Effect of magnetic and nonmagnetic nano metal oxides doping on the critical temperature of a YBCO superconductor

The co-precipitation method was used for the preparation of nano metal oxides [23–25]. The chemicals used were of analytical grade and used without further purification. Cobalt chloride (CoCl₂·6H₂O), manganese chloride (MnCl₂·4H₂O), tin chloride (SnCl₂·5H₂O), copper chloride (CuCl₂·2H₂O), cobalt chloride (CoCl₂·6H₂O), chromium chloride (CrCl₃·6H₂O) and oxalic acid (C₂H₂O₄·2H₂O) with purity 99.5% were used. After which, 5 ml ammonia solution (NH₄OH) was added drop by drop in the solution to increase the pH value of the solution prepared with 0.1 M oxalic acid and metal chlorides and then the two solutions interacted and the corresponding metal oxalate preciptated in water. During precipitation the solution was subjected to magnetic stirring at the rate of 550 rpm. The resultant precipitated oxalate solution was kept in an oven for drying over night at 90 °C then the dried product was ground in powder form and put in a furnace at 700 °C for 5 h to obtain nano metal oxide powder.

YBCO powder is prepared by the solid state reaction route by mixing a stochiometric amount of Y₂O₃, BaCO₃, CuO followed by grinding, calcination and sintering according to figure 1.

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A series of polycrystalline composite samples of YBCO+x wt% where x = 0.2 of each metal oxides SnO₂, Mn₃O₄, Cr₂O₃, Co₃O₄ and CuO were mixed separately and pressed into pellets. The pellets were sintered according to the same curve of heat treatment figure 1. X-ray diffraction (XRD) was done for phase conformation and structural analysis using a Brukur D8 Advance Cu-Kα target with a secondary monochromator (λ = 1.5411 nm) powered at 40 kV and 40 mA. Scanning electron microscopy (SEM) was done for microstructural analysis using Philips XL 30 equipped with an EDX unit, accelerating voltage of 30 KV, magnification up to 400 000×, and resolution of 3.5 nm. Temperature dependent resistivity ρ(T) was measured using the standard four-probe techniques with a Nanovoltmeter (182-KEITHLEY) and constant current source (224-KEITHLEY), with the voltage resolution of 10–8 V of the Nanovoltmeter, and a constant current source of 1 mA flowing through the samples. A closed cycle helium refrigerator consists of a Cryo-compressor, (model: 531-120-IBARA), cooled head, (model: CCS-100EB-JAUS RESEARCH CO.), supplied with a suitable holder, vacuum pump (model: RVB-EDWARDS) and a temperature controller (321-Autotuning Temperature Controller-LAKESKORE); a temperature resolution of ±0.1 K was used for temperature variation.

Figure 2 shows the XRD patterns of the obtained nano oxides Co₃O₄, Cr₂O₃, CuO, Mn₃O₄ and SnO₂.

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XRD is a convenient method for determining the mean size of nano crystallites in nano crystalline bulk materials. The first scientist, Paul Scherrer, published his results in a paper that included what became known as the Scherrer equation in 1981 [26]. This can be attributed to the fact that 'crystallite size' is not synonymous with 'particle size', while XRD is sensitive to the crystallite size inside the particles. From the well-known Scherrer formula equation (1) the average crystallite size, L, is:

$$\begin{eqnarray*}&&L=\displaystyle \frac{K\lambda }{\beta \;\mathrm{cos}\;\theta },\end{eqnarray*}$$

where λ is the x-ray wavelength in nanometer, β is the peak width of the diffraction peak profile at half maximum height resulting from small crystallite size in radians and K is a constant related to crystallite shape, normally taken as 0.9. The value of β in 2θ axis of diffraction profile must be in radians. The θ is the Bragg angle and can be in degrees or radians, since the cosθ corresponds to the same number. Table 1 shows the average crystal size in a nanometer obtained from the Scherrer equation [27] and the values of the magnetic moment of each metal oxide.

Figure 3 shows the powder XRD patterns of the YBCO samples doped with different metal oxides. The analysis of the data indicates a predominantly single phase perovskite structure YBCO with orthorhombic Pmmm symmetry and small quantities of secondary phases.

