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Observation of attractive forces in zone-melted YBa2Cu3O7−x
Volume 9, number
MATERIALS
5,6
OBSERVATION
OF ATTRACTIVE
FORCES
March
LETTERS
IN ZONE-MELTED
1990
YBa&u@_,
W.H. CHEN, N. ZHU and P.J. McGINN Department of Materials Science and Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Received
1 December
1989
Attractive forces between zone-melted YBa2Cus0,_, (YBCO) samples and a magnet beneath the samples were observed. Attractive forces were observed only among the samples with large magnetic hysteresis loops. The magnitude of the observed attractive forces is related to the sample’s critical current density, J,, and the magnetic flux gradient the sample experiences.
1. Introduction Suspension phenomena have been observed in silver-doped YBCO [ I] and melt-textured YBCO [ 2 ] high-T, superconductors. Since the high-temperature superconductors are type II superconductors, magnetic flux can be trapped in the materials. Suspension of a superconductor below a magnet occurs when the attractive forces due to flux trapped by the sample equal the gravitational and repulsive forces [ 3 1. However, attractive forces for the geometric arrangement of high-temperature superconductors above a permanent magnet were not observed until recently. Nguyen et al. [ 4 ] quantitatively measured the forces between a sample of a YBCO composite and a magnet. Marshall et al. [ 5 ] measured the forces between both YBCO and Tl-Ba-Ca-Cu-0 superconductors and a magnet. Repulsive forces between the superconductors and the magnet were observed for all the samples but attractive forces were observed only between the Tl-Ba-Ca-Cu-0 superconductor and the magnet. In this Letter we report the observation of attractive forces between zone-melted YBCO samples and a magnet.
and cooled by liquid nitrogen at a distance of about 50 cm from a 2.2 kG permanent magnet. The magnet is disk-like in shape with 38 mm in diameter and 6 mm in thickness. The experimental apparatus, as shown in fig. 1, permits the container with the sample to be moved up and down relative to the magnet of 70 g which rests on an electric balance. The balance, Fisher Scientific, model S-400, with an accuracy of 0.01 g is calibrated by standard weights and zeroed prior to the experimental measurement. The insulating container and supporting materials are
2. Experimental Aligned samples were prepared by zone melting YBCO bars as described earlier [ 6 1. The cylindrical shaped samples of 2 mm in diameter and 15 mm in length were fixed to a thermal insulating container 0167-577x/90/$
03.50 0 Elsevier
Science Publishers
Fig. 1. Schematic diagram of the experimental measure the attractive and repulsive forces.
B.V. (North-Holland)
apparatus
used to
189
Volume 9, number 5,6
MATERIALS LETTERS
March 1990
non-magnetic in order to minimize possible errors. The empty insulating container has been subjected to the experimental conditions of the force measurements and as expected, no force (zero reading) has been recorded by the balance.
3. Results and discussion The microstructure of the sample was studied by X-ray diffraction, SEM and polarized optical microscopy. The cleaved surface of a grain in the textured structure is shown in fig. 2a in a polarized light micrograph. Both heavy twinning and fine precipitates are observed in the samples. In fig. 2b, a welloriented structure of the sample is shown in an SEM micrograph of a fracture surface of the zone-melted sample. Fig. 3 shows the magnetic hysteresis loop of an aligned sample at 77 K measured on a vibrating sample magnetometer with the c axis of the textured sample parallel to the applied field. Also shown for reference are the hysteresis loops of a sintered Agdoped sample and a sintered YBCO sample before texturing. Critical current densities of the textured sample are approximately 20 000 and 18 000 A/cm* at 1 T and 2 T magnetic fields respectively, according to Bean’s model [7] using the assumption of strong links. Transport dc measurement of J, = 1000 A/cm* at 77 K at 1.5 T has been recorded, using the criterion of 1 uV/cm. The low-field hysteresis loop of the sample is shown in fig. 4. When a textured YBCO sample is moved toward the magnet from where it was cooled to 77 K at zero field, a repulsive (positive) force is observed on the balance. If the sampIe is stopped after moving towards the magnet, the repulsive forces initially decrease rapidly and then decrease more slowly, finally approaching a nearly constant value. However, the largest force value observed occurs during the motion. This repulsive force continues to decrease over a period of approximately 30 min, presumably due to flux creep. When the sample is moved away from the permanent magnet from the previous position, a negative force is shown by the balance, an indication of an attractive force. The attractive force behaves similarly to the repulsive force, in that its maximum value is recorded during the motion, it decreases quickly initially, then decreases slowly to a nearly 190
Fig. 2. (a) Polarized optical micrograph for a fracture surface of the zone-melted YBCO sample. Twin planes, { 1lo}, {1iO}, are perpendicular to each other, which indicates the fracture surface is a cleaved plane of (001). (b) SEM mi~o~aph of a fracture surface of zone-melted YBCO (showing a well-oriented structure). The surface is fractured perpendicular to the surface shown in (a).
