Patent Number: 
Section: description

In the present invention, a beta cell is provided comprising a semiconductor junction device made of an icosahedral boride semiconductor, a radioisotope source of beta radiation, and means for transmitting electrical energy to an outside load. Because of the use of the icosahedral boride semiconductor, the beta cell of the present invention does not suffer the long-term conventional radiation-induced damage to a degree to significantly degrade the performance of the beta cell. Carrard et al. (M. Carrard, D. Emin; and L. Zuppiroli, Physical Review B, 1995, 51, 270-274) demonstrated that some boron compounds do not suffer accumulating damage from high-energy electron bombardment even at temperatures as low as 91K. These solids are called icosahedral borides. Icosahedral borides are solids primarily composed of boron atoms that form clusters whose atoms reside at the twelve vertices of icosahedra. Carrard et al. thus find that beta-induced damage to icosahedral borides spontaneously self-heals. The present invention relates to the use of icosahedral boride semiconductors in beta cells. In particular, the self-healing of beta-induced damage in icosahedral boride semiconductors permits beta cells based on icosahedral boride semiconductors to utilize sources that emit high-energy beta particles such as 90Sr or 170Tm. Because of self-healing, the lifetimes of icosahedral boride beta cells are limited by the rate of decay of the radioisotope energy source rather than by radiation damage to the semiconductor. Examples of icosahedral boride semiconductors include B12As2, B12P2, elemental boron in both its xcex1-rhombohedral and xcex2-rhombohedral structures, and boron carbides, B12-xC3-x, where 0.15 less than x less than 1.7 (the single phase region of B12-xC3-x). Room-temperature carrier mobilities in several icosahedral boride semiconductors, B12As2, B12P2, and xcex1-rhombohedral boron, are comparable to those of semiconductors that are commonly utilized in solar cells. These mobilities are high enough for these icosahedral borides to operate efficiently in beta cells. Sources of beta radiation include the radioisotopes 90Sr, 147Pm, 170Tm, 3H, 63Ni, 137Cs, 141Ce, and 204Tl, and compounds containing these radioisotopes. One embodiment of the present invention, shown in FIG. 1, comprises a Schottky-barrier junction device 10, a beta-emitting radioisotope stratum 21 that emits beta radiation 22, and means 31 for transmitting the produced,electrical energy to a load. The Schottky-barrier device can be formed by depositing a thin metal contact 11 that serves as a Schottky barrier (a non-Ohmic contact), for example Au, on an icosahedral boride semiconductor 12. The thickness of the metal contact, typically 0.1 to 0.5 microns, is kept small to minimize loss of beta-particles"" energy 22 as it passes through the metal. The icosahedral boride semiconductor may be a film of typical thickness 0.1 to 100 microns deposited on a substrate such as SiC or a metal diboride (such as NbB2, TiB2, ZrB2, HfB2, TaB2) or a free-standing icosahedral boride semiconductor. Another, Ohmic, metal contact 13 on the unirradiated back side of the sandwich completes the electrical circuit. The layer of the beta-emitting radioisotope, for example 90Sr or compounds containing 90Sr such as 90SrTiO3, is typically of thickness 0.1 to 50 microns. The maximum thickness of the beta-emitting radioisotope stratum will be fixed for each radioisotope by the self-absorption depth of the radioisotope, beyond which no beta particles can escape the stratum. Another embodiment of the invention, a variation of the beta cell illustrated in FIG. 1, comprises a p-n junction icosahedral boride semiconductor device and a beta-emitting radioisotope stratum (see FIG. 2). Boron carbides and xcex2-rhombohedral boron are intrinsically p-type and native defects in both B12As2 or B12P2 frequently render them p-type. The p-type region 15 can also be established by incorporating a p-dopant, for example substituting Si, Ge, or C, for As or P in B12As2 or B12P2. The n-type region 16 is established by incorporating an n-dopant, for example S, Se, or Te for P or As in B12As2 or B12P2. The thickness of the n- and p-type regions are typically between 0.1 and 100 microns. The beta-emitting radioisotope is selected from for example 90Sr, 147Pm, and 170Tm. As with the prior embodiment, the optimal thickness of the radioisotope stratum is determined by its self-absorption length. Electrical leads from the p-type region and the n-type region connect the p-n junction device with the rest of the electrical system. Another embodiment of the invention, illustrated in FIG. 3, comprises a 3-dimensional stack 40 incorporating alternate strata (generally approximately uniform layers) of beta-emitting radioisotope 41 and icosahedral boride junction devices that can comprise both p-type regions 42 and n-type regions 43. As illustrated in FIG. 3, a 3-dimensional stack utilizes beta particles 44 emitted from both faces of radioisotope strata. Furthermore, beta particles emitted from radioisotope layers can have ranges that allow them to traverse several semiconductor junction devices in the stack. The 3-dimensional stack thus permits more efficient collection of beta-particles energies. Individual junction devices within this stack can be either Schottky-barrier or p-n junction devices as described in the prior embodiments. The means for transmitting the produced electrical energy are not shown in FIG. 3. The beta cell is generally enclosed within a metal shield (case), as illustrated in FIG. 4. The thickness and type of material of the shield. 53 is such that radiation produced by the beta cell is attenuated to desired levels outside the case. The electrical output of the stack (such as the stack 40 illustrated in FIG. 3) is established across a positive terminal 51 and a negative terminal 52. In general, at least one terminal projects through an opening in an electrically insulating cap 54 in the shield. The shield material and its thickness is selected based on the beta source and the application. For example, for when minimal or no shielding is required, an aluminum case could be appropriate. For other embodiments that require shielding, the shield could be made from such metals as lead or depleted uranium. The beta cell of the present invention can be utilized in a variety of configurations, including both series and parallel combinations to achieve the desired output currents and voltages. Additionally, the type of beta source and the configuration of the beta source with respect to the type of icosahedral boride material and layer thickness and number of layers of icosahedral bride material can be varied to achieve the desired electrical energy output. One embodiment of the present invention is specifically illustrated by a configuration of the cell of FIG. 5. A Schottky-barrier device 60 was created by depositing separate gold contacts 61 onto the surface of a film of semiconducting B12As2 64, which was layered on top of a SiC substrate 65. Electrical leads 66 were attached to the Au contacts to transmit the produced electrical energy to a load. In this example, the B12As2 was a p-type semiconductor. In this example, the thickness of the B12As2 film was approximately 0.1 micron. A beam of energetic electrons 63, from a source 62, was caused to impinge upon one of the Au contacts. The thickness of this Au contact was approximately 0.1 micron. When the energy of the impinging electrons was insufficient to penetrate this thickness of Au, less than about 10 keV, no emf was measured across the bombarded junction. When the energy of the incident electron beam was sufficient to penetrate through the Au contact, an emf was produced. This emf was caused by the separation of the electron-beam induced electron-hole pairs in the B12As2 semiconductor. The open-circuit emf of the Schottky-barrier device was measured relative to the non-bombarded junction. A 40 keV beam generated an open circuit emf of 800 mV. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.