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Process-induced flux pinning in melt-textured YBCO superconductor
Ill
Physlca C 209 (1993) 187-190 North-Holland
Process-induced
flux pinning in melt-textured YBCO superconductor
K. Salama, V. Selvamanickam, and D. F. Lee Department of Mechanical Engineering, and Texas Center for Superconductivity University of Houston, Houston, TX, U.S.A. In this paper, we summarize two processing approaches which have been used to produce pinning centers in melt-textured YBCO resulting in the enhancement of current density. The first approach is the addition of nonsuperconducting inclusions such as Y2BaCuO5, and the second is high temperature mechanical deformation to generate dislocations and stacking faults.
1.
PINNING BY Y2BaCuO5 PARTICLES
It is well k n o w n that normal-phase particles in conventional superconductors can be effective pinning centers In melt-textured Y B a 2 C u 3 0 x (123), however the Y2BaCuO 5 (211) particles themselves are unlikely to be pinning sites since their dimensions are much larger than the coherence length of the HTS. Yet, the interfaces between particles and their associated defects such as stacking faults and dislocations may be able to provide pinning. N u m e r o u s studies have shown that the presence of 211 particles are effective in flux pinning [1], and the current density (Jc) increases with increasing 211 content. Other mvestigators, h o w e v e r , have d e t e r m i n e d that Jc is either independent of the amount of 211 inclusions [2] or even decreases with 211 content [3]. In order to clarify the influence of 211 on Jc, m e a s u r e m e n t s of the angular dependence of ]c have been conducted on melttextured 123 containing different 211 particle size and content. Melt-textured specimens with 0 to 50 vol.% 'large' rod-shaped 211 inclusions of 4 to 20 larn size were manufactured by varying the slow cooling rate of the liquid phase processing technique [4]. Also, melt-textured 123 containing 'small' spherical 211 mclusions of 0.5 - 2 lam size were fabricated through the addition of 211 particles to the start material [5]. The dependence of Jc of these samples on 0921-4534/93/506 00 © 1993 - Elsevier Science Pubhshers B V
the orientation of an external magnetic field w i t h r e s p e c t to their a-b p l a n e was determined by tranport current measurements. These measurements were performed at 77 K and 1.5 T with the field aligned at different angles to the a-b plane. The results obtained for the specimens with different 'large' 211 contents are shown in Fig. 1. It can be seen from this figure that the Jc at all field orientations decreases with increasing a m o u n t of 211 particles. The angular dependence of Jc for the specimens with 'small' 211 inclusions at 1.5 T are shown m Fig. 2. It is evident from this figure that there is a consistent improvement in the c u r r e n t c a r r y i n g capability with i n c r e a s i n g m i c r o n - s i z e 211 a d d i t i o n throughout the whole angular range. The opposite effects of 211 on Jc in these two types of specimens m a y be reconciled if the influences of particle size and magnetic field are taken into consideration. If the 123/211 Interfaces are effective pinning sites, then Jc should increase with the surface area of the inclusions, i.e., with 211 content of a fixed particle size, as in the case of conventional superconductors. However, this increase in Jc will not be monotonic because as the n u m b e r of particles is increased, the superconducting volume is reduced. A balance between these two influences will necessitate an optimal second-phase content for a given particle size, where further increase in the amount of parhcles will result in a decrease in All rights reserved
188
K Salama et al / Process-mduced flux pruning m melt-textured YBCO superconductor
the Jc of the material [5]. This optimal second-phase content will vary with the particle size, shifting to a higher percentage when the inclusions are small. Based on these results and the probable existence of an optimal 211 content which varies with the 30000
size of the inclusions, it is proposed that the d e p e n d e n c e of Jc on 211 content in melttextured 123 is not unique, but changes according to the regime of the 211 content, the size of the 211 inclusions, and the strength of the applied magnetic field.
................................... 77K, 1 5 T 'Large' 211 inclusions
25000
•
•
17vo1.%211
o
30 vol % 211
•
50voL%211
•
•
~-- 20000 • d~e "- 15000q
0 0 •
100O0
•
•
•
•
o oo oooo
•
0
o o oO
0
(3°
o •
o
•
•
•
~oo o
•
•
, o , o , o, oo o
5000 .
