Millimeter-wave dielectric properties of single-crystal ferroelectric and dielectric materials

Transmittance measurements on various single crystal ferroelectric and dielectric materials, BaTiO(3), SrTiO(3), LiNbO(3), LiTaO(3), (PbMg(1/3)Nb(2/3)O(3))0.73-(PbTiO(3))0.27, LaAlO(3), and Bi(4)Ge(3)O(12), over a broad millimeter-wave (MMW) frequency range have been performed. Frequency dependence of the complex dielectric permittivity has been measured in the MMW region using high-power sources for the first time, using a free-space, quasi-optical MMW spectrometer equipped with high-power backward wave oscillators (BWOs) as sources of coherent radiation, tunable in the range from 30 to 120 and 180 to 260 GHz. These results are compared with MMW permittivity of these materials obtained by other methods as well as to RF, microwave, and optical frequency permittivities for all the materials tested. The effects of both crystallographic orientation and quality of the surface polishing of the crystals have been examined. Uncertainties and possible sources of instrumentation and measurement errors related to the freespace MMW technique are discussed. This work demonstrates that precise MMW permittivity data can be obtained even on relatively small and thin crystals of different surface conditions and orientations using the high-power BWO-based quasioptical approach.     

Effect of frequency on dielectric properties of liquid crystal doped with side-chain liquid crystalline polymer

The frequency and applied bias voltage dependence of the dielectric properties and dielectric anisotropy of liquid crystal (LC) doped with side-chain liquid crystalline polymer (SLCP) mixture have been investigated using the admittance spectroscopy method (C–V and G/ω–V) in the frequency range of 10 kHz to 10 MHz at room temperature. The liquid crystal used in this experiment is E63. The doping material used in this study is SLCP and its concentration is ensured 1 wt % in E63. Dielectric constant (?′), dielectric loss (?), dielectric loss tangent (tan δ) and real and imaginary parts of electrical modulus (M′ and M″) of the E63/SLCP mixture was also calculated. Moreover, dielectric anisotropy (Δ?) as a function of frequency was obtained. Results show that the values of the all dielectric parameters are strong functions of frequency and applied bias voltage. After a critical frequency, dielectric anisotropy has negative values according to p/n type changing.     

Relaxor ferroelectric thin-film heterostructures: Scaling of dielectric properties

An experimental study of the dielectric behavior of relaxor ferroelectric (RFE) thin-film capacitor heterostructures and a separate analysis of the dielectric properties of the RFE films are presented. Epitaxial heterostructures of RFE PbMg_1/3Nb_2/3O₃, PbSc_0.5Nb_0.5O₃, PbMg_1/3Nb_2/3O₃-PbTiO₃ and PbSc_0.5Nb_0.5O₃-PbTiO thin films (100–500 nm) with La_0.5Sr_0.5CoO₃ bottom and Pt top electrodes were fabricated by in situ pulsed laser deposition on MgO (1 0 0) and LaAlO₃(1 0 0). Dielectric properties of the heterostructures were studied as a function of frequency (10²–10⁶ Hz), temperature (77–725 K), and amplitude of applied a.c. field (10²–10⁶ V m^–1). The contribution of the film/electrode interfaces to the properties of the heterostructures was evaluated using a model of a series capacitor connection of the film and passive interface layers. The characteristics of the interface layers were experimentally found from the temperature evolution of the dielectric response of the heterostructures, and the true properties of the films were reconstructed. In the films, all typical features of RFE were found to be essentially similar to those in single crystals. Also, it was shown that in heterostructures, both a relaxor-like behavior and an apparent dielectric nonlinearity can be determined by the film/electrode interface rather than by the films.     

Second-harmonic generation in distributed feedback ferroelectric liquid crystal cells

The helicoidal structures in ferroelectric liquid crystals can be utilized to realize a special phase matching for the second-harmonic generation (SHG) when two counter-propagating fundamental waves are normally incident and the wavelength of the second-harmonic wave is near the selective reflection band edge. In this paper, we report the thickness and polarization dependences of the SHG thus observed. By comparing the experimental results with the theoretical ones developed, good agreement was confirmed.     

