Effects of Vertical Temperature Gradient on the Growth Morphology

Article pubs.acs.org/crystal

Effects of Vertical Temperature Gradient on the Growth Morphology and Properties of Single Domain YBCO Bulks Fabricated by a New Modified TSIG Technique Yu-Xia Guo, Wan-Min Yang,* Jia-Wei Li, Li-Ping Guo, Li-Ping Chen, and Qiang Li College of Physics and Information Technology, Shaanxi Normal University, Xi’an, 710062, China ABSTRACT: Single domain YBCO bulk superconductors have been fabricated with different vertical temperature gradients (VTG) by a modified top-seeded infiltration and growth (TSIG) process with a new solid phase (Y2O3+BaCuO2) and a new liquid phase (Y2O3+6CuO +10BaCuO2). It is found that the angle α (between the upper surface and interface of a- and c-growth sectors) is very sensitive to the VTG, which means that the growth rate in caxis direction (Rc) can be changed by the VTG, according to tan α = Rc/Ra; Ra is the growth rate in the a-axis direction. It is also found that the 4-fold growth sectors did not cover the whole surface of the sample grown with a positive VTG, and the volume fraction of the single c-axis growth sector (Vfc) is 37.5% of the sample grown with a zero VTG, but it is reduced to 25% by a positive VTG, and enlarged to 53.6% by a negative VTG. The results of levitation force and trapped field of the samples show that a negative (or positive) VTG can improve (or reduce) levitation force and trapped field of the sample compared with that of the sample grown under a zero VTG. The results provide a very important way to fabricate large-size YBCO bulks with higher Vfc and better physical properties.

1. INTRODUCTION Since the discovery of high-Tc cuprate superconductors in 1986 by Bednorz and Müller,1 a significant research effort has been made worldwide to understand the fundamental mechanism for the high-Tc-superconductivity2 and find effective ways to enhance the properties of this kind of material for applications, such as in magnetic bearing,3,4 motors,5 trapped flux magnets, and power lines.6−8 The YBCO superconductor has become one of the most important and practical high temperature superconductors because of its high Tc, the secondary peak effect, high levitation force, and larger critical current density (Jc) in high magnetic field. The enhancement in these properties can be achieved by the introduction of flux pinning centers and the optimization of fabrication conditions.9−11 Though the top seeded melt textured growth (TSMTG) method is one of the most popular methods for the fabrication of single domain REBCO bulk superconductors with high superconducting properties, there are some disadvantages of the TSMTG method, such as leakage of the Ba−Cu−O liquid phase, shape distortion, and shrinkage. To overcome these problems, the top seeded infiltration and growth (TSIG) process has been developed and widely used in many laboratories.12−17 Recently, a new and very effective modified TSIG method has been developed to simplify the growth process of REBCO bulk superconductors in our laboratory, so that only one kind of precursor powder BaCuO2 should be prepared during the TSIG process of REBCO bulk superconductors.18 Compared to conventional TSIG, a new solid phase composed of BaCuO2 and RE2O3, instead of conven© 2015 American Chemical Society

tional solid phase made by RE2BaCuO5, and a new liquid phase composed of BaCuO2, Y2O3, and CuO, instead of conventional liquid phase made by REBa2Cu3O7‑δ and Ba3Cu5O8, can significantly improve the working efficiency and reduce the fabrication costs of the REBCO bulk superconductors by the TSIG method. It is known that a single domain REBCO bulk consists of five growth sectors (GSs), including of four a-GSs and one c-GS, and all the GSs are separated by the growth sector boundaries (GSBs), 4 a/a-GSBs between a-GSs, 4 a/c-GSBs between aGSs and c-GS,19 and the position of a/c-GSB is defined by tan α = Rc/Ra,20 as shown in Figure 1.

Figure 1. GSs and GSBs of single domain YBCO bulk samples. Received: December 13, 2014 Revised: February 5, 2015 Published: February 20, 2015 1771

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Crystal Growth & Design

separately put in the area with a positive VTG (1.0 °C/cm), a zero VTG (0 °C/cm), and a negative VTG (−1 °C/cm) of the furnace, and the vertical temperature distribution of the furnace is shown in Figure 3. Then, the samples were heated to 900 °C at a rate of 150 °C/h and

Here we proposed a new partition method according to the physical properties and the GSs morphology of Figure 1, one is the lower part (below the AB line) with only a single c-GS (SGS) in cylindrical shape, and the other is the upper part (above the AB line) with five growth sectors (FGS) in cylindrical shape. In general, the physical properties of a sample with a SGS are much better than those of a sample with multiple GSs, such as Jc, so it is easy to think that high levitation force and trapped field can be obtained in a single domain REBCO bulk with high volume fraction of SGS (Vfc) and lower volume fraction of FGS. Therefore, in order to fabricate highquality single domain REBCO bulks, it is very necessary to enlarge Vfc, but the key problem is how to carry it out? However, it is also difficult to find any studies on how to enlarge Vfc and control the position of a/c-GSB. So in this paper, the effects of VTG on the growth morphology, the Vfc, the levitation force, and the trapped field of single domain YBCO bulks have been investigated.

