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Kondo-like 4f delocalization in Gd at high pressure

Description: We present resonant inelastic x-ray scattering (RIXS) and x-ray emission spectroscopy (XES) results which suggest Kondo-like aspects in the delocalization of 4f electrons in Gd metal to 113 GPa. Analysis of the RIXS data reveal a prolonged and continuous process throughout the entire pressure range, so that the volume collapse transition at 59 GPa is only part of the delocalization phenomenon. Moreover, the L{sub {gamma}1} XES spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.
Date: November 28, 2005
Creator: Maddox, B R; Lazicki, A; Yoo, C S; Iota, V; Chen, M; McMahan, A K et al.
Partner: UNT Libraries Government Documents Department

New cubic phase of lithium nitride to 200 GPa

Description: We present a new cubic ({gamma}) Li{sub 3}N phase discovered above 40({+-}5) GPa. Structure and electronic bands are examined at high pressure with synchrotron x-ray diffraction and inelastic x-ray scattering in a diamond anvil cell, and also with first-principles calculations. We observe a dramatic band-gap widening and volume collapse at the phase transition. {gamma}-Li{sub 3}N remains extremely stable and ionic to 200 GPa, with predicted metallization near 8 TPa. The high structural stability, wide band-gap and simple electronic structure of {gamma}-Li{sub 3}N are analogous to that of such lower valence closed-shell solids as NaCl, MgO and Ne, meriting its use as a low-Z internal pressure standard.
Date: July 19, 2005
Creator: Lazicki, A; Maddox, B; Evans, W; Yoo, C S; McMahan, A K; Pickett, W E et al.
Partner: UNT Libraries Government Documents Department

Pressure-Induced Electronic Spin Transition of Iron in Magnesiow?stite-(Mg,Fe)O

Description: An electronic transition of iron in magnesiowuestite has been studied with synchrotron Moessbauer and X-ray emission spectroscopies under high pressures. Synchrotron Moessbauer studies show that the quadrupole splitting disappears and the isomer shift drops significantly across the spin-paring transition of iron in (Mg{sub 0.75},Fe{sub 0.25})O between 62 and 70 GPa, whereas X-ray emission spectroscopy of the Fe-K{sub {beta}} fluorescence lines in dilute (Mg{sub 0.95},Fe{sub 0.05})O also confirms that a high-spin to low-spin transition occurs between 46 GPa and 55 GPa. Based upon current results and percolation theory, we reexamine the high-pressure phase diagram of (Mg,Fe)O and find that iron-iron exchange interaction plays an important role in stabilizing the high-spin state of iron in FeO-rich (Mg,Fe)O.
Date: October 5, 2005
Creator: Lin, J F; Gavriliuk, A G; Struzhkin, V V; Jacobsen, S D; Sturhahn, W; Hu, M Y et al.
Partner: UNT Libraries Government Documents Department

Low cost hydrogen/novel membrane technology for hydrogen separation from synthesis gas

Description: The production of hydrogen from synthesis gas made by gasification of coal is expensive. The separation of hydrogen from synthesis gas is a major cost element in the total process. In this report we describe the results of a program aimed at the development of membranes and membrane modules for the separation and purification of hydrogen from synthesis gas. The performance properties of the developed membranes were used in an economic evaluation of membrane gas separation systems in the coal gasification process. Membranes tested were polyetherimide and a polyamide copolymer. The work began with an examination of the chemical separations required to produce hydrogen from synthesis gas, identification of three specific separations where membranes might be applicable. A range of membrane fabrication techniques and module configurations were investigated to optimize the separation properties of the membrane materials. Parametric data obtained were used to develop the economic comparison of processes incorporating membranes with a base-case system without membranes. The computer calculations for the economic analysis were designed and executed. Finally, we briefly investigated alternative methods of performing the three separations in the production of hydrogen from synthesis gas. The three potential opportunities for membranes in the production of hydrogen from synthesis gas are: (1) separation of hydrogen from nitrogen as the final separation in a air-blown or oxygen-enriched air-blown gasification process, (2) separation of hydrogen from carbon dioxide and hydrogen sulfide to reduce or eliminate the conventional ethanolamine acid gas removal unit, and (3) separation of hydrogen and/or carbon dioxide form carbon monoxide prior to the shift reactor to influence the shift reaction. 28 refs., 54 figs., 40 tabs.
Date: October 1, 1990
Creator: Baker, R.W.; Bell, C.M.; Chow, P.; Louie, J.; Mohr, J.M.; Peinemann, K.V. et al.
Partner: UNT Libraries Government Documents Department

300 Degree C GaN/AlGaN Heterojunction Bipolar Transistor

Description: A GaN/AIGaN heterojunction bipolar transistor has been fabricated using C12/Ar dry etching for mesa formation. As the hole concentration increases due to more efficient ionization of the Mg acceptors at elevated temperatures (> 250oC), the device shows improved gain. Future efforts which are briefly summarized. should focus on methods for reducing base resistance.
Date: October 14, 1998
Creator: Abernathy, C. R.; Baca, A. G.; Cho, H.; Chow, P. P.; Han, J.; Hichman, R. A. et al.
Partner: UNT Libraries Government Documents Department