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Substitution of Sm at Ca site in Bi1.6Pb0.4Sr2Ca2-xSmxCu3Oy superconductors
ARTICLE IN PRESS
Physica B 399 (2007) 94–100 www.elsevier.com/locate/physb
Substitution of Sm at Ca site in Bi1:6 Pb0:4Sr2 Ca2xSmxCu3Oy superconductors O. Ozturka, M. Akdogana, H. Aydina, M. Yilmazlarb, C. Terzioglua,, I. Belenlia a
Department of Physics, Faculty of Arts and Sciences, Abant Izzet Baysal University, 14280 Bolu, Turkey b Faculty of Education, Sakarya University, 54300 Hendek, Sakarya, Turkey Received 2 March 2007; accepted 23 May 2007
Abstract We have investigated the effect of the partial substitution of Ca by Sm in the Bi-2223 superconducting samples prepared by standard solid-state reaction method. Our investigations consisted of DC electrical resistivity, AC susceptibility, X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. We measured the critical transition temperatures, activation energies and irreversibility lines from the resistivity versus temperature curves under DC magnetic fields in the range of 0 and 0.6 T. The superconducting transition temperature, T c , and activation energy, U 0 , were found to decrease with increasing Sm concentration and with increasing applied magnetic field. Increasing the Sm content shifted the irreversibility temperature to lower values. The AC susceptibility measurements were carried out at 80 A/m field strength and f ¼ 211 Hz frequency. XRD patterns and SEM micrographs are given to provide information about Bi-2223 and Bi-2212 phases and their grain size. The possible reasons for the observed degradation in microstructural and superconducting properties due to Sm substitution were discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.62.c; 61.10.Nz Keywords: AC susceptibility; Activation energy; Irreversible temperature; Magnetoresistivity; Intergranular critical current density
1. Introduction It is well known that superconductivity is suppressed by the presence of magnetic ions in the conventional metallic superconductors. This behaviour can be understood in terms of the pair-breaking mechanism [1]. In high-T c superconductors, it is well known that the superconducting properties are related to the chemical dopings, preparation conditions and hole concentrations. The objectives of doping works are to optimize the hole concentration, to introduce pinning centres, to enhance the formation of Bi2223 phase [2]. Variation in the T c , J c , and lattice parameters are obtained via doping the system with other elements at different levels and under various preparation conditions [3–11]. Terzioglu et al. [3] have recently investigated the effect of substitution of Sm for Ca on Corresponding author. Tel.: +90 3742541000; fax: +90 3742534642.
E-mail address: [email protected] (C. Terzioglu). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.05.028
superconducting, microstructure and mechanical properties of Bi1:6 Pb0:4 Sr2 Ca2x Smx Cu3 Oy [3,4,6]. It was obtained that superconducting and mechanical properties degrade with increasing Sm content. It was also reported that for x ¼ 1:0 and 1.5 the compounds showed semiconducting behaviour, resistivity increased with decreasing temperature. The irreversibility line can be determined from various measurements such as DC resistivity and AC susceptibility [12]. Muller et al. [13] first reported the irreversibility line in high temperature superconducting ceramics. The measurements of AC susceptibility are commonly used to determine magnetic and superconducting properties of these materials. In particular, the AC susceptibility measurement is useful in distinguishing between inter- and intragrain properties of the specimens. Two peaks, T p and T g , reflect inter- and intragranular losses in the ceramic superconductors. They can be distinguished from the temperature dependence of the imaginary part of the complex AC susceptibility only
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when high AC fields were used in the measurement [14,15]. The first peak appears at a temperature T g slightly below T c and indicates a maximum hysteresis loss for the motion of intragranular loss [15]. The second loss peak appears at a temperature T p lower than T g caused by the motion of intergranular vortices [16]. They both depend on the sample’s composition. In this work, we report X-ray diffraction (XRD), scanning electron microscopy (SEM), magnetoresistivity, and AC susceptibility measurements on Sm substituted Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy samples. We also investigated the effect of the partial substitution of Ca by Sm on the activation energy, the irreversibility line (or temperature), and inter- and intragrain properties of Bi-2223 superconductor. 2. Experimental details The Sm substituted Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy samples with x ¼ 0; 0:0005, 0.001 and 0.005 were prepared by the standard solid-state reaction method using high purity chemicals Bi2 O3 (99.99%), PbO ð99:9 þ %Þ, SrCO3 ð99:9 þ %Þ, CaCO3 (99+%), CuOð99 þ %Þ and Gd2 O3 ð99 þ %Þ [3]. Rectangular bars were cut from the sintered samples for electrical resistivity and AC susceptibility measurements. The typical sample size was ð2:9 2:1 12:1Þ mm3 . The calcinations and annealing processes of the samples were carried out using a programmable tube furnace from PROTHERM (Model PTF 12/75/200). The samples annealed at 860 C for 200 h with different Sm substitutions in Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy (x ¼ 0; 0:0005; 0:001 and 0.005) will be hereafter denoted as B0, B1, B2, and B3, respectively. We measured temperature dependence of resistivity of the samples using usual four-probe method with 5 mA DC current in a cryostat. Magnetoresistivity measurements were made under different DC magnetic fields (0, 0.3 and 0.6 T). The