- Email: [email protected]
The effect of cooling rates on properties of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy superconductors produced by solid-state reaction method
Physica C 451 (2007) 113–117 www.elsevier.com/locate/physc
The effect of cooling rates on properties of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy superconductors produced by solid-state reaction method O. Ozturk a, D. Yegen a, M. Yilmazlar b, A. Varilci a, C. Terzioglu a
a,*
Department of Physics, Faculty of Arts and Sciences, Abant Izzet Baysal University, 14280 Bolu, Turkey b Department of Physics, Faculty of Arts and Science, Kirikkale University, 71450 Kirikkale, Turkey Received 28 August 2006; received in revised form 1 November 2006; accepted 6 November 2006 Available online 13 December 2006
Abstract We have investigated the effect of the cooling rates in the Bi-2223 superconducting samples prepared by standard solid-state reaction method using electrical resistivity, transport critical current density, and XRD (X-ray diffraction) measurements. We estimated the transition temperature values from the DC resistivity measurements. We observed that transition temperature, Tc, and transport critical current density, J trans , depend on the cooling rates of the samples. They both increase with increasing cooling rates. We estimated the peak c temperature, Tp, from our previous ac susceptibility measurements. The pinning force density increased with increasing the cooling rate. XRD patterns are used to calculate lattice parameter c and obtain information about Bi-2223 phase ratio. 2006 Elsevier B.V. All rights reserved. Keywords: Transition temperature; Intergranular critical temperature; Pinning force; Peak temperature; Bean model
1. Introduction A tremendous amount of work on Bi(Pb)–Sr–Ca–Cu–O system has been done since its discovery. Both doping with different elements at various amounts and adjusting preparation condition affect phase formation and physical properties in this system. Different cooling speeds are employed in fabrication process, which interacts with other processing parameters such as heat treatment duration and temperature. Different cooling speeds are also related to adjustment of the oxygen content during the cooling stage [1]. It is useful to observe the variation of both the superconducting properties as well as the normal state properties of materials with changing cooling rates in order to better understand superconductivity. AC susceptibility and DC resistivity measurements are useful tool to investigate the magnetic and superconducting
*
Corresponding author. E-mail address: [email protected] (C. Terzioglu).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.11.003
properties. In particular, they are indispensable for making the distinction between intergrain and intragrain properties. At high ac fields, two loss peaks are usually present in the imaginary part of susceptibility data; a broad peak at low temperature (coupling losses) due to the motion of intergranular (Josephson) vortices [2] and a narrower peak (intrinsic losses) due to the motion of intragranular (Abrikosov) vortices inside the superconducting grains near T onset [3]. These two peaks depend on the sample proc cessing variables as well as the samples composition for the Bi-2223 system [4,5]. Early study showed that the transition temperature and room temperature resistivity of the Bi0.7Pb0.3Sr1Cu1.8Oy depend on the rate of cooling from the sintering temperature [6]. Yegen et al. [7] performed low field AC susceptibility versus temperature measurements on the same samples that are used in this study. They observed that peak temperatures, Tp, and intergranular critical current density, J inter ðT p Þ, increase with increasing cooling rate. In the latter c decade, the cooling rate was found to be an important parameter for optimizing performance of Ag-sheathed
114
O. Ozturk et al. / Physica C 451 (2007) 113–117
Bi-2223 superconductors [8,9]. They found that the value of J trans for the Ag-sheathed samples increases with c decreasing cooling rate. It was interpreted that the enhancement of J trans with decreasing cooling rate is due c to increases in the flux pinning strength. Terzioglu et al. [1] have recently investigated the effect of cooling rates on oxygen content of Bi-2223 samples with and without silver sheathing. They obtained that J trans of bare bulk samples c increases while that of Ag-sheathed samples decreases with increasing cooling rate. Cooling rates are important for improving superconducting properties such as transport critical current density, transition temperature and intergranular connectivity. This importance is closely related to changing oxygen content during cooling rate. In this work, we report measurements of electrical resistivity as a function of temperature, and transport critical current density in order to investigate the effect of the cooling rates on the activation energy in the mixed state and intergrain properties of Bi-2223 superconductors. We also reported XRD measurements to calculate the lattice parameter and the relative portion of Bi-2223 and that of Bi-2212 phases. 