Avneesh Mishra

Electrical and electro-optical parameters of 4ʹ-octyl-4-cyanobiphenyl nematic liquid crystal dispersed with gold and silver nanoparticles

Thermodynamical, dielectric, optical and electro-optical characterisation of pure 8CB and its composites with gold and silver nanoparticles have been studied. Thermodynamical studies suggest a decrease in clearing temperature of the nanocomposite systems as compared to the pure system. Dielectric parameters of pure nematic liquid crystal and nanocomposites in the homeotropic and planar aligned samples have been measured in the frequency range of 1–35 MHz. Ionic conductivity increases significantly in nematic and smectic Ad (SmAd) phases, whereas dielectric anisotropy is almost unchanged for both the nanocomposites. Threshold voltage for Freederick transition, switching voltage and splay elastic constant have decreased in the case of nanocomposite systems. Relaxation frequency and activation energy of an observed relaxation mode corresponding to molecular rotation about the short axis increase in the SmAd phases of both the nanocomposites. The optical study suggests that due to dispersion of nanoparticles, the optical band gap has decreased.

Dielectric and switching parameters of para-, ferro- and antiferroelectric phases of (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(6-perfluorooctanoyloxyhex-1-oxy) biphenyl-4-carboxylate

Dielectric and electro-optical response of the title compound (abbreviated as (S) 7F6Bi) have been investigated. The dielectric studies have been carried out in the frequency range of 1 Hz to 35 MHz under planar anchoring conditions of the molecules. Material exhibits paraelectric SmA*, ferroelectric SmC* and a wide temperature range (∼83°C) antiferroelectric phases. The SmA* phase shows soft mode relaxation due to tiltfluctuation of molecules in the MHz region. Its relaxation frequencies decrease with decrease in the temperature. Dielectric strength increases (∼0.2–3.4) with decrease in temperature and follows Curie–Weiss law. The SmC* phase shows Goldstone mode relaxation due to phase fluctuation of molecules with relaxation frequency ∼10 kHz. Only one mode of relaxation has been observed in the phase presumably due to antiferroelectric ordering of the molecules. Its dielectric strength varies from ∼0.55 at 104.3°C to 2.20 at 37.6°C. Switching parameters, namely spontaneous polarisation, switching time and rotational viscosity, have also been determined.

Effect of silver nanoparticles on frequency and temperature-dependent electrical parameters of a discotic liquid crystalline material

The effect of silver nanoparticles (AgNPs) of diameters 6 and 100 nm on a discotic liquid crystalline material, namely 2,3,6,7,10,11-hexabutyloxytriphenylene (in short HAT4), has been observed in thermodynamic, electrical and optical texture studies. Silver nanoparticles (0.6 wt%) of diameter ~6 nm demonstrate a negligible (but ~100 nm shows appreciable) effect on the broad temperature range plastic columnar hexagonal (Colhex) phase (~65.0°C) of pure HAT4. The dielectric studies have been carried out in the frequency range of 10 Hz–35 MHz under homeotropic anchoring conditions of the molecules. In the low frequency region of pure HAT4 and its AgNP composites, a relaxation mode has been observed. AgNPs of 6 nm elevate the value of dielectric permittivity of the plastic columnar hexagonal phase of pure HAT4. The dc conductivity of pure HAT4 and its AgNP composite (6 and 100 nm) material has been determined. The optical band gap for pure and AgNP composites of HAT4 has been determined by the ultraviolet-visible study. Due to insertion of AgNPs, the optical band gap of HAT4 has reduced.

Dielectric properties of a highly polar liquid crystalline material showing various types of layered structures

ABSTRACT Liquid crystal compound N-(4-n-pentyloxybenzalidene) 4′-n-pentylaniline is very interesting because of its rich phase variant, i.e. nematic, smectic A, smectic C, smectic F and smectic G phases. In the present work, temperature- and frequency-dependent dielectric studies on this compound in the homeotropic and planar orientation of molecules have been carried out in the frequency range of 1 Hz–10 MHz. The compound shows negative dielectric anisotropy (i.e. Δε < 0). Dielectric studies have shown three relaxation modes of different origin in various phases of the material. Relaxation frequencies of the observed modes follow Arrhenius behaviour. The relaxation due to interfacial polarisation, i.e. Maxwell–Wagner–Sillars effect has been observed in the nematic and isotropic phases. A relaxation mode due to the fluctuation of molecular tilt resembling the soft mode has been detected in the smectic A phase with pre- and post-transitional effects. Another mode due to the fluctuation of molecules about their short axes has been detected in the nematic and smectic A, C, F and G phases. Activation energies and ionic conductivity of these phases have also been determined. GRAPHICAL ABSTRACT

