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Effects of Y2BaCuO5 content and the initial temperature of slow-cooling on the growth of YBCO bulk
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Physica C 468 (2008) 435–441 www.elsevier.com/locate/physc
Effects of Y2BaCuO5 content and the initial temperature of slow-cooling on the growth of YBCO bulk Xing-da Wu *, Ke-Xi Xu, Jing-he Qiu, Peng-jun Pan, Keran Zhou Physics Department, Shanghai University, Shangda Road 99, Shanghai 200444, People’s Republic of China Received 20 November 2007; received in revised form 8 January 2008; accepted 15 January 2008 Available online 8 February 2008
Abstract In order to grow large mono-domain YBa2Cu3O7 d bulk in the tubular furnace, the effect of Y2BaCuO5 (Y211) content and initial temperature of the slow-cooling process on the growth of YBa2Cu3O7 d (Y123) bulk has been investigated. Single grain Y123 bulk samples with 20 mol% and 30 mol% Y211 addition have higher original crystallization temperature than that with 40 mol% Y211; however, the melt-textured growth rate of the single domain seems to be faster for the system with higher Y211 concentrations. In addition, it seems that the crystal growth of YBCO is extremely sensitive to the initial temperature of slow-cooling. The optimal initial temperature of slow-cooling process, which should be chosen according to the Y211 phase content, has been brought to the fore in the growth of large grain bulk. The above data is very useful for the fabrication of large grain (RE)–Ba–Cu–O bulk, and a group of YBCO single domain samples with diameter of 53 mm have been successfully produced, all of which present excellent magnetic levitation performance in the axial direction. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.81.Bd; 74.62.Bf Keywords: Y2BaCuO5 content; The initial temperature of slow-cooling; Large grain YBCO
1. Introduction Yttrium-based high temperature single domain bulk superconductors prepared by top-seeded melt-texturing growth (TSMTG) method show strong levitation force on a permanent magnet and the ability to trap high magnetic fields, which are attractive for various engineering applications, for example in magnetic bearings (flywheels energy storage system, etc.) [1–3] or as trapped flux magnets (magnetic separation device, etc.) [4]. The levitation force and the field trapping capability of the bulk superconductors is related to the critical current density (Jc) and the current loop size (d). Jc can be enhanced by increasing the flux pinning by means of doping [5,6] or irradiation [7,8]. If *
Corresponding author. Tel.: +86 21 66132515. E-mail addresses: [email protected] (X.-d. Wu), kxxu@staff.shu. edu.cn (K.-X. Xu). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.01.007
the Jc properties were equal in each sample and if the geometric effect could be ignored, the levitation force and the trapped field of the bulk superconductors would be proportional to the radius of the sample. Therefore, an enlargement of the grain size is very important for the applications of these bulk samples. It is well known that the TSMTG method is a very effective process to fabricate YBCO mono-domains, in which the Y123 domain grown from a seed is promoted by peritectic recombination of the Y211 particles and the Ba- and Cu-rich liquid phases. However, it is difficult to grow large sample with high performance mainly for the following two reasons. The first one is a slow dissolution rate of coarsening Y211 particles in the melt, which results in reduction of the growth rate or even earlier termination of crystal growth [9,10]. The other one is the temperature window for the stable growth of single grain monoliths, which is relatively narrow [11]. In this work, we will focus our investigation on the effect of Y211
X.-d. Wu et al. / Physica C 468 (2008) 435–441
inclusions as well as the slow-cooling stage, since both of them play significant role in the growth of large grain bulk. Influence of Y211 particles on the microstructure or superconducting properties of an YBCO single domain has been widely studied. It has been well known that the residual solidified liquid which is deleterious to the superconducting properties can be obviously reduced by adding extra Y211 particles deliberately. Generally around 10– 40 mol% of extra Y211 particles are added in the system [9,11,12]. It has also been known that the Y211 particles in the final single domain have a positive effect on the superconducting properties as the Y123/Y211 interfaces are good pinning centers [13,14]. However, effect of Y211 content on the initial crystallization behavior of YBCO is still unrevealed. The slow-cooling process is another essential parameter that has to be controlled in order to achieve a large grain sample with high performance. There is a small temperature window exists between the temperature where the heterogeneous nucleation is promoted from the seed and the temperature where a homogeneous nucleation appears [15]. It is important to carefully control the slow-cooling process in order to keep the system in this temperature window [16,17]. It has also been reported that slow-cooling rate is a crucial factor affecting the microstructure, texture growth and superconducting properties of the sample [12,16]. However, not enough attention has been paid to effects of the initial temperature of slow-cooling on the growth of YBCO single grains. In this paper, influence of Y211 content and the initial temperature of slow-cooling on the growth of melt-texture single domain bulk have been studied using four batches of Y123 pellets with 20 mol%, 30 mol% and 40 mol% Y211 addition. The first two batches of specimens were quenched at the temperatures from which the initial growth process is suggested to start, in order to observe the influence of Y211 content on the growth process of the sample. The next two batches of specimens were processed in the same tubular furnace with two different thermal processes to study the influence of initial temperature of the slow-cooling on the growth of the sample. Finally, based on the data acquired from these four batches of specimens, a group of large grain YBCO bulk with diameter of 53 mm and high magnetic levitation performance has been successfully fabricated.
