Saturday, 3G-09, 12:25-12:30

Thickness dependence of critical current density in thick MgB₂ films

Mina Hanna

Shufang Wang^2,3, Joan M. Redwing³, X. X. Xi^2,3 and Kamel Salama¹

¹ Department of Mechanical Engineering and Texas Center for Superconductivity, The University of Houston, Houston, TX 77204, USA Tel: 713-743-4081. Email: mwhanna@uh.edu
² Department of Physics, The Pennsylvania State University, University Park, PA 16801, USA
³ Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16801, USA

Coated conductors have been the focus of study of scientists and researchers for the past two decades since the discovery of YBCO in 1987 at the University of Houston. For coated-conductor applications, it is necessary to deposit thick films in order to maximize the critical current in the wire or tape and enhance the engineering critical current density. However, the main obstacle to higher current in thicker films is that the critical current density (J_c) in the superconductor drops dramatically as the coating thickness is increased. This limitation was first observed over 18 years ago in YBCO coated-conductors, yet only recently it became one of the most important remaining challenges in the coated conductors field as they are approaching the commercialization stage. As MgB₂ coated conductors started hitting the road since the discovery of superconductivity in MgB₂ in 2001, a study on J_c thickness dependence in MgB2 films is deemed to be necessary.

MgB₂ coated conductors have many potential applications and are processed by many different techniques, the most common two are the hybrid physical-chemical vapor deposition (HPCVD) and the ex-situ high temperature annealing of Boron films in Mg vapor. We succeeded in fabricating high quality MgB₂ films that carry high J_c using the ex-situ annealing fabrication technique and we examined the J_c behavior as a function of thickness in MgB₂ films. The study shows that critical current density in MgB₂ films decreases with increasing film thickness, similar to that observed in YBCO coated conductors, which is probably due to impurity diffusion during annealing and microstructural degradation for thicker films. The high J_c values carried by our films show that the ex-situ fabrication method can produce high quality MgB₂ films at low processing temperature, which is promising for resonance frequency cavity applications and coated-conductor wires and tapes.

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