Morphology of ice droxtals grown from supercooled water droplets

This study describes the growth modes of polyhedral ice crystals grown from spherical ice crystals formed by freezing of supercooled water droplets. Frozen water droplets were grown on a thin glass plate at -15 and -7°C and at humidities above and below water saturation. In the case of growth at -15°C and at a humidity above water saturation, a frozen water droplet grows into an 8-faced ice crystal through a 20-faced ice crystal, while in the case of growth at -15°C and at a humidity between the ice saturation and the water saturation, it grows into an 8-faced ice crystal with truncated edges and corners through an 8-faced ice crystal with curved and flat surfaces. The growth modes at -7°C and at humidities above and below water saturation are almost the same as those at -15°C, except that the axial ratios c/a of the droxtals at -15°C are about 0.95 but those at -7°C are about 1.04.     

Dielectric Studies in Solid Phase EBBA

Dielectric measurements have been made in the stable and metastable solid phases of EBBA in the frequency range 0.1 KHz to 100 KHz and in the temperature range - 170 to - 10°C. The dielectric dispersion was absent in the stable phase while in the metastable phase, obtained by rapid cooling, a clear dielectric dispersion was seen. In addition to the primary dispersion extending above about - 125°C there also appears a clear secondary dispersion region below - 125°C. The strength of the second dispersion is lower by one order of magnitude than the primary dispersion. The primary dispersion is of the Debye type with a symmetric distribution of relaxation times. The Cole-Cole distribution parameter varies with temperature and shows a maximum at about -64°C. The results in the primary dispersion region can be qualitatively explained by assuming the free rotation of dipoles above -64°C and a restricted rotation of dipoles at low temperatures.     

Structure of the Nematic Phase of Pbaba

X-ray diffraction investigations of the nematic phase (191°C - 276°C) of p-n-propyloxybenzylidene-p-amino benzoic acid have shown that this phase is a skewed cybotactic nematic phase. The diffraction patterns, obtained with aligned samples, reveal an additional surprising structural feature, - the presence of one dimensional molecular correlation in a direction parallel to the nematic director. At 192°C, it is found that the molecular unit, thus correlated, is about 37 Å in length (dimer length) and the correlation range extends up to about seven such units. This l-d correlation persists up to the N-I transition.     

Synthesis and some Physical Properties of 1-Cyclohexyl-2-(4”-Halobiphenyl-4'-yl)Ethanes

A series of 1-(trans-4'-n-alkylcyclohexyl)-2-(4”-halobiphenyl-4'-yl)ethanes, which show nematic phases, was prepared. Their transition temperatures, enthalpies and entropies were measured. Their bulk viscosities, birefringences and dielectric constants were determined by extrapolation. 1-(trans-4'-n-propylcyclohexyl)-2-(4”-fluorobiphenyl-4'-yl)ethane has a high clearing of 125° C, a low viscosity of 25 c.p. at 20 °C, a relatively high birefringence of 0.166 at 25 °C, and a positive dielectric anisotropy of 4.9. The transition temperatures, the N-I transition entropies, the bulk viscosities and the birefringences for the 1-(trans-4'-n-alkylcyclohexyl)-2-(4”-fluorobiphenyl-4'-yl)ethanes exhibited odd-even effects in relation to the number of carbon atoms in the alkyl chain. The bulk viscosity as a function of the third power of the van der Waals radius of the halogeno group, the birefringence as a function of the van der Waals radius of the halogeno group, and the dielectric constants as a function of the dipole moment of the halogeno group are discussed for the 1-(trans-4'-n-propylcyclohexyl)-2-(4”-halobiphenyl-4'-yl)ethanes.     

The Mesogenic Behavior of Isomeric and Nonisomeric Series of Novel Azoesters. 4-(4′-n-Alkoxy Benzoyloxy) Phenylazo-2″-Bromobenzenes

A novel homologous series: 4-(4′-n-alkoxy benzoyloxy) phenylazo-2^″-bromobenzenes is synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesogenic behavior in a series. The mesogenic property commences from fifth homologue to the last homologue. The transition temperatures of the series are relatively low ranging between 68°C and 143°C. The mesogenic range varies between 6°C and 22°C. The novel azoester series is nematogenic without exhibition of any smectogenic property and an average thermal stability of 89.4°C. The mesogenic behavior of the novel series is compared with structurally similar isomeric/nonisomeric other known series.     

