Sequential electron beam evaporation of YBCO thin films through the barium fluoride route

Superconducting YBCO thin films have been fabricated by sequential electron beam evaporation of metallic constituents for yttrium and copper while barium is replaced with BaF2. The conversion of BaF2 BaO has been attained through wet oxygen annealing. Correlations of stoichiometry with microstructural, as well as superconducting, properties of the films have been examined. Results are given for YBa2Cu3O7– thin films on SrTiO3 (1 0 0) and YSZ (1 0 0) single crystal substrates. The usefulness of the BaF2 preparation route for the fabrication of mixed phase (Y123 + Y124) and pure Y124 is also demonstrated. Microstructural properties of Y123 thin films indicate different growth processes for the two substrates.     

Progress in Processing of Thin Wall HTc Bulk Superconducting Cryomagnets

We report on our progress in the preparation of single domain of high temperature superconducting (HTS) bulk YBCO and GdBCO cryomagnets with multiple holes. Three mains steps have been explored and discussed in this study:

1. Gd₂BaCuO₅ (Gd211) and Y₂BaCuO₅ (Y211) and thin wall preforms shaping by the extrusion technique.
2. GdBa₂Cu₃O₃ (Gd123) YBa₂Cu₃O₃ (Y123) single domain obtained from Y211 using the Seed Infiltration Growth (SIG) process.
3. Annealing under high oxygenation pressure in order to improve the superconducting properties.

The thin wall samples with larger specific areas offer some advantages as better thermal exchange, shorter oxygen diffusion paths, possibility to reinforce the material to overcome the mechanically stresses during magnetization. The process and the microstructural features at various stages of processing, with particular emphasis on the characteristics of Gd211 and Y211 inclusions are discussed. Properties like the magnetic trapped field at different temperatures, the superconducting transition temperatures, and critical current densities of the resulting composites are presented and compared with their values in samples processed by other variants of the melt textured growth process.     

Superconductor-Insulator Transition in Composite Granular (1−<Emphasis Type="Italic">n</Emphasis>)Gd123-<Emphasis Type="Italic">n</Emphasis>Pr123 and (1−<Emphasis Type="Italic">m</Emphasis>)Gd123-<Emphasis Type="Italic">m</Emphasis>PrBa123

Granular composite samples of GdBa₂Cu₃O_7– (Gd123), PrBa₂Cu₃O_7– (Pr123), and Pr_0.5Ba_0.5Ba₂Cu₃O_7– (PrBa123) have been prepared by the solid state reaction technique. The characterization of samples has been done by SEM and XRD measurements. We have investigated the effect of Pr123 and PrBa123 insulating grains on the superconductor-insulator transition and the normal state resistivity of the (1–n)Gd123-nPr123 and (1–m)Gd123-mPrBa123 systems. The dominant diffusion of Pr ions onto the neighboring Gd123 grains in (1–n)Gd123-nPr123 causes high rate of suppression of superconductivity similar to the chemical-doped GdPr123 system. For (1–m)Gd123-mPrBa123 system, the suppression rate is slower and we have superconducting sample with T_c^mid = 41 K for the second phase of m = 0.6 sample. Comparison of the superconductor-insulator and metal-insulator transitions in the granular samples and the chemical substituted GdPr123 indicates optimization of the superconducting state in (1–n)Gd123-mPrBa123 relative to (1–n)Gd123-nPr123 and GdPr123 systems. The more stable characteristic of PrBa123 relative to Pr123, when they are mixed with Gd123 grains, makes PrBa123 more suitable for any superconductor-insulator application. The mechanism of suppression of superconducting state for Pr123 has been discussed by comparison of experimental results against the hole filling and hole localization models presented for the insulating Pr123.     

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa₂Cu₃O₇ indicate that this compound is strictly isostructural with its superconducting RBa₂Cu₃O₇ (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa₂Cu₃O₇ series as a function of R³⁺ ionic radius. The sole structural anomaly attributable to the presence of Pr³⁺ in the YBa₂Cu₃O₇ structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa₂Cu₃O₇ supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa₂Cu₃O₇ structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr³⁺ (f ²) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa₂Cu₃O₇ remains unknown; however, superconductivity is turned back on for Nd³⁺ (f ³), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa₂Cu₃O₇ and nonmetallic PrBa₂Cu₃O₇ are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.     

Annealing dependence of the superconductivity of Pb0.65Sr2.35Y0.5Ca0.5Cu2Oy

The effect of annealing conditions on the superconductivity of the Pb-based 1 2 1 2 phase Pb_0.65Sr_2.35Y_0.5Ca_0.5Cu₂O _y system was studied. The weight change of the samples was measured by thermogravimetric analysis (TGA). The phases occurring in the samples were detected by X-ray diffraction (XRD) and their superconducting transitions were determined by alternating current (a.c.) susceptibility measurements. The results showed that the oxygen occupancy sites of the samples annealed under various conditions were different, and the oxygen content or oxygen occupancy site affected the phase formation of the sample. It has been found that there is an optimum value of oxygen content at which TC is the highest, and the highest TC can be obtained only within a very narrow range of oxygen content. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the stereochemistry of cations in the doping block of superconducting copper oxides

Metal-oxygen complexes containing Cu, Tl, Hg, Bi, and Pb cations are electronically active in superconducting copper oxides by stabilizing single phases with enhancedT _c, whereas other metal-oxygen complexes deteriorate copper oxide superconductivity. Cu. Tl, Hg, Bi, and Pb in their actual oxidation states are closed-shelld ¹⁰ or inerts ² pair ions. Their electronic configurations have a strong tendency to polarize the oxygen environment. The closed shelld ions with low-lyingnd ¹⁰nd ⁹(n+1)s excitations form linear complexes through hybridization polarizing the apical oxygens. Comparatively lownd ⁹(n+1)s excitation energies distinguish Cu^1+,3+, Tl³⁺, Hg²⁺, from other closed-shelld ¹⁰ ions deteriorating copper oxide superconductivity, e.g., Zn²⁺.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the linear term is presented and discussed.     

Degradation of Superconductivity in Annealed Irradiated Bi2Sr2CaCu2O8+δ Single Crystals During Exposure to Air

As for irradiated Bi₂Sr₂CaCu₂O₈₊ single crystals annealed in flowing N₂ atmosphere, flowing O₂ atmosphere, and air, their superconductivity was degraded during exposure to air. The degradation of superconducting transition temperature T _c is attributed to the doping of oxygen. The small recovery of T _c may be related to the redistribution of oxygen. The superconductivity of samples annealed in N₂ atmosphere and air is more stable than that of the sample annealed in O₂ atmosphere during exposure to air.     

Synthesis and Superconductivity of New Cuprates (Pb0.8W0.2)Sr2(Nd1 ? xCax)Cu2O7 ? δ

New cuprates with nominal composition (Pb_0.8W_0.2)Sr₂(Nd_{1 – x} Ca _x )Cu₂O_{7 –} (0x1) were synthesized by solid-state reaction in N₂. The crystal structure was characterized by X-ray powder diffraction (XRD) as tetragonal. Direct current electrical resistance measurements were applied to check the existence of superconductivity in these cuprates. Superconductivity with T _c (onset) up to 82 K is observed when x = 0.6. Synthesis in N₂ is necessary in obtaining superconductivity in these cuprates. Preparation in air or post-treatment in flowing oxygen destroys superconductivity. A comparison is made with previous Pb-based 1212 superconducting oxides. The valence of Pb and the possible position of W in the lattice are discussed.     

Analysis of indentation size effect (ISE) behavior in low-load Vickers microhardness testing of (Sm123)1−x(Nd123)x superconductor system

Indentation size effect (ISE) for (Sm123)_1?x(Nd123)_x superconducting samples which were fabricated by the solid state reaction technique for values of x = 0.00, 0.05, 0.10, 0.20, and 0.30 was investigated by analyzing the theoretical models. When the experimental data of a number of single crystals which have the different crystal structure and different chemical bonding inside the polycrystallined samples were analyzed with the ISE models, the sample encountering with resistance and elastic deformation was observed as well as plastic deformation. The microhardness values on different surfaces of materials were calculated by using Meyer Law, proportional specimen resistance model, modified proportional specimen resistance model, elastic/plastic deformation model and the Hays–Kendall (HK) approach. The results showed that the HK approach was determined as the most successful model. Furthermore, X-ray powder diffraction and scanning electron microscope measurements were analyzed for superconducting properties of (Sm123)_1?x(Nd123)_x superconductor system. The results showed that microhardness values at the minimum load and averaged plateau region of load increased with increase of Nd123 concentration. Nd123 content can be used as to be estimated the microhardness value of (Sm123)_1?x(Nd123)_x superconducting sample in the range of 0.878–2.717 GPa. The control of the microhardness value by using Nd123 content in (Sm123)_1?x(Nd123)_x superconducting structure can be useful in technological applications in superconductivity industry.     

