Bibliothek

Notizen: This work reports the temperature dependence of electronic conduction in thin nitrided oxides for temperatures ranging from 298 to 423 K. It was found that the conduction currents in the nitrided oxides have different temperature dependencies from those of conventional thermal oxides. At low electric fields (∼6 MV/cm), the temperature dependencies of the nitrided oxides can be divided into two segments which are attributed to the shallow trap-assisted conduction for temperature less than 400 K and Poole–Frenkel or thermionic emission for temperatures greater than 400 K, whereas the temperature dependence of thermal oxide is governed by exp(−0.143/kT) for the entire temperature range of this study. At high electric fields (∼12 MV/cm), the conduction current of nitrided oxides is mainly governed by Fowler–Nordheim tunneling for temperatures less than 340 K. In studying the temperature dependencies of the electronic trapping in nitrided oxides, we found that the density of trapped charge decreases as large as 27.8% for temperatures rising from 298 to 423 K. It indicates that the density of traps at energy levels less than 0.036 eV (measured from the conduction band of the nitrided oxide) is significantly larger than that of thermal oxides.

Notizen: This research is a multifaceted study which investigates not only the role of melanin in providing photostability to natural hair color and hair proteins, but also the claim that the presence of specific artificial colors in hair slows down the rate of photodegradation of hair proteins. In earlier studies, the extractability of protein from photodegraded hair was investigated and showed that many of the cleaved proteins could not be extracted because of photo-oxidative cross-linking. The current study investigates the effect of the amount of melanin in hair of different ethnicity and the presence of artificial hair colors on the extractability of the main classes of hair proteins. Furthermore, the data are used in the interpretation of the effect of these components in being able to prevent photo-oxidative damage to hair proteins.When exposed to sunlight, hair undergoes changes in chemical, mechanical and morphological properties. The UVB and UVA regions of the solar spectrum are the most damaging to human hair. Of these two, the UVA region is predominant in the solar spectrum at low altitudes. Hair of different ethnicity responds differently to the damaging radiation of the solar spectrum, because of different amounts of melanin present in hair. Melanin absorbs the impinging radiation (especially at the lower wavelengths (254–350 nm), and converts it by some complex internal mechanism into heat. Because of this, melanin provides a photochemical protection to natural hair color and hair proteins and prevents their photodegradation. However, the melanin pigments act sacrificially and become themselves degraded in the process of protecting the proteins from light. As a result, this «protective» effect of the melanin pigments does not last during long-term intense exposure, when, regardless of the amount of melanin in hair, most matrix, intermediate filament and high molecular weight hair proteins undergo photo-oxidative cross-linking into higher molecular weight species, and their extractability from hair decreases significantly.The goal of this study is to demonstrate how UV-radiation affects natural and artificial hair color during long-term exposures. Bright-field and UV-microspectrophotometry and an electrophoretic separation technique (SDS-PAGE) were chosen as investigative techniques for these studies, because they are well-suited to accurately and reproducibly investigate the initial properties of a specific hair sample and the changes in these properties as a result of long-term light-exposure. The goal of this paper is not to relate this to the content and type of melanin in hair. Electrophoresis, while not measuring the exact quantitative amount of protein extracted, is a semi-quantitative method, where increases in brightness of the bands represent increased amounts of proteins that were extracted of that specific protein from hair. This electrophoretic study attempts to determine whether the presence of natural or artificial color in hair influences the protein extractability in unaltered hair and the photo-oxidative cross-linking during light-exposure.The bright-field microspectrophotometric study showed that high concentrations of melanin provide protection to the melanin itself and that they prevent loss of natural hair color during light-exposure. However, neither large amounts of melanin in hair of different ethnicity, nor artificial hair colors (even a dye with an absorption in the UV region) provide protection to the hair proteins against photodegradation under the conditions used in this study.UV-microspectrophotometry has suggested the formation of high levels of photo-oxidized proteins as a result of light-exposure. Electrophoresis revealed photo-oxidative cross-linking of most matrix, intermediate filament and high molecular weight hair proteins into their higher molecular weight analogues, rendering them less extractable due to their lowered diffusivity. Only very low levels of low molecular weight matrix proteins could be extracted.

Notizen: We present an efficient and accurate continuum-mechanics approach for the numerical prediction of displacement, stress, strain, and strain energy density fields in an anisotropic substrate (modeled as a half-space) due to a buried quantum dot (QD). Our approach is based on Green's function solution in anisotropic and linearly elastic half-space combined with the Betti's reciprocal theorem. Numerical examples clearly show that the crystalline anisotropy of the III–V semiconductor group has great influence on the elastic fields, as compared to the isotropic solution. In particular, it is found that the hydrostatic strain and strain energy density on the surface of anisotropic half-space made of different crystalline materials due to a cubic QD can be substantially different, and therefore, the isotropy approximation neglecting their differences should not be used in general. Furthermore, the hydrostatic strains on the surface of an anisotropic half-space due to a finite-size (cubic) QD and an equal-intensity point QD at relatively large depth (about twice the side length of the cubic QD) can still be quite different, in contrast to the corresponding isotropic result. These observations indicate that in modeling and analyzing the mechanical and electronic behaviors of QD semiconductor structures, the effect of crystalline anisotropy should be considered in general. © 2001 American Institute of Physics.

