Refinement of the atomic structure of specimens cut out from different growth pyramids of a K(H0.052D0.948)2 PO4 single crystal

The structures of two crystalline specimens cut out from the pyramidal and prismatic growth sectors of a K(H_0.052D_0.948)₂PO₄ single crystal have been studied by diffuse neutron scattering and precision diffuse X-ray scattering. Diffuse scattering is concentrated in the vicinity of the Bragg reflections and is practically the same in specimens cut out from different growth sectors of a single crystal. X-ray diffraction analysis using the extinction parameters provided the establishment of a higher perfection of the specimen cut out from the prismatic growth sector. The precision X-ray studies revealed different configurations of hydrogen bonds in the specimens.     

Thickness dependence of the crystallization process of wedge-shaped Zr70Cu27.5Rh2.5 metallic glass prepared by ion milling

Crystallization happens when metallic glass is annealed at elevated temperature. It has been believed that the crystallization process is primarily determined by the metallic glass composition with no report and expectation that it may depend on the physical dimension. In the present work, in situ annealing experiment was performed on a Zr₇₀Cu_27.5Rh_2.5 metallic glass under transmission electron microscopy observation. The specimen is prepared by ion milling and is wedge-shaped with its edge thin enough to be transparent for 200 keV electrons. At annealing condition well beyond that required for full crystallization for the regular ribbons, crystallization was not observed in areas with thickness of approximately 250 nm while crystalline grains in the thin and thick neighboring areas were observed.     

Determination of Molar Ratio in AB1−x Rocksalt-type Compounds by X-ray Powder Diffraction Method. Application to TiC1−x

The B/A molar ratio in AB_1−x rocksalt-type compounds is determined by X-ray powder diffraction method by measuring of integrated intensities I_hkl. The strategy of measurement is presented to avoid all effects influencing the integrated intensities of X-ray powder diffraction as primary extinction, preferred orientation and surface roughness effect. This paper deals with application of this method to the determination of the deviation from stoichiometry of TiC_1−x compounds.     

Further Experimental Evidence for the Wall Structure of the Flexoelectric Domains in Symmetrically Weakly-Anchored MBBA Layers

An attempt for measuring of the penetration non-critical length of MBBA flexoelectric surface-induced domains is made. The disappearance of the domains into subcritical regions is related to the strong nonhomogeneity of the electric field around the gap which is able to inhibit the flexoelectric domain formation. The disappearance of the electrohydrodynamic domains generated from the flexoelectric ones, on the other hand, shows their secondary character as well. The existence of the penetration non-critical length of the flexoelectric domains is further confirmed with the observation of the corresponding supercritical influence length of side walls or air bubbles.

The behavior of the flexoelectric domains in an additionally applied magnetic field reveals their wall structure and permits the determination of the value and sign of the two MBBA flexoelectric coefficients of bend e _3x and splay e _1z, when the value and sign of the total flexoelectric coefficient ( e _z, + e _3x) are known.     

Consolidation and mechanical properties of Cu46Zr42Al7Y5 metallic glass by spark plasma sintering

Cu₄₆Zr₄₂Al₇Y₅ metallic glass with nearly 100% relative density was obtained by spark plasma sintering (SPS) with a diameter of 15 mm, which was larger than the largest size of 10 mm for the as-cast specimen. The fracture strength of the sintered specimen reached 2044 MPa, which was 15% higher than that of the as-cast Cu₄₆Zr₄₂Al₇Y₅ glassy specimen. The densification and compressive properties of the sintered specimens were related to sintering temperature. Structural changes of the specimens sintered at various sintering temperatures resulted in the difference of macro-mechanical properties.     

