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We have measured creep relaxation in α-Al 2 O 3 , thermally grown on stoichiometric β-NiAl, and on β- and γ'-phase alloys of Ni–Al–Pt at temperatures between 950–1100 °C. Creep was monitored using in-situ measurements of strain relaxation in the oxide following the sudden imposition of a stress. A stress was imposed by abruptly changing the sample temperature, exploiting the thermal expansion difference between oxide and substrate. The in-plane elastic strain was obtained using a sin 2 ψ X-ray diffraction technique exploiting synchrotron radiation. For oxides grown on β-NiAl and Ni–Al–Pt samples, we found that strain relaxation rates are comparable to those observed in fine grained α-Al 2 O 3 bulk ceramics, when bulk results are extrapolated to the lower temperatures examined here. When Hf was added to the Ni–Al–Pt alloy, creep rates in the thermally grown oxide were substantially slowed. Creep rates at stress levels of 100 MPa, or less, are proportional to σ n , with n ≤ 2, consistent with a diffusional creep mechanism.

Stress evolution in Al2O3 films grown on a Fe3Al-based alloy, with or without a prior H2-anneal heat treatment, was followed in situ with a synchrotron X-ray source at 1000 and 1100 °C and during cooling. These results are complimented by studies of oxidation kinetics, the scale/alloy interfacial chemistry, and the oxide, alloy and interface microstructures. The H2-anneal reduced the sample sulfur impurity to less than 1 ppm, significantly improved scale adhesion, maintained a planar scale/alloy interface, but did not demonstrably affect the oxidation rate or the growth stress development. On the untreated alloy, interface wrinkling was a major stress relief mechanism, but for the H2-annealed samples it was oxide creep with contribution from alloy grain boundary sliding. The interface strength and the extent of oxide spallation were closely related to the amount of sulfur segregated at the interface.

This paper examines the demagnetization of a trapped magnetic field in a high TC superconducting (HTS) monolith using a single, short, high-amplitude field pulse. With a single optimized pulse, the peak trapped field can be reduced to one-third of the maximum saturated value. Alternatively, the spatial average of the trapped field could be fixed very close to zero. After demagnetization, the trapped field has both positive and negative values at various locations in the HTS monolith. Transient responses of the HTS to pulsed fields with different amplitudes are also reported and discussed. It does not appear that the residual trapped field can be reduced to zero throughout the monolith using pulsed field demagnetization methods in a fixed sample/coil geometry. Bean's critical state model is used to explain the measured equilibrium field distributions. Transient response of the HTS to an externally applied field is qualitatively explained by magnetic diffusion. Drive circuit characteristics and design feasibility issues are also addressed

Using a glancing synchrotron X-ray beam (Advanced Photon Source, Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from thermally grown oxides on NiAl samples were recorded during oxidation at 1000 or 1100°C in air. The diffraction patterns were analyzed to determine strain and phase changes in the oxide scale as it developed and evolved. Strain was obtained from measurements of the elliptical distortion of the Debye-Scherrer rings, where data from several rings of a single phase were used. Results were obtained from α-Al2O3 as well as from the transition alumina, in this case θ-Al2O3, which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in α-Al2O3 was tensile, with a magnitude high enough to cause Al2O3 fracture. New α-Al2O3 patches nucleated at the scale/alloy interface and spread laterally and upward. This transformation not only puts the alpha alumina in tension, but can also cause the tr...

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K for a set of YBCO single-domain rings. These were fabricated from melt-textured cylindrical YBCO monoliths that were densified to nearly 100%, and then oriented from a single seed. The rings were surrounded by a drive coil that can, under pulse conditions, achieve applied magnetic fields in excess of 1 T and induce currents in excess of 50 kA. Simultaneous magnetic characterization with a Rogowski coil and Hall probe was used to determine the induced current in the sample and the magnetic field in the center of the sample. Magnetic fields trapped in the samples were mapped with a scanning Hall probe. When compared with similar measurements on multidomain c-axis-oriented YBCO rings, the flux penetration is faster and more uniform around the circumference of the ring. The observed critical current density, /spl ap/ 15,000 A/cm/sup 2/ at 77 K, is suitable for application in penetration-type fault current limiters. Separate measurements of the trapped magnetic field and critical current density in the rings are compared with results obtained by analysis of magnetic diffusion characteristics.

Alumina scales that grow during oxidation of FeCrAl alloys can develop a convoluted morphology. Although convolution relieves the overall growth stress, high thermal stresses develop locally and can be detrimental to the scale or interface integrity. Ruby fluorescence measurements and finite element simulations are used to examine residual thermal stresses and strains that result when the convoluted scales are cooled to room temperature. Unlike a flat scale that is in biaxial compression, a convoluted scale contains significant gradients, with tensile stress components along the outside and near the interface of the convoluted peaks. The experimental results are in good agreement with model calculations and provide much needed verification of the model assumptions. Because the ruby fluorescence technique provides only the hydrostatic stress averaged over an excited volume that includes the entire alumina scale thickness, modeling provides detail and insight to the experimental measurements.

