Characteristics of electrical properties of wide temperature range TGB phases in liquid crystal dimers

4-n-decyloxy-4′-(cholesteryloxycarbonyl-1-butyloxy) chalcone and its two successive homologous are optically active dimeric compound derived from cholesterol. They possess wide temperature ranges of two twist grain boundary (TGB) phases namely TGBA and TGBC*. Comprehensive dielectric studies have been carried out for these compounds in the frequency range of 1 Hz to 10 MHz for different conditions of molecular anchoring. Various electrical parameter viz. dielectric permittivity, dielectric anisotropy, dc conductivity, and activation energy have been determined for these two TGB phases. Weak relaxation processes have been detected under planar anchoring of molecules in the TGBA and TGBC* phases presumably due to amplitude (soft mode) and phase (Goldstone mode) fluctuations.     

Thermodynamic and electrical characteristics of the twisted grain boundary phases of 4-n-dodecyloxy-4′-(cholesteryloxycarbonyl-1-butyloxy) chalcone

The investigated optically active dimeric compound, 4-n-dodecyloxy-4′-(cholesteryloxycarbonyl-1-butyloxy) chalcone, shows wide temperature ranges of two twisted grain boundary (TGB) phases, namely TGBA and TGBC*. The dielectric spectroscopy has been carried out for this compound in the frequency range of 1?Hz to 35?MHz for two different conditions of molecular anchoring. A relaxation mode has been detected in the TGBA and TGBC* phases for the planar anchoring of the molecules. This mode lies in the MHz region and has the characteristics of amplitude fluctuation of directors (soft mode). Results are supportive of the theory developed for the soft modes of TGBA and TGBC* phases. Dielectric anisotropy shows an unusual but characteristic (of TGB phases) variation with temperature.     

Electrical and electrochemical properties of nanocrystalline LiFePO4 cathode

Lithium iron phosphate (LiFePO₄) cathode material has been prepared by hydrothermal synthesis. The XRD spectrum exhibited different characteristic peaks along with (311) predominant orientation corresponding to orthorhombic crystal structure with Pnma space group. Electric and dielectric properties were studied over a frequency range of 1 Hz–1 MHz at different temperatures. The conductivity was found to be increased with increasing temperature following Arrhenius relation with an estimated activation energy of 0.44 eV. The dielectric properties were analyzed in the framework of complex dielectric permittivity and complex electric modulus formalisms. The complex permittivity as a function of frequency and temperature was investigated. Several important parameters, such as activation energy, ionic hopping frequency, carrier concentration, ionic mobility, and diffusion coefficient, etc., were determined. The electrochemical characteristics of LiFePO₄ are examined in aqueous region. It exhibited a good reversible cyclic voltammogram on sweeping the potential upward and downward with discharge capacity of 140 mAh/g.     

Effect of a magnetic field on the permittivity of 80%La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/20%GeO<Subscript>2</Subscript> composite

The dielectric properties of a magnetoresistive conducting two-phase 80%La_0.7Sr_0.3MnO₃/20%GeO₂ (wt %) composite have been studied near the percolation threshold in magnetic fields from 0 to 15 kOe at frequencies of the measurement field from 5 kHz to 1 MHz. The samples have inductive impedances; i.e., their permittivities can be considered negative due to a high conductivity in this frequency range. The permittivity increases in magnitude in magnetic field, and the values of the magnetodielectric coefficient reach 23% at room temperature. The reasons for the effect of magnetic field on the dielectric permittivity of samples are discussed.     

System of two dielectric cylinders involving charge sources: I. Calculation of the electric field

The electric field of two dielectric straight circular cylinders containing line charges on their axes is investigated. The cylindrical bodies in question are parallel and touch each other from the exterior. They have different dielectric permittivities, charges, and radii. Also, they are surrounded by an arbitrary dielectric medium. An analytic solution to the respective boundary-value problem is given, and the most important particular cases are analyzed. Special features of the electric field in the region around the contact of the dielectric cylinders are studied versus the permittivity of the material used and the relationship between electric charges.     

Specific features of the approximation of the dielectric spectra of alkylcyanobiphenyl liquid crystals

The frequency dependences of the longitudinal and transverse permittivities of oriented nematic liquid crystals belonging to the alkylcyanobiphenyl group nCB (n=5–8) are measured in the relaxation region in the meter and decimeter wavelength ranges. It is established that the dispersion of the longitudinal permittivity is well approximated by the sum of two Debye processes with different relaxation times. The frequency dependence of the transverse permittivity is represented by the dispersion relation with a continuous distribution of relaxation times in a specified time range. It is demonstrated that, in the high-frequency range (f>200 MHz), in which the dielectric spectra exhibit a number of weakly pronounced dispersion features, the total dispersion of the permittivity is adequately described by the sum of relaxation and resonance processes.     

