J Supercond Nov Magn (2012) 25:273–291 DOI 10.1007/s10948-011-1296-0
O R I G I N A L PA P E R
Ion Beam Analysis and Normal-State Conduction Mechanisms for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 Superconducting Phases Substituted by Ruthenium R. Awad · M. Roumié · A.I. Abou-Aly · S.A. Mahmoud · M.M. Barakat
Received: 8 April 2011 / Accepted: 8 August 2011 / Published online: 13 September 2011 © Springer Science+Business Media, LLC 2011
Abstract Superconducting samples of type Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ , (Bi, Pb)-2223, with 0.0 ≤ x ≤ 0.4 and type Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ , (Tl, Pb)/ Sr-1212, with 0.0 ≤ x ≤ 0.525 were synthesized using the standard solid-state reaction technique. The lattice parameters and the surface morphology for these samples were determined using X-ray powder diffraction (XRD) and scanning electron microscope (SEM) measurements, respectively. All element-contents of the samples prepared were estimated from the electron dispersive X-ray (EDX) technique, and their results were compared with those obtained from particle-induced X-ray emission (PIXE). In addition, the oxygen-content was determined using elastic Rutherford backscattering spectroscopy (RBS) technique at 3 MeV proton beam. The superconducting transition temperature Tc and the hole carrier concentration P were determined from the electrical resistivity measurement. The data of both Tc and P for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ phases increased up to x = 0.05 and 0.075, respectively and then they decreased as x increased. The superconductivity was completely destroyed around x = 0.4 and 0.525 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. The normal-state electrical resistivity data were analyzed using the two and three dimensional variable range hopping (2D-VRH and 3D-VRH) and the Coulomb gab (CG). R. Awad () · A.I. Abou-Aly · S.A. Mahmoud · M.M. Barakat Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt e-mail:
[email protected] M. Roumié Accelerator Laboratory, Lebanese Atomic Energy Commission, National Council for Scientific Research, Airport Road, PO Box 11, 8281 Beirut, Lebanon
Keywords Ru-content · (Bi, Pb)-2223 · (Tl, Pb)/Sr-1212 · PIXE · RBS · Conduction mechanism 1 Introduction Since the discovery of high-temperature superconductors HTSCs systems, (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 are interesting phases as they can exhibit different electrical behaviors through the partial chemical substitutions. (Bi, Pb)2223 phase is the most promising phase to synthesis tapes, wires and thin films for large-scale and high-current applications [1]. It is characterized by having high Tc , critical current density Jc and upper critical magnetic field Bc2 of order of 110 K, 8000 A/cm2 and 150 T, respectively [2]. The major limitations of this phase for the current superconductor applications are the intergrain weak links, weak flux pinning capability and presence of residual secondary phases. These secondary phases are located mainly between the superconducting grains, preventing the flow of the supercurrent [3]. On the other hand, (Tl, Pb)/Sr-1212 phase has the shortest insulating distance between the superconducting CuO2 -planes among all Tl-based superconductors [4] and its structure is quite similar to that of YBCO phase [5]. This short insulating distance reduces the anisotropy through the stronger interlayer coupling, less severe thermally activated flux motion and better performance in magnetic field [4]. Its Tc value ranges from 70 to 80 K [6] depending mainly on the processing conditions, resulting oxygen-content and other deviations from stoichiometry. This phase is characterized by having Bc2 (0) = 89 T [7] and intragranular Jc of order of 106 A/cm2 at 40 K in self field [8]. Chemical substitutions in HTSCs have a great importance for developing new superconducting materials conceived for diverse applications. In fact, it is the most effective way to improve the physical properties of un-substituted
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samples and to create pinning centers [9–11], which lead to enhance Jc value. This depends on the ionic radii, the bonding characteristics of the substituted elements and the excess of oxygen-content. Both elements-content and the structural details were performed using the ion beam analysis (IBA), including both PIXE and RBS techniques. PIXE technique usually checks the contamination of the prepared samples, whereas RBS signal determines the metal concentrations and oxygen-content. Recently, many authors estimated the elements- and O-content for TlBa2 Ca2−x Scx Cu3 O9−δ [12] and (Tl0.8 Hg0.2−x Sbx )Ba2 Ca2 Cu3 O9−δ [13] phases, as well as for TlBa2 Ca3−x Erx Cu4 O11−δ and Cu0.25 Tl0.75 Ba2 Ca3−x Erx Cu4 O12−δ [14] phases using PIXE and RBS techniques. PIXE spectra indicated that all the real elements-content as weight % were nearly close to those of the nominal values. The oxygen-content, obtained from non-Rutherford backscattering using 3 MeV H+ , was very close to the nominal value. Moreover, the normal state of HTSCs showed many unusual properties which are far from the standard Fermi liquid behavior. A typical anomalous property is the linear temperature dependence of the electrical resistivity. This was in contrast with the T 2 dependence that was expected from Fermi liquid model [15]. Among different models for describing the charge transport in these materials, the variable range hopping conduction mechanism between localized states was widely used for the normal state of HTSCs [16]. Many reports showed that three dimensions VRH (3D-VRH) [17], two dimensions VRH (2D-VRH) [18] and Coulomb gap regime [19] were the proper mechanisms for the normal-state conductivity of HTSCs. In this work, we studied the effect of Ru substitution on the elements-content and electrical resistivity of (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 superconducting phases. Superconducting samples of type Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ with 0.0 ≤ x ≤ 0.4 and type0 Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ with 0.0 ≤ x ≤ 0.525 were prepared at ambient pressure. The prepared samples were investigated using XRD, SEM, EDX, PIXE, RBS and electrical resistivity measurements. Furthermore, the normalstate electrical resistivity was also discussed according to the VRH and CG conduction mechanism models.
