Formation of High-Tc Superconducting Bi-Pb-Sr-Ca-Cu Oxide Films by Spray Pyrolysis of an Oxalate Suspension

A Bi-Pb-Sr-Ca-Cu oxide superconducting thick film, exhibiting T_c(onset) = 120 K and T_c(zero) = 100 K, can be obtained by spray pyrolysis of oxalate suspension on the (100) MgO substrate, using the partial-melting state at 860°C for 15 h, a rapid-quenching process to 500°C, and a postannealing treatment for 1 h at 835°C. These processes are believed to enhance the volume fraction of 2223 phase and increase the T_c(zero) . The films show preferred orientation and adhere well to the substrate.     

Atomistic Simulation of High-Tc Oxides

Atomistic lattice simulations are reported of the normalstate crystal properties of high- T _c oxides. Nonstoichiometry in La₂CuO₄ and Nd₂CuO₄ associated with oxygen interstitial defects is examined in some detail. The lattice structure and defect chemistry of Ca₂CuO₃, Sr₂CuO₃, La₂SrCu₂O₆, and Sr₂CuO₄ are also considered. Finally, Jahn-Teller effects and bipolaron formation are addressed in relation to high- T _c behavior.     

Preparation of High-Tc Superconducting Oxides by the Amorphous Citrate Process

The high-T_c superconducting oxide Yba₂Cu₃O_7–y has been prepared by the amorphous citrate process. A citrate-nitrate solution was prepared at room temperature and dehydrated at around 80°C to yield a solid precursor material. The precursor was fired to high temperature to form the desired compound. The decomposition mechanism was studied with thermogravimetric analysis and X-ray diffraction. Intermediate products were formed during the decomposition reactions. Nearly phase-pure Yba₂Cu₃O_7-y powders were obtained by firing the precursor in air to 900°Cfor2 h. Sintered samples were typically over 90% dense and exhibited good superconducting properties.     

High-Tc Hydroxide- and Fluoride-Containing Ceramic Superconductors

Experimental data for maximum T _c , up to 125 K with temperature cycling, are reported with regard to Y-Ba-Cu-O.OH systems. This increase in T _c is presumably associated with structural transition or phase equilibrium for [CuO₆] local octahedra, occupied by hydroxyl or fluorine ions in the oxygen vacancy sites. Possible improvement in T _c using hydroxide ceramic compounds is described briefly.     

Coaxial Lines Containing Ceramic High-Tc Superconducting-Center Conductors

A superconducting line should be essentially lossless and dispersionless and have a purely resistive impedance up to at least 10 GHz. In fact we expect that a long (>1 km) line could carry nanosecond rise time pulses with zero distortion and low loss (<1 dB/km). We have measured the losses in some short sections of coaxial line containing ceramic high-T_c superconducting-center conductors over the frequency range from 5 to 50 MHz. There is only a slight improvement over a completely copper coaxial line.     

Effect of Purity of Raw Materials and Processing on the Microstructure and Properties of High-Tc Ba2YCu3O7-x

Microstructure and properties of high- T _c Ba₂YCu₃O_{7- x} ceramics were studied as a function of purity of raw materials and processing conditions. Technical-grade (TG) and reagent-grade (RG) raw materials were used to prepare the precursor superconducting powders, the characteristics of which were monitored by XRD, chemical analysis, and SEM. Bulk specimens were then prepared with the two master batches. XRD microphase analysis and SEM-EDS investigation did not show phase differences. J _c and normal-state resistivity measurements indicated that the TG specimens had more resistive grain boundaries than did RG specimens.     

High-Tc Superconductor Coating on Metal Substrates by an Electrophoretic Technique

High-T_c superconductor wire and sheet were fabricated by electrophoretic deposition of superconductor powders on silver-metal substrates followed by an appropriate sintering schedule. The technique has some promise for technological applications.     

