bateria nuclear

bateria nuclear

(Parte 2 de 2)

40IEEE Spectrum |September 2004 |NA

Polonium-210 in a nuclear battery**57 0

ENERGY DENSITY Lithium-ion in a chemical battery 0.3 Methanol in a fuel cell*3 Tritium in a nuclear battery**850

*Assuming 50 percent efficiency **Assuming 8 percent efficiency and 4 years of operation

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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 altogetherand 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 chargedischarge 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 MICROBATTERIESdepends 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 20percent. 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 1millicurie 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.

Nuclear microbatteries contain only small amounts of radioactive material, but safety is nonetheless a crucial issue. It is important first to note that not all radioisotopes are alike. The level of radioactivity depends on the type and amount of the radioisotope.

Radioisotopes are unstable atoms that spontaneously emit high-energy particles as they decay to a more stable state. Most emit gamma rays, which are essentially high-energy X-rays that can penetrate most materials, including human flesh. But other radioisotopes emit alpha particles (an aggregate of two protons and two neutrons) and beta particles (high-energy electrons) that can’t penetrate as deeply and therefore pose less risk.

The nuclear microbatteries we are developing contain 1to 10 millicuries of nickel-63 or tritium, whose beta particles have relatively low energy and can be blocked by a layer of 25to100 micrometers of plastic, metal, or semiconductor; they are also blocked by the thin dead-skin layer covering our bodies.

Other than shielding considerations, safety concerns also involve the possibility of a release of the radioisotope into the environment and subsequent inhalation or ingestion.

Again, by limiting the amount of radioisotope and by using the proper packaging, it is possible to ensure that such nuclear microbatteries offer minimal risk to the public.

In fact, radioisotopes have been used for decades in commercial applications. Many smoke detectors contain 1to5 microcuries of americium-241, used to ionize the air between a pair of parallel plates. (The detector measures the degree of ionization between the plates; when smoke enters the gap, it changes the ionization, which activates the alarm.) And some emergency exit signs in public buildings, schools, and auditoriums that have to remain visible during power outages contain 8to10 curies of tritium, whose emitted electrons excite phosphor atoms, illuminating the sign.

The amount of radioactive material in the nuclear batteries we are developing falls between those in a smoke detector and in an exit sign. And for whatever amount, any commercial application of such nuclear batteries would have to take into account all required safety measures, including designing safe packaging and following regulations about handling and disposing of the device and its components. —A.L. & J.B.

September 2004|IEEE Spectrum |NA41 Authorized licensed use limited to: UNIV ESTADUAL PAULISTA JULIO DE MESQUITA FILHO. Downloaded on July 13, 2009 at 16:56 from IEEE Xplore. Restrictions apply.

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