A review of micro radioisotopic batteries Presenter: Robert Walton

Contents 1 Why micro nuclear batteries 2 How to create these devices 3 What batteries are possible 4 What the future may hold Robert Walton 11/19/2018

Alternative Uses of Radioisotopes 1 Alternative Uses of Radioisotopes Why micro nuclear batteries First known battery example developed by Henry Moseley in 1913 – direct charge type NASA have utilized Radioisotope Thermoelectric Generators (RTGs) in long term missions since 1961 RTGs have also been used to power pacemakers between 1970 and the mid 1980s Radioisotopic decay utilized in commercial smoke detectors since the 1970s (Am-241) Self powered exit signs (Tritium: H-3) Robert Walton 11/19/2018

Advantages relative to conventional batteries 1 Why micro nuclear batteries Advantages relative to conventional batteries Very high energy density Very long life (potentially) Continuous operation devices No (or very few) moving parts Very little sensitivity to environmental changes High reliability Scalable to microns (generally) Robert Walton 11/19/2018

Ragone Plot 1 Why micro nuclear batteries Higher energy density devices contain greater overall levels of energy, while higher power density devices can discharge their energy more quickly. Robert Walton 11/19/2018

Key considerations when designing a nuclear battery 2 How to create these devices Key considerations when designing a nuclear battery How much power is needed? What isotope to use? Emission Beta or Alpha? Try to avoid Gamma Half life of the isotope Energy of decay – penetrative depth of the emissions Robert Walton 11/19/2018

Isotope Selection 2 How to create these devices Robert Walton 11/19/2018

STS Etcher in University of Birmingham clean room 2 MEMS Possibilities How to create these devices MicroElectroMechanical Systems From the electronics industry Ability to “etch” multiple copies of a structure simultaneously As easy to etch 1 million as to etch 10 given the correct scale Process: Design Mask Etch STS Etcher in University of Birmingham clean room Robert Walton 11/19/2018

Types of Nuclear Batteries 3 Types of Nuclear Batteries What batteries are possible Two types of power source Thermal Types Thermoelectric Thermophotovoltaic Thermionic AMTEC Non Thermal Types Direct Charge Direct Conversion Optoelectric Mechanical Robert Walton 11/19/2018

Thermoelectric 3 What batteries are possible Single module RTG with dual thermal piles NASA RTG used for the Galileo and Ulysses missions (DOE/NASA/JPL) 235W per RTG system 1.14m long X 0.42m ⦰ at 28V for 4 years + operating temperature: 1200K – 1500K (1350K usual) Robert Walton 11/19/2018

Direct Charge 3 What batteries are possible Charge is collected from radioactive decay with one electrode of a capacitor High Voltage (100kV+) Low Current (< 100nA) Alpha or Beta decay utilized Vacuum or dielectric between the electrodes Isotopes : H-3, Sr-90, Ni-63 etc. Power Density: 0.6W/kg – 78W/kg Energy Density: 2.9X105Wh/kg - 2.8X107Wh/kg Efficiency: < 2.5% Overall Efficiency Robert Walton 11/19/2018

Direct Conversion 3 What batteries are possible The radiation incident on the p-n junction generates multiple electron holes and free electrons through transfer of kinetic energy practical efficiency < 2% source: Ni-63, Pm-147, H-3, and Sr-90 lower voltage (< 5V) higher current ( up to 0.1mA) beta or alpha decay utilized converter: Si, Ge, 4H SiC, AlGaAs, etc. power: energy density: 1 electron (β particle) generates ~ 1000 electron hole pairs Open Circuit Voltage Robert Walton 11/19/2018

Indirect Conversion 3 What batteries are possible Wide variety of designs but basic principle is that the energy from radioactive decay is converted into optical energy and is then converted into electrical energy in photovoltaic cells Alpha or Beta Phosphor Screens Semiconductor Kinetic Energy Photonic Electrical Robert Walton 11/19/2018

State of the art 3 What batteries are possible Cornell University energy generation by combination of direct charge and piezoelectric 5.1% overall efficiency 22nW continuous and 750µW cycled (0.08% duty cycle) Mechanical/Radioisotopic hybrid system University of Illinois University of Missouri direct conversion device utilizing a liquid Schottky diode the liquid receiver should minimise radiation damage 16nW maximum power Cylindrical direct charge devices utilizing Pm-147 and H-3 Efficiencies up to 15% Powers up to 140µW Robert Walton 11/19/2018

4 What the future may hold Challenges/Issues Miniaturize a thermal battery type which doesn’t lose its heat too rapidly Protect photovoltaic cells from radiation damage (direct conversion) Recreate theoretical high efficiencies in practical experiments Improve surface geometry of direct collection (and direct charge) receivers Effective power conditioning of the very high voltages generated in direct charge devices Robert Walton 11/19/2018

Summary Nuclear batteries have been around for a long time and in may forms Application of MEMS manufacturing techniques may make micro nuclear batteries feasible At this scale it is possible that the traditional disadvantages of nuclear batteries may be overcome In the future nuclear batteries may be powering sensors in nearly every object around you Robert Walton 11/19/2018