外加电压频率对液晶介电各向异性的影响

液晶材料的介电各向异性通常与频率有关。为进一步研究频率对液晶材料介电常数的影响,首先,使用紫外可见分光光度计(METASH UV-9000S)和表面轮廓仪(Contor GK-T)分别测量液晶盒厚度以及聚酰亚胺(PI)取向层厚度,通过精密热台(LTS 350)控制实验温度20℃,使用精密LCR表(Agilent E4980A)测定4种不同液晶材料在100～2 000Hz的频率内的平行和垂直排列向列相液晶盒电容;然后,利用液晶盒电容模型计算出不同频率下液晶的平行和垂直介电常数,并绘制频率-介电各向异性曲线;最后,分析频率对液晶介各向异性的影响。实验结果表明:温度一定,正性液晶的介电各向异性随频率的升高而减小,然后逐渐趋于平缓,负性液晶的介电各向异性随频率变化基本保持不变。此项研究对进一步分析液晶材料的介电特性具有一定的指导意义。     

侧位含氟苯乙炔类液晶化合物的微波介电性能

近年来基于液晶材料的微波通信器件研究发展迅速,液晶材料的介电损耗成为制约微波器件发展的瓶颈,然而目前对微波用液晶材料性能报道较少。本文以低熔点高双折射侧位含氟苯乙炔类液晶作为研究对象,将其按一定比例掺杂到母体液晶MA中,采用矩形谐振腔微扰法测试所选液晶化合物在微波频段(10～30GHz)下的介电性能,探讨分子结构对微波频段液晶介电性能的影响作用。实验结果表明:在高频时的液晶介电各向异性与分子极性和双折射率相关,侧位含氟苯乙炔类和端基异硫氰基苯乙炔类液晶化合物均具有较大的介电各向异性(Δ_(ε_r)0.85);对于具有较高双折射率的对称含氟三苯二炔类和三苯乙炔异硫氰基类液晶化合物表现出较低的介电损耗(tanδ_(ε_r⊥)8.0×10~(-3),18GHz),而异硫氰基的含氟二苯乙炔类和不对称含氟三苯二炔类液晶化合物则表现出较高的介电损耗(tanδ_(ε_r⊥)8.0×10~(-3),18GHz)。     

液晶的介电各向异性分布对液晶微带线特性的影响

基于向列液晶的微带传输线模型,研究了其模场解和1～100 GHz频段内传输特性,采用有限元法较为精确和全面地分析了各向异性非均匀分布的液晶介质对微带传输线模场解和S参量的影响,并且分析了微带电极上施加偏置电压对该传输线S参数的影响。仿真结果表明:该微波传输线模型的回波损耗在考虑液晶的介电各向异性非均匀分布时比传统仿真方法所得结果更低,随着偏置电压在一定范围内连续增大其S参数连续变化,同时谐振频率连续移动,并且这种精确的分析方法计算得到的谐振频率点的偏移量相比传统分析方法修正误差2.4 GHz,这为液晶微波可调器件的进一步研究奠定了理论基础。     

基于液晶材料的带宽可重构毫米波滤波器

根据液晶材料在毫米波段良好的介电特性和调谐能力,设计了一款基于液晶材料的毫米波带宽可重构宽带带通滤波器。滤波器使用一个高通滤波器和一个低通滤波器级联实现带通效果;在低通部分加载液晶材料,通过调谐液晶材料的等效介电常数改变低通滤波器的响应频率,实现带宽的可重构。仿真结果表明,当调谐液晶介电常数从2.4变化到3.8时,滤波器的高频截止频率从52 GHz下降至48 GHz,相对带宽从84.9%变为78.3%。     

含氟三环NCS液晶材料的介电性能研究

为了探究液晶材料的介电性能,本文研究了4PPTGS和4PUTGS两种含氟三环NCS类液晶材料的介电各向异性和介电损耗。首先用精密LCR表(Agilent E4980A)测量液晶盒的电容并用双盒模型和液晶盒电容模型得到4PPTGS和4PUTGS两种液晶材料的平行和垂直介电常数,再由电压-电容特性曲线得到它们的阈值电压,并进一步探讨了介电各向异性和阈值电压对温度的依耐性;然后,在20 Hz~10kHz范围内研究了外加电压频率对液晶材料介电损耗的影响,两种液晶材料在1kHz左右都存在介电损耗峰值,为了减小器件的功耗和提升器件的质量,液晶材料应选择在介电损耗小的频率下工作;最后,通过对平行和垂直排列向列相盒中液晶材料在不同电压下介电损耗的测试与分析,介电损耗的变化是由于在外加电场下液晶分子固有偶极矩的取向极化引起的,介电损耗值的大小与液晶分子的排列状态密切相关。此项研究对提升液晶材料在应用中的介电性能具有一定的指导意义。     

