9th Annual International Energy Conversion Engineering Conference
31 July - 03 August 2011, San Diego, California
AIAA 2011-5981
A Nanopore Multilayer Isotope Battery Using Radioisotopes
from Nuclear Wastes
George H. Miley1 and Nie Lou2
NPL Associates, INC, Champaign, IL 61821
2
University of Illinois, Urbana, Illinois, 61801
1
Nuclear (“isotope”) batteries of various sorts based on a variety of radioisotopes have
been studied for over 50 years. They can play a mission-critical role when other power
sources cannot fulfill the demanding requirements for maintenance free ultra long life
operation. However, the conversion efficiency and power level are often far from that
desired so that their applications in the civilian market have been quite limited, although
some military and NASA space uses have occurred. The typically high costs of prior isotope
batteries have further exacerbated the problem. Meanwhile, the handling and storage of
nuclear waste have become both a technological and a political issue. In the present paper
we discuss use of isotopes from nuclear wastes to power nuclear batteries, lowering their
costs and providing an important use for key isotopes from nuclear waste. However, these
isotopes are not “ideal” for such use, so improved nuclear battery design must be developed
to provide a useful battery. The present reference design is based on the most abundant
isotope pair available in fission products, Sr-90/Y-90. Other waste isotopes could be used in
later designs once this approach is successfully demonstrated from Sr-90/Y-90.
Nomenclature
A
= beta radioisotope activity
Al
= aluminum
Ci
= Curie
cm
= centimeters
CSDA
= continuous slowing down approximation
CVD
= chemical-vapor-deposited
e
= charge of an electron
Eβ-avg
= average energy of the beta particles emitted
Η
= Hamiltonian
keV
= kiloelectron volt
mm
= millimeters
η
= conversion efficiency
Ni
= nickel
p
= radioisotope density
P
= power output
Pu
= Plutonium
p-n
= positive-negative doped junction
Pm
= Promethium
um
= micrometer
V
= radioisotope volume
Si
=silicon
Sr-90/Y-90 = strontium-90/yittrium-90 isotopes
µ
=
1
Professor Emeritus, Nuclear, Plasma and Radiological Engineering, 214 Talbot Laboratory, 104 S. Wright Street,
Urbana, IL 61801
2
Postdoctoral Associate, Nuclear, Plasma, and Radiological Engineering, 214 Talbot Laboratory, 104 S. Wright
Street, Urbana, IL 61801
1
American Institute of Aeronautics and Astronautics
Copyright © 2011 by George H Miley. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
W
=
I. Introduction
The applications of Sr-90/Y-90 isotope battery are multifold. For example, in large stack designs, it could
replace the Pu-238 based thermoelectric generator for deep space explorations. In the sizes described her, however,
it is mainly of interest for tickle changing in remote electronics or for low power requirements in circuits. This
nuclear battery has intrinsic high conversion efficiency since it utilizes direct energy conversion of the isotope decay
energy to electricity. The U.S. currently has limited facilities to produce Pu-238. Since 1993, all Pu-238 that the U.S.
has used in space probes, some 16.5 kg, has been purchased from Russia. NASA is requesting funding to restart
domestic production, but it will take another 10 years to produce tangible amounts. Therefore, high power
betavoltaic battery based on the proposed nanopore/multilayer technology is an interesting alternative and gives the
nation self reliance in a critical technology area, while saving taxpayer money. This approach has the added benefit
of consuming at least one vital part of nuclear waste. Possible consumer applications include long-life maintenancefree smoke detectors with the nano-nuclear battery in place of ordinary batteries. This should give the smoke
detector up to 100 years of life, nicely matching that of Am-241 isotope that is widely adopted in household smoke
detectors. Finally, various civilian and military electronic systems, e.g. remote control systems for airfields and for
radar are in remote locations. Often located where harsh weather conditions challenge use of conventional batteries,
however nuclear batteries can stand wide variations in temperature and inherently offer ultra long lifetimes. Thus,
this too offers a good market for nuclear batteries.