Optical memory effect in a deformed helix ferroelectric liquid crystal

Optical memory in a deformed-helix ferroelectric liquid crystal is proposed by deforming the helix under the application of a square-voltage pulse of known magnitude and frequency. This effect is based on the electromechanical effect of helix deformation due to the electric field. When the interaction between the electric field and the dipole is sufficiently strong, all of the dipoles align along the electric field. In such a situation the interlayer dipole-dipole interaction is strong enough to balance the elastic deformation energy. When the electric field is switched off, the molecules remain in a static, balanced state owing to the dipole-dipole interaction and hence the memory effect.     

Optical switching of a metal-clad waveguide with a ferroelectric liquid crystal

Optical switching based on waveguide optics with a ferroelectric liquid crystal (FLC) is reported. The FLC cell was prepared as a prism coupler on which the liquid-crystal layer was sandwiched between two gold cladding layers. The role of the gold layer was examined, and the optimum thickness of the top gold layer for obtaining high contrast was determined by use of the Fresnel equation. Various optical modulations of reflectivity were predicted on the basis of theoretical calculation, taking into account the molecular reorientation of the FLC, and examined at an appropriate angle of incidence and rotational angle of the FLC cell with respect to the plane of incidence.     

Characterization and studies on grafting of ferroelectric liquid crystal onto polybutylacrylate

Ferroelectric liquid crystalline (FLC) elastomer as a class of electroactive polymers, exhibiting high strain at very low voltage compared to other materials. This giant eletrostriction exploring new applications for ferroelectric materials and these materials are suitable candidates for designing nanoscale artificial muscles, sensors, actuators, pumps and motors. Due to these properties, considerable attention has been paid to these materials in recent years. In this study, ferroelectric liquid crystalline (FLC) monomer was synthesized in a mini reactor in a four‐step procedure. The synthesized materials were characterized with HNMR and FTIR in each step. Then the monomer in the presence of benzoylperoxide as an initiator and n‐butyl acetate as a solvent was grafted onto polybutylacrylate. Grafting was carried out in different processing conditions and products were characterized with HNMR and DSC. The level of initiator, method of initiator's injection to reactor, reaction time, and reactants concentration were part of parameters that were investigated. The analysis of results shows that the reaction time is an important parameter and appropriate grafting needs a long time. Components concentration illustrated important effect on the level of grafting and final structure of grafted chains.     

Sintering characteristics and dielectric properties of silver-doped PMN-PZN-PT relaxor ferroelectric ceramics

The effects of silver doping on PMN-PZN-PT relaxor ferroelectric ceramics were investigated in order to demonstrate the roles of the inner electrodes in the cofiring of MLCCs, because silver could diffuse from an electrode layer into a dielectric layer. Even if the addition of silver promoted the sintering of ceramics, the insulation resistance and the dielectric properties were changed to a certain extent so that they had a great influence on the reliability of MLCCs. Defect chemistry principles were used to analyze the mechanism of action of silver doping. The relationship of silver migration and microstructural formation was also discussed     

The threshold voltage,dielectric and conductivity properties of C60-doped smectic A liquid crystal

Abstract

Influence of fullerenes C₆₀ on the threshold voltage, dielectric and conductivity properties of smectic A liquid crystal 4-hexyloxyphenyl ether 4′-hexyloxy 3′-nitrobenzoic acid is investigated. It is shown that the transverse component of the real part of dielectric permittivity increases at the additive of fullerenes while the longitudinal component decreases. In this case, a maximum of dielectric absorption shifts to the high-frequency region. Conductivity increases at low frequencies and decreases at the high ones. Experimental results are explained by location of fullerenes between liquid crystal molecules reducing their interaction. As a result, the order parameter and viscosity of the matrix decrease.     