2. Experimental We prepared superconducting powders with Bi1.7Pb0.35Sr1.9Ca2.1Cu3Ox chemical stoichiometry. Several calcination steps were applied with intermediate grindings at temperatures up to 820 C. Each calcination step took 12 h. A number of tablets were made under 1 GPa uni-axial pressure. They were then heat treated twice at 830 C for 48 h with grinding and re-pelletising after the first heat treatment. After re-pressing, these samples were then heat treated at 835 C for 48 h. The furnace was heated at 100 C/h to the annealing temperature and at the end of the heat treatment 25 C/h, 50 C/h and 100 C/h cooling rates were employed to cool down to the room temperature. After cooling to room temperature, the pellets were cut into rectangular bars (3 mm · 2 mm · 12 mm) for resistivity and critical current measurements. The measurements of DC resistivity and transport critical current density J trans were performed with the fourc probe method on all samples. Both voltage and current contacts were made with silver paint, the contact resistance being in the order of 0.1 X or lower. The outside pin-points feed the current and the inner ones (the distance between voltage probes is approximately 7 mm) allow the voltage drop measurement. We determined J trans property from c the I–V curves at 77 K, following the 1 lV/cm criteria. Resistive contacts caused heating and the test was completed in possible short time, in order to avoid this effect. We measured temperature (40–130 K) dependence of resistivity of the samples running 5 mA dc current through the samples in the cryostat. A Keithley 220 programmable current source and a Keithley 2182A nano-voltmeter were used for the resistivity and I–V measurements. The transition temperature, Tc, is the temperature at which a resis-
tance-less percolation path is established. In bulk samples of Bi-2223, this path goes through many grain boundaries providing valuable information about them. XRD data were taken using a Rigaku D/Max-IIIC diffractometer with CuKa radiation in the range 2h = 4– 60 with a scan speed of 3/min and a step increment of 0.02 at room temperature. Phase purity and the lattice parameters were obtained from these XRD patterns. The accuracy in determining the lattice parameter c was ˚ . The mean values of lattice parameter c for Bi±0.001 A 2223 phase are determined from (0 0 2), (0 0 10), (0 0 12) and (0 0 14) peaks of the XRD measurements for different cooling rate. 3. Results and discussion The samples of Bi-2223 tablets were coded A25, A50 and A100, numbers referring to cooling speeds in unit of degrees per hour. In our previous work, we investigated the cooling rates in Bi(Pb)SrCaCuO high temperature superconductors by using the low field AC magnetic susceptibility method [7]. The investigations consisted of ac susceptibility as a function of temperature (77–120 K) under different ac field amplitudes, the temperature dependence of intergranular critical current density, J inter ðT p Þ; c and SEM for the samples. It was observed that the overall susceptibility curves of the samples are shifted to lower temperature by increasing the ac field amplitude. The intensity of the imaginary parts increased with increasing the ac field amplitude. The susceptibility–temperature curves shifted to lower temperatures and considerably increased the transition width in v00 –T curves, and also decreased the shielding fraction of the superconducting phase in the samples with decreasing the cooling rates. For sintered high-Tc superconductors the temperature dependence of the imaginary part of the complex ac susceptibility shows two peaks, intergrain peak temperature, Tp, and intragrain peak temperature, Tg, at high ac fields [10]. The first peak appears at a temperature Tg slightly bellow T onset due to intragranular supercurrents, and a second c loss peak appears at a temperature Tp lower than Tg due to sample-circulating intergranular supercurrents. The maximum of v00 appears at a temperature Tp where the intergranular field just penetrates the center of sample [11]. We did not see any intrinsic loss peaks in our samples. The absence of intragrain peak, Tg, is due to the presence of smaller decoupled grain in the samples [12]. It was also observed that peak temperatures, Tp, shift to lower temperature with increasing ac field amplitudes. The shift of Tp with field can be explained in terms of the strength of the pinning force. In the calculation of J inter ðT p Þ as a function c of the peak temperature from our previous work [7], it was found that the value of J inter ðT p Þ increases with increasing c the cooling rates and with decreasing temperature. From the previous SEM analysis, it was observed that the grain size of the fast cooled sample is considerably larger than the grain size of the slow cooled sample.
O. Ozturk et al. / Physica C 451 (2007) 113–117
In the present study, we have studied the peak temperature dependence as a function of ac magnetic field, Hac, in order to investigate the effect of cooling rates on the intergranular pinning force. The values of Tp were estimated from our previous ac susceptibility versus temperature measurements for the ac field amplitudes at 40, 80, 150, 250, 400 and 550 A/m. As can be seen from Fig. 1 a linear dependence of Tp as a function of Hac is observed for all the samples. Mu¨ller critical state model assumes a magnetic flux independent pinning force densities, and aJ and ag for inter- and intra-granular vortices described by the relation [13] ð1Þ T p ¼ T p0 T p0 U 1=2 H ac h i 0 leff ð0Þ where U is l2aa , a is the height of the samples, leff(0) is J ð0Þ the effective permeability of the ceramic, and aJ(0) is the intergranular pinning force density. From a least squares fit of this expression (Eq. (1)) to the data Tp0 and U were extracted for all samples (Table 1). The values of Tp0 increased while U decreased with increasing the cooling rates. This means that the values of aJ(0) increases with increasing the cooling rates. The effective pinning force density decreases with increasing ac magnetic field causing Tp to shift to lower temperature. The XRD patterns from the surface of the A25 and A100 samples are shown in Fig. 2. Miller indices are indicated in the figure. There is a significant increase in the
100
(c) A100 (b) A50 (a) A25
98
Tp(K)
96
94
92
90 0
100
200
300
400
500
600
Hac (A/m) Fig. 1. Intergranular peak temperature as a function of ac magnetic field amplitude for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of: (a) 25 C/h, (b) 50 C/h and (c) 100 C/h samples.