Two types of ―direct conversion‖ have used in nuclear batteries – high voltage direct collection and a solid state
p-n junction.1 For present purposes we elect to use a p-n junction convertor to allow low voltages compatible with
conventional electrical circuits. Use of Sr-90/Y-90 relies on conversion of the beta particle energy, and such devices
have been labeled ―betavoltaic‖ batteries. The basic principle for operation of a betavoltaic type nuclear battery is
relatively straightforward. A radioisotope beta particle source is mated with a semiconductor p-n junction. When an
energetic beta particle enters the junction, it creates a multitude of electron-hole pairs (with the exact number
depending on the semiconductor and beta energy involved). The intrinsic electric field of the semiconductor’s
depletion region sweeps the electrons and holes through this region, allowing them to be collected at the device
contacts, creating a current.
The study of betavoltaic cells began in the 1950s.1-6 Some of the most successful early Betavoltaic work was
performed by L.C. Olsen in the early 1970s.7 Olsen built several versions of 147Pm betavoltaic cells employing
planar silicon diodes. The final version was the Model 400 Betacel. It used 66 Ci of 147Pm to create 400 µW of
power at an efficiency of 1.7%. Additionally, the Betacel was small —roughly half the size of a C-cell battery. At a
low current load, the Model 400 Betacel could easily perform for 10 years. While very successful, this unit and
following concepts had limited use due to their low power.
II.
Description of the Betavoltaic Battery
One obvious approach to increase the betavoltaic power output is to increase the depletion region surface area.
Researchers at Cornell University experimented with this idea by depositing 63Ni in a grooved Si substrate . More
recently, W. Sun et al.8 reported a novel betavoltaic technology involving a porous silicon substrate. The researchers
created nanopores in silicon and doped the sample to create a high surface area p-n diode. They then added gaseous
tritium (a beta emitting radioisotope) to create a betavoltaic device out of the porous silicon. Although the resulting
power output was very small (mainly due to the low decay energy density of gaseous tritium), their work
demonstrates that a porous silicon device offers order of magnitude efficiency increase over an otherwise similar
planar device. (The efficiency increased from 0.023% (planar) to 0.22% (porous) in terms of power per activity
(W/Ci)). This approach is expected to achieve yet higher efficiencies while increasing the output power by
incorporating a higher density isotope such as 63Ni along with an advanced pore structure.
The first steps in the transition from a planar cell to the porous silicon cell can be seen in Figure 1. In a planar
cell, the source is unidirectional, and efficiency losses are caused by emission of beta particles in the ―wrong‖
direction and self-absorption. In the porous cell of W. Sun et al., the source was bidirectional and self- absorption
was decreased, thus increasing efficiency. However, the low activity and density of tritium limited the output power
to a low level. In the porous silicon cell, the advantages of prior porous cell designs in terms of surface area will be
retained, but the output power will be higher due to the higher activity of Sr-90/Y-90.
2
American Institute of Aeronautics and Astronautics
The order of magnitude increase in
efficiency attributed to porous devices
leads to the basis of this project. The
researchers propose to create a porous
silicon betavoltaic that uses a solid metal
radioisotope rather than gaseous tritium
used by Sun et al. in order to increase the
power and efficiency.
In preliminary
studies under a Nuclear Engineering
Education Research (NEER) project, we
have been employing Nickel-63 for initial
demonstrations that a metallic isotope
could be deposited in porous-like
structure.10 This has been quite successful
and lays the ground work for moving to Sr90/Y-90. The switch in isotope will require
some important changes in the method
used for deposition in porous-like
structures, but based on the prior work, this
seems feasible.
