ResoSOL: Réseau sol(ID)aire des énergies ! ENERGIES RENOUVELABLES
Microchip can turn heat into electricity

19:00 30 January 02
Duncan Graham-Rowe
    A microchip that can transform heat into electric current is now working on a lab bench at the Massachusetts Institute of Technology, US. Its inventors say it could harness heat from a car's engine and provide power for its electronics, charge laptop batteries by recycling heat from the computer's microprocessor, or simply bask in the baking desert sun generating electricity.
    The device may be clever, but it has a decidedly unprepossessing name: a thermal diode. Nonetheless, it marks an important step in thermal electronics - or thermionics - which has seen little innovation since the inventor Thomas Edison first observed the thermionic effect in 1883.
    In a thermionic vacuum tube, an electrically heated electrode "boils" off free electrons, which jump across a gap, drawn by a voltage applied to another electrode. But there's another type of vacuum tube that doesn't need to have electricity fed into it. Instead, it generates electricity - albeit very inefficiently - by using heat from the environment.
    The heat gives a few electrons enough kinetic energy to boil off and jump a tiny gap, creating a minuscule electric current. But the temperatures needed to generate this current are very high--around 1000 °C. "They are not very efficient, and tend to be expensive," says Gao Min at the University of Cardiff. As a result thermal diodes have found only limited applications, such as making electricity from nuclear sources in space probes or satellites.
Better idea
    Attempts to make semiconductor versions of these devices have always been foiled by the technical difficulties of creating a very narrow vacuum gap between chip layers. But Peter Hagelstein, a physicist at MIT, and Yan Kucherov at energy conversion start-up ENECO in Salt Lake City, Utah, have a better idea. Their first attempt at a semiconductor version of a thermal diode operates at the comparatively low temperature of 200 °C.
    Hagelstein and Kucherov's big idea is to replace the vacuum gap with layers of an electron-rich semiconducting material. They found that this significantly boosted the current generated.
    In an experiment funded by the US military, they used an indium antimonide-based semiconductor. This comprised an electron "emitter" doped with electron-donating impurities to give a surplus of electrons. On the opposite side, they placed an electron "collector" doped with electron-deficient impurities. These have lots of missing electrons, effectively creating positive "holes".
Highly doped
    By placing an additional highly doped electron-rich layer between the emitter and collector, the team got more electrons to traverse the gap, though they are not quite sure how yet. "The details of the mechanism are still under discussion," says Hagelstein.
    They speculate that high-energy electrons reaching the electron-rich layer cause a scattering "chain reaction" whose overall effect is to turn more of the heat into current.
    The pair are now refining their device to see if it can work at even lower temperatures. But their major challenge will be making them affordable, says Min.