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Title: Investigations on magnetic nanoparticle liquid crystal composites
Authors: Khushboo
Supervisor: Sharma, Puneet
Raina, K. K.
Keywords: Liquid cystal, magneti nanoparticle
Issue Date: 20-Apr-2018
Abstract: Most of the liquid crystal display (LCD) devices are based on the reorientation of LC molecules in external electric or magnetic fields. In practice, majority of LC devices are driven by electric field. This is because the sensitivity of LC to an external field is too high: only small amount of applied voltage is required to trigger the LC reorientation and optical responses. In contrast, the sensitivity of LC in magnetic field is very high. Although, previous reports suggest that optical and other properties of LC can be controlled by magnetic field but usually it cannot be employed due to (i) Low sensitivity to magnetic field and (ii) very week anisotropy of diamagnetic susceptibility (χa ~ 10−7). Therefore, rather a high magnetic fields (H > 1 kOe) must be applied to get a magneto-optical response comparable to its electro-optical analog. As a consequence, increasing the magnetic sensitivity of liquid crystals is an important issue for the development of new devices. In this thesis efforts are made to understand and investigate the effect of magnetic nanoparticles dispersion on the physical properties of liquid crystals. Three different type of nanoparticles (Iron, nickel ferrite and nickel) in very small concentration are used for dispersion purpose into nematic liquid crystal (6OCB) and ferroelectric liquid crystals mixtures. Dispersion of Fe NPs (0.25 wt. %) in 6OCB on textures, isotropic- nematic transition temperature (TI-N), electro-optical and dielectric properties are investigated in planar aligned cell. The threshold voltage and TI-N decrease after dispersion of Fe NPs. Dielectric spectroscopy show that in nematic phase, relaxation frequency also decreases in Fe NPs doped 6OCB composite. The band gap and AC conductivity in case of 6OCB-Fe sample increase over pure 6OCB sample. An improvement in spontaneous polarization, response time in nickel ferrite- FLC doped samples compared to FLC is observed and is explained on the basis of dipole moment and anchoring phenomena. The Goldstone mode (GM) is detected in all samples and follows a Debye-type relaxation behavior. A twofold increase in relaxation frequency for doped sample rather than pure sample has been observed. The bandgap was found more or less independent of doping concentration. The activation energy also decreases with increasing the amount of dopant. Ferromagnetic nanoparticles, nickel of size ~ 20, 40 nm were also studied in FLC mixture. A minor improvement in spontaneous polarization, rotational viscosity and faster response time in NiNPs/ FLC dispersed samples than pure FLC was observed. A Goldstone mode of relaxation frequency ~100 Hz is detected in all samples and follow a Debye type relaxation behavior. In addition, it is observed that size of NiNPs does not have any remarkable effect on spontaneous polarisation, relaxation frequency and dielectric strength. A single relaxation peak at 363, 362 Hz is also noticed in pure FLC and NiNPs/ FLC samples.
Appears in Collections:Doctoral Theses@SPMS

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