Scripta Materialia 116 (2016) 36–39
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Polythermal method of Top-Seeded Solution-Growth for large-sized single crystals of REBa2Cu3O7 − δ Hui Xiang a,1, Linshan Guo a,1, Haochen Li a, Xiangxiang Cui a, Jun Qian a, Ghulam Hussain a, Yan Liu a, Xin Yao a,b,⁎, Qunli Rao c, Z.Q. Zou c a State Key Lab for Metal Matrix Composites, Key Lab of Artificial Structures & Quantum Control (Ministry of Education), Dept. of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b Collaborative Innovation Center of Advanced Microstructures, Nanjing, China c Instrumental Analysis Center, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
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Article history: Received 27 December 2015 Received in revised form 20 January 2016 Accepted 23 January 2016 Available online 5 February 2016 Keywords: Czochralski technique High-temperature superconductors Crystallization Metastable phases Polythermal
a b s t r a c t To overcome demerits of the low production efficiency in the prior art isothermal method (constant supercooling) of Top-Seeded Solution-Growth (TSSG), we applied the polythermal method (increasing supercooling), and a large YBa2Cu3O7 − δ single crystal with a dimension of a × b × c = 13.8 × 13.5 × 12.3 mm was obtained via a cooling rate of 0.5 K/h, gaining a growth rate nearly 2 times faster than that by constant supercooling. The optimal growth mode was proposed for more REBa2Cu3O7 − δ (RE = Sm, Nd) materials basing on their characteristics of solubility curves. © 2016 Elsevier Ltd. All rights reserved.
For deep investigations on superconductivity, large single crystals of REBa2Cu3O7 − δ (RE123, RE = Y, Sm, Nd) with good quality have been highly requested towards and provided from our crystal growth lab, achieving excellent collaborative work recently [1–8]. As an advanced and classic method, Top-Seeded Solution-Growth (TSSG) is of significant importance in producing sizeable and superior crystals of RE123 materials for fundamental studies and device applications [9–13]. In the former case, the Y123 single crystal is preferable to other RE123 ones, particularly, for the investigation on magnetic order and related phenomena by the neutron scattering experiment [3,12], in which Sm123 and Nd123 are not suitable due to the neutron absorption and cation off-stoichiometric effect [14–18]. By modifying conventional TSSG, the solute-rich-liquid crystalpulling method (SRL-CP) was successfully developed to produce constantly high supersaturation [19–21], in which Y2BaCuO5 (Y211) powder was settled at the bottom of the crucible as Y supply source and a temperature gradient in the solution was kept between the surface and the bottom to induce the Y transportation to the surface via natural and forced convections. Thus the large-sized Y123 single
⁎ Corresponding author at: State Key Lab for Metal Matrix Composites, Key Lab of Artificial Structures & Quantum Control (Ministry of Education), Dept. of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China. E-mail address:
[email protected] (X. Yao). 1 Co-first author.
http://dx.doi.org/10.1016/j.scriptamat.2016.01.037 1359-6462/© 2016 Elsevier Ltd. All rights reserved.
