Structure and properties of melt-textured YBa₂Cu₃O₇  , high pressure–high temperature treated and oxygenated under evaluated oxygen pressure

Abstract

MT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current density jc = 85 kA cm−2 in zero field and more than 10 kA cm−2 up to 5 T field when the external magnetic field was perpendicular to the ab-plane of Y123, and a jc = 23 kA cm−2 in zero field and jc close to 1 kA cm−2 in 10 T field when the magnetic field was perpendicular to the c-axis of Y123. The microstructure of these samples contained an unusually high density of twins (about 30 twins µm−1) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatic high pressure–high temperature (HP–HT) treatment we may vary the twin and dislocation densities in the material by changing the sample orientation in high pressure apparatus, while the oxygen content of Y123 phase as well as the lattice parameters remain unchanged. The microstructure of the material in the case where the highest pressure has been applied in the direction perpendicular to the ab-plane of Y123 is characterized by a very low twin density, perfect dislocations stepped along directions and small faulted loops corresponding to CuO intercalating in the matrix. For this material jc = 10 and 8 kA cm−2 in zero field were observed (when the external magnetic field was perpendicular to the ab-plane and perpendicular to the c-axis of Y123, respectively). High pressure–high temperature treatment causes an increase in the material density (up to near the theoretical one), microhardness and fracture toughness.

Modern superconductive materials for electrical machines and devices working on the principle of levitation

“…The material structure is characterized by a comparatively low density of macrocracks parallel to the ab-plane of 5.3 mm -1 ; high density of microcracks parallel to the ab-plane of 0,5 m -1 and density of twins of 30 m -1 (Fig. 8,c) [11]. The density of stacking faults around Y211 inclusions was high (Fig.…”

“…The coherence length in type-II the superconductors, as a rule, is determined by the size of the defects or inclusions that can be pinning centers of the magnetic flux and thus can promote an increase in critical current density of the material. Because of this, in the Y123-based superconductors the grain boundaries with the high off-orientation angles are a significant obstacle for the superconductive current flow, and their presence in the material leads to a substantial decrease of critical current density, j c , and the pinning centers in the material can be dislocations, stacking faults, and twins [4]. In order to get higher critical current densities, the MT-YBCO ceramics are grown, where the high j c is attained due to the textured structure growing with the help of seed crystal.…”

High-Pressure Oxygenation of Mt-Ybco: The Way to Reduce the Oxygenation Time, to Prevent Macrocracking, and to Obtain Materials With High Critical Currents.

“…For the investigation of the oxygenation process, large MT-YBCO blocks were cut into rectangular parallelepipeds of about 2.5-3×2.5-3×6.5-8.5 mm 3 (that could be inserted into the magnetometer as a whole) with a diamond wire in such a manner that the longer side of the parallelepiped was approximately parallel to the c-axis of the Y123 domain (FIGUREs 1d, 2d). The Bulk MT-YBCO ceramic from Y123 and Y211 melt-textured in the same manner as the thin-walled MT-YBCO and oxygenated under a controllable oxygen pressure of 0.5 kPa-16 MPa at 700°C for 72 h [6] is given for comparison (Type 4). In addition, the whole block of thinwalled MT-YBCO (Type 2) 16 mm in diameter and 10 mm in thickness (FIGURE 1 b) has been oxygenated for the field mapping study in the same process together with the small parallelepipeds.…”

“…The amount of oxygen in the Y123 structure of MT-YBCO has been determined by estimating the c-parameter and using its correlation with the amount of to 900 o C oxygen replaces gradually nitrogen so that at 900 o C samples are in flowing oxygen at 0.1 MPa and kept under these conditions for 0.5 h; then the temperature has been reduced to 800 o C at a rate of 10 K/h; on reaching 800 o C, the oxygen pressure was gradually increased (according to an exponential law) to 16 MPa, and at 16 MPa and 800 o C samples were kept for 48 h , after which the temperature was decreased with a constant rate to the room one. oxygen [6,7]. The j c was estimated from magnetization hysteresis loops obtained using an Oxford Instrument 3001 vibrating sample magnetometer (VSM) and Bean's model.…”

“…Initial, our studies of oxygenation under pressure were aimed only at increasing the oxygenation rate. In the course of the studies the possibility has been found to oxygenate the Y123 structure under 16 MPa at 700 o C and to get extremely high j c , especially in the ab-plane [6]. The material structure contained a very high twin density that can enhance pining.…”