A family of proposed miniaturesources of power would exploit the di-rect conversion of the kinetic energy ofα particles into electricity in diamondsemiconductor diodes. These powersources would function over a widerange of temperatures encountered interrestrial and outer-space environ-ments. These sources are expected tohave operational lifetimes of 10 to 20years and energy conversion efficiencies>35 percent.

A power source according to the pro-posal would include a pair of devices likethat shown in the figure. Each device wouldcontain Schottky and p/n diode devicesmade from high-band-gap, radiation-harddiamond substrates. The n and p layers inthe diode portion would be doped sparsely(<1014 cm-3) in order to maximize the vol-ume of the depletion region and therebymaximize efficiency. The diode layerswould be supported by an undoped dia-mond substrate.

The source of α particles would be athin film of 244Cm (half-life 18 years) sand-

wiched between the two paired devices.The sandwich arrangement would forcealmost every α particle to go through theactive volume of at least one of the de-vices. Typical α particle track lengths inthe devices would range from 20 to 30 mi-crons. The α particles would be made tostop only in the undoped substrates toprevent damage to the crystalline struc-tures of the diode portions.

The overall dimensions of a typicalsource are expected to be about 2 by 2by 1 mm. Assuming an initial 244Cmmass of 20 mg, the estimated initial out-

put of the source is 20 mW (a current of20 mA at a potential of 1 V).

This work was done by Jagdish U. Patel,Jean-Pierre Fleurial, and Elizabeth Kolawa ofCaltech for NASA’s Jet Propulsion Labo-ratory. Further information is contained ina TSP (see page 1).

This invention is owned by NASA, and apatent application has been filed. Inquiriesconcerning nonexclusive or exclusive licensefor its commercial development should be ad-dressed to the Patent Counsel, NASA Manage-ment Office–JPL at (818) 354-7770. Refer toNPO-30323.

NASA Tech Briefs, July 2006 33

This Diamond Diode is one of two that would be sandwiched together with a thin film of 244Cm, whichis a source of α particles. The sandwich structure would constitute an alpha-voltaic device.

Ohmic Contact Al All Around

P-Doped Diamond Boron 1019/cm3

N- Doped Silicon Substrate 5 × 1015/cm3

NPO30323

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Top Ohmic Contact Gold

Alpha-Voltaic Sources Using Diamond as Conversion MediumCompact, long-lived solid-state devices would tolerate wide temperature ranges. NASA’s Jet Propulsion Laboratory, Pasadena, California

White-Light Whispering-Gallery-Mode Optical ResonatorsOverlapping resonator modes are exploited to obtain wide, high-Q spectra.NASA’s Jet Propulsion Laboratory, Pasadena, California

Whispering-gallery-mode (WGM) opti-cal resonators can be designed to exhibitcontinuous spectra over wide wavelengthbands (in effect, white-light spectra), withultrahigh values of the resonance qualityfactor (Q) that are nearly independent offrequency. White-light WGM resonatorshave potential as superior alternatives to(1) larger, conventional optical res-onators in ring-down spectroscopy, and(2) optical-resonator/electro-optical-modulator structures used in coupling ofmicrowave and optical signals in atomicclocks. In these and other potential appli-cations, the use of white-light WGM res-onators makes it possible to relax the re-quirement of high-frequency stability oflasers, thereby enabling the use ofcheaper lasers.

In designing a white-light WGM res-onator, one exploits the fact that thedensity of the mode spectrum increasespredictably with the thickness of the

Figure 1. Cleaved Optical Fibers tangent to the rim of the resonator disk are used to couple light intoand out of the disk. The fibers are shifted with respect to the middle of the rim to obtain a high de-gree of interaction with all the WGM modes.

ResonatorDisk

Cleaved Optical Fiber

Input Output

NPO42221 Fig 1 ABPI

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