127 Matching Results

Search Results

Advanced search parameters have been applied.

Coherence techniques at extreme ultraviolet wavelengths

Description: The renaissance of Extreme Ultraviolet (EUV) and soft x-ray (SXR) optics in recent years is mainly driven by the desire of printing and observing ever smaller features, as in lithography and microscopy. This attribute is complemented by the unique opportunity for element specific identification presented by the large number of atomic resonances, essentially for all materials in this range of photon energies. Together, these have driven the need for new short-wavelength radiation sources (e.g. third generation synchrotron radiation facilities), and novel optical components, that in turn permit new research in areas that have not yet been fully explored. This dissertation is directed towards advancing this new field by contributing to the characterization of spatial coherence properties of undulator radiation and, for the first time, introducing Fourier optical elements to this short-wavelength spectral region. The first experiment in this dissertation uses the Thompson-Wolf two-pinhole method to characterize the spatial coherence properties of the undulator radiation at Beamline 12 of the Advanced Light Source. High spatial coherence EUV radiation is demonstrated with appropriate spatial filtering. The effects of small vertical source size and beamline apertures are observed. The difference in the measured horizontal and vertical coherence profile evokes further theoretical studies on coherence propagation of an EUV undulator beamline. A numerical simulation based on the Huygens-Fresnel principle is performed.
Date: October 1, 2002
Creator: Chang, Chang
Partner: UNT Libraries Government Documents Department

Stress-corrosion fatigue-crack growth in a Zr-based bulk amorphousmetal

Description: Electrochemical and mechanical experiments were conducted to analyze the environmentally-influenced cracking behavior of a bulk amorphous metal, Zr41.2Ti13.8Cu12.5Ni10Be22.5. This study was motivated by a scientific interest in mechanisms of fatigue-crack propagation in an amorphous metal, and by a practical interest in the use of this amorphous metal in applications that take advantage of its unique properties, including high specific strength, large elastic strains and low damping. The objective of the work was to determine the rate and mechanisms of subcritical crack growth in this metallic glass in an aggressive environment. Specifically, fatigue-crack propagation behavior was investigated at a range of stress intensities in air and aqueous salt solutions by examining the effects of loading cycle, stress-intensity range, solution concentration, anion identity, solution de-aeration, and bulk electrochemical potential. Results indicate that crack growth in aqueous solution in this alloy is driven by a stress-assisted anodic reaction at the crack tip. Rate-determining steps for such behavior are reasoned to be electrochemical, stress-dependent reaction at near-threshold levels, and mass transport at higher (steady-state) growth rates.
Date: September 21, 2005
Creator: Schroeder, V. & Ritchie, R. O.
Partner: UNT Libraries Government Documents Department

Surface Energy Anisotropy Effects on Pore-Channel Stability:Rayleigh Instabilities in m-Plane Sapphire

Description: Internal, high-aspect-ratio pore channels with their long axes parallel to the m(10{bar 1}0) plane of sapphire were generated through sequential application of photolithography, ion-beam etching and solid-state diffusion bonding. The axial orientation of channels within the m plane was systematically varied to sample a range of bounding-surface crystallographies. The morphologic evolution of these pore channels during anneals at 1700 C was recorded by postanneal optical microscopy. The development and growth of periodic axial variations in the pore channel radius was observed, and ultimately led to the formation of discrete pores. The wavelength and average pore spacing, assumed to reflect the kinetically dominant perturbation wavelength, varied with the in-plane pore channel orientation, as did the time for complete channel breakup. Results are compared to those previously obtained when pore channels were etched into c(0001)-plane sapphire and annealed under similar conditions. The results indicate a strong effect of surface stability on the evolution behavior.
Date: September 7, 2005
Creator: Santala, Melissa K. & Glaeser, Andreas M.
Partner: UNT Libraries Government Documents Department

Segregation phenomena at growing alumina/alloy interfaces

Description: The chemistry and structure at the scale/alloy interface are important factors governing scale adhesion. The chemical changes can occur from segregation of impurities in the alloy, such as sulphur and carbon, or alloying elements such as chromium, aluminium and reactive elements. This paper reviews studies of the changes of interfacial composition with oxidation time for Al{sub 2}O{sub 3} formed on several model alumina-forming alloys, and tries to relate that to the interfacial strength. Results show that sulphur segregation to oxide/metal interfaces can indeed occur, but the type and amount of segregants at the interface depend on the alloy composition and the interface structure. Co-segregation of impurities with alloying elements can also occur, resulting in multi-layer segregants at the interface. Sulphur-containing interfaces are indeed weaker, but the major role of sulphur is to enhance interfacial void formation. Reactive elements in the alloy not only gather sulfur but also exert an additional positive effect on scale adhesion.
Date: March 30, 2005
Creator: Hou, Peggy Y.
Partner: UNT Libraries Government Documents Department

