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.     

Formation of Yttrium Barium Cuprate Powder at Low Temperatures

Yttrium barium cuprate can be formed at temperatures below 800°C from mixed oxides and carbonate if a low-oxygen-pressure atmosphere is used. This eliminates the formation of a high-oxygen-content nonsuperconducting phase which has been previously shown to form at lower temperatures. Reaction of barium carbonate and the oxides of copper and yttrium at temperatures as low as 750°C in reducing atmospheres is shown to form tetragonal YBa₂Cu³O_x with x about 6. This compound can then be oxidized by cooling in an oxidizing atmosphere to obtain the desired x value of about 7.     

Jonker "Pecir" Analysis of Oxide Superconductors

Data for the normal state of the superconducting oxide systems YBa₂Cu₃O_6+y, and the nonsuperconducting system La₃Ba₃Cu₆O_12.5+y, when plotted as thermoelectric coefficient versus logarithm of conductivity, exhibit Jonker "pear" behavior, confirming the semiconducting character of these materials. The symmetry of the plots indicates similar conduction parameters (density-of-states (DOS), transport constants, and mobilities) for p - and n -type mechanisms. Band gaps on the order of 0.5 eV are obtained. DOS—mobility products for the above-mentioned systems and La_2-xBa_xCuO₄ and Bi₂Sr₂CaCu₂O₈ scale roughly according to the density of copper ions in these materials and fall in the range 10²⁰ to 10²¹ (cm·V·s)⁻¹. Assuming DOS equal to copper content results in mobilities on the order of 0.15 cm²·V⁻¹· s⁻¹. These results are discussed in terms of possible semiconduction mechanisms.     

Reactivity of Ceramic Superconductors with Palladium Alloys

Palladium alloy compositions were investigated for suitability as a nonreactive material for the processing of ceramic superconductors. Barium-based superconductors were tested on Pd-Au and Pd-Ag alloys for reactivity. Bismuth-based superconductors were tested on a Pd-Ag alloy. The least reactive was found to be 70% Pd-30% Ag for barium-based high-temperature superconductors (HTSC), whereas 30% Pd-70% Ag was found to be least reactive for bismuth-based HTSC.     

Reaction Sequencing During Processing of the 123 Superconductor

Reaction sequencing studies were conducted for two precursors to the 123 superconductor in air and helium environments. In air, the reactants in both precursor systems produce an intermediate 123 phase at 1000 K without going through the BaCuO₂ and Y₂BaCuO₅ intermediate phases, observed in earlier work with mixed powder precursors. In helium, the fully developed 123 tetragonal phase is formed at temperatures as low as 880 K, but it also decomposes to Y₂BaCuO₅, BaCu₂O₂, and BaCuO₂; the extent of decomposition depends on the temperature. The 123 tetragonal phase completely decomposes by 1075 K, but it can be reformed if the atmosphere is switched back to air at a temperature above 950 K.     

Fabrication and Microstructure of Composite Metal-Clad Ceramic Superconducting Wire

Sheathed high- T _c superconducting ceramic wire composites have been fabricated by wire drawing. Superconducting powders of YBa₂Cu₃O_7-x (Y-123) and Bi₄(Sr, Ca)₆Cu₄O_16+x (Bi-464) have both been prepared and successfully fabricated into continuous wire form. Under certain circumstances, a displacement reaction occurs at the interface between the ceramic superconductor and the containment tube; the nature of this displacement reaction and its prevention are discussed. Microcracking of the ceramic core is attributable predominantly to relative shrinkage effects during sintering. Failure of the metal wall during wire fabrication is attributed to drawing conditions and the mechanical properties of core and cladding.     

High-Temperature Defect Structure of Lanthanum Cuprate

High-temperature (650° to 8507deg;C) electrical conductivity and Seebeck coefficient measurements as functions of oxygen partial pressure and temperature in polycrystalline lanthanum cuprate support a defect model consisting of oxygen interstitials charge compensated by electron holes. The La:Cu ratio was therefore estimated to be 2.000 ± 0.001. By comparison with existing oxygen nonstoichiometry data, the high-temperature electron hole mobility and density-of-states were estimated.     

Traveling Reaction Zone Method for Preparation of Textured Ceramic Superconductor Thick Films

Textured thick films of superconducting YBa₂Cu₃O_7−x (YBCO) have been prepared on ceramic substrates using a traveling reaction zone method. The technique utilizes the rapid reaction between Y₂Cu₂O₅ and BaCuO₂ to form YBCO as the film passes through a steep temperature gradient furnance. The films consist of a single continuous superconducting phase with strong c -axis orientation normal to the translation direction of the film.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

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.     

