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Microstructure and transport properties of melt-textured joint of YBCO
Physica C 372–376 (2002) 1187–1190 www.elsevier.com/locate/physc
Microstructure and transport properties of melt-textured joint of YBCO J.G. Noudem
a,*
, E.S. Reddy a, E.A. Goodilin a, M. Tarka a, M. Noe b, G.J. Schmitz a
a
b
ACCESS e.V. Materials & Processes, Intzestr. 5, D-52072 Aachen, Germany Forschungszentrum Karlsruhe, Institut f€ur Technische Physik, D-76344 Karlsruhe, Germany
Abstract The engineering applications of high temperature superconducting materials require long length and/or complex shape bulk textured materials. The size and shape limitations for single domain melt texturing of bulk monolith highlights the importance of joining processes. In this paper the various factors influencing the formation of the joint and its microstructure during solidification of the YbBa2 Cu3 O7 x solder phase between two single domain bulks have been investigated. Transport current measurements across the joints being configured in a brick wall structure under optimized conditions are presented. 2002 Published by Elsevier Science B.V. Keywords: Melt-texture joining; Microstructure; Shaping; Electrical properties
1. Introduction Various applications based on bulk superconductors [1–3] require materials of large sizes and complex shapes. Melt-processed Y123 materials have emerged with properties suitable for most of the applications. However, melt-processing techniques by now, owing to slow growth rates of the 123 phase and the high viscosity of the melt
* Corresponding author. Present address: Faculty of Sciences and Techniques, Laboratory of Electrodynamics of Advanced Materials, Francois Rabelais University, Parc de Grandmont, F-37200 Tours, France. Fax: +33-247-36-69-47. E-mail address: [email protected] (J.G. Noudem).
during texturing, limit the preparation of bulk materials in required sizes and shapes. This makes the development of reliable joining techniques important in realizing various engineering applications. The texturing of a solder phase at joining interfaces has been widely used for joining of 123 bulks [4–11]. However, the microstructure of the interface region is often observed to associate with pores, cracks and residual liquid phases reducing the performance of the joint. In the present paper an attempt is made to understand the solidification process and isolate the causes for the defect formation in the interface region during texturing of the solder phase. The transport current measurements across the joints being configured in a brick wall structure are also discussed.
0921-4534/02/$ - see front matter 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 6 9 - 3
1188
J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
2. Experimental The bulk pieces used for joining experiments are parts of top-seeded melt-grown Y123 samples. The solder phases used for joining is basically the YbBa2 Cu3 Oy or YbBa2 Cu3 Oy with 40 wt.% Yb2 BaCuO5 , 0.5 wt.% PtO2 in the form of a paste prepared using glycerin. The parts to be joined were glued with the solder paste. The texturing of the solder phase for joining was conducted in an isothermal box-type furnace in ambient atmosphere with a thermal schedule similar to the one reported elsewhere [6].
3. Results and discussion The various configurations investigated in the present work and the corresponding microstructures of the joints are shown in Fig. 1. The important findings made from the microstructures are essentially the following: • The absence of external pressure results in a large number of defects, Fig. 1(a). • The application of an external pressure on the joining faces minimizes the residual impurity phases at the impingement boundary, Fig. 1(b), but some residual liquid phases and pores still remain.
• The fine size of 211 particles in the liquid phase, refined by the PtO2 addition, results in a sharp planar growth fronts and minimizes the amount of residues upon impingement, Fig. 1(c). • The polishing of bulk surfaces further improves the microstructure of joint, Fig. 1(d). This is due to the fact that nucleation and epitaxy on the polished surfaces result in a sharp planar growth front and yield a good joint upon impingement. • Fig. 1(e) shows that it is important to decrease the width of the interface region to confine the liquid phases within the joining region during texturing. The conditions for texturing of the solder phase at the interface of the bulk pieces for joining are very different to that of growing a single domain during top-seeded melt-growth process. The growth mechanism in the joining zone with respect to various configurations is described elsewhere [6]. The room temperature resistivities measured on the complex brick wall-shape joint (Fig. 2) across the voltage taps are illustrated in Fig. 3. The resistivity values obtained in the artificial grain boundary are about 30% higher than the resistivity of the parent monoliths. The increased resistivity of the joints may occur due to the presence of pores and residual liquid phases. A similar observation is also reported by Maeda et al. [12]. The
Fig. 1. Back-scattered images of joined regions in the cases of (a) no external pressure applied, (b) applied pressure of 20 Pa, (c) using a PtO2 containing solder, (d) optically polished surfaces, (e) inclined surfaces. The corresponding experimental configurations are shown schematically below the respective microstructure. V: voids, P: pores, LP: liquid phases.
