John Clifford Jones

Fast, high-contrast ferroelectric liquid crystal displays and the role of dielectric biaxiality

Abstract The operation of ferroelectric liquid crystal displays depends strongly on the material properties and the electrical addressing method. Fast, high-contrast displays with high levels of multiplexing and wide operating ranges are possible using materials showing a minimum in their response time-voltage characteristic operated with JOERS/Alvety and related drive schemes. These utilize materials with significant dielectric biaxialities and relatively low values of the ferroelectric polarization so that a minimum is observed in the switching characteristic. The route for material improvement is explored.

Double minimum in the surface stabilized ferroelectric liquid crystal switching response

A double minimum has recently been observed in the time–voltage switching response for a smectic C* liquid crystal layer in the surface stabilized geometry (“Ferroelectric Liquid Crystal Device,” K. P. Lymer and J. C. Jones, U.K. Patent No. GB2338797, 17th June 1999). Liquid crystal continuum theory is used to demonstrate that this unusual switching behavior arises if the equilibrium orientation of the molecular director rotates around the smectic cone as a function of distance through one half of the layer only. The double minimum is shown to evolve for large differences between the e2 and e1 components of the smectic C biaxial permittivity tensor.

OPTICAL STUDIES OF THIN LAYERS OF SMECTIC-C MATERIALS

The optical behaviour of surface-stabilized ferroelectric liquid crystal samples cannot be described using a uniform director profile. The simplest model based on the chevron layer structure and the surface alignment is a triangular director profile. Analytical expressions for the resulting transmitted light were calculated using the Jones matrix formalism and fitted to experimental data results of a smectic-C host made using an optical multichannel analyser (OMA). This simple model gives an excellent fit, with results that are consistent with X-ray measurements of the chevron structure.

The importance of dielectric biaxiality for ferroelectric liquid crystal devices

Abstract The dielectric biaxiality of SCE13, a commercially available ferroelectric liquid crystal (FLC) mixture, has been measured. The results have been used to compare theoretically predicted AC field stabilization effects and the voltages for response time minima with experiment.

Measurement of the biaxial permittivities for several smectic C host materials used in ferroelectric liquid crystal devices

Abstract Dielectric biaxiality plays an important role in the electric field behaviour of ferroelectric liquid crystal devices, providing the necessary driving force for AC field stabilization and playing an important role in device dynamics. The static electric permittivities have been measured as a function of temperature for several mixtures with an N[sbnd]SA[sbnd]SC phase sequence. Three permittivities are defined for the biaxial SC phase, and were determined from measurements of homeotropic and planar cells and from the average permittivity extrapolated from the uniaxial nematic and smectic A phases. The SC cone angle is assumed to be equivalent at dielectric and optical frequencies. Results are reported for five common ferroelectric liquid crystal hosts and for two racemic mixtures, including the racemic analogue of the commercially available SCE 13. The dielectric biaxiality is sufficiently large below the SA to SC transition in each of the studied mixtures to have a strong influence on the electro...

The effect of the biaxial permittivity tensor and tilted layer geometries on the switching of ferroelectric liquid crystals

Abstract Using a simple uniform switching model, we investigate the behaviour of the voltage dependent switching time of surface stabilized ferroelectric liquid crystals as a function of the biaxial permittivity tensor and the layer tilt angle. We show that the dielectric biaxiality can markedly effect the response time (τ) of the device and is the origin of the minimum in τ as a function of voltage (V) in tilted layer systems. The dielectric biaxiality should, therefore, be optimized for multiplexing schemes which use the τ—V minimum.

Dielectric Biaxiality in SC Host Systems

Abstract AC field stabilisation of ferroelectric SC* liquid crystals with the chevron layer geometry requires that there is a significant dielectric biaxiality.1,2 To investigate this, the electric permittivities of an SC host mixture based on the phenyl pyrimidines and exhibiting an N-SA-SC phase sequence are reported. A large relaxation of e∥ occurred, which caused a sign reversal of the dielectric anisotropy with a crossover frequency of 25 kHz at 25°C. The host electro-optic behaviour was then characterised using extinction angle measurements as a function of applied voltage at frequencies corresponding to Δ≈ > 0, Δe ≈ 0 and δe < 0, and the results explained using a tilted layer model with a biaxial dielectric tensor. The magnitude of the SC dielectric biaxiality, the layer tilt angle in the planar homogeneous geometry and the cone angle are determined. Measurements for several other hosts commonly used in commercial FLC mixtures are reported and it is shown that in each case the dielectric biaxiality...

On the influence of short range order upon the physical properties of triphenylene nematic discogens

Abstract The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence of ∊ and the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the ND phase. The strong temperature dependence of |k 33/Δχ|, the large value of k11/k 33 and the rise in k 11/k 33 as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole cor...

11.2: Zenithal Bistable Device (ZBD™) Suitable for Portable Applications

Zenithal Bistable display devices have been made which are compatible in size and functionality with current mobile phone or pager displays. Radical improvements in the viewing characteristics particularly in reflective mode are shown. Furthermore this paper describes: low voltage operation; fast latching times; and broad-margin matrix addressing.

Assessing ferroelectric materials for application in τVMIN mode devices

Abstract Improved operation of FLC devices which operate in the [tacute]uVmin mode require materials with high dielectric biaxialities and low viscosities. Measurements of these properties are difficult and so a method for comparing materials using the [tacute]uV characteristic itself is proposed. Three figures of merit are suggested that allow comparison of host materials independently of the ferroelectric spontaneous polarisation, the cone angle and cell spacing. The temperature dependence for the figures of merit are analysed in detail.