Atomic battery

The terms atomic battery, nuclear battery, tritium battery and radioisotope battery are used to describe a device which uses the emissions from a radioactive isotope to generate electricity.

Devices for converting natural radioactive decay directly into electricity have a long history. Nuclear battery technology began in 1913, when Henry Moseley first demonstrated the Beta Cell. The field received considerable research attention for applications requiring long-life power sources for space needs during the 50s and 60s. Over the years many types and methods have been developed. The scientific principles are well known, but modern nano-scale technology and new wide bandgap semiconductors have created new devices and interesting material properties not previously available.

Batteries using the energy of radioisotope decay to provide long-lived power (10-20 year) 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 betavoltaics have 6–8%.

Thermionic converter
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 contact ionization) to neutralize the electron space charge.

Radioisotope Thermoelectric Generator
A thermoelectric converter connects pairs of thermocouples in series. Each thermocouple is formed by the junction of two dissimilar materials. One of each pair is heated and the other cooled. 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 cells
Thermophotovoltaic 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 cell concurrently with thermocouples to provide a 3 to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators.

Alkali-metal thermal to electric converter
The alkali-metal thermal to electric converter (AMTEC) is an electrochemical system which is based on the electrolyte used in the sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline β-alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900 - 1300 K and a low pressure region containing a condenser for liquid sodium at 400 - 700 K. Efficiency of AMTEC cells has reached 16% in the laboratory and is predicted to approach 20%.

Non-thermal converters
Non-thermal converters extract a fraction of the nuclear energy as it is being degraded into heat. Their outputs are not functions of temperature differences as are thermoelectric and thermionic converters. Non-thermal generators can be grouped into three classes.

Direct charging generators
In the first type, the primary generators consists of a capacitor which is 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.

Betavoltaics
In May 2005, a group including researchers from the University of Rochester and from the University of Toronto announced  a small battery powered by the beta-particle-emitting decay of tritium and positioned the product as suitable for pacemakers or low-current electrical household devices. The device gathers energy from the beta-particles that pass through a silicon diode, in a manner analogous to photovoltaic cells. This technique is called betavoltaics and has the potential to radically increase atomic battery efficiency and energy production densities.

Optoelectric
An optolectric nuclear battery has also been proposed by researchers of the Kurchatov Institute in Moscow. A beta-emitter (such as technetium-99) would stimulate an excimer mixture, and the light would power a photocell. The battery would consist of an excimer mixture of argon/xenon in a pressure vessel with an internal mirrored surface, finely-divided Tc-99, and an intermittent ultrasonic stirrer, illuminating a photocell with a bandgap tuned for the excimer. If the pressure-vessel is carbon fiber/epoxy, the weight to power ratio is said to be comparable to an air-breathing engine with fuel tanks. The advantage of this design is that precision electrode assemblies are not needed, and most beta particles escape the finely-divided bulk material to contribute to the battery's net power.

Reciprocating Electromechanical Atomic Batteries
Electromechanical atomic batteries use the build up 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 (35Hz). 

Radioisotopes Used
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.

In fiction
In the live action TV series Batman, atomic batteries are used to power the Batmobile, as Robin's deployment checklist includes "Atomic batteries to power. Turbines to speed.". In the original cartoon, The Justice League, a nuclear battery from a JLA communicator buried on the JLA building site in the Cretaceous period enables the members to rescue others sent back in time by the original Legion of Doom. In space combat simulation game Wing Commander 3, atomic batteries were also used to power the spacecraft using the same phrase from the Batman series "Atomic Batteries to Power, Turbines to Speed". 

In the French cartoon Code Lyoko, the supercomputer in the series uses a nuclear battery. It is believed by some fans to be uranium-based, but this is unlikely as the half-life of the battery seems to be 20 years or so, judging from the show's established timeline, and a uranium battery would last for thousands of years. Some fans currently theorize that the fuel for the battery is Lead-210, with a half life of 22.3 years.