Dielectric Relaxation Processes Research Articles

Structural elucidation, morphological properties, dielectric properties and their physical interpretation of cerium-substituted cobalt and barium-based M-type hexagonal nano ferrites

This research uses the sol–gel method to look into how adding Co2+ and Ce3+ dopant cations changes the structure, shape, and electrical properties of M-type Ba hexagonal ferrites that have been synthesized. x-ray diffraction (XRD) analysis confirms the successful formation of the targeted hexagonal M-type crystal structure. We observed a reduction in unit cell volume and lattice parameters as the dopant concentration increased, indicating the effective incorporation of dopant ions into the crystal lattice. When the doping process happened, needle-like grain shapes appeared, which could be seen with a field emission scanning electron microscope (FESEM). As the concentration of the dopant increased, the dielectric spectroscopic measurements revealed an increase in the loss tangent (tan δ) from 0.05 to 3.68, and a decrease in the dielectric constant (ε′) from 283 to 3.41. This suggests a reduction in polarization and dielectric permittivity, as well as increased energy dissipation within the material. The electric modulus spectra showed relaxation behaviour that was non-Debye-type, which is another sign that there are complex and multifaceted ways for charges to move. The measurements of relaxation time and AC conductivity showed that the relaxation intervals were not regular and that the conductivity dropped from 2.22*10–4 Ω−1m−1 to 1.4*10–7 Ω−1m−1 as the doping concentration increased. Based on these findings, it seems that the processes of conductivity and dielectric relaxation play a big role in how charges move around in the doped ferrites. We validated complex impedance data obtained through electrochemical impedance spectroscopy (EIS) software against calculated impedance values. The derived grain and grain boundary characteristics also agreed with the observed grain distribution and boundaries from the micrographs, further corroborating the analysis.

Investigation of the temperature dependence of the dielectric relaxation of chlorobenzene, bromobenzene and iodobenzene

This paper deals with the investigation results of temperature dependence of complex permittivity’sε∗ = έ − iε˝ of chloro, bromo and iodo- benzene at wavelength 11,50 cm and 12,80 cm within the temperature range 20 ÷ -80˚. It is established that dielectric properties of given liquids are characteristic of one relaxation time. The dielectric spectra were studied taking into account other frequencies given in the literature. In all investigated liquids, a jump appears in the real and imaginary parts of the complex permittivity at the point of the phase transition. The temperature dependence of the dielectric relaxation time of molecules in the liquid state has been determined. It was found that with an increase in the amount of the halogen substituent, an increase in the relaxation time occurs. Macroscopic and molecular relaxation times were calculated from the experimental values. The thermodynamic quantities characterizing the process of dielectric relaxation are calculated. The activation enthalpy of dielectric relaxation processes is compared. The studies were carried out using the dielectric spectroscopy method. This method allows a more detailed study of the dielectric properties of the objects of study due to the large equilibrium (“static”) dielectric constant of the object. It has been determined that the height of the potential barrier separating the two equilibrium positions of a polar molecule is greatest in the state of a pure polar liquid and decreases with dilution in a non-polar solvent.

Effect of lattice defects on crystal structure and microwave properties of Ba(Ga1/2Ta1/2)O3 doped Ba(Zn1/3Nb2/3)O3 ceramics

Traditionally, B-site cation ordering structure in Ba(B′1/3B″2/3)O3 based ceramics with complex perovskite structure is considered to be the dominant factor in determining the dielectric loss. The crystal structure of BZN-BGT ceramics changes to the B-site disordering from B-site 1:2 ordering accompanying with remarkably reduced dielectric loss. Ba(Ga1/2Ta1/2)O3 doping also inhibits the formation of Nb-rich secondary phases induced by Zn volatilization on the surface of BZN-BGT ceramics as well. The conductive activation energy Ea, an energy required for electrons to escape from the traps, increases to 0.55eV from 0.40eV, indicating the decrease of lattice defects concentration. As Ba(Ga1/2Ta1/2)O3 doping concentration increases, the dielectric thermal relaxation process of BZN-BGT ceramics weakens and the electrical conductivity decreases. These results indicate that the dielectric loss of BZN-BGT ceramics containing volatile Zn could be controlled by the lattice defects and secondary phases rather than B-site 1:2 ordering structure.