Microwave dielectric dispersion in ferroelectric telluric acid ammonium phosphate

The paper presents the results of the investigation of dielectric dispersion in monoclinic telluric acid ammonium phosphate crystal over the frequency range 1 MHz to 77 GHz. It is shown that ferroelectric dispersion of the Debye type along the direction perpendicular to the plane (101) occurs in the frequency range 10⁸ to 10¹¹ Hz and is caused by a single relaxational soft mode. The relaxational soft mode is caused by the flipping motion of protons between two potential minima in a hydrogen bond. The frequency of the flipping proton mode in the paraelectric phase varies according to v_s = 0.94 (T – T_o ) GHz and decreases to 1.75 GHz at T_c = 317.3 K. The relaxational mode gives the main contribution to the high static dielectric permittivity of this crystal which fits the Curie-Weiss law. The results confirm the order-disorder nature of a proper ferroelectric phase transition of second order.     

Observation of dielectric relaxation modes in chiral smectic C liquid crystal mixture

The dielectric parameters in the frequency range 50 Hz to 1 MHz have been measured in a SmC* liquid crystal mixture in planar alignment. The relaxation frequency (f_r), distribution parameter (α) and dielectric strength (δε) have been evaluated and their corresponding values were found to be 206.62 Hz, 0.06 and 114.6 respectively. The Goldstone mode, domain mode, soft mode and a new relaxation mode have been observed.     

Effect of conductivity on the dielectric characteristics of cyano-ethyl ester of poly(vinyl alcohol)

Frequency dependences of the real and imaginary parts of the dielectric permittivity of films prepared from cyano-ethyl ester of poly(vinyl alcohol) have been studied within a frequency range of 25 Hz to 1 MHz at temperatures from 266 to 323 K. The frequency dependence of the total conductivity of the polymer dielectric has been calculated. At T < T _g , it has been established to follow a power law course with an exponent close to unity, which evidences hopping pattern of the conductivity. The anomalous increase of the dielectric losses and dielectric permittivity in the low-frequency range at T ≥ T _g is attributed to formation of double electric layers at the electrode-polymer boundary.     

Dielectric spectroscopy and viscosity studies of aqueous poly(ethylene oxide) and poly(vinyl pyrrolidone) blend

The complex dielectric function, electric modulus, impedance and ac electrical conductivity behaviour of aqueous solutions of 5 wt% poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP) and their different volume percent blends were investigated in the frequency range 20 Hz to 1 MHz at 15, 30 and 45 °C. It is found that the real part of dielectric function of these blends at 1 MHz decreases with the increase of PEO concentration and their dc electrical conductivity has strong correlation with the electrode polarization relaxation time. The static permittivity, ionic conductivity, electrode polarization relaxation time and apparent viscosity have linear behaviour with temperature variation at fixed volume concentration of the aqueous polymers blend. The viscosity of these aqueous polymeric blends increases with the increase of PEO concentration. The behaviour of hydrogen bond interactions between the polar segments of PEO and PVP were explored from the comparative change in dielectric parameters and viscosity of the two phase aqueous polymeric systems.     

基于介质谐振器原理的左手材料设计

本文通过对高介电常数介质基于介质谐振器理论进行分析,明确了利用高介电常数介质产生负介电常数或负磁导率的途径在于介质中产生具有Lorentz谐振形式的电磁响应的电偶极子或磁偶极子,指出了这种偶极子的产生来源于电磁波在介质中形成的驻波,而左手通带的形成正是由于电偶极子和磁偶极子之间的相互影响,破坏了驻波形成的条件所实现的. 模拟结果表明,通过将尺寸相同,介电常数不同的介质进行组合,使二者电谐振和磁谐振的频率点重合从而实现左手通带,最后利用高介电常数,低损耗的陶瓷进行样品制作并测试,测试结果证实了基于这一原理实 关键词： 全介质左手材料 介质谐振器 磁偶极子 电偶极子     

A study of the microwave dielectric permittivity of liquid crystals in electric and magnetic fields

A microwave detector based on a self-sustained oscillator circuit is proposed as a means to investigate the real and imaginary components of the dielectric permittivity of liquid crystals in external electric and magnetic fields. Results are given for measurements of a 500 MHz oscillator frequency for two types of nematic crystals, 5CBP and MBBA. Fundamental regularities are identified in the behavior of the microwave dielectric permittivity of samples in electric and magnetic fields. It is shown that the minimum of the high-frequency dielectric loss in liquid crystals correspond to a situation in which the long axes of the molecules are oriented parallel to the direction of the microwave electric field. Zh. Tekh. Fiz. 68, 117–121 (January 1998)     

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

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 f_NRM, δε^NRM and distribution parameter α_NRM 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.     

