Blocky and spheroidal growth in Y1.5Ba2Cu3Ox system

Solidification of Y_1.5Ba₂Cu₃O_x system was studied with the aim to find the temperature of self-nucleation. The microstructure after cooling from isothermal dwells in the temperature range 984–995 °C was analysed. It is shown that blocky YBa₂Cu₃O_7?δ crystals were self-nucleated and grown during isothermal dwells below 988 °C and spheroidal multicrystals were formed during cooling from isothermal dwells at higher temperatures. The symmetrical and radial YBa₂Cu₃O_7?δ crystal branches of spheroids grow in the 〈110〉 crystal direction.     

Magneto‐transport properties of polycrystalline YBa2(Cu1‐xMx)3O7‐δ (M = B and Mn)

The magnetic and transport properties of polycrystalline YBa₂ (Cu_1‐xM_x)₃ O_7‐δ (M = B and Mn) superconductor was investigated. Samples of YBa₂(Cu_1‐xB_x)₃O_7‐δ doped with several concentrations of boron B(x = 0.05 and 0.1) were investigated using magnetization measurements. A YBa₂(Cu_1‐xMn_x)₃O_7‐δ sample doped with Mn with concentration of x = 0.02 was investigated using current‐voltage (I‐V) measurements. Our results on the YBa₂(Cu_1‐xB_x)₃O_7‐δ samples reveal a considerable increase in the hysterisis width of the magnetization, M versus the applied magnetic field H with increasing boron concentration. The lower critical field was also found to be enhanced by boron doping. The critical current density, J_c was found to be significantly enhanced in the Mn‐doped sample. The enhancement of J_c was found to be more significant at the lower temperatures for all applied magnetic fields used (0 Oe, 300 Oe, and 500 Oe). Thus, chemical doping is suggested to enhance the vortex pinning forces in the YBCO samples. From the resistivity (R‐T) measurements, chemical doping of the samples was found to have no significant effect on the critical temperature, T_c. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization features of YBa2Cu3O7‐δ in the Y2BaCuO5‐BaCuO2‐CuO and Y2Cu2O5‐BaCuO2 systems

Based on the data of X‐ray phase and microstructure analysis, the sample composition was optimized in order to provide maximum size of the textured macrograins of YBa₂Cu₃O_7‐δ and of the crystallites in the Y₂BaCuO₅‐BaCuO₂‐CuO, Y₂Cu₂O₅‐BaCuO₂ systems. The growth rate has been studied and the YBa₂Cu₃O_7‐δ growth activation energy has been calculated for the samples of Y₂BaCuO₅+3BaCuO₂+2.3CuO, Y₂BaCuO₅+3BaCuO₂+0.6CuO, and Y₂Cu₂O₅+3.5BaCuO₂ compounds in the temperature range of 1240‐1270K for the case of use of the Y₂Cu₂O₅ and Y₂BaCuO₅ precursors with an average grain diameter of 10 μm and 1mm. A crystallization mechanism of YBa₂Cu₃O_7‐δ in the Y₂BaCuO₅‐BaCuO₂‐CuO and Y₂Cu₂O₅‐BaCuO₂ systems in the case of different sizes of Y₂BaCuO₅ and Y₂Cu₂O₅ precursor grains was proposed and validated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic analysis and technology of preparation of high-Tc superconducting system YBa2Cu3O7−x

By modeling the equilibrium state of the phase formation reaction of High-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) and using the results of P-T−x correlation investigation of some authors, the variation of Gibbs free energy (ΔGm) versus temperature has been calculated. On the basis of the variation of ΔGm, the technology procedure of preparation of phase 1:2:3 with desired x has been established to meet requirement of basic investigation and application of High-T_c superconductors YBa₂Cu₃O_7−x.     

Phase Transformations in the Y2BaCuO5, «Ba3Cu5O8» and Y2BaCuO5‐«BaCuO2» Systems

Peritectic interaction of Y₂BaCuO₅ and Y₂BaCuO₉‐d phases with the Y‐Ba‐Cu‐O system flux melt has been modeled. The sequence of phase transformations in the Y₂BaCuO₅‐«Ba₃Cu₅O₈» and Y₂BaCuO₅‐«BaCuO₂» systems when forming the YBa₂Cu₃O₇‐d single crystal compound has been established.     

Oxygen potentials,stoichiometry, superconducting,and magnetic properties in the Y?Ba?Cu?O system

The relationship between stoichiometry in YBa₂Cu₃O_x and oxygen potential was investigated by barometry and compared with in-situ X-ray diffraction experiments between 300 and 1000 °C (6 < x < 7). The physical properties are influenced even by small deviations in x. Y₂BaCuO₅ was included in the investigations and found to be antiferromagnetic with T_N ≈ 30 K.     

The thermodynamic equilibrium of the formation process of High-Tc superconducting phase YBa2Cu3O7−x

Analysing the equilibrium state of the formation reaction of the high-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) on the base of the pressure-temperature-oxygen content (P-T−x) correlation research and using the Van't-Hoff equation, the Gibbs free energy (ΔG_m) of the reaction has been calculated for the first time. It is seen that the absolute value of ΔG_m is not large and of the order of 2.5 kcal · mole⁻¹. This leads to an instability of the 1:2:3 phase. Also, from the calculated values of ΔG_m relative stability regions of the 1:2:3 phase are determined for different values of x. Combinatively analysing the relation lg P = A−B/T (where A – the x-dependent constant, and value of B was determined to be equal to 10518 atm. K) and the value of ΔG_m, we come to the conclusion that the synthesis of YBa₂Cu₃O_{7 − x} when x → 0 must be carried out under high oxygen pressure and low temperature for a lasting time, because the reaction rate is limited by slow diffusion in the solid phase.     

Nucleation of an MTG YBa2Cu3Oy

The kinetics of the appearance of the superconducting 123 phase in partially melted at 1200 °C YBCO samples has been studied in the temperature range of low undercoolings from 1040 °C to 920 °C. An EDS analysis of the phases formed at different stages of the cooling of the melt has been performed and the experimental conditions for crystallization of 123 phase with finely dispersed 211 phase in it have been established. The results have been interpreted in terms of the classical theory of nucleation and they have been applied to the MTG process for formation of highly textured YBa₂Cu₃O_y materials.     

Growth of ultrathin twin-free <Emphasis Type="Italic">b</Emphasis>-oriented YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 – <Emphasis Type="Italic">x</Emphasis></Subscript> films

Twin-free b-oriented YBa₂Cu₃O_{7 – x} films with a thickness less than 40 nm have been epitaxially grown on (100)SrLaGaO₄ crystals. Based on the temperature dependence of resistance, the onset temperature of the transition to the superconducting state is found to be 90 K; the transition width is 4 K. The film growth has been performed in two stages. A (100)PrBa₂Cu₃O_{7 – x} buffer layer was previously grown on a (100)SrLaGaO₄ substrate by rf magnetron sputtering in an Ar–O₂ gas mixture at a continuous and monotonic increase in temperature from 660 to 830°C. The main YBa₂Cu₃O_{7 – x} film was grown on the buffer layer surface by pulsed laser deposition in an oxygen medium at a fixed temperature (800°C). The above processes were implemented in different chambers, which were connected by a vacuum channel for transporting samples. Both films were grown in situ, without contacting atmosphere in all growth stages. An X-ray diffraction study has shown that the YBa₂Cu₃O_{7 – x} films are single-crystal and free of precipitates of other phases and domains of other orientations.     

The influence of small deviations from stoichiometry on the properties of Y?Ba?Cu?O superconductor ceramics

According to a central composite design samples with deviations from stoichiometry YBa₂Cu₃O_7−x of ±2.5 wt% in the ingredients were prepared. The influence of these deviations on phase content, superconducting properties and microstructure of the ceramics was studied.     

Mechanical Damping of YBa2Cu3O7 –x at Temperatures above 300 K

Damping of ultrasonic waves with frequencies in the 100 kHz region was measured in the temperature region 300 … 1000 K. Two peaks could be observed (near 400 K or near 700 K, respectively). Occurrence of these peaks depends on oxygen content of the sample. The higher temperature peak is attributed to phase transition Pmmm 〈– – –〉 P 4/mmm of YBa₂Cu₃O_{7 – x}.     

