Raytheon Technology Today 2011 Issue 1

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Feature Continued from page 13 These types of betavoltaics generally develop power levels that can approach 1 milliwatt. Radiation-tolerant, wide band gap semiconductors are ideal candidates for direct-conversion betavoltaics. Several semiconductors have been identified as ideally suited for these applications. They include GaN, aluminum gallium nitride, silicon carbide and diamond. Since electrons are rapidly absorbed as they emerge from the radioactive plated surface, the useful isotope plating thickness is limited to a few micrometers at best. Therefore, methods to scale up the output of these devices depend primarily on increasing direct contact surface area. Honsberg, et al., described a conceptualized approach to address this issue. It consists of mating GaN layers on each side of thin Ni-63 wafers in order to maximize output power. Using this GaN- isotope sandwich design to capture a large fraction of emitted beta particles, the ability to develop a 2.3 volt open circuit voltage with a short circuit current of 1.1 microamperes was reported. 1 Raytheon currently produces GaN devices for highpower microwave applications and also has an established and demonstrated capability for growing thin-film chemical vapor deposition diamond. With this established presence in developing materials that are highly desirable for betavoltaics-based power sources, Raytheon is in a good position to drive this technology forward. In light of the limited range of low-energy beta particles considered here, beta sources for direct conversion devices are considered to be relatively safe since they are literally stopped by the outermost dead skin layer. There are a number of radiologically-safe pure beta emitting isotopes with half-lives ranging from 2.6 to 100.3 years and with energy densities as high as 10 11 kilojoules per cubic meter (kJ/m 3 ). In comparison, diesel fuel has an energy density of approximately 4x10 7 kJ/m 3 , illustrating that betavoltaic sources can have very high energy densities and, consequently, long lives. The choice of the appropriate isotope is dictated primarily by operational 14 2011 ISSUE 1 RAYTHEON TECHNOLOGY TODAY Radioactive Source Half-life (year) Specific Activity (GBq/g) considerations, where the isotope is selected based in part by matching its half-life to the application’s expected operational life. Use of a pure beta emitter is also preferred, since generating other decay products can lead to a significant dose to the operator and possible damage to the direct conversion device semiconductor when long term exposures are required. In general, a long half-life pure beta isotope provides prolonged battery life at the expense of generating low power, while a short halflife isotope provides higher power at a more limited sustainable life span. Table 2 shows some candidate beta-emitting isotopes and their relevant properties. Self-Reciprocating Cantilever Another unique betavoltaic application is based on a vibrating piezoelectric cantilever concept, the self-reciprocating cantilever. This functions initially as a charge-to-motion Maximum Decay Energy Average Decay Energy Specific Power mWatt/Ci Tritium, 3H 12.3 357,000 18.6 keV 5.7 keV 33.7 63Ni 100 2,190 67 keV 17 keV 100 147Pm 2.6 36,260 230 keV 73 keV 367 Table 2. Pure beta particle emitters. Source: M. Mohamadian, et al., “Conceptual Design of GaN Betavoltaic Battery use in Cardiac Pacemaker,” Proceedings ICENES (2007). 63 Ni radioisotope emitter Vout conversion process followed by mechanical energy conversion to electrical energy. In this rendering, the device contains a beta particle-emitting isotope-coated surface designed to continuously deliver a negative charge to a nearby piezoelectric materialcoated cantilever. This conceptual design is shown in Figure 3, where the self-reciprocating cantilever process begins by charging the opposing surface with a large fraction of the charge emitted by the isotope. Once a sufficient negative charge builds up on the cantilever surface, the resulting electrostatic force field begins to draw the cantilever to the fixed location, positively charged lower surface. When an adequate charge is accumulated, the cantilever bends to the point where it contacts the radioisotopecoated surface. Upon contact, electrons flow from the negatively charged surface, causing surface charge neutralization, collapsing the electrostatic field to zero and Piezoelectric plate (PZT) Atomic Batteries Silicon beam Collector, thick enough to capture all emitted particles 63 Ni radioisotope emitter Figure 3. Self-reciprocating piezoelectric cantilever. One betavoltaic design configuration is based on vibrating piezoelectric cantilevers that convert electrostatic energy to kinetic energy and back into electric energy. Amit Lal, Rajesh Duggirala and Hui Li, “Pervasive Power: A Radioisotope-Powered Piezoelectric Generator,” PERVASIVEcomputing, Jan-Mar 2005.