(Bi2-xZnx)(Ti2-xNbx)O7陶瓷的结构与介电性能

研究了(Bi2-xZnx)(Ti2-xNbx)O7(0.4≤x≤1.0)陶瓷材料的结构与介电性能.X-射线衍射结果表明,该组分体系在950～1 100 ℃烧结,可得到单相立方焦绿石结构陶瓷.扫描电子显微镜观察样品形貌发现,x越大,晶粒尺寸越大.室温介电性能的测试表明,在1 MHz条件下,随x值的增大,介电常数从218下降到122,损耗为(1～4)×10-4.介电温谱测试发现,该组分体系在低温下出现明显的介电弛豫峰,峰形随x增大逐渐宽化.微波特性的测试表明,在谐振频率2～3 GHz,样品的品质因数与谐振频率之积Q×f为112～158 GHz.     

BaTiO3掺杂羰基铁粉复合材料吸波性能研究

BaTiO3介电材料在微波频段具有良好的介电频散特性,可通过在羰基铁粉吸收剂中掺杂BaTiO3改良其频散特性。采用固相合成法在不同温度下制备了BaTiO3粉体,测试表明,制备的BaTiO3粉体形成了单一的四方相晶体结构,具有很高的纯度和结晶度、较好的频散特性。在羰基铁粉中掺杂不同含量的BaTiO3粉体,使用同轴线方法测试了复合粉体在2～18GHz频段范围内的介电常数和磁导率,基于传输线理论计算获得了复合粉体的反射系数,对比结果显示：BaTiO3粉体的掺入,明显改进了羰基铁粉介电常数的频散特性,有效地提高了低频波段（2～5GHz）的吸收性能,含有4％BaTiO3（质量分数）的样品在3GHz处的反射系数达到-8dB。适当比例BaTiO3和羰基铁的混合粉体有望成为性能优越的低频波段吸波材料。     

基于人工局域表面等离激元的液晶微波介电常数测量传感器

液晶材料在微波频段具有良好的调制特性，在微波可调谐器件领域具有巨大的应用潜力。本文针对液晶材料微波介电常数的测量需求，提出了一种基于人工局域表面等离激元谐振的传感器。通过设计环形谐振器结构，在sub-6 GHz频段形成局域表面等离激元窄带谐振峰。通过给液晶施加外加电场，能够实现对液晶介电常数的调控。通过谐振频点位置的拟合，能够得到对应的液晶的介电常数大小，从而实现液晶材料在微波频段的介电常数的测量。本文研究了不同液晶层厚度、不同液晶介电常数对人工局域表面等离激元谐振频点的影响。随着液晶层厚度增加或者液晶介电常数的减小，谐振频点f₁和f₂都逐渐增大。当液晶层厚度大于或等于0.5 mm时，谐振频点f₁和f₂随介电常数的变化具有良好的线性度，且具有高灵敏度(>400 MHz/Δε)，远大于基于目前报道的其他形式介电常数传感器。同时，本传感器结构可以在液晶层上下施加电场，从而实现在不同外加电场作用下液晶材料微波介电常数的测量，在液晶微波特性研究领域具有应用潜力。     

Li_2O-Nb_2O_5-TiO_2微波介电陶瓷掺杂改性研究

采用传统的固相反应法制备Li-Al-B(LAB)掺杂立方晶系Li2O-Nb2O5-TiO2(LNT)微波介电陶瓷。运用XRD、SEM和微波介电性能测试等手段,研究了LAB掺杂对样品烧结性能及微波介电性能的影响。结果表明,在LNT陶瓷中添加LAB,有效促进LNT陶瓷烧结,使材料的介电常数和品质因数显著提高。当掺入LAB的质量分数为4%时,样品在900℃保温2h后烧结致密,并获得最佳微波性能:介电常数εr=18.05,品质因数与频率的乘积Q×f=22 040GHz(f=6.41GHz),频率温度系数τf=-20.74×10-6/℃。     

卫星接收机高频头微波振荡器的稳频方法

1 引言 在卫星电视接收机的高频头中,本机振荡器的工作频率很高,处在微波频段.根据下变频的要求,本机振荡频率与输入信号频率相差一个中频,第一中频的频率范围为950～1 450 MHz,对于C波段,通常采用高本振方案,输入信号的频率范围为3.70～4.20 GHz,本振频率为5.15 GHz;对于Ku波段,通常采用低本振方案,输入信号的频率范围为11.70～12.20 GHz,本振频率为1.075 GHz.     

Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters

We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

On p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode). We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

Plasma Modification of Fluoropolymers for Aligning Liquid Crystals

In this paper we describe methods to control liquid crystal (LC) alignment using plasma discharge on ferroelectric fluoropolymers. Two different plasma modification techniques were investigated: corona discharge and RF plasma in Ar gas. Corona discharge is a proven technique known to reorient the dipoles in poly (vinylidene fluoride) and its copolymers resulting in a strong remnant polarization. The polarization was patterned resulting in preferential LC alignment in selected regions. RF plasma in Ar gas defluorinates the polymer surface leading to planar alignment of positive dielectric anisotropy LCs. The defluorination of the alignment layer also causes low voltage switching of the LC.     