Nonequilibrium kinetics of electrons and phonons in superconductors in intense UHF fields

The dependence of the superconducting gap of a thin superconducting film under irradiation by a UHF field is investigated for different values of the amplitude and frequency of the electromagnetic field and arbitrary temperatures. It is shown that the maximal stimulation of superconductivity does not exceed several percent. The distribution functions of nonequilibrium quasiparticles are found for different values of the parameters of the problem. A theoretical investigation of phonon emission from the film under UHF pumping is given. It is shown that in a small spectral interval of frequencies near the double gap value the film does not emit, but absorbs phonons from the substrate. This result is closely related to the stimulation of superconductivity by the UHF field and is due to the effective supercooling of the electron subsystem. The role of heating processes due to inelastic electron-electron collisions, which increase the total number of quasiparticles, is considered and their influence on the kinetics of nonequilibrium electrons and phonon emission is investigated.     

Synthesizing and Physical Properties of GdPr-123 High-Tc Superconductor

Ceramic Gd_1-xPr_xBa₂Cu₃O_7-y (GdPr-123) high-T_c superconductors have been synthesized by the standard solid state reaction method and characterized by XRD, SEM, TGA, and DT techniques in the range of x, 0.0 ≤ x ≤ 1.0 Samples have orthorhombic structure with Pmmm symmetry and there is a small percentage (less than 1%) of the Ba sites occupied by Pr ions in some Pr-rich samples. In these samples a small trace of BaCuO₂ and Pr BaO₃ secondary phases persist with the main peaks at 2θ = 29.3° and 28.9° respectively. Microstructure analysis indicates a uniform grain size distribution with a mean size of 5 μm. No significant change of grain size is shown throughout the range of x studied. The valence of Pr and Cu were determined by magnetization measurements in the temperature range 100 to 250°K. These data reveal a valence of 3.86+ for Pr in all samples independant of x. The similarity of the superconducting and insulating properties in this system to those of the oxygen deficient RBa₂Cu₃O₇ (R-123) (R: Y or rare earth) system implies that the mechanism of superconductivity in high-T_c. superconductors cannot be two-dimensional, even though the superconductive current occurs in two dimensions.     

Three-dimensional shape of the Y2BaCuO5 pattern in melt-textured Y–Ba–Cu–O oxide

Crystallographic orientations of the Y211 tracks that were trapped within the melt-textured Y123 domains were determined from the orientation relationship among the Y211 pattern, twin traces and a microcrack. Analysing the orientation of the two-dimensional Y211 patterns that were observed on various crystallographic planes of the Y123, the three-dimensional shape of the Y211 pattern was inferred as follows: only one x-type Y211 pattern was included within one Y123 domain and three sets of diagonal Y211 tracks met with the corners of the Y123 domain. The diagonal Y211 tracks lie on the (1 1 0), ( 1 0), (0 1 1), (0 1), (1 0 1) and (1 0 ) planes of the Y123 domain. The planes where the Y211 tracks are located are considered to be the boundary planes where local growing parts of the Y123 domain impinge upon each other. The formation mechanism of the Y211 pattern is discussed on the basis of anisotropic growth behaviour and the shape of the Y123 domain. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of long Y123 superconducting filament by solution spinning

The preparation of a long superconducting Y123 filaments is developed by using solution spinning for the precursor of the oxide. Long precursor filaments are prepared by dry spinning with a starting homogeneous aqueous PVA solution containing Y, Ba and Cu acetates. The as-drawn filament with 250 m in diameter and 300 mm in length is pyrolyzed and partial-melted by using a tube furnace with a constant heating zone of 500 mm. A superconducting filament of 60 m in diameter and 200 mm in length with high J_c of more than 10⁴A/cm² at 77 K and 0 T is obtained by controlling the partial melting.     

Fabrication of multi-filamentary Y123 superconductor

The precursor Y₁Ba₂Cu₃O_x (Y123) filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed Y, Ba and Cu acetates. The as-drawn filaments were heated to remove volatile components and to generate a superconducting phase. The filamentary superconductors were passed through Ag paste and sintered. By controlling the heating condition, the composite with excellent connection between the filaments and metallic Ag matrix was obtained. The overall transport critical current density (J_e) of more than 2000 A/cm² at 77 K and 0 T was achieved for the composite with partially melted Y123 filaments. Although the J_e value decreased by applying a low magnetic field less than 0.1 T, the superconductivity of the composite maintained at 10 T at 77 K.     

Exploring Multi-Component Superconducting Compounds by a High-Pressure Method and Ceramic Combinatorial Chemistry

In this short review, we provide some new insights into the material synthesis and characterization of modern multi-component superconducting oxides. Two different approaches such as the high-pressure, high-temperature method and ceramic combinatorial chemistry will be reported with application to several typical examples. First, we highlight the key role of the extreme conditions in the growth of Fe-based superconductors, where a careful control of the composition-structure relation is vital for understanding the microscopic physics. The availability of high-quality LnFeAsO (Ln = lanthanide) single crystals with substitution of O by F, Sm by Th, Fe by Co, and As by P allowed us to measure intrinsic and anisotropic superconducting properties such as H _c2, J _c. Furthermore, we demonstrate that combinatorial ceramic chemistry is an efficient way to search for new superconducting compounds. A single-sample synthesis concept based on multi-element ceramic mixtures can produce a variety of local products. Such a system needs local probe analyses and separation techniques to identify compounds of interest. We present the results obtained from random mixtures of Ca, Sr, Ba, La, Zr, Pb, Tl, Y, Bi, and Cu oxides reacted at different conditions. By adding Zr but removing Tl, Y, and Bi, the bulk state superconductivity got enhanced up to about 122 K.     