The mixtures were uniaxially pressed into small disks (20 mm in diameter and 12 mm in thickness), hexagonshaped pellets (with a face diagonal length of 38 mm and a thickness of 15 mm), and large disks (60 mm in diameter and 20 mm in thickness), respectively. Small SmBaCuO (Sm123) crystals prepared by the TSMTG process were used as seeds, and were centrally placed on top of the pellets before melt processing. Nine batches of samples were processed in all, for which the heat schedules are shown in Fig. 1. The precursor pellets were heated to 940 °C at a rate of 100 °C h 1, held for 5 h, and then heated quickly to 1050 °C and dwelled for 1.5–3 h according to the volume of the pellets, from where they were cooled to T1 at a rapid cooling rate, and then slowly cooled to T2, followed by the quenching at a rate of about 270 °C h 1 to room temperature. The temperatures T1 and T2, as well as the slow-cooling rates, are different from batch to batch. Details of the heat treatment are also shown in Table 1. For comparative investigation, there were three pellets with different Y211 concentration (20 mol%, 30 mol% and 40 mol%, respectively) in each batch from Batch 1 to 2, and there were two pellets with 30 mol% and 40 mol% Y211, respectively, in Batch 3 and 4. As for the Batch 5–9, only one large disk with 30 mol% Y211 added was processed in each batch, which was used for evaluating the efficiency of the process method and the magnetic properties such as levitation force.
1050ºC
T1
Temperature
436
T2
940ºC
Time Fig. 1. Heat treated schedules for nine batches of samples.
2. Experimental details YBa2Cu3O7 d and Y2BaCuO5 precursor powders were fabricated through repeating calcinations and grinding of stoichiometric mixtures of high-purity Y2O3, BaCO3 and CuO at 950–970 °C. The precursor powders were mixed thoroughly according to a nominal composition of Y123 + xY211 in a molar ratio with x = 0.2, 0.3 and 0.4. To reduce the size of Y211 precipitate particles dispersed in the melt-processed superconducting matrix, 0.5 wt.% CeO2 was also added.
Table 1 Important parameters of the heating cycles for each batch Batch number
T1 (°C)
T2 (°C)
Slow-cooling rate (°C h 1)
Sample types
1 2 3 4 5–9
1010 1010 1005 997 997
997 994 976 976 973
0.3 0.3 0.3 0.3 0.18
Small disks Small disks Hexagon-shaped pellets Hexagon-shaped pellets Large disks
X.-d. Wu et al. / Physica C 468 (2008) 435–441
3. Results and discussion 3.1. Influence of Y211 content on single domain growth Fig. 2 shows top views of the small size disc samples: Samples (a) and (d) with Y211 inclusions of 20 mol%, samples (b) and (e) with Y211 inclusions of 30 mol%, and samples (c) and (f) containing 40 mol% Y211. Samples (a), (b) and (c) which belong to the first batch were melt-processed in a tubular furnace and quenched from 997 °C, while samples (d), (e) and (f), belonged to the second batch processed in the same furnace, were quenched from 994 °C. First of all, it is easily to find that the morphologies of samples (a), (b) and (c) are significantly different from each other. Sample (b) displays a poly-grain mode with many Y123 grains randomly distributed regardless of the seed orientation. Sample (a) presents an appearance quite similar to that of sample (b), but a small Sm123-seeded grain can be seen at the center. As for sample (c), only several grains distribute on the surface, and no single domain grown from the seed can be seen. It is safe to conclude that the difference among these three samples is attributed to the different content of Y211 in each of them, since all of them are processed in the same condition. It is known that the seeded growth process mainly consists of three steps: the peritectic decomposition process which occurs above the peritectic temperature Tp, the onset of heterogeneous nucleation around the seed when the system is cooled down below a certain temperature denoted as Ts, and the subsequent directional solidification process during slow-cooling stage. There is a thermodynamic nucleation barrier window within which the Y123 phase grows from the position of the seed crystal without compe-
437
tition from grains nucleated homogeneously elsewhere in the incongruent melt [18]. When the melt is cooled down to Ts, the barrier against nucleation around the seed is low enough and heterogeneous nucleation takes place. Cooling down the melt continually, the nucleation barrier for the whole system reduces further and the peritectic recombination of Y123 may be accelerated. However, multiple grain nucleation would be caused if the under cooling was so large that the barrier was too low to avoid spontaneous nucleation in the system. According to our analysis presented above, it is natural to think that the reason why no textured single domain around the seed can be seen in samples (a), (b) and (c) may be that the quench temperature (997 °C) is above the temperature Ts. The melt processing was interrupted prior to the onset of nucleation or growth around the Sm123 seed. But the nucleation and growth of Y123 grains might also occur during the initial stage of quenching process if the system was quenched from the temperature just above Ts. With the nucleation barrier in the system rapidly reducing, lots of nucleation site besides the position of seed occurred and each grain grew competitively at the same time, resulting in the case of samples (a) and (b). As for sample (c), a pre-solidification morphology is presented except for several Y123 grains taken place, which implies that the heterogeneous nucleation temperature Ts of this sample is still far below 997 °C, and that the spontaneous nucleation temperature is also lower than that in samples (a) and (b). Therefore, an assumption can be made that system with 20 mol% and 30 mol% Y211 addition has a higher nucleation temperature than that with 40 mol% Y211 addition, although the mechanism should be unveiled in further investigation.
