W. T. Elam

A new atomic database for X-ray spectroscopic calculations

Abstract The authors undertook to compile a database of recent values of the atomic parameters required for fundamental parameters (FP) calculation of X-ray fluorescence (XRF) spectra, calculation of X-ray absorption in crystals and other samples, and correction of X-ray absorption spectra for self-absorption effects. All values were obtained from published sources and include the elements hydrogen (atomic number 1) through californium (atomic number 98). The data were collected into a single unstructured ASCII text file.

On the formation of nanocrystals in the soft magnetic alloy Fe73.5Nb3Cu1Si13.5B9

Studies were conducted on alloys of atomic mass composition Fe73.5Nb3Cu1Si13.5B9 and Fe76.5Cu1Si13.5B9. TEM studies demonstrated that Fe-rich nanocrystals precipitated from the amorphous starting materials exhibited the DO3 structure from very early in the transformation process. This result was corroborated by extended X-ray absorption fines structure (EXAFS) studies which showed evidence of ordering in the Fe-rich precipitates from early in the transformation process. EXAFS studies also showed that Cu clusters with near-f.c.c. structure were present from very early stages of the crystallization process. A model is proposed which describes the role of the Cu clusters as catalyzing nucleation of the DO3 crystallites by providing a low energy interface upon which the activation energy for nucleation of the DO3 phase is lowered.

On the role of Cu and Nb in the formation of nanocrystals in amorphous Fe73.5Nb3Cu1Si13.5B9

Extended x‐ray absorption fine structure analysis of amorphous and partially crystallized samples of the soft magnetic alloy Fe73.5Nb3Cu1Si13.5B9 reveals that, even before heat treatment, a portion of the Cu is present in the form of tiny, close packed clusters. Analysis of the Nb‐free alloy Fe76.5Cu1Si13.5B9 shows that the Cu clusters are not present in the quenched ribbons, but that fcc Cu precipitates form during heat treatment. Results suggest that the Cu clusters act to catalyze nucleation of Fe‐rich nanocrystals, but that these clusters are formed on a finer scale when Nb is added to the alloy, perhaps because it helps to lower the solubility of Cu in the amorphous phase.

Multielement spectrometer for efficient measurement of the momentum transfer dependence of inelastic x-ray scattering

Nonresonant x-ray Raman scattering (XRS) is the inelastic scattering of hard x rays from the K shell of low-Z elements or the less tightly bound shells of heavier elements. In the limit of low momentum transfer q, XRS is determined by the same transition matrix element as is measured by x-ray absorption spectroscopies. However, XRS at higher q can often access higher order multipole transitions which help separate the symmetry of various contributions to the local density of states. The main drawback of XRS is its low cross section—a problem that is compounded for a q-dependent study. To address this issue, we have constructed a multielement spectrometer to simultaneously measure XRS at ten different values of q. By means of example, we report new measurements of the XRS from the L- and K-edges of Mg. This instrument is now available to general users at the Advanced Photon Source as the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer.

Optical performance of the Naval Research Laboratory's Materials Analysis Beam Line at the NSLS

Abstract The optical performance of the Naval Research Laboratorys Materials Analysis Beam Line at the National Synchrotron Light Source is reported. Photon fluxes into the experimental hutch were measured over the energy range from 3 to 11 keV. Two sets of flux data were taken, one with no slits in the beam and the other with a 2.5 mm diameter slit which passed only the best focused portion of the beam. At 6 keV, a total flux in excess of 2 × 109 photons/(s mA) of stored current was observed. The focal spot was mapped at several energies using a pinhole and a detector which were mounted on a two dimensional translation stage. At 6 keV the fwhm dimensions of the focal spot were 1.0 mm horizontally, and 3.0 mm vertically. The energy spread in the monochromatic beam was estimated by measuring the width of a narrow pre-edge feature of manganese in KMnO4 at 6.5 keV. The spread was minimized by adjusting the curvature and incident angle of a collimating mirror upstream of the beam lines monochromator. At 6.5 keV an energy spread of 2.6 eV was observed. These measured quantities of flux, spot size and energy resolution are compared to their expected values [1,2] to give an overall view of the performance of the optics.