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The lattice parameters of YBCO with orthorhombic structure are a₀ = 3.823 Å, b₀ = 3.885 Å, c₀ = 11.7 Å. It should be mentioned that no peaks corresponding to any other compounds were detected by XRD. The lattice parameters and unit cell volumes of these samples are presented in table 2. It may be noticed that the lattice constants a, b and c change slightly with the different doping of oxides as shown in table 2. For Mn₃O₄ there is no significant change in a and b parameters while c decreases slightly. The smaller c value could be responsible for a better interlayer exchange and consequently, could give better superconducting properties. For YBCO doped with SnO₂ the a parameter increases slightly while b remains constant with a slight decrease in the c parameter. As for doping with Co₃O₄ and Cr₂O₃, we find that there is no significant difference in a and b parameters but the c parameter is almost the same. While for SnO₂ b and c parameters are only slightly affected but a increased. The difference between a and b parameters reduces the orthorhombicity (δ), and the high value of orthorhombicity of the YBCO samples is the result of high oxygen content and consequently could give better superconducting properties [31].

The transmission electron microscopy (TEM) images of YBCO doped concentration of 2% of Co₃O₄, Mn₃O₄, CuO, SnO₂ and Cr₂O₃ are shown in figure 4.

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The microstructure characterization, i.e. grain size distribution of the composites is shown in figure 5. It shows that the YBCO samples in figure 3 exhibit large grains randomly oriented in all directions with the presence of pores between the grains. From the graphs it is obvious that the grains of all samples are in the same range 1–4 μm. For every doping of nano oxides, it is shown that nano dots are distributed randomly mostly over the YBCO grains. Nano oxide doping of SnO₂ tends to gather slightly in some spots between the grains, while Co₃O₄ and Mn₃O₄ and CuO show slightly tilted randomization dots on the grains. Cr₂O₃ tends to form dots in the nanosize range but greater than the other doping oxides and acts as a cover over the grain.

Nanoscale defects shown in the SEM graphs act as pinning centers which determine the current flow and the dissipation of energy through the superconductor. Nanostructures play a dramatic role in optimizing pinning without destroying an unnecessary volume fraction of the superconductor, thereby allowing for a significant increase in J_c [32–35].

Figure 6 shows the normalized resistance versus temperature plots of YBCO doped with 0.2 wt% of nano metal oxides. Firstly it is characterized by the normal state that shows a metallic behavior (above 2T_c). The normal state resistivity is found to be linear from room temperature to a certain temperature and follows the Anderson and Zou relation ρ_m(T) = A + BT, where ρ_m(T) is calculated by using the values of A and B parameters which are obtained from the linear fitting of resistivity in the temperature range 2T_c to 300 K and extrapolated to 0 K gives a resistivity slope (dρ/dT) and residual resistivity ρ₀ as seen from figure 6.

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The second is the region characterized by the contribution of Cooper pair fluctuations to the conductivity below T_c where ρ(T) deviates from linearity. This is mainly due to the increasing rate of Cooper pair formations on decreasing temperature. Therefore the fluctuation induced conductivity in their region follows all models to yield the dimensional exponent appropriate to fluctuation-induced conductivity. The resistivity decreases with the doping of non magnetic nano metal oxides (Co₃O₄, Cr₂O₃, CuO and SnO₂) indicating that it adheres to grain boundary forming weak links. The transition temperature from the normal to superconducting state is severely damaged by the addition of non magnetic nano metal oxides (shown in table 2). The depression of T_c with non magnetic nano metal oxide addition may be either due to a decrease in oxygen content in the CuO chains [36] or due to the trapping of mobile holes or some mechanism connected with the oxygen vacancy disorder [37]. Additionally, a close investigation of the resistivity plots shows that there is an increase in T_c for the sample with Mn doping. The increase in the transition temperature for Mn doping (T_c = 112 K) is probably due to the Mn inclusion which reduced the formation of other YBCO phases and enhanced the formation of Y-123. Cu doping shows the highest depression rate followed by Sn, Cr and Co doping because of the fact that Sn and Cu substitute Cu (+2) sites on CuO₂ planes in the lattice, whereas Co and Cr occupy Cu (+1) sites along Cu-O chains and they have different magnetic moments.

Table 3 indicates that the values of ρ₀ depend on the type of doping and it is obvious that the value of ρ₀ increases from the pure YBCO sample. For nano nonmagnetic doping the value of ρ₀ increased greater than nano magnetic doping.