Calculation of the parameters of the X-ray diffraction station with adaptive segmented optics on the side beam from the wiggler of the Sibir’-2 storage ring

The mounting of an X-ray diffraction station on the side beam of a 19-pole superconducting wiggler makes it possible not only to use the central synchrotron radiation beam with a wavelength of 0.5 ?, but also to solve problems requiring softer X rays at a synchrotron radiation (SR) intensity exceeding that for the beams from the bending magnet. A numerical simulation of the formation of photon beams from a source and their transmission through the elements of the station (and through the station as a whole) allows one to calculate the parameters of the station, compare it with the existing analogs, determine its potential and actual efficiency of its elements, and estimate the adjustment quality. A numerical simulation of the SR source on the side beam from the wiggler and the focusing channel (segmented condenser mirror, monochromator with sagittal focusing by the segmented second crystal, and segmented focusing mirror) has been performed. The sizes of the focus and the divergence of rays in it are determined with allowance for the finite sizes of segments. The intensity of radiation with a wavelength ?? = 1.0 ? in the focus is determined taking into account the loss in the SR extraction channel and in the focusing channel. The values of the critical wavelength for the side beam from the wiggler and the wavelength resolution are calculated. The intensities in the X-ray diffraction pattern and its angular resolution are found.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Effects of the Li-evaporation on the Czochralski growth of γ-LiAlO2

Two-inch-diameter γ-LiAlO₂ single crystals were grown from the melt by Czochralski method. The crystals were examined by optical methods, high-resolution X-ray diffraction and transmission electron microscopy (TEM). Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the Li/Al ratio in the residual melts. The Li-evaporation from both melt and grown crystal is the main problem in the γ-LiAlO₂ growth and has to be controlled by acting on the vertical temperature gradient. Shallow gradients increase the Li-evaporation from the crystal surface resulting in boules with a milky rim. On the other hand, steep gradients may induce cracks in the boule and enhance the Li₂O escape from melt with consequent variation of the composition. ICP-OES investigations reveal that melt compositions can vary in the range from 46.5 to 50 mol% Li₂O to obtain transparent LiAlO₂ crystals. Beyond this value, the formation of inclusions inside the crystals is probable. We have established an optimized growth assembly, which allows remaining the melt composition stoichiometric. The as-grown crystals exhibit defects like subgrains, twins and a core of voids and fine-grained inclusions. The latter could be characterized by TEM as submicron LiAl₅O₈ crystallites.     

Numerical investigation of the effect of heat shield shape on the oxygen impurity distribution at the crystal–melt interface during the process of Czochralski silicon crystal growth

In this study, a numerical simulation is performed to investigate the effect of the shape of the heat shield on the oxygen concentration in the melt. The results show that the oxygen concentration in the melt can be significantly decreased by increasing the speed of the argon gas near the crucible wall. This can be achieved by enlarging the horizontal length of the heat shield. The oxygen concentration at the melt–crystal interface varies with the length of the crystal growth. In the initial stage, there is a significant decrease in the oxygen concentration as the growth length increases. There is also a significant reduction in the emission of oxygen from the crucible wall due to the lower melt depth and crucible temperature. The transportation of oxygen impurity towards the melt–crystal interface is suppressed by the vortex motion in the melt. When the crystal exceeds a certain length, the oxygen concentration in the melt–crystal interface starts to increase with increasing crystal length, due to the drop in vortex motion in the melt.     

What is the Shape of a Polymer Chain near the Theta Point?

We address the question of the conformation of a polymer near the θ point. To this end, we present an argument that the statistics of polymer rings at the θ point in two dimensions is exactly given by the statistics of the external perimeter (“hull”) of a percolation cluster. As a consequence, the fractal dimension d_f (θ) of a polymer chain at the θ point coincides with that of the hull of the percolating cluster, d_f (θ) - d_H . We also perform extensive simulations of the conventional θ point model—the interacting self-avoiding walk (ISAW)—and the smart kinetic walk (SKW). We demonstrate that the SKW predicts a higher order critical point, termed the θ′ point, which has the same critical behavior as the ISA W. We conclude that the SKW is a well-defined walk that gives the conformation of a polymer near the θ point in two dimensions. In particular, we report accurate calculations of all three tricritical point exponents that are needed to fully describe the θ point.     

