The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
K. Urban - One of the best experts on this subject based on the ideXlab platform.
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spherical aberration correction in tandem with Exit Plane wave function reconstruction interlocking tools for the atomic scale imaging of lattice defects in gaas
Microscopy and Microanalysis, 2004Co-Authors: Karsten Tillmann, Andreas Thust, K. UrbanAbstract:With the availability of resolution boosting and delocalization minimizing techniques, for example, spherical aberration correction and Exit-Plane wave function reconstruction, high-resolution transmission electron microscopy is drawing to a breakthrough with respect to the atomic-scale imaging of common semiconductor materials. In the present study, we apply a combination of these two state-of-the-art techniques to investigate lattice defects in GaAs-based heterostructures at atomic resolution. Focusing on the direct imaging of stacking faults as well as the core structure of edge and partial dislocations, the practical capabilities of both techniques are illustrated. For the first time, we apply the technique of bright-atom contrast imaging at negative spherical aberration together with an appropriate overfocus setting for the investigation of lattice defects in a semiconductor material. For these purposes, the elastic displacements associated with lattice defects in GaAs viewed along the [ 1 10] zone axis are measured from experimental images using reciprocal space strain map algorithms. Moreover, we demonstrate the benefits of the retrieval of the Exit-Plane wave function not only for the elimination of residual imaging artefacts but also for the proper on-line alignment of specimens during operation of the electron microscope—a basic prerequisite to obtain a fair agreement between simulated images and experimental micrographs.
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investigation of the cluster substructure of icosahedral al pd mn quasicrystals by means of Exit Plane wavefunction reconstruction in high resolution electron microscopy
Philosophical Magazine Letters, 1998Co-Authors: R Rosenfeld, A Thust, Wenge Yang, M Feuerbacher, K. UrbanAbstract:The Exit-Plane wave reconstruction technique has been used for the first time to investigate the structure of icosahedral Al-Pd-Mn quasicrystals. The complexvalued Exit-Plane wavefunction along a fivefold direction was reconstructed with a resolution of 0.135 nm on the basis of a series of high-resolution electron microscopy images taken at different defocus values of the objective lens. In contrast with the original electron microscopic images, the Exit-Plane wavefunction is free from imaging artefacts. The result fits well with that expected from the Mackay-type cluster model for the Al-Pd-Mn icosahedral quasicrystal derived from X-ray and neutron diffraction structure investigations. Different columnar arrangements of clusters were identified; clusters whose projections barely interfere with those of other clusters display almost perfect tenfold symmetry, while the local symmetry of strongly overlapping projected clusters is degraded.
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reconstruction of the projected crystal potential from a periodic high resolution electron microscopy Exit Plane wave function
Ultramicroscopy, 1996Co-Authors: Markus Lentzen, K. UrbanAbstract:A method based on a simulated annealing algorithm is applied for the reconstruction of the projected crystal potential belonging to a periodic high-resolution electron microscopy Exit Plane wave function. Using simulated Exit Plane wave functions of GaAs at different specimen thicknesses, the convergence behaviour and the accuracy of the algorithm are investigated. It is demonstrated that the reconstruction is possible even under strongly non-linear scattering conditions at small specimen thicknesses. Further, the convergence of the algorithm to an ambiguous solution beyond a certain specimen thickness is discussed.
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Non-linear reconstruction of the Exit Plane wave function from periodic high-resolution electron microscopy images
Ultramicroscopy, 1994Co-Authors: Andreas Thust, Markus Lentzen, K. UrbanAbstract:Abstract Two different numerical procedures, an algorithm based on the method of simulated annealing as well as a genetic algorithm, are applied for the full non-linear reconstruction of the Exit Plane wave function belonging to periodic areas of high-resolution electron microscopy images. Using simulated high-resolution images of the intermetallic alloy Ni 4 Mo as a test case, the convergence behaviour and the accuracies of both algorithms are investigated. Based on processed experimental high-resolution images of this alloy, a practical example of the reconstruction of the Exit Plane wave function is given.
Graham J. Nathan - One of the best experts on this subject based on the ideXlab platform.
