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The effect of the compaction pressure on the microwave surface resistance of the Pb doped BSCCO system
Physlea C 235-240 (1994) 1995-1996 North-Holland
PHYSICA
The effect of the compaction pressure on the microwave surface res(stancc of the Pb doped BSCCO system R. Torres, C. de Francisco, J.M. Mufioz, M. Zazo*, L. Torres* and J. Ifiiguez* Departamento Electricidad y Electr6nica, Facui~ad de Ciencias, Universidad de Valladolid, 47071-Valladolid, Spain *Departamento Ffsica Aplicada, Facultad de Ffsicas, Universidad de Salamanca, 37071-Salamanca, Spain Microwave surface resistance is analyzed in Bi-based sample superconductors prepared under different compaction pressures (250 to 1500 MPa). The measurement is performed with the help of a new automatic measuring system based on a resonant cavity. It was found that the best results are obtained for samples initially sintered at low compaction pressure, then crushed and resintered for a short time at larger pressure.
1. INTRODUCTION
2. EXPERIMENTAL ASPECTS
Superconductor samples are obtained from BiSr-Ca-Cu-O system, which contain at least two highT c phases, called 2212 and 2223. An important effort has been focused to establish the mechanisms that promote the formation of the phase with the highest T c. In the present work, we focus our attention to the effect that the compaction pressure has on the superconducting properties after sintering the bulk material. In order to get the zero resistance state, it is necessary not only to obtain intrinsic, granular superconductivity but also a good coupling between the particles. Microwave surface resistance measurement constitutes one of the most interesting procedures for the characterization of these oxides, because of the possible applications of these materials in the high frequency range. Moreover, it should be remembered that the rmcrowave measurements are contactiess, and they permit to investigate the resistive behavior below the dc-zero resistance temperature. In this way, the surface resistance measured with the help of a new automatic measuring system based on a resonant cavity, is investigated. Finally, we try to stablish the best way of synthesize a good BSCCO superconductor in a short sintering time.
2.1. Sample preparation
0921-4534/94/S07.00 © 1994 - Elsevier Sclcnce B V. All rights rcservcd SSDI 0921-4534(94)01564-3
For our investigation a series of polycrystalline samples with Bi I 7Pbo 4Sr I 6Ca2 4Cu 3 6Ox nominal composition has been prepared from stoichiometncally rmxed powders of Bi203, PbO, SrCO 3, CaCO 3 and CuO. As it has been reported previously by Huang [1], this compositton gets better superconducting properties than others, with the same sintering time. The mixture was ball milled and calcined at 810°C for 30 h The samples were then pressed into small cylinders 5 mm in diameter at pressures varying from 250 to 1500 MPa (see table 1) and sintered at 852°C for 20 h in mr.
2.2. Measuring techniques The crystal structure of the samples was examined by X-ray powder &ffraction measurement The resistivity was measured by the usual low ttequeilcy
m~uiou~,
ouHa~c
resistance
measure-
ments were carried out with a computer-aided system based on the temperature record of losses within a resonant microwave cavity containing a small superconducting pellet, cut out from the cylindrical sample and placed in a maximum of the nucrowave magnetic field. A varactor tuned Gunn generator feeds the TE l i 1 resonant cavity The power reflected from the cavity goes back to a circulator and drives a power sensor. The frequency
R Torrt,s et al /Phvstca C 235-240 (1994) 1995-1996
1996
Table 1 Compaction pressure, critical temperatures and surface resistance of samples.
represent (fig. 1) the dependence of surface resistance of all the samples on temperature, a decrease with increasing pressure can be observed.
Sample Pressure ~MPa) _Te onset (K) T c zero (K) R s at 80K (m f~)
4. DISCUSSION
A 250 107.5 86 126
B 500 108 86.5 101
C 1000 108 82 107
D 1500 108 82 78
of operation is measured by means of a frequency counter coupled to the waveguide. The automatization of this system is achieved with the help of a computer that locks the oscillator at the resonant frequency of the cavity, carries out a small sweep and then calculates the quality factor. The experimental results are then processed using the analysis strategies described in [2]. Then, we can find the surface resistance of the superconductor material by measuring the loaded Q with and without the sample, and applying these results B
~ r
0
I .fl
I
~
L
~6
D
9O
qO~
q~O
TEMPERATURE (K) Figure 1. Temperature dependence of the m~crowave surface resistance. 3. E X P E R I M E N T A L RESULTS From the x-ray diffraction spectra, it can be observed that all the samples are composed by several phases such us CuO, the Iow-T c phase and the htgh-T c phase. The 2223 to 2212 phase ratio decreases w~th increasing pressure The resistw~ty data (table 1) show that all the samples have a T c onset at about i 10 K, but samples A and B reach the zero resistw~ty above 85 K whereas samples C and D reach this state at about 82 K. Finally if we
The oxidation of all the superconducting sample will be complete in a shorter sintering time if the structure is porous enough to allow the free exchange of oxygen atoms between the inner sample and the surrounding atmosphere. Therefore, the lowest pressed sample (in our case, the sample A) must contain the biggest amount of the high-T c phase of all the specimens. This explanation is coherent with the XRD patterns obtained. On the other hand a porous structure leads to a great electrical resistivity due to the coupling effects between neighboring grains. This coupling is controlled by the grain boundaries and non superconducting phases present in the bulk sample and results in the formation of a multiconnected network consisting of Josephson-junction-like weak links. Accordingly, it can be assumed that samples highly pressed will have bigger intergranular links and so, a greater conductivity. In this context, the sample D, in spite of having a great amount of the low-T c phase in its inner, has the high-T c grains intimately related on the surface region, and then its surface resistance will be the smallest of all the samples (see Fig. i) Fmally, we can conclude that in order to get a good hJgh-Tc BSCCO superconductor with a short smtering t~me, ~t is necessaD the sintering of the sample with small initial pressing, then crushing it, and reslntering for a small time with higher compression pressure. The resistivity of the sample at the normal state is much io~er after the resintermg and the zero resistance is reached near 87 K. The surface resistance is also reduced, indicating the decrease of the effective grainboundary thickness between the superconductirg particles REFERENCES 1 Y T IAuang, R G Lm, S W Lu, P . T . W u a n d W N Wang, App Phys Lett. 56 (1990) 779 2 R. Torres, J M Mufioz and C. de Francisco, Appl Phys A 54 (1992) 511
PHYSICA
The effect of the compaction pressure on the microwave surface res(stancc of the Pb doped BSCCO system R. Torres, C. de Francisco, J.M. Mufioz, M. Zazo*, L. Torres* and J. Ifiiguez* Departamento Electricidad y Electr6nica, Facui~ad de Ciencias, Universidad de Valladolid, 47071-Valladolid, Spain *Departamento Ffsica Aplicada, Facultad de Ffsicas, Universidad de Salamanca, 37071-Salamanca, Spain Microwave surface resistance is analyzed in Bi-based sample superconductors prepared under different compaction pressures (250 to 1500 MPa). The measurement is performed with the help of a new automatic measuring system based on a resonant cavity. It was found that the best results are obtained for samples initially sintered at low compaction pressure, then crushed and resintered for a short time at larger pressure.
1. INTRODUCTION
2. EXPERIMENTAL ASPECTS
Superconductor samples are obtained from BiSr-Ca-Cu-O system, which contain at least two highT c phases, called 2212 and 2223. An important effort has been focused to establish the mechanisms that promote the formation of the phase with the highest T c. In the present work, we focus our attention to the effect that the compaction pressure has on the superconducting properties after sintering the bulk material. In order to get the zero resistance state, it is necessary not only to obtain intrinsic, granular superconductivity but also a good coupling between the particles. Microwave surface resistance measurement constitutes one of the most interesting procedures for the characterization of these oxides, because of the possible applications of these materials in the high frequency range. Moreover, it should be remembered that the rmcrowave measurements are contactiess, and they permit to investigate the resistive behavior below the dc-zero resistance temperature. In this way, the surface resistance measured with the help of a new automatic measuring system based on a resonant cavity, is investigated. Finally, we try to stablish the best way of synthesize a good BSCCO superconductor in a short sintering time.
2.1. Sample preparation
0921-4534/94/S07.00 © 1994 - Elsevier Sclcnce B V. All rights rcservcd SSDI 0921-4534(94)01564-3
For our investigation a series of polycrystalline samples with Bi I 7Pbo 4Sr I 6Ca2 4Cu 3 6Ox nominal composition has been prepared from stoichiometncally rmxed powders of Bi203, PbO, SrCO 3, CaCO 3 and CuO. As it has been reported previously by Huang [1], this compositton gets better superconducting properties than others, with the same sintering time. The mixture was ball milled and calcined at 810°C for 30 h The samples were then pressed into small cylinders 5 mm in diameter at pressures varying from 250 to 1500 MPa (see table 1) and sintered at 852°C for 20 h in mr.
2.2. Measuring techniques The crystal structure of the samples was examined by X-ray powder &ffraction measurement The resistivity was measured by the usual low ttequeilcy
m~uiou~,
ouHa~c
resistance
measure-
ments were carried out with a computer-aided system based on the temperature record of losses within a resonant microwave cavity containing a small superconducting pellet, cut out from the cylindrical sample and placed in a maximum of the nucrowave magnetic field. A varactor tuned Gunn generator feeds the TE l i 1 resonant cavity The power reflected from the cavity goes back to a circulator and drives a power sensor. The frequency
R Torrt,s et al /Phvstca C 235-240 (1994) 1995-1996
1996
Table 1 Compaction pressure, critical temperatures and surface resistance of samples.
represent (fig. 1) the dependence of surface resistance of all the samples on temperature, a decrease with increasing pressure can be observed.
Sample Pressure ~MPa) _Te onset (K) T c zero (K) R s at 80K (m f~)
4. DISCUSSION
A 250 107.5 86 126
B 500 108 86.5 101
C 1000 108 82 107
D 1500 108 82 78
of operation is measured by means of a frequency counter coupled to the waveguide. The automatization of this system is achieved with the help of a computer that locks the oscillator at the resonant frequency of the cavity, carries out a small sweep and then calculates the quality factor. The experimental results are then processed using the analysis strategies described in [2]. Then, we can find the surface resistance of the superconductor material by measuring the loaded Q with and without the sample, and applying these results B
~ r
0
I .fl
I
~
L
~6
D
9O
qO~
q~O
TEMPERATURE (K) Figure 1. Temperature dependence of the m~crowave surface resistance. 3. E X P E R I M E N T A L RESULTS From the x-ray diffraction spectra, it can be observed that all the samples are composed by several phases such us CuO, the Iow-T c phase and the htgh-T c phase. The 2223 to 2212 phase ratio decreases w~th increasing pressure The resistw~ty data (table 1) show that all the samples have a T c onset at about i 10 K, but samples A and B reach the zero resistw~ty above 85 K whereas samples C and D reach this state at about 82 K. Finally if we
The oxidation of all the superconducting sample will be complete in a shorter sintering time if the structure is porous enough to allow the free exchange of oxygen atoms between the inner sample and the surrounding atmosphere. Therefore, the lowest pressed sample (in our case, the sample A) must contain the biggest amount of the high-T c phase of all the specimens. This explanation is coherent with the XRD patterns obtained. On the other hand a porous structure leads to a great electrical resistivity due to the coupling effects between neighboring grains. This coupling is controlled by the grain boundaries and non superconducting phases present in the bulk sample and results in the formation of a multiconnected network consisting of Josephson-junction-like weak links. Accordingly, it can be assumed that samples highly pressed will have bigger intergranular links and so, a greater conductivity. In this context, the sample D, in spite of having a great amount of the low-T c phase in its inner, has the high-T c grains intimately related on the surface region, and then its surface resistance will be the smallest of all the samples (see Fig. i) Fmally, we can conclude that in order to get a good hJgh-Tc BSCCO superconductor with a short smtering t~me, ~t is necessaD the sintering of the sample with small initial pressing, then crushing it, and reslntering for a small time with higher compression pressure. The resistivity of the sample at the normal state is much io~er after the resintermg and the zero resistance is reached near 87 K. The surface resistance is also reduced, indicating the decrease of the effective grainboundary thickness between the superconductirg particles REFERENCES 1 Y T IAuang, R G Lm, S W Lu, P . T . W u a n d W N Wang, App Phys Lett. 56 (1990) 779 2 R. Torres, J M Mufioz and C. de Francisco, Appl Phys A 54 (1992) 511