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Development of Brazing Technology for Use in High- Temperature Gas Separation Equipment

Description: The development of high-temperature electrochemical devices such as oxygen and hydrogen separators, fuel gas reformers, solid oxide fuel cells, and chemical sensors is part of a rapidly expanding segment of the solid state technology market. These devices employ an ionic conducting ceramic as the active membrane that establishes the electrochemical potential of the device, either under voltage (i.e. to carry out gas separation) or under chemical gradient (to develop an electrical potential and thereby generate electrical power). Because the device operates under an ionic gradient that develops across the electrolyte, hermiticity across this layer is paramount. That is, not only must this thin ceramic membrane be dense with no interconnected porosity, but it must be connected to the rest of the device, typically constructed from a heat resistant alloy, with a high-temperature, gas-tight seal. A significant engineering challenge in fabricating these devices is how to effectively join the thin electrochemically active membrane to the metallic body of the device such that the resulting seal is hermetic, rugged, and stable during continuous high temperature operation. Active metal brazing is the typical method of joining ceramic and metal engineering components. It employs a braze alloy that contains one or more reactive elements, often titanium, which will chemically reduce the ceramic faying surface and greatly improve its wetting behavior and adherence with the braze. However, recent studies of these brazes for potential use in fabricating high-temperature electrochemical devices revealed problems with interfacial oxidation and subsequent joint failure [1,2]. Specifically, it was found that the introduction of the ceramic electrolyte and/or heat resistant metal substrate dramatically affects the inherent oxidation behavior of the braze, often in a deleterious manner. These conclusions pointed to the need for an oxidation resistant, high-temperature ceramic-to-metal braze and consequently lead to the development of the novel reactive air brazing ...
Date: April 23, 2003
Creator: Weil, K.S.; Hardy, J.S. & Kim, J.Y.
Partner: UNT Libraries Government Documents Department

Theory of self-organized critical transport in tokamak plasmas

Description: A theoretical and computational study of the ion temperature gradient and {eta}{sub i} instabilities in tokamak plasmas has been carried out. In toroidal geometry the modes have a radially extended structure and their eigenfrequencies are constant over many rational surfaces that are coupled through toroidicity. These nonlocal properties of the ITG modes impose strong constraint on the drift mode fluctuations and the amciated transport, showing a self-organized characteristic. As any significant deviation away from marginal stability causes rapid temperature relaxation and intermittent bursts, the modes hover near marginality and exhibit strong kinetic characteristics. As a result, the temperature relaxation is self-semilar and nonlocal, leading to a radially increasing heat diffusivity. The nonlocal transport leads to the Bohm-like diffusion scaling. The heat input regulates the deviation of the temperature gradient away from marginality. The obtained transport scalings and properties are globally consistent with experimental observations of L-mode charges.
Date: July 1, 1995
Creator: Kishimoto, Y.; Tajima, T.; Horton, W.; LeBrun, M.J. & Kim, J.Y.
Partner: UNT Libraries Government Documents Department

Kinetic resonance damping rate of the toroidal ion temperature gradient mode

Description: The linear damping rates of the toroidal ion temperature gradient ({eta}{sub i}) mode due to the toroidal resonance are calculated in the local kinetic limit. The well-known Landau contour method is generalized to treat the analytic continuation problem of the guiding center dispersion function in the toroidal resonance system where the resonance occurs from both the magnetic {del}B-curvature drift and the parallel ion transit drift. A detailed numerical analysis is presented for the dependence of the damping rate of the toroidal {eta}{sub i} mode on various parameters such as {epsilon}{sub n}, {kappa}{sub y}, and the rapped electron fraction. In addition, a consideration is presented on the decay problem of the ballistic response by phase mixing in the toroidal system, which is directly related to the kinetic damping problem of the wave normal modes by the toroidal resonance.
Date: November 1, 1993
Creator: Kim, J. Y.; Kishimoto, Y.; Horton, W. & Tajima, T.
Partner: UNT Libraries Government Documents Department

Toroidal kinetic {eta}{sub i}-mode study in high temperature plasmas

Description: A new kinetic integral equation for the study of the ion temperature gradient driven mode in toroidal geometry is developed that includes the ion toroidal (curvature and magnetic gradient) drift motion {omega}{sub D}, the mode coupling from finite {kappa}{sub {parallel}} due to the toroidal feature of the sheared magnetic configuration. The integral equation allows the stability study for arbitrary {kappa}{sub {parallel}} {upsilon}{sub i}/({omega} {minus} {omega}{sub D}) and {kappa}{perpendicular}{rho}{sub i}. A systematic parameter study is carried out for the low {beta} circular flux surface equilibrium. Possible correlations between the unstable mode characteristics and some experimental results such as fluctuation spectrum and anomalous ion transport measurements are discussed.
Date: November 1, 1991
Creator: Dong, J. Q., Horton, W. & Kim, J. Y.
Partner: UNT Libraries Government Documents Department

