Several studies have elaborated the Bi-based superconductor ceramics by simplest method solid state reaction. The preparation of Bi-based high-T_c superconductors is difficult compared to YBCO. Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure. For Bi–Sr–Ca–Cu–O, it is relatively simple to prepare the Bi₂Sr₂CaCu₂O_y (Bi-2212) (T_c ≈ 85 K) phase, whereas it is very difficult to prepare a single phase of Bi-2223 (T_c ≈ 110 K). The Bi₂Sr₂CaCu₂O_y (Bi2212) superconductor is known to be one of the most stable compounds among the copper oxide-based high –T_c superconductors. However, two crucial factors may limit the transport properties of Bi-2212 superconductors. One is the inter-grain weak links due to the low extent of texture resulting in^[1], more or less extended, inter-granular space filled, or no, by secondary phases. The other is the weak flux pinning, which may results from intrinsic defects in the lattice structure^[2,3]. This issue can, however, be resolved up to some extent by filling the pores with some suitable nanostructures at inter-granular sites in the bulk superconductor. Several works were directed towards to improve its superconducting properties by employed various methods^[4-6], including introducing artificial flux pinning. Enhancement or destruction of the superconducting properties depends on the characteristics of the dopant in the crystal structure. In order to explore the effects of the doping on the physical and electrical properties of the BPSCCO system extensive studies have been done by using different doping elements.

Numerous studies show that addition of elements as BaSO₄, SrSO₄, Nd₂O₃, Y₂O₃, ZrO₂, Al₂O₃, SiC, PbO, PbO₂, Pr₆O₁₁, SnO₂, Ag₂O, NiF₂, LiCl₄ and Pr₂O₃ to Bi2212, Bi2223 and MgO etc., is a suitable method for the optimization of Bi based HTS materials^[7-18]. Slight changes in the lattice structure, microstructure, thermodynamic properties, electric and/or magnetic properties producethis effect.

On the other hand, there are a lot of parameters affecting the formation of these phases such as synthetic method sintering temperature, valences, oxygen content as well asphysical properties of the dopants and whole numbers in CuO₂ layers. A metal-insulator transition corresponding to an opening of a pseudo-gap occurs. The compound goes continuously from an over doped to an under doped state, passing through an optimal doping corresponding to a maximum value of T_c^[19-23]. α and proton irradiation have also an effect on T_c in Bi-2212, increasing it by, probably, kicking off oxygen in over doped system^[24-26]. In Bi-2223, addition of TiO₂ has effects on grain connectivity and alignment, Jc value and cation stoichiometry^[27]. Increasing percentage of TiO₂ may result in aggregation of TiO₂ as impurity phases and not become part of the structure and decreased critical temperature^[28]. Addition of up to 5% of weight percentage of TiO₂ decreases of superconducting properties of the irradiated samples^[29].

In present work, we have investigated the effects of anti-ferromagnetic Ti addition on structural, compositional, morphological and superconducting transport properties of Bi-2212 matrix by different experimental techniques.

Experiment

Precursor samples of Bi-2212 phase were prepared using a conventional solid-state reaction method. For that, starting powders of high purity grade (99.9 %) of Bi₂O₃, SrCO₃, CaCO₃ and CuO weighed in stoichiometric proportion corresponding to Bi₂Sr₂CaCu₂O_8+δ, were mixed in an agate mortar, then fired in air at 810°C for 30 hours. Successively, the prepared powders were ground and formed in pellets of 12 mm diameter under a force of 50 KN. Then, the obtained pellets undergo two heat treatments at 840°C and 860°C successively, reached at 5°C/min, for 120 h in two steps (60 hours for each one) separated by a grinding and forming in pellet shape of the samples. After that, they obtained pellets are ground and mixed to TiO₂. The blend of both powders is ground for 1 h. The additional amount of Mg in these cases represents, in mass, 0%, 1%, 2%, 3%, 4% and 5% of the mass of the precursor pellet. The obtained powders (noted Bi₂Sr₂CaCu₂Ti_xO_8+δ where x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) are successively formed in pellet shape under the same pressure and then sintered for 60 h at 860°C. The reference precursor (x = 0) was also ground and sintered as others to ensure identical physical conditions of preparation for all the samples.

Use of X-ray powder diffraction (XRD), at room temperature and using CuK_α radiation, allows characterizing the crystalline structure of the samples. Varying the 2θ angle from 10° to 63° with a step of 0.021°, a Siemens D8-Advance diffractometer records the diffraction patterns. Identification of the existing phases was obtained with help of JCPD data file^[30]. Lattice parameters of the samples were refined using JANA06 software^[31]. Microphotographs were taken on a JEOL7100F Scanning Electron Microscope (SEM). Energy dispersive X-ray spectrometer (EDS) equipping the SEM gives the elements present in the samples. The temperature dependence of the electrical resistivity was recorded using a closed cycle helium refrigerator Cryodine CTI-Cryogenics and a standard four probes technique with a measuring current of 5 mA. AC susceptibility was measured on Lake Shore 7130-model AC susceptometer using a mutual inductance coil system with a closed cycle refrigerator.

Results and Discussion

The XRD patterns of the samples are displayed in Figure 1 indicating the dominant phase of Bi2212. The indexed peaks corresponds to the 2212 phase. The introduction of Ti with x = 0.05 induced a significant increase of the intensity of the main peaks of theBi(Ti)2212 phase due to enhanced grain growth and better orientation of grains with Ti diffusion. It should be noted that sample with x = 0.02 showed the broadening of some peaks due to the spacing between the peaks or overlapping of 2223-phase with 2212-phase as corroborate well with XRD results patterns. The appearance of more than two phases could be related to the stacking faults along the c-axis^[32].

In all the spectra, the intensities of the main peaks (008), (0010) and (0012) are much higher than the other ones. The increase in the intensities of the peaks of doped samples may testify the enhanced grain growth and orientation of grains.This is an indication that the grains of the samples have a preferential orientation along the (00l) direction. There is a consistent variation in the intensity and the position of the peaks which indicate the change in the phase composition and the lattice parameters of the sample^[33]. The samples studied do not exhibit any different phases including Ti atoms, indicating its complete solubility in the Bi-2212 superconducting crystal structure.

Figure 1: (a) X ray diffraction patterns of Ti doped Bi-2212. (b) Lattice parameters as a function of Ti doping content.

The experimental intensities are fitted using the pseudo-Vought function. A 36 terms polynomial of Legendre was used to describe the background and the Simpson method was applied for the asymmetry correction. The results of the refinement of cell parameters and the corresponding agreement factors (R_p, R_wp) and the goodness of fit (GOF) are presented in Table 1.

Table 1: Cell parameters (a, b, c), cell volume V, agreement factors (R_p, R_wp) and goodness of fit (GOF) of Bi₂Sr₂CaCu₂Ti_xO_8+δ

The variations of the lattice parameters versus the doping element are reported in Table 1. The system have an orthorhombic structure. The c-axis value decreases from 30.813Å to 30.779Å. The behaviour of the lattice parameters can be explained by the increase of the oxygen content in the unit cell by the replacement of Cu²⁺ by Ti⁴⁺ in the structure, it is quite possible that Ti goes to the Cu site. Also, Ti doping imparts additional charge carriers which incorporates excess oxygen into Bi-O layer leading to an increase in oxygen content in the (Bi, Pb)-2212 structure. This induces a contraction in Bi-O layers.This can be attributed to the contraction of c-axis length.

On the other side, the ionic radius of Ti⁺⁴ (0.68Å) is lower than that of Cu⁺² (0.72Å), Ti⁺⁴ ions with outer electronic shell 2s²2p⁶ configuration are without magnetic spin moment (spineless). The c-axis length is expected to decrease which is observed in our XRD results. It is well known that the lattice parameter a is controlled by the length of in-plane Cu-O bond^[34]. The table 1 shows that introducing TiO₂ increases parameter a and decreases parameter b. Length of both a-axis and b-axis are controlled by the length Cu-O bond. This bond depends on the local Jahn Teller distortion of the oxygen octahedron around Cu²⁺. The contraction of c-axis may change that distortion and consequently changes a and b axis.The volume of unit cell in these samples also contracted to 899.882 to 895.777Å. Obviously, the attractive electrostatic interaction between the positively charged Ti⁺⁴ and the negatively charged oxygen O⁻² ions results in a compression of the Cu-O and Sr-O atomic layers along the c direction, and consequently in an decrease of the lattice parameter b. The redistribution of the hole concentration between the Bi-O layers and the Cu-O₂ planes would lead to changes in lattice parameters and the hole concentration in the Cu-O₂ planes.

Cell parameters (a, b, c), cell volume V, agreement factors (R_p, R_wp) and goodness of fit (GOF) of Bi₂Sr₂CaCu₂Ti_xO_8+δ

The morphology was examined by SEM micrographs of pure and doped samples as shown in Figure 2. They were taken at the same magnification (5000). The shape of the grains is affected by Ti doping suggesting that the doping element may change the melting point of Bi-2212 phase. The samples exhibit a common feature of plate-like layered grains (2212 phase) randomly distributed^[35]. Due to the presence of the pores in between the grains, the samples are less compact.Pores were observed generally in all the samples.Except for the samplesundoped and dopedwith x=0.02 shows more dense, have more uniform surface appearance and granular structure due to the agglomeration and enlargement of grains which causes to the decreasing of voids.Indicating rich formation of thesuperconductingphase. Small white nodules, due to some precipitate of the starting powders, can be noticed for pure sample. Each sample was polycrystalline and grain orientation was anisotropic. Increasing Ti content degrade grain morphology and connectivity. Grain size is estimated to less than 7 μm.

Also, in figure 3 shows EDS analysis patterns of the samples. The results demonstrate that there is no undesirable element in all samples. The element contents reported in Table 2 show that, in all samples, Cu is lower, while Sr and Ca are higher than the ones of the stoichiometric formula of Bi-2212. The content of oxygen is also higher but, due its low atomic mass, cannot be considered in this kind of analysis. Ti is detected in all doped samples. Ti content is not monotonic with x due to the limited surface of detection and the low number of grains analyzed. The values of oxygen content are gradually increased which leads to an increase in the carrier concentration. When tetravalent Ti ion substitutes for divalent Cu one, the excess of positive charge is compensated by incorporation of extra oxygen atoms in the BiO planes.

Table 2: Quantitative EDS results of Bi₂Sr₂CaCu₂Ti_xO_8+δ for 0 ≤ X ≤ 0.05

Upon increasing Ti content, it is found that the Cu content decrease which confirms that the doping element is effectively incorporated into crystalline structure of the samples.

Figure 2: SEM micrographs f Bi₂Sr₂CaCu₂Ti_xO_8+δ compounds, x= 0.0, x= 0.01, x= 0.02, x= 0.03, x= 0.04, x= 0.05.

Figure 3: EDS spectrum of Bi₂Sr₂CaCu₂Ti_xO_8+δ compounds, x= 0.0, x= 0.01, x= 0.02, x= 0.03, x= 0.04, x= 0.05.

Figure 4: Mapping of the elements of undoped sample.

Figure 5: Mapping of the elements of sample with x = 0.01.

Figure 6: Mapping of the elements of sample with x = 0.02.

Figure 7: Mapping of the elements of sample with x = 0.03.

Figure 8: Mapping of the elements of sample with x = 0.04.

Figure 9: Mapping of the elements of sample with x = 0.05.

Figure 10: Temperature dependence of the electrical resistivity of the samples

EDS analysis allows also a mapping of the elements present in the samples. Figure 3 shows this kind of mapping obtained for samples undoped and doped. Figures show the mapping of Ti in the doped samples. The black areas in the mapping indicate the maximum content of the element. Figures confirm the incorporation of titanium in the grains and its distribution is practically non uniform on the whole explored area. Normalized resistivity versus temperature for all samples with Ti content (x) is given in Figure 10. All samples exhibit metallic behaviour at high temperatures, followed by a superconducting transition as the temperature is lowered. The resistivity in the normal state decreases for x=0.01 and 0.04 and increases for x=0.02, 0.03 and 0.05. Porosity and disorientation of grains may reduce drastically the resistivity^[33]. Table 3 summarize the values of the parameters characterizing the ρ(T) curves: onset (T_c,onset) and offset ( T_c,off) critical transition temperatures, transition width ΔT_c and residual resistivity ρ₀.

Table 3: Onset ( T_{c, onset}) and offset ( T_{c, off}) critical transition temperatures, transition width ΔT_c , residual resistivity ρ₀ and hole-carrier concentration (P ) of Bi₂Sr₂CaCu₂Ti_xO_8+δ samples.

