Hardware


Radioisotopes for FPGAs? Sure thing.


Atomic technology powers crypto keys for the long haul 20-year betavoltaic batteries rely on tritium electrons


I remember the first time I played with a solar cell as a child experimenter. I used a flashlight, solar cell, and another flashlight bulb to create a weakly lit filament. It would take me another 10 years and a college education before I understood the principle behind using photons to create an electrical current. City Labs has performed a similar miracle, creating a nanocurrent betavoltaic battery that not only holds a charge for 20 years, it meets NRC standards so anyone can buy it. This technology is a genuine breakthrough and I’m particularly enthused at the horizons opened by the company’s NanoTritium batteries. I spoke with City Labs founder Peter Cabauy to get the inside glow, so to speak. Edited excerpts follow. Chris A. Ciufo, Editor


VME: I am thoroughly unfamiliar with your company. Let’s start there.


CABAUY: We started in this industry in 2005 as a couple of scientists and engi - neers just saying, “Well, we want to do some cool technology.” But we didn’ t necessarily fall in lo ve with batteries. However, we saw the need for advanced battery technology in FPG As. Early on, we realized there w as a market here because Lockheed Martin, Raytheon, and Northrop Grumman wanted to buy these batteries, but at the time these types of batteries didn’t exist. Even then, Lock- heed Martin, Orlando, FL gave us a pur- chase order for prototype batteries.


VME: OK. City Labs issued a press release in December saying that the company was awarded a million- dollar Air Force Research Labs [AFRL] contract for your 20-year lifespan NanoTritium batteries for microelectronics.


CABAUY: Yes. We are making a betavol- taic battery. Betavoltaics are very much like photovoltaics, except that instead of receiving sunlight to mak e a current out of a solar cell, we are recei ving beta radiation, caused by beta electrons from tritium. Tritium is the most benign radio- isotope, and is commonly used for medi- cal tracers, exit signs, and luminous watch dials, for example.


VME: Why use betavoltaic batteries? What are the advantages?


CABAUY: Instead of having the “betas” or electrons coming off the tritium and hit- ting a phosphor so that it glo ws, they are hitting a semiconductor PN junction and


thereby creating a current just lik e a solar cell would. The advantage is that while none of the chemical batteries work well through all temperatures, betavoltaic bat- teries are very resilient and can withstand temperatures of -50 °C up to +150 °C. Also, tritium’s half-life is 12.5 years. So the battery could last more than 20 years.


VME: So tritium emits electrons that strike the PN junction and then a current flows?


CABAUY: Well, the electron strik es the PN junction and creates electron-hole pairs. And a PN junction is able to separate those electron-hole pairs and capture them and make them into usable electrical cur - rent. As I mentioned, it’s very similar to the way a solar cell would work. In fact, it is a very tweaked-up solar cell that can receive these very low-energy electrons coming off of tritium. Tritium is such a good radioiso- tope for this type of application because the electrons – or as they call them, “betas” – can be stopped with just a thin sheet of paper. So, for example, someone wear- ing a di ver’s wristwatch isn’t worried about radiation affecting them in any way. Tritium is a very benign radioisotope, as far as radioisotopes go.


VME: No current flows until there’s a load put on the device?


CABAUY: Since we are creating elec - tron-hole pairs, there always needs to be a load, but it is very different from a chemical battery. A solar cell has an open- circuit voltage and a short-circuit current. If the solar cell is connected to an appli - cation or load, the maximum voltage and maximum current will rise and are contin- gent upon the connected device.


16 VME and Critical Systems / Spring 2011


VME: With a typical beta hitting a typical PN junction, are we talking nano-amps of currents?


CABAUY: Just like solar cells, you can connect as many as needed to get the desired current or desired v oltage, but typically it’s somewhere between 50 and 100 nano-amps per square centimeter , depending on tritium density and semi - conductor efficiency. At City Labs, we’ve achieved 25 to 50 nanowatts per centi- meter squared. The semiconductor can be made quite thin, so it’s just a matter of stacking. Right now, it would be about 10 microwatts per cubic centimeter by stacking, but we expect to achieve up to 50 microwatts per cubic centimeter.


Keep in mind that I w as referring to the power density, but the energy density is very large. The average energy density over 20 years is about 5x the energy den- sity of a lithium thionyl chloride battery, which is the highest-energy density lith- ium battery.


VME: Over 20 years, what changes within the battery?


CABAUY: Imagine that you’ re just pumping out little nano-amps and micro- amps through time, so it’ s like a little trickle. But if it’s a continuous trickle – and with radioisotopes it’s always con- tinuous – even continuous trickling of water can create a Grand Canyon over a long period of time. It’s the same thing with these betavoltaics – a few drops of this trickle charging current will create a lot of energy over time. Connecting these to a lithium thin-film battery or an ultacapacitor can do quite well over time for powering up a chip or RF device.


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