Nuclear power—with its high energy density and low carbon footprint—is a source with which the international community has decades of experience. However, the challenges that come along with the technology have kept it from becoming a more dominant factor in the global energy mix. Geopolitical issues lie at the center of many of these challenges. Matters of safety, waste management, and proliferation are intrinsic to the technology. Although the carbon footprint of using nuclear fuels is smaller, there are still disadvantages of using nuclear fuel. The waste, while a much lower volume must be handled very carefully because of its radioactivity. Nuclear fuels require far more complicated systems to extract their energy, which calls for greater regulation.
One of the solutions to Nuclear waste is an atomic battery, nuclear battery, radioisotope battery or radioisotope generator is a device which uses energy from the decay of a radioactive isotope to generate electricity. Like nuclear reactors, they generate electricity from nuclear energy, but differ in that they do not use a chain reaction. Although commonly called batteries, they are technically not electrochemical and cannot be charged or recharged. In comparison they are very costly, but have an extremely long life and high energy density, and so they are mainly used as power sources for equipment that must operate unattended for long periods of time, such as spacecraft, pacemakers, underwater systems and automated scientific stations in remote parts of the world.
In 1953, Paul Rappaport proposed the use of semiconducting materials to convert the energy of beta decay from radioactive substances into electricity. Beta particles—electrons and positrons—emitted by a radioactive source ionize atoms of a semiconductor, creating uncompensated charge carriers. In the presence of a static field of a p-n structure, the charges flow in one direction, resulting in an electric current. Batteries powered by beta decay came to be known as betavoltaics. When compared to chemical batteries, nuclear batteries are characterized by higher volumetric energy density (therefore longer battery life) and stronger endurance in harsh conditions.
The technology requires a large space to capture escaping heat inside semiconductors effectively. The shortcomings of RTG technology are its poor efficiency of 6%, its low power density, and its large size. Betavoltaic cells, also known as betavoltaic devices, are a nuclear battery technology used in small devices that cannot use Radioactive Thermoelectric Generators.
Conventional chemical or “galvanic” batteries, like the lithium-ion cells in a smartphone or the alkaline batteries in a remote, are great at putting out a lot of power for a short amount of time. A lithium-ion battery can only operate for a few hours without a recharge, and after a few years it will have lost a substantial fraction of its charge capacity.
The chief advantage of betavoltaic cells over galvanic cells is their longevity. Radioactive isotopes used in nuclear batteries have half-lives ranging from tens to hundreds of years, so their power output remains nearly constant for a very long time. Unfortunately, the power density of betavoltaic cells is significantly lower than that of their galvanic counterparts. Despite this, betavoltaics were used in the 1970s to power cardiac pacemakers, before being phased out by cheaper lithium-ion batteries, even though the latter have shorter lifetimes.
In 2009, MIT review reported on a new version of the batteries, called betavoltaics, being developed by an Ithaca, NY-based company and tested by Lockheed Martin. The batteries could potentially power electrical circuits that protect military planes and missiles from tampering by destroying information stored in the systems, or by sending out a warning signal to a military center. The batteries are expected to last for 25 years. The company, called Widetronix, is also working w ith medical-device makers to develop batteries that could last decades for implantable medical devices.
The technology is now reemerging, says Peter Cabauy, CEO of another betavoltaic company, Miami-based City Labs, because semiconducting materials have improved so much. Early semiconducting materials weren’t efficient enough at converting electrons from beta decay into a usable current, so they had to use higher energy, more expensive–and potentially hazardous–isotopes. More efficient semiconducting materials can be paired with relatively benign isotopes such as tritium, which produce weak radiation.
The University of Bristol posted a press release in 2016 introducing another possible next generation nuclear battery technology using carbon isotopes in the form of diamonds. Nuclear power generation produces radioactive waste that cannot be easily disposed. In United Kingdom alone, 95,000 tons of radioactive C-14 are deposited and decaying. Researchers at the University of Bristol discovered a way to heat and gasify the radioactive C-14 concentration on the surface of deposited nuclear graphite wastes, and condense the gas into artificial diamonds.
