Tiny nuclear power sources

[John Jacobus <crispy_bird@YAHOO.COM>]

I saw this through another list server, and thought it

would be of interest.  It is a long article, so here

is the link: 

http://www.spectrum.ieee.org/WEBONLY/publicfeature/sep04/0904nuc.html



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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 



For 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 generator. 



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 generators. 



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

developing. 



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

direct-conversion approach. 



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 light variations. 



-------------------------

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 chamber. 



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 more efficiently. 



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 per day. 



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 material lasts. 



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. 



-------------------



=====

+++++++++++++++++++

"Everyone is ignorant, only on different subjects."

Will Rogers



-- John

John Jacobus, MS

Certified Health Physicist

e-mail:  crispy_bird@yahoo.com





		

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