The local atomic order of Cu and Fe in heat treated Fe/sub 73.5/Nb/sub 3/Cu/sub 1/Si/sub 13.5/B/sub 9/ ribbons

EXAFS analysis of amorphous and partially crystallized samples of the soft magnetic alloy Fe/sub 73.5/Nb/sub 3/Cu/sub 1/Si/sub 13.5/B/sub 9/ reveals that a portion of the Cu is present in the form of tiny, close packed clusters even before heat treatment. The results suggest that the strong role of Cu in modifying the microstructure of this alloy is due to the clusters acting to catalyze nucleation of Fe rich nanocrystals. >

Examination of coherent surface reflection coefficient (CSRC) approximations in shallow water propagation

The parabolic wave equation (PE) code of Rosenberg [J. Acoust. Soc. Am. 105, 144–153 (1999)] is used as a benchmark to study acoustic propagation in an ocean waveguide with a rough air/water interface. The PE results allow a close examination of the ability of a ray code [i.e., Gaussian RAy Bundle (GRAB)] to accurately estimate coherent field propagation using a coherent reflection coefficient derived from scattering theory. Comparison with PE implies that the Beckmann–Spizzichino model, as given within the GRAB software package, does not give accurate predictions of the coherent field at long ranges. Three other coherent reflection coefficient approximations are tested: the perturbation, the small slope, and the Kirchhoff approximations. The small slope approximation is the most accurate of the models tested. However, the Kirchhoff approximation is perhaps accurate enough for some purposes and would be simpler to implement as a module within GRAB.

Plasma sprayed superconducting oxides

Abstract A technique for plasma spray processing of high temperature superconducting oxides is discussed. Powdered superconducting material is fed into a thermal, high pressure plasma flame and deposited onto suitably prepared substrates. A post-spray anneal yields deposits with transition temperatures above the boiling point of liquid nitrogen.

A model for nucleation of nanocrystals in the soft magnetic alloy Fe73.5Nb3Cu1Si13.5B9

Abstract Melt-spun ribbons of the alloy Fe73.5Nb3Cu1Si13.5B9 are amorphous, but heat treating them at 550°C for one hour generates a microstructure which consists of DO3 structure FeSi crystallites about 10 nm in diameter which are separated from one another by remnant amorphous films that are enriched in B and Nb. Numerous investigators have proposed that nucleation on this exceedingly fine scale occurs at sites which are locally enriched in Fe, this enrichment being caused by partitioning of Cu during the heat treatment. Extended x-ray absorption fine structure (EXAFS) results show that Cu rich clusters with a near fcc structure form early in the precipitation process, and that the size of these clusters is influenced by the presence of Nb. These results, and an extensive transmission electron microscopy study, support an argument that the DO3 precipitates form in intimate contact with the Cu clusters, rather than at sites displaced from them. This model is consistent with results from other experimental studies and with theoretical analyses which show that the surface energy barrier to nucleation of subsequent phases is lowest at the first-formed precipitate.

Development of an electrospray approach to deposit complex molecules on plasma modified surfaces

Two established techniques have been coupled to allow surfaces to be precision engineered. Electrospray ionization to bring large, complex, intact molecular ions into the gas phase has been interfaced with a radio frequency (rf) plasma reactor to treat surfaces making them receptive to the deposition of active biomolecules. The new instrument has been designed and used successfully to deposit a number of high molecular weight molecules including the polysaccharide, sodium hyaluronan (HA), that has an important role in a number of physiological functions. Substrate material is treated using a rf glow discharge plasma chamber, to clean and activate the surface in a controlled manner, then exposed to a beam of multiply charged ions in the gas phase that have been generated using electrospray techniques. The ions are deposited gently onto the substrate and become covalently bound. The molecular integrity and stability of HA surfaces prepared in this way was established using x-ray photoelectron spectroscopy, ...