Dielectric Relaxation Spectroscopy of Liquid Crystal in Nematogenic Mesophase

In the present study, we report the dielectric behavior of a high temperature nematic liquid crystal (NLC) in both the planar as well as homeotropic alignments. To understand the mechanism of such high temperature NLC, we have performed dielectric spectroscopy in frequency range 100 Hz to 10 MHz. The different dielectric parameters, viz., effective dielectric permittivity and dielectric anisotropy of the sample have been determined with the variation of frequency and temperature. It is found that the dielectric parameters depend on dipole moment and rotation of side chain of molecule. The phase transition of this high temperature NLC has been confirmed by the DSC study.     

Modification in dielectric properties of SWCNT doped ferroelectric liquid crystals

Effect of single walled carbon nanotubes (SWCNT) in ferroelectric liquid crystal (FLC) geometry has been studied for low and high concentrations of dopant. The results are compared with the characteristics of pure FLC system. An appreciable change in dielectric permittivity, relaxation behavior and other vital parameters has been observed for SWCNT doped FLC system, which shows the adaptability of carbon nanotubes with that of pure FLC molecule. Under high electric field, highly concentrated SWCNT doped FLC shows enormously large values of dielectric loss. This is due to the high conductivity of SWCNT at higher electric field. Interestingly, such effect is not much prominent for the low concentration of the SWCNT doped FLC system.     

Influence of Electric and Magnetic Fields on the Molecular Alignment in EBBA

The effects of external de and af electric fields of strengths up to 25kV/cm on the nematic liquid crystal p-ehtoxybenzylidene-[pacute]-n-butyl aniline (EBBA) are investigated. The effectiveness of electric fields and of high magnetic fields in producing molecular alignment are compared. Changes in the molecular alignment are observed in terms of changes in the rf dielectric constant. These measurements are as fruitful as microwave measurements in studying the degree of ordering of molecules in bulk samples of liquid crystals. The threshold voltages for chevron formation in the dielectric regime are found to be considerably lower than the threshold voltages necessary induce changes in dielectric constant in the same regime.     

On the Dielectric Behavior of Unaligned Samples of 4-n-Octyl-4′-cyanobiphenyl (8CB)

Data on the dielectric properties of the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been determined onto unaligned samples, for a large scale of frequency, as function of temperature. The components of the complex dielectric permittivity as well as the corresponding dielectric parameters were evaluated using Havriliak-Negami (HN) functions. The dielectric characteristics were discussed and were compared with the literature, a good agreement being observed. The method allows to obtain for the first time the dielectric strength for a large range of the temperature and gives the two functions describing the relaxation processes in the isotropic range.     

Dielectric relaxation in ferroelectric TlInS2 layered crystals within metastable chaotic state

The results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS₂ crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS₂ is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Field and Frequency Effects in a Nematic Mixture of Negative and Positive Dielectric Anisotropy

The molecular alignment in the nematic liquid crystal mixture of p-ethoxy benzylidene p′-n-butyl aniline (EBBA) and p-ethoxy benzylidene p′-cyano aniline (EBCA) has been investigated by measuring the dielectric constant at 1 MHz. Detailed investigation has been carried out for a composition of 2.74%, of EBCA in EBBA for different electric and magnetic field strengths and at different frequencies of the applied electric field. Both parallel and perpendicular electric and magnetic field configurations have been employed. Temperature and frequency dependence of the dielectric anisotropy have been studied.

Two interesting features have been observed. I) In the crossed field configuration complete molecular alignment could be observed by suitably changing the electric field strength. The range of electric field strength required to change the molecular ordering, is frequency dependent. This range shows a minimum in the neighbourhood of 2 kHz. 2) It has also been observed that in the parallel field configuration minimum dielectric constant change is observed apparently independent of frequency and electric field strength. Detailed results of the field and frequency effects and of optical studies in this mixture are reported.     

Dielectric characteristics of Ce3+ doped Sr0.61Ba0.39Nb2O6 with Cole‐Cole plots technique

In this paper, we have investigated two‐relaxator mechanism and dielectric characteristics of Ce³⁺ doped Sr_0.61Ba_0.39Nb₂O₆ with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginer part of the complex dielectric susceptibility in vicinity of ferroelectric‐paraelectric phase transition has been studied in the frequency region 0.1 kHz–10 MHz. The measurements of the dielectric constant of the real and imaginer parts show strongly frequency dependence. The investigations of the dielectric constant revealed a non‐Debye type dielectric relaxation for Ce⁺³ doped SBN61 by using Cole‐Cole plots. It reveals the coexistence of the two dielectric relaxators in vicinity of the phase transition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Debye type dielectric relaxation in carbon nano-balls’ and 4-DMAABCA acid doped E7 coded nematic liquid crystal

The electrical properties of carbon nano-balls’ and 4-dimethylaminoazobenzene-2′-carboxylic acid doped dispersed nematic liquid crystal composite were investigated by impedance spectroscopy technique. The conductivity and capacitance were measured in the frequency range 100 kHz-1 MHz and temperature range 300-380 K. The loss peak was observed in the dielectric loss spectra and was identified as nearly-Debye type relaxation. Cole-Cole plots have been used to describe the characteristic changes of electrical properties in mentioned temperature interval. The sample presents monodispersive relaxation behavior with a relaxation time of ∼10⁻⁷ s. The relaxation process is attributed to the dipolar rotation of the long molecular axis and the activation energy is found to be 0.097 eV.     

Dielectric Study on the Alignment or Orientation of N(4-n-heptyloxy benzylidene)4´-n-butylaniline and the Anomalous Dielectric Behavior in the Smectic G Phase

The present article reports on the alignment or orientation of a particular compound N(4-n-heptyloxy benzylidene)4´-n-butylaniline through dielectric study. The effect of using two different alignment layers (for homogenous alignment) and doping silver nanoparticles (0.25% by weight) on the behavior in dielectric property with varying temperature and biasing electric field was studied in terms of liquid crystal alignment or orientation using the mechanism proposed by earlier workers. The study includes the anomalous dielectric behavior in SmG phase by presenting a technique for the analysis of the absorption process, which exists only in SmG phase at fixed frequency while varying temperature, is also presented.     

Relaxation and charge transport phenomena in polyaniline nanofibers: Swift heavy ion irradiation effects

Relaxation and charge transport mechanisms in hydrochloric acid (HCl) doped polyaniline (PAni) nanofibers synthesized using interfacial polymerization technique and the swift heavy ion irradiation effects have been investigated by dielectric relaxation spectroscopy. Non-Debye type relaxation with a distribution of relaxation times observed in PAni nanofibers is dominated by hopping of trapped charges. Swift heavy ion irradiation induces a benzenoid to quinoid transition in PAni nanofibers. Dynamic processes occurring at different frequencies in PAni nanofibers exhibit the same activation energy which reveals an enhanced coupling among the localized short range dipolar motions. The increase in conductivity relaxation time upon irradiation indicates an increase in the carrier hopping length thereby impeding charge transport. The ac conductivity of the pristine and irradiated PAni nanofibers has been found to obey the universal power law of frequency. Although charge transport in both the pristine and irradiated PAni nanofibers follows the correlated barrier hopping (CBH) model with polarons as major charge carriers, significant decrease in conductivity has been observed upon irradiation.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

Growth and high frequency dielectric study of pure and thiourea doped KDP crystals

Non linear optical (NLO) materials have acquired new significance with the advent of a large number of devices utilizing solid‐state laser sources. Several NLO materials have been used for this kind of technological applications. The Potassium di‐hydrogen phosphate (KDP) one of NLO material having superior non linear optical properties has been exploited for variety of applications. In the present investigation we have grown KDP crystals from aqous solution with thiourea, an organic non linear optical material. We could enhance the SHG efficiency of thiourea doped KDP crystal. It was 1.99 times more that of pure KDP. We observed more enhancements in nonlinearity for low concentration of thiourea.The crystal structure and cell parameters of grown crystal were determined from Powder XRD.The incorporation of thiourea in the grown crystals was qualitatively analyzed from FT‐IR study. The absorption spectra of pure and thiourea doped KDP crystal reveal that thiourea doped KDP crystals would be a better nonlinear optical (NLO) material for second harmonic generation (SHG) than pure KDP. The thermal decomposition and weight loss of pure and thiourea doped KDP crystal was observed by thermogravimetric (TGA) analysis and Differential Scanning Calorimetry (DSC). The high frequency dielectric study of pure KDP crystal, thiourea doped KDP crystals and organic additive thiourea was carried out using X‐band at frequency 8GHZ and 12GHZ by transmission line wave guide method. We observed low dielectric constant of thiourea doped KDP crystal when it is doped with 2mole% of thiourea. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

PHYSICAL PROPERTIES OF SOME NOVEL NEMATIC BIMESOGENS FOR THE FLEXOELECTRIC EFFECT

Abstract

In a chiral nematic liquid crystal, the flexoelectric effect consists of a fast and linear coupling with an applied electric field. One difficulty to overcome is the unwinding of the helix that occurs at higher fields due to dielectric coupling. The use of bimesogens, which possess very low molecular dielectric anisotropy can improve flexoelectric characteristics. New bimesogen compounds have recently been synthesised that exhibit switching angles of 45° for applied fields of about 9 V.μm^?1. In this paper, results from dielectric, electro-optic and dynamic light scattering measurements are reported for the new bimesogenic mixture. The dielectric anisotropy Δε changes sign with temperature and its values range between -0.2 and 0.3 for the temperature range studied. For Δε weakly positive, no electric field Freedericksz transition could be induced but Williams domains are observed instead. The large decrease in the bend elastic constant to viscosity coefficient ratio is attributed to a large increase in the bend viscosity coefficient.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

Growth,spectral and thermal properties of KAP single crystals in the presence of DL‐Alanine and L‐Methionine amino acid dopants

Single crystals of potassium acid phthalate (KAP) and amino acid (DL‐Alanine, L‐Methionine) doped KAP were grown from aqueous solutions by slow cooling method. The grown crystals were characterized using powder X Ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis. The thermal stability of KAP in the presence of dopants was analysed using Thermogravimetric and Differential Scanning caloriemetric (TGA/ DSC) studies and the maximum temperature for non linear optical application of this compound in the presence of dopants was found out. The transmittance of KAP was found to increase in the presence of dopants. Etch pits were observed for all the crystals using different etchants. Vickers microhardness tests were performed to study the mechanical stability of the crystals. The hardness of DL‐alanine doped KAP is more than that of L‐alanine doped KAP crystal. The dielectric constant and loss were determined as a function of temperature. Frequency response of the dielectric constant and dielectric loss factor have been studied over the frequency range of 50Hz – 5MHz. Second harmonic generation (SHG) was confirmed in all the crystals using the Kurtz and Perry powder technique. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

β Dielectric relaxation mode in side-chain liquid crystalline polymer film

In this paper, side-chain liquid crystalline polymer (SLCP) film was studied by dielectric relaxation spectroscopy (DRS). Dielectric properties were measured in the frequency range from 1 kHz to 10 MHz at some fixed temperatures. The experimental data were fit non-Debye type Cole-Cole dielectric relaxation function. A single dielectric relaxation mode was observed in polymer film. It is observed that relaxation characteristics show Arrhenius type behavior. Activation energy value obtained from the relaxation data was found to be 0.1 eV. The relaxation process is interpreted in terms of side-chain dipole group motion (β relaxation).     

Some special dielectric characteristics of TlGaSe2 layered crystals

TlGaSe₂ layered crystals were characterized with dielectric spectroscopy measurements. We have studied the memory effect within soliton regime, time relaxation in annealing temperature, and the effect of different cooling rate on conductivity and dielectric constant. The effects connected with the annealing time and cooling rate processes may be explained the theory of DDW. The measurements were carried out in a narrow temperature region (95‐135 K). The temperature dependence of dielectric constant measurements show that the crystal reveals a weak dielectric anomaly at 101 K. A well‐defined ferroelectric behaviour of first order transition was observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

添加LCB玻璃的CSLST微波介质陶瓷低温烧结及其性能研究

采用传统电子陶瓷工艺制备了添加Li2CO3-CuO-B2O3(LCB)玻璃为烧结助剂的(Ca0.9375 Sr0.0625)0.3(Li0.5Sm0.5)0.7TiO3 (CSLST)微波介质陶瓷,并对其烧结特性、晶相组成和介电性能进行了系统的研究.结果表明:通过液相烧结,LCB玻璃能有效降低CSLST烧结温度至900℃.XRD分析结果显示添加LCB玻璃后材料中均产生了杂相.性能分析结果发现随着LCB添加量的增大,陶瓷的体积密度、介电常数εr、品质因数与谐振频率乘积Q×f呈现先上升后下降的趋势,频率温度系数Tf则单调降低.添加质量分数为12.5;的LCB玻璃的CSLST陶瓷在900℃下保温5h可以完全烧结,并具有最佳的微波介电性能:εr=77.7,Q×f=1845 GHz,Tf=21.35 × 10-6/℃.     

Polystyrene Blend Alignment Layers for High Pretilt Angle Control

We prepared blend alignment layer based on 4-ethylphenoxymethyl-substituted polystyrene (P4EP) and polystyrene containing 7-hydroxycoumarin (P7COU) for liquid crystal (LC) alignment. Stable and uniform vertical LC cells having pretilt angles adjustable from 90° to 81.2° were obtained from these polystyrene films having molar contents of P4EP and P7COU in the ranges of 100–70 and 0–30 mol%, respectively. The LC alignment behavior was well correlated with the wettability of the polymer films. These polystyrene blends having good solubility in many organic solvents and their films having low process (curing) temperature can be candidates for LC alignment layer in the flexible display. This study contributes to the latest efforts to develop new alignment layers for pretilt angle control.     

Polarisation‐independent liquid crystal devices

Liquid crystal (LC) devices can be used as amplitude modulators and phase modulators. Most LC devices are polarisation‐dependent and require at least one polariser. As a result, the optical efficiency is greatly reduced. In this paper, we review some of our recently developed polarisation‐independent LC devices. For amplitude modulation, we report two polariser‐free devices which combine light scattering with dye absorption: dye‐doped polymer‐dispersed liquid crystal (PDLC) and dye‐doped LC gels. For phase modulation, we also present two examples: residual phase type, e.g., voltage‐biased PDLC, Polymer‐Stabilised Cholesteric Texture (PSCT), and homeotropic LC gels; and double‐layered structure, such as thin polymer film‐separated orthogonal LC layers, and double‐layered LC gels. Potential applications of these polarisation‐independent LC devices for displays, laser beam steering and adaptive optics are emphasised.