NASA's latest rover on Mars depends on a sandwich of semiconducting material that can turn heat into electricity. In the case of Curiosity, the steady radioactive decay of plutonium 238 warms such thermoelectric material and turns roughly 4 percent of that heat into a steady flow of electrons. A similar radioisotope thermoelectric generator (RTG) on the moon's Sea of Tranquility is still working after decades, as are the RTGs in the two Voyager spacecraft launched 35 years ago; such enduring reliability is the main reason NASA employed the inefficient technology. Now researchers have discovered a way to at least double the efficiency of such power generators—suggesting that thermoelectrics might find a home in applications outside of aerospace and back here on Earth.

The most common core of new and old thermoelectrics is a compound called lead telluride. When exposed to heat on only one side—whether it be from a radioactive isotope or another source—it will induce an electric current as long as the temperature differential is maintained. The challenge of improving thermoelectrics has been to keep heat from transferring across the material without also interfering with its ability to conduct electricity.

Chemist Mercouri Kanatzidis of Northwestern University and his colleagues report in Nature on September 20 that by precisely engineering the material from the atomic to the individual grain scale, the thermal conductivity of lead telluride can be impeded without affecting its electrical conductivity. The result is a material that can convert at least 8 percent of the heat into electricity—and could theoretically convert as much as 20 percent. (Scientific American is part of Nature Publishing Group.)

The researchers first melted the lead telluride and then froze it, creating nanoscale crystalline structures out of the atoms. These precisely oriented nanostructures scatter the medium wavelength vibrations, or phonons, that carry heat while allowing electrons to pass unobstructed.

But longer wavelength phonons continue to pass through as well, because their wavelengths are longer than the size of the nanostructures. So Kanatzidis and his colleagues went further, grinding the nanostructured material into powder. The powder was then subjected to spark plasma sintering—squeezing the powder while also passing "a very large amount of [electrical] current," in the words of Kanatzidis, through it briefly—to consolidate the grains into a larger block. Because the sintering occurs so quickly, the material does not melt but does compact, making it hard enough to be cut and manufactured into the core of a thermoelectric device. And these grains then effectively block the longer wavelength heat while still allowing electricity to flow.

This combination of what Kanatzidis calls "panoscopic" processes results in a lead telluride material that is more than twice as efficient at converting heat into electricity at high temperatures. "It's pretty significant and it makes the whole thing smaller," Kanatzidis notes.

7 Comments

1. 1. SigmaEyes 08:04 PM 9/20/12

   I presume its not practical or even possible, but imagine if they could incorporate this technology into solar PV panels. Ridding the existing PV panels of heat raises their efficiency. By sinking that heat into such a thermo-electric surface and extracting an additional 4% of that heat, I wonder if they could increase efficiencies by a total of 3 or 4 percent.
   
   Of course it would probably raise the cost of each panel more than what could be justified by the additional output. But I can still dream...
   
   I wonder if the "extreme heat" required is 400 degrees F or 1200 degrees, or what. I guess it would have to be less than the melting point of the lead.
   
   I wonder if the effect is affected by water, moisture, or vapor, such as would be encountered by rain.
   
   If it simply requires a temperature differential like a common thermocouple does, imagine every coastal community with cool sea water cooling one side and the sun and a magnifying lens on the other. lol- it would probably take a mall rooftop sized apparatus to make enough power for one household.
   
   If they get this up to 15 to 20% efficiency, perhaps it would give PV pannels a run for the future in renewable energy production. Glad there are people working on this kind of research and development.
   
   It does raise one's curiosity. Great subject for your article, David Biello.

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2. 2. R.Blakely 05:09 AM 9/21/12

   There is really a lack of "precision science" in current thermoelectric devices that convert heat into electricity. For example, in general, a solar cell is a well-engineered triple layer device that converts photons into energetic electrons, and it incorporate ohmic contacts, but thermoelectric cells, based upon flawed science, are used in opposing pairs of two-layer devices to overcome the "ohmic contact problem". In reality, solar cells and thermoelectric cells should be similar since they both harness energetic electrons, which are generated by photons or phonons. Solar cells do not use a heat sink. Similarly, thermoelectric cells should be designed to operate without a heat sink.

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3. 3. hotvocals 08:48 AM 9/21/12

   Phonons, huh? Oh brother....a little low on the scale there aren't ya? The resonance of a group of constituents is a far cry from the release of hadron particle bonding energy. 4 to 8%? That's less secondary energy than a battery powered guitar amplifier's speaker can put out, and on a tiny bayyery for 2 hrs yet....resonent vibrations are better off in your microwave oven...at least it don't take much to exite a dipole into a higher efficiency than that of a calrod oven....

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4. 4. Tony_Who 10:21 AM 9/21/12

   "Entropy is created in volumes, and reduced by surfaces."
   
   Is this true as a general statement?
   
   Consider this at the nano scale compared to the macro scale, where the surface area to volume ratios are much, much greater.
   
   Thanks,
   -Tony
   

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5. 5. jerryd 05:43 PM 9/21/12

   
   they'd get far more output either with Infer Red PV cells or a Rankine/steam engine, both doing about 20% eff in that size.
   
   And when one is so ineff you need much larger radiators to get rid of the waste heat, thus extra weight.

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6. 6. Feliks 03:10 PM 9/27/12

   I am now 15 years deals with this issue. Very promising experience mam.Mozna utilize high efficiency of the temperature difference between the ground (say 2 m below the surface) and the exterior air. And now, the bigger the frost on the surface, the more enegi thus produced. Lead is toxic, but the batteries for cars on doing ..

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7. 7. kienhua68 06:29 PM 9/29/12

   All this talk about efficiency of converting one energy form to another. Yet for as many years as automobile engines have been in existence, they only achieve 25-30% efficiency.
   For each gallon used only about one quart does work. Heat and friction along with the engine design consume the rest.
   If its going to take another 50yrs to get alternative power
   up and running we might do better with more efficient transportation while we wait. Public transport to reduce
   the billions of gallons of wasted fuel in traffic jams.
   The country has not seen an infrastructure upgrade since the 50's or 60's.
   

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