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Transition behavior of magnetic susceptibility and electrical resistivity of Y-Ba-Cu-O superconductor
Physica 148B (1987) 449-452 North-Holland, Amsterdam
T R A N S I T I O N BEHAVIOR OF M A G N E T I C S U S C E P T I B I L I T Y AND E L E C T R I C A L
RESISTIVITY OF Y - B a - C u - O SUPERCONDUCTOR H. M O R I T A , K. W A T A N A B E , Y. M U R A K A M I , S. K O N D O , Y. O B I , K. N O T O , H. F U J I M O R I and Y. M U T O Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan
Received 10 August 1987 The difference between the onset critical temperatures determined by the measurements of a dc-electrical resistivity (Toot) and of an ac-susceptibility (Tcox) was examined. In the early stage of the calcination, the resistivity can be zero even when the fraction of the superconductive volume (V) is about 3-6%, but the difference between T¢or and Toox is large. When V is larger than 60%, Toor and Tooxcoincide well with each other. The V dependence of Jc is drastically large around V= 100%, therefore, a very high completeness of the superconductive compound is indispensable to a high Jc-
I. Introduction
2. Experimental procedure
It has been pointed out for the high-T c superconducting oxides that there are apparent differences between the superconducting characteristics determined by a susceptibility and by a dc-electrical resistivity measurement. For example, a susceptibility-temperature curve gives a lower critical temperature than a resistivitytemperature curve [1]. Furthermore, a critical current density estimated from the susceptibility data is about 100-1000 times larger than that measured by the dc four probe method [2]. These differences are considered to originate from the fact that the susceptibility measurement detects the sum of partial susceptibilities, whereas the resistivity measurement detects the resistivity of a conductive path which is a connection of small parts with the highest critical temperature. In this experiment, we have examined such differences in more detail. A series of YiBa2Cu3-oxide samples with a different fraction of the superconductive volume were prepared, and their dc-electrical resistivity, acsusceptibility and critical current were measured. On the basis of these data, the reasons for the difference and the formation process of the superconducting c o m p o u n d are discussed.
A series of YiBa2Cu3-oxide samples were prepared from the mixture of Y203(99.9%), BACO3(99.99% ) and CUO(99.99%). The raw oxide mixture of 40 g was ground by the Labo. mill (Yamato Nitto UT-21) for 3 hours and pressed to disks of 18 mm in diameter and about 3 m m in thickness. Then it was calcined at 1170 K for various hours in air and slowly cooled at a rate of 100 K/h. After the annealing of 14 h, some disk samples were reground and pressed to disks and annealed again under the same condition. The heat treatments were performed both in air and in flowing oxygen. T c and Jc were determined by usual dc four terminal electrical measurements. Electrical contact was made by ultrasonic soldering. Temperature below 100 K was measured by precalibrated carbon glass resistor with the accuracy of 0.5 K. Magnetic fields were produced by a water-cooled magnet [3]. Susceptibility was measured by a mutual induction ac-bridge method [4].
0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
3. Results and discussion In order to examine the relation of the difference in T c to the volume fraction of supercon-
450
H. Morita et al. / Transition behavior in YtBa2Cu3-oxide
ducting compound (V), the samples with different V were prepared by changing the annealing time, and the electrical resistivity (R) and the susceptibility (Xg) were measured. Fig. 1 shows the annealing time dependences of R and Xg. In this paper, the critical temperatures are defined as shown in fig. lc. The R - T curve deviates from the linear normal resistive curve at Tooh. And the tangent to the superconducting transition curve intersects the extrapolated normal resistive curve and the horizontal axis at T~or and Tcf , respectively. The Xg- T curve deviates from the linear line at Tcox. The specimen, which is annealed at l 1 7 0 K for 4 h after grinding the raw oxide mixture, shows very small Xg which begins to appear around 85 K and increases only to 0.0018 e m u / g at 5 K (fig. la). On the other hand, R increases with decreasing temperature down to 91 K like the resistance of a semiconductor. The negative Xg cannot be observed around Tco h (91 K), and the value of Xg ( 0 . 0 0 0 8 e m u / g ) at T~f means that the volume fraction V is only about 6% (the Xg of a 100% superconductive volume is 0.0135emu/g). By 0 ,..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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the annealing at l 1 7 0 K for 14h, the V value increases to about 70% at 5 K, but the V value is about 2% at Tcf where R reduces to zero sharply (fig. lb). These V values at Tcf are very small compared to the value deduced by the percolation theory [5]. The regrinding and the subsequent annealing enhance the compound formation (fig. lc). The value of Xg means that the superconductive volume is nearly 100% below about 65 K and 20% at Tcf of 88 K. However it should be noticed that the grinding damages the superconductive compound severely. Fig. ld shows that the value of Xg is reduced to 0.0055 from 0.0135 e m u / g by one grinding, that is, V is reduced to 40% from 100%. The R - T curve behaves like that of semiconductor and T c can not be observed down to 77 K. Fig. 2 shows the diffraction patterns for the same specimens as fig. 1. After the annealing for 4 h (fig. 2a), the considerably sharp diffraction lines of the Y]Ba2Cu3-oxides are observed, though fig. la implies that the superconductive volume is about 13%. The diffraction lines bejo~
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Fig. 1. Heat treatment dependences of the ac-susceptibility (Xg) and the electrical resistivity (R) for Y1Ba2Cu3-oxide.
