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Magneto-optical and microstructural investigations on KClO3-doped YBCO HTSC
Physica C 357±360 (2001) 201±204
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Magneto-optical and microstructural investigations on KClO3-doped YBCO HTSC Anjela Koblischka-Veneva a, Michael R. Koblischka a,b,*, Masato Murakami a a
SRL/ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105, Japan b NST A/S, Priorparken 685, DK-2605 Brùndby, Denmark Received 16 October 2000; accepted 17 January 2001
Abstract The in¯uence of KClO3 addition on the magnetic properties and microstructure of YBCO HTSC materials with nominal composition (A): Y1 Ba2 x Kx Cu3 Oy , (B): Y1 0:2x Ba2 0:2x Kx Cu3 Oy , (C): Y1 Ba2 Cu3:5 x Kx Oy x 0±0:75 were investigated. Magneto-optic imaging of ¯ux distributions was employed to study the grain connectivity in the doped and undoped samples in a direct way. The obtained ¯ux patterns taken at various temperatures reveal that KClO3 doped samples x 0:30 exhibit ¯ux distributions being close to those being observed in melt-textured superconductors. This demonstrates that the grain connectivity in the doped samples is considerably improved as compared to pure ones, which is an important issue for the fabrication of coated conductors of YBCO. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.h; 74.60.Jg; 74.62.Dh Keywords: YBCO HTSC; KClO3 doping; MO imaging; Grain boundaries
The magnetic behavior of YBCO high-temperature superconductors is largely dominated by processes in the intergranular medium [1±4]; being a major obstacle to the current ¯ow. In earlier work [5,6], it was found that the presence of some alkali metal-containing additions during the preparation of YBCO aects the properties of the intergranular medium of the end product. Very recently, the critical currents across grain boundaries in YBCO thin ®lms were found to be en-
* Corresponding author. Address: Experimentalphysik, Universitaet des Saarlandes, P.O. Box 15 11 50, D-66041 Saarbruecken, Germany. Tel./fax: +49-681-302-4867. E-mail address: [email protected] (M.R. Koblischka).
hanced by means of doping by Ca [7,8]. Therefore, studies of doping of polycrystalline YBCO play an important role for the development of YBCO coated conductors. Three series of YBCO were prepared with the nominal compositions (A): Y1 Ba2 x Kx Cu3 Oy , (B): Y1 0:2x Ba2 0:2x Kx Cu3 Oy , (C): Y1 Ba2 Cu3:5 x Kx Oy where x 0, 0.20, 0.30, 0.40, 0.60, 0.75. The samples were obtained following classic ceramic technology described in Ref. [9]. The phase content, microstructure and morphology of the crystalline grains were characterized using X-ray powder diraction (XRD) and scanning electron microscopy (SEM) coupled with an electron probe microanalyzer (EPMA). The AC complex susceptibility was measured with a Lake Shore AC susceptometer (Model 7000).
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 5 - 2
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A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
In this paper, we report on results obtained by magneto-optic (MO) imaging of ¯ux distributions of samples from batch A. AC susceptibility measurements showed that the superconducting transition temperature for KClO3 -doped samples (x 0:20±0.60) was found to be higher (Tc;onset 94:5±93.8 K) than for undoped YBCO (Tc;onset 91 K) [5,6,9]. Furthermore, measurements of the DC susceptibility in ®elds between 10 mT and 7 T showed a very interesting feature: At temperatures below 20 K, the curves of the sample A (with x 0:3) exhibited a clear downturn of the DC susceptibility [10], which we attributed to a change of length scale of the shielding currents. Therefore, the MO imaging at these temperatures should show a clear dierence between the undoped and doped samples. Using SEM photographs, we carried out a statistical analysis of the grain size in the samples. Fig. 1 shows that the grain size reaches a maximum for x 0:3, and higher concentrations of the dopant x 0:60 lead to a small grain structure and changes in the grain boundaries, and thus, decoupling of the grains. Fig. 2 presents one such SEM image (sample B, x 0:40). Here, a highly homogeneous large-grain ceramic structure is achieved. The grains have the speci®c shape of parallelepipeds. Fig. 3 presents the ¯ux patterns obtained by MO imaging on the KClO3 -doped sample A with x 0:30 at three dierent temperatures, (a) T 10 K, (b) T 50 K and (c) T 77 K. At low temperatures, the sample does not exhibit the typical ¯ux pattern of a polycrystalline sample [11,12],
Fig. 1. Grain-size distribution of polycrystalline KClO3 -doped YBCO, samples (A), (B), and (C).
