Effect Of Current Density On The Preparation Of MgB₂Film Via Molten Salt Electrochemistry

Since the discovery of the simple MgB2inter-metallic compound superconductivity in2001, researchers have taken extensive interest in investigating it’s structure, superconducting mechanism, synthesis methods and application prospects for its commercial use because it has a series of advantages, such as a higher superconducting transition temperature(Tc)(an order of39K), larger critical current densities (Jc)(an order of105A·cm-2), larger coherence length of5-6nm and higher upper critical field. Meanwhile, it has simple crystal structure; low cost, easy to processe and especially does not have weak links of grain boundaries. MgB2superconductor might be the first superconductor material that will be applied for industry in large-scale.At present, with the development and perfection of thin film fabrication methods, almost all methods can be used for the fabrication of MgB2thin films. So far, the relatively mature processing technique reported to prepare MgB2thin film is Physical or chemical vapor deposition. However it is hindered in large scale application owing to the disadvantages of expensive equipments and low production efficiency. The molten-salts electrochemical technique is considered as a desirable method for the preparation of MgB2thin film because the installations of the molten-salts electrochemical technique are very simple, the cost is low, and the productivity is high, moreover it can prepare MgB2thin films with various sizes and shapes on cathode. The promising technique, however, is still at the experimental stage. Up to date, as the theory investigation is scare many experimental parameters are difficult to be decided, In addition, the MgB2phase purity and crystallinity of films derived are undesirable. Therefore, to obtain uniform, dense and better-crystallinity superconducting thin films has significant research and application value.In this paper, molten salt electrochemical was employed for the fabrication of MgB2thin film and the influence of different molten salts on the quality of the prepared MgB2thin film were discussed in the perspective of the properties of molten salts. The studies of different molten salts showed that the MgB2thin films generated at520℃,3.5V with3hours in the molten salt with the molar ratio of MgCl2:KCl:B2O3=3:2:0.9had the better quality. And the molar ratio of Mg and B elements in the film was1:1.83.The studies of current density showed that the electrolyte temperature, voltage, time and the contents of supporting electrolyte KCl all significantly affect the current density. When the cathode potential was between-2V and-3V, cation such as Mg2+and B3+begin to discharge on the cathode. In about10minutes, MgB2nano-crystal nucleation was formed on the cathode surface and began to grow closer to each other to form micron particles. When the electrolysis time was more than3hours, the grains stopped grow, so3hours was suitable, and largest current density was achieved at the temperature485. Supporting electrolyte KCl had a dual role on the current density, on one hand, it would reduce the melting point, resistance and the viscosity of the molten salt, thereby increase the current density, on the other hand, it might prevent the transmission of ions and thus induced the current density. Current density which may affect the growth mechanism played an important role in the process of the preparation of the MgB2thin film. If the current density was larger, the MgB2nucleation rate is faster than the growth rate, which resulted in the fromation of fine grains. But if the current density was too high, there would be worse density, rough surface and particle agglomeration in the film. Experimental results indicated that in MgCl2+KCl+B2O3molten salts when the molar contention of KCl was35.5%, the prepared film had higher purity and relatively smooth surface.