The superconducting transition temperature T_c, which displays the superconducting transition within the grains, is determined as the temperature corresponding to the crest in dρ/dT versus T curve. T_c (onset) is the temperature at which grains become superconducting. The granular T_c is controlled by the lattice oxygen content. Hence, T_c (onset) is affected by x, the excess oxygen, whereas T_c (off) is controlled by the intergranular links too. Introduction of Ti increases the T_c,onset and induced destruction of weak intergranular links in polycrystalline samples that causes an increase in the transition width and fast decrease in Tc (off). The value of Tconset (K) was increased, which showed the increase in superconducting volume fraction and cooper pairs formation across transition from normal state to superconducting state.

The transition width δT_c is a picture of the thermally activated flux creep. The creep of the vortices depends on the extent of the structural defects in the grains of the sample. The creep is in first time collective then in second time individual. In other words, vortices move firstly by bundles then individually. The change of resistivity is proportional to the number of vortices moving and to their speed. When the vortices move by bundles, the speed is low and the resistivity has low variation. This situation correspond to the beginning of the transition near T_c,off. Thus, a grand part of the transition corresponds to individual moving of the vortices. Near, T_c,onset the movement of the vortices correspond to a flux flow regime. In doped samples with x = 0.03 and 0.05, there is a change in the slope of the transition, a kind of knee. This is probably an early beginning of flux flow regime. This result is confirmed of the low structural quality of the samples (x = 0.03, 0.05) shown by the corresponding SEM photo.

The enhancement of superconducting properties in BSCCO system is attributed to the optimization of hole concentration in the CuO₂ layers. The charge carrier concentration, p per Cu, in the CuO₂ planes is found by using the empirical relation^[36];

Tc⁄T_c^max =1-82.6(p-0.16)²;

Where, T^max_c is taken as 95 K for Bi-2212 system. It was observed that hole carrier concentration increases from 0.126 to 0.14 with increasing TiO₂ addition and increasing T^onset_c, as shown in Table 3. The sample with x = 0.05 has the highest value of hole carrier concentration which is 0.140.

Figure 11 shows the temperature dependence of the magnetic susceptibility χ(T) of the samples. Measurements were recorded from 20 to 120 K, with a zero-field cooling (ZFC) of the samples. According to collected ZFC data, all samples (where x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) became diamagnetic below their onset temperatures which were evaluated at 81.17, 83, 86.45, 83.37, 87.4 and 88 K, respectively. These results confirm those obtained from resistivity. The Tconesetis enhanced by the introduction of Ti and its variation versus x is the same as in resistivity. This variation, deduced from susceptibility is monotonic with x. The low temperature part of χ shows that the superconducting volume fraction does not vary as T_c. In susceptibility measurements the broadening of the transition is caused by the inhomogeneities and also a weak connection at grain the boundaries. It can be observed that the real part of the AC susceptibility,χ(T), of the pure sample shows two drops. The first drop at T_c is due to the superconducting transition within grains.The second drop at T_cJ is due to the occurrence of the superconducting coupling between grains. This second transition was not observed for the doped samples, probably the larger amounts of impurity phases, particularly SrTiO₃, result in a separation of the Bi(2212) crystals, therefore preventing the appearance of a large weak-link contribution to the diamagnetic response of the sample^[37].

Onset (T_c,onset) and offset (T_c,off) critical transition temperatures, transition width ΔT_c, residual resistivity ρ₀ and hole-carrier concentration (P) of Bi₂Sr₂CaCu₂Ti_xO_8+δ samples.

Figure 11: Temperature dependence of the susceptibilities  (real part) and of the samples.

Conclusion

In summary, polycrystalline Bi₂Sr₂CaCu₂Ti_xO_8+δ superconducting samples (where x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05)prepared by the conventional solid-state reaction method were investigated. The results indicate that the doping of TiO₂ into BSCCO has modified the microstructure of Bi 2212 where the grain size of the samples is reduced, grain connectivity, hole carrier concentration, superconducting behavior and diamagnetism.

It is possible to say that the raise of the onset temperature may arise from the increasing of hall carrier concentration and confirm that is at optimum value in the doped samples. TiO₂ added samples showed a significant enhancement of T_c compared with the non-added samples.

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