First, a simple calculation will highlight
the possibilities of a Sr-90/Y-90 porous
silicon cell. The power generated by a
betavoltaic device can be found with the
following equation:
P  AE  avgeV
Here
A
Figure 1. Boost the current of isotope batteries with a p-n
junction on pores. Black arrows show the direction of the beta.
(in W).
(1)
= the beta radioisotope activity (decays/sec-cm3),
E avg
= the average energy of the beta particles emitted (eV),
e = the charge of an electron (J/eV),
V = the radioisotope volume (cm3), and

= the radiation-to-electrical energy conversion efficiency.
A calculated power output for a theoretical Sr-90/Y-90 betavoltaic cell can be found in Table 1.
Table 1. Output power calculation for a theoretical porous silicon cell based on Sr-90/Y-90.
Parameter
Symbol
/
Equati
on
Units
Value
Sr-90
Y-90
Sr/Y-90 average
140
200
550,000
930
~280
~570
Activity
Average Beta Particle
Energy
A
E avg
Specific power
Radioisotope Density

3
g/cm
0.166
~2.6
3028
4.47
~0.94
~2.6
Power density
Conversion Efficiency

%
0.43
~1
13560
~1
2.49
~1
Power Output
P
4.3
1.4x105
25
Ci/g
keV
mW/cm3
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American Institute of Aeronautics and Astronautics
The proposed Sr-90/Y-90 isotope battery builds on our previous experiences gained in Ni-63 studies. For that
design we investigated pore-structured isotope batteries with the state-of-the-art silicon lithography techniques. A
typical device structure is shown in Fig. 2
Figure 2. (a) and (b) Cutaways of existing UIUC pored isotope battery. (c) top view of etched Si
Another major improvement that increases the battery’s efficiency is a stacked multi-p-n junction structure. The
average electron energy from Sr-90 is ~ 200 keV, which has a continuous slowing down approximation (CSDA)
range of 240 um in Si. In contrast, the thickness of an average p-n junction is roughly 1 µm. Therefore, most of the
beta radiation escapes with a planar junction. With the pored junction as in Fig. 2, the efficiency improves but still
leaves much to be desired. The pore diameter is preferably on the µm scale and for practical reasons the
corresponding pore cannot be too deep, typically with ~ 20 µm. Most of the Sr-90 betas would still be lost in this
thin layer.
However, if 12 layers of pored-p-n junctions, each 20 um thick, are stacked in series, in Fig. 2(c), the overall
stack will nicely match the range of Sr-90 beta, resulting in maximal efficiency. The daughter Y-90 has even higher
range, ~ a few mm in Si, and constitutes another compelling reason for the multilayer design. The additional benefit
of the stacked structure is higher voltages that match practical electronics. A single betavoltaic p-n junction typically
gives 100 mV at normal load. A 30-layer stack should raise this up to 3 V, obviating the need for additional voltage
step-up circuits.
III. Issues
The radiation shield of the battery are chiefly for the higher energy beta from the Y-90. The shield structure can
be primary constructed from stainless steel of 5 mm in thickness. Some extraneous Bremsstrahlung from the betas
slowing down occurs with Sr-90/Y-90. However, these are of relatively low energy and emission rates are low, so a
thin layer of lead x-ray shield of 0.5 mm in thickness suffices to stop most of them. This low-costing layer will bring
external levels well within regulatory limits.
Another issue is that high energy betas cause radiation damage in crystalline Si, but with the amorphous Si in our
device, the performance degradation is largely avoided. It has been well known in the literature 10,11 that the a-Si:H
films and structures have a superior radiation tolerance as compared to the conventional crystalline silicon
technologies. It can also be readily annealed with mild conditions or even at room temperature.
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American Institute of Aeronautics and Astronautics
A. Goals for Battery
Sr-90 beta
emitter
A prior representative betavoltaic
battery that had one of the best performing
} multilayers
specifications generated 400 microwatt on
single-device level. That was achieved in
the 1970s. More recent endeavors in this
Planarized
field resulted in devices on the order of
Al w/ reflow
microwatt or lower due to weaker beta
sources, despite the innovation in
Ni
structures. The overall goal of the current
n-Si
proposed research is then to demonstrate
p-Si
that 1) monolithic stacked p-n junction
Al
betavoltaic on the 1 mW level, and the
+
feasibility to go higher to the watt level.
