Defect formation, distribution and size reduction of in melt-processed YBCO superconductors

The effects of MgB 4 addition on the superconducting properties and the microstructure of in situ processed MgB 2 bulk superconductors were studied. MgB 4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB 4 ) were heat-treated at 650 ℃ for 1 h in flowing argon. The powder X-ray diffraction (XRD) analysis for the heat-treated samples showed that the major formed phase in all samples was MgB 2 and the minor phases were MgB 4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB 2 decreased as the amount of MgB 4 addition increased. MgB 4 particles included in a MgB 2 matrix is considered to suppress the grain growth of MgB 2 . The onset temperatures (T c,onset ) of MgB 2 with MgB 4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB 4 addition slightly reduced the T c,onset of MgB 2 to 36.5 K. This result indicates that MgB 4 addition did not influence the superconducting transition temperature (T c ) of MgB 2 significantly. On the other hand, the small additions of 1-5 wt.% MgB 4 increased the critical current density (J c ) of MgB 2 . The J c enhancement by MgB 4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB 4 particles dispersed in a MgB 2 matrix.

Section: Introductionmentioning

confidence: 99%

Enhanced critical current density of in situ processed MgB₂ bulk superconductors with MgB₄ additions

S.H.Kim¹,

Kang²,

Jun³

et al. 2017

“…As already reported in many references [5][6][7], there are many microdefects such as twins, dislocations, stacking faults, inclusion particles and oxygen deficiencies inside superconductors produced through the melt growth process. They act effectively in trapping the magnetic flux inside the superconducting grains of the FC samples.…”

Section: Crystallographic Orientation Vs Trapped Magnetic Fieldmentioning

confidence: 77%

“…These four lines start at the seed located at the center of the sample and meet the four corners of the sample. The orientations of the x-shaped lines are the YBCO <110> orientations, which is known to be caused by the anisotropic growth of YBCO grain [5]. A single x-shaped pattern is formed on the top surface of the sample, which indicates that the sample is a single grain and the top surface is perpendicular to the c-axis of YBCO.…”

Section: Methodsmentioning

confidence: 99%

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Orientation and thickness dependence of magnetic levitation force and trapped magnetic field of single grain YBa₂Cu₃O_7-y bulk superconductors

Jung¹,

Go²,

Joo³

et al. 2017

The effects of the crystallographic orientation and sample thickness on the magnetic levitation forces (F) and trapped magnetic field (B) of single grain YBCO bulk superconductors were examined. Single grain YBCO samples with a (001), (110) or (100) surface were used as the test samples. The samples used for the force-distance (F-d) measurement were cooled at 77 K without a magnetic field (zero field cooling, ZFC), whereas the samples used for the B measurement were cooled under the external magnetic field of a Nd-B-Fe permanent magnet (field cooling, FC). It was found that F and B of the (001) surface were higher than those of the (110) or (100) surface, which is attributed to the higher critical current density (J c ) of the (001) surface. For the (001) samples with t=5-18 mm, the maximum magnetic levitation forces (F max s) of the ZFC samples were larger than 40 N. About 80% of the applied magnetic field was trapped in the FC samples. However, the F and B decreased rapidly as t decreased below 5 mm. There exists a critical sample thickness (t=5 mm for the experimental condition of this study) for maintaining the large levitation/trapping properties, which is dependent on the material properties and magnitude of the external magnetic fields.

“…Defects such as vacancy, dislocation, twins, stacking faults and non-superconducting lattice are developed during processing and oxygenation heat treatment [3][4][5][6][7][8]. They can effectively act as flux pinning sites which pin the flux in the superconducting matrix.…”

Section: Introductionmentioning

confidence: 99%

Effects of electron beam irradiation on the superconducting properties of YBCO thin films

Lee¹,

Choi²,

Jun³

et al. 2016

The effects of electron beam (EB) irradiation on the superconducting critical temperature (T c ) and critical current density (J c ) of YBCO films were studied. The YBCO thin films were irradiated using a KAERI EB accelerator with an energy of 0.2 MeV and a dose of 10 15 -10 16 e/cm 2 . A small T c decrease and a broad superconducting transition were observed as the EB dose increased. The value of J c s (at 20 K, 50 K and 70 K) increased at doses of 7.5×10 15 and 2.2×10 16 e/cm 2 . However, J c s decreased as the dose increased further. The X-ray diffraction (XRD) analysis showed that the c axis of YBCO was elongated and the full width at half maximum (FWHM) increased as the dose increased, which is strong evidence of the atomic displacement by EB irradiation. The transmission electron microscopy (TEM) showed that the amorphous layer formed in the vicinity of the surfaces of the irradiated films. The amorphous phase was often present as an isolated form in the interior of the films. In addition to the formation of the amorphous phase, many striations running along the a-b direction of YBCO were observed. The high magnification lattice image showed that the striations were stacking faults. The enhancement of J c by EB irradiation is likely to be due to the lattice distortion and the formation of defects such as vacancies and stacking faults. The decrease in J c at a high EB dose is attributed to the extension of the amorphous region of a non-superconducting phase.