This is quite clever work andwill be quickly adapted to a variety of materials for a variety ofapplications. Right now we are justbeginning to grasp the scope of this methods potential, but it is sufficient tosay we have another type of switch here which is not obviously over precise andcan be readily applied to circumstances not needing such precision.
I suspect that this technologywill soon become commonly applied as we get some experience with it. Imagine just how much one could use this toaffect the performance of steel.
It will mostly be applied toexotic materials in the early days and perhaps in something as prosaic as thefuse box were a simple metal fork would be rather welcome. In fact having a conductor that can beinserted anywhere and counted on to cut of current on overheating would behugely efficient and put the whole difficulty out of mind. Systems could then be engineered to acceptonly a certain load before merely choking of supply.
Today the fuse component is amajor part of the overall circuit design in terms of hardware. Replacing it all with simple cable insertsthat did the same thing is a much better option.
New method found for controlling conductivity
by David L. Chandler
MIT News Office
An artistic rendering of the suspension as it freezes shows graphiteflakes clumping together to form a connected network (dark spiky shapes atcenter), as they are pushed into place by the crystals that form as the liquidhexadecane surrounding them begins to freeze. Image: Jonathan Tong
A team of researchers at MIT has found a way to manipulate both the thermalconductivity and the electrical conductivityof materials simply by changing the external conditions, such as thesurrounding temperature. And the technique they found can change electricalconductivity by factors of well over 100, and heat conductivity by more thanthreefold.
"It's a new way of changing and controlling the properties"of materials - in this case a class called percolated composite materials - bycontrolling their temperature, says Gang Chen, MIT's Carl Richard SoderbergProfessor of Power Engineering and director of the Pappalardo Micro and NanoEngineering Laboratories. Chen is the senior author of a paper describing theprocess that was published online on April 19 and will appear in a forthcomingissue of Nature Communications.
The paper's lead authors are former MIT visiting scholars Ruiting Zhengof Beijing Normal University and Jinwei Gao of South China Normal University
The system Chen and his colleagues developed could be applied tomany different materials for either thermal or electrical applications. Thefinding is so novel, Chen says, that the researchers hope some of their peerswill respond with an immediate, "I have a use for that!"
One potential use of the new system, Chen explains, is for a fuse toprotect electronic circuitry. In that application, the material would conduct electricitywith little resistance under normal, room-temperature conditions.But if the circuit begins to heat up, that heat would increase the material'sresistance, 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 downthe resistance decreases and the circuit automatically resumes its function.
Another possible application is for storing heat, such as from a solarthermal collector system, later using it to heat water or homes or to generateelectricity. The system's much-improved thermal conductivity in the solid statehelps it transfer heat.
Essentially, what the researchers did was suspend tiny flakes of onematerial in a liquid that, like water, forms crystals as it solidifies. Fortheir initial experiments, they used flakes of graphite suspended in liquidhexadecane, but they showed the generality of their process by demonstratingthe control of conductivity in other combinations of materials as well.
The liquid used in this research has a melting point close to roomtemperature - advantageous for operations near ambient conditions - but theprinciple should be applicable for high-temperature use as well.
The process works because when the liquid freezes, the pressure ofits forming crystal structure pushesthe floating particles into closer contact, increasing their electrical andthermal conductance. When it melts, that pressure is relieved and theconductivity goes down.
In their experiments, the researchers used a suspension thatcontained just 0.2 percent graphite flakes by volume. Such suspensions areremarkably stable: Particles remain suspended indefinitely in the liquid, aswas shown by examining a container of the mixture three months after mixing.
By selecting different fluids and different materials suspended withinthat liquid, the critical temperature at which the change takes place can beadjusted at will, Chen says.
"Using phase change to control the conductivity of nanocompositesis a very clever idea," says Li Shi, a professor of mechanical engineeringat the University of Texas at Austin
"I think this is a very crucial result," says JosephHeremans, professor of physics and of mechanical and aerospace engineering at Ohio State University
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