In general, a radioisotope is an element which emits radiation having a specific energy and then decays into a stable isotope. Herein, decay modes include so-called EC decay, in which an atomic nucleus captures K-orbital electrons, in addition to α, β−, and β+ decay. Most of radioisotopes emit extra energy as alpha rays, beta rays, or gamma rays to become stable isotopes. The amount of the radioisotope is represented by radioactive intensity, that is, the number of decays per unit time. The time required for the amount of an radioactive element to be reduced to half its initial value by radioactive decay is denoted as a half-life period, wherein since the period is constant depending on a radioisotope, the radioisotope may emit radiation from a few years to a few hundred years depending on the half-life period.
A beta voltaic battery is a battery which is realized by using a P—N semiconductor that absorbs beta rays emitted from a radioisotope such as nickel (Ni-63) and promethium-147 (Pm-147). The beta voltaic battery, as a conventional isotope battery using beta rays, may generate a current by disposing a Ni-63 foil or sealed ray source, as a beta-ray emitter, on a semiconductor having a silicon P—N junction structure to absorb beta rays. As described above, the semiconductor P—N junction process and the fabrication of the beta-ray source are separately performed, wherein, since beta rays are emitted to the outside in this case, a separate shielding package must be used in the outside of the isotope battery in order to shield the beta rays. In this case, absorption rate and dose of the beta rays absorbed at the P—N junction and energy conversion efficiency of the semiconductor structure directly affect output efficiency.
When techniques related to the above-described beta voltaic battery are examined, Korean Patent Application Laid-open Publication No. 10-2014-0129404 discloses a radioisotope battery and a method of manufacturing the same. Specifically, in the above prior art document, provided are a radioisotope battery, in which the manufacture of the radioisotope battery as well as the shielding of radiation emitted by radioisotope Ni-63 from the outside are achieved, and a method of manufacturing the same. However, in a case in which coating is performed on a metal seed layer as described above, the shielding may occur in the seed layer. Accordingly, output may be reduced due to the low absorption of beta-rays.
As described above, the radioisotope, as an energy source of the battery, is mainly used in the form of a sealed ray source or in the form in which it is electroplated on the surface of the metal seed layer. Particularly, with respect to Ni-63 among the radioisotopes, since its energy is low at 66.945 keV, it may not damage semiconductor chips. However, there are limitations in that self-shielding may occur and a penetration depth of beta rays may be relatively small. Accordingly, since the absorption of beta rays is low, the output may be reduced. With respect to the sealed ray source, sealing is performed by coating the surface of a Ni-63 layer with Ni, wherein, in a case in which the sealing layer is plated on the seed layer, shielding of beta rays may occur in the seed layer, and thus, the absorption of beta rays may almost not occur.
Accordingly, while studying a beta voltaic battery having an excellent energy conversion efficiency, the present inventors developed a beta voltaic battery having a sandwich structure by combining two semiconductor layers with a beta-ray generator, in which both sides of a metal substrate are coated with a radioisotope ray source so as to be directly in contact with the semiconductor layer in the structure of the sealed ray source, and found that, since the beta voltaic battery having a sandwich structure has no sealing layer, the absorption of beta rays by the semiconductor may be improved and the output is improved due to the radioisotope ray source coated on the both sides, thereby leading to the completion of the present invention.