115
intensities of particular peaks between the A25 and A100 samples. The determined lattice parameters from the (0 0 l) peaks of the XRD data are given in Table 2. It is observed that the lattice parameter c increases with increasing the cooling rates. A larger unit cell means less oxygen in the unit cell and this is consistent with reported mass loss upon fast cooling [1,14]. Variation of the Ca content due to impurity phase formation during slow cooling may also have contributed to the lattice parameter change although there is possibility that Ca insufficiency may be compensated by Sr–Ca inter-substitutions [15]. The impurity phase (Ca2PbO4) is observed around 2h = 18 in the A25 sample. The intensity of this peak decreases with increasing cooling rates and disappears in the A100 sample. It has already been suggested that precipitation of Ca2PbO4 can be avoided by using relatively faster cooling rates [16]. The relative proportions of the Bi-2223 phase were determined from (0 0 2), (0 0 8), (0 0 10), (1 1 5), (0 0 12), (2 0 0), (1 1 11) and (0 2 12) peaks and the relative proportions of Bi-2212 phase were determined from (1 1 7), (2 2 8) and (1 1 17) peaks, using the following well-known expressions [17,18]: P I H ðhklÞ P fð2223Þ ¼ P ; ð2Þ I H ðhklÞ þ I LðhklÞ P I LðhklÞ P : ð3Þ fð2212Þ ¼ P I H ðhklÞ þ I LðhklÞ Here IH(hkl) and IL(hkl) are the intensities of the (h k l) diffraction lines for Bi-2223 and Bi-2212 phases, respectively (Fig. 2). The calculated relative portion of the samples is listed in Table 2. As seen in the table, with increasing cooling rates, the relative portion of Bi-2223 phase increased and that of Bi-2212 phase decreased. The electrical resistivity was measured using the standard four-probe dc technique, in the temperature range between 70 and 130 K. Resistivity versus temperature characteristics of the samples was investigated in a closed cycle crystat using a dc current of 5 mA. Typical dimensions of the samples for transport measurements were 3 · 2 · 12 mm3 and the distance between voltage probes was approximately 7 mm. The temperature dependence of resistivity for all samples is shown in Fig. 3. In agreement with the literature, XRD patterns obtained from the slow cooled samples (25 C/h) have the characteristic impurity peak due to Ca2PbO4 with a higher reflection count in comparison with fast cooled samples (100 C/h). Presence of the higher XRD peak of this impurity phase is consistent with the experimental finding that fast cooled sample had lower normal state resistivity and higher zero resistivity
Table 1 Peak temperature Tp, ac field amplitude Hac, intergranular critical current density Jc(Tp), U (inversely proportional to aJ(0) of the samples) Samples
Tp0 (K)
U(1/aJ(0))
J inter ðT p Þ (A/cm2) at 92 K c
Tp (K) (at 150 A/m)
A25 A50 A100
97.67 98.26 98.35
0.0122 0.0118 0.0110
68.22 85.48 114.38
95.82 96.45 96.65
0.004 0.0035
1121 2214
0024
1119 311 L(1117) 317
200 1117 0020
0212
(a) A25
L(228)
(a) A25
0.003
(b) A50
ρ (ohm-cm)
115
Ca2PbO4 008
002
Intensity (a.u)
L(117) 0012 119 200 0014 1111
O. Ozturk et al. / Physica C 451 (2007) 113–117
0010
116
(b) A100
0.0025 0.002
(c) A100
0.0015 0.001 0.0005
5
10
15
20
25
30
35
40
45
50
55
60
0 90
2θ (degree)
95
100
105
110
115
120
125
130
Temperature (K) Fig. 2. The XRD patterns for (a) the A25 and (b) A100 samples. The peaks indexed (h k l)L and (h k l)H represent the Bi-2212 and Bi-2223 phases respectively.
transition. Formation of the impurity phase during slow cooling has also caused (through consumption of Ca) phase segregation resulting in the appearance of higher XRD peaks belonging to low Tc Bi-2212 phase. This may also have contributed to the above-mentioned observed results from transport resistivity measurements. It is also apparent that formation of Ca2PbO4 will disturb Pb stoichiometry of the sample which in turn will elevate phase segregation from Bi-2223 to Bi-2212, resulting in the appearance of higher low Tc phase peaks in XRD patterns. These stoichiometric changes have influence on the other structural properties such as oxygen content, lattice parameters, transition temperature and grain connectivity, all of which are also somehow interrelated. On the other hand, the transition temperature and the lattice parameter c increases with increasing the cooling rates (see Table 2). These results agree with previous work [6]. This behaviour is due to the change in the Meissner effect as a function of the cooling rate, and is also consistent with the result of the XRD and our previous AC susceptibility measurements [7]. It is observed that the broadening of the resistivity transition width decreases with increasing the cooling rate. As can be seen from the figure, the transition curves from normal to superconducting state do not indicate double step transition. The silver wrapped samples exhibited double step R–T transition [1]. From this result, we conclude that grain boundaries act as oxygenation channels and their
Fig. 3. Temperature dependence of resistivity for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of (a) 25 C/h, (b) 50 C/h and (c) 100 C/h samples.
