Papers by Author: Pavel Diko

Abstract: To prepare bulk single-crystal REBCO superconductors by the new single-direction growth method (SDMG: Single-Direction Melt-Growth), it is necessary to produce a large-area seed of high quality, for example, based on EuBCO. Since the samples prepared by the SDMG method copy the structure of the seed, for the production of large-area seeds it is necessary to optimize the time-temperature regime in order to grow seeds with a suitable structure and composition and minimize structural defects (limiting the amount of subgrains and others). A higher growth rate was used in comparison with the standard growth rates used to produce EuBCO seeds of larger dimensions. The increased growth rate in the crystal growth window reduces the outflow of the melt from the sample, and thus it is possible to achieve a single-crystal sample in the entire volume of the precursor. The samples were produced at different growth rates: 1; 2; 3 and 5 °C/h. The microstructure of the samples was studied by polarized light microscopy and scanning electron microscopy. The size and distribution of Eu211 particles in the sample volume and the subgrain structure were studied on the fabricated samples.

Abstract: Single grain YBCO bulk superconductors were prepared from different starting precursors. In the first case, Y₂Ba₁Cu₁O₅ (Y211) powder as the solid phase and the liquid phase of mixture of YBa₂Cu₃O_7-x+BaCuO₂+CuO powders were used. In the second case the solid phase was prepared from the mixture of Y₂O₃+BaCuO₂ instead of the conventionally used Y211. The microstructure of samples was studied by polarized light microscopy and scanning electron microscope (SEM). The influence of different staring compositions on the final size of Y211 particles is shown. Also the influence of BaCuO₂ concentration in the Y₂O₃+x·BaCuO₂ (x=0.2; 0.5; 0.8; 1; 1.2; 1.5; 1.8) mixture on the Y211 size was studied.

Abstract: 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.

Abstract: 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.

Abstract: The influence of Sm addition on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied. Precursor powders YBa₂Cu₃O_7-δ (Y-123), Y₂O₃ and CeO₂ were enriched with different amounts of SmBa₂Cu₃O_y (Sm-123) or Sm₂O₃ powders with the aim to increase critical current density, J_c, by introducing additional pinning centers. YBCO bulk superconductors with SmBa₂Cu₃O_y (Y123-Sm) or Sm₂O₃ (Y123-SmO) powder addition were prepared by the optimized top seeded melt growth process in the form of single grains. Microstructure analysis revealed that Sm₂O₃ addition leads to a higher amount of smaller Y₂BaCuO₅ (Y-211) particles, what is related to high critical current densities (J_c ~ 7 x 10⁴ A/cm²) of the YBCO samples with Sm₂O₃ addition in low magnetic fields. The effect of Sm addition in the form of SmBa₂Cu₃O_y as well as Sm₂O₃ powder on Y₂BaCuO₅ particle size, critical temperature, T_c, and critical current density, J_c, is reported.

Abstract: The influence of different sintering temperatures during Spark Plasma Sintering (SPS) process on microstructure of La-Ca-Sr-Mn-O ceramics has been studied. The powders of La_0.67Ca_0.33-xSr_xMnO₃ (x = 0.33; 0.03) (LCSM) perovskite were prepared by milling of the stoichiometric amounts of the starting materials - lanthanum oxide (La₂O₃), calcium oxide (CaO), strontium carbonate (SrCO₃) and manganese oxide (MnO₂), and subsequently calcinated twice. After the second calcinations the LCSM powders were treated by SPS method at four different temperatures (1000°C, 1150°C, 1200°C and 1250°C), at uniaxial pressure of 50 MPa in a vacuum. The microstructure characterizations were done by polarized light microscopy and scanning electron microscopy. The microstructural observations showed that increasing sintering temperature leads to an increase of grain size. The energy dispersive spectral (EDS) analysis confirmed that higher sintering temperatures cause changes in the phase composition of the investigated LCSM perovskite materials. The benefits of the LCSM samples preparation by SPS process are discussed.

Abstract: A composite material of graphite/polystyrene sulfonate was prepared by solvothermal method. The dissolution of polystyrene sulfonate in water ensures homogeneous distributions of graphite and creates composition material at room temperatures. Water removing by the drying, permit to prepare films. Structure of the film was analyzed by scanning electron microscopy. The influence of different ratio between graphite and polystyrene sulfonate is discussed.

Abstract: YBCO (YBa₂Cu₃O₇/ Y₂BaCuO₅) bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. CeO₂, BaCeO₃ and BaO₂ powders were added to nominal composition Y_1.5Ba₂Cu₃O_y (Y-123) with the aim to refine Y₂BaCuO₅ (Y-211) secondary particles, which provide pining. Added powders were refined by friction milling. The system behaviour was characterized by thermal analyses. Microstructure of prepared samples was studied by polarized light microscopy and the influence of additions is described.

Abstract: The YBCO bulk single-grain superconductors were prepared by Top-Seeded Melt-Growth (TSMG) process. Y₂BaCuO₅ (Y211) fine powder was added to YBa₂Cu₃O_7-δ (Y123) nominal composition. Powder refinement was characterized by X-ray powder difractometry, laser granulometry and scanning electron microscopy. The system behaviour was characterized by thermal analysis. Microstructure of prepared samples was studied by polarized light microscopy. The influence of Y211 addition the sample microstructures are shown.

Abstract: A La_0.67Ca_0.33-xSr_xMnO₃ (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.