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experimental investigation of acoustic forcing on temperature soot volume fraction and primary particle diameter in non premixed laminar flames
Combustion and Flame, 2017Co-Authors: Kae Ken Foo, Zhiwei Sun, Paul R Medwell, Zeyad T Alwahabi, Bassam B Dally, Graham J. NathanAbstract:Abstract The influence of the acoustic-driven oscillation frequency on the structure and the soot formation of a laminar non-premixed ethylene flame is presented. The flames were forced at either 20 and 40 Hz, corresponding to Strouhal numbers of 0.23 and 0.46. Two-dimensional phase-resolved measurements of gas temperature, soot volume fraction and the diameter of primary particles were measured simultaneously for one steady and two forced flames with non-linear excitation regime Two-Line Atomic Fluorescence, Laser-Induced Incandescence and Time-Resolved LII, respectively. Simultaneous measurements of gas temperature and soot volume fraction provide details of the difference between the flame structure and the soot distribution in the forced and unforced flames. The distinctive features of the forced flames are the occurrence of necking near to the fuel tube and the formation of a hollow soot “shell”. Despite the distinctive structure, the soot region is confined to a restricted range of temperatures, approximately 1700–1800 K. The strong spatial correlation between the presence of soot and a narrow range of temperature is well established for steady laminar flames. The present measurements also reveal the complex relationship between diameter of primary particles and soot volume fraction for these flames. Planar gas temperature images and OH* chemiluminescence profiles show that the reaction zone extends periodically upstream from the Exit Plane of the fuel tube for the flame with S t = 0.23 . This phenomenon, which only occurs for some conditions, is an important consideration for the modelling of these flames because the boundaries of the computational domain typically starts at the fuel flow Exit Plane.
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the effect of stokes number on particle velocity and concentration distributions in a well characterised turbulent co flowing two phase jet
Journal of Fluid Mechanics, 2016Co-Authors: Timothy C. W. Lau, Graham J. NathanAbstract:Simultaneous measurements of particle velocity and concentration (number density) in a series of mono-disperse, two-phase turbulent jets issuing from a long, round pipe into a low velocity co-flow were performed using planar nephelometry and digital particle image velocimetry. The Exit Stokes number, , was systematically varied over two orders of magnitude between 0.3 and 22.4, while the Reynolds number was maintained in the turbulent regime ( ). The mass loading was fixed at , resulting in a flow that is in the two-way coupling regime. The results show that, in contrast to all previous work where a single Stokes number has been used to characterise fluid–particle interactions, the characteristic Stokes number in the axial direction is lower than that for the radial direction. This is attributed to the significantly greater length scales in the axial motions than in the radial ones. It further leads to a preferential response of particles to gas-phase axial velocity fluctuations, , over radial velocity fluctuations, . This, in turn, leads to high levels of anisotropy in the particle-phase velocity fluctuations, , throughout the jet, with increasing as is increased. The results also show that the region within the first few diameters of the Exit Plane is characterised by a process of particle reorganisation, resulting in significant particle migration to the jet axis for and away from the axis for . This migration, together with particle deceleration along the axis, causes local humps in the centreline concentration whose value can even exceed those at the Exit Plane.
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Influence of Stokes number on the velocity and concentration distributions in particle-laden jets
Journal of Fluid Mechanics, 2014Co-Authors: Timothy C. W. Lau, Graham J. NathanAbstract:The first measurement of the influence of the Stokes number on the distributions of particle concentration and velocity at the Exit of a long pipe are reported, together with the subsequent influence on the downstream evolution of these distributions through a particle-laden jet in co-flow. The data were obtained by simultaneous particle image velocimetry (PIV) and planar nephelometry (PN), using four cameras to provide high resolution through the first 30 jet diameters and also correction for optical attenuation. These data provide much more detailed information than is available from previous measurements. From them, a new understanding is obtained of how the Stokes number influences the flow at the jet Exit Plane and how this influence propagates throughout the jet.
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the effect of initial conditions on the Exit flow from a fluidic precessing jet nozzle
Experiments in Fluids, 2004Co-Authors: Chong Y Wong, Graham J. Nathan, Timothy OdohertyAbstract:The fluidic precessing jet (FPJ) is a member of a family of self-excited oscillating jet flows that has found application in reducing oxides of nitrogen (NOx) from combustion systems in the high-temperature process industries. Its flow field is highly three-dimensional and unsteady, and many aspects of it remain unresolved. Velocity data, measured close to the Exit Plane, are presented for a variety of FPJ nozzles with three different inlet conditions, namely, a long pipe, a smooth contraction and an orifice. The results indicate that jet inlets that are known to have nonsymmetrically shedding initial boundary layers, namely those from the orifice or long pipe, cause jet precession to be induced more easily than the smooth contraction inlet, which is known to have a symmetrically shedding initial boundary layer. The nature of the Exit flow is dominated by the degree to which a given configuration generates precession. Nevertheless, the three different inlet conditions also produce subtle differences in the Exit profiles of mean velocity and turbulence intensity when the flow does precess reliably.
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effect of small vortex generators on scalar mixing in the developing region of a turbulent jet
International Journal of Heat and Mass Transfer, 1999Co-Authors: Graham J. NathanAbstract:Abstract This paper investigates the effect of small tabs placed at the Exit Plane of an axisymmetric smooth contraction on passive scalar mixing in a slightly heated turbulent jet. It is shown that the presence of tabs profoundly distorts the jet flow field and consequently modifies the scalar mixing characteristics significantly. Tabs cause the mean temperature to decrease more rapidly with downstream distance, implying an increased mixing rate. Furthermore, it is found that two tabs distort the jet from the axisymmetric state more dramatically than four tabs.
