Nuclear, or radioisotope, or atomic batteries are self-contained power sources capable of operating without recharging for years. The creation of such a source, which is in demand in various industries from space to medicine, is one of the most promising areas in physics. Scientists at MEPhI have come close to realizing this task.
Nuclear batteries are current sources that convert the energy of radioactive decay of metastable nuclei into electricity. The radioactive elements in nuclear batteries are a- and b-active nuclei with half-lives of T1/2 ranging from hundreds of days to hundreds of years depending on the problem to be solved. The choice of nucleus for a nuclear battery from the wide range of radionuclides used in radioisotope power depends on the specific purpose for which the power source is being built, its mode of operation and a number of other conditions.
Choice of radioisotope and conversion scheme
The areas of application are varied: in the near future nuclear batteries will be indispensable in remote locations away from infrastructure, such as in the Arctic, in deep water, on long-distance gas and oil pipelines, in space, as well as in communication and medical applications where long term monitoring is required without the need to recharge or swap energy sources. Apart from high power density, simplicity and ease of use of the radionuclide (for instance, in a nuclear reactor) and the absence of gamma radiation are also important features - so, for example, only plutonium-238 and nickel-63 are suitable for nuclear batteries in pacemakers or blood pressure monitors and blood values. The requirement for a safe radioisotope sharply narrows the pool of potential candidates, since nuclei must either all decay to the ground state of the daughter nucleus or populate the excited states of the daughter nucleus with a very low probability.
In addition to the choice of radioisotope, the choice of a scheme for converting nuclear decay energy into electricity is also of fundamental importance in developing radioisotope energy sources. In practice, the conversion of nuclear energy into electrical energy is carried out mainly according to the indirect step principle: the kinetic and Coulomb energy of alpha and beta particles is first converted into other energy, such as thermal, chemical, mechanical, light, etc., and then into electrical energy.
Why nickel-63?
Scientists at the NRNU MEPhI have been investigating the possibility of using 63Ni as a radioisotope for nuclear batteries in the civilian sector. It is the most promising radionuclide in beta-voltaics - the average beta particle energy of 63Ni is 17.5 keV, its half-life is 100.1 years - and it could easily be made a soft source beta shield in a miniature power cell.
A group of scientists from the LaPlase Institute led by Peter Borysiuk has proposed an original 63Ni-based physics system that allows efficient generation of secondary electrons directly inside nanostructured nickel films and significantly increases the current signal caused by a cascade of multiple inelastic beta-particle collisions. This system is relatively simple in terms of experimental implementation and is an ensemble of densely packed nickel nanoclusters with a gradient size distribution of nanoparticles deposited on the surface of a broadband silicon oxide dielectric. The key feature of the system is based on the fact that due to the size dependence of the Fermi energy, the presence of a spatially non-uniform size distribution of metallic nanoparticles leads to a spatial charge redistribution in such a system. This means that in an electrically conducting system of metallic nanoparticles contacting each other, the average size of which changes monotonically in the selected direction, a potential difference should be registered in the same direction. Thus, the formation of nickel-63 nanocluster films with a gradient size distribution of nanoparticles offers a unique opportunity and enables the combination of two important processes: firstly, the formation of coatings with a fixed potential difference (determined by the size difference of nanoparticles in the selected direction); secondly, the conversion of 63Ni beta decay energy into electron current without using additional complex to implement semiconductor systems.
A discovery made during development
However, it was found that these nanostructured films can be used as a selective photo-emitter system with a redistributed emission spectrum in a given spectral range. Experiments have shown that the oxidation process of this film leads to the formation of an oxide shell on top of the metal core of the nanocluster. Thus, oxidation of a metal film forms an ensemble of metallic nanoclusters with a spatial distribution of nanoclusters in size and having a layer (shell) of oxide. Small size of nano-clusters (2-15 nm) leads to manifestation of quantum properties, so the ensemble of such nano-clusters with the oxide shell is a set of semiconductor materials with a wide spread of band gap width. This allows the emission of photons of a given wavelength when heated and, therefore, makes it possible to "tune" the emission spectrum of the proposed system to the desired wavelength range. This is a fundamentally important point that raises, within the framework of the proposed concept, the energy efficiency and energy conservation of modern thermal power sources to a whole new level.
The development of thermophotovoltaic converters is currently being actively pursued in the USA and Europe with the aim of increasing efficiency for use in space exploration vehicles. Increasing the efficiency of solar cells through the use of (external link) thermophotovoltaic materials will give a new impetus to the improvement of nuclear batteries. So, for the time being, the way to create high-efficiency radioisotope power sources is to find new or modified materials, such as (nano)materials, which could replace silicon, germanium and other narrow-zone semiconductors in their semiconducting properties.
The idea proposed by MEPhI scientists is an original alternative approach to solving the problem of converting nuclear decay energy into electricity. The idea, put into practice, makes it possible to use energy conversion throughout the whole volume of the material, which increases conversion efficiency and opens up a wide range of scalability of these cells to gain more power or miniaturisation. This fact gives the right to consider this approach to the development of nuclear batteries with energies up to sub-kW as a universal one.