Fig. 2. SEM image of a polished surface; sample B with x 0:40.
where the ¯ux enters individually into the superconducting grains. Only in the lower part of the sample, a set of grain boundaries can be observed which act as channels for the magnetic ¯ux. These ¯ux patterns of the KClO3 -doped sample resemble the patterns found in melt-textured bulks [13]. With increasing temperature, the role of the grain boundaries becomes more and more dominant, which clearly indicates a reduction of the grain coupling. This behavior corresponds directly to the observations of the DC susceptibility [10]. In contrast to this doped sample, Fig. 4 presents the ¯ux pattern obtained on sample from batch A with x 0:60. The observation temperature was 50 K, and the applied magnetic ®eld was 60 mT. Here, the grain coupling is practically destroyed, and the ¯ux penetrates only into some superconducting grains; as it is expected in polycrystalline materials. The observed peculiarities resulting from adding KClO3 can be explained by changes in the morphology of grain size and grain boundaries of the microstructure of YBCO. After the second heat treatment the XRD data showed that in all cases the KClO3 -doped samples are pure 123 phase materials with only an indication of traces of CuO for the sample with x 0:40. No chlorine or other impurity phases related with chlorine or potassium are found in the KClO3 doped samples. The values of the lattice parameters of KClO3 -doped samples are comparable to the data published for the desired Y1 Ba2 Cu3 Oy orthorhombic structure for y 6:8±7 [14]. For the monophase KClO3 -doped samples (x 0:2) we used chemical analysis (iodometric titration [15]) to determine the oxygen content and we found
A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
203
Fig. 4. Flux pattern of sample A with x 0:60, which exhibits the typical pattern of a polycrystalline sample, thus indicating only a poor grain coupling. T 50 K, applied ®eld 60 mT.
Fig. 3. MO ¯ux patterns of the KClO3 -doped sample A with x 0:30 at dierent temperatures: (a) 10 K, (b) 50 K and (c) 77 K, but at the same applied ®eld of 100 mT. The magnetic ®eld is imaged as bright areas; the Meissner phase remains dark. Only in the lower part of the images, grain boundaries can be observed. The sample does not exhibit the typical ¯ux pattern of a polycrystalline sample, see e.g. Refs. [12,13], but resembles instead the pattern of melt-textured bulks. The marker is 500 mm long.
that y is 6.9±7, which is in good agreement with our XRD data (relation between crystal lattice parameters and oxygen content). We associated the in¯uence of the additive on the microstructural morphology with the presence
of a liquid phase during the sintering stage [9]. A very important microstructural result is that SEM±EPMA revealed traces of K and Cl in the grain boundaries of the samples with KClO3 additions. This observation is a clear indication of the improved grain coupling by KClO3 addition, which presents an important result for e.g. the preparation of coated conductors using YBCO. Furthermore, our result is in accordance with the observations from Ref. [7,8]. The results obtained showed that the presence of K- and Cl-containing impurities on the grain boundaries changes the intergranular coupling considerably, and, within certain concentration limits, improves the superconducting characteristics. Therefore, the doping of YBCO will be an important step for the fabrication of e.g. coated conductors of YBCO.
Acknowledgements This work is partially supported by NEDO. AV and MRK gratefully acknowledge support from STA (Japanese Science and Technology Agency) during the stay at SRL/ISTEC.
References [1] J.R. Clem, Physica C 153±155 (1988) 50. [2] V. Calzona, et al., Physica C 157 (1989) 425.
204 [3] [4] [5] [6] [7] [8] [9] [10]
A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
H. Dersch, G. Blatter, Phys. Rev. B 38 (1988) 11391. J.W. Li, et al., Phys. Rev. B 46 (1992) 9190. A. Veneva, et al., Adv. Supercond. 11 (1999) 689. A. Veneva, et al., J. Low Temp. Phys. 117 (1999) 939. A. Schmehl, et al., Europhys. Lett. 47 (1999) 110. J. Mannhart, et al., Phil. Mag. B 80 (2000) 827. A. Veneva, et al., Physica C 308 (1998) 175. M.R. Koblischka, A. Veneva, M. Murakami, Adv. Supercond. 12 (2000) 245. [11] M.R. Koblischka, R.J. Wijngaarden, Supercond. Sci. Technol. 8 (1995) 199.
