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.     

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.     

Reaction of YBa2Cu3O7-δ Powder with NOx Gas in an Aerosol Reactor

Thermodynamic calculations predict, and experiments verify, that YBa₂Cu₃O₇-₈ (123) powder is unstable in the presence of NO_x-containing aerosol reactor exhaust gases at temperatures below about 600°C. Powders collected above the stability temperature are single-phase 123, while powders collected at lower temperature contain Ba(NO₃)₂ formed by reaction of the powder with NO_x, after exit from the hot zone.     

A Process for Cf/SiC Composites Using Liquid Polymer Infiltration

A continuous carbon fiber/silicon carbide matrix composite material has been produced by a low-cost process. In this process the space in a two-dimensional carbon fiber preform is filled with a SiC powder by a pressure infiltration method. High particle packing densities are achieved within the fiber preform in this way. The compact body is heat-treated at 400°C to form a porous framework, which is then infiltrated with a liquid preceramic polymer, Ceraset^TM SN. Subsequently the infiltrated polymer is pyrolyzed in argon at 1300°C. The microstructure of the final composite is characterized, and mechanical properties of these composites 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.     

Superconductive Properties of the YBa2Cu4O8 Superconductor

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa₂Cu₄O₈ (124) at 850°C in flowing oxygen at 1 atm (≅10⁵ Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were characterized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be ∼5 × 10⁴ A/cm², and the superconducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase.     

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.     

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.     

Alignment of YBa2Cu3O7-x and Ag─YBa2Cu3O7-x Composites at ∼930°C by Eutectic Formation

The present work describes a new technique to synthesize aligned YBa₂Cu₃O_{7- x} and Ag─YBa₂Cu₃O_{7- x} superconducting composites from Ba- and Cu-deficient compositions (relative to YBa₂Cu₃O_{7- x} ) plus BaCuO₂. For YBa₂Cu₃O_{7- x} , high transition temperature midpoint T_c (91 K), temperature of zero resistivity T ₀ (90 K), and critical current density J_c (>3000 A°Cm⁻² at 77 K) were achieved by using this technique. This procedure provides the potential for using a reliable and reproducible densification and alignment technique alternative to partial or full melting. The composite is highly aligned, with an average grain size of ∼1 to 2 mm and domains of width greater than 5 mm. The initial phase assemblage consists of YBa₂Cu₃O_{7- x} (123) as the major phase plus YBa₂CuO₅ (211) CuO as minor phases. The BaCuO₂ is added to the Ba- and Cu-deficient starting composition in order to assist in the formation of a CuO-rich liquid as well as to compensate for the Ba and Cu deficiences in 123. Since the liquid forms at ∼900°C and is compatible with 123, it can be used to facilitate alignment of 123 at ∼930°C. The addition of Ag to the system results in eutectic formation with the (solidified) liquid, substantial filling of the pores during sintering, and improved alignment.     

Type II Magnetic Levitation on Sinter-Forged YBa2Cu3Ox Superconductor

Submicrometer powder of singlephase YBa₂Cu₃O_x (YBC), prepared from nitrate salts, were free-sintered and sinter-forged at 1223 K. The levitation force on magnets exerted by the superconductor was found to depend on the thickness of the superconductor, the shear strain imposed during sinter-forging, and the time and temperature of the high-pressure oxygen anneal after sinter-forging. The first result is direct proof of the type II nature of the YBC superconductor. In the second experiment a shear strain of 300% increased the levitation force by a factor of 3. The third result confirmed that YBC transforms to the tetragonal structure at 1223 K, which must be restored to the orthorhombic structure by annealing at lower temperature. Optimum results were obtained when the specimens were annealed in pressurized oxygen (3 MPa) for 50 h at 673 K.     

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.     

Dependence of YBaCuO Superconductor Properties on Constituent Oxide Preparation: I, CuO and BaCO3 Pretreatment

A systematic study has been made of the influence of different solid-state decomposition preparations of copper oxide and barium carbonate on the synthesis of YBa₂Cu₃O_7−x superconductor. Considerable morphological control on the end product can be achieved through a monitored calcining of starting materials. The ceramic particle size is found to be primarily dependent on initial CuO morphology, whereas uniformity of the ceramic and the occurrence of an intergranular phase is sensitive to the form of the BaO precursor. The use of accoustic testing for characterizing the ceramic quality is suggested.     

