I-V characteristic behavior of BSCCO-2223 superconductor under low intensity DC magnetic fields

Resumo Electrical characterization of superconductor materials exposed to external magnetic field play an important role for many technological applications. In this paper, the electrical characterization of Bi-2223 pellet prepared by conventional route was performed. The electrical resistance temperature dependence (RxT), showed a superconductor transition at around 105 K. The current-voltage (I-V) behavior under magnetic field and temperature has been investigated, the results point to a power-law dependence between the electrical current (I) and applied voltage (V), at different conditions, as described by the literature. The external DC magnetic field, was produced by a simple home-made apparatus, where a simple copper coil was used to produce an external DC magnetic field between 2,0 mT and 8,0 mT. Then, the dependence of the critical current (Ic) on magnetic field and temperature has been studied, revealing a double step behavior.

The Bi2Sr2Ca2Cu3Ox (Bi-2223) can be cited as the most interesting system, exhibiting critical temperature at around 110 K [8], many studies are devoted to improve the understanding of the dominant mechanism that limit the Ic in applications under external magnetic fields.
The oxide superconductors are commonly called Type 2. The critical temperatures (Tc) associated with them are much higher than those of superconductors based on metallic alloys [1,9].The oxide superconductors were discovered in 1986 by Georg Bednorz and Alex Mueller.They realized that the league had distinct characteristics of conventional superconductors, Type 1.The transition to the superconducting state was gradual, with the presence of an intermediate state.Moreover, the Meissner effect was not perfect: the material allowed the penetration of a magnetic field, hence contrary to the type-1 superconductors.
The Type 1 presents the complete Meissner effect for values of applied magnetic field (Ha) below of Hc (critical magnetic field).For Ha over Hc the sample behaves like a normal material allowing the penetration of the field.While, type 2 presents the complete effect until the lower critical field Hc1.From Hc1 the penetration of the field into the sample begins and the fluxoids appear [2].
The density of the fluxoids increases when approaching the upper critical field Hc2, at which, the superconductivity disappears completely from the material.Between Hc1 and Hc2, the superconductor is in a "mixed state", where the nucleus of the fluxoids is in the normal state, and the rest of the material remains in a superconducting state.In this field range, it is possible to say that an incomplete Meissner effect is observed [9,10].
In this paper, a study is performed based on the effect discovered by physicists Walter Meissner and Robert Ochsenfeld, better known as the Meissner effect [9].The superconductors are not only perfect conductors, but also exhibit perfect diamagnetism.

2.1-Sample preparation.
Superconducting ceramics of the Bi1.6Pb0.4Sr2.0Ca2.0Cu3.0Oxsystem were prepared, by chemical process [11].The precursor polymeric resin was dried in an electric furnace at 100 o C for 24 hours, the resulting powder was submitted to a several thermal treatments at 600 o C/24hs, intercalated by milling steps in an agate mortar.
Finally, 6.0 g of the obtained powder was used to produce pellets with 1,0 mm thick and 20 mm of diameter, by uniaxial pressing applying 9 Tons at room temperature.Finally, the pellets were sintered at 840 o C for 100 hours, in air atmosphere, by using a heating hate of 120 o C/h and a cooling rate of 300 o C/h.

2.2-Sample Characterization.
The temperature dependence of electrical resistance (RxT) and current density (IxV) measurements were performed by using the DC four probes method.First, the electrodes were painted with conductor silver ink over the BSCCO pellet surface, then the RxT measurements were carried out from room to liquid nitrogen temperature (300K -77K), by using an automatic home-made system which employs a temperature sensor DT470 SD from Lake Shore Cryotronics and a Dewar with liquid nitrogen to temperature control.
The electrical measurements were carried out by using a programmable voltage/current source model 228A, a nanovoltimeter model 2182, and a high performance multimeter DMM model 2000, all of them from Keithley Instruments.The external magnetic fields were applied perpendicular pellet surface direction, Figure 1, by using a small homemade copper coil, which was previously characterized by using a gauss meter model MG 2000-20, the final dimensions of the coil were of around 8,75 mm in thickness and 2,24 in diameter.

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Figure 1: Plot of the magnetic field generated by the coil according to the applied electric current.

3.-Results and Discussion
The temperature dependence of DC electrical resistance (RxT) for the BSCCO ceramic pellet is shown at

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The electrical characterization performed at 80K, Figure 3, revealed a loss of superconductivity occurs approximately 50 mA at zero magnetic field, and it is successively interrupt around 40 mA, 30 mA, 20 mA and 15 mA when external magnetic fields of 2,0 mT, 4,0 mT, 6,0 mT and 8,0 mT are applied, respectively.The critical current (Ic), showed to decreases considerably with increasing magnetic field, this result is in accordance with the behavior described theoretically, which describe a polynomial type dependency between the applied voltage and electrical current [9].A similar behavior were observed at 85K, Figure 4, the applied current of loss superconductivity was around of 34 mA at zero external magnetic field.The critical current (Ic) decreases considerably with increasing magnetic field.The observed Ic was around 20 mA, 17 mA, 3 mA and 1 mA when a external magnetic field of 2,0 mT, 4,0 mT, 6,0 mT and 8,0 mT was applied respectively.Complementary measurements taken at 90K, shown in Figure 5, show an Ic of loss superconductivity around 17mA at zero external magnetic field.
The critical current (Ic) also decreases considerably with increasing magnetic field.
The estimated Ic value at 2,0 mT, 4,0 mT, 6,0mT and 8,0 mT was around of 15 mA, 11 mA, 2 mA and 1 mA, respectively.In the latter case, we observed that the material began to behave JETI, v.1, n.2, 2018 somewhat like a normal conductor, it can be understood as material is still partly in a superconducting state, but a small fraction is going to exhibit a normal conduction state.behavior, because as the temperature increases, there is a greater vibration of the atoms, thus there is a greater loss of Cooper pairs limiting the amount of pairs in the material [12][13][14][15][16].

Figure 2 .
By decreasing of the temperature a one step drop of the resistance value was observed at 105 K (as evidenced by the inset RxT derivative curve), this critical temperature can be attributed to Bi-2223 superconductor transition.

Figure 3 :
Figure 3: Characteristic curves of voltage versus electrical current for different external

Figure 4 :
Figure 4: Characteristic curves of the critical current as function of (a) Applied external magnetic field at different temperatures, and (b) Temperature at different magnetic fields.