Fig. 2. Top views of the six small disks with Y211 inclusions of 20 mol% for (a) and (d), 30 mol% for (b) and (e), 40 mol% for (e) and (f), and quenched from 997 °C for (a), (b) and (c), from 994 °C for (d), (e) and (f), respectively.
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It is interesting to observe the growth modes of the systems quenched from the temperature below Ts. The second batch consisting of samples (d), (e) and (f) with 20 mol%, 30 mol% and 40 mol% Y211, respectively, was quenched from a lower temperature 994 °C, and the top views of which have been shown in Fig. 2. All of them are polycrystalline, but a prominent Sm123-seeded YBCO grain at the centre can be seen, which suggests that heterogeneous nucleation and growth of single domain have taken place in the temperature range from 997 °C to 994 °C. In addition, the size of the seeded single domain of sample (f) seems to be equal to that of sample (e), which indicates a higher growth rate in sample (f), since the 40 mol% Y211 system has a lower nucleation temperature as suggested above and hence it has a shorter effective growing time comparing with sample (e). As for the sample (d), the seeded YBCO grain is smaller than that of samples (e) and (f). This result indicates that the slowest growth rate in sample with 20 mol% Y211 addition, since sample (d) has the highest crystallization temperature. Effect of RE2BaCuO5 (RE211) content on growth rate has also been studied in other REBaCuO systems [19,20]. Generally the growth rate increases as the RE211 content increases. This can be explained as follows. As the RE211 content increases, the inter-particle distance decreases and the diffusion distance of RE ions through the liquid phase reduces, resulting in a large growth rate.
3.2. Influence of the initial temperature of slow-cooling on single domain growth Photographs of the four hexagon-shaped pellets of Batch 3 and 4 are shown in Fig. 3. The slow-cooling process started at 1005 °C for samples (a) and (b), and at 997 °C for samples (c) and (d). Samples containing varying amounts of added Y211 have also been used for comparative observations, with 30 mol% for samples (a) and (c), and 40 mol% for samples (b) and (d). It can be observed from Fig. 3 that systems with different Y211 concentration are extremely sensitive to the initial temperature of slowcooling process. At a first glance it may appear incredible that single grains grown in a larger temperature range (slow-cooling from 1005 °C to 976 °C) have a smaller size than that grown in a narrower one (slow-cooling from 997 °C to 976 °C). Actually, the effective growing time is the same for these two batches, since the initial nucleation temperature locates at or below the temperature of 997 °C as suggested in the previous section. But this is still not enough to explain the phenomenon, and there must be some other mechanisms. It is well known that the Y211 particles are subjected to the Oswald ripening phenomenon at high temperature in the melt [21]. In the Oswald ripening process, the smaller Y211 particles are dissolved in the melt, and dissolved ions
Fig. 3. Top surfaces of YBCO grains with Y211 inclusions of 30 mol% for (a) and (c) and 40 mol% for (b) and (d), and with two different initial temperature of slow-cooling which is 1005 °C for (a) and (b) and 997 °C for (c) and (d), respectively.
X.-d. Wu et al. / Physica C 468 (2008) 435–441
migrate to grow the biggest particles. The coarsening of Y211 inclusions is enhanced when the delayed time at elevated temperature is long. In particular, in the case of a large volume fraction of Y211 in the liquid, the coarsening rate becomes larger. Large Y211 inclusions cause difficulty in completion of the peritectic reaction, resulting in the possibility of the coexistence of low-temperature stable phases (BaCuO2 and CuO) because diffusion through the solid is much slower [22]. As for the two samples of Batch 3, the coarsening of Y211 particles had been enhanced in liquid phase during the initial stage of slow-cooling process (from 1005 °C to 997 °C). Then when the systems were cooled down to the temperature from which heterogeneous nucleation and growth of single grain started, the growth rate of Y123 crystal would be limited due to the small rate of yttrium diffusion in the liquid. The situation seems to be more severe for sample (b) with higher Y211 concentration, which has a smaller size of single domain. In contrast, when the initial temperature of slow-cooling is set at 997 °C, a temperature very near the initial nucleation one, perfect single domain like samples (c) and (d) can be obtained. In order to confirm that different growth morphologies between Batch 3 and Batch 4 is caused by the different delayed time at elevated temperature, and the longer one has enhanced the coarsening of Y211 inclusions which has a negative effect on the growth of single grain, the microstructure of samples (b) and (d) in Fig. 3 was observed with a scanning electron microscope (SEM). Fig. 4 shows the typical microstructure of these two samples with different initial temperature of slow-cooling: (a) 1005 °C, (b) 997 °C. The light gray inclusions are Y211 particles embedded in the superconducting matrix. It turns out that the size of Y211 inclusions in these two samples is very different. In Fig. 4a, the Y211 inclusions are distributed non-uniformly and some of which have grown to a very large size. While in Fig. 4b, the Y211 inclusions are dispersed in the Y123 matrix much more uniform, and no large Y211 particles can be found. These results suggest that delayed time at elevated temperature should be shorted by selecting an appropriate initial temperature of slow-cooling.