Effects of Material Transfer by Liquid and Vapor on the Microstructure of Sintered Y-Ba-Cu-O Superconductors

Effects of a slight compositional change on the microstructure of sintered Y-Ba-Cu-O superconducting ceramics were investigated. The compositional change was provided either by the liquid which has been formed at the hot end of the rectangular bar specimen sintered under gradient temperature or by the vapor from Y-Ba-Cu-O melt. The results indicated that the Cu and Ba vapor pressure of the sintering atmosphere must be closely controlled to obtain an enhanced densification and a desired single-phase microstructure.     

Oxygen Pressure Effect on the Y2O3—Bao—Cuo Liquidus

Concurrent DTA, TGA, and oxygen pressure measurement were used to determine liquidus temperature for compositions near YBa₂Cu₃O_x. Liquidus temperatures were measured for oxygen partial pressures from 2 × 10⁻⁴ to 1 bar. Liquidus temperatures decreased for lowered oxygen pressure. The ternary eutectic is 930°C for 1 bar and 854°C for 0.02 bar oxygen pressure. Incongruent melting of YB₂Cu₃O_xoccurs at 1034°C for 1 bar and 938°C for 2 × 10⁻⁴ bar oxygen pressure. Some implications for processing are also discussed.     

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.     

Experimental and Thermodynamic Study of Nonstoichiometry in 〈YBa2Cu3O7-x〉

The dependence of the nonstoichiometry of 〈YBa₂Cu₃O_7−x〉 (solid) has been studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen-reduction methods for determining the absolute oxygen to-metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were used to derive a chemical thermodynamic representaltion of the experimental variables. The data were also compared with several other investigations to indentify the selfconsistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram.     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: II, Nonstoichiometry

Aspects of nonstoichiometry for the Y-Ba-Cu (1: 2: 3) system are considered. The general formula YBa₂Cu₃O_7-x has been assumed for considerations of nonstoichiometry in 1: 2:3 oxide cuprates. Assuming that copper ions may occupy different lattice positions (independently of their valency), the equilibrium constants for oxygen intercalation were determined:     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: III, Defect Model and Conductivity Mechanism

Based on the measurements of electrical conductivity and thermopower (Seebeck coefficient) as a function of oxygen partial pressure, the defect structure and corresponding conduction mechanisms at elevated temperatures are considered for the Y-Ba-Cu-O (1:2:3) system. It has been postulated that the simple hopping model is not applicable to YBa₂Cu₃O_7-x. A modified conduction hopping mechanism has been proposed and equations describing the mobility of charge carriers in the studied system are derived. The most important advantage of the present model, in comparison to previous models, involves considering interactions between both electrons and electron holes and the resulting effect on their mobility terms. The observed departure of experimental data from the Heikes formula is explained by a very high concentration of defects and resulting substantial interactions between the defects which must be taken into account. The proposed transport model exhibits good agreement with experimental data of nonstoichiometry and the presently determined electrical conductivity and thermopower. It has been argued that the charge transfer mechanism depends substantially on the oxygen partial pressure and resulting oxygen content in the oxide lattice.     

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 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.     

Calculation of Phase Diagrams for the YO1.5—BaO—CuOx System

The thermodynamic data for the YO_1.5–BaO, BaO-CuO_x, and YO_1.5–CuO_x quasi-binary systems were optimized from experimental phase diagrams. They were used to calculate tentative phase diagrams for the YO_1.5–BaO—CuO_x quasi-ternary system. The equilibrim liquidus surface and the isothermal sections of the ternary system at 900°, 925°, 950°, 975°, and 1000°C were calculated. The isopleths containing YBa₂Cu₃O_7-δ were also calculated.     

Novel Solution-Soaking Method for Adding Dopants to YBa2Cu3O7−x to Achieve Higher Sintered Densities

YBa₂Cu₃O_7−x was doped with various metal ions by a new technique in which a pellet (after binder burnout) was soaked in a solution containing the appropriate ions and then dried. The sintered density of the treated pellets depended on the dopant in the solution, and in many cases it was much higher than that obtained for pure YBa₂Cu₃O_7−x (93% to 96% as compared with 85% to 90%). A study of the microstructure revealed that, in those cases where higher sintered densities were obtained, the grain size was much smaller. The grain morphology, however, did not change. It is likely that the high concentration of dopant adsorbed on the grain surface during solution soaking enhanced the rate of sintering.