2. EXPERIMENTAL SECTION The precursor powder, BaCuO2, was prepared by conventional the solid-state reaction route in air using high purity powders of BaCO3 and CuO, which were weighed in the molar ratio of BaCO3:CuO = 1:1 and calcined at 910 °C for 24 h after initial grinding by a ball-milling machine. In order to get X-ray purity BaCuO2 powders, the calcining and grinding process mentioned above should be repeated three times. The new solid phase was composed of Y2O3 and BaCuO2 in the molar ratio of Y2O3:BaCuO2 = 1:1.2,21 well mixed by a ball-milling machine, then pressed into pellets in batches of 15 g with the diameter of 20 mm. The liquid phase was composed of Y2O3, CuO, and BaCuO2, well mixed in the molar ratio of Y2O3:CuO:BaCuO2 = 1:6:10, then pressed into pellets in batches of 25 g with the diameter of 30 mm. The bigger diameters of liquid source pellets than that of solid phase can increase the supporting stability of the liquid source pellets.22 After that, Yb2O3 powders were pressed into a plate in batches of 5 g with the diameter of 30 mm and the thickness of 2 mm to support the liquid phase during the TSIG process. The pressed solid phase pellet, liquid phase pellet, and Yb2O3 pellet were laid up concentrically from top to bottom in order. Then the well-stacked pellets were put on an alumina plate; between them there were some MgO single crystals with the same height. Finally, a self-made NdBCO seed crystal was placed on the top surface of the solid phase pellet with ab-plane parallel to the solid phase surface, as shown in Figure 2. Three samples, named S1, S2, and S3, made of the same precursor powders, and with the same arrangement, were put in a self-designed furnace with different VTG. The samples S1, S2, and S3 were

Figure 3. Vertical temperature distribution of the furnace. held for 10 h, and afterward heated up to 1045 °C at a rate of 120 °C/ h and held for 2 h to ensure the liquid phase infiltrated into the solid phase completely, subsequently cooled to 1012 °C at a rate of 60 °C/ h, then cooled to 1002 °C at a fast rate of 0.5 °C/h, after that cooled to 990 °C at a slow rate of 0.2 °C/h, and finally cooled to room temperature. The as-grown samples were oxygenated for 200 h in flowing oxygen at temperatures ranging from 470 to 400 °C, so that the samples S1, S2, and S3 could have superconducting properties. The microstructure of the samples was investigated by a scanning electron microscope (SEM). The transition temperatures (Tc) of the samples were measured with 0.01 T magnetic field applied by a vibration sample magnetometer (VSM). The levitation force and the trapped field of the samples were measured by a self-made system at the temperature of liquid nitrogen.23 A NdFeB permanent magnet with a diameter of 20 mm and 0.5 T at the top surface is used for the levitation force measurement, and the permanent magnet used for magnetization is 40 mm in diameter and 0.5 T at the top surface.

3. RESULTS AND DISCUSSION 3.1. Morphology of YBCO Bulks Grown under Different VTG. Figure 4 shows the morphology of the samples S1, S2, and S3 fabricated with the positive, zero, and negative VTG regions in the furnace. The typical single-domain

Figure 4. (a,b,c) Top views of samples S1, S2, and S3. (d,e,f) Side views of samples S1, S2, and S3.

Figure 2. Arrangement diagram of the samples before TSIG process. 1772

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Crystal Growth & Design morphology of samples can be seen from the top views of the bulks, as shown in Figure 4a,b,c, i.e., clear, 4-fold growth sectors on the top surface; there is no random nucleation observed in the samples but the 4-fold growth sectors in S1 do not cover the whole surface. An example of the X-ray diffraction (XRD) pattern of the single domain YBCO bulk (S1) is shown in Figure 5. Only some 00l (l = 2, 3, 4, 5, 6, 7) diffraction peaks

Figure 7. (a,b,c) Cross-section views of S1, S2, and S3. (d,e,f) Corresponding cross-section sketch diagrams of S1, S2, and S3.