magnetic field was applied parallel to the current direction, by an electromagnet. The transition temperature T c was determined as the temperature at which zero resistivity was achieved. The reversibility temperature was determined as zero resistance temperature at various selected magnetic fields. The effective activation energy serves as an important parameter for the description of the flux dynamics in the mixed state for high-T c superconductors. A commonly accepted method for probing the magnetic field dependence of the effective activation energy is to measure the resistive transition in various applied magnetic fields. More accurate values of activation energy, U 0 , can be obtained from transport rather than magnetic measurements due to the widening of the magnetic transition. We have calculated U 0 in this study using the line pinning model [17] by doing approximations from resistivity measurements [18–22] via an Arrheniustype method. We have assumed that the resistivity r has following dependence: rðTÞ ¼ r0 expðU 0 =kB TÞ,
(1)
95
where kB is Boltzmann’s constant. When we plot ln r versus 1/T curve, the slope of the low resistivity part gives the activation energy U 0 . Magnetic susceptibility as a function of temperature ð752120 KÞ measurements were performed using a 7130 AC susceptometer of Lake Shore with a closed cycle refrigerator at 211 Hz frequency and 80 A/m field strength. The values of the T onset and T p were estimated from the c real and imaginary parts of AC susceptibility curves, respectively. We could observe only one peak in each of the w00 versus T plots. The values of intergranular critical current densities J c ðT p Þ in our samples were calculated using Bean’s model from the acquired data. XRD data were collected using a Rigaku D/Max-IIIC diffractometer with CuKa radiation in the range 2Y ¼ 4260 with a scan speed of 31 per minute and a step increment of 0.021 at room temperature. The lattice parameter c was determined from (0 0 2), (0 0 8), (0 ,1 0), and (0 0 1 4) peaks. The accuracy in determining the lattice parameter c was ˚ 0:001 A. The surface morphologies of the pure and Sm substituted samples were studied by using a JEOL JST-6400 scanning electron microscope. 3. Results and discussion 3.1. Electrical resistivity and activation energy We performed the electrical resistivity as a function of temperature between 77 and 130 K to investigate the effect of Sm substitution on the superconducting properties of the Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy samples with 0pxp 0.005. Fig. 1 displays the temperature variation of the normalized resistance (that each sample’s resistance normalized at 130 K) of the pure sample (B0) as well as Sm added samples (B1, B2, and B3). Zero-resistivity transition temperatures of the B0, B1, B2 and B3 samples are
Fig. 1. Temperature dependence of normalized resistance for the samples.
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determined as 107 102, 97, and 93 K, respectively. The estimated T c values of the samples decreased with increasing Sm content. The lower zero resistance transition temperature with increasing Sm content could be interpreted as a result of the suppression of superconductivity by the Sm3þ ions. It was observed that Sm substituted samples showed a broader transition, which indicates presence of impurities and weak-links between superconducting grains. The transition width of the sample B0 is 9 K but that of the sample B3 is 14 K. Broadening of the transition width indicates that the Sm substituted samples have lower percentage of the Bi-2223 phase compared to B0 sample [3]. If a substitution causes weak-link behaviour only, we usually expect beginning of the transition to remain same. Deviation form the metallic behaviour occurred at a few Kelvins lower temperature when Sm addition was increased, from B1 to B2. The difference between B2 and B3 is not at the high temperature end of the transition but at the low end of the transition. By looking at the R2T curves for B0 and B1, we can conclude that Sm addition, at first, did only cause weak-link behaviour. When it is increased to x ¼ 0:001, Bi-2223 phase was modified to have lower onset critical temperature besides weak-links. When we further increase Sm content of our samples, we observed excessive weak-link behaviour. The transition from normal to the superconducting state for Sm substituted samples has double step nature: We believed that the double step resistive transition is an indication of weak-links. All samples show a linear temperature dependence of the electrical resistivity, r ¼ rð0Þ þ aT, in normal state. As the Sm content increases, much broader superconducting transitions, higher room temperature resistivities and lower critical transition temperatures were observed. This degradation can be attributed to the worsened grain boundary properties and modified Bi-2223 phase caused by Sm content. The results obtained from the DC resistivity measurements are tabulated in Table 1. Fig. 2 displays the temperature dependence of resistivity at magnetic fields of 0, 0.3 and 0.6 T for B1 and B2. We observed that the broadening of the resistivity transition width increases with increasing external DC magnetic field and with increasing Sm concentration. It was also observed that, with increasing DC magnetic field up to 0.6 T, T offset c ranges from 107 to 79 K for B0, 102 to 71 K for B1, 97 to 63 K for B2, and 93 to 56 K for B3 sample. Below T c , the Table 1 Some characteristics of superconducting samples Sample
Lattice parameter c (A˚)
T offset (K) c From DC resistivity
(K) T onset c From AC susceptibility
T p (K) at 80 A/m
B0 B1 B2 B3
37.236 37.206 37.197 37.190
107 102 97 93
110 108 105 102
97 92 86 81
Fig. 2. The temperature dependence of resistivity at magnetic fields of 0, 0.3 and 0.6 T for the B1 and B2 samples.