A man-made diamond generates an electric current when placed in a radiation field, and a diamond made of C-14 produces a radioactive field spontaneously. Hence, the diamond battery can create a constant electric current as long as it remains radioactive. Although C-14 can deliver only 15 joules per gram (compared to 700 joules per gram of standard alkaline battery), the C-14 diamond battery can generate power for 7746 years before reaching 50% charge (compared to a single day usage of standard alkaline battery). The C-14 diamond can be encapsulated in a non-radioactive diamond shell which will block all radiation and protect the battery under harsh conditions. The resulting battery is made of the hardest material on Earth, so the industry might finally overcome the psychological resistance of sensitive clients such as patients using IMDs.
Nuclear Battery Applications
The emerging IoT revolution demand smart, integrated, miniaturised and low-energy wireless nodes, typically powered by non-renewable energy storage units (batteries). The latter aspect poses constraints as batteries have a limited lifetime and often their replacement is impracticable. When the cost of manufacturing nuclear batteries decreases, low-power internet-of-things devices could also be powered cord-free for hundreds of years with a single charge using this revolutionary technology one day.
Space exploration poses unique challenges that are not faced when working with electronics on Earth. It is impossible or extremely costly to access a device once it has been launched into the space. Because only a small percentage of sunlight reaches the outer perimeter of the solar system compared to the orbit of Earth, solar energy is not a practical solution to powering electronic equipments when exploring the outer planets. NASA uses a specific type of nuclear battery technology called Radioactive Thermoelectric Generator (RTG) to power their spacecrafts in missions that last over 10 years.
Implantable medical devices (IMDs) also utilize the unique characteristics of nuclear batteries, for example, pacemakers and hearing aids. Just like in spacecrafts, batteries used to power IMDs must function reliably over a long period of time without being accessed for recharge or maintenance. Unlike in spacecrafts, however, batteries used in IMDs must be limited in size and radioactivity. Hence, a different nuclear battery technology called betavoltaic cell is used in IMDs. Although the technology was invented and widely used for patients in the 1970s, the potential risk of radiation convinced the medical industry to shift to lithium ion batteries in the 1980s. Only with the recent advancement in safety of nuclear batteries, the option with a considerable advantage in battery life is being reconsidered.
In 2018, a team of researchers from the University of Bristol built a robot volcanologist and used a drone to ferry it to the top of the volcano, where it could passively monitor its every quake and quiver until it was inevitably destroyed by an eruption. The robot was a softball-sized sensor pod powered by microdoses of nuclear energy from a radioactive battery the size of a square of chocolate. The researchers called their creation a dragon egg. Scott and a small group of collaborators have been developing a souped-up version of the dragon egg’s nuclear battery that can last for thousands of years without ever being charged or replaced.
In Nov 2020, Scott and his collaborator, a chemist at Bristol named Neil Fox, created a company called Arkenlight to commercialize their nuclear diamond battery. Morgan Boardman, Arkenlight’s CEO said the company is looking at applications where it is either impossible or impractical to regularly change a battery, such as sensors in remote or hazardous locations at nuclear waste repositories or on satellites. Boardman also sees applications that are closer to home, like using the company’s nuclear batteries for pacemakers or wearables. He envisions a future in which people keep their batteries and swap out devices, rather than the other way around. “You’ll be replacing the fire alarm long before you replace the battery,” Boardman says.
Russian Prototype nuclear battery packs 10 times more power
Team of Russian scientists have come up with a new nuclear design based on nickel-63, which is said to be more efficient than other commercially available batteries. Their new battery prototype packs about 3,300 milliwatt-hours of energy per gram, which is more than in any other nuclear battery based on nickel-63, and 10 times more than the specific energy of commercial chemical cells.
