories.

The system that Chen and his colleagues developed could be applied to many different materials for either thermal or electrical applications. One potential use of the new system, Chen explains, is for a fuse to protect electronic circuitry. In that application, the material would conduct electricity with little resistance under normal, room-temperature conditions. But if the circuit begins to heat up, that heat would increase the material’s resistance, until at some threshold temperature it essentially blocks the flow, acting like a blown fuse. Instead of needing to be reset, as the circuit cools down the resistance decreases and the circuit automatically resumes its function. Heat switches exist, but involve separate parts made of different materials, whereas this system has no macroscopic moving parts, says Joseph Heremans, professor of physics and of mechanical and aerospace engineering at Ohio State University.

Another possible application is for storing heat, such as from a solar thermal collector system, later using it to heat water or homes or to generate electricity. The systems much-improved thermal conductivity in the solid state helps it transfer heat.

"Using phase change to control the conductivity of nanocomposites is a very clever idea," says Li Shi, a professor of mechanical engineering at the University of Texas at Austin. MIT is interested in developing other applications for the process now.

Gang Chen, MIT’s Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories, is the senior author of a paper describing the process that was published online on April 19 http://www.nature.com/ncomms/journal/v2/n4/full/ncomms1288.html and will appear in a forthcoming issue of Nature Communications. Lead authors are former MIT visiting scholars Ruiting Zheng of Beijing Normal University and Jinwei Gao of South China Normal University, along with current MIT graduate student Jianjian Wang.

The research was partly supported by grants from the National Science Foundation.

Learn more at http://web.mit.edu/

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