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					                      ACKNOWLEDGEMENT

         I express my sincere thanks to Dr.P.M.S.Nambisan, Head of the Department for

 providing me with the guidance and facilities for the seminar
.
         I extend my sincere gratitude to seminar coordinator Mr.Gylson Thomas, for his

 cooperation for presenting the seminar
.
         I thank my seminar guide Mrs.Jayasree.M.S. for

her great help and guidance in preparing and presenting my seminar
.
         I also extend my sincere thanks to all other faculty members of Electrical and

Electronics Department and my friends for their support and encouragement
.
                                                     TINU JO. M
                ABSTRACT

            Nuclear batteries harvest energy from radioactive
specks and supply power to micro electromechanical systems
(MEMS). This paper describes the viability of nuclear batteries for
powering realistic MEMS devices. Nuclear batteries are not
nuclear reactors in miniatures, but the energy comes from high-
energy particles spontaneously emitted by radioactive elements.
Isotopes currently being used include alpha and low energy beta
emitters.Gama emitters have not been considered because they
would require a substantial amount of shielding. The sources are
available in both soil and liquid form.
            Several approaches are being explored for the
production of MEMS power sources. The first one is a junction
type battery. The second concept involves a more effective use of
the charged particles produced by the decay: the creation of a
resonator by inducing movement due to attraction or repulsion
from the collection of charge particles.
CONTENTS

  INTRODUCTION

  NUCLEAR BATTERIES

  JUNCTION TYPE BATTERY

  SELF-RECIPROCATING CANTILEVER

  ISOTOPE SELECTION

  SAFETY ASSESSMENT

  ADVANTAGES

  DISADVANTAGES

  APPLICATIONS

  CONCLUSION

  REFERENCES
  INTRODUCTION

             Micro electro mechanical systems (MEMS) comprise
a rapidly expanding research field with potential applications
varying from sensors in air bags, wrist-warn GPS receivers, and
matchbox size digital cameras to more recent optical applications.
Depending on the application, these devices often require an on
board power source for remote operation, especially in cases
requiring for an extended period of time. In the quest to boost
micro scale power generation several groups have turn their
efforts to well known enable sources, namely hydrogen and
hydrocarbon fuels such as propane, methane, gasoline and diesel.
Some groups are develo ping micro fuel cells than, like their
micro scale counter parts, consume hydrogen to produce
electricity. Others are developing on-chip combustion engines,
which actually burn a fuel like gasoline to drive a minuscule
electric generator. But all these approaches have some difficulties
regarding low energy densities, elimination of by products, down
scaling and recharging. All these difficulties can be overcome up
to a large extend by the use of nuclear micro batteries.

                Radioisotope thermo electric generators (RTGs)
exploited the extraordinary potential of radioactive materials for
generating electricity. RTGs are particularly used for generating
electricity in space missions. It uses a process known as See-beck
effect. The problem with RTGs is that RTGs don‟t scale down
well. So the scientists had to find some other ways of converting
nuclear energy into electric energy. They have succeeded by
developing nuclear batteries.
  NUCLEAR BATTERIES

             Nuclear batteries use the incredible amount of energy
released naturally by tiny bits of radio active material without any
fission or fusion taking place inside the battery. These devices use
thin radioactive films that pack in energy at densities thousands of
times greater than those of lithium-ion batteries. Because of the
high energy density nuclear batteries are extremely small in size.
Considering the small size and shape of the battery the scientists
who developed that battery fancifully call it as “DAINTIEST
DYNAMO”. The word „dainty‟ means pretty.

 Types    of nuclear batteries
             Scientists have developed two types of micro nuclear
batteries. One is junction type battery and the other is self-
reciprocating cantilever. The operations of both are explained
below one by one.
  1. JUNCTION TYPE BATTERY

              The kind of nuclear batteries directly converts the
high-energy particles emitted by a radioactive source into an
electric current. The device consists of a small quantity of Ni-63
placed near an ordinary silicon p-n junction – a diode, basically.

