Stress mechanism of Y1-xGdxBCO thin film with Gd substitution prepared by F-free metal organic deposition method

Abstract

The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction. Recently, the second-generation YBa2Cu3O7- (YBCO) high temperature superconducting materials have attracted much attention and become a hot research point. The YBCO coated conductors are widely used in transmission cables, motors, generators and magnetic energy storage systems due to their high critical current densities and high irreversible fields. To obtain high critical current, it is necessary to increase the thickness of YBCO film. However, as the thickness increases, the cracking of the film appears and the a-axis grains form, which causes the critical current density to decrease drastically, hence the critical current declines, i.e., the so called thickness effect appears. In order to overcome the thickness effect, a great many of efforts have been devoted to it. It is realized gradually that the growth orientation of the c-axis can be controlled by the stress of film, which can be achieved through the substitution of Y by Gd and Sm each with a larger ionic radius. However, the systematical study of the evolution of the stress mechanism with the substitution ratio is still lacking due to the extreme complexity of the stress manipulation. Therefore, a series of Y1-xGdxBCO thin films with different substitution ratios is deposited on lanthanum aluminate substrates by the fluorine-free metal organic deposition method in order to reveal the evolution of the stress mechanism with Gd substitution. The growth orientations, microstructures and lattice vibration characteristics of the films are analyzed by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The results show that the lattice constant of the film increases and the orientation of the c-axis changes with the Gd substitution ratio for x increasing to a value less than 0.5, and the blue shift of the O(2)/O(3) mode of the Raman spectrum decreases with increasing x. For x=0.5, the blue shift of the O(2)/O(3) mode vanishes, indicating the free standing film with optimal c-axis orientation. However, with the further increase of Gd content, the film structure is deteriorated, and the performance is degraded as well. The red shift of the O(2)/O(3) mode occurs and the frequency decreases with increasing x. Our results indicate that the stress mechanism can be manipulated by controlling the content of various ionic radii in Y1-xGdxBCO films. The free standing film with optimal c-axis orientation can be obtained through adopting an appropriate substitution ratio, i.e., the ratio of m Y:Gd equaling 1:1. These results suggest that manipulation of the stress mechanism is a promising method to overcome the thickness effect effectively.