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Large domain growth of Ag-doped YBaCuO-system superconductor
MATERIALS SCIENCE & ENGINEERING
ELSEVIER
Materials
Science
and Engineering
B53 (1998)
70-74
B
Large domain growth of Ag-doped YBaCuO-system superconductor S. Kohayashi a.*, S. Yoshizawa b, H. Miyairi
c, H. Nakane c, S. Nagaya d
Abstract
The effect of Ag addition on the phasediagram,solidification conditionsand fabrication of large domain YBaCuO-systemhas been studied. Calcined powder of compositionY,,sBaz,sCu3~jOs-,with 0.5 wt.% PI and 0, 1, 5, 10, 20 and 30 wt.% Ag was prepared. The melting point of each powder was measuredby differential thermal analysis(DTA). A large domain (45 mm diameterand 20 mm thick) of the Ag-doped YBaCuO-systemsuperconductorwas preparedfor each composition by a seeding and unidirectional growth techniquewhich involved controlling the furnace temperaturegradient. In order to assess the quality of the crystal orientation, the distribution of trapped flux density wasmeasured.The entire samplewasmagnetizeduniformly and the trapped magneticfield observedto form a singledome shape.0 1998Elsevier ScienceS.A. All rights reserved. Ke~‘lt’ods: YBaJu,O,
-,y superconductor;
Y,BaCuO,;
Melt
process;
1. Introduction
Since the discovery of superconducting materials with high-critical transition temperature (T,), considerable effort has been expended both to understand the reason for the high T, mechanism and to improve the critical current density (J,) of high T, samples. It is known that superconducting YBaCuO-system prepared by a melt process has few weak links because it grows in the form of large grains, which exhibited dramatically improved values of J, [l]. Recently many different applications of melt processed YBaCuO have been developed. such as magnetic bearings, flywheels, noncontact transport systems, magnetic shieldings, superconducting motors, bulk magnets, current leads and current limiters. In some of these applications. however, it is necessary to improve the mechanical strength of the material. The mechanical properties of melt processed YBaCuO can be improved by adding Ag [2]. However. a large domain of the Ag-doped YBaCuO superconductor could not be fabricated readily due to the change in crystal growth conditions caused by the
* Corresponding author. 0921-5107/98/$19.00 PIISO921-5107(97)00304-8
0 1998 Elsevier
Science
S.A. All
rights
reserved.
Ag; Crystal -
growth
addition of Ag. To determine the optimal conditions for crystai growth, the phase diagram of Y,.,Ba,,Cu,,,O, _ ,-Ag system was studied by differential thermal analysis (DTA) and X-ray diffraction of quenched samples. A large domain of the Ag-doped YBaCuO-system superconductor was subsequently prepared on the basis of this phase diagram.
2. Phase diagram of Y 1.8Ba,~,Cu,~,0p_ x-Ag system
Powders of Y,O,, BaCO, and CuO were mixed with a nominal ratio of Y:Ba:Cu= l&2.4:3.4, of 0.5 wt.% Pt, and 1, 3, 5, 10, 20 and 30 wt.% Ag. The mixture was calcined at 930°C for 24 h. The calcined powders were then pulverized and pressed into pellets. The pellets were melted partially at 1100°C for 0.5 h and cooled slowly from 1000 to 850°C at 1°C min - ‘. These pellets were pulverized and analysed by differential thermal analysis (DT.4) and the X-ray diffraction. The DTA was measured from room temperature to 1050°C at a heating rate of 2°C min- ‘. The DTA curves in the Y,,,Ba,,,Cu,,,O,,,-Ag system are shown in Fig. 1. A shift to a lower temperature in proportion to the amount of Ag was observed for the endothermic peak
S. Kol~cyxhi
e! al. /Materials
Science
of the un-doped system at 1000°C. A small endothermic peak in the system containing 3 wt.% Ag. is apparent at ca. 950°C of which the signal intensity increased with increasing Ag content. From these results, we conclude the peak at ca. 950°C is caused by the melting of the Ag in the system which suggests that there is some kind of phase transition between 970 and 1000°C. The quenching experiments were performed by the following process to study the influence of Ag on growth phases. The specimens were reheated in a furnace at 960 and 990°C for 30 min. They were removed for rapid cooling to room temperature. X-ray diffraction of the samples was performed before and after reheating and quenching to identify the phases present, as shown in Fig. 2. In both samples there were YBa,Cu,O,-,y phases, Y,BaCuO, phases and Ag metal present before reheating and quenching. Y2BaCu05, BaCuO, phases and Ag metal were observed in the sample quenched from 990°C. The results obtained from DTA and X-ray diffraction
==----I
i TEMPERATUREYCI Fig. 1. Differential thermal analysis curves in the Y, RBa2,4Cu3,409_ x-Ag system.
and Engineering
B53 (1998)
70- 74
71
yielded the composition-temperature diagram shown in Fig. 3. The transformation temperature was defined by extrapolation of the base line and the peak line in the lower temperature. The melting point of the YBa,Cu,O,-,y phase decreases rapidly as the Ag content varies from 0 to 5 wt.%, and saturates at concentrations over 5 wt.O/o.
3. Crystal growth of Ag-doped YBaCuO-system superconductors The most important step in the large domain fabrication is to cool slowly from a temperature just higher which YBa,Cu,O,-,y phase solidifies. If it is cooled a lower temperature than this, the nucleation occurs at several positions. By the cooling from the too high temperature, particles of YzBaCuOj phase are condensed and enlarged [3]. The critical current density of melt processed YBaCuO is decreased by coarsening YzBaCuOj particles. The crystallization temperatures of the YBa,Cu,O, _ x phase for an Ag content of 5, 10, 20 and 30 wt.% are approximately equal, as shown in Fig. 3. Therefore, large domain YBaCuO of the above compositions were fabricated by the following process manner as follows: the calcined powders were pulverized and pressed into pellets of 53-mm diameter and 25-mm thickness. The pellets were melted partially at 1100°C for 0.5 h and cooled at 5°C min- ’ until the temperature of the top surface was lowered to 970°C which is just before the crystallization temperature of YBa,Cu,O, _ ,y phase. A temperature gradient of 5°C cm - l was then applied within the furnace to maintain the lowest temperature at the top of the sample. When the temperature at the upper sample was reduced to lOOO”C, the pellets were seeded with a single grain cut from an un-doped YBaCuO bulk sample prepared by melt processing. The size of the seed was about 2 mm in diameter and 0.5 mm thick. The ab-plane of the seed was placed in contact with the top surface of the pellet to induce growth along the c-axis of the YBaCuO sample. The samples were then cooled at 0.5 and 40°C h- ‘, from 970 to 890°C and from 890°C to room temperature, respectively. Ag-doped YBaCuO samples up to 45 mm in diameter and 20 mm thick were prepared by this tecnique. Photographs of the Ag doped YBaCuO samples are shown in Fig. 4. It appears that the nucleation occurred only at the seeded point and then the crystal grew in the radial direction. The entire sample was oriented in the direction of the c-axis. Ag was exuded from the bulk YBaCuO material and condensed on the surface of the sample with 30 wt.% of Ag. Finally, the crystallized samples were annealed for 200 h from 600 to 300°C under oxygen atmosphere.
72
5’. Kohuynshi
et al. / Mate~iais
Science
nad Engitzeering
Before
B.53 (1998)
76 74
reheating
(bMgiOwt%
-rA
&fore
reheating
4
VI
(dMg30wt% Fig. 2. X-ray diffraction patterns of the samples with an Ag content of (a) 5 wt.%, (b) 10 wt.%, (c) 20 wt.% and (d) 30 wt?/ before reheating and quenching at 960 and 990°C. 123, Yba,Cu,O,-, phase; 21 I. Y,BaCuO, phase.
