Electromechanical effect in surface stabilized and unwound SC* liquid crystals

Applying an A.C. electric field to a planar oriented S_C* liquid crystal sample, a shear flow occurs parallel to the bounding plates and perpendicular to the helical axis. We have investigated this so-called electromechanical effect by applying different D.C. bias electric fields superposed on a constant A.C. field. The amplitude of the mechanical vibration was measured versus the bias field in different samples. From the results, structural models of the surface stabilized and unwound samples were tested.     

Experimental study of a SmA*-SmC*A phase transition

We have performed a detailed experimental study of the electroclinic behaviour in the SmA* phase above a SmA*-SmC*_A phase transition. The tilt angle and polarization were measured as a function of the applied a.c. electric field, and the dielectric constant was obtained under different values of the bias field (0≤E _d.c.≤ 3 V μm^-1). In the region of linear regime, the behaviour observed for this SmA*-SmC*_A phase transition is very similar to the one previously described for SmA*-SmC* phase transitions. The experimental results obtained under high bias field are in good agreement with the predictions of the simple theoretical model considered.     

Measuring internal electric fields in organic light-emitting diodes using electroabsorption spectroscopy

A widely applicable electroabsorption technique to measure internal electric fields in organic light-emitting diodes is presented. The technique exploits the change in the a.c. electroabsorption response in the presence of a d.c. electric field. The electroabsorption signal is modulated at the fundamental frequency of the a.c. test signal, in addition to the usual modulation at the second harmonic frequency, when a d.c. bias is present. In metal/organic film/metal devices employing different metal contacts there is a built-in electric field in the organic film caused by the difference in work function between the two contacts. The electroabsorption response at the fundamental frequency of the applied a.c. bias is measured as a function of an external d.c. bias. The electroabsorption signal is nulled when the applied d.c. bias cancels the built-in electric field established by the different metals. We apply this technique to measure changes in metal–polymer Schottky barrier heights as a function of the contact metal. In metal/multiple organic films/metal structures the electroabsorption signals from the constituent organic films are identified spectroscopically and measured at both the fundamental and second harmonic frequency of the a.c. test signal. The amplitudes of the electroabsorption responses are then used to determine the a.c. and d.c. electric fields present in the organic layers. We apply this technique to determine the d.c. electric field distribution within a multi-layer organic light-emitting diode. These results highlight the general applicability of electroabsorption methods to probe internal electric fields in organic light-emitting diodes. © 1997 John Wiley & Sons, Ltd.     

Dielectric properties of four room temperature ferroelectric and antiferroelectric multi-component liquid crystalline mixtures

Dielectric properties of four recently formulated room temperature multi-component liquid crystalline mixtures with paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC*_A) phases have been studied as a function of temperature and frequency. Under planer anchoring condition, dielectric spectroscopy revealed all the characteristic modes: low frequency P_L and high frequency P_H mode in SmC*_A phase, Goldstone mode (GM) in SmC* phase and soft mode (SM) in SmA* phase. Dielectric behaviour has also been studied under the application of DC bias electric field. With bias electric field, we have been able to study the soft mode dielectric behaviour in the SmC* phase. An unknown high frequency mode (X-mode) with and without bias is also observed in SmC* phase. Dielectric results are explained in the light of generalised Landau theory. The mixtures show very high soft mode electroclinic coefficient in the SmA* phase in addition to fast switching in SmC*_A and SmC* phases [30].     

The buildup of the electric field during positron annihilation lifetime spectroscopy measurement

Positron annihilation lifetime spectra were measured on pure polystyrene (PS) and PS samples whose film surfaces were coated with gold, graphite (GR‐PS), or M_oS₂. The results showed the longest lifetime remained constant with measured time in all experimental samples, whereas the corresponding intensity decreased with time at different rates. The experimental phenomena were associated with the buildup of an electric field inside the polymer during extended positron annihilation lifetime spectroscopy (PALS) measurement. The decrease in the rate was attributed to the presence of conductive film causing the neutrality between positive charges and negative charges, thereby reducing the buildup of the electric field. Additionally, we also performed PALS measurement on GR‐PS under different experimental conditions, such as the conductive film being grounded or not grounded or the presence of an external electric field. These results further indicated that the buildup of the electric field was responsible for the decrease in the intensity with time. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 332–336, 2001     

Spectroscopic studies on the effect of field strength upon the curie transition of a VDF/TrFE copolymer

