Investigation of Niobium ( Nb ) Substitution on Structural and Superconducting Properties of ( Bi , Pb )-Based Superconductors

In this study, the effects of Nb substitution on the Bi-based superconducting materials have been investigated. The X-ray diffraction measurement indicated coexistence of Bi-2212, Bi-2223 phases and some impurity phases of CuO, CuNb2O6, CaNb2O6, CaCuO2, and Sr5Nb5O16. With increasing Nb content, impurity phases consistent with the Nb element appeared in the samples. Also with increasing Nb content, the Bi-2223 phase of samples gradually was decreased and in contrast, the Bi-2212 phase was increased. From the SEM, results have been seen that with increasing of Nb contents, the crystal structure of the samples was slightly changed because of the disrupted grain growth. From the electrical resistivity measurements, it has been found that with increasing of Nb contents, critical temperature decreases and the superconducting transition width (∆T) increases. Estimated critical current density showed that Jc decreases with increasing Nb content, as expected.


Introduction
In high-temperature superconductors, there is a group which is called the Bi-based superconductors.Following the discovery of Bi-based superconductors [1], researchers work on this type of high-temperature superconductors.A lot of works have been done in the field of high-temperature superconductors [2]- [10] in order to improve their critical current density (J c ), critical temperature (T c ), and better understand the structural properties of Bi-based superconductors.
The Bi-based superconductors are defined by the Bi 2 Sr 2 Ca n−1 Cu n O 2n+4+y general formula and with abbreviations of the BSCCO.In this formula, n is determined with values 1, 2 and 3 which indicates the number of CuO 2 layers in the crystal structure of Bi-based superconductors.In general, Bi-based superconductors have three phases Bi-2201, Bi-2212 and Bi-2223 with around 20, 85, and 110 K critical temperatures, respectively [11] [12] [13].Among these phases, the Bi-2223 phase with the chemical formula of Bi 2 Sr 2 Ca 2 Cu 3 O 10+y appears to be suitable for superconducting devices which are operating at a temperature of liquid nitrogen (T > 77 K) [14].But the formation of pure Bi-2223 phase is a critical issue in fabricating BSCCO superconductors.
Before, the researchers have studied the effect of the high-valency cations, like Ta 5+ , V 5+ , Nb 5+ and Nd 3+ for obtaining of Bi-2223 single phase and found that doping any one of them can significantly enhance the formation of Bi-2223 phase.These cation roles are quite similar to Pb for stabilizing the Bi-2223 phase [15] [16] [17].Double substitution of Pb with other elements is being attempted for further improvement in Bi-based superconductors so that enhances the transition temperature or the critical current density (J c ) [18].Ekicibil et al. studied the effect of Gd concentration on the properties of Bi 1.7 Pb 0.35−x Gd x Sr 2 Ca 3 Cu 4 O 12+y superconductors.They found that with increasing of Gd content, the Bi-2223 phase decreases and the insulating phase increases.In the other words, even with very small amount of Gd 3+ substation, Pb 2+  Gd 3+ , superconducting properties degrades by decreasing the hole concentration [19].
In this research, our aims are: 1) To answer this question that substitution of Pb with Nb in BPSCCO systems increases or decreases the critical temperature.
2) To explore the magnetic behavior of the modified BPSCCO systems.In other words, this substitution improves or depresses the superconductivity of BPSCCO systems.

Experimental
Bi 1.65 Pb 0.35−x Nb x Sr 2 Ca 2 Cu 3 O 10+δ samples, with x = 0.0, 0.05, 015, 0.25 and 0.35 substitutions were prepared from ultra-fine and high grade purity powders Bi2O3 (99.99%),PbO (99.99%),SrCO 3 (99.99%),CaCO3 (99.9%),Nb 2 O 5 (99.99%) and CuO (99.99%) using the conventional solid-state reaction method [20].The starting powders in stoichiometric proportions were weighted and well mixed (the precursors have milled approximately for 1 h) and pressed to pellets by using hydraulic press type.The pellets were first calcined at 810˚C for 48 h in order to start the formation of the superconducting phases.Then the pellets were ground and mixed in an agate mortar pestle, the resulting powder from the calcination process pressed into pellets, and sintered at 825˚C in the air for 48 h.For the preparation of pellets from powders, dry pressing at 250 MPa in calcination processes and pressing at 450 MPa in the final process were employed.Our samples were disc-shaped pellets with 13 mm diameter and 2 mm thickness.Finally, the pellets (samples) were sintered at 845˚C for 100 h in air and slowly furnace cooled to room temperature.We labeled our samples A (x = 0.00), B (x = 0.05), C (x = 0.15), D (x = 0.25) and E (x = 0.35).The heat treatment schedule for (Bi, Pb)-2223 pellets in programmable temperature controlled furnace is shown in Figure 1.
For measurement of the DC resistivity of samples as a function of temperature, the standard four-point probe method with silver point contacts was used.
The transition temperature T C was determined as the temperature at zero resistivity.The structure of the samples was checked by SIMENS X-ray diffractometer with CuKα (1.54 Å) radiation in the range of 2θ = 2˚ -60˚.Lattice parameters were determined from the XRD patterns by using Match 3.3 software based on Cohen's least square method.Scanning electron microscope (SEM) photographs for the study of the microstructure were taken by using MIRA3 TESCAN.Magnetic measurements of samples were done by a Quantum Design PPMS model 6000.