crystal with a size of 14.5 × 14. mm2 in the a–b plane and 13 mm in the c-axis direction (for short, 14.5 × 14.5 × 13 mm3) was gained via a long growth period of 214 h [23]. In comparison with the 23 × 22 × 19 mm3-sized Sm123 one grown via 146 h [24] and the 24 × 24 × 21 mm3-sized Nd123 one grown via 67 h [25], it is clear that the large Y123 single crystal is more difficult to prepare due to its slow growth rate caused by the low Y solubility in the Ba–Cu–O melt and the steep liquidus slope near the peritectic temperature (Tp) [26]. Furthermore, it was found that the Y solubility increased with increasing oxygen partial pressure [27,28]. Thus the largest Y123 single crystal was fabricated under 1 atm. oxygen pressure with an area 19.8 × 19.5 mm2 in the a–b plane and 16.5 mm in the c-axis direction [14,27], possessing a growth rate nearly 2 times faster than that under air atmosphere. In this technique, the control of pO2 needs a supporting equipment and much more cost. For growing RE123 single crystals, the modified TSSG process has been comprehensively studied, where the isothermal method (constant supercooling) was commonly used [19–23]. However, as a conventional technology, TSSG is less investigated. Particularly, there have been no reports on the polythermal method (increasing supercooling), in which the temperature is slowly lowered during crystal growing. In this work, the polythermal method was applied to promote the crystal growth. A 13.8 × 13.5 × 12.3 mm3-sized Y123 single crystal was produced with a growth rate nearly 2 times faster than that in the isothermal method. Additionally, considering the supersaturation in initial
H. Xiang et al. / Scripta Materialia 116 (2016) 36–39
and following growth stages, the optimal growth mode was proposed for various RE123 materials on the basis of their characteristics of solubility curves. The experiments were carried out in air-processed TSSG. The apparatus contained a furnace to melt the solvent 3BaO-5CuO and a rod to connect the seed. The rod acts as a heat leak, making the seed slightly cooler than the nearby liquid. The growth initiated on the film seed Y123, which was coated on the MgO substrate, with the size of 3 × 3 mm2. The powder 3BaO-5CuO was acquired through a solid state reaction by mixing the BaCO3 and CuO powders in the ratio of Ba:Cu = 3:5. Then the mixture of raw powders was calcined at 900 °C in air for 48 h. The RE2O3 crucible was used to provide the RE solute and reduce contamination. The solvent in the RE2O3 crucible was first heated from the room temperature (Tr) to a high temperature (Th) which is usually 10–30 K higher than Tp to melt the 3BaO-5CuO solvent in 12 h. After the solution was kept at Th for about 40 h till all the solvents were molten, the liquidsurface temperature was calibrated by a Pt-Rh thermocouple at the growth temperature (Tg). The suitable Tg is several K lower than Tp. The rotation rate of the crystal growth was normally set as 100 rpm to facilitate the forced convection, transporting the solute ions to the growth interface. The difference between the isothermal and polythermal methods is how to conduct the supercooling from Tg. In the prior art isothermal method, temperature was held at Tg to keep constant supercooling. While in this study, after holding at Tg for several hours, a polythermal method was applied to conduct increasing supercooling from Tg to the end temperature (Te). The brief temperature– time profile for the growth of RE123 single crystals is presented in Fig. 1. X-ray diffraction (XRD) measurement was conducted by a D8 Advance Da Vinci diffractometer (Bruker Co. Ltd., German) with Cu Kα radiation (λ = 1.5418 Å) at a tube voltage of 40 kV and a tube current of 40 mA. The XRD spectrum was collected with a scanning speed of 1° min−1 within the 2θ range of 10–60°. After a specimen with approximate dimensions of 2.0 × 2.0 × 1.0 mm3 was cut from the as-grown crystal and oxygenated at 500 °C for 72 h, the critical transition temperature (Tc) was detected by a Quantum Design PPMS vibrating sample magnetometer. The sample was cooled from 300 K to 70 K without an applied field, then, a magnetic field of 20 Oe was applied and the magnetization of the sample was measured on warming. The polythermal method of growing Y123 single crystals was conducted with the cooling rate ranging from 0.5 K/h to 1.0 K/h. Fig. 2 displays the growth time dependence of the Y123 single crystal length along the c-axis direction under various conditions, including the growth method, the cooling rate and the atmosphere. It is clear that in the polythermal method, the growth rate increases with increasing the cooling rate. When the cooling rate is up to 1.0 K/h, the average growth rate is nearly 3 times larger than that by constant cooling and
Fig. 2. Growth time dependence of the Y123 single crystal length along the c-axis direction under various conditions, including the growth method, the cooling rate and the atmosphere.
close to the growth rate in the pure O2 environment by the SRL-CP method [24]. It should be notice that the growth period cannot last for a long time with high cooling rate because the solution will quickly reach the unstable region, under which circumstance, a spontaneous nucleation catastrophe occurs in the solution, leading to the polycrystals growth rather than single crystal growth. Fig. 3(a) and (b) shows the top and side morphologies of a large Y123 single crystal grown in the polythermal method at the cooling rate of 0.5 K/h. The sample exhibits a typical pyramid shape, with the bottom a–b plane of 13.8 × 13.5 mm2, and a height of 12.3 mm in the c-axis. The growth rate in the c-axis direction is about 0.07 mm/h, which is faster than that by constant cooling. In short, increased supercooling has a profound effect in facilitating the crystal growth. Fig. 3(c) shows the XRD pattern of the aforementioned crystal on the a–b plane, presenting its well c-axis-oriented feature. Fig. 3(d) shows the temperature dependence of normalized magnetization for the sample grown in air by the polythermal method after oxygenating at 500 °C for 72 h. An external field of 20 Oe was applied parallel to the c-axis from 80 to 100 K. A high Tc of 92 K with a sharp transition width within 1 K is clearly exhibited. To prepare single crystals, an appropriate supersaturation which is defined as Δ c = c - c0 of the solution is required as the growth driving force, where c and c0 are the actual and the equilibrium concentration of the solute, respectively. There are many ways to produce supersaturation, such as by supercooling of the solution, by a temperature gradient, by additional solute and by evaporation of solvent [29]. In TSSG, the furnace is arranged to produce a well-defined temperature gradient, and the temperature in the vicinity of the melt surface is maintained near the temperature for the onset of solidification. The seed is attached to a support rod working as a heat leak, which makes the seed slightly cooler than the nearby liquid. The growth initiates on the seed [30]. Conducting the polythermal method in this work, the temperature was slowly lowered to induce an additional supersaturation Δcadd continuously and steadily, as shown in Eq. (1). Δcadd ¼
Fig. 1. Temperature—time profile for the growth of RE123 single crystals. In the prior art isothermal method, the temperature was usually held at Tg to keep constant supercooling. In this work, after holding at Tg for several hours, the polythermal method is applied to conduct increasing supercooling from Tg to Te.
37
∂c ∂T
ΔT add
ð1Þ
T
∂c ÞT is the temperature coefficient of solubility (the t.c.s value), where ð∂T and ΔTadd is the increased supercooling. Due to the slow cooling process, the above equation can be divided by time Δt on the both side. Then
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H. Xiang et al. / Scripta Materialia 116 (2016) 36–39
Fig. 3. Y123 single crystal grown in air by the polythermal method at the cooling rate of 0.5 K/h. (a) The size of the a-b plane is 13.8 × 13.5 mm2. (b) The length in the c-axis direction is 12.3 mm. (c) The XRD pattern of Y123 single crystal on the a–b plane. (d)Temperature dependence of magnetization for the Y123 single crystals grown in air by the polythermal method.
the left side ΔcΔtadd is called Jadd, which represents the additional solute flux for the crystal growth. Accordingly, on the right side, which represents the cooling rate, as illustrated in Eq. (2). J add ¼
Δcadd ¼ Δt
ΔT add Δt
∂C ΔT add ∂c ¼ R Δt ∂T T ∂T T
is R,
ð2Þ
So with the adoption of increasing supercooling, an additional solute flux is gradually and gently supplied, providing a continuous supersaturation for the growth of Y123 single crystals. It is clear that the polythermal process is an efficient and convenient method to enhance the growth rate. Furthermore, the cooling rate should be set in a suitable range. Below the cooling rate of 0.5 K/h, it seems to have no obvious effect in stimulating the growth. On the other hand, above the cooling rate of 1.5 K/h, the solution may quickly shift from the metastable into the unstable region, leading to the spontaneous nucleation. At the beginning of the growth process, the initial supersaturation is sufficiently obtained by the cooling from Th to Tg. In order to study the effect of the initial cooling on the growth rate of Y123 single crystals, temperature was cooled down from various Th (1282 K, 1296 K and 1308 K, respectively) to Tg. As shown in Table 1, with increasing Th, the growth rates are enhanced from 0.042 to 0.075 mm/h in the isothermal method while from 0.065 to 0.0875 mm/h in the polythermal