High Pressure Scanning Tunneling Microscopy and High PressureX-ray Photoemission Spectroscopy Studies of Adsorbate Structure,Composition and Mobility during Catalytic Reactions on A Model SingleCrystal

Description: Our research focuses on taking advantage of the ability of scanning tunneling microscopy (STM) to operate at high-temperatures and high-pressures while still providing real-time atomic resolution images. We also utilize high-pressure x-ray photoelectron spectroscopy (HPXPS) to monitor systems under identical conditions thus giving us chemical information to compare and contrast with the structural and dynamic data provided by STM.
Date: May 12, 2006
Creator: Montano, M. O.
Partner: UNT Libraries Government Documents Department

Cyclotron resonance and quasiparticles

Description: This introductory paper contains personal perspectives about the importance of cyclotron resonance in forming our modern view of solids. The papers following this one will discuss the discovery, refinements, and some of the latest developments. Although I will touch on some of these subjects, I leave the details to the other authors and in the main focus on the conceptual impact of the work. I propose that it was experiments based on cyclotron resonance which established the quasiparticle concept.
Date: January 15, 2005
Creator: Cohen, Marvin L.
Partner: UNT Libraries Government Documents Department

Energy levels of isoelectronic impurities by large scale LDA calculations

Description: Isoelectronic impurity states are localized states induced by stoichiometric single atom substitution in bulk semiconductor. Photoluminescence spectra indicate deep impurity levels of 0.5 to 0.9eV above the top of valence band for systems like: GaN:As, GaN:P, CdS:Te, ZnS:Te. Previous calculations based on small supercells seemingly confirmed these experimental results. However, the current ab initio calculations based on thousand atom supercells indicate that the impurity levels of the above systems are actually much shallower(0.04 to 0.23 eV), and these impurity levels should be compared with photoluminescence excitation spectra, not photoluminescence spectra.
Date: November 22, 2002
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

One-Angstrom microscope update

Description: The One-Angstrom Microscope project has attained its goal, and is now producing images down to 1 Angstrom resolution. We have demonstrated transmission electron microscopy of defect structures down to a resolution of 1.1 Angstrom, with evidence that 0.89 Angstrom will be possible. This level of resolution will soon be made available to all those NCEM users who have a requirement for sub-Angstrom resolution.
Date: April 4, 1999
Creator: O'Keefe, Michael A.
Partner: UNT Libraries Government Documents Department

A kicker design for the rapid transfer of the electron beam between radiator beamlines in LUX

Description: I present in this paper preliminary design concepts for a fast kicker magnet and driver for the rapid transfer of the electron beam between radiator beam lines in LUX. This paper presents a feasibility study to find a roughly optimized subset of engineering parameters that would satisfy the initial design specifications of: Pulse width < 30 mu s, rise / fall time < 10 mu s, time jitter < 1ns, magnetic length < 0.5meter, gap height = 15mm, gap width = 25mm, peak field = 0.6Tesla, bend angle = 1.7 deg. for beam energy of 3.1 Gev, repetition rate = 10KHz. An H magnet core configuration was chosen. Through an iterative mathematical process employing Mathcad 11 [1] a realizable design was chosen. Peak current, Peak voltage across the coils, conductor losses due to proximity and skin effects, and basic circuit topology were investigated. Types and losses of core material were only briefly discussed. The final topology consists of two magnets in series running at 10KHz, .3Tesla, 630 amp peak current, 10 mu s pulse width, 693 Watts per coil section, driven by fast solid state switch with an energy recovery inductor.
Date: June 30, 2004
Creator: Stover, Gregory D.
Partner: UNT Libraries Government Documents Department

New waveguide-type HOM damper for ALS storage ring cavities.

Description: The ALS storage ring 500 MHz RF system uses two re-entrant accelerating cavities powered by a single 320kW PHILLIPS YK1305 klystron. During several years of initial operation, the RF cavities were not equipped with effective passive HOM damper systems. Longitudinal beam stability was achieved through cavity temperature control and the longitudinal feedback system (LFB), which was often operating at the edge of its capabilities. As a result, longitudinal beam stability was a significant operations issue at the ALS. During two consecutive shutdown periods (April 2002 and 2003) we installed E-type HOM dampers on the main and third harmonic cavities. These devices dramatically decreased the Q-values of the longitudinal anti-symmetric HOM modes. The next step is to damp the rest of the longitudinal HOM modes in the main cavities below the synchrotron radiation damping level. This will hopefully eliminate the need for the LFB and set the stage for a possible increase in beam current. The ''waveguide'' type of HOM damper was the only option that didn't significantly compromise the vacuum performance of the RF cavity. The design process and the results of the low level measurements of the new waveguide dampers are presented in this paper.
Date: June 28, 2004
Creator: Kwiatkowski, Slawomir; Baptiste, Kenneth & Julian, James
Partner: UNT Libraries Government Documents Department