A research team led by Vladimir Blank, the director of TISNCM and chair of nanostructure physics and chemistry at MIPT, came up with a way of increasing the power density of a nuclear battery almost tenfold. The physicists developed and manufactured a betavoltaic battery using nickel-63 as the source of radiation and Schottky barrier-based diamond diodes for energy conversion. The prototype battery achieved an output power of about 1 microwatt, while the power density per cubic centimeter was 10 microwatts, which is enough for a modern artificial pacemaker. Nickel-63 has a half-life of 100 years, so the battery packs about 3,300 milliwatt-hours of power per 1 gram—10 times more than electrochemical cells.
The work reported in this story has prospects for medical applications. Most state-of-the-art cardiac pacemakers are over 10 cubic centimeters in size and require about 10 microwatts of power. This means that the new nuclear battery could be used to power these devices without any significant changes to their design and size. “Perpetual pacemakers” whose batteries need not be replaced or serviced would improve the quality of life of patients.
The space industry would also greatly benefit from compact nuclear batteries. In particular, there is a demand for autonomous wireless external sensors and memory chips with integrated power supply systems for spacecraft. Diamond is one of the most radiation-proof semiconductors. Since it also has a large bandgap, it can operate in a wide range of temperatures, making it the ideal material for nuclear batteries powering spacecraft.
Russian scientists have developed an economical atomic battery with power increased by 10 times
Scientists from NUST MISIS have presented an innovative autonomous power source – a compact atomic battery that can last up to 20 years. Due to the original 3D-structure of the beta-voltaic element, its dimensions have decreased by three times, the specific power has increased by 10 times, and the cost has decreased by 50%. The results have been published in the international scientific journal Applied Radiation and Isotopes.
The original patented microchannel 3D structure of a nickel beta-voltaic element is used in the device. Its peculiarity is that the radioactive element is applied on both sides of the so-called planar p-n junction, which simplifies the cell manufacturing technology, as well as the control of the reverse current that “steals” the battery power. The special microchannel structure provides an increase in the effective conversion area of the beta radiation by 14 times, which results in an overall increase of current.
“The output electrical parameters of the proposed design were: short-circuit current IKZ – 230 nA /cm2 (in the usual planar construction – 24 nA), the final power – 31nW /cm2, (in the planar one – 3nW). The design allows to increase the efficiency of converting the energy released during the decay of a β-source into electricity by an order of magnitude, which in the future will reduce the cost of the source by about 50% due to the rational use of an expensive radioisotope,” said one of the developers Sergey Legotin, Associate Professor of the Department of Semiconductor Electronics and Physics semiconductors at NUST MISIS.
At the same time, the development will make it possible to increase the specific power by an order of magnitude, due to which the weight and dimensions of the batteries based on them will decrease three times while maintaining the required output power level. The battery can be used in several functional modes: as an emergency power supply and temperature sensor in devices used at extreme temperatures and in hard-to-reach (or completely inaccessible) places: in space, under water, in high-altitude areas.
At the moment, the developers are completing the procedure for international patenting of the invention, and the device itself has already been recognized by foreign experts. In particular, in the review of the international marketing research agency Research and Markets, NUST MISIS has been named one of the key players in the global betavoltaic batteries market. The university ranks among such companies as City Labs, BetaBatt, Qynergy Corp and Widetronix. The review indicates that the development of NUST MISIS scientists – a battery based on beta-voltaic cells (BVE) – has great potential since the demand for reliable batteries with a long service life is growing in all industries. Taking into account the unique characteristics – small size and safety – the development of NUST MISIS scientists will be able to occupy a significant share of the power supply market.
There is an alternative radioisotope for use in nuclear batteries: Dimond converters could be made using radioactive carbon-14, which has an extremely long half-life of 5,700 years. Work on such generators was earlier reported by physicists from the University of Bristol. Producing a large amount of nickel-63 is a bit difficult, but the team has assured that the bulk production on the industrial level will roll out in the next decade. They have also established methods for the large scale production of the thin diamond layers. The team plans to improve the design in the future for increasing the power density.
Widetronix’s batteries are powered by the decay of a hydrogen isotope called tritium into high-energy electrons. This type of nuclear decay is called “beta decay,” for the high-energy electrons, called beta particles, that it produces. The lifetimes of betavoltaic devices depend on the half-lives, ranging from a few years to 100 years, of the radioisotopes that power them.
