Local structure of nanopowders of refractory nitrides used to increase the critical current of high-temperature superconductors
The effect of dispersivity on the local-structure features in ZrN nanocrystals has been investigated by X-ray absorption (EXAFS) spectroscopy. The Debye-Waller factors and bond lengths have been determined for Zr-N and Zr-Zr coordination spheres. It is found that a decrease in the nanoparticle size leads to an increase in the static Debye-Waller factor for the Zr-Zr bond. The scale of changes is comparable with the increase in the Debye-Waller factor upon heating from 15 to 77 K.
Physicochemical methods of enhancing the performance of high-Tc superconductors
Experimental data are summarized on the optimization of the performance of YBa2 Cu3O7–x, Bi2Sr2CaCu2OP8+δ, and (Bi,Pb)2Sr2Ca2Cu3O10 +δhigh–Tc ceramics by producing structural defects via irradiation or doping with metals and different inorganic compounds. A classification of additions is proposed according to their interaction with the superconductors. For additions having an advantageous effect on the superconducting and mechanical properties of the high–Tc materials, the optimum concentration and size ranges are indicated.
Enhancement of the point defect pinning effect in Mo-doped Bi2212 single crystals of reduced anisotropy
High quality Bi 2 Sr 2 CaCu 2-x Mo x O y (x = 0, 0.01 and 0.02) single crystals have been grown by a self-flux method in a horizontal temperature gradient and their flux pinning and irreversibility behaviour have been investigated. The irreversibility lines of the undoped and Mo-doped Bi2212 crystals have been greatly improved by reducing the anisotropy parameter y. However, this improvement is much more pronounced for Mo-doped crystals than for the undoped ones. The peak effect of magnetization loops also changes with both Mo-doping and y. The results provide strong evidence that the point defect pinning served by Mo is greatly enhanced when the anisotropy of the system is reduced.