Orientational Distribution Function and Order Parameters of Two 4′-Alkoxy-4-Cyanobiphenyls in Mesomorphic Phase

X-ray diffraction studies on samples of 4′-Pentyloxy-4-Cyanobiphenyl and 4′-Octyloxy-4-Cyanobiphenyl over the entire mesomorphic range are reported in this paper. A magnetic field was used to align samples. The orientational distribution functions f(β) have been calculated from the angular distribution of X-ray intensities. Orientational order parameters ( P ₂) and ( P ₄) have been calculated and compared with Maier-Saupe theoretical values. Expressions for the angular part of the pseudopotentials have been obtained from the temperature variation of f(β). The intermolecular distances D have been calculated from the experimental data and it is found that D is significantly smaller for well aligned samples. The apparent molecular length in the nematic phase and the layer thickness in the smectic phase have also been determined and are found to be about 1.4 times larger than the true molecular length in the fully extended form. This is in accordance with the findings from other cyano compounds.     

Problems and results of diffusion length investigations by energy-dependent EBIC measurements

The fundamentals of the method are described. From applications of this measuring method the following results were obtained: The determination of the diffusion length on glass-plate bevels with evaporated Schottky contacts is frequently quite difficult and sometimes even impossible. Besides a reduction of the diffusion length with increasing microdefect density surprisingly also the opposite behaviour can be found. Comparative PEM and EBIC investigations yield differences between both diffusion lengths (L_EBIC < < L_PEM) depending on the sample pre-history.     

Effect of ion beam irradiation on the corrosion behavior of the melt-spun ribbon Ti60Ni40

Potentiodynamic polarization studies were carried out on unirradiated and irradiated Ti₆₀Ni₄₀ specimens in 1 M HNO₃ aqueous medium at room temperature. The irradiation was carried out using N⁺ 150 keV ions at a fluence of 1 × 10¹⁶ ions/cm². Polarization results revealed that the irradiated specimen exhibits less corrosion current density as compared to the unirradiated Ti₆₀Ni₄₀ specimen. X-ray photoelectron spectroscopy (XPS) studies were also carried out on the irradiated and unirradiated specimens after corrosion test. It was observed that the absence of Ti³⁺ species in the oxide film of the irradiated specimen as compared to the unirradiated specimen results in the improvement of the corrosion resistance. Polarization results are also corroborated by weight loss data.     

Experimental Determination of the Curvature-Induced Reduction in the Smectic A-Nematic Transition Poin

On the basis of an analogy between smectic A-nematic and superconductor-normal metal transitions, de Gennes has predicted that a twist or bend distortion should reduce T _AN with respect to that of a curvature-free sample. We report measurements of the reduction of T _AN on samples prepared in two different geometries. In the first, a magnetically induced twist distortion has been used to obtain preliminary data on 8 OCB and CBOOA samples. In the second, a wedge-shaped twisted nematic cell has been used to get quantitative data on the reduction of T _AN of CBOOA as a function of twist distortion.     

Amorphous to crystalline transition of selenium thin films of different thicknesses

Thin films of amorphous selenium obtained by vacuum evaporation display an increase of “optical gap” E_g^opt with an increase of thickness of the film. From the observed dependence of E_g^opt on the thickness of the film, the influence of the thickness on the gap states is interpreted in terms of the density of states model proposed by Mott and Davis. The amorphous to crystalline transition obtained by heat treatment of the specimen is also investigated. The minimum temperature for an appreciable change in crystallisation determined by the transmission of light through selenium films is also a function of the thickness and binding energy of the films. The crystalline structures resulting from heat treatment at different temperatures have been identified by scanning electron microscopy. The generation of different crystalline structures is reported in terms of the thickness and preparation conditions of the amorphous films.     

Electron beam induced surface modification of amorphous Sb2Se3 thin film

The change in the surface morphology of amorphous Sb₂Se₃ thin films during the electron beam irradiation has been studied mainly by atomic force microscopy (AFM). Electron beam at accelerating voltages 30 kV is focused onto the surface of the specimens of 100-μm thickness, and then the surface morphology of each specimen has been observed by AFM in air. The modification of the film surface includes lateral and vertical resizing which is typically in the micrometer and sub-micrometer range. Protrusions above the surface as high as 90 nm are observed at 180 pA electron beam current, whilst trenches as deep as 97 nm are observed at 800 pA electron beam current (total thickness of thin film is 100 nm). The dependence of patterns characteristics on irradiation parameters such as exposure time and beam current has also been studied. Physical mechanisms for trench and mound formation are proposed.     

