Real-Time Observation and Analysis of Single-Domain YBCO Bulk

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Real-Time Observation and Analysis of Single-Domain YBCO Bulk Superconductor by TSIG Process Wanmin Yang,* Xiaodan Guo, Feng Wan, and Guozheng Li Department of Physics, Shaanxi Normal University, Xi'an, Shaanxi, 710062, China ABSTRACT: The growth process of single domain YBCO bulk superconductor by top-seeded infiltration and growth technique has been observed in real time by an in situ high temperature video camera. The initial epitaxial growth temperature from the NdBCO seed and growth rate of a single domain YBCO bulk were obtained by measuring the growth distance between the seed and the growing interface at different times. It was found that the initial epitaxial growth temperature of YBCO crystal is about 1008 °C; the average growth rate (Ra) is increasing with time at first, reaches a highest value about 0.28 mm/h in the range of under cooling temperature(ΔT) 18
20 °C, and decreases to 0.26 mm/h until the single domain YBCO bulk grown up to the whole sample at about ΔT = 22 °C. In addition, It is also discovered that the instantaneous growth rate(Ri) is a random scattering data with time and ΔT, Ri varies from 0.04 to 1.14 mm/h and without any order. These results were observed for the first time.

1. INTRODUCTION High levitation force and auto-stabilized levitation of REBCO bulk superconductors make it possible for various applications, such as high-field permanent magnets, magnetic bearing, flywheel, levitated transportation systems, motors, and generators. These applications are based on the quality of the superconductors, now single domain REBCO bulk superconductors can be fabricated by top-seeded melt growth (TSMG) method1
4 and top-seeded infiltration growth (TSIG) process5
10 in many laboratories. But it is still difficult to fabricate larger (such as 80 mm in diameters) samples with a single domain up to the whole volume because of the competition between the growth of single domain REBCO bulk from the seed and the random nucleation of other grains in the rest of the samples, if the temperature window for the growth of YBCO bulks and the cooling rate in this range is not designed scientifically. To overcome this problem, it is necessary to make clear the growth mechanism of a single domain of REBCO bulk and to determine the key parameters for optimizing the growth technology Cima et al11 proposed a solidification model based on the result of direct microstructural observation of quenched solidification interfaces, which indicates that the growth of YBCO crystal is not encountered in conventional solidification, the peritectic reaction occurs by dissolution of the Y2BaCuO5(Y211) particles into the liquid followed by reprecipitation on the YBCO crystal surface, and predicated that the planar growth rate of YBCO crystal may be limited to about 3 μm/s. Endo et al12 reported that the growth length increased with increasing the holding time, but the growth rates were found to decrease with the holding times longer than 105 seconds. The growth of YBa2Cu3Oy (Y123) depends not only on the undercooling but also on the amount of the excess Y211 in the sample. In addition, pushing/trapping of the Y211 r 2011 American Chemical Society

phase particles at the solidification front of YBCO crystal should be considered during the melt growth process. Donglu Shi et al13 reported that the growth rate of YBCO crystal is of anisotropy and the growth rate along ab plane(Rab) was larger than that of c axis(Rc); Rab increased from 0.2 to 1.1 mm when the undercooling temperature ΔT increased from 2 to 10 °C;but Rab increased with from 0.3 to 0.44 mm between ΔT of 25 and 40 °C. All the above results are obtained from quenched samples. Recently, Cao et al14 have investigated the growth mechanism of YBCO bulk by seeded infiltration growth (SIG) method using both the magnetic susceptibility measurements and in situ video observation at high temperature. Huang et al15 have observed the crystal growth process of YBCO superconductor under an external electric field, it is found that the growth length of each Y123 single grain increased almost linearly with time. Mori et al16 have studied the growth process of faceted for Sm123 film by in situ observation, It is found that the maximum growth rate along ab plane (Rab) is about 0.0018 mm/h at undercooling ΔT = 25K. Yan et al17 have enhanced the growth rate of melt-textured YBCO bulk to about 0.32 mm/h by employment of 1 atm oxygen pressure during the TSMG process, which is 1.5 times larger than that in air. But up to now, most of these results are obtained from quenching method, there is not any report on real averaged growth rate(Ra) and instantaneous growth rate(Ri) of YBCO bulk superconductors. In this paper, the real time growth process of single domain YBCO bulk superconductor by TSIG has been observed and recorded by an in situ high temperature video Received: March 15, 2011 Revised: May 11, 2011 Published: May 16, 2011 3056