2 Experimental Technique Conventional solid-state reaction technique was used to prepare superconducting samples of nominal composition of Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ with x = 0.0, 0.025, 0.05, 0.075, 0.15, 0.3 and 0.4 [20]. On the other hand, starting oxides of Tl2 O3 , PbO2 , SrO2 , BaO2 , CaO, CuO and RuO2 (purity ≥ 99.9) were used to prepare superconducting samples of nominal composition Tl0.5 Pb0.5 Sr1.6 Ba0.4 Ca
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Cu2−x Rux O7−δ with x = 0.0, 0.075, 0.15, 0.3, 0.45 and 0.525, using one step solid-state reaction technique. The constituted oxides were weighted according to the stoichiometric ratios, except for Tl2 O3 which was weighted 10% above the stoichiometric amount to compensate the vapor loss during the preparation process. These oxides were mixed using an agate mortar to make fine powder and then they were sifted in 125 μm sieve to obtain a homogeneous mixture. The powder was pressed in a form of a disc of dimensions 1.5 cm in diameter and 0.3 cm thickness. The samples were then wrapped in silver foil, in order to reduce thallium evaporation during the preparation. The samples were heated in a sealed quartz tube to 850 °C with a heating rate of 4 °C/min and held at this temperature for five hours. Finally, the samples were left to cool slowly in the furnace. The prepared samples were characterized by XRD using Shimadzu-7000 powder diffractometer with Cu-Kα radiation (λ = 1.54056 Å) in the range 4◦ ≤ 2θ ≤ 70°. The surface morphology of these samples was identified using a Jeol scanning electron microscope JSM-5300 operated at 25 kV. The elements-content of the prepared samples were determined using an Oxford X-ray micro-probe analysis connected to Jeol scanning microscope JSM-5300. The elements-content of the investigated samples were also measured by the conventional vacuum PIXE. The 1.7 MV tandem accelerator of the Lebanese Atomic Energy Commission [21] was used to deliver 3 MeV proton beam on the samples with 3 μC of fluence. This fluence sufficed to have less than 2–3% of statistical error in the peak area of the measured elements. The beam intensity was around 2 nA to keep the counting rate at <1000 cps. The target chamber contains a retractable collimated Si (Li) detector with 30 mm2 of active area and a 175 eV measured energy resolution at 5.9 keV. The thickness of the beryllium detector window was 13 μm which allowed the detection of elements with atomic number Z > 10. The analyzed samples were mounted on a rotating wheel controlled by a PC and a stepping motor. In order to detect and qualify all the composed elements of the investigated samples in one run, an Al funny filter of 250 μm thickness is placed in front of Si (Li) as Xray absorber [22, 23]. Simultaneously, elastic backscattering measurements were taken under normal incident beam. A partially depleted PIPS (Passivity Implanted Planar Silicon) detector from Canberra, with 14 keV energy resolutions and 25 mm2 active areas, detected the backscattered particles of the 3 MeV proton beam at a scattering angle θ of 165°, where the solid angle of the detector is 5.45 × 10−3 sr. For IBA analysis, a mass of 0.3 g of each sample was very finely ground to ensure homogeneity and then a thick target pellet of an approximate dimensions 1 × 1 × 0.2 cm3 was formed. Borax powder was pressed with the ground powder in order to improve the adhesion of the obtained thick pellet and to keep it free from cracks. The sample was hold in
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an Al holder frame using ultra-pure carbon layer in order to ensure good electrical contact and hence to be able to take an accurate direct measurement of the beam fluence. Different PIXE spectra were processed with the GUPIX package (Guelph PIXE software package) [24], while the RBS ones were analyzed and fitted using the SIMNRA code [25]. The Guelph PIXE program is based on the fundamental parameters including X-ray production cross sections, X-ray attenuation, proton stopping powers, detector efficiency, collected charge and geometry effect to produce an output of elements-content in ppm. The electrical resistivity of the prepared samples was measured by a conventional four-probe technique from room temperature down to zero resistivity temperature T0 using a closed cryogenic refrigeration system. The samples used for resistivity measurement had dimensions of about 1.5 × 0.2 × 0.3 cm3 . The connection of copper leads with the samples was made using a conductive silver paint. The temperature of the samples was monitored by a Chromel versus Fe–Au thermocouple and stabilized with the aid of a temperature controller to within ±0.1 K. 3 Results and Discussion Figures 1 and 2 show the XRD patterns for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ with x = 0.0, 0.05 and 0.15, and for
Fig. 1 XRD patterns for (Bi, Pb)-2223 with x = 0.0, 0.05 and 0.15
275
Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ with x = 0.0, 0.075 and 0.15, respectively. As shown in Fig. 1, the most highintensity peaks for x = 0.0 belong to a nearly single tetragonal phase of (Bi, Pb)-2223 with few low-intensity peaks which belong to (Bi, Pb)-2212 and Ca2 PbO4 phases. The notations H (h k l), L (h k l) and (+ ) indicate the peaks of (Bi, Pb)-2223, (Bi, Pb)-2212 and Ca2 PbO4 phases, respectively. The intensity of the typical peak of (Bi, Pb)-2223 phase at 2θ = 4.77° increases up to x = 0.05, meaning that the lower content of Ru can stabilize the formation of (Bi, Pb)-2223 phase. This peak decreases at x > 0.05 and the peaks intensity of (Bi, Pb)-2212 phase increases. The patterns of (Bi, Pb)-2212 and Ca2 PbO4 phases predominate for x ≥ 0.15. These results indicate that the high Ru-content degrades formation of (Bi, Pb)-2223 phase and favors the formation of (Bi, Pb)-2212 phase. The relative volume fraction of (Bi, Pb)-2223, (Bi, Pb)-2212 and Ca2 PbO4 phases were calculated [20] and they are listed in Table 1 versus Rucontent. The relative volume fraction of (Bi, Pb)-2223 phase increases while it decreases for (Bi, Pb)-2212 and Ca2 PbO4 phases as x increases from 0.0 to 0.05. This behavior has a reverse trend with further increase in x. The lattice parameters a and c were calculated using the least square method through d values and (h k l) planes for tetragonal unit cell structure. Their variations with Ru-content are also listed in Table 1. The lattice parameter a increases slightly and the
