K.C. Goretta

Synthesis of phase-pure orthorhombic YB2Cu3Ox under low oxygen pressure

Abstract Reaction of Y 2 O 3 , BaCO 3 and CuO for 4 h at 800°C in flowing O 2 with a total pressure of about 2.7×10 2 Pa, followed by cooling in O 2 at ambient pressure, has produced phase-pure orthorhombic YBa 2 Cu 3 O x . Keeping the ratio of O 2 to evolved CO 2 above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and sintered pellets with improved super-conducting properties.

Effect of Y2BaCuOx precipitates on flux pinning in melt-processed YBa2Cu3Ox

Abstract Magnetic hysteresis data have been taken at 4.2, 50, and 77 K over a wide range of magnetic field on three types of melt-textured YBa 2 Cu 3 O x (123) samples: stoichiometric 123, 123+6 mol% Y 2 BaCuO 5 (211) particles, and quench-melt-growth-processed (QMGP) 123+6 mol% 211. In order to obtain accurate comparisons of the intragranular critical current densities, the magnetization measurements were performed on finely powdered samples, the average particle size of which was less than the grain size of the original sample. The QMGP samples exhibited substantially larger hysteresis at 4.2 and 50 K, indicating significantly enhanced flux pinning at these temperatures. At 77 K, however, although the hysteresis loop of the QMGP samples was largest at low fields, the hysteresis difference, Δ M , decreased more rapidly with field than did the Δ M of the stoichiometric melt-processed sample. A possible pinning mechanism related to the observed hysteretic behavior is discussed.

Solid particle erosion of a graphite-fiber-reinforced bismaleimide polymer composite

Abstract Erosion rate measurements and scanning electron microscopy observations were made for the steady state erosion of a graphite-fiber-reinforced bismaleimide polymer composite and compared with results obtained on the unreinforced matrix polymer. For erodents with high kinetic energies, the material removal rate of the composite is dominated by the brittle graphite fibers, resulting in erosion rates much higher than those of the matrix polymer.

High‐temperature deformation of YBa2Cu3O7−δ

Steady‐state flow stress of dense (99% of theoretical) polycrystalline YBa2Cu3O7−δ has been measured in direct compression at temperatures between 870 and 980 °C in oxygen partial pressures of 0.03, 0.18, and 1.0×105 Pa for strain rates between 5×10−6 and 4×10−5 s−1. The stress exponent was close to one, indicating that diffusional flow was the dominant deformation mechanism. The flow stress decreased with decreasing oxygen partial pressure, obeying a power law with an exponent of approximately 1/2 . The activation energy was about 800 kJ/mol at an oxygen partial pressure 1×105 Pa, but decreased to 600 kJ/mol at a pressure of 3×103 Pa.

Transport critical current density and microstructure in extruded YBa2Cu3O7−x wires processed by zone melting

YBa2Cu3O7−x compounds were extruded into long wires with the diameter of 1 mm after sintering. The sintered wires were subsequently zone melted to develop a highly textured microstructure. Magnetization experiments at 77 K indicated a Jc value of 1×105 A/cm2 at 1 T. Transport measurements at 77 K showed a greatly enhanced field dependence of the critical current density. Transmission electron microscopy revealed an important grain‐boundary feature which eliminated the weak‐link behavior. Large amounts of dislocations have also been found in the zone‐melted sample which may contribute to flux pinning in the system.

Influence of oxygen concentration on processing YBa2Cu3O7−x

The sintering kinetics of YBa2Cu3O7−x have been studied from 870 to 930 °C in oxygen/argon mixtures ranging from 0.2% to 100% oxygen. For sintering at temperatures below that required for formation of any liquid phase, densification rates increased with decreasing oxygen concentration, and transport critical current densities increased with increasing density. The activation energy for sintering was 190±30 kJ/mol.

Sintering of YBa2Cu3O7−x compacts

Abstract Heating of YBa 2 Cu 3 O 7− x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm 2 ) have been obtained in these solid-state sintered samples.

Effects of nanosize MgO additions to bulk Bi2.1Sr1.7CaCu2Ox

Abstract Nanosize MgO particles were added to Bi 2.1 Sr 1.7 CaCu 2 O x (Bi-2212) powder in various molar fractions. Bulk samples were made from the MgO/Bi-2212 and heat treated by partial-melt processing. Scanning electron microscopy and energy-dispersive X-ray spectra were used to study the phases present in the samples. The effects of the MgO additions on the flux pinning behavior as a function of magnetic field and temperature were investigated by magnetization hysteresis measurements obtained with a SQUID magnetometer and a 33 T vibrating sample magnetometer. It was found that the nanosize MgO addition did not affect T c of Bi-2212, while significantly expanding the magnetization hysteresis loops and increasing the irreversibility field. X-ray and SEM analysis suggested that MgO is inert with respect to Bi-2212.

Solid-particle erosion of bismaleimide polymers

Abstract Solid-particle erosion of bismaleimide (BMI) polymers of various compositions was studied and compared with measured mechanical properties. For angular aluminum oxide erodents of mean diameter 42, 63, 143 or 390 μm impacting normal to the surface at 60 m s −1 , the erosion rate was found to be a strong function of the size of the impacting particle. Material removal occurred primarily by a process of brittle-fracture-induced damage. However, the results suggest that for the BMI specimens, degradation and plasticity occurred in addition to fracture, and that the occurrence of these phenomena reduced the erosion rates.

Thermomechanical response of polycrystalline BaZrO3

Thermomechanical properties of fine-grained, 96% dense, polycrystalline BaZrO{sub 3} specimens were studied. The average thermal expansion coefficient of BaZrO{sub 3} at 2511000{sup o}C was 7.72x10{sup -6}{sup o}C{sup -1}. At 25{sup o}C, Youngs modulus was 240.5 GPa and the shear modulus was 97.2 GPa. These values decreased by {approx}10% with heating to 1015{sup o}C. Poissons ratio was 0.237 and was independent of temperature. The BaZrO{sub 3} specimens exhibited a thermal expansion coefficient of about one half, and elastic moduli more than twice, those of polycrystalline high-temperature superconductors. Diffusional creep was observed when BaZrO{sub 3} was compressed at 1300-1400{sup o}C; the activation energy for steady-state deformation was 460{+-}30 kJ/mole. The mechanism was probably grain-boundary sliding, controlled by diffusion of one of the cations along grain boundaries. Deformation rates below 1000{sup o}C were too low to relax thermally generated stresses.