Andreas Thust - One of the best experts on this subject based on the ideXlab platform.
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Spherical-aberration correction in tandem with the restoration of the Exit-Plane wavefunction: synergetic tools for the imaging of lattice imperfections in crystalline solids at atomic resolution
Journal of Materials Science, 2006Co-Authors: Karsten Tillmann, Andreas Thust, Lothar Houben, Knut UrbanAbstract:With the availability of resolution-boosting and delocalization-minimizing techniques, aberration-corrected high-resolution transmission electron microscopy is currently enjoying great popularity with respect to the atomic scale imaging of lattice imperfections in crystalline solid-state materials. In the present review, the most striking practical benefits arising from the synergetic combination of two sophisticated state-of-the-art techniques, i.e. spherical-aberration-corrected imaging as well as the numerical restoration of the Exit-Plane wavefunction from a focal series of high-resolution micrographs, are illustrated by highlighting their combined use for the atomic-scale characterization of misfit dislocations, stacking faults and grain boundaries in common semiconductor materials and metastable metal phases. For these purposes recent progress is reviewed in the atomic-scale characterization of (i) Lomer-type misfit dislocations at In_ x Ga_1- x As/GaAs heterointerfaces and extrinsic stacking fault ribbons in GaAs together with the associated lattice displacements [Tillmann et al. (2004) Microsc Microanal 10:185], (ii) the core structure of chromium implantation-induced Frank partial dislocations in GaN [Tillmann et al. (2005) Microsc Microanal 11:534] as well as (iii) tilt boundaries between β-phase Ta crystallites in thin metallization layers [Tillmann et al. (2006) Phil Mag, in press]. In addition, practical advantages are demonstrated of the retrieval of the Exit-Plane wavefunction not only for the measurement and subsequent elimination of residual lens aberrations still present in aberration-corrected microscopy, but also for the proper alignment of specimens during operation of the electron microscope.
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spherical aberration correction in tandem with Exit Plane wave function reconstruction interlocking tools for the atomic scale imaging of lattice defects in gaas
Microscopy and Microanalysis, 2004Co-Authors: Karsten Tillmann, Andreas Thust, K. UrbanAbstract:With the availability of resolution boosting and delocalization minimizing techniques, for example, spherical aberration correction and Exit-Plane wave function reconstruction, high-resolution transmission electron microscopy is drawing to a breakthrough with respect to the atomic-scale imaging of common semiconductor materials. In the present study, we apply a combination of these two state-of-the-art techniques to investigate lattice defects in GaAs-based heterostructures at atomic resolution. Focusing on the direct imaging of stacking faults as well as the core structure of edge and partial dislocations, the practical capabilities of both techniques are illustrated. For the first time, we apply the technique of bright-atom contrast imaging at negative spherical aberration together with an appropriate overfocus setting for the investigation of lattice defects in a semiconductor material. For these purposes, the elastic displacements associated with lattice defects in GaAs viewed along the [ 1 10] zone axis are measured from experimental images using reciprocal space strain map algorithms. Moreover, we demonstrate the benefits of the retrieval of the Exit-Plane wave function not only for the elimination of residual imaging artefacts but also for the proper on-line alignment of specimens during operation of the electron microscope—a basic prerequisite to obtain a fair agreement between simulated images and experimental micrographs.
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Non-linear reconstruction of the Exit Plane wave function from periodic high-resolution electron microscopy images
Ultramicroscopy, 1994Co-Authors: Andreas Thust, Markus Lentzen, K. UrbanAbstract:Abstract Two different numerical procedures, an algorithm based on the method of simulated annealing as well as a genetic algorithm, are applied for the full non-linear reconstruction of the Exit Plane wave function belonging to periodic areas of high-resolution electron microscopy images. Using simulated high-resolution images of the intermetallic alloy Ni 4 Mo as a test case, the convergence behaviour and the accuracies of both algorithms are investigated. Based on processed experimental high-resolution images of this alloy, a practical example of the reconstruction of the Exit Plane wave function is given.
Alec D Gallimore - One of the best experts on this subject based on the ideXlab platform.
