Published on Jan 10, 2016
A burgeoning need exists today for small, compact, reliable, lightweight and self-contained rugged power supplies to provide electrical power in such applications as electric automobiles, homes, industrial, agricultural, recreational, remote monitoring systems, spacecraft and deep-sea probes.
Radar, advanced communication satellites and especially high technology weapon platforms will require much larger power source than today’s power systems can deliver. For the very high power applications, nuclear reactors appear to be the answer. However, for intermediate power range, 10 to 100 kilowatts (kW), the nuclear reactor presents formidable technical problems.
Because of the short and unpredictable lifespan of chemical batteries, however, regular replacements would be required to keep these devices humming. Also, enough chemical fuel to provide 100 kW for any significant period of time would be too heavy and bulky for practical use. Fuel cells and solar cells require little maintenance, and the latter need plenty of sun.
Thus the demand to exploit the radioactive energy has become inevitably high. Several methods have been developed for conversion of radioactive energy released during the decay of natural radioactive elements into electrical energy. A grapefruit-sized radioisotope thermo- electric generator that utilized heat produced from alpha particles emitted as plutonium-238 decay was developed during the early 1950’s.
Since then the nuclear has taken a significant consideration in the energy source of future. Also, with the advancement of the technology the requirement for the lasting energy sources has been increased to a great extent. The solution to the long term energy source is, of course, the nuclear batteries with a life span measured in decades and has the potential to be nearly 200 times more efficient than the currently used ordinary batteries. These incredibly long-lasting batteries are still in the theoretical and developmental stage of existence, but they promise to provide clean, safe, almost endless energy.
Betavoltacis is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Betavoltaics are generators of electrical current, ineffect a form of a battery, which use energy from a radioactive source emitting beta particles (electrons). The functioning of a betavoltaics device is somewhat similar to a solar panel, which converts photons (light) into electric current.
Betavoltaic technique uses a silicon wafer to capture electrons emitted by a radioactive gas, such as tritium. It is similar to the mechanics of converting sunlight into electricity in a solar panel. The flat silicon wafer is coated with a diode material to create a potential barrier. The radition absorbed in the vicinity of and potiential barrier like a p-n junction or a metal-semiconductor contact would generate separate electron-hole pairs which inturn flow in an electric circuit due to the voltaic effect. Of course, this occurs to a varying degree in different materials and geometries.
A pictorial representation of a basic Betavoltaic conversion as shown in figure 1. Electrode A (P-region) has a positive potential while electrode B (N-region) is negative with the potential difference provided by me conventional means.
The junction between the two electrodes is comprised of a suitably ionisable medium exposed to decay particles emitted from a radioactive source.
The energy conversion mechanism for this arrangement involves energy flow in different stages:
Before the radioactive source is introduced, a difference in potential between to electrodes is provided by a conventional means. An electric load R L is connected across the electrodes A and B. Although a potential difference exists, no current flows through the load R L because the electrical forces are in equilibrium and no energy comes out of the system. We shall call this ground state E 0 .
Next, we introduce the radioactive source, say a beta emitter, to the system. Now, the energy of the beta particle E b generates electron- hole pair in the junction by imparting kinetic energy which knocks electrons out of the neutral atoms. This amount of energy E 1 , is known as the ionization potential of the junction.
Further the beta particle imparts an amount of energy in excess of ionization potential. This additional energy raises the electron energy to an elevated level E 2. Of course the beta [particle dose not impart its energy to a single ion pair, but a single beta particle will generate as many as thousands of electron- hole pairs. The total number of ions per unit volume of the junction is dependent upon the junction material.
next, the electric field present in the junction acts on the ions and drives the electrons into electrode A. the electrons collected in electrode A together with the electron deficiency of electrode B establishes Fermi voltage between the electrodes. Naturally, the electrons in electrode A seek to give up their energy and go back to their ground state (law of entropy).
the Fermi voltage derives electrons from the electrode A through the load where they give up their energy in accordance with conventional electrical theory. A voltage drop occurs across the load as the electrons give an amount of energy E 3. Then the amount of energy available to be removed from the system is
E 3 = E b - E 1 - L 1 -L 2
Where L 1 is the converter loss and L 2 is the loss in the electrical circuit.
the electrons, after passing to the load have an amount of energy E 4 .from the load, the electrons are then driven into the electrode B where it is allowed to recombine with a junction ion, releasing the recombination energy E 4 in the form of heat this completes the circuit and the electron has returned to its original ground state.
The end result is that the radioactive source acts as a constant current generator. Then the energy balance equation can be written as
E 0 =E b -E 1 -E 3 -L 1 -L 2
Until now betavoltaics has been unable to match solar-cell efficiency. The reason is simple: when the gas decays, its electrons shoot out in all directions. Many of them are lost. A new Betavoltaic device using porous silicone diodes was proposed to increase their efficiency. The flat silicon surface, where the electrons are captured and converted to a current, and turned into a 3- dimensional surface by adding deep pits. Each pit is about 1 micron wide. That is four hundred-thousandths of an inch. They are more than 40 microns deep. When the radioactive gas occupies these pits, it creates the maximum opportunity for harnessing the reaction.
An optoelectric nuclear battery has been proposed by researchers of the kurchatov institute in Moscow. A beta emitter such as technetium-99 are strontium-90 is suspended in a gas or liquid containing luminescent gas molecules of the exciter type, constituting “dust plasma”. This permits a nearly lossless emission of beta electrons from the emitting dust particles for excitation of the gases whose exciter line is selected for the conversion of the radioactivity into a surrounding photovoltaic layer such that a comparably light weight low pressure, high efficiency battery can be realized.
These nuclides are low cost radioactive of nuclear power reactors. The diameter of the dust particles is so small (few micrometers) that the electrons from the beta decay leave the dust particles nearly without loss. The surrounding weakly ionized plasma consists of gases or gas mixtures (e.g. krypton, argon, xenon) with exciter lines, such that a considerable amount of the energy of the beta electrons is converted into this light the surrounding walls contain photovoltaic layers with wide forbidden zones as egg. Diamond which converts the optical energy generated from the radiation into electric energy.
The battery would consist of an exciter of argon, xenon, or krypton (or a mixture of two or three of them) in a pressure vessel with an internal mirrored surface, finely-ground radioisotope and an intermittent ultrasonic stirrer, illuminating photocell with a band gap tuned for the exciter. When the electrons of the beta active nuclides (e.g. krypton-85 or argon-39) are excited, in the narrow exciter band at a minimum thermal losses, the radiations so obtained is converted into electricity in a high band gap photovoltaic layer (e.g. in a p-n diode) very efficiently the electric power per weight compared with existing radionuclide batteries can then be increased by a factor 10 to 50 and more. If the pressure-vessel is carbon fiber / epoxy the weight to power ratio is said to be comparable to an air breathing engine with fuel tanks.
The advantage of this design is that precision electrode assemblies are not needed and most beta particles escape the finely-divided bulk material to contribute to the batteries net power. The disadvantage consists in the high price of the radionuclide and in the high pressure of upto 10MPa (100bar) and more for the gas that requires an expensive and heavy container.
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