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Professor Peidong Yang led the University of California, Berkley team who discovered silicon nanowire's thermoelectric properties.  (Source: University of California, Berkley)

A scanning electron microscope image of a thin silicon nanowire stretching between two heating pads, one as a heat source, the other as a sensor.  (Source: A. Hochbaum)
Silicon heat capture could allow cheap refrigeration and energy saving

Among the many valuable properties of silicon is its ability to capture solar energy to create electricity via its photoelectric character.  Now scientists are discovering that silicon, when properly prepared, can form a very good thermoelectric as well.  This opens the door to a plethora of uses, including refrigeration, solar heat power generation, and power generation from other heat sources, such as computer waste heat or car heat.

A thermoelectric device has two  basic modes of operation.  When a thermoelectric is placed over a heat gradient, it generates an electric current.  The other mode is the reverse; when a thermoelectric is exposed to an electric current, it creates a heat difference, cooling one side of it, and warming the other side.  Thus thermoelectrics are applicable to power generation, refrigeration and heating.

Traditional thermoelectrics, which have been around since the 1960s, rely on either bismuth telluride or lead telluride.  These materials are relatively expensive due to scarcity and lack of a large manufacturing infrastructure.  They are also bulky and require more material, which further increaese their cost.  While thermoelectric coolers have achieved modest commercial usage in seat coolers and picnic coolers, they have yet to realize their full potential.

A new breakthrough may change that.  Professor Peidong Yang and his colleagues at the University of California, Berkley published in last week's Nature journal the results of years of research into using silicon as thermoelectrics.  Their results show that silicon can be a viable thermoelectric.

The key is in the preparation.  The researchers prepared thin nanowires of silicon.  When these wires are exposed to a temperature difference, they generate electricity.  Standard silicon is a poor thermoelectric, but according to Dr. Yang, "the performance of the nanowires is already comparable to the best existing thermoelectric material."

A good thermoelectric needs to have two key properties -- good electrical conduction, and poor heat conduction.  Silicon typically conducts both very well, but by producing 50 nm nanowires, the heat conduction of silicon is reduced to one hundreth of its normal levels, while electrical conduction remains unchanged.  The material is comparable to commercial thermoelectrics.

Two possible uses of the technology are to generate electricity from waste heat of car engines.  Current thermoelectrics are too expensive and large to make this a practical possibility.  Nanowire silicon layers, though could provide a means to recapture some of the energy lost to heat during the conversion to mechanical energy in a car engine.  This extra savings could be stored in batteries, to give next generation electric vehicles, such as the Chevrolet Volt, even better efficiency.

It could also find a home in solar power cells.  By coupling it with traditional photoelectric cells, much higher efficiencies could possibly be reached.  Yet another application is to put the materials in computers to provide energy savings, which would be particularly valuable to mobile computing.  Further, it could be used in refrigeration applications, as well.

Much work needs to be done before the process is perfected.  The physics behind why nanowires of silicon lose their heat conduction is not understood, which stands in the way of refining the efficiency of this class of devices.  Further creating a thermoelectric on the macroscopic scale, by creating a network of nanowires, has yet to be accomplished.  Still, the discovery of these properties in silicon promise a way to eventually use replace current less ideal thermoelectrics with an abundant material with a large processing infrastructure.

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RE: Not that universal
By MrTeal on 1/22/2008 1:25:47 AM , Rating: 3
Let's suppose we have a CPU that is cooled by water, so a high wattage of heat dissipated by the CPU equals a low temperature increase in the water. But instead of some thermal compound, let's stick a piece of cardboard between the CPU and the waterblock. Our CPU when turned off is at ambient. We turn it on, and within a second the CPU temp rises by some wattage, which equals 5 degrees. Now there is a differential of 5 degrees between it and the water. A differential of 5 degrees means 1 degree's worth of heat wattage can leak past the cardboard and into the water, which for the purposes of this remains the same. The CPU has been coolde by 1 degree so it is 4 degrees above ambient. In the next second that the CPU is on it again rises by 5 degrees worth of heat wattage. This time the differential is 9, and therefore 1.8 degrees can leak over. It's at 7.2, we add 5, and a 12.2 differential means we dissipate 2.44 degrees. And so on until the CPU is 25 degrees hotter than the ambient water temperature and now the CPU dissipates as much heat into the water as it generates.

Ok, I think here's where you're getting it confused a little. In the simple cardboard example you have, the cardboard will have a thermal conductivity, k (kappa), given in W/(m*K). The thermal resistance (theta) is = length/(k*Area). So, in this case, say the cardboard is 1mm thick (l) and has a 4cm^2 area. The whole thing has a thermal resistance of 10K/W, so for every watt of power it dissipates, the temperature will rise 10 Kelvin (or degrees C).

There will be some delay involved due to the (small) thermal mass of the CPU die, but assuming that the CPU die has perfect contact with the top of the package, and the waterblock stays at ambient the whole time (ie, it has no thermal resistance), the temperature of the CPU die will be...
Tcpu = 20C + 10(K/W)*P, where P = power dissipated and T is in celcius. So, if your CPU is dissipating 40 watts, with a crappy piece of cardboard in between it and the waterblock it'll eventually reach an equilibrium, at 420C. Ouch.

For reference here, when you're insulating a transistor in a power amplifier or some such thing, you often have to electrically isolate the back of the transistor from the heatsink, since the transistor might be hooked to a voltage and you don't want to zap someone. One of the best materials was beryllium oxide, until they stopped using it on account of the cancer and all. It had a thermal conductivity of about 0.25K/W. With just one of those crappy elastomer pads you see on heatsinks, it's about 0.1K/W. Hence why people buy the fancy thermal grease.

Let's do the same thing with a regular silicone nanowire TEC with poor heat conductivity. The CPU turns on and it raises 5 degrees. 1 degree of heat on the CPU side leaks out so it's again at +4. Now here comes the tricky part - where it leaks out to - because I don't understand the seebeck effect. One of 2 things could happen.

If you think of it as a heat engine, you have power going in from the CPU, say 100W. You have electrical power out, and being exceedingly generous I'll say 10%, so 10W. The other 90W are heat that goes to the water block. You would still be dissipating 90W, and even if the element had a resistance of 1K/W, you'd still be looking at a 100C cpu once it stabilized. All that for 10 lousy watts of power.

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