To make a battery that lasts 25 years from tritium, which has a half-life of 12.3 years, Widetronix loads the package with twice as much tritium as is initially required. These devices can withstand much harsher conditions than chemical batteries. This, and their long lifetimes, is what makes betavoltaics attractive as a power source for medical implants and for remote military sensing in extremely hot and cold environments.
Widetronix’s batteries are made up of a metal foil impregnated with tritium isotopes and a thin chip of the semiconductor silicon carbide, which can convert 30 percent of the beta particles that hit it into an electrical current. “Silicon carbide is very robust, and when we thin it down, it becomes flexible,” says Widetronix CEO Jonathan Greene. “When we stack up chips and foils into a package a centimeter squared and two-tenths of a centimeter high, we have a one microwatt product.” The prototype being tested by Lockheed Martin produces 25 nanowatts of power.
Betavoltaics aren’t very powerful. They don’t have nearly enough power to drive a laptop or a cell phone. But their energy density is high: they store a lot of energy in films just micrometers thick and can be made in very small packages. “We’re focusing on places where you need a very long life and energy density,” says Greene.
One such place is the monitoring of military equipment. “Everything the Department of Defense puts out has to have antitamper protection so that if someone gets their hands on the seeker head of a missile, or an entire aircraft, it would be very difficult to reverse-engineer it,” says Christian Adams, a chemist at Lockheed Martin Missiles and Fire Control. The memory chips that control such antitamper systems, says Adams, require very low continuous power over a long time. Military specifications also require that these devices withstand extreme conditions that normal batteries can’t tolerate: they must operate in temperatures from -65 to 150 ˚C and withstand high-frequency vibrations, high humidity, and blasts of salt. “If the battery freezes out or dies out, the memory circuit loses its configuration,” and the device fails, says Adams.
Lockheed is also working with the company to develop higher-power betavoltaics for remote monitoring of missiles. Sending out a radio signal to say “I’m healthy” requires microwatts of power, says Adams. Widetronix is also testing its batteries with medical-device company Welch Allyn. It expects to sell the batteries for $500.
The future promise of betavoltaics might be in very cheap sensors embedded in buildings and bridges where “you don’t ever want to change the battery,” says Amit Lal, professor of electrical and computer engineering at Cornell University. However, this would require companies such as Widetronix to move to longer half-life materials, such as nickel isotopes that last 100 years.
While tritium has a half-life of only 12.3 years, one of its chief advantages, besides safety, is that it can be secured cheaply from Canadian nuclear reactors that produce heavy water as a by-product. Longer half-life isotopes such as nickel-63 must be purchased abroad at high prices. “Since the end of the Cold War, there is no government support for radioisotope infrastructure in the United States,” says Lal. “Making batteries that last forever is probably good reason to build that infrastructure.”
Nuclear Battery Technology
Radioactive Thermoelectric Generator (RTG)
Radioactive Thermoelectric Generator uses heat generated spontaneously from radioactive substances. The technology requires a large space to capture escaping heat inside semiconductors effectively. The shortcomings of RTG technology are its poor efficiency of 6%, its low power density, and its large size.
NASA calls their technology Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), and in 2016, NASA announced the next generation Enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTG). As Fig. illustrates, eMMRTG improves the original MMRTG with a new thermoelectric technology called Thermoelectric Couple Assembly. eMMRTG’s improved efficiency will also help NASA save plutonium which is in extreme shortage in the United States.
Betavoltaic cells, also known as betavoltaic devices, are a nuclear battery technology used in small devices that cannot use Radioactive Thermoelectric Generators. Betavoltaic cells utilize beta-decay of isotopes such as tritium. Tritium is a byproduct of nuclear power plants, so manufacturing betavoltaic cells with tritium is an excellent way to turn nuclear wastes into useful goods. The shortcoming of betavoltaic cells in, comparison to chemical batteries, is the low power output. According to Jonathane Greene, the CEO of Widetronix which manufactures betavoltaic cells, a package that is one centimeter-squared wide and two-tenths of a centimeter tall generates one microwatt of power. In comparison, a smartphone using 50% CPU, Wi-Fi connection, and white display will use 1857 mW, so a nuclear battery is not suitable for consumer electronics.
