R esearch on pervasive computing systems has focused on algorithms, system architectures, and software issues associated with systems that compute for extended time periods and that are incorporated into everyday objects. However, as the time period for system use increases, hardware reliability becomes an important design factor. A possible reliability metric for pervasive computing systems could be the degree to which the systems are immune to power loss and wide variations in operating conditions. The goal is to achieve power sources that operate over a wide temperature range and for extended time periods with high reliability. To reach this goal, researchers have investigated technologies for miniature micropower applications and developed radioisotope power generators (see the "Related Energy Sources" sidebar). Unfortunately, the former usually suffer from low energy densities, low conversion efficiencies, or unreliability, while the latter suffer from low energy conversion efficiencies, taking away from the high energy densities that radioisotope thin films offer. To help remedy this, we've created a power source employing radioactive thin films and piezoelectric unimorphs, using a nonthermal energy conversion cycle that enables much higher energy conversion efficiency.The kinetic energy of the particles emitted from radioisotopes is temperature insensitive up to fusion temperatures (radioactive thin films emit electrons as beta particles, helium nuclei as alpha particles, and photons as x-rays). This can extend a pervasive computing system's operating temperature range, possibly from milliKelvin to thousands of Kelvin, assuming the hardware is composed of the appropriate materials. Moreover, radioactive thin films emit energy over a time governed by the half-life, which can be very long. For the 63 Ni ␤-source we used, the half-life is 100.2 years. Hence, the power sources we describe could extend a system's operating life by several decades or even a century, during which time the system could gain learned behavior without worrying about the power turning off.Radioactive thin-film-based power sources also have energy density orders of magnitude higher than chemical-reaction-based energy sources. This enables submillimeter-scale power sources, which is significant given the crucial role that metrics of power and energy density play in determining pervasive computing systems' usefulness in applications limited by size.For example, although pacemakers and diabetes-monitoring equipment are already available, no system is small enough to fit inside a prostrate or brain for long-term monitoring A long-lasting radioisotope micropower generator for self-powered sensor microsystems promises to make pervasive computing systems more reliable. Its higher energy conversion efficiency enables microsystems with small amounts of radioactivity to realize sensor and basic computation operations.

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The change in open-circuit voltage of a 4H SiC p-n diode betavoltaic cell in response to temperature was used to sense temperature. A linear sensitivity of 2.7mV∕K was obtained from 24to86°C. This was achieved with only 2.5μCi of active nickel-63 as the β source, giving a short circuit current of 21pA, a low-enough activity for civilian applications. The measured sensitivity of 2.7mV∕K was lower than the 5.5mV∕K predicted from the theory. The 28GΩ shunt resistance of the betavoltaic cell was used to explain the lower sensitivity.

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