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Magnetic field dependence of critical current density of BiPbSrCaCuO polycrystalline superconductor
Solid State Communications, Printed in Great Britain. MAGNETIC
Vol. 73, No. 5, pp. 349-352, 1990.
0038-1098/90 $3.00 + .OO Pergamon Press plc
FIELD DEPENDENCE OF CRITICAL CURRENT DENSITY OF Bi-Pb-Sr-Ca-Cu-0 POLYCRYSTALLINE SUPERCONDUCTOR V. P&ha&k
National Research Institute for Materials, Opletalova 25, 113 12 Prague 1, Czechoslovakia and F. Gomory Institute of Electrical Engineering, Electra-Physical Research Centre, Slovak Academy of Sciences, 842 39 Bratislava, Czechoslovakia (Received 3 July 1989; in revised form 6 October 1989 by P.H. Dederichs) Macroscopic critical current densities &,, of the (B&,9Pb,,, ),SrZCaZCusO,,,+, polycrystalline strongly oriented sample have been measured by two inductive methods - AC susceptibility measurements and magnetic flux profile meaurements - up to superimposed DC magnetic field of 140mT. It is shown that the sample exhibits high i,(77 K) of 870 A/cm* and 200A/cmZ at magnetic field of zero and 50 mT, respectively.
1. INTRODUCTION THE CRITICAL current densities of polycrystalline (often multiphased) samples of the Bi-(Pb)-Sr-CaCu-0 system have been usually rather low particularly as compared with values of properly sintered samples of the Y-Ba-Cu-0 system (a few hundred A/cm* at 77 K). The possibility of solving this problem consists in preparing single-high T,( 110 K)-phase samples [l-4] or, at least, dominant-high T,-phase samples with a low content of the low T,(80 K) phase without other impurity phases. Remarkable improvements in the critical current density j, (700 A/cm* and 100 A/cm* at 77 K and zero magnetic field and 20 mT, resp.) have been achieved by a modified combination of heat treatment and double pressing [5]. Single-high T,phase sample prepared in a similar way exhibited j, (77 K, 0 T) of 1070 A/cm* [3]. This paper presents a preparation (by single pressing) of the Bi-Pb-Sr-Ca-Cu-0 strongly orientated sample with a major portion of the high TCphase and inductive measurements of its critical current densities and further improvements ofjc, especially in the magnetic field. 2. PREPARATION AND DESCRIPTION THE SAMPLE
OF
The sample with the nominal composition of (B&.9Pb0.,)2Sr2CaZCu30,,,+xwas prepared by common
powder metallurgical method from reagents of Bi20,, Pb02, SrCOa , CaCO, and CuO (for more details see another paper [4]). Appropriate amounts of the reagents were mixed and calcinated at 780°C for 100 h in air. Subsequent heating at 84OOC for 500 h in a special oxygen atmosphere [l] followed the calcination to form the high T, phase. The atmosphere contained oxygen with partial pressure of 7 kPa. The reacted powder with the content of the high T, phase and the low T, phase of approx. 85% and 15%. resp. (according to the results of X-ray diffraction analysis) was then pressed at an uniaxial pressure into a disk-shape pellet of 16mm diameter and 2 mm thickness and eventually sintered at 835°C for 32 h in the mentioned atmosphere. The cylindrical specimen of 2mm diameter and 15 mm height was ground off from this pellet. The results of the X-ray diffraction analysis have shown that the Bi-Pb-Sr-Ca-Cu-0 cylinder of the density of 5.5g/cm3 contains more than 90% of the high T, phase, the residual low T, phase and no other impurity phase, It has been also observed that the microstructure of the sample is strongly orientated with the crystallic c axis parallel to the direction of the pressing, i.e. perpendicular to the cylinder axis. The SEM observations have revealed that the grains are in a form of rather thin plates with following dimensions: length and width approximately from 2 pm to 15 pm and thickness h about 0.3 pm. 349
350
CURRENT
Vol. 73, No. 5
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
0.5
o 0
X” 0.25
‘cm
q
AC susceptibility flux profiles extrapolated
[A/cm’]
-1 80
90 T
kl
100
110 0
Fig. 1. Temperature dependencies of complex AC susceptibility for four different AC field amplitudes b measured in zero superimposed DC field. 3. INDUCTIVE
MEASUREMENTS
Two types of inductive measurements on the BiPb-Sr-Ca-Cu-0 cylinder have been carried out: (1) AC susceptibility measurements with successive determination of macroscopic critical current density
[61;
(2) magnetic flux profile measurements by Campbell’s method [7] at 77.3K to determine the field dependence of the macroscopic critical current density j, and the volume fraction of the superconducting grains in the sample [8]. A plot of the AC susceptibility parts dependence on the temperature is given in Fig. 1 (at a zero superimposed DC field Bd) and in Fig. 2 (at Bd = 50 mT). In Fig. 1, the interval of the grain transition is evident. It lies between 106 and 108 K. In this interval,
