MIT Scientists Make a Polyethylene Heatsink 153
arcticstoat calls our attention to MIT research that has produced a version of polyethylene that can conduct heat away from computer chips. Polyethylene is the most widely used plastic. It's not clear how practical this research is for industrial-scale use, involving as it does an atomic-force microscope. The work is detailed in a paper published in Nature Nanotechnology this month. "The new process causes the polymer to conduct heat very efficiently in just one direction, unlike metals, which conduct equally well in all directions. ... The key to the transformation was getting all the polymer molecules to line up the same way, rather than forming a chaotic tangled mass, as they normally do. The team did that by slowly drawing a polyethylene fiber out of a solution, using the finely controllable cantilever of an atomic-force microscope, which they also used to measure the properties of the resulting fiber. This fiber was about 300 times more thermally conductive than normal polyethylene along the direction of the individual fibers, says the team’s leader..."
Re:Article is wrong. (Score:5, Informative)
"The new process causes the polymer to conduct heat very efficiently in just one direction,"
I was thinking, wow, is this even possible? If this is true, I think they've just created a material that could behave like a passive air-conditioner, heater, refridgerator, etc., while using NO power, ever. That alone must be breaking some serious laws of thermodynamics..
"One dimension" or "one axis," would have been more appropriate than "one direction."
Dimension, Not Direction (Score:3, Informative)
I think they mean in one dimension, not direction. The plastic will conduct heat longitudinally a lot better than laterally, but it will conduct heat longitudinally equally well both to and fro. If they ever come up with a material that only conducts heat in one direction (a thermal "diode", if you will) then that solves our energy woes.
Thermal conductivity (Score:5, Informative)
Since neither the summary nor the article has been kind enough to expand on "300 times more thermally conductive than normal polyethylene", I figured I'd look it up.
Thermal Conductivity of some common Materials: [engineeringtoolbox.com]
Polyethylene HD: 0.42 - 0.51 W/mK
Aluminium: 250W/mK
Copper: 401 W/mK
Best case scenario: 153 W/mK or 61% as conductive as aluminium, 38% as conductive as copper. Not exactly impressive for a heat sink
Re:Article is wrong. (Score:3, Informative)
This definition of "direction" may seem counterintuive at first, but it's really quite useful, because in physics you often encounter things which have to do with a direction in this sense (as is the case with this new material). That also explains why the writers of the article used this definition.
Note that you cannot simply use "dimension" in its stead. There are three dimensions in three-dimensional space (by definition), but infinite directions, so it's clear that they cannot be the same thing.
Re:Article is wrong. (Score:4, Informative)
Yes, I was really intrigued and confused, after reading the line: "The new process causes the polymer to conduct heat very efficiently in just one direction,"
You said:
You connect a heat source to the bottom of a water tank, as it heats water on the bottom, the density of water in vicinity decreases and flow upward in one direction.
He was talking about conduction. You're talking about convection.
Re:What about therm interface Re:Thermal conductiv (Score:4, Informative)
Re:Can't it degrade over time? (Score:2, Informative)
That phrase you're looking for is "let loose the dogs of war".
The verb in that quote is "to let loose", not "to loose".
Re:Article is wrong. (Score:3, Informative)
Just for argument's sake, the jury is still out on 'thermal rectification'. The key is just that you can't ignore certain parts of entropy generation that will exist in such a device. Here's an abstract link from a young professor at UC-Riverside, currently getting a DARPA Young Investigator Award.
Solid-State Thermal Rectification With Existing Bulk Materials
http://dx.doi.org/10.1115/1.3089552 [doi.org]
As long as the system results in a net entropy increase, some versions of the theory say its possible.
Re:Thermal conductivity (Score:5, Informative)
don't forget the rest, though:
Density:
copper: 8.96g/cm3
aluminum: 2.7 g/cm3
silicon: 2.33 g/cm3
AluminumNitride (high thermal conductivity insulating ceramic, k~160to190W/mK): 3.33g/cm3
LDPE and HDPE: 0.92-0.97 g/cm3.
So, you're getting a factor of 2-10x in weight savings. Tell that to a aerospace designer and he'll make it work. It's also a cheap material (well, feedstock's cheap. and normal PE is cheap, especially relative to copper these days). Who knows how expensive this stuff might be if they can make more than single fibers.
Re:Competitive, but still not better than (Score:3, Informative)
and between 1/6 and 1/15 that of diamond (900–2,320 W/mK)
A very apt comparison since I only use diamond heat sinks for my gaming machines.
Diamond is widely considered to be one of if not the most thermally conductive material available. This comparison was included because for those familiar, it is a handy reference. It was as if, because your arms are too short to touch the ceiling, you believe it doesn't matter how high it is.
On a side note, there actually IS diamond thermal paste available for sale [innovationcooling.com]! Huzzah.
The ability to direct the heat flow can make up for a somewhat lower conductivity for many applications, and can also allow for layouts and applications which wouldn't work with metal heat sinks.
Since the primary issue with metal heat sinks is generally getting the heat wicked off of them, I'd be more apt to consider Finite Element based Thermal Analysis when designing my heat sinks, instead of considering a non isothermal material. That is to say, the problem probably isn't with your material, it's probably with your airflow.
Re:Article is wrong. (Score:3, Informative)
The little spiny thingy is a perfectly "normal" heat engine in that it exploits a thermal heat difference that is created by an influx of energy from an external source i.e. the photons that heat up the dark side.
The difference with a ideal uni-directional heat conductor is that it allows to create the heat imbalance out of thin air i.e. without putting in additional energy the entropy of the system is lowered. The wikipedia article that I linked to explains this in a bit more detail:
A perpetual motion machine of the second kind is a machine which spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However it does violate the more subtle second law of thermodynamics (see also entropy). Such a machine is different from real heat engines (such as car engines), which always involve a transfer of heat from a hotter reservoir to a colder one, the latter being warmed up in the process. The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, as stated by the second law of thermodynamics. In contrast, a hot reservoir inside an internal combustion engine is created by a spark igniting fumes which contain stores of chemical potential energy. The temperature of the fumes increases above that of the surroundings. This is not a perpetual motion machine since the increase in temperature is a result of the release of a finite available amount of chemical energy, which is always much less than the total heat energy and mass-energy contained within the system. As explained by statistical mechanics, there are far more states in which heat distribution is close to thermodynamic equilibrium than states in which heat is concentrated in small regions, so temperatures will tend to even out over time, reducing the amount of free energy available for conversion to mechanical energy.
Re:Heat Diode (Score:4, Informative)
No, they did not.
They built a material that allows heat to flow along one axis. It can go either way through it, but only in that one dimension.