Dielectric and switching properties of a highly tilted AFLC material (S)-(+)-4-(1-methylheptyloxycarbonyl)-2,3-difluorophenyl 4′-[3-(2,2,3,3,4,4,4-heptafluorobutoxy)prop-1-oxy]biphenyl-4-carboxylate

Thermodynamic, dielectric, optical and switching parameters of a single-phase antiferroelectric (AF) liquid crystalline material (S)-(+)-4-(1-methylheptyloxycarbonyl)-2,3-difluorophenyl 4′-[3-(2,2,3,3,4,4,4-heptafluorobutoxy)prop-1-oxy]biphenyl-4-carboxylate have been studied. These studies show wide temperature range (~97.8°C–25.3°C) of AF SmC*A phase in the material. The dielectric studies have been carried out in the frequency range of 1 Hz–35 MHz under planar anchoring conditions of the molecules. The dielectric spectrum of the SmC*A phase exhibits three relaxation modes due to the collective as well as individual molecular processes. Relaxation frequencies of these modes lie in the range of kHz–MHz regions. Relative permittivity of the material (at 10 kHz) varies from ~8.8 at 98.8°C to 9.9 at 41.0°C. Maximum tilt of the molecule in the SmC*A phase is ~43°C. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of PS is ~439 nC/cm2 and switching time is the order of 1–5 millisecond, whereas viscosity is moderate.

Dielectric and electro-optical response of a room temperature tri-component antiferroelectric mixture

Frequency- and temperature-dependent dielectric and switching parameters of a room temperature tri-component antiferroelectric liquid crystal mixture W-287 have been determined. Dielectric, optical texture and thermodynamic studies show wide room temperature range antiferroelectric SmC*a (˜91.1°C to <–25°C) phase in addition to high temperature paraelectric SmA* (˜2.6°C) and ferroelectric SmC* (˜4.4°C) phases. The dielectric studies carried out in the frequency range of 1–35 MHz under planar anchoring condition of the molecules show five different relaxation modes appearing in the SmA*, SmC* and SmC*a phases. Using Curie–Weiss law fit, ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 91.8°C. The dielectric response of SmC*a phase exhibits unusually three relaxation modes due to collective as well as individual molecular processes in addition to phason mode in the SmC* phase and amplitudon mode in the SmA* phase. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of PS is ˜300 nC/cm2, whereas viscosity is moderate. Switching time is of the order of few milli seconds.

Relaxation phenomena of a highly tilted ferroelectric liquid crystalline material (S)-(+)-4'-(3-pentanoyloxy prop-1-oxy)biphenyl-4-yl-4-(1-methylheptyloxy)benzoates

Titled ferroelectric liquid crystalline material (in short, 4H3R) exhibits cholestric (N*), ferroelectric (SmC*) and hexatic (SmB*) phases, when it is cooled from isotropic liquid phase to crystal phase. Dielectric studies have been carried out in the frequency range of 1 Hz to 35 MHz under planar anchoring conditions of the molecules. The soft mode relaxation due to the amplitude fluctuation of the polarisation vector has appeared in the N* phase, whose relaxation frequencies decrease but dielectric strength marginally increases (~0.30–0.54) with decrease in the temperature. The Goldstone relaxation mode has appeared in SmC* phase with a constant relaxation frequency. The relaxation frequency of hexatic SmB* phase follows Arrhenius behaviour. The conductivity of the material is also measured in all the exhibited phases.

Thermodynamical, electrical and electro-optical studies of a room temperature tri-component antiferroelectric liquid crystalline material

Thermodynamic and electric studies show the wide room temperature range antiferroelectric SmC*a (91.1 to −20.0 ºC) phase in addition to narrow temperature range paraelectric SmA* (∼2.1 ºC) and ferroelectric SmC* (∼4.3 ºC) phases in thestudied antiferroelectric material. Frequency-dependent dielectric studies show five different relaxation modes appearing in SmA*, SmC* and SmC*a phases under the planar anchoring condition of the molecules. The ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 97.3 ºC using Curie–Weiss law. The dielectric response of SmC*a phase exhibits three relaxation modes in addition to Goldstone mode in SmC* phase and soft mode in SmA* phase. Polarizations, switching time and rotational viscosity have also been determined. Maximum value of polarization (P) is ∼300 nC/cm2. Switching time is the order of few milliseconds.