439
3.3. Growth and levitation properties of large single domains The experiments above have provided important information which is very useful for the growth of large YBCO single domain. Firstly, the initial nucleation temperature can be adjusted by changing the content of Y211 in precursor powders. Secondly, the crystal growth of YBCO single grain is extremely sensitive to the initial temperature of slow-cooling process. Finally, optimal initial temperature of slow-cooling process should be chosen according to the Y211 phase content in order to grow large Y123 grains. Based on these data, five sets of large grain samples with 30 mol% Y211 additions have been successfully produced in the tubular furnaces which have been calibrated, using the parameters of the heating cycles shown in Fig. 1 and Table 1. It should be emphasized here that a slower cooling rate (0.18 °C h 1) and wider temperature range of slowcooling process have been used, since the growth time needed for large sample is much longer than that of small ones. Fig. 5 shows the photographs of one of the large grain samples 53 mm in diameter and 15 mm in thickness. Well-grown grain from the seed crystal with fourfold growth sectors extending from a seed toward the sample edge and the bottom is presented, which shows that single domain growth was realized. Similar growth morphologies were also obtained for the other four large grain samples. The X-ray diffraction pattern of one of the large grain samples is shown in Fig. 6, in which only (0 0 l) peaks are visible, indicating the single domain characteristic of the samples. The levitation forces were measured in zero field cooled (ZFC) state, using a NdFeB magnet of 50 mm in diameter, the magnetic field density on the surface is 0.5 T. Firstly, the sample was immersed in liquid nitrogen in a container below the magnet with an appropriate initial distance between them (about 60 mm). After the sample got into the superconductivity state, the magnet was driven by a controlling motor towards to the sample. The central axes of the samples and magnet were fixed on the same line during the levitation force measurements. In the process of magnet moving toward the sample, the distance between
Fig. 4. SEM photographs of the cross section for 40 mol% Y211 added samples with different initial temperature of slow-cooling: (a) 1005 °C, (b) 997 °C.
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Fig. 5. Photographs of a large single domain YBCO bulk 53 mm in diameter.
Intensity (arb.unit)
250000
299 N, 276 N, 276 N and 274 N, respectively, for these five large grains. Fig. 7 shows the levitation forces versus the distance between the YBCO bulks and the magnet for two samples (070210A and 070216A) among these five large grains, from which it can be found that the repulsion forces range from 88 N and 94 N when the distance between the two surfaces of the sample and magnet is 10 mm. Hence, all our large samples exhibit robust levitation ability.
(006)
200000 (005)
150000 100000 (003)
(007)
50000 (002)
(004)
4. Conclusion
0 20
40
60
80
2θ (degree) Fig. 6. X-ray pattern measured on the top surface of large YBCO single domain.
the sample and magnet was measured by a displacement sensor, and the levitation force is measured by the pressure sensor. The maximum levitation forces were taken at the gap about 0.2 mm wide between the two nearest surfaces of the sample and the magnet, and they are 302 N, 350
Levitation force (N)
300
The effect of Y2BaCuO5 content and initial temperature of the slow-cooling process on the growth of YBa2Cu3O7 d bulk has been investigated in this paper. A higher nucleation temperature can be obtained in the sample with 20 mol% and 30 mol% Y211 addition. However, samples with 40 mol% Y211 get a larger growth rate. In addition, it seems that the crystal growth of YBCO is extremely sensitive to the initial temperature of slow-cooling. Therefore, in order to grow large single grain bulk, an optimal initial temperature of slow-cooling process should be chosen according to the Y211 phase content. Based on these important data, a group of large YBCO bulks 53 mm in diameter has been successfully produced, all of which present excellent magnetic levitation performance in the axial direction. Acknowledgments
250 200
This work was supported by the ‘‘Shanghai Academic Discipline Project, No. 06ZZ01”, and ‘‘Shanghai Leading Academic Discipline Project, Project No. T0104”.
150 100
070216A
50
References
070210A
0 -50 0.1
1 10 Distance (mm)
100
Fig. 7. The levitation forces versus distance at 77 K for two large samples with the maximum levitation force are 302 N and 274 N, respectively.