Infrared spectroscopic studies of the effects of field strength upon the ferroelectric phase transition behavior of a VDF/TrFE (75/25) copolymer upon heating and cooling in an electric field have revealed new findings. The paraelectric phase in the absence of an electric field resembles the α phase of PVDF with a trace of short trans sequences distributed randomly along the chain axis. The paraelectric phase in a high electric field is very different from that in the absence of an electric field. The paraelectric phase under an electric field has much longer trans sequences. The Curie transition temperature upon heating is a first-order transition temperature (T^↑_c) and is dramatically elevated from 120 to 135°C under the field of more than 0.4 MV/cm. Upon cooling, the paraelectric phase in an electric field does not show a clear transition. The field-induced phase transition and the loss of dipole switchability observed below a cooling temperature of 120°C, and their dependence on time and filed strength when exposed to a cyclic bipolar electric field are discussed. © 1993 John Wiley & Sons, Inc.     

Analysis of the hysteresis curve of ferroelectric polymers by a phenomenological relaxation theory

A simple relaxation theory for the displacement versus electric field hysteresis of ferroelectric polymers is developed in which the relaxation time is assumed to be a function of electric field, as has been experimentally evidenced by polarization-reversal switching. The theory gives an analytical expression for the hysteresis curve. The coercive field E_c predicted by the theory agrees well with data on E_c as a function of temperature for poly(vinylidene fluoride) from ?60 to 20°C and with data on E_c as a function of frequency for vinylidene fluoride/trifluoroethylene copolymer (73/27 molar ratio) at 20°C over the range 0.01–0.7 Hz.     

Electroconformational coupling (ECC): An electric field induced enzyme oscillation for cellular energy and signal transductions

Previous work has shown that membrane ATPases can extract free energy from applied oscillating electric fields for doing chemical work, e.g. to synthesize ATP from ADP and P_i or to transport Rb and Na ions against their respective electrochemical gradient. Data of these experiments are briefly reviewed. Electroconformational Coupling (ECC) is used to interpret these results. Computer analysis of a four state cyclic enzyme mechanism reproduces many experimental features. It is shown that a coulombic interaction between an enzyme and an alternating electric field (ac) can cause the enzyme to oscillate between different conformational states. If the frequency of the applied field matches the kinetic characteristics of the system and the amplitude matches the energy level required for inducing productive catalytic cycling, a phenomenological resonance between catalytic reaction and the periodic field is generated. A condition necessary for achieving energy coupling is the kinetic bias arising from the binding energy of the ligand. Analysis indicates that only dynamic electric fields, i.e. oscillating or fluctuating fields, can propel the cyclic reaction of the enzyme catalysis, and thus be effective for transducing energy. A stationary transmembrane electric field must be modulated, e.g. by opening and closing of an ion channel, to become oscillatory in order o produce the same effect. We propose that ECC is a fundamental process of cellular energy and signal transductions. Here, many membrane associated events are reduced to Michaelis-Menten types of enzyme catalytic reactions and they are thus amenable to the quantitative analysis of chemical kinetics.     

Electromechanical Responses of a Crosslinked Polydimethylsiloxane

A high molecular weight polydimethylsiloxane, PDMS, gel was prepared and investigated as an electroactive polymer actuator. Electromechanical properties of the PDMS gels were measured under an oscillatory shear mode at the temperature of 27 °C to determine the effects of crosslink ratio and electric field strength. The storage modulus, G′, of PDMS gel increases linearly with crosslink density but nonlinearly with electric field. The increase in the storage modulus with crosslink density is due to the increase in the number of junction points and strands. With increasing electric field strength, the storage modulus increases as the electric field induces dipole moments generating the electrostatic forces within the matrices. The gel with the crosslink ratio of 0.01 possesses the highest G′ sensitivity of 41% at 2 kV/mm. The temporal response of PDMS gels upon repeated applications of electric field strength of 2 kV/mm was investigated. For the crosslink PDMS (N_c/N_m = 0.01) system, at the electric field of 2 kV/mm, G′ immediately increases and rapidly reaches a steady-state value. With electric field off, G′ decreases and nearly recovers its original value. The crosslinked PDMS (N_c/N_m = 0.01) is nearly a reversible system. Finally, we investigated the bending response of the PDMS films, suspended in silicone oil between copper electrodes. From the deformation data, we estimated the dielectrophoresis force, F_D, to be a linear function of electric field strength.     