thermally assisted flux flow (TAFF) is an important dissipation mechanism causing a long resistive tail. TAFF theory predicts that in the low current limit, the resistivity obeys the relation of Eq. (1). By rewriting Eq. (1) in the form r U0 . (2) ln ¼ r0 kB T We obtain U 0 from the slope of the Arrhenius plot for ln r versus 1/T. In order to achieve this aim we use linear fits to apparently linear parts of the curves in the low resistivity region of the graph. Fig. 3 shows one of these graphs from which we calculated activation energy for B2 at 0.3 T. From the slopes of the lines, one of which is shown in Fig. 3 as example, we calculated the activation energy as a function of external magnetic field. The activation energies of the other samples calculated likewise are listed in Table 2. In literature, a large number of researchers used
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Fig. 3. Plot of ln r versus 1=T for B2 sample at 0.3 T. Slope of this graph gives activation energy. Fig. 4. Applied DC magnetic field as a function of critical temperature for B1 sample. Table 2 Activation energy values of superconducting samples Sample
U 0 (K) at 0 T
U 0 (K) at 0.3 T
U 0 (K) at 0.6 T
B0 B1 B2 B3
11 700 10 069 7733 7239
1702 1613 1531 1380
1596 1470 1362 1253
this technique to determine the activation energy [12,23–26]. As seen in Table 2, the activation energy decreases significantly with increasing DC magnetic field and with increasing Sm content, being consistent with a previous work [26]. The relationship between T p and H AC was investigated for these samples and found to be linear [27]. In that study, the value of pinning force decreased with increasing Sm amount, which is consistent with the present results. We want to investigate the temperature dependence of peak field H p , deduced from the measurements of the electrical resistivity. Fig. 4 shows the corresponding data of zero resistance temperature at various selected magnetic fields for B1. These temperature values appear in Fig. 4 are comparable to irreversibility or depinning temperatures [28]. It is well to depin the flux line since lowering the temperature increases the pinning force. This results the zero resistivity or the irreversibility temperature to shift downwards as the magnetic field increases. It is observed that the irreversible region of H irr versus T graph extends to higher temperatures for pure sample. For the irreversibility, a representation: H irr ¼ H 0 ð1 T=T c Þn ,
(3)
near T c has been established. Both pre-factor H 0 and n are model-dependent parameters [29,30]. As can be seen from
Fig. 5. AC susceptibility (w ¼ w0 þ iw00 Þ versus temperature plots for the samples.
Fig. 4, the power low relation gives a good fit to experimental data for all samples being in good agreement with previous works [12,24,26]. 3.2. AC susceptibility In order to investigate the effect of Sm substitution on superconducting properties of the samples, we performed AC susceptibility measurements. We used H AC ¼ 80 A=m field amplitude and f ¼ 211 Hz frequency. Fig. 5 shows AC susceptibilities (w0 and w00 ) as a function of temperature for
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samples B0 and B1, B2, and B3. As seen from the real parts of the plots, the onset critical temperature appeared at 110 K for B0 sample and 108, 105, 102 K for B1, B2, and B3 samples, respectively. It shows that the transition temperature, defined as the temperature at which the Meissner signal begins to appear, decreases progressively with increasing x. The maximum peak of w00 appears at a temperature T p where the intergranular field just penetrates the centre of sample [31]. The overall susceptibility curves are shifted to lower temperature with increasing the Sm content. Imaginary part of B0, B1, B2 and B3 samples show peaks at temperatures 97, 92, 86, and 81 K for magnetic field strength of 80 A/m, respectively. At higher AC field amplitudes (typically greater than 400 A/m), two loss peaks can usually be seen in the imaginary part of susceptibility data; a broad peak at low temperature (coupling losses) and a narrower peak (intrinsic losses) near T onset . We did not see any intrinsic loss peaks in our c
(0024 )H
(31 5)L (317)L
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(021 2)H
(00 12)H
x=0.0
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(001 0)H (115)H (115)L
(004)H
(00 8)H
(002 )H
sample because of the low value of the applied field. This may be due to small grain size and no flux penetration into the grains [32]. Amplitude of the imaginary part of AC susceptibilities decreased with increasing Sm content. As can be seen in Fig. 5, Sm substitution shifts the susceptibility–temperature curves to lower temperatures and considerably increases the transition width, and also decreases the shielding fraction of the superconducting phase in the samples. This similar effect of the Smsubstitution in transition width is revealed for the DC resistivity measurements [33,34]. In the light of these, the broadening of the peaks with increasing Sm substitution can be said to be due to gradual penetration of flux into the centre of the intergranular regions. The real and imaginary parts are zero (w0 ¼ w00 ¼ 0) when the sample is in normal state (full penetration). Below T p , the amplitude of w00 falls due to decreasing amount of flux penetration. When the temperature reaches to a certain lower value, in other
(200 )H (0014 )H (111 1)H
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(0012 )H
20
25
30
(00 24)H
(315 )L
(3 17)L
(1119 )H
(220 )L (220 )H (2012 )L
(021 2)H
(0024 )H
(315)L (317)L
(11 19)H
(0212 )H
(111 1)H
(220)L (220)H (2012 )L
(0014 )H (119)H (20 0)H
35
40
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(002 4)H
(111 9)H
(220)L (220)H (20 12)L (111 5)L
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(1111 )H
(115 )H (115)L
(008)L
(008 )H
15
x=0.005
(119 )H (200)H
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(001 2)H
(115)H (115)L
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(004)H (00 4)H
10
x=0.001
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(0010 )H (002 )H (002)H
5
(119)H (200)H (00 14)H (11 11)H
(115)H (115 )L
(008)H
(004 )H
Intensity (a.u.)
(002 )H
(00 10)H
(001 2)H
x=0.0005
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60
2θ ( Degree) Fig. 6. The XRD patterns for (a) B0, (b) B1, (c) B2 and (d) B3 samples. The peaks indexed ðh k lÞL and ðh k lÞH represent the Bi-2212 and Bi-2223 phases, respectively.
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words when w0 ¼ 1 and w00 ¼ 0, the whole body starts to shield. This indicated that destructive effect in the superconducting properties brought about by Sm–Ca replacement in the Bi-2223 system. This is in line with previous work [35–37]. We estimated the temperature dependence of intergranular critical current density using Bean’s model [38]. Accordingly, when flux lines fully penetrate to the sample, losses reaches to highest point. The critical current density is calculated for our sample using the relation J inter ðT p Þ ¼ c H a =a for the sample having cross section of the rectangular bar shaped, like 2a 2b where aob. J inter is the c intergranular current density and H a is the amplitude of applied field at T p . Therefore, the value of intergranular critical current density of the samples as a function of the peak temperature were calculated by using Bean model and critical current density value is about 76 A=cm2 at 97, 92, 86 and 81 K for B0, B1, B2, and B3, respectively. This result is consistent with the transport critical current density measurements at 77 K reported in a previous work [3]. 3.3. XRD and SEM analyses The XRD patterns of B0, B1, B2 and B3 samples are shown in Fig. 6. The lattice parameters determined from the (0 0 l) peaks of the XRD data are given in Table 1. It is observed that the lattice parameter c decreases significantly with increasing Sm content as listed in Table 1. A similar result has been reported by Zandbergen et al. [39]. As pointed out by Zandbergen et al., the behaviour of lattice parameter can be explained by the increase of the oxygen content in the unit cell by the replacement of Ca2þ by Sm3þ in the structure. It was speculated that the excess of oxygen goes into the bismuth oxide layers causing a decrease in lattice parameter c. It is also believed that the decrease of lattice parameter c is due to incorporation of Sm ions into the interstitial sites in the unit cell rather than occupation of the Ca sites. It was obtained that the intensities of the peaks corresponding to the Bi-2223 phase decrease and the intensities of the peaks corresponding to the Bi-2212 phase increase with increasing Sm concentration. Fig. 7 represents the SEM micrographs of B0, B1, B2 and B3 samples. The grain size of pure sample is relatively bigger than that of the Sm substituted Bi(Pb)SrCaCuO samples. Smaller grain size of Sm substituted samples may have contributed to low intergranular J c of these samples.
Fig. 7. SEM micrographs of (a) B0, (b) B1, (c) B2 and (d) B3 samples.
4. Conclusions This paper reports on the experimental results of substitution of Sm in Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy superconductor. Our investigations revealed that the substitution of Sm might have weakened the coupling of the grains, leading to deterioration of the microstructure and superconducting properties of the samples. When Sm substituted samples of Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy phase, prepared
by solid-state reaction methods, are compared with the undoped sample, following statements are concluded:
The surface morphology of the microstructure is degraded. Transformation to the Bi-2223 phase inhibited. The critical transition temperature decreased. Room temperature resistivity increased. The intergranular critical current density also decreased.
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Degradation of superconducting properties is attributed to worsening of the grain boundaries and modification of the high-T c phase.
Acknowledgements This work is supported partly by The Scientific and Technological Council of Turkey (Project No: 104T325) and in part by the Turkish State Planning Organization (DPT) (Project No: 2004K120200). References [1] A. Coskun, A. Ekicibil, B. Ozcelik, K. Kiymac, Chin. J. Phys. 43 (2005) 372. [2] I. Belenli, Ph.D. Thesis, 1993. [3] C. Terzioglu, M. Yilmazlar, O. Ozturk, E. Yanmaz, Physica C 423 (2005) 119. [4] M. Yilmazlar, H.A. Cetinkara, M. Nursoy, O. Ozturk, C. Terzioglu, Physica C 442 (2006) 101; O. Gorur, T. Kucukomeroglu, C. Terzioglu, A. Varilci, M. Altunbas, Physica C 418 (2005) 355. [5] C. Terzioglu, O. Ozturk, A. Kilic, A. Gencer, I. Belenli, Physica C 434 (2006) 153. [6] O. Ozturk, D. Yegen, M. Yilmazlar, A. Varilci, C. Terzioglu, Physica C 451 (2007) 113. [7] A. Biju, R.P. Aloysius, U. Syamaprasad, Supercond. Sci. Technol. 18 (2005) 1454. [8] N. Udomkan, P. Vinotai, R. Suryanarayanan, N. Charoenthai, Supercond. Sci. Technol. 18 (2005) 1294. [9] V.G. Prabitha, A. Biju, R.G. Abhilashkumar, P.M. Sarun, R.P. Aloysius, U. Syamaprasad, Physica C 433 (2005) 28. [10] A. Biju, R.G. Abhilash Kumar, R.P. Aloysius, U. Syamaprasad, Physica C 449 (2006) 109. [11] A. Ekicibil, A. Coskun, B. Ozcelik, K. Kiymac, J. Low Temp. Phys. 140 (2005) 105. [12] S. Nezir, S. Celebi, M. Altunbas, J. Alloys Compd. 302 (2000) 235. [13] K.H. Muller, M. Takashige, J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [14] J.R. Clim, Physica C 50 (1991) 153.