Results and Discussion
where I(Bi-2223) and I(Bi-2212)) are the intensity of present phases in the samples which were indicated in Figure 2. The percentage volume fraction of the present BSCCO phases of the samples listed in Table 2.As seen in this table,   Many properties of the polycrystalline materials depend on the grains size.So, by using of the Debye-Scherer formula in Match 3.3 software and the XRD data, grains size were calculated [26].

cos
where β is the full width at half maximum of X-ray peaks (radians), λ is the wavelength of the incident radiation, and θ is the angle of the peak.With using Debye-Scherer formula, the size of the grains calculated lies between 250 Å and 360 Å.With calculation of crystal lattice parameters by using Match 3.3 software, were found that all of the samples have an orthorhombic structure which has been reported also by other groups [27] [28].As seen in Table 3, with increasing Nb substitution, c-parameter decreases simultaneously, but in a and b-parameters there are not regular changes.These changes in the parameter of c correlate with decreasing holes concentration in the CuO 2 plane, as suggested by  T with Nb content (x), is given in Figure 5.All samples exhibit metallic behavior, this means that resistivity decreases with decreasing temperature.Table 3  is further decreased in the samples D and E. This result may be due to a decrease of the Bi-2223 phase and increasing of the Bi-2212 phase and especially increasing impurity phases in these samples as extracted from XRD result analysis.
A sharp drop of resistivity was seen for samples A (x = 0.00) and B (x = 0.05), this means that these samples consist of predominantly of Bi-2223 phase.As was indicated by Bolat et al., once the volume fraction of Bi-2223 within the sample is sufficient to make this possible that a one-step resistivity transition is observed even in the samples which contain a rather large amount of Bi-2223 phase [31].Samples with x = 0.15, 0.25 and 0.35 showed long tail (a high transition width), the reason may existence of the small amount of the Bi-2212 phase and impurity phases or non-superconducting regions or multi superconducting phases in the samples.
Sample with x = 0.25 showed the two-step resistivity transition.It is possible that the both Bi-2223 and Bi-2212 phases exist within one grain in such a way that low-T c (Bi-2212) phase can play important role in the weak link [27].The zero field cooled (ZFC) magnetization versus temperature curve of samples were measured under external applied magnetic field of 50 Oe.These results are illustrated in Figure 6.The diamagnetic behavior of the samples below their onset temperatures is seen from M-T curves.Additionally, from M-T curves it can be concluded that samples A (x = 0.00) and E (x = 0.35) show the maximum and minimum diamagnetic property among the samples and a reducing tendency on the superconducting properties with the increasing of Nb content.
For determining the intergranular critical current density, the magnetic-hysteresis cycles were measured at 10 K for all samples between applied fields of ±9000 Oe.These results are shown in Figure 7.The obtained results for M-H curves like the M-T curves prove that the sample A with no substitution of Nb is a much better superconductor than the others.By using Bean's critical state model, critical current density (J c ) of samples were calculated from the hysteresis loops [32].In Bean's critical state model, critical current density proportional to the width of hysteresis loop M M M + − ∆ = − and calculates with this formula: In this formula, J c is the critical current density in amperes per square centimeter of a sample and M + and M − magnetizations are obtained from the in-   In ceramic high-temperature superconductors, one of the most important properties is their grain structure.These grain structures can be illustrated and explained by the SEM micrographs.These SEM micrographs provide us with data about the formation of the surface morphology of the samples.Surface morphology micrographs taken by SEM for all samples are shown in Figure 9.
Other researchers indicated that morphology of BSCCO systems composed of randomly plate-like grains with unfilled spaces between them [33]

Conclusion
In summary, the nominal composition of the Bi For this reason, with increasing of Nb content impurity phases of samples, Pb substitution with Nb in BPSCCO system decreases superconducting properties.