Notizen: The thermal stress effects of the inter-level dielectric (ILD) layer on the ferroelectric performance of integrated Pt/SrBi2Ta2O9(SBT)/Pt capacitors were investigated. Two different thin film materials, pure SiO2 grown at 650 °C and B- and P-doped SiO2 grown at 400 °C by chemical vapor deposition techniques, were tested as an ILD layer. The ILD layer encapsulated the SBT capacitor array. During high temperature thermal cycling (up to 800 °C) after ILD deposition, which is used for both densifying the ILD and curing of the various damage imposed on the SBT capacitors, a large thermal stress occurred in the bottom Pt layer due to the thermal expansion mismatch between the various layers. In particular, the pure SiO2 ILD layer between the capacitors did not allow thermal expansion of the Pt layers, which led to a large accumulation of compressive stress in the layer. This resulted in hillock formation in the bottom Pt layer and eventual capacitor failure. However, the B- and P-doped SiO2 ILD layer contracted during thermal cycling by removing residual impurities, which allowed greater expansion of the Pt layer. Therefore, compressive stress accumulation did not occur and excellent ferroelectric properties were thus obtained from the integrated capacitor array. © 2001 American Institute of Physics.

Notizen: Low temperature thermoluminescence data from rare earth doped lanthanum fluoride is characterized by numerous glow peaks. These differ in temperature as a function of the substituted rare earth ion, dopant concentration and thermal annealing. Analysis of the component bands in terms of activation energy, frequency factor and kinetic order reveals simple patterns in which the movement of the glow peaks is related to the ionic radii of the rare earth ions. Models for the thermoluminescence signals resulting from close association of the dopant recombination sites and the charge traps are discussed. Further consideration of a preference for impurity clustering, in order to reduce lattice strain, is used to explain the concentration and annealing induced changes. © 2000 American Institute of Physics.

Notizen: To extend the measurement range to 100 GPa, a modified manganin gauge was prepared and examined. The modifications were focused on the package materials and technique. The manganin sensing elements were first deposited by magnetron sputtering on Al2O3 substrates, and then covered by a layer of Al2O3 films several μm thick by electron beam evaporation. After being placed on slabs of Al2O3 ceramic, the gauges were shocked to 40–80 GPa with a light gas gun. The stress-time profiles showed a good plateau of about 1 μs in duration. The initial calibration resulted in a linear curve with piezoresistance coefficient of 0.0198±0.0002 GPa−1. © 2002 American Institute of Physics.

Notizen: We present development of microfocusing Kirkpatrick–Baez mirrors GeoSoilEnviroCARS. The mirror is made of single-crystal silicon and the desired elliptical shape is formed by elastic bending with a two-parameter bender. We present data from long trace profiler measurements of the bent shape, which verifies the design principle, and motivates a new bender design. We tested the re-engineered mirror bender focusing white radiation from a National Synchrotron Light Source bending magnet, achieving a 2.5 μm focus with a gain (using only one mirror) of 91. © 1995 American Institute of Physics.

Notizen: We designed, created, and tested a device for measuring electron bunch length in the picosecond region. The basic idea behind the method is a circular scanning of the electron bunch in the rotating magnetic field of a TM110 rf cavity. As a result, the longitudinal position of the electrons is transformed into an angular position in a plane orthogonal to the axis. The resolution limit of the instrument is 4 ps (rms). © 1995 American Institute of Physics.

Notizen: The commissioning of the 7-GeV Advanced Photon Source (APS) storage ring began in early 1995. Characterization of the stored particle beam properties involved time-resolved transverse and longitudinal profile measurements using optical synchrotron radiation (OSR) monitors. Early results include the observation of the beam on a single turn, measurements of the transverse beam sizes after damping using a 100 μs integration time (σx∼150±25 μm, σy∼65±25 μm, depending on vertical coupling), and measurement of the bunch length (στ ∼25 to 55 ps, depending on the charge per bunch). The results are consistent with specifications and predictions based on the 8.2 nm-rad natural emittance, the calculated lattice parameters, and vertical coupling less than 10%. The novel, single-element focusing mirror for the photon transport line and the dual-sweep streak camera techniques, which allow turn-by-turn measurements, will also be presented. The latter measurements are believed to be the first of their kind on a storage ring in the USA. © 1996 American Institute of Physics.