Uniform Alignment of Liquid Crystalline Cubic Blue Phase II via Rubbing Treatment

The uniform alignment of liquid crystalline cubic blue phase II (BPII) has been achieved using a conventional surface rubbing treatment. Through comparison of thick and thin cells of assembled rubbed glass, which were covered with commercially available polyimide (PI), we confirmed that rubbed surfaces play an important role in the fabrication of monodomain PBII. These well-aligned cubic BPII materials can be promising for photonics devices.     

Blue Phase in Liquid Crystal Polymer Mixtures

Blue phases are observed in mixtures of a low molecular weight cholesteric and a nematic polymer. Phase diagrams, optical properties and textures of these mixtures are reported and compared to the case of low molecular weight cholesteric-low molecular weight nematic mixtures.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Kinetics of Overall Phase Transition in Changing Size System

Kinetics of phase transition is studied for the more general case when the size of the system is assumed time dependent. In the three dimensional case typical examples in this respect could be connected with the solution of cosmological problems. In the two dimensional case could be the spreading and crystallization of undercooled water on oil or the experiments concerning phase transition in monolayers in a Lengmuir balance. The time dependence of the degree of overall transformation α(t) is formulated in a generalized form corresponding to above assumption. Explicit solutions are given for a constant rate of expanding (or shrinking) and for a constant acceleration of the size change of the system under the assumption that both nucleation rate I and linear growth rate G are constants. It is demonstrated that the process of phase transition proceeds much faster in a shrinking systems and is impeded in expanding ones. The reason for this effect is that parts of the new phase, formed in the initial parts of the system, are transferred into the diminishing volume (or surface). Thus the concentration of the new phase is mechanically increased.     

Optical Studies on Smectic-Isotropic Phase Transition Behavior of Two Thermotropic Liquid Crystals

The binary mixture of two compounds, viz., Cholesteryl laurate (CL) and diethyl 4, 4′-azoxy benzoate shows the cholesteric and smectic phases sequentially when the specimen is cooled from its isotropic phase. Refractive indices, birefringence, and optical transmittance have been measured by the optical technique. With the help of measured data, the macroscopic ordered parameter has been discussed. The temperature dependence of these parameters has also been discussed.     

Refractive Indices and Dielectric Studies of Three Phenylcyclohexane Liquid Crystals in the Nematic Phase

The refractive indices n_e, n_o and densities and the dielectric primittivities ε_∥ and ε_⊥ of three P-cyano-P-alkylcyclohexanes (PHC-3, PCH-5, PCH-7) have been measured at different temperatures. The order parameters calculated by using Neugebauer and Vuks' methods were comparable. A comparison of order parameters of the PCH with the corresponding alkylcyanobiphenyls at reduced temperatures τ (= T-T_NI/T_NI) shows that the replacement of a phenyl ring with a cyclohexyl ring decreases the order parameter appreciably. The experimental values of dielectric permittivities ε_⊥ in each of the PCHs are found to be less than half of the ε_⊥-value calculated from Maier and Meier equations using the polarizability and the order parameter obtained from refactive index measurements. This was explained as due to short range antiparallel ordering of strongly polar molecules in the nematic phase, as proposed by Chandrasekhar and Madhusudan, which was not taken into consideration in Maier and Meiers' theory     

Phase transition and thermal stability of reentrant smectic phase in mixture of liquid crystalline materials

In the present work, our investigation is to study the optical and thermal properties of the binary mixture of cholesteric and nematic compounds namely, decyloxy benzoic acid (DBA), and cholesteryl chloride (ChCl), which exhibits different liquid crystalline phases with reentrant smectic-A phase. The reentrant smectic-A phase has been observed at different concentrations and at different temperatures. The existence of reentrant smectic-A phase has been observed by optical microscopic studies. The temperature variation of optical anisotropy, X-ray and helical pitch of the cholesteric (N^*) phase has also been discussed.     

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.     

NMR Study of the Phase Transitions in Some Derivatives of Cholesterol and B-Nor Cholesterol

In this paper we analyze by broad line NMR spectroscopy method, the influence of modifying the cholesterol skeleton upon mesomorphic properties and characteristics (phase transition points, mesomorphic ranges, etc.) in some derivatives of cholesterol. From our studies regarding the phase transition points by NMR spectroscopy method, we can assert that derivatives of the B-nor cholesterol have the same behavior as derivatives of the cholesterol.     

Elements of the Phase Diagram of YbInCu4

To clearify how the valence transition temperature of YbInCu₄ is connected with the composition of the grown crystals, we investigated the phase diagram. Since elements of the liquidus curve of the system Yb-In-Cu were already known we focused on the solidus curve in the surrounding of the valence changing YbInCu₄. Bridgman-technique for crystal growth was used. The composition of the solidified crystals was determined by means of wavelength-dispersive X-ray analysis. The valence transition temperature is measured via the lattice constant by low temperature X-ray diffraction. Results are, that the assumed exchange of Yb and In, which is indicated in the formula Yb_xIn_2-xCu₄ seems to be not correct. Not the quasi-binary section for constant Copper content should be used, but one for constant Ytterbium content. Starting compositions with an excess of Ytterbium lead to the substitution of Indium by Copper. The valence transition temperature of these crystals is shifted from 40K to 70K.     

Order Parameter and Refractive Index of Cholesteryl Linoleate

Refractive indices of cholesteryl linoleate are determined at different temperatures and order parameters have been calculated in smectic and cholesteric phases.     

Phase Diagram and Dielectric Studies in Hydrogen-Bonded Liquid Crystal System

The thermal and dielectric behaviors of mixtures were investigated for systems of p-n-hexyloxybenzoic acid (6OBA) and p-n-octyloxybenzoic acid (8OBA) by differential scanning calorimetry, polarized optical microscopy, and dielectric measurements. The T-X phase diagram was obtained for this system. Eutectic composition was calculated and received experimentally. Enthalpies of mixing were calculated to describe phase transitions physically. All mixtures show enantiotropic smectic and nematic phases. Dielectric permittivity has higher value for the mixture with fractional proportion of 30 to 70% mol for 8OBA than that for initial acid. It is assumed that co-crystallization of initial compound occurs when the ratio of components is close to equimolar ratio.     

掺杂VO2薄膜的相变机理和研究进展

掺杂VO2薄膜是一种具有相变特性的功能材料,具有广阔的应用前景.本文综述了掺杂VO2薄膜的相变机理、制备方法和应用前景,并指出掺杂VO2薄膜的发展趋势,以期更好地探讨掺杂VO2薄膜的应用研究.     

The Smectic A and the Smectic C Phase: A Coherent Molecular Picture

A molecular field theory of the smectic A and smectic C phase is presented which is based on an interaction derived in terms of the molecular polarizability, its anisotropy and the components of the static molecular quadrupole tensor. It is shown that this attractive interaction strongly depends on the anisotropy of the excluded molecular volume. The results obtained explain the stability of the A phase, the appearance of the tilt in the C phase and the A-C phase transition, without resource to adjustable parameters which indeed have no molecular significance.     

四方相区NBT-KBT无铅铁电介电弛豫与铁电相变研究

本文研究了具有四方结构的(Na1-xKx)0.5Bi0.5TiO3体系中x＝0.22,0.26和0.30陶瓷材料不同温度下的电滞回线,结合变温XRD和介电温谱分析,发现该体系四方结构的组成为反铁电体,但紧靠准同型相界的四方结构由于场诱导下引起的反铁电-铁电相变,表现出铁电体特性,材料在升温过程中由于反铁电宏畴向微畴的转变导致了介电峰的产生,且在介电温谱上表现出强烈的介电常数-频率依赖性,为弛豫铁电体特征.     

纳米薄膜材料的蓝光发射特性及其研究进展

蓝光发射是纳米半导体材料发光特性的一个重要研究方向.本文首先简要介绍了蓝光发射的物理机制,接着重点评述了几种主要纳米薄膜材料,如ZnO、GaN、CdS和Si纳米结构等,近3～5年内在蓝发光射特性方面所取得的最新研究进展.最后,对存在问题和发展前景进行了讨论与展望.     

高压下Pca21相TiO2直接相变的第一性原理研究

利用第一性原理的GGA+U方法对TiO2的金红石相(P42/mnm)、铌铁矿相(Pbcn)和Pca21相(Pca21)进行了计算.首先,优化了这几个相的晶体结构及相关参数,分析了焓曲线图,发现在减压至8.4 GPa时,Pca21相直接相变为铌铁矿相.其次,通过拟合分析了三阶Birch-Murnaghan状态方程得到Pca21相的体弹模量B0与类萤石结构实验值(202 GPa)非常符合.最后,通过讨论Pca21相的相变压强和体弹模量,进一步证明Pca21相结构更符合类萤石实验结果.     

SrTiO3铁电体反铁畸变序参量的研究

钛酸锶(SrTiO3)的反铁畸变是氧八面体绕[001]轴转过一微小角度θ引起的,由于前人对相变中晶体学原胞的选取不合适,所以不能正确说明晶体结构对称性的变化.本文提出一种晶体学原胞的选取方法,能很好地反映相变前后对称性的变化.由居里原理,分析其序参量是一个三阶全对称的极张量,并给出具体的数学形式.     

Electro-Optical Properties of Electric Field-Induced-Polymer Stabilization at Blue Phase of Chiral Nematic Liquid Crystal

Polymer stabilization of the blue phase induced a wide temperature range of more than 60 K, but the high driving voltage and the large switching hysteresis compared to the low molecular weight blue phase of the chiral nematic liquid crystal. In this study, we newly proposed electric field-induced-polymer stabilization of blue phase (EFIPSBP) which the electric field was applied to the blue phase during polymer stabilization and investigated the driving voltage and the switching hysteresis using various voltage-applied EFIPSBP cells. We confirmed that the driving voltage and the switching hysteresis of the EFIPSBP cell were decreased compared to those of conventional polymer stabilized BP at 8OCB chiral nematic liquid crystal mixture. This could be suggested as a potential method for improving the driving voltage and the switching hysteresis of conventional polymer stabilized BP.     

SiO2气凝胶小球热处理过程中的相变研究

以无机硅溶胶为先驱物,通过溶胶-凝胶过程和常压干燥工艺制备出SiO2气凝胶.将SiO2气凝胶在不同温度条件下进行热处理,采用DSC、XRD、TEM、BET和IR等手段表征了SiO2气凝胶由无定形态向多晶态转化过程中,其多孔结构特征的变化.研究结果表明,在600℃以下进行热处理,样品表现出良好的热稳定性;当热处理温度在600～1060℃,随着温度的升高,多孔结构特征因无定形SiO2微粒团聚而逐渐消失;当热处理温度高于1060℃,无定形SiO2发生相变形成多晶态,导致SiO2气凝胶多孔结构特征完全消失.