Toroidal kinetic. eta. sub i -mode study in high temperature plasmas

Description: A new kinetic integral equation for the study of the ion temperature gradient driven mode in toroidal geometry is developed that includes the ion toroidal (curvature and magnetic gradient) drift motion {omega}{sub D}, the mode coupling from finite {kappa}{sub {parallel}} due to the toroidal feature of the sheared magnetic configuration. The integral equation allows the stability study for arbitrary {kappa}{sub {parallel}} {upsilon}{sub i}/({omega} {minus} {omega}{sub D}) and {kappa}{perpendicular}{rho}{sub i}. A systematic parameter study is carried out for the low {beta} circular flux surface equilibrium. Possible correlations between the unstable mode characteristics and some experimental results such as fluctuation spectrum and anomalous ion transport measurements are discussed.
Date: November 1, 1991
Creator: Dong, J.Q., Horton, W. & Kim, J.Y.
Partner: UNT Libraries Government Documents Department

Anomalous ion thermal transport in hot ion plasmas by the ion temperature gradient mode

Description: Experiments show that the observed radial profiles of the ion thermal conductivity {chi}{sub i} have the opposite shapes with those obtained from the ion temperature gradient mode ({eta}{sub i} mode) turbulence model by the traditional mixing length estimate. In this work, this radial profile problem is reconsidered with an electromagnetic study of the linear stability of the toroidal {eta}{sub i} mode and a new rule for choosing the mixing length. It is first shown that the electromagnetic effect gives a significant stabilizing effect on the toroidal {eta}{sub i} mode, and that the observed reduction of {chi}{sub i}(r) in the core region can be explained by this electromagnetic effect. Secondly, in view of earlier numerical simulations showing the transfer of fluctuation energy to larger scales that those for the fastest growth rate, as well as fluctuation measurements indicating longer radial correlation lengths, a new mixing length formula is proposed to explain the radial increase of the {chi}{sub i}. It is shown the new formula fits well the observed {chi}{sub i}(r) profiles in two TFTR supershot discharges and also gives the scaling law in the current and the magnetic field which agrees better with experiment than the conventional formula.
Date: June 1992
Creator: Kim, J. Y.; Horton, W. & Coppi, B.
Partner: UNT Libraries Government Documents Department

Kinetic resonance damping rate of the toroidal ion temperature gradient mode

Description: The linear damping rates of the toroidal ion temperature gradient ({eta}{sub i}) mode due to the toroidal resonance are calculated in the local kinetic limit. The well-known Landau contour method is generalized to treat the analytic continuation problem of the guiding center dispersion function in the toroidal resonance system where the resonance occurs from both the magnetic {Delta}B-curvature drift and the parallel ion transit drift. A detailed numerical analysis is presented for the dependence of the damping rate of the toroidal {eta}{sub i} mode on various parameters such as {var_epsilon}{sub n}, {kappa}{sub y}, and the trapped electron fraction. In addition, a consideration is presented on the decay problem of the ballistic response by the phase mixing in the toroidal system, which is directly related to the present damping problem of the wave normal modes by the toroidal resonance.
Date: September 1, 1993
Creator: Kim, J. Y.; Kishimoto, Y.; Horton, W. & Tajima, T.
Partner: UNT Libraries Government Documents Department

Self-Organized Profile Relaxation by Ion Temperature Gradient Instability in Toroidal Plasmas

Description: Toroidal effects on the ion-temperature gradient mode are found to dictate the temperature evolution and the subsequent relaxed profile realization according to our toroidal particle simulation. Both in the strongly unstable fluid regime as well as in the near-marginal kinetic regime we observe that the plasma maintains an exponential temperature profile and forces the heat flux to be radially independent. The self-organized critical relaxed state is sustained slightly above the marginal stability, where the weak wave growth balances the wave decorrelation.
Date: February 1, 1993
Creator: Kishimoto, Y.; Tajima, T.; LeBrun, M. J.; Gray, M. G.; Kim, J. Y. & Horton, W.
Partner: UNT Libraries Government Documents Department

Self-organized profile relaxation by ion temperature gradient instability in toroidal plasmas

Description: Toroidal effects on the ion-temperature gradient mode are found to dictate the temperature evolution and the subsequent relaxed profile realization according to our toroidal particle simulation. Both in the strongly unstable fluid regime as well as in the near-marginal kinetic regime we observe that the plasma maintains an exponential temperature profile and forces the heat flux to be radially independent. The self-organized critical relaxed state is sustained slightly above the marginal stability, where the weak wave growth balances the wave decorrelation.
Date: February 1, 1993
Creator: Kishimoto, Y.; Tajima, T.; LeBrun, M.J.; Gray, M.G.; Kim, J.Y. & Horton, W.
Partner: UNT Libraries Government Documents Department