276
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Fig. 2 XRD patterns for (Tl, Pb)/Sr-1212 with x = 0.0, 0.075 and 0.15 Table 1 The variation of the relative volume fraction of (Bi, Pb)-2223, (Bi, Pb)-2212 and Ca2 PbO4 phases, and the lattice parameters with Ru-content for Bi1.8 Pb0.4 Sr2 Ca2 Cu3−x Rux O10+δ , 0.0 ≤ x ≤ 0.4 x
Relative volume fraction (%) (Bi, Pb)-2223
Lattice parameters (Å) (Bi, Pb)-2212
Ca2 PbO4
a
c
0
86.37
12.59
1.04
5.3945
37.1916
0.025
87.95
10.64
1.41
5.4015
37.1879
0.05
88.84
10.22
0.94
5.4049
37.1830
0.075
73.17
25.57
1.26
5.4073
37.1634
0.15
–
96.92
3.08
5.4111
30.6158
0.3
–
95.61
4.39
5.4194
30.5218
0.4
–
95.66
4.34
5.4295
30.4854
Table 2 The variation of the relative volume fraction of (Tl, Pb)/Sr-1212 and other impurities, and the lattice parameters with Ru-content for Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ , 0.0 ≤ x ≤ 0.525 x
0
Relative volume fraction (%)
Lattice parameters (Å)
(Tl, Pb)/Sr-1212
Other
a
c
88.39
11.61
3.8042
12.1865
0.075
92.28
7.72
3.8050
12.1783
0.15
88.66
11.34
3.8059
12.1712
0.3
81.31
18.69
3.8064
12.1637
0.45
75.87
24.13
3.8070
12.1557
0.525
61.23
38.77
3.8076
12.1481
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lattice parameter c decreases significantly with increasing Ru-content. For x ≥ 0.15, the lattice parameters a and c are closer to those of (Bi, Pb)-2212 phase, indicating the presence of phase change from (Bi, Pb)-2223 to (Bi, Pb)-2212 phase [20]. This phase change is due to the distortion between (Bi, Pb)-2223 slabs and the formation of weak links at grain boundaries through the higher partial substitution of Cu2+ ions by Ru4+ ions [20]. On the other hand, XRD patterns for (Tl, Pb)/Sr-1212 phase are well indexed by a tetragonal lattice with a space group P4/mmm as shown in Fig. 2. For un-substituted sample, all the peaks belong to the main tetragonal peaks of (Tl, Pb)/Sr-1212 phase with other impurities such as (Ca,Sr)-cuprate and small amounts of unidentified impurities. The relative volume fraction of (Tl, Pb)/Sr-1212 phase and other impurities are listed in Table 2 versus Ru-content. The relative volume fraction of (Tl, Pb)/Sr-1212 phase increases as x increases from 0.0 to 0.075, followed by a decrease in its value with further increase in x. In addition, the other impurities increase by increasing Ru-content for x ≥ 0.15. This means that low Rucontent, 0.0 < x ≤ 0.075, enhances (Tl, Pb)/Sr-1212 phase formation. The lattice parameters a and c were also calculated and their values are listed in Table 2. a and c, for unsubstituted sample, are closer to those obtained by Lebbou et al. [26] for Tl0.5 Pb0.5 (Sr1−x Bax )2 CaCu2 O7−δ phase with x = 0.2. a increases slightly and c decreases significantly with increasing Ru-content. A similar behavior was found by Kandyel et al. [27] for Tl0.5 Pb0.5 Sr2 CaCu2−x Nix O7−δ phase. This variation could be due to the excess oxygencontent in the unit cell, resulting from the partial replacement of Cu2+ ions by Ru4+ ions. This can enhance the average oxidation-state, leading to smaller Cu–O distance within the copper-oxygen sheet (c-axis) [28]. In addition, the lattice parameter a is controlled by the length of in-plane Cu–O bond which may be expanded as a result of electrons addition into antibonding orbital that was also caused by the partial replacement of Cu2+ ions by Ru4+ ions [29]. Figures 3(a)–(c) and 4(a)–(c) show the SEM micrographs of the fractured surface for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ with x = 0.0, 0.05 and 0.15, and for Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ with x = 0.0, 0.075 and 0.15, respectively. For (Bi, Pb)-2223 phase, the granular morphology of the un-substituted sample consists of flaky layers of large platelet-like structure with random alignment distribution which is a signature of (Bi, Pb)-2223 phase formation [30]. The platelet grains are linked well for low Ru-content, up to x = 0.05, in which the platelike grains still have a flaky structure. The grains started to degrade by random orientation, showing a weak links between them for x > 0.05. This increases the level of impurities, voids and hence porosity associated with the formation of smaller plat-like grains at x = 0.15, belonging to (Bi, Pb)-2212 phase [31]. We also observed a small amount of
277
Fig. 3 SEM images for (Bi, Pb)-2223 with x = 0.0 (a), 0.05 (b) and 0.15 (c)
278
Fig. 4 SEM images for (Tl, Pb)/Sr-1212 with x = 0.0 (a), 0.075 (b) and 0.15 (c)
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randomly distributed sphere-like grains, indicating the presence of Ca2 PbO4 phase [20]. On the other hand for (Tl, Pb)/Sr-1212 phase, the micrographs show generally porous microstructure accompanied by a gradual reduction in the average grain size with increasing Ru-content as shown in Figs. 4(a)–(c). The un-substituted sample shows relatively rounded and irregular grains. These grains are well linked up to x = 0.075 as shown in Fig. 4(b), then they become less rounded with smaller average grains size with further increase in Ru-content. For x = 0.15, the grain boundaries gradually become less visible possibly due to partial melting during high-temperature sintering. Since all the samples were prepared under the same heating condition, it is possible that the morphological changes are related to lowering of partial melting temperature due to substitution of Ru. A similar result was observed by Yahya et al. [32] in Tl0.5 Pb0.5 Sr2−x Ybx CaCu2 O7−δ superconducting phase. Figures 5(a)–(d) display EDX analysis for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ phases with x = 0.0 and 0.15, respectively. It is clear that, for each phase, no difference appears between the two samples except the appearance of ruthenium peak for x = 0.15 at energy ranges from 2.477 to 2.688 keV as shown in Figs. 5(b) and (d). This indicates that the Ru was successfully introduced into the microstructure of (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases. Typical PIXE spectra for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ phases with x = 0.0 and 0.15 are shown in Figs. 6(a)–(d), respectively. The K X-ray lines of Ca, Cu, Sr and Ru are seen for each phase, in addition to Ag and Ba which are seen for (Tl, Pb)-1212 phase. L X-ray lines of Ba are appeared in (Tl, Pb)/Sr1212 spectra. Furthermore, L and M X-ray lines of Pb, Bi and Tl, Pb are observed for (Bi, Pb)-2223 and (Tl, Pb)/Sr1212 phases, respectively. The appearance of Ag K X-ray lines, at the ppm level, in PIXE spectra for (Tl, Pb)/Sr-1212 phase as impurity is not affect the stoichiometry of the investigated samples. It arisen from the surface contamination from the silver sheets which were used for wrapping the samples during the preparation process. This indicates that PIXE technique is more useful for detecting the low contamination than EDX technique. The real elements-content, determined from EDX and PIXE techniques, with their experimental errors are listed in Tables 3 and 4 for (Bi, Pb)2223 and (Tl, Pb)/Sr-1212 phases, respectively. It is clear that the most of the real elements-content from EDX results are close to those of the nominal valves except for Ru. This could be due to the disability for EDX technique to detect the lower elemental-content. Although thallium is volatile at T = 710 °C, which is lower than the sintering temperature, its real content is nearly close to the nominal ones. This means that the addition of 10% of Tl2 O3 more than the stoichiometric ratio is suitable to compensate both vapor
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279
Fig. 5 EDX analysis for (Bi, Pb)-2223 with x = 0.0 (a) and 0.15 (b), and for (Tl, Pb)/Sr-1212 with x = 0.0 (c) and 0.15 (d)
loss and the deposition on the silver wrap during the sintering process. On the other hand, a marvelous agreement between the real and nominal elements-content is observed from PIXE results, except for Ca and Ba. The accurate detection of low Ru-content could be due to the large cross section of X-ray and low background contribution. So, PIXE is a highly sensitive method for the multi-element analysis and a large number of elements that may be seen simultaneously. Meanwhile, the lower observed values for Ca and Ba than those that expected could be attributed to the overlapping of heavier elements over the lighter one. It is well known that PIXE technique is insensitive to oxygen as its value was calculated indirectly by completing the stoichiometry to 100 wt.% of different constituents that appeared in the PIXE spectra [13]. It is mentioned that the real O-content as weight %, resulting from PIXE measurements, is far enough from the nominal one as shown in Table 5 for each phase. Rutherford backscattering spectrometry RBS presents an alternative technique for an accurate determination of Ocontent. It was found from our previous work [14] that the O-content could be determined more accurately at 3 MeV
H+ than at 2 MeV He++ . This is because, at 2 MeV He++ , the heavier elements overlapped over the lighter elements, especially O [12]. Nevertheless, protons have lower stopping cross section and generally higher scattering cross section for light elements than alpha particles. Therefore, proton elastic backscattering spectrometry (BS) could provide a sensitive tool for analyzing light elements such as oxygen, where highly non-Rutherford cross sections result above proton energies of 1000 keV. The O-content, for the different samples, was determined using 3 MeV proton elastic backscattering by fitting non-Rutherford backscattering cross section of oxygen [25] in the SIMNRA simulation. Figures 7(a) and (b) illustrate a characteristic RBS spectrum for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu2.85 Ru0.15 O10+δ and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu1.85 Ru0.15 O7−δ phases, respectively. Unlike the O signal, in all obtained spectra, the ruthenium signal was indistinguishable due to its presence between Sr and Bi, Pb for (Bi, Pb)-2223 phase, as well as its presence between Sr and Tl, Pb for (Tl, Pb)/Sr-1212 phase. The energy difference regarding of these elements is almost corresponding to the detector resolution (∼14 keV). In fact,
280
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Fig. 6 PIXE spectra for (Bi, Pb)-2223 with x = 0.0 (a) and 0.15 (b), and for (Tl, Pb)/Sr-1212 with x = 0.0 (c) and 0.15 (d)
for up to 3 MeV the cross section energy dependence is rather smooth for the oxygen (p,p) elastic scattering, except for the narrow resonance at 2.66 MeV, which made it very suitable for analytical purposes. Furthermore, the theoretical model used in the SIMNRA simulations to fit the experimental spectra, was described by Gurbich [33] and found to be in a good agreement with most of the posterior measurements and benchmark experiments. The evaluation of the cross sections by combining a large number of different data sets in the framework of the theoretical model made it possible to calculate excitation functions for analytical purposes for any scattering angle, with reliability
exceeding that of any individual measurement. Ramos et al. [34] measured the oxygen (p,p) cross sections, between 500 and 2500 keV, which agreed with the Gurbich simulations. Therefore, based on the Gurbich simulation model used in SIMNRA, the real and nominal values of O-content as weight %, as well as the real O-content are listed in Table 5 versus Ru-content for each phase. The real O-content as weight %, resulting from the RBS measurements, is higher than the nominal one for (Bi, Pb)-2223 phase which has a reverse trend for (Tl, Pb)/Sr-1212 phase. The real Ocontent, determined from PIXE and RBS measurements for each phase, increases with increasing Ru-content and its val-
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281
Fig. 6 (Continued)
ues are listed in Table 5 with corresponding experimental errors. This could be due to the excess of oxygen, resulting from the partial replacement of Cu2+ ions by Ru4+ ions. This increase confirmed the variation of the lattice parameters with Ru-content for each phase as discussed before. Figures 8(a)–(d) display the temperature dependence of the electrical resistivity for (Bi, Pb)-2223 with x = 0.0, 0.025, 0.05, 0.075 and 0.15 and with x = 0.3 and 0.4, and for (Tl, Pb)/Sr-1212 with x = 0.0, 0.075, 0.15 and 0.3 and with x = 0.45 and 0.525, respectively. The un-substituted sample, for each phase, is characterized by a nearly sharp resistive transition, indicating its high purity. The broadening of resistive transition decreases by increasing Ru-content up to x = 0.025 and 0.075 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. Samples with 0.0 ≤ x ≤ 0.15 for each
