a modular design for nuclear battery technology
TRANSCRIPT
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A Modular Design for Nuclear Battery Technology
A Senior Project
presented to
the Faculty of the Physics Department
California Polytechnic State University, San Luis Obispo
In Partial Fulfillment
of the Requirements for the Degree
Bachelor of the Arts in Physics
by
Randy Lao
June, 2011
©2011 Randy Lao
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Section 1: Introduction
The search for alternative energy sources has led to research in many different fields
to find a more efficient way to power every day devices such as phones, laptops, and anything
else that requires a battery to function. One modern concept is the idea of a battery that, instead
of utilizing the energy in a chemical gradient like a dry cell battery, uses the energy given off by
natural radioactive decay of an isotope to create an electric current. This type of cell is called a
nuclear battery. In the past, nuclear batteries were used to power electronics for extended
periods of time such as unmanned spacecraft and remote measurement stations. For a short
period of time in the 1970’s cardiac pacemakers used a nuclear battery for power, capable of
producing enough energy to last over the lifetime of the patient. Lithium-ion batteries soon
replaced these, though, as the large size of the battery was considered impractical to implant in
the human body with an average length of 13 cm and cylindrical design1. With the application of
new technology, a new, modular case design can be proposed while drastically reducing size.
In order for a nuclear battery to function, the power source has to be a radioactive
isotope. An isotope is a nuclide of an element with a different mass due to a different number of
neutrons. For example, chlorine (Cl, Z=17) has 76% of nuclei containing N=18 neutrons while
the other 24% has N=20 neutrons. These isotopes may be unstable structures that decay to form
other nuclides by emitting different types of particles, electromagnetic radiation, as well as heat,
which is a process known as radioactivity2. The stability of an isotope depends on the ratio of
protons to neutrons and a Line of Stability that describes the radioactivity of isotopes can be seen
in Figure [1]. The time scale for radioactivity to occur is different among isotopes and their
decay can be measured in terms of how long it takes for half of the original element to decay to
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another element. This value is known as the half-life or
!
t 12
3. A half-life can be calculated with
the equation:
!
t 12
=ln2"
(1)
Generally, a decay rate can be described as:
!
R = "dN(t)dt
= #N(t) (2)
Fig [1] Line of Stability
(ned.caltech.edu)
As seen in the figure above,
!
"# decay is a type of radiation that certain elements may go through
depending on the ratio of protons to neutrons inside the nucleus. For example, hydrogen-1 and
hydrogen-2 (deuterium) are stable isotopes, but hydrogen-3 (tritium) is radioactive. Tritium has
too many neutrons in its nucleus and is not stable. Therefore one or more of its nuclides will
undergo a nuclear reaction that can be represented in the equation:
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neutron
!
" proton + electron (3)
where the mass of the particles in this equation is conserved. When this reaction occurs in
tritium, the product electron is ejected with some kinetic energy from the nucleus and is known
as a
!
"-particle. Since the resulting nucleus has an additional proton, the atom becomes an
element that is one atomic number higher on the periodic table3. In this case, the resulting
product from a tritium nucleus undergoing beta decay is a helium atom and a beta particle, as
illustrated in Figure [2]. This overall reaction can be written as:
!
13H"2
3He+#10e (4)
with the subscript on the element representing the number of protons and the superscript
representing the atomic mass. The negative subscript on the e represents the negative charge on
that electron.
Fig. [2] Tritium Decay
(library.thinkquest.org/3471/radiation_types.html)
This idea becomes vital for the function of the modular nuclear battery because the
natural decay of a
!
"#-emitting particle is the primary source of energy. This electron can be
used to cause a type of chain reaction, or flow of other electrons, that can be harnessed as
electrical energy. The rate of flow of electrons through a medium is known as a current. Another
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variable to identify when examining the power of an electrical source is voltage. Voltage is the
electrical driving force of a charged object or an electric potential difference4. It can also be
thought of as “electrical pressure” of a system. The higher the voltage, the more driving force
there is to move from a high electric potential to a low electric potential. Combined with current,
there is a quantifiable measure of the rate in which a system is capable of doing work. This idea
is known as power. Power, P, is the rate that work is performed or energy is converted, and can
be written in terms of electricity as:
!
P =VI (5)
where V is the voltage and I is the current of the system. This means that the decay of
radioisotope with beta emission can theoretically drive electrons through a medium, and the
output power of a device that harness such a process depends on the potential difference and
current caused by the radioactivity.
The effectiveness of a nuclear battery is also determined by the ability for the
medium that the beta particle travels through to carry an electric current. This is also known as
the conductivity of the material. On the contrary, resistance is the amount that a certain medium
is able to impede the ability for an electric current to travel through it. There are several
different classes of materials that are separated by their ability to carry current. First, there are
insulators or nonconductors. These materials may carry little to no current through them due to
high resistance or low conductivity. Examples of insulators are wood or plastic. The next class
is conductors. These materials have relatively low resistance and/or very high conductivity.