0
.
i
.
15
.
,
.
.
a
.
.
I
.
.
i
.
.
i
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.
i
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.
i
.
.
i
.
.
i
.
.
i
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.
30 45 60 75 90 105 120 135 150 165 180 Angle between field and c-axis (degrees)
Figure 1 Angular dependence of Jc on field angle of melt-textured 123 with varying amount of 'large' 211 inclusions at 77 K and 1.5 T. 50000 77K, 1 5 T 'Small' 21 !mclustons
40000 O
w" 30000
~+~O 0
0
[] 0
• • x + X o D
0% 211 added (#1) 0% 211 added (#2) 10% 211 added (#1) 10% 211 added (#2) 10% 211 added (#3) 15%211 added(#1) 15% 211 added (#2)
0
0
0
00 0
0
~
0
0
O~
00o001
oo 0o~ +
10000
0
0
15
30 45 60 75 90 105 120 135 150 165 180 Angle between field and c-axis (degrees)
Figure 2 Angular dependence of Jc on field angle of melt-textured 123 containing varying amount of "small" 211 inclusions at 77 K and 1.5 T.
189
K Salama et al /Process-mducedfluxpmnmg m melt-textured YBCO superconductor
40000
.
~
oo
35000
Ii
~o
30000
{,#
°°
\ o
•
e/ -
o/
-
25000 0~)0 0
20000
~
(cos ¢) 0.~ fit
0
o oOODOO O O O O ooOO
~Oo O O O O O OGODo
1500(]
-30
0
30
60
90
120
150
180
210
Angle between magnetic field and c-axis (degrees) Figure 3. Angular dependence of Jc of undeformed and uniaxially deformed melt-textured samples at 1.5 T and 77 K. The expected (cos 0) 0.5 behavior from point pinning by dislocations is also included. 2600(3
' ' 1 ' ' 1 ' ' 1 '
I
I
2400(3
4[
2200(3
@
I
I
'1
I
14
1
01t~-t~
1 -60
Figure 4
-30
0 30 > 60 90 120 150 180 210 Angle between field and c-axis (degrees)
240
Angular dependence of Jc of an isostatically deformed melt-textured sample at 1.5 T and 77K.
190
2.
K Salama et al / Process-tnduced flux pmnmg m melt-textured YBCO superconductol
P I N N I N G BY DISLOCATIONS
Dislocations and stacking faults are two common crystal defects that exist within melttextured 123 superconductors. If the density of these defects can be increased, it could be p o s s i b l e to e n h a n c e the Jc of these superconductors in magnetic field orientations other than B I I a-b and decrease the strong sensitivity of Jc to field orientation. It has been shown that a high density of dislocations can be generated in the a-b plane of melttextured YBCO s u p e r c o n d u c t o r s b y high temperature uniaxial and isostatic deformation processing [6]. The d e f o r m e d samples were found to exhibit a dislocation density of 1010 cm -2 which is an order of magnitude higher than that in undeformed samples. The dependence of Jc on the angle between the magnetic field and the c-axis at 1.5 T and 77 K in undeformed and uniaxially deformed samples are s h o w n in Fig. 3. As seen in the figure, the Jc of the uniaxially d e f o r m e d sample is substantially higher than that of the u n d e f o r m e d 123 throughout the whole range of 0. In addition, the Jc value at B I I c is found to be as high as the Jc value at B I I a-b. Moreover, the peak at B II c is extremely broad and extends all the w a y up to 10 ° away from the a-b plane peak. The broad peak at B I I c can be explained b y considering point pinning by dislocations created b y the deformation process. Since the dislocations are confined to the a-b planes, the flux lines will intersect the dislocations at discrete points when the field is directed at an angle to the a-b planes. Since the dislocations are a few mtcrons long, each of them can be anticipated to interact with several flux lines. When melt-textured 123 is penetrated by a magnetic field, the projected flux line spacing on the a-b plane, ao*, can be expressed as