dx.doi.org/10.1021/cg2003222 | Cryst. Growth Des. 2011, 11, 3056–3059

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Figure 1. in situ high temperature video camera device for monitoring the morphology of growing YBCO bulk crystal. (a) Schematic sketch, (b) the device photograph. (1) Cooling vessel, (2) digital video camera, (3) high temperature lens, (4) quartz glass, (5) halogen lamps, (6) Pyrex window, (7) furnace, (8) sample.

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Figure 2. Some pictures captured from the video recording of a growing single domain YBCO pellet (20 mm in diameter). The growth is consisted of 3 stages:epitaxial nucleation, developed to a square shape and homothetic growth.

camera, the initial epitaxial growth temperature from the NdBCO seed and growth rate of a single domain YBCO bulk have been studied in detail.

2. EXPERIMENTAL SECTION 2.1. Sample Fabrication. The Y2BaCuO5(Y211), YBa2Cu3Oy (Y123), and Ba3Cu5O8(Y035) powders were prepared by solid state reaction method form initial powders Y2O3(99.99%),BaCO3(99%)and CuO(99%) ; The XRD results confirmed that all the Y211, Y123 and Y035 were single-phase compounds. The average size of the Y211 particles is about 1.5 mm. The solid phase Y211(12 g) and liquid phase (Y123/Y035 = 1:1, 9 g) powders, were pressed into pellets of the same diameter 20 mm respectively. Furthermore, Y2O3 powder(2 g) was pressed into a pellet of the same diameter 20 mm used to support the liquid phase at elevated temperature. The Y2O3 pellet, liquid phase pellet and Y211 pellet were superposed in order with Y211 pellet on the top. The well arranged pellets were placed on an alumina plate with some MgO single crystals between them. Finally, an NdBCO crystal of dimensions 2 mm 1.5 mm 1.2 mm was placed on the top surface of the Y211 pellet with ab-plane parallel to the surface, the details is similar to that in reference.18 The sample, put into a vertical tube furnace with near uniformly distributed temperature, was heated up to 1045 °C and dwelled for 2 h, then rapidly cooled to 1015 °C, after that, the sample was cooled to 985 °C in 72 h, later cooled to room temperature at a rate of 120 °C/h, and finished the growth process of single domain YBCO bulk sample. 2.2. In Situ High-Temperature Video Camera Device. Three holes were drilled through the cover of the tube furnace along one of its diameter; each hole was covered by a Pyrex window. Two of the windows near to the edge of the furnace are used to let lights from the two halogen lamps projecting toward the sample; the hole at the center is used to observe the interior of the tube furnace. A high temperature digital video camera is fixed above the tube furnace, and used to monitor the morphology of the growing YBCO bulk crystal, as shown in Figure 1. The images of the YBCO bulk are digitized and saved in a computer. Therefore the size and growth speed of the YBCO bulk crystal can be monitored and recorded in real time during the TSIG process, the growth length (L) is evaluated by measuring the distance between the seed edge and the growth front of the single domain YBCO crystal.

Figure 3. Relationship between the growth length (L) of single domain YBCO crystal and time (t) during the slowly cooling process after temperature of 1008 °C.