Specific heats of FCC solid solutions of the Bi2O3-Y2O3 system

For the first time specific heats of the face centered cubic solid solutions (Bi₂O₃)_1−x(Y₂O₃)_x in the temperature range 350–1000 K were measured by means of Differential Scanning Calorimetry. Samples were also investigated with thermal expansion. The results showed the phase transitions for (Bi₂O₃)_0.75 (Y₂O₃)_0.25, which is acsribed to δ → δ* change in lattice mentioned in literature and for (Bi₂O₃)_0.58(Y₂O₃)_0.42, which is connected with a metastable behaviour of this sample.     

Growth of YBa2Cu3O7 films with [110] tilt of CuO planes to surface on SrTiO3 crystals

YBa₂Cu₃O₇ films with the CuO plane tilted to the surface have been grown on SrTiO₃ crystalline substrates by pulsed laser deposition. This tilt was obtained by rotating the film lattice with respect to the substrate surface around its [110] axis oriented parallel to the surface. The zero tilt of the CuO plane was implemented at the orientation of the SrTiO₃ crystal surface parallel to the (100) plane. The rotation angles were varied in the range from 0° to 70°. It is found that, being tilted at any angle, the CuO planes of the film remain parallel to one or several crystallographic planes of the (100)-type substrate. In the range of tilt angles from 0° to 49°, the film has a single-crystal structure. Above 49° the film is transformed into a three-domain texture and its surface roughness sharply increases.     

Structure and morphological stability of (Lu1–xEux)2O3 thin films

(Lu_1–xEu_x)₂O₃ smooth, crack‐free, transparent films were prepared by the Pechini sol–gel method and a spin‐coating technique. Thermogravimetric analysis, differential thermal analysis and FITR spectroscopy were used to study the chemical processes during annealing of the films. Film structure, morphology and optical properties were investigated. X‐ray diffraction (XRD) analysis reveals the cubic phase of (Lu_1–xEu_x)₂O₃ films annealed in the 600–1000 °C temperature range. Smooth and crack‐free films with thicknesses of 250–1000 nm were obtained in the 600–800 °C temperature range. The thickness upper limit (1000 nm) of morphological stability of films (Lu_1–xEu_x)₂O₃ on sapphire substrates has been studied.     

Optical properties of non‐centrosymmetric mixed crystals K2(La1‐x Cex)(NO3)5 · 2 H2O (x = 0.0 – 1.0)

Large single crystals of optical quality of the non‐centrosymmetric orthorhombic potassium rare earth nitrate mixed crystals K₂(La_1–x Ce_x)(NO₃)₅ · 2 H₂O were grown at 38 °C from diluted HNO₃. For crystals with x = 0.0, 0.19, 0.38 and 0.66 refractive indices and their dispersion were determined with an error less than 1 · 10^–4 in the wavelength range 0.404 – 1.083 μm by the prism method. Phase matching conditions for collinear SHG frequency conversion were analysed in detail, including calculation of the effective nonlinear optical susceptibility. By an appropriate choice of the fraction x of cerium the mixed crystals K₂(La_1–x Ce_x)(NO₃)₅ · 2 H₂O allow an adjustment of non‐critical type I phase matching conditions to a desired wavelength of the fundamental wave within the range 1.055(4) – 1.107(6) μm. Non‐critical type II phase matching can be tuned in the wavelength range 0.949(2) – 0.931(2) μm. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature variation of the structural and thermal characteristics of YBa2Cu3O7−x in the range from 90 to 400 K

The X-ray studies of the tetragonal YBa₂Cu₃O_7−x compound are performed at different temperatures from 400 to 90 K and the temperature variation of the thermal expansion coefficients and the Debye characteristic temperature is determined. The anisotropy of thermal expansion is investigated. It is found that the two dynamical characteristics monotonously decrease with temperature lowering whereas for the superconducting orthorhombic modification their temperature dependences are anomalous. The mean thermal expansion coefficient as well as the overall Debye temperature for the tetragonal phase are smaller than those for the orthorhombic one.     