equation, it can be seen that at a given B, the n u m b e r of flux lines intersected b y a n y dislocation will increase with decreasing 0 due to the reduction in ao*. Therefore, the flux pinning force can be expected to increase with decreasing flux line lattice spacing (at a given field), and the Jc will then be proportional to (cos 0) 0"5. As a result, Jc at B I I c will be a maximum (0 = 0) and its value decreases as 0 is increased. This (cos 0) 0.5 relationship is plotted in Fig. 3 and can be seen to fit the peak in Jc over an angular range up to 50 ° from B I I c. From this analysis, it can be concluded that dislocations created by uniaxial deformation are effective point pinning centers and increase the Jc at B II c and intermediate field orientations The angular dependence of Jc of a sample deformed by hot isostatic pressing is plotted In Fig. 4. As in the case of the unlaxially d e f o r m e d sample, the Jc behavior of the hipped sample reveals a broad peak in Jc centered at B I I c which extends to about 10° from the a-b plane. It can also be seen that this peak can be described by a (cos 0) 0.5 relationship. Therefore, u s i n g the same a r g u m e n t as in the case of uniaxmally d e f o r m e d samples, the broad peak in Jc centered at B I I c in Fig. 4 is considered to be due to point pinning by dislocations lying on the a-b plane that were created b y hot isostatlc pressing. ACKNOWLEDGMENT : This w o r k is s u p p o r t e d b y the Texas C e n t e r for S u p e r c o n d u c t i v i t y at the University of H o u s t o n under Prime Grant MDA 972-88-G-0002 from DARPA and the state of Texas. REFERENCES
1. 2. 3. 4.
where [3 is a parameter related to the flux-line geometry and ~o is the flux quantum. From this
5. 6.
:
M. Murakaml et al. Supercond. Sci. and Technol. 4 (1991) $49. S. Jin et al. Physica C. 181 (1991) 57. P. McGinn et al. Physlca C. 176 (1991) 203. K. Salama et al. Appl. Phys. Lett. 54 (1989) 2352. D.F. Lee et al. Physlca C. 202 (1992) 83. V. Selvamanickam et al. J. Mater. Res. 8 (1993) 249.
Physlca C 209 (1993) 187-190 North-Holland
Process-induced
flux pinning in melt-textured YBCO superconductor
K. Salama, V. Selvamanickam, and D. F. Lee Department of Mechanical Engineering, and Texas Center for Superconductivity University of Houston, Houston, TX, U.S.A. In this paper, we summarize two processing approaches which have been used to produce pinning centers in melt-textured YBCO resulting in the enhancement of current density. The first approach is the addition of nonsuperconducting inclusions such as Y2BaCuO5, and the second is high temperature mechanical deformation to generate dislocations and stacking faults.
1.
PINNING BY Y2BaCuO5 PARTICLES
It is well k n o w n that normal-phase particles in conventional superconductors can be effective pinning centers In melt-textured Y B a 2 C u 3 0 x (123), however the Y2BaCuO 5 (211) particles themselves are unlikely to be pinning sites since their dimensions are much larger than the coherence length of the HTS. Yet, the interfaces between particles and their associated defects such as stacking faults and dislocations may be able to provide pinning. N u m e r o u s studies have shown that the presence of 211 particles are effective in flux pinning [1], and the current density (Jc) increases with increasing 211 content. Other mvestigators, h o w e v e r , have d e t e r m i n e d that Jc is either independent of the amount of 211 inclusions [2] or even decreases with 211 content [3]. In order to clarify the influence of 211 on Jc, m e a s u r e m e n t s of the angular dependence of ]c have been conducted on melttextured 123 containing different 211 particle size and content. Melt-textured specimens with 0 to 50 vol.% 'large' rod-shaped 211 inclusions of 4 to 20 larn size were manufactured by varying the slow cooling rate of the liquid phase processing technique [4]. Also, melt-textured 123 containing 'small' spherical 211 mclusions of 0.5 - 2 lam size were fabricated through the addition of 211 particles to the start material [5]. The dependence of Jc of these samples on 0921-4534/93/506 00 © 1993 - Elsevier Science Pubhshers B V