20
40 2e(deg)
60
Fig. 2. Heat treatment dependence of the X-ray diffraction pattern for YjBa2Cu3-oxide.
451
H. Morita et al. / Transition behavior in Y~Ba2Cu3-oxide
come sharper with increasing annealing time or with increasing volume of YxBa2Cu3-oxide, and the (013) line characteristic to the orthorhombic phase begins to appear at the low angle side of (103) line in fig. 2c. The Y~Ba2Cu3-oxide compound is mainly orthorhombic, but the tetragonal compound also coexists since the Xgtemperature curves are step like forms. The diffraction lines of the elemental oxides and carbonate are observed in all patterns, though their intensities reduce with increasing annealing time. In order to make clear the difference between the critical temperatures determined by the resistivity and by the susceptibility measurements, the annealing time dependence of the critical temperatures and V at Tcf are shown in fig. 3. As can be seen in the figure, the annealing time dependence of T~ox is small but that of T~oh, T~o~ and Tel are large, and the difference among them is large for the specimens annealed for 2 and 4 h. The V values of these specimens at T~f (Vr~) of 3.3 and 6.3% are very small compared to the expected value by the percolation theory. Therefore, it is considered that the formation of the superconductive compound starts on the surface of the elemental oxide particles. Then the electrical path between the electrodes can be completed even when the V value is very small. In
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Fig. 4. Critical current density under the magnetic field of 10 T (J~(10T)) as a function of the superconductive volume (V) for Y1Ba2Cu3-oxide.
the case of the 2 h annealing, the high-Tc region is considered to be connected insufficiently since the V value is too small and T~ox is higher than T~oh and T~or. It should be noticed that Tcox and T~or of the specimen annealed more than 14 h coincide well each other, that is, /'co x and T~or coincide well when the V value at 5 K is more than 60%. Fig. 4 shows the critical current (J~) measured at 4.2 K under the magnetic field of 10T as a function of the fraction of the superconductive volume (V4.2K). Though the specimens with small V4.2K less than 60% have Toor and Tcox higher than 90 K, their Jc (10 T) is nearly zero. Therefore, superconducting area of these specimens is considered to be isolated and the connection between them is very weak. Around the V value of about 100%, the V dependence of J~ is drastic. Therefore, the very small fraction of the imperfect volume existing between the superconducting area is considered to play a very important role for high Jc.
s l
(~ 2 4
i
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i
1~2 Ann. Time ( h o u r s )
4
0
Fig. 3. Annealing time dependences of the critical temperatures (T~) and the fraction of the superconducting volume at T~ (Vr~f) for Y1Ba2Cu3-0xide. T~ is defined in fig. lc.
4. Summary From the examination of the critical current (Jc) and of the critical temperatures determined by the measurements of an ac-susceptibility (/'cox) and of a dc-electrical resistivity (Tcor) for
452
H. Morita et al. / Transition behavior in Y1Ba2Cu3-oxide
the specimens with the various fractions of superconductive volume (V), the following results are obtained. In the early stage of the calcination, the resistivity can be zero even when the V is about 3 - 6 % , since the superconductive c o m p o u n d is formed only on the surface of the elemental oxide particles. Therefore, Jc is nearly zero and the difference between Toox and Toor appears because of the incomplete connection of the superconductive parts. With increasing V, the difference becomes small. The V dependence of Jc is drastic around 100%, therefore, the
completeness of the superconductive compound is very important for the high Jc.
References [1] D.C. Larbalestier et al., Submitted to J. Appl. Phys. (1987). [2] D.K. Finnemore et al., Proc. ICMC 1987 (Illinois). [3] Y. Nakagawa et al., Sci. Rep. RITU A33 (1986) 251. [4] C.M. Brodbeck, R.R. Bukrey and J.T. Hoeksema, Rev. Sci. Instrum. 49 (1978) 1279. [5] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574.