2) overall efficiency greater than 2%. Both
Figure 3. The battery with multi-p-n junction structure. The flat
specs will break the historical records
top of Al layers is formed by reflow.13 The ohmic contacts are
achieved in the 1970s, if successfully
Al-on-n-type and Ni-on-p-type Si. The Si is amorphous,
achieved.
chemical-vapor-deposited (CVD) on top of underlining metal
Tightly
associated
with
the
layers. The pores are plasma etched14 and can be irregular in
performance goals is the breakthrough in
shapes.
the fabrication process. We know that
multi-p-n-junction is a key feature to high
efficiency and high power in betavoltaics. However, it had also been a labor/capital/time intensive processing effort
with the current semiconductor fabrication procedures. In general, a gas phase deposition technique that can grow
semiconductor films with thickness up to 100 microns in a reasonable time is difficult today with the current CVD
methods. However, we believe this can be tackled reasonably well with our atmosphere pressure corona discharge
CVD that we have demonstrated in our laboratory recently. Therefore, the other goal of the project is to develop this
new
B. Technological challenges
This battery is intended to overcome three major technological challenges:
1) Verification of (nano-pore) multi-layer structure with existing processing techniques
2) Developing a low-cost method
for the growth of amorphous Si.
3) Addressing radiation safety and
shielding
To resolve the first challenge, we
are developing units that demonstrate
the device structure of or Fig. 4 which
is a simplified version of Fig. 3.
This monolithic structure represents
a breakthrough in betavoltaic if proved
to work. Some issues are expected to
exist like the surface morphology
degradation after too many layers.
Others may include proper ohmic
contacts and doping. These are
expected to be probed by experiment
with the proper choice of different
metals. A potential challenge in the
Sr-90 beta
emitter
} multilayers
Simple Al
5
+
American Institute of Aeronautics and Astronautics
Ni
n-Si
p-Si
Al
Figure 4. The multi-p-n junction structure with simplified
processing.
process outlined in Fig. 3 is the reflow of Al to planarize the surface. In Fig. 3 the pores might be formed by
electrochemical etch, which gives multiple increase in surface area compared to Fig. 4. The downside though is the
deterioration in the smoothness of the surface, which should cause problem for the next layer. The reflow might
involve very precise control of temperature and cleanliness to reach desired result. If the experiment deemed it nonpractical in the real world, we have a back-up plan as shown in Fig. 4. Instead of relying on pores this structure
adopts an all-planar structure. The loss in pore is compensated by gain in the much tighter single layer that gives
more p-n junction layer for the same amount of thickness. The end result then is essentially the same as the pored
type. We will choose the optimal design based on experimental findings during the execution of the project.
The final multilayer structure is thick, on the order of 100 microns. This requires a very high rate deposition to
make the structure viable. The traditional plasma enhanced chemical vapor deposition PECVD of a-Si has a
deposition rate around 5 A/s. Therefore a single p-n junction would take more than half an hour. A whole nuclear
battery, with possibly up to 100 layers, would take more than 50 hours for deposition of the silicon part alone. This
is clearly a show-stopper to large scale application. On the other hand, we have developed an ambient pressure
corona discharge CVD that speeds up the deposition rate tenfold, with the added benefit of doing away the
expensive vacuum chamber.
The large number of vacuum in/outs (required for convention methods to deposit the metals needed for the
present design) hampers the fabrication turnaround time somewhat. Therefore we are currently considering other
deposition methods. The corona discharge sputtering technique is also promising for deposition of Al and Ni. This
would significantly cut the number of vacuum in/outs required.
