This work presents a study on the electrical and structural properties of percolating composites based on graphite (Gt) particles dispersed with various concentrations into an insulating polyester matrix (PES). Their structural characterization was performed using small-angle neutron scattering (SANS), providing information about the dispersion of fillers within the matrix. Electrical measurements were carried out in the frequency range from 1 Hz to 10 MHz and temperature from 30°C to 100°C. It was found that when the filler concentration is above the percolation threshold and the temperature above the glass transition, the positive temperature coefficient of resistance is identified. The mechanism responsible for this behavior was attributed to the tunneling effect. The Nyquist representations of the complex impedance spectra were modeled using the Cole–Cole model. The obtained values of the α exponent that gauges the broadening of the loss spectrum suggest a behavior close to a model of a single relaxation time.

The aim of this work is to investigate the electric properties of carbon nanotube-reinforced epoxy polymer composites, using impedance spectroscopy, in the frequency range from 1 to 10 and over the temperature range from 25 to 105 degrees C. The dielectric response was analyzed using the complex permittivity and the electrical modulus formalisms, depending on temperature and filler concentration in the polymer matrix. Furthermore, an equivalent circuit model is proposed to describe the impedance response of carbon nanotubes/epoxy composites. The impedance studies disclosed the appearance of grain and grain-boundary effects, as confirmed by the Nyquist plot.

The electrical properties of epoxy polymer/carbon nanotubes composites were characterized using impedance spectroscopy in the frequency range between 1Hz and 10MHz and temperature range between 25 degrees C and 105 degrees C. We report the analysis of the experimental data using the electric modulus formalisms to understand the dielectric relaxation mechanisms. The variation of the real and imaginary parts of the electric modulus versus frequency and temperature were suggestive of two relaxation processes, associated with dipolar relaxation and CNT-polymer interfaces. The Havriliak-Negami model of dielectric relaxation was used for modelling the relaxation processes, extracting the relaxation parameters.

A binary mixture of bent-core and rod-shaped liquid crystals was chosen as a model substance combining the properties of both types of liquid crystals. The mixture was doped with a small amount of spherical and rod-like magnetic nanoparticles. Differential scanning calorimetry experiments were performed for the pure as well as for the doped mixture at different heating rates ranging from 1 to 16 degrees C/min. The addition of the magnetic nanoparticles lowered the phase transition temperature. This effect is more intensive in the case of the rod-like magnetic nanoparticles. The kinetics of the nematic to isotropic phase transition was evaluated in the framework of the differential isoconversional method. The calculated apparent activation energy showed non-monotonic behaviour and a sensitivity on the shape of added magnetic nanoparticles.

In this work the 4-n-hexyl-4'-cyanobiphenyl liquid crystal was doped with differently shaped magnetite nanoparticles. The structural changes were observed by capacitance measurements. Influence of the shape of magnetic particles on magnetic Freedericksz transition depends on the type of anchoring, which is characterized by the density of the anchoring energy and by the initial orientation between the liquid crystal molecules and the magnetic moment of the magnetic particles. It was observed that in the case of doping with spherical particles, the critical magnetic field is shifted to higher values with increase of volume concentration of the magnetic nanoparticles but decreases with increase of biasing voltage. In the case of doping with rod-like particles, the critical magnetic field is almost independent of the volume concentration of the magnetic nanoparticles.

This work is devoted to the study of composite systems of the liquid crystal 4-n-hexyl-4-cyanobiphenyl (6CB) doped with differently shaped magnetite nanoparticles. The ferronematic samples were prepared with the volume concentration of spherical, as well as of rod-like magnetic particles. The structural transitions in ferronematic samples were observed by capacitance measurements in a capacitor made of indium-tin-oxide-coated glass electrodes in combined electric and magnetic fields.

We have investigated the electrical properties of carbon-nanotubes-loaded DGEBA polymer composites in the frequency range between 1Hz and 10 MHz and temperature range between 25 degrees C and 105 degrees C. The frequency dependence of electrical data have been analyzed in two frameworks: the electrical modulus formalism with the Kohlrausch-Williams-Watts stretched exponential function (KWW) and the electrical conductivity by using the Jonscher's power law. The stretching exponent beta(kww) and the Jonscher exponent n are found to be temperature dependent for all carbon nanotubes concentrations and show a very slight variation with increasing the amount of filler percentage at room temperature.

