Microstructural Nonuniformity of GdBCO Superconductor Bulk

High-temperature superconducting materials REBa2Cu3O7-δ(REBCO or RE123, RE is a rare earth), can widely be used in electric motors, nuclear magnetic resonance and other fields, because these can trap strong magnetic fields. The top-seeded melt-texture growth method is widely chosen in the preparation and performance research of various REBCO superconductor bulks, which can effectively suppress random nucleation and reduce weak connections between grains, etc. However, the superconductor bulks prepared by this method often have the problem of uniformed microstructure. A single-domain GdBCO superconductor bulk with NdBCO single-seed guidance and Y123 liquid source was successfully growth using the top-seeded melt-texture growth method, and the microscopic morphology of the samples were studied, which exhibits the microstructural nonuniformity of GdBCO superconductor bulk. The specimens at different positions, C1, C2 below the seed, P1, P2 below the cross pattern, of the bulk exhibit critical transition temperature, TC , higher than 94.5 K, with a maximum value of 95.9 K, demonstrating superior superconductivity. Cracks and pores are unevenly distributed in GdBCO superconductor bulk, and the pores mainly gather in the area below the seed and far from the upper surface. At the same distance from the upper surface, specimens below the cross pattern have higher self-field JC values (P1>C1, P2>C2) than specimens below the seed, which should be related to the nonuniformity of the pores.


Introduction
High-temperature superconducting materials REBa 2 Cu 3 O 7-δ (REBCO or RE123, RE is a rare earth), can widely be used in electric motors, nuclear magnetic resonance and other fields, because these can trap strong magnetic fields [1,2].As a preparation method that can effectively inhibit the random nucleation of REBCO superconductor bulk and reduce the weak connection between grains, researchers usually choose the top-seeded melt texture growth method in the preparation and performance research of various REBCO superconductor bulks.However, the superconductor bulks prepared by this method usually have the problem of microstructure inhomogeneity [3,4,5].The non-uniform microstructure may be caused by the seed crystals becoming the new nucleation center, the non-uniform distribution of the second phase particles caused by the trapping/pushing theory in the process of melt growth, and the infiltration and diffusion of the liquid phase sources during the growth process.The analysis of the microstructure heterogeneity of REBCO superconductor bulks is of great significance for understanding the growth mechanism of REBCO superconductor bulks and improving their superconducting properties.In our previous work [6,7], the nonuniformity distribution have been reported owing to the distribution of the second phase particles, Gd 2 BaCuO 5 (Gd211).Therefore, in this study, the basic top-seeded melt-texture growth method was utilized to growth undoped GdBCO superconductor bulk guided by NdBCO single-seed crystal growth and Y123 liquid source, and the microstructure and superconducting properties of the bulk at different positions were studied.