Compact Swelling in Bi1.4Pb0.6Sr2Ca2Cu3.6Oy During the Formation of High-Tc Superconducting Phase

Compact swelling in Pb-doped Bi-Sr-Ca-Cu-O superconductor has been studied by observing the effects of the size of calcined powders, volatilization of materials, and sintering of high- T _c (2223) powders. The bulk density increases at the early stage of sintering, for about 20 h, and then decreases. Densification occurs when the low- T _c (2212) phase and a liquid phase exist, whereas dedensification occurs with the formation of the 2223 phase regardless of the presence of the liquid. Gas evolution from specimens does not appear to be responsible for compact swelling. Compact swelling is explained by anisotropic growth of thin, platelike 2223 grains in random orientation. When 2223 grains grow in a preferred direction, compact swelling is suppressed.     

The Use of Nitrogen Dioxide to Improve the Superconducting Properties of YBa2Cu3O7

The elemental homogeneity of YBa₂Cu₃O₇ powders can be improved substantially by heating the powder in a nitrogen dioxide-containing atmosphere (e.g., 950°C), followed by annealing in oxygen above 750°C, and slow cooling to room temperature. The improved homogeneity results in a substantially larger flux exclusion signal for the NO₂-treated powder, as measured by ac susceptibility. Moreover, the NO₂-processed powder exhibits a slablike morphology which should be more suitable for grain alignment. A substantial advantage of the NO₂ process is that this process is easily scaled to larger batches and the results are highly reproducible. This is not the case for solid-state reaction processes requiring repeated heating and grinding. The experimental results suggest a mechanism which involves the formation of a small amount of molten Ba(NO₃)₂ which acts as a flux that dissolves the constituents and reprecipitates them as highpurity YBa₂Cu₃O₇. The effects of the various process variables on the properties of the treated powder, and the reproducibility of the process, are discussed.     

Humidity Effect on the High-Tc (Pb,Bi)SrCaCuO Superconductor

The effect of humidity on the bismuth superconductor is investigated under 90% humidity at 30°C for 250 h. Nearly none of the peaks of high- T _c phase (2223) can be observed in the X-ray pattern after the test. The silver contact resistance of the sample also increases prominently in the time interval from 60 h to 100 h. When the humidity-tested sample is heated again under sintering conditions (845°C for 50 h in air), the high- T _c phase appears again.     

Metal Dopants in Bi-Pb-Ca-Sr-Cu-O High-Tc Superconductor Thick Films Prepared by Melt Solidification

Metal-doped superconductor thick films Bi_1.6Pb_0.4M_0.1 Ca₂-Sr₂Cu₃O_x (M = V, Nb, Mo, In, Sn, Sb, and W) were prepared using a melt–solidification procedure. The optimum heat-treatment temperature T _opt was observed in all the films with different dopants; T _opt was related to the temperature at which a partial melting of the films occurred as determined by differential thermal analysis. The dopants in the melt-solidified films lower the partial melting temperature and thus lower T _opt for the formation of the 110 K phase.     

Stability of YBa2Cu3O7-x in Molten Chloride Salts

The decomposition products of YBa₂Cu₃O_7-x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl₂, CaCl₂, or MgCl₂ promote formation of CuO, Cu₂Y₂O₅, and loss of barium to the chloride salt as BaCl₂. Salts based on BaCl₂ or containing LiCl result in YBa₂Cu₃O_7-x decomposition products of Y₂BaCuO₅, CuO, and BaCl₂. High barium activity in the salt supports formation of the Y₂BaCuO₅ phase and reaction of CO₂ with the salt producing BaCO₃. Decomposition is most sluggish in binary NaCl-KCl salts where minimal amounts of reaction or decomposition products are observed.     

Preparation of the High-Quality Superconductor La1Ba2Cu3Oy

The relationship between the preparation procedure and superconducting properties of La₁Ba₂Cu₃O_y was studied. A series of samples was sintered in an N₂-gas atmosphere for various lengths of time ranging from 1 to 40 h, followed by a fixed annealing procedure in O₂. It was found that the shorter the sintering period, the higher was the oxygen content. The samples sintered for a period of less than 15 h contained excess oxygen compared with La₁Ba₂Cu₃O₇ and exhibited poor superconducting properties. The sample sintered for 40 h had an oxygen content y equal to 6.95, and had excellent superconducting properties. The mechanism for preparing high-quality La₁Ba₂Cu₃O_y is discussed.     

