Noncrystalline Medium Research Articles

Non-local mathematical models for aggregation processes in dispersive media

Particles aggregation is widespread in different technological processes and nature, and there are many approaches to modeling this phenomenon. However, the time non-locality effects with witch these processes are often   accompanied leave to be none well elaborated at present. This problem is justified especially in reference to nano-technological processes. The paper is devoted to the non-local modification of Smoluchowski equation that is the key point for describing influence of synchrony and asynchrony delays in aggregation processes for clusters of different orders. The main scientific contribution consists in deriving the non-linear wave equation describing the evolution of different orders clusters concentration under aggregation processes in polydispersed systems with following for the mentioned non-locality. The practical significance lies in the fact that the results obtained can serve as the basis for the engineering calculation of the kinetics of aggregation in polydisperse nano-systems. The research methodology is based on mathematical modeling with the help of the relaxation transfer kernels approach. Succeeding analysis of aggregation processes on the base of submitted ideology can be directed to generalizing master equations with allowing for space non-locality too. The submitted approach opens up fresh opportunities for detailed study of influence of relaxation times hierarchy on the intensity of aggregation and gelation processes in non-crystalline media containing dispersed solid phase.

A crystal-processing machine using a deep-ultraviolet laser: application to long-wavelength native SAD experiments.

While native SAD phasing is a promising method for next-generation macromolecular crystallography, it requires the collection of high-quality diffraction data using long-wavelength X-rays. The crystal itself and the noncrystalline medium around the crystal can cause background noise during long-wavelength X-ray data collection, hampering native SAD phasing. Optimizing the crystal size and shape or removing noncrystalline sample portions have thus been considered to be effective means of improving the data quality. A crystal-processing machine that uses a deep-UV laser has been developed. The machine utilizes the pulsed UV laser soft ablation (PULSA) technique, which generates less heat than methods using infrared or visible lasers. Since protein crystals are sensitive to heat damage, PULSA is an appropriate method to process them. Integration of a high-speed Galvano scanner and a high-precision goniometer enables protein crystals to be shaped precisely and efficiently. Application of this crystal-processing machine to a long-wavelength X-ray diffraction experiment significantly improved the diffraction data quality and thereby increased the success rate in experimental phasing using anomalous diffraction from atoms.

Phase-contrast imaging of multiply-scattering extended objects at atomic resolution by reconstruction of the scattering matrix

Three-dimensional phase contrast imaging of multiply-scattering samples in X-ray and electron microscopy is extremely challenging, due to small numerical apertures, the unavailability of wavefront shaping optics, and the highly nonlinear inversion required from intensity-only measurements. In this work, we present a new algorithm using the scattering matrix formalism to solve the scattering from a non-crystalline medium from scanning diffraction measurements, and recover the illumination aberrations. Our method will enable 3D imaging and materials characterization at high resolution for a wide range of materials.

Characteristics of Refractive-Index Sensor in Graphene-Modified Long-Range Surface Exciton-Polaritons

The organic non-crystalline medium of 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)-benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulfobutyl)-benzimidazolium hydroxide (TDBC) is emerging as possible alternative plasmonic material for noble metal in visible region. In this paper, a novel long-range surface exciton-polariton (LRSEP) sensor based on TDBC film covered with graphene is reported. To enhance the imaging sensitivity, the thickness of TDBC film and the number of graphene layers are optimized. The result shows that the optimized imaging sensitivity is enhanced to 3243 RIU−1 when ns = 1.34. Compared with the traditional noble metal film-based sensor, the proposed LRSEP sensor demonstrates that the imaging sensitivity has been greatly improved. This is the first study of the TDBC film-based LRSEP sensor, which we hope to support the potential development of chemical sensing and bio-sensing.