By exploiting the strain dependence of the ruby luminescence line, we have measured room-temperature residual strains in thermally-grown alumina scales. Measurements were made on two alloys Fe-5Cr-28Al and Fe-18Cr 10Al (at.% bal. Fe), oxidized between 300–1300°C. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (RE) are also presented. Scales formed on RE-containing alloys (Zr or Hf) could support significantly higher strains at T ≥ 1000°C. Strain relief associated with spallation thresholds is readily observed. In early-stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early-stage formation of α-Al2O3. It appears that, in the presence of Cr2O3 or Fe2O3, the α-phase of Al2O3 can form at anomalously low temperatures.

Using Raman scattering we have investigated the oxidation, in air, of the Fe-Cr-Ni stainless steels Fe-25Cr-20Ni, Fe-25Cr-20Ni-3Zr, and Fe-24Cr-3Zr (wt.%) as a function of temperature in the range 300 to 1000°C. The Raman technique is very sensitive to, and provides a clear identification of, the oxides Fe2O3 and Cr2O3. However, the technique is insensitive to NiO, FeO, and does not give a clear identification of spinels. The Fe−Cr−Ni alloys form chromia scales at temperatures greater than ∼800°C. At lower oxidation temperatures, transient phases are observed. With a 1-h heat treatment at 300°C, we observe the formation of an unidentified scale; we speculate that it is either amorphous or consists of disordered spinel(s). Near 400°C we begin to observe hematite (Fe2O3). The intensity of the Fe2O3 signal increases with temperature to ∼600°C and then decreases, being largely replaced by the signal from Cr2O3. The thickness of the Cr2O3 scale increases with temperature up to ∼1000°C above which spallation becomes apparent. Spinel phases also apparently persist in the scale to 1000°C.

We discuss first-principles angle-resolved photoemission (ARPES) intensity calculations for the (001)-surface of YBa 2 Cu 3 O 7 , with focus on spectra for k ∥ -values along the ΓS symmetry direction. The theoretical spectra show the sensitivity of the spectral shape to the specific surface termination used in the computations. The experimental spectra however appear to be well-described by assuming the first layer to be BaO followed by the CuO 2 plane layer. Effects of the direction of polarization vector of the incident light on the spectra are examined; in particular, we show that the predicted behavior of the spectral feature arising from the CuO 2 plane bands is in remarkable accord with experiments. This result gives confidence in the reasonableness of the LDA wave functions implicit in the computations.

We report AC susceptibility and resistivity measurements of the vortex motion in single crystal YBa_2Cu_3O_(7−δ), with δ ~ 0.62. The superconducting transition temperature was varied from 7 K to 17 K by altering the oxygen vacancy ordering in the chains. We find that both the slope of the irreversibility line and its extrapolated zero temperature value scale linearly with T_c, providing strong evidence for the intrinsic nature of the irreversibility line. Moreover, the reduced magnetic field scale of oxygen deficient crystals allows us to obtain new results at previously inaccessible reduced temperatures, well below T_c.

The structure of the “211” or “green phases,” Yb 2 BaCuO 5 and Lu 2 BaCuO 5 , have been refined from time-of-flight powder neutron diffraction data. Both crystallize in the orthorhombic system, space group Pnma , with lattice parameters a = 12.0588(1), A, b = 5.6123(1), A, c = 7.0535(1), A, and a = 12.0314(1), A, b = 5.5989(1), A, c = 7.0380(1), A, respectively. The phases are isostructural with Y 2 BaCuO 5 , consisting of a three-dimensional framework of M O 7 units ( M = Yb, Lu) enclosing cavities occupied by the Ba and Cu atoms. Ba is 11-coordinate and Cu 5-coordinate, in a distored squarepyramidal environment.

Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.

The superconducting transition temperature, T c , of YBa 2 Cu 3 O 6.41 quenched from 500°C into liquid nitrogen increases from 0 to 20 K while annealing at room temperature during the first few days following the quench. Using neutron powder diffraction and Rietveld refinement we show that this time-dependent increase of T c is accompanied by changes in the structural properties. The a -and c -axes contract by 0.04%, while the percentage shortening of the b -axis is three times smaller. We attribute this behavior to charge transfer between the chains and planes, with the smaller contraction along the b -axis arising from oxygen ordering in the chains. Clear evidence for this charge transfer is seen in the relevant Cu-O and Cu-Cu distances. No significant changes in the average occupancies of oxygen sites are observed. We attribute the increase in T c to local ordering of oxygen atoms around the Cu(1) atoms with no change in the average site occupancies.

