- Email: [email protected]
Critical current density of filamentary Y123 superconductor
PHYSICA ELSEVIER
Physica C 263 (1996) 450-452
Critical current density of filamentary Y 123 superconductor Tomoko Goto * Nagoya Institute of Technology, Dept. of Materials Science & Engineering, Gokiso-cho. Showa-ku, Nagoya, Japan
Abstract
Long precursor filaments are prepared by dry spinning with a starting homogeneous aqueous PVA solution containing Y, Ba and Cu acetates. By controlling the spinning process and partial-melting, a high Jc value of more than 105 A/cm 2 at 77 K and 0 T was obtained. The filament with high Jc does not always have a texture well aligned to the direction of the fiber axis. The field dependence of the transport Jc of the filament at 77 K is small if the grain boundary is clean.
The widespread commercial applicability of Y 123 superconducting wires depends on the development of processing methods to produce a non-weak-linked and strongly flux-pinning microstructure in reasonable periods of time. Although a textured microstructure can be produced by a directional solidification process that exhibits transport Jc values in excess of 10 s A / c m 2, higher Jc and flux trapping properties are desirable for applications. We have studied the preparation of superconducting oxides in a long filament using textile fiber spinning technology for the precursor of the filament. A transport Jc value of more than 10 4 A / c m 2 at 77 K and 0 T is attained for melt-processed Y123 filament by solution spinning [1]. In this paper, the transport Jc of the partial-melted Y123 filament is examined and a peculiarity of the filamentary Y123 superconductor is clarified. Long filaments were prepared by dry spinning with a starting homogeneous aqueous solution of Y, Ba and Cu acetates, as reported in a separate paper
* Fax: +81 52 735 5294; e-mail: [email protected].
[2]. An aqueous solution is prepared in advance by dissolving poly(vinyl alcohol), propionic acid and 2-hydroxy isobutyric acid. A mixture of the acetates of a stoichiometric composition of Y : Ba: Cu = 1 : 2 : 3 is dissolved in the aqueous solution. The resultant solution is concentrated and deaerated to obtain a stable viscous homogeneous spinning dope. The dope is extruded as a filament into a hot air zone and coiled on a winding drum. The as-drawn filament with diameter of 250 l~m is pyrolyzed to remove the volatile components and partially melted under 02 flow. The electrical resistivity of the heated filament is measured by a standard four-probe method. Ag paint is used to connect Ag sputtered parts of the filament with Ag electrodes of 75 Ixm diameter. The transport Jc measurement is performed at 77 K and 0 T using a DC current with a criterion of 1 p N / c m . The effect of the pyrolyzed condition on the partial-melting of the filament was examined to enhance the reproducibility of the high Jc [3]. The as-drawn filament was heated to 450°C and 500°C at a heating rate of 30°C/h in air to remove volatile components. The structure of the filament pyrolyzed at 450°C turns to a mixed phase of CuO and
0921-4534/96/$15.00 © 1996Elsevier Science B.V. All fights reserved SSDI 0921 -4534(96)00031-7
T. Goto / Physica C 263 (1996) 450-452
metastable tetragonal phase of a = 0.520 nm and c = 0.806 nm. In addition, BaCO 3 phase appeared by heating up to 500°C. The filament pyrolyzed at 150°C for 1 h and 500°C for 1 h in air consisted of BaCO 3, C u t and Y203. Partial-melting involves heat treatment of the filament above the solidus temperature for decomposition into a copper-rich liquid and Y2BaCuOs, and slow cooling from the peritectic temperature to about 850°C for nucleation and growth of the 123 phase, followed by furnace cooling for oxygenation. An additional slow cooling for the oxygenation was not needed owing to the fineness and large surface area of the filaments. As the present filament starts from the chemical liquid precursor of the oxide, the solidus and peritectic temperatures are considerably low. It is well known for the bulk 123 oxide that a crystal growth rate less than l ° C / h is needed for good alignment and obtaining the high Jc [4]. The optimum cooling rate for