Planar and textured surface optimization for a tritium‐based betavoltaic nuclear battery

Abstract

Remote, terrestrial, and space sensors require sources that have high enough power and energy densities for continuous operation for multiple decades. Conventional chemical sources have lower energy densities and lifetimes of 10 to 15 years depending on environmental conditions. Betavoltaic (βV) nuclear batteries using β--emitting radioisotopes possess energy densities approximately 1000 times greater than conventional chemical sources. Their electrical power density (Pe,vol in W/cm3) in a given volume is a function of β--flux surface power density , surface interface type between radioisotope and transducer, β- range, and transducer thickness and conversion efficiency (ηs). Tritium is the most viable β--emitting radioisotope because of its commercial availability, low biotoxicity, half-life, and low energy, which minimizes the penetration depth and damage of transducer. To maximize Pe,vol, tritium in solid or liquid form must be used in the βV nuclear battery. A Monte Carlo source model using MCNP6 was developed to maximize the Pe,vol of a tritium-based βV nuclear battery. First, a planar coupling configuration with different tritiated compounds (ie, titanium tritide and tritiated nitroxide) and a semiconductor transducer (4H-SiC) with thicknesses of 1 and 100 μm were modeled. The results showed that β--source efficiency (ηβ), which is the percentage of energy deposited in the transducer, decreased as the tritiated compound's mass density increased. The highest Pe,vol was dependent on a combination of characteristics: specific activity (Am in Ci/g), mass density, and 4H-SiC layer thickness. The tritiated nitroxide with the highest Am at 2372 Ci/g produced the highest Pe,vol at 2.46 mW/cm3. Second, a 3-D coupling configuration was modelled to increase surface interfacing between the radioisotope source and textured transducer surface. 3-D coupling configuration increased the percentage of energy deposited into the transducer because of more surface interfacing between the transducer and source in the same volume. The tritiated nitroxide was selected as the radioisotope source coupled with five different textured surface feature types. The Pe,vol as a function of textured surface feature and gap, where the radioisotope is located, width was calculated for 1- and 100-μm 4H-SiC layer thicknesses. Results showed that ηβ increased compared with planar coupling configuration (ie, approximately 56.2% increase over planar with cylindrical hole array) except with the rectangular pillar array. Still, the rectangular pillar array produced the highest Pe,vol at 4.54 mW/cm3 with an increasing factor of 2.29 compared with the planar coupling configuration.