Y 2 BaCuO 5 cristallise dans Pbmm avec a=7,1342, b=12,1811, c=5,6580 A, Z=4; affinement jusqu'a R=0,0293. La structure cristalline de ce compose a ete reaffinee en utilisant les valeurs determinees par Michel et Raveau (1982) comme valeurs de depart. La structure consiste en chaines paralleles a l'axe de prismes rhomboedriques de YO 7 , ayant des aretes communes. Les chaines sont liees de facon transversale par les atomes de Ba et Cu. Chaque atome Ba est donc coordine par cinq atomes O, formant une pyramide quadratique deformee

Several alumina-forming Fe-Al intermetallic alloys have been oxidized in oxygen for 100 h at 1000 C to understand the scaling kinetics, scale morphology, scale adhesion, and strain accommodation in the scales. Oxidation studies were conducted by thermogravimetry, followed by analyses of the surfaces of oxide scales. In addition, samples were cooled to 77 K and then fractured; then, their scale/metal interfaces were analyzed. Some of the scales were adhesion-tested by applying a tensile load to pull the scale away from the substrate. Finally, ruby fluorescence was used to measure in-plane strains in the oxide scales and values correlated with scale microstructures.

An experimental program has been initiated to evaluate the chemical, microstructural, and mechanical integrity of thermally grown oxide scales to establish requirements for improved corrosion performance in terms of composition, structure, and properties. Iron aluminides of several compositions were selected for the study. Oxidation studies were conducted in air and oxygen environments at 1000{degrees}C. The results showed that the scaling kinetics followed a parabolic rate law but that the rates in early stages of oxidation were significantly greater than in later stages; the difference could be attributed to the presence of fast-growing transient iron oxides in the layer during the early stages. Further, scale failure occurred via gross spallation, scale cracking, and nodule formation and was influenced by alloy composition. Auger electron spectroscopy of Ar-exposed specimens of ternary Fe-Cr-Al alloy showed sulfur on the gas/scale side of the interface; the sulfur decreased as the exposure time increased. Raman spectroscopy and ruby fluorescence were used to examine the scale development as a function of oxidation temperature. Ruby-line shift is used to examine phase transformations in alumina and to calculate compressive strains in thermally grown scales.

The authors present first-principles computations together with corresponding angle-resolved photoemission measurements in order to delineate the shape and polarization dependence of the spectral feature associated with the CuO{sub 2} plane-related bands from the (001) surface of YBa{sub 2}Cu{sub 3}O{sub 6.9}. Theoretical predictions are found to be in remarkable agreement with the observed character of the spectral feature between 0 and 0.3 eV binding energy (for k{sub {parallel}} values along the {Gamma}-S line), and indicate that the local-density-approximation based wave-functions implicit in the theory provide a reasonable description of the CuO{sub 2} plane band states near the Fermi energy. The computations also show that of the six possible surface terminations, the ARPES spectra from Y123(001) surface are reasonably described by the BaO/CuO{sub 2} ideal surface termination, i.e. by assuming a BaO layer followed by a CuO{sub 2} plane layer below.

This paper shows that CaO added to silicate glasses behaves much like Na/sub 2/O in converting bridging oxygen sites to nonbridging sites. Good correspondence with model predictions was obtained but deviations were still sufficiently large to warrant attention. We speculate that some CaO may remain unreacted or that small-scale phase separation may occur. XPS core level shifts were monitored and possible charge-transfer effects were considered. Preliminary theoretical calculations utilizing a molecular-cluster approach were presented. XPS spectra for various glass compositions were simulated by appropriately combining local densities of state calculated for individual atoms. Dominant spectral features observed experimentally were found in the simulations. In conjunction with experiments, more carefully refined calculations will be subsequently examined. 7 figures.

X-ray photoemission spectroscopy (XPS) studies have been undertaken on a series of soda aluminosilicate glasses with Al/Na ratio varying from 0 to 1.33 to test the current structural models for these glasses. By monitoring the amount of bridging and nonbridging oxygens as a function of Al/Na ratio, XPS results indicate that the amount of nonbridging oxygens in the glass structure is directly proportional to the sodium concentration and independent of the Al/Na ratio, thus contradicting the expectation of all the proposed structural models. Changes in the charge distribution of the aluminosilicate glass network and the sodium ions are clearly evident in the XPS results.

Sodium silicate glasses containing hexavalent uranium ions have been studied using the EXAFS technique. The U 6+ ions appear in the uranyl configuration with two oxygen atoms at 1.85 A and four to five at 2.2–2.3 A. In the glasses (0.25Na 2 O·0.75SiO 2 ) 1− x (UO 3 ) x with x = 0.02 to x = 0.1, planar (or nearly planar) uranium containing clusters, wi U-U distances of 3.3 A, are observed. A layered model is proposed to describe these glasses.