the present filament of 40°C/h was one order of magnitude higher than that for the bulk sample owing to the filamentary morphology. It is found that the optimum cooling rate increased with decreasing diameter of the filament [3]. This is beneficial for the practical production of an Y123 wire by this method. Partial-melting of the filament pyrolyzed under various conditions was examined and the magnetic field dependence of the transport J~ for the filaments was measured [5]. Fig. 1 shows the dependence of J¢ on the magnetic field at 77 K for the partially melted filaments. A current was applied to the direction of the fiber axis and the flow direction of the transport current is always normal to the applied magnetic field. The hysteresis effect on the magnetization curve is small. In the case of the filament pyrolyzed at 150°C for 1 h and 500°C for 1 h, the transport J~ decreases rapidly to 1 x 103 A / c m 2 on applying a field less than 0.5 T, and then decreases monotonically with the field and reaches 5 A / c m 2 at 5 T. Similar magnetization curves were measured for the filament pyrolyzed at 500°C at a heating rate of 30°C/h. For both cases, the melting process of the filament starts after the precipitation of B a C O 3 phase by the pyrolyzing process. This shows that the weak-link behavior at grain boundaries remained. On the other hand, for the filament pyrolyzed up to 450°C with slow heating and without the precipita-
451
3
4t o
~2 o
•J
I I
I0
I
I
I
I
20
30
40
50
H
(kOe)
Fig. 1. Jc vs. applied field H: O , [] up; Q, • down. O , O : The filament pyrolyzed at 150°C for 1 h and 500°C for 1 h. [], • : The filament pyrolyzed at 450°C at a heating rate of 30°C/h.
tion of BaCO 3, a Jc of more than 103 A / c m 2 is maintained on applying 4 T and the weak-link behavior is small. This is attributed to the fact that fine grains of the metastable tetragonal phase are formed during pyrolysis at 450°C and several hagstroms of effective pinning centers may be introduced into the sample during partial melting. The nature of the effective pinning centers is not yet clear. Thus, the field dependence of the Jc of the filament can be improved by controlling the pyrolyzing conditions and one can introduce effective flux pinning at 77 K by the solution method. The Jc of the heated filament depends on the spinning condition as well as the heat treatment [6]. The effect of the spinning dope for the precursor filament on the Jc of the heated filament was examined. The solution spinning was successfully performed through a solution containing partially hydrolyzed PVA with the degree of polymerization (DP) of 2450 and 1700. A higher Jc was observed for the filament spun from PVA solution with DP = 2450 and a lower content of the mixed acetates. A maximum J~ value of 1.04 X 105 A / c m 2 at 77 K and 0 T was attained for the filament spun from PVA solution of D P - - 2 4 5 0 with the acetates of twice PVA content. The polished and etched surface of the longitudinal cross-section of the filament with the highest Jc in this study is shown in Fig. 2. The filament consists of plate-like grains and has a sheath and core type of texture. The fine grains in the
452
T. Goto / Physica C 263 (1996)450--452
Fig. 2. Polished and etched surface on a longitudinal cross-section of the filament with J¢ of 1.04× 105 A / c m 2 at 77 K and 0 T.
sheath are well aligned, whereas irregular bulky grains align loosely in the core of the filament. The effect of Ar atmosphere during partial-melting of the filament was also examined [7]. The melting temperature can be reduced by 70°C in Ar flow as compared with 02 flow. The window for the optimum melting condition became narrow for the case of Ar flow. The highest J~ of 3.4 × l 0 4 A / c m 2 at 77 K and 0 T is attained for the filament melted at 940°C in Ar flow. The longitudinal cross-section after chemical etching of the filament with the high J~ is shown in Fig. 3. The filament is dense and consists of irregular, large grains. The grain is composed of rod-like grains. Any preferred orientation of
Fig. 3. Polished and etched surface on a longitudinal cross-section of the filament with Jc of 3.4× 104 A / c m 2 at 77 K and 0 T.
