Papers by Author: A.E. Vorobyova

Abstract: The structure of high-purity copper and stabilizing copper of Cr-plated Nb₃Sn-strands with different RRR (residual resistance ratio) values has been studied. Cr diffusion into peripheral layers of stabilizing Cu has been revealed. The variations of RRR and Cr content in Cu at annealing have been estimated. It is demonstrated that an additional drop of RRR may be caused by oxygen diffusion from Cr coating obtained by electroplating. Optimal regimes of the diffusion annealing of Cr-plated Nb₃Sn-strands for ITER (International Thermonuclear Experimental Reactor) have been determined.

Abstract: The structure of Nb3Sn-based, bronze-processed Ti-doped multifilamentary superconducting wires has been studied by TEM and SEM after the first (5750C,100 h) and the second (6500C,100 h) stage of the diffusion annealing. The Nb3Sn layers formation in all the composites proceeds by one and the same mechanism and starts with nucleation of particles and very fine grains of this phase in Nb filaments where Sn diffuses from the bronze matrix. Ti, inserted both in the bronze matrix, or Nb filaments, diffuses into the growing superconducting layer and promotes its more active formation. At the first stage of annealing (5750C, 100 h) Nb3Sn grains have an average size of 40 nm, and at the second stage (6500C, 100 h) they increase by a factor of 1.5 and the grain size distribution gets wider. After the two-stage annealing the amount of the residual niobium is small, and some Nb filaments, especially in doped composites, almost completely transform into Nb3Sn. In the Nb3Sn layers of a zone of columnar grains is adjacent to the residual Nb.

Abstract: Bronze-processed Nb3Sn-based multifilamentary composites with external diameter of 0.8 and 0.5 mm and coupled Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy. After the two-staged annealing, 575°С, 150 h + 650°С, 200 h, commonly used for ITER conductors, a nanocrystalline layer of superconducting Nb3Sn compound is formed in every Nb filament as a result of solid-state reactive diffusion of Sn from the bronze matrix. It is demonstrated that in the wires of smaller external diameter the Nb filaments transformation into the Nb3Sn compound is more pronounced, that is the amount of the residual Nb is smaller. Besides, the nanocrystalline structure of the Nb3Sn diffusion layers is more perfect in 0.5 mm diameter wires, namely, the Nb3Sn grains are finer (their average size being 60 nm compared to 70 nm in 0.8 mm diameter wires) and are more uniform in sizes (the root mean square deviation of grain size distribution is correspondingly 15 and 17 nm).

Abstract: Multifilamentary Nb3Sn-based superconducting composites manufactured by an internal-tin method have been studied by transmission (TEM) and scanning (SEM) electron microscopy. The main goal of this study is to reveal the effect of diffusion annealing regimes as well as the external diameter of the wires on the structure of nanocrystalline Nb3Sn layers (average grain size, grain size distribution, layer thickness, amount of Sn, etc.). It is demonstrated that multistep diffusion annealing results in quite a complete transformation of Nb filaments into Nb3Sn though some amount of the residual Nb remains in the filaments center. With an external diameter decrease the superconducting layers structure has been found to refine and get somewhat more uniform. An additional high-temperature annealing results in marked growth of Nb3Sn grain sizes and their scattering in sizes, which may negatively affect the current-carrying capacity of a wire.

Abstract: Multifilamentary bronze-processed Nb3Sn-based composites have been studied by the methods of TEM and SEM. Ti as a doping element required for an enhancement of superconducting characteristics, especially in high magnetic fields, was inserted either in a bronze matrix, or in Nb filaments of a composite. It has been found that Ti diffuses into the growing Nb3Sn layer in both cases, and affects positively its structure and superconducting characteristics of a composite as a whole, especially in case of the doped matrix. When Ti is added to Nb filaments, it forms fine particles of intermetallic compounds with Sn in the nanocrystalline diffusion layer. When these particles are formed, grain boundaries of the diffusion Nb3Sn layer purify from segregations, and grains in the vicinity of these particles coarsen, which negatively affects the current-carrying capacity of a composite. That’s why an optimal amount of Ti in Nb should be chosen, when Ti mainly dissolves in the Nb3Sn phase increasing its superconducting properties and not deteriorating its grain structure.

Abstract: Kinetics of formation of superconducting Nb3Sn layers and the structure of bronzeprocessed Nb/Cu-Sn composites with Zr, Zn or Mg-doped matrixes or Ti-doped Nb filaments of different geometry have been studied by the methods of TEM, SEM and electron-probe microanalysis. All the doping elements have been found to accelerate the rate of growth of the diffusion Nb3Sn layers and consequently their thickness. Correlation between the diffusion annealing schedules, the geometry of Nb filaments, the structure of the diffusion Nb3Sn layers and the current-carrying characteristics of multifilamentary Nb/Cu-Sn composites has been established.