Saturday, January 15, 2011

Long Lasting Alpha Voltaic Batteries for MEMS and Deep Space

The concept of an alpha voltaic battery was proposed in 1954 by W. G. Pfann and W. van Roosbroeck, (Journal of Applied Physics, Volume 25, No. 11, pp. 1422-1434, November 1954). In an alpha voltaic battery a radioactive substance that emits energetic alpha particles is coupled to a semiconductor p/n junction diode. As the alpha particles penetrate into the p/n junction, they decelerate and give up their energy as electron-hole pairs. These electron-hole pairs are collected by the p/n junction and converted into useful electricity much like a solar cell. 

The main reason alpha voltaic batteries are not commercially successful is that the alpha particles damage the semiconductor material so as to degrade its electrical performance in just a matter of hours as 

NASA and Rochester Institute of Technology ((RIT) Rochester, NY) have developed alpha voltaic batteries whose performance does not degrade in a matter of hours because of damage to the layer of semiconductor material from the emitted alpha particles. The present invention also provides power supplies which are both small and have a long life span and thus are suitable for a variety of technologies, including micro electrical mechanical systems (MEMS). NASA and RIT share U.S. Patents 7,867,640 and 7,867,639.

Further, the alpha voltaic batteries can be scaled to higher power levels which make them useful in another wide range of technologies, such as a power source of deep space missions. 

The emerging technologies of micro electrical mechanical systems (MEMS) are a perfect application for alpha voltaic batteries. The NASA and Rochester Institute battery provides a long life power source that simply did not exist for these devices prior to this invention. Additionally, it is very suitable for integrating batteries directly on the semiconductor for a "battery-on-a-chip" concept. These alpha voltaic batteries could produce power on the order of micro-Watts, sufficient for many MEMS applications.

With the present battery, scaling to higher power levels suitable for deep space missions (100's of Watts) is also possible. Alpha voltaic batteries have at least two unique properties when compared to conventional chemical batteries that make them outstanding candidates for deep space missions: 1) The alpha emitting materials have half-lives from months to 100's of years, so there is the potential for "everlasting" batteries; and 2) Alpha voltaic batteries can operate over a tremendous temperature range. Ordinary chemical batteries all fail at temperatures below C., while alpha voltaic batteries have been demonstrated to work at C. in a prototype of an alpha voltaic battery. 

The absorption and conversion layer comprises a layer of phosphor, such as ZnS:Ag, which fluoresces photons of approximately 2.66 eV (465 nm wavelength) in energy, although other types and numbers of absorption and conversions layers could be used. By way of example only, other materials which could be used for the absorption and conversion layer  include rare earth oxides or rare earth doped garnet crystals and nanoscale materials known as "quantum dots" that exhibit flourescence under particle radiation, although other types of materials could be used. Materials that fluoresce under particle radiation, collectively known as phosphors, can convert particle radiation into photons with very high efficiency. 


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