X-Ray-Interferometric Determination of Å-scale Lattice Shifts at the Surface of Silicon Crystals — the Analogue to Light-Optical Interference Microscopy

The paper presents a new imaging method based on a refined and better stabilised X-ray Michelson interferometer [Appel , Bonse ]. Interference fringes are detected which mark local Å-scale shifts of the lattice at the surface of the interferometer's phase-shifter crystal. Conventional double-crystal topography (DCT), lacking phase information of the beam reflected by the specimen crystal, does not detect mere lattice shifts. The new method appears to be also more sensitive to any combined lattice strain δ ≡ Δd/d + cot θ_B Δθ (θ_B: Bragg angle, d: lattice parameter) present in the crystal layer contributing to the surface Bragg reflection. δ is known to be the dominant part of the DCT image [Bonse , 1958; Bonse , Hartmann ]. The technique presented here is the X-ray analogue to the well known light-optical interference microscope. A possible application is the testing of silicon crystals for minute lattice imperfections causing lattice-incoherent positions of atoms in a surface layer of typical 1 to 10 μm thickness. For instance striations, which under certain conditions arise during crystal growth, cause such incoherence.     

On the characterization of electrically active inhomogeneities in semiconductor silicon by charge collection at schottky barriers using the SEM-EBIC (II). Contrast due to defects

The influence of the control variables (E₀, I_b, U_r) as well as the semiconductor parameters (ϱ, L) on the defect contrast of direct irradiated Schottky barriers is investigated. The resolution of the subsurface regions is enhanced by increasing the primary beam energy E₀. The sharply imaged region can be extended into the depth of the crystal by applying a reverse bias U_r. Contrast width broadenings are observed in high-ohmic material, if increasing the beam current I_b, which may be annuled by switching on U_r. Against the generally held view the diffusion-induced contrast width broadening of defects (or parts of them) located outside the depletion layer was found as an extremely small fraction of the diffusion length L. Thus in rough approximation the contrasts can be interpreted as a superposition of the in general club-shaped generation volume and the defect shape.     

Characteristics of GaAs?AlGaAs (SCH) lasers grown by low-temperature LPE technique

In this paper we report the performance and study of GaAs AlGaAs (SCH) laser. The structures were grown by a new variant of the LPE-technique with temperatures regimes lower than 600 °C. The Low Temperature LPE-technique, recently propoused by ANDREEV and coworkers at the IOFFE-Physico-Technical Institute exhibit growth rates of the growing layers as low as that corresponding to MBE or MOCVD techniques. In this way, it is possible to obtain and control layer thicknesses in the structure of the order of tens to hundreds of Ångströms. Laser heterostructures with active layer thickness of 30 nm, were performed and the lowest threshold current density of 437 A/cm² for a cavity length of 1.33 mm were obtained. A typical growth process is described and a SEM structure microphotograph is shown. The dependences of the threshold current density J_th and the differential quantum efficiency η_d with the cavity length L are reported. From these data, other laser parameters are evaluated such as internal optical losses α₁, internal quantum efficiency η_i and the dependence of the maximum of the gain factor against the nominal current density J_nom at threshold. This dependence was compared with the theoretical predictions and a good agreement was found.     