J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
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surements no voltage drop is observed across any of the voltage taps even up to 700 A, corresponding to a Jc at a minimum of 12 kA/cm2 at 77 K in the self-field. The high currents being passed through the configuration without any transition indicate a clean microstructure of the artificial joints.
4. Conclusion Fig. 2. Brick wall-shape structure prepared by joining of three independent melt-textured blocks. The sample is equipped with current leads and different voltage taps.
The microstructural features due to texturing of the Yb123 solder phase with a lower peritectic decomposition temperature applied for joining of bulk melt-textured samples are discussed for various configurations. It has been argued that the conditions and texturing of the solder phase are different from that of the single domain growth. Accordingly, modifying the conditions during joining viz., polishing the surfaces meant for joining and reducing the 211 particle size in the solder phase is found to yield an interface with minimum defects. The transport Jc of a sample fabricated as a brick wall structure exceeds 12 kA/cm2 at 77 K in self-field.
Acknowledgements Fig. 3. The Jc values measured for the brick wall configuration. The inset shows the contact resistance values at high currents.
resistivity value of one of the parent bulk is observed to be lower than the other bulk pieces. This might be due to the variation in the microstructural features of single domain monoliths; for instance the lower 211 particle density, which means that fewer platelet cracks leads to a lower. The transport measurements investigated at 77 K and 0 T on the brick wall configuration are shown in Fig. 3. The inset in the figure shows the contact resistivity values measured at high applied currents of up to 700 A. It can be observed that a low contact resistivity of around 500 nX cm2 is obtained even at high currents exceeding 500 A. The typical linear behavior of the curve is due to the contact heating. During the transport Jc mea-
This work is supported by German Federal Ministry of Higher Education and Research under grants 13N7490 and 13N7571/1 and the Alexander von Humboldt foundation (EAG).
References [1] B. Gromoll, G. Reis, W. Schmidt, H.P. Kr€amer, P. Kummeth, H.W. Neum€ uller, IEEE Trans. Appl. Supercond. 7 (1997). [2] J.R. Hull, T.M. Mulcahy, R.G. Abboud, IEEE Trans. Appl. Supercond. 5 (1995) 626. [3] Y. Itho et al., Jpn. J. Appl. Phys. 34 (1995) 5574. [4] S.M. Mukhopadhyay, N. Mahadev, S. Sengupta, Physica C 329 (2000) 95. [5] G.J. Schmitz, A. Tigges, J.C. Schmidt, Supercond. Sci. Technol. 11 (1998) 73. [6] J.G. Noudem, E.S. Reddy, M. Tarka, M. Noe, G.J. Schmitz, Supercond. Sci. Technol. 14 (2001) 363.
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J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
[7] A.D. Bradley, W. Lo, M. Mironova, et al., J. Mater. Res. 16 (2001) 2298. [8] T. Prikhna, W. Gawalek, V. Moshchil, et al., Physica C 354 (2001) 333. [9] M.P. Delamare, C. Jooss, B. Bringmann, et al., IEEE Trans. Appl. Supercond. 11 (2001) 3788.
[10] H. Zheng, H. Claus, L. Chen, et al., Physica C 350 (2001) 17. [11] W. Lo, D.A. Cardwell, A.D. Bradley, et al., IEEE Trans. Appl. Supercond. 9 (1999) 2042. [12] J. Maeda et al., Advances in superconductivity, in: Proceedings of the 12th ISS’99, October 17–19, Morioka, vol. 12, 2000, p. 449.
