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A study of stability of tungstophosphoric acid, H{sub 3}PW{sub 12}O{sub 40}, using synchrotron XPS, XANES, Hexane cracking, XRD and IR spectroscopy

Description: Tungstophosphoric Acid (HPW) has been investigated using different spectroscopic and chemical techniques. Bulk sensitive techniques such as x-ray diffract ion (XRD) and infrared (IR) spectroscopy indicate that the acid is stable at temperatures as high as 300 degrees C or higher. However, our work suggests that HPW starts loosing stability at temperature as low as 200 degrees C. For instance, P 2p peak was not detected in the XPS spectrum of HPW preheated at 100 degrees C, but was clearly observed after preheating the acid at 200 degrees C and 400 degrees C. This suggests the destruction of the molecules of the surface leading to the enrichment of surface with phosphorus. These results may explain why HPW deactivates very fast, e.g., 8 min at 200 degrees C, in hexane cracking experiments. This could limit the use of HPW in surface reactions that even require moderate temperatures. Detailed infrared spectroscopic investigation of the HPW as a function of temperature showed a gradual in crease in absorbance of the W-O-W corner shared vibration relative to the absorbance of the other bands. This indicates that the symmetry, and hence the stability, of the molecule was decreased upon heating.
Date: June 7, 2002
Creator: Jalil, Pasl A.; Faiz, M.; Tabet, N.; Hamdan, N.M. & Hussain, Z.
Partner: UNT Libraries Government Documents Department

High resolution XPS study of oxide layers grown on Ge substrates

Description: High resolution X-ray Photoelectron Spectroscopy (XPS) was used to analyze thin layers of germanium oxide grown on germanium substrates under various conditions. The results reveal the presence of high density of electron states located at the oxide/germanium interface that lead to the energy band bending. The surface of native oxide layers and that of thin oxide layer grown under dry oxygen correspond to GeO2 composition. Under Ar etching, lower oxidation states were revealed. Short in-situ heat treatment at T=400 degrees C under ultra high vacuum leads to the removal of the oxide layer. In addition, the analysis of the layer grown at T=380 degrees C under dry oxygen suggest that carbides form at the oxide/substrate interface.
Date: July 29, 2002
Creator: Tabet, N.; Faiz, M.; Hamdan, N.M. & Hussain, Z.
Partner: UNT Libraries Government Documents Department

Magnetic properties of fluorinated Pb-doped Hg-1223 high Tc superconductors

Description: Fluorination of Pb-doped HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub y} (Hg-1223) system was investigated. Structural and magnetic measurements reveal that fluorine addition promotes the phase formation, optimize the charge carrier concentration, and enhance flux pinning. We report an increase in both the transition temperature and the critical current density through controlled fluorine incorporation. Even a possible change in the pinning mechanism in this technologically important system is suggested.
Date: September 23, 2001
Creator: Hamdan, N.M.; Sastry, P.V.P.S.S. & Schwartz, J.
Partner: UNT Libraries Government Documents Department

Soft X-ray photoemission studies of Hf oxidation

Description: Soft X-Ray Photoemission Spectroscopy using surface sensitive Synchrotron Radiation has been applied to accurately determine the binding energy shifts and the valence band offset of the HfO2 grown on Hf metal. Charging of oxide films under x-rays (or other irradiation) is circumvented by controlled and sequential in-situ oxidation. Photoemission results show the presence of metallic Hf (from the substrate) with the 4f7/2 binding energy of 14.22 eV, fully oxidized Hf (from HfO2) with the 4f7/2 binding energy of 18.16 eV, and at least one clear suboxide peak. The position of the valence band of HfO2 with respect to the Hf(m) Fermi level is determined as 4.05 eV.
Date: February 1, 2002
Creator: Suzer, S.; Sayan, S.; Banaszak Holl, M.M.; Garfunkel, E.; Hussain, Z. & Hamdan, N.M.
Partner: UNT Libraries Government Documents Department

Understanding the magnetic anisotropy in Fe-Si amorphous alloys

Description: The origin of the magnetic anisotropy in a very disordered Fe-Si alloy has been investigated. The alloy containing 40 percent at. Si was prepared in the form of a thin film in a DC magnetron sputtering chamber. Structural disorder was obtained from Extended X-ray Absorption Fine Structure spectroscopy. The uniformity and lack of inhomogeneities at a microscopic level was checked by measuring their transverse magnetic susceptibility and hysteresis loops. The orbital component of the magnetic moment was measured by X-ray Magnetic Circular Dichroism spectroscopy. The orbital moment was extraordinary high, 0.4mB. Such a high value contrasted with the relatively small uniaxial anisotropy energy of the thin film (2kJ/m3). This suggests that the cause of the magnetic anisotropy in this alloy was a small degree of correlation in the orientation of the local orbital moments along a preferential direction.
Date: August 1, 2002
Creator: Diaz, J.; Hamdan, N.M.; Jalil, P.; Hussain, Z.; Valvidares, S.M. & Alameda, J.M.
Partner: UNT Libraries Government Documents Department