the grains is not detected. It is reported that with a 10° grain boundary mismatch, the Jc value drops by a factor of 20 in Y123 superconducting films [8]. However, another work has shown that some specific high-angle grain orientation can carry a relatively high critical current in the bulk scale flux-grown bicrystals [9]. It is found that during single-crystal growth, crystals will rotate to a low-energy orientation which supports relatively high critical currents. The reason for the high J~ in the present filament without grain alignment is not yet clear. However, it is suggested that the spiral supercurrent path for the low-energy orientation builds up on the cylindrical filamentary morphology if the grain boundaries are clear.
Acknowledgements The author would like to express her thanks to the High Field Laboratory for Superconducting Materials at Tohoku University for measuring the transport Jc in magnetic field. This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
References [1] T. Goto and T. Sugishita, Jpn. J. Appl. Phys. 30 (1991) L997. [2] T. Goto, T, Sugishita and K. Kojima, Physica C 171 (1991) 441. [3] T. Goto, J. Mater. Sci. 30(1995)6070. [4] M. Murakami, Supercond. Sci. Technol. 5 (1992) 185. [5] T. Goto, K. Watanabe, S. Awaji and H. Tomita, Proc. 6th Int. Symp. Superconductivity ISS'93, Hiroshima, (1993) p. 831. [6] H. Tomita, T. Goto and K. Takahashi, J. Mater. Res., submitted. [7] T. Goto and T. lzuka, Supercond. Sci. Technol. 8 (1995) 705. [8] D. Dimos, P. Chaudhari J. Mannhart and F. LeGoues, Phys. Rev. Lett. 61 (1989) 211. [9] D. Larbalestier, S. Babcock, X. Cai, M. Field, Y. Gao, N. Heinig, D. Kaiser, K. Merkel, L. William and N. Zhang, Physica C 185-189 (1991) 315.
Physica C 263 (1996) 450-452
Critical current density of filamentary Y 123 superconductor Tomoko Goto * Nagoya Institute of Technology, Dept. of Materials Science & Engineering, Gokiso-cho. Showa-ku, Nagoya, Japan
Abstract
Long precursor filaments are prepared by dry spinning with a starting homogeneous aqueous PVA solution containing Y, Ba and Cu acetates. By controlling the spinning process and partial-melting, a high Jc value of more than 105 A/cm 2 at 77 K and 0 T was obtained. The filament with high Jc does not always have a texture well aligned to the direction of the fiber axis. The field dependence of the transport Jc of the filament at 77 K is small if the grain boundary is clean.
The widespread commercial applicability of Y 123 superconducting wires depends on the development of processing methods to produce a non-weak-linked and strongly flux-pinning microstructure in reasonable periods of time. Although a textured microstructure can be produced by a directional solidification process that exhibits transport Jc values in excess of 10 s A / c m 2, higher Jc and flux trapping properties are desirable for applications. We have studied the preparation of superconducting oxides in a long filament using textile fiber spinning technology for the precursor of the filament. A transport Jc value of more than 10 4 A / c m 2 at 77 K and 0 T is attained for melt-processed Y123 filament by solution spinning [1]. In this paper, the transport Jc of the partial-melted Y123 filament is examined and a peculiarity of the filamentary Y123 superconductor is clarified. Long filaments were prepared by dry spinning with a starting homogeneous aqueous solution of Y, Ba and Cu acetates, as reported in a separate paper
* Fax: +81 52 735 5294; e-mail: [email protected].
