[FoRK] The munchkins now proliferate nuclear
shri.fork at enablery.org
Mon Sep 27 23:37:56 PDT 2004
A few years ago, it was one of Rohit's posts to this list,
refering to the iPic as "munchkins", which had first
introduced me to F-of-RK.
Ubiquity is all fine and dandy to talk about, till we have
to go swapping batteries on them. What could be a more
fitting way to power these proliferating devices ... than
to go nuclear.
Thought folks here may find this interesting.
The Daintiest Dynamos
By harvesting energy from radioactive specks, nuclear microbatteries could
power tomorrow's microelectromechanical marvels?and maybe your cellphone, too
By Amit LaL & James Blanchard
0904nuc01.jpgFor several decades, electronic circuitry has been shrinking at a
famously dizzying pace. Too bad the batteries that typically power those
circuits have not managed to get much smaller at all.
In today's wrist-worn GPS receivers, matchbox-size digital cameras, and
pocketable personal computers, batteries are a significant portion of the
volume. And yet they don't provide nearly enough energy, conking out seemingly
at the worst possible moment.
The reason is simple: batteries are still little cans of chemicals. They
function in essentially the same way they did two centuries ago, when the
Italian physicist Alessandro Volta sandwiched zinc and silver disks to create
the first chemical battery, which he used to make a frog's leg kick.
Now, with technologists busily ushering in a new age of miniaturization based
on microelectromechanical systems (MEMS), batteries have arrived at a critical
juncture. MEMS are finding applications in everything from the sensors in cars
that trigger air bags to injectable drug delivery systems to environmental
monitoring devices. Many of these systems ideally have to work for long
periods, and it is not always easy to replace or recharge their batteries. So
to let these miniature machines really hit their stride, we'll need smaller,
longer-lasting power sources.
For several years our research groups at Cornell University and the University
of Wisconsin-Madison have been working on a way around this power-source
roadblock: harvesting the incredible amount of energy released naturally by
tiny bits of radioactive material.
The microscale generators we are developing are not nuclear reactors in
miniature, and they don't involve fission or fusion reactions. All energy comes
from high-energy particles spontaneously emitted by radioactive elements. These
devices, which we call nuclear microbatteries, use thin radioactive films that
pack in energy at densities thousands of times greater than those of
lithium-ion batteries [see table, "Energy Content"].
A speck of a radioisotope like nickel-63 or tritium, for example, contains
enough energy to power a MEMS device for decades, and to do it safely. The
particles these isotopes emit, unlike more energetic particles released by
other radioactive materials, are blocked by the layer of dead skin that covers
our bodies. They penetrate no more than 25 micrometers in most solids or
liquids, so in a battery they could safely be contained by a simple plastic
package [see sidebar, "Not All Radioisotopes Are Equal."]
Our current prototypes are still relatively big, but like the first transistors
they will get smaller, going from macro- to microscale devices. And if the
initial applications powering MEMS devices go well, along with the proper
packaging and safety considerations, lucrative uses in handheld devices could
be next. The small nuclear batteries may not be able to provide enough electric
current for a cellphone or a PDA, but our experiments so far suggest that
several of these nuclear units could be used to trickle charges into the
conventional chemical rechargeable batteries used in handheld devices.
Depending on the power consumption of these devices, this trickle charging
could enable batteries to go for months between recharges, rather than days, or
possibly even to avoid recharges altogether.
"IT IS A STAGGERINGLY SMALL WORLD THAT IS BELOW," said physicist Richard P.
Feynman in his famous 1959 talk to the American Physical Society, when he
envisioned that physical laws allowed for the fabrication of micro- and
nanomachines and that one day we would be able to write the entire
Encyclopaedia Britannica on the head of a pin.
Feynman's vision has finally begun to materialize, thanks to ever more
sophisticated microelectronics. Micro- and nanoscale machines are poised to
become a multibillion-dollar market as they are incorporated in all kinds of
electronic devices. Among the revolutionary applications in development are
ultradense memories capable of storing hundreds of gigabytes in a
fingernail-size device, micromirrors for enhanced displays and optical
communications equipment, and highly selective RF filters to reduce cellphone
size and improve the quality of calls.
But, again, at very small scales, chemical batteries can't provide enough juice
to power these micromachines. As you reduce the size of such a battery, the
amount of stored energy goes down exponentially. Reduce each side of a cubic
battery by a factor of 10 and you reduce the volume?and therefore the energy
you can store?by a factor of 1000. In fact, researchers developing sensors the
size of a grain of sand had to attach them to batteries they couldn't make
smaller than a shirt button.