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Physica B 399 (2007) 94–100 www.elsevier.com/locate/physb
Substitution of Sm at Ca site in Bi1:6 Pb0:4Sr2 Ca2xSmxCu3Oy superconductors O. Ozturka, M. Akdogana, H. Aydina, M. Yilmazlarb, C. Terzioglua,, I. Belenlia a
Department of Physics, Faculty of Arts and Sciences, Abant Izzet Baysal University, 14280 Bolu, Turkey b Faculty of Education, Sakarya University, 54300 Hendek, Sakarya, Turkey Received 2 March 2007; accepted 23 May 2007
Abstract We have investigated the effect of the partial substitution of Ca by Sm in the Bi-2223 superconducting samples prepared by standard solid-state reaction method. Our investigations consisted of DC electrical resistivity, AC susceptibility, X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. We measured the critical transition temperatures, activation energies and irreversibility lines from the resistivity versus temperature curves under DC magnetic fields in the range of 0 and 0.6 T. The superconducting transition temperature, T c , and activation energy, U 0 , were found to decrease with increasing Sm concentration and with increasing applied magnetic field. Increasing the Sm content shifted the irreversibility temperature to lower values. The AC susceptibility measurements were carried out at 80 A/m field strength and f ¼ 211 Hz frequency. XRD patterns and SEM micrographs are given to provide information about Bi-2223 and Bi-2212 phases and their grain size. The possible reasons for the observed degradation in microstructural and superconducting properties due to Sm substitution were discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 74.72.h; 74.62.c; 61.10.Nz Keywords: AC susceptibility; Activation energy; Irreversible temperature; Magnetoresistivity; Intergranular critical current density
1. Introduction It is well known that superconductivity is suppressed by the presence of magnetic ions in the conventional metallic superconductors. This behaviour can be understood in terms of the pair-breaking mechanism [1]. In high-T c superconductors, it is well known that the superconducting properties are related to the chemical dopings, preparation conditions and hole concentrations. The objectives of doping works are to optimize the hole concentration, to introduce pinning centres, to enhance the formation of Bi2223 phase [2]. Variation in the T c , J c , and lattice parameters are obtained via doping the system with other elements at different levels and under various preparation conditions [3–11]. Terzioglu et al. [3] have recently investigated the effect of substitution of Sm for Ca on Corresponding author. Tel.: +90 3742541000; fax: +90 3742534642.
E-mail address: [email protected] (C. Terzioglu). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.05.028
superconducting, microstructure and mechanical properties of Bi1:6 Pb0:4 Sr2 Ca2x Smx Cu3 Oy [3,4,6]. It was obtained that superconducting and mechanical properties degrade with increasing Sm content. It was also reported that for x ¼ 1:0 and 1.5 the compounds showed semiconducting behaviour, resistivity increased with decreasing temperature. The irreversibility line can be determined from various measurements such as DC resistivity and AC susceptibility [12]. Muller et al. [13] first reported the irreversibility line in high temperature superconducting ceramics. The measurements of AC susceptibility are commonly used to determine magnetic and superconducting properties of these materials. In particular, the AC susceptibility measurement is useful in distinguishing between inter- and intragrain properties of the specimens. Two peaks, T p and T g , reflect inter- and intragranular losses in the ceramic superconductors. They can be distinguished from the temperature dependence of the imaginary part of the complex AC susceptibility only
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when high AC fields were used in the measurement [14,15]. The first peak appears at a temperature T g slightly below T c and indicates a maximum hysteresis loss for the motion of intragranular loss [15]. The second loss peak appears at a temperature T p lower than T g caused by the motion of intergranular vortices [16]. They both depend on the sample’s composition. In this work, we report X-ray diffraction (XRD), scanning electron microscopy (SEM), magnetoresistivity, and AC susceptibility measurements on Sm substituted Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy samples. We also investigated the effect of the partial substitution of Ca by Sm on the activation energy, the irreversibility line (or temperature), and inter- and intragrain properties of Bi-2223 superconductor. 2. Experimental details The Sm substituted Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy samples with x ¼ 0; 0:0005, 0.001 and 0.005 were prepared by the standard solid-state reaction method using high purity chemicals Bi2 O3 (99.99%), PbO ð99:9 þ %Þ, SrCO3 ð99:9 þ %Þ, CaCO3 (99+%), CuOð99 þ %Þ and Gd2 O3 ð99 þ %Þ [3]. Rectangular bars were cut from the sintered samples for electrical resistivity and AC susceptibility measurements. The typical sample size was ð2:9 2:1 12:1Þ mm3 . The calcinations and annealing processes of the samples were carried out using a programmable tube furnace from PROTHERM (Model PTF 12/75/200). The samples annealed at 860 C for 200 h with different Sm substitutions in Bi1:6 Pb0:4 Sr2 ðCa2x ÞSmx Cu3 Oy (x ¼ 0; 0:0005; 0:001 and 0.005) will be hereafter denoted as B0, B1, B2, and B3, respectively. We measured temperature dependence of resistivity of the samples using usual four-probe method with 5 mA DC current in a cryostat. Magnetoresistivity measurements were made under different DC magnetic fields (0, 0.3 and 0.6 T). The magnetic field was applied parallel to the current