Influence of Ca doping and addition of nano-particles of Si and SiC on critical transition temperature of YBa₂Cu₃O₇-8 superconductor

Currently, intensive research has been carried out to understand the mechanism of superconductivity in YBa2Cu307-o (YBCO) system. However, there is a need to study the influence of Calcium (Ca) on the superconductivity of YBCO. Therefore, it is our aim in this study to investigate the effect of Ca on superconducting transition temperature (Tc) of YBCO by selective doping at Y, Ba and Cu sites, respectively. This study is also focused on the potentiality of the nano-particles of Si and SiC as effective pinning centres in YBCO. Finally the superconductivity of YBCO doped by Ca and nano-particles of Si and SiC was investigated. The polycrystalline samples were prepared via solid state reaction method and they were analyzed by XRD, SEM and four-point electrical measurements. The XRD results of the Ca doping indicate that all samples can be indexed to Y123 phase with the highest dominant peaks of (103) and (013). The Y211 phase appeared slightly in all site of Ca doping system. SEM micrographs showed larger grain size grving nse to the reduction of porosity In (Y l-xCax)Ba₂Cu₃O₇-8 and Y(Bal-xCax)2Cu307_osystems. However, the increasing of Ca concentration doped into Cu site system leads to smaller grain size. It was found that T, decreased in all the systems. But then it dropped drastically in YBa2(Cul-xCax)307-ofollowed by (Yl-xCax)Ba₂Cu₃O₇-8 and finally by Y(Bal-xCax)2CU307-8. In order to study the influence of nano-particles on Te, 0.5, 1.0, 1.5 and 2.0 weight percentages (wt.%) of Si and SiC were added into YBCO samples respectively. XRD showed the dominance of Y123 phase in all the samples. The changes in a, b and c lattice parameters caused a nonsystematic trend for Si and a reduction in SiC of the orthorhombicity. Meanwhile, the grain size is decreased with the addition of nano-particles. The depression of T, in nano-Si from 91 K to 77 K is larger than that of nano-SiC which is from 91 K to 80 K attributed to that Si is easier to ionize and become Si4+ which may disturbs the overall structure of YBCO. Finally, the T, of (Yl-xCax)Ba₂Cu₃O₇-8 reacted with nano-Si was 74 K while that reacted with nano-SiC the T, was 78 K. Thus, it revealed nano-Si has a stronger effect on T, suppression than nano-SiC. Besides, lattice defect is increased in the nano-Si case contributing to the larger lattice strain. Therefore, in this research we have discovered that the Te degradation is determined by several factors which are the ionic radii, charge valency and hole concentration.

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