Electrical conductivity of pyrolyzed polyacrylonitrile

Abstract

Using ultrapure samples of polyacrylonitrile (PAN) of 485,000 or 150,000 average molecular weight solution cast in dimethylformamide, the dc conductivity ([sgrave]) of pyrolyzed PAN (PANP) films has been studied for pyrolysis temperatures (T_p) of 280-435°C. Conductivity measurements made during pyrolysis indicate the onset of a dramatic increase in [sgrave] for T_p of 390-435°C. Conductivities as high as 5 (ohm-cm)^?1 have been observed for T_p < 435°C. This situation contrasts sharply with previous literature which had indicated that a increased uniformly and monotonically with T_p for 200°C < T_p < 900°C and that values of [sgrave] > 1 (ohm-cm)^?1 were observed only for T_p > 600°C. Our results indicate that the maximum value of [sgrave] obtained is not a strong function of T_p (390°C < T_p < 435°C) or of molecular weight. However, the rate of increase of [sgrave](t) is strongly dependent on T_p in this range. After pyrolysis, repeated heating and cooling below T_p do not alter [sgrave](T). IR spectra show that the sudden increase in [sgrave] is correlated to the formation of conjugated C=C and C=N bonds. The shape of [sgrave](T) suggests that conduction is probably due to hopping.     

Novel Cinnamate Esters—Synthesis and Mesomorphic Properties in Relation to Molecular Structure

A novel homologous series 4-[4′-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting of 11 homologs was synthesized and studied with a view to understanding and establishing the relation between molecular structure and mesomorphic behavior of a substance. Mesomorphic behavior of the series commences from the third homolog and then continues until the last hexadecyloxy homolog. The first and second members of a series are non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic transitions are enantiotropic in nature. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type and that of the smectic mesophases are focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series shows a normal behavior of the transition curves with a minor abnormality of the last three homologs for the nematic-isotropic transition curve. The average thermal stability for smectic and nematic mesophases is 176.0°C and 219.3°C, respectively. Analytical and spectral data agree with the molecular structures of homologs. Mesomorphic phase length varies between 22°C and 76°C. Smectic phase length varies from 6°C to 33°C and nematic phase length varies from 22°C to 54°C. Thus, the novel present series is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the novel series are compared with structurally similar other known homologous series.     

Mesomorphism dependence on molecular rigidity with reference to ‒CH˭CH‒ unit of central bridge

A novel ester homologous series of rich mesomorphism and low temperatures with unexpected phase behaviors of eleven homologues was synthesized and studied with a view to understanding and establishing the relation between mesomorphic behaviour and the molecular structure of a series 4-(4′-n-alkoxybenzoyloxy)-4″-chlorobenzyl cinnamates. All the members of the novel series are enantiotropically smectogenic and the octyloxy (C₈) to hexadecyloxy (C₁₆) homologues are enantiotropically nematogenic, in addition to smectogenic. Odd–even effect is observed for Sm?I/Sm?N transition curve but it is absent for N?I transition curve. Textures of nematic phase are threaded or Schlieren and that of the smectic phase are fan shaped or batonates of smectic-A type phase or Smectic-C type for C₁₆ homologues as judged directly from a heating stage of an optical polarizing microscopy. Analytical and spectral data confirmed the molecular structures of novel homologues. Mesomorphic properties of present series are compared with the structurally similar other known series. The average smectic and nematic thermal stabilities are 92.78°C and 100.8°C, respectively. Mesophase length minimum to maximum for smectic and nematic are 21.0°C to 31.1°C and 8.4°C to 42.6°C respectively. Thus, the present novel series is partly nematogenic and fully smectogenic with considerable degree of mesomorphism and low melting type.     

Synthesis and Nonlinear Optical Properties of Novel Polyurethane Containing Cyanovinylnitroresorcinoxy Group

Novel X-type polyurethane 5 containing 4-(2′-carbomethoxy-2′-cyano)vinyl-6-nitroresorcinoxy groups as nonlinear optical (NLO) chromophores, which constitute part of the polymer backbone, was prepared and characterized. Polyurethane 5 is soluble in common organic solvents such as acetone and N,N-dimethylformamide. It shows thermal stabilities up to 260°C from thermogravimetric analysis with glass transition temperature obtained from differential scanning calorimetry near 108°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at 1064 nm fundamental wavelength is 9.83 × 10^?9 esu. Polymer 5 exhibits a high thermal stability even at 2°C higher than T_g, and no significant SHG decay is observed below 110°C, which is acceptable for nonlinear optical device applications.     

