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.

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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.