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Beta particles (electrons) released from a thin film of radioactive material are absorbed by the cantilever, giving it a negative charge. The cantilever is pulled down toward the positively charged film until it is near enough for a current to flow and equalize the charge. The cantilever springs back up, and the process repeats. ITHACA, N.Y. -- While electronic circuits and nanomachines grow ever smaller, batteries to power them remain huge by comparison, as well as short-lived. But now Cornell University researchers have built a microscopic device that could supply power for decades to remote sensors or implantable medical devices by drawing energy from a radioactive isotope.

The device converts 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, says Amit Lal, Cornell assistant professor of electrical and computer engineering.

Lal and Cornell doctoral candidate Hui Li described a prototype of the device at a U.S. Department of Defense meeting of Defense Advanced Research Projects Agency (DARPA) investigators in Detroit in August. The prototype is the first MEMS (micro-electromechanical systems) version of a larger device that Lal designed and built while a member of the faculty at the University of Wisconsin, Madison, working with nuclear engineering professors James Blanchard and Douglas Henderson.

The prototype device uses a copper cantilever 2 centimeters long. Future nanofabricated versions could be smaller than one cubic millimeter. The prototype is made up of a copper strip 1 millimeter wide, 2 centimeters long and 60 micrometers (millionths of a meter) 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). Radioactive materials can emit beta particles, alpha particles or gamma rays, the last two of which can carry enough energy to be hazardous. Lal has chosen only isotopes that emit beta particles, whose energy is small enough not to penetrate skin, to be used in his device.

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, equalizing the charge. The rod springs up, and the process repeats. The principle is much like that underlying an electric doorbell, in which a moving bar alternately makes and breaks the electric circuit supplying an electromagnet that moves the bar.

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 says a battery using this isotope might continue to supply useful energy for at least half that time. (The half-life is the time it takes for half the atoms in an element to decay.) 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 magnetized 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 says. 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.

*Radioisotope piezoelectric generator*

A Radioisotope piezoelectric generator converts energy stored in the radioactive material directly into motion to generate electricity by the repeated deformation of a piezoelectric material. This approach creates a high-impedance source and, unlike chemical batteries, the devices will work in a very wide range of temperatures.

[edit] Description A piezoelectric cantilever is mounted directly above a base of the radioactive isotope nickel-63. The Milli-Curie-level nickel-63 thin film generates electrons alone. As the isotope decays, it emits beta particles. As the 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. 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. As the cantilever directly generates electricity when deformed, a charge pulse is released each time the cantilever cycles.

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. Thus, a battery using this iso­tope might continue to supply useful energy for at least half that time. Researchers have demonstrated devices with about 7% efficiency with high duty cycles of 120 Hz to low duty (every three hours) cycle self-reciprocating actuators

**Charge amplifier**

A charge amplifier is a circuit whose equivalent input impedance is a capacitance that provides a very high value of impedance at low frequencies. Thus contrary to what its name may suggest, a charge amplifier does not amplify the electric charge present at its input. Its function is actually to obtain a voltage proportional to that charge and yield a low output impedance. Hence it is a charge-to-voltage converter. Common applications include piezoelectric sensors and photodiodes, in which the charge output from the transducer is converted into a voltage. Charge amplifiers are often found in instrumentation, and in the readout circuitry of CCD imagers and flat-panel X-ray detector arrays. In read-out circuits the objective is usually to measure the very small charge stored within an in-pixel capacitor, despite the capacitance of the circuit-track to the readout circuit being a couple of orders of magnitude greater than the in-pixel capacitor.

Advantages include:

Enables quasi-static measurements in certain situations, such as constant pressures on a piezo lasting several minutes[1] Piezo element transducer can be used in much hotter environments than those with internal electronics[1] Gain is dependent only on the feedback capacitor, unlike voltage amplifiers, which are affected greatly by the input capacitance of the amplifier and the parallel capacitance of the cable[1][2] Contents [hide]

1 Design 2 Applications 3 Precautions 4 References 5 External links

Design Charge amplifiers are usually constructed using op amps with a feedback capacitor. They thus act in a similar manner to an integrator. Since the transducer acts in a similar manner to a differentiator, the two transfer functions cancel and the output voltage is proportional to the charge produced by the transducer. Stray capacitance at the input to the amplifier is not detrimental to operation because this capacitance is always at a virtual ground.

Applications Accelerometer signal conditioning Guitar pickup amplifiers Vibration transducers Nuclear instrumentation

Precautions Cable from the transducer to the amplifier should not be subject to vibration as extra charges can be generated by friction and lead to unwanted noise output.