Diffraction reflection of a focused X-ray wave by a multilayer crystal

The Bragg diffraction of a narrow X-ray beam in a multilayer crystal is studied in the reflection geometry. It is shown by computer simulation of the experiment that strong integrated kinematic reflection occurs at any boundary of a multilayer crystal at the point of intersection of a narrow beam with the boundary. In contrast to the dynamic diffraction of a plane wave, attenuation of a narrow incident beam due to the reflection is small in comparison with the conventional absorption. As an example of a multilayer crystal, a Fabry-Perot interferometer is considered, which consists of two 70-μm-thick crystal Si layers separated by a 100-μm-thick air layer. The 12 4 0 reflection is studied at a photon energy of 15 keV. Three methods of formation of a narrow beam are considered: wavefront limitation by a slit, focusing with a parabolic lens, and focusing with a zone plate. It is shown that, for a local analysis of the thickness of layers and scattering parameters, the method of parabolic lens focusing has some advantages in comparison with the other methods.     

Nematic Liquid Crystals with a Trifluoromethyl Group

Abstract

We have synthesised a new class of compounds incorporating a trifluoromethyl group in the terminal alkoxy or alkenyloxy chain. Compounds containing several aromatic rings were synthesised with a view to producing compounds of high birefringence. This also included the synthesis of compounds containing a carbon-carbon triple bond. Compounds with three 1,4-disubstituted rings were synthesised in order to produce materials with a high nematic clearing point and as low a melting point as possible. Laterally fluoro-substituted compounds were prepared to generate a high positive value of the dielectric anisotropy, a low melting point and no smectic mesophases. Substances containing a cyclohexyl ring, which tend to induce a lower viscosity and compounds incorporating a bicyclo[2.2.2]octane ring were also synthesised in order to produce a high nematic clearing point and to influence the elastic constants. The (E)-trifluorobut-2-enyloxy-compounds were prepared in attempts to produce compounds with appropriate elastic constants and a high birefringence.     

Biaxial nematic phase of crank-like bis(β-diketonato)copper(II) complexes established by X-ray diffraction and Z-value calculations

We have synthesized a series of six novel bis(β-diketonate)copper (II) complexes, 1a, 1b, 2a, 2b, 3a, and 3b, substituted by two bulky substituents in the short molecular axis direction to investigate their mesomorphism. The m,p,m'-trimethoxylphenyl-substituted derivatives, 2a and 2b, and the m,p-dimethoxylphenyl-substituted derivatives, 3a and 3b, did not show mesomorphism, whereas each of the p-methoxyphenyl-substituted derivatives 1a and 1b only showed a nematic phase, which was revealed from polarizing microscopic observations. We established from X-ray diffraction and Z-value calculations that each of the crank-like derivatives 1a and 1b forms a rectangular parallelepiped dimer and shows a biaxial nematic phase.     

Computersimulation einer eindimensionalen polykristallinen Gefügebildung mit konstanter Keimbildungsrate und konstanter Kristallisationsgeschwindigkeit

For the process of isothermic freezing of a melt in a capillary tube with a constant nucleation rate and for all the crystallites with the same and constant rate of crystallization a mathematical model and computer programs for simulation are elaborated. In the result of simulation of the model by a digital computer one gets frequency distributions of the grain lengths. In a lot of simulated cases the cumulative frequency distributions of the grains could well be approximated by a Rosin-Rammler-Sperling distribution.     

Nature of luminescence and strain in gallium nitride nanowires

Photo- and cathodo-luminescence measurements of a variable-diameter ensemble of GaN nanowires revealed a diameter-dependent, spectral emission distribution between 350 nm and 850 nm. Spectral analysis indicated that wires with a diameter less than 400 nm were dominated by a yellow luminescence with a weaker near UV/violet emission also present. Examination of this ensemble showed that there was a general trend in the ratio of near-UV-to-yellow emission intensities with increasing nanowire diameter. Additionally, a broad green emission appears in the nanowires with a diameter above approximately 200 nm. A calculation based on the nanoheteroepitaxy model indicates that this diameter represents a transitional thickness where strain is relieved by defect formation mechanisms with a characteristic green emission.     

Estimation of the effect of relative humidity on protein crystallization

The effect of relative humidity in a crystallization box on the rate of establishment of supersaturation conditions during protein crystallization by diffusion of solvent vapors is estimated. A modified crystallization box is designed, which provides the formation of a stable air flow with a specified relative humidity and its measurement directly in the closed space between a drop and a reservoir. The range of relative humidities necessary to obtain the supersaturation conditions in a drop with a protein crystallization solution is determined.     

A New Organic Solid State Reaction: Hydrogenation of Phenols

During the past fifteen years there has been a number of detailed studies devoted to the chemistry of organic molecular solids and made by a number of groups involving chemists and crystallographers. These workers were interested in the reactions of the organic solids subjected either to a physical agent like light or heat or to a chemical one which can be a solid, a liquid or a gas.¹     

In situ microscopic observations of an electroless silver deposition process under high magnetic fields

The mechanism of the morphological effect of magnetic fields on electroless silver deposition was investigated through in situ microscopic observation by using a periscope system that was developed on the basis of a confocal scanning laser microscope. At the growth front of a silver dendrite, under a 12 T magnetic field applied perpendicularly to the sample plane, a silver branch was grown straightly for a while; then, a certain length of the branch at the neighbor of the tip started moving rapidly and was bent in an integrated manner. As a result of the process, a dense silver dendrite in the shape of a vortex was formed. When the sample space was narrowed, the branch did not bend due to the increase in the static friction between the branch and the glass plates. Judging from these observations, the mechanism of the formation of a dense vortex dendrite was thought to be the effect of a Lorentz force acting on the branch due to the electric current flowing through the branch itself accompanied by the silver deposition and the copper dissolution reactions.     

A Positive Contrast Guest-Host Display Using a Liquid Crystal of Negative Dielectric Anisotropy

Recently we described a novel guest-host cell which exhibits a positive contrast (dark digits on a clear background) employing a mixture of a nematic liquid crystal with negative dielectric anisotropy, a chiral component, and a dichroic dye. In this cell, the cell thickness and the alignment layers are chosen such that in the absence of a field, the mixture shows a uniform homeotropic structure. When a voltage greater than the threshold value is applied, a helicoïdal structure is adopted, which absorbs the incident illumination.

In this work the expression for the threshold voltage is derived and compared with experimental results. Experimental results are also presented which indicate, for a given cell thickness, the influence of the pitch of the mixture on the contrast and the response time.     

Kinetics of the Incorporation of Dopants into Epitaxial CVD Silicon

Starting from the gas-phase-transport-controlled and reaction-controlled incorporation mechanisms and the equilibrium incorporation of the dopants a critical study of the adsorption model is carried through, as a result of which a model of adsorption is proposed. This modified model reflects the incorporation concentration of the dopants as a function of the layer growth rate and contains the equilibrium incorporation of dopants as a border-line case for a layer growth rate of zero.     

The influence on a stability analysis involving a prototype solidification problem of including an interfacial surface entropy effect in the heat balance relationship at the interface

A reexamination of the conservation of heat boundary condition usually imposed at the solid-liquid interface during controlled solidification by means of a careful application, to energy conservation at that interface, of a general balance equation from continuum mechanics for surfaces of discontinuity yields a new conservation of heat condition containing an extra term involving interfacial curvature and surface entropy. A standard linear stability analysis by a normal mode technique of the uniform growth of a planar interface of a pure solid into a thermally undercooled liquid shows that the consideration of this extra term has a destabilizing effect on a model which does not include it for this prototype priblem if surface entropy is negative and a stabilizing influence if it is positive.     

Field-Induced Orientation Transition in Ferroelectric Liquid Crystals

Static and dynamic effects of a fast bistable device containing a ferroelectric liquid crystal are investigated theoretically. The switching mechanisms are different at low and high electric fields. There is a critical voltage which separates a bulk switching regime from the regime of domain wall motion. This threshold is determined as a function of physical device parameters.     

Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

We report the details of the construction and calibration of a sensitive surface rheometer, inspired by an instrument described in the literature, adapted to the study of photosensitive polymeric materials. By high resolution video tracking of the motion of a floating magnetized needle we are able to measure the viscoelastic complex shear modulus G of a Langmuir monolayer with an accuracy of 5*10^− 6 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The shear modulus is measured as a function of temperature, illumination power and wavelength. The reversible rheological changes induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G mainly real) to a viscoelastic regime (real and imaginary parts of G comparable). These results are confirmed by a comparison with independent measurements performed using other rheological techniques. Finally a discussion is made, taking into account the results of a recent X-ray photon correlation spectroscopy (XPCS) experiment on the same polymer in equilibrium and out of equilibrium.     

Light Scattering from Nematic Liquid Crystals in a Nonuniform Velocity Field

Fluctuations about a nonequilibrium steady state of a nematic liquid crystal subject to small, linear shear, are studied by means of a generalized Langevin method. We find a symmetric broadening of the director power spectrum and estimate the numerical size of the effect for a typical material.     

Numerical modeling of optical properties of a system of two zone plates for focusing hard synchrotron radiation

Results of numerical experiments on focusing of a monochromatic spherical wave by a system of two linear zone plates are reported. Calculations were performed for a photon energy of 12.3985 keV and zone plates with a radius of the first zone of 5 μm, a number of zones of 628, and an aperture of 250 μm. To calculate the Kirchhoff integrals, the double Fourier transform method was used and the fast Fourier transform procedure on a grid with a number of points 65 536 = 2¹⁶ was applied. On the basis of the calculation results, a conclusion was drawn that two zone plates operate as one with a doubled phase shift in zones with a material if the longitudinal distance between them is smaller than 1/3 of the focus depth and the transverse displacement is smaller than 1/3 of the outermost zone width (the focus size). If the distance (displacement) exceeds the focus depth (size), the two zone plates operate independently, similar to refracting lenses with a set of different focusing orders, including the zero order. The nature of the moiré pattern at a transverse displacement of the zone plates is discussed.     

Fictive temperature dependence of subcritical crack growth rate of normal glass and anomalous glass

Subcritical crack growth rates of soda–lime–silicate glass, which is a typical normal glass, and silica glass, which is a typical anomalous glass, with different fictive temperatures were measured by the double-cleavage-drilled-compression (DCDC) fracture mechanics technique under both dry and humid atmospheres in order to clarify the effect of the fictive temperature on mechanical strength and fatigue. In the humid atmosphere, the soda–lime–silicate glass with a higher fictive temperature showed a slower crack growth rate than the same glass with a lower fictive temperature while the silica glass with a higher fictive temperature showed a faster crack growth rate than the silica glass with a lower fictive temperature. These results imply that normal glass with a higher fictive temperature is expected to show a higher mechanical strength compared with the same glass with a lower fictive temperature and anomalous glass with a higher fictive temperature is expected to show a lower mechanical strength than the same glass with a lower fictive temperature when tested in ambient air if the flaw size is the same. In the dry atmosphere, the fictive temperature effects on the crack growth rate in both glasses were small and within the experimental error.     

Color Gamut of Liquid Crystal Polysiloxanest

Polysiloxanes with suitable side-chains form a cholesteric liquid crystal phase that can be applied to a substrate to produce a durable coating which exhibits strong colours below the glass transition temperature. The peak reflectance is almost 0.5 and a nearly 1.0 peak reflectance can be obtained using a half-wave retardation plate coated on both sides with the material and viewed from one direction against a black background. Values below 0.5 and between 0.5 and 1.0 have been also obtained using suitable solvents and alignment techniques. Many desired spectral reflectance distributions can be then obtained because of the additive color properties of such cholesteric coatings. As a result it is possible to produce a larger colour gamut than with real surface colours such as pigments and dyes. Employing commercially available material experimental work was carried out demonstrating a greatly expanded colour gamut on the chro-maticity diagram. Also it is possible to modify the spectral reflectance distribution of coloured substrates by coating them with a material having a reflectance peak at the desired band of wavelength.     

A stroll among effective temperatures in aging systems: Limits and perspectives

In this paper we present a short survey on the concept of effective temperature, on its onset as a glass former vitrifies, on the various definitions in literature and their limits of applicability. An exactly solvable model glass is employed to compare effective temperatures among them and to set a criterion for the occurrence of an universal extra temperature in the framework of a ‘two-temperature thermodynamics’ for off-equilibrium aging systems. It will be shown that aging in glass formers is not a sufficient requirement. As an instance, memory effects typical of glasses are not compatible with a unique effective temperature. Yet, a reduced range of applicability can still be established and investigated.     

Photoluminescence dynamics in chalcogenide glasses and crystals

Time-resolved photoluminescence studies reveal distinct differences between the recombination processes in a chalcogenide glass and in its crystalline counterpart. Here the three luminescence bands of a-As₂S₃ are interpreted in terms of the recombination of an excition, a self-trapped exciton and a pair of electron- and hole-like small polarons. The two luminescence bands observed in the crystal are attributed to the recombination of two types of excitons composed respectively of a hole bound to a self-trapped electron, and a hole which is induced to self-trap in the presence of a self-trapped electron.