Nd3+:Y3Al5O12 laser ceramics: Influence of the size of yttrium oxide particles on sintering

The influence of the size of Y₂O₃ powder particles on the structure formation and densification of Nd³⁺:Y₃Al₅O₁₂ laser ceramics has been studied. It is shown that the use of 50- and 100-nm yttrium oxide particles makes it possible to synthesize single-phase yttrium aluminum garnet at temperatures of 1200 and 1500°C, respectively, whereas in the case of 5000-nm yttrium oxide particles 2-h exposure at a temperature of 1500°C yields only 80 wt % of the Nd³⁺:Y₃Al₅O₁₂ phase. Bulk swelling of pressed samples during sintering of 2.94Y₂O₃-0.06Nd₂O₃-5Al₂O₃ powders with the size ratio of the initial particles R(Al₂O₃/Y₂O₃) ～ 5 is observed. The application of different-sized powders (R ～ 2.5) provides quantitative ratios between phases in the 3Y₂O₃-5Al₂O₃ system at which shrinkage in a temperature range of 20–1500°C is dominant. Laser ceramics 0–2 at % Nd³⁺:Y₃Al₅O₁₂ have been obtained by the solid-phase sintering of oxide powders (R ～ 2.5). The slope efficiency for 1 at % Nd³⁺:Y₃Al₅O₁₂ laser ceramics is found to be 33%.

     

Interface-induced microstrain in La0.67Ca0.33MnO3/YBa2Cu3O7 superlattices

Epitaxial La_0.67Ca_0.33MnO₃/YBa₂Cu₃O₇ superlattices with varying modulation lengths are produced by pulsed laser deposition and their detailed structural characterization is performed by X-ray diffraction. It is shown here that YBa₂Cu₃O₇ layers in all superlattice specimens have considerably high microstrain values, which are probably interface induced. The large distortions in the vicinity of the interfaces may alter the electronic structure and, hence, may have important consequences on the electrical and magnetic properties of the superlattice systems.     

Direct observations of pseudo-binary NdBa2Cu3Ox–Ba3Cu10O13 phase diagram and NdBa2Cu3Ox crystallization under reduced oxygen atmosphere

The pseudo-binary NdBa₂Cu₃O_x–Ba₃Cu₁₀O₁₃ phase diagrams and the crystallization of NdBa₂Cu₃O_x have been in situ observed using high-temperature optical microscope under three different oxygen atmospheres namely 1%, 0.1% and 0.0097% oxygen in argon. It has been observed that the liquidus line becomes narrower both in composition and temperature with decreasing oxygen pressure. This result suggested that under reduced oxygen atmosphere, the NdBa₂Cu₃O_x crystals could only be grown from a peritectic melt consisting of Nd₄Ba₂Cu₂O₁₀ and liquid. The crystallization temperature of NdBa₂Cu₃O_x was found to decrease logarithmically with decreasing oxygen content in the atmosphere.     

X‐ray diffraction analysis of powder and thin film of (Gd1‐x Yx)2O3 prepared by sol‐gel process

The mixed oxide (Gd_1‐xY_x)₂O₃ (0.0 ≤ x ≤ 1.0) were synthesized, as powder and thin film, by a sol‐gel process. X‐ray diffraction data were collected and crystal structure and microstructure analysis were performed using Rietveld refinement method. All samples were found to have the same crystal system and formed solid solutions over the whole range of x. The cationic distribution, Gd³⁺ and Y³⁺, over the two non‐equivalent sites 8b and 24d of the space group Ia3 is found to be random for all values of (x). The lattice parameter is found to vary linearly with the composition (x). Replacing Gd³⁺ and Y³⁺ by each other introduces a systemic decrease in the x‐coordinate of cation position (24d) and slight changes in the oxygen coordinates. Crystallite size and microstrain analysis is performed along different crystallographic directions and anisotropic changes are found with the composition parameter (x). The average crystallite size ranges from 75 to 149 nm and the r.m.s strain from 0.027 to 0.068 x10^‐2. Textured Gd_1.841Y_0.159O₃ (400) buffer layers, with a high degree of alignment in both out‐plane and in‐plan, are successfully grown on cube textured Ni (001) tape substrates by sol–gel dip coating process. The resulting buffer layers are crack‐free, pinhole‐free, dense and smooth. YbBa₂Cu₃O_7‐x (YbBCO) thin film could be (00l) epitaxially grown on the obtained buffer layer using sol–gel dipping technique. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)