Investigation on optical,thermal, mechanical,dielectric and ferroelectric properties of non linear optical single crystal guanidinium manganese sulphate

Semi organic nonlinear optical crystal of guanidinium manganese sulphate hydrate (GuMnS) was grown from its aqueous solution by slow evaporation solution growth technique. Formation of the crystalline compound was confirmed by powder X-ray diffraction analysis (PXRD).The compound crystallizes in the triclinic crystal system with space group Pī. The functional groups present in the crystal have been identified by FTIR spectroscopic analysis. UV–Vis–NIR spectral study indicates that the grown crystal is transparent in the entire visible region with a lower cut off wavelength 229 nm and the band gap value is found to be 5.06 eV. Nonlinear refractive index (n₂), absorption coefficient (β) and third order nonlinear susceptibility (χ⁽³⁾) were determined using Z-scan technique. Thermo gravimetric and differential thermo gravimetric analyses (TG-DTG) were performed to analyse the thermal behaviour of the grown crystal. The dielectric constant and dielectric loss were studied as a function of frequency of the applied field. The ferroelectric nature of the grown crystal was analysed by P–E hysteresis loop. The mechanical behaviour of the grown crystal was studied by Vickers’s micro hardness test. The laser induced surface damage threshold (LDT) of the grown crystal was found to be 1.157 GW/cm². The obtained results show that guanidinium manganese sulphate crystals are potential materials in NLO device applications.     

铁电液晶空间光调制器响应特性的研究

以铁电液晶为非线性介质建立了光寻址空间光调制器的等效电路,并以写入光和擦除光为控制参变量,利用电路分析软件Pspice模拟了该光调制器的光电响应特性。结果表明,电路方法得到的上升时间在微秒(μs)量级上,擦除效应的临界值在毫瓦每平方米(mW／cm^2)量级上;光电响应速度随写入光强(擦除光强)的增大而加快(减慢),均与相关文献的实验结果吻合。     

液固混合介质的隔振性能分析

液固混合介质以一种全新的工作机理形成优良的隔振系统动力学特性.此介质由一类几乎不可压缩液体和许多可压缩的固体单元混合而成;当振动、冲击发生时,液体将动压力瞬间传递到所有单元体上,使它们同时参与变形,可有效隔离振动,冲击能量也可被大幅度吸收、损耗;若设计得当,这类隔振器可同时具有卓越的隔振、缓冲性能.为研究其隔振机理与隔振性能,以空心橡胶球作为固体单元体,分析了该单元体在有限变形情况下的变形规律以及隔振器的非线性刚度特性,建立了系统的非线性动力学方程;采用精细时程积分方法,计算了主共振的时域响应.并和试验结果进行了对比,两者有较好的一致性;还用此数值方法研究了系统主共振时的稳定性.在一定条件下.液固混合介质隔振系统主共振响应存在多解和跳跃现象;最后分析了液固混合介质隔振器的隔振性能,发现了一些有趣的新现象.     

High-temperature dielectric response of ferroelectric relaxors

It has long been considered that polar nanoregions in relaxors form at Burns temperature T(d) ≈ 600K. High-temperature dielectric investigations of Pb(Mg(1/3)Nb(2/3)) O(3) (PMN) single crystal, PMN-PbTiO(3) ceramics, and (Pb,La) (Zr,Ti)O(3) ceramics reveal, however, that dielectric dispersion, detected around 600K, is due to the Maxwell-Wagner-type contributions of surface layers. The intrinsic response was analyzed in terms of the universal scaling, taking into account the asymptotic and the correction-to-scaling behavior, and the results imply much higher T(d) or formation of polar nanoregions in a broad temperature range. High values of the dielectric constant indicate, however, that polar order already exists at the highest measured temperatures of 800K. The obtained critical exponents indicate critical behavior associated with universality classes typically found in spin glasses.     

Enhanced dielectric properties of ferroelectric polymer composites induced by metal-semiconductor Zn-ZnO core-shell structure

The effect of metal-semiconductor Zn-ZnO core-shell structure on dielectric properties of polyvinylidene fluoride (PVDF) composites was investigated. Zn-ZnO fillers were obtained by the heat-treatment of raw Zn particles under air. The enhanced dielectric constant of Zn-ZnO/PVDF composites results from the duplex interfacial polarizations induced by metal-semiconductor interface and semiconductor-insulator interface. The dielectric loss is still low because of the presence of ZnO semiconductor shell between Zn metal core and insulator PVDF matrix. Furthermore, the dielectric performance of as-prepared composites could be further optimized through adjusting the thickness of semiconductor shell.