M. Grimsditch

The structure of phosphorus-selenium glasses I. Concentration dependence of the short- and intermediate-range order☆

Abstract The structures of phosphorus-selenium glasses PxPe1−x with x=0 (pure selenium), 0.05, 0.15, 0.25, 0.40 and 0.50 have been investigated with neutron diffraction, extented, X-ray absorption fine structure measurements, and Raman and infrared spectroscopy. The behavior of the near-neighbor coordination at r∼2.3 A as a function of x suggests a picture in which P4Sen molecular units are linked together by Se atoms or chains. The diffraction patterns exhibit anomalous peaks at Q ∼ 1.2A which rise dramatically as P is added to the Se glass; these peaks can also be understood on a molecular picture. The Raman spectra at the larger P concentrations are characterized by series of relatively sharp peaks which also suggest molecular units and for x = 0.5 bear a close correspondence to the spectrum of crystalline P4Se3.

Strain determination in thermally-grown alumina scales using fluorescence spectroscopy

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.

Residual stresses in convoluted oxide scales

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.

Alkali carbonates: Raman spectroscopy, ab initio calculations, and structure

Abstract Equilibrium structures of Li 2 CO 3 and K 2 CO 3 were calculated using ab initio molecular orbital calculations carried out at the Hartree-Fock (HF) level. Of the four structures considered for Li 2 CO 3 and K 2 CO 3 , the most stable was a structure with all five atoms in a plane. The harmonic frequencies were also calculated and found to be in agreement with the present Raman measurements. Structure factors, calculated from the ab initio data for each of the four structures considered, are compared with existing X-ray results.

Effect of Surface Roughness on Oxidation: Changes in Scale Thickness, Composition, and Residual Stress

The effect of surface roughness on the properties of the oxide scale formed on Fe–Cr–Al alloys during oxidation in air at high temperatures has been investigated. Large and systematic differences in scale thickness, in the composition of the oxides forming the scale, and in the residual stess levels are found.

Transient oxidation in Fe-Cr-Ni alloys: A Raman-scattering study

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.

Elastic constants of metal‐insulator superlattices

A Brillouin scattering study of elastic properties of NbN/AlN superlattices is presented. Because the coupling of light to surface waves in this system is very weak, we describe a modification of the technique which enhances the scattering efficiency, thereby allowing measurements of a system which is otherwise inaccessible. This particular multilayer superlattice is one of the very few that does not exhibit any elastic anomalies as a function of layer thickness in accordance with the idea that electron transfer may be the mechanism responsible for elastic anomalies in superlattices.

Phase diagram and oxygen stoichiometry of Y‐Ba‐Cu‐O thin films

We have performed an exhaustive study of the phase diagram of Y‐Ba‐Cu‐O thin films using chemical analysis, energy dispersive x‐ray spectroscopy, Auger electron spectroscopy, and x‐ray diffraction. We show that Raman scattering can provide information regarding impurity phases and oxygen stoichiometry in thin films

Raman study of phonons in K2Al2B2O7 crystals

A Raman scattering study of the vibrational modes in K2Al2B2O7 crystals has been performed at room temperature. The vibrational modes are classified by the nuclear site group analysis method and the internal vibrational modes are assigned to vibrations of the BO3 triangle and AlO4 tetrahedron.

Anisotropic uniaxial pressure effects in YBa2Cu3O7−δ

We present measurements of the uniaxial pressure dependence ofTc of untwinned YBa2Cu3O7−δ crystals with various oxygen stoichiometries. For all samples investigated,Tc decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La2−xSrxCuO4 and YBa2Cu4O8 systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.