Electron effects of the substituents on the structure of 4,4′-dipyridylium dications

The crystal structure of N,N′-di(2-hydroxyethyl)-4,4′-dipyridylium diperchlorate C₁₄H₁₈Cl₂N₂O₁₀ is determined by X-ray diffraction at 293 K. The crystals are monoclinic, a = 6.446(1) Å, b = 14.479(3) Å, c = 9.912(1) Å, β = 99.24(2)°, space group P2₁/c, and Z = 2; 1562 reflections measured; and wR2 = 0.086 and R1 = 0.033 for 1209 reflections with F > 4σ(F). It is found that, in the absence of charge transfer from the counterion to the dication, the planarity of the dipyridylium skeleton of the molecule is primarily due to the conjugation between the lone electron pairs of the hydroxyl groups and the π system of the dication. This inference is supported by the intramolecular O?N contact (2.847 Å).     

Morphology prediction of crystals grown in the presence of impurities and solvents — An evaluation of the state of the art

Computational methods enable to calculate relative face growth rates and crystal shape from structural information alone. Even if these models are sufficient for the calculation of the habit of a vapor grown crystal, most of them fail to correctly reproduce the habit of crystal growth from solution. In recent years, new approaches have been proposed based on the substitution of additive molecules in the crystal lattice or on the surface of the crystal. The new computer-based approaches provide a fundamental understanding of processes of crystal growth from solution. The number of methods proposed in morphology prediction is enormous. Herein, an overview of these methods and approaches is provided.     

Simulation of the spinodal phase separation dynamics of the Bi–Zn system

In the phase separation occurring at the miscibility gap (at the spinodal region) of an alloy a discrete symmetry is spontaneously broken and a domain wall network is formed. Field theory simulations are often used to study the dynamics of topological defects networks appearing in different physical contexts. In this work, we focus on the dynamics of the two immiscible liquids appearing on the phase diagram of the Bi–Zn system, one of the basic systems of lead free solders. We use phase field simulations to quantitatively simulate the dynamics of the two liquids separation in the Bi–Zn system, at different temperatures and for different concentrations. We obtain the miscibility gap curve and the domain morphologies of the system as a function of time, temperature and component concentrations using simulations.     

Change of the Prenematic Order in the Isotropic Phase Close to the Isotropic–Nematic Phase Transition with Increasing Aerosil Nanoparticles Density

The influence of the hydrophilic and hydrophobic aerosil nanoparticles density on the formation of the prenematic order close to the isotropic – nematic (I–N) phase transition is presented. The lowering of the activation energy, with the increase of aerosil nanoparticles density, suggests a change of the prenematic order in the isotropic phase. The temperature dependence of the static dielectric permittivity for 4–n–pentyl–4′–cyanobiphenyl (5CB) exhibits an anomaly near the I–N phase transition. With the increase of aerosil density, a substantial modification in the anomaly of the static permittivity was observed in the isotropic phase. Indicated changes are different for hydrophilic and hydrophobic aerosil as a result of different surface interaction between nematic molecules and aerosil nanoparticles.     

An analysis of the applicability of the Scheil equation in the determination of segregation in monocrystals of the manganese-zinc ferrites Mn1?xZnxFe2O4

Theoretical assumptions concerning the segregation phenomenon of alloying components during formation of monocrystals of the manganese-zinc ferrite Mn_{1 x}Zn_xFe₂O₄ have been presented. Taking into account the case of the unidirectional solidification of the above ferrite by means of the Bridgman furnace, which is working in:

• Closed system.
• Open system, with adding of the pure component (melting at an elevated temperature).
• Open system, with adding of ferrite of a nominal composition.
• The method for calculation of alloying components segregation has been analysed. A possibility of modelling the segregation by means of the choice of the size of the melting zone has been considered for a given monocrystal.

     

On the role of fluctuations at the boundary of Earth’s solid core

The fluctuation effects at the boundary between the internal solid and external liquid regions of the Earth’s core are considered within the vitrification model. If the internal core is characterized by a relatively small static shear modulus and shear acoustic oscillations, this phase transition becomes slightly stepwise and can be accompanied by critical phenomena. The corresponding fluctuation corrections to the thermodynamic derivatives are calculated with arbitrary critical indices. The propagation and absorption of seismic waves, which become weakly anisotropic due to the anisotropic “liquid” inclusions (which arise in the hysteretic boundary region due to the Earth’s rotation and viscous stress effect), are considered. The resulting estimates are compared with the existing geophysical data.     

