With very high energy densities of 1-10 MJ/cc (compared to 1-20 kJ/cc for conventional electrochemical and hydrocarbon fuels), and a long half-life of 1-100 years, radioisotope fueled batteries are ideal for applications requiring compact, long lifetime power supplies, such as remote sensing and implantable devices. Furthermore, low energy β emitters (63Ni, 147Pm, etc.) have little or no safety concerns, and Promethium-147 powered betavoltaics have been implanted inside humans for powering cardiac pacemakers in the past.

     To achieve compact radioisotope batteries, the power density of the device should be as high as possible. The power output density of a betavoltaic battery can be expressed as follows Pout = PfuelFFFη fuelηβ (1) where Pfuel is the fuel power density, FFF is the fuel fill factor (volume percentage of the radioisotope fuel), ηfuel is radioisotope thin-film emission efficiency, and ηβ is betavoltaic conversion efficiency. Pfuel and ηfuel are determined by the radioisotope material, but FFF and ηβ are not.

     In the current invention, FFF and ηβ are improved to maximize the power density of a betavoltaic battery. To improve FFF without sacrificing conversion efficiency, only non-active substrate regions are etched away. The thicknesses of the SiC and silicon wafers range from 300μm to 500μm, where only the top ~ 20μm is the active functioning region for a betavoltaic battery. A FFF improvement of 8X can be achieved by thinning down the non-active substrate to 30μm. Furthermore, in a planar device, all of the electrons irradiated away from substrate are wasted, which is 50% of all electrons. By stacking thinned-down devices together, all of the electrons will be utilized, which decreases the radioactivity needed for a battery. Besides higher FFF values, less radioactivity also provides a safer device.

Figure 1. Comparison of optimized betavoltaic battery with traditional.

Potential Applications

• Long-term embedded power sources for medical, construction, defense, and other industries

Advantages

• Greatly improved power density
• Lower radioactivity

File Number: 4941