direction, by an electromagnet. The transition temperature T c was determined as the temperature at which zero resistivity was achieved. The reversibility temperature was determined as zero resistance temperature at various selected magnetic fields. The effective activation energy serves as an important parameter for the description of the flux dynamics in the mixed state for high-T c superconductors. A commonly accepted method for probing the magnetic field dependence of the effective activation energy is to measure the resistive transition in various applied magnetic fields. More accurate values of activation energy, U 0 , can be obtained from transport rather than magnetic measurements due to the widening of the magnetic transition. We have calculated U 0 in this study using the line pinning model [17] by doing approximations from resistivity measurements [18–22] via an Arrheniustype method. We have assumed that the resistivity r has following dependence: rðTÞ ¼ r0 expðU 0 =kB TÞ,
(1)
95
where kB is Boltzmann’s constant. When we plot ln r versus 1/T curve, the slope of the low resistivity part gives the activation energy U 0 . Magnetic susceptibility as a function of temperature ð752120 KÞ measurements were performed using a 7130 AC susceptometer of Lake Shore with a closed cycle refrigerator at 211 Hz frequency and 80 A/m field strength. The values of the T onset and T p were estimated from the c real and imaginary parts of AC susceptibility curves, respectively. We could observe only one peak in each of the w00 versus T plots. The values of intergranular critical current densities J c ðT p Þ in our samples were calculated using Bean’s model from the acquired data. XRD data were collected using a Rigaku D/Max-IIIC diffractometer with CuKa radiation in the range 2Y ¼ 4260 with a scan speed of 31 per minute and a step increment of 0.021 at room temperature. The lattice parameter c was determined from (0 0 2), (0 0 8), (0 ,1 0), and (0 0 1 4) peaks. The accuracy in determining the lattice parameter c was ˚ 0:001 A. The surface morphologies of the pure and Sm substituted samples were studied by using a JEOL JST-6400 scanning electron microscope. 3. Results and discussion 3.1. Electrical resistivity and activation energy We performed the electrical resistivity as a function of temperature between 77 and 130 K to investigate the effect of Sm substitution on the superconducting properties of the Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy samples with 0pxp 0.005. Fig. 1 displays the temperature variation of the normalized resistance (that each sample’s resistance normalized at 130 K) of the pure sample (B0) as well as Sm added samples (B1, B2, and B3). Zero-resistivity transition temperatures of the B0, B1, B2 and B3 samples are
Fig. 1. Temperature dependence of normalized resistance for the samples.
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determined as 107 102, 97, and 93 K, respectively. The estimated T c values of the samples decreased with increasing Sm content. The lower zero resistance transition temperature with increasing Sm content could be interpreted as a result of the suppression of superconductivity by the Sm3þ ions. It was observed that Sm substituted samples showed a broader transition, which indicates presence of impurities and weak-links between superconducting grains. The transition width of the sample B0 is 9 K but that of the sample B3 is 14 K. Broadening of the transition width indicates that the Sm substituted samples have lower percentage of the Bi-2223 phase compared to B0 sample [3]. If a substitution causes weak-link behaviour only, we usually expect beginning of the transition to remain same. Deviation form the metallic behaviour occurred at a few Kelvins lower temperature when Sm addition was increased, from B1 to B2. The difference between B2 and B3 is not at the high temperature end of the transition but at the low end of the transition. By looking at the R2T curves for B0 and B1, we can conclude that Sm addition, at first, did only cause weak-link behaviour. When it is increased to x ¼ 0:001, Bi-2223 phase was modified to have lower onset critical temperature besides weak-links. When we further increase Sm content of our samples, we observed excessive weak-link behaviour. The transition from normal to the superconducting state for Sm substituted samples has double step nature: We believed that the double step resistive transition is an indication of weak-links. All samples show a linear temperature dependence of the electrical resistivity, r ¼ rð0Þ þ aT, in normal state. As the Sm content increases, much broader superconducting transitions, higher room temperature resistivities and lower critical transition temperatures were observed. This degradation can be attributed to the worsened grain boundary properties and modified Bi-2223 phase caused by Sm content. The results obtained from the DC resistivity measurements are tabulated in Table 1. Fig. 2 displays the temperature dependence of resistivity at magnetic fields of 0, 0.3 and 0.6 T for B1 and B2. We observed that the broadening of the resistivity transition width increases with increasing external DC magnetic field and with increasing Sm concentration. It was also observed that, with increasing DC magnetic field up to 0.6 T, T offset c ranges from 107 to 79 K for B0, 102 to 71 K for B1, 97 to 63 K for B2, and 93 to 56 K for B3 sample. Below T c , the Table 1 Some characteristics of superconducting samples Sample
Lattice parameter c (A˚)
T offset (K) c From DC resistivity
(K) T onset c From AC susceptibility
T p (K) at 80 A/m
B0 B1 B2 B3
37.236 37.206 37.197 37.190
107 102 97 93
110 108 105 102
97 92 86 81
Fig. 2. The temperature dependence of resistivity at magnetic fields of 0, 0.3 and 0.6 T for the B1 and B2 samples.