Aqueous Nuclear Battery
Baek Hyun Kim and Jae Won Kwon at University of Missouri published a paper in 2014 proposing one possible next generation nuclear battery technology. Aqueous Nuclear Battery, which is also known as water-based nuclear battery, uses liquid medium for radiolysis, absorbing the kinetic energy of beta particles which is lost in betavoltaic cells. In Kim and Kwon’s design using nanoporous titanium dioxide semiconductors coated in platinum, a high efficiency of 53.88% was reached at a potential of 0.9 volts. Using an aqueous solution for radiolytic energy conversion results in higher energy level and lower temperature than using a solid state material does.
Diamond Nuclear Battery
The University of Bristol posted a press release in 2016 introducing another possible next generation nuclear battery technology using carbon isotopes in the form of diamonds. Nuclear power generation produces radioactive waste that cannot be easily disposed. In United Kingdom alone, 95,000 tons of radioactive C-14 are deposited and decaying. Researchers at the University of Bristol discovered a way to heat and gasify the radioactive C-14 concentration on the surface of deposited nuclear graphite wastes, and condense the gas into artificial diamonds. A man-made diamond generates an electric current when placed in a radiation field, and a diamond made of C-14 produces a radioactive field spontaneously. Hence, the diamond battery can create a constant electric current as long as it remains radioactive. Although C-14 can deliver only 15 joules per gram (compared to 700 joules per gram of standard alkaline battery), the C-14 diamond battery can generate power for 7746 years before reaching 50% charge (compared to a single day usage of standard alkaline battery). The C-14 diamond can be encapsulated in a non-radioactive diamond shell which will block all radiation and protect the battery under harsh conditions. The resulting battery is made of the hardest material on Earth, so the industry might finally overcome the psychological resistance of sensitive clients such as patients using IMDs.
Development of Nano-Diamond Batteries by NDB
In 2020, NDB announced two proof-of-concept tests conducted at the Cavendish Laboratory at Cambridge University and Lawrence Livermore National Laboratory in California. As stated above, the nano-diamond battery from the NDB used nuclear waste to generate power. The radioactive core is protected with multiple layers of synthetic diamonds or polycrystalline diamond.
The polycrystalline diamond is an exceptionally thermally conductive material. This material also can contain the radiation within the device. The use of a polycrystalline diamond makes the nano-diamond battery immensely tough and tamperproof.
Technologies behind the development of nano-diamond batteries that ensure radiation, thermal, and mechanical safety are discussed below:
- Diamond Nuclear Voltaic (DNV) is a device that consists of a semiconductor. Individual units are connected to form a stack arrangement and fabricated to create a positive and negative contact surface analogous to a standard battery system. This design improves the system’s overall efficiency, which includes the generation of a substantial amount of electricity and a multi-layer safety shield for the product.
- All radioactive isotopes can produce high amounts of heat energy. A single crystalline diamond (SCD) in the DNV unit and the strategic placement of radioactive source between the DNV units prevents self-absorption of heat by the radioisotope.
- NDB technology has utilized alpha, beta, and neutron radiations using boron-10 doping, helping to convert the extra neutron into the alpha ray. This design also enables the rapid conversion of radiation to usable electricity.
- The advanced flexible structural design enables it to take any shape based on its application. This feature makes NDB extremely market-friendly.
- The utilization of radioactive waste is a subject that many have not researched. NDB uses radioactive waste and reuses them by reprocessing and recycling. This technology ensures sustainability and gives rise to a clean energy source, and Achieving this has the added advantage of ensuring environmental safety.
Researchers believe that this technology would reduce the costs and challenges of storing nuclear waste in the most useful form. NDB envisioned the coexistence of innovation and restoration of a healthy environment. Implementing their innovative technology would improve the standards of living and pave the way towards the development of eco-friendly, green, and sustainable energy.