4. Evaluation of the magnetic properties
In order to assess the quality of the crystal orientation (i.e. whether there is a high angle grain boundary or not) the distribution of trapped flux density was measured in the specimens. The sample was field-cooled
0
10
20 Ag
30
40
[wt%l
Fig. 3. Pseudo-binary phase diagram for the Y,,,Ba,,,Cu,,,O,- ,-Ag system. 123, YBa#u,O,-, phase; 211, Y,BaCuO, phase; L, liquid phase.
to 77 K by immersing it in liquid nitrogen under 0.45 T. The axial component of the trapped magnetic flux density in the sample was then measured by scanning a Hall element sensor (F.W. Bell, BHT-921) which has an active area of 0.5 mm in diameter over its surface.~This was moved step by step over an area of 50 x 50 mm2 at a height of 0.5 mm above the top surface of the sample with a pitch of 2 mm. The contour maps of the trapped magnetic flux density of the Ag-doped and non-doped YBaCuO samples magnetized at 0.45 T are shown in Fig. 5. The entire sample was magnetized uniformly and the trapped magnetic field formed a single dome shape for each of the samples. However: some hollows on the dome are apparent for the Ag-doped YBaCuO samples. Since such a hollow may be formed by the absence of magnetic flux in regions of small defects, such as cracks or high angle grain boundaries, there appears to be a few of these in the Ag-doped samples [4]. Although the origin of the defects has not been identified, they often appear in the rapidly crystallized sample. Therefore, they can be reduced by reducing the rate of crystal growth.
S. Kol~tryrwl~i
er al. / B4uterials
Scieizce and Engineering
B53 (1998)
70-74
13
Xhml
(a)Ag Owt%
x Lllml (e)Ag 3Ovrt?h Fig. 5. Contour map of the trap@ magnetic flux density of the YBaCuO-system superconductors with Ag contents of 0 wt.“/u, (b) 5 wt.%, (c) 10 wt.‘%, (d) 20 wt.‘% and (e) 30 wLX field cooled at 77 K under 0.45 T.
5. Conclusions
The effect of Ag addition on the phase diagram, solidification conditions, and fabrication of large domain YBaCuO has been studied. Calcined powder of composition Y,,,Ba&u,,~O, _ .\’ with 0.5 wt.% Pt powder 0, 1, 5, 10, 20 and 30 wt.% Ag were prepared and the associated phase diagram was determined by DTA, quenching experiments and by X-ray diffraction. It was found that the melting point of YBa,Cu,O,x phase decreases rapidly for an Ag content between 0 and 5 wt.% and saturates over 5 wt.%. YBaCuO containing 5, lo1 20 and 30 wt.% Ag were prepared by a seeding and unidirectional growth technique. Ag-doped YBaCu0 samples were 45 mm in diameter and 20 mm thick. Although few hollows are apparent in the distribution of trapped magnetic field for the Ag-doped sample, the majority of the specimen was uniformly magnetized. References
1
6 Cd)
5
4
3
30~1%
Fig. 4. Photographs of the YBaCuO-system superconductors with Ag content of (a) 5 wt.‘!& (b) 10 wt.‘%. (c) 20 wt.% and id) 30 wt.!/,.
[I] M. Murakami. M. Morita. K. Doi, K. Miyamoto, .Jpn. J. Appl. Phys. 28 (1989) 1189. [2] Y. Yanagi, Y. Itoh, T. Oka, H. Tanaka, S. Takashima, Y. Yamada U. Mizutani. Advances in Superconductivity 5, Proc. of the 5th Internationai Symposium on Superconductivity (ISS‘92), Springer-Verlag, Berlin. 1992, p. 799. [3] K. Salama, V. Seivamdnickam, L. Gao, K. Sun, Appl. Phys. Lea 54 (1989) [4]
2352.
S. Kohayashi. Y. ishikdwa. S. Yoshizawa and H. Kojima, Advances in Superconductivity 5, Pros, of the 5th International Symposium on Superconductivity (ISS.92). Springer-Verlag, Berlin, 1992, p. 795.