[12] M.R. Koblischka, T. Schuster, H. Kronm uller, Physica C 219 (1994) 205. [13] M.R. Koblischka, T. Schuster, G. Ravi Kumar, in: H.W. Weber (Ed.), Proceedings of the 7th IWCC, Alpbach, Austria, 24±27 January, 1994, World Scienti®c, Singapore, 1994, p. 399. [14] JCPDS powder diraction, Alphabetical Indexes (1997) ®le nos. 38-1433, 39-486, 39-1434. [15] H.A. Flaschka, A.J. Barnard, P.E. Sturrock, Quantitative Analytical Chemistry, vols. 1 & 2, Barnes and Noble, NY, 1969.
www.elsevier.com/locate/physc
Magneto-optical and microstructural investigations on KClO3-doped YBCO HTSC Anjela Koblischka-Veneva a, Michael R. Koblischka a,b,*, Masato Murakami a a
SRL/ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105, Japan b NST A/S, Priorparken 685, DK-2605 Brùndby, Denmark Received 16 October 2000; accepted 17 January 2001
Abstract The in¯uence of KClO3 addition on the magnetic properties and microstructure of YBCO HTSC materials with nominal composition (A): Y1 Ba2 x Kx Cu3 Oy , (B): Y1 0:2x Ba2 0:2x Kx Cu3 Oy , (C): Y1 Ba2 Cu3:5 x Kx Oy x 0±0:75 were investigated. Magneto-optic imaging of ¯ux distributions was employed to study the grain connectivity in the doped and undoped samples in a direct way. The obtained ¯ux patterns taken at various temperatures reveal that KClO3 doped samples x 0:30 exhibit ¯ux distributions being close to those being observed in melt-textured superconductors. This demonstrates that the grain connectivity in the doped samples is considerably improved as compared to pure ones, which is an important issue for the fabrication of coated conductors of YBCO. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.h; 74.60.Jg; 74.62.Dh Keywords: YBCO HTSC; KClO3 doping; MO imaging; Grain boundaries
The magnetic behavior of YBCO high-temperature superconductors is largely dominated by processes in the intergranular medium [1±4]; being a major obstacle to the current ¯ow. In earlier work [5,6], it was found that the presence of some alkali metal-containing additions during the preparation of YBCO aects the properties of the intergranular medium of the end product. Very recently, the critical currents across grain boundaries in YBCO thin ®lms were found to be en-
* Corresponding author. Address: Experimentalphysik, Universitaet des Saarlandes, P.O. Box 15 11 50, D-66041 Saarbruecken, Germany. Tel./fax: +49-681-302-4867. E-mail address: [email protected] (M.R. Koblischka).
hanced by means of doping by Ca [7,8]. Therefore, studies of doping of polycrystalline YBCO play an important role for the development of YBCO coated conductors. Three series of YBCO were prepared with the nominal compositions (A): Y1 Ba2 x Kx Cu3 Oy , (B): Y1 0:2x Ba2 0:2x Kx Cu3 Oy , (C): Y1 Ba2 Cu3:5 x Kx Oy where x 0, 0.20, 0.30, 0.40, 0.60, 0.75. The samples were obtained following classic ceramic technology described in Ref. [9]. The phase content, microstructure and morphology of the crystalline grains were characterized using X-ray powder diraction (XRD) and scanning electron microscopy (SEM) coupled with an electron probe microanalyzer (EPMA). The AC complex susceptibility was measured with a Lake Shore AC susceptometer (Model 7000).
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 0 5 - 2
202
A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
In this paper, we report on results obtained by magneto-optic (MO) imaging of ¯ux distributions of samples from batch A. AC susceptibility measurements showed that the superconducting transition temperature for KClO3 -doped samples (x 0:20±0.60) was found to be higher (Tc;onset 94:5±93.8 K) than for undoped YBCO (Tc;onset 91 K) [5,6,9]. Furthermore, measurements of the DC susceptibility in ®elds between 10 mT and 7 T showed a very interesting feature: At temperatures below 20 K, the curves of the sample A (with x 0:3) exhibited a clear downturn of the DC susceptibility [10], which we attributed to a change of length scale of the shielding currents. Therefore, the MO imaging at these temperatures should show a clear dierence between the undoped and doped samples. Using SEM photographs, we carried out a statistical analysis of the grain size in the samples. Fig. 1 shows that the grain size reaches a maximum for x 0:3, and higher concentrations of the dopant x 0:60 lead to a small grain structure and changes in the grain boundaries, and thus, decoupling of the grains. Fig. 2 presents one such SEM image (sample B, x 0:40). Here, a highly homogeneous large-grain ceramic structure is achieved. The grains have the speci®c shape of parallelepipeds. Fig. 3 presents the ¯ux patterns obtained by MO imaging on the KClO3 -doped sample A with x 0:30 at three dierent temperatures, (a) T 10 K, (b) T 50 K and (c) T 77 K. At low temperatures, the sample does not exhibit the typical ¯ux pattern of a polycrystalline sample [11,12],
Fig. 1. Grain-size distribution of polycrystalline KClO3 -doped YBCO, samples (A), (B), and (C).
