SOLAR CLUB du CERN
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.