Low-frequency dielectric relaxation of Cs+-ions in single-crystals of mica-type layer-silicates

Measurements of the permittivity ?′ and of the dielectric loss ?″ have been made on oriented single-crystals of mica-type layer silicates (vermiculites) in the frequency range 30 Hz–10 MHz. A low-frequency region of dielectric absorption has been observed which has been assigned to the migration of Cs⁺-interlayer cations on free cation sites. In a close relation to this migration we have to assume the reorientation of large electric dipoles—originated in interstitial cation and negative layer charges. Applying the BAUER equation there results an activation enthalpy of diffusion across the lattice of 11.21 $\text{kcal}\text{mol}$. Estimates of an effective dipole moment and of the number of cations contributing to the process agree well with the structural and our experimental data.     

Dielectric relaxation and anisotropy of nematic mixture E-24

The dielectric anisotropy and dispersion of the real and imaginary part of permittivity of commercially important nematic mixture E-24 were investigated in the frequency range from 1 kHz to 10 MHz and temperature range 287-328 K. The measurements in nematic phase indicate Debye-type dispersion with relaxation time of 1.07 μs at 313 K and activation energy 41.01 kcal/mol. The results have been explained by assuming the molecular rotation about the long molecular axis under a hindering nematic potential. The dielectric anisotropy Δε is positive and the mean dielectric permittivity falls with rising temperature. Δε is also used to determine the order parameter for varying temperature.     

Twisted Grain Boundary Phases in the Binary Mixtures of 3ß-Chloro-5-Cholestene and 4-N-Decyloxybenzoic Acid

From the thermodynamical, optical texture and dielectric studies of the binary mixtures of 3β-chloro-5-cholestene (ChCl) and 4-n-decyloxybenzoic acid (DOBA), the phase diagram has been drawn. It has been observed that low concentrations of ChCl (1 to 7 mol%) in DOBA induce various types of twisted grain boundary (TGB) submesophases, whereas higher concentrations induce a smectic A (SmA) mesophase. Various optical textures of the TGB phases under different conditions of molecular anchoring have been observed. Weak transitions related with TGB phases have been detected from the temperature dependence of dielectric permittivity. The observed phase diagram of ChCl-DOBA binary system is in complete conformity with the theoretically predicted mean-field phase diagram derived by Renn within the framework of the chiral Chen-Lubenski model     

2维光子晶体谐振腔的分析与模拟

 为指导光子晶体谐振腔的设计,运用3维电磁场仿真软件HFSS模拟了2维光子晶体谐振腔,分析了影响2维光子晶体谐振腔的主要特性参数,主要包括所插介质杆的排列结构、介电常数及其半径和间距。研究表明,在其它条件保持不变时,若增大介质杆半径,则同一模式频率没有同一的变化规律;若增大介质杆介电常数,则出现的规则模式减少,并且没有基模出现,同一个模式,频率明显降低;若增大介质杆间距,则计算的频率间隔减小,对其它参数影响不大,只是同一模式的频率略有减小。     

Fluctuation-dissipation theorem in liquid and glassy glycerol: frequency-dependent dielectric permittivity and dielectric polarization fluctuation measurements

We show frequency-dependent dielectric permittivity and dielectric polarization fluctuation measurements of liquid and glassy glycerol. This allows a direct comparison of both quantities determined independently. After cooling the glycerol sample to 179 K with a cooling rate of 0.85 K/min we studied the aging time dependence of the dielectric permittivity and the polarization fluctuations using the identical glycerol sample. A cross-correlation technique allows measurements of noise levels below the amplifier noise. In the frequency range between 0.3 and 300 Hz we find the measured data to be in agreement with the fluctuation-dissipation theorem for the liquid and glassy state not depending on the aging time.     

Dielectric relaxation of <Emphasis Type="Italic">trans</Emphasis>-4-propyl-(4-cyanophenyl)-cyclohexane liquid crystals

The frequency dependences of the permittivity of a trans-4-propyl-(4-cyanophenyl)-cyclohexane (3PCH) liquid crystal in the nematic phase are measured in the frequency range 1–2000 MHz for different orientations of the director of liquid-crystal molecules with respect to the polarization of an ac electric field. The temperature dependences of the dielectric and optical characteristics are compared in the range of the transition from the nematic phase to the isotropic liquid phase. The dielectric spectra are approximated numerically, and the times of dipole relaxation upon rotation of the molecules about the short and long axes are determined. It is demonstrated that intramolecular motions make a significant contribution to the relaxation. The activation enthalpies are calculated.     

Infrared dispersion of lead phosphate at room temperature

Infrared reflection spectra [15–4000 cm^-1] along the b (binary axis) and c axes (A_u and B_u-type modes) are reported at room temperature as obtained with a Fourier-transform scanning interferometer. The spectra are analyzed with the factorized form of the dielectric function and the infrared-active mode parameters are reported.