<P> 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 betavoltaics have 6--8% . </P> <P> 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 . </P> <P> 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 to generate a larger voltage 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 . </P> <P> 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 cells concurrently with thermocouples to provide a 3 - to 4-fold improvement in system efficiency over current thermoelectric radioisotope generators . </P>

Who invented the atomic battery what purpose did it serve