3. RESULTS AND DISCUSSION A φ20 mm single-domain YBCO bulk superconductor has been fabricated by TSIG process, the whole process was monitored by the in situ high temperature video camera device. The video camera takes 30 pictures every minute and saved in a computer, so that we can make clearly the growth process of YBCO bulk crystal. Figure 2 shows some pictures taken from the recorded images of the growing single domain YBCO bulk during TSIG process by the video camera process. As we can see from these figures, the growth procedure is consisted of 3 steps: epitaxial nucleation, developed to a square shape and homothetic growth. Combined with the growth length, times and the TSIG temperature profile mentioned above, it is found that the epitaxial nucleation and the formation of square shape YBCO crystal from the seed is observed at 1008 °C, even though the slowly cooling process starts from the peritectic temperature Tp = 1015 °C, and then the square shape YBCO crystal grows up to the whole samples until the temperature reaches 993 °C; the corresponding undercooling temperature (ΔT = Tp
T) for the starting growth and the ending growth are 7 and 22 °C respectively. In order to study conveniently, the times corresponding to 1008 and 993 °C are denoted as the growth starting time ts = 0 and the growth ending time te = 36 h. 3057

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Figure 4. Average growth rate (Ra) of single domain YBCO crystal evaluated by L/t, the starting time is corresponding to the initial growth temperature of YBCO crystal at 1008 °C.

Figure 5. Instantaneous growth rate (Ri) of single domain YBCO crystal evaluated by ΔL/Δt, Δt is the time interval between t and t þ Δt, ΔL is the growth length increment of the single domain YBCO crystal during Δt times.

The growth length (L) and the corresponding time (t) has been taken from the video recording images after temperature of 1008 °C, as plotted in Figure 3. Where L is measured from the seed edge to the nearest growth front of the growing YBCO crystal during the slow cooling process. As we can see from Figure.3, L is increasing monotonously with time, but it is an nonlinear function of time, this indicates that the growth of YBCO crystal can be divided into three zones: (1) an accelerating growth stage, the size and the growth rate of the growing YBCO crystal is slowly increasing at the time interval between 0 and 12.5 h, corresponding to the temperature cooling from 1008 °C to about 1003 °C; (2) a steady growing stage, the size of the YBCO crystal increases fast and stably in the period of time between 12.5 and 31.5 h, corresponding to the temperature cooling from 1003 °C to about 995 °C; (3) the decelerating growth stage, the size and growth rate of the YBCO crystal increases slowly in the interval between 31.5 and 36 h, corresponding to the temperature cooling from 995 °C to about 993 °C. This result is different with the results obtained at fixed undercooling temperature those are of linear relationship between the growth length and time.15,16 Figure 4 shows the average growth rate of single domain YBCO crystal along a (or b) axis at different times, Ra is evaluated by L/t. As we can see from this figure, Ra is not increasing monotonously and consisted of three zones: (1) the starting growth and formation of square shape stage, Ra is increasing rapidly at first 1.5 h and keeping around 0.05 mm/h until t = 6 h in this stage, this happens in the time from ts to 6 h, and temperature between 1008 and 1005.5 °C; (2) the accelerating growth stage, Ra increases linearly from 0.05 to 0.27 mm/h during the time interval between 6 to 25 h, corresponding to the temperature cooling from 1005.5 °C to about 997.6 °C; (3) the near constant growth rate stage, Ra is of nearly fixed value 0.28 mm/h with time increasing from 25 to 36 h, corresponding to the temperature cooling from 997.6 °C to about 993 °C. This is different with the result obtained at fixed undercooling temperature. The growth rate is generally evaluated by the total growth length divided by the corresponding time, so it is difficult to give the instantaneous growth rate of the YBCO bulk superconductor if we can not observe its real growth process, especially for the quenching method. The instantaneous growth rate (Ri) at time t of single domain YBCO crystal is evaluated by ΔL/Δt, as shown in Figure 5. Δt is the time interval between t and t þ Δt, Δt is

taken about 0.5 to 1.5 h according to whether it is easy to measure the growth length ΔL or not. ΔL is the growth length of the single domain YBCO crystal during Δt times. As we can see from Figure.5, the growth rate is much different from the reported articles,15
17 the data is so randomly scattered and impossible to be described by a formula, the growth rate varies from 0.04 to 1.14 mm/h, these results were observed for the first time. This maybe related with the undercooling temperature, microstructure, Y211 particles distribution, the concentration of Y3þ, Ba2þ, Cu2þ in the liquid phase, the temperature, the released latent heat, etc. The details will be reported elsewhere later.