Transport in the self-organized relaxed state of ion temperature gradient instability

Description: We investigate the anomalous heat conduction in a tokamak plasma analytically and computationally. Our toroidal particle simulation shows a new emerging physical picture that the toroidal plasma exhibits marked properties distinct from a cylindrical plasma: (1) the development of radially extended potential streamers localized to the outside of the torus, (2) more robust ion temperature gradient instability, (3) radially constant eigenfrequency, (4) global temperature relaxation, and (5) radially increasing heat conductivity {chi}{sub i}. These results are analyzed by linear and quasilinear kinetic theory. A relaxation theory based on the reductive perturbation theory in the quasilinear equation is developed. The theory constrains the thermal flux so that {chi}{sub i} increases radially. The Bohm-like scaling is found in connection with the radially extended mode structure.
Date: June 1, 1993
Creator: Tajima, T.; LeBrun, M. J.; Gray, M. G.; Kim, J. Y.; Horton, W.; Wong, V. et al.
Partner: UNT Libraries Government Documents Department

Microstructure and pinning properties of hexagonal-disc shaped single crystalline MgB2

Description: We synthesized hexagonal-disc-shaped MgB2 single crystals under high-pressure conditions and analyzed the microstructure and pinning properties. The lattice constants and the Laue pattern of the crystals from x-ray micro-diffraction showed the crystal symmetry of MgB2. A thorough crystallographic mapping within a single crystal showed that the edge and c axis of hexagonal-disc shape exactly matched the [10-10] and the [0001] directions of the MgB2 phase. Thus, these well-shaped single crystals may be the best candidates for studying the direction dependences of the physical properties. The magnetization curve and the magnetic hysteresis curve for these single crystals showed the existence of a wide reversible region and weak pinning properties, which supported our single crystals being very clean.
Date: December 12, 2002
Creator: Jung, C.U.; Kim, J.Y.; Chowdhury, P.; Kim, Kijoon H.P.; Lee, Sung-Ik; Koh, D.S. et al.
Partner: UNT Libraries Government Documents Department

Anomalous ion thermal transport in hot ion plasmas by the ion temperature gradient mode

Description: Experiments show that the observed radial profiles of the ion thermal conductivity {chi}{sub i} have the opposite shapes with those obtained from the ion temperature gradient mode ({eta}{sub i} mode) turbulence model by the traditional mixing length estimate. In this work, this radial profile problem is reconsidered with an electromagnetic study of the linear stability of the toroidal {eta}{sub i} mode and a new rule for choosing the mixing length. It is first shown that the electromagnetic effect gives a significant stabilizing effect on the toroidal {eta}{sub i} mode, and that the observed reduction of {chi}{sub i}(r) in the core region can be explained by this electromagnetic effect. Secondly, in view of earlier numerical simulations showing the transfer of fluctuation energy to larger scales that those for the fastest growth rate, as well as fluctuation measurements indicating longer radial correlation lengths, a new mixing length formula is proposed to explain the radial increase of the {chi}{sub i}. It is shown the new formula fits well the observed {chi}{sub i}(r) profiles in two TFTR supershot discharges and also gives the scaling law in the current and the magnetic field which agrees better with experiment than the conventional formula.
Date: January 1, 1992
Creator: Kim, J.Y.; Horton, W. (Texas Univ., Austin, TX (United States). Inst. for Fusion Studies) & Coppi, B. (Massachusetts Inst. of Tech., Cambridge, MA (United States). Research Lab. of Electronics)
Partner: UNT Libraries Government Documents Department

Magnetism as a main origin for the diverse magnetotransport.

Description: Magnetotransport properties of pure Mg metal and MgB{sub 2} samples with varying amounts of unreacted Mg are systematically studied in magnetic fields up to 18 T. With increasing quantity of Mg, the inhomogeneous MgB{sub 2} samples show greatly decreased residual resistivity, enhanced residual resistance ratio (RRR) and enhanced magnetoresistance (MR), gradually approaching the transport behaviors of pure Mg metal. We use the generalized effective medium theory to show that the large RRR and MR of the inhomogeneous MgB{sub 2} samples can be quantitatively explained by a two-phase model in which the two phases are MgB{sub 2} and pure Mg.
Date: January 1, 2001
Creator: Betts, J. B. (Jonathan B.); Jaime, M. (Marcello); Lacerda, A. H. (Alex H.); Boebinger, G. S. (Gregory S.); Jung, C. U. (Chang Uk); Kim, H. J. (Hoen-Jung) et al.
Partner: UNT Libraries Government Documents Department