the orientation of an external magnetic field w i t h r e s p e c t to their a-b p l a n e was determined by tranport current measurements. These measurements were performed at 77 K and 1.5 T with the field aligned at different angles to the a-b plane. The results obtained for the specimens with different 'large' 211 contents are shown in Fig. 1. It can be seen from this figure that the Jc at all field orientations decreases with increasing a m o u n t of 211 particles. The angular dependence of Jc for the specimens with 'small' 211 inclusions at 1.5 T are shown m Fig. 2. It is evident from this figure that there is a consistent improvement in the c u r r e n t c a r r y i n g capability with i n c r e a s i n g m i c r o n - s i z e 211 a d d i t i o n throughout the whole angular range. The opposite effects of 211 on Jc in these two types of specimens m a y be reconciled if the influences of particle size and magnetic field are taken into consideration. If the 123/211 Interfaces are effective pinning sites, then Jc should increase with the surface area of the inclusions, i.e., with 211 content of a fixed particle size, as in the case of conventional superconductors. However, this increase in Jc will not be monotonic because as the n u m b e r of particles is increased, the superconducting volume is reduced. A balance between these two influences will necessitate an optimal second-phase content for a given particle size, where further increase in the amount of parhcles will result in a decrease in All rights reserved
188
K Salama et al / Process-mduced flux pruning m melt-textured YBCO superconductor
the Jc of the material [5]. This optimal second-phase content will vary with the particle size, shifting to a higher percentage when the inclusions are small. Based on these results and the probable existence of an optimal 211 content which varies with the 30000
size of the inclusions, it is proposed that the d e p e n d e n c e of Jc on 211 content in melttextured 123 is not unique, but changes according to the regime of the 211 content, the size of the 211 inclusions, and the strength of the applied magnetic field.
................................... 77K, 1 5 T 'Large' 211 inclusions
25000
•
•
17vo1.%211
o
30 vol % 211
•
50voL%211
•
•
~-- 20000 • d~e "- 15000q
0 0 •
100O0
•
•
•
•
o oo oooo
•
0
o o oO
0
(3°
o •
o
•
•
•
~oo o
•
•
, o , o , o, oo o
5000 .
0
.
i
.
15
.
,
.
.
a
.
.
I
.
.
i
.
.
i
.
.
i
.
.
i
.
.
i
.
.
i
.
.
i
.
.
30 45 60 75 90 105 120 135 150 165 180 Angle between field and c-axis (degrees)
Figure 1 Angular dependence of Jc on field angle of melt-textured 123 with varying amount of 'large' 211 inclusions at 77 K and 1.5 T. 50000 77K, 1 5 T 'Small' 21 !mclustons
40000 O
w" 30000
~+~O 0
0
[] 0
• • x + X o D
0% 211 added (#1) 0% 211 added (#2) 10% 211 added (#1) 10% 211 added (#2) 10% 211 added (#3) 15%211 added(#1) 15% 211 added (#2)
0
0
0
00 0
0
~
0
0
O~
00o001
oo 0o~ +
10000
0
0
15
30 45 60 75 90 105 120 135 150 165 180 Angle between field and c-axis (degrees)
Figure 2 Angular dependence of Jc on field angle of melt-textured 123 containing varying amount of "small" 211 inclusions at 77 K and 1.5 T.
189
K Salama et al /Process-mducedfluxpmnmg m melt-textured YBCO superconductor
40000
.
~
oo
35000
Ii
~o
30000
{,#
°°
\ o
•
e/ -
o/
-
25000 0~)0 0
20000
~
(cos ¢) 0.~ fit
0
o oOODOO O O O O ooOO
~Oo O O O O O OGODo
1500(]
-30
0
30
60
90
120
150
180
210
Angle between magnetic field and c-axis (degrees) Figure 3. Angular dependence of Jc of undeformed and uniaxially deformed melt-textured samples at 1.5 T and 77 K. The expected (cos 0) 0.5 behavior from point pinning by dislocations is also included. 2600(3
' ' 1 ' ' 1 ' ' 1 '
I
I
2400(3
4[
2200(3
@
I
I
'1
I
14
1
01t~-t~
1 -60
Figure 4
-30
0 30 > 60 90 120 150 180 210 Angle between field and c-axis (degrees)
240
Angular dependence of Jc of an isostatically deformed melt-textured sample at 1.5 T and 77K.