Nematic Anisotropic Liquid‐Crystal Gels—Self‐Assembled Nanocomposites with High Electromechanical Response

The uniqueness of liquid crystals (LCs) lies in the large anisotropies of their properties, which can be utilized to generate high electromechanical responses. In a properly oriented LC polymer system, an external electric field can induce reorientation of the mesogenic units possessing a dielectric anisotropy, which, when coupled with the shape anisotropy of the mesogenic units, can in turn produce large mechanical strain. Anisotropic LC gels, which can be obtained by in‐situ photopolymerization of the reactive LC molecules in the presence of non‐reactive LC molecules in an oriented state, are an example of such liquid‐crystal polymer systems. It is shown here that a homeotropically aligned LC gel in its nematic phase exhibits high electrically induced strain (> 2 %) with an elastic modulus of 100 MPa and a high electromechanical conversion efficiency (75 %) under an electric field of 25 MV/m. These anisotropic LC polymeric materials could provide a technologically compatible system for such applications as artificial muscles and as microelectromechanical devices.     

Preparation of poly (N-vinylcarbazole)-co-poly(2-(dimethylamino)ethyl methacrylate) based hydrogen bonded side-chain liquid crystal copolymer

In this study, new side chain liquid crystalline copolymers were prepared from N-vinyl carbazole (NVC) and 2-(Dimethylamino)ethyl methacrylate) (DMAEM) as a hydrogen bond acceptor copolymer and 8-(4-cyanobiphenyl-4′-oxy)octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between nitrogen of (DMAEM) and hydroxyl group of the LC8. The formation of H bond was confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the copolymers and homopolymer of the (DMAEM) was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. The dielectric relaxation properties of H-bonded Side Chain LC Copolymers (HB-LCP) doped 8-(4-cyanobiphenyl-4′-oxy)octan-1-ol (LC8) and pure LC8 liquid crystals have been investigated by the dielectric spectroscopy (DS) method. The dielectric behavior of the LCs shows a dielectric relaxation process. The relaxation frequency of the LCs was changed by the addition of HB-PLC. It is evaluated that the dielectric strength and relaxation properties of LC8 and LC8/HB-PLC LCs can be controlled by 1% HB-PLC dopant.     

Focusing properties of liquid crystal lens cells with stack-layeredstructure in the millimeter-wave region

Liquid crystal (LC) lens cells are fabricated using a nematic LC material with a positive dielectric anisotropy and semicircle-shaped metal substrates as quasioptical millimeter-wave devices. The millimeter-wave focusing properties of the LC lens are measured at 94 GHz and its convergent effects caused by the lens-shaped configuration are then observed. Changes in the focusing properties by applying an external electric field are confirmed     

Photogeneration of High Pretilt Angles of Nematic Liquid Crystals by Non‐Polarized Light Irradiation of Azobenzene‐Containing Polymer Films

A vertical‐alignment (VA) cell of nematic liquid crystals (LCs) was prepared using photoirradiated thin films of a poly(methacrylate) with mesogenic moieties of 4‐trifluoromethoxyazobenzene as the side chains. Optical anisotropy was generated by oblique irradiation of the azobenzene‐containing polymer films with non‐polarized UV light, followed by annealing treatment to enhance the photodichroism, which displayed thermal stability. The combination of oblique exposure to non‐polarized UV light and subsequent annealing treatment brought about high pretilt angles of nematic LCs so that a photoaligned VA LC cell was fabricated. The photopatterned LC cell exhibited electro‐optical properties with excellent optical quality when a voltage was applied even after heating at 100 °C for several hours.     

Addition of single-wall carbon nanotubes to a liquid crystal material: Impact on dielectric properties

In this study, the effect of carbon nanotube (CNT) on the dielectric parameters of (11-(4-cyanobiphenyl-4′-oxy)undecan-1-ol) (coded as) LC11 has been investigated by dielectric spectroscopy measurements. According to analysis of ac dielectric parameters׳ analysis, while relaxation frequency increases with CNT doping, the magnitude of energy loss decreases. In this respect, CNT doped LC11 material which can be utilized in the wide working frequency interval with low energy requirements, has a great potential for technological applications. On the other hand, CNT adding changes the relaxation type from nearly Debye to non-Debye type. In addition to these remarkable modifications on the dielectric parameters, it has been determined that CNT doping extends the frequency interval where the liquid crystal (LC) system shows negative dielectric anisotropy (NDA). In other words, observation of the NDA at the higher frequency region relative to LC11 means that the molecular alignment can be controlled by CNT. Hence, tuning the alignment of the LC11 molecules by CNT doping may lead its utility for various LC device applications.