RESUMO
Unattended, compact, terrestrial and space sensors require sources that have high energy and power densities to continuously operate for 3 to 99 years depending on application. Currently, chemical sources cannot fully satisfy these applications, especially in solid state form. Betavoltaic (ßV) nuclear batteries using ß--emitting radioisotopes possess energy densities 1000 times greater than conventional chemical sources. Their power density is a function of ß- flux saturation point relative to the planar (2D) configuration, ß- emission range, and the semiconductor converter, the betavoltaic (ßV) cell, properties. The figure of merit is the beta (ß-)-flux surface power density ( [Formula: see text] in µWn per cm2 footprint), where an optimal portion of incident beta particles penetrates the surrounding semiconductor depletion region. Tritiated nitroxides are favorable radioisotope sources with the potential to have the highest specific activity (Am in Ci/g) and [Formula: see text] for an organic compound in solid form. The goal of this research is to demonstrate a tritiated nitroxide nuclear battery using the planar (2D) coupling configuration. The reproducible tritiation procedure produced stable product with a Am of approximately 635 Ci/g, which was 70% of the theoretical Am. For the nuclear battery demonstration, the tritiated nitroxide, dissolved in methanol, was deposited on a 4H-SiC ßV and InGaP photovoltaic (PV) cell using a dispensing apparatus and micropipette. Both devices' characteristics were measured beforehand using a controlled electron beam source to approximate the surface radioactivity from the deposited radioisotope. The maximum power point (MPP) of the 4H-SiC and InGaP were 7.77 nW/cm2 and 1.63 nW/cm2 with 100 mCi and 67 mCi, respectively. The power and total efficiency were lower than expected due to partial solvent evaporation and droplet thickness. Numerical models using MCNP6 Monte Carlo code were used to simulate an optimal nuclear battery prototype. The models' accuracy was confirmed with the device calibration curves and a previous metal tritide model based on empirical results. Based on optimal model results, the tritiated nitroxide saturation layer thickness (D0.99) and [Formula: see text] (D0.99) were 10 µm and 558 nW/cm2, respectively, using a 4H-SiC.
RESUMO
Beta radioisotope energy sources, such as tritium (3H), have shown significant potential in satisfying the needs of a sensor-driven world. The limitations of current beta sources include: (i) low beta-flux power, (ii) intrinsic isotope leakage and (iii) beta self-absorption. The figure of merit is the beta-flux power (dPß/dS in µWn/cm2), where an optimal portion of incident beta particles penetrates the semiconductor depletion region. Thus, the goal of this research was to identify a compound to contain a beta emitter that can permit beta-flux power of at least 0.73 µWn/cm2 from one side, where it can be used for both planar and textured semiconductor structures. Nitroxides were chosen because of previous demonstrated deuteration, ease of synthesis, diversity of structure, and pliability. As a proof-of-principle, nitroxide [1] was prepared and tritiated with a specific activity of 103Ci/g. The corresponding tritiated nitroxide in toluene was found to have no measurable 3H2 outgassing after 27 days, thus it was considered stable. After 256 days in solution, analysis of the compound showed only 2% tritium loss, whereas in solid form, there was approximately 50% of tritium loss after 21 days. To compare with the performance of a typical metal tritide carrier, the standard MCNPX Monte Carlo code was used to calculate the beta-flux power of tritiated nitroxide and titanium tritide (0.2 µWn/cm2 and 0.70 µWn/cm2), respectively. The difference between numerical and empirical results of titanium tritide was 4%, showing the model validity. For the tritiated nitroxide to be comparable to titanium tritide in a planar configuration (2-D), the gravimetric density (3H weighted percentage) would need to be at least 9%.
