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. Image: Jonathan Tong
team of researchers at MIT has found a way to manipulate both the thermal
conductivity and the electrical conductivity of materials simply by changing
the external conditions, such as the surrounding temperature. And the technique
they found can change electrical conductivity by factors of well over 100, and
heat conductivity by more than threefold.
a new way of changing and controlling the properties” of materials—in this case
a class called percolated composite materials—by controlling their temperature,
says Gang Chen, MIT’s Carl Richard Soderberg Professor of Power Engineering and
director of the Pappalardo Micro and Nano Engineering Laboratories. Chen is the
senior author of a paper describing the process that was published online in Nature
Communications. The paper’s lead authors are former MIT visiting
scholars Ruiting Zheng of Beijing Normal Univ. and Jinwei Gao of South China
Normal Univ., along with current MIT graduate student Jianjian Wang. The
research was partly supported by grants from the National Science Foundation.
system Chen and his colleagues developed could be applied to many different
materials for either thermal or electrical applications. The finding is so
novel, Chen says, that the researchers hope some of their peers will respond
with an immediate, “I have a use for that!”
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. But then, instead of needing to be reset, as the
circuit cools down the resistance decreases and the circuit automatically
resumes its function.
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 system’s much-improved thermal conductivity in the solid state
helps it transfer heat.
Graduate student Jianjian Wang holds a flask containing the suspension of graphite flakes in hexadecane, as Gang Chen looks on. Photo: Melanie Gonick
what the researchers did was suspend 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—advantageous for
operations near ambient conditions—but the principle should be applicable for
high-temperature use as well.
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. Such
suspensions are remarkably stable: Particles remain suspended indefinitely in
the liquid, as was shown by examining a container of the mixture three months
selecting different fluids and different materials suspended within that
liquid, the critical temperature at which the change takes place can be
adjusted at will, Chen says.
phase change to control the conductivity of nanocomposites is a very clever
idea,” says Li Shi, a professor of mechanical engineering at the Univ. of Texas
at Austin. Shi
adds that as far as he knows “this is the first report of this novel approach”
to producing such a reversible system.
think this is a very crucial result,” says Joseph Heremans, professor of
physics and of mechanical and aerospace engineering at Ohio State Univ. “Heat
switches exist,” but involve separate parts made of different materials,
whereas “here we have a system with no macroscopic moving parts,” he says. “This is excellent work.”