"Electronic circuits and nanomachines grow ever smaller, but the batteries to power them are huge in comparison," said Lal, an assistant professor of electrical and computer engineering. "We have built a tiny battery that can supply power for decades to remote sensors or implantable medical devices by drawing energy from a radioactive isotope."

Cornell doctoral candidate Hui Li assisted Lal in his work. Lal downsized the battery from a larger design he built while a member of the faculty at the University of Wisconsin, Madison, with nuclear-engineering professors James Blanchard and Douglas Henderson.

The MEMS battery translates the stored energy in the radioactive isotope directly into the physical motion of a microscopic cantilever, enabling it to move MEMS components directly or to generate electricity for circuitry. Lal argues that atomic batteries are the best solution for "always on" sensors and other devices for long-term monitoring. His atomic battery was designed under a Darpa contract.

A copper cantilever is mounted directly above a thin film of the radioactive isotope nickel-63. Isotopes are heavier versions of an element that have an excess of neutrons in their nuclei. As the isotope decays, it emits high-energy particles. In this case, they are known as beta particles, free electrons that are biologically harmless compared with the alpha particles and gamma rays produced by other isotopes when they decay.

As the copper cantilever accumulates the emitted electrons, it builds up a negative charge at the same time that the isotope film becomes positively charged. The beta particles essentially transfer electronic charge from the thin film to the cantilever. The opposite charges cause the cantilever to bend toward the isotope film.

Decades to decay

Just as the cantilever touches the thin-film isotope, the charge jumps the gap. That permits current to flow back onto the isotope, equalizing the charge and resetting the cantilever. As long as the isotope is decaying - a process that can last for decades - the tiny cantilever will continue its up-and-down motion.

The half-life of nickel-63 - the time it takes to radiate halve its mass - is more than 100 years, but Lal says that an atomic battery will operate properly only during the first half of its half-life, or about 50 years in this case. Other isotopes with which Lal has experimented offer different operating points regarding the amount of energy they can supply vs. their lifetime. Some isotopes are also immune to environmental factors, such as temperature, that severely affect the lifetime of normal batteries.

"The military has many applications that could benefit from our atomic batteries, such as sensors that monitor stored missiles or battlefield sensors that must be concealed and left unattended," said Lal. "Civilian uses include medical devices that can be implanted inside the body for long periods."

MEMS applications could transfer the motion of the cantilever directly into an actuator for a linear motion, or move a cam or ratcheted wheel for rotary motion. Electricity can be generated by putting a magnet on the end of the cantilever, which moves through a coil as it oscillates. Lal has also built prototypes using a piezoelectric cantilever that directly generates electricity when deformed, releasing a charge pulse each time the cantilever cycles.

Working with sensors

"You could also use the pulses from a piezoelectric material to generate a radio-frequency pulse to transmit information," he said, or to drive an LED to generate an optical signal.

For sensors, Lal is also experimenting with using the motion of the tiny cantilevers to directly sense gases. By designing the material of the cantilever to be sensitive to particular gases, its cycle would change speed when that gas was present. Different cantilever materials could change the period or amplitude of the oscillation in the presence of different gases, temperatures or pressure changes.

Currently, Lal, Li and another doctoral candidate, Hung Guo, are downsizing the atomic battery even further. The current prototype squeezes all the working parts into a cube smaller than 1 mm on a side.