Fig. 2. SEM image of a polished surface; sample B with x 0:40.
where the ¯ux enters individually into the superconducting grains. Only in the lower part of the sample, a set of grain boundaries can be observed which act as channels for the magnetic ¯ux. These ¯ux patterns of the KClO3 -doped sample resemble the patterns found in melt-textured bulks [13]. With increasing temperature, the role of the grain boundaries becomes more and more dominant, which clearly indicates a reduction of the grain coupling. This behavior corresponds directly to the observations of the DC susceptibility [10]. In contrast to this doped sample, Fig. 4 presents the ¯ux pattern obtained on sample from batch A with x 0:60. The observation temperature was 50 K, and the applied magnetic ®eld was 60 mT. Here, the grain coupling is practically destroyed, and the ¯ux penetrates only into some superconducting grains; as it is expected in polycrystalline materials. The observed peculiarities resulting from adding KClO3 can be explained by changes in the morphology of grain size and grain boundaries of the microstructure of YBCO. After the second heat treatment the XRD data showed that in all cases the KClO3 -doped samples are pure 123 phase materials with only an indication of traces of CuO for the sample with x 0:40. No chlorine or other impurity phases related with chlorine or potassium are found in the KClO3 doped samples. The values of the lattice parameters of KClO3 -doped samples are comparable to the data published for the desired Y1 Ba2 Cu3 Oy orthorhombic structure for y 6:8±7 [14]. For the monophase KClO3 -doped samples (x 0:2) we used chemical analysis (iodometric titration [15]) to determine the oxygen content and we found
A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
203
Fig. 4. Flux pattern of sample A with x 0:60, which exhibits the typical pattern of a polycrystalline sample, thus indicating only a poor grain coupling. T 50 K, applied ®eld 60 mT.
Fig. 3. MO ¯ux patterns of the KClO3 -doped sample A with x 0:30 at dierent temperatures: (a) 10 K, (b) 50 K and (c) 77 K, but at the same applied ®eld of 100 mT. The magnetic ®eld is imaged as bright areas; the Meissner phase remains dark. Only in the lower part of the images, grain boundaries can be observed. The sample does not exhibit the typical ¯ux pattern of a polycrystalline sample, see e.g. Refs. [12,13], but resembles instead the pattern of melt-textured bulks. The marker is 500 mm long.
that y is 6.9±7, which is in good agreement with our XRD data (relation between crystal lattice parameters and oxygen content). We associated the in¯uence of the additive on the microstructural morphology with the presence
of a liquid phase during the sintering stage [9]. A very important microstructural result is that SEM±EPMA revealed traces of K and Cl in the grain boundaries of the samples with KClO3 additions. This observation is a clear indication of the improved grain coupling by KClO3 addition, which presents an important result for e.g. the preparation of coated conductors using YBCO. Furthermore, our result is in accordance with the observations from Ref. [7,8]. The results obtained showed that the presence of K- and Cl-containing impurities on the grain boundaries changes the intergranular coupling considerably, and, within certain concentration limits, improves the superconducting characteristics. Therefore, the doping of YBCO will be an important step for the fabrication of e.g. coated conductors of YBCO.
Acknowledgements This work is partially supported by NEDO. AV and MRK gratefully acknowledge support from STA (Japanese Science and Technology Agency) during the stay at SRL/ISTEC.
References [1] J.R. Clem, Physica C 153±155 (1988) 50. [2] V. Calzona, et al., Physica C 157 (1989) 425.
204 [3] [4] [5] [6] [7] [8] [9] [10]
A. Koblischka-Veneva et al. / Physica C 357±360 (2001) 201±204
H. Dersch, G. Blatter, Phys. Rev. B 38 (1988) 11391. J.W. Li, et al., Phys. Rev. B 46 (1992) 9190. A. Veneva, et al., Adv. Supercond. 11 (1999) 689. A. Veneva, et al., J. Low Temp. Phys. 117 (1999) 939. A. Schmehl, et al., Europhys. Lett. 47 (1999) 110. J. Mannhart, et al., Phil. Mag. B 80 (2000) 827. A. Veneva, et al., Physica C 308 (1998) 175. M.R. Koblischka, A. Veneva, M. Murakami, Adv. Supercond. 12 (2000) 245. [11] M.R. Koblischka, R.J. Wijngaarden, Supercond. Sci. Technol. 8 (1995) 199.
[12] M.R. Koblischka, T. Schuster, H. Kronm uller, Physica C 219 (1994) 205. [13] M.R. Koblischka, T. Schuster, G. Ravi Kumar, in: H.W. Weber (Ed.), Proceedings of the 7th IWCC, Alpbach, Austria, 24±27 January, 1994, World Scienti®c, Singapore, 1994, p. 399. [14] JCPDS powder diraction, Alphabetical Indexes (1997) ®le nos. 38-1433, 39-486, 39-1434. [15] H.A. Flaschka, A.J. Barnard, P.E. Sturrock, Quantitative Analytical Chemistry, vols. 1 & 2, Barnes and Noble, NY, 1969.