共查询到20条相似文献,搜索用时 31 毫秒
1.
L. S. Bezhanova Z. V. Bagdasaryan M. L. Baburyan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2007,42(3):120-124
We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC*LC properties. 相似文献
2.
Maja Stojanović Alexej Bubnov Dusanka Ž. Obadović Vĕra Hamplová Miroslav Kašpar Miroslav Cvetinov 《Phase Transitions》2013,86(4):380-390
Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed. 相似文献
3.
J. M. Czerwiec M. Żurowska K. Garbat R. Dąbrowski M. Marzec S. Wróbel 《Opto-Electronics Review》2011,19(3):270-276
Influence of the electric field upon alignment of molecules in ferro- and antiferroelectric phases has been studied for two
fluoro-substituted compounds exhibiting antiferroelectric phase at room temperature. Two different relaxation processes have
been revealed in the ferroelectric as well as antiferroelectric phase. Low temperature value of spontaneous polarization is
ca. 130 nC/cm2 for both substances studied. The substances align very well in the external electric field — a mono-domain of the ferroelectric
SmC* phase can be obtained in about 3.5 hours. 相似文献
4.
Anamika Pramanik Malay Kumar Das Vĕra Hamplová Miroslav Kašpar Alexej Bubnov 《Phase Transitions》2015,88(7):745-757
The effect of several polar ester linkage groups incorporated in the molecular core of a chiral lactic acid derivative on self-assembling properties has been investigated by polarizing optical microscopy, small angle X-ray diffraction, differential scanning calorimetry, optical and electro--optical studies. The compound possesses the paraelectric smectic A* (SmA*) and ferroelectric smectic C* (SmC*) phases over a broad temperature range. Mesomorphic behaviour, spontaneous polarization, birefringence, optical transmission, dielectric anisotropy and structural properties of the self-assembled chiral material have been determined. The obtained results are discussed and compared with that of other liquid crystalline materials. Experimentally determined spontaneous polarization and tilt angle values are also used to elucidate the nature of SmC* to SmA* phase transition. The effect of polar ester linkages in the molecular core has also been discussed. 相似文献
5.
E. S. Filimonova A. V. Emel’yanenko J. H. Liu 《Moscow University Physics Bulletin》2017,72(4):369-375
The spontaneous polarization processes in synclinic and anticlinic smectic liquid crystals are discussed in the context of molecular statistical theory. The effects of interaction between molecules, external electric fields, and bending of molecules via polarization are elucidated as well. 相似文献
6.
Tolédano P Figueiredo Neto AM Boulbitch AA Roy A 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,59(6):6785-6797
The stable antiferroelectric and ferrielectric smectic phases which may arise below a chiral SmA* phase are investigated theoretically. The symmetry and physical properties of the bilayer and multilayer configurations are worked out. Antiferroelectric and ferrielectric bilayer and multilayer configurations, possessing an induced spontaneous ferroelectric polarization component perpendicular to the smectic layers, are shown to take place, as the result of a nonlinear piezoelectric effect. These states of low polar symmetries occur when the angle between the inlayer projections of the dipoles and the director of the molecules is different from 90 degrees. 相似文献
7.
Bretanha LC Teixeira VE Ritter M Siqueira GM Cunico W Pereira CM Freitag RA 《Ultrasonics sonochemistry》2011,18(3):704-707
The alternative synthesis of 12 1,2,4-oxadiazoles using ultrasound irradiation from trichloroacetoamidoxime and acyl chlorides is reported. Seven of them are novel compounds. The 3-trichloromethyl-5alkyl(aryl)-1,2,4-oxadiazoles have been synthesised in better yields and shorter reaction times compared to the conventional method. This protocol can be applicable for preparation of 1,2,4-oxadiazoles containing aryl or alkyl groups attached at their C-5 side-chain. 相似文献
8.
Link DR Clark NA Ostrovskii BI Soto Bustamante EA 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(1):R37-R40
Thin freely suspended films of a mixture of an achiral side-chain liquid crystal polymer and its monomer have been studied with depolarized reflected light microscopy. We observe that regions with an odd number of bilayers exhibit a net spontaneous polarization in the tilt plane of the molecules, while regions with an even number of bilayers have no net polarization. These odd-even effects are direct evidence that the tilted smectic bilayers are anticlinic at the polymer backbone and synclinic at bilayer interface and confirm that the phase is bilayer-by-bilayer antiferroelectric. 相似文献
9.
10.
Organic compounds exhibiting the smectic C phase are made of rod-like molecules that have dipolar groups with lateral components. We argue that the off-axis character of the lateral dipolar groups can account for tilt in layered smectics (SmC, SmC*, SmI etc.). We develop a mean-field
theory of the smectic C phase based on a single-particle potential of the form U
C ∝ sin(2θ)cosφ, consistent with the biaxial nature of the phase, where θ and φ are the polar and azimuthal angles, respectively.
The hard-rod interactions that favour the smectic A phase with zero tilt angle are also included. The theoretical phase diagrams
compare favourably with experimental trends. Our theory also leads to the following results: i) a first-order smectic C to
smectic A transition above some value of the McMillan parameter α, leading to a tricritical point on the smectic C to smectic
A transition line and ii) a first-order smectic C to smectic C transition over a very small range of values of the model parameters.
We have also extended the theory to include the next higher-order term in the tilting potential and to include the effect
of different tilt angles for the molecular core and the chain in the SmC phase.
Received 3 August 2002
RID="a"
ID="a"Present address: Department of Physics, Vijaya College, R. V. Road, Bangalore - 560 004, India.
RID="b"
ID="b"e-mail: nvmadhu@rri.res.in 相似文献
11.
In most homologous series of compounds made of bent-core (BC) molecules, the B2 B1 and B6 phases occur as the chain length decreases. We have studied binary mixtures of the compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4-n-dodecylbiphenyl 4’-carboxylate (BC12) which exhibits the B2 phase with the compound 4-biphenylyl 4’-n-undecyloxybenzoate (BO11) made of rod-like (R) molecules. We find the above sequence
of occurrence of the B phases with increasing concentration of BOH. In this paper we describe the physical origin for the
formation of these phases in both pure compounds and in the mixtures. We have also found the occurrence of the biaxial smectic
A phase when the BO11 concentration is increased to 87–95.5 mol%. We also report on another binary system composed of BC12
and 4-n-octyloxy 4’- cyanobiphenyl (8OCB) made of R molecules. This system exhibits the biaxial smectic A phase down to 30°C. Using polarized
infrared spectroscopy we find that the mutual orientation of the R and BC molecules in the SmAdb liquid crystal is such that the arrow axes of the BC molecules are along the layer normal of the partial bilayer smectic
structure formed by the rods. We also describe unusual growth patterns obtained when the nematic phase transforms to the SmAdb phase in a mixture with 24 mol% of BC12. 相似文献
12.
Mottram NJ Ul-Islam N Elston SJ 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(1):613-619
We have included the inherent molecular biaxiality of the smectic C phase in a model of the chevron structure. This molecular biaxiality is related to a hindered rotation about the molecular long axis which for chiral, polar molecules induces a spontaneous polarization. Through the coupling between biaxiality and the smectic cone angle, continuity of the molecular distribution at the chevron interface leads to changes in the cone angle. Under certain approximations we are able to find analytic expressions for the chevron structure and consequently estimate the width of the chevron interface. There are in fact two correlation lengths which govern variations in the cone angle and the biaxiality. 相似文献
13.
Swadesh Kumar Gupta Dharmendra Pratap Singh Rajiv Manohar 《Current Applied Physics》2013,13(4):684-687
Effect of single wall carbon nanotubes (SWCNTs) on electro-optical performance of a ferroelectric liquid crystal (FLC) has been studied. Voltage dependent spontaneous polarization and response time measurement has been made for the pure and SWCNT doped FLC system. Dielectric measurement has also been performed to understand the existing interaction between SWCNTs and FLC molecules. The results have shown increase in the value of spontaneous polarization and relative permittivity with slight slower response for the doped system. The observed properties of doped system revealed that the SWCNTs can perform well with FLC at low applied electric field to enhance the performance of LC devices. 相似文献
14.
Boyko Katranchev 《Phase Transitions》2016,89(2):115-132
Microtextural polarization, phase transitions, and electro-optical effects are studied in a series of nanocomposites, grown by mixing alkyloxybenzoic acids (nOBAs), displaying hydrogen-bonded dimeric liquid crystal (LC) state, with non-mesogens (single-walled carbon nanotubes (SWCNTs), perfluorooctanoic acid) or mesogens (bent-core LC compound D14F3). Each of the studied nanocomposites, in which the nOBA serves as a matrix, forms complexes with bent-shaped dimeric, caused by the interaction between the dopant structural units and the dimer rings. This feature, coordinated with the surface anchoring, bulk and electrical effects, leads to drastic reduction of the LC system symmetry. As a result, transitions from achiral (characteristic for the pristine nOBA) to chiral states (including ferroelectric smectic C with C2 symmetry and ferroelectric smectic CG with the lowest C1 triclinic one) take place. The functionalization of the SWCNTs causes drastic increase of the ferroelectricity. 相似文献
15.
Priyadarshini S. Sahoo Anuradha Panigrahi Sunanda K. Patri Ram N. P. Choudhary 《Central European Journal of Physics》2008,6(4):843-848
Polycrystalline samples of Ba4SrRTi3V7O30 (R=Sm and Dy), members of the tungsten-bronze family, were prepared using a high-temperature, solid-state reaction technique
and studied their electrical properties (using complex impedance spectroscopy) in a wide range of temperature (31–500°C) and
frequency (1 kHz-1 MHz). Preliminary structural (XRD) analyses of these compounds show the formation of single-phase, orthorhombic
structures at room temperature. The scanning electron micrographs (SEM) provided information on the quality of the samples
and uniform distribution of grains over the entire surface of the samples. Detailed studies of the dielectric properties suggest
that they have undergone ferroelectric-paraelectric phase transition well above the room temperatures (i.e., 432 and 355°C
for R= Sm and Dy, respectively, at frequency 100 kHz). Measurements of electrical conductivity (ac and dc) as a function of
temperature suggest that the compounds have semiconducting properties much above the room temperature, with negative temperature
coefficient of resistance (NTCR) behavior. The existence of ferroelectricity in these compounds was confirmed from a polarization
study.
相似文献
16.
A ferroelectric phase transition is observed in Langmuir-Blodgett films prepared from substituted copper phthalocyanine molecules.
The linear and nonlinear dielectric properties of the films and the switching of their spontaneous polarization are investigated
in the temperature range of the phase transition. The observed features can be explained by the Landau-Ginzburg model of a
first-order phase transition.
Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 625–631 (10 November 1999) 相似文献
17.
A. L. Andreev I. N. Kompanets T. B. Andreeva Yu. P. Shumkina 《Physics of the Solid State》2009,51(11):2415-2420
The director reorientation in smectic liquid crystals with ferroelectric properties has been considered in the case where
the interaction of liquid-crystal molecules with the surface leads to a partial unwinding of the helical structure of the
liquid crystal and the reorientation occurs as a result of the domain-wall motion. The dependences of the velocity of domain-wall
motion on the electric field strength, electric field variation frequency, boundary conditions, spontaneous polarization,
and viscosity of the liquid crystal have been determined. It has been demonstrated that an increase in the electric field
variation frequency or the polar part of the anchoring energy and the spontaneous polarization of the liquid crystal at a
constant field frequency results in an increase of the velocity of domain-wall motion. As a consequence, the time of the electro-optic
response of the liquid crystal in weak electric fields (from 0.4 to 2.0 V/μm) decreases by a factor of more than three. 相似文献
18.
The first experimental evidence for triclinic symmetry of bulk smectic liquid-crystal samples of achiral banana-shaped molecules is presented. This phase corresponds to the so-called Sm-CG phase consisting of biaxial molecules and characterized by two tilt directions with respect to the layer normal: tilt of the molecular plane (clinic) and tilt of the molecular kink direction (leaning). Each smectic layer has a polarization component normal to the smectic layers (C1 symmetry). The observations suggest that the phase tentatively labeled as B7 is identical with the Sm-CG phase. 相似文献
19.
We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase. 相似文献
20.
S. A. Pikin E. I. Demikhov E. S. Pikina M. V. Gorkunov 《Journal of Experimental and Theoretical Physics》1997,84(3):506-515
The dynamics of spontaneous polarization switching of the ferroelectric smectic C* in a variable electric field are examined theoretically and experimentally with the help of polarized light scattering.
The observed effect of quasiresonant scattering both in freely suspended smectic films and in ordinary electro-optical cells
is interpreted within the framework of the nonlinear model of isolated movable kinks in the director orientation distribution.
It is shown that the maximum of the scattering intensity at the characteristic frequency of the applied electric field disappears
at low temperatures and for small thicknesses of the smectic film. The dependence of the “resonant” frequency on the electric
field amplitude, the proximity to the phase transition temperature, the film thickness and thickness of the ferroelectric
domains, and also various material parameters is found. Estimates are made of such important characteristics as the dielectric
anisotropy, viscosity, and elasticity of the smectic films. The effect of film thickness on the density distribution of the
polar anisotropy energy in the film and on the corresponding shape of the moving orientation front within the film are discussed.
Zh. éksp. Teor. Fiz. 111, 919–937 (March 1997) 相似文献