IN THE QUEST TO BOOST MICROSCALE POWER GENERATION, several groups have turned
their efforts to well-known energy sources, namely hydrogen and hydrocarbon
fuels such as propane, methane, gasoline, and diesel. Some groups are
developing microfuel cells that, like their macroscale counterparts, consume
hydrogen to produce electricity. Others are developing on-chip combustion
engines, which actually burn a fuel like gasoline to drive a minuscule electric
There are three major challenges for these approaches. One is that these fuels
have relatively low energy densities, only about five to 10 times that of the
best lithium-ion batteries. Another is the need to keep replenishing the fuel
and eliminating byproducts. Finally, the packaging to contain the liquid fuel
makes it difficult to significantly scale down these tiny fuel cells and
The nuclear microbatteries we are developing won't require refueling or
recharging and will last as long as the half-life of the radioactive source, at
which point the power output will decrease by a factor of two. And even though
their efficiency in converting nuclear to electrical energy isn't high?about 4
percent for one of our prototypes?the extremely high energy density of the
radioactive materials makes it possible for these microbatteries to produce
relatively significant amounts of power.
For example, with 10 milligrams of polonium-210 (contained in about 1 cubic
millimeter), a nuclear microbattery could produce 50 milliwatts of electric
power for more than four months (the half-life of polonium-210 is 138 days).
With that level of power, it would be possible to run a simple microprocessor
and a handful of sensors for all those months.
And the conversion efficiency won't be stuck at 4 percent forever. Beginning
this past July we started working to boost the efficiency to 20 percent, as
part of a new Defense Advanced Research Projects Agency program called Radio
Isotope Micro-power Sources.
Space agencies such as NASA in the United States have long recognized the
extraordinary potential of radioactive materials for generating electricity.
NASA has been using radioisotope thermoelectric generators, or RTGs, since the
1960s in dozens of missions, like Voyager and, more recently, the Cassini
probe, now in orbit around Saturn. Space probes like these travel too far away
from the sun to power themselves with photovoltaic arrays.
RTGs convert heat into electricity through a process known as the Seebeck
effect: when you heat one end of a metal bar, electrons in this region will
have more thermal energy and flow to the other end, producing a voltage across
the bar. Most of NASA's washing-machine-size RTGs use plutonium-238, whose
high-energy radiation can produce enormous heat.
But as it turns out, RTGs don't scale down well. At the diminutive dimensions
of MEMS devices, the ratio between an object's surface and its volume gets very
high. This relatively large surface makes it difficult to sufficiently reduce
heat losses and maintain the temperatures necessary for RTGs to work. So we had
to find other ways of converting nuclear into electric energy.
ONE OF THE MICROBATTERIES WE DEVELOPED early last year directly converted the
high-energy particles emitted by a radioactive source into an electric current.
The device consisted of a small quantity of nickel-63 placed near an ordinary
silicon p-n junction?a diode, basically. As the nickel-63 decayed, it emitted
beta particles, which are high-energy electrons that spontaneously fly out of
the radioisotope's unstable nucleus. The emitted beta particles ionized the
diode's atoms, creating paired electrons and holes that are separated at the
vicinity of the p-n interface. These separated electrons and holes streamed
away from the junction, producing the current.
Nickel-63 is ideal for this application because its emitted beta particles
travel a maximum of 21 µm in silicon before disintegrating; if the particles
were more energetic, they would travel longer distances, thus escaping the
battery. The device we built was capable of producing about 3 nanowatts with
0.1 millicurie of nickel-63, a small amount of power but enough for
applications such as nanoelectronic memories and the simple processors for
environmental and battlefield sensors that some groups are currently
The new types of microbatteries we are working on now can generate
substantially more power. These units produce electricity indirectly, like
minute generators. Radiation from the sample is converted first to mechanical
energy and then to oscillating pulses of electric energy. Even though the
energy has to go through the intermediate, mechanical phase, the batteries are
no less efficient; they tap a significant fraction of the kinetic energy of the
emitted particles for conversion into mechanical energy. By releasing this
energy in brief pulses, they provide much more instantaneous power than the
For these batteries, which we call radioactive piezoelectric generators, the
radioactive source is a 4-square-millimeter thin film of nickel-63 [see
illustration, "Power From Within"]. On top of it, we cantilever a small
rectangular piece of silicon, its free end able to move up and down. As the
electrons fly from the radioactive source, they travel across the air gap and
hit the cantilever, charging it negatively. The source, which is positively
charged, then attracts the cantilever, bending it down.
A piece of piezoelectric material bonded to the top of the silicon cantilever
bends along with it. The mechanical stress of the bend unbalances the charge
distribution inside the piezoelectric crystal structure, producing a voltage in
electrodes attached to the top and bottom of the crystal.
After a brief period?whose length depends on the shape and material of the
cantilever and the initial size of the gap?the cantilever comes close enough to
the source to discharge the accumulated electrons by direct contact. The
discharge can also take place through tunneling or gas breakdown. At that
moment, electrons flow back to the source, and the electrostatic attractive
force vanishes. The cantilever then springs back and oscillates like a diving
board after a diver jumps, and the recurring mechanical deformation of the
piezoelectric plate produces a series of electric pulses.
