Sunhurov M. Physical basis for the creation of textured substrates based on paramagnetic Ni-W alloys for the second generation high-temperature superconductors

The important scientific problem is solved in the thesis – investigation of the fundamental properties of Ni(1-x)Wx alloys and the development on this basis the principles to creating the textured substrates based on Ni-W alloys with paramagnetic structure for the second generation high-temperature superconductors. The strategy of creating elongated ribbon-type substrates can be developed due to study of the structure, electrophysical, magnetic and other properties of Ni-W system; investigation of the influence of the chemical composition, the conditions of mechanical and thermal treatment on the structure and properties of Ni(1-x)Wх tapes; the establishment of the nature and mechanisms of the processes of phase and texture formation, proceeding at the boundary of the interface "substrate – buffer coating". The newest methods for investigating the fundamental physical properties of multilayer and multicomponent materials based on the combination of diffraction and absorption spectroscopy in a single experiment and low temperature resistometry using three types of data are developed. Based on the research of the crystal structure and the electrophysical properties of Ni(1-x)Wx alloys in the wide range of concentrations (x = 0 – 0.5) and temperatures (50 – 270 K), the main mechanisms of the flow of electric current in eutectic mixture of crystallites with an FCC and BCC crystal structures are established. At relatively high temperatures at concentrations of tungsten below the value corresponding to the percolation threshold, electric current flows exclusively through the grains of the FCC phase with an increased specific electrical resistance. When the percolation threshold is reached the electric charge transfer occurs mainly through continuous channels formed by Ni-W grains with BCC structure of lower resistance. The effect of temperature on the percolation mechanism of electric current flow in a two-phase system FCC Ni-W + BCC Ni-W is found and developed the insights about its nature. When the temperature decreases, on the one hand, the resistance of the FCC phase decreases, on the other hand, the percolation channels of current flow expand, i.e. the contribution of the BCC phase to the total electrical conductivity of alloys increases significantly. The following factors have been shown to influence the crystalline structure and morphology of the two-layer Ni(1-x)Wx / TiN system: the chemical composition, magnetic state, and the stacking fault energy. It is shown that the X-ray diffraction and absorption spectra for the Ni(1-x)Wx / TiN systems based on ferromagnetic Ni0.95W0.05 and paramagnetic Ni0.905W0.095 alloys have fundamentally different behavior. It is shown that the dynamics of the change in the character of the diffraction pattern in the Ni0.95W0.05 / TiN system which possesses stable cubic texture are almost completely determined by the absorption processes of characteristic X-rays, while the morphology of the system naturally does not change. In the Ni0.905W0.095 / TiN system, for the first time were observed the anomalous X-ray optical effect – an increase in the intensity of the diffraction lines of the substrate with growth of the coating thickness; and the effect of counter epitaxy — the correlated formation of a cubic texture of <100> (001) type in both components of a two-layer composition. It is established that the process of structural adjustment takes place in a very narrow range of coating thickness TiN (ΔhTiN = 0 – 1 μm). The nature and mechanisms of the detected effects are established. The driving force of the process associated with changes in the morphology of the system Ni0.905W0.095 / TiN, that is the reorientation of the crystallites in the laboratory coordinate system are strains arising at the interface between materials with different values of the parameters of the crystal lattices.