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[14] K. Inoue, S. Nariki, M. Murakami, Physica C 378–381 (2002) 755. [15] W. Lo, D.A. Cardwell, C.D. Dewhurst, S.L. Dung, J. Mater. Res. 11 (1995) 786. [16] L. Xiao, H.T. Ren, Y.L. Jiao, M.H. Zheng, Y.X. Chen, Physica C 386 (2003) 262. [17] R. Tournier, E. Beaugnon, O. Belmont, X. Chaud, D. Bourgault, D. Isfort, L. Porcar, P. Tixador, Supercond. Sci. Technol. 13 (2000) 886. [18] E. Sudhakar Reddy, N. Hari Babu, K. Iida, T.D. Withnell, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) 64. [19] D.A. Cardwell, N. Hari Babu, W. Lo, A.M. Campbell, Supercond. Sci. Technol. 13 (2000) 646. [20] Hiroyuki Fujimoto, IEEE Trans. Appl. Supercond. 15 (2005) 3098. [21] C.J. Kim, I.H. Kuk, G.W. Hong, T.H. Sung, S.C. Han, J.J. Kim, Mater. Lett. 34 (1998) 392. [22] Y. Shiohara, A. Endo, Mater. Sci. Eng. R19 (1997) 1.
Physica C 468 (2008) 435–441 www.elsevier.com/locate/physc
Effects of Y2BaCuO5 content and the initial temperature of slow-cooling on the growth of YBCO bulk Xing-da Wu *, Ke-Xi Xu, Jing-he Qiu, Peng-jun Pan, Keran Zhou Physics Department, Shanghai University, Shangda Road 99, Shanghai 200444, People’s Republic of China Received 20 November 2007; received in revised form 8 January 2008; accepted 15 January 2008 Available online 8 February 2008
Abstract In order to grow large mono-domain YBa2Cu3O7 d bulk in the tubular furnace, the effect of Y2BaCuO5 (Y211) content and initial temperature of the slow-cooling process on the growth of YBa2Cu3O7 d (Y123) bulk has been investigated. Single grain Y123 bulk samples with 20 mol% and 30 mol% Y211 addition have higher original crystallization temperature than that with 40 mol% Y211; however, the melt-textured growth rate of the single domain seems to be faster for the system with higher Y211 concentrations. In addition, it seems that the crystal growth of YBCO is extremely sensitive to the initial temperature of slow-cooling. The optimal initial temperature of slow-cooling process, which should be chosen according to the Y211 phase content, has been brought to the fore in the growth of large grain bulk. The above data is very useful for the fabrication of large grain (RE)–Ba–Cu–O bulk, and a group of YBCO single domain samples with diameter of 53 mm have been successfully produced, all of which present excellent magnetic levitation performance in the axial direction. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 74.81.Bd; 74.62.Bf Keywords: Y2BaCuO5 content; The initial temperature of slow-cooling; Large grain YBCO
1. Introduction Yttrium-based high temperature single domain bulk superconductors prepared by top-seeded melt-texturing growth (TSMTG) method show strong levitation force on a permanent magnet and the ability to trap high magnetic fields, which are attractive for various engineering applications, for example in magnetic bearings (flywheels energy storage system, etc.) [1–3] or as trapped flux magnets (magnetic separation device, etc.) [4]. The levitation force and the field trapping capability of the bulk superconductors is related to the critical current density (Jc) and the current loop size (d). Jc can be enhanced by increasing the flux pinning by means of doping [5,6] or irradiation [7,8]. If *
Corresponding author. Tel.: +86 21 66132515. E-mail addresses: [email protected] (X.-d. Wu), kxxu@staff.shu. edu.cn (K.-X. Xu). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.01.007
the Jc properties were equal in each sample and if the geometric effect could be ignored, the levitation force and the trapped field of the bulk superconductors would be proportional to the radius of the sample. Therefore, an enlargement of the grain size is very important for the applications of these bulk samples. It is well known that the TSMTG method is a very effective process to fabricate YBCO mono-domains, in which the Y123 domain grown from a seed is promoted by peritectic recombination of the Y211 particles and the Ba- and Cu-rich liquid phases. However, it is difficult to grow large sample with high performance mainly for the following two reasons. The first one is a slow dissolution rate of coarsening Y211 particles in the melt, which results in reduction of the growth rate or even earlier termination of crystal growth [9,10]. The other one is the temperature window for the stable growth of single grain monoliths, which is relatively narrow [11]. In this work, we will focus our investigation on the effect of Y211
X.-d. Wu et al. / Physica C 468 (2008) 435–441
inclusions as well as the slow-cooling stage, since both of them play significant role in the growth of large grain bulk. Influence of Y211 particles on the microstructure or superconducting properties of an YBCO single domain has been widely studied. It has been well known that the residual solidified liquid which is deleterious to the superconducting properties can be obviously reduced by adding extra Y211 particles deliberately. Generally around 10– 40 mol% of extra Y211 particles are added in the system [9,11,12]. It has also been known that the Y211 particles in the final single domain have a positive effect on the superconducting properties as the Y123/Y211 interfaces are good pinning centers [13,14]. However, effect of Y211 content on the initial crystallization behavior of YBCO is still unrevealed. The slow-cooling process is another essential parameter that has to be controlled in order to achieve a large grain sample with high performance. There is a small temperature window exists between the temperature where the heterogeneous nucleation is promoted from the seed and the temperature where a homogeneous nucleation appears [15]. It is important to carefully control the slow-cooling process in order to keep the system in this temperature window [16,17]. It has also been reported that slow-cooling rate is a crucial factor affecting the microstructure, texture growth and superconducting properties of the sample [12,16]. However, not enough attention has been paid to effects of the initial temperature of slow-cooling on the growth of YBCO single grains. In this paper, influence of Y211 content and the initial temperature of slow-cooling on the growth of melt-texture single domain bulk have been studied using four batches of Y123 pellets with 20 mol%, 30 mol% and 40 mol% Y211 addition. The first two batches of specimens were quenched at the temperatures from which the initial growth process is suggested to start, in order to observe the influence of Y211 content on the growth process of the sample. The next two batches of specimens were processed in the same tubular furnace with two different thermal processes to study the influence of initial temperature of the slow-cooling on the growth of the sample. Finally, based on the data acquired from these four batches of specimens, a group of large grain YBCO bulk with diameter of 53 mm and high magnetic levitation performance has been successfully fabricated.
The mixtures were uniaxially pressed into small disks (20 mm in diameter and 12 mm in thickness), hexagonshaped pellets (with a face diagonal length of 38 mm and a thickness of 15 mm), and large disks (60 mm in diameter and 20 mm in thickness), respectively. Small SmBaCuO (Sm123) crystals prepared by the TSMTG process were used as seeds, and were centrally placed on top of the pellets before melt processing. Nine batches of samples were processed in all, for which the heat schedules are shown in Fig. 1. The precursor pellets were heated to 940 °C at a rate of 100 °C h 1, held for 5 h, and then heated quickly to 1050 °C and dwelled for 1.5–3 h according to the volume of the pellets, from where they were cooled to T1 at a rapid cooling rate, and then slowly cooled to T2, followed by the quenching at a rate of about 270 °C h 1 to room temperature. The temperatures T1 and T2, as well as the slow-cooling rates, are different from batch to batch. Details of the heat treatment are also shown in Table 1. For comparative investigation, there were three pellets with different Y211 concentration (20 mol%, 30 mol% and 40 mol%, respectively) in each batch from Batch 1 to 2, and there were two pellets with 30 mol% and 40 mol% Y211, respectively, in Batch 3 and 4. As for the Batch 5–9, only one large disk with 30 mol% Y211 added was processed in each batch, which was used for evaluating the efficiency of the process method and the magnetic properties such as levitation force.
1050ºC
T1
Temperature
436
T2
940ºC
Time Fig. 1. Heat treated schedules for nine batches of samples.
2. Experimental details YBa2Cu3O7 d and Y2BaCuO5 precursor powders were fabricated through repeating calcinations and grinding of stoichiometric mixtures of high-purity Y2O3, BaCO3 and CuO at 950–970 °C. The precursor powders were mixed thoroughly according to a nominal composition of Y123 + xY211 in a molar ratio with x = 0.2, 0.3 and 0.4. To reduce the size of Y211 precipitate particles dispersed in the melt-processed superconducting matrix, 0.5 wt.% CeO2 was also added.
Table 1 Important parameters of the heating cycles for each batch Batch number
T1 (°C)
T2 (°C)
Slow-cooling rate (°C h 1)
Sample types
1 2 3 4 5–9
1010 1010 1005 997 997
997 994 976 976 973
0.3 0.3 0.3 0.3 0.18
Small disks Small disks Hexagon-shaped pellets Hexagon-shaped pellets Large disks
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3. Results and discussion 3.1. Influence of Y211 content on single domain growth Fig. 2 shows top views of the small size disc samples: Samples (a) and (d) with Y211 inclusions of 20 mol%, samples (b) and (e) with Y211 inclusions of 30 mol%, and samples (c) and (f) containing 40 mol% Y211. Samples (a), (b) and (c) which belong to the first batch were melt-processed in a tubular furnace and quenched from 997 °C, while samples (d), (e) and (f), belonged to the second batch processed in the same furnace, were quenched from 994 °C. First of all, it is easily to find that the morphologies of samples (a), (b) and (c) are significantly different from each other. Sample (b) displays a poly-grain mode with many Y123 grains randomly distributed regardless of the seed orientation. Sample (a) presents an appearance quite similar to that of sample (b), but a small Sm123-seeded grain can be seen at the center. As for sample (c), only several grains distribute on the surface, and no single domain grown from the seed can be seen. It is safe to conclude that the difference among these three samples is attributed to the different content of Y211 in each of them, since all of them are processed in the same condition. It is known that the seeded growth process mainly consists of three steps: the peritectic decomposition process which occurs above the peritectic temperature Tp, the onset of heterogeneous nucleation around the seed when the system is cooled down below a certain temperature denoted as Ts, and the subsequent directional solidification process during slow-cooling stage. There is a thermodynamic nucleation barrier window within which the Y123 phase grows from the position of the seed crystal without compe-
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tition from grains nucleated homogeneously elsewhere in the incongruent melt [18]. When the melt is cooled down to Ts, the barrier against nucleation around the seed is low enough and heterogeneous nucleation takes place. Cooling down the melt continually, the nucleation barrier for the whole system reduces further and the peritectic recombination of Y123 may be accelerated. However, multiple grain nucleation would be caused if the under cooling was so large that the barrier was too low to avoid spontaneous nucleation in the system. According to our analysis presented above, it is natural to think that the reason why no textured single domain around the seed can be seen in samples (a), (b) and (c) may be that the quench temperature (997 °C) is above the temperature Ts. The melt processing was interrupted prior to the onset of nucleation or growth around the Sm123 seed. But the nucleation and growth of Y123 grains might also occur during the initial stage of quenching process if the system was quenched from the temperature just above Ts. With the nucleation barrier in the system rapidly reducing, lots of nucleation site besides the position of seed occurred and each grain grew competitively at the same time, resulting in the case of samples (a) and (b). As for sample (c), a pre-solidification morphology is presented except for several Y123 grains taken place, which implies that the heterogeneous nucleation temperature Ts of this sample is still far below 997 °C, and that the spontaneous nucleation temperature is also lower than that in samples (a) and (b). Therefore, an assumption can be made that system with 20 mol% and 30 mol% Y211 addition has a higher nucleation temperature than that with 40 mol% Y211 addition, although the mechanism should be unveiled in further investigation.
Fig. 2. Top views of the six small disks with Y211 inclusions of 20 mol% for (a) and (d), 30 mol% for (b) and (e), 40 mol% for (e) and (f), and quenched from 997 °C for (a), (b) and (c), from 994 °C for (d), (e) and (f), respectively.
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It is interesting to observe the growth modes of the systems quenched from the temperature below Ts. The second batch consisting of samples (d), (e) and (f) with 20 mol%, 30 mol% and 40 mol% Y211, respectively, was quenched from a lower temperature 994 °C, and the top views of which have been shown in Fig. 2. All of them are polycrystalline, but a prominent Sm123-seeded YBCO grain at the centre can be seen, which suggests that heterogeneous nucleation and growth of single domain have taken place in the temperature range from 997 °C to 994 °C. In addition, the size of the seeded single domain of sample (f) seems to be equal to that of sample (e), which indicates a higher growth rate in sample (f), since the 40 mol% Y211 system has a lower nucleation temperature as suggested above and hence it has a shorter effective growing time comparing with sample (e). As for the sample (d), the seeded YBCO grain is smaller than that of samples (e) and (f). This result indicates that the slowest growth rate in sample with 20 mol% Y211 addition, since sample (d) has the highest crystallization temperature. Effect of RE2BaCuO5 (RE211) content on growth rate has also been studied in other REBaCuO systems [19,20]. Generally the growth rate increases as the RE211 content increases. This can be explained as follows. As the RE211 content increases, the inter-particle distance decreases and the diffusion distance of RE ions through the liquid phase reduces, resulting in a large growth rate.
3.2. Influence of the initial temperature of slow-cooling on single domain growth Photographs of the four hexagon-shaped pellets of Batch 3 and 4 are shown in Fig. 3. The slow-cooling process started at 1005 °C for samples (a) and (b), and at 997 °C for samples (c) and (d). Samples containing varying amounts of added Y211 have also been used for comparative observations, with 30 mol% for samples (a) and (c), and 40 mol% for samples (b) and (d). It can be observed from Fig. 3 that systems with different Y211 concentration are extremely sensitive to the initial temperature of slowcooling process. At a first glance it may appear incredible that single grains grown in a larger temperature range (slow-cooling from 1005 °C to 976 °C) have a smaller size than that grown in a narrower one (slow-cooling from 997 °C to 976 °C). Actually, the effective growing time is the same for these two batches, since the initial nucleation temperature locates at or below the temperature of 997 °C as suggested in the previous section. But this is still not enough to explain the phenomenon, and there must be some other mechanisms. It is well known that the Y211 particles are subjected to the Oswald ripening phenomenon at high temperature in the melt [21]. In the Oswald ripening process, the smaller Y211 particles are dissolved in the melt, and dissolved ions
Fig. 3. Top surfaces of YBCO grains with Y211 inclusions of 30 mol% for (a) and (c) and 40 mol% for (b) and (d), and with two different initial temperature of slow-cooling which is 1005 °C for (a) and (b) and 997 °C for (c) and (d), respectively.
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migrate to grow the biggest particles. The coarsening of Y211 inclusions is enhanced when the delayed time at elevated temperature is long. In particular, in the case of a large volume fraction of Y211 in the liquid, the coarsening rate becomes larger. Large Y211 inclusions cause difficulty in completion of the peritectic reaction, resulting in the possibility of the coexistence of low-temperature stable phases (BaCuO2 and CuO) because diffusion through the solid is much slower [22]. As for the two samples of Batch 3, the coarsening of Y211 particles had been enhanced in liquid phase during the initial stage of slow-cooling process (from 1005 °C to 997 °C). Then when the systems were cooled down to the temperature from which heterogeneous nucleation and growth of single grain started, the growth rate of Y123 crystal would be limited due to the small rate of yttrium diffusion in the liquid. The situation seems to be more severe for sample (b) with higher Y211 concentration, which has a smaller size of single domain. In contrast, when the initial temperature of slow-cooling is set at 997 °C, a temperature very near the initial nucleation one, perfect single domain like samples (c) and (d) can be obtained. In order to confirm that different growth morphologies between Batch 3 and Batch 4 is caused by the different delayed time at elevated temperature, and the longer one has enhanced the coarsening of Y211 inclusions which has a negative effect on the growth of single grain, the microstructure of samples (b) and (d) in Fig. 3 was observed with a scanning electron microscope (SEM). Fig. 4 shows the typical microstructure of these two samples with different initial temperature of slow-cooling: (a) 1005 °C, (b) 997 °C. The light gray inclusions are Y211 particles embedded in the superconducting matrix. It turns out that the size of Y211 inclusions in these two samples is very different. In Fig. 4a, the Y211 inclusions are distributed non-uniformly and some of which have grown to a very large size. While in Fig. 4b, the Y211 inclusions are dispersed in the Y123 matrix much more uniform, and no large Y211 particles can be found. These results suggest that delayed time at elevated temperature should be shorted by selecting an appropriate initial temperature of slow-cooling.
