Determination of the Rotational Viscosity and of the Elastic Constant from Measurements of Switching Times and Threshold Voltage on Planar Oriented Sc Phases

The Freedericksz transition of planar oriented S_c phases was studied experimentally with an electrooptical method and the theoretically predicted reorientation along the S_c tilt cone confirmed. Measurements of the threshold voltage and the switching times lead to the determination of the rotational viscosity and the elastic constant for five selected compounds with positive dielectric anisotropy.     

Texture growth and screw dislocations in smectic A phases

The formation of the textures of smectic A phases was investigated. In planar oriented samples the A phases grow in the shape of rods without defects. The velocity of growth is strongly anisotropic with its maximum normal to the director orientation. In non oriented samples the smectic A texture is growing in batonnets with defect structure to give focal conic textures. The maximum growth velocity of the batonnets is parallel to the director orientation. Since this would contradict to the Wulff theorem we assume a growth mechanism with a screw dislocation.     

First Truly Ferroelectric Liquid Crystal

The first faroelectric liquid crystalline phase (in 8SI* (ΔS-(+)-(4-(2′methylbuty1) phenyl-4′-n-octylbiphenyl-4 carboxylate) is reported and its physical properties are discussed. The close connection between the phase sequences of 8SI* and its racemate are established. Both compounds show a number of smectic phases above the newly discovered ferroelectric phase and its analogue in the racemate. These phases (CI and G') are tilted in the racemate 8SI and tilted with a helicoidal structure in 8SI*. In the truly ferroelectric phase, X, the director of the molecules can be oriented in a bistable way in an electric field Furthermore when the field is turned off the director orientation remains unchanged. This behaviar is fundamentally different from that in bulk smectic phases which support the helix structure in the absence of external forces. A model far ‘racemates’ in higher smectic phases like G, G', H’ etc as well as a the newly discovered ferroelectric phase is presented. The response of cholesteric blue phases of 8SI* to an external field is discussed in an appendix.     

The Dynamics of Chiral 5*CB Molecules Confined in a Porous Matrix

The dielectric permittivity of chiral isopentyl-cyanobiphenyl 5*CB confined to an Anopore membrane with a diameter of 100 nm for the two orientations of molecules, radial, and axial with respect to the pore axis, was studied. We report the different polymorphism of the 5*CB from that of the bulk material. The phases sequence depends on the forced orientation of molecules. The cholesteric phase does not appear during cooling of the axially oriented sample. Moreover, we observe tumbling of the molecules in metastable solid phase.     

Parabolic Approximation to the Shape of Oriented Eutectic Interface

The calculation of interface shape for oriented eutectic growth has been presented for regular structure formation. The parblic approximation to the interface shape has been assume in the proposed model. The differences between the modes of both phases growth have been distinguished. The proposed calculation of the interface shape provides a justification for the existence of lead distance. The obtained equations contain the physical factors which affect the formation of concave-convex interface. The periodic character of interface shape shape has been taken into account. The proposed model has been related to the theory of completely coupled growth.     

Structure of Polycrystalline Cu Films as Solar Selective Coatings

Copper thin films (5–150 nm) were prepared by vacuum deposition with different rates (0.7, 1.5 and 3 nm/s). The position, intensity and profile of X-ray diffraction lines were analysed to study the phases, the crystallographic preferred orientation as well as the residual strain and crystallite size. The fcc polycrystalline Cu phase was revealed and no oxide phases were identified. The films were highly oriented with 〈111〉 fiber texture. The ratio of P₁₁₁/P₂₀₀ increased with the film thickness. Thus, in case of amorphous substrate, the type of the crystallographic texture of a film depends mainly on the structure of the deposited material. The crystallite size increases while the residual strain decreases, as the film thickness or the deposition rate is increased. The crystallite size was very small compared with the film thickness. The effect of deposition rate was pronounced specially from 0.7 to 1.5 nm/s.     

Modern preparation methods of oriented thick films of superconducting cuprates

Modern trends in the development of methods of manufacturing biaxially textured large-grained coatings ～ 100 μm thick based on superconducting rare-earth-barium cuprate elements, as well as bismuth-containing high-temperature superconductors, are considered. It is shown that the formation of a symmetric artificial surface relief makes it possible to control the mutual orientation of crystallites on polycrystalline metal substrates with high efficiency. In this case, the fraction of superconductor crystallites with an angle of mutual misorientation <5% may be as high as 75–90%. The high efficiency of this method of forming oriented films of multicomponent oxide phases indicates that it can be used to fabricate new-generation superconducting composite materials.     

