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.     

Investigation of the Mechanism of Influence of Stress Corrosion on the Development of Macroplastic Instabilities of Aluminum–Magnesium Alloy

Crystallography Reports - Macroscopic jumps of plastic deformation (few percent in amplitude) on creep curves of aluminum–magnesium alloy, caused by a local effect of concentrated solution of...     

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.     

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).     

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.     

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.     

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.     

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.     

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.     

Peculiarities of the microstructure of a nanoscale modification of η-TiO2

Samples with nanoscale η-TiO₂ phase have been obtained by sulfate and modified sulfate methods and are characterized by a complex of techniques: X-ray diffraction, electron diffraction, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and X-ray energy-dispersive analysis. The effect of sample formation conditions on the size and shape of crystallites, content of the amorphous component in the samples, and their elemental composition has been established. A significant change (depending on the synthesis conditions) in the parameters of the diffraction reflection with d ～ 17–21 Å (intensity and interplanar spacing d, Å), pronounced for η-TiO₂, is revealed. This change is most likely related to the variation in the content of water molecules in the interlayer space of η-TiO₂ structure and/or the change in crystallite shape.     

Electron Microscopy of the Microstructure of Nb–Ti Tapes

Crystallography Reports - The microstructure of a cold-rolled tape made of Nb–50 wt % Ti superconducting alloy before and after heat treatment has been investigated by transmission electron...     

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.     

Crystallization of mutant forms of the γ subunit of archaeal translation initiation factor 2

Archaeal translation initiation factor 2 (aIF2) is homologous to its eukaryotic counterpart (eIF2). It is a heterotrimeric protein consisting of α, β, and γ subunits. The protein e/aIF2 forms a ternary complex with guanosine 5′-triphosphate and the initiator methionyl-tRNA (Met-tRNA_i) and delivers the latter to the ribosome. In archaea, translation initiation factor 2 has an additional function. The γ subunit of aIF2 binds mRNAs with a triphosphate at the 5′-end and prevents 5′-to-3′ directional mRNA decay. To determine the mRNA-binding site on the surface of aIF2γ, mutations were introduced into the protein sequence at sites of possible interactions with mRNA. The crystals of the mutant forms of aIF2γ were obtained, and X-ray diffraction data sets suitable for structure determination at atomic resolution were collected.     

Structure of the interfaces of the InxGa1 − x As quantum well from X-ray diffraction data

The structure of the interfaces of a 10-nm-thick In_xGa_{1 ? x} As quantum well buried in the semiconductor GaAs matrix has been studied by the method of double-crystal X-ray diffractometry. It has been shown that, in comparison with the well-known photoluminescence method, the X-ray diffraction method has considerable advantages in characterization of multilayer systems. The detailed analysis of the rocking curves provided the reconstruction of the profiles of indium distributions in quantum wells for specimens with different indium concentrations.     

Effect of germanium on the microstructure and the magnetic properties of Fe–B–Si amorphous alloys

In this work, we present a systematic study on the crystallization kinetics and the magnetic properties of melt-spun Fe₈₀B₁₀Si_10 ? xGe_x (x = 0.0 ? 10.0) amorphous alloys. The activation energy for crystallization, determined by differential scanning calorimetry, displayed a strong dependence on the Ge content, reflecting a deleterious effect on the alloys' thermal stability and their glass forming ability with increasing Ge concentration. On the other hand, the alloys exhibited excellent soft magnetic properties, i.e., high saturation magnetization values (around 1.60 T), alongside Curie temperatures of up to 600 K. Complementary, for increasing Ge substitution, the ferromagnetic resonance spectra showed a microstructural evolution comprising at least two different magnetic phases corresponding to a majority amorphous matrix and to Fe(Si, Ge) nanocrystallites for x ≥ 7.5.     

The Wannier-Mott exciton’s dynamic properties without limitations on the magnitudes of the wave vectors of carriers

Abstract

Most often, the excitation of the Wannier-Mott excitons’ type is generated by electromagnetic action when the electron from the valence band passes into the conduction band. The influence of a magnitude wave vector for the exciton dynamic on the wave vector of the dynamics of the internal relative motion of the carriers is considered. It is shown that due to the effect of the field of intra-crystalline stresses on the exciton movement, a substantial displacement of the exciton spectrum may occur. Estimates of the possible magnitude of such a displacement were made.     

The influence of the cooling rate of A 2PbO · SiO2 melt on the constitution of silicate anions

The kinetics of crystallization in a 2Pbo · SiO₂ melt have been investigated. A TTT-diagram was constructed, which describes the kinetic parameters for the formation of crystalline phases in the system. By means of silicate anion analysis the relationship between the cooling rates of the melt and the structure of silicate units in solid 2PbO · SiO₂ has been studied. Substantial differences in the thermal treatment of the melt lead to alterations of the silicate anion constitution, which cause changes in the crystallization behavior of glassy 2PbO · SiO₂.     

Dependence of the Tilt Angle on External Forces for Smectic-C And Chiral Smectic-C Liquid Crystals—Measurement of the Heat Capacity of DOBAMBC

A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement.

The dependence of the tilt angle on applied electric and magnetic fields is discussed for both smectic-C and chiral (ferroelectric) smectic-C liquid crystals. Numerical estimates of the Landau coefficients are used to show that the effect of external forces on the tilt angle is negligible except when being less than approximately 0.2 K from the transition to the smectic-A phase. The effect on the smectic-C-smectic-A phase-transition temperature is also discussed.

Boundary effects can also affect the tilt angle, and an expression for the penetration depth of a boundary-induced disturbance is calculated. For thin, planar samples, a coupling between the two boundaries can induce a transition to the smectic-A phase. It is demonstrated that this transition is analogous to the ordinary Freederickz transition in nematic liquid crystals.