Exploring dielectric properties and polarization relaxation processes in ionic liquid crystals with synthesized carbon and gold nanoparticles

The main goal is to study the structural and dielectric characteristics of advanced composite materials based on ionic metal alkanoates with various types of nanoparticles. The matrix Cd+2(C7H15COO)−2 formed as a result of the reaction of cadmium with the carboxylate groups of alkanoate (C7H15COO)−2 is used for the synthesis of nanoparticles. At room temperature, the matrix of the ionic crystalline material represents multilayer structure with each layer thickness of 1.8 nm. However, when heated in the temperature range from 100 °C to 180 °C, the matrix undergoes a phase transition into the liquid crystalline smectic A phase, preserved after cooling to room temperature. This transition opens up possibilities for the synthesis of carbon and gold nanoparticles. The temperature treatments of the matrix allow to control of the size and shape of nanoparticles during synthesis and create highly stable nanocomposites. The structural features and dielectric properties of these nanocomposites are studied using transmission electron microscopy, enabling to determine of the sizes of nanoparticles and their dispersion and impedance spectroscopy, respectively. Dielectric properties of nanocomposite materials were studied at different temperatures corresponding to two material phases. We carried out an analysis of the dielectric characteristics of the pure matrix in comparison with nanocomposite materials, containing carbon and gold nanoparticles to identify the role of the nanoparticles. Based on our experimental data, we proposed a model to explain the charge transfer process, taking into account relaxation processes and polarization of nanocomposite materials.

Evidence of self-biased magnetoelectric coupling in eco-friendly (Na0.41K0.09Bi0.5TiO3-Ba0.85Ca0.15Zr0.1Ti0.9O3)-(CoFe2O4) particulate composites

The magnetoelectric (ME) properties of eco-friendly 1-x(0.94Na0.41K0.09Bi0.5TiO3-0.06Ba0.85Ca0.15Zr0.1Ti0.9O3)-x (CoFe2O4) (NKBCZT-CFO) with x = 0, 0.1, 0.2, 0.3 and 0.4 particulate composites were investigated. The X-ray diffraction studies provided evidence for the simultaneous existence of constituent phases (ferroelectric and ferromagnetic) in the composite. The structural and surface morphology analysis of the composite affirmed the homogeneous and densely packed distribution of CFO grains within the NKBCZT matrix. The composite's multiferroic behavior at room temperature was confirmed through the characterization of P vs E and M vs H hysteresis loops. The observed values of remnant polarization (Pr) and coercive field (Ec) are 26.7 μC/cm2 and 35.5 kV/cm, respectively, for 10 mol% of CFO. The increase in CFO content resulted in a decrease in the ferroelectric properties, possibly due to the conductive behaviour exhibited by CFO, and correlated with leakage current measurement. For the composite containing 40 mol% of CFO, the estimated values of remanent (Mr), saturation (Ms) magnetization, coercive field (Hc), and magnetic moment (µB) are 2.35 emu/g, 21.85 emu/g, 104.2 Oe, and 0.86 µB, respectively. The non-Debye type dielectric relaxation processes have been observed in the composites, and the electron hopping mechanism resulted in a higher dielectric constant (εʹ). Enhancement in relaxor nature had been observed through Vogel-Fulcher analysis with CFO content. The evidence of ME coupling was revealed through the direct ME voltage coefficient (αME) and magnetocapacitance (MC) measurement. The observed maximum value of MC measured at 1 kHz for the 20 mol% CFO is ∼5.6 %. Self-biased ME coupling (non-zero αME at HDC = 0 Oe) was observed in 20 and 30 mol% of CFO composite. The highest measured value of αME for 20 and 30 mol% CFO is 3.63 mV/cm.Oe and 4.09 mV/cm.Oe at HDC = 0 Oe respectively. The strong ME interaction and significant value of self-biased ME αME suggest that these composites can be explored for magnetic sensor and ME memory device applications.

A comprehensive study of dielectric, modulus, impedance, and conductivity of SrCeO3 synthesized by the combustion method

AbstractIn this work, perovskite oxide SrCeO3 was synthesized using the solution combustion method (SCM). Synthesized samples have been characterized using X‐ray diffraction (XRD), Raman, ultra violet (UV)–visible spectroscopy, and scanning electron microscopy (SEM) techniques. The XRD and Rietveld refinement has confirmed the orthorhombic structure with space group Pnma. The average grain size of this ceramic sample is 1.2 µm. The dielectric constant, dissipation factor, modulus, impedance, and alternating current (AC) and direct current (DC) conductivities were analyzed in the temperature range RT to 600°C and frequency range 20 Hz–2 MHz. Further, it was observed that the role of grain boundaries in dielectric and electrical properties is significant, at low temperatures. The origin of the dielectric relaxation and conduction process is the same. The activation energy for DC conduction (Edc) was found to be .79 eV. Electrical conduction/relaxation is assigned to the diffusion of doubly ionized oxygen (Vo··). The value of DC conductivity (1.2 × 10−3 Ω−1 cm−1 at 600°C) and activation energy of .79 eV is very close to the values reported for doped CeO2‐based oxide ion conductors for SOFCs. This study has demonstrated that SrCeO3 can be an alternative candidate for the oxide ion conductor electrolyte application and the potential to be used as a thermally stable capacitor application for microwave dielectric applications.

Synthesis of sodium silicate and its application of dielectric relaxation spectroscopy — A report

This study aims to explain the dielectric relaxation in sodium silicate (Na2SiO3) and its direct dependence on temperature across various frequencies. The study examines and elucidates the involvement of various types of polarization in the thermo-active dielectric relaxation process. Both low-frequency dielectric dispersion and complex impedance play pivotal roles in controlling the thermo-active dielectric relaxation process. Utilizing complex data plots, the study actively, and prominently illustrates the relaxation process.

The impact of architecture on the behavior of siloxane-grafted polymethacrylate

In this paper, we studied for the first time the impact of the molecular weight, Mn, and the topology on the molecular dynamics of a series of grafted poly(poly(dimethylsiloxane methacrylate)) (polyPDMSMA). The behavior of grafted (molecular brushes) polyPDMSMA and their three-armed counterparts of tailor-made properties prepared by Atom Transfer Radical Polymerization (ATRP) was monitored by means of broadband dielectric spectroscopy (BDS), differential scanning calorimetry (DSC) and comprehensive rheology analysis. Surprisingly, the dielectric loss spectra of all studied polyPDMSMA exhibit the presence of two dielectric relaxation processes above their glass transition temperature, Tg. A comparison of dielectric, calorimetric and mechanical data indicated that the faster and slower relaxation processes identified in loss spectra can be related to the segmental and chain dynamics, respectively. Interestingly, we observed that the “local” segmental dynamics of studied polyPDMSMA, involving the mobility of few repeatable units creating a segment, is almost unaffected by neither the length of polymer chain, number of grafts nor its topology. This is also reflected in a weak molecular weight-dependences of Tg of studied polyPDMSMA independently to their topology. On the other hand, the time-scale separation between segmental and the chain modes as well as the amplitude of the latter process increase with growing Mn of all examined polyPDMSMA. Additionally, it should be mentioned that the mechanical data collected for samples characterized by high molecular weight (of Mn > 280 kg/mol) reveal the emergence of additional process (most likely related to the grafts and/or the star arm relaxation) at the intermediate regime, independently to their topology. Interestingly, this mode was observed to be much slower for the star-shaped macromolecules when compared to the grafted polymers, despite their lower Mn. This implies that the observed differences are not a result of different molecular weights, but rather originate from a various molecular mechanism due to the different topology of the studied samples. In this context, one can assume that in the case of high-molecular-weight macromolecules, the backbone architecture has a significant influence on the behavior of polymers, including their molecular dynamics.

Low temperature dielectric study and photoconductive analysis of ZnO/NiO composite material

The sol-gel process was used to synthesize a ZnO–NiO composite material in this study. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the material, which confirmed the existence of both ZnO nanoparticles and NiO sheets in the composite. The composite material contained two distinct crystalline phases, the face-centered cubic phase of NiO and the hexagonal phase of ZnO. The composite's average crystallite size was determined to be 19.0 nm. The particle size of the composite material was calculated to be ∼121 nm. The band edge emission at 393 nm (3.15 eV) was measured using photoluminescence (PL) spectroscopy. The PL spectrum revealed other emissions as well. The dielectric properties of the material were studied at low temperatures in the frequency range of 20 Hz to 1 MHz, and it was found to have a low dielectric dissipation at high frequencies, making it suitable for use in high frequency microwave devices. Impedance data was fitted using Z-View software with a fitting error of <5%. Impedance spectroscopy revealed that the composite material had two dielectric relaxation processes related to the bulk and the grain boundary. Analysis of frequency dependent ac conductivity revealed Overlapping large-polaron hopping (OLPH) is responsible conduction mechanism in our case indicated by variation of temperature (200–280 K) dependent parameter “s” with temperature. Photoconductivity studies showed that the composite material had enhanced electrical conductivity and photoconductivity compared to pure ZnO and NiO. This makes it a promising candidate for use in optoelectronic applications such as solar cells, optical switches, and photodetectors. The responsivity of composite (0.86 mA/Watt) is found to be 10 times greater than that of the responsivity of NiO (0.085 mA/Watt).

Effect of the iron content on the thermal and dielectric relaxation process of sodium zinc-phosphate quaternary glassy systems

To explore the consequence of Na2O substitution by Fe2O3 on the physical, thermal and dielectric properties of zinc-phosphate-based glassy systems, a series of samples with chemical compositions of 25ZnO-40P2O5-(35-x)Na2O-xFe2O3 where, (x = 0.10, 0.15, 0.20, 0.25) have been prepared by melt quenching technique. The well-known Archimedes principle is deployed to measure the value of densities of the materials. DSC thermographs specify that the glass transition temperature (Tg) declines with the incorporation of Fe2O3 content. While thermal stability increases upon adding the Fe2O3 content. The dielectric constant (ε/) and dielectric loss (ε//) rise with an increase in temperature and fall as frequency rises sharply. The obtained relaxation time exhibits Arrhenius behaviour, and the dynamic conductivity relaxation process is not of the Debye type because the Kohlrausch stretching exponent (β) value is not equal to unity. According to the scaled electric modulus (complex) spectra, composition instead of temperature affects the dynamic conductivity relaxation mechanism.

Electric field modulation of ERK dynamics shows dependency on waveform and timing

Different exogenous electric fields (EF) can guide cell migration, disrupt proliferation, and program cell development. Studies have shown that many of these processes were initiated at the cell membrane, but the mechanism has been unclear, especially for conventionally non-excitable cells. In this study, we focus on the electrostatic aspects of EF coupling with the cell membrane by eliminating Faradaic processes using dielectric-coated microelectrodes. Our data unveil a distinctive biphasic response of the ERK signaling pathway of epithelial cells (MCF10A) to alternate current (AC) EF. The ERK signal exhibits both inhibition and activation phases, with the former triggered by a lower threshold of AC EF, featuring a swifter peaking time and briefer refractory periods than the later-occurring activation phase, induced at a higher threshold. Interestingly, the biphasic ERK responses are sensitive to the waveform and timing of EF stimulation pulses, depicting the characteristics of electrostatic and dissipative interactions. Blocker tests and correlated changes of active Ras on the cell membrane with ERK signals indicated that both EGFR and Ras were involved in the rich ERK dynamics induced by EF. We propose that the frequency-dependent dielectric relaxation process could be an important mechanism to couple EF energy to the cell membrane region and modulate membrane protein-initiated signaling pathways, which can be further explored to precisely control cell behavior and fate with high temporal and spatial resolution.

Investigations on the optical, dielectric, electrical, and rheological properties of PEG200/ZnO semiconducting nanofluids for soft matter based technological innovations

Semiconducting nanofluids (SNFs) consisting of poly (ethylene glycol) (PEG200) as a green biocompatible base fluid with homogeneous dispersion of eco-friendly zinc oxide (ZnO) nanomaterial, concentrations ranging from 0.01 to 0.20 wt%, are prepared by state-of-the-art ultrasonic cavitation homogenized process. These PEG200/ZnO SNFs are characterized by employing an ultraviolet-visible (UV-Vis) spectrophotometer, precision inductance-capacitance-resistance (LCR) meter, and rotational viscometer. A detailed analysis of 200–800 nm range UV–Vis absorbance spectra of these SNFs revealed longer stability of suspended ZnO nanoparticles in PEG200 fluid, and their nanomaterial concentrations dependent absorbance and extinction coefficient with tunable dual energy band gaps ranging from 3 to 5 eV. Optical performance recognized the potential applications of these SNFs in the advances of soft matter based optoelectronic and photosensitive devices and also as an excellent UV radiation shielding material. The study of 20 Hz – 1 MHz range dielectric and electrical spectra explained that the ZnO concentrations have a significant impact on the electrical conductivity of these SNFs, while the dielectric permittivity and electrode polarization relaxation process are marginally altered. High static dielectric permittivity (∼ 22) and low electrical conductivity of about μS/cm order, at 303.15 K, evidence the suitability of these SNFs in advances of energy storage devices. The rheological behaviour of PEG200/ZnO materials is carried out at different temperatures ranging from 303.15 to 323.15 K, which illustrates the Newtonian characteristics of these nanofluids with moderate dynamic viscosity and activation energy, and therefore, they could be utilized as effective heat transfer materials in photovoltaic/thermal devices and systems.

Solid-state impedance spectroscopy studies of dielectric properties and relaxation processes in Na2O-V2O5-Nb2O5-P2O5 glass system

Solid-state impedance spectroscopy studies of dielectric properties and relaxation processes in Na2O-V2O5-Nb2O5-P2O5 glass system

The Effect of Dielectric Relaxation Processes on the Complex Dielectric Permittivity of Soils at Frequencies From 10 kHz to 8 GHz—Part II: Broadband Analysis

The Effect of Dielectric Relaxation Processes on the Complex Dielectric Permittivity of Soils at Frequencies From 10 kHz to 8 GHz—Part II: Broadband Analysis

Dielectric relaxation in solutions chlorobenzene–benzene and chlorobenzene–hexane

This paper presents the results of a study of the dielectric constant of solutions of a polar liquid in a nonpolar solvent: chlorobenzene–benzene, chlorobenzene–hexane. The measurements were carried out at a wavelength [Formula: see text][Formula: see text]cm in the temperature range from [Formula: see text]C to [Formula: see text]C. The studies were carried out using the dielectric spectroscopy method. This method allows a more detailed study of the dielectric properties of the objects of study due to the large equilibrium (“static”) dielectric constant of the object. The temperature dependence of the dielectric relaxation time of molecules in the liquid and solid states of the studied solutions is determined. It has been established that with increasing concentration (0.300, 0.562, 0.794, 1.000 for a chlorobenzene–hexane solution and 0.179, 0.368, 0.567, 0.778, 1.000 for a chlorobenzene–benzene solution) of the halogen substituent, the relaxation time increases. The measurement results of dielectric constant [Formula: see text] and absorption coefficient [Formula: see text] obtained for concentrated solutions chlorobenzene–benzene, chlorobenzene–n–hexane at wavelengths [Formula: see text], 80 and [Formula: see text][Formula: see text]cm at temperature [Formula: see text]C are given in the paper. The static dielectric constant is obtained at a frequency of 1[Formula: see text]MHz. The obtained experimental values [Formula: see text], [Formula: see text] and [Formula: see text] of investigated systems in ([Formula: see text], [Formula: see text]) plane locate on the semi-circle the center of which is on [Formula: see text] axis. In this case, the high-frequency limit value of [Formula: see text] dielectric coefficient exceeds the corresponding n2 refraction index square. The macroscopic and molecular relaxation times are calculated on the base of experimental data. The thermodynamic quantities characterizing the process of dielectric relaxation are calculated for solutions of chlorobenzene–benzene, chlorobenzene–hexane. It has been determined that the height of the potential barrier separating the two equilibrium positions of a polar molecule is greatest in the state of a pure polar liquid and decreases with dilution in a nonpolar solvent.

The Origin of Collective Dielectric Relaxation Process in Ferroelectric and Antiferroelectric Phases of Liquid Crystals

Dielectric spectroscopy has proven as an important tool for the characterization of ferroelectric and antiferroelectric phases observed in the thermotropic chiral liquid crystals compounds. These compounds generally have paraelectric smectic A (SmA*) phase, ferroelectric smectic C (SmC*) phase and antiferroelectric smectic C (SmCA*) phase. In some compounds, ferroelectric (SmCg*) phase is also found. Dielectric spectra of these phases under planar alignment of the molecules are sometimes complex but rich in information and are very helpful in the determination of the structure of these phases. In the SmA* phase, a mode in high kHz region is observed. In the SmC* phase, a relaxation mode observed in low kHz region having high dielectric strength. However, in the SmCg* phase observed mode relaxation frequencies lies in same frequency window as SmC* phase but their dielectric strength was less than at least one order magnitude in comparison to observed mode of SmC* phase. In the SmCA* phase, dielectric spectrum contains two relaxation modes observed in kHz and MHz regions respectively. The dielectric strengths of these two modes are very small and less than at least one/two order magnitude in comparison to observed mode of SmA*/ SmC* phases. These observed relaxation processes are discussed as a function of temperature and bias electric field and their origin is also examined in the light of the experimental work. Dielectric parameters of the above phases observed in such material are discussed with regards to their structure property relationship.

Structural, electrical, and magnetic characteristics of eco-friendly Dy modified BiFeO3 ceramics

The study of basic crystal structure, dielectrics, magnetic and electrical characteristics of a high temperature sintered dysprosium (Dy) substituted bismuth ferrite (BFO) ceramic material, Bi(Fe0.90Dy0.10)O3 (abbreviated as BFDO10) has been reported in this work. The solid-state reaction (SSR) technique is adopted for synthesizing the material. Study of the X-ray diffraction (XRD) data shows an orthorhombic symmetry. The approximate crystallite size is found to be 29 nm. The field emission scanning electron micrograph (FESEM) shows a high density grains of the sample and energy dispersive X-ray micro-analysis (EDXMA) reveals the presence of its basic components. The extensive analysis of the dielectric and electrical properties of the sample was carried out within a frequency from 1 kHz to 1000 kHz and temperature ranges from 25 to 500 °C. A study of the impedance spectroscopy reveals a negative temperature coefficient of resistance (NTCR) behavior and a non-Debye type of dielectric relaxation process of the compound. The ac-conductivity (σac ) of the material follows Jonscher’s universal power law. A study of the electric field polarization confirms the enhancement in remnant polarization and coercive field. The magnetic property shows an enhancement in ferromagnetic orders by virtue of this doping. Moreover, several other interesting features which could be utilized for modern device applications also specified in this article.

Dielectric relaxation and conductivity phenomena in ferroelectric ceramics at high temperatures

The issues related to microstructure and its effects on functional properties of electroceramics become increasingly prominent with the progressive downscaling of electronics. In this study, temperature-dependent dielectric and impedance data of BaTiO3 (BT), Sr-doped BT (BST) and layer-structured BaBi4Ti4O15 (BBTO) ceramics are systematically investigated to reveal the effects of doping and perovskite layering on the dielectric relaxation and conduction processes at high temperatures. By using complex impedance spectroscopy, it is demonstrated that the ceramics are electrically heterogeneous structures displaying a non-ideal Debye-like behavior at high temperatures. Using a suitable equivalent circuit model, the impedance diagrams at different temperatures are simulated and the refined values of macroscopic resistance and capacitance are used for the identification and characterization of particular microstructural regions (grains, grain boundaries, interfaces, etc.). The approach of combining the impedance, electric modulus and permittivity formalisms is demonstrated to be effective in explaining the peculiarities of the electrical charge transport mechanism and relaxation in the grains, grain boundaries and interface layers of the ceramics.

Molecular Dynamics and Near-Tg Phenomena of Cyclic Thioethers.

This article presents the synthesis and molecular dynamics investigation of three novel cyclic thioethers: 2,3-(4'-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2-ene (compound 1), 2,3,14,15-bis(4',4″(5″)-methylbenzo)-1,4,7,10,13,16,19,22,25-octathiacyclotetracosa-2,14-diene (compound 2), and 2,3,8,9-bis(4',4″(5″)-methylbenzo)-1,4,7,10-tetrathiacyclododeca-2,8-diene (compound 3). The compounds exhibit relatively high glass transition temperatures (Tg), which range between 254 and 283 K. This characteristic positions them within the so-far limited category of crown-like glass-formers. We demonstrate that cyclic thioethers may span both the realms of ordinary and sizeable molecular glass-formers, each featuring distinct physical properties. Furthermore, we show that the Tg follows a sublinear power law as a function of the molar mass within this class of compounds. We also reveal multiple dielectric relaxation processes of the novel cyclic thioethers. Above the Tg, their dielectric loss spectra are dominated by a structural relaxation, which originates from the cooperative reorientation of entire molecules and exhibits an excess wing on its high-frequency slope. This feature has been attributed to the Johari-Goldstein (JG) process. Each investigated compound exhibits also at least one intramolecular secondary non-JG relaxation stemming from conformational changes. Their activation energies range from approximately 19 kJ/mol to roughly 40 kJ/mol. Finally, we analyze the high-pressure molecular dynamics of compound 1, revealing a pressure-induced increase in its Tg with a dTg/dp coefficient equal to 197 ± 8 K/GPa.

Smectic layers reorientation with stripes and heliconical structure in surface stabilised ferroelectric liquid crystal

ABSTRACT High-tilt angle ferroelectric liquid crystal material in surface stabilised geometry (SSFLC) is investigated for the smectic layer reorientation and its molecular dynamics by electro-optical and dielectric spectroscopy techniques. Smectic layer reorientation (5 o to 45 o ) is obtained by controlled cooling rate and applying electric field in the vicinity of transition temperature from chiral nematic ( N ∗ ) to chiral smectic C ( Sm C ∗ ) phase. After slow cooling down to room temperature, the stripe domain textures along the rubbing are observed with quasi-bistable states. Dielectric spectroscopy has revealed the unusual increment in the dielectric permittivity. A dielectric relaxation process at lower frequency than Goldstone mode with the increase in DC bias up to 2 V is observed in the sample cooled without any bias. This increment is found to be originated by the heliconical structure within the stripes which is seen in the optical textures recorded at the time of dielectric scan. However, this increment is not observed in the sample cooled under DC bias. Thus, there exist different phenomena in the high tilt angle SSFLC sample cooled in different conditions of DC bias; stripe domains, smectic layers reorientation, heliconical stricture and twisted structure within stripes which are significant to explore deeply in high-tilt angle ferroelectric liquid crystals.