Innovative nanotechnologies for modern technical superconductors
Superconductors are characterized by several critical parameters (see table), for example, by the critical temperature at which a material goes into superconducting state. The critical temperature of the currently known superconductors varies in very wide limits from 0.0005 K by magnesium (Mg) and 39 K by magnesium diboride (MgB2) to about 135 K by mercury-containing compound HgBa2Ca2Cu3O8. The current flowing through the superconductor cannot be increased indefinitely, because when reaching a certain value (the magnitude of the critical current) the state of superconductivity can disappear. In addition, the conductor ceases to be a superconductor under the action of an external magnetic field if its intensity is above a certain value, called the critical field.
Since 70-ies of the last century, the VNIINM and Kurchatov institute have begun to actively develop technical superconductors. Technologies developed in VNIINM have been implemented in industrial production, organized in Kazakhstan (at that time part of the USSR), where technical LTS based on superconducting alloy NbTi and superconducting intermetallic compound Nb3Sn were produced on the scale from tens to hundreds of tons. These materials were used to create in the USSR the world's first large tokamaks (toroidal chambers with magnetic coils) with superconducting magnet systems T-7 and T-15, which were successfully commissioned and have shown full compliance with material requirements.
Low-temperature composite NbTi- and Nb3Sn-superconductors, as a rule, represent a composite wire (strand) with diameter of 0.5–2.0 mm and a length up to 50 km, containing a metal matrix of up to several tens of thousands of continuous filaments with a diameter of 1.5–5 μm each. Structures of both types of superconductors are characterized by certain location of the superconducting filaments in a matrix of copper or copper alloy. In addition, the strand construction contains a stabilizing copper sheath and a diffusion barrier, typically made of niobium or tantalum (fig.1).
It should be emphasized that the high electrical characteristics of low temperature NbTi- and Nb3Sn-based superconductors are provided by features of the nanostructures. In particular, the share of titanium in a NbTi should be from 46.5 to 50 wt. % because at lower level in the structure of the alloy decreases the amount of the α-Ti nanoparticles, which provide a high current carrying capacity of the superconductor, and at greater level drastically increases the number of nanoparticles that decorating the grain boundaries of the alloy, which significantly reduces the ductility of the material and makes it difficult to obtain long-length composite wires. In addition, the number and morphology of α-Ti nanoparticles are determined by the thermomechanical processing of the material. To ensure a required critical current density in superconducting alloy the nanostructure is created that represents a combination of superconducting and not superconducting phases. As shown by the experiments, three combinations of cold deformation and subsequent heat treatment is sufficient to achieve current-carrying capacity of 3000 A/mm2 in an external magnetic field of 5 T at a temperature 4.2 K. In NbTi-superconductors with high current-carrying capacity the α-phase particles with a thickness of 1–3 nm are surrounded by crescent subgrains of the NbTi solid solution (fig.2). The average distance between particles is 7–8 nm, and volumetric quantitative proportion of α-particles is 16–17 %.
Creating a regular heterogeneous nanostructure with different distances between superconducting and not superconducting areas, it is possible to improve NbTi-superconductors to obtain maximum critical current densities in various fields.
To increase current-carrying capacity of Nb3Sn-superconductors, especially in external magnetic fields over 12 Tesla, a a special nanostructure of superconducting intermetallic compound is formed. According to the theory, to improve the critical current it is necessary to obtain fine and homogeneous grain structure of Nb3Sn. This can be achieved by applying a special technology of strands manufacturing. A special attention is paid to the final reactionary heat treatment, in which process a superconducting layer is formed due to diffusion of tin from the surrounding CuSn matrix to niobium fibers. Fig.3 shows typical nanostructure of Nb3Sn-layer of developed in VNIINM superconductor and a histogram of the distribution of the grains of the intermetallic compound. The histogram shows that the majority of the grains of the intermetallic compound Nb3Sn has a size less than 100 nm.
Another way to increase current-carrying capacity of Nb3Sn-strands is the alloying of the matrix or fibers with different elements, most often with titanium. Electron-microscopic studies showed that the titanium atoms diffuse into the superconducting layer, forming nanoparticles of the Cu-Ti compound (fig.4), which increases current-carrying capacity of the composite superconductor.