ELSEVIER
Materials
Science
and Engineering
B53 (1998)
70-74
B
Large domain growth of Ag-doped YBaCuO-system superconductor S. Kohayashi a.*, S. Yoshizawa b, H. Miyairi
c, H. Nakane c, S. Nagaya d
Abstract
The effect of Ag addition on the phasediagram,solidification conditionsand fabrication of large domain YBaCuO-systemhas been studied. Calcined powder of compositionY,,sBaz,sCu3~jOs-,with 0.5 wt.% PI and 0, 1, 5, 10, 20 and 30 wt.% Ag was prepared. The melting point of each powder was measuredby differential thermal analysis(DTA). A large domain (45 mm diameterand 20 mm thick) of the Ag-doped YBaCuO-systemsuperconductorwas preparedfor each composition by a seeding and unidirectional growth techniquewhich involved controlling the furnace temperaturegradient. In order to assess the quality of the crystal orientation, the distribution of trapped flux density wasmeasured.The entire samplewasmagnetizeduniformly and the trapped magneticfield observedto form a singledome shape.0 1998Elsevier ScienceS.A. All rights reserved. Ke~‘lt’ods: YBaJu,O,
-,y superconductor;
Y,BaCuO,;
Melt
process;
1. Introduction
Since the discovery of superconducting materials with high-critical transition temperature (T,), considerable effort has been expended both to understand the reason for the high T, mechanism and to improve the critical current density (J,) of high T, samples. It is known that superconducting YBaCuO-system prepared by a melt process has few weak links because it grows in the form of large grains, which exhibited dramatically improved values of J, [l]. Recently many different applications of melt processed YBaCuO have been developed. such as magnetic bearings, flywheels, noncontact transport systems, magnetic shieldings, superconducting motors, bulk magnets, current leads and current limiters. In some of these applications. however, it is necessary to improve the mechanical strength of the material. The mechanical properties of melt processed YBaCuO can be improved by adding Ag [2]. However. a large domain of the Ag-doped YBaCuO superconductor could not be fabricated readily due to the change in crystal growth conditions caused by the
* Corresponding author. 0921-5107/98/$19.00 PIISO921-5107(97)00304-8
0 1998 Elsevier
Science
S.A. All
rights
reserved.
Ag; Crystal -
growth
addition of Ag. To determine the optimal conditions for crystai growth, the phase diagram of Y,.,Ba,,Cu,,,O, _ ,-Ag system was studied by differential thermal analysis (DTA) and X-ray diffraction of quenched samples. A large domain of the Ag-doped YBaCuO-system superconductor was subsequently prepared on the basis of this phase diagram.
2. Phase diagram of Y 1.8Ba,~,Cu,~,0p_ x-Ag system
Powders of Y,O,, BaCO, and CuO were mixed with a nominal ratio of Y:Ba:Cu= l&2.4:3.4, of 0.5 wt.% Pt, and 1, 3, 5, 10, 20 and 30 wt.% Ag. The mixture was calcined at 930°C for 24 h. The calcined powders were then pulverized and pressed into pellets. The pellets were melted partially at 1100°C for 0.5 h and cooled slowly from 1000 to 850°C at 1°C min - ‘. These pellets were pulverized and analysed by differential thermal analysis (DT.4) and the X-ray diffraction. The DTA was measured from room temperature to 1050°C at a heating rate of 2°C min- ‘. The DTA curves in the Y,,,Ba,,,Cu,,,O,,,-Ag system are shown in Fig. 1. A shift to a lower temperature in proportion to the amount of Ag was observed for the endothermic peak
S. Kol~cyxhi
e! al. /Materials
Science
of the un-doped system at 1000°C. A small endothermic peak in the system containing 3 wt.% Ag. is apparent at ca. 950°C of which the signal intensity increased with increasing Ag content. From these results, we conclude the peak at ca. 950°C is caused by the melting of the Ag in the system which suggests that there is some kind of phase transition between 970 and 1000°C. The quenching experiments were performed by the following process to study the influence of Ag on growth phases. The specimens were reheated in a furnace at 960 and 990°C for 30 min. They were removed for rapid cooling to room temperature. X-ray diffraction of the samples was performed before and after reheating and quenching to identify the phases present, as shown in Fig. 2. In both samples there were YBa,Cu,O,-,y phases, Y,BaCuO, phases and Ag metal present before reheating and quenching. Y2BaCu05, BaCuO, phases and Ag metal were observed in the sample quenched from 990°C. The results obtained from DTA and X-ray diffraction
==----I
i TEMPERATUREYCI Fig. 1. Differential thermal analysis curves in the Y, RBa2,4Cu3,409_ x-Ag system.