The charge-discharge cycle of the cantilever repeats continuously, and the
resulting electric pulses can be rectified and smoothed to provide
direct-current electricity. Using this cantilever-based power source, we
recently built a self-powered light sensor [see photo, "It's Got the Power"].
The device contains a simple processor connected to a photodiode that detects
Nuclear batteries can pack in energy at densities THOUSANDS OF TIMES greater
than those of lithium-ion batteries
Also using the cantilever system, we developed a pressure sensor that works by
"sensing" the gas molecules in the gap between the cantilever and the source.
The higher the ambient pressure, the more gas molecules in the gap. As a
result, it is more difficult for electrons to reach and charge the cantilever.
Hence, by tracking changes in the cantilever's charging time, the sensor even
detects millipascal variations in a low-pressure environment like a vacuum
To get the measurements at a distance, we made the cantilever work as an
antenna and emit radio signals, which we could receive meters away?in this
application the little machine was "radio active" in more ways than one. The
cantilever, built from a material with a high dielectric constant, had metal
electrodes on its top and bottom. An electric field formed inside the
dielectric as the bottom electrode charged. When it discharged, a charge
imbalance appeared in the electrodes, making the electric field propagate along
the dielectric material. The cantilever thus acted like an antenna that
periodically emitted RF pulses, the interval between pulses varying accordingly
to the pressure.
What we'd like to do now is add a few transistors and other electronic
components to this system so that it can not only send simple pulses but also
modulate signals to carry information. That way, we could make MEMS-based
sensors that could communicate with each other wirelessly without requiring
complex, energy-demanding communications circuitry.
NUCLEAR MICROBATTERIES MAY ULTIMATELY CHANGE the way we power many electronic
devices. The prevalent power source paradigm is to have all components in a
device's circuitry drain energy from a single battery. Here's another idea:
give each component?sensor, actuator, microprocessor?its own nuclear
microbattery. In such a scheme, even if a main battery is still necessary for
more power-hungry components, it could be considerably smaller, and the
multiple nuclear microbatteries could run a device for months or years, rather
than days or hours.
One example is the RF filters in cellphones, which now take up a lot of space
in handsets. Researchers are developing MEMS-based RF filters with better
frequency selectivity that could improve the quality of calls and make
cellphones smaller. These MEMS filters, however, may require relatively high dc
voltages, and getting these from the main battery would require complicated
electronics. Instead, a nuclear microbattery designed to generate the required
voltage?in the range of 10 to 100 volts?could power the filter directly and
Another application might be to forgo the electrical conversion altogether and
simply use the mechanical energy. For example, researchers could use the motion
of a cantilever-based system to drive MEMS engines, pumps, and other mechanical
devices. A self-powered actuator could be used, for instance, to move the legs
of a microscopic robot. The actuator's motion?and the robot's tiny steps?would
be adjusted according to the charge-discharge period of the cantilever and
could vary from hundreds of times every second to once per hour, or even once
THE FUTURE OF NUCLEAR MICROBATTERIES depends on several factors, such as
safety, efficiency, and cost. If we keep the amount of radioactive material in
the devices small, they emit so little radiation that they can be safe with
only simple packaging. At the same time, we have to find ways of increasing the
amount of energy that nuclear microbatteries can produce, especially as the
conversion efficiency begins approaching our targeted 20 percent. One
possibility for improving the cantilever-based system would be to scale up the
number of cantilevers by placing several of them horizontally, side by side. In
fact, we are already developing an array about the size of a postage stamp
containing a million cantilevers. These arrays could then be stacked to achieve
even greater integration.
Another major challenge is to have inexpensive radioisotope power supplies that
can be easily integrated into electronic devices. For example, in our
experimental systems we have been using 1 millicurie of nickel-63, which costs
about US $25?too much for use in a mass-produced device. A potentially cheaper
alternative would be tritium, which some nuclear reactors produce in huge
quantities as a byproduct. There's no reason that the amount of tritium needed
for a microbattery couldn't cost just a few cents.
Once these challenges are overcome, a promising use for nuclear microbatteries
would be in handheld devices like cellphones and PDAs. As mentioned above, the
nuclear units could trickle charge into conventional batteries. Our
one-cantilever system generated pulses with a peak power of 100 milliwatts;
with many more cantilevers, and by using the energy of pulses over periods of
hours, a nuclear battery would be able to inject a significant amount of
current into the handheld's battery.
How much that current could increase the device's operation time depends on
many factors. For a cellphone used for hours every day or for a power-hungry
PDA, the nuclear energy boost won't help much. But for a cellphone used two or
three times a day for a few minutes, it could mean the difference between
recharging the phone every week or so and recharging it once a month. And for a
simple PDA used mainly for checking schedules and phone numbers, the energy
boost might keep the batteries perpetually charged for as long as the nuclear
Nuclear microbatteries won't replace chemical batteries. But they're going to
power a whole new range of gadgetry, from nanorobots to wireless sensors.
Feynman's "staggeringly small world" awaits.
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