Influence of water on the viscosity and thermal expansion of sodium trisilicate glasses

The transformation range viscosity, activation energy for viscous flow and the dilatometric critical temperature of sodium trisilicate glasses are all decreased by addition of water, whereas the thermal expansion coefficient in the elastic region is not altered to a significant extent. Although these effects are similar to those observed in vitreous silica, they are considerably reduced in magnitude. Since sodium trisilicate glasses are already highly depolymerized by the presence of Na₂O, the additional depolymerization due to small additions of water does not seem adequate to explain these results. It is suggested that water behaves similarly to the alkali oxides and that the results of this study may be due to an extreme case of the mixed-alkali effect.     

The effects of oxygen and boron functionalization on the optical properties of dithienothiophenes

We study the optical properties of dithienothiophene functionalized with lateral dimesitylboryl groups and/or oxygen atoms at the central thienyl sulphur atom. The excitation energies of these systems are computed using first-principles time-dependent density functional theory, and the results are discussed and compared with measured absorption spectra. The agreement between theoretical and experimental results is very good. It is found that the oxygen and the boron functionalization modifies the lowest unoccupied molecular orbital, and thus optical properties, in a different way. Moreover if the boron and the oxygen functionalization are combined together, the effects of the former dominate.     

Viewing Angle Dependence and Optimisation of the SSFLCD for Full Colour LCD TV

If the Surface Stabilised Ferroelectric LCD¹ is to seriously threaten the position of the CRT it is essential that it offers full colour performance over a wide angle of view.

The dependence of transmission as a function of wavelength on viewing angle has been calculated by solving numerically the equation for the effective refractive indices and path length using polar coordinates. Experimental measurements on a thick (8.1 micron) and a thin (2.1 micron) cell are presented in agreement with the theoretical predictions.

The deviation in hue and colourfulness (saturation) as a function of cell spacing variation is also studied and results plotted on the CIE 1931 Chromaticity diagram. They indicate that very high tolerances in cell spacing uniformity are required and that even with optimum cell spacing the viewing cone is restricted.

A solution is proposed to greatly improve the viewing cone and ease the constraint on cell spacing tolerance by introducing a multigap structure of the type suggested to reduce the effect of dispersion in the TN displays. Theoretical results are presented.     

EINSTEIN model and modern methods in the statistical theory of crystals

A connection between various methods of the atomic theory of crystals and the EINSTEIN model is analyzed. It is pointed out that despite the oversimplified choice of a unique mode of vibrations for all atoms, in many cases it provides better results than the DEBYE model does. What is more, a number of methods of the theory of strongly anharmonic crystals can be considered as extensions of the EINSTEIN model which in its turn, is their limiting case when the anharmonic terms and interatomic correlations are neglected. A comparison between some peculiarities and results of such methods is drawn.     

Does the entropy and volume dependence of the structural α-relaxation originate from the Johari–Goldstein β-relaxation?

Conventional theories of glass transition are focussed on reproducing the thermodynamic and dynamic properties of the structural α-process. However, in the last decade a class of secondary relaxations, called the Johari–Goldstein (JG) β-relaxations, has attracted attention in view of the existence of various correlations and similarities of its properties with the structural α-process. These experimental evidences suggest that the secondary process may play a fundamental role in glass transition, and any theory of glass transition would be incomplete until the inter-relations between the JG β-relaxation and the structural relaxation have been considered. In this paper, some significant experimental results showing several general and nontrivial connections between structural relaxation and JG β-relaxation are reported. In particular, these connections indicate that, like the structural relaxation, the JG β-relaxation is entropy and volume dependent. The experimental facts are consistent with the results of the coupling model.     

Possible differences between the surface and bulk structure of glasses

Changes in cation co-ordination at glass surfaces (indicating an increase of the oxygen-to-cation ratio within the topmost superficial layers) are evidenced by comparing data acquired by spectroscopic techniques of different excitation depth. Based on these experimental results, a theoretical model is developed that predicts structural discrepancies between surface and bulk structure. Such reconstructed surfaces are of great importance for all kind of surface-related crystallization phenomena.     

Analysis of the applicability of the modified kinematic approximation to describe the off-specular neutron scattering from the surface of micro- and nanostructured objects

The applicability of the modified kinematic approximation to describe the off-specular neutron scattering from interfaces between media is analyzed. It is demonstrated that in some cases one can expect not only a qualitative but also a quantitative agreement between the data and the results of experiments and calculations based on more accurate techniques. Diffuse scattering from rough surfaces and thin films with correlated and noncorrelated roughness of the upper and lower interfaces and the neutron diffraction by stripe magnetic domains and magnetic domains with a random size distribution (magnetic roughness) are considered as examples.     

An Investigation of the Dynamic Structures of Ferrocene,Ruthenocene and Dibenzenechromium in the Solid State and in Solution

Pulse nuclear magnetic resonance measurements have been used to determine the activation enthalpies for the reorientation of the organic rings in ferrocene, ruthenocene and dibenzenechromium. The contribution from non-bonded interactions to the total potential for reorientation of the rings has been calculated and is compared to the experimental activation enthalpies. The results indicate that there is no contribution to the potential from the non-bonded interactions between the two rings on a single molecule and that the observed potential barrier is due to a combination of crystal packing forces and bonding forces within the molecule.