Electric field-induced patterns in liquid crystals have been observed and studied for about 50 years. During this time, a great variety of structures, detected under different conditions, have been described; theoretical descriptions were also developed parallel with the experiments and a huge number of papers have been published. The non-vanishing interest in the topic is due to several factors. First, most experimentalists working with new (or even well-known) liquid crystals apply sooner or later an electric field for different purposes and, as a response, often (maybe undesirably or unexpectedly) have to face with emergence of patterns. Second, understanding the complexity of the formation mechanism of regular patterns in a viscous, anisotropic fluid is an extremely challenging theoretical task. Third, specialists in display fabrication or in other applications are also interested in the results; either to make use of them or in order to avoid field-induced patterns. In this review, we attempt to provide a systematic overview of the large amount of published results, focusing on recent achievements, about the three main types of electric field-induced patterns: transient patterns during the Freedericksz transition, flexoelectric domains and electroconvection. As a result of different instability mechanisms, a variety of pattern morphologies may arise. We address the physical background of the mechanisms, specify the conditions under which they may become effective, discuss the characteristics of the patterns, and summarize the possibilities of morphological transitions induced by frequency, voltage or temperature variations. Special emphasis is given to certain topics, which recently have gained enhanced interest from experimental as well as theoretical point of view, like driving with ultra-low frequencies or non-sinusoidal (superposed) waveforms, and the dynamics of defects and embedded colloidal particles. Assisting newcomers to the field, we also mention some, yet unresolved, problems, which may need further experimental and/or theoretical studies.

The main objective of this work was to study the electric response of the carbon nanotubes/epoxy composites upon varying the concentrations of the nanotubes, in order to obtain a comprehensive understanding of the influence of filler's percentage and temperature on the impedance parameters. Dielectric spectra of the carbon nanotubes-epoxy resin composites were recorded in the frequency range of 1 Hz-10 MHz and over the temperature range of 25 degrees C-105 degrees C. In a first part, the thermal properties of the composites were analyzed by differential scanning calorimetry, which reveals a decrease in the glass transition temperatures with the concentration of the nanotubes. The phenomenon of the positive temperature coefficient in resistivity that is recognizable for the carbon nanotube concentrations above the percolation threshold has been the interest of the second part of the work. The results reveal that gradually, as the filler concentration approaches the percolation threshold of the composite, the positive temperature coefficient in the resistivity effect becomes even more pronounced. Finally, the structure of the filled polymer samples is characterized using a small angle neutron scattering technique.

The primary research goal within contemporary liquid crystalline display industry is to discover new materials exhibiting phase temperature range and properties useful for display application. In this work, we studied five binary mixtures composed of different concentrations of bent-core and calamitic compounds. Pure compounds and selected mixtures were studied by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffractometry, as well as by semi-empirical quantum-chemical calculations. Phase transition types, temperatures, and enthalpies were determined. Molecular packaging in the nematic mesophase is proposed. All studied mixtures exhibit the nematic phase of wide temperature range near to the room temperature.

We report on precise impedance measurements, with the aim of exploring the influence of a dc bias voltage on the dielectric permittivity and electrical conductivity of liquid crystals and on their anisotropies. We prove that the dielectric permittivity is not affected by a dc bias; however, the electrical conductivity suffers a substantial reduction upon increasing the superposed do voltage. Moreover, we show that the relative conductivity anisotropy also diminishes at increasing do bias.

We have investigated the influence of doping with spherical magnetic nanoparticles on the mixture of a bent-core and a calamitic liquid crystal. Results showed a reduction of the critical field of the magnetic Freedericksz transition by more than a factor of two after the doping. Moreover, we give for the first time experimental evidence of the theoretically predicted magnetically induced negative shift of the isotropic to nematic phase transition temperature.

In the work phase transitions in bent-core liquid crystals were studied using differential scanning calorimetry. For the binary mixture of bent-core molecules with 50 wt% of rod-shaped compound, the nematic to smectic transition occured below 40 degrees C and the crystallization temperature shifted to sub-ambient temperatures. The influence of doping of the bent-core liquid crystals with magnetic nanoparticles on the kinetics of observed phase transitions was studied. The phase transition temperatures were shifted depending on the nanoparticle type and changed with varying cooling rate for all studied liquid crystal samples.

Liquid crystals, due to their large dielectric anisotropy, respond very sensitively to application of an external electric field, whereas they are only weakly sensitive to the magnetic field. A possible way of improving that sensitivity is doping liquid crystals with magnetic nanoparticles. As a result, stable colloidal suspensions of liquid crystals with relatively low concentrations of magnetic nanoparticles (called ferronematics, ferrocholesterics, ferrosmectics, etc.) can be produced. We illustrate some examples of the influence of the magnetic field, as well as of a superposition of magnetic and electric fields on the structural transitions (e.g. on the Freedericksz transition) in ferronematics based on the calamitic liquid crystal 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). It is shown that the samples respond to the applied magnetic field of low strength. The effects of the magnetic particles and magnetic field on the nematic to isotropic phase transition temperature are discussed as well.

Phase transitions of different binary mixtures of a bent-core (10DClPBBC) and a rod-shaped (6OO8) liquid crystal were studied using differential scanning calorimetry. For the binary mixture with 50:50 weight ratio of bent-core and rod-shaped molecules, the nematic to smectic transition occurred below the temperature of 40 degrees C and crystallization was shifted to sub-ambient temperature. It was found that crystallization was the phase transition with the lowest apparent activation energy.

In this work, five mixtures with different concentrations of banana-shaped and calamitic compounds have been prepared and subsequently studied by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction on non-oriented samples. The phase sequences and molecular parameters of the binary systems are presented.

Properties of magnetic nanoparticles significantly depend on their size, shape and structure. Doping liquid crystals with nanoparticles (NPs) in low volume concentrations has been shown to be a promising method to modify the properties of liquid crystals. The presence of nanoparticles in liquid crystal changes the existing properties or introduce some new features for the composite mixtures. Recently a mean-field theory has been developed to describe the influence of embedded nanoparticles on the orientation order and on the isotropic–nematic phase transition of the host liquid crystal [1]. It was shown that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the isotropic–nematic transition temperature. On the contrary, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. Here we report on the experimental justification of some predictions of the above theory. The nematic liquid crystal 4-(trans-4‘-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) was doped with spherical and rod-like magnetic nanoparticles. The phase transitions from the isotropic to the nematic phase were observed by polarizing microscope (see figure) as well as by capacitance measurements. We have found a significant influence of the magnetic particles shape as well as their volume concentration on the temperature of the isotropic-nematic phase transition. Moreover, the obtained results are in accordance with the theoretical expectations described in [1].

We investigated experimentally the magneto-optical and dielectric properties of magnetic-nanoparticle-doped nematic liquid crystals (ferronematics). Our studies focus on the effect of the very small orienting bias magnetic field Bbias, and that of the nematic director pretilt at the boundary surfaces in our systems sensitive to low magnetic fields. Based on the results we assert that Bbias is not necessarily required for a detectable response to low magnetic fields, and that the initial pretilt, as well as the aggregation of the nanoparticles play an important (though not yet explored enough) role.

The behaviour of an electric field induced pattern forming instability, the electroconvection in a nematic liquid crystal, was studied under the influence of superposed dc and ac electric voltages. The onset parameters (threshold voltages and critical wave numbers) were determined. It was found that the superposition of voltages inhibits the pattern forming mechanism; therefore the patternless region extends to much higher voltages than the individual ac or dc thresholds. A dc bias induced reduction of the electrical conductivity and a shift of the crossover frequency from the conductive to dielectric electroconvection regimes were also detected. Nematic liquid crystals are anisotropic fluids with an orientational order characterized by the preferred direction of their molecules, the director n(r), which can be (re)oriented by an electric field E [1]. In most experiments and applications thin (5-20 μm) liquid crystal films are sandwiched between transparent electrodes and a uniform quiescent state is ensured by proper surface aligning techniques. Upon applying an electric voltage V exceeding some critical value Vc, director distortions may occur [1]. Display applications require this distortion be uniform in the cell plane; however, under certain conditions complex spatio-temporal structures, patterns, can also be induced. Here we will deal exclusively with a particular dissipative pattern forming phenomenon, the standard electroconvection (EC) [2]. The resulting patterns correspond to a spatially periodic system of convection rolls of a wave vector q, which appear as dark and bright stripes in a polarizing microscope. Standard EC is mostly observed in planar nematics having a negative dielectric anisotropy εa < 0 and a positive electrical conductivity anisotropy σa > 0. EC can be induced by dc (Vdc) as well as by ac (Vac) voltages. In the latter case Vac corresponds to the rms value of the driving sinusoidal voltage of frequency f . Its driving feedback mechanism was invented by Carr and Helfrich [2]: a spatial director fluctuation n(r) leads to space charge separation e(r) due to σa; the Coulomb force induces a flow v(r) forming vortices due to the constraining

The influence of the shape anisotropy of magnetic particles on the isotropic-nematic phase transition was studied in ferronematics based on the nematic liquid crystal 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The liquid crystal was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarizing microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic-nematic phase transition in liquid crystal is demonstrated. The results are in a good agreement with recent theoretical predictions.