Melt Processing of YBa2Cu3O7-x

Sintering YBa₂Cu₃O_{7- x} bulk forms at 1050°C followed by annealing at 980°C causes the development of a thick oriented surface layer (Lotgering factor = 0.7). The thickness of the layer depends on the thermal treatment, which is a two-step sintering process. Firing at 1050°C for 2.5 h followed by 30 h at 980°C leads to the development of a 0.1-mm-thick surface layer, with clear indication that longer annealing would result in a thicker film. Some orientation develops during un-axial compaction of the powders. Lotgering orientation factor calculation from X-ray diffraction analysis. SEM, and TEM were used to characterize the microstructure of these samples. T _c was similar to that of conventionally processed high-density samples, between 83 and 87 K. Some thermal treatments resulted in samples that displayed high resistivity above T _c , possibly caused by segregation of Cu to the grain boundaries.     

Thin-Layer Dielectrics in the Pb[(Mg1/3Nb2/3)1-xTix]O3 System

Thin layers of Pb[(Mg_1/3Nb_2/3)_{1– x} Ti _x ]O₃ (PMNT) were prepared by spin casting alkoxide-based solutions on platinized Si. The effects of additives, heat treatment, and composition ( x = 0 to 0.9) on perovskite phase development, ceramic microstructure, and dielectric properties are reported. Depending upon the processing conditions, ceramic thin layers could be formed in a nonferroelectric pyrochlore phase (A₂B₂O₆) or in a ferroelectric perovskite phase (ABO₃). The dimensions of the pyrochlore and perovskite units cells were related and increased with Mg and Nb contents. To minmize pyrochlore formation, the most effective processing method involved rapid heat treatment between successive solution depositions. Phase development and microstructure were also affected by solution additives. Additions of benzoic acid were found to affect the structure in solution and the later organic pyrolysis behavior from thin layers. The effect of composition on the dielectric and ferroelectric properties is also reported.     

Enhancement of Tc(0) by Substitution of Gallium in the Bismuth-Based High-Tc Superconducting Material

We report the enhancement of the zero resistivity T _c(0) by 5.5 K i.e. from 104 to 109.5 K by substitution of gallium 1.34% of copper in the bismuth 2223 compound. A series of Ga-containing compounds Bi₂Pb_0.4Sr₂Ca₂Cu_{3− x} Ga _x O _y ( x =0.00, 0.02, 0.04, 0.06, and 0.08) are synthesized by the solid-state reaction method. The samples are characterized by measurements of their dc electrical resistivity and ac magnetic susceptibility and by the powder X-ray diffraction analysis. It is noted that the high- T _c (2223) phase increases from 57.55% in an undoped sample to 92.99% in samples containing a low concentration of gallium i.e. x ≤0.04.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

Preparation of Orthorhombic Ba2YCu3O7 Powder by Single-Step Calcining

A single calcination step, solid-state process that provides orthohombic Ba₂YCu₃O₇ powder is described. BaCO₃, Y₂O₃, and CuO are used as precursor materials. The only phase identifiable by X-ray diffraction is the orthorhombic Ba₂YCu₃O₇. The use of a vacuum during the inital stages of the calcining process promotes complete decomposition of the carbonate, and no residual carbonate is observed. An oxygen atmosphere during the later stages of calcining ensures proper oxidation to Ba₂YCu₃O₇. The use of a similar combination vacuum-oxygen calcining schedule should also be beneficial in the preparation of chemically derived powders.     

Solid-Solution Ranges of the n= 2 and n= 3 Superconducting Phases in Bi2(SrxCa1 −x)n+1CunOy and the Effect on Tc

The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi₂(Sr _x Ca_{1 − x} ) _{n +1}Cu _n O _y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T _c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter.