bateria nuclear, Notas de estudo de Engenharia de Produção

Baixe bateria nuclear e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! 36 IEEE Spectrum | September 2004 | NA DYNAMOS +NUCLEAR BATTERIES THE D A I N T I E S T or 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 pock- etable 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 BY HARVESTING ENERGY FROM RADIOACTIVE SPECKS, NUCLEAR MICROBATTERIES COULD POWER TOMORROW’S MICROELECTROMECHANICAL MARVELS—AND MAYBE YOUR CELLPHONE, TOO B Y A M I T L A L & J A M E S B L A N C H A R D F 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. B R YA N C H R IS TIE September 2004 | IEEE Spectrum | NA 37 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. generating electricity. NASA has been using radioisotope thermo- electric generators, or RTGs, since the 1960s in dozens of mis- sions, 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, elec- trons 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 diminu- tive dimensions of MEMS devices, the ratio between an object’s surface and its volume gets very high. This relatively large sur- face makes it difficult to sufficiently reduce heat losses and main- tain 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 sili- con 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 disintegrat- ing; if the particles were more energetic, they would travel longer distances, thus escaping the battery. The device we built was capa- ble of producing about 3 nanowatts with 0.1 millicurie of nickel-63, a small amount of power but enough for applications such as nano- electronic 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 electric- ity 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 effi- cient; they tap a significant fraction of the kinetic energy of the emitted particles for conversion into mechanical energy. By releas- ing this energy in brief pulses, they provide much more instan- taneous power than the direct-conversion approach. For these batteries, which we call radioactive piezoelectric gen- erators, 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, charg- ing 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 sil- icon 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 attrac- tive force vanishes. The cantilever then springs back and oscil- lates 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 con- nected to a photodiode that detects light variations. Also using the cantilever system, we developed a pressure sen- sor 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 diffi- cult 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 mate- rial 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 imbal- ance appeared in the electrodes, making the electric field propa- gate 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 communi- cate 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 com- ponent—sensor, actuator, microprocessor—its own nuclear microbattery. In such a scheme, even if a main battery is still nec- essary for more power-hungry components, it could be consid- erably 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 40 IEEE Spectrum | September 2004 | NA ENERGY CONTENT ENERGY DENSITY TECHNOLOGY (MILLIWATT-HOURS/MILLIGRAM) Lithium-ion in a chemical battery 0.3 Methanol in a fuel cell* 3 Tritium in a nuclear battery** 850 Polonium-210 in a nuclear battery** 57 000 *Assuming 50 percent efficiency **Assuming 8 percent efficiency and 4 years of operation 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. 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 compli- cated electronics. Instead, a nuclear microbattery designed to gen- erate 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 radi- ation 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 possi- bility for improving the cantilever-based system would be to scale up the number of cantilevers by placing several of them horizon- tally, 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 alterna- tive 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 cell- phones 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 sig- nificant amount of current into the handheld’s battery. How much that current could increase the device’s opera- tion 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 recharg- ing 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.  TO P : S R B TEC H N O LO G IES IN C .; B O TTO M : H EM EVA TEC H N O LO G IES /A LA M Y 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 1 to 10 millicuries of nickel-63 or tritium, whose beta particles have relatively low energy and can be blocked by a layer of 25 to 100 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 micro- batteries offer minimal risk to the public. In fact, radioisotopes have been used for decades in commercial applications. Many smoke detectors contain 1 to 5 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 8 to 10 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. NOT ALL RADIOISOTOPES ARE EQUAL September 2004 | IEEE Spectrum | NA 41 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.