phase show a metallic-like behavior at higher temperatures (dρ/dT > 0.0) followed by a transition to superconducting state as the temperature decreases. The electrical resistivity data ρ(T ) for these samples were well fitted in the temperature range 130 K ≤ T ≤ 300 K and 110 K ≤ T ≤ 300 K for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively according to Matthiessen rule: ρ(T ) = ρ ∗ + αT ,
(1)
where ρ ∗ is the residual resistivity and α ≡ dρ/dT is the resistivity temperature coefficient. A small curvature in the electrical resistivity data above the superconducting transition temperature is observed for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases with 0.0 ≤ x ≤ 0.15. This behavior is
(%)
(%)
content
Exp. errors Element-
x = 0.05 (%)
content
Exp. errors Element-
x = 0.075 (%)
content
Exp. errors Element-
x = 0.15 (%)
Exp. errors
content
Element-
x = 0.3 (%)
content
Exp. errors Element-
x = 0.4 (%)
Exp. errors
2.077 1.291 1.11 38.51 2.053 1.217 2.25 42.05 2.026 1.193 3.52 43.20 2.000 1.190 4.78 43.35 1.978 1.271 5.83 39.47 1.952 1.256 7.04 40.17 1.931 1.256 8.06 40.88
2.954 2.975 1.54
0.000 0.000 0.00
Ca
Cu
Ru
3.38 2.488 2.581 7.86 4.39 2.431 2.581 6.48 3.46
1.71 1.936 1.948 3.20 2.60 1.927 1.948 3.65 3.12
0.00 0.010 0.025 −2.59 −1.73 0.031 0.050 −0.55 −0.03 0.051 0.078 −0.83 −3.69 0.093 0.167 −2.25 −11.50 0.214 0.307 −3.39 −2.40 0.279 0.307 −5.73 −1.57
0.85 2.909 2.913 2.21 2.10 2.850 2.868 3.39 2.78 2.791 2.828 4.59 3.33 2.733 2.754 4.09
1.83 1.945 1.948 2.77 2.60 1.944 1.955 2.82 2.26 1.972 1.978 1.42 1.11 1.954 1.966 2.28
9.35 0.366 0.375 8.42 6.21 0.370 0.375 7.51 6.34
1.954 1.963 2.29
Sr
4.04 0.372 0.377 7.00 5.77 0.377 0.380 5.84 5.00 0.381 0.383 4.68 4.35 0.359 0.363 10.26
1.791 1.861 0.52 −3.41 1.784 1.810 0.89 −0.57 1.776 1.863 1.32 −3.49 1.769 1.852 1.75 −2.88 1.785 1.846 0.81 −2.53 1.777 1.846 1.30 −2.54 1.795 1.846 0.28 −2.11
0.379 0.384 5.14
Bi
Pb
EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE EDX PIXE
content
Exp. errors Element-
content
x = 0.025
Element-
x = 0.0
Element Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ
Table 3 The elements-content of Bi, Pb, Sr, Ca, Cu and Ru, determined from EDX and PIXE, with their experimental errors versus Ru-content for (Bi, Pb)-2223 phase
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(%)
content
0.000 0.000 0.00
Ru
1.27 0.70 1.12
2.71
26.00 0.993 0.782
1.33
2.39 −3.94 0.480 0.486
0.57 1.682 1.693
21.78 0.988 0.751
21.74 0.392 0.337
0.58 1.584 1.589
1.78 −10.22 0.483 0.530
0.30 −1.59 1.829 1.839
23.59 0.389 0.313
(%)
Exp. errors
1.08
1.19
2.08
1.02
(%)
Exp. errors
2.70 0.472 0.470 0.68 1.591 1.618
0.39 1.508 1.517
24.91 0.975 0.758
15.65 0.383 0.305
x = 0.525
6.06 0.466 0.487
2.53 2.74
2.10 1.427 1.463
24.21 0.986 0.741
23.72 0.394 0.303
0.55 −1.15 1.566 1.563 4.30
(%)
Exp. errors
2.67
3.23
1.36
1.45
2.14
0.82
25.86
24.25
2.34
2.27 −1.95
6.72
EDX PIXE EDX PIXE
content
Element-
0.98 −6.63 0.489 0.510
5.62
EDX PIXE EDX PIXE
content
Element-
x = 0.45
3.47 −5.91 0.495 0.533
4.05
EDX PIXE EDX PIXE
content
Element-
x = 0.3
0.00 0.060 0.083 20.00 −11.28 0.105 0.179 30.00 −19.40 0.189 0.363 37.00 −21.08 0.324 0.519 28.00 −15.37 0.360 0.599 31.43 −14.03
1.94 1.919 1.956
20.65 0.987 0.740
0.998 0.793 0.23
1.956 1.961 2.19
Ca
Cu
3.25
0.83 −1.04 1.580 1.591
25.53 0.387 0.306
0.399 0.298 0.35
−1.49 1.587 1.617
1.596 1.624 0.25
Sr
2.71 −7.55 0.488 0.520 2.27 −9.01 0.491 0.551
0.498 0.550 0.47 −10.07 0.486 0.538
0.499 0.558 0.28 −11.55 0.489 0.545
Ba
(%)
Exp. errors
EDX PIXE EDX PIXE
content
Element-
x = 0.15
Tl
EDX PIXE EDX PIXE
(%)
Exp. errors
Pb
EDX PIXE EDX PIXE
Element-
Elementcontent
x = 0.075
Exp. errors
x = 0.0
Element Tl24 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ
Table 4 The elements-content of Tl, Pb, Sr, Ba, Ca, Cu and Ru, determine from EDX and PIXE, with their experimental errors versus Ru-content for (Tl, Pb)/Sr-1212 phase
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Table 5 The real and nominal values of oxygen as weight %, determine from PIXE and RBS, and the real O-content with its experimental errors versus Ru-content for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases x
(Bi, Pb)-2223
(Tl, Pb)/Sr-1212
0.0
O-element Nominal
PIXE
(wt. %)
Real (wt. %)
15.42
15.04
RBS O-content
Exp. Errors (%)
Real (wt. %)
O-content
Exp. Errors (%)
9.658
3.42
15.77
10.123
−1.23
0.025
15.44
15.04
9.665
3.35
15.83
10.172
−1.72
0.05
15.47
15.04
9.672
3.28
15.97
10.272
−2.72
0.075
15.49
15.08
9.707
2.93
15.98
10.287
−2.87
0.15
15.61
15.20
9.816
1.84
16.01
10.322
−3.22
0.3
15.80
15.31
9.935
0.65
16.17
10.498
−4.98
0.4
15.98
15.40
10.029
−0.29
16.19
10.544
−5.44
0.0
15.88
16.53
7.026
−0.38
15.38
6.537
6.62
0.075
16.06
16.70
7.128
−1.83
15.38
6.565
6.21
0.15
16.17
17.00
7.288
−4.11
15.51
6.646
5.05
0.3
16.45
17.68
7.638
−9.12
15.76
6.809
2.73
0.45
16.66
18.30
7.969
−13.85
15.97
6.959
0.59
0.525
16.70
19.00
8.312
−18.74
15.99
6.994
0.09
characterized by the superconducting thermodynamic fluctuations [35] or the opening of spin-gap that appears in the HTSCs due to magnetic impurities substitutions [36]. The resistivity data behave like a semiconductor at hightemperature range and become superconducting as the temperature decreases with x = 0.3 for (Bi, Pb)-2223 phase and with x = 0.3 and 0.45 for (Tl, Pb)/Sr-1212 phase. This upturn in the resistivity behavior could be explained according to two different reasons. The first one is due to the disorder of Ru4+ ions at Cu sites [37], while the second one is attributed to the magnetic disorder of magnetic impurities including an uncompensated local magnetic moment on the CuO2 -planes. A semiconductor-like behavior, in the whole temperature range, is observed with x = 0.4 and 0.525 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. This behavior was explained according to Anderson impurity model [38]. This model indicated that the electronic states near the Fermi surface are inerrant due to a large overlap of the Cu 3d and O 2p wave functions, but become localized when this overlap is reduced. The transition width ΔT (ΔT = Tc − T0 ), the room-temperature resistivity ρn , ρ ∗ and dρ/dT values are listed in Table 6 versus Ru-content for each phase. The superconducting transition width is calculated to examine the purity of the prepared samples. The lower ΔT is attributed to the existence of percolative path which became first superconducting, thereby shorting out the current. In addition, the higher ΔT is due to the presence of macroscopic inhomogeneity and secondary phases. For each phase, both ρn and ρ ∗ increase with increasing Rucontent, reflecting the enhancement of the scattering mechanism through the partial replacement of Cu2+ ions by Ru4+ ions. dρ/dT value increases as x increases, followed by a