thermally assisted flux flow (TAFF) is an important dissipation mechanism causing a long resistive tail. TAFF theory predicts that in the low current limit, the resistivity obeys the relation of Eq. (1). By rewriting Eq. (1) in the form r U0 . (2) ln ¼ r0 kB T We obtain U 0 from the slope of the Arrhenius plot for ln r versus 1/T. In order to achieve this aim we use linear fits to apparently linear parts of the curves in the low resistivity region of the graph. Fig. 3 shows one of these graphs from which we calculated activation energy for B2 at 0.3 T. From the slopes of the lines, one of which is shown in Fig. 3 as example, we calculated the activation energy as a function of external magnetic field. The activation energies of the other samples calculated likewise are listed in Table 2. In literature, a large number of researchers used
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Fig. 3. Plot of ln r versus 1=T for B2 sample at 0.3 T. Slope of this graph gives activation energy. Fig. 4. Applied DC magnetic field as a function of critical temperature for B1 sample. Table 2 Activation energy values of superconducting samples Sample
U 0 (K) at 0 T
U 0 (K) at 0.3 T
U 0 (K) at 0.6 T
B0 B1 B2 B3
11 700 10 069 7733 7239
1702 1613 1531 1380
1596 1470 1362 1253
this technique to determine the activation energy [12,23–26]. As seen in Table 2, the activation energy decreases significantly with increasing DC magnetic field and with increasing Sm content, being consistent with a previous work [26]. The relationship between T p and H AC was investigated for these samples and found to be linear [27]. In that study, the value of pinning force decreased with increasing Sm amount, which is consistent with the present results. We want to investigate the temperature dependence of peak field H p , deduced from the measurements of the electrical resistivity. Fig. 4 shows the corresponding data of zero resistance temperature at various selected magnetic fields for B1. These temperature values appear in Fig. 4 are comparable to irreversibility or depinning temperatures [28]. It is well to depin the flux line since lowering the temperature increases the pinning force. This results the zero resistivity or the irreversibility temperature to shift downwards as the magnetic field increases. It is observed that the irreversible region of H irr versus T graph extends to higher temperatures for pure sample. For the irreversibility, a representation: H irr ¼ H 0 ð1 T=T c Þn ,
(3)
near T c has been established. Both pre-factor H 0 and n are model-dependent parameters [29,30]. As can be seen from
Fig. 5. AC susceptibility (w ¼ w0 þ iw00 Þ versus temperature plots for the samples.
Fig. 4, the power low relation gives a good fit to experimental data for all samples being in good agreement with previous works [12,24,26]. 3.2. AC susceptibility In order to investigate the effect of Sm substitution on superconducting properties of the samples, we performed AC susceptibility measurements. We used H AC ¼ 80 A=m field amplitude and f ¼ 211 Hz frequency. Fig. 5 shows AC susceptibilities (w0 and w00 ) as a function of temperature for
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samples B0 and B1, B2, and B3. As seen from the real parts of the plots, the onset critical temperature appeared at 110 K for B0 sample and 108, 105, 102 K for B1, B2, and B3 samples, respectively. It shows that the transition temperature, defined as the temperature at which the Meissner signal begins to appear, decreases progressively with increasing x. The maximum peak of w00 appears at a temperature T p where the intergranular field just penetrates the centre of sample [31]. The overall susceptibility curves are shifted to lower temperature with increasing the Sm content. Imaginary part of B0, B1, B2 and B3 samples show peaks at temperatures 97, 92, 86, and 81 K for magnetic field strength of 80 A/m, respectively. At higher AC field amplitudes (typically greater than 400 A/m), two loss peaks can usually be seen in the imaginary part of susceptibility data; a broad peak at low temperature (coupling losses) and a narrower peak (intrinsic losses) near T onset . We did not see any intrinsic loss peaks in our c
(0024 )H
(31 5)L (317)L
(1119 )H
(220)L (220)H
(021 2)H
(00 12)H
x=0.0
(119 )H
(001 0)H (115)H (115)L
(004)H
(00 8)H
(002 )H
sample because of the low value of the applied field. This may be due to small grain size and no flux penetration into the grains [32]. Amplitude of the imaginary part of AC susceptibilities decreased with increasing Sm content. As can be seen in Fig. 5, Sm substitution shifts the susceptibility–temperature curves to lower temperatures and considerably increases the transition width, and also decreases the shielding fraction of the superconducting phase in the samples. This similar effect of the Smsubstitution in transition width is revealed for the DC resistivity measurements [33,34]. In the light of these, the broadening of the peaks with increasing Sm substitution can be said to be due to gradual penetration of flux into the centre of the intergranular regions. The real and imaginary parts are zero (w0 ¼ w00 ¼ 0) when the sample is in normal state (full penetration). Below T p , the amplitude of w00 falls due to decreasing amount of flux penetration. When the temperature reaches to a certain lower value, in other