High field electrostrictive response of polymers

It is well known that electrostrictive strains are proportional to the square of the applied electric field. It therefore appeared reasonable to assume that for some polymeric materials, a large acoustic thickness response, d_T, \[ \left( {d_T = \frac{{dS}}{{dE}}} \right) \] might be obtained by application of high dc bias fields, ～ 20 MV/m, to a film while driving the film with an ac signal to access the high slope region of the electrostrictive strain vs. applied field curve. Previous studies of crystallizing poly(vinylidene fluoride) (PVF₂) from solution under high electric fields have demonstrated that gel-like samples of PVF₂ with high content of the plasticizer tricresyl phosphate (TCP) could be subjected to electric fields as high as ～ 100 MV/m. Using this type of heavily plasticized PVF₂ d_T values ～ 4 Å/V were obtained. Values of 9 Å/V were obtained for a certain class of thermoplastic elastomer (i.e., a polyurethane). These d_T values are considerably greater than those obtained from conventional piezoelectric ceramic materials. In addition, large elastic strains (> 3%) were observed as a function of applied dc field. © 1994 John Wiley & Sons, Inc.     

Spin-Crossover Assisted Spin-Switching and Rectification Action in Half-Metallic Graphitic Carbon Nitride(g-C4N3)

Herein, we report on the potential multifunctional spintronic action of half-metallic graphitic carbon nitride (g-C₄N₃). We observed electrostatic spin-crossover action at an applied electric field of −0.77 V nm⁻¹, which eventually leads to spin-switching action and change in sign of bias dependent spin injection coefficient. The system also acts as a spin polarized charge current rectifier with rectification ratio of 65.41 in spin-up channel only. This electric field-controlled spin switching action and simultaneous existence of rectification action makes graphitic carbon nitride a perfect multifunctional spintronic system-an ideal material for quantum logic gate design. Results obtained have been substantiated through transmission spectra and transmission pathways analyses. An analysis of projected device density of states of the system and molecular projected self consistent Hamiltonian states analysis reveals that the electron flow of the system is mainly facilitated by 2p orbitals of C and N atoms.     

Phase modulation and ellipticity of the light transmitted through a smectic C* layer with short helix pitch

A chiral ferroelectric smectic C* liquid crystal (FLC) with the helix pitch p ₀?=?330 nm was developed to avoid any scattering of visible light when the helix is not unwound over a certain limit. Planar cells with different FLC layer thickness (16 and 44 μm) have been assembled with helix axis parallel to the glass plates and aligned along the rubbing direction. The ellipticity of the light passing through the cells vs. the electric field was investigated, and a method for evaluating the electrically controlled birefringence via ellipticity measurements has been established. We have found that the FLC cell is an optical retardation layer driven by the electric field, the effective birefringence being proportional to the square electric field. The physical origin of the electrically controlled phase shift of the light passing through the FLC layer has been analysed.     

The Goldstone mode in mixtures containing ferroelectric liquid crystals

Abstract

Measurements of complex electric permittivity of room temperature ferroelectric liquid crystal mixtures have been made on aligned samples with the electric measuring field being parallel to the layer planes. The spontaneous polarization, the tilt angles and pitch have been measured in these mixtures. By theoretical fitting of the experimental points of electric permittivity for the Cole–Cole modification of the Debye equation dielectric parameters, the dielectric strength, relaxation frequency, and distribution parameter for the Goldstone mode have been computed. The dielectrically observed Goldstone mode in our mixtures is shown to have both DC bias field and AC field dependences.     

ROLE OF ELECTRIC POLARIZATION IN THE THERMOLUMINESCENCE OF CHLOROPLASTS

Abstract Effect of an external electric field on thermoluminescence and thermodepolarization currents were studied in suspensions and dry films of pea chloroplasts. The external electric field was applied either during interrupted heating at a given temperature ("direct effect") or was "stored" in the sample during cooling in the range –25°C and –70°C ("electret effect").
It is shown that in chloroplast suspension both the luminescence burst upon "direct effect" and the new band peaking between –40°C and –50°C induced by the "electret effect" are accompanied by a preferential decrease in the intensity of the thermoluminescence band around +10°C.
A correlation was established between the polarized state of thylakoids and the thermoluminescence burst induced by the external field. In electret samples of chloroplast suspension the intensity of field induced low temperature thermoluminescence band under different experimental conditions varied in parallel with the intensity of thermodepolarization current. In dry films of chloroplasts, in addition to parallel changes in intensities of thermoluminescence and thermodepolarization current, also the peak positions shifted in the same manner when either humidity of samples or temperature of illumination and of application of external field were varied.
We conclude that the polarization state of thylakoid membrane, which can be governed by an external electric field, plays an important role in determining charge stabilization and recombination properties of photosynthetic units. Our results implicate that electric field induces conformational changes in the membranes which increases the frequency factor ascribed to the recombination processes.     