80
100
90 T [Kl
Fig. 3. Temperature dependencies of macroscopic critical current density j,, at three levels of superimposed DC field (0, 10 and 50mT). shielding is apparent due to supercurrents (x’ < 0), but the loss component x” remains zero. Under 106 K, macroscopic shielding caused by intergrain currents is dominant. At Bd = 50mT, effects of intragrain currents are not observable. The height of x” peaks is lower here than in the zero DC field, because at Bd $ b (AC field amplitude), the current density is practically constant in the sample cross section in contrary to the zero DC conditions [9]. From the set of AC susceptibility curves recorded at different b, the temperature dependence of the
b [mfl
7’0.5
-1 I
80
I
I
90
I
100
I
110
T [Kl
Fig. 2. Temperature dependencies of complex AC susceptibility for three different AC field amplitudes b measured in superimposed DC field of 50mT.
Fig. 4. Relation between AC field amplitude b and normalized penetration depth 6/R measured for different values of superimposed DC field Bd. Zero field curve cannot be used for evaluations of critical current density and volume content of superconducting grains due to evidently sharp field dependence of critical current density on local field. _
CURRENT
Vol. 73, No. 5
351
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
1000
I
f-7 D .
km
q
field cooling zero field cooling extrapolated value
I
T = 77.3 K
50 -
[A /cm*]
“g
0
0
50
100
150
Bd [mfl
Fig. 5. Dependence of macroscopic critical current density j, on superimposed DC magnetic field B,, at 77.3 K. Field cooling regime always gave higher values ofj,, than zero field cooling. macroscopic critical current density can be determined [6]. Data obtained at three levels of Bd are given in Fig. 3. The points determined from the profile measurements at 77.3 K (triangles) are inserted to the same figure, showing excellent agreement between both methods. The value at Bd = 0 could be extrapolated, because maximum b = 7mT is the upper limit of our apparatus [lo]. The set of flux profiles, i.e. the curves relating AC field amplitude b to the normalized penetration depth 6/R (R is the sample radius) is plotted in Fig. 4. Zero field profile exhibits peculiarities caused by nonconstant j, in the sample cross section at a low DC field [9]. Other profiles carry the character typical for high T, polycrystalline samples: they consist of two parts, related to the intergrain and intragrain supercurrents. At lower AC fields, the macroscopic intergrain currents shield the whole sample. At certain b, kink occurs, followed by an interval with a much higher slope. It is ascribed to the intragrain currents, by several orders of magnitude higher than the intergrain ones. From the intergrain part of profiles, j, can be obtained. We have determined in this way its field dependence at 77.3 K (see Fig. 5). Besides the points obtained at the zero field cooling, those determined in the field cooling regime are given for comparison. The difference is much lower than that observed in YBaCuO polycrystalline samples. From the position of the kink in the flux profile, the volume fraction of superconducting grains in the sample can be determined [8]. In our sample, it
0
1 50
1 100 Bd [mTI
150
Fig. 6. Relative content of superconducting grains in the sample volume VPdetermined from magnetic flux profile measurements. depends relatively weakly on the magnetic field and can be estimated at 25% (see Fig. 6).This value, however, gives a volume fraction of superconducting grain not penetrated by the magnetic field. As in our sample the half-thickness of grains (h/2 = 0.15 pm) is comparable to the penetration depth A of Bi,Pb-Sr-CaCu-0 superconductors (,l 5 0.14pm [l l]), a great portion of the volume of superconducting grains is not included in this value. That is why the presence of the relatively high fraction (90%) of high T, phase measured by X-ray diffraction in the sufficient dense sample does not contradict the relatively low value (25%) obtained from the magnetic flux profile measurements. 4. CONCLUSIONS Taking into account the polycrystalline character of the sample with the nominal composition of (B~,Pbo,l)2SrzCa~Cu~0,0+,, surprisingly high macroscopic critical current densitiesj,, have been obtained. At 77.3 K, j,, = 870 A/cm* in the zero superimposed DC field Bd, while at Bd = 50mT j,, it still remains 200 A/cm*. The magnetic field dependence of the macroscopic critical current density indicates that the weak links between the superconducting grains play an essential role in the electric current transport in the sample. REFERENCES 1. 2. 3.
4.
U. Endo, S. Koyama & T. Kawai, Jpn J. Appl. Phys. 27, L1476 (1988). S. Koyama, U. Endo 8z T. Kawai, Jpn J. Appl. Phys. 27, L1861 (1988). C.G. Cui, J.L. Zhang, S.L. Li, J. Li, F. Shi, S.Z. Zhou, Z.H. Shi & J. Dou, Solid State Commun 70, 287 (1989).
V. Plechheek, H. Hejdovii & 2. Trejbalovh, Cryogenics (in press).
352 5. 6. 7. 8.
CURRENT
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
Y. Tanaka, T. Asano, K. Jikihara, M. Fukutomi, J. Machida t H. Maeda, Jpn J. Appl. Phys. 27, 1655 (1988). F. Giimiiry & P. Lobotka, Solid State Commun. 66, 645 (1988). A.M. Campbell, J. Phys. C2, 1492 (1969). H. Kiipfer, I. Apfelstedt, R. Fliikiger, C. Keller, R. Meier-Hirmer, B. Run&h, A. Turowski, U. Wiech & T. Wolf, Cryogenics 28, 650 (1988).
9. 10, 11.
Vol. 73, No. 5
F. Giimory, S. Takacs, P. Lobotka, K. Friilich & V. P&h&k, Physicu CM, 1 (1989). P. Lobotka 8z F. Gomiiry, Phys. Status Solid (a),, 109, 205 (1988). S. Martin, A.T. Fiory, R.M. Fleming, L.F. Schneemeyer Jz J.V. Waszczak, Phys. Rev. Lett. 60,2194 (1988).
Vol. 73, No. 5, pp. 349-352, 1990.
0038-1098/90 $3.00 + .OO Pergamon Press plc
FIELD DEPENDENCE OF CRITICAL CURRENT DENSITY OF Bi-Pb-Sr-Ca-Cu-0 POLYCRYSTALLINE SUPERCONDUCTOR V. P&ha&k
National Research Institute for Materials, Opletalova 25, 113 12 Prague 1, Czechoslovakia and F. Gomory Institute of Electrical Engineering, Electra-Physical Research Centre, Slovak Academy of Sciences, 842 39 Bratislava, Czechoslovakia (Received 3 July 1989; in revised form 6 October 1989 by P.H. Dederichs) Macroscopic critical current densities &,, of the (B&,9Pb,,, ),SrZCaZCusO,,,+, polycrystalline strongly oriented sample have been measured by two inductive methods - AC susceptibility measurements and magnetic flux profile meaurements - up to superimposed DC magnetic field of 140mT. It is shown that the sample exhibits high i,(77 K) of 870 A/cm* and 200A/cmZ at magnetic field of zero and 50 mT, respectively.
