Nuclear Microbatteries

Jake BlanchardANS – Hollywood FL

June 2002

What is MEMS?

Micro Electro Mechanical SystemsSensorsActuatorsMotors

Power Sources

How do we run these devices?External powerFuel CellsChemical BatteriesFossil FuelsRadioisotopes

Nuclear Batteries for MEMS

Radioisotopes have been used for power for a long time (space, pacemakers)Advantages are long life (decades without refueling)…and high energy density

Comparison

Consider 1 mg for power source

3000210-Po (5% - 4 years)

5003-H (5% - 4 years)

3Fuel Cell (methanol, 50%)

0.3Chemical Battery (Li-ion)

Energy Content (mW-hr)Source

Isotope Selection

Type of radiationAlphaBeta

Half-LifeLong -> Long battery lifeShort -> Higher power

Avoid gammas in the decay chainEnergy of particles

Isotope SelectionIsotope Average

energy Half life

Specific activity

Specific Power

Estimated Range in Cu

(KeV) (year) (Ci/g) (W/g) (microns) 63Ni 17.4 100.2 57 0.006 14 32Si 68.8 172.1 65 0.03 107 90Sr 195.8 28.8 138 0.16 332 106Ru 10.03 1.06 3300 0.0002 5 3H 5.7 12.3 9664 32.5 3 210Po 5304.3 0.38 4493 137 0.5 32P 694.9 0.04 285700 1.18 1344

Incorporation of Sources

Three WaysActivation in reactorAddition of radioactive liquid to deviceAddition of radioactive solid to device

Electroless plating of Ni-63H-3 microspheres

Self Absorption in liquid

Liquid sources will have reduced efficiency due to beta absorptionWe measured power using liquid source, then dried and remeasuredPower increased by 25% after dryingActivity measurement indicated no evaporation of active species

Tritium from MicrospheresGlass beads containing Li-6 were irradiated in UW reactor (radius about 20 microns)2 mCi produced in 37 minutes from 0.25 gCapsule was filled with water to prevent heatingMaximum temperature estimated to be 360 C (worst case)Beads were then placed in diode

Junction Type Battery

Energy from radiation used to create ion-hole pair in the junction.Excessive energy might damage the junction (> 250KeV)Current proportional to contact area

•13 micromachined channels(55% more than planar)• 8 µl of liquid 63Ni (64 µCi)

N

P

A different view

Performance

-1.4E-09

-1.2E-09

-1E-09

-8E-10

-6E-10

-4E-10

-2E-10

00 0.01 0.02 0.03 0.04 0.05 0.06

VOLTAGE [V]

CU

RR

ENT

[A]

30 minutes2 hours16 hours

Beads Must Be Small Enough to Avoid Significant Absorption

Escape Probability

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.00E-05 1.00E-04 1.00E-03 1.00E-02

diameter (cm)

Pe

18.6 keV

15 keV

10.7 keV

5.7 keV

Sphere with uniform production of betas

Optimization of DiodeIf junction is too deep, most particles will stop shortIf too shallow, performance will be poorWe conducted test of range of depths (0.5 to 10 microns deep)Voltage depends on diode characteristics, so current is key variableJunction width ~ 1 micronResult is isotope dependent (and diode)

Optimum depth ~ 2 microns

0.00E+00

2.00E-10

4.00E-10

6.00E-10

8.00E-10

1.00E-09

0 2 4 6 8 10 12

Junction depth (Micron)

Shor

t circ

uit c

urre

nt (A

)

Self-Reciprocating Cantilever

1 mCi of 63Ni

Beam discharges on contact.Resulting movement is periodic.

Accumulated charge created by ionizing radiation leads to attractive force.

Photograph

Modeling

The charge collecting process is governed by:

Beam stiffness k gives:

RAV

AI

dV

dtd

−=

αε0

2

20

0 )(dAVddk ε

=−

Comparison to Experiment

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

3.0E-05

3.5E-05

0 100 200 300 400

time (s)

gap

(m)

Measured

Calculated

Comparison of Time to Contact

0

50

100

150

200

250

300

350

400

0.0E+00 5.0E-06 1.0E-05 1.5E-05 2.0E-05

Initial Separation (m)

Perio

d (s

econ

ds)

Measured

Calculated

Conclusions

Radioisotopes can provide practical power for MEMSPrimary advantage is long life