The crystal structure of ferrocene is thought to be disordered above 135[ddot]K, and a model is proposed for the order-disorder transition in the solid state. On this basis, dynamic models are proposed for the structures of the three metallocenes in solution.     

Investigation of the structure and properties of quartz in the α-β transition range by neutron diffraction and mechanical spectroscopy

The paper reports on the results of complex investigations into the physical properties of synthetic quartz single crystals and quartz powders in the temperature range of the α-β transition with the use of neutron diffraction and mechanical spectroscopy. The crystal structure of quartz powders with different average sizes of grains is determined in the temperature range up to 620°C and in the α-β transition temperature range. The temperature dependences of the internal friction and the resonant frequency for quartz samples in the vicinity of the phase transition temperature are obtained upon excitation of vibrations in the planes parallel and perpendicular to the Z axis of the quartz crystal. The temperatures at the maxima of the internal friction in the range 560–620°C are determined. The assumptions regarding the possible reasons for the shift of the phase transition temperature are made. It is revealed that the internal friction is characterized by a maximum that is observed in the vicinity of 350°C and is not related to the structural transformations in quartz.     

Kinetics of protein crystal nucleation and growth in the batch method

We have applied the Johnson–Mehl–Avrami–Kolomogorov (JMAK) theory of crystal nucleation and growth to the problem of protein crystallization in the batch method. Without integrating the JMAK equation explicitly, we use dimensional analysis to derive a general formula for the half-life for decay of the protein supersaturation. This formula includes a dimensioned group and an arbitrary dimensionless function. We integrate the JMAK equation exactly for the special case where the growth rate is independent of the supersaturation and the nucleation rate is proportional to its square. This gives an equation for the time decay of the supersaturation and a formula for the half-life in which all arbitrary dimensionless functions are evaluated. The results are consistent not only with Von Weimarn's rule, which asserts that the average size of a crystal increases as the supersaturation decreases, but also with our experimental results for crystallization of lysozyme, in which the half-life at fixed pH decreases with increasing ionic strength and decreasing temperature.     

Surface Deformation and Walls in the Conduction Regime of Nematic Liquid Crystals

Reflection of a laser beam from the free surface of a nematic liquid crystal shows that the surface is periodically deformed when a high electric field (conduction regime) is continuously applied. Walls which appear to be of the alignment-inversion type have also been observed for these fields. These results are associated with flow cells that were discussed earlier.     

高温高压与CVD金刚石单晶衬底质量对比研究

本文通过高分辨X射线衍射(HRXRD)、激光拉曼光谱(Raman)、晶格畸变检测等测试分析方法对多组高温高压(HTHP) Ⅰb、HTHP Ⅱa和化学气相沉积(CVD)型(100)面金刚石单晶样品进行对比研究。HRXRD和Raman的检测结果均表明HTHP Ⅱa型金刚石单晶的结晶质量接近天然金刚石,其XRD摇摆曲线半峰全宽和Raman半峰全宽分别为0.015°~0.018° 和1.45~1.85 cm^-1。晶格畸变检测仪的检测结果表明,HTHP Ⅱa型金刚石单晶的应力分布主要有两种:一种几乎无明显应力分布,另一种沿<110>方向呈对称的放射状分布,其他区域无晶格畸变。HTHP Ⅰb和CVD型金刚石单晶应力分布均相对分散,晶格畸变复杂,与其HRXRD和Raman的检测结果相符。进一步利用等离子体刻蚀法对三种类型金刚石单晶(100)面位错缺陷进行对比分析,结果表明,HTHP Ⅱa型金刚石位错密度为三者中最低,仅为1×10³ cm^-2。本研究为制备高质量大尺寸CVD金刚石单晶的衬底选择提供了实验依据。     

The impact of pressure and temperature on growth rate and layer uniformity in the sublimation growth of AlN crystals

To effectively design a large furnace for producing large-size AlN crystals, a fully coupled compressible flow solver was developed to study the sublimation and mass transport processes in AlN crystal growth. Compressible effect, buoyancy effects, flow coupling between aluminum gas and nitrogen gas, and Stefan effect are included. Two sets of experimental data were used to validate the present solver. Simulation results showed that the distributions of Al and N₂ partial pressures are opposite along the axial direction due to constant total pressure and Stefan effect, with the Al and nitrogen partial pressures being highest at the source and seed crystals positions, respectively. The distributions of species inside the growth chamber are obviously two-dimensional, which can curve a flat crystal surface. Simulation results also showed that AlN crystal growth rate can be increased by reducing total pressure or by increasing seed temperature or by increasing source-seed temperature difference. High nitrogen pressure causes decrease in growth rate, but it is beneficial for obtaining uniform growth rate in the radial direction. Results of simulation also showed that there is an optimized temperature difference (40 °C) in the present furnace for obtaining good homogeneity of growth rate.     