Tilt Angle Determination of a Smectic C Phase by Field-Induced Freedericksz Transition and x ray Investigations

A smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates. When an electric field was applied normal to the director direction a dielectric reorientation (Freedericksz transition) occurred. Dissolving a dichroic dye in the smectic C phase and applying an electric field, a guest host effect similar to nematic phases was observed. From the dielectric reorientation the tilt angle of the smectic C phase was determined at different temperatures. The tilt angles obtained by the electrooptical measurements agree sufficiently with the values measured by x ray investigations.     

Optical Rotatory Dispersion Measurements of the Liquid Crystalline Blue Phases in Thick Layers

The results of optical rotatory dispersion (ORD) measurements in liquid crystalline blue phases (BP) are reported for pure compounds and mixtures. They have been performed on polycrystalline samples (thick layers). Two features distinguish their spectra from those obtained from liquid single crystals:

(1) The magnitude of the rotation is much smaller, and

(2) no sharp anomalous ORD region is present.

The spectra possess a weak, broad anomalous region. Their sign and wavelength dependence follows the sense and pitch of the cholesteric helix. ORD experiments with mixtures of the same cholesteric esters with a nematic substance yield results typical of well oriented samples. The spectra exhibit larger rotation angles and a significantly smaller anomalous region.     

Struktur von AIIBVI-Epitaxieschichten in Abhängigkeit von den Aufdampfbedingungen

The structure of epitaxial films of A^IIB^VI-compounds is discussed. Especially the results of investigations concerning the epitaxy of CdSe on cleavage planes of NaCl and CaF₂ using high-vacuum evaporation are treated. Epitaxial films of A^IIB^VI-compounds usually show defect structures due to the co-existence of the zincblende and wurtzite structures, oriented overgrowth of these two phases, twinning and stacking faults. The resulting complex diffraction patterns (for electron diffraction in transmission) are discussed. The nature of the substrate, substrate temperature, and deposition rate besides experimental factors decisively influence the orientation, structure, and phase composition of the films. An influence of deviations from the stoichiometric composition has to be taken into account.     

Erratum

The meaning of the rotational friction and diffusion tensor in Liquid crystalline phases characterized by anisotropic viscosities is discussed, by generalizing the hydrodynamic Stokes-Einstein relationships. Corrections due to the viscosity anisotropy effects to the rotational correlation times, relevant for the interpretation of magnetic and dielectric relaxation experiments, have been computed by solving the appropriate Fokker-Planck equation.     

Effect of structural transformations on the texture of Ni?Fe base alloys

1. Processes of texture formation were studied in three Ni Fe system alloys containing Mo and Ti (alloy No. 1), Cr, Ti and Al (alloy No. 2) and Co, Ti and Al (alloy No. 3). The deformation texture is identical in all alloys which agrees with the proximity of stacking fault energies as estimated from X-ray data. 2. Annealing leads to a pronounced diffusion and alteration of the texture in alloy No. 1, but does not alter the texture type and intensity in alloy No. 2, whereas in alloy No. 3 the typical strain texture components are intensified. 3. Structure processes governing texture formation were identified as recrystallization in alloy No. 1, and oriented γ–γ′ transformation in alloys No. 2 and 3 where the intergranular boundaries were blocked by heterogenous precipitation of phases not possessing fcc structure.     

Melt nonstoichiometry and defect structure of ZnGeP<Subscript>2</Subscript> crystals

The defect structure of ZnGeP₂ crystals grown from a melt by the vertical Bridgman method has been investigated. A deviation of the melt composition from stoichiometric leads to the formation of striations and the inclusions of other phases which are observed as structures (chains) oriented parallel to the growth axis. According to the microanalysis data, the inclusion composition corresponds to a mixture of ZnGeP₂, Zn₃P₂, and Ge. Nanoinclusions of germanium phosphide are detected by transmission electron microscopy. X-ray topography reveals defects of four types. The main defects in the central part of an ingot are related to the composition fluctuations, and the newly formed dislocations are basically single ones. Most dislocations are formed at the crystal periphery.     

TSD of Liquid Crystals in the Supercooled Nematic Phase

MBBA and EBBA are used to apply the techniques of thermally stimulated depolarization (TSD) to liquid crystals in their glassy state. The orientational mobilities of the butyl and methoxy resp, ethoxy group show significant differences compared to the crystalline phases. A small TSD band in MBBA at 69.5 K and in EBBA at 64.5 K probably comes from relaxations of the butyl group. A ten times stronger TSD band at 117.5 K resp. at 124 K is due to relaxations of the methoxy respect to the ethoxy group. Using the partial heating technique and the thermal sampling technique a detailed analysis of this band allows more insight into the distribution of reorientation enthalpies and relaxation times.     

