Thermal investigations of reactive powder mixtures as precursors for melt-processing of YBa2Cu3O7−x (original) (raw)
Microstructural development of partial-melt processed YBa2Cu3O7−δ
Proceedings of the Samahang Pisika ng Pilipinas, 2002
YBa 2 Cu 3 O 6.5 (YBCO123) was synthesized, subjected to a pre-melt process, and its microstructures were characterized via SEM. The microstructural evolution was observed by pre-melting YBCO123 at the melting temperature onset of 1020°C and cooling it down with crystallization cooling rates (CR) 100, 50, 33.3, 20, 10, 5, and 2°C/hr. Microstructural development from a brittle fissured faceted phase with no defined grain boundary and densified melted features to column-like grains and biphasic grains with hexagonal grain boundaries were observed as the CR was varied from fastest to slowest CR.
Physica C: Superconductivity, 2003
The influence of the ratio of small and large size YBa 2 Cu 3 O x (Y123) starting powders on the microstructure development in the sintered and melt-textured YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 (Y211) bulk superconductors with the CeO 2 addition was studied. It was shown that Y211 low concentration regions in the microstructure of the melt-textured samples are formed due to the presence of large multigrain particles and/or agglomerates of small 123 particles in the starting Y123 powder. Moreover, it was observed that large Y211 particles in the melt-textured samples are nucleated on the surface of large Y123 grains during the sintering stage. It is suggested that the insufficient mixing of the powders with higher portion of small Y123 particles causes that the CeO 2 concentration in the sample is more inhomogeneous. Consequently the Y211 particles can locally grow during peritectic melting and temperature dwell.
Fabrication of melt textured YBa2Cu3O7 samples
Cryogenics, 1994
Textured YBCO samples were produced following the partial-melt method with sample transport. Structural and compositional characterizations, performed by means of XRD and SEM/EDS, show the presence of the 123 with a small amount of 211 inclusions. The investigated samples show a non weak link behaviour in the irreversible magnetic properties at 77 K, with critical current densities Jc = 104 A/c m2 at 2 kOe.
Solid state reactions in the formation of YBa2Cu3O7−δ high Tc superconductor powders
Solid State Ionics, 1989
The synthesis of high Tc superconducting YBa2Cu307 ~ powders obtained by three different schemes has been investigated. The decomposition of metal carboxylates, mixed metal oxalates, and the conventional ceramic method of mixed oxide/carbonate are contrasted. Thermal analysis (TGA, DTA) results as well as X-ray diffraction data are used to follow the progress of the decomposition and solid state reactions leading to the formation of the required perovskite. The effect of cation mixing at the molecular level in the carboxylates and oxalates is compared to the physical mixing of coarser particles in the conventional approach. In the case of the carboxylates and mixed oxalate it is suggested that Ba and Y form a double carbonate that decomposes into barium carbonate and yttrium oxide in the progress of the reaction.
Thermal reactivity of YBa2Cu3O7-δ with Al2O3 addition in air atmosphere
Ceramics International, 2019
The high-temperature thermal reactivity of YBa 2 Cu 3 O 7-δ (Y123) compound with Al 2 O 3 addition (0.5-10 wt%) was investigated and the thermal processes were described. Experimentally obtained results showed that Al 2 O 3 firstly reacted with Y123 at temperatures far below the peritectic melting point of Y123. During this solid state reaction Al partially diffused into Y123 and formed a solid solution. Also, the formation of Al-rich complex oxides, mainly with barium, and basic secondary phases as Y 2 BaCuO 5 and CuO, was observed with increasing Al 2 O 3 content. The formation of additional secondary phases with Al 2 O 3 provoked their reactivity with Y123 or between themselves. After increasing the temperature of the powder mixtures above the peritectic melting point of Y123 it was found that Al was incorporated in Y123 at higher concentrations than during the solid state reaction. The reason for that could be a partial or full dissolution of Al-rich complex oxides formed at lower temperatures which in turn increased the source of Al for the incorporation in Y123. The thermal behaviour of powder mixtures was investigated with the help of simultaneous differential thermal analysis and thermogravimetry. Morphology and microstructure were examined using powder X-ray diffraction and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy.