method at the cooling rate of 0.8 K/h. In combination with the liquidus of Y in the Ba3Cu5O8 solution under air atmosphere, it can be deduced that the larger initial cooling is, the higher supersaturation is obtained. We can draw the conclusion that the higher initial cooling enhances of the growth rate. On the other hand, overlarge cooling leads to the spontaneous nucleation in the solution because of the excessive driving force, especially for the growth of Sm123 and Nd123 single crystals due to the smooth liquidus slope of Sm and Nd near Tp. The polythermal method plays a critical role in facilitating the growth rate of Y123 single crystal, meanwhile it also works in the growth of large-sized Sm123 and Nd123 single crystals. The average growth rate of air-processed Nd123 in the polythermal method at the
Table 1 The growth rates of Y123 single crystals in air corresponding to different initial cooling in the isothermal and the polythermal (R = 0.8 K/h) methods, respectively. Initial cooling (K)
14
28
40
Growth rate (mm/h) in the isothermal method Growth rate (mm/h) in the polythermal method with R = 0.8 K/h
0.042 0.065
0.0625 0.085
0.075 0.0875
Fig. 4. The growth rates of Y123 and Nd123 single crystals corresponding to the cooling rate in air. The inset is the temperature dependence of Y, Sm and Nd solubilities in the Ba3Cu5O8 solution under air atmosphere.
H. Xiang et al. / Scripta Materialia 116 (2016) 36–39 Table 2 The suitable growth modes for various RE123 materials on the basis of their characteristics of solubility curves. Rare Earth element
Y
Sm
Nd
Peritectic temperature (Tp) (K) Suitable growth temperature (Tg) (K) Solubility of RE at Tp (at. %) Liquidus slope of RE123 at Tp (K/at.%) Temperature Coefficient of Solubility (t.c.s)(/K) Suggested range of cooling rate (K/h)
1278 ± 5 1268 0.6 90.8 1.10 × 10−2
1333 ± 5 1330 1.8 33.3 3.00 × 10−2
1359 ± 5 1354 3.2 26.3 3.82 × 10−2
0.5–1.5
0.2–0.5
0.2–0.4
cooling rate of 0.25 K/h is 2 times larger than that in the isothermal method. Besides, the growth rate of Nd123 single crystals is much faster than Y123 as shown in Fig. 4. It is worth mentioning that the cooling rates in the polythermal method for the growth of Sm123 and Nd123 single crystals are suggested less than that of Y123 because the solubility of Sm and Nd in the Ba–Cu–O melt is higher and the liquidus slope of Sm and Nd near Tp is smoother [20]. As shown in the inset of Fig. 4, the solubilities of Sm and Nd are several times higher than that of Y near Tp, so that the initial supersaturation is sufficient to provide large driving force to grow Sm123 and Nd123 single crystals. Moreover, near and below Tp, the liquidus slopes of Sm and Nd are smoother, which ∂c ÞT of Sm and Nd are larger than that of Y, so that the cooling means ð∂T rate should be set lower, to prevent a large Jadd and a following spontaneous nucleation. The suitable growth modes proposed for various RE123 materials are comprehensively summarized on the basis of their characteristics of solubility curves [31,32], as shown in Table 2. In summary, the polythermal method was applied to induce an additional solute flux and continuously provide the driving force to grow RE123 single crystals. A large-sized Y123 single crystal with an a–b plane of 13.8 × 13.5 mm2 and a height of 12.3 mm in the c-axis direction was obtained under the cooling rate of 0.5 K/h in airprocessed TSSG. Furthermore, the optimal growth mode was proposed for various RE123 materials on the basis of their characteristics of solubility curves. Additionally, in order to maintain the high speed of crystal proceeding in the liquid metastable region, the supersaturation in the initial and the following growth stage was also taken into consideration.
Acknowledgments The authors are grateful for financial support from the MOST of China (Grant No. 2012CB821404), and NSFC (Grant No. 51172143, 51572171).
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