I. B. Bobylev

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.

Effect of low-temperature annealing on the critical parameters of highly textured YBa2Cu3Oy

The effect of treatment at a temperature of 200°C and the natural aging on the critical parameters of a highly textured yttrium barium cuprate YBa2Cu3O6.9 has been investigated. It has been shown that non-superconducting (at T = 77 K) particles precipitated during phase decomposition of this compound are effective pinning centers. At 200°C, the YBa2Cu3Oy compound interacts with atmospheric moisture. This inter-action results in the formation of stacking faults, which also provide pinning of magnetic vortices. The structural changes occurring during low-temperature annealing and natural aging of the compound lead to an increase in the critical current density and the first critical field. The presence of pinning centers of different nature in the structure causes a synergistic effect, which significantly increases the current-carrying capacity of materials, including those in strong magnetic fields.

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.

Effect of low-temperature treatment and subsequent high-temperature annealing on the critical current density of YBa 2 Cu 3 O y

The field dependences of the critical current density of the HTSC compound YBa2Cu3Oy recovered at T = 920–950°C after the low-temperature treatment have been investigated. At T = 200°C, structural defects are formed in a wet environment, which are capable of initiating pinning of magnetic vortices. A short-term (1–3 h) recovery annealing performed at T = 930–950°C leaves in the samples a fairly large amount of structural defects formed during the low-temperature treatment, which results in a substantial increase in the critical current density in magnetic fields of ∼2 T as compared to the ceramics not subjected to double annealing. A longer high-temperature treatment removes the structural defects formed and brings the electrophysical properties of YBa2Cu3Oy to the level characteristic of the ceramics produced by standard technology.

Effect of water vapor and low-temperature decomposition on the structure and electrophysical properties of nonstoichiometric compound Ba2YCu3O7 − δ

The effect of water vapor and decomposition into phases with different oxygen contents on the structure and electrophysical properties of the nonstoichiometric compound Ba2YCu3O7 − δ has been studied. Both these factors were shown to lead to the formation of structural defects that can be pinning centers and improve the current-carrying capacity of materials. At the same time, the reaction with water leads to a deterioration of superconducting properties of grain boundaries. The optimum electrophysical properties of Ba2YCu3O7 − δ can be reached by choosing appropriate heat-treatment conditions, namely, the temperature and time of annealing and the water-vapor content in the annealing atmosphere. The phase diagram of the Ba2YCu3O6-Ba2YCu3O7 system below 400°C is refined.

Electrophysical properties and the structure of the YBa2Cu3Oy compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa2Cu3Oy restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.

Influence of chemical composition on the stability of YBa2Cu3Oy in a humid atmosphere

The influence of replacements of yttrium and barium in YBa2Cu3Oy and doping of this compound with CeO2, ZrO2, and Pr2O3 on the capability of absorbing water from the annealing atmosphere at T = 200°C has been studied. The complication of the chemical composition of the 123-type compounds leads to an enhancement of their stability to aggressive components of the gaseous phase (H2O, CO2). These compounds have a variable chemical composition both in oxygen and water and can exist in three modifications: the oxygen-ordered ortho-phase, the oxygen-disordered tetra-phase, and the pseudocubic phase with a variable water content, which at T = 200°C separates into an oxygen-deficient water-enriched phase and an oxygen-rich anhydrous phase.

Effect of the atmosphere of low-temperature annealing on the structure and electrophysical properties of nonstoichiometric YBa2Cu3O7 − δ ceramics

The fine structure and electrophysical properties of nonstoichiometric YBa2Cu3O7 − δ ceramics and the effect of low-temperature annealing (t ⩾ 200°C) in various atmospheres on these parameters have been studied. It has been shown that, during annealing in a vacuum, the decomposition is quite sluggish; structures typical of initial stages of decomposition are observed. The decomposition in an inert-gas atmosphere occurs more actively, and structures typical of stages of deep decomposition are realized. It has been found that, during low-temperature annealing, the structure and properties are affected by two factors; these are the decomposition into phases differing in the oxygen content, and water absorption, leading to the transformation with the formation of a pseudo-cubic lattice. The annealing atmosphere substantially affects the kinetics of both processes.

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.

Effect of the oxygen partial pressure on the kinetics of decomposition of the Ba2YCu3O7−δ phase at temperatures <400°C

The effect of the oxygen partial pressure on the spinodal decomposition in the Ba2YCu3O6—Ba2YCu3O7 system has been studied. A decrease in the oxygen partial pressure was found to increase the decomposition rate. The most probable cause of the effect consists in the decrease of oxygen concentration in the near-boundary regions of grains.