4. SUMMARY The growth properties of single domain YBCO bulk superconductor have been observed by an in situ high temperature video camera. It was found that the average growth rate Ra is increasing with time at first, reaches a highest value about 0.28 mm/h at an under cooling temperature ΔT between 18 to 20 °C, and decreases to 0.26 mm/h at ΔT = 22 °C. It is discovered that the instantaneous growth rate Ri is of a random scattering data with time and ΔT, Ri varies from 0.04 to 1.14 mm/h. These results were observed for the first time. ’ AUTHOR INFORMATION Corresponding Author

*Email: [email protected], [email protected].

’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No.50872079), the National High Technology Research and Development Program of China (863 Program, Grant No. 2007AA03Z241) and the Fundamental Research Funds for the Central Universities (Grant No. GK200901017). ’ REFERENCES (1) Gawalek, W.; Habisreuther, T.; Zeisberger, M.; Litzkendorf, D.; Surzhenko, O.; Kracunovska, S.; Prikhna, T. A.; Oswald, B.; Kovalev, L.; Canders, W. Supercond. Sci. Technol. 2004, 17, 1185–1188. 3058

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(2) Litzkendorf, D.; Habisreuther, T.; Bierlich, J.; Surzhenko, O.; Zeisberger, M.; Kracunovska, S.; Gawalek, W. Supercond. Sci. Technol. 2005, 18, S206–S208. (3) Cardwell, D. A. Mater. Sci. Eng., B 1998, 53, 1–10. (4) Shi, Y.; Babu, N. H.; Iida, K.; Yeoh, W. K.; Dennis, A. R.; Pathak, S. K.; Cardwell, D. A. Phys. C 2010, 470 (17
18), 685–688. (5) Meslin, S.; Noudem, J. G. Supercond. Sci. Technol. 2004, 17, 1324–1328. (6) Iida, K.; Babu, N. H.; Shi, Y.; Cardwell, D. A. Supercond. Sci. Technol. 2006, 19, S478–S485. (7) Babu N. H.; Iida K.; Shi Y.; Cardwell D. A. Appl. Phys. Lett. 2005, 87. 202506 (3pp). (8) Meslin, S.; Harnois, C.; Chateigner, D.; Ouladdiaf, B.; Chaud, X.; Noudem, J. G. J. Eur. Ceram. Soc. 2005, 25, 2943–2947. (9) Meslin, S.; Iida, K.; Babu, N. H.; Cardwell, D. A.; Noudem, J. G. Supercond. Sci. Technol. 2006, 19, 711–718. (10) Noudem, J. G.; Meslin, S.; Horvath, D.; Harnois, C.; Chateigner, D.; Eve, S.; Gomina, M.; Chaud, X.; Murakami, M. Phys. C 2007, 463
465, 301–307. (11) Cima, M. J.; Fleming, M. C.; Figueredo, A. M.; Nakade, M.; Ishii, H.; Brody, H. D.; Haggerty, J. S. J. Appl. Phys. 1992, 72, 179–190. (12) Endo, A.; Chauhan, H. S.; Nakamura, Y.; Shiohara, Y. J. Mater. Res. 1996, 11, 1114–1119. (13) Shi, D.; Qu, D.; Tent, B. Mater. Sci. Eng. B 1998, 53, 18–22. (14) Cao, Hitao; Chaud, X.; Noudem, J. G.; Zhang, Cuiping; Hu, Rui; Li, Jinshan; Zhou, Lian Ceram. Int. 2010, 36, 1383–1388. (15) Huang, X.; Uda, S.; Yao, X.; Koh, S. J. Cryst. Growth 2006, 294, 420–426. (16) Mori, N.; Maebatake, T.; Teranishi, R.; Yamada, K.; Mukaida, M.; Miura, M.; Yoshizumi, M.; Izumi, T. Phys. C 2010, 470, 1266–1270. (17) Yan, S.; Chen, Y.; Ikuta, H.; Yao, X. IEEE Trans. Appl. Superconductivity 2010, 20 (2), 66–70. (18) Li, G. Z.; Yang, W. M.; Tang, Y. L.; Ma, J. Cryst. Res. Technol. 2010, 45 (3), 219–225.

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