New method found for controlling conductivity

Apr 29, 2011 by David L. Chandler
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

A 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.

“It’s a new way of changing and controlling the properties” of — in this case a class called percolated composite — 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 on April 19 and will appear in a forthcoming issue of Nature Communications. The paper’s 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.

The 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!”

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. But then, instead of needing to be reset, as the circuit cools down the resistance decreases and the circuit automatically resumes its function.

Graduate student Jianjian Wang holds a flask containing the suspension of graphite flakes in hexadecane, as Gang Chen looks on. Photo: Melanie Gonick

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 system’s much-improved in the solid state helps it transfer heat.

Essentially, 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.

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 percent 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 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, Chen says.

“Using phase change to control the of nanocomposites is a very clever idea,” says Li Shi, a professor of mechanical engineering at the University 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.

“I think this is a very crucial result,” says Joseph Heremans, professor of physics and of mechanical and aerospace engineering at Ohio State University. “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.”

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More information: Reversible temperature regulation of electrical and thermal conductivity using liquid–solid phase transitions, Nature Communications 2, Article number: 289 doi:10.1038/ncomms1288 www.nature.com/ncomms/journal/… full/ncomms1288.html

Abstract
Reversible temperature tuning of electrical and thermal conductivities of materials is of interest for many applications, including seasonal regulation of building temperature, thermal storage and sensors. Here we introduce a general strategy to achieve large contrasts in electrical and thermal conductivities using first-order phase transitions in percolated composite materials. Internal stress generated during a phase transition modulates the electrical and thermal contact resistances, leading to large contrasts in the electrical and thermal conductivities at the phase transition temperature. With graphite/hexadecane suspensions, the electrical conductivity changes 2 orders of magnitude and the thermal conductivity varies up to 3.2 times near 18 °C. The generality of the approach is also demonstrated in other materials such as graphite/water and carbon nanotube/hexadecane suspensions.

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Rdavid
1 / 5 (2) Apr 29, 2011
"Particles remain suspended indefinitely in the liquid, as was shown by examining a container of the mixture three months after mixing."

Three months equals indefinely?
bugmenot23
5 / 5 (3) Apr 29, 2011
Three months equals indefinely?


For all intents and purposes, Yes.
unknownorgin
3 / 5 (5) Apr 30, 2011
These guys do not get out much. There are already devices constructed of plastic and silver particles that open a circut when overloaded and then reconnect when it cools down.
Changing electrical resistance with temperture has been done for 50 years with a device called a thermister (no moving parts) . If you want a change in electrical conductivity VS temperture of factor over 100 there are chips for sale and simple transistor circuts that do this. There is a differance in thermal condutivity between solids and liquids ,No kidding this has been known for the last 100 years.
Megadeth312
4 / 5 (4) Apr 30, 2011
These guys do not get out much. There are already devices constructed of plastic and silver particles that open a circut when overloaded and then reconnect when it cools down.
Changing electrical resistance with temperture has been done for 50 years with a device called a thermister (no moving parts) . If you want a change in electrical conductivity VS temperture of factor over 100 there are chips for sale and simple transistor circuts that do this. There is a differance in thermal condutivity between solids and liquids ,No kidding this has been known for the last 100 years.


Nothing that you describe is anywhere near what this paper is about. Obviously this same feat can be accomplished in more complex ways, you're missing the point entirely.

Oh, and the following words are spelled wrong:

circut, temperture, thermister, differance, condutivity

If you want anyone to take you seriously, start with spell check, then maybe move on to knowing what your talking about.
unknownorgin
3 / 5 (7) Apr 30, 2011
Megadeth312 I think your spell check is malfunctioning and whatever you were smoking caused you to miss the point that the two researchers are reinventing the wheel because there are simple electronic componets manufatured in thier very own country that already do what they claim as new discovery. Spelling is secondary to conveying information and I dont care if you or anyone else is impressed I just call them like I see them. Every time someone gets some carbon atoms to do somthing they write a paper and act like they just brought the world out of the dark ages and it gets old after awhile. Oh and megadeth is okay but Korn and disturbed are better.
ubavontuba
1 / 5 (1) Apr 30, 2011
So before these guys came along, how did the temperature sensor, oxygen sensor, circuit breakers, and fuses in my car work? Is it magic?

Just because you invent a different method to accomplish something doesn't mean you invented a better, cheaper, faster, or otherwise more practical method.

Megadeth312
1 / 5 (1) May 01, 2011
"electronic componets"

Components

and since when do "componets" equal a single medium?

"I think your spell check is malfunctioning"

Circut - Circuit
temperture - Temperature
thermister - Thermistor
differance - Difference
condutivity - Conductivity

English a second language bro?

Put down the crack pipe, Mr. Sheen.
Megadeth312
2 / 5 (2) May 01, 2011
Just because you invent a different method to accomplish something doesn't mean you invented a better, cheaper, faster, or otherwise more practical method.


Nobody is arguing that, the point here is new methodology, nothing more. Who are you to say what this tiny bit of discovery will lead to?

I won't argue that the discovery may not mean much for science, but its a discovery nonetheless. Assuming anything is useless simply because there are other ways to do it is asinine.

In all honesty, these results speak more about the nature of matter then any new practical technology.

@unknownorgin

one more thing.

manufatured - Manufactured
thier - Their
somthing - Something

Its hard to communicate with someone who has no respect for language, clearly you have no respect for the information you want to convey. You can't spell every technical term you use wrong and think anyone will take it seriously, have you seen these words before?? Welcome to academia.
rawa1
not rated yet May 02, 2011
..A 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..
Did they reinvented thermistor?