constant value. These phenomena cannot be observed in the system when the superconductor is replaced by a permanent magnet. In this case, only an attractive force (negative reading) or repulsive force (positive reading) is observed. The reading only depends on the relative distance between the magnets and is independent of the speed of motion. Fig. 5 shows the measurement of the force as a function of distance and direction of motion between the superconductor and the magnet starting from a zeroforce position 50 cm away from the magnet. Data
Volume 9, number
MATERIALS
5,6
Magnetic
Field
(KOe)
Fig. 3. The hysteresis loops at 77 K of as-sintered doped (B ) and zone-melted (C) YBCO samples.
~1
Fig. 4. The low-field 77 K.
1
0 Magnetic
hysteresis
(A), silver-
2
F,eld (KOe)
loop of a zone-melted
sample
at
March
LETTERS
used in fig. 5 were the readings of the quasi-constant forces at the respective distances from the magnet. Even at a distance at which the repulsive forces were zero, attractive forces were still recorded. The attractive forces were observed only in the YBCO samples with a relatively large hysteresis loop, like that shown in fig. 3, curve C. The large hysteresis loop is due to the large amount of flux trapped by pinning sites in the large textured grains. Features that may be responsible for flux pinning in these samples include precipitates, twins and dislocations. The large grains contain both precipitates and twins, as evidenced by the polarized optical micrograph as shown in fig. 2. Greater dislocation densities were observed in textured samples than in sintered samples both in our samples and other work [ 2 1. Such dislocations may be generated by impurity phases due to different thermal expansion coefficients from the YBCO matrix. When there is relative motion of the superconductor away from the magnet, the sample will behave like a magnet, even though the trapped field will decrease slowly due to flux creep, causing a negative (attractive) force as observed. The attractive forces can be considered as having two components. One component is the contribution of the magnetization of the superconductor by an external field. It can be calculated from the hysteresis loop (fig. 4) and the gradient of the magnetic field [ 31. This is the contribution of the trapped magnetic field. It is independent of the speed of sample motion and can be described mathematically as F, = c, MdH/dz
121
9 6
to \
--o--dH/dt>
0
-*-dH/dt<
0
\ .
0
\
k, 9
-3 I 0
0.0, O-o-oy~a-La&e.
:
._A---
l *a, 10
20
I 30 Distance
II 40
50
l 60
,
(1)
where M is the magnetization, H is the value of the external field, z is the direction of the motion, and c, is a constant. The other component of the force is due to the magnetic flux change of the superconductor. Any flux change will cause a variation of electric potential in the vicinity of the flux and induce a current in the conductor according to the Maxwell equation
10
3
1990
70
(mm)
Fig. 5. Forces between a liquid-nitrogen-cooled YBCO sample and a permanent magnet as a function of distance from the magnet as the sample is moved toward the magnet (aH/af > 0) and away from the magnet (aH/at
V’x E= - aB/& . The magnetic field generated by the induced superconducting current will oppose the flux change. This force, therefore, can be presented F~ =c,PaBlat
,
(2) 191
Volume 9, number 5,6
MATERIALS LETTERS
where c2 is a constant, P is associated with the contribution of all the non-superconducting phases in the superconductor. Due to the change of flux, which is proportional to the term aBlat and is a function of motion speed, the force Ii2 changes with the speed at which the magnet and superconductor move relative each other. The total force, F, is the sum of the two, F=F, +F2 =~~MaH/az+c,PaBl&
.
(3)
The experimental observations can thus be explained according to eq. (3): The force is dominated by aB/& whenever there is relatively fast motion between the sample and the magnet; when the relative motion is stopped, the force proportional to aBlat is zero, so the remaining force which is recorded in fig. 5 is the contribution of the pinning force. This force is relatively constant for a fixed position, and because flux decay occurs slowly, a quasi-constant force has been recorded.
4. Conclusions
An attractive force can be observed between a highly textured YBCO superconductor and a permanent magnet situated below it. Highly oriented, large grain samples prepared by zone melting show large hysteresis loops. The large loop is crucial to observe the attractive force. The attractive force is pro-
192
March 1990
portional to the relative motion, decreases with time and can be detected at a greater distance than can repulsive forces.
Acknowledgement
This research has been supported by the Indiana Center for Innovative Superconductor Technology through funding from the Indiana Corporation for Science and Technology.
References [l] P.N. Peters, R.C. Sisk, E.W. Urban, C.Y. Huang and M.K. Wu, Appl. Phys. Letters 52 ( 1988) 2066. [2] S. Jin, R.C. Sherwood, E.M. Gyorgy, T.H. Tiefel, R.B. van Dover, S. Nakahara, L.F. Schneemeyer, R.A. Fastnacht and M.E. Davis, Appl. Phys. Letters 54 ( 1989) 584. [3] C.Y. Huang, Y. Shapira, E.J. McNiff Jr., P.N. Peters, B.B. Schwartz, M.K. Wu, R.D. Shull and C.K. Chiang, Mod. Phys. Letters B 2 (1988) 869. [4] H.V. Nguyen, T.T. Srinivasan, R.E. Newnham and A.S. Bhalla, in: 1989 Talk Summaries, Electronic Division of the American Ceramic Society Annual Meeting, Paper Number 112-SVI-89. [5] D.B. Marshall, R.E. Dewanes,P.E.D. MorganandJ.J.Ratto, in: 1989 Talk Summaries, Electronic Division of the American Ceramic Society Annual Meeting, Paper Number 114~SVI-89. [6] P.J. McGinn, W. Chen and M. Black, Physica C 161 (1989) 198. [7] C.P. Bean, Phys. Rev. Letters 8 (1962) 250.
MATERIALS
5,6
OBSERVATION
OF ATTRACTIVE
FORCES
March
LETTERS
IN ZONE-MELTED
1990
YBa&u@_,
W.H. CHEN, N. ZHU and P.J. McGINN Department of Materials Science and Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Received
1 December
1989
Attractive forces between zone-melted YBa2Cus0,_, (YBCO) samples and a magnet beneath the samples were observed. Attractive forces were observed only among the samples with large magnetic hysteresis loops. The magnitude of the observed attractive forces is related to the sample’s critical current density, J,, and the magnetic flux gradient the sample experiences.
1. Introduction Suspension phenomena have been observed in silver-doped YBCO [ I] and melt-textured YBCO [ 2 ] high-T, superconductors. Since the high-temperature superconductors are type II superconductors, magnetic flux can be trapped in the materials. Suspension of a superconductor below a magnet occurs when the attractive forces due to flux trapped by the sample equal the gravitational and repulsive forces [ 3 1. However, attractive forces for the geometric arrangement of high-temperature superconductors above a permanent magnet were not observed until recently. Nguyen et al. [ 4 ] quantitatively measured the forces between a sample of a YBCO composite and a magnet. Marshall et al. [ 5 ] measured the forces between both YBCO and Tl-Ba-Ca-Cu-0 superconductors and a magnet. Repulsive forces between the superconductors and the magnet were observed for all the samples but attractive forces were observed only between the Tl-Ba-Ca-Cu-0 superconductor and the magnet. In this Letter we report the observation of attractive forces between zone-melted YBCO samples and a magnet.
and cooled by liquid nitrogen at a distance of about 50 cm from a 2.2 kG permanent magnet. The magnet is disk-like in shape with 38 mm in diameter and 6 mm in thickness. The experimental apparatus, as shown in fig. 1, permits the container with the sample to be moved up and down relative to the magnet of 70 g which rests on an electric balance. The balance, Fisher Scientific, model S-400, with an accuracy of 0.01 g is calibrated by standard weights and zeroed prior to the experimental measurement. The insulating container and supporting materials are
2. Experimental Aligned samples were prepared by zone melting YBCO bars as described earlier [ 6 1. The cylindrical shaped samples of 2 mm in diameter and 15 mm in length were fixed to a thermal insulating container 0167-577x/90/$
03.50 0 Elsevier
Science Publishers
Fig. 1. Schematic diagram of the experimental measure the attractive and repulsive forces.