T R A N S I T I O N BEHAVIOR OF M A G N E T I C S U S C E P T I B I L I T Y AND E L E C T R I C A L
RESISTIVITY OF Y - B a - C u - O SUPERCONDUCTOR H. M O R I T A , K. W A T A N A B E , Y. M U R A K A M I , S. K O N D O , Y. O B I , K. N O T O , H. F U J I M O R I and Y. M U T O Institute for Materials Research, Tohoku University, Katahira, Sendai 980, Japan
Received 10 August 1987 The difference between the onset critical temperatures determined by the measurements of a dc-electrical resistivity (Toot) and of an ac-susceptibility (Tcox) was examined. In the early stage of the calcination, the resistivity can be zero even when the fraction of the superconductive volume (V) is about 3-6%, but the difference between T¢or and Toox is large. When V is larger than 60%, Toor and Tooxcoincide well with each other. The V dependence of Jc is drastically large around V= 100%, therefore, a very high completeness of the superconductive compound is indispensable to a high Jc-
I. Introduction
2. Experimental procedure
It has been pointed out for the high-T c superconducting oxides that there are apparent differences between the superconducting characteristics determined by a susceptibility and by a dc-electrical resistivity measurement. For example, a susceptibility-temperature curve gives a lower critical temperature than a resistivitytemperature curve [1]. Furthermore, a critical current density estimated from the susceptibility data is about 100-1000 times larger than that measured by the dc four probe method [2]. These differences are considered to originate from the fact that the susceptibility measurement detects the sum of partial susceptibilities, whereas the resistivity measurement detects the resistivity of a conductive path which is a connection of small parts with the highest critical temperature. In this experiment, we have examined such differences in more detail. A series of YiBa2Cu3-oxide samples with a different fraction of the superconductive volume were prepared, and their dc-electrical resistivity, acsusceptibility and critical current were measured. On the basis of these data, the reasons for the difference and the formation process of the superconducting c o m p o u n d are discussed.
A series of YiBa2Cu3-oxide samples were prepared from the mixture of Y203(99.9%), BACO3(99.99% ) and CUO(99.99%). The raw oxide mixture of 40 g was ground by the Labo. mill (Yamato Nitto UT-21) for 3 hours and pressed to disks of 18 mm in diameter and about 3 m m in thickness. Then it was calcined at 1170 K for various hours in air and slowly cooled at a rate of 100 K/h. After the annealing of 14 h, some disk samples were reground and pressed to disks and annealed again under the same condition. The heat treatments were performed both in air and in flowing oxygen. T c and Jc were determined by usual dc four terminal electrical measurements. Electrical contact was made by ultrasonic soldering. Temperature below 100 K was measured by precalibrated carbon glass resistor with the accuracy of 0.5 K. Magnetic fields were produced by a water-cooled magnet [3]. Susceptibility was measured by a mutual induction ac-bridge method [4].
0378-4363/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation
3. Results and discussion In order to examine the relation of the difference in T c to the volume fraction of supercon-
450
H. Morita et al. / Transition behavior in YtBa2Cu3-oxide
ducting compound (V), the samples with different V were prepared by changing the annealing time, and the electrical resistivity (R) and the susceptibility (Xg) were measured. Fig. 1 shows the annealing time dependences of R and Xg. In this paper, the critical temperatures are defined as shown in fig. lc. The R - T curve deviates from the linear normal resistive curve at Tooh. And the tangent to the superconducting transition curve intersects the extrapolated normal resistive curve and the horizontal axis at T~or and Tcf , respectively. The Xg- T curve deviates from the linear line at Tcox. The specimen, which is annealed at l 1 7 0 K for 4 h after grinding the raw oxide mixture, shows very small Xg which begins to appear around 85 K and increases only to 0.0018 e m u / g at 5 K (fig. la). On the other hand, R increases with decreasing temperature down to 91 K like the resistance of a semiconductor. The negative Xg cannot be observed around Tco h (91 K), and the value of Xg ( 0 . 0 0 0 8 e m u / g ) at T~f means that the volume fraction V is only about 6% (the Xg of a 100% superconductive volume is 0.0135emu/g). By 0 ,..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-4
lo
.." ......