forcing the cantilever to spring back and oscillate around its initial equilibrium position. This approach allows for the continuous transfer of energy from mechanical to electrical, generated by the vibrating piezoelectric-based cantilever. As the platform continues to oscillate, the piezoelectric- attached structure generates useful electricity that can be harvested for various applications. Its oscillation frequency can be fine-tuned by modifying the length of the cantilever and the strength of the radiation source. Depending on its intended application, the output of this device can be utilized as a source of tunable rapid current spikes, or filtered to produce a continuous DC output stream. An approach to implementing radio frequency identification (RFID), for example, is based on a modification of the self-reciprocating cantilever design described above. Tin, et al. 2 , report the generation of 264 MHz wireless signals induced directly by vibration of the cantilever. Another RFID design employs a surface acoustic wave (SAW) resonator connected to and excited by the vibrating cantilever. This concept has been shown to produce a frequency modulation found useful as a CMOS-compatible wireless communications beacon. The authors report on the design of a SAW transponder that can transmit an RF signal at 800 microwatts, with a 10 microsecond pulse duration, every three minutes at a frequency locked to a 315 megahertz SAW resonator. 3 In addition to RFID, Raytheon is pursuing applications such as power sources for autonomous sensors and long-lived, miniature, wireless transmitters. The benefit provided to these applications is long-term unattended, reliable operation achieved using devices that contain embedded high energy density betavoltaic power sources. • Bernard Harris 1 C. Honsberg, et al., ”GaN Betavoltaic Energy Converters,” IEEE Photovoltaics Specialist Conference. Orlando, Fla., 2005. 2 S. Tin, et al.,”Self-Powered Discharge-Based Wireless Transmitter,” IEEE International Conference on MicroElectroMechanical Systems, 2008. 3 S. Tin and A. Lal, “A radioisotope-powered surface acoustic wave transponder,” J. Micromech. Microeng., 19 (2009). Creating Compact, Reliable and Clean Power With Fuel Cell Technology Feature The demand for compact, reliable and clean power is an important driver and constraint for large and small systems. Rapid technological advances have been made in fuel cells, which are of interest to Raytheon and our customers as a more effective source of power for key products and as a more sustainable source of power for facilities and large-scale systems. Raytheon is employing new developments in fuel cell technology that are likely to be part of the next generation of power solutions. How Fuel Cells Work: The Basics Fuel cells produce direct current (DC) electrical power through an electrochemical reaction in which a fuel (hydrogen or a hydrocarbon) is oxidized, typically with pure oxygen or air. Some designs use chemicals such as chlorine as the oxidizing agent. A fuel cell consists of two electrodes, the anode and cathode, separated by an electrolyte. The electrolyte may be liquid, such as an aqueous alkaline solution, or solid, such as those using polymer membranes or solid oxide fuel cells (SOFC). This distinction defines the two basic classes of fuel cells — liquid and solid — with numerous variants of each. Fuel cells are distinct from batteries, the other major class of electrochemical power cells. A fuel cell is a thermodynamically open system into which fuel is continuously injected to generate power. In contrast, a battery is a closed system that stores power, though many battery types can have power added through recharging. Figure 1 illustrates the electrochemistry that powers fuel cells. Because fuel cells create electrical power through an electrochemical reaction rather than through combustion, their efficiency is not limited by the Carnot cycle efficiency of traditional heat engines (e.g., internal combustion engines or turbines), but is potentially higher. Advances in fuel cell technologies during the past decade are helping to realize this theoretical efficiency advantage. Anode Current Carbon + Hydrogen C H + zO � nCO + mH O + e x y 2 2 2 − Fuel positive ions (H Air + Heat + ) or negative ions (O2- ) Electrolyte Cathode Oxygen Figure 1. Schematic of a fuel cell. Hydrocarbon fuel or pure hydrogen combines with oxygen in the fuel cell to generate electricity. (Source: Bloom Energy) Reaction byproducts from fuel cells include water, carbon dioxide (in the case of hydrocarbon fuels), and waste heat as well as useful electrical power. Some fuel cell schemes harness the waste heat to boost efficiency. Fuel cells can be considered a green technology Continued on page 16 RAYTHEON TECHNOLOGY TODAY 2011 ISSUE 1 15
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Page 5 and 6: Applications • Infrastructures
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