Yu. V. Blinova

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

The resistance of the nonstoichiometric compounds YBa2Cu3O6.8 (single crystals, ceramics) and YBa2Cu3O6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in Tc from 75 K in the initial state to 90 K. The retained superconductivity and the increase in Tc are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments.

Phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below 400°C

X-ray diffraction analysis and transmission electron microscopy have been used to study the low-temperature decomposition of the nonstoichiometric (in oxygen) HTSC compound Ba2YCu3O7 − δ. The phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below ≤400°C has been constructed. The temperature range corresponding to phase separation has been found to be divided into two portions. At T > 250°C, two orthorhombic phases characterized by different oxygen contents are formed; at the higher temperatures, the phase separation of the compound into a tetragonal and an orthorhombic phase takes place. The separation was also found to observe at T = 100°C; this indicates the possibility of natural aging for the Ba2YCu3O7 − δ compound at room temperature.

Nature of heavy-atom disordering in the YBa2Cu3O6.8 (1.5 at % Ce) single crystal: X-ray photoelectron spectroscopy

Photoelectron spectroscopy was used to study nonstoichiometric 123 single crystals in the initial state and after annealing at 200°C in different atmospheres (vacuum, argon, and air). The main cause for the disordering of heavy atoms (Y and Ba), which occurs during annealing in an argon atmosphere and air, was determined by a comparative analysis of the structure and spectra. The disorder is caused by the OH− group that penetrates into the lattice of the compound through oxygen-vacancy chains.

Spinodal decomposition of the nonstoichiometric YBa2Cu3O7−δ compound at room temperature

Electron and polarized-light optical microscopy have been used to study the spinodal decomposition of the YBa2Cu3O7−δ compound depending on the oxygen content. Experimental data reported indicate that whatever the state —initial or preliminarily aged at a high temperature—of the YBa2Cu3O7−δ (δ ≈ 0.2 and 0.4) nonstoichiometric oxide, it undergoes natural aging typical of metallic alloys.

Mechanism of thermal decomposition of the nonstoichiometric compound YBa2Cu3O6.8

Thermal decomposition of the nonstoichiometric high-temperature superconductor YBa2Cu3O6.8 at a temperature of 200°C in air has been investigated using the full-profile analysis of X-ray diffraction lines. Two mechanisms of decomposition are revealed. The first mechanism, i.e., separation into two phases with a different oxygen content, occurs continuously. The second mechanism, i.e., disordering of the heavy atoms Y, Ba, Ba, Y along the crystallographic axis c, begins to occur after a 20- to 35-h annealing and progresses with a further annealing.

Mechanisms of the formation of a bulk superconducting phase MgB2 at high temperatures

The structure of MgB2 samples synthesized from magnesium flakes and a boron powder at temperatures of 900–1000°C has been investigated. It has been found that the samples have “dendrite-like” and layered structures formed as a result of the dissolution of solid boron in liquid magnesium followed by crystallization. The obtained structures have been analyzed within the theoretical concepts of crystallization from melt.

Mechanism of the formation and specific features of the structure of massive samples of compound MgB2

Using different methods, it has been revealed that two MgB2 phases with the same hexagonal lattice, which differ in the contents of Mg and B (in the limits of the homogeneity range), as well as in the concentration of impurity oxygen and in the microstructure, are formed. The regions that correspond to these two phases of MgB2 have relatively large sizes (100–500 μm) and, alternatingly, fill the entire volume of the sample. It is assumed that the two-phase state of MgB2 is due to the specific features of the mechanism of its formation (when synthesizing at 800–1000°C), which includes the stage of the melting of magnesium, the dissolution of solid boron in the melt to a concentration that corresponds to the composition of the MgB2 compound, and the subsequent crystallization of the MgB2 compound in the melt with the formation of a dendrite-like structure, which is accompanied by an appropriate redistribution of the main components and impurities.

Structure and stability of superconducting core of single-core MgB2/Cu,Nb tube composite with a high critical current

The core of a single-core MgB2/Cu,Nb composite, which has been prepared by the ex-situ technique and exhibits a high critical current equal to 427 A (at 0 T and 4.2 K, jc ≥ 105 A/cm2), has been studied using various structural methods. Two kinds of MgB2 crystals were observed; those of the first kind is large, highly dense crystals characterized by a low oxygen content (2–8 at %) and the others are fine, weakly coupled crystallites characterized by high oxygen content (4–21 at %). To perform a comparative analysis of the structures, we have also studied an MgB2 bulk sample synthesized at 1000°C. It was found that two phases with the same lattice are formed; they differ in the magnesium and boron contents (within the homogeneity range), impurity oxygen content and microstructure as well but differ slightly in the lattice parameters. The two-phase state of MgB2 bulk sample is due to the mechanism of its formation, which includes the melting of magnesium, the dissolution of solid boron in it, and the crystallization of MgB2 from the melt with the formation of dendrite-like structure characterized by corresponding redistribution of components and impurities. To a certain degree, the two-phase structure of MgB2 bulk sample is inherited by the MgB2/Cu,Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the MgB2 compound is the destabilizing factor and leads to the transformation of the superconductor into MgO.

Structural state of second-generation HTSCs obtained by laser ablation

The structure of buffer layers and deposited YBa2Cu3Ox (Y123) films with a high critical current density (∼106 A/cm2) has been investigated by different methods. These superconducting films and buffer layers are found to have a fine-grained structure, which, along with the high texture of Y123 films of the (001) type, is believed to be responsible for the high critical current density. An unusual texture is revealed in buffer layers, which differs from that of substrates and Y123 films. The superconducting films deposited on buffer CeO2 layers exhibit a system of orthogonal lines; in the case under consideration, this is a manifestation of a domain structure with Y123 particles at boundaries.

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

The structure and properties of synthesized massive MgB2-based samples subjected to deformation with Bridgman anvils have been studied. Deformation results in the formation of fine-grained structure of the MgB2 phase, enhancement of interconnection of grains, complete disappearance of friable MgB2-phase areas, and abrupt increase in the microhardness.