This battery could bring about a revolution in the world of electric cars. Researchers believe that this technology will benefit the electric car industry due to its immense longevity and efficiency, unlike any other existing batteries. Recent advancements in space technology include electric aircraft development that has created the demand for batteries with longevity and safety. Space vehicles and satellites are currently supported by solar power, which is subjected to an unsettling space environment. NDB powers electric aircraft, drones, and space stations for a more extended period.
The use of NDB for powering standard electronic devices such as laptops and smartphones negates the need to charge such devices continually. NDB claims the use of their product would benefit the consumers by providing them with power outlet independent devices and increasing personal quantum computing and the device’s computational power
A thermionic converter consists of a hot electrode, which thermionically emits electrons over a space-charge barrier to a cooler electrode, producing a useful power output. Caesium vapor is used to optimize the electrode work functions and provide an ion supply (by surface ionization) to neutralize the electron space charge.
Radioisotope-powered cardiac pacemaker being developed by the Atomic Energy Commission, is planned to stimulate the pulsing action of a malfunctioning heart. Circa 1967. A radioisotope thermoelectric generator (RTG) uses thermocouples. Each thermocouple is formed from two wires of different metals (or other materials). A temperature gradient along the length of each wire produces a voltage gradient from one end of the wire to the other; but the different materials produce different voltages per degree of temperature difference. By connecting the wires at one end, heating that end but cooling the other end, a usable, but small (millivolts), voltage is generated between the unconnected wire ends. In practice, many are connected in series (or in parallel) to generate a larger voltage (or current) from the same heat source, as heat flows from the hot ends to the cold ends. Metal thermocouples have low thermal-to-electrical efficiency. However, the carrier density and charge can be adjusted in semiconductor materials such as bismuth telluride and silicon germanium to achieve much higher conversion efficiencies.
Thermophotovoltaic (TPV) cells work by the same principles as a photovoltaic cell, except that they convert infrared light (rather than visible light) emitted by a hot surface, into electricity. Thermophotovoltaic cells have an efficiency slightly higher than thermoelectric couples and can be overlaid on thermoelectric couples, potentially doubling efficiency. The University of Houston TPV Radioisotope Power Conversion Technology development effort is aiming at combining thermophotovoltaic cells concurrently with thermocouples to provide a 3- to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.
A Stirling radioisotope generator is a Stirling engine driven by the temperature difference produced by a radioisotope. A more efficient version, the advanced Stirling radioisotope generator, was under development by NASA, but was cancelled in 2013 due to large-scale cost overruns.
Non-thermal converters extract energy from emitted radiation before it is degraded into heat. Unlike thermoelectric and thermionic converters their output does not depend on the temperature difference. Non-thermal generators can be classified by the type of particle used and by the mechanism by which their energy is converted.
Energy can be extracted from emitted charged particles when their charge builds up in a conductor, thus creating an electrostatic potential. Without a dissipation mode the voltage can increase up to the energy of the radiated particles, which may range from several kilovolts (for beta radiation) up to megavolts (alpha radiation). The built up electrostatic energy can be turned into usable electricity in one of the following ways.
A direct-charging generator consists of a capacitor charged by the current of charged particles from a radioactive layer deposited on one of the electrodes. Spacing can be either vacuum or dielectric. Negatively charged beta particles or positively charged alpha particles, positrons or fission fragments may be utilized. Although this form of nuclear-electric generator dates back to 1913, few applications have been found in the past for the extremely low currents and inconveniently high voltages provided by direct-charging generators. Oscillator/transformer systems are employed to reduce the voltages, then rectifiers are used to transform the AC power back to direct current.
English physicist H. G. J. Moseley constructed the first of these. Moseley’s apparatus consisted of a glass globe silvered on the inside with a radium emitter mounted on the tip of a wire at the center. The charged particles from the radium created a flow of electricity as they moved quickly from the radium to the inside surface of the sphere. As late as 1945 the Moseley model guided other efforts to build experimental batteries generating electricity from the emissions of radioactive elements.