optimum oxygen content is greatly affected by cooling rates. It can be said that optimum oxygen contents for grains and grain boundaries may be achieved by different processing routes. Oxygen content of the grain boundaries inevitably affects their transport electric properties. As a result there should be optimum oxygen content for grain boundaries for the best transport electric properties. When we achieve optimum oxygen content for intragrain regions, it is possible that oxygen content of the grain boundaries is far from optimum. Every sample shows the linear temperature dependence characteristic of Cu-oxide-based (high-Tc) superconductors above the transition, and the resistivity value in the normal state increases with decreasing cooling rates. Room temperature resistivities were calculated from room temperature I–V curves (Table 2). The value of the resistivity at room temperature was decreased about 37% upon the cooling rates. The transport critical current densities as a function of the cooling rates were measured in liquid nitrogen at zero magnetic fields. The direct determination of J trans (I–V c curve) is somewhat uncertain due to the problems related to measurements of low voltage on short samples and the self-heating effects at high currents. Actual dc currents were passed through the samples up to 7 A. Resistive contacts caused heating and the test were completed in possible
Table 2 Critical temperature Tc, critical current Ic, transport critical current density Jc, lattice parameter c, volume fraction, and room temperature resistivity of the samples Samples
Critical temperature T offset (K) c
Lattice parameter ˚) c (A
Critical current density J trans (A/cm2) c
Volume fraction (%) 2212
2223
A25 A50 A100
98.9 100.4 101.8
37.133 37.145 37.164
40 49 67
13 10 6
87 90 94
Room temperature resistivity q (mX cm) 8.50 7.05 5.35
O. Ozturk et al. / Physica C 451 (2007) 113–117 0.014
4. Conclusions
0.012
Increasing cooling rates of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy increased the transport critical current density from 40 to 67 A/cm2 and the critical transition temperature by about 3 K compared with slow cooled sample. The temperature dependence of the electrical resistivity is approximately linear in the normal state as expected. The normal state resistivity decreases with increasing cooling rates. It is observed that the both J trans and aJ(0) are decreased with decreasing c the cooling rates. The lattice parameter c of the samples increased with increasing the cooling rates.
0.01
Voltage (V)
117
0.008
0.006
0.004
(c) A100 (b) A50 (a) A25
0.002
0 0
2
4
6
8
Acknowledgement 10
Current (A) Fig. 4. Voltage versus current graph for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of (a) 25 C/h , (b) 50 C/h and (c) 100 C/h samples.
shortest time to avoid extensiveness of this effect. An I–V curve for the samples is shown in Fig. 4. It is observed that the J trans of the samples increases from 40 to 67 A/cm2 with c increasing the cooling rates (Table 2). This result is consistent with the values of aJ(0). The increase of J trans in the c samples by increasing cooling rates may be caused by the increase of grain sizes and by improved coupling between superconducting grains. This result is consistent with our previous intergranular critical current density measurements [7]. It was observed that the value of J trans for the c Ag-sheathed samples increases with decreasing cooling rate [1,9]. It was shown by Tetenbaum and Maroni [19] that slow cooling increases the oxygen content of the Bi-2223 phase. It was interpreted that this increase in oxygen content may contribute to the enhanced critical current density observed during the slow cooling for the Ag-sheathed samples. Weight measurements of the samples with and without silver sheathing revealed a systematic change in masses [1]. It was observed that bare samples (without silver sheathing) had lost more mass than silver wrapped samples. There may be two factors contributing to this difference; there is no silver sheathing so that out diffusion of oxygen is faster and sublimation of Pb and Bi is also easier. Since the out diffusion of oxygen does not happen during cooling, the difference can be attributed to loss of oxygen during the annealing and sublimation Bi and Pb.
This work is supported by The Scientific and Technological Council of Turkey (Project Numbers: 104T323, 104T324 and 104T325). References [1] C. Terzioglu, O. Ozturk, A. Kilic, A. Gencer, I. Belenli, Physica C 434 (2006) 153. [2] K.H. Mu¨ller, J.C. Macfarlane, R. Driver, Physica C 69 (1989) 158. [3] K.H. Mu¨ller, J.C. Macfarlane, R. Driver, Physica C 203 (1991) 178. [4] A.V. Pop, Supercond. Sci. Technol. 12 (1999) 672. [5] A.V. Pop, G. Ilonca, D. Ciurchea, M. Ye, I.I. Geru, V.G. Kantser, M. Todic, R. Delteur, J. Alloys Comput. 116 (1996) 241. [6] U. Balachandran, Donglu Shi, D.I. Dos Santos, S.W. Graham, M.A. Patel, B. Tani, K. Vandervoort, H. Claus, R.B. Poeppel, Physica C 156 (1988) 649. [7] D. Yegen, C. Terzioglu, O. Gorur, I. Belenli, A. Varilci, Chin. J. Phys. 44 (3) (2006) 233. [8] J.A. Parrell, D.C. Larbalestier, Appl. Phys. Lett. 69 (1996) 2915. [9] M. Lelovic, T. Deis, N.G. Eror, U. Balachanran, P. Haldar, Supercond. Sci. Technol. 9 (1996) 965. [10] J.R. Clim, Physica C 50 (1991) 153. [11] K.H. Mu¨ller, M. Nikolo, N. Savvides, R. Driver, in: K. Kajimura, H. Hayakawa (Eds.), Adv. Superconduct., III, Springer Verlag, Tokyo, 1991, p. 119. [12] K.H. Mu¨ller, S.J. Collocott, R. Driver, N. Savvides, Supercond. Sci. Technol. 4 (1991) 325. [13] K.H. Mu¨ller, D.N. Matthews, R. Driver, Physica C 191 (1992) 339. [14] I. Belenli, D.Phil. Thesis, Oxford Univ. (UK), 1993. [15] T.G. Holesinger, D.J. Miller, L.S. Chumbley, M.J. Kramer, K.W. Dennis, Physica C 202 (1992) 109. [16] Y.L. Chen, R. Stevens, J. Am. Ceram. Soc. (1992) 1162. [17] C.W. Chiu, R.L. Meng, L. Gao, Z.J. Huang, F. Chen, Y.Y. Xue, Nature 365 (1993) 323. [18] S.A. Halim, S.A. Khawaldeh, S.B. Mohammed, H. Azhan, Mater. Chem. Phys. 61 (1999) 251. [19] M. Tetenbaum, V.A. Maroni, Physica C 260 (1996) 71.
