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To create the windings of the magnetic system of the Future Circular Collider, it is necessary to modernize the Nb3Sn-based composite superconductors obtained by the method of internal tin sources, which also involves an increase in the thermal conductivity and electrical conductivity of the matrix copper as determined using the residual resistance ratio (RRR) parameter (the ratio of the values of the electrical resistivity of copper at 273 and 20 K). The achievement of high RRR values is controlled by the balance between the geometric dimensions, the volume fractions of the superconductor components, and the mode of reaction heat treatment (RHT) used for the formation of the Nb3Sn phase. The results of an experimental and theoretical study of the effect of the strand diameter and the RHT mode on the RRR parameter are given. The diffusion of tin from the zone of subelements into the stabilizing copper in the course of RHT in strands of various diameters has been investigated by scanning electron microscopy and electron probe microanalysis. The study was carried out on experimental samples of multifilamentary superconductors based on Nb3Sn with a distributed Nb barrier.

When twisted into a cable, reduced, and wrapped, Nb3Sn composite superconductors are subject to significant deformation and can sustain damage that will affect their electrophysical properties. This article examines the effect of artificially induced surface damage on the electrophysical characteristics and microstructure of 0.82 and 0.7 mm in diameter Nb3Sn superconductors fabricated by the bronze technology and the internal tin source method. It is shown that the critical current of the deformed samples decreases with increasing depth, amount, and extent of the damage. The current-carrying capacity of a superconductor decreases with increasing deformation produced by flattening wire samples to thickness 0.6–0.4 mm.

To construct magnetic systems in modern high-energy physics projects, such as creation of the Future Circular Collider (FCC) and modernization of the Large Hadron Collider (HL-LHC), Nb3Sn superconductors with a high level of electrophysical properties, above all being a high current carrying capacity, are requisite. Such multifilament Nb3Sn superconductors are obtained by the technique of internal feeding of tin (IFT), consisting in deformation of a composite including high-purity niobium, copper, and tin to make a wire smaller less than 1 mm in diameter. The presence of such components as tin-based alloys or pure tin with Tm = 232°C in the composite wire composition precludes the possibility of Nb recrystallization annealing, which imposes strict requirements on the Nb mechanical properties, which determine the Nb capability for plastic deformation. In the study, the influence of the design and conditions of creating bimetal Nb/Cu rods on the grain structure of niobium and the state of the boundary between it and a copper sheath is discussed. The mechanical properties of rods at different steps of their fabrication are investigated. The results will be applied in development of the technological conditions of manufacture of Nb3Sn superconductors by the IFT technique.

The texture of niobium in composite Nb3Sn superconductors prepared by internal-tin-source and bronze techniques, and in a subelement, which is required for fabricating the composite using the internal-tin-source technique, is studied in this work. It is shown that prior to the reaction heat treatment, the structural components of Nb have the (110) drawing texture; after the reaction heat treatment, no preferred orientation of Nb3Sn superconducting compound grains is observed.

The structure and mechanical properties of composite Cu/Nb/Sn rods of various designs used in the fabrication of multifiber Nb3Sn superconductors prepared by the internal tin source method are studied. It is shown that, upon reaching a high degree of deformation (ln μ ~ 9), the tensile strength of the rods is ~700 MPa, while the plasticity drops to 2%. The structural features of the fracture surface of composite semifinished products are studied. The results will be used in the development of technological modes for the fabrication of Nb3Sn superconductors.

The growth kinetics of a layer of intermetallic compound Nb3Sn in bronze-processed multifilamentary superconductors of different designs and diameters, developed and fabricated at the A.A. Bochvar High-Technology Research Institute of Inorganic Materials (JSC “VNIINM”), has been studied. The influence of diffusion annealing conditions in a wide range of temperatures and exposure times on the formation of Nb3Sn phase in superconductors has been investigated by methods of metallographic analysis using optical microscopy, electron microscopy, and microanalysis. The structural features of Nb3Sn layers in the initial formation stage and after long-term diffusion annealing are analyzed, their thickness is measured, and the grain structure and composition are studied. Recommendations on improvement of the diffusion annealing conditions for Nb3Sn superconductors of different designs, aimed at obtaining high current-carrying ability, are elaborated.

—The effect of synthesis conditions exerted on the phase composition of MgB2 samples synthesized in a temperature range from 650 to 900°C and the features of the microstructure thereof are studied. It is shown that the maximum processing level of the initial boron and magnesium mixture into magnesium boride MgB2 with a minimum content of impurity phases of magnesium oxide and higher magnesium borides can be achieved in the samples after annealing in a temperature range from 650 to 750°C for 1 h upon a preliminary compaction pressure amounting to 630–640 MPa.

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.

The required combination of performance properties for internal tin Nb3Sn wires for high energy physics includes high critical current density (J c) and RRR values as well as low magnetization and effective diameters (Deff ) value. It is quite a challenge to achieve them all at once. A comprehensive study has been carried out on two internal tin Nb3Sn wires with essentially different layouts and 37 sub-elements each. The first type of wires has distributed diffusion barriers. The second one has common diffusion barrier with sub-elements internally divided by resistive separators. Two sets of data for two types of wires have been obtained, which include transport measurements of J c(B), magnetization loops M(B), superconducting transition temperature (Tc), RRR values, Sn-concentration by EDAX measurements and XRD analysis. XRD analysis of the superconducting layers has shown that the compressive macrostress is larger for the wire with overall barrier than for the wire with distributed diffusion barriers. This observation correlates with a shift of the stress-dependent curves of the superconducting transition. The difference in the volume fractions and the homogeneities of the Nb3Sn phase leads to a significant difference of the J c(B) dependences and the values of the Kramer critical field (BKr) for these types of wires. In summary, the high critical current density for the first type of wires and the low hysteresis losses for the second type of wires have achieved with high RRR values in both cases.

Superconducting materials with high current carrying capacity operating in magnetic fields up to 18 T at 4.2 K are required for promising applications in magnetic systems of high-energy physics devices, NMR-spectroscopes and others. The paper presents the study results of the Nb3Sn strands manufactured by the bronze route differed in the filament composition. The filaments were made from the alloys Nb-3.5 wt. %Ta and Nb-7.0wt.%Ta and the matrix was produced from the Cu-15.5wt.%Sn-0.25wt.%Ti alloy. To form the superconducting Nb3Sn phase a series of reaction heat treatments of conductor samples were carried out by two-stage regimes with the temperature of the second stage varying from 650 to 720 °C. The microstructure of the Nb3Sn layer was studied by scanning electron microscopy and their electro-physical characteristics were also measured. It was shown that after reaction at 575 °C, 100 h + 650 °C, 200 h samples with different Ta content had almost the same current -carrying capacity in the field range 12-18 T at 4.2 K. The critical temperature of superconductors decreased from 17.5 to 16.5 K with an increase of the second stage of reaction temperature from 650 to 720 °C.