Microwave cavity technique to study the dielectric response in 4′-n-Heptyl-4-biphenyl nematic liquid crystal at 20.900 GHz and 29.867 GHz
Introduction
Liquid crystals are materials with many unique physical, optical and electro-optical properties. New compounds and mixtures of liquid crystal are chemically stable, with low absorption, very large optical anisotropy, a liquid crystalline phase in a wide range of temperatures, are easily oriented at boundaries and are also easily reoriented by electrical or magnetic fields. Therefore, they are important optical materials for numerous applications in modern optoelectronics [1]. Nematic liquid crystals (LCs) are fluid phases found by anisometric molecules which, though free to rotate as in ordinary liquids, are preferentially aligned along a common axis, and called director. The dielectric properties play an important role [2], [3], [4], [5], [6] to understand the macroscopic structure of the matter. The dielectric properties measurements by the resonance techniques have higher accuracy than measurements by the transmission techniques especially for the dielectric loss. Therefore the resonance techniques are still widely used. Various resonance techniques to conduct dielectric properties measurement and accuracy of microwave cavity perturbation technique have been described elsewhere [7], [8]. This technique had been used by various workers for studying dielectric response of bio-molecule such as water [9], [26], ionic and aqueous solutions [10], [11], organic liquids [12] and different types of liquid crystals [13], [14], [15], [16]. Most recently Huang Ming et al. used this technique for measuring the moisture content of sulfide minerals concentrates [17]. Further it has also been used to study dielectric response of various types of solid state materials [18], [19] as it provides the results with accuracy especially at low perturbation. Liquid crystals are very interesting materials to infiltrate photonic crystal fibers with special reference to nematics.
The objective of the present work is to study and compare the effect of temperature variation on the variation of dielectric response of 4′-n-Heptyl-4-biphenyl Nematic Liquid Crystal at two different frequencies. The Slater’s perturbation equations [3], [4] have been used for the purpose of attainment of simple treatment and avoiding complexity of computer program and to compare the relaxation time (τ) by the analysis of measured data of relative shift of the resonant frequency (Δf), relative width of resonant profile (Δw) and amplitude of the resonance profile (Δh). The frequency of the applied field is fixed at 20.900 GHz and 29.867 GHz and cavity is operated in the TM010 mode. The measurements have been carried out in the temperature range 293–308 K
Section snippets
Experimental details and theory
The fundamental concept of the perturbation technique is that the presence of a small piece of the dielectric sample in the resonant cavity will cause a shift of resonance frequency and decrease of the quality factor of the cavity. The “small” means that the volume of the sample is much smaller than the volume of the cavity. It is also assumed the presence of the specimen to the change in the overall geometrical configuration of the electromagnetic must be very small. The complex permittivity
Results and discussion
We have measured shift in resonance frequency (Δf) and width of resonance profile (Δw) at two different frequencies 20.900 GHz and 29.867 GHz using Microwave cavity spectrometer. Since this technique does not give absolute value of permittivity and dielectric loss due to involvement of the form factor F(E) defined in Eq. (3), we have determined the relaxation time (τ). The plots of observed data of Δf, Δw, and τ with respect to temperature are given in Fig. 4a, Fig. 4b, Fig. 4c respectively. The
Conclusion
It is concluded that the relaxation time and other parameters do not change with temperature only but also with change in frequency. It is also concluded that the microwave cavity spectrometer is one of the best techniques which is accurate to the extent better than 3% [9] monitor phase transition in liquid crystals, relative change in dielectric response and it needs very small quantity of sample (<10−9 m3). Further unlike the dielectric resonance techniques, where computer programs are
Acknowledgment
The part of present work has been carried out within the framework of the Associate ship Scheme of the Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy. The authors (MJ and HP) are grateful to ICTP for providing the necessary facilities and literature for the present work. One of author (MJ) thanks Prof. James A. Robert, (UNT, USA) for measurements.
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