The effect of cooling rates on properties of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy superconductors produced by solid-state reaction method O. Ozturk a, D. Yegen a, M. Yilmazlar b, A. Varilci a, C. Terzioglu a
a,*
Department of Physics, Faculty of Arts and Sciences, Abant Izzet Baysal University, 14280 Bolu, Turkey b Department of Physics, Faculty of Arts and Science, Kirikkale University, 71450 Kirikkale, Turkey Received 28 August 2006; received in revised form 1 November 2006; accepted 6 November 2006 Available online 13 December 2006
Abstract We have investigated the effect of the cooling rates in the Bi-2223 superconducting samples prepared by standard solid-state reaction method using electrical resistivity, transport critical current density, and XRD (X-ray diffraction) measurements. We estimated the transition temperature values from the DC resistivity measurements. We observed that transition temperature, Tc, and transport critical current density, J trans , depend on the cooling rates of the samples. They both increase with increasing cooling rates. We estimated the peak c temperature, Tp, from our previous ac susceptibility measurements. The pinning force density increased with increasing the cooling rate. XRD patterns are used to calculate lattice parameter c and obtain information about Bi-2223 phase ratio. 2006 Elsevier B.V. All rights reserved. Keywords: Transition temperature; Intergranular critical temperature; Pinning force; Peak temperature; Bean model
1. Introduction A tremendous amount of work on Bi(Pb)–Sr–Ca–Cu–O system has been done since its discovery. Both doping with different elements at various amounts and adjusting preparation condition affect phase formation and physical properties in this system. Different cooling speeds are employed in fabrication process, which interacts with other processing parameters such as heat treatment duration and temperature. Different cooling speeds are also related to adjustment of the oxygen content during the cooling stage [1]. It is useful to observe the variation of both the superconducting properties as well as the normal state properties of materials with changing cooling rates in order to better understand superconductivity. AC susceptibility and DC resistivity measurements are useful tool to investigate the magnetic and superconducting
*
Corresponding author. E-mail address: [email protected] (C. Terzioglu).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.11.003
properties. In particular, they are indispensable for making the distinction between intergrain and intragrain properties. At high ac fields, two loss peaks are usually present in the imaginary part of susceptibility data; a broad peak at low temperature (coupling losses) due to the motion of intergranular (Josephson) vortices [2] and a narrower peak (intrinsic losses) due to the motion of intragranular (Abrikosov) vortices inside the superconducting grains near T onset [3]. These two peaks depend on the sample proc cessing variables as well as the samples composition for the Bi-2223 system [4,5]. Early study showed that the transition temperature and room temperature resistivity of the Bi0.7Pb0.3Sr1Cu1.8Oy depend on the rate of cooling from the sintering temperature [6]. Yegen et al. [7] performed low field AC susceptibility versus temperature measurements on the same samples that are used in this study. They observed that peak temperatures, Tp, and intergranular critical current density, J inter ðT p Þ, increase with increasing cooling rate. In the latter c decade, the cooling rate was found to be an important parameter for optimizing performance of Ag-sheathed
114
O. Ozturk et al. / Physica C 451 (2007) 113–117
Bi-2223 superconductors [8,9]. They found that the value of J trans for the Ag-sheathed samples increases with c decreasing cooling rate. It was interpreted that the enhancement of J trans with decreasing cooling rate is due c to increases in the flux pinning strength. Terzioglu et al. [1] have recently investigated the effect of cooling rates on oxygen content of Bi-2223 samples with and without silver sheathing. They obtained that J trans of bare bulk samples c increases while that of Ag-sheathed samples decreases with increasing cooling rate. Cooling rates are important for improving superconducting properties such as transport critical current density, transition temperature and intergranular connectivity. This importance is closely related to changing oxygen content during cooling rate. In this work, we report measurements of electrical resistivity as a function of temperature, and transport critical current density in order to investigate the effect of the cooling rates on the activation energy in the mixed state and intergrain properties of Bi-2223 superconductors. We also reported XRD measurements to calculate the lattice parameter and the relative portion of Bi-2223 and that of Bi-2212 phases. 