V. Kavečanský

The influence of annealing in flowing argon on the microstructural and superconducting properties of Al doped YBCO bulks

The superconducting properties of Y Ba2(Cu1 − xAlx)3O7 − δ/Y2BaCuO5 bulk superconductors with Al concentrations x = 0.0025, 0.005, 0.02 and 0.05 were studied. The influence of annealing in flowing oxygen (SO) and argon at 800 °C with subsequent oxygenation at 400 °C on the superconducting and microstructural properties was investigated. The clearest peak effect at 77 K was observed at the lowest Al concentration (x = 0.0025) for SO and at the highest Al concentration (x = 0.05) for annealing in argon. No decrease of the transition temperature, Tc, with increasing Al concentration was observed for Al doped samples annealed in argon up to x = 0.02 in contrast to Al doped samples after standard oxygenation, where Tc decreases continuously with Al concentration. The twin structures and the unit-cell lattice parameters of the crystals were investigated by optical microscopy and by x-ray diffraction. The results indicate the possibility of clustering of the Al atoms during annealing in flowing argon at 800 °C.

Structural dependence of the spin-disorder resistivity of thin dysprosium films

Abstract An increase of the spin-disorder resistivity with decreasing film thickness has been observed in dysprosium films of thickness below ≈ 120 nm. X-ray diffraction experiments revealed this dependence is caused by a structural change.

Properties and structure of (Tl0.5Pb0.5)Sr2(Ca1?xNdx)Cu2Oz (Tl-1212) superconductors

Bulk material of Nd-doped Tl-1212 superconductors was prepared via a two-step procedure based on a wet chemical method to prepare precursor material followed by heat-treatment at 895??C for 10?h. The samples with Nd stoichiometry of x = 0.05, 0.1, 0.15, 0.20 0.25 and 0.30 were characterized by XRD, electron microscopy and by resistance measurements. Oxygen stoichiometry of the specimens was analyzed. Lattice parameters were derived from x-ray diffraction and neutron diffraction measurements. The increase of the a-axis and the decrease of the c-axis as a function of the Nd content are discussed.

The microstructure changes in Tl2Ba2Ca2Cu3Ox samples during annealing

On etudie par micrographie optique les microstructures des echantillons recuits a 840-900 o C pendant 15 a 25 mn

Thermal stability of NdBCO/YBCO/MgO thin film seeds

Thermal stability of the Nd1+x Ba2−x Cu3O7−δ (Nd-123 or NdBCO) thin films deposited on MgO substrate, with YBa2Cu3O7−δ (Y-123 or YBCO) buffer layer (NdBCO/YBCO/MgO thin film), has been experimentally studied in order to determine the optimal film thickness acting as seed for bulk YBCO growth. YBCO bulk superconductors with Y2BaCuO5 (Y-211) and CeO2 addition were prepared by the top seeded melt growth process in a chamber furnace using NdBCO/YBCO/MgO thin film seeds of different thicknesses (200–700 nm with 20 nm YBCO buffer layer) and different maximum temperatures, T max. The maximum temperatures varied in the range of 1040 °C–1125 °C. The highest thermal stability 1118 °C was observed in the case of NdBCO/YBCO/MgO thin film of 300 nm thickness. These results are corroborated with differential scanning calorimetry and high temperature x-ray diffraction measurements, as well as microstructure observations.

[Ni(bpy)(ox)]: a candidate in the class of Haldane gap systems (bpy = 2,2ʹ-bipyridine, ox = oxalate)

[Ni(bpy)(ox)] (bpy = 2,2ʹ-bipyridine; ox = oxalate) was solvothermally prepared in the microcrystalline form and identified by chemical analyses and IR spectroscopy. X-ray powder diffractometry indicates its isostructural character with analogous complexes [M(bpy)(ox)], and thus, its chain-like crystal structure formed of {Ni(bpy)} building units linked by bridging oxalate anions coordinated in a bis(chelate) fashion. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization reveal that the studied compound belongs to the class of systems with a Haldane gap. The estimated magnitude of the gap Eg ≈ 13.4 cm−1 is comparable with those found in archetypal Haldane gap systems. Graphical abstract

The Influence of Sintering Conditions on Microstructure of La-Ca-Sr-Mn-O Magnetocaloric Ceramics

A La0.67Ca0.33-xSrxMnO3 (x=0.33; 0.03; 0) perovskite magnetocaloric powders were prepared by solid state synthesis in air. Phase transformations were characterized by thermal analyses and the phase composition was confirmed by X-ray powder diffraction. Pressed pellets were sintered at different temperatures. Microstructure analyses were done by polarized light microscopy and scanning electron microscopy. The influence of sintering conditions on porosity, grain size and content of secondary phases is shown.

Microstructure of the NiMnGa Heusler Alloys Prepared by Suction Casting and Melt-Spinning

The microstructure of the Ni50Mn25Ga25 Heusler alloy (HA) rod cast to a copper mould and melt-spun ribbon was studied. According to X-ray diffraction analyses in both cases, the solidified alloy is a single-phase. In suction casting, the radial columnar crystals grow by cellular or dendritic growth, which leads to chemical inhomogeneity of the whole cast rod on the microscopic scale. The melt-spun ribbon exhibits a homogeneous alloy at the wheel side and an inhomogeneous alloy on the free surface side of the ribbon formed by cellular and dendritic growth. At both types of casting, the columnar crystals growth along the <100> crystal direction. According to texture analyses by EBSD, the fine grain structure at the wheel side of the ribbon exhibits no texture, while the columnar grain structure on the free surface side exhibits the <100> fibre texture with a declination by about 10 degrees in the spinning direction. Melt-spinning leads to significant refinement of the HA microstructure.

Study of Synthesis and Crystal Structure of Barium Cerate at High Temperatures

Crystalline barium cerate was synthesized by oxalate coprecipitation from nitrates of barium and cerium [1]. The oxalate precursor prepared by chemical methods was calcined at different temperatures up to 950°C. The barium cerate was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray diffraction investigation enables the determination of the phases that originate at different stages of synthesis and the crystal structure of final barium cerate, as well. From XRD patterns the average size of coherent regions was estimated by using Halder-Wagner method [2]. Both size and shape of crystallites were also studied by scanning electron microscopy. It was found that crystallites of barium cerate arise within the initial particles of the oxalate precursor.