Examples of conductors include aluminum and copper4. The most important type of material
relative to the function of a nuclear battery is the semiconductor. The special properties of
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semiconductors allow the harnessing of radioactive energy from the decay of radioisotopes that
release of
!
"# particles.
Semiconductors are materials that contain properties that fall in between conductors
and insulators. Conduction in semiconductors can occur in multiple ways. First, electrons may
move throughout a semiconductor in opposite direction to conventional flow. Holes have a
positive charge and move in the conventional flow direction. Holes are a property of only
semiconductor devices, and can be thought of as an absence of an electron in a particular orbital
of an atom4. A process called doping allows semiconductors to be used for the construction of
electronic devices. Doping adds impurities that are either donors or acceptors, either resulting in
an addition of negatively charged (electron) carriers or positively charged holes. P-type material
has an excess of positive charge carriers and N-type material has an excess of negative charge
carriers. Layering these two types of materials together will allow movement of charge,
resulting in the generation of an electric current when a potential is created. This is known as a
PN-Junction. A nearby material that emits electrons toward a PN-junction that is oriented a
specific way will cause a potential gradient and induce an electric current as seen in Figure [3].
This process is known as the Voltaic Effect14.
Fig. [3] Voltaic Effect on PN-Junction Semiconductor Material
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Converging all of the aforementioned ideas, a new source of energy can be utilized.
Many types of these batteries have been designed with these same basic ideas but have slight
modification in mechanics. These designs are proprietary and are made for very specific
applications. The next section will explore some of these technologies, but there will an
emphasis on betavoltaics as it pertains to the proposed modular design.
Section 2: Types of Nuclear Batteries
Nuclear batteries can be separated into two main categories. The first type of
nuclear battery is Thermal Converting Batteries. Thermal Converting Batteries have energy
outputs that directly depend on the temperature difference of certain components that constitute
the energy transfer mechanism. An example is NASA’s Radioisotope Thermo-electric Generator
(RTG) shown in Figure [4].
Fig. [4] Radioisotope Thermo-electric Generator Schematic
(Department of Energy—Office of Nuclear Energy)
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RTGs, like other nuclear batteries, essentially have no moving parts. They generate
electricity by utilizing the heat released from radioactive decay. Heat from decay is generated by
many unstable isotopes, including byproducts of nuclear fission capable of radiating heat for
decades. This heat source conducts onto a thermo-electric generator. The heat-to-electricity
conversion occurs when two different conducting metals are joined together to create a closed
circuit and the two junctions are kept at different temperatures. In the case above, the
thermocouple is made of silicon and germanium. The temperature differential is brought about
by utilizing the radioisotope to transfer more heat to one metal conductor while the other is
cooled (in this case, by heat radiation in space). The result of the system is an electrical current
through the closed circuit5. Other Thermal Converting Batteries work in a similar way, with
minor changes in the conversion mechanism. Although Thermal Converting batteries like RTGs
allow the harnessing of heat energy from radioactive decay, their implementation is not practical
in small, everyday devices.
Non-thermal Converting Batteries extract incident energy of radioactive decay and
does not depend on a temperature differential. There are three existing types of non-thermal
converting atomic batteries that have been developed. The notions for these batteries have come
about in recent decades and the technology for each can be considered in their infancy.
The first type is the Optoelectric Battery, which involves turning the beta decay of
radionuclides to light and then the conversion of light into electrical energy (photovoltaics).
Isotopes are suspended in gas that is capable of being excited to a specific excimer line by
emitted electrons. The chosen excimer gases then emit light that is of certain wavelength, which
is then used to excite a PN-junction in a photovoltaic cell to produce an electric current.
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Technetium-99 and Strontium-90 have both been used as the radioisotope and the gases used
may be a mixture of argon, xenon, and krypton for the production of this type of battery6.
The second type is the Radioisotope Piezoelectric Battery (RPB), developed at
Cornell University. Piezoelectricity is the linear accumulation of electric charge due to
mechanical stress or pressure directly due to the Piezoelectric Effect. The reverse Piezoelectric
Effect is what drives the RPB, using an electric field and mechanical energy to produce a current.
A piezoelectric cantilever is suspended over a thin film of the radioactive isotope nickel-63. This
isotope emits beta particles towards the cantilever, which gradually builds an electric charge. At
the same time, the film of nickel isotope builds a positive charge. As the potential gets stronger,
the piezoelectric cantilever bends toward the film until they come into contact. The charge then
jumps the gap and the current flows back to the isotope, equalizing the potential and resetting the
cantilever7.