Semi-interpenetrating polymer networks based on polyurethane and poly(2-hydroxyethyl methacrylate): Dielectric study of relaxation behavior

The study of molecular dynamics by broadband dielectric spectroscopy (BDS) is presented for polyurethane (PU), poly(2-hydroxyethyl methacrylate) (PHEMA) and for semi-IPNs based on PU and PHEMA synthesized by photopolymerization. The dielectric properties were performed in wide range of frequencies and temperatures with the goal to establish the relation between the relaxations and the structure. Five relaxation phenomena were finally detected for PHEMA : γ-, βsw-, β-relaxations at low temperatures and α-relaxation at 150 °C at high frequencies plus ionic conductivity relaxation which starts at 0 °C. For semi-IPNs the overlapping of γ- and βsw-relaxations of PHEMA (?125/?75 °C), then with increasing the temperature α-relaxation in PU (?75/0 °C), next ionic conductivity relaxation which starts at 0 °C, and finally the α-relaxation of PHEMA (+125/+170 °C) were detected. The α-relaxation of PHEMA in semi-IPNs shifts to lower temperatures and became broader with increasing amount of PU due to incomplete phase separation in the system and formation of interphases. The dielectric relaxation phenomena were fitted with Havriliak–Negami equation. Activation energy, τ_o and α parameters were calculated. For α-relaxations corresponding dielectric characteristics have been determined from Vogel–Fulcher–Tammann equation. The relaxation map for investigated PU, PHEMA and semi-IPNs was built.     

Synthesis and Properties of Novel Nonlinear Optical Polyester Containing Cyanovinylnitroresorcinoxy Group

Novel X-type polyester 5 containing 4-(2’-carbomethoxy-2’-cyano)vinyl-6-nitroresorcinoxy group as nonlinear optical (NLO) chromophore, which constitutes parts of the polymer backbone, was prepared and characterized. Polyester 5 is soluble in common organic solvents such as N,N-dimethylformamide and acetone. Polyester 5 shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of near 116°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at the 1064 nm fundamental wavelength is 4.25 × 10^?9 esu. The dipole alignment exhibits a thermal stability even at 4°C higher than glass-transition temperature (T_g), and no significant SHG decay is observed below 120°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.     

Formation and oxidation of porous silicon by anodic reaction

Porous silicon film (PSF) was formed by anodic reaction in aqueous hydrofluoric acid (HF) in the range 2 ? n < 4, n being the average number of electrons flowing through the external circuit per atom of silicon dissolved. Electron diffraction pattern of as-grown PSF changes to a very broad Kikuchi pattern from a sharp Kikuchi pattern for smaller values of n. The disproportionation reactions occur at a local point of the silicon surface for n between 2 and 4. When n is nearly equal to 2, the PSF structure changes to be amorphous and the reactions occur over the entire area of the silicon surface. The oxidation of PSF proceeds with the increase of the absorption intensities of three Si-O bands at 1070, 455, and 800 cm^-1 below 600°C, and two Si-O bands which are 455 and 800 cm^-1 above 600°C. Activation energies of Si-O bands at 1070, 455, and 800 cm^-1 at oxidation below 600°C are 0.1, 0.3, and 0.6 eV respectively. The activation energy changes remarkably below and above 600°C. The oxygen distributions in oxidized porous silicon of p-type and n-type PSF are uniform and nonuniform respectively in the thickness direction.     

The effect of cooling rate on size,quality and morphology of KTiOPO4 (KTP) crystals grown by different nucleation techniques

KTP crystals have been grown by two nucleation techniques namely spontaneous nucleation in flux medium and nucleation on Pt rod using K₆P₄O₁₃ flux.10 °C/h, 7 °C/h, 0.8 °C/h, 0.4 °C/h and 0.2 °C/h cooling rates were applied for spontaneous nucleation and crystals up to 15 × 7 × 4 mm³ in size were grown. 1 °C/h and 0.8 °C/h cooling rates were also used for nucleation on Pt rod and crystals up to 8 × 6 × 3 mm³ in size were grown. The effect of cooling rate on size, morphology and optical quality of grown crystals by both techniques were studied. For nucleation on Pt rod upper and lower rotation rates limits and an optimum rotation rate were distinguished for each cooling rate. Quality of the grown crystals by both techniques was characterized by optical transmission analysis.     

Crystal Growth in Gels at Elevated Pressures: The Upper Limit of Temperature for Metastable Formation of Aragonite

Both rapid precipitation and diffusion controlled gel growth were applied to crystallize calcium carbonate at temperatures in the range of 100 °C to 270 °C. The amount of aragonite was determined by means of X-ray diffraction data. The morphology of the aragonite crystals are described. Metastable formation of aragonite was observed only at temperatures below 270 °C.     

Characterization of alkoxy-derived iron silicate

Amorphous iron silicates, x Fe₂O₃ · (1 ? x) SiO₂ (0.1 ? x ? 0.4), were prepared by calcining alkoxy-derived gels obtained by the hydrolysis of metal alkoxide mixtures. Structural studies based on X-ray diffratometry. Mössbauer effect and infrared spectroscopy indicated that the materials were indeed amorphous up to 500°C in the composition range x ? 0.3 and that the structure is formed by SiOFe bonds. Crystallization took place on calcining the samples with the composition x ? 0.3 above 600°C. Fayalite deposited at 600°C and hematite and α-cristobalite deposited above 700°C. In the case of the composition x = 0.4 the product was amorphous up to 300°C and only hematite was detected above 400°C.     

Synthesis of nano‐crystalline zeolite β: Effects of crystallization parameters

The effects of variation in Si/Al ratio (25 and 100) and crystallization temperature (80 °C to 180 °C, at an interval of 20 K) on crystal size of zeolite β were studied. Products obtained at different synthesis parameters were characterized by powder X‐ray diffraction, IR spectroscopy, thermal analysis, scanning electron microscopy and nitrogen adsorption. Increase in crystal size with crystallization temperature and Si/Al molar ratio was observed. Crystal morphology at 140 °C was spherical whereas at 180 °C it was of irregular shape. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Crystal Structures of Methyl and Ethyl N-(2,3-Dimethoxybenzyl)-N-(p-toluenesulfonyl)-2-phenylglycinates

The crystal structures of methyl and ethyl N-(2,3-dimethoxybenzyl)-N-(p-toluenesulfonyl)-2-phenylglycinates 4a and 4b respectively, have been determined by X-ray diffraction analysis. The presence of different alkyl ester groups in both molecules does not affect the geometry retaining a synclinal conformation for the N atom and the O atom of the C=O group [N—C17—C18—O5 38.1(5)° for 4a and 33.3(4)° for 4b ].     

Synthesis and Mesomorphic Properties of Ferroelectric Liquid Crystals with a Fluorinated Asymmetric Frame (1)

New ferroelectric liquid crystals with a fluorinated asymmetric frame were synthesized utilizing optically active S-(-)-2-fluoroalkanols prepared from R-(+)-1,2-epoxyalkanes with pyridinium poly(hydrogen fluoride), and their mesomorphic and physical properties (phase behavior, spontaneous polarization, response time) were studied. The biphenyl-phenyl derivatives of three-ring systems exhibited enantiotropic chiral smectic C phase in a broad temperature range (108°C-185°C). The phenyl benzoate of two-ring systems exhibited chiral smectic C phase at temperature relatively close to room temperature, and were found to possess the spontaneous polarization larger than 35 nC/cm².     

High performance and low level of image sticking fringe field switching mode liquid crystal display with photo-alignment layers

Abstract

A novel photo-alignment fringe field switching mode liquid crystal display (PA-FFS-LCD) with a wide nematic temperature range from ?40?°C to 95?°C was developed. The new developed PA-FFS-LCD shows fast gray-to-gray response time (<20?ms at 25?°C and < 300?ms at ?30?°C), wide viewing angle, and high contrast ratio about 1600 owing to use an isomerization-type photo-alignment material. Image sticking was not recognized owing to a suitable arrangement of pixel and common electrodes.