A 25-Year Battery

Long-lived nuclear batteries powered by hydrogen isotopes are in testing for military applications.

Batteries that harvest energy from the nuclear decay of isotopes can produce very low levels of current and last for decades without needing to be replaced. A new version of the batteries, called betavoltaics, is being developed by an Ithaca, NY-based company and tested by Lockheed Martin. The batteries could potentially power electrical circuits that protect military planes and missiles from tampering by destroying information stored in the systems, or by sending out a warning signal to a military center. The batteries are expected to last for 25 years. The company, called Widetronix, is also working with medical-device makers to develop batteries that could last decades for implantable medical devices.

Nuclear power: The package inside this prototype betavoltaic battery contains layers of silicon carbide and metal foil embedded with the radioactive isotope tritium. When high-energy electrons emitted by the decay of tritium hit the silicon carbide, it produces an electrical current that exits the cell through the metal pins. Such batteries are designed to last 25 years.  Credit: Widetronix

Widetronix's batteries are powered by the decay of a hydrogen isotope called tritium into high-energy electrons. While solar cells use semiconductors such as silicon to capture energy from the photons in sunlight, betavoltaic cells use a semiconductor to capture the energy in electrons produced during the nuclear decay of isotopes. This type of nuclear decay is called "beta decay," for the high-energy electrons, called beta particles, that it produces. The lifetimes of betavoltaic devices depend on the half-lives, ranging from a few years to 100 years, of the radioisotopes that power them. To make a battery that lasts 25 years from tritium, which has a half-life of 12.3 years, Widetronix loads the package with twice as much tritium as is initially required. These devices can withstand much harsher conditions than chemical batteries. This, and their long lifetimes, is what makes betavoltaics attractive as a power source for medical implants and for remote military sensing in extremely hot and cold environments.

The concept of betavoltaics is about 50 years old. The first pacemakers used betavoltaics based on the radioactive element promethium, but these betavoltaics were phased out when cheaper lithium-ion batteries were developed. The technology is now reemerging, says Peter Cabauy, CEO of another betavoltaic company, Miami-based City Labs, because semiconducting materials have improved so much. Early semiconducting materials weren't efficient enough at converting electrons from beta decay into a usable current, so they had to use higher energy, more expensive--and potentially hazardous--isotopes. More efficient semiconducting materials can be paired with relatively benign isotopes such as tritium, which produce weak radiation.

Widetronix's batteries are made up of a metal foil impregnated with tritium isotopes and a thin chip of the semiconductor silicon carbide, which can convert 30 percent of the beta particles that hit it into an electrical current. "Silicon carbide is very robust, and when we thin it down, it becomes flexible," says Widetronix CEO Jonathan Greene. "When we stack up chips and foils into a package a centimeter squared and two-tenths of a centimeter high, we have a one microwatt product." The prototype being tested by Lockheed Martin produces 25 nanowatts of power.

Betavoltaics aren't very powerful. They don't have nearly enough power to drive a laptop or a cell phone. But their energy density is high: they store a lot of energy in films just micrometers thick and can be made in very small packages. "We're focusing on places where you need a very long life and energy density," says Greene.

One such place is the monitoring of military equipment. "Everything the Department of Defense puts out has to have antitamper protection so that if someone gets their hands on the seeker head of a missile, or an entire aircraft, it would be very difficult to reverse-engineer it," says Christian Adams, a chemist at Lockheed Martin Missiles and Fire Control. The memory chips that control such antitamper systems, says Adams, require very low continuous power over a long time. Military specifications also require that these devices withstand extreme conditions that normal batteries can't tolerate: they must operate in temperatures from -65 to 150 ˚C and withstand high-frequency vibrations, high humidity, and blasts of salt. "If the battery freezes out or dies out, the memory circuit loses its configuration," and the device fails, says Adams.

"Widetronix is the first out of the gate with something that can be tested to military specifications," says Adams. Lockheed received the company's prototypes last week. If the betavoltaics pass the test, Lockheed will probably have them in antitamper products in about a year's time, he says. Lockheed is also working with the company to develop higher-power betavoltaics for remote monitoring of missiles. Sending out a radio signal to say "I'm healthy" requires microwatts of power, says Adams. Widetronix is also testing its batteries with medical-device company Welch Allyn. It expects to sell the batteries for $500.

City Lab's Cabauy says that though the prospect of nuclear batteries, especially for medical implants, may raise eyebrows, tritium is safe. Besides the beta particle, other products of tritium's decay are an antineutrino and an isotope of helium that is not radioactive. "A piece of paper can stop the radiation from tritium," says Cabauy.

The future promise of betavoltaics might be in very cheap sensors embedded in buildings and bridges where "you don't ever want to change the battery," says Amit Lal, professor of electrical and computer engineering at Cornell University. However, this would require companies such as Widetronix to move to longer half-life materials, such as nickel isotopes that last 100 years. While tritium has a half-life of only 12.3 years, one of its chief advantages, besides safety, is that it can be secured cheaply from Canadian nuclear reactors that produce heavy water as a by-product. Longer half-life isotopes such as nickel-63 must be purchased abroad at high prices. "Since the end of the Cold War, there is no government support for radioisotope infrastructure in the United States," says Lal. "Making batteries that last forever is probably good reason to build that infrastructure.
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