[2]. An aqueous solution is prepared in advance by dissolving poly(vinyl alcohol), propionic acid and 2-hydroxy isobutyric acid. A mixture of the acetates of a stoichiometric composition of Y : Ba: Cu = 1 : 2 : 3 is dissolved in the aqueous solution. The resultant solution is concentrated and deaerated to obtain a stable viscous homogeneous spinning dope. The dope is extruded as a filament into a hot air zone and coiled on a winding drum. The as-drawn filament with diameter of 250 l~m is pyrolyzed to remove the volatile components and partially melted under 02 flow. The electrical resistivity of the heated filament is measured by a standard four-probe method. Ag paint is used to connect Ag sputtered parts of the filament with Ag electrodes of 75 Ixm diameter. The transport Jc measurement is performed at 77 K and 0 T using a DC current with a criterion of 1 p N / c m . The effect of the pyrolyzed condition on the partial-melting of the filament was examined to enhance the reproducibility of the high Jc [3]. The as-drawn filament was heated to 450°C and 500°C at a heating rate of 30°C/h in air to remove volatile components. The structure of the filament pyrolyzed at 450°C turns to a mixed phase of CuO and
0921-4534/96/$15.00 © 1996Elsevier Science B.V. All fights reserved SSDI 0921 -4534(96)00031-7
T. Goto / Physica C 263 (1996) 450-452
metastable tetragonal phase of a = 0.520 nm and c = 0.806 nm. In addition, BaCO 3 phase appeared by heating up to 500°C. The filament pyrolyzed at 150°C for 1 h and 500°C for 1 h in air consisted of BaCO 3, C u t and Y203. Partial-melting involves heat treatment of the filament above the solidus temperature for decomposition into a copper-rich liquid and Y2BaCuOs, and slow cooling from the peritectic temperature to about 850°C for nucleation and growth of the 123 phase, followed by furnace cooling for oxygenation. An additional slow cooling for the oxygenation was not needed owing to the fineness and large surface area of the filaments. As the present filament starts from the chemical liquid precursor of the oxide, the solidus and peritectic temperatures are considerably low. It is well known for the bulk 123 oxide that a crystal growth rate less than l ° C / h is needed for good alignment and obtaining the high Jc [4]. The optimum cooling rate for the present filament of 40°C/h was one order of magnitude higher than that for the bulk sample owing to the filamentary morphology. It is found that the optimum cooling rate increased with decreasing diameter of the filament [3]. This is beneficial for the practical production of an Y123 wire by this method. Partial-melting of the filament pyrolyzed under various conditions was examined and the magnetic field dependence of the transport J~ for the filaments was measured [5]. Fig. 1 shows the dependence of J¢ on the magnetic field at 77 K for the partially melted filaments. A current was applied to the direction of the fiber axis and the flow direction of the transport current is always normal to the applied magnetic field. The hysteresis effect on the magnetization curve is small. In the case of the filament pyrolyzed at 150°C for 1 h and 500°C for 1 h, the transport J~ decreases rapidly to 1 x 103 A / c m 2 on applying a field less than 0.5 T, and then decreases monotonically with the field and reaches 5 A / c m 2 at 5 T. Similar magnetization curves were measured for the filament pyrolyzed at 500°C at a heating rate of 30°C/h. For both cases, the melting process of the filament starts after the precipitation of B a C O 3 phase by the pyrolyzing process. This shows that the weak-link behavior at grain boundaries remained. On the other hand, for the filament pyrolyzed up to 450°C with slow heating and without the precipita-
451
3
4t o
~2 o
•J
I I
I0
I
I
I
I
20
30
40
50
H
(kOe)
Fig. 1. Jc vs. applied field H: O , [] up; Q, • down. O , O : The filament pyrolyzed at 150°C for 1 h and 500°C for 1 h. [], • : The filament pyrolyzed at 450°C at a heating rate of 30°C/h.