Design study of the bending sections between harmonic cascade FEL stages

Description: The present design of LUX (linac based ultra-fast X-ray facility) includes a harmonic cascade FEL chain to generate coherent EUV and soft X-ray radiation. Four cascade stages, each consisting of two undulators acting as a modulator and a radiator, respectively, are envisioned to produce photons of approximate wavelengths 48 nm, 12 nm, 4 nm and 1 nm. Bending sections may be placed between the modulator and the radiator of each stage to adjust and maintain bunching of the electrons, to separate, in space, photons of different wavelengths and to optimize the use of real estate. In this note, the conceptual design of such a bending section, which may be used at all four stages, is presented. Preliminary tracking results show that it is possible to maintain bunch structure of nm length scale in the presence of errors, provided that there is adequate orbit correction and there are 2 families of trim quads and trim skew quads, respectively, in each bending section.
Date: June 30, 2004
Creator: Wan, Weishi; Corlett, John; Fawley, William & Zholents, A.
Partner: UNT Libraries Government Documents Department

Determining the surface and interface structure of nanomaterials

Description: This paper informally speculates on the challenges of determining the atomic-scale surface and interface structure of nanomaterials. The relative capabilities of different techniques are compared. This includes discussion of theoretical methods needed to interpret experimental techniques.
Date: June 14, 2004
Creator: Van Hove, Michel A.
Partner: UNT Libraries Government Documents Department

Femtosecond nonlinear spectroscopy at surfaces: Second-harmonic probing of hole burning at the Si(111)7x7 surface and fourier-transform sum-frequency vibrational spectroscopy

Description: The high temporal resolution and broad bandwidth of a femtosecond laser system are exploited in a pair of nonlinear optical studies of surfaces. The dephasing dynamics of resonances associated with the adatom dangling bonds of the Si(111)7 x 7 surface are explored by transient second-harmonic hole burning, a process that can be described as a fourth-order nonlinear optical process. Spectral holes produced by a 100 fs pump pulse at about 800 nm are probed by the second harmonic signal of a 100 fs pulse tunable around 800 nm. The measured spectral holes yield homogeneous dephasing times of a few tens of femtoseconds. Fits with a Lorentzian spectral hole centered at zero probe detuning show a linear dependence of the hole width on pump fluence, which suggests that charge carrier-carrier scattering dominates the dephasing dynamics at the measured excitation densities. Extrapolation of the deduced homogeneous dephasing times to zero excitation density yields an intrinsic dephasing time of {approx} 70 fs. The presence of a secondary spectral hole indicates that scattering of the surface electrons with surface optical phonons at 570 cm{sup -1} occurs within the first 200 fs after excitation. The broad bandwidth of femtosecond IR pulses is used to perform IR-visible sum frequency vibrational spectroscopy. By implementing a Fourier-transform technique, we demonstrate the ability to obtain sub-laser-bandwidth spectral resolution. FT-SFG yields a greater signal when implemented with a stretched visible pulse than with a femtosecond visible pulse. However, when compared with multichannel spectroscopy using a femtosecond IR pulse but a narrowband visible pulse, Fourier-transform SFG is found to have an inferior signal-to-noise ratio. A mathematical analysis of the signal-to-noise ratio illustrates the constraints on the Fourier-transform approach.
Date: November 24, 2004
Creator: McGuire, John Andrew
Partner: UNT Libraries Government Documents Department

Enhanced vibrations at surfaces with back-bonds nearly parallel to the surface

Description: It has been discovered that several very different surfaces exhibit a common property: unusually large vibration amplitudes of the outermost atoms, well beyond the enhancement normally expected at typical clean surfaces. These special surfaces are: ice H2O(0001), alpha-Al2O3(0001), alpha-Ga(010) and Si(111)-(2x1). The root-mean-square vibration amplitudes in these surfaces are at least double the bulk values. The common cause that may explain these vibration amplitudes is that the surface atoms (or molecules in the case of ice) only have back-bonds that are nearly parallel to the surface. In this geometry, vibrations, especially perpendicular to the surface, involve primarily bond bending rather than bond stretching/compression: since bond bending is relatively soft, the corresponding vibration modes can have larger amplitudes. It is suggested that theory examine and confirm this cause of enhanced surface vibration amplitudes, and explore its implication for other phenomena such as adsorption and catalysis.
Date: March 16, 2004
Creator: Van Hove, M.A.
Partner: UNT Libraries Government Documents Department

