Paramagnetic centres in X-irradiated LiNaSO4 single crystals

The EPR and optical absorption spectra of paramagnetic centers produced by X-irradiation in LiNaSO₄ single crystals were investigated. 7 EPR lines are observed at room temperature and 6 more lines – at LNT. The angular dependences of line positions at LNT are studied and the principal g-factor values are defined. The comparison with published data permitted to assign six the most anisotropic lines to ion-radical SO with different orientations in the lattice; two lines to ion-radical SO; slightly anisotropic doublet line – to O ozonide ion; isotropic line with g = 2. 0045 – to ion radical SO.     

Strained Lattice Organic Thin Film Growth by Molecular Beam Epitaxy

Abstract

Strained lattice organic thin film which possessed distinct lattice structure from the bulk crystal was formed by molecular beam epitaxy (MBE). In situ infrared (IR) spectroscopic characterization was carried out to clarify the as-grown film structure under several growth conditions. The growth condition for parallel orientation of the flat molecules was discussed under the interrelation between the molecule-substrate interactions and the intermolecular interactions.     

Analytical calculation of binding energies of highly symmetrical particles

Analytical formulae of binding energies per atom are derived for icosahedral, cuboctahedral and face-centered cubic particles A_N by means of summation of pair potentials. Asymptotical relations for larger particles E/N = V (A – BN^−1/3) as well as the topological model of clusters are discussed.     

The Effective Temperature of Surfaces of Liquid Metals and Some Non-Metals

An estimation of decrease of atomic bond energy by raising temperature for surfaces of liquid metals and some non-metals had been made. The calculation values of the derivative du_sdu_b where u_s and u_b, are bond energy of atoms in surface and bulk of a liquid, are significantly above 1 and in limits from 2 to 10. It was supposed that this results from anomalously intensive thermal expansion of surface layers of liquids – the effect which is well known for crystal surfaces. This expansion was explained by the effective temperature of these surfaces which, thus, was found to be by several times above those for bulk of liquids.     

Investigation of the accuracy of modeling the orientation distribution function of polycrystalline materials from individual orientations

The accuracy of reconstructing the orientation distribution function of grains in polycrystalline materials by the kernel method from a set of individual orientations has been studied. In particular, the influence of choosing the smoothing kernel in the kernel method and the influence of the dependence on the resulting distribution are taken into account. The rotations obeying the normal distribution on the group SO(3) and obtained by the statistical test method are used as an orientation set. Some aspects of electron backscattering diffraction measurements are considered. This problem is urgent in view of the expanding potential of the experimental study of the texture of polycrystalline materials, especially for electron microscopy, which makes it possible to measure the orientations of individual grains.     

Bulk modulus — volume — ionicity relationship in tetrahedrally bonded semiconductors

It is shown that the bulk modulus of all diamond, zincblende and wurtzite structure compounds can be evaluated by means of the simple relation B = b₀a^−m(1 – g₀f_i) where a is the lattice parameter, f_i the spectroscopically defined bond ionicity and b₀, m and g₀ are constants having the same values for all compounds. This bulk modulus relation is used to estimate the bulk moduli for those zincblende and wurtzite structure compounds for which no experimental measurements have been made until now.     

Quench Hardness Anisotropy of Rhombohedral Crystals: Calcite

Variation of microhardness with orientation of indenter (A) and quenching temperature T_q is studied on freshly cleaved surfaces {100} of natural rhombohedral crystals of calcite by producing Knoop indentations for various applied loads. The relation between hardness H_A and quenching temperatures for the high loads (40–80 g) is graphically established. It also depends on the crystal orientation. The anisotropic indices are also determined.     