Magnetoelectric interactions in polycrystalline multiferroic antiferromagnets CuFe1−xRhxO2 (x = 0.00 and x = 0.05)

Magnetoelectric coupling in the polycrystalline antiferromagnets CuFe_0.95Rh_0.05O₂ and CuFeO₂ has been investigated. For both samples, electric polarization was observed in the absence of an applied external magnetic field demonstrating that for multiferroic research ceramics are worth to be studied. The observed magnetodielectric effect for CuFe_0.95Rh_0.05O₂ in the electrically polar phase supports the existence of a noncollinear antiferromagnetic state.Interestingly, the electric polarization of this sample can be suppressed by a magnetic field. The temperature dependence of the relative magnitude of the magnetodielectric effect shows a discontinuity, clearly indicating different mechanisms of the magnetodielectric couplings in polar and paraelectric antiferromagnetic states.     

A determination of the effective temperatures for the dissociation of the proton bound dimer of dimethyl methylphosphonate in a planar differential mobility spectrometer

The dissociation of the proton bound dimer of dimethyl methylphosphonate to the protonated and neutral molecule has been studied in a planar differential mobility spectrometer. The internal energy of the ions is the sum of the thermal component and the electric field component and results in an effective temperature, T_eff, that is significantly greater than that of the ambient atmosphere temperature, T. The measured rate constant for dissociation, which rises exponentially with field strength, together with activation energy and pre-exponential factor previously determined under thermal conditions, allow the calculation of T_eff as a function of electric field strength. T_eff is a linear function of electric field intensity at constant T over the range of T from 60 °C to 140 °C. The efficiency of the available collision energy in causing dissociation decreases with increasing T, from 52% at 60 °C to 40% at 140 °C.     

直流电场诱导下镓铟基氟磷玻璃的微晶化研究

采用高温熔融法制备了20GaF3-15InF3-17CdF2-15ZnF2-20PbF2-10SnF2-3P2O5体系透明玻璃,并利用电场的诱导作用,对其微晶化进行了系统的研究。利用DSC、SEM、XRD等手段对材料进行了表征。结果表明,在热处理过程中加载一个静电场,能促进ZnF2晶粒的形成,其尺寸约为20～30 nm。而且发现,在正负电极至少有一个没有接触材料的情况下,电场也能在远低于析晶起始温度的恒温环境中起到诱导析晶作用。     

Dielectric and impedance properties of Sr(Sm0.5Nb0.5)O3 ceramics

Perovskite types Sr(Sm_0.5Nb_0.5)O₃, (SSN) ceramics have been prepared through solid state reaction route. The scanning electron microscopy provides information on the quality of the samples and uniform grain distribution over the surface of the samples. The field dependence of the dielectric response was measured in a frequency range from 50 Hz to 1 MHz and in a temperature range from 60 °C to 420 °C indicates polydispersive nature of the materials. An analysis of the dielectric constant (?′) and tangent loss (tanδ) with frequency is performed assuming a distribution of relaxation times as confirmed by the scaling behavior of electric modulus spectra. The frequency dependence of the electric modulus peak is found to obey Arrhenius law with activation energy of ∼0.026 eV. The complex plane impedance plot shows the grain boundary contribution for higher value of dielectric constant in the law frequency region. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures in SSN.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

Electric conductivity of active carbons from phenol-formaldehyde oligomers and its relationship with the sorption of noble metals

The electric resistance of samples of active carbons, obtained from phenolformaldehyde oligomers of different nature and activated at different conditions, has been investigated. A clear relationship has been found to exist between the specific electric resistance of the carbons and the temperature of their thermal treatment. The structure of the polymer, from which the carbon had been obtained, as well as the nature of the activator affect its electric conductivity. A relationship has been established between the sorption characteristics of carbon sorbents towards substances, absorbed by the carbon, based on a donor-acceptor mechanism, and the electric conductivity of these samples. A symbatic relationship exists between the sorption of [Au(CN)₂]^– and the electric resistance.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 4, pp. 508–511, July–August, 1989.