Aluminum-containing intergranular phases in hot-pressed silicon carbide

Description: Aluminum-containing intergranular phases, forming intergranular films and secondary phase particles at triple-junctions in SiC hot-pressed with aluminum, boron, and carbon additions, were studied by transmission electron microscopy. Statistical high-resolution electron microscopy study of intergranular films indicated that a large fraction of the vitreous intergranular films in the s-hot-pressed SiC crystallized during postannealing in argon above 1000 C. However, brief heating to 1900 C indeed re-melted 25 percent of the crystallized intergranular films. The structural transitions were reflected in the statistical width distributions of the amorphous grain boundary layers. At triple-junctions, Al2O3, Al2OC-SiC solid solution, and mullite phases were newly identified. These phases,together with others reported before are represented in a quaternary phase diagram for 1900 C. It is proposed that a SiC-Al2OC liquid domain is to be included in this phase diagram.
Date: January 12, 2003
Creator: Zhang, Xiao Feng & De Jonghe, Lutgard C.
Partner: UNT Libraries Government Documents Department

HRTEM at half-Angstrom resolution: From OAM to TEAM

Description: Transmission electron microscopy (TEM) at sub-Angstrom resolution is important for nanotechnology. Identifying atom positions requires appropriate resolution, the ability to separate distinct objects in images. With Cs corrected, the information limit of the TEM controls resolution. The OAM has demonstrated that a resolution of 0.78A is possible. The TEAM (transmission electron achromatic microscope) will be a TEM using hardware correction of Cs with a monochromator to improve its information limit beyond that of the OAM by improvement of the electron-beam energy spread. It is shown that A 300keV HRTEM TEAM does not require a Cc corrector to reach 0.5A as long as beam energy spread and objective-lens current ripple are lowered sufficiently. A lower-voltage TEAM will require stricter limits on objective-lens current ripple to reach the targeted 0.5A resolution. No improvement in HT ripple or noise is required to improve the information limit per se since the monochromator determines the energy spread in the beam. However, improved HT ripple and noise will improve the beam current statistics (number of electrons passing through the monochromator) by placing more of the electrons closer to the center of the energy-spread distribution
Date: February 17, 2003
Creator: O'Keefe, Michael A.
Partner: UNT Libraries Government Documents Department

Defocus step size of the LBNL One Angstrom Microscope

Description: The change in focus of a high-resolution electron microscope is generally assumed to be linear with change in objective lens current. Thus the defocus step size should be constant for a constant step in lens current. Measurements on the LBNL One-Angstrom Microscope show that the step size increases with increasing underfocus (reduced lens current). Differentiation of the best-fit quadratic shows that the defocus step size varies linearly as defocus changes.
Date: July 2, 2002
Creator: O'Keefe, Michael A. & Nelson, E. Chris
Partner: UNT Libraries Government Documents Department

Sub-Angstrom resolution with aberration-corrected TEM: Present and future

Description: Resolution is the ability to determine if a feature in an image represents two objects rather than one. Rayleigh's resolution criterion, an accepted standard in optics, was derived as a means for judging when two sources of light (stars) were distinguishable from a single source. In microscopy, resolution is the ability to determine if detail in an image represents distinct (separated) objects. In high-resolution TEM, these objects are atoms. Resolution of |d| is achieved when atoms separated by a (projected) distance |d| can be perceived as separate objects. TEM images are able to depict projected atom columns because they are interference patterns of the directly transmitted electron beam with beams diffracted from the specimen imaged under well-established conditions.
Date: July 1, 2004
Creator: O'Keefe, Michael A.
Partner: UNT Libraries Government Documents Department

Theory and modeling in nanoscience: Report of the May 10-11, 2002 Workshop

Description: On May 10 and 11, 2002, a workshop entitled ''Theory and Modeling in Nanoscience'' was held in San Francisco, California, sponsored by the offices of Basic Energy Science and Advanced Scientific Computing Research of the Department of Energy. The Basic Energy Sciences Advisory Committee and the Advanced Scientific Computing Advisory Committee convened the workshop to identify challenges and opportunities for theory, modeling, and simulation in nanoscience and nanotechnology, and additionally to investigate the growing and promising role of applied mathematics and computer science in meeting those challenges. This report is the result of those contributions and the discussions at the workshop.
Date: June 28, 2002
Creator: McCurdy, C. William; Stechel, Ellen; Cummings, Peter; Hendrickson, Bruce & Keyes, David
Partner: UNT Libraries Government Documents Department