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method for analyzing e b probe spectra from hall thruster plumes
Review of Scientific Instruments, 2009Co-Authors: Rohit Shastry, Richard R Hofer, Bryan M Reid, Alec D GallimoreAbstract:Various methods for accurately determining ion species’ current fractions using E×B probes in Hall thruster plumes are investigated. The effects of peak broadening and charge exchange on the calculated values of current fractions are quantified in order to determine the importance of accounting for them in the analysis. It is shown that both peak broadening and charge exchange have a significant effect on the calculated current fractions over a variety of operating conditions, especially at operating pressures exceeding 10 -5 torr. However, these effects can be accounted for using a simple approximation for the velocity distribution function and a one-dimensional charge exchange correction model. In order to keep plume attenuation from charge exchange below 30%, it is recommended that pz � 2, where p is the measured facility pressure in units of 10 -5 torr, and z is the distance from the thruster Exit Plane to the probe inlet. The spatial variation of the current fractions in the plume of a Hall thruster and the error induced from taking a single-point measurement are also briefly discussed.
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method for analyzing e b probe spectra from hall thruster plumes
Review of Scientific Instruments, 2009Co-Authors: Rohit Shastry, Richard R Hofer, Bryan M Reid, Alec D GallimoreAbstract:Various methods for accurately determining ion species’ current fractions using E×B probes in Hall thruster plumes are investigated. The effects of peak broadening and charge exchange on the calculated values of current fractions are quantified in order to determine the importance of accounting for them in the analysis. It is shown that both peak broadening and charge exchange have a significant effect on the calculated current fractions over a variety of operating conditions, especially at operating pressures exceeding 10 -5 torr. However, these effects can be accounted for using a simple approximation for the velocity distribution function and a one-dimensional charge exchange correction model. In order to keep plume attenuation from charge exchange below 30%, it is recommended that pz � 2, where p is the measured facility pressure in units of 10 -5 torr, and z is the distance from the thruster Exit Plane to the probe inlet. The spatial variation of the current fractions in the plume of a Hall thruster and the error induced from taking a single-point measurement are also briefly discussed.
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neutral density map of hall thruster plume expansion in a vacuum chamber
Review of Scientific Instruments, 2005Co-Authors: Mitchell L R Walker, Alec D GallimoreAbstract:A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09mg∕s. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46mg∕s for discharge voltages of 300 and 500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140 000 and 240000l∕s. The pressure map is performed with a rake consisting of five calibrated Bayard–Alpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTF’s centerline shows that the plume pressure decreases from a maximum at the thruster Exit Plane down to the facility background pressure at approximately 2m downstream of the Exit Plane. Comparison of axial pressure profiles on the LVTF’s centerline shows tha...
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internal plasma potential profiles in a laboratory model hall thruster
Physics of Plasmas, 2001Co-Authors: James M Haas, Alec D GallimoreAbstract:The Plasmadynamics and Electric Propulsion Laboratory High-speed Axial Reciprocating Probe system is used in conjunction with a floating emissive probe to measure plasma potential in the discharge chamber of the P5 Hall thruster. Plasma potential measurements are made at a constant voltage, 300 V, at two different discharge current conditions: 5.4 and 10 A. The plasma potential contours for the 5.4 A case indicate that the acceleration region begins several millimeters upstream of the Exit Plane, extends several centimeters downstream, and is uniform across the width of the discharge chamber. The 10 A case is similar to the 5.4 A case with the exception that the acceleration region is shifted downstream on centerline. Axial electric field profiles, computed from the measured potential, show a double peak structure in the 5.4 A case, indicating a zone of ion deceleration. Perturbations to the discharge current are shown to correspond spatially with the location of the peak electric field indicating that th...
Markus Lentzen - One of the best experts on this subject based on the ideXlab platform.
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reconstruction of the projected crystal potential from a periodic high resolution electron microscopy Exit Plane wave function
Ultramicroscopy, 1996Co-Authors: Markus Lentzen, K. UrbanAbstract:A method based on a simulated annealing algorithm is applied for the reconstruction of the projected crystal potential belonging to a periodic high-resolution electron microscopy Exit Plane wave function. Using simulated Exit Plane wave functions of GaAs at different specimen thicknesses, the convergence behaviour and the accuracy of the algorithm are investigated. It is demonstrated that the reconstruction is possible even under strongly non-linear scattering conditions at small specimen thicknesses. Further, the convergence of the algorithm to an ambiguous solution beyond a certain specimen thickness is discussed.
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Non-linear reconstruction of the Exit Plane wave function from periodic high-resolution electron microscopy images
Ultramicroscopy, 1994Co-Authors: Andreas Thust, Markus Lentzen, K. UrbanAbstract:Abstract Two different numerical procedures, an algorithm based on the method of simulated annealing as well as a genetic algorithm, are applied for the full non-linear reconstruction of the Exit Plane wave function belonging to periodic areas of high-resolution electron microscopy images. Using simulated high-resolution images of the intermetallic alloy Ni 4 Mo as a test case, the convergence behaviour and the accuracies of both algorithms are investigated. Based on processed experimental high-resolution images of this alloy, a practical example of the reconstruction of the Exit Plane wave function is given.