Preparation of Single-Phase High Tc Superconductor (Bi,Pb)2Sr2Ca2Cu3Ox (2223) with Hg Substitution

The successful preparation of single-phase Hg-doped (2223) using a nitrate decomposition/solid state reaction method is described in this paper. The effects of Hg doping on the formation of single-phase (2223) have been investigated by X-ray and synchrotron radiation diffraction. When compared with Pb-doped system, the use of Hg as a dopant has resulted in the reduction of both the final sintering temperature and the annealing time of the single-pure (2223) material, without any obvious deterioration of the superconducting properties.     

Effects of Temperature and Strain Rate on the Plastic Deformation of Fully Dense Polycrystalline Y1Ba2Cu3O7−x Superconductor

The knowledge of the steady-state stress for plastic deformation as a function of temperature and strain rate is essential for hot-forming superconducting material into commercially useful shapes. In this paper, results are presented on the experimental determination of the rheology of fully dense polycrystalline Y₁Ba₂Cu₃O_7−x superconducting material at temperatures ranging from 750° to 950°C and strain rates of 10⁻⁴, 10⁻⁵, and 10⁻⁶ s⁻¹. The data are best fitted by a power law: ε(s⁻¹)=8.9 × 10⁻¹⁷. (s⁻¹) σ^2.5 (Pa) exp [−2.01 × 10⁵(J·mol⁻¹)|RT]. X-ray analysis shows that the superconducting material retains its phase composition after nearly 70% total strain of the sample. A strong anisotropy in the resistivity of the deformed samples is observed because of the development of a preferred orientation of the a or b axis of Y₁Ba₂Cu₃O_7−x orthorhombic perovskite single crystals perpendicular to the principal maximum compressive stress.     

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.     

Thermodynamic Study of Reduced Phases in the BaLa4Cu5O13.1−x System

The thermodynamic stabilities of reduced, perovskiterelated cuprates with the BaLa₄Cu₅O_{13.1− x} structure were investigated using differential scanning calorimetry techniques. For 0 < x 0.6 the partial molar enthalpy of oxygen in the structure is constant at -187.4 kJ/mol O₂. This value is almost identical to those reported for other perovskite cuprates. These results suggest that −190 kJ is a reliable value for the oxidation of Cu from 2⁺ to 3⁺ and can be used with confidence in calculations of the stabilities of the p -doped perovskite cuprates. Anomalously large enthalpies measured for samples prepared with higher values of x under forming gas probably result from trace contaminants of Cu metal.     

Influence of Undercooling and Nd4Ba2Cu2O10 Content on the Growth Rate of Large-Grain Bulk Nd-Ba-Cu-O Superconductor

The dependence of growth rate on isothermal undercooling and composition has been investigated for Nd-Ba-Cu-O single grains containing various amounts of nonsuperconducting Nd₄Ba₂Cu₂O₁₀ (Nd-422) phase inclusions and fabricated under a 1% O₂ in N₂ atmosphere using a top seeded melt growth technique. The growth rate along the crystallographic c -direction is observed to exceed that along the a / b direction at all undercooling temperatures and exhibits a maximum for a Nd-422 content of ∼10 mol%. The samples have been examined by optical microscopy and the results interpreted within an established planar solidification model.     

Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics

Purified air is passed over a specimen of YBa₂Cu₃O_{7– x} at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10⁻⁵ Pa for BaO( g ), 1.2 × 10⁻⁴ Pa for Cu( g ), and 2.2 × 10⁻⁵ Pa for CuO( g ) are obtained under 2.1 × 10⁴ Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.     

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.     

Elevated-Temperature Toughening Mechanisms in a SiCw/Al2O3 Composite

Crack growth resistance studies of a Sic-whisker-reinforced Al₂O₃-matrix composite have been correlated with the composite microstructure to determine the active fracture toughening mechanism, at each of three test temperatures through 1400°C. Evidence of cumulative toughening at all temperatures, as reported in the literature, was validated by R -curves; however, isolation of the following wake zone effects from that of the frontal process zone elicits a departure from published assumptions. A frontal zone mechanism, presumably microcrack toughening, dominates at room temperature, while a following wake zone mechanism of crack face whisker-bridging controls at temperatures near 1200°C.