2. Experimental We prepared superconducting powders with Bi1.7Pb0.35Sr1.9Ca2.1Cu3Ox chemical stoichiometry. Several calcination steps were applied with intermediate grindings at temperatures up to 820 C. Each calcination step took 12 h. A number of tablets were made under 1 GPa uni-axial pressure. They were then heat treated twice at 830 C for 48 h with grinding and re-pelletising after the first heat treatment. After re-pressing, these samples were then heat treated at 835 C for 48 h. The furnace was heated at 100 C/h to the annealing temperature and at the end of the heat treatment 25 C/h, 50 C/h and 100 C/h cooling rates were employed to cool down to the room temperature. After cooling to room temperature, the pellets were cut into rectangular bars (3 mm · 2 mm · 12 mm) for resistivity and critical current measurements. The measurements of DC resistivity and transport critical current density J trans were performed with the fourc probe method on all samples. Both voltage and current contacts were made with silver paint, the contact resistance being in the order of 0.1 X or lower. The outside pin-points feed the current and the inner ones (the distance between voltage probes is approximately 7 mm) allow the voltage drop measurement. We determined J trans property from c the I–V curves at 77 K, following the 1 lV/cm criteria. Resistive contacts caused heating and the test was completed in possible short time, in order to avoid this effect. We measured temperature (40–130 K) dependence of resistivity of the samples running 5 mA dc current through the samples in the cryostat. A Keithley 220 programmable current source and a Keithley 2182A nano-voltmeter were used for the resistivity and I–V measurements. The transition temperature, Tc, is the temperature at which a resis-
tance-less percolation path is established. In bulk samples of Bi-2223, this path goes through many grain boundaries providing valuable information about them. XRD data were taken using a Rigaku D/Max-IIIC diffractometer with CuKa radiation in the range 2h = 4– 60 with a scan speed of 3/min and a step increment of 0.02 at room temperature. Phase purity and the lattice parameters were obtained from these XRD patterns. The accuracy in determining the lattice parameter c was ˚ . The mean values of lattice parameter c for Bi±0.001 A 2223 phase are determined from (0 0 2), (0 0 10), (0 0 12) and (0 0 14) peaks of the XRD measurements for different cooling rate. 3. Results and discussion The samples of Bi-2223 tablets were coded A25, A50 and A100, numbers referring to cooling speeds in unit of degrees per hour. In our previous work, we investigated the cooling rates in Bi(Pb)SrCaCuO high temperature superconductors by using the low field AC magnetic susceptibility method [7]. The investigations consisted of ac susceptibility as a function of temperature (77–120 K) under different ac field amplitudes, the temperature dependence of intergranular critical current density, J inter ðT p Þ; c and SEM for the samples. It was observed that the overall susceptibility curves of the samples are shifted to lower temperature by increasing the ac field amplitude. The intensity of the imaginary parts increased with increasing the ac field amplitude. The susceptibility–temperature curves shifted to lower temperatures and considerably increased the transition width in v00 –T curves, and also decreased the shielding fraction of the superconducting phase in the samples with decreasing the cooling rates. For sintered high-Tc superconductors the temperature dependence of the imaginary part of the complex ac susceptibility shows two peaks, intergrain peak temperature, Tp, and intragrain peak temperature, Tg, at high ac fields [10]. The first peak appears at a temperature Tg slightly bellow T onset due to intragranular supercurrents, and a second c loss peak appears at a temperature Tp lower than Tg due to sample-circulating intergranular supercurrents. The maximum of v00 appears at a temperature Tp where the intergranular field just penetrates the center of sample [11]. We did not see any intrinsic loss peaks in our samples. The absence of intragrain peak, Tg, is due to the presence of smaller decoupled grain in the samples [12]. It was also observed that peak temperatures, Tp, shift to lower temperature with increasing ac field amplitudes. The shift of Tp with field can be explained in terms of the strength of the pinning force. In the calculation of J inter ðT p Þ as a function c of the peak temperature from our previous work [7], it was found that the value of J inter ðT p Þ increases with increasing c the cooling rates and with decreasing temperature. From the previous SEM analysis, it was observed that the grain size of the fast cooled sample is considerably larger than the grain size of the slow cooled sample.
O. Ozturk et al. / Physica C 451 (2007) 113–117
In the present study, we have studied the peak temperature dependence as a function of ac magnetic field, Hac, in order to investigate the effect of cooling rates on the intergranular pinning force. The values of Tp were estimated from our previous ac susceptibility versus temperature measurements for the ac field amplitudes at 40, 80, 150, 250, 400 and 550 A/m. As can be seen from Fig. 1 a linear dependence of Tp as a function of Hac is observed for all the samples. Mu¨ller critical state model assumes a magnetic flux independent pinning force densities, and aJ and ag for inter- and intra-granular vortices described by the relation [13] ð1Þ T p ¼ T p0 T p0 U 1=2 H ac h i 0 leff ð0Þ where U is l2aa , a is the height of the samples, leff(0) is J ð0Þ the effective permeability of the ceramic, and aJ(0) is the intergranular pinning force density. From a least squares fit of this expression (Eq. (1)) to the data Tp0 and U were extracted for all samples (Table 1). The values of Tp0 increased while U decreased with increasing the cooling rates. This means that the values of aJ(0) increases with increasing the cooling rates. The effective pinning force density decreases with increasing ac magnetic field causing Tp to shift to lower temperature. The XRD patterns from the surface of the A25 and A100 samples are shown in Fig. 2. Miller indices are indicated in the figure. There is a significant increase in the
100
(c) A100 (b) A50 (a) A25
98
Tp(K)
96
94
92
90 0
100
200
300
400
500
600
Hac (A/m) Fig. 1. Intergranular peak temperature as a function of ac magnetic field amplitude for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of: (a) 25 C/h, (b) 50 C/h and (c) 100 C/h samples.
