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| Fig. 1: Diagram of a reverse-biased P-N junction diode. Since this is a reverse-biased junction, electrons remain on the p-side of the junction, while holes remain on the n-side. A standard p-n junction would invert the sides of the diode to which the electrons and holes flow. (Source: Wikimedia Commons) |
Betavoltaic fuel cells harvest power from the β decay of a nuclear isotope, enabling the development of power sources with decades-long lifetimes of operation. [1] A new form of nuclear battery, made from diamond, promises to significantly extend the lifetime of nuclear batteries and provide resistance against radiation damage to the cell during its use. [1] Nuclear batteries have applications in electronic systems which do not require frequent maintenance and can trade off energy supply quantity for consistency of energy supply. Examples include electronics operating in remote environments (such as spacecraft) or medical implants (such as pacemakers). [2,3]
The electron-voltaic effect was discovered by Paul Rappaport, when he was working at RCA labs. By bombarding silicon wafers with β particles from a 50 mCi Sr-90/Y-90 radioactive source, he deduced that the silicon wafers produced an electric potential, the maximum voltage of which is governed by the equation
| V | = | kT e |
ln( | Is I0 |
+1) |
In this equation, k is Boltzmann's constant, T is temperature, e is the electronic charge, I0 is the reverse saturation current (a parameter of the device which remains constant), and Is is the short-circuit current, which is dependent on radiation flux incident on the device. [4]
Rappaport's iteration of a betavoltaic cell delivered 0.8 μW of electric power from a total 200 μCi of radioactive power emitted by the source, giving a cell conversion efficiency of 0.4%. Rappaport found that an optimized wafer of the same design has the potential to produce electric potential with an efficiency of 2%. [4,5]
In 1972, L.C. Olsen characterized generated current from the electron-voltaic effect as the production of p-n electron-hole pairs. As we see in Fig. 1, upon being bombarded with radiation, electrons in a p-n junction diode are promoted from the valence band to the conduction band. The separation of electrons on the n-side of the junction and holes on the p-side, allows current to flow unidirectionally through the semiconductor and connected circuit. [6,7]
Though this effect can be likened to the production of energy from sunlight incident on photovoltaic cells, there are several important distinctions to be made. Regarding photovoltaics, the solar spectrum is largely comprised of photons of approximately 1.8-3.1 eV, from which we can derive the Shockley-Queisser limit for solar cell energy efficiency of 30% for an energy gap of between 0.54 eV and 0.93 eV. [8] Electron-voltaic cells are not bounded by this limitation: while photons only produce one p-n electron-hold pair, each emitted β particle can produce many because the average energy of an emitted β particle is thousands of times that of an optic photon. [7] Despite this, the energy potential of a betavoltaic is far from unbounded: the current state of the art betavoltaic cell, fabricated from Ni63-4H SiC attained a conversion efficiency of 6%. [9] Also of note, the electric potential produced by betavoltaic cells is dwarfed by that of solar cells, since the incident flux of beta particles is on the order of 106 less than solar flux. This severely limits the amount of energy that can be produced from electron-voltaic cells. [5,7] Additionally, when designing betavoltaic cells, designers must take into consideration isotope half-life (longer half-lives produce energy for a longer period of time), and the possible effects of radiation damage on the semiconductor; this is less of a concern in the design of photovoltaic cells. [7]
The concern of radiation damage to the semiconductor in betavoltaic cells spurred research into materials that exhibited the betavoltaic effect. Work by Bauer et al. shows that diamond is a radiation-hard material. [10] Furthermore, diamond is a wide-width semiconductor: wide-bandwidth semiconductors serve well as energy converters because they exhibit greater maximum conversion efficiencies. [11]
These observations prompted Bormashov et al. to design a nuclear battery based on Schottky-barrier diamond diodes. They sought to test the extent to which Schottky-barrier diamond diodes were promising materials (from the standpoints of energy output and cell degradation) in the design of nuclear batteries. They synthesized 130 energy cells based on Schottky-barrier diamond diodes, the energy output of which they then tested by exposing the cells to Ni-63 (β), Pm-147 (β), Sr-90/Y-90 (β), and Pu-236 (α) radiation sources. Of particular note are the results obtained with the Ni-63 source. The sample contained total internal activity of 5 mCi cm-2 and an average β emission energy of 17.4 keV. From a total source energy of 0.52 μW cm-2, their cell produced a maximum output power of 45 nW, for an overall energy conversion of 0.6%. Due to the long lifetime of Ni-63, however, they find that the battery produces a specific energy of 120 Wh/kg, which is on par with commercial cells currently in production. Furthermore, they exposed their cells to 1,400 hours of radiation from the Sr-90/Y-90 and observed no radiation degradation, which led them to conclude that the constraint on the lifetime of their cells is the half-life of the isotope, rather than the rate of cell degradation. [2]
In recognition of the select applications of betavoltaic cells being in compact environments (like medical implants), Bormashov et al. then worked to the output power density of betavoltaic cells. [12] To do so, they leveraged diamond's radiation-hardness to design a battery consisting of ultrathin diamond cells, layered with radioactive with Ni-63. While standard batteries would require a conductor, such as a foil, to act as an electrical connector between the diamond cells, the battery designed by Bormashov et al. leveraged the Ni-63 both as a power source and as an electrical connector between diamond cells. [12] In this way, their battery produced a power output density of 10 W/cm3, which is the highest known output power density for a nuclear battery powered by Ni-63. [12]
Though a promising area of research, nuclear batteries remain prohibitively expensive - although, as the cost of manufacturing decreases, they may well serve the applications stated earlier. [3,12] Diamond-based betavoltaic cells dramatically reduce radiation damage to the cell, which massively increases the longevity and output power density of betavoltaic batteries.
© Michael Cooper. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. Lanham, "Diamonds are Forever: Nuclear Diamond Batteries," Physics 241, Stanford University, Winter 2017.
[2] V. Bormashov et al., "Development of Nuclear Microbattery Prototype Based on Schottky Barrier Diamond Diodes", Phys. Status Solidi A 212, 2539 (2015).
[3] J. Park, "Review and Preview of Nuclear Battery Technology," Physics 241, Stanford University, Winter 2017.
[4] P. Rappaport, "The Electron-Voltaic Effect in p-n Junctions Induced by Beta-Particle Bombardment," Phys. Rev. 93, 246 (1954).
[5] L. C. Olsen et al., "Betavoltaic Power Sources," Physics Today 65, No. 12, 35 (December 2012).
[6] L. C. Olsen, "Betavoltaic Energy Conversion," Energ. Convers. 13, 117 (1973).
[7] S. Harrison, "Betavoltaic Devices," Physics 241, Stanford University, Winter 2013.
[8] W. Shockley and H. J. Queisser, "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells," J. Appl. Phys. 32, 510 (1961).
[9] B. Liu et al., "Power-Scaling Performance of a Three-Dimensional Tritium Betavoltaic Diode," Appl. Phys. Lett. 95, 233112 (2009).
[10] C. Bauer, et al., "Radiation Hardness Studies of CVD Diamond Detectors," Nucl. Instrum. Meth. A 367, 207 (1995).
[11] K. E. Bower et al. eds. Polymers, Phosphors and Voltaics for Radioisotope Batteries (CRC Press, 2002).
[12] V. S. Bormashov eta l., "High Power Density Nuclear Battery Prototype Based on Diamond Schottky Diodes," Diam. Relat. Mater. 84, 41 (2018).
[13] Y. Yemane, "Atomic Batteries, Physics 241, Stanford University, Winter 2011.