Fig. [5] Mechanical Action of a Piezoelectric Cantilever With a Radioisotope
(Cornell News, Cornell University)
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This type of battery has been shown to convert the energy of beta decay into mechanical energy
with about 7% efficiency and can vary its frequency from 120 Hz to a cycle every 3 hours. Since
the half-life of nickel-63 is over 100 years, it will be able to keep a reliable conversion of
electrical to mechanical energy in this form for more than half that time. The cantilever may
mechanically actuate a linear device or can move a cam or ratcheting wheel as examples of how
it may be incorporated to produce movement. Also, a magnet may be placed on the cantilever
and surrounded by a coil in order to produce a current as the magnet passes7.
Fig [6] Diagram of Betavoltaic Cell
The third kind of Non-thermal Converting Nuclear Battery is the betavoltaic cell.
Analogous to a photovoltaic cell, the betavoltaic cell operates on the same principle, but
generates energy from a beta-emitting radioisotope instead of light. When the isotope radiates
toward a PN-junction, it creates hole pairs in the semiconductor material, which in turn generates
a current due to the voltaic effect represented in Figure [6]. Beta-emitting isotopes such as
strontium-90 can provide continuous energy for over a decade, with a half-life of 28.8 years. It
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also decays to yttrium-90, which is another
!
"# particle emitter with a half-life of 64 hours. This
means that there is a potential for more energy from one isotope. There are multiple stages to the
energy conversion of the radioisotope into usable electrical energy.
Stage 1: The ground state, E0 is made from a potential difference between two electrodes that is
provided by an electrical load. This could be from any mechanism requiring power to operate.
Although a potential exists there is no current flow because the pn-junction of the system is in
equilibrium, and there is no energy source.
Stage 2: A beta-emitting radioisotope is introduced into the system. The energy Eb of an emitted
beta particle collides with another electron in the PN-junction and generates electron-hole pairs
by transferring kinetic energy. The energy required to strip an electron from a neutral atom in
the junction will be defined as E1.
Stage 3: The beta particle transfers energy in excess to the ionization potential, which raises
electron energy to an excited level. This excited level is defined as E2. The beta particle will
collide multiple times before effectively transferring all of its energy and returning to a ground
state electron. The number of collisions and electron-hole pairs generated depends the junction
material. More efficient doping will generate a larger amount of excited electrons.
Stage 4: Excited electrons are driven out circuit toward Electrode A in Figure [6] due to the
induced electric field present. Excited electrons in the electrode seek to give up their ionized
energy and go back to ground state. This creats a Fermi potential across the Electrodes in the
circuit.
Stage 5: The Fermi Voltage drives electrons from Electrode A down the circuit and into the
load, RL, where they give up their energy E3. The amount of energy able to be removed from the
system and into the load is
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!
E3 = EB " E1 " L1 " L2 (6)
where L1 is the loss in the conversion and L2 is the losses in the electrical circuit.
Stage 6: After passing through RL, the electrons have the energy E4. From there, the electrons
are driven into Electrode B and into the P-end of the junction where it may fill the hole of a
junction ion, releasing E4 in the form of heat. The electron has then returned to its original
ground state and the circuit is completed. The radioactive source continues to supply this
process constantly at a rate that matches the decay (equation 2) above. The total energy equation
of the system can be written as:
!
E0 = EB " E1 " E3 " L1 " L2 (7)
The result is that the potential difference provides a constant voltage, while the radioisotope is
the generator of the work that moves the electrons since the ground state energy is constant8.
Recent research in betavoltaics has also increased the efficiency of the conversion
system. Traditional semiconductors that accept energy from a voltaic effect are manufactured to
have flat, planar surfaces. Modern technology allows the production of “porous” silicon
semiconductors. The pores in the material allow a three-dimensional well to be in place of a flat
surface. This dramatically increases the surface area of the semiconductor material to absorb
!
"#
particles, resulting in higher efficiency. This works because
!
"# decay occurs in all directions
relative to the isotope. Surrounding the radioactive material within close proximity allows the
maximum amount of energy to be converted. The new type of silicon comes about from an
etching technique that creates wells about 1 micron wide and 40 microns deep in random
locations. In order to maximize this kind of design, the radioactive isotope will have to be in the
gaseous form. Tritium gas has been used to show a ten fold increase in efficiency with silicon
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etched in a random fashion and an estimated 160 fold increase in efficiency once manufacturing
techniques can create etched wells in a uniform lattice9.
Another recent breakthrough in betavoltaics is the idea of using a liquid or
amorphous semiconductor. High-energy
!
"# emission from radioactive isotopes can slowly
deteriorate semiconductors in a solid crystalline or lattice structure10. This results in reduced
efficiency over time. To circumvent this outcome, liquid semiconductors have been used.
