Effect of CO2 on the Processing of Y-Ba-Cu-O Superconductors

The superconducting properties of YBa2Cu3O6+x reacted with various known ratios of O2/CO2 gas mixtures during sintering at different temperatures were studied. Jc was found to decrease drastically upon reaction with CO2, becoming zero at certain CO2 activities. The stability region for the 123 superconductor, as a function of CO2 activity and temperature, was empirically formulated as follows: log pCO2 \u3c (−45,000)/T + 33.4. The grain boundaries in sintered samples with Jc = 0 were investigated with HRTEM in conjunction with EDS. Two distinct types of grain boundaries were observed. Approximately 10% of the grain boundaries were wet by a thin layer of a second phase, deduced to be BaCuO2. The remaining boundaries were sharp grain boundaries. The grain structure near the sharp grain boundaries was tetragonal. These two types of grain boundaries are thought to be responsible for Jc being zero

Defect production by energetic particle bombardment

Aspects of low-dose, low-temperature irradiation experiments, and elevated-temperature experiments are reviewed. Information and data are included on methods of determining lattice atom displacement thresholds, lattice damage rates measured by changes in resistivity, ion damage, and neutron damage. (JRD

HIGH-RESOLUTION ELECTRON MICROSCOPY OF GRAIN BOUNDARIES IN FCC MATERIALS

La structure atomique des joints de grains (JG) de flexion et à grand angle a été étudiée dans Au et dans NiO par microscopie électronique à haute résolution. En plus des joints symétriques, on observe plusieurs facettes dissymétriques dans Au et dans NiO. Les JG dissymétriques sont importants, non seulement à cause du plus grand nombre de configurations dissymétriques possibles, mais aussi parce-que ces facettes dissymétriques ont souvent une faible énergie. La translation solide perpendiculaire aux JG symétriques Σ= 5 dans le NiO est plus petite que celle prévue par simulation à l'ordinateur, alors que dans l'or, les JG semblent être en général plus dilatés que prévu par des calculs fondés sur des potentiels atomiques encastrés. Le volume supplémentaire total dans les oxydes peut être considérablement plus grand que celui déduit des seules translations rigides, du fait de la présence de défauts lacunaires sur le JG. Des multiplicités structurelles et une tendance à préserver la cohérence à travers l'interface ont été observés dans les joints de NiO et d'or. On a aussi observé dans Au plusieurs structures nouvelles à caractère tri-dimensionnel. Toutes ces observations soulignent l'importance de la localisation de fautes d'alignement, et la tendance qu'ont les solides compacts à préserver autant que possible des régions qui concordent à l'échelle atomique.The atomic structure of and large-angle tilt grain boundaries(GBs) has been studied in Au and NiO by high-resolution electron microscopy. In addition to symmetric boundaries a number of asymmetric facets are found in both Au and NiO. Asymmetric GBs are important, not only because of the much greater multitude of possible asymmetric configurations, but also because asymmetric facets often may be low in energy. The rigid-body translation normal to the Σ=5 symmetric GBs in NiO is smaller than predicted from computer simulation, while GBs in Au largely appear to have greater expansions than expected from calculations based on embedded atom potentials. The total excess volume in the oxides may be substantially larger than deduced from the rigid-body translations alone, due to the presence of vacancy-type defects at the GB. Structural multiplicities and a tendency for maintaining coherence across the interface have been observed in both NiO and Au grain boundaries. A number of novel GB structures that assume a 3-dimensional character have been observed in Au. These observations underline the importance of misfit localization and the tendency in close-packed solids to maintain, whenever possible, atomically well-matched regions

TEM investigation of 14 MeV neutron damage

From joint meeting of the American Nuclear Society and the Atomic Industrial Forum and Nuclear Energy Exhibition; San Francisco, California, USA (11 Nov 1973). 14-MeV neutron damage has been investigated in gold by means of transmission electron microscopy (TEM). The sites of energetic displacement cascades are visible due to the presence of vacancy clusters formed by the collapse or rearrangement of vacancies within the depleted zones. In 14-MeV n- induced cascades, a strong tendency towards subcascade formation is found. On the average 1.8 clusters are formed per cascade. Individual cascades with as many as 6 subcascades have been found. The number densities of clusters as well as the number densities of cascades are proportional to the fluence. The cross section for the formation of visible cascades is sigma /sub c/ = 3.3 x 10/sup - 24/ cm/sup 2/. It can be shown that recoi ls from elastic neutron scattering events can account for only about 20% of the visible cascades. The cross section corresponding to the balance of the observed cascades is, within experimental error, equal to the nonelastic neutron scattering cross section. This indicates that all nonelastic scattering events lead to the formation of a visible cascade. Quantitative agreement with what is expected from heavy ion bombardments regarding the cross sections involved is found, however, estimates of the average cascade energy in the 14MeV neutron bombardments are somewhat higher than expected. (10 figures, 29 referenccs) (auth

Atomic relaxation modes in grain boundaries

High-resolution electron microscopy (HREM) in combination with computer simulations of the fully relaxed atomic structures and energies of symmetric and asymmetric grain boundaries (GBs) in Au <110> tilt bicrystals has been applied to a study of atomic relaxation modes. These investigations indicate that: (1) Atomic relaxations are typically dominated by short-range interactions and the tendency of the solid to assume a local atomic environment similar to the bulk. (2) Misfit localizations are likely within a structural unit whenever the GB unit cell is large. (3) When there is a length mismatch along the GB the atomic relaxations can assume quasiperiodic character, generating densely spaced regions of structural disorder, akin to misfit dislocations. (4) Atomic relaxations can take the form of stacking disorder

GRAIN BOUNDARY STRUCTURE IN NiO

Microscopie électronique par transmission est utilisée pour étudier la structure des joints de grains dans NiO. En général, on a trouvé que les joints de flexion sont facettés. Les périodicités structurales dans les facettes ont été observées pour les joints à faible et grande désorientations. Une densité atomique réduite effective dans une région de 0.5-0.8 nm en largeur au joint de grains est suggérée par le comportement de contraste défocalisé des joints de flexion et de torsion.Transmission electron microscopy is used to study the structure of grain boundaries (GBs) in NiO. tilt GBs in NiO are generally found to be faceted. Structural periodicities within facets have been observed at low and high misorientation. An effective reduced atomic density at the GB within a region of 0.5-0.8 nm in width is suggested by the defocus contrast behavior of high-angle tilt and twist GBs

Atomic matching across internal interfaces

The atomic structure of internal interfaces in dense-packed systems has been investigated by high-resolution electron microscopy (HREM). Similarities between the atomic relaxations in heterophase interfaces and certain large-angle grain boundaries have been observed. In both types of interfaces localization of misfit leads to regions of good atomic matching within the interface separated by misfit dislocation-like defects. It appears that, whenever possible, the GB structures assume configurations in which the atomic coordination is not too much different from the ideal lattice. It is suggested that these kinds of relaxations primarily occur whenever the translational periods along the GB are large or when the interatomic distances are incommensurate. Incorporation of low index planes into the GB appears to lead to preferred, i.e. low energy structures, that can be quite dense with good atomic matching across a large fraction of the interface. 20 refs., 9 figs