Tiny atomic battery could run for decades

Researchers at Cornell University have built a micro-electromechanical system that could supply decades worth of power to remote sensors or implantable medical devices by drawing energy from a radioactive isotope.

The device is said to convert the energy stored in the radioactive material directly into motion. It could directly move the parts of a tiny machine or could generate electricity in a form more useful for many circuits than has been possible with earlier devices.

This new approach creates a high-impedance source (the factor that determines the amplitude of the current) better suited to power many types of circuits, said Amil Lal, Cornell assistant professor of electrical and computer engineering.

The prototype is made up of a copper strip 1 millimetre wide, 2 centimetres long and 60 micrometers thick that is cantilevered above a thin film of radioactive nickel-63 (an isotope of nickel with a different number of neutrons from the common form). As the isotope decays, it emits beta particles (electrons), whose energy is small enough not to penetrate skin.

The emitted electrons collect on the copper strip, building a negative charge, while the isotope film, losing electrons, becomes positively charged. The attraction between positive and negative bends the rod down. When the rod gets close enough to the isotope, a current flows, equalising the charge. The rod springs up, and the process repeats itself.

Radioactive isotopes can continue to release energy over periods ranging from weeks to decades. The half-life of nickel-63, for example, is over 100 years, and Lal said a battery using this isotope might continue to supply useful energy for at least half that time.

Other isotopes offer varying combinations of energy level versus lifetime. And unlike chemical batteries, the devices will work in a very wide range of temperatures. Possible applications include sensors to monitor the condition of missiles stored in sealed containers, battlefield sensors that must be concealed and left unattended for long periods, and medical devices implanted inside the body.

The moving cantilever can directly actuate a linear device or can move a cam or ratcheted wheel to produce rotary motion. A magnetised material attached to the rod can generate electricity as it moves through a coil.

Lal also has built versions of the device in which the cantilever is made of a piezoelectric material that generates electricity when deformed, releasing a pulse of current as the rod snaps up. This also generates a radio-frequency pulse that could be used to transmit information. Alternatively, Lal suggests, the electrical pulse could drive a light-emitting diode to generate an optical signal.

In addition to powering other devices, the tiny cantilevers could be used as stand-alone sensors, Lal said. The devices ordinarily operate in a vacuum. But the sensors might be developed to detect the presence or absence of particular gases, since introducing a gas to the device changes the flow of current between the rod and the base, in turn changing the period or amplitude of the oscillation. Temperature and pressure changes also can be detected.

Lal, Cornell doctoral candidate Hui Li and Cornell doctoral candidate Hang Guo are now building and testing practical sensors and power supplies based on the concept. The prototype shown in August was gigantic by comparison with the latest versions, Lal said. An entire device, including a vacuum enclosure, could be made to fit in less than one cubic millimetre, he concluded.

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