共查询到20条相似文献,搜索用时 343 毫秒
1.
Ashwani Kumar Singh Amir Iqbal Upendra Bahadur Singh Roman S. Dabrowski 《Phase Transitions》2018,91(8):811-820
Thermodynamic and electro-optical characterization of a ferroelectric liquid crystalline material, namely ((S) (+) 4-(1-methylheptyloxy) phenyl 4′-octyloxybiphenyl-4-carboxylate) possessing paraelectric SmA*, ferroelectric SmC*, hexatic SmBh* and SmI* phases has been carried out. Phase identification has been done by optical and thermodynamic studies. Switching parameters viz. spontaneous polarization, switching time and rotational viscosity have been determined. The spontaneous polarization has been found to increase with decreasing temperature in SmC* phase. The switching time is found of the order of few milliseconds. 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.
Dielectric spectroscopy investigations in the frequency range 50?Hz to 1?MHz have been carried out on a new ferroelectric liquid-crystalline material (S-(-)-4-(2-n-hexylpropionyloxy)biphenyl-4′-(3-methyl-4-decyloxy)benzoate) possessing a relatively large spontaneous polarization (P s?~?240?nC?cm?2) and containing a lateral methyl group on the aromatic ring of the alkoxybenzoate unit. The effect of temperature on the dielectric relaxation modes has been investigated in the SmC* and N* phases. From dielectric dispersion data, relaxation frequency and dielectric strength of all detected relaxation modes have been evaluated and discussed. A new surface-like mode of relaxation frequency ~11?kHz and dielectric strength 3.8, is seen to appear in the SmC* phase. 相似文献
6.
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
7.
8.
Experimental investigations on the ferroelectric liquid crystal, R-4′(1-butoxycarbonyl-1-ethoxy) phenyl 4-(4-octyloxy phenyl) benzoate (1BC1EPOPB) of large spontaneous polarization (P S(+) = 240?nC?cm?2), using dielectric and calorimetric techniques, are reported. The temperature range of 25.0–125.0°C has been chosen for dielectric measurements. Dielectric dispersion studies are carried out in the temperature range 45.0–75.0°C and in the frequency range 2?Hz to 2?MHz for the smectic A, smectic C* and smectic X phases. A new phase called ‘smectic X’ has been found around 56.3°C. The transition temperatures identified by the dielectric dispersion studies for different phases and those given by DSC techniques are in close agreement. 相似文献
9.
10.
J. Hemine C. Legrand N. Isaert A. El Kaaouachi H.T. Nguyen 《Physica B: Condensed Matter》2007,390(1-2):34-39
Structural (helical pitch), electro-optical (tilt angle, spontaneous polarization and response time) and dielectric (Goldstone mode) characterizations have been performed on two pure ferroelectric liquid crystals of a biphenyl alkyloxy benzoate series and they show the N*–SmA*–SmC* phase sequence. The different results are discussed: the helical pitch, the spontaneous polarization and the rotational viscosity which is determined as a function of temperature by two methods using electro-optical or dielectric measurements. An Arrhenius behaviour of the rotational viscosity is found for the two compounds. The corresponding activation energies are determined. 相似文献
11.
The ferroelectric phase transition and its relation to the spontaneous strain in ferroelastic NH4HSO4 crystals were investigated using VO2+ ions as an EPR probe. The impurity ions were found to be interstitially trapped at sites surrounded by crystallographically inequivalent NH4+ and SO42? ions. The polar VO2+ axes exhibited temperature-dependent displacements in two distinct directions with different energies. The differential properties of VO2+ ions in NH4HSO4 crystals were used to verify the presence of internal stress in the ferroelastic phases, and the corresponding strain was studied in the range between ?120 and + 100°C. The results indicate that the ferroelectric phase transition occurs as a consequence of lattice instability caused by the internal strain. At the second-order structural transition a dipolar lattice emerges in the crystal and the spontaneous polarization appears as a result of internal entropy transfer to the strained lattice. 相似文献
12.
Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures 总被引:1,自引:0,他引:1
The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in
the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation
mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode
appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition
temperature (T
C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency
and temperature. The calculated values for fNRM, δεNRM and distribution parameter αNRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT
C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous
polarization. 相似文献
13.