1. INTRODUCTION THE CRITICAL current densities of polycrystalline (often multiphased) samples of the Bi-(Pb)-Sr-CaCu-0 system have been usually rather low particularly as compared with values of properly sintered samples of the Y-Ba-Cu-0 system (a few hundred A/cm* at 77 K). The possibility of solving this problem consists in preparing single-high T,( 110 K)-phase samples [l-4] or, at least, dominant-high T,-phase samples with a low content of the low T,(80 K) phase without other impurity phases. Remarkable improvements in the critical current density j, (700 A/cm* and 100 A/cm* at 77 K and zero magnetic field and 20 mT, resp.) have been achieved by a modified combination of heat treatment and double pressing [5]. Single-high T,phase sample prepared in a similar way exhibited j, (77 K, 0 T) of 1070 A/cm* [3]. This paper presents a preparation (by single pressing) of the Bi-Pb-Sr-Ca-Cu-0 strongly orientated sample with a major portion of the high TCphase and inductive measurements of its critical current densities and further improvements ofjc, especially in the magnetic field. 2. PREPARATION AND DESCRIPTION THE SAMPLE
OF
The sample with the nominal composition of (B&.9Pb0.,)2Sr2CaZCu30,,,+xwas prepared by common
powder metallurgical method from reagents of Bi20,, Pb02, SrCOa , CaCO, and CuO (for more details see another paper [4]). Appropriate amounts of the reagents were mixed and calcinated at 780°C for 100 h in air. Subsequent heating at 84OOC for 500 h in a special oxygen atmosphere [l] followed the calcination to form the high T, phase. The atmosphere contained oxygen with partial pressure of 7 kPa. The reacted powder with the content of the high T, phase and the low T, phase of approx. 85% and 15%. resp. (according to the results of X-ray diffraction analysis) was then pressed at an uniaxial pressure into a disk-shape pellet of 16mm diameter and 2 mm thickness and eventually sintered at 835°C for 32 h in the mentioned atmosphere. The cylindrical specimen of 2mm diameter and 15 mm height was ground off from this pellet. The results of the X-ray diffraction analysis have shown that the Bi-Pb-Sr-Ca-Cu-0 cylinder of the density of 5.5g/cm3 contains more than 90% of the high T, phase, the residual low T, phase and no other impurity phase, It has been also observed that the microstructure of the sample is strongly orientated with the crystallic c axis parallel to the direction of the pressing, i.e. perpendicular to the cylinder axis. The SEM observations have revealed that the grains are in a form of rather thin plates with following dimensions: length and width approximately from 2 pm to 15 pm and thickness h about 0.3 pm. 349
350
CURRENT
Vol. 73, No. 5
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
0.5
o 0
X” 0.25
‘cm
q
AC susceptibility flux profiles extrapolated
[A/cm’]
-1 80
90 T
kl
100
110 0
Fig. 1. Temperature dependencies of complex AC susceptibility for four different AC field amplitudes b measured in zero superimposed DC field. 3. INDUCTIVE
MEASUREMENTS
Two types of inductive measurements on the BiPb-Sr-Ca-Cu-0 cylinder have been carried out: (1) AC susceptibility measurements with successive determination of macroscopic critical current density
[61;
(2) magnetic flux profile measurements by Campbell’s method [7] at 77.3K to determine the field dependence of the macroscopic critical current density j, and the volume fraction of the superconducting grains in the sample [8]. A plot of the AC susceptibility parts dependence on the temperature is given in Fig. 1 (at a zero superimposed DC field Bd) and in Fig. 2 (at Bd = 50 mT). In Fig. 1, the interval of the grain transition is evident. It lies between 106 and 108 K. In this interval,
80
100
90 T [Kl
Fig. 3. Temperature dependencies of macroscopic critical current density j,, at three levels of superimposed DC field (0, 10 and 50mT). shielding is apparent due to supercurrents (x’ < 0), but the loss component x” remains zero. Under 106 K, macroscopic shielding caused by intergrain currents is dominant. At Bd = 50mT, effects of intragrain currents are not observable. The height of x” peaks is lower here than in the zero DC field, because at Bd $ b (AC field amplitude), the current density is practically constant in the sample cross section in contrary to the zero DC conditions [9]. From the set of AC susceptibility curves recorded at different b, the temperature dependence of the
b [mfl
7’0.5
-1 I
80
I
I
90
I
100
I
110
T [Kl
Fig. 2. Temperature dependencies of complex AC susceptibility for three different AC field amplitudes b measured in superimposed DC field of 50mT.
Fig. 4. Relation between AC field amplitude b and normalized penetration depth 6/R measured for different values of superimposed DC field Bd. Zero field curve cannot be used for evaluations of critical current density and volume content of superconducting grains due to evidently sharp field dependence of critical current density on local field. _
CURRENT
Vol. 73, No. 5
351
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
1000
I
f-7 D .