Crystal Structure of an Adduct of Sarcosine with Sulfuric Acid (at 140 K)

The crystal structure of an adduct of sarcosine with sulfuric acid, (C₃NO₂H₈)₂SO²⁻₄, has been determined at low temperature (140 K). The crystals are triclinic, space group = P1 with the unit cell dimensions, a= 7.623(1) Å, b = 11.538(2) Å, c = 14.214(2) Å, α = 71.46(2)°, β = 74.36(2)°, γ = 86.46(2)°. Based on 4924 reflections with intensities larger than 3σ(I), the structure was refined to a conventional R factor of 0.038 giving e.s.d.s. in bond lengths and angles of 0.003 Å and 0.2°, respectively. There are two formula units in the asymmetric unit and both sulfate ions exist in two disordered orientations. The sulfate group 1 occupies the two orientations in the proportion 0.82/0.18 and sulfate group 2 in proportion 0.69/0.31. Both Orientations of the sulfate groups satisfy the hydrogen bonding scheme equally well. There is also a pseudo-symmetry element within the asymmetric unit as one half of this unit is transformcd into the other by a translation of approximately one half along the b- and c-axes. Thus, sarcosine molecule 1 is translated to sarcosine IV and sarcosine II to III. In the same manner, the sulfate group I in its main orientation is translated to the sulfate group 2 in its alternative orientation and vice versa.     

Mechanical properties of single crystalline and glassy lithium triborate

Mechanical properties of LiB₃O₅ single crystal plates with different orientation as well as of glass with the same composition have been investigated. The nano‐ (H) and microhardness (H_M), the reduced Young's modulus (E_r) and the crack behaviour of the samples were studied. Both hardness and Young's modulus of glass appeared smaller in comparison to corresponding single crystal data (H ∼ 7 – 8 GPa, H_M ∼ 6 GPa, E_r ∼ 70 – 80 GPa for glass and H ∼ 10 – 15 GPa, H_M ∼ 6 –11 GPa, E_r ∼ 93 – 155 GPa for single crystal). H, E_r, and the plane of crack propagation proved orientation‐dependent. Cracks in the glass sample were not observed up to 0.49 N microindentation load, whereas for the single crystal the cracks appeared already at 0.098N. In single crystals the observed cleavage planes {211} and/or {412} are oriented nearly parallel to planes of B‐O rings. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of a Banded Texture in Solutions of Liquid Crystalline Polymers: 1. Hydroxypropylcellulose in H2O

Abstract

The formation of a banded texture was investigated for sheared anisotropic solutions of hydroxypropylcellulose (HPC) in H₂O. A cone and plate rheometer made of highly polished glass allowed the exploration of the shear rate (γ) range between 0.1 and 150 sec^?1 and the study of the effect of shear time t_s. Texture, monitored with a video-camera, appeared after a time t_b following cessation of shear. t_b is usually a decreasing function of γ and t_s. In most cases a precursor banded texture precedes the banded texture, and t_b attains zero at high enough γ or t_s. However, when a particularly good orientation of the flowing solution is achieved, the precursor texture is not observed, t_b is finite, and the bands are particularly well formed. We speculate that bands are the normal relaxation mode for well oriented solution. For less oriented solutions flow instability may lead to bands even during flow.     

Raman spectroscopy study of the structure of gallium nitride epitaxial layers of different orientations

The composition nonstoichiometry and structural quality of undoped gallium nitride layers grown by the hydride vapor phase epitaxy on sapphire substrates of different orientations have been estimated using Raman spectroscopy. It is found for the first time that the peak position of the phonon mode E₂(high) in the Raman spectra of gallium nitride films at a wave vector of 572 cm^–1 depends on the initial orientation of sapphire substrate and is low-frequency shifted when passing from Ga-polar to partially N-polar orientation. Additional modes are found in the spectra of GaN layers grown on substrates with m and r orientations. It is shown that a decrease in the composition deviation from stoichiometry, caused by reducing the HCl flow through the gallium source during the growth of GaN layers, leads to an increase in the phonon-mode intensity in the Raman spectrum.     