B.V. (North-Holland)
apparatus
used to
189
Volume 9, number 5,6
MATERIALS LETTERS
March 1990
non-magnetic in order to minimize possible errors. The empty insulating container has been subjected to the experimental conditions of the force measurements and as expected, no force (zero reading) has been recorded by the balance.
3. Results and discussion The microstructure of the sample was studied by X-ray diffraction, SEM and polarized optical microscopy. The cleaved surface of a grain in the textured structure is shown in fig. 2a in a polarized light micrograph. Both heavy twinning and fine precipitates are observed in the samples. In fig. 2b, a welloriented structure of the sample is shown in an SEM micrograph of a fracture surface of the zone-melted sample. Fig. 3 shows the magnetic hysteresis loop of an aligned sample at 77 K measured on a vibrating sample magnetometer with the c axis of the textured sample parallel to the applied field. Also shown for reference are the hysteresis loops of a sintered Agdoped sample and a sintered YBCO sample before texturing. Critical current densities of the textured sample are approximately 20 000 and 18 000 A/cm* at 1 T and 2 T magnetic fields respectively, according to Bean’s model [7] using the assumption of strong links. Transport dc measurement of J, = 1000 A/cm* at 77 K at 1.5 T has been recorded, using the criterion of 1 uV/cm. The low-field hysteresis loop of the sample is shown in fig. 4. When a textured YBCO sample is moved toward the magnet from where it was cooled to 77 K at zero field, a repulsive (positive) force is observed on the balance. If the sampIe is stopped after moving towards the magnet, the repulsive forces initially decrease rapidly and then decrease more slowly, finally approaching a nearly constant value. However, the largest force value observed occurs during the motion. This repulsive force continues to decrease over a period of approximately 30 min, presumably due to flux creep. When the sample is moved away from the permanent magnet from the previous position, a negative force is shown by the balance, an indication of an attractive force. The attractive force behaves similarly to the repulsive force, in that its maximum value is recorded during the motion, it decreases quickly initially, then decreases slowly to a nearly 190
Fig. 2. (a) Polarized optical micrograph for a fracture surface of the zone-melted YBCO sample. Twin planes, { 1lo}, {1iO}, are perpendicular to each other, which indicates the fracture surface is a cleaved plane of (001). (b) SEM mi~o~aph of a fracture surface of zone-melted YBCO (showing a well-oriented structure). The surface is fractured perpendicular to the surface shown in (a).
constant value. These phenomena cannot be observed in the system when the superconductor is replaced by a permanent magnet. In this case, only an attractive force (negative reading) or repulsive force (positive reading) is observed. The reading only depends on the relative distance between the magnets and is independent of the speed of motion. Fig. 5 shows the measurement of the force as a function of distance and direction of motion between the superconductor and the magnet starting from a zeroforce position 50 cm away from the magnet. Data
Volume 9, number
MATERIALS
5,6
Magnetic
Field
(KOe)
Fig. 3. The hysteresis loops at 77 K of as-sintered doped (B ) and zone-melted (C) YBCO samples.
~1
Fig. 4. The low-field 77 K.
1
0 Magnetic
hysteresis
(A), silver-
2
F,eld (KOe)
loop of a zone-melted
sample
at
March
LETTERS
used in fig. 5 were the readings of the quasi-constant forces at the respective distances from the magnet. Even at a distance at which the repulsive forces were zero, attractive forces were still recorded. The attractive forces were observed only in the YBCO samples with a relatively large hysteresis loop, like that shown in fig. 3, curve C. The large hysteresis loop is due to the large amount of flux trapped by pinning sites in the large textured grains. Features that may be responsible for flux pinning in these samples include precipitates, twins and dislocations. The large grains contain both precipitates and twins, as evidenced by the polarized optical micrograph as shown in fig. 2. Greater dislocation densities were observed in textured samples than in sintered samples both in our samples and other work [ 2 1. Such dislocations may be generated by impurity phases due to different thermal expansion coefficients from the YBCO matrix. When there is relative motion of the superconductor away from the magnet, the sample will behave like a magnet, even though the trapped field will decrease slowly due to flux creep, causing a negative (attractive) force as observed. The attractive forces can be considered as having two components. One component is the contribution of the magnetization of the superconductor by an external field. It can be calculated from the hysteresis loop (fig. 4) and the gradient of the magnetic field [ 31. This is the contribution of the trapped magnetic field. It is independent of the speed of sample motion and can be described mathematically as F, = c, MdH/dz
121
9 6
to \
--o--dH/dt>
0
-*-dH/dt<
0
\ .