Microstructure and transport properties of melt-textured joint of YBCO J.G. Noudem
a,*
, E.S. Reddy a, E.A. Goodilin a, M. Tarka a, M. Noe b, G.J. Schmitz a
a
b
ACCESS e.V. Materials & Processes, Intzestr. 5, D-52072 Aachen, Germany Forschungszentrum Karlsruhe, Institut f€ur Technische Physik, D-76344 Karlsruhe, Germany
Abstract The engineering applications of high temperature superconducting materials require long length and/or complex shape bulk textured materials. The size and shape limitations for single domain melt texturing of bulk monolith highlights the importance of joining processes. In this paper the various factors influencing the formation of the joint and its microstructure during solidification of the YbBa2 Cu3 O7 x solder phase between two single domain bulks have been investigated. Transport current measurements across the joints being configured in a brick wall structure under optimized conditions are presented. 2002 Published by Elsevier Science B.V. Keywords: Melt-texture joining; Microstructure; Shaping; Electrical properties
1. Introduction Various applications based on bulk superconductors [1–3] require materials of large sizes and complex shapes. Melt-processed Y123 materials have emerged with properties suitable for most of the applications. However, melt-processing techniques by now, owing to slow growth rates of the 123 phase and the high viscosity of the melt
* Corresponding author. Present address: Faculty of Sciences and Techniques, Laboratory of Electrodynamics of Advanced Materials, Francois Rabelais University, Parc de Grandmont, F-37200 Tours, France. Fax: +33-247-36-69-47. E-mail address: [email protected] (J.G. Noudem).
during texturing, limit the preparation of bulk materials in required sizes and shapes. This makes the development of reliable joining techniques important in realizing various engineering applications. The texturing of a solder phase at joining interfaces has been widely used for joining of 123 bulks [4–11]. However, the microstructure of the interface region is often observed to associate with pores, cracks and residual liquid phases reducing the performance of the joint. In the present paper an attempt is made to understand the solidification process and isolate the causes for the defect formation in the interface region during texturing of the solder phase. The transport current measurements across the joints being configured in a brick wall structure are also discussed.
0921-4534/02/$ - see front matter 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 9 6 9 - 3
1188
J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
2. Experimental The bulk pieces used for joining experiments are parts of top-seeded melt-grown Y123 samples. The solder phases used for joining is basically the YbBa2 Cu3 Oy or YbBa2 Cu3 Oy with 40 wt.% Yb2 BaCuO5 , 0.5 wt.% PtO2 in the form of a paste prepared using glycerin. The parts to be joined were glued with the solder paste. The texturing of the solder phase for joining was conducted in an isothermal box-type furnace in ambient atmosphere with a thermal schedule similar to the one reported elsewhere [6].
3. Results and discussion The various configurations investigated in the present work and the corresponding microstructures of the joints are shown in Fig. 1. The important findings made from the microstructures are essentially the following: • The absence of external pressure results in a large number of defects, Fig. 1(a). • The application of an external pressure on the joining faces minimizes the residual impurity phases at the impingement boundary, Fig. 1(b), but some residual liquid phases and pores still remain.
• The fine size of 211 particles in the liquid phase, refined by the PtO2 addition, results in a sharp planar growth fronts and minimizes the amount of residues upon impingement, Fig. 1(c). • The polishing of bulk surfaces further improves the microstructure of joint, Fig. 1(d). This is due to the fact that nucleation and epitaxy on the polished surfaces result in a sharp planar growth front and yield a good joint upon impingement. • Fig. 1(e) shows that it is important to decrease the width of the interface region to confine the liquid phases within the joining region during texturing. The conditions for texturing of the solder phase at the interface of the bulk pieces for joining are very different to that of growing a single domain during top-seeded melt-growth process. The growth mechanism in the joining zone with respect to various configurations is described elsewhere [6]. The room temperature resistivities measured on the complex brick wall-shape joint (Fig. 2) across the voltage taps are illustrated in Fig. 3. The resistivity values obtained in the artificial grain boundary are about 30% higher than the resistivity of the parent monoliths. The increased resistivity of the joints may occur due to the presence of pores and residual liquid phases. A similar observation is also reported by Maeda et al. [12]. The
Fig. 1. Back-scattered images of joined regions in the cases of (a) no external pressure applied, (b) applied pressure of 20 Pa, (c) using a PtO2 containing solder, (d) optically polished surfaces, (e) inclined surfaces. The corresponding experimental configurations are shown schematically below the respective microstructure. V: voids, P: pores, LP: liquid phases.
