Oxygen ordering and superconductivity in La(Ba2−xLax)Cu3O7+δ
The observation of superconductivity above 90 K in the Y–Ba–Cu–O system1,2 has prompted a search for other structures that display high-temperature superconductivity. The discovery of superconductivity above 70 K in La3−xBa3+xCu6Ol4+y3 raises some questions concerning its relationship to the YBa2Cu3O7−δ structure and the origins of the high-temperature superconductivity. The original X-ray diffraction study of La3Ba3Cu6O14+y, indicated a tetragonal structure4,5, on the basis of which it has been proposed that La3−xBa3+xCu6O14+y is a new class of high-temperature superconductor which, in contrast to YBa2Cu3O7−δ, contains no Cu–O chains3. More recently, a neutron diffraction study of non-superconducting La3Ba3Cu6O14+y6has indicated that the correct structure of this compound is simply the tetragonal YBa2Cu3O7−δ structure with partial substitution of lanthanum on the barium site7–12. In the present neutron powder diffraction study of a series of La(Ba2−xLax)Cu3O7+δ compounds, we show that the superconducting phase is a similarly disordered isomorph of the orthorhom-bic YBa2Cu3O7−δ structure13–19. The region of solid solubility for this compound extends from x = 0.5 (La:Ba:Cu ratios of 3:3:6) to x≈ 0.25 and does not include the stoichiometric compound x = 0 (LaBa2Cu3O7−δ). The presence of Cu–O chains, and not just a formal copper charge state of ≥2, is vital for the occurrence of superconductivity.
Critical parameters of high Tc compounds
Abstract Upper critical fields of XBa2Cu3O7−δ with X = Y, Er, Gd, Ho, Eu, or Y1−xPrx hava been measured using pulsed magnetic fields up to 26 T. As long as the high temperature superconductivity is present, the superconducting coherence length calculated from the critical field data, remains comparable to the size of the unit cell (≈ nm). Oxygen evolution experiments have been used to control the depletion of oxygen sites corresponding to different activation energies. The variation of the critical parameters has been determined as a function of the oxygen deficiency δ for the YBa2Cu3O7-δ compound.
Finite Difference Modeling on the Temperature Field of Substrate in Friction Surfacing
Friction surfacing is an advanced process of great potential, especially in the field of repair and reclamation of worn and damaged components. As a kind of thermal processing technology, the temperature profile and changes of the substrate in friction surfacing are important factors to get coatings of high quality. In this paper, the temperature field of the substrate during the process is simulated by finite difference method. The results are consistent with experiments, reappearing the temperature change of the substrate during friction surfacing, allowing further study on figuration and quality of coatings.