On the centrifugal granulation characteristics by rotary disk: Effect of outer edge structure

Three distinct kinds of rapid variations have been detected in the light curves of dwarf novae: rapid flickering, short period coherent oscillations, and quasi-periodic oscillations. The rapid flickering is seen in the light curves of most, if not all, dwarf novae, and is especially apparent during minimum light between eruptions. The flickering has a typical time scale of a few minutes or less and a typical amplitude of about .1 mag. The flickering is completely random and unpredictable; the power spectrum of flickering shows only a slow decrease from low to high frequencies. The observations of U Gem by Warner and Nather (1971) showed conclusively that most of the flickering is produced by variations in the luminosity of the bright spot near the outer edge of the accretion disk around the white dwarf in these close binary systems.

Download Full-text

Oxygen K near edge structures studied with multiple scattering calculations

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100104583 ◽

1988 ◽

Vol 46 ◽

pp. 504-505

Author(s):

Xudong Weng ◽

Peter Rez

Keyword(s):

Cross Section ◽

Cross Sections ◽

Partial Cross Section ◽

Energy Window ◽

Edge Structure ◽

Fine Structures ◽

Hydrogenic Model ◽

Electron Energy Loss ◽

Quantitative Chemical

In electron energy loss spectroscopy, quantitative chemical microanalysis is performed by comparison of the intensity under a specific inner shell edge with the corresponding partial cross section. There are two commonly used models for calculations of atomic partial cross sections, the hydrogenic model and the Hartree-Slater model. Partial cross sections could also be measured from standards of known compositions. These partial cross sections are complicated by variations in the edge shapes, such as the near edge structure (ELNES) and extended fine structures (ELEXFS). The role of these solid state effects in the partial cross sections, and the transferability of the partial cross sections from material to material, has yet to be fully explored. In this work, we consider the oxygen K edge in several oxides as oxygen is present in many materials. Since the energy window of interest is in the range of 20-100 eV, we limit ourselves to the near edge structures.

Download Full-text

Difference mode EXELFS with parallel-detection EELS

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100130948 ◽

1992 ◽

Vol 50 (2) ◽

pp. 1262-1263

Author(s):

Michael K. Kundmann ◽

Ondrej L. Krivanek

Keyword(s):

Edge Detection ◽

Important Distinction ◽

Edge Structure ◽

Parallel Detection ◽

Gain Variation ◽

Ideal Solutions ◽

Detector Cell ◽

The Difference ◽

Elemental Detection ◽

Partial Correction

Parallel detection has greatly improved the elemental detection sensitivities attainable with EELS. An important element of this advance has been the development of differencing techniques which circumvent limitations imposed by the channel-to-channel gain variation of parallel detectors. The gain variation problem is particularly severe for detection of the subtle post-threshold structure comprising the EXELFS signal. Although correction techniques such as gain averaging or normalization can yield useful EXELFS signals, these are not ideal solutions. The former is a partial throwback to serial detection and the latter can only achieve partial correction because of detector cell inhomogeneities. We consider here the feasibility of using the difference method to efficiently and accurately measure the EXELFS signal.An important distinction between the edge-detection and EXELFS cases lies in the energy-space periodicities which comprise the two signals. Edge detection involves the near-edge structure and its well-defined, shortperiod (5-10 eV) oscillations. On the other hand, EXELFS has continuously changing long-period oscillations (∼10-100 eV).

Download Full-text

Electron energy-loss near-edge structure in silicate minerals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100130924 ◽

1992 ◽

Vol 50 (2) ◽

pp. 1258-1259

Author(s):

D W McComb ◽

R S Payne ◽

P L Hansen ◽

R Brydson

Keyword(s):

Solid State ◽

Energy Loss ◽

Electron Energy ◽

State Energy ◽

Local Environment ◽

Edge Structure ◽

Silicate Minerals ◽

Two Samples ◽

Electron Energy Loss ◽

Careful Processing

Electron energy-loss near-edge structure (ELNES) is an effective probe of the local geometrical and electronic environment around particular atomic species in the solid state. Energy-loss spectra from several silicate minerals were mostly acquired using a VG HB501 STEM fitted with a parallel detector. Typically a collection angle of ≈8mrad was used, and an energy resolution of ≈0.5eV was achieved.Other authors have indicated that the ELNES of the Si L2,3-edge in α-quartz is dominated by the local environment of the silicon atom i.e. the SiO4 tetrahedron. On this basis, and from results on other minerals, the concept of a coordination fingerprint for certain atoms in minerals has been proposed. The concept is useful in some cases, illustrated here using results from a study of the Al2SiO5 polymorphs (Fig.l). The Al L2,3-edge of kyanite, which contains only 6-coordinate Al, is easily distinguished from andalusite (5- & 6-coordinate Al) and sillimanite (4- & 6-coordinate Al). At the Al K-edge even the latter two samples exhibit differences; with careful processing, the fingerprint for 4-, 5- and 6-coordinate aluminium may be obtained.