decrease in its value to be negative at x = 0.4 and x ≥ 0.3 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. The increase in dρ/dT is attributed to the decrease in the carrier concentration [39], or due to an unsuspected temperature dependent scattering contribution by Ru center. The variation of Tc with Ru-content is also listed in Table 6 for each phase. The superconducting transition temperature was determined from the maximum point in the first derivative curve of resistivity versus temperature. Tc increases with increasing Ru-content up to x = 0.05 and 0.075 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively and beyond which it decreases with further increase in x. This parabolic dependence of Tc on x was considered to be a typical character of the substituted high-temperature superconductors [40]. This relation indicates that the unsubstituted sample lies in the under-doped region. The addition of Ru4+ ions increases the oxygen-content, leading Tc goes up to reach its optimum value at x = 0.05 and 0.075 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. This enhancement in Tc is confirmed with the enhancement of phase formation for (Bi, Pb)-2223 [20] and (Tl, Pb)/Sr-1212 phase as displayed in Tables 1 and 2, respectively. The further increase in Ru-content leads to depress Tc , corresponding to the presence of over-doped region effect by Ru at x > 0.05 and >0.075 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. This behavior is confirmed with the calculated O-content, from RBS measurements, which increases with increasing Ru-content as shown in Table 5 for each phase. Also, there are two different explanations for the strong depression in Tc for each phase. The first was explained by Abrikosov-Gor’kov theory [41] for a Cooper pair-breaking mechanism that resulted
J Supercond Nov Magn (2012) 25:273–291
285
(a) (a)
(b)
(b) Fig. 7 RBS spectrum for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu2.85 Ru0.15 O10+δ (a) and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu1.85 Ru0.15 O7−δ (b)
from magnetic ions substitution, creating a disorder in the internal magnetic state. The second one is due to the scattering between the Cooper pair and magnetic ions according to spin-flip process [42]. This process is characterized by total spin conservation in the scattering event, so the spin of the Ru atom must flip when the Cooper pairs are broken. Furthermore, the decrement of Tc value for (Bi, Pb)-2223 phase can be understood due to a strong phase change from (Bi, Pb)-2223 phase to (Bi, Pb)-2212 phase at x = 0.15 and 0.3 [20]. The hole carrier concentration per Cu ion, P , is calculated by using the following relation [43]: P = 0.16 −
1−
Tc Tcmax
0.5 82.6
,
(2)
where Tcmax was taken as 110 and 80 K for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. The parabolic relationship between the superconducting transition temperature and the hole carrier concentration is observed for each
(c) Fig. 8 The temperature dependence of the electrical resistivity for (Bi, Pb)-2223 with x = 0.0, 0.025, 0.05, 0.075 and 0.15 (a) and with x = 0.3 and 0.4 (b), and for (Tl, Pb)/Sr-1212 with x = 0.0, 0.075, 0.15 and 0.3 (c) and with x = 0.45 and 0.525 (d)
phase as shown in Fig. 9. P enhances up to x = 0.05 and 0.075 for (Bi, Pb)-2223 [20] and (Tl, Pb)/Sr-1212 phases as shown in the insert of Fig. 9. Then, it decreases for further increase in Ru-content. In order to investigate the normal-state electrical resistivity, the data were fitted according to two and three dimensional variable range hopping (2D-VRH, 3D-VRH), and
286
J Supercond Nov Magn (2012) 25:273–291
Coulomb gap (CG) regimes. The temperature dependence of resistivity in the hopping conduction mechanism is expressed through the following relation [44]: ρ(T ) = ρ0
T T∗
2p
∗ p T exp , T
(3)
where ρ0 is temperature independent coefficient and T ∗ is a characteristic temperature. p = 1/(D + 1) with D = 2 and 3, where D is the dimensionality of the conduction mechanism in the VRH regime. Furthermore, p = 1/2 for both two and three dimensions in the CG regime. We had fitted the ln(ρ/T 2p ) versus 1/T p curves by a linear relation for different values of p. This fitting was applied in the temperature interval from room temperature down to 120 and 100 K with x ≤ 0.3 and ≤0.45, and down to 50 K with x = 0.4 and 0.525 for (Bi, Pb)-2223 and (Tl,
(d)
Pb)/Sr-1212 phases, respectively. Figures 10(a) and (b) show the typical curves for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu2.6 Ru0.4 O10+δ and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu1.475 Ru0.525 O7−δ samples, respectively. The fitting parameters T ∗ , ρ0 and χ 2 , derived from the fittings for all samples under investigation, are listed in Table 7 for each phase. The χ 2 parameter, the squared standard deviation for this fitting, indicates the accuracy of the best fitting. As it is seen in Table 7 for each phase, the samples with 0.0 ≤ x ≤ 0.3 are well fitted with p = 1/2, meaning that the CG is the domain conduction mechanism for these samples. Many experimental and theoretical investigations indicated that the CG mechanism can govern the normal-state electrical resistivity of low substituted HTSCs phases [45]. The conduction mechanism of 3D-VRH is found to be dominant with x = 0.4 and x ≥ 0.45 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively.
