In this review, we consider the current state of development of both low-temperature superconductors based on Nb 3 Sn and high-temperature superconductors. The effect of the preparation method and the composition of alloying elements on the microstructure of the superconducting layer and the current-carrying capacity of superconductors based on Nb 3 Sn and high-temperature superconductors of the second generation based on yttrium and bismuth cuprates, magnesium diboride, and iron-containing compounds is analyzed.

A design has been developed for three-layer composite substrate tapes based on a nickel-tungsten alloy and corrosion-resistant steel. Materials have been picked and experimental samples of three-layer composite tapes obtained. It has been shown that the substitution of corrosion-resistant steel for ferromagnetic nickel–tungsten alloy in 60% of the tape volume decreases the saturation magnetization of the substrate tape by a factor of 2. The mechanical properties of the composite three-layer tape have been investigated. It has been shown that composite tapes are close in terms of the mechanical properties to tapes made from a homogeneous nickel–tungsten alloy. The conditions for the formation of a sharp cubic texture have been determined.

The main applications and trends in the development of modern high-current superconductors, the characteristics of niobium-titanium superconductors for the FAIR (Germany) and NICA (Dubna, Russia) accelerator complexes, medical tomographs, and Nb3Sn superconductors for high-resolution spectrometers and modernization of the large hadron collider are examined. Advanced composite superconductors developed at VNIINM for different applications are described. The main results of development work on technologies for the fabrication of targets for spraying buffer and superconducting layers and for substrate tapes of different types for the fabrication of second-generation high-temperature superconductors are examined.

Substrate materials are the key element of the coated conductors. There are two types of substrates-nontextured (stainless steel (SS) or Hastelloy alloys) and textured ones which are based on the Ni-W alloys. The main requirements for the textured tapes (RABiTS) are the presence of the highly pronounced biaxial cubic texture {100} <;100> and low values of magnetic susceptibility. To realize these requirements RABiTS tapes of various kinds were developed-homogenous tapes from Ni-W alloys with W content 5.5-9 at.% and composite ones. Composite tapes consist of three layers-one of SS and two of Ni-W alloy with W content 6 at.%. Mechanical and magnetic properties of the tapes were investigated and the dependence of tapes texture quality from total accumulated deformations was studied. The ultimate tensile strength value of composite tapes was 750± 10 MPa. The conditions necessary to form the required {100} <;100> texture are different for the homogeneous and composite tapes. According to the magnetic measurements saturation magnetization of composite tapes is about a factor of two lower than for similar homogeneous tapes.

The effect of the degree of strain due to cold rolling on subsequent formation of texture in ribbons from Ni-W alloys is studied. A quality texture is obtained in all the tungsten-containing alloys studied, which makes it possible to use the ribbons as an initial material for high-temperature superconductors.

The core of a single-core MgB 2 /Cu,Nb composite, which has been prepared by the ex-situ technique and exhibits a high critical current equal to 427 A (at 0 T and 4.2 K, j c ≥ 10 5 A/cm 2 ), has been studied using various structural methods. Two kinds of MgB 2 crystals were observed; those of the first kind is large, highly dense crystals characterized by a low oxygen content (2–8 at %) and the others are fine, weakly coupled crystallites characterized by high oxygen content (4–21 at %). To perform a comparative analysis of the structures, we have also studied an MgB 2 bulk sample synthesized at 1000°C. It was found that two phases with the same lattice are formed; they differ in the magnesium and boron contents (within the homogeneity range), impurity oxygen content and microstructure as well but differ slightly in the lattice parameters. The two-phase state of MgB 2 bulk sample is due to the mechanism of its formation, which includes the melting of magnesium, the dissolution of solid boron in it, and the crystallization of MgB 2 from the melt with the formation of dendrite-like structure characterized by corresponding redistribution of components and impurities. To a certain degree, the two-phase structure of MgB 2 bulk sample is inherited by the MgB 2 /Cu,Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the MgB 2 compound is the destabilizing factor and leads to the transformation of the superconductor into MgO.

The pulsed laser deposition (PLD) targets used for HTSC 2G fabrication should be uniform and chemically suitable to make long tapes with reproducible critical properties. We investigated inner density distribution of flat YBCO targets using non-destructive neutron introscopy and tomography techniques. The methods are based on absorption of monochromatic neutron beam which is going through the specimen. In first set of targets we measured variance of material density (up to 10-25%) with maxima density located in central layer of the target body. After updating of target fabrication technology no significant density inhomogeneity were detected. The dependence of critical current of superconducting films with thickness of 300 nm deposited on sapphire substrate with appropriate buffer layers using PLD YBCO targets with various oxygen content was also investigated. Oxygen index of YBCO targets vary from 6.12 to 6.85. No significant influence of target oxygen index on critical current density of fabricated films was detected.

