反式环己基(三)联苯类液晶化合物微波介电性能分析

Analysis for the dielectric properties of trans-cyclohexyl(tri)biphenyl liquid crystal compounds in microwave frequency bands

目前微波移相器用液晶材料大多以高双折射率多联苯类或多苯乙炔类液晶化合物作为主要组分,但这些液晶化合物在使用时不可避免地存在粘度大、介电损耗偏高或光热稳定性较差等缺点。反式环己基(三)联苯类液晶化合物具有相对较低的粘度、高热稳定性等特点,普遍用在显示器件之中,但在微波介电性能方面的研究报道较少。本文选择性合成了6个异硫氰基反式环己基联苯和三联苯类液晶化合物,测试了它们的相态和微波(10~30 GHz)介电性能,利用密度泛函理论(DFT),通过分子模拟计算了它们的偶极矩和极化率,并与相关结构的多联苯类液晶化合物的性能进行比较,探讨其反式环己基结构对液晶分子的微波介电性能的影响。实验结果表明,异硫氰基反式环己基(三)联苯类液晶化合物不仅其熔点相对较低(<90℃)、向列相温度范围宽(86~160℃),而且在10~30 GHz时的介电损耗低(≤7.25×10^(-3)),使其微波的品质因素相对较高(η≥30),适于作为微波用液晶材料的有效组份。

At present,the liquid crystal materials used in microwave phase shifters are mostly high birefringence polybiphenyls or polyphenylacetylene liquid crystal compounds as the main components.But these liquid crystal compounds inevitably have some disadvantages such as large viscosity,slightly higher dielectric loss or poor photothermal stability.The trans-cyclohexyl(tri)biphenyl liquid crystal compounds with relatively low viscosity and high thermal stability are widely used in displays devices,but their microwave dielectric properties studies have been fewer reported.In this paper,six compounds of trans-cyclohexylbiphenyl and tri-biphenyl isothiocyanate were synthesised selectively,and their liquid crystal phase and microwave(10~30 GHz)dielectric properties were tested.Their dipole moments and polarizabilities were calculated by molecular simulation using density functional theory(DFT),and compared with the properties of the related polybiphenyl liquid crystal compounds to explore the effect of the trans-cyclohexyl structure on the microwave dielectric properties of liquid crystal molecules.The experimental results show that the isothiocyanato-trans-cyclohexyl(tri)biphenyl liquid crystal compounds not only have relatively low melting points(<90℃)and wide nematic temperature ranges(86~160℃),but also have low maximum dielectric losses(≤7.25×10^(-3))at 10~30 GHz,which makes their microwave quality factors relatively high(η≥30)and suitable to be used as effective components in microwave-usable liquid crystal materials.