WORKING:

              As the Ni-63 decays it emits beta particles, which are
high-energy electrons that spontaneously fly out of the
radioisotope‟s unstable nucleus. The emitted beta particles ionized
the diode‟s atoms, exciting unpaired electrons and holes that are
separated at the vicinity of the p-n interface. These separated
electrons and holes streamed away form the junction, producing
current.

               It has been found that beta particles with energies
below 250KeV do not cause substantial damage in Si [4] [5]. The
maximum and average energies (66.9KeV and 17.4KeV
respectively) of the beta particles emitted by Ni-63 are well below
the threshold energy, where damage is observing silicon. The long
half-life period (100 years) makes Ni-63 very attractive for remote
long life applications such as power of spacecraft instrumentation.
In addition, the emitted beta particles of Ni-63 travel a maximum
of 21 micrometer in silicon before disintegrating; if the particles
were more energetic they would travel longer distances, thus
escaping. These entire things make Ni-63 ideally suitable in
nuclear batteries.

CONSTRUCTION:
               Since it is not easy to micro fabricate solid
radioactive materials, a liquid source is used instead for the micro
machined p-n junction battery. The diagram of a micro machined
p-n junction is shown below




    As shown in figure a number of bulk-etched channels have been
micro machined in this p-n junction. Compared with planar p-n
Junctions, the three dimensional structure of our device allows for
a substantial increase of the junction area and the macro machined
channels can be used to store the liquid source. The concerned p-n
junction has 13 micro machine channels and the total junction area
is 15.894 sq.mm (about 55.82% more than the planar p-n junction).
This is very important since the current generated by the powered
p-n junction is proportional to the junction area.

In order to measure the performance of the 3-dimensional p-n
junction in the presence of a radioactive source, a pipette is used to
place 8 l of liquid Ni-63 inside the channels micro machined on
top of the p-n junction. It is then covered with a black box to shield
it from the light. The electric circuit used for these experiments is
shown below.
The following figure shows the I-V curves measured at 30
minutes, 2 hours and 16 hours after pouring the radioactive source
on the micro channels of the p-n junction.
In the figure we can observe that the maximum current generated
in the micro machined p-n junction by the ni-63 source, i.e. the
short circuit current is 1.31A.

ANALYTICAL MODEL
              In Si, the generation of one electron hole pair (EHP)
requires about 3.2eV, even though its energy gap between the
conduction band and the covalent band is just 1.12eV. So the
theoretical maximum current generated by Ni-63 can be calculated
as

Imax = 64Ci * (3.7*(10^10) dps) * (17.4KeV)/(3.2eV)*(1.6 * 10)
     = 2.06 * (10^-9) A
     = 2.06 nA

According to this estimate, the measured current value is 64.07%
of the theoretical maximum value. This illustrates the effectiveness
of making the boron-diffused depth around 40m during micro
fabrication, which is approximately the traveling distance of Ni-63
beta particles in Si. Normal p-n junctions, which have ultra shallow
junctions (less than m), would result in very low currents since
most of the beta particles would stop in the p or in the n regions,
instead of the depletion region.
  On the other hand, the open circuit voltage is very small only
53mv.this is partly due to the large p and n contact resistances. In
the device, the metal used for the n contact is aluminium alloy and
both the p and n regions are all covered by metal (the open circuit
voltage for a normal junction type battery is in the range of 0.2vto
0.5v). A very effective way of increasing the open circuit voltage
in a junction type battery is by reducing the contact area.
   The maximum power can be approximately estimated by:
  Pmax = Is*Vo=1.31nA*0.053v = 0.069nW
2.SELF-RECIPROCATING CANTILEVER

              This concept involves a more direct use of the
charged particles produced by the decay of the radio active source:
the creation of a resonator by inducing movement due to attraction
or repulsion resulting from the collection of charged particles. As
the charge is collected, the deflection of a cantilever beam
increases until it contacts a grounded element, thus discharging the
beam and causing it to return to its original position. This process
will repeat as long as the source is active.
This has been tested experimentally. The following figure shows
the experimental setup.



