侧位含氟苯乙炔类液晶化合物的微波介电性能

Microwave dielectric properties of fluorine substituted phenylacetylene liquid crystal compounds

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(10~30 GHz)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(Δεr>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(tanδεr⊥<8.0×10-3,18 GHz),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(tanδεr⊥>8.0×10-3,18 GHz)。

In recent years,the research of liquid crystal microwave communication devices has developed rapidly.The high dielectric loss of liquid crystal materials has become a bottleneck restricting the development of microwave devices.There are few reports on the properties of liquid crystal materials at microwaves.Therefore,we take the fluorine substituted tolane-type compounds with low melting point and high birefringence as the research object,doping them into the liquid crystal matrix MA in a certain proportion respectively,and testing them at microwave k-band(10~30 GHz)through the rectangular cavity perturbation method.And then,the influence of molecular structure on the dielectric properties of liquid crystals at microwave frequency is discussed.The experimental results show that dielectric anisotropy at high-frequency is related to the polarity of liquid crystal molecules and birefringence.The lateral fluorophenyl phenylacetylene and terminal isothiocyano phenylacetylene liquid crystal compounds exhibit high dielectric anisotropy(Δεr>0.85).Triphenyl diacetylenes substituted by F-atom symmetrically and triphenyl acetylene liquid crystal compounds with terminal isothiocyanate exhibit lower dielectric loss(tanδεr⊥<0.008,18 GHz)due to higher birefringence.Fluorinated diphenylacetylenes with terminal isothiocyanates and triphenyldiyne substituted with fluorine asymmetrically liquid crystal compounds show higher dielectric loss(tanδεr⊥>0.008,18 GHz).