Validity of the second Fick's law for modeling ion-exchange diffusion in non-crystalline viscoelastic media (glasses)

It is shown that, in general case, the diffusion equation (or the second Fick's law) does not provide an adequate description of ion-exchange transport phenomena in viscoelastic media, including glassy or any other non-crystalline media. In this connection the general phenomenological model of ion-exchange diffusion in viscoelastic media has been developed. A theoretical analysis of the model shows that, in the case of a linear dependence of medium density on the concentration of diffusing ions, the necessary and sufficient condition of the absolute validity of the diffusion equation in viscoelastic media is Φ ≫ 1, where Φ = τD/τR is the dimensionless value (or criterion of similarity), with τD = L2/D being the characteristic time of diffusion and τR = η/G being the characteristic time of stress relaxation, where L, D, η, and G are the characteristic length of diffusion, the diffusivity, the viscosity, and the shear modulus, respectively. The value of 1/Φ characterizes the accuracy which is provided if the second Fick's law is used in the simulation of ion-exchange diffusion in viscoelastic media. We have demonstrated the applicability of this criterion experimentally. Our experimental studies on ion-exchange diffusion in an oxide glass (typical viscoelastic media) have shown that under the condition the Φ > 105 the experimental concentration profiles are close to those predicted by the second Fick's law to within an accuracy of 1%.

Cation-cation interactions between uranyl(VI) ions

Data on crystalline U(VI) compounds with mutual coordination of uranyl ions in their structure (cation-cation interactions, CCIs) are summarized. The topology and geometry of CCIs, including the characteristics of Voronoi-Dirichlet polyhedra of uranium atoms, are analyzed. 15 types of complexes [UO22+]n formed by CCIs are revealed. The influence of CCIs on the stereochemical features of U(VI) are considered, and common and distinctive features of CCIs in uranyl(VI) and neptunyl(V) compounds are discussed. Factors favoring the occurrence of CCIs in uranyl compounds are analyzed. The spectroscopic manifestations of CCIs are considered. Data on CCIs in some noncrystalline media are presented. The strength of cation-cation bonds in uranyl complexes is analyzed.

Molecular dynamics study of crystallization of polymer systems confined in small nanodomains

We study the ordering dynamics of polymer crystallization using molecular dynamics simulations, in which the polymers are confined to small nanodomains surrounded by a noncrystalline medium. Although the adsorption process and surface diffusion of polymer chains play a major role in the case of the crystalline interface, the domain interface which consists of noncrystalline medium does not have such a crystal substrate to induce the growth of a crystal layer, which may lead to different ordering dynamics. We found that existence of a noncrystalline domain interface has two opposite effects. In the case of semiflexible polymer systems, the domain interface accelerates crystallization in the initial period, whereas it suppresses crystallization in the intermediate or late period. When the rigidity of polymer chains increases, the acceleration effect of the initial crystallization induced by the domain interface is sometimes hidden by spontaneous homogeneous nucleation. These ordering behaviors can be explained by the restriction on the local chain direction near the domain surface in the crystal nucleation stage and the restriction of large orientation relaxation and coalescence in the crystal growth stage. Simulation results reveal that the confinement by the noncrystalline medium does not have a simple effect on the polymer crystallization dynamics, but has various conflicting effects combined with the time stage, the rigidity of polymer chains, and the strength of the surrounding domain interface.

Multiple scattering and functional integrals

A functional integral method for performing the statistical averages implicit in problems involving multiple scattering is developed. The scattering fields in a noncrystalline medium are represented as a fixed set of fields for each incident particle, but are assumed to vary randomly from one particle in the beam to the next. The method is applied to eikonal treatments of both multiple scattering and photon radiation. This latter problem involves the Landau-Pomeranchuk-Migdal effect, and correction terms to earlier treatments are evaluated. {copyright} {ital 1997} {ital The American Physical Society}

The conduction properties of Plasma-Enhanced Low-Pressure Chemical Vapour Deposited (PELPCVD) SIPOS

The electrical characteristics of PELPCVD SIPOS-silicon heterojunctions are presented. The d.c. current-voltage characteristics as a function of temperature for SIPOS-silicon heterojunctions, for SIPOS having silicon contents of from 31 to 79 atomic percent and film thickness of from 1400 to 6800 Å are reported. At high temperatures, carrier transport occurs by thermal excitation over a mobility edge, giving an activation energy of half the bandgap. At lower temperatures, the tunneling mechanism of Variable Range Hopping (VRH) dominates the conduction process. The current density-voltage characteristics of all the samples over the entire measured temperature range are accurately predicted by the use of a variation of Mott's model for conduction in a non-crystalline medium.