(200 )H (0014 )H (111 1)H
98
(0012 )H
20
25
30
(00 24)H
(315 )L
(3 17)L
(1119 )H
(220 )L (220 )H (2012 )L
(021 2)H
(0024 )H
(315)L (317)L
(11 19)H
(0212 )H
(111 1)H
(220)L (220)H (2012 )L
(0014 )H (119)H (20 0)H
35
40
45
50
(002 4)H
(111 9)H
(220)L (220)H (20 12)L (111 5)L
(0212 )H
(1111 )H
(115 )H (115)L
(008)L
(008 )H
15
x=0.005
(119 )H (200)H
(00 10)H
(001 2)H
(115)H (115)L
(00 8)H
(004)H (00 4)H
10
x=0.001
(00 14)H
(0010 )H (002 )H (002)H
5
(119)H (200)H (00 14)H (11 11)H
(115)H (115 )L
(008)H
(004 )H
Intensity (a.u.)
(002 )H
(00 10)H
(001 2)H
x=0.0005
55
60
2θ ( Degree) Fig. 6. The XRD patterns for (a) B0, (b) B1, (c) B2 and (d) B3 samples. The peaks indexed ðh k lÞL and ðh k lÞH represent the Bi-2212 and Bi-2223 phases, respectively.
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words when w0 ¼ 1 and w00 ¼ 0, the whole body starts to shield. This indicated that destructive effect in the superconducting properties brought about by Sm–Ca replacement in the Bi-2223 system. This is in line with previous work [35–37]. We estimated the temperature dependence of intergranular critical current density using Bean’s model [38]. Accordingly, when flux lines fully penetrate to the sample, losses reaches to highest point. The critical current density is calculated for our sample using the relation J inter ðT p Þ ¼ c H a =a for the sample having cross section of the rectangular bar shaped, like 2a 2b where aob. J inter is the c intergranular current density and H a is the amplitude of applied field at T p . Therefore, the value of intergranular critical current density of the samples as a function of the peak temperature were calculated by using Bean model and critical current density value is about 76 A=cm2 at 97, 92, 86 and 81 K for B0, B1, B2, and B3, respectively. This result is consistent with the transport critical current density measurements at 77 K reported in a previous work [3]. 3.3. XRD and SEM analyses The XRD patterns of B0, B1, B2 and B3 samples are shown in Fig. 6. The lattice parameters determined from the (0 0 l) peaks of the XRD data are given in Table 1. It is observed that the lattice parameter c decreases significantly with increasing Sm content as listed in Table 1. A similar result has been reported by Zandbergen et al. [39]. As pointed out by Zandbergen et al., the behaviour of lattice parameter can be explained by the increase of the oxygen content in the unit cell by the replacement of Ca2þ by Sm3þ in the structure. It was speculated that the excess of oxygen goes into the bismuth oxide layers causing a decrease in lattice parameter c. It is also believed that the decrease of lattice parameter c is due to incorporation of Sm ions into the interstitial sites in the unit cell rather than occupation of the Ca sites. It was obtained that the intensities of the peaks corresponding to the Bi-2223 phase decrease and the intensities of the peaks corresponding to the Bi-2212 phase increase with increasing Sm concentration. Fig. 7 represents the SEM micrographs of B0, B1, B2 and B3 samples. The grain size of pure sample is relatively bigger than that of the Sm substituted Bi(Pb)SrCaCuO samples. Smaller grain size of Sm substituted samples may have contributed to low intergranular J c of these samples.
Fig. 7. SEM micrographs of (a) B0, (b) B1, (c) B2 and (d) B3 samples.
4. Conclusions This paper reports on the experimental results of substitution of Sm in Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy superconductor. Our investigations revealed that the substitution of Sm might have weakened the coupling of the grains, leading to deterioration of the microstructure and superconducting properties of the samples. When Sm substituted samples of Bi1:6 Pb0:4 Sr2 ðCa2x Smx ÞCu3 Oy phase, prepared
by solid-state reaction methods, are compared with the undoped sample, following statements are concluded:
The surface morphology of the microstructure is degraded. Transformation to the Bi-2223 phase inhibited. The critical transition temperature decreased. Room temperature resistivity increased. The intergranular critical current density also decreased.
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Degradation of superconducting properties is attributed to worsening of the grain boundaries and modification of the high-T c phase.
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