IMAGINE how much energy we could save if the waste heat produced by the
countless machines we use could be converted into electricity. Well, it’s no
longer a dream. A microchip that can transform heat into electric current is now
working on a lab bench at the Massachusetts Institute of Technology. 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.
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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
“hDZ”.
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 aren’t 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.