Having properties of solid semiconductors, their liquid counterparts can be used because of their
higher versatility in the application of betavoltaics. Liquid semiconductors are made by melting
selenium at high temperatures, then compounding the material with sulfur resulting in a
compound such as Se65S35. The radioactive isotope, no matter solid or gas, can be directly added
to this compound11. This allows the absorption of
!
"# particles from all angles, since the isotope
is completely surrounded by the liquid semiconductor while not deteriorating from constant
bombardment of beta particles.
Either of these methods can be employed into the design of a battery. Because
existing electrical devices already employ some degree of modulation for batteries, the proposed
design is an example of how to utilize the aforementioned betavoltaic technologies.
Section 3: Novel Application
When designing a battery case for every day use with this type of technology the
first consideration must be safety. There is an emphasis on shielding in order to contain
radioactive materials as well as stop any unabsorbed or unconverted radiation. The mass of the
radioisotope can vary depending on application, but there is a threshold for this design that
cannot be determined until further testing, either in a lab or a computer simulation such as
Geant4. The design for the components, including calculations for material properties and
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volumes were all performed in SolidWorks 3D CAD12. The design is based on the standard AA
dry cell battery used in everyday electronic devices.
The first component of the battery casing is the housing of the radioactive material
and semiconductor apparatus seen in Figure [7].
Fig. [7] Housing Tube
The material chosen for the housing tube is solid polyurethane foam. High-energy
!
"# particle
shielding requires a low-density material13, and polyurethane is meets this criterion while still
maintaining rigidity. The outer diameter of the Housing Tube is 0.6 cm, and length is 42.9 mm.
The 1 mm thick walls can be changed to meet the adequate amount of initial shielding for the
battery, depending on what type and amount of beta emitter is used (some beta emitters eject
particles at higher energy8). As it is designed, the Housing Tube weighs 0.06 grams.
Aluminum Alloy 1060 is used for the terminal caps because of its high conductivity,
and conversely, low resisitivity of 2.8
!
"# m . The aluminum caps are also thick (2 mm) in order
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to ensure proper shielding. Both caps have a base diameter of 10 mm. The positive and negative
caps can be seen in Figures [8] and [9], respectively.
Fig. [8] AA1060 Positive Cap
Fig. [9] AA 1060 Negative Cap
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The outer case is the most vital structure to ensure the safety of the battery. It is
responsible for the absorption of any stray particles that are not absorbed by the low-density
polyurethane layer. Also, the outer layer is responsible for containment of the inner materials.
This means the material should be able to prevent a leak of radioisotope from compromised
structure by having a high tensile strength and high compressive strength. Industrial epoxy has
these properties while still maintaining a light weight. The tensile strength of unfilled epoxy is
28 million N/m2 and the compressive strength is even larger at a rated value of 104 million N/m2.
A model of a medial cross-section of the outer casing can be seen in Figure [10].
The epoxy case will be molded over the base flange of each cap to further ensure
structural stability of the whole apparatus. Additionally, the epoxy enclosure will provide
additional shielding and prevent removal of the terminal caps. The total weight of the epoxy
outer case is 7.74 grams.
Fig. [10] Epoxy Outer Case
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Fig [11] Tube, Caps, and Cross-Sectioned Epoxy Case
Once the betavoltaic energy conversion system is inserted into the Housing Tube, the
semiconductors are attached to their respective aluminum alloy caps. The terminal caps are then
mated to the ends of the Housing Tube. The epoxy case is then cast over the tube and cap
apparatus to make the complete battery (Fig. [12]).
Altogether, the casing system for the betavoltaic battery weighs only 8.86 grams. The
battery will weigh more with the radioisotope energy conversion system inside the Housing
Tube, but it will be less than the 23 grams of a standard AA dry cell battery. This provides for
more versatility in applications dependent on weight. This technology can be incorporated into
every day technologies by adding a larger mass of beta emitting isotopes and/or putting more
power cells in series. Since there is no mainstream use for radioactive wastes, beta emitters that
are byproducts of nuclear fission such as strontium-90 can be utilized and recycled.
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Figure [12] Complete Betavoltaic Battery with Modular Case
Figure [13] Negative Side of Betavoltaic Battery
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Although this technology looks promising, little research has been done to increase the
efficiency of the conversion system. Betavoltaics has the potential to provide decades of
constant energy, but without a high efficiency or a dangerous amount of radioisotopes, the
battery is still not capable of powering mobile devices such as phones and laptops that require
high-energy output (LED screens, speaker systems, large mechanical systems, etc.). Although,
without investing into the exploration of the new technology, beta emitters will continue to
naturally decay and release energy that can be harnessed and utilized as an alternative energy
source.
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References
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