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45 results on '"BARR, T. L."'

28. ESCA Studies of Yttrium Orthoaluminum Perovskites

Author

Pawlak, D. A., Wozniak, K., Frukacz, Z., Barr, T. L., Fiorentino, D., and Hardcastle, S.

Abstract

Room temperature X-ray photoelectron spectroscopy (XPS or ESCA) has been employed to investigate pure single crystals of yttrium aluminum perovskite (YAlO3 (YAP)) and YAlO3 doped with several rare earth elements, Y1-xLnxAlO3 (Ln = Pr, Er, Ho). XPS peak shapes of the main elements forming the YAP structure, in structures that have been doped with ions of varying oxidation states, are rationalized as a function of structural environment and the oxidation state of doping ions. The significant changes in chemical shifts and full widths at half-maximum of the signals are caused by doping and the presence of color centers. The most distinctive change in chemical shifts is observed for the O(1s) peaks. Annealing in H2/N2 atmosphere (1200 °C) changes the color of the sample, removing color centers due to relocation of oxygen ions in the structure. Although the total amount of oxygen in the samples does not change, the relative ratio of oxygen ions occupying two independent positions in the structure changes on annealing. Only in the case of YAlO3 without color centers is the composition of crystal close to the stoichiometric one. Also the Y(3d) and Al(2p) signals are shifted to higher binding energies for doped YAP crystals.

Published
1999

29. ESCA Studies of Yttrium Aluminum Garnets

Author

Pawlak, D. A., Wozniak, K., Frukacz, Z., Barr, T. L., Fiorentino, D., and Seal, S.

Abstract

X-ray photoelectron spectroscopy (XPS/ESCA) has been employed to investigate pure single crystals and powdered samples of yttrium aluminum garnet, Y3Al5O12 (YAG), and YAG crystals doped with several rare earth elements (Ln = Pr, Er, Ho, Tm) and a transition metal (Cr). Core level XPS peak shapes of the main elements forming the garnet structure can be rationalized due to different structural environments of particular ions modified by doping. The change of dopant oxidation state also results in variation of XPS peaks and helps to identify the sites in which it takes place. Single-crystal and powder samples give different XPS spectra. Possible sources of these differences are discussed. Similarities and differences between simple and mixed oxides are shown. The structure of YAG suggests the presence of only one independent oxygen ion; however the O(1s) spectra of all YAG systems exhibit two readily discerned peaks. An explanation for this dichotomy is discussed, involving the possible polarization of the oxygen valence electron density between the aluminum and yttrium. Alternative explanations are also considered.

Published
1999

44. X-ray diffraction studies of oxidized high-palladium alloys.

Author

Brantley WA, Cai Z, Papazoglou E, Mitchell JC, Kerber SJ, Mann GP, and Barr TL

Subjects
Copper chemistry, Gallium chemistry, Oxides chemistry, X-Ray Diffraction, Dental Alloys chemistry, Palladium chemistry
Abstract

Objective: The purpose of this study was to use x-ray diffraction (XRD) to obtain new information about the oxide layers on four representative oxidized high-palladium alloys., Methods: Cast specimens of two Pd-Cu-Ga alloys and two Pd-Ga alloys, with both polished and etched surfaces and air-abraded surfaces, were subjected to oxidation procedures recommended by the manufacturers. The specimens were analyzed by x-ray diffraction using CuK alpha radiation, and the peaks were compared to appropriate Joint Committee on Powder Diffraction Standards (JCPDS)., Results: The surface preparation procedure had a profound effect on the phases present in the oxide layers. For the specimens that had been polished and etched, CuGa2O4 and beta-Ga2O3 were detected on the 79Pd-10Cu-9Ga-2Au alloy, whereas SnO2 and CuGa2O4 were detected on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy. The oxide layers on both Pd-Cu-Ga alloys contained Cu2O1 and the oxide layer on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy may contain beta-Ga2O3. The principal phase in the oxide layers on both Pd-Ga alloys that had been polished and etched was ln2O3, which exhibited extreme preferred orientation. No other phase was detected in the oxide layer on the 85Pd-10Ga-2Au-1Ag-1 ln alloy, whereas beta-Ga2O3 was found in the oxide layer on the 75Pd-6Ga-6Au-6Ag-6.5ln alloy. For the air-abraded specimens, beta-Ga2O2 was not present in the oxide layers on the Pd-Cu-Ga alloys, and beta-Ga2O3 was the major phase in the oxide layers on the Pd-Ga alloys. Palladium oxide(s) in varying amounts were detected for both surface preparations of the Pd-Cu-Ga alloys and for the air-abraded Pd-Ga alloys. Except for the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy, the oxide layer caused minimal change in the lattice parameter of the palladium solid solution compared to that for the as-cast alloy., Significance: Knowledge of the phases found in the oxide layers on these high-palladium alloys is of fundamental importance for interpreting differences in the adherence of dental porcelain to the metal substrates under static and dynamic conditions, and may provide guidance in the development of new high-palladium alloys with improved metal-ceramic bonding.

Published
1996
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45. Surface chemistry and biological pathogenicity of silicates: an X-ray photoelectron spectroscopic study.

Author

Seal S, Krezoski S, Barr TL, Petering DH, Klinowski J, and Evans PH

Subjects
Animals, Cell Division drug effects, Mice, Molecular Structure, Silicates toxicity, Spectrometry, X-Ray Emission, Tumor Cells, Cultured, Silicates chemistry
Abstract

We extend our electron spectroscopy for chemical analysis studies of the chemistry of silicates to provide direct surface chemical information on the interactions involved in silicate-induced lung and tissue pathology. A total of five fibrous and non-fibrous silicate minerals, primarily amphiboles, have been studied: anthophyllite, tremolite, cummingtonite, hornblende and actinolite. We have followed the 'inlattice' surface chemistry of these materials and monitored features such as the simultaneous presence of four- and six-coordinate (with respect to oxygen) structural aluminium, and the presence of iron in the M4 octahedral positions. In vitro experiments involving contact of the silicate with cultured murine Ehrlich cells have identified modifications in the surface chemistry of Al, Mg and Fe in the silicates and changes in cellular iron content.

Published
1996
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