Download Full-text

Chemical and orientational imaging of polymeric samples

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100168074 ◽

1994 ◽

Vol 52 ◽

pp. 68-69

Author(s):

H. Ade ◽

B. Hsiao ◽

G. Mitchell ◽

E. Rightor ◽

A. P. Smith ◽

...

Keyword(s):

Ethylene Terephthalate ◽

National Laboratory ◽

Brookhaven National Laboratory ◽

Carbonyl Groups ◽

Aromatic Rings ◽

Edge Structure ◽

X Ray ◽

Scanning Transmission ◽

Polymeric Samples ◽

X Ray Absorption

We have used the Scanning Transmission X-ray Microscope at beamline X1A (X1-STXM) at Brookhaven National Laboratory (BNL) to acquire high resolution, chemical and orientation sensitive images of polymeric samples as well as point spectra from 0.1 μm areas. This sensitivity is achieved by exploiting the X-ray Absorption Near Edge Structure (XANES) of the carbon K edge. One of the most illustrative example of the chemical sensitivity achievable is provided by images of a polycarbonate/pol(ethylene terephthalate) (70/30 PC/PET) blend. Contrast reversal at high overall contrast is observed between images acquired at 285.36 and 285.69 eV (Fig. 1). Contrast in these images is achieved by exploring subtle differences between resonances associated with the π bonds (sp hybridization) of the aromatic groups of each polymer. PET has a split peak associated with these aromatic groups, due to the proximity of its carbonyl groups to its aromatic rings, whereas PC has only a single peak.

Download Full-text

Electron energy loss near edge structures of intermetallic alloys and grain boundaries in NiAl

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100165070 ◽

1996 ◽

Vol 54 ◽

pp. 522-523

Author(s):

G.A. Botton ◽

C.J. Humphreys

Keyword(s):

High Temperature ◽

Transition Metal ◽

Energy Loss ◽

Grain Boundaries ◽

Industrial Applications ◽

Edge Structure ◽

Structural Applications ◽

Yield Behaviour ◽

Late Transition ◽

Metal Aluminides

Transition metal aluminides are of great potential interest for high temperature structural applications. Although these materials exhibit good mechanical properties at high temperature, their use in industrial applications is often limited by their intrinsic room temperature brittleness. Whilst this particular yield behaviour is directly related to the defect structure, the properties of the defects (in particular the mobility of dislocations and the slip system on which these dislocations move) are ultimately determined by the electronic structure and bonding in these materials. The lack of ductility has been attributed, at least in part, to the mixed bonding character (metallic and covalent) as inferred from ab-initio calculations. In this work, we analyse energy loss spectra and discuss the features of the near edge structure in terms of the relevant electronic states in order to compare the predictions on bonding directly with spectroscopic experiments. In this process, we compare spectra of late transition metal (TM) to early TM aluminides (FeAl and TiAl) to assess whether differences in bonding can also be detected. This information is then discussed in terms of bonding changes at grain boundaries in NiAl.

Download Full-text

Acquisition of EELS spectra for high-resolution spectroscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100148721 ◽

1993 ◽

Vol 51 ◽

pp. 578-579

Author(s):

Maoxu Qian ◽

Mehmet Sarikaya ◽

Edward A. Stern

Keyword(s):

Energy Loss ◽

Detection System ◽

High Resolution Spectroscopy ◽

High Signal ◽

Short Report ◽

Edge Structure ◽

High Background ◽

Gain Variation ◽

Sufficient Energy ◽

On Line

It is difficult, in general, to perform quantitative EELS to determine, for example, relative or absolute compositions of elements with relatively high atomic numbers (using, e.g., K edge energies from 500 eV to 2000 eV), to study ELNES (energy loss near edge structure) signal using the white lines to determine oxidation states, and to analyze EXELFS (extended energy loss fine structure) to study short range ordering. In all these cases, it is essential to have high signal-to-noise (S/N) ratio (low systematical error) with high overall counts, and sufficient energy resolution (∽ 1 eV), requirements which are, in general, difficult to attain. The reason is mainly due to three important inherent limitations in spectrum acquisition with EELS in the TEM. These are (i) large intrinsic background in EELS spectra, (ii) channel-to-channel gain variation (CCGV) in the parallel detection system, and (iii) difficulties in obtaining statistically high total counts (∽106) per channel (CH). Except the high background in the EELS spectrum, the last two limitations may be circumvented, and the S/N ratio may be attained by the improvement in the on-line acquisition procedures. This short report addresses such procedures.

Download Full-text