Fig. 9 The variation of Tc with P for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases. The insert shows the variation of P versus Ru– content
Fig. 8 (Continued)
Table 6 The variation of Tc ΔT , ρn , ρ ∗ and dρ/dT with Ru-content for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases x
Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ
ΔT (K)
ρn
ρ∗
dρ/dT × 10−4
(m ·cm)
(m ·cm)
(m ·cm·K−1 )
0.0
103.50
7.50
0.309
0.046
9.13
0.025
104.75
6.25
0.372
0.048
11.14
0.05
106.00
15.50
0.419
0.075
11.89
0.075
99.75
20.25
0.450
0.123
11.43
0.15
66.63
7.13
0.633
0.271
12.52
0.3
50.50
30.50
1.721
–
12.13
–
2.907
–
−41.10
76.75
14.25
0.766
0.176
19.91
0.075
79.50
11.50
1.907
0.834
35.97
0.15
74.00
14.50
1.957
1.471
16.52
0.3
71.00
20.50
3.148
–
−2.98
0.45
65.25
36.00
18.366
–
−377.16
–
35.207
–
−397.35
0.4 Tl0.5 Pb0.5 Sr1.6 Pb0.4 CaCu2−x Rux O7−δ
Tc (K)
0.0
0.525
–
–
J Supercond Nov Magn (2012) 25:273–291
287
and (Tl, Pb)/Sr-1212 phases with x = 0.4 and ≥ 0.45, respectively which confirms their absence in our ρ–T curves. This could be due to its occurrence at a high temperature T > 300 K. This assumption was supported by the hightemperature measurements [47]. In the opposite limit, where Tmin is very low, only the metallic-like behavior can be observed for each phase with 0.0 ≤ x ≤ 0.15. Using the Mott parameterization for 3D-VRH regime [48], the T ∗ in (3) is related to both the density of state DOS at Fermi energy N (EF ), N (EF ) ≈ 1021 states/(eV·cm3 ) [49], and the localization length of the carriers d ∗ through the following expression: ∗
d = (a)
21 ∗ kB T3D N (EF )
1/3 .
(4)
On the other hand, the localization length for CG regime is given by [50] ∗ d ∗ = e2 /kB TCG ε
(b) Fig. 10 The linear fittings of ln(ρ/T 2p ) versus 1/T p with p = 1/2, p = 1/3 and p = 1/4 for Bi1.8 Pb0.4 Sr2 Ca2.1 Cu2.6 Ru0.4 O10+δ (a) and Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu1.475 Ru0.525 O7−δ (b)
(5)
where ε is the dielectric constant, ε = 10 [51]. The calculated localization length for each mechanism is listed in Table 7 for each phase. It is clear that, for each phase, d ∗ is higher than the distance of the neighboring atoms for unsubstituted sample [52]. So, the overlap of the carriers’ wave functions is enough for performing the conduction easily. Therefore the resistivity of un-substituted sample is lower than the substituted samples, confirming with the variation of ρn with Ru-content as displayed in Table 6. As the localization length decreases with increasing x, the hopping range R over which the carriers can hop decreases too according to the following relations: ∗ /T )1/2 ]/4 RCG = [d ∗ (TCG
(6)
and Similar results were reported for the crossover from CG to 3D-VRH with increasing the substitution content [45]. As the normal state is interrupted by the occurrence of superconductivity, the CG fitting is deviated from the experimental data at such temperatures when there is a drop in resistivity. In addition, VRH is the responsible mechanism for the normal-state behavior, whereas the superconductivity occurred at much lower temperatures [46]. With increasing Ru-content, the change in resistivity from the metallic-like to semiconductor-like behavior is separated by a shallow minimum ρmin at Tmin , Tmin = T ∗ /21/p . The values of Tmin are listed in Table 7 versus x for CG and 3D-VRH regimes. We noticed that for x = 0.3, Tmin (CG) values are nearly equal to those observed in ρ–T curves for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases as shown in Figs. 8(a) and (c), respectively. Tmin (3D-VRH) is higher than the temperature range measurements 18 K ≤ T ≤ 300 K for (Bi, Pb)-2223
R3D = [3d ∗ /(2πN(EF )kB T )]1/4
(7)
The calculated hopping range values for CG and 3D-VRH regimes versus Ru-content at different temperature ranges, 50 K ≤ T ≤ 300 K, are listed in Table 8 for each phase. T ∗ increases with increasing Ru-content, leading to decrease the conductivity. This means that the term exp(T ∗ /T )p in (3) is more effective than (T /T ∗ )2p term. While the decrease in both d ∗ and R indicates the localization of the carriers in the normal state which needs higher energy to hop to other sites. These points emphasize that the semiconducting state overcomes the normal state for each phase after a further increase in Ru-content. When the localization length is very large, the extended carriers can do the conduction process easily. Therefore, for low Ru-content, the hopping range is less than the localization length. But for VRH mechanism, R should be larger than d ∗ . For x = 0.4
Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ
Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ
99.45 216.44 586.37
0.075 (CG) 0.15 (CG) 0.3 (CG)
1.510 8.265
681.00 784.67 1257.84
0.3 (CG) 0.45 (3D-VRH) 0.525 (3D-VRH)
26.134
0.914
0.603
224.19 490.71
0.15 (CG)
0.111
1.629
0.843
0.199
0.086
0.045
0.033
0.020
0.075 (CG)
70.10
0.0 (CG)
1091.44
47.07
0.05 (CG)
0.4 (3D-VRH)
26.88 35.32
0.025 (CG)
(m ·cm)
0.9919
0.9914
0.9996
0.9993
0.9992
0.9953
0.9963
0.9992
0.9978
0.9935
0.9929
0.9956
0.9955
314.46
196.17
170.25
122.68
56.05
17.52
272.86
146.59
54.11
24.86
11.77
8.83
6.72
Tmin (K)
13.25
21.23
24.47
33.95
74.32
237.69
15.27
28.41
76.98
167.54
353.98
471.71
619.83
d ∗ (Å)
4862.89
3213.62
1623.89
542.48
173.36
51.60
5038.03
950.79
153.51
45.38
29.68
24.20
15.98
23.173
7.245
2.004
1.111
0.733
0.286
1.430
0.847
0.258
0.147
0.116
0.105
0.064
(m ·cm)
ρ0
T ∗ (K)
χ2
2D-VRH (P = 1/3)
ρ0
CG (P = 1/2) T ∗ (K)
0.0 (CG)
x (Best fit)
Table 7 The fitting parameters obtained from CG and VRH conduction mechanisms for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases
1255.44
173.46
59.99
33.18
10.92
6.24
(m ·cm)
T ∗ (K)
1842.87
579.97
113.89
33.06
0.9938 22592.37
0.9940 11202.73
0.9989
0.9988
0.9843
0.9891
21.563
5.952
1.712
1.245
0.928
0.447
1.182
0.831
0.513
0.364
0.205
0.170
0.125
ρ0
3D-VRH (P = 1/4)
0.9981 28508.31
0.9918
0.9899
0.9783
0.9814
0.9808
0.9758
χ2
0.9972
0.9946
0.9966
0.9954
0.9667
0.9636
0.9993
0.9767
0.9790
0.9676
0.9895
0.9730
0.9657
χ2
2824.05
1400.34
230.36
72.50
14.24
4.13
3563.54
156.93
21.68
7.50
4.15
1.37
0.78
Tmin (K)
22.09
27.91
50.93
74.88
128.82
194.56
20.44
57.88
111.97
159.52
194.32
281.44
339.26
d ∗ (Å)
288 J Supercond Nov Magn (2012) 25:273–291
Tl0.5 Pb0.5 Sr1.6 Ba0.4 CaCu2−x Rux O7−δ
Bi1.8 Pb0.4 Sr2 Ca2.1 Cu3−x Rux O10+δ
10.86 9.22 8.58 6.78
0.3 0.45 0.525
0.4
0.15
7.28
0.3
16.06
9.93
0.15
0.075
16.35
0.075
28.72
24.12
0.05
0.0
40.46 35.05
0.025
25.27
26.79
31.14
34.28
39.27
43.53
24.78
32.15
37.91
41.42
43.52
47.74
50.02
7.43
9.40
10.09
11.89
17.59
31.47
7.97
10.88
17.91
26.42
38.40
44.33
50.81
RCG
46.38
T = 250 K
RCG
R3D
T = 300 K
0.0
x
26.45
28.04
32.59
35.88
41.10
45.56
25.94
33.65
39.68
43.35
45.54
49.96
52.35
R3D
8.30
10.51
11.29
13.30
19.67
35.18
8.92
12.16
20.02
29.53
42.93
49.56
56.81
RCG
T = 200 K
27.96
29.65
34.46
37.94
43.45
48.17
27.43
35.58
41.96
45.84
48.16
52.83
55.36
R3D
9.59
12.14
13.03
15.35
22.71
40.62
10.29
14.04
23.12
34.10
49.57
57.22
65.60
RCG
T = 150 K
30.05
31.86
37.03
40.77
46.69
51.76
29.47
38.23
45.09
49.26
51.75
56.77
59.48
R3D
Table 8 The variation of R with Ru-content for both CG and 3D-VRH regimes at 50 K ≤ T ≤ 300 K for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases
11.74
14.87
15.96
18.80
27.82
49.75
12.61
17.20
28.31
41.77
60.71
70.09
80.34
RCG
T = 100 K
33.25
35.26
40.98
45.12
51.68
57.29
32.62
42.31
49.90
54.51
57.27
62.83
65.83
R3D
16.61
21.03
22.57
26.59
39.34
70.36
17.83
24.33
40.04
59.07
85.86
99.12
113.62
RCG
T = 50 K
39.55
41.93
48.73
53.66
61.45
68.13
38.79
50.31
59.34
64.83
68.11
74.71
78.29
R3D
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290
and ≥0.45 and at T ≤ 300 K, the 3D-VRH occurs probably in the CuO2 -planes as indicated in Table 8 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. Also, the decrease in R with increasing T is probably due to thermal fluctuations. This result is in consistent with the change in the hopping energy versus temperature in perovskite structure [53].
4 Conclusion Series of (Bi, Pb)-2223 phase with 0.0 ≤ x ≤ 0.4 and (Tl, Pb)/Sr-1212 phase with 0.0 ≤ x ≤ 0.525 were successfully prepared via conventional solid-state reaction technique. The elements-content of prepared samples were estimated from both EDX and PIXE techniques. The results indicated that PIXE technique is more sensitive for detecting the low elements-content than EDX technique. On the other hand, RBS is more accurate technique for determination the oxygen-content than PIXE technique. XRD and SEM analysis showed improvement of the phase formation up to x = 0.05 and 0.075 for (Bi, Pb)-2223 and (Tl, Pb)/Sr1212 phases, respectively. This improvement was accompanied by the increase in both the number of hole carrier concentration and the superconducting transition temperature for each phase. The superconducting transition temperature decreased at x > 0.05 and 0.075 and the superconductivity was completely destroyed at x = 0.4 and 0.525 for (Bi, Pb)-2223 and (Tl, Pb)/Sr-1212 phases, respectively. In addition, the variation of the lattice parameters and Tc with Ru-content, for each phase, was in consistent with the calculated O-content. The analysis of the normal-state electrical resistivity, for each phase, indicated that CG was dominant for x ≤ 0.3, whereas 3D-VRH was dominant for x > 0.3. Acknowledgements This work was performed in the superconductivity and metallic-glass lab, Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt. The authors are grateful for the support of Accelerator Laboratory, Lebanese Atomic Energy Commission, CNRS, Beirut, Lebanon for PIXE and RBS measurements.
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