How to Optimize Critical Current Performance of RE123 Materials by Controlling Oxygen Content
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How to Optimize Critical Current Performance of RE123 Materials by Controlling Oxygen Content Jun-ichi Shimoyama, Shigeru Horii, Kenji Otzschi and Kohji Kishio Department of Superconductivity, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ABSTRACT
Oxygen nonstoichiometry behaviors of REBa2Cu3Oy (RE123: RE= Nd, Sm, Eu, Gd, Dy. Ho and Y) compounds have been precisely determined by the thermogravimetric measurements. Dependence of oxygen content on temperature and oxygen partial pressure were found to slightly depend on the RE element. Thermodynamic quantities of oxygen, such as hO2 and sO2, also varied with RE element. Relationship between Tc and oxygen content is strongly dependent on the RE species. This suggests that pinnig effect due to the oxygen defects is different in each RE123 compound and, therefore, critical current properties of RE123 must be optimized by precise control of oxygen content as well as selection of suitable RE elements.
INTRODUCTION
Soon after the discovery of RE123 superconductor in 1987, their oxygen nonstoichiometry have been eagerly studied as functions of temperature and partial pressure of oxygen [1], because oxygen content was found to be most important variables to produce 90K class superconductors. In those days, numerous phase diagrams relating to the nonstoichiometric oxygen content were proposed for RE123 compounds with various RE elements. However, significant change in oxygen nonstoichiometry behavior was not found due to different RE elements, while partial melting points of RE123 was confirmed to be apparently dependent on the ionic size of RE. Recent precise studies on the RE123 single crystals revealed that vortex states and pinning mechanisms are quite sensitive to the oxygen content, i.e. concentration of oxygen defects, besides twin structures [2]. The 90K-class Tc does not change largely in the oxygen composition, y = 6.85~7.00, however, carrier concentration affecting electromagnetic anisotropy and penetration depth and defect change largely, resulting in various critical current properties. This strongly indicates that critical current properties of RE123 materials, such as conductors and bulk magnets, must be optimized by post-annealing under suitable conditions, because resulting oxygen defect concentration, 7-y, determines Tc, pinning sites and electromagnetic anisotropy. We should not satisfy with 90K-class Tc of the products. Unfortunately, old studies on the oxygen nonstoichiometry of RE123 were not precise enough for suggesting better annealing conditions for various RE123 materials. Therefore, more systematic, quantitative and extensive information on their oxygen nonstoichiometry has been required. In the present study, oxygen nonstoichiometry of RE123 compounds with various RE elements was precisely investigated as functions of temperature and partial pressure of oxygen, PO2, and dependence of their superconducting properties on oxygen content was systematically studied. Pinning mechanism of RE123 materials and guiding principle for E8
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