upper surface and a/c-GSB, is defined by tan α = Rc/Ra, and it is easy to see that the angle α is very sensitive to the VTG, such as α2 = 40° for the sample grown in a uniformly distributed temperature zone, but the positive VTG enlarges the α to about α1 = 45°, while the negative VTG reduces the α to about α3 = 35°. As we know, the temperature distribution is uniform in the lateral direction during the TSIG process, so the Ra for samples S1, S2, and S3 is of the same value, i.e., Ra1 = Ra2 = Ra3; so, according to tan α = Rc/Ra, it is clear that Rc can be really inncreased (reduced) by the positive (negative) VTG. It is easy to see from Figure 7d,e,f that α1 > α2 > α3, which means tan α1 > tan α2 > tan α3, indicating that Rc1 > Rc2 > Rc3. Hence, the smaller the VTG, the smaller the Rc, and the smaller α, the larger Vfc is and the higher physical properties of YBCO bulk. 3.4. Designed Model for Growth of a Large YBCO Bulk with High Volume Fraction of SGS. Though the angle α and Rc can be relatively reduced by a negative VTG to get larger Vfc, it is still not so effective for us to fabricate large-size single domain REBCO bulk samples with high Vfc and high quality physical properties. For example, if we want to prepare a single domain YBCO bulks with 100 mm diameter and 30 mm thickness with only one seed, even under the negative VTG (α3 = 35°), the thickness of the FGS can be calculated to be 28.7 mm (h4 = 28.7) and almost close to the whole thickness (30 mm) of the sample, so the Vfc will be nearly zero, as shown in Figure 8a, which is significantly unfavorable for us to improve the physical properties of the large-size samples. In addition, it will also take too much time to fabricate large-size YBCO bulks

Figure 5. XRD pattern of the top surface of the sample.

can be observed in Figure 5, which means that the c-axis of the sample is perpendicular to its top surface and the YBCO grains are well oriented in the same direction; this is the typical characteristic of the single domain. We have also analyzed samples S2 and S3 by XRD, and the XRD patterns of the samples S2 and S3 are the same as the XRD patterns of sample S1. All the growth traces of the a/a-GSB boundaries on the side surface have reached the bottom of the bulk. However, there are some small regions not grown on the side surface of the samples, and the sizes of them are different: the size is biggest for S1, then S2, and it is smallest for S3, as indicated by the red line loops in Figure 4d,e,f. 3.2. Microstructure of YBCO Bulks Grown under Different VTG. Figure 6a,b,c shows the SEM micrographs of

Figure 6. (a,b,c) SEM micrographs of a/c planes of the samples S1, S2, and S3.

a/c planes of the samples; as we can see from these figures, all three samples S1, S2, and S3 have the same type of microstructure consisting of the lamellar Y123 crystal grain and Y211 particles, but there are still some differences. It can be seen that the thickness of the lamellar Y123 crystal grain is the thinnest for S1, the middle for S2, and the thickest for S3; the average thicknesses are approximately 0.82, 1.15, and 1.59 μm, respectively. It is considered that the thicker the lamellar Y123 crystal grain, the better the physical properties of the single domain YBCO bulk. 3.3. Cross-Section Views of YBCO Bulks Grown under Different VTG. Figure 7 shows the cross-section views of S1, S2, and S3 and their sketch diagrams. The angle α, between the

Figure 8. (a) Single seed model for growth of a large YBCO bulk. (b) Designed model for growth of a large YBCO bulk with high volume fraction of SGS. 1773

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Crystal Growth & Design due to the relatively low growth rate, and it is also very difficult to avoid any random nucleation during the TSIG process. If multiple seeds are used for the fabrication of large-size YBCO bulks, TSIG process time can be reduced, but it is difficult to grow samples with sufficiently good properties:24 this is mainly due to the real existence of boundaries between the neighboring single domains and the GSBs in each single domain of multiseeding large YBCO bulks. Therefore, with an attempt to shorten the growth time, enlarge the Vfc, and improve the physical properties of large-size YBCO bulks, we put forward a new method here to fabricate large-size YBCO bulks by combination of the VTG and multiple seeding techniques. For example, if we prepare a single domain YBCO bulk with 100 mm diameter and 30 mm thickness by this new method with 5 seeds with the same negative VTG (α3 = 35°), the thickness of the FGS (h5) can be greatly reduced and the Vfc can be significantly improved, as shown in Figure 8b. It can be calculated that h5 = 5.7 mm, so it is very clear that h5 is much smaller than h4 = 28.7 mm, and the Vfc (about 81%) is much higher than that of the sample fabricated by a single seed. It is easy to conclude that the physical properties of the multiseeded YBCO bulk will be much better than those of the single seed YBCO bulk by taking into account the negative VTG advantage and reasonable number of seeds in the design and arrangement. 3.5. Transition Temperature of YBCO Bulks Grown under Different VTG. The Tc of the samples was measured by a VSM. Small specimens (about 2.5 mm × 2.5 mm × 1.5 mm) cut from large single domain samples were measured with an external magnetic field of 0.01 T applied perpendicular to the a−b plane of the specimens. After the measurement, it is found that the transition temperatures of the samples S1, S2, and S3 are almost the same. As seen in Figure 9 the transition temperature of the three samples is about 92.7 K.