More than 40 years of experience in the research, development and production of composite LTS has allowed Russia to take part in the international project of thermonuclear experimental reactor ITER. Russia, having won the tender, became one of the manufacturers of superconductors, along with leading companies from Europe, USA and Japan. To ensure the supply of superconducting materials for magnet systems of ITER, on the basis of the Chepetsky mechanical plant (ChMP) was organized industrial production of LTS (fig.5). VNIINM specialists has prepared more than 100 technical specifications for specialized equipment, has developed a technological route and methods of control of semi-finished and finished products. At the plant was established cryogenic test laboratory for measurements of electrical characteristics of samples of obtained superconducting materials.
Within 5 years after the commencement of LTS for ITER was obtained about 220 tons of Nb3Sn- and NbTi-superconducting strands, that meets all your requirements. VNIINM specialists constantly carried out the optimization and control of technological regimes at all stages of production, and also performed verification tests of the strands samples as the reference laboratory, officially recognized by ITER.
Along with the already implemented in industrial production so-called "bronze" manufacturing technology of Nb3Sn-strands, VNIINM continues development in other promising areas. One of solutions is so-called "powder in tube " (PIT). Foreign manufacturers of Nb3Sn-superconductors have achieved in the experimental samples the critical current density on the cross section of copper over 2500 A/mm2 in a field of 12 T at 4.2 K. VNIINM develops superconductors using this method and the prototypes are already produced (fig.6).
Another solution to achieve the highest critical current density, is the method of "internal source of tin", according to which individual sources of tin are located in the copper matrix, in addition to distributed niobium fibers. This significantly increases the content of superconducting phase and allows to get a record values of critical current density on the cross section without copper (more than 3000 A/mm2 in 12 T at 4.2 K). VNIINM developes designs and technologies for manufacture of such superconductors.
HTS attract the attention of developers of electrical products due to possibility to use the phenomenon of superconductivity at nitrogen temperatures (77 K), which is very promising from the point of view of their economic efficiency. Initially, the high-temperature superconductors of 1st generation were developed, the composite wires sheathed in silver-based alloys based on Bi2Sr2Ca2Cu3Ox (Bi-2223/Ag) (fig.7a). Since the mid 2000-ies the high-temperature superconductors of 2nd generation (HTS-2) are of the growing interest, which are based on a multi-layer tape – a thin metal substrate with sequentially deposited oxide buffer layers and superconducting functional layer of YBa2Cu3Oх (Y-123) (fig.7b).
One of the most important tasks of the developers of HTS-2 is to increase the critical current flowing through the superconductor, and this is achieved with use of nanotechnology. In particular, one of the effective methods to increase current-carrying capacity is the introduction and uniform distribution in the superconducting layer of nanoparticles of oxides of rare earth and transition metals. To solve this problem, specialists of VNIINM have developed targets for deposition of superconducting layers, doped with oxide particles of barium zirconate. Appropriate techniques allow to avoid the formation of coarse conglomerates of nanoparticles of oxides and to distribute them evenly across the target volume (fig.8).
Nanotechnologies are used by the specialists of VNIINM also in the production of metal belts for HTS-2, a substrate onto which a buffer and superconducting layers are deposited. According to the requirements, the surface roughness of this tape should not exceed 20 nm (Ra≤ 20 nm). VNIINM has perfect technology for manufacturing of texturized tapes on the basis of Ni-W alloy with a length of up to several hundreds of meters and strips on the basis of stainless steel with a length of over 1000 meters (fig.9).
Superconductive magnesium diboride MgB2 is also the object of intense research, due to the high critical temperature (39 K), which is two times higher than that of Nb3Sn and four times higher than that of NbTi. The relatively low anisotropy of properties, simple chemical composition, low cost of components for synthesis open new possibilities for the practical use of MgB2-based superconductors (fig.10) in the magnetic and electrical devices.
And in this case, the technologists and scientists use nanotechnologies. In particular, the doping with SiC nanoparticles or carbon nanotubes is used to improve the critical currents which can transfer the MgB2 superconductor. To obtain alloyed MgB2 specialists of VNIINM at first mixed raw powder of boron with carbon nanotubes (fig.11) and, after addition of magnesium, carried out heat treatment.
The future of physics, energy and medicine is inconceivable without the use of superconductors. To create a powerful magnetic systems of thermonuclear fusion devices, particle accelerators, high-resolution spectrometers, medical magneto-resistive scanners and other equipment need it is necessary to improve superconducting materials, using innovative technologies. Only with the use of the latest achievements of materials science it is possible to create nanoscale structure of superconducting compounds to obtain the necessary high current-carrying capacity of low-temperature and high-temperature superconductors.