and Engineering
B53 (1998)
70- 74
71
yielded the composition-temperature diagram shown in Fig. 3. The transformation temperature was defined by extrapolation of the base line and the peak line in the lower temperature. The melting point of the YBa,Cu,O,-,y phase decreases rapidly as the Ag content varies from 0 to 5 wt.%, and saturates at concentrations over 5 wt.O/o.
3. Crystal growth of Ag-doped YBaCuO-system superconductors The most important step in the large domain fabrication is to cool slowly from a temperature just higher which YBa,Cu,O,-,y phase solidifies. If it is cooled a lower temperature than this, the nucleation occurs at several positions. By the cooling from the too high temperature, particles of YzBaCuOj phase are condensed and enlarged [3]. The critical current density of melt processed YBaCuO is decreased by coarsening YzBaCuOj particles. The crystallization temperatures of the YBa,Cu,O, _ x phase for an Ag content of 5, 10, 20 and 30 wt.% are approximately equal, as shown in Fig. 3. Therefore, large domain YBaCuO of the above compositions were fabricated by the following process manner as follows: the calcined powders were pulverized and pressed into pellets of 53-mm diameter and 25-mm thickness. The pellets were melted partially at 1100°C for 0.5 h and cooled at 5°C min- ’ until the temperature of the top surface was lowered to 970°C which is just before the crystallization temperature of YBa,Cu,O, _ ,y phase. A temperature gradient of 5°C cm - l was then applied within the furnace to maintain the lowest temperature at the top of the sample. When the temperature at the upper sample was reduced to lOOO”C, the pellets were seeded with a single grain cut from an un-doped YBaCuO bulk sample prepared by melt processing. The size of the seed was about 2 mm in diameter and 0.5 mm thick. The ab-plane of the seed was placed in contact with the top surface of the pellet to induce growth along the c-axis of the YBaCuO sample. The samples were then cooled at 0.5 and 40°C h- ‘, from 970 to 890°C and from 890°C to room temperature, respectively. Ag-doped YBaCuO samples up to 45 mm in diameter and 20 mm thick were prepared by this tecnique. Photographs of the Ag doped YBaCuO samples are shown in Fig. 4. It appears that the nucleation occurred only at the seeded point and then the crystal grew in the radial direction. The entire sample was oriented in the direction of the c-axis. Ag was exuded from the bulk YBaCuO material and condensed on the surface of the sample with 30 wt.% of Ag. Finally, the crystallized samples were annealed for 200 h from 600 to 300°C under oxygen atmosphere.
72
5’. Kohuynshi
et al. / Mate~iais
Science
nad Engitzeering
Before
B.53 (1998)
76 74
reheating
(bMgiOwt%
-rA
&fore
reheating
4
VI
(dMg30wt% Fig. 2. X-ray diffraction patterns of the samples with an Ag content of (a) 5 wt.%, (b) 10 wt.%, (c) 20 wt.% and (d) 30 wt?/ before reheating and quenching at 960 and 990°C. 123, Yba,Cu,O,-, phase; 21 I. Y,BaCuO, phase.
4. Evaluation of the magnetic properties
In order to assess the quality of the crystal orientation (i.e. whether there is a high angle grain boundary or not) the distribution of trapped flux density was measured in the specimens. The sample was field-cooled
0
10
20 Ag
30
40
[wt%l
Fig. 3. Pseudo-binary phase diagram for the Y,,,Ba,,,Cu,,,O,- ,-Ag system. 123, YBa#u,O,-, phase; 211, Y,BaCuO, phase; L, liquid phase.
to 77 K by immersing it in liquid nitrogen under 0.45 T. The axial component of the trapped magnetic flux density in the sample was then measured by scanning a Hall element sensor (F.W. Bell, BHT-921) which has an active area of 0.5 mm in diameter over its surface.~This was moved step by step over an area of 50 x 50 mm2 at a height of 0.5 mm above the top surface of the sample with a pitch of 2 mm. The contour maps of the trapped magnetic flux density of the Ag-doped and non-doped YBaCuO samples magnetized at 0.45 T are shown in Fig. 5. The entire sample was magnetized uniformly and the trapped magnetic field formed a single dome shape for each of the samples. However: some hollows on the dome are apparent for the Ag-doped YBaCuO samples. Since such a hollow may be formed by the absence of magnetic flux in regions of small defects, such as cracks or high angle grain boundaries, there appears to be a few of these in the Ag-doped samples [4]. Although the origin of the defects has not been identified, they often appear in the rapidly crystallized sample. Therefore, they can be reduced by reducing the rate of crystal growth.
S. Kol~tryrwl~i
er al. / B4uterials
Scieizce and Engineering
B53 (1998)
70-74
13
Xhml
(a)Ag Owt%
x Lllml (e)Ag 3Ovrt?h Fig. 5. Contour map of the trap@ magnetic flux density of the YBaCuO-system superconductors with Ag contents of 0 wt.“/u, (b) 5 wt.%, (c) 10 wt.‘%, (d) 20 wt.‘% and (e) 30 wLX field cooled at 77 K under 0.45 T.
5. Conclusions
The effect of Ag addition on the phase diagram, solidification conditions, and fabrication of large domain YBaCuO has been studied. Calcined powder of composition Y,,,Ba&u,,~O, _ .\’ with 0.5 wt.% Pt powder 0, 1, 5, 10, 20 and 30 wt.% Ag were prepared and the associated phase diagram was determined by DTA, quenching experiments and by X-ray diffraction. It was found that the melting point of YBa,Cu,O,x phase decreases rapidly for an Ag content between 0 and 5 wt.% and saturates over 5 wt.%. YBaCuO containing 5, lo1 20 and 30 wt.% Ag were prepared by a seeding and unidirectional growth technique. Ag-doped YBaCu0 samples were 45 mm in diameter and 20 mm thick. Although few hollows are apparent in the distribution of trapped magnetic field for the Ag-doped sample, the majority of the specimen was uniformly magnetized. References
1
6 Cd)
5
4
3
30~1%
Fig. 4. Photographs of the YBaCuO-system superconductors with Ag content of (a) 5 wt.‘!& (b) 10 wt.‘%. (c) 20 wt.% and id) 30 wt.!/,.
[I] M. Murakami. M. Morita. K. Doi, K. Miyamoto, .Jpn. J. Appl. Phys. 28 (1989) 1189. [2] Y. Yanagi, Y. Itoh, T. Oka, H. Tanaka, S. Takashima, Y. Yamada U. Mizutani. Advances in Superconductivity 5, Proc. of the 5th Internationai Symposium on Superconductivity (ISS‘92), Springer-Verlag, Berlin. 1992, p. 799. [3] K. Salama, V. Seivamdnickam, L. Gao, K. Sun, Appl. Phys. Lea 54 (1989) [4]
2352.
S. Kohayashi. Y. ishikdwa. S. Yoshizawa and H. Kojima, Advances in Superconductivity 5, Pros, of the 5th International Symposium on Superconductivity (ISS.92). Springer-Verlag, Berlin, 1992, p. 795.