Table of contents

This review examines surface alignment of liquid crystals by exposure to light. Two distinct effects are discussed: illumination with polarized light induces a surface anisotropy to an alignment layer and hence a preferred in-plane orientation of the overlying liquid crystal director. Alternatively, a photochemical reaction of the alignment surface changes the liquid crystal anchoring conditions from homogeneous to homeotropic. We discuss how cis/trans isomerization, crosslinking and photodegradation are used to produce photoalignment layers and we show how the performance of the materials can be optimized by molecular design. Pretilted alignment is discussed and theoretical models are introduced to explain the photoalignment process. A range of display and non-display applications for photoalignment is presented.

The presence of defects in thin-film dielectrics often leads to dielectric relaxation as a function of frequency, in which the dielectric constant decreases and the loss tangent increases with increasing frequency. Dielectric relaxation results in charge storage capacity reduction under dynamic random access memory operating conditions. In this work, the dielectric relaxation behaviour of dc reactive sputtered Ta₂O₅ thin film was investigated. Using dