Osman Gorur

Role of diffusion-annealing time on the mechanical properties of bulk Bi-2223 superconductors diffusion-doped with Au

In order to investigate the role of Au doping and diffusion-annealing durationxa0on the mechanical properties of Bi–Pb–Sr–Ca–Cu–O, Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy superconductors were prepared by standard solid-state reaction methods. Doping of Bi-2223 was carried out by means of gold diffusion during sintering from an evaporated gold film on pellets. The effect of the diffusion-annealing duration on the mechanical and superconducting properties of these compounds was investigated using scanning electron microscopy, dc resistivity and hardness measurements. These measurements indicated that the Au doping and diffusion-annealing time increased the superconducting transition temperature, Vickers hardness, Youngs modulus, yield strength, fracture toughness values and improved the grain connectivity. The mechanical properties of the samples were found to be load dependent. In addition, we calculated the load independent hardness, Youngs modulus, yield strength and fracture toughness of the samples. These mechanical properties of the Au-doped samples were dependent on the diffusion-annealing time. Possible reasons for the observed improvements in the superconducting and mechanical properties due to Au diffusion are discussed.

Investigation of some physical properties of silver diffusion-doped YBa2Cu3O7?x superconductors

Silver diffusion in superconducting YBa2Cu3O7?x (YBCO) has been studied over the temperature range 500?800??C by x-ray diffraction measurements. The diffusion coefficient of Ag in YBCO was determined from the observed increase (0.12%) in the lattice parameter c as compared with the undoped sample. It was shown that the diffusion coefficient of Ag in the temperature range 500?800??C changed from 1 ? 10?10 to 5 ? 10?9?cm2?s?1 with an activation energy of about 1.06?eV. For the Ag-doped sample, the critical transition temperature and the critical current density were increased from 90 to 92.5?K and from 60 to 98?A?cm?2 respectively in comparison with those of undoped YBCO. The possible reasons for the observed increases in Tc and Jc due to Ag diffusion were discussed.

Production of BiPbSrCaCuO Thin Films on MgO and Ag/MgO Substrates by Electron Beam Deposition Techniques

Superconducting BiPbSrCaCuO thin films were prepared on MgO(001) and Ag/MgO substrates using an electron beam (e-beam) evaporation technique. The effects of annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties, respectively, were investigated by X-ray diffraction, atomic force microscopy, and by measurements of the critical temperature and the critical current density. It was shown that an annealing of both types of films at 845 or 860 °C resulted in the formation of mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. The slight increase of the critical temperature from 103 K to 105 K, the enhancement of the critical current density from 2 x 10 3 to 6 x 10 4 A/cm 2 , and the improved surface smoothness are due to a possible silver doping from the substrate.

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO, CaO, CuO)

The thermal expansion of superconducting Bi1.6Pb0.4Sr2Ca2Cu3Ox (BiPbSrCaCuO) and its oxide components Bi2O3, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is α=6.4×10−6K−1. The temperature dependences of ΔL/L of pressed Bi2O3 reveals sharp changes of length on heating (T1=712°C), and on cooling (T2=637°C and T3=577°C), caused by the phase transition monoclinic-cubic (T1) and by reverse transitions via a metastable phase (T2 and T3). By thermal expansion measurements of melted Bi2O3 it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi2O3. The thermal expansion of red PbO reveals a sharp decrease in ΔL/L, on heating (T1=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi2O3 (αm=3.6 × 10−6 and αc=16.6×10−6K−1 for monoclinic and cubic Bi2O3 respectively), the melted Bi2O3 (αm=7.6×10−6 and αc=11.5×10−6K−1), PbO (αt=9.4×106 and ωor=3.3×10−6K−1 for tetragonal and orthorhombic PbO respectively), CaO (α=6.1×10−6K−1) and CuO (α=4.3×10−6K−1) are presented.

Anomalies of thermal expansion in Ag diffusion doped BiPbSrCaCuO superconductors

Abstract Thermal expansion of Ague5f8Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O x structures was measured in the interval 20–830°C using an inductive dilatometer. Two anomalies in the thermal-expansion curves of samples were discovered: a decrease in length ΔL / L on heating (740–810°C) and an increase in ΔL / L on cooling (810–660°C). Changes of composition, structure and electrical properties of Ag doped superconductor caused by heat treatments during thermal-expansion runs were examined using XRD, DTA, optical microscopy and resistivity measurements. A model explaining the observed results was discussed. Thermal-expansion coefficients of Ag doped and undoped BiPbSrCaCuO samples are 7.5 × 10 −6 K −1 (at 60–680°C) and 6.4 × 10 −6 K −1 (at 30–830°C), respectively.

High-temperature thermal expansion of BiPbSrCaCuO superconductors

Abstract The linear thermal expansion in the range 30–800°C, X-ray diffraction and resistivity versus temperature of BiPbSrCaCuO samples depending on the cooling rate (at the end of sintering) were investigated. Two types of samples were prepared: quenched ones (about 1000°C/min) and slowly cooled ones (3°C/min) in the thermal synthesis process. X-ray patterns corresponding mainly to the Bi-2223 phase were observed for the quenched samples. The slowly cooled samples exhibited patterns attributed mainly to the Bi-2212 phase and also the additional reflections caused by the inclusions (PbO 2 , Bi 2 O 3 and PbBi). The coefficients of thermal expansion (in the range 30–800°C) of the quenched and slowly cooled samples were 6.2 × 10 −6 and 5.1 × 10 −6 K −1 respectively. The thermal expansion curve of the slowly cooled samples showed some peculiarities at about 300°C. The critical temperature of the quenched sample (108 K) is higher than that for the slowly cooled sample (92 K). The degradation of the Bi-2223 phase (in the form of increasing the content of Bi-2212 phase and inclusions of the second phases) due to slow cooling of the samples at the sintering process was attributed to the precipitation phenomenon on cooling. The peculiarities on the thermal expansion curves around 300°C of the slowly cooled samples were interpreted as a result of the influence of inclusions.