A potential challenge in the process outlined in Fig. 3 is the reflow of Al to planarize the surface. This might
involve very precise control of temperature and cleanliness to reach desired result. If the experiment deemed it nonpractical in the real world, we have a back-up plan as shown in Fig. 4. Instead of relying on pores this structure
adopts an all-planar structure. The loss in pore is compensated by gain in the much tighter single layer that gives
more p-n junction layer for the same amount of thickness. The end result then should be the same as the pored type.
We will choose the optimal design based on experimental findings during the execution of the project. Because the
radiation spectrum is harder in Sr/Y-90 than Ni-63 we previously investigated, attention must be paid to the shield
design of the device packaging.
IV. Conclusion
The typically high costs of prior isotope batteries have been a factor preventing their widespread use. At the
same time, the handling and storage of nuclear waste has become a technological and a political issue. A partial
solution to the two somewhat different issues comes together if isotopes from nuclear wastes can be used to power
nuclear batteries, lowering their costs and providing an important use for key isotopes from nuclear waste. However,
these isotopes are not ―ideal‖ for such use, so improved nuclear battery using a stacked p-n junction concept was
presented here to provide a useful battery. The present reference design is based on the most abundant isotope pair
available in fission products, Sr-90/Y-90. Other waste isotopes could be used in later designs once this approach is
successfully demonstrated from Sr-90/Y-90.
References
1
Miley, G. H., ―Direct Conversion of Nuclear Radiation Energy,‖ American Nuclear Society, 1971.
Flicker, H., Loferski, J., and Elleman, T. S., ―Construction of a promethium-147 atomic battery,‖ IEEE Transactions on
Electron Devices, Vol. 11, No. 1 1964, pp. 2-8.
3
Greensboro, J., Lewis, M., Matheson, W. E. et al., ―Betacel nuclear battery development and qualification,‖ Transactions of
the American Nuclear Society, Vol. 16, 1973, pp. 55-56.
4
Pfann, W. G. and Van Roosbroeck, W., ―Radioactive and Photoelectric p-n Junction Power Sources,‖ Journal of Applied
Physics, Vol. 25, No. 11, 1954, pp. 1422-1434.
5
Rappaport, P., ―The Electron-Voltaic Effect in p-n Junctions Induced by Beta-Particle Bombardment,‖ Physical Review, Vol.
93, No. 1, pp. 246, 1954.
6
Manasse, F. K., Pinajian, J. J., and Tse, A. N., ―Schottky Barrier Betavoltaic Battery,‖ IEEE Transactions on Nuclear
Science, Vol. 23, No. 1, 1976, pp. 860-870.
7
Olsen, L. C., ―Betavoltaic energy conversion,‖ Energy Conversion, Vol. 13, No. 4, 1973, pp. 117-124.
8
Sun, W., Kherani, N. P., Hirschman, K. D. et al., ―A three-dimensional porous silicon p-n diode for betavoltaics and
photovoltaics,‖ Advanced Materials, Vol. 17, No. 10, 2005, pp. 1230.
2
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American Institute of Aeronautics and Astronautics
9
Ulmen, B., Desai, P., Moghaddam, S., et al., ―Development of diode junction nuclear battery using 63Ni,” Journal of
Radioanalytical and Nuclear Chemistry, Vol. 282, No. 2, 2009, pp. 601-604.
10
Kishimoto, N., Amekura, H., Kono, K., et al., ―Radiation resistance of amorphous silicon in optoelectric properties under
proton bombardment,‖ Journal of Nuclear Materials, Vol. 258-263, No. 2, 1998, pp. 1908-1913.
11
Srour, J. R., Vendura, Jr., G. J., Lo, D. H., et al., ―Damage mechanisms in radiation-tolerant amorphous silicon solar cells,‖
IEEE Transactions on Nuclear Science, Vol. 45, No. 6, 1998, pp. 2624-2631.
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