Electromechanical atomic batteries use the buildup of charge between two plates to pull one bendable plate towards the other, until the two plates touch, discharge, equalizing the electrostatic buildup, and spring back. The mechanical motion produced can be used to produce electricity through flexing of a piezoelectric material or through a linear generator. Milliwatts of power are produced in pulses depending on the charge rate, in some cases multiple times per second (35 Hz).
A radiovoltaic (RV) device converts the energy of ionizing radiation directly into electricity using a semiconductor junction, similar to the conversion of photons into electricity in a photovoltaic cell. Depending on the type of radiation targeted, these devices are called alphavoltaic (AV, αV), betavoltaic (BV, βV) and/or gammavoltaic (GV, γV). Betavoltaics have traditionally received the most attention since (low-energy) beta emitters cause the least amount of radiative damage, thus allowing a longer operating life and less shielding. Interest in alphavoltaic and (more recently) gammavoltaic devices is driven by their potential higher efficiency.
Alphavoltaic devices use a semiconductor junction to produce electrical energy from energetic alpha particles.
Betavoltaic devices use a semiconductor junction to produce electrical energy from energetic beta particles (electrons). A commonly used source is the hydrogen isotope tritium. Betavoltaic devices are particularly well-suited to low-power electrical applications where long life of the energy source is needed, such as implantable medical devices or military and space applications.
Gammavoltaic devices use a semiconductor junction to produce electrical energy from energetic gamma particles (high-energy photons). They have only recently (in the 2010s) been considered. A gammavoltaic effect has been reported in perovskite solar cells. Another patented design involves scattering of the gamma particle until its energy has decreased enough to be absorbed in a conventional photovoltaic cell. Gammavoltaic designs using diamond and Schottky diodes are also being investigated.
Radiophotovoltaic (optoelectric) conversion
In a radiophotovoltaic (RPV) device the energy conversion is indirect: the emitted particles are first converted into light using a radioluminescent material (a scintillator or phosphor), and the light is then converted into electricity using a photovoltaic cell. Depending on the type of particle targeted, the conversion type can be more precisely specified as alphaphotovoltaic (APV or α-PV), betaphotovoltaic (BPV or β-PV) or gammaphotovoltaic (GPV or γ-PV).
Radiophotovoltaic conversion can be combined with radiovoltaic conversion to increase the conversion efficiency.
Atomic batteries use radioisotopes that produce low energy beta particles or sometimes alpha particles of varying energies. Low energy beta particles are needed to prevent the production of high energy penetrating Bremsstrahlung radiation that would require heavy shielding. Radioisotopes such as tritium, nickel-63, promethium-147, and technetium-99 have been tested. Plutonium-238, curium-242, curium-244 and strontium-90 have been used.
Nuclear engineers at the University of Wisconsin, Madison have explored the possibilities of producing minuscule batteries which exploit radioactive nuclei of substances such as polonium or curium to produce electric energy. As an example of an integrated, self-powered application, the researchers have created an oscillating cantilever beam that is capable of consistent, periodic oscillations over very long time periods without the need for refueling. Ongoing work demonstrate that this cantilever is capable of radio frequency transmission, allowing MEMS devices to communicate with one another wirelessly. These micro-batteries are very light and deliver enough energy to function as power supply for use in MEMS devices and further for supply for nanodevices.
The radiation energy released is transformed into electric energy, which is restricted to the area of the device that contains the processor and the micro-battery that supplies it with energy. Batteries using the energy of radioisotope decay to provide long-lived power (10–20 years) are being developed internationally. Conversion techniques can be grouped into two types: thermal and non-thermal. The thermal converters (whose output power is a function of a temperature differential) include thermoelectric and thermionic generators. The non-thermal converters (whose output power is not a function of a temperature difference) extract a fraction of the incident energy as it is being degraded into heat rather than using thermal energy to run electrons in a cycle. Atomic batteries usually have an efficiency of 0.1–5%. High-efficiency betavoltaic devices can reach 6–8% efficiency.