Figure 10. Levitation forces of S1, S2, and S3.

levitation forces of the samples show that a negative (or positive) VTG can improve (or reduce) levitation force of the sample to 38.3 N (or 32.4 N). 3.7. Trapped Fields of YBCO Bulks Grown under Different VTG. Figure 11a,b,c shows the trapped field

Figure 11. (a,b,c) Trapped field mapping of S1, S2, and S3. (d) Trapped fields along radial direction of S1, S2, and S3.

mapping on the top surface of samples S1, S2, and S3. They were measured at a distance of 0.5 mm from the surface of the samples using a homemade device with a Hall probe. Before measurements, the samples were cooled to 77 K in the presence of a 0.5 T magnet field perpendicular to the surfaces, and held for 10 min. The permanent magnets used for magnetization have a diameter of 40 mm and a surface field of 0.5 T, and the scanning area of the Hall probe is 30 mm × 30 mm. The trapped field distributions of all the samples show typical single-peak field profiles, indicating that they are of single magnetic domain. The maximum trapped fields (Btr, max) are present at the centers of the samples, and they are 0.304, 0.322, and 0.378 T for S1, S2, and S3, respectively. Figure 11d shows the trapped fields along the radial direction of the samples. The maximum trapped fields of the samples show that a negative (or positive) VTG can improve (or reduce) the trapped field of the sample to 0.378 T (or 0.304 T), compared with the trapped field of 0.322 T for the sample grown with the zero VTG.

Figure 9. Temperature dependent normalized magnetic moment curve of the sample.

3.6. Levitation Forces of YBCO Bulks Grown under Different VTG. Figure 10 shows the levitation forces versus the distance between the samples and the cylindrical NdFeB permanent magnet with the same diameter as the samples (20 mm) and a surface field of 0.5 T at the temperature of liquid nitrogen measured by a self-made device. The maximum levitation force measured in this experiment is achieved at the smallest distance (0.5 mm) between the two nearest surfaces of the sample and the magnet. It can be seen from Figure 10 that the maximum levitation forces of S1, S2, and S3 are 32.4, 34.8, and 38.3 N, respectively. Compared with the levitation force 34.8 N of the sample grown with a zero VTG, the maximum 1774

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3.8. Effects of Vfc on Levitation Forces and Trapped Field of Single Domain YBCO Bulks. According to the experiment results in Figure 10 and Figure 11, the maximum levitation forces and trapped fields of the samples grown with different VTG are much different. This phenomenon is closely related with the different VTG, because all the other experimental conditions are the same, so the difference among the maximum levitation forces and trapped fields of the three samples is only dependent on the different VTG. As we mentioned in section 3.3, the different VTG can result in different α and Rc, and finally lead to different Vfc values of YBCO bulks, so it is easy to think that the different levitation forces and trapped field are closely related to the Vfc of YBCO bulks. So, according to Figure 7d,e,f, it is easy to see that the heights (h) of FGS are different for samples S1, S2, and S3, and h1 > h2 > h3, implying Vfc1 < Vfc2 < Vfc3. It is calculated that Vfc2 is 37.5% of the sample grown with a zero VTG, but it is reduced to 25% by a positive VTG and enlarged to 53.6% by a negative VTG. The result is consistent with levitation force and trapped field results F1 < F2 < F3, Btr, max1 < Btr, max2 < Btr, max3, indicating that a negative VTG is more suitable to growing high quality YBCO bulks. The results above clearly indicate that a negative VTG is of great importance in fabricating YBCO bulks with higher Vfc and better physical properties; at the same time, we propose a new method for the fabrication of large-size YBCO bulks with higher Vfc and better physical properties by the combination of the VTG and multiple seeding techniques, as shown in Figure 8.

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4. CONCLUSION Single domain YBCO bulk superconductors have been fabricated with different VTG of the furnace by a modified TSIG process. After investigating the top and side morphology, microstructure, cross-section views, transition temperatures, levitation forces, and trapped fields of the samples, it is found that the angle α was changed by different VTG, and Rc in the sample was smallest with a negative VTG, and then the uniformly distributed temperature zone and a positive VTG, which finally lead to different Vfc values of the samples. The results of levitation force and trapped field show that a negative VTG is more helpful to fabricate YBCO bulk superconductors with large Vfc and high physical properties. The results can be used to fabricate large-grain YBCO bulk by combination of multiple seeding techniques.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel./fax: +86 29 81530738. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation in China (No. 51342001, 50872079), the Keygrant Project of Chinese Ministry of Education (No. 311033), Research Fund for the Doctoral Program of Higher Education of China (No.20120202110003), the Key Program of Science and Technology innovation team of Shaanxi Province (2014KTC-18) and The Fundamental Research Funds for the Central Universities (No. GK201305014, GK201503023). 1775

DOI: 10.1021/cg501817z Cryst. Growth Des. 2015, 15, 1771−1775