RABiTS production process was developed for tapes from Ni-W alloys with W content in the range of 4-10.5 at.%. Tapes' composition, structure and magnetic properties were investigated. High purity Ni-W ingots were smelted and tapes with thickness of 0.06-0.09 mm and various total accumulated deformations in the range of 90-99.2% were fabricated. Texture quality dependence on accumulated deformation in the tapes with W various content was studied. X-ray analysis data including pole figures show that high quality cube texture can be formed in the tapes with W content up to 9 at.% inclusive, but not formed in the tapes with 10.5 at.%. According Curie temperature values and saturation magnetization measurements of alloys with W content more than 8 at.% are paramagnetic at liquid nitrogen temperature. Summarizing it can be stated that RABiTS tapes with 8-9 at.% W content have optimal combination of structure and magnetic properties for using in coated conductors.

The structure of buffer layers and deposited YBa 2 Cu 3 O x (Y123) films with a high critical current density (∼10 6 A/cm 2 ) has been investigated by different methods. These superconducting films and buffer layers are found to have a fine-grained structure, which, along with the high texture of Y123 films of the (001) type, is believed to be responsible for the high critical current density. An unusual texture is revealed in buffer layers, which differs from that of substrates and Y123 films. The superconducting films deposited on buffer CeO 2 layers exhibit a system of orthogonal lines; in the case under consideration, this is a manifestation of a domain structure with Y123 particles at boundaries.

X-ray diffraction and transmission and scanning electron microscopy have been used to study ceramics of Bi,Pb-2223/Ag composites (with small deviations from stoichiometry) after standard treatment and annealings with some variations in temperatures compared to standard treatment (optimum annealings). The data obtained have been analyzed together with the results of measurements of the critical currents of the composites. A nonuniform distribution of the structure and components over the transverse section of ceramic filaments has been revealed after standard treatment. The content of carbon in the center of the filaments was higher by a factor of two or even greater than that in the subsurface layers adjacent to the Ag sheath. It is shown that each composition of the composite has its own optimum annealing temperature, which ensures a decrease in the carbon content in the center of the ceramic filaments and the creation of a structure that is more favorable for obtaining high critical currents.

X-ray photoelectron spectroscopy (XPS) has been used to study (001) textured ceramic composites Bi,Pb-2223/Ag after high-temperature annealing in an atmosphere with a reduced oxygen content (7.5% O2 + 92.5% N2). Discussion is performed of the chemical and charge states of heavy atoms (Bi, Pb, Sr, Ca, Cu) and light atoms (of the main element (oxygen) and impurity element (carbon)) under conditions of oxygen deficit in the lattice.

After annealing of Bi,Pb-2223/Ag composites in a nitrogen-oxygen atmosphere with a low oxygen content in the 2223 phase, cross-polarized waves of oxygen atomic displacements have been revealed by electron diffraction in the [010] 2223 direction. It has been demonstrated using the methods of X-ray photo-electron spectroscopy and nuclear microanalysis that nitrogen does not interact with the 2223 lattice and that the oxygen index (9.67 ± 0.20) becomes less than the stoichiometric index ( x = 10). It has been found that the generation of atomic displacement waves is due to the deficiency of oxygen in the double Bi-O layers.

The growth of the high-temperature superconductors (HTSCs) on flexible films for the fabrication of nanoelectronic devices is considered. A method for the creation of the HTSC films on the nickeltungsten substrates is proposed. The deposition of the CeO2 and YBCO epitaxial films that are grown on short substrates made of flexible nickel-tungsten ribbon manufactured using the RABiTS technology is comprehensively analyzed, and the properties of the resulting HTSC films are studied. The electrophysical properties of the HTSC films on flexible substrates are demonstrated.

Superconducting properties of MgB2 wires with and without carbon doping up to 10 at.% have been investigated. Carbon particles consisted of 10 - 40nm nano-tubes or 10 – 20nm size SiC. Wires were made by “in-situ” technique. The regimes of ultrasound homogenization of the mixture of nano- and micro-particles were elaborated. Mixture homogeneity was determined by SEM analysis. The influence of carbon on MgB2 synthesis con ditions (temperature and anneal time), lattice parameters was studied by metallographic and X-ray analysis. Superconducting properties (Ic, Tc) of MgB2 wires were also studied. Tc value dropped from 39K for pure MgB2 to 35K in the case of carbontubes doping and to 32K in the case of SiC doping. Critical current density values in magnetic fields 2 – 12 T (4.2K) were compared for wires with pure and carbon-doped MgB2. Ic significant improvement was fixed in the fields higher than 4 T.

X-ray diffraction, transmission electron microscopy, and scanning electron microscopy with microanalysis have been used to study filamentary superconducting composites Bi,Pb-2223/Ag with different values of the critical current (54 and 89 A), prepared by the same method. It is shown that the lowered values of the critical current are connected with decreasing diffusion flow of Ag atoms from the sheath into the ceramics and with a decrease in the oxygen content in the superconducting 2223 phase. This, in turn, is caused by a weakening of the diffusion contact between the Ag sheath and the ceramics. At the Ag-ceramics boundaries, small aggregates of carbon and, sometimes, oxygen atoms are revealed. For explaining the origin of these aggregates, the initial composites (without heat treatment) were investigated by the method of Auger spectroscopy. It has been found that in the composites which upon subsequent annealing exhibit bubbling (disturbance of the contact between the Ag sheath and the ceramics), there is present a noticeable quantity of bound oxygen (water vapors) with an admixture of carbon dioxide.