Application of Modified Kick's Law to Rhombohedral Crystals: Calcite

Optical study of mechanical response indicated by Knoop indentation of cleavage faces of natural rhombohedral clacite crystals based on MEYER'S law/KICK'S law (P adⁿ) and also on modified KICK'S law (P–W) = bd² is made. The response of cleavage faces for low load region (LLR) and high load region (HLR) of applied loads P to produce indentation mark of longer diagonal length is different. In LLR the Newtonian resistance pressure W is negative and varies with orientation and quenching temperature whereas in HLR it is positive and almost constant at room temperature and quenching temperatures. In LLR, W indicates crystal anisotropy. The values of resistance pressure W₂ and standard hardness b₂ in HLR are independent on thermal treatment and are constant within experimental limits. Since they are independent on indenter orientations, they are not associated with crystal anisotropy.     

3D simulation of the effects of growth parameters on the growth of sapphire crystals using heat exchanger method

3D simulations using the commercial CFDRC and FIDAP code, which are based on finite element techniques, were performed to investigate the effects of anisotropic conductivity on the convexity of the melt–crystal interface and the hot spots of sapphire crystal in a heat‐exchanger‐method crystal growth system. The convection boundary conditions of both the energy input to the crucible by the radiation as well as convection inside the furnace and the energy output through the heat exchanger are modeled. The cross‐sectional flow pattern and the shape of the melt–crystal interface are confirmed by comparing the 3‐D modeling results with previous 2D simulation results. In the 3D model, the “hot spots” in the corners of the crucible are donut shaped, and the shape changes with the value of the conductivity of anisotropic crystal. The outline of the crystal becomes more convex as the conductivity in the z direction (k_sz) increases. The outline of melt–crystal interface is elliptical when the anisotropic conductivity is moving in the radial direction (k_sx and k_sy). The portion at the outline touching the bottom of the crucible is smaller than the maximum outline of the crystal, meaning that the shape at the “hot spot”, changes with the value of the conductivities of anisotropic crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory of Systems of RodIike Particles: II. Thermotropic systems with orientation-dependent interactions

Abstract

A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos² Ψ_ij, where Ψ_ij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T? measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T? and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of foremost importance. The reduced temperature [Ttilde]_ni = T_ni/xT? at which the nematic-isotropic transition takes place in the neat liquid decreases with decrease in x below its athermal limit x_erit = 6.417 for [Ttilde]^–1 _ni = 0. Both the entropy difference between isotropic and nematic phases and the orientational heat capacity C^p, are monotonic through the transition; C_p, diverges at a temperature appreciably above T_nl, where the metastable anisotropic state becomes unstable. Comparison of theory with experiments on those nematogens whose molecules can be approximated by rigid rods lends encouragement to the prospect of relating characteristics of the nematic-isotropic transition to molecular structure.     

Simulating vapour growth morphology of crystalline urea using modified attachment energy model

We report a computational model to simulate vapour growth morphology of urea crystal by considering molecular anisotropy and surface relaxation of different crystal faces. It has been argued that the faces' growth occurs through the adsorption of molecular layers rather than a slice of thickness d_hkl. The molecular layer is a 2-D periodic arrangement of molecules in which each molecule has same the orientation. The molecular orientations in a slice of thickness d_hkl may differ from each other and depend on crystallographic orientation of the slice. The discussed approach has been employed to simulate vapour growth shape of crystalline urea by calculating attachment energy of molecular layers using Hartee–Fock and density functional theories. The calculated growth morphology is in good agreement with the vapour grown shape of urea crystal. The role of thermal and growth kinetics affecting the vapour growth morphology has been discussed. The observed polar growth morphology of urea crystal has also been discussed particularly in the context of different atomic environments of (111) and (?1?1?1) faces.     