190
2.
K Salama et al / Process-tnduced flux pmnmg m melt-textured YBCO superconductol
P I N N I N G BY DISLOCATIONS
Dislocations and stacking faults are two common crystal defects that exist within melttextured 123 superconductors. If the density of these defects can be increased, it could be p o s s i b l e to e n h a n c e the Jc of these superconductors in magnetic field orientations other than B I I a-b and decrease the strong sensitivity of Jc to field orientation. It has been shown that a high density of dislocations can be generated in the a-b plane of melttextured YBCO s u p e r c o n d u c t o r s b y high temperature uniaxial and isostatic deformation processing [6]. The d e f o r m e d samples were found to exhibit a dislocation density of 1010 cm -2 which is an order of magnitude higher than that in undeformed samples. The dependence of Jc on the angle between the magnetic field and the c-axis at 1.5 T and 77 K in undeformed and uniaxially deformed samples are s h o w n in Fig. 3. As seen in the figure, the Jc of the uniaxially d e f o r m e d sample is substantially higher than that of the u n d e f o r m e d 123 throughout the whole range of 0. In addition, the Jc value at B I I c is found to be as high as the Jc value at B I I a-b. Moreover, the peak at B II c is extremely broad and extends all the w a y up to 10 ° away from the a-b plane peak. The broad peak at B I I c can be explained b y considering point pinning by dislocations created b y the deformation process. Since the dislocations are confined to the a-b planes, the flux lines will intersect the dislocations at discrete points when the field is directed at an angle to the a-b planes. Since the dislocations are a few mtcrons long, each of them can be anticipated to interact with several flux lines. When melt-textured 123 is penetrated by a magnetic field, the projected flux line spacing on the a-b plane, ao*, can be expressed as
equation, it can be seen that at a given B, the n u m b e r of flux lines intersected b y a n y dislocation will increase with decreasing 0 due to the reduction in ao*. Therefore, the flux pinning force can be expected to increase with decreasing flux line lattice spacing (at a given field), and the Jc will then be proportional to (cos 0) 0"5. As a result, Jc at B I I c will be a maximum (0 = 0) and its value decreases as 0 is increased. This (cos 0) 0.5 relationship is plotted in Fig. 3 and can be seen to fit the peak in Jc over an angular range up to 50 ° from B I I c. From this analysis, it can be concluded that dislocations created by uniaxial deformation are effective point pinning centers and increase the Jc at B II c and intermediate field orientations The angular dependence of Jc of a sample deformed by hot isostatic pressing is plotted In Fig. 4. As in the case of the unlaxially d e f o r m e d sample, the Jc behavior of the hipped sample reveals a broad peak in Jc centered at B I I c which extends to about 10° from the a-b plane. It can also be seen that this peak can be described by a (cos 0) 0.5 relationship. Therefore, u s i n g the same a r g u m e n t as in the case of uniaxmally d e f o r m e d samples, the broad peak in Jc centered at B I I c in Fig. 4 is considered to be due to point pinning by dislocations lying on the a-b plane that were created b y hot isostatlc pressing. ACKNOWLEDGMENT : This w o r k is s u p p o r t e d b y the Texas C e n t e r for S u p e r c o n d u c t i v i t y at the University of H o u s t o n under Prime Grant MDA 972-88-G-0002 from DARPA and the state of Texas. REFERENCES
1. 2. 3. 4.
where [3 is a parameter related to the flux-line geometry and ~o is the flux quantum. From this
5. 6.
:
M. Murakaml et al. Supercond. Sci. and Technol. 4 (1991) $49. S. Jin et al. Physica C. 181 (1991) 57. P. McGinn et al. Physlca C. 176 (1991) 203. K. Salama et al. Appl. Phys. Lett. 54 (1989) 2352. D.F. Lee et al. Physlca C. 202 (1992) 83. V. Selvamanickam et al. J. Mater. Res. 8 (1993) 249.