Structural and energetic aspects of the differences between real and predicted polymorphs

In crystal structure prediction simulations based on lattice energy minimization, usually hundreds of structures within a reasonable range of lattice energy and density are found, whereas in practice, it is very rare to find more than a few polymorphs of the same compound. In the work presented here, this discrepancy is investigated from a structural and energetic point of view. 56 crystal structures of 26 polymorphic mono‐ and disubstituted aromatic compounds, extracted from the Cambridge Structural Database, have been analysed with respect to inter‐polymorphic structural similarity. For comparison, potential crystal packing arrangements of the substances have been predicted with molecular mechanics simulations using a generic force field. The predicted structures are analysed with respect to structural features and similarity, and with respect to the number of structures and their lattice energy. It is found that the real polymorphs studied in this work tend to be structurally quite dissimilar with regard to hydrogen bonding and spatial packing of structural motifs, while many of the predicted structures of a given compound are very similar to each other. The results suggest that structure and lattice energy alone cannot explain why so few polymorphs are found in practice compared to the very large numbers predicted in simulations. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Competition between photorelaxation and photoexcitation in chalcogenide glasses and the effect of aging

Measurements of transient photodarkening and permanent Bragg reflector formation were performed on fragile As₁₈S₆₅Se₁₇ glass and indicate that photodarkening and photorefraction can only be induced in previously aged glasses while they are prevented in freshly annealed glasses. These phenomena are associated with the photorelaxation process that is revealed to be prominent in fresh glasses and is shown to effectively anneal out the defects produced during photoexcitation thereby resulting in no net permanent changes in the structure. On the other end, aged glasses do not undergo any photorelaxation and exhibit large transient and permanent changes associated with an increase in structural enthalpy as confirmed by Differential Scanning Calorimetry. These results demonstrate that photorelaxation can be a significant problem in fragile glass compositions as it cancels out the effect of photoexcitation and impedes the photodarkening and photorefractive process.     

A comparison of the structures of vapour-deposited and bulk arsenic sulphide glasses

Accurate comparative measurements have been made of the X-ray diffraction pattern of a bulk arsenic sulphide glass prepared from the melt and a 250 μm film prepared by vapour deposition of the evaporated bulk. By using Ag Kα radiation, measurements have been made up to $\text{Q = 21}\text{A}\text{?}{}^{\mathrm{?1}}\text{(Q = 4π}\text{sin}\text{θ/λ)}$ so as to give good spatial resolution. Significant differences are found between bulk and film specimens, and in particular a shoulder is found on the high r side of the first peak in the distribution function for the film. Also, the film is richer in arsenic than the parent bulk. The results are compared in detail with a number of models, all of which involve covalent bonding requirements being fully satisfied, and it is concluded that there are more AsAs bonds in the film than the minimum required by stoichiometry and that a significant fraction ($\text{≈}\text{2}\text{3}$) of these are unusually long ($\text{≈2.6}\text{A}\text{?}$ compared to the more usual 2.5 Å). The most probable explanation is that the vapour contains a significant fraction of As₄S₄ species (in which the AsAs distances are $\text{≈2.6}\text{A}\text{?}$) and that the film contains a mixture of molecular species which have undergone considerable polymerization. The extent to which this results in a sheet-like structure similar to that proposed for the bulk glass remains unclear.     

Solute segregation in a lid driven cavity: Effect of the flow on the boundary layer thickness and solute segregation

Our objective in the present work is to study the effect of convective flows, ranging from laminar to fully turbulent, on solute segregation in directional solidification configurations. To do so, numerical simulations performed in a model 2D lid driven cavity; the problem parameters, apart from the species molecular diffusion coefficient, are the lid and growth velocities. Purely diffusive to fully convective mass transport conditions are modelled in our parametric study. In parallel, a scaling analysis aiming at the determination of the solute boundary layer thickness is proposed. The results show that a single non-dimensional number, based on the interface stress, is able to capture the physics of the solute transport phenomena.     

尿素晶体线性和非线性光学系数的计算

本文使用从头计算平面波赝势法计算了尿素晶体的电子能带结构、线性和非线性光学系数,折射率和倍频系数的计算结果与实验结果基本符合.计算发现尿素晶体中C、N和O元素的2p轨道电子态发生了明显杂化,而能量较高的2p轨道电子态的杂化是其非线性光学效应的主要来源.     

Y2O3掺杂ZrO2配合物的分子结构和光谱特性的理论研究

本文应用密度泛函理论指导人们深入理解氧化锆溶胶中分子的结构和光谱性能,探讨了溶胶中锆配合物的结构参数、光谱特性和原子的Mulliken 电荷布局. 理论结果表明掺杂引起粒径尺寸减少是由于掺杂引起氧桥聚合速度减慢和颗粒间吸引力的减弱.此外,振动光谱分析表明锆配位前驱体的有序化和第二相掺杂剂的引入明显减少了单斜氧化锆的特征光谱.理论分析结果很好地符合了实验结果.