Bulk non-radiative recombination reduction in nGaAs due to an annealing under Ga-Al melt

It is shown by microcathodoluminescence and electron beam induced current investigations that efficiency of radiative recombination and the values of hole diffusion lengths in nGaAs substrate increase (up to L_p ∽ 5 μm) due to an annealing under Ga Al melt. The values of the annealing critical temperature exceeding of which leeds to bulk non-radiative recombination drop as well as optimal values of annealing temperature and duration are determined. In pAlGaAs–(p-n)GaAs solar cells based on annealed substrates the values of open circuit voltage U_oc = 1.16 – 1.18 V under 500-fold concentrated solar radiation as well as values of photocurrent comparable with the best results of solar cells with epitaxial nGaAs are achieved.     

On existence of one-partial bulk waves in semi-infinite piezoelectric media

The conditions for the existence of one-partial bulk waves satisfying the boundary conditions on a mechanically free surface of a semi-infinite piezoelectric medium have been analyzed. In purely elastic media, similar wave solutions are known to exist along the propagation directions m which form lines on the sphere m ? m = 1, passing through all the available degeneracy points (acoustic axes). It is shown that in a triclinic piezoelectric half-space with a metallized surface, one-partial bulk waves may exist solely along isolated propagation orientations, whereas for a nonmetallized surface, such waves can exist only if an additional condition for the material constants of the medium is fulfilled. It is also shown that the one-partial bulk solution may not exist along an arbitrary acoustic axis in a piezoelectric. These conclusions are valid in the general case of unrestricted anisotropy, i.e., they do not take into account the material symmetry. In addition, the orientations providing the propagation of one-partial bulk waves because of the existing symmetry are specified for piezoelectric media of various symmetry classes.     

Germanium Incorporation in Ga1–xInxAs LPE Layers and GaAs Bulk Crystals

Atomic absorption spectroscopy (AAS) is used to determine the Ge concentration in Ga_1–xIn_xAs and GaAs. The orientation dependence of Ge incorporation in (111)A-and (111)B oriented samples has been studied. The distribution coefficients for both the orientations were determined to be k_Ge^(111)A = 2.6 · 10⁻² and k_Ge^(111B = 1.4 · 10⁻² for Ga_1–xIn_xAs with 0 ≦ x ≦ 0.13. The differences between the two orientations are explained with the aid of the band bending model. Doping gradients in thick epitaxial layers and along crystal length of polycristalline TGS-grown GaAs ingots have been investigated too. In Ga_1–xIn_xAs layers any Ge concentration gradient couldn't be observed, but in TGS compact crystals Ge concentration increases with crystal length because the melt composition changes significantly during solidification. The results are compared with those of electrical measurements.     

Texturuntersuchungen von Metallen mittels Neutronenbeugung

The most important advantages of the application of the neutronographic diffraction in texture investigations as compared to the X-ray interference method are described. Comments on the experimental technology used with Rossendorf's reactor and the applied methodology of investigation for the fully automatical measurement of complete pole figures follow. Some of the recent results of texture research at fcc and bcc metals and alloys are discussed on the base of the three-dimensional orientation distribution functions. – Investigations into the texture development in cold-rolled copper reveal that the preferred orientations are represented by a characteristic tube that is shortened at the transition to α-brass with concentration of zink increasing and the rolling degree raising respectively. – As the measurements at iron demonstrate the ideal orientations used for the specification of the rolling-texture development in bcc metals correspond to those of the texture of copper if the indexes of rolling and normal direction are interchanged. – According to the Taylor theory, textures appear at the surfaces of aluminium wires that are similar to a rolling texture whereas in the wire interior ordinary fibre textures are present.     

Wetting of InP by indium- and indium-tin melts

Orientation dependent wetting of InP substrates by In- and In_xSn_1−x melts saturated with InP was studied using the sessile drop method. The contact angle was found to depend on the substrate orientation as follows: It is shown that a tin admixture to the indium melt improves the wetting behaviour. Additional to the dependence on substrate orientation it was found an anisotropic wetting on all investigated faces.