115
intensities of particular peaks between the A25 and A100 samples. The determined lattice parameters from the (0 0 l) peaks of the XRD data are given in Table 2. It is observed that the lattice parameter c increases with increasing the cooling rates. A larger unit cell means less oxygen in the unit cell and this is consistent with reported mass loss upon fast cooling [1,14]. Variation of the Ca content due to impurity phase formation during slow cooling may also have contributed to the lattice parameter change although there is possibility that Ca insufficiency may be compensated by Sr–Ca inter-substitutions [15]. The impurity phase (Ca2PbO4) is observed around 2h = 18 in the A25 sample. The intensity of this peak decreases with increasing cooling rates and disappears in the A100 sample. It has already been suggested that precipitation of Ca2PbO4 can be avoided by using relatively faster cooling rates [16]. The relative proportions of the Bi-2223 phase were determined from (0 0 2), (0 0 8), (0 0 10), (1 1 5), (0 0 12), (2 0 0), (1 1 11) and (0 2 12) peaks and the relative proportions of Bi-2212 phase were determined from (1 1 7), (2 2 8) and (1 1 17) peaks, using the following well-known expressions [17,18]: P I H ðhklÞ P fð2223Þ ¼ P ; ð2Þ I H ðhklÞ þ I LðhklÞ P I LðhklÞ P : ð3Þ fð2212Þ ¼ P I H ðhklÞ þ I LðhklÞ Here IH(hkl) and IL(hkl) are the intensities of the (h k l) diffraction lines for Bi-2223 and Bi-2212 phases, respectively (Fig. 2). The calculated relative portion of the samples is listed in Table 2. As seen in the table, with increasing cooling rates, the relative portion of Bi-2223 phase increased and that of Bi-2212 phase decreased. The electrical resistivity was measured using the standard four-probe dc technique, in the temperature range between 70 and 130 K. Resistivity versus temperature characteristics of the samples was investigated in a closed cycle crystat using a dc current of 5 mA. Typical dimensions of the samples for transport measurements were 3 · 2 · 12 mm3 and the distance between voltage probes was approximately 7 mm. The temperature dependence of resistivity for all samples is shown in Fig. 3. In agreement with the literature, XRD patterns obtained from the slow cooled samples (25 C/h) have the characteristic impurity peak due to Ca2PbO4 with a higher reflection count in comparison with fast cooled samples (100 C/h). Presence of the higher XRD peak of this impurity phase is consistent with the experimental finding that fast cooled sample had lower normal state resistivity and higher zero resistivity
Table 1 Peak temperature Tp, ac field amplitude Hac, intergranular critical current density Jc(Tp), U (inversely proportional to aJ(0) of the samples) Samples
Tp0 (K)
U(1/aJ(0))
J inter ðT p Þ (A/cm2) at 92 K c
Tp (K) (at 150 A/m)
A25 A50 A100
97.67 98.26 98.35
0.0122 0.0118 0.0110
68.22 85.48 114.38
95.82 96.45 96.65
0.004 0.0035
1121 2214
0024
1119 311 L(1117) 317
200 1117 0020
0212
(a) A25
L(228)
(a) A25
0.003
(b) A50
ρ (ohm-cm)
115
Ca2PbO4 008
002
Intensity (a.u)
L(117) 0012 119 200 0014 1111
O. Ozturk et al. / Physica C 451 (2007) 113–117
0010
116
(b) A100
0.0025 0.002
(c) A100
0.0015 0.001 0.0005
5
10
15
20
25
30
35
40
45
50
55
60
0 90
2θ (degree)
95
100
105
110
115
120
125
130
Temperature (K) Fig. 2. The XRD patterns for (a) the A25 and (b) A100 samples. The peaks indexed (h k l)L and (h k l)H represent the Bi-2212 and Bi-2223 phases respectively.
transition. Formation of the impurity phase during slow cooling has also caused (through consumption of Ca) phase segregation resulting in the appearance of higher XRD peaks belonging to low Tc Bi-2212 phase. This may also have contributed to the above-mentioned observed results from transport resistivity measurements. It is also apparent that formation of Ca2PbO4 will disturb Pb stoichiometry of the sample which in turn will elevate phase segregation from Bi-2223 to Bi-2212, resulting in the appearance of higher low Tc phase peaks in XRD patterns. These stoichiometric changes have influence on the other structural properties such as oxygen content, lattice parameters, transition temperature and grain connectivity, all of which are also somehow interrelated. On the other hand, the transition temperature and the lattice parameter c increases with increasing the cooling rates (see Table 2). These results agree with previous work [6]. This behaviour is due to the change in the Meissner effect as a function of the cooling rate, and is also consistent with the result of the XRD and our previous AC susceptibility measurements [7]. It is observed that the broadening of the resistivity transition width decreases with increasing the cooling rate. As can be seen from the figure, the transition curves from normal to superconducting state do not indicate double step transition. The silver wrapped samples exhibited double step R–T transition [1]. From this result, we conclude that grain boundaries act as oxygenation channels and their
Fig. 3. Temperature dependence of resistivity for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of (a) 25 C/h, (b) 50 C/h and (c) 100 C/h samples.
optimum oxygen content is greatly affected by cooling rates. It can be said that optimum oxygen contents for grains and grain boundaries may be achieved by different processing routes. Oxygen content of the grain boundaries inevitably affects their transport electric properties. As a result there should be optimum oxygen content for grain boundaries for the best transport electric properties. When we achieve optimum oxygen content for intragrain regions, it is possible that oxygen content of the grain boundaries is far from optimum. Every sample shows the linear temperature dependence characteristic of Cu-oxide-based (high-Tc) superconductors above the transition, and the resistivity value in the normal state increases with decreasing cooling rates. Room temperature resistivities were calculated from room temperature I–V curves (Table 2). The value of the resistivity at room temperature was decreased about 37% upon the cooling rates. The transport critical current densities as a function of the cooling rates were measured in liquid nitrogen at zero magnetic fields. The direct determination of J trans (I–V c curve) is somewhat uncertain due to the problems related to measurements of low voltage on short samples and the self-heating effects at high currents. Actual dc currents were passed through the samples up to 7 A. Resistive contacts caused heating and the test were completed in possible
Table 2 Critical temperature Tc, critical current Ic, transport critical current density Jc, lattice parameter c, volume fraction, and room temperature resistivity of the samples Samples
Critical temperature T offset (K) c
Lattice parameter ˚) c (A
Critical current density J trans (A/cm2) c
Volume fraction (%) 2212
2223
A25 A50 A100
98.9 100.4 101.8
37.133 37.145 37.164
40 49 67
13 10 6
87 90 94
Room temperature resistivity q (mX cm) 8.50 7.05 5.35
O. Ozturk et al. / Physica C 451 (2007) 113–117 0.014
4. Conclusions
0.012
Increasing cooling rates of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy increased the transport critical current density from 40 to 67 A/cm2 and the critical transition temperature by about 3 K compared with slow cooled sample. The temperature dependence of the electrical resistivity is approximately linear in the normal state as expected. The normal state resistivity decreases with increasing cooling rates. It is observed that the both J trans and aJ(0) are decreased with decreasing c the cooling rates. The lattice parameter c of the samples increased with increasing the cooling rates.
0.01
Voltage (V)
117
0.008
0.006
0.004
(c) A100 (b) A50 (a) A25
0.002
0 0
2
4
6
8
Acknowledgement 10
Current (A) Fig. 4. Voltage versus current graph for the Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy annealed at 830 C for 48 h with cooling rate of (a) 25 C/h , (b) 50 C/h and (c) 100 C/h samples.
shortest time to avoid extensiveness of this effect. An I–V curve for the samples is shown in Fig. 4. It is observed that the J trans of the samples increases from 40 to 67 A/cm2 with c increasing the cooling rates (Table 2). This result is consistent with the values of aJ(0). The increase of J trans in the c samples by increasing cooling rates may be caused by the increase of grain sizes and by improved coupling between superconducting grains. This result is consistent with our previous intergranular critical current density measurements [7]. It was observed that the value of J trans for the c Ag-sheathed samples increases with decreasing cooling rate [1,9]. It was shown by Tetenbaum and Maroni [19] that slow cooling increases the oxygen content of the Bi-2223 phase. It was interpreted that this increase in oxygen content may contribute to the enhanced critical current density observed during the slow cooling for the Ag-sheathed samples. Weight measurements of the samples with and without silver sheathing revealed a systematic change in masses [1]. It was observed that bare samples (without silver sheathing) had lost more mass than silver wrapped samples. There may be two factors contributing to this difference; there is no silver sheathing so that out diffusion of oxygen is faster and sublimation of Pb and Bi is also easier. Since the out diffusion of oxygen does not happen during cooling, the difference can be attributed to loss of oxygen during the annealing and sublimation Bi and Pb.
This work is supported by The Scientific and Technological Council of Turkey (Project Numbers: 104T323, 104T324 and 104T325). References [1] C. Terzioglu, O. Ozturk, A. Kilic, A. Gencer, I. Belenli, Physica C 434 (2006) 153. [2] K.H. Mu¨ller, J.C. Macfarlane, R. Driver, Physica C 69 (1989) 158. [3] K.H. Mu¨ller, J.C. Macfarlane, R. Driver, Physica C 203 (1991) 178. [4] A.V. Pop, Supercond. Sci. Technol. 12 (1999) 672. [5] A.V. Pop, G. Ilonca, D. Ciurchea, M. Ye, I.I. Geru, V.G. Kantser, M. Todic, R. Delteur, J. Alloys Comput. 116 (1996) 241. [6] U. Balachandran, Donglu Shi, D.I. Dos Santos, S.W. Graham, M.A. Patel, B. Tani, K. Vandervoort, H. Claus, R.B. Poeppel, Physica C 156 (1988) 649. [7] D. Yegen, C. Terzioglu, O. Gorur, I. Belenli, A. Varilci, Chin. J. Phys. 44 (3) (2006) 233. [8] J.A. Parrell, D.C. Larbalestier, Appl. Phys. Lett. 69 (1996) 2915. [9] M. Lelovic, T. Deis, N.G. Eror, U. Balachanran, P. Haldar, Supercond. Sci. Technol. 9 (1996) 965. [10] J.R. Clim, Physica C 50 (1991) 153. [11] K.H. Mu¨ller, M. Nikolo, N. Savvides, R. Driver, in: K. Kajimura, H. Hayakawa (Eds.), Adv. Superconduct., III, Springer Verlag, Tokyo, 1991, p. 119. [12] K.H. Mu¨ller, S.J. Collocott, R. Driver, N. Savvides, Supercond. Sci. Technol. 4 (1991) 325. [13] K.H. Mu¨ller, D.N. Matthews, R. Driver, Physica C 191 (1992) 339. [14] I. Belenli, D.Phil. Thesis, Oxford Univ. (UK), 1993. [15] T.G. Holesinger, D.J. Miller, L.S. Chumbley, M.J. Kramer, K.W. Dennis, Physica C 202 (1992) 109. [16] Y.L. Chen, R. Stevens, J. Am. Ceram. Soc. (1992) 1162. [17] C.W. Chiu, R.L. Meng, L. Gao, Z.J. Huang, F. Chen, Y.Y. Xue, Nature 365 (1993) 323. [18] S.A. Halim, S.A. Khawaldeh, S.B. Mohammed, H. Azhan, Mater. Chem. Phys. 61 (1999) 251. [19] M. Tetenbaum, V.A. Maroni, Physica C 260 (1996) 71.