J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
1189
surements no voltage drop is observed across any of the voltage taps even up to 700 A, corresponding to a Jc at a minimum of 12 kA/cm2 at 77 K in the self-field. The high currents being passed through the configuration without any transition indicate a clean microstructure of the artificial joints.
4. Conclusion Fig. 2. Brick wall-shape structure prepared by joining of three independent melt-textured blocks. The sample is equipped with current leads and different voltage taps.
The microstructural features due to texturing of the Yb123 solder phase with a lower peritectic decomposition temperature applied for joining of bulk melt-textured samples are discussed for various configurations. It has been argued that the conditions and texturing of the solder phase are different from that of the single domain growth. Accordingly, modifying the conditions during joining viz., polishing the surfaces meant for joining and reducing the 211 particle size in the solder phase is found to yield an interface with minimum defects. The transport Jc of a sample fabricated as a brick wall structure exceeds 12 kA/cm2 at 77 K in self-field.
Acknowledgements Fig. 3. The Jc values measured for the brick wall configuration. The inset shows the contact resistance values at high currents.
resistivity value of one of the parent bulk is observed to be lower than the other bulk pieces. This might be due to the variation in the microstructural features of single domain monoliths; for instance the lower 211 particle density, which means that fewer platelet cracks leads to a lower. The transport measurements investigated at 77 K and 0 T on the brick wall configuration are shown in Fig. 3. The inset in the figure shows the contact resistivity values measured at high applied currents of up to 700 A. It can be observed that a low contact resistivity of around 500 nX cm2 is obtained even at high currents exceeding 500 A. The typical linear behavior of the curve is due to the contact heating. During the transport Jc mea-
This work is supported by German Federal Ministry of Higher Education and Research under grants 13N7490 and 13N7571/1 and the Alexander von Humboldt foundation (EAG).
References [1] B. Gromoll, G. Reis, W. Schmidt, H.P. Kr€amer, P. Kummeth, H.W. Neum€ uller, IEEE Trans. Appl. Supercond. 7 (1997). [2] J.R. Hull, T.M. Mulcahy, R.G. Abboud, IEEE Trans. Appl. Supercond. 5 (1995) 626. [3] Y. Itho et al., Jpn. J. Appl. Phys. 34 (1995) 5574. [4] S.M. Mukhopadhyay, N. Mahadev, S. Sengupta, Physica C 329 (2000) 95. [5] G.J. Schmitz, A. Tigges, J.C. Schmidt, Supercond. Sci. Technol. 11 (1998) 73. [6] J.G. Noudem, E.S. Reddy, M. Tarka, M. Noe, G.J. Schmitz, Supercond. Sci. Technol. 14 (2001) 363.
1190
J.G. Noudem et al. / Physica C 372–376 (2002) 1187–1190
[7] A.D. Bradley, W. Lo, M. Mironova, et al., J. Mater. Res. 16 (2001) 2298. [8] T. Prikhna, W. Gawalek, V. Moshchil, et al., Physica C 354 (2001) 333. [9] M.P. Delamare, C. Jooss, B. Bringmann, et al., IEEE Trans. Appl. Supercond. 11 (2001) 3788.
[10] H. Zheng, H. Claus, L. Chen, et al., Physica C 350 (2001) 17. [11] W. Lo, D.A. Cardwell, A.D. Bradley, et al., IEEE Trans. Appl. Supercond. 9 (1999) 2042. [12] J. Maeda et al., Advances in superconductivity, in: Proceedings of the 12th ISS’99, October 17–19, Morioka, vol. 12, 2000, p. 449.