    .
In the above setup the changes emitted from the source are
collected in the beam to generate the electrostatic force that drives
it.

CONSTRUCTION:

         The self-reciprocating cantilever consists of a radioactive
source of thickness very small and of area 4square mm. above this
thin film there is a cantilever beam. It is made of a rectangular
piece of silicon. Its free end is able to move up and down. On this
cantilever beam there is a copper sheet attached to it. Also above
this cantilever there is a piezoelectric plate. So the self-
reciprocating cantilever type nuclear batteries are also called as
radioactive piezoelectric generator.

WORKING
        First the beta particles, which are high-energy electrons, fly
spontaneously from the radioactive source. These electrons get
collected on the copper sheet. Copper sheet becomes negatively
charged. Thus an electrostatic force of attraction is established
between the silicon cantilever and radioactive source. Due to this
force the cantilever bends down.

        The piece of piezoelectric material bonded to the top of the
silicon cantilever bends along with it. The mechanical stresses of
the bend unbalances the charge distribution inside the piezoelectric
crystal structure, producing a voltage in electrodes attached to the
top and bottom of the crystal.

        After a brief period – whose length depends on the shape
and material of the cantilever and the initial size of the gap- the
cantilever come close enough to the source to discharge the
accumulated electrons by direct contact. The discharge can also
take place through tunneling or gas breakdown. At that moment,
electrons flow back to the source, and the electrostatic attractive
force vanishes. The cantilever then springs back and oscillates like
a diving board after a diver jumps, and the recurring mechanical
deformation of the piezoelectric plate produces a series of electric
pulses.

        The charge-discharge cycle of the cantilever repeats
continuously, and the resulting electric pulses can be rectified and
smoothed to provide direct-current electricity.

The following series of figures shows the functions of a radioactive
piezoelectric generator step by step.
Power from within
How a nuclear micro generator converts radioactivity into
electricity

1. Beta particles (high-energy electrons) fly spontaneously from
the radioactive source and hit the copper sheet, where they
accumulate.

2. Electrostatic attraction between the copper sheet and the
radioactive source bends the silicon cantilever and the piezoelectric
plate on top of it.

3. When the cantilever bends to the point where the copper sheet
touches the radioactive source, the electrons flow back to it, and
the attractive force ceases.

4.The cantilever then oscillates, and the mechanical stress in the
piezoelectric plate creates an imbalance in its charge distribution,
resulting in an electric current.
ANALYTICAL MODEL

         In order to understand the behavior of the system an
analytical model has been developed for both the charge collection
and deflection of the cantilever. The charge collecting process is
governed by,

 dQ=adt-V/Rdt

Where dQ is the amount of charge collected by the beam during a
given time dt, a represents the current emitted by the radioactive
source, V is the voltage across the source and the beam and R is
the effective resistance between them. The second term on the
right hand side represents the current leakage arising from the
ionization of the air. Since V = Q / C, where C is the capacitance
of the beam and the source, the previous equation can be
rearranged to obtain:

           dQ/dt+Q/RC=a

           Solving,

           Auxilliary equation,(D+1/RC)Q=0

           D=-!/RC

           Complementary function CF=C1exp(-t/RC)

            Particular integral PI=a(1/(D+1/RC)),at D=0

                                  =a/(1/RC) =aRC

            Complete solution CS=CF+PI=C1exp(-t/RC)+aRC=Q

             At t=0, Q=0

              Then,C1+aRC=0
               C1=-aRC

              Then, Q=-aRCexp(-t/RC)+aRC

               The solution is Q=aRC(1-exp(-t/RC))




This equation can be readily solved to get,
           Deflection vs. time at a pressure of 50 mTorr

        The above figure shows a typical experimental result, in
which the initial distance is 3.5 mm and the vacuum is 50 mTorr.