Ix]0-;
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the annealing at l 1 7 0 K for 14h, the V value increases to about 70% at 5 K, but the V value is about 2% at Tcf where R reduces to zero sharply (fig. lb). These V values at Tcf are very small compared to the value deduced by the percolation theory [5]. The regrinding and the subsequent annealing enhance the compound formation (fig. lc). The value of Xg means that the superconductive volume is nearly 100% below about 65 K and 20% at Tcf of 88 K. However it should be noticed that the grinding damages the superconductive compound severely. Fig. ld shows that the value of Xg is reduced to 0.0055 from 0.0135 e m u / g by one grinding, that is, V is reduced to 40% from 100%. The R - T curve behaves like that of semiconductor and T c can not be observed down to 77 K. Fig. 2 shows the diffraction patterns for the same specimens as fig. 1. After the annealing for 4 h (fig. 2a), the considerably sharp diffraction lines of the Y]Ba2Cu3-oxides are observed, though fig. la implies that the superconductive volume is about 13%. The diffraction lines bejo~
3.OH 2a
2 151~
- 8
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-12
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.|0
lb
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~-8
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.... "
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4
2
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0/2d
......
20 40 60 80 T(K)
60 20°
Fig. 1. Heat treatment dependences of the ac-susceptibility (Xg) and the electrical resistivity (R) for Y1Ba2Cu3-oxide.
20
40 2e(deg)
60
Fig. 2. Heat treatment dependence of the X-ray diffraction pattern for YjBa2Cu3-oxide.
451
H. Morita et al. / Transition behavior in Y~Ba2Cu3-oxide
come sharper with increasing annealing time or with increasing volume of YxBa2Cu3-oxide, and the (013) line characteristic to the orthorhombic phase begins to appear at the low angle side of (103) line in fig. 2c. The Y~Ba2Cu3-oxide compound is mainly orthorhombic, but the tetragonal compound also coexists since the Xgtemperature curves are step like forms. The diffraction lines of the elemental oxides and carbonate are observed in all patterns, though their intensities reduce with increasing annealing time. In order to make clear the difference between the critical temperatures determined by the resistivity and by the susceptibility measurements, the annealing time dependence of the critical temperatures and V at Tcf are shown in fig. 3. As can be seen in the figure, the annealing time dependence of T~ox is small but that of T~oh, T~o~ and Tel are large, and the difference among them is large for the specimens annealed for 2 and 4 h. The V values of these specimens at T~f (Vr~) of 3.3 and 6.3% are very small compared to the expected value by the percolation theory. Therefore, it is considered that the formation of the superconductive compound starts on the surface of the elemental oxide particles. Then the electrical path between the electrodes can be completed even when the V value is very small. In
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60
80
V~2K(%)
100
Fig. 4. Critical current density under the magnetic field of 10 T (J~(10T)) as a function of the superconductive volume (V) for Y1Ba2Cu3-oxide.
the case of the 2 h annealing, the high-Tc region is considered to be connected insufficiently since the V value is too small and T~ox is higher than T~oh and T~or. It should be noticed that Tcox and T~or of the specimen annealed more than 14 h coincide well each other, that is, /'co x and T~or coincide well when the V value at 5 K is more than 60%. Fig. 4 shows the critical current (J~) measured at 4.2 K under the magnetic field of 10T as a function of the fraction of the superconductive volume (V4.2K). Though the specimens with small V4.2K less than 60% have Toor and Tcox higher than 90 K, their Jc (10 T) is nearly zero. Therefore, superconducting area of these specimens is considered to be isolated and the connection between them is very weak. Around the V value of about 100%, the V dependence of J~ is drastic. Therefore, the very small fraction of the imperfect volume existing between the superconducting area is considered to play a very important role for high Jc.
s l
(~ 2 4
i
I
i
1~2 Ann. Time ( h o u r s )
4
0
Fig. 3. Annealing time dependences of the critical temperatures (T~) and the fraction of the superconducting volume at T~ (Vr~f) for Y1Ba2Cu3-0xide. T~ is defined in fig. lc.
4. Summary From the examination of the critical current (Jc) and of the critical temperatures determined by the measurements of an ac-susceptibility (/'cox) and of a dc-electrical resistivity (Tcor) for
452
H. Morita et al. / Transition behavior in Y1Ba2Cu3-oxide
the specimens with the various fractions of superconductive volume (V), the following results are obtained. In the early stage of the calcination, the resistivity can be zero even when the V is about 3 - 6 % , since the superconductive c o m p o u n d is formed only on the surface of the elemental oxide particles. Therefore, Jc is nearly zero and the difference between Toox and Toor appears because of the incomplete connection of the superconductive parts. With increasing V, the difference becomes small. The V dependence of Jc is drastic around 100%, therefore, the
completeness of the superconductive compound is very important for the high Jc.
References [1] D.C. Larbalestier et al., Submitted to J. Appl. Phys. (1987). [2] D.K. Finnemore et al., Proc. ICMC 1987 (Illinois). [3] Y. Nakagawa et al., Sci. Rep. RITU A33 (1986) 251. [4] C.M. Brodbeck, R.R. Bukrey and J.T. Hoeksema, Rev. Sci. Instrum. 49 (1978) 1279. [5] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574.