km
q
field cooling zero field cooling extrapolated value
I
T = 77.3 K
50 -
[A /cm*]
“g
0
0
50
100
150
Bd [mfl
Fig. 5. Dependence of macroscopic critical current density j, on superimposed DC magnetic field B,, at 77.3 K. Field cooling regime always gave higher values ofj,, than zero field cooling. macroscopic critical current density can be determined [6]. Data obtained at three levels of Bd are given in Fig. 3. The points determined from the profile measurements at 77.3 K (triangles) are inserted to the same figure, showing excellent agreement between both methods. The value at Bd = 0 could be extrapolated, because maximum b = 7mT is the upper limit of our apparatus [lo]. The set of flux profiles, i.e. the curves relating AC field amplitude b to the normalized penetration depth 6/R (R is the sample radius) is plotted in Fig. 4. Zero field profile exhibits peculiarities caused by nonconstant j, in the sample cross section at a low DC field [9]. Other profiles carry the character typical for high T, polycrystalline samples: they consist of two parts, related to the intergrain and intragrain supercurrents. At lower AC fields, the macroscopic intergrain currents shield the whole sample. At certain b, kink occurs, followed by an interval with a much higher slope. It is ascribed to the intragrain currents, by several orders of magnitude higher than the intergrain ones. From the intergrain part of profiles, j, can be obtained. We have determined in this way its field dependence at 77.3 K (see Fig. 5). Besides the points obtained at the zero field cooling, those determined in the field cooling regime are given for comparison. The difference is much lower than that observed in YBaCuO polycrystalline samples. From the position of the kink in the flux profile, the volume fraction of superconducting grains in the sample can be determined [8]. In our sample, it
0
1 50
1 100 Bd [mTI
150
Fig. 6. Relative content of superconducting grains in the sample volume VPdetermined from magnetic flux profile measurements. depends relatively weakly on the magnetic field and can be estimated at 25% (see Fig. 6).This value, however, gives a volume fraction of superconducting grain not penetrated by the magnetic field. As in our sample the half-thickness of grains (h/2 = 0.15 pm) is comparable to the penetration depth A of Bi,Pb-Sr-CaCu-0 superconductors (,l 5 0.14pm [l l]), a great portion of the volume of superconducting grains is not included in this value. That is why the presence of the relatively high fraction (90%) of high T, phase measured by X-ray diffraction in the sufficient dense sample does not contradict the relatively low value (25%) obtained from the magnetic flux profile measurements. 4. CONCLUSIONS Taking into account the polycrystalline character of the sample with the nominal composition of (B~,Pbo,l)2SrzCa~Cu~0,0+,, surprisingly high macroscopic critical current densitiesj,, have been obtained. At 77.3 K, j,, = 870 A/cm* in the zero superimposed DC field Bd, while at Bd = 50mT j,, it still remains 200 A/cm*. The magnetic field dependence of the macroscopic critical current density indicates that the weak links between the superconducting grains play an essential role in the electric current transport in the sample. REFERENCES 1. 2. 3.
4.
U. Endo, S. Koyama & T. Kawai, Jpn J. Appl. Phys. 27, L1476 (1988). S. Koyama, U. Endo 8z T. Kawai, Jpn J. Appl. Phys. 27, L1861 (1988). C.G. Cui, J.L. Zhang, S.L. Li, J. Li, F. Shi, S.Z. Zhou, Z.H. Shi & J. Dou, Solid State Commun 70, 287 (1989).
V. Plechheek, H. Hejdovii & 2. Trejbalovh, Cryogenics (in press).
352 5. 6. 7. 8.
CURRENT
DENSITY OF Bi-Pb-Sr-Ca-Cu-0
Y. Tanaka, T. Asano, K. Jikihara, M. Fukutomi, J. Machida t H. Maeda, Jpn J. Appl. Phys. 27, 1655 (1988). F. Giimiiry & P. Lobotka, Solid State Commun. 66, 645 (1988). A.M. Campbell, J. Phys. C2, 1492 (1969). H. Kiipfer, I. Apfelstedt, R. Fliikiger, C. Keller, R. Meier-Hirmer, B. Run&h, A. Turowski, U. Wiech & T. Wolf, Cryogenics 28, 650 (1988).
9. 10, 11.
Vol. 73, No. 5
F. Giimory, S. Takacs, P. Lobotka, K. Friilich & V. P&h&k, Physicu CM, 1 (1989). P. Lobotka 8z F. Gomiiry, Phys. Status Solid (a),, 109, 205 (1988). S. Martin, A.T. Fiory, R.M. Fleming, L.F. Schneemeyer Jz J.V. Waszczak, Phys. Rev. Lett. 60,2194 (1988).