Figure 2 Figure 1 .
Figure 2 shows the XRD patterns of samples A to E, which Nb was substituted in the form of Nb 2 0 5 .Obtained results from XRD patterns indicated coexistence of the Bi-2212 and Bi-2223 phases and some impurity phases.Impurity phases such as CuO, CaCuO 2 , CuNb 2 O 6 , CaNb 2 O 6 , and Sr 5 Nb 5 O 15 were detected in the

Figure 2 .
Figure 2. The XRD patterns of the samples A to E.
Figure 3. Small impurity peaks of samples were identified by using Match 3.3 Software.For example, Figure 4(a) and Figure 4(b) show the match Ritveld refinement

Figure 3 .
Figure 3. Volume fraction of Bi-2223 and Bi-2212 phases' as a function of Nb substitution.

Figure 4 .
Figure 4. Match ritveld refinement of the C and E samples.
summarize the value of critical temperature offset .Samples A and E have the maximum and minimum critical temperature offset c T (R = 0) with the amount of 95.1 K and 50.3K respectively.The offset c T

Figure 5 .
Figure 5. Electrical resistivity versus temperature curves for all samples between 40 and 120.

Figure 6 .
Figure 6.Magnetization versus temperature curves for all samples with an applied field of 50 Oe.

Figure 7 .
Figure 7. M-H hysteresis curves for all samples at temperature of 10 K.

Figure 9 .
Figure 9. SEM micrograph for samples A and E.
1.65 Pb 0.35−x Nb x Sr 2 Ca 2 Cu 3 O 10+δ (x = 0.0, 0.05, 0.15, 0.25 and 0.35) compounds has been prepared by the solid-state reaction method and the effects of Pb 2+ substitution by Nb 5+ in (Bi-Pb)-2223 superconducting samples have been investigated.The obtained results from XRD, resistance measurements, DC magnetization and hysteresis measurements showed that with increasing of Nb content volume fraction of the high-T c (Bi-2223) phase, the critical temperature ( ) offset c T and critical current density (J c ) of the samples decreased.In the other words, Pb 2+ substitution by Nb 5+ degrades the superconducting properties of the samples.In the sintering temperature range of 840˚C -860˚C, two phases of Bi-2212 and Bi-2223 are formed.For the high-T c (Bi-2223) phase formation and large growth of grain size, sintering temperature must be close to the partial melting point of composing elements of BSCCO system.The melting point of Nb 2 O 5 (1512˚C) is much higher than the sintering temperature of BSCCO system and a melting point of PbO (888˚C).

Table 1 .
Comparison of five highest peaks and impurity phase of Bi 1.65 Pb 0.35−x Nb x Sr 2 -Ca 2 Cu 3 O 10+δ .Advances in Materials Physics and Chemistry

Table 2 .
Percentage volume fraction of Bi-2223 and Bi-2212 phases in the samples.
Satyavathi et al. and Zandbergen et al.[29] [30].If the valance states of the Bi and Nb are supposed to be unchanged in the material (Bi 3+ and Nb 5+ ), since the ionic radius of Nb 5+ (0.7 Å) is different from Bi 3+ (0.96 Å), probably this is a reason for changing of the unit cell parameters.Normalized resistivity versus temperature for all samples and the variation of 079 Advances in Materials Physics and Chemistry [35].SEM micrographs indicated that the grain size and the distribution of grains on the surfaces of the samples are quite different.The structure of sample A with no Nb 5+ has large plate-like grains, which is characteristic of the grain structure of Bi-2223 phase.In contrast, SEM micrograph of sample E with no Pb 2+ contains the mixture of different sized flaky and plate-like grains which are connected with each other while there are some unfilled spaces among them as expected for Bi-2212 phase.Consistent with XRD results, substitution of Nb 5+ ions with Pb 2+ , the Bi-2223 phase has gradually transformed into the Bi-2212.Superconducting transition temperature has a parabolic relationship with the hole concentration p (the number of holes per Cu atom) can be calculated by using this relation which is given by Presland et al.[35].