Growth of large Bi2Sr2CaCu2Oy single crystals by a modified vertical Bridgman method

Growth of large Bi2Sr2CaCu2Oy single crystals by a modified vertical Bridgman method

Journal of Crystal Growth 237–239 (2002) 749–752 Growth of large Bi2Sr2CaCu2Oy single crystals by a modified vertical Bridgman method H. Tanaka*, S. K...

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Journal of Crystal Growth 237–239 (2002) 749–752

Growth of large Bi2Sr2CaCu2Oy single crystals by a modified vertical Bridgman method H. Tanaka*, S. Kishida Department of Electrical and Electronic Engineering, Tottori University, 4-101, Koyama-Minami, Tottori 680-8552, Japan

Abstract We prepared Bi2Sr2CaCu2Oy (Bi-2212) single crystals by a modified vertical Bridgman method, where the single crystals were grown rotating crucibles. We investigated the dependences of crystal growth on the temperature gradients at the growth region, the starting temperatures of growth (Ts ) and the rotation speeds of the crucibles. When the Bi-2212 single crystals were grown under the temperature gradient of around 4.01C/mm, the Ts of more than 9751C and the rotation speed of about 25 rpm, large plate-like Bi-2212 single crystals with good crystallinity were obtained. r 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.h; 74.25.q; 81.10.Fq Keywords: A2. Bridgman technique; A2. Single crystal growth; B2. Oxide superconducting materials

1. Introduction Large and high-quality single crystals are usually required in measurements of accurate physical and chemical properties of high-Tc superconductors. Bi2Sr2Can1CunOy superconductors are technologically important materials and were chosen to study a member of this family, Bi2Sr2CaCu2Oy (Bi-2212). The growth of Bi-2212 single crystals has been carried out using a traveling solvent floating zone (TSFZ) method [1], a KCl flux method [2,3], a self-flux method [4–8] and a vertical Bridgman method [9–12] up to the present. However, it is difficult to obtain large Bi-2212 single crystals with high degree of crystal*Corresponding author. Tel./fax: +81-857-31-5244. E-mail address: [email protected] (H. Tanaka).

lization. In general, the control of crystal nucleation is difficult using a self-flux method, whereas it is easy using a vertical Bridgman (VB) method. As a result, large and high-quality Bi-2212 single crystals are expected to be obtained by the use of a VB method, since the seed crystals are automatically produced. In addition, we expect that the single crystal become larger with the size of crucible, in comparison with a TSFZ method. Recently, it have been reported that large size Bi-2212 single crystals have been grown by a VB method [13]. However, there are no reports regarding the effects of rotating crucibles on crystal growth. In this study, we prepared Bi-2212 single crystals by a VB method rotating crucibles (modified VB method), and clarified the optimum conditions for obtaining large Bi-2212 single crystals with good crystallinity (hereafter, high-quality Bi-2212 single crystals).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 0 0 - 0

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2. Experimental procedure The starting materials with the compositions of Bi:Sr:Ca:Cu=2:2:1:2 were prepared by mixing and grinding the powders of Bi2O3 (purity: 98.0%), SrCO3 (95.0%), CaCO3 (99.5%) and CuO (90.0%). In order to suppress the overflow of the materials from alumina crucibles at temperatures beyond the melting point of a Bi-2212 superconductor, the starting materials were calcined at 7601C in the form of a pellet. Then, the pellet was ground again and transferred into an alumina crucible. The crucible was heated in an electric furnace, and then pulled down rotating crucibles for crystal growth. We prepared Bi-2212 single crystals changing the temperature gradients at growth region, starting temperatures of growth (Ts ) and rotation speeds of crucibles. After the crucible was mechanically broken up with a vise, the single crystals were carefully collected. The X-ray diffraction (XRD) patterns and the resistance–temperature (R–T) characteristics of prepared single crystals were measured. In addition, the electron probe microanalysis (EPMA) measurement and the scanning electron microscope (SEM) observations were also carried out.

3. Results and discussion Fig. 1 shows an optical photograph of the single crystals prepared by a modified VB method. From

the photograph, we found that the plate-like single crystals grew from the bottom of the crucible to the top along the pulling direction. Table 1 summarizes the maximum sizes, the superconducting phases, the FWHM values of XRD (0 0 10) peak and the Tc (zero-resistance temperature) of single crystals. The single crystals were prepared at the temperature gradients of 1.51C/mm, 2.51C/mm and 4.01C/mm, where the mass of starting materials, the bottom angle of crucible, the pulling down rate, the Ts and the rotation speed were 25 g, 451, 1 mm/h, 9751C and 50 rpm, respectively. All single crystals were plate-like, and the largest single crystal with a size of 6  2.5 mm2 was obtained at the temperature gradients of 4.01C/ mm. Fig. 2 shows the XRD patterns of the single crystals in Table 1. As shown in the figure, the (0 0 1) XRD peaks from a Bi-2212 superconductor were observed. From the results, we found that the superconducting phases of the single crystals were independent of the temperature gradients from 1.51C/mm to 4.01C/mm. The FWHM values of the XRD line at 2y ¼ 291; which corresponds to (0 0 10) XRD peaks of a Bi-2212 superconductor, were also measured. The FWHM values of the single crystals at the temperature gradients of 2.51C/mm and 4.01C/mm were about 0.071 which was the smallest in the single crystals prepared at various temperature gradients. However, they were larger than those of high-quality YBa2Cu3Oy single crystals [14,15]. This may be caused by the microscopic intergrowth between different

20

Fig. 1. Optical photograph of the Bi-2212 single crystals prepared by a modified vertical Bridgman method.

