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INTROUCTION
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.
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