Angelina K. Stoyanova-Ivanova

Spectrophotometric determination of oxygen content in calcium substituted RBCO (R=Eu, Gd, Er) superconductors

The oxygen stoichiometry in R1−xCaxBa2Cu3Oy (R=Eu, Er, Gd; x=0, 0.2, 0.25, 0.3) superconducting bulk samples was determined spectrophotometrically. The dependence of the critical temperature on the y-oxygen coefficient and the x-coefficient of the included calcium was studied.

Microstructure and superconductivity of bulk BPSCCO/LPMO composite

Bulk superconducting (SC) ceramics containing BPSCCO and LPMO (Lanthanum/Lead – manganite phase) have been produced. The initial components were prepared by a low-temperature Pechini method. The submicron powders in weight ratio 90/10 Bi1.6Pb0.4Sr2Ca2Cu3Oz/La0.6Pb0.4MnO3 (nominal composition) were heat treated at 840°C. The duration of the heat treatment effect (60 and 100 h) of the composite on the transformation of the microstructure was studied. The obtained composites were analyzed by scanning electron microscopy (SEM) and by the method of energy-dispersive X-ray spectroscopy (EDX). They contain several phases. It was established that the SC 2212 phase is predominate in the composite. The phase La0.6Pb0.4MnO3 transforms in solid solution with preliminary composition La0.5(Sr + Ca)0.5Mn1− z Cu z O3, which after full replacement of the La and Mn ions leads to the appearance of phases with nominal composition Sr1− x Ca x CuO y . AC and DC magnetization measurements were used to study the SC and magnetic properties of the samples. Both samples are SC with critical temperatures 75 and 77 K, respectively. It was concluded that the SC and magnetic phases stably coexist in the composite sintered at 60 h heat treatment at 840°C.

Synthesis and superconducting properties of Nd0.33Eu0.08Gd0.58Ba2Cu3Oz materials

We have studied the effect of the ratio of different rare-earth element on the superconducting properties and phase formation of (Nd0.33Eu0.08Gd0.58)Ba2Cu3Oz ceramic. Bulk NEG samples were prepared using the solid-state reaction process. The superconducting transition for Nd0.33Eu0.08Gd0.58Ba2Cu3Oz is Tc ≈90 K and the value of the critical current density (Jc) is 13.9 A/cm2 at 77 K under zero magnetic fields. This value is twice as high when compared with the (Jc) value of YBCO systems (Jc = 7.31 A/cm2). The obtained bulk sample was used for the production of superconducting Ag-sheathed tapes by OPIT method including hot rolling. The critical current density of the obtained tape (337 A/cm2) is one order higher than the one of the bulk sample.

Increasing the sensitivity of the spectrophotometric determinations of the oxygen content in YBCO superconducting samples using the I3−-starch compound

The conditions for formation of the I(3)(-)-starch compound and measuring its absorbance have been found, and a spectrophotometric method has been developed for the determination of the oxygen content in YBa(2)Cu(3)O(y) superconducting bulk samples. The method involves the following stages: a decomposition of the sample in an acid medium in the presence of iodide ions under inert atmosphere; formation of a complex between Cu(II) and glycine; binding the I(3)(-)-complex with a starch and the absorbance measurement of the colored I(3)(-)-starch compound. The coefficient of the active oxygen is calculated by the ratio of the absorbances of two solutions and the method does not require both calibration and precise measuring sample mass. The accuracy of the results is confirmed applying the comparative spectrophotometric method that uses the yellow I(3)(-)-complex. The precision of the results evaluated by the relative standard deviation is 2%. The developed method is sensitive and allows a sample mass about 2mg to be used. The analysis is rapid and requires a simple and inexpensive apparatus. Thus the new method would be useful for an express analytical control of the oxygen content of YBCO-superconducting materials produced for the electronics.

Phase formation and microstructure in Re1-xCaxBa2Cu3Oz (Re=Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics

We have investigated the influence of Ca-substitution and different rare earths on the microstructure of RE1-xCaxBa2Cu3O7-δ (RE= Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics. Scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy have been used to study the microstructure and the chemical composition of the samples. A correlation was established between the polycrystalline microstructure and phase formation depending on the additive content. We observed that calcium is distributed uniformly in the crystals. The formation of minor impurity phases improved the sintering conditions.

Spectrophotometric method for rapid estimation of the oxygen stoichiometry in YBCO superconducting samples

Detailed investigations on the possibility for determination of the oxygen stoichiometry in YBa2Cu3Oy superconducting samples by measuring the absorbance of the colored Co(III)-EDTA and Cu(II)-EDTA complexes in one solution have been performed. The absorptivity of the complexes at pH 2.5 and 4.9 as well as Beer’s law have been studied. The content of the solution for the sample dissolving has been established. The stoichiometry of the oxidation-reduction reaction between Cu(III) from the sample and Co(II) in the solution has been proved. The possibility to dissolve the sample in air has been tested. As a result a simple and rapid spectrophotometric method for the determination of the δ non-stoichiometric oxygen coefficient in YBa2Cu3O6.5+δ samples has been developed without precise measurement of the sample mass. The standard deviation of the δ- and y-coefficient results is 0.017. The reproducibility of the y-values in relative units (sr) is 0.2%. The accuracy of the results has been confirmed by the method, which is based on the absorbance measurement of the I3−-starch compound.Graphical abstract

Microstructure and thermal properties of quasi-equal rare earth substitution Y0.5Gd0.5Ba2Cu3O6.94 superconductor

Sintering effects in YBa2Cu3Oz samples with quasi-equal rare earth substitution have been investigated. It has been shown that the Y-123 type compound can be formed when gadolinium is partially substituted (in this case 50% atomic substitution) for yttrium. The superconducting compound was obtained by the optimized ceramic method, including solid-state reaction, melting and sintering, controlled by X-ray diffraction. The microstructure was investigated by SEM and energy dispersive X-ray analyses. An AC susceptibility measurement has shown that Tc =93 K. Temperature-dependece of the thermal conductivity (4.5–300 K) of the polycrystalline Y0.5Gd0.5Ba2Cu3O6.94 sample was also measured.