|Fig. 1: Types of Nuclear/Atomic batteries.|
Natural radioactivity produces radiation that has energy. Atomic/Nuclear Batteries harness this energy. Quantitative first principles that enable this are discussed elsewhere.  This document takes the discussion forward to methods of harnessing this energy. The properties like energy and power density, domains of applications and the cost per unit energy depend primarily on the material in use, as noted in ; while the efficiency and output potential depend on the type of conversion used, as seen in Fig. 1.
Thermal converters (radioisotope thermoelectric generators - RTGs) use the thermal energy of the radioisotope decay to generate electricity. One such RTG is shown in Fig. 2. Methods to accomplish this include heating up a thermocouple, producing infrared radiation from hot metals to power "solar cells", using the Stirling Engine and many more. [2-3] These have reached an efficiency of upto 23% experimentally and 30% in theory. [4-5]
|Fig. 2: A photograph of the RTG that NASA's Apollo 14 mission carried to the Moon. The RTG is the gray colored device with cooling fins. Source: Wikimedia Commons. NASA still uses RTG based Stirling Engines. |
Indirect conversion typically involves two steps of conversion. The radioactive decay consisting of either alpha or beta particles is impinged on some radio luminescent material like phosphor to produce photons and then is collected using photodiodes or 'solar cells'. Optimization has to be done on the structure of the photo diode, the phosphor filling, the method to collect photons and the placement of the radioactive material. Also, the optical properties of the conversion processes need to be matched. At best, we can expect an overall efficiency of 2% at 3.5V open circuit voltage. Theoretically this efficiency can be 25%. [6-7]
Direct conversion uses the radioisotope decay to directly drive a device that converts these charged particles to electricity. The betavoltaic effect uses beta particles to generate electricity as in the photovoltaic effect, say using a photodiode. The electrons can be collected wither using a 'solar cell' configuration or using a contact potential difference (using metal contacts of different work functions to drive the electrons). [8-9] We could also generate secondary electrons from an irradiated surface and use them to drive a load. This works with gamma radiations too. The efficiencies range from 0.35% for secondary electron method to 2% for betavoltaic method. The secondary elctron method can produce upto 500,000V at 50mW. [10-12]
|Fig. 3: Working of the direct charge induction MEMS cantilever.|
Two things to note for the above two methods. First, the Shockley-Queisser limit is a fundamental limit to the efficiency. Second, when high energy radiation hits a solid, it damages it, so the rate of degradation of solid generally overwhelms the rate of radioactive decay. So some people use liquids. 
Direct charge batteries rely on using the charge on beta or alpha particles to directly derive a current. Historically this was done by collecting the charges of radioactive decay on a metal and using a capacitive structure to drive a current.  Lately, micro electro mechanical system (MEMS) are used. As seen in Fig. 3, we can charge the tip of a cantilever and use the electrostatic force between the charged tip and ground to force a contact and hence drive a current; this is a repeating process. [14-15] This category of converters is quite popular today in academic literature and they hold promise for and on-chip battery if integration in an IC process is achieved.
RTGs are used a lot in space and MEMS-direct charge converters promise an on-chip battery. Other methods are mostly of academic interest.
© Suhas Kumar. 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.
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