Thermodynamic Evaluation of the Nickel-rich Part of the Ni?Zn-System

Optimization of thermodynamic and phase diagram data about the solid and liquid Ni Zn solutions is performed. The temperature and concentration dependence of the Gibbs free excess energy is described by means of second degree polynomials. The calculated liquidus and solidus curves have been compared to the experimental ones.     

Influence of crystallisation conditions on the microtexture of the directionally solidified eutectic of the MgF2?CaF2 system

The influence of high-frequency (HF) and resistive (R) heating of a vertical crystallizer furnace on the directional solidification process of the MgF₂ CaF₂ system eutectic has been compared. The turbulent flow induced by HF heating given rise to inhomogeneities in microtexture, which do not occur under R heating conditions. Variations in the growth rate (v) and temperature gradient (ΔT) lead to characteristic changes of the eutectic microstructure: from lamellar to rod-like. Quantitative analysis of these changes was advantageously performed using a digital image analysis of the area (AR), maximum cross-section (D-MAX), direction of the maximum cross-section (ANGLE D-MAX) of MgF₂ particles of the microtexture. This was recorded on SCAN images of transverse cuts through the given directionally solified samples as well as the analysis of the shape of MgF₂ particles.     

On the origin of the high Kerr coefficient measured in thallium–zinc–tellurite glasses

Measurements of Kerr electrooptical sensitivity of several zinc–thallium–tellurite glasses are presented, and composition dependence of Kerr sensitivity is compared with the dependence of the second harmonic generation efficiency collected for optically poled TeO₂–TlO_0.5–ZnO glasses. These data being analyzed jointly with Raman measurements data allowed us to conclude that the high electrooptical Kerr coefficient and nonlinearity of Tl₂O–ZnO–TeO₂ glasses, and their sharp increase with augmenting concentration of thallium oxide TlO_0.5 above 15% should be attributed to the presence of Tl⁺ cations having very high non-linear polarizability most likely related to their electronic lone pairs.     

Study of the Gilding Technology of the “Idol” from the 10th Century Mound “Chernaya Mogila” (“Black Grave”)

Crystallography Reports - A unique artifact—the “idol” from the 10th century mound “Chernaya Mogila” (“Black Grave”), a famous Old Rus’ site, has...     

Investigation of the photovoltaic performance of the polycrystalline silicon p–n junction by a photothermal measurement

For establishing a new methodology for evaluating an effect of the grain boundaries, both the piezoelectric photo-thermal (PPT) and the surface photo-voltage (SPV) measurements of polycrystalline Si p–n junction samples with different volume fractions of grain boundaries were carried out. We could define the signal intensity ratio of SPV/PPT as the key indicator of photovoltaic performance. This is because the PPT signal implies the phonon emitting carrier loss, whereas the SPV denotes the photo-excited carrier accumulation at the surface and the junction interface. It was found that the SPV/PPT ratio and solar cell efficiency decreased with increasing volume fraction of the grain boundaries. Present experimental results demonstrated that one can directly estimate the photovoltaic performance of in-process polycrystalline Si p–n junction wafer by adopting the combination of the PPT and the SPV methodologies without electrodes. Since the PPT detects the non-radiative recombination process, present methodology and the laser-beam-induced current and the photoluminescence imaging methods are complementary. By complementary use of these methods, it becomes possible to investigate the characteristic of grain boundary.     

The influence of the Mg content and the quenching conditions on the decomposition kinetics of Al?Zn?Mg alloys

By means of resistivity measurements, TEM and microhardness measurements the decomposition kinetics of A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 0.2; 0.5; 1.0; 1.5; 2.25; 3.0) were investigated. For T_a < T_{a, crit} < T_h (T_h: temperature of the rapid homogenous nucleation) in A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 1.0; 1.5; 2.25; 3.0) an inversion of the decomposition kinetics for T_q > T_inv: (T_inv: inversion temperature) was established. This result can be explained with reference to the binding of excess vacancies in dislocation loops. Proceeding from the discrepancy between the experimentally estimated concentrations of vacancies bound in dislocation loops and the concentrations of free vacancies at T_q, calculated by means of the Lomer equation, a model is proposed, which explains the high vacancy concentration of the loops. In terms of this model the values of the migration energy, obtained for A1-3 at.% Zn — 1.5 at.% Mg in the T_a-range 23 °C ≦ T_a ≦ 100 °C, and the shift of the inversion temperatures T_inv towards higher values with increasing T_a are interpreted. The same applies to the influence of the Mg content on the decomposition kinetics in A1-4.5 at.% Zn-x at.% Mg alloys.