          In this figure we can observe that the beam bends very
slowly. Therefore, the electrostatic force on the beam can be taken
as balanced by the elastic force from the beam itself. Since the
electrostatic force is proportional to Q2 and the distance between
the beam and the source can be taken as a constant, being  the
deflection of the beam, we have:



K is the elastic constant of the beam,  can be assumed to be
constant since 63Ni has a half life of more than 100 years (Ni-63 is
considered as the ideal radioactive source for radioactive
piezoelectric generator), R in the experiment is also a constant
because the pressure is maintained and no breakdown of the air
happened, otherwise the beam will bounce back. C can also be
assumed to be constant because it has been observed
experimentally that the deflection of the beam is very small
compared to the initial distance between the beam and the
radioactive source. Therefore



The following figure compares the deflection measured
experimentally with the values obtained analytically according to
the discussion shown above, by fitting R. We observe a very good
match between them.




Comparison between the experimental and analytical values for
thedeflection.to be able to design the device with the period and
energy release
         For this model, however, does not include the periodic
behavior of this device. Current experiments show a minimum
period of about 30 minutes, at which time the electrostatic energy
is released as electric current. Further studies are being done in this
area, trying to identify the key characteristics of the system in
order level appropriate for each particular application.

         From all the above explanations on the working of
radioactive piezoelectric generator it is obvious that electricity is
produced indirectly like minute generators. Radiation from the
sample is converted first to mechanical energy and then to
oscillating pulses of electric energy. Even though the energy has to
go through the intermediate mechanical phase, the batteries are no
less efficient; they tap a significant fraction of the kinetic energy of
the emitted particles for conversion into mechanical energy. By
releasing this energy in brief pulses, they provide much more
instantaneous power than direct conversion approach.
SELECTION OF ISOTOPE

          Selection of a suitable isotope as the radioactive source in
nuclear batteries is very critical. There are many factors
considering which we can make a decision. These factors include
safety, reliability, cost, and activity. We need the nuclear batteries
as small as possible. Since gamma rays are very energetic gamma
emitters would require a substantial amount of shielding. So
gamma emitters have not been considered. Both pure alpha and
low energy beta emitters have been used. The alpha emitters have
an advantage due to the short range of the alpha particles. This
short range allows increased efficiency and thus provides more
design flexibility, assuming that a sufficient activity can be
achieved.

          Half-life of radioactive source is crucial. It should be high
enough so that the useful life of the battery is sufficient for typical
applications, and low enough to provide sufficient activity. In
addition, the new isotope resulting after decay should be stable, or
it should decay without emitting gamma radiation. The isotopes
currently in use for this work are compared below:

                                                  Max          Average
                 Radiation
   Isotope                       Half-Life      Energy         Energy
                   Type
                                                 [keV]          [keV]
      3
       H            Beta           12.3 y         18.6            5.7
     63
          Ni        Beta          100.2 y         66.9           17.4
     210
           Po       Alpha         138.4 d        5304.3            -

                        Isotopes used for this work.

         To explore the viability of the nuclear micro battery
concept, some scoping calculations need to be carried out. Using
210
   Po as an example, one can analyze the best-case scenario,
assuming that the nuclear battery is created using pure 210Po. In
this case, the activity would be approximately 4,500 Ci per gram or
43,000 Ci per cubic centimeter. Thus, for a characteristic source
volume of 10-5 cubic centimeters (0.1 cm x 0.1cm x 10 microns),
one obtains approximately 0.5 Ci. Based on the results of previous
experimental studies, the available power would be on the order of
0.5 mW (about 1 mW/Ci). The power required for MEMS devices
can range from nanowatts to microwatts. A typical case is that of a
low power CMOS driven mechanical cantilever forming an air-gap
capacitor with the substrate.