Interface reactions between YBa2Cu3O7−x and W,Si or WSi2

The chemical compatibility between YBa₂Cu₃O_7–x (Y123) and W, Si or WSi₂ was studied using quasi-infinite diffusion couples which were encapsulated and annealed at 650 to 800C for 5–80 h. The phases formed at the interface and the morphology and growth kinetics of the reaction zone were analysed in cross-sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. A Si/Y123 sample was also studied using secondary ion mass spectroscopy (SIMS). In addition, bulk powder mixtures of Y123 with W, Si or WSi₂ were equilibrated at 800C for 100–300 h and phase analysis was performed using X-ray diffraction.The following primary reactions are identified, for tungsten: 6YBa₂Cu₃O₇+7W 2BaWO₄+5Ba₂WO₅+3Y₂O₃+18Cu and for silicon: 2YBa₂Cu₃O_6.5 +3Si2BaSiO₃+Ba₂SiO₄+Y₂O₃+6Cu. The reactivity of WSi₂ is not much smaller compared to that of elemental W and Si. The results of diffusion couples and bulk samples are consistent and complement each other, the dominant product phases are observed in both experiments. A diffusion controlled mechanism with a parabolic growth law and a relatively small activation energy (Q = 44 kJ mol^–1) is verified for the W/Y123 reaction. In both the W/Y123 and Si/Y123 couples the dominantly diffusing species are barium and oxygen. Barium diffuses far through the reaction zone to form double oxides, leaving a fine grained Y-Cu-O layer behind (mostly Y₂O₃+Cu), and eventually the entire superconductor phase Y123, being depleted of Ba, turns into this Y-Cu-O mixture.     

Composition and superconductivity in Bi-Sr-Ca-Cu-O and substituted systems

The composition of the superconducting phase Bi₄(Sr_3±x Ca_3x )₌₆Cu₄O₁₆, has been determined by means of a sysmetic investigation of the relationship between composition and superconductivity, and phase analysis. The superconducting phase belongs to tetragonal system witha=0.3825 nm,c=3.082 nm, space group (S.G.) 14/mmm. The effect of strontium and calcium contents in the Bi(Sr_1±x Ca_1x )₌₂Cu₂O_5.5, system and substitution in the Bi_1–x A _x SrCaCu₂O₅ (A=V, Gd, Y) and BiSrCa_1–x Pb _x Cu₂O_5.05 systems on superconductivity, have been investigated. In the Bi-Sr-Ca-Cu-O system, there are two superconducting phases corresponding to transition temperatures of 110 and 80 K, respectively. The difference between the two superconducting phases may be due to the stacking difference of atoms along thec direction in the structure and their oxygen content.     

Superconductivity of heat-treated Nb-65 at.% Ti alloy

Brief reviews are given of the effect of heat-treatment on the microstructure of cold-worked bcc metals and the superconducting properties of niobium alloys. Particular attention is paid to the influence of interstitial impurities in these processes. The annealing effects in microstructure and superconducting properties of a cold-worked Nb-65 at.% Ti alloy, containing oxygen as a major impurity, have been studied. The precipitation process takes the form TiNb + O₂ various Ti oxides Ti + TiO. Differences in precipitation sequence are described for vacuum-annealing and annealing in impure argon. Flux-pinning is related to the microstructural observations. At least three pinning mechanisms appear to operate; dislocation pinning and two types of precipitate pinning. These observations are in accord with previously proposed pinning models.     

Ion migration in high-T c superconductors and the nature of the conducting state ind-wave superconductors

The available experimental data on the ion migration in superconducting Y-123 and Bi-2223 films under the effect of high transport currents are summarized. Irrespective of structural perfection, electrostimulated diffusion of heavy ions and oxygen gives rise to the formation of new, stable phases and partial degradation of superconductivity. A model taking into account the proximity of the superconducting, structural, and magnetic transitions in high-T _c materials is used to explain ion migration as the response of the system of ions and coherentd electrons to the external electric field. Thed-wave symmetry superconducting state is shown to possess combined, rotational + translational invariance, rotational symmetry being due to the motion of electrons in the curl field of the ions, playing the role of topological defects.