The valence-band structures of the light actinides thorium, uranium, and their dioxides have been investigated by means of x-ray photoemission spectroscopy. This study shows that the electronic structures of Th${\mathrm{O}}_{2}$ and U${\mathrm{O}}_{2}$ are quite similar, except for the presence of $5f$ electrons in U${\mathrm{O}}_{2}$ which lie close to the Fermi level. It is these low-binding-energy $5f$ electrons that give rise to the most dramatic differences in physical properties, notably the magnetism, electrical conductivity, and color. Conduction in U${\mathrm{O}}_{2}$ apparently occurs via holes created by excitation of rather localized $5f$ electrons from the uranium to interstitial oxygen-acceptor sites. Sputtering experiments on thin oxide film and bulk samples show Fermi-level shifts in x-ray photoemission spectra associated with doping changes that result from preferential removal of interstitial ("impurity") oxygen atoms. The deep-trap and polaron conduction models have been reexamined. It is our view, at this time, that neither model can be ruled out. For the pure metal thorium, valence-band spectra were compared with the available density-of-states calculations and satisfactory agreement was obtained. For $\ensuremath{\alpha}\ensuremath{-}\mathrm{uranium}$, no detailed band calculation exists. However, the results agree with the qualitative density-of-states picture presented by Friedel.

In recent years, substantial gains have been made in the development of spectroscopic techniques for electronic properties studies. These techniques have seen relatively small, but growing, application in the field of actinide research. Photoemission spectroscopies, reflectivity and absorption studies, and x-ray techniques will be discussed and illustrative examples of studies on actinide materials will be presented.

L'analyse des donnees EXAFS necessite la connaissance des fonctions d'amplitude et de deplacement de phases. On presente des procedures pour obtenir ces fonctions de facon experimentale et theorique. Application au cas de metaux de transition 3d

XANES measurements are reported for a number of transition metal oxides. Oxide phases, in which the transition element could be widely varied (within the 3d series) while preserving the crystal structure, were systematically examined. The materials examined include monoxides, perovskites, zircons and spinels. For those samples of a given oxide phase, the near edge spectra are nearly identical but spectra for different phases are dissimilar. These observations are consistent with the simplest view of the x-ray absorption process, namely that dipole selection rules are obeyed and spectral features predominately result from transitions between the K shell and empty states with p-character.

X-ray photoemission spectra of valence and bonding bands of UO 2 , U 3 O 8 and UO 3 are presented. The gradual transfer of localized 5f electrons into the bonding band as a function of oxygen content has been studied in these oxides.

We report structure, resistivity, and Meissner effect measurements on YBa/sub 2-//sub x/Sr/sub x/Cu/sub 3/O/sub 7-//sub delta/ for 0

Using XPS as the principal investigative tool, the bonding properties of selected metal oxides added to silicate glass are being examined. In this paper, the results of XPS studies of uranium, plutonium, and neptunium in binary and multicomponent silicate-based glasses are presented. Models are proposed to account for the very diverse bonding properties of 6/sup +/ and 4/sup +/ actinide ions in glass. 15 references, 3 figures, 1 table.

Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi(2)Sr(2)CaCu(2)O(8). Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

X-ray photoemission spectroscopy (XPS) studies have been undertaken on amorphous SiO2 and on silicate glasses containing varying amounts of Na2O. The XPS results enable one to distinguish the “bridging” oxygens linking adjacent silicon atoms from the “nonbridging” oxygens that appear in the glass when Na2O is added. Charge readjustment with Na2O addition can also be qualitatively monitored.

We report here the results obtained for powder diffraction studies on four YBa{sub 2}Cu{sub 3}O{sub x} samples in which a model with two orthorhombic phases was used to obtain an improved agreement. The implications of this structural model on the systematic variation of {Tc} with oxygen content will be discussed. 28 refs., 3 figs., 2 tabs.

Photoelectron spectroscopic measurements of core-electron levels in cations of transition-series insulators commonly show satellite features in the vicinity of the main photoelectron peaks. In this paper, the relationship between main lines and adjacent satellites is systematically examined for 3 d-series insulators. The spectral features are interpreted using a relaxation model that allows for the possibility of different final-state screening conditions of the photoinduced hole. Systematic trends are examined by employing atomic calculations and utilizing transition-state theory to calculate electron removal energies for atoms in appropriately simulated chemical environments. It is demonstrated that the main-line and satellite energies are both dependent on the cation charge environment but with different degrees of sensitivity. The consequence is that sensitivity to chemical change can be monitored in the observed main-line to satellite separation. Model predictions are tested by monitoring the dependence of the satellite separations on the cation valence state and on the electronegativity of both cations and ligated anions. Systematic tests are applied to 3 d-series insulators but more general applicability of the relaxation model is also discussed. 93 references, 19 figures, 1 table.