The most important goal of our research is to show the influence of the ‘guest’ (bent-core mesogen, 1,3-phenyldicarboxylatebis{4-[(4-octylbenzoyl)sulphanyl]phenyl} [IFOS8], banana-shaped liquid crystal [BLC]) on the ‘host’ (calamitic liquid crystal [CLC], (S)-(+)-1-methylheptyloxybiphenyl-(4-n-octylphenyl)thiobenzoate [MHOBS8]), on the stability and the destabilization of the antiferroelectric B2 and the ferroelectric smectic C* (SmC*) phases, and change of the temperature ranges of other phases in the binary liquid crystal mixtures. This work is focused on polymorphism of three new binary liquid crystal mixtures, exhibiting a ‘guest–host’ (guest liquid crystal–host liquid crystal [GH-LC]) effect. MHOBS8 has, among others, a ferroelectric SmC* phase, and IFOS8 assumes the B2 phase with antiferroelectric properties. The observed properties of the mixtures, such as variation of the phase transition temperatures, spontaneous polarization, tilt angle and switching time, are characteristic of a ‘guest–host’ mixture. The influence of BLC on the character of the interactions within the CLC host is discussed, with particular attention paid to electro-optical properties of the GH-LC mixtures. 相似文献
14.
The aim of this paper is to study the influence of electric field on alignment of para-, ferro- and antiferroelectric phases
in the vicinity of SmA* — SmC* or SmC* — SmC
A
*
phase transitions as to obtain mono-domain cells. Four mixtures studied (W-193B, W-193B-1, W-201, W-204D) show the SmC
A
*
phase in a wide room temperature range. Measurements of the spontaneous polarization versus temperature by using reversal
current method give an answer to the question, what kind of the transitions take place between para-, ferro- or antiferroelectric
phases using the Landau mean field theory. Optimal electrooptic parameters for different compositions of the mixtures such
as tilt angle, spontaneous polarization and saturation voltage have been measured to compare parameters of the mixtures studied. 相似文献
15.
In this work, the physical properties of newly synthesized liquid crystalline compound exhibiting two liquid crystalline phases (ferroelectric and antiferroelectric) were studied. Based on the results of differential scanning calorimetry, polarizing microscopy, and photoelastic modulator methods, the temperature dependences of spontaneous polarization, tilt angle, switching time, and birefringence in the ferroelectric, as well as antiferroelectric phases were determined. Furthermore, the influence of the external electric field on the liquid crystalline textures was studied and the phase sequences at heating and cooling were revealed. The temperature dependence of spontaneous polarization was analysed by means of Landau mean-field theory, and the critical parameter β obtained for ferroelectric liquid crystalline and isotropic liquid transition was 0.21 which is close to 0.25, the value characteristic for tri-critical point. 相似文献
16.
3,5-Disubstituted 1,2,4-oxadiazoles are a new type of liquid crystalline (LC) compounds with asymmetrical five-membered heterocycle
as a central unit. They have a bent shape and are very convenient model-compounds for studying the dependence of the LC properties
on the molecular design. We have also synthesized and investigated ‘banana-shaped’ 1,2,4-oxadiazoles using the ester groups
as the linkage units. The new compounds exhibit spontaneous polarization in the smectic phase, even if there is no chiral
group in the molecules. Preliminary experimental data suggest the presence of spontaneous polarization in the nematic phase
as well. In order to study the structural properties of the LC phases, X-ray diffraction (XRD) measurements on powder samples
have been carried out. Based on the XRD data, a model of the structural arrangement of the bent molecules in the smectic phase
is provided, which accounts for the macroscopic spontaneous polarization as well as the ferroelectric switching behavior. 相似文献
17.
D. Pociecha E. Gorecka J. Przedmojski 《The European physical journal. E, Soft matter》2000,1(2-3):137-140
Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds
belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature
dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical
points have been observed on phase transition lines in binary mixtures of respective materials having first and second order
phase transitions between hexatic phases.
Received 21 June 1999 相似文献
18.
A short review of the published experimental data on various physical properties and some new results of the measurement of dielectric and electromechanical properties of Cs2HgCI4 crystals in a wide temperature range are presented. The crystal is shown to reveal anomalies at seven temperatures associated with structural phase transitions and hence to have eight phases. In two temperature regions there exists a small spontaneous polarization and the anomalous dielectric permittivity is a nonlinear function of the electric biasing field. Within the phase followed by the first polar phase on cooling the crystal, the effects of thermal, electrical and mechanical memory, specific for incommensurate phases, are observable. 相似文献
19.
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
20.
In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C* transition is always of first order. The theoretical predictions are compared with the available experimental results.-1 相似文献