Entrapment of Y2BaCuO5 particles in melt-textured YBa2Cu3O7−δ crystals and its effect on J
Physica C: Superconductivity, 1996
In order to investigate the entrapment phenomenon of Y2BaCuO5 (Y211) particles and its effect on the properties of the critical current density (Jc) in YBa2Cu307_~ (Y123) crystals, Pt-added Y123 crystals were grown by two different methods; the constant undercooling method and the continuous cooling method, with top-seeded growth. It was revealed that the amounts and size distribution of trapped Y211 particles in solidified Y123 crystals depended on the growth direction ((001), (100)) and the growth rate (R) as a function of undercooling (AT). These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. The Jc values of the Y123 crystals were found to vary with their Y211 distributions, which in turn depended on growth conditions such as growth direction and/or growth rate even when the nominal composition was kept constant. Considering the effect of R or AT on the Y211 particle entrapment/pushing and Jc properties, it was shown that the conventional continuous cooling methods (i.e. MPMG/QMG methods) were favorable for Y211 particle entrapment, leading to high Jc values.
Experimental study of the decomposition of Y1Ba2Cu3O7−x into tetragonaland orthorhombic phases
Physica C: Superconductivity, 1988
Infrared absorption spectroscopy and X-ray diffraction measurements have been used to show that a single phase oxygen deficient Y~BazCu307_x (YBCO) when annealed at low temperatures (200°C) is unstable towards decomposition into a mixture of orthorhombic and tetragonal phases. This is in qualitative agreement with the calculated phase diagram of YBCO. The degradation in the superconducting properties (Meissner signal reduced by a factor of 2 ) that accompanies this annealing treatment is understood as being a direct consequence of the decomposition.
Journal of Alloys and Compounds, 1998
The resistive transitions of large grained and textured YBa Cu O polycrystals fabricated by the flame-quench-melt-growth (FQMG) 2 3 7 2 x process were studied in various selected magnetic fields. It was found that the appropriate amount of silver addition enhances the formation of the superconducting phase and slightly increases the transition temperature. The magnetic behaviour of YBCO with silver and without silver were determined by measurements of the resistivity (r) in an external magnetic field. It was found that samples with silver showed a lower transition temperature in a magnetic field. The activation energy, U , for thermally activated flux flow (TAFF) was o 21 estimated for all samples from the slope of the Arrhenius plot of ln r versus T. It was found that the activation energy of the samples decreases with increasing silver content and magnetic field.
Fabrication of pure YBaCuO powders with controlled microstructure
Journal of the European Ceramic Society, 1999
A fabrication process has been optimized to successfully produce ®ne and pure high temperature superconducting YBaCuO powders. The resulting powders have been examined by X-ray diraction, scanning and transmission electron microscopy with Energy Dispersive X-ray (EDXS) and Electron Energy Loss (EELS) Spectroscopies. Results show that this process allows the preparation of samples with a correct stoichiometry and a controlled microstructure without detectable secondary phases at grain boundaries to the scale of the transmission microscopy. A comparison of the powder quality is made with powders prepared by conventional methods. #
Thermochemistry of the Y2O3-BaO-Cu-O system
High temperature reaction calorimetry using molten lead borate as a solvent is used to study the thermochemistry of phases in the Y 2 O 3 -B a O -C u -O system. Enthalpies of formation of YBa 2 Cu 3 0^ (6 < x < 7), YBa 2 Cu 4 0 g , Y 2 BaCu0 5 , Y 2 Cu 2 0 5 , and BaCuO 2 oi phases have been determined. The thermodynamic relationships among these phases at room temperature have been assessed. The results show that the 123 phase is not thermodynamically stable with respect to assemblages containing combinations of Y 2 BaCu0 5 , Y 2 Cu 2 0 5 , BaCuO 2 , Y 2 O 3 , and CuO. The fully oxidized 123 phase is not stable with respect to the 124 and the above assemblages. Reactions at room temperature are shown as follows: YBa 2 Cu 3 07 (s) + CuO (s) -» YBa 2 Cu 4 0 8 (s); AH = -31.9 ± 22.3 kJ mol" 1 and YBa 2 Cu 3 0 676 (s) -5YBa 2 Cu 4 0 8 (s) + Y 2 BaCu0 5 (s) + jBaCu0 2 .oi (s); AH = -78.9 ± 21.0 kJ mol" 1 .