On the morphology of <100>-textured Germanium Crystallites in Zn—Ge Eutectic Alloy

Different geometry parameters for the cross-like germanium crystallites (eutectic dendrites) of the Zn-Ge eutectic alloys unidirectionally solidified have been related to the growth rate R and liquid temperature gradient G using linear regression analysis. It has been found that for eutectic dendrite spacing λ, balk length d and balk thickness b follow the relationship of the type w = C_wR^−rwG^−gw; C_w is a constant. The spacing relationship of eutectic dendrites (r_λ = 0.21; g_λ = 0.48) shows a similarity with published experimental and theoretical results on primary dendrite arm spacing (r_λ = 0.25; g_λ = 0.5). It has been shown in terms of a profile ration λ/d that for geometrical similar morphologies the relationship λ²R = const. is valid.     

Neutron Scattering by Copolymers

Abstract

Neutron scattering is a very efficient tool for studying polymers in bulk as well as in solution. This is mainly due to the difference of scattering length between hydrogen and deuterium which allows the possibility of changing the contrast without too serious alteration in the conformation and thermodynamics of the systems.

In order to take full advantage of this possibility, one has to remember the general rules which allow the evaluation of the total scattering function S(q) as a function of the partial scattering functions S _pq (q) which describes the contributions of the interferences between scattering center of species p and q.^1.2 In a first part we shall briefly establish these rules for incompressible systems introducing what has been called by J. Koberstein³ the molecular contrast and the phase contrast.

In the second part of this talk we shall discuss the problem of copolymers. S(o) the intensity scattered at zero angle depends on the heterogeneity in composition of the sample,⁴ for homogeneous composition S(o) = o. This gives the possibility of studying trans-esterification in polyester, for example.

At finite q for a monodisperse system, S(q) is not zero and it will be shown how it depends on the length of the sequences and the architecture of the copolymers. If the number of blocks is large S _pq (q) is practically not affected by the total length of the polymer.⁵

The condition I ^?1(q) = o allows one to write the equation of a q dependent generalized spinodal and therefore to determine the parameters which govern the stability of a multiphase system.

In the last part we shall discuss the problem of the application of this formalism to networks and gels made of different types of units. It will be shown that the scattering is mainly sensitive to the local conformation and that the introduction of translational order between the units does not affect significantly the scattering curves but can explain the occurrence of secondary maxima which have been observed in polyurethane and ionomers.⁶     

Refinement of the structure of cyclohexylammonium chloride (cyclohexylamine·HCl)

The crystal structure of cyclohexylammonium chloride has been refined to a conventionalR value of 0.029 using X-ray data obtained with molybdenum radiation. The title compound crystallizes in the orthorhombic space group Pca2₁ witha=9.339(2),b=11.449(2),c=7.546(1) Å, andD _c=1.117 g cm^–3 forZ=4. An earlier structure determination has been essentially verified with the exception of some important features: a significantly longer and more reasonable C(1)–N bond distance, increased values for all the endocyclic torsion angles and an increase in the length of thea axis.     

Photoconductivity of rare-earth-doped (CdSe,ZnS)

Photoconductivity rise and decay curves of (CdSe, ZnS) doped with La and Dy separately are investigated as a function of ambient temperature and pressure, electrode material and cell thickness. Using a number of metal electrodes Fe is found to give maximum I_PC/I_DC ratio for (CdSe, ZnS): La and Cu for (CdSe, ZnS): Dy. Due to change in temperature, maximum I_DC and I_PC are found to 70 °C. A possible mechanism is proposed for thermal and photo-excitations in terms of these results. At lower ambient pressure the photocurrent is found to be low. Cell thickness effect also shows changes in I_PC.     

Growth of Ge1—xSix (x= 0 to 0.5) Single Crystals by a Zone Melting Method

Ge_1—xSi_x graded crystals with 0 < x < 0.66 were grown by a zone melting method in a double ellipsoid mirror furnace. They have a diameter of 9 mm and grown lengths between 27 and 80 mm. Single crystalline regions reached a maximum Si concentration of 50 at%, determined by EDX and ECP measurements. Starting with a Ge seed, a defined concentration profile was obtained by a wedge-shaped arrangement of the feed material. The concentration profiles could be predicted by a simulation program.