Published on Nov 30, 2023
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 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.
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.
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.
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. 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.
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 figure 1.
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.
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.
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