(0012) (0012)

(0010)

30 2  (deg.)

(0012)

(008)

(c)4.0˚C/mm

(0010)

(008)

(b)2.5˚C/mm

(0010)

Intensity (arb.units)

(008)

(a)1.5˚C/mm

40

Fig. 2. X-ray diffraction patterns of the Bi-2212 single crystals prepared at the temperature gradients of (a) 1.51C/mm, (b) 2.51C/mm, and (c) 4.01C/mm.

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Table 1 Maximum sizes, superconducting phases, FWHM values of XRD lines and Tc of the BSCCO single crystals prepared at various temperature gradients Temperature gradients (1C/mm)

Maximum sizes (mm2)

Superconducting phases

FWHM values of XRD lines (1)

Tc (K)

1.5 2.5 4.0

21 2.5  1 6  2.5

2212 2212 2212

0.10 0.07 0.07

Fa 52 56

Bi2Sr2Can1CunOy superconducting phases as well as the interchange between constituent elements, which are easily produced in Bi2Sr2CaCu2Oy than in YBa2Cu3Oy superconductors. In addition, lowpurity starting materials, especially CuO (90%), may had effects on the crystallinity of single crystals. The Tc of single crystals prepared at the temperature gradients of 2.51C/mm and 4.01C/mm were 52 and 56 K, respectively. The single crystals prepared at the temperature gradient of 1.51C/mm were too small to measure their R–T characteristics. As the R–T measurements of as-grown single crystals were carried out, the Tc values were largely dependent on the oxygen content in the surface of the single crystals. This is because the Tc values are sensitive to small variations of oxygen content in the surface. The low Tc (52 and 56 K) of the single crystals may be caused by the nonoptimized oxygen content of the crystals. From the maximum sizes and the FWHM values of the (0 010) peak, we found that the optimum temperature gradient for obtaining high-quality Bi-2212 single crystals was around 4.01C/mm. Fig. 3 shows the maximum sizes and the FWHM values of the (0 010) XRD lines of the Bi-2212 single crystals. The single crystals were prepared at the Ts of 9001C, 9501C, 9751C and 10001C, where the mass of starting materials, the bottom angle of crucible, the pulling down rate, the temperature gradients and the rotation speed were 25 g, 451, 1 mm/h, 2.51C/mm and 100 rpm, respectively. As shown in the figure, we could obtain the large plate-like single crystals at the Ts of 9751C and 10001C, which were 16  4 and 13  4 mm2, respectively. Moreover, the FWHM values of (0 010) XRD lines were about 0.061 in the single crystals prepared at the Ts of 9751C and 10001C. Therefore, we concluded that the optimal Ts for high-quality Bi-2212 single crystals was

80

60

0.12

FWHM values of the XRD lines

0.08 40

20

0

0.04 Maximum sizes

900

940 Ts (˚C)

980

1020

0

FWHM values of the XRD lines (deg.)

‘‘F’’means immeasurable.

Maximum sizes (mm2)

a

Fig. 3. Maximum sizes and FWHM values of the XRD lines of the Bi-2212 single crystals prepared at various Ts :

Fig. 4. R–T characteristics of Bi-2212 single crystals prepared at the Ts of (a) 9001C, (b) 9501C, (c) 9751C, and (d) 10001C.

more than 9751C. Although the figure of the XRD patterns was not shown, the single crystals had Bi-2212 single phase. Fig. 4 shows the R–T characteristics of the single crystals of Fig. 3. The temperature dependence of resistance in the normal state showed a metallic behavior in the single crystals prepared at the Ts of 9501C, 9751C and 10001C. As shown in the figure, the Tc values of single crystals prepared

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structure of a Bi-2212 superconductor. The thickness of the single crystals was estimated to be in the range from 1 to 100 mm from the SEM images.

4. Conclusions

Fig. 5. SEM image of Bi-2212 single crystal prepared with the rotation speed of 50 rpm.

at various Ts were different from each other. This may be caused by the oxygen content of the single crystals which were not optimized or the cation composition which was different from the stoichiometry of a Bi-2212 superconductor. We also grew single crystals with the rotation speeds of 10, 25, 50 and 100 rpm, and investigated the maximum sizes, the superconducting phases, FWHM values of XRD lines and Tc : The single crystals were prepared under the following conditions; the mass of starting materials, the bottom angle of crucible, the pulling down rate, the temperature gradients and the Ts were 25 g, 451, 1 mm/h, 2.51C/mm and 9751C, respectively. Although the figure was not shown, the largest single crystal was obtained with the rotation speed of 25 rpm and its size was 19  4 mm2. In addition, the shape of single crystals grown with the rotation speed of 100 rpm was bulky. This may be caused by the centrifugal force which increased with rotation speeds. The FWHM values were independent of the rotation speed and were about 0.061. Therefore, we concluded that the optimum rotation speed for growing high-quality Bi-2212 single crystals was around 25 rpm. Fig. 5 shows the SEM image of single crystals grown with the rotation speed of 50 rpm. This image indicated that the air-cleaved surface of the single crystal was very flat, and that the layer structure was observed reflecting the crystal

We prepared Bi-2212 single crystals by a VB method rotating crucibles, and investigated the optimum conditions for obtaining high-quality Bi-2212 single crystals. We found that the size and the crystallinity of single crystals were dependent on the temperature gradients, the Ts and the rotation speeds. The optimum temperature gradients, Ts and rotation speed for high-quality Bi-2212 single crystals were around 4.01C/mm, more than 9751C and about 25 rpm, respectively.

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