Electro-optical, calorimetric, x-ray, and dielectric measurements are reported on a mixture containing a bent-core liquid crystal 4-chloro-1,3-phenylinebis [4-(4-n-tetradecyloxyphenyliminomethyl) benzoate] mixed with the nonpolar solvent n-hexadecane (HEX). It is observed that the addition of HEX depresses the isotropic-to-smectic phase transition temperature, but the crystallization temperature does not change considerably. Instead, the texture of the crystalline phase changes and, at sufficiently high concentrations of HEX (>20 wt %), an optically isotropic phase appears. Above 40 wt % HEX concentrations, the mesophase completely disappears and a direct isotropic solid-to-isotropic liquid transition takes place. At increasing HEX concentration, the transition enthalpies, the layer ordering, and the magnitude of the electric polarization decrease. X-ray studies reveal that HEX molecules pack in between the smectic layers, resulting in an increase of the layer spacing by about 3 A. The increase of the layer spacing saturates at 5 wt % of HEX. The phase segregation observed seems to be due to steric interactions between the flexible HEX molecules and the rigid bent cores of the liquid crystal molecules.

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We describe a microwave plasma-asssisted chemical vapour deposition facility with some novel features including the ability to adjust the microwave cavity tuning during operation, thereby maximizing the overall device efficiency. The initial application of the facility was for diamond thin film synthesis. Microwave power, generated by a magnetron obtained from a commercial microwave oven, enters the cylindrical resonant cavity from the top face, which can be adjusted to position the maximum electric field at the desired location within the cavity; this tuning can be done while the equipment is running, allowing fine adjustment of the field distribution when the cavity is plasma-loaded. The microwave power activates the reactant gases, which are at low pressure inside a quartz bell jar specimen chamber, and the cavity base plate has a window through which the rear face of the hot substrate can be viewed, allowing pyrometric temperature measurement. A moveable sample holder allows control of the sample position in relation to the plasma ball. The system is stable and the deposition rate is about 1.25 j&m h-'.

The authors investigate the effect of the surface treatment on the electrical response of a nematic liquid crystal cell. The theoretical analysis is performed by describing, from the electrical point of view, the liquid crystal and the surface treatment by means of the dielectric constants and the electrical conductivities. The predictions of the model are compared with the measurements performed on a nematic cell submitted to an external voltage steplike or linear in time. They show that to correctly interpret the experimental data it is necessary to take into account the surface layer of Debye, connected with the presence of the ions, responsible for the electrical conduction in nematics.

We investigate the influence of the electric field on the conductivity of an antiferroelectric liquid crystal formed by bananalike molecules. Our experimental analysis shows that large deviations of the current from a pure Ohmic behavior are observed for applied voltage of the order of the ones used for the determination of the spontaneous polarization in these types of materials. The measurements are performed in the isotropic and B2 phases. In the isotropic phase these deviations are interpreted in terms of the model proposed by Onsager, according to which, for weak electrolyte, the conductivity depends on the actual electric field present in the liquid. In the ordered phase we show that they are partially due to the external field induced reorientation of the director structure. A good agreement between the model and the experimental data is obtained, and an estimate of the coefficients taking into account the effect of the electric field on the conductivity is deduced. The relevance of this effect on the experimental determination of the spontaneous polarization is found to be of the order of 10%.

We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed.

A dielectric mode in electroclinic liquid crystalsA new method to measure the spontaneous polarization density P S of ferroelectric liquid crystals as a function of the temperature is described. It is based on the spectral decomposition of the polarization and measurement of the different temporal harmonics using a lock-in amplifier. It is particularly adapted to measure small values of P S in regions where different nonlinear process compete. This condition is fulfilled at temperatures around the SmC*-SmA transition temperature in liquid crystals. The method has a quantitative procedure to evaluate the saturation state of the sample's polarization at a given applied electric field. It overcomes the difficulty of the subjective evaluation of the hysteresis loop present in the usual methods. The accuracy in the measurement of P S of the ZLI-3654 ferroelectric liquid crystal in the region of about 0.5 nC/cm 2 is approximately one order of magnitude larger than that obtained with conventional hysteresis loop methods.

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