C̆erenkov radiation at the resonance frequency of an amplifying medium

The propagation of C\ifmmode \breve{}\else \u{}\fi{}erenkov radiation through a medium in which the population of two energy levels is inverted has been studied. The population inversion is represented by an imaginary part of the index of refraction of the medium at the resonance frequency. The resonant atoms are embedded in a host medium which has the shape of a slab of finite width $L$ and dielectric constant ${\ensuremath{\epsilon}}_{h}$, which is real and varies slowly for frequencies close to the resonance frequency of the embedded atoms. It is assumed that the host medium and the embedded atoms inside the slab behave as an isotropic noncrystalline medium. The slab is covered by a mirror on each of its surfaces and the medium outside the slab and the mirrors has dielectric constant ${\ensuremath{\epsilon}}_{1}$ (${\ensuremath{\epsilon}}_{1}=1$ for vacuum). While each individual charged particle can stimulate coherent emission by different resonant atoms, the phase of radiation which is due to different particles is uncorrelated. It is shown, however, that radiation of high intensity in the direction of the C\ifmmode \breve{}\else \u{}\fi{}erenkov cone can be produced by highcurrent particle beams. Exact expressions, in integral form, are obtained for the electromagnetic field everywhere in space. Using the method of stationary phase approximate expressions are derived for the C\ifmmode \breve{}\else \u{}\fi{}erenkov fields as well as the radiated energy in the foward and backward direction, far away from the slab.

Electronic Processes in Noncrystalline Materials

N F Mott and E A Davis London: Oxford University Press 1971 pp xiii + 437 price £750 It is hard to think of a physicist other than Professor Mott who would be able to attempt successfully a book of such a broad compass and in Dr Davis, a specialist in the field of amorphous semiconductors, he has found an ideal co-author. The book is divided into two parts: the first deals with the theory of electrons in a noncrystalline medium and its application to liquid metals, amorphous semimetals and semiconductors.

Theory of the Propagation of the Plane-Wave Disturbances in a Distribution of Thermal Neutrons

The propagation of plane-wave disturbances in a neutron gas in thermal equilibrium with a moderator is studied using the linearized Boltzmann equation. The eigenvalue spectrum of the appropriate Boltzmann transport operator is investigated for both polycrystalline and noncrystalline media, and several necessary conditions for the existence of a point spectrum (and hence for the existence of plane-wave propagation) are discussed. Techniques are presented which allow the solution of various boundary-value problems using this spectral representation.

Pumping Induced Imperfections in Glass:Nd3+ Lasers

AbstractA barium crown glass:Nd3+ laser rod which developed permanent internal imperfections during laser pumping was investigated by optical metallography, transmission electron microscopy, and electron diffraction techniques. The imperfections were found to be microcracks resulting from differential thermal expansion between crystallites (resulting from devitrification) and the surrounding noncrystalline medium. An analysis of the kinetics of devitrification, the radial temperature distribution during pumping and stresses resulting from differential thermal expansion are in agreement with a proposed mechanism for the development of permanent internal imperfections.

A test of diamond's method for estimating size distribution of aromatic ring systems

Diamond's method of estimating the size distribution of aromatic planes in a non-crystalline medium has been applied to a number of liquid hydrocarbon specimens and the results shown to suffer from both inaccuracy and inconsistency. It would appear that similar results for coals may be quite misleading, while those from cokes and similar materials should be treated with caution.