0
\
k, 9
-3 I 0
0.0, O-o-oy~a-La&e.
:
._A---
l *a, 10
20
I 30 Distance
II 40
50
l 60
,
(1)
where M is the magnetization, H is the value of the external field, z is the direction of the motion, and c, is a constant. The other component of the force is due to the magnetic flux change of the superconductor. Any flux change will cause a variation of electric potential in the vicinity of the flux and induce a current in the conductor according to the Maxwell equation
10
3
1990
70
(mm)
Fig. 5. Forces between a liquid-nitrogen-cooled YBCO sample and a permanent magnet as a function of distance from the magnet as the sample is moved toward the magnet (aH/af > 0) and away from the magnet (aH/at
V’x E= - aB/& . The magnetic field generated by the induced superconducting current will oppose the flux change. This force, therefore, can be presented F~ =c,PaBlat
,
(2) 191
Volume 9, number 5,6
MATERIALS LETTERS
where c2 is a constant, P is associated with the contribution of all the non-superconducting phases in the superconductor. Due to the change of flux, which is proportional to the term aBlat and is a function of motion speed, the force Ii2 changes with the speed at which the magnet and superconductor move relative each other. The total force, F, is the sum of the two, F=F, +F2 =~~MaH/az+c,PaBl&
.
(3)
The experimental observations can thus be explained according to eq. (3): The force is dominated by aB/& whenever there is relatively fast motion between the sample and the magnet; when the relative motion is stopped, the force proportional to aBlat is zero, so the remaining force which is recorded in fig. 5 is the contribution of the pinning force. This force is relatively constant for a fixed position, and because flux decay occurs slowly, a quasi-constant force has been recorded.
4. Conclusions
An attractive force can be observed between a highly textured YBCO superconductor and a permanent magnet situated below it. Highly oriented, large grain samples prepared by zone melting show large hysteresis loops. The large loop is crucial to observe the attractive force. The attractive force is pro-
192
March 1990
portional to the relative motion, decreases with time and can be detected at a greater distance than can repulsive forces.
Acknowledgement
This research has been supported by the Indiana Center for Innovative Superconductor Technology through funding from the Indiana Corporation for Science and Technology.
References [l] P.N. Peters, R.C. Sisk, E.W. Urban, C.Y. Huang and M.K. Wu, Appl. Phys. Letters 52 ( 1988) 2066. [2] S. Jin, R.C. Sherwood, E.M. Gyorgy, T.H. Tiefel, R.B. van Dover, S. Nakahara, L.F. Schneemeyer, R.A. Fastnacht and M.E. Davis, Appl. Phys. Letters 54 ( 1989) 584. [3] C.Y. Huang, Y. Shapira, E.J. McNiff Jr., P.N. Peters, B.B. Schwartz, M.K. Wu, R.D. Shull and C.K. Chiang, Mod. Phys. Letters B 2 (1988) 869. [4] H.V. Nguyen, T.T. Srinivasan, R.E. Newnham and A.S. Bhalla, in: 1989 Talk Summaries, Electronic Division of the American Ceramic Society Annual Meeting, Paper Number 112-SVI-89. [5] D.B. Marshall, R.E. Dewanes,P.E.D. MorganandJ.J.Ratto, in: 1989 Talk Summaries, Electronic Division of the American Ceramic Society Annual Meeting, Paper Number 114~SVI-89. [6] P.J. McGinn, W. Chen and M. Black, Physica C 161 (1989) 198. [7] C.P. Bean, Phys. Rev. Letters 8 (1962) 250.