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3.3. Growth and levitation properties of large single domains The experiments above have provided important information which is very useful for the growth of large YBCO single domain. Firstly, the initial nucleation temperature can be adjusted by changing the content of Y211 in precursor powders. Secondly, the crystal growth of YBCO single grain is extremely sensitive to the initial temperature of slow-cooling process. Finally, optimal initial temperature of slow-cooling process should be chosen according to the Y211 phase content in order to grow large Y123 grains. Based on these data, five sets of large grain samples with 30 mol% Y211 additions have been successfully produced in the tubular furnaces which have been calibrated, using the parameters of the heating cycles shown in Fig. 1 and Table 1. It should be emphasized here that a slower cooling rate (0.18 °C h 1) and wider temperature range of slowcooling process have been used, since the growth time needed for large sample is much longer than that of small ones. Fig. 5 shows the photographs of one of the large grain samples 53 mm in diameter and 15 mm in thickness. Well-grown grain from the seed crystal with fourfold growth sectors extending from a seed toward the sample edge and the bottom is presented, which shows that single domain growth was realized. Similar growth morphologies were also obtained for the other four large grain samples. The X-ray diffraction pattern of one of the large grain samples is shown in Fig. 6, in which only (0 0 l) peaks are visible, indicating the single domain characteristic of the samples. The levitation forces were measured in zero field cooled (ZFC) state, using a NdFeB magnet of 50 mm in diameter, the magnetic field density on the surface is 0.5 T. Firstly, the sample was immersed in liquid nitrogen in a container below the magnet with an appropriate initial distance between them (about 60 mm). After the sample got into the superconductivity state, the magnet was driven by a controlling motor towards to the sample. The central axes of the samples and magnet were fixed on the same line during the levitation force measurements. In the process of magnet moving toward the sample, the distance between
Fig. 4. SEM photographs of the cross section for 40 mol% Y211 added samples with different initial temperature of slow-cooling: (a) 1005 °C, (b) 997 °C.
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Fig. 5. Photographs of a large single domain YBCO bulk 53 mm in diameter.
Intensity (arb.unit)
250000
299 N, 276 N, 276 N and 274 N, respectively, for these five large grains. Fig. 7 shows the levitation forces versus the distance between the YBCO bulks and the magnet for two samples (070210A and 070216A) among these five large grains, from which it can be found that the repulsion forces range from 88 N and 94 N when the distance between the two surfaces of the sample and magnet is 10 mm. Hence, all our large samples exhibit robust levitation ability.
(006)
200000 (005)
150000 100000 (003)
(007)
50000 (002)
(004)
4. Conclusion
0 20
40
60
80
2θ (degree) Fig. 6. X-ray pattern measured on the top surface of large YBCO single domain.
the sample and magnet was measured by a displacement sensor, and the levitation force is measured by the pressure sensor. The maximum levitation forces were taken at the gap about 0.2 mm wide between the two nearest surfaces of the sample and the magnet, and they are 302 N, 350
Levitation force (N)
300
The effect of Y2BaCuO5 content and initial temperature of the slow-cooling process on the growth of YBa2Cu3O7 d bulk has been investigated in this paper. A higher nucleation temperature can be obtained in the sample with 20 mol% and 30 mol% Y211 addition. However, samples with 40 mol% Y211 get a larger growth rate. In addition, it seems that the crystal growth of YBCO is extremely sensitive to the initial temperature of slow-cooling. Therefore, in order to grow large single grain bulk, an optimal initial temperature of slow-cooling process should be chosen according to the Y211 phase content. Based on these important data, a group of large YBCO bulks 53 mm in diameter has been successfully produced, all of which present excellent magnetic levitation performance in the axial direction. Acknowledgments
250 200
This work was supported by the ‘‘Shanghai Academic Discipline Project, No. 06ZZ01”, and ‘‘Shanghai Leading Academic Discipline Project, Project No. T0104”.
150 100
070216A
50
References
070210A
0 -50 0.1
1 10 Distance (mm)
100
Fig. 7. The levitation forces versus distance at 77 K for two large samples with the maximum levitation force are 302 N and 274 N, respectively.
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