Experimental
Using commercial high-purity Gd123 powder and Gd211 powder as the initial powder, a molar ratio of 5:2 was used.In order to improve the mechanical properties of GdBCO superconductor bulk and suppress the coarsening of Gd211 particles, Ag 2 O and Pt powder were added to the initial powder, respectively.The overall ratio of precursor powder is: Gd123+40 mol% Gd211+10 wt% Ag 2 O+0.5 wt% Pt.Put the precursor powders into a ball mill for mixed grinding, and then press them into a cylinder with a diameter of 25 mm and a thickness of 12 mm.Press commercial pure Y123 powder into a cylinder as liquid phase source with a diameter of 25 mm and a thickness of 3 mm.
The assembly method is as follows: place Y123 liquid phase source pellet below the GdBCO precursor pellet to provide sufficient liquid phase source.Then the size of 2 mm × 2 mm ×0.5 mm NdBCO seed crystal is placed at the center of the upper surface of the precursor pellet, and Y 2 O 3 pellet is added below the Y123 liquid source pellet to prevent the reaction between the Al 2 O 3 substrate and the liquid source.Finally, the entire sample is placed in a box furnace for the top-seeded melt-texture growth.Then, the sample is annealed in high-purity oxygen atmosphere, which becomes an orthogonal superconductor bulk from a tetragonal non-superconducting phase.The annealing process has been reported [6,7].To confirm the microstructure of GdBCO superconductor bulk, Firstly, we cut the sample in a rectangular area enclosed by abcd in Figure 1, parallel to the c-axis direction, and then place the rectangular sheet under a metallographic microscope to observe the microstructure of the larger surface area.
In order to further investigate the superconducting properties of GdBCO superconductor bulk in different regions, the superconductor bulk was cut again with the positions shown in Figure 1 as C1, C2, P1, and P2, respectively.Among them, C1 and C2 specimens are taken from the area below the seed crystal, and the specimens closer to the upper surface is defined as C1, while specimen farther from the upper surface is defined as C2.Unlike other studies, P1 and P2 were taken from the area below the cross pattern of the bulk, rather than from the four growth areas of the bulk.The specimen closer to the upper surface was defined as P1, while the specimen farther from the upper surface is defined as P2.These cut specimens are in the size, 2 mm × 2 mm ×1 mm.The magnetic properties of rectangular specimens were measured using physical property measurement system (PPMS).An external magnetic field was applied in the direction parallel to the c-axis of the specimens to obtain the temperature dependent curve and  Figure 2 shows the macroscopic topography of GdBCO superconductor bulk with the guidance of NdBCO single seed.It can be seen that the bulk after the process of sintering and annealing still maintains the shape of a cylinder, and there is no collapse phenomenon or obvious cracks.The seeds used to guide the growth of the bulk remained intact and did not melt.Moreover, the cross pattern and four growth regions on the surface of the GdBCO superconductor bulk are clear, which shows that GdBCO superconductor bulk has been successfully prepared, and has good single-domain property [9].
Optical micrograph of GdBCO single-domain bulk is shown in Figure 3.The testing range is large and the overall microstructure of the sample can be observed.From Figure 3, it can be observed that there are a small number of cracks parallel to the ab plane (as shown by the red arrow) and a large number of pores (as shown by the blue box) scattered inside the GdBCO superconductor bulk.In GdBCO superconductor bulk, pores are mainly distributed on the lower surface of the bulk and the area below the seed crystal, and the content of pores decreases with the increase of distance from the area below the seed crystal.These microstructures demonstrate the nonuniformity of GdBCO superconductor bulk.Figure 4 shows the relationship between the measured magnetization and the temperature.All the specimens show a narrow transition width and a high critical temperature, indicating that these specimens have good superconducting properties.The critical transition temperature, T C , of all specimens is higher than 94.5K, and the highest T C value is 95.9K, which is obtained below the seed crystal and far from the upper surface (C2).The T C values of these specimens show that the critical transition temperatures of the specimens below the seed (C1 and C2) were higher than those below the cross pattern (P1 and P2).The difference in T C between the specimens under the cross pattern is smaller than that under the seed.Figure 5 shows the curve of the critical current desity, J C , with the magnetic field of each specimen under the temperature of 77 K, and it was observed that the J C of each specimen also shows obvious differences.At the same distance from the upper surface, the specimens below the cross pattern have higher self-field J C values (P1>C1, P2>C2) than the specimens below the seed, which may result from the nonuniformity of distribution of the pores [10].Especially in the region close to the upper surface, the self-field J C value of the specimens below the cross pattern (P1) is significantly higher than that of the specimen below the seed crystal (C1).In the region closer to the upper surface, the specimens show second peak effect, and the C1 specimen below the seed crystal is more obvious, which makes the sapecimen still maintain good superconductivity under the middle field.In the region far from the upper surface, the specimen shows high J C value under the high field, especially the P2 specimen under the cross pattern, whose value still maintains 3.8 kA/cm 2 under the high field.It is of great significance for the practical application of superconductor bulks.

Conclusion
Single-domain GdBa 2 Cu 3 O 7-δ (GdBCO) superconductor bulk with NdBa 2 Cu 3 O 7-δ (NdBCO) single-seed guidance and YBa 2 Cu 3 O 7-δ (Y123) liquid source was successfully prepared by the top-seeded melt-texture growth method, and the macro and micro morphologies of the sample were preliminatively studied.Specimens at different locations, C1, C 2 below the seed, P1, P2 below the cross pattern, of the bulk exhibit the critical transition temperatures, T C , higher than 94.5 K, with a maximum value of 95.9 K. Cracks and pores are distributed unevenly in the superconductor bulk, and the pores mainly gather in the lower part of the seed crystal and far from the upper surface.By the analysis of the superconductivity at the different locations of the bulk, it is observed that the T C of the specimen below the seed is higher than that below the cross pattern, and the self-field J C value of the specimen below the cross pattern is higher than that below the seed crystal.The specimens closer to the upper surface have more advantages in the middle field, while the specimens farther away from the upper surface can still maintain good superconductivity at high field.The experimental results show that it is necessary to improve the preparation method of GdBCO superconductor bulk, so as to reduce cracks and pores and improve the microstructural nonuniformity of superconductor bulk and growth GdBCO superconductor bulks with high performance, such as, a buffer was added between NdBCO seed and GdBCO precursor.

Figure 1 .
Figure 1.Schematic illustration of the positions of the photographs and the locations of the sub-specimens used to measure T C and J C of GdBCO single-domain bulk.
the specimens.Based on the hysteresis loop, the J C value of the specimens was calculated using the extended Bean critical state model[8].

2 Figure 5 .
Figure 5. Applied magnetic field dependence of J C at 77 K with B//c for the GdBCO single-domain bulk at the different positions, C1, C2, P1, P2, respectively.