Discontinuous Dielectric Reorientation in the Smectic C Phase of 3-n-Heptyl-6-[4-n-hexyloxyphenyl]-1,2,4,5-tetrazine

In the nematic phase and smectic C phase of 3-n-Heptyl-6-[4-n-hexyloxyphenyl]- 1,2,4,5-tetrazine the Freedericksz transition was studied. In both phases the threshold voltage and the switching times were measured. In the smectic C phase two thresholds were observed which can be explained by an asymmetric chevron structure.     

Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was normal to the AC magnetic field. In both cases, the ribbon’s axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.     

GaN grown on (1 1 1) single crystal diamond substrate by molecular beam epitaxy

GaN epilayers are grown on (1 1 1) oriented single crystal diamond substrate by ammonia-source molecular beam epitaxy. Each step of the growth is monitored in situ by reflection high energy electron diffraction. It is found that a two-dimensional epitaxial wurtzite GaN film is obtained. The surface morphology is smooth: the rms roughness is as low as 1.3 nm for 2×2 μm² scan. Photoluminescence measurements reveal pretty good optical properties. The GaN band edge is centred at 3.469 eV with a linewidth of 5 meV. These results demonstrate that GaN heteroepitaxially grown on diamond opens new rooms for high power electronic applications.     

On Packing in Smectics: X-Ray Diffraction Study on the Homologous Series 4, 4′-di-n-alkyl-azoxybenzenes

The smectic layer spacings of the homologous series 4,4′-di-n-aLkyl-azoxybenzenes with alkyl chain length from m=6 to m=10 were determined and a mean layer spacing increment of 0.109 nm per methylen group was found.

The results are discussed in relation to two different models:

(I) The diffuse cone model of A. de Vries. Differences between layer spacing and molecular length are attributed to orientational disorder leading to mean tilt angles up to 18° of the molecules.

(II) The second model differentiates between the directions of the rigid aromatic core and that of the alkyl chains. Orthogonality of the rigid core long axis to the smectic layer planes and therefore tilting of the alkyl chains results in coincidence between calculated layer thickness and measured smectic layer spacings.

X-ray film erposures of oriented samples clearly demonstrate the S_A -type of the phases and confirm a slight odd-even effect of layer spacing increments.     

Thermal behavior of biogenic apatite crystals in bone: An X‐ray diffraction study

The thermal behavior of the bovine bone mineral and synthetic stoichiometric hydroxyapatite was investigated by X‐ray diffraction. The bone samples in solid (planar oriented pieces) and in powder form were examined to elucidate how the microstructural and textural properties of bone mineral are modified under heating. As could be expected, the thermal behavior of the bone mineral depends not only on the structural distortions, but also on the crystal habit, texture and ordering of biocrystals in tissue. The temperature growth of biogenic apatite crystals, unlike synthetic hydroxyapatite, is seen to be nonmonotonic and multi‐staged. At 600 to 700°C the biomineral crystallites grow rapidly due to disappearance of the mosaic structure as the lattice imperfections are annealed. After heating between 700°C and 900°C the bone mineral appears to be composed of roughly equidimensional ≥200 nm crystals. The further growth of the crystals in the range from 900 to 1300°C occurs by the mass transport mechanism, supporting the idea that the bone mineral is not a discrete aggregation of crystals, but rather a continuous mineral phase with direct crystal‐crystal bonding. Estimates are presented to show the important role of the surface mass transport mechanism in the growth of apatite crystals. The material obtained by heating a cortical bone fragment between 900°C and 1300°C turns out to be composed of two crystal types: crystals oriented along the bone axis (major morphology) and those of differing shape and orientation (minor morphology). The heating‐induced variations in the longitudinal and transverse dimensions of differing‐morphology crystals are found to be coherent. Small amounts of CaO, MgO and other crystalline phases are seen to be formed in the bone mineral under heating. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Orientation of Rod-Shape Molecule, 2,2′-Bis[4-(Trifluoromethyl)Phenyl]-5,5′-Bithiazole in Films Deposited in a Vacuum on Oriented α-Sexithiophene Films

We demonstrated orientation control of n-type rod-shape, 2,2′-Bis[4-(trifluoromethyl)phenyl]-5,5′-bithiazole (BTMPB) molecules when deposited on the in-plane oriented polythiophene (PT) and α-sexithiophene (6T)/PT films in vacuum. The results showed that the π-π stacking direction of BTMPB molecules on the PT and 6T/PT films is perpendicular to the substrate plane. In addition, a molecular long axis of BTMPB oriented parallel to the friction direction of the PT. Moreover, in-plane orientation of BTMPB was more enhanced by in-plane oriented 6T molecules than in-plane oriented PT underneath. On the other hand, the molecular long axis of BTMPB deposited on the substrates stood on the substrates.