tion of BaCO 3, a Jc of more than 103 A / c m 2 is maintained on applying 4 T and the weak-link behavior is small. This is attributed to the fact that fine grains of the metastable tetragonal phase are formed during pyrolysis at 450°C and several hagstroms of effective pinning centers may be introduced into the sample during partial melting. The nature of the effective pinning centers is not yet clear. Thus, the field dependence of the Jc of the filament can be improved by controlling the pyrolyzing conditions and one can introduce effective flux pinning at 77 K by the solution method. The Jc of the heated filament depends on the spinning condition as well as the heat treatment [6]. The effect of the spinning dope for the precursor filament on the Jc of the heated filament was examined. The solution spinning was successfully performed through a solution containing partially hydrolyzed PVA with the degree of polymerization (DP) of 2450 and 1700. A higher Jc was observed for the filament spun from PVA solution with DP = 2450 and a lower content of the mixed acetates. A maximum J~ value of 1.04 X 105 A / c m 2 at 77 K and 0 T was attained for the filament spun from PVA solution of D P - - 2 4 5 0 with the acetates of twice PVA content. The polished and etched surface of the longitudinal cross-section of the filament with the highest Jc in this study is shown in Fig. 2. The filament consists of plate-like grains and has a sheath and core type of texture. The fine grains in the
452
T. Goto / Physica C 263 (1996)450--452
Fig. 2. Polished and etched surface on a longitudinal cross-section of the filament with J¢ of 1.04× 105 A / c m 2 at 77 K and 0 T.
sheath are well aligned, whereas irregular bulky grains align loosely in the core of the filament. The effect of Ar atmosphere during partial-melting of the filament was also examined [7]. The melting temperature can be reduced by 70°C in Ar flow as compared with 02 flow. The window for the optimum melting condition became narrow for the case of Ar flow. The highest J~ of 3.4 × l 0 4 A / c m 2 at 77 K and 0 T is attained for the filament melted at 940°C in Ar flow. The longitudinal cross-section after chemical etching of the filament with the high J~ is shown in Fig. 3. The filament is dense and consists of irregular, large grains. The grain is composed of rod-like grains. Any preferred orientation of
Fig. 3. Polished and etched surface on a longitudinal cross-section of the filament with Jc of 3.4× 104 A / c m 2 at 77 K and 0 T.
the grains is not detected. It is reported that with a 10° grain boundary mismatch, the Jc value drops by a factor of 20 in Y123 superconducting films [8]. However, another work has shown that some specific high-angle grain orientation can carry a relatively high critical current in the bulk scale flux-grown bicrystals [9]. It is found that during single-crystal growth, crystals will rotate to a low-energy orientation which supports relatively high critical currents. The reason for the high J~ in the present filament without grain alignment is not yet clear. However, it is suggested that the spiral supercurrent path for the low-energy orientation builds up on the cylindrical filamentary morphology if the grain boundaries are clear.
Acknowledgements The author would like to express her thanks to the High Field Laboratory for Superconducting Materials at Tohoku University for measuring the transport Jc in magnetic field. This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
References [1] T. Goto and T. Sugishita, Jpn. J. Appl. Phys. 30 (1991) L997. [2] T. Goto, T, Sugishita and K. Kojima, Physica C 171 (1991) 441. [3] T. Goto, J. Mater. Sci. 30(1995)6070. [4] M. Murakami, Supercond. Sci. Technol. 5 (1992) 185. [5] T. Goto, K. Watanabe, S. Awaji and H. Tomita, Proc. 6th Int. Symp. Superconductivity ISS'93, Hiroshima, (1993) p. 831. [6] H. Tomita, T. Goto and K. Takahashi, J. Mater. Res., submitted. [7] T. Goto and T. lzuka, Supercond. Sci. Technol. 8 (1995) 705. [8] D. Dimos, P. Chaudhari J. Mannhart and F. LeGoues, Phys. Rev. Lett. 61 (1989) 211. [9] D. Larbalestier, S. Babcock, X. Cai, M. Field, Y. Gao, N. Heinig, D. Kaiser, K. Merkel, L. William and N. Zhang, Physica C 185-189 (1991) 315.