1. A review of micro radioisotopic batteries
Presenter: Robert Walton

2. 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

3. Alternative Uses of Radioisotopes1
Alternative Uses of Radioisotopes
Why micro nuclear batteries
First known battery example developed by Henry Moseley in – 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 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

4. Advantages relative to conventional batteries1
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

5. 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

6. Key considerations when designing a nuclear battery2
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

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

8. STS Etcher in University of Birmingham clean room2
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

9. Types of Nuclear Batteries3
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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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