SAFETY ASSESMENT

          The analysis demonstrates that current laws well below
the limits establish the potential amounts of radiation that a worker
could be exposed to due to the fabrication of these nuclear micro
batteries. The study includes the external exposure as well as the
internal exposure due to ingestion or inhalation of the radioactive
materials.

        Since the work involves the use of small amounts of
radiation and radioactive materials, it is necessary to comply with
current Radiation Protection Standards. The potential health and
environmental effects of fabricating, using and disposing of these
nuclear micro-batteries have been studied in detail.

        Current radiation protection regulations are based on the
Linear Non-Threshold model (LNT), which assumes that any
amount of radiation exposure, no matter how small, may have
negative health effects. This model was derived by extrapolating
known acute (high dose and high dose rate) exposure data points in
a linear or curvilinear fashion through the origin. Lately, however,
there has been a movement among the Medical and Health Physics
communities encouraging the review of the current regulations by
using the Non-Linear Threshold model (NLT), that establishes that
there are no detectable harmful health effects to humans at
radiation levels below 100 mSv (10 rem). Therefore, even though
it is possible to prove that our devices do comply with current
regulations, the actual health effects may be even more
insignificant.

        The analysis   proves that the environmental impact of
disposing of these micro-devices once their useful life has ended,
as well as the associated costs are minimal. Since after three half-
lives the activity of the isotope has decayed to about 10% of the
original activity, the micro-batteries would be below background
radiation level.

       The MEMS devices like sensors, actuators resonators, etc,
with their integrated nuclear micro-battery will be ensured that
their use does not result in any unsafe exposure to radiation. The
radioactive material in the device will be encapsulated in a way
that the probability of being released into the environment, and
being accidentally inhaled or digested is extremely small. The
analysis has been performed in the worst-case scenario, assuming
that the full amount of the radioactive material contained by the
micro battery would be accidentally ingested or inhaled.

        In addition to safety considerations, the design of the
shielding and encapsulation of these micro devices is aimed to
minimize the size and weight of the devices, and their cost.

       The external dose associated with these sources is zero,
because an alpha particle needs to have energy of more than 7.5
MeV to penetrate the protective layer of the skin (0.07 mm think),
and a ß particle needs to have energy of more than 70 keV. Since
our sources have energies lower than those they are unable to
penetrate the skin.
.
                   INHALATION                   INGESTION
   DOSE
   LIMIT
                         44.57                       5.479
     [2]
  [mrem/d]
     3
       H          2.4            18         0.0346           158
     64
          Ni      5.69            7         0.0813           67
    210
          Po     13540           -           193.4            -
                               # OF
                                             DOSE
                            DEVICES                       # OF
                 DOSE                       EQUIV.
                            INHALED                     DEVICES
                EQUIV.                        TO
                                TO                     INGESTED
               TO LUNG                     WHOLE
                             REACH                     TO REACH
               [mrem/d]                     BODY
                               THE                     THE LIMIT
                                           [mrem/d]
                              LIMIT

 Radiation Effects on a worker after the inhalation or ingestion of
a 5 μCi micro-battery.

                   INHALATION                   INGESTION
   DOSE
   LIMIT
                         2.28                        0.273
     [2]
  [mrem/d]
     3
       H          2.4            0.95       0.0346           7
     64
          Ni      5.69           0.40       0.0813           3
    210
          Po     13540          -           193.4           -
                              # OF          DOSE          # OF
                 DOSE
                            DEVICES        EQUIV.       DEVICES
                EQUIV.
                            INHALED          TO        INGESTED
               TO LUNG
                               TO          WHOLE       TO REACH
               [mrem/d]
                             REACH         BODY        THE LIMIT
                              THE        [mrem/d]
                             LIMIT

Radiation Effects on a member of the public after the inhalation or
               ingestion of a 5 μCi micro-battery.
ADVANTAGES OF NUCLEARBATTERIES

The main advantages of nuclear batteries are listed below:

  1. High energy density
  2. Harmless; even though deals with radioactive materials
  3. Requires no refueling or recharging till the half life period of
     the radioactive source used
  4. Compactness
  5. Light weight

          Compared to almost all other energy sources like
  chemical batteries, on chip combustion engines, micro fuel cells
  etc the energy density of nuclear batteries is found to be very
  high. So nuclear batteries can pack in energy at thousands of
  times greater than that of lithium-ion battery. This high energy
  density helps to reduce the size of nuclear batteries. But if we
  reduce the size of chemical batteries the energy storage will be
  reduced exponentially.

         Usually, devices dealing with radioactive materials are
  harmful to human body; but nuclear batteries are not. Nuclear
  batteries are made harmless by careful selection of suitable
  isotope. The selected isotope will not have gamma radiations,
  which is harmful among the three-alpha, beta and gamma
  radioactive emissions. Also thin, but strong shielding is
  provided for preventing alpha and beta radiations from coming
  out.

        Until the half-life period of the radioactive source we are
  using we need not recharge the battery or refill the fuel. The
  battery can supply the same power without failure up to that
  period. After the half-life of the radio active source the power
  output will be reduced by a factor of two. Then we should refill
  the battery with fuel. If we are using radioactive source like Ni-
  63 with half-life 100 year or more we need not recharge the
battery at all. This is because the MEMS devise to which the
battery is supplying power will not be expected to have a half-
life more than 100 years.

      The high-energy density and thin shielding requirement
helps nuclear battery to be compact and lightweight.




DISADVANTAGES OF NUCLEAR BATTERIES

     The main disadvantages of nuclear batteries are listed
below;

1. Low conversion efficiency.
2. High cost

       Conversion efficiency simply means the efficiency of
converting nuclear energy into electrical energy. For nuclear
batteries the value of conversion efficiency is found to be 4%
only. The targeted value of conversion efficiency is now 20%.
One possibility for improving the conversion efficiency of
cantilever based system would be to scale up the number of
cantilevers by placing several of them horizontally, side by side.

       Another major challenge is to have inexpensive
radioisotope power supplies that can be easily integrated into
electronic devices. For example, in experimental systems 1mCi
of Ni-63 costs about US$25-too much for use in mass produced
device. A potentially cheaper alternative would be tritium,
which some nuclear reactors produce, in huge quantities as a by-
product. So the amount of tritium needed for a micro battery
couldn‟t cost just a few cents.
APPLICATIONS

        Nuclear batteries are going to power a whole new range
of gadgetry, from nano robots to wireless sensors. Nuclear
batteries can be used in handheld devices like cell phones,
PDAs etc, very small pocketable computers, digital cameras,
MEMS engines, pumps and all. Some revolutionary applications
are

1. Ultra dense memories capable of storing hundreds of
gigabytes in a finger nail-size device

2. Micromemories for enhanced displays

3. Optical communication equipment

4. Highly selective RF filters to reduce cell phone size and
improve the quantity of calls.
CONCLUSION

         Nuclear batteries may be considered as a milestone in
the field of micro scale power generation. The disadvantages of
nuclear batteries outweigh its disadvantages. Hence nuclear
batteries are going to supply power in a wide range of MEMS
devices.

       As a result of high energy density nuclear batteries are
extremely small in size. The half life of radioactive source helps
to increase the period between two consecutive recharges.
Among the two types of nuclear batteries, radioactive
piezoelectric generators are more efficient.

       The world of MEMS devices is eagerly waiting for the
arrival of nuclear batteries in the market to make a revolutionary
change in size of MEMS.
REFERENCE

1.   IEEE SPECTRUM September 2004
2.   http://homepages.cae.wiscedu/blanchar/res/iconebattery.pdf
3.   www.bugsoft.com/ieee/meet2002.html
4.   http//www.nae.edu/nae/bridge.nsf/weblinks/MKEZ.5HUQ7J
     ?open document

				
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