Various types of chemical doping have been reported as very effective methods to improve the superconducting properties of MgB2 superconductor. Specially, carbon doping via liquid type of carbon-containing compounds have been shown better superconducting properties. In this work, the liquid type of glycerin (C3H8O3) was used as a carbon dopant in MgB2 synthesis. The glycerin was mixed with a liquid media at a different ratio and then pretreated with refined boron powder. Then carbon doped MgB2 superconductor was synthesized through subsequent heat treatment of the pretreated boron powder with magnesium powder. Variation of the amount of carbon dopants and viscosity of the liquid media was correlated with critical current densities and other superconducting properties of MgB2 bulk. The effects of liquid carbon dopants on the superconducting properties also compared with those of solid dopants.

Compared to entrapped spherical pores in YBa2Cu3O7−y (123) crystals melt-textured without additives, the pores entrapped within 123 crystals melt-textured with 5 wt% BaCeO3 are elongated and aligned parallel to (100), (010), and (001) growth planes of the 123 crystals. The front side of the pores that meets first the growth front of the 123 crystal is faceted but the backside is wavy. Many BaCeO3 particles are segregated at the wavy surface. The crystallographic alignment of the elongated pores and the segregation of BaCeO3 particles are discussed in terms of the contact angle of the pores on the growth front and interfacial energy relationships between the related phases.

Effects of the heating rate (100−6000 °C/h) to a peritectic temperature (Tp – 1015 °C) on conversion of Y2O3−BaCuO2−CuO and Y2BaCuO5−BaCuO2−CuO precursor powders into YBa2Cu3O7−y (Y123) were studied. Both precursor powders were rapidly converted into the Y123 phase. A volume fraction of the Y123 phase was more than 50%, even at a relatively fast heating rate of 3000 °C/h. At about 100−200 °C/h, which correspond to the rates of the first heating cycles of practical melt processes, almost 100% of the precursor powders were converted into a Y123 phase. Microstructures were studied in respect to Y2BaCuO5 (Y211) particle size, distribution, gas evolution, and nucleation mechanism of Y211 particles.

The particle segregation mode of two different inclusion phases of Y2BaCuO5 (Y211: a dissolving phase in a Ba–Cu–O liquid phase) and BaCeO3 (a nondissolving phase) was investigated in the melt-textured YBa2Cu3O7−y (Y123) with BaCeO3 addition (0–20 wt. %), and with 30 wt.% Y211 plus BaCeO3 (0–20 wt. %). The segregation mode of the inclusion phases is dependent not only on the type of the inclusion phases but also their amounts. When the trapped amount of the Y211 is small, they make an X-like pattern on the diagonal planes of the Y123 crystal. When the amount of the Y211 is large, meanwhile, the Y211 particles are trapped within four tetrahedral spaces (normal to the c-axis) bounded by the diagonal planes of the Y123 crystal, with no Y211 trapping within two tetrahedral spaces parallel. On the other hand, the nondissolving BaCeO3 particles make linear tracks normal to the {100} growth fronts of the Y123 crystal.

In order to understand the formation mechanism of the Y2BaCuO5 free region, microstructures concerning incongraent mehing and peritectic reaction were studied in melt-textured Y–Ba–Cu–O oxides. It is found that spherical pores form during incongruent melting of the YBa2Cu3O7−y phase into Y2BaCuO3 and a Ba–Cu–O liquid phase. As the melting goes on, liquid phase flows into the pores and then produces spherical liquid pockets containing a few Y2BaCuO5 particles. During slow cooling of the sample from the peritectic temperature to the temperature where YBa2Cu3O7−y phase is formed, the liquid pockets are converted into YBa2Cu3O7−y phase containing a few Y2BaCuO5 particles. Sometimes, remnant Ba–Cu–O liquid phase is present at the center part of the Y2BaCuO5 free regions due to the incomplete peritectic reaction. It is concluded that formation of spherical pores during incongruent melting of YBa2Cu3O7−y is responsible for the formation of the Y2BaCuO5 free regions.

Segregation of second-phase particles within Y1Ba2Cu3O7−y domain was investigated in melt-textured Y-Ba-Cu-O with metal oxide (CeO2, SnO2, and ZrO2) addition. It is found that coarse particles (Y2Ba1Cu1O5) are trapped with a special pattern in the interior of Y1Ba2Cu3O7−y domain, while fine BaCeO3 and BaSnO3 particles are present within the remnant liquid-phase region. During the growth of Y1Ba2Cu3O7−y domain, fine particles appear to be pushed out of the advancing Y1Ba2Cu3O7−y /liquid interface toward the liquid phase. The particle segregation that occurred during peritectic growth of the Y1Ba2Cu3O7−y domain was explained in terms of the Uhlmann-Chalmers-Jackson theory based on the particle interaction at solid/liquid interface.

The formation of BaCeO3 and its effects on microstructure were studied in sintered/melt-textured Y-Ba-Cu-O oxides containing 5 wt. % CeO2 and various amounts of Y2Ba1Cu1O5. The added CeO2 was converted to fine particles of BaCeO3 near 930 °C, which is the conventional sintering temperature for Y-Ba–Cu-O. Y2Ba1Cu1O5 and CuO are formed as by-products of the reaction between CeO2 and Y1Ba2Cu3O7−y phase. The CeO2 addition reduced the particle size of Y2Ba1Cu1O5 which was trapped in the Y1Ba2Cu3O7−y matrix after the melt-texture growth. During the peritectic decomposition stage of Y1Ba2Cu3O7−y phase into Y2Ba1Cu1O5 and liquid phase, the morphology of the decomposed Y2Ba1Cu1O5 was changed from a blocky shape in the undoped sample to an acicular shape of high anisotropy in the CeO2-added sample. The formation of the highly anisotropic Y2Ba1Cu1O5 particles appears to be responsible for the refinement of Y2Ba1Cu1O5 particle after the melt-texture processing.

In order to investigate microstructural variation by 2-1-1 addition in partially melted Y–Ba–Cu–O, a specimen resulting from 2-1-1 added to 1-2-3 was heat-treated through the peritectic temperature. Microstructure was observed on the directionally solidified region near the interface of the two samples. The 2-1-1 addition results in a homogeneous microstructure where no remnant liquid phase is present. It reduced the 1-2-3 plate thickness, as well as suppressed the formation of microcracks due to the tetragonal-to-orthorhombic phase transition or the thermal contraction during cooling from the peritectic temperature. The formation of microcracks induced by the phase transition seems to be closely related to the process of oxygen diffusion into a sample. We discuss the formation of microcracks in terms of the oxygen diffusion along the plate boundaries and of the thickness of 1-2-3 plates. The decrease in the plate thickness and the fine dispersion of 2-1-1 particles contribute suppression of the formation of microcracks and their propagation.

The CeO2-added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO2 was converted to a form of BaCeO3, but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO2. All the added CeO2 in these samples was consumed to form only BaCeO3 which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (Tc) of the solid-state reacted sample gradually decreased with increasing CeO2 content due to the phase change and the formation of BaCeO3, whereas the Tc of the partially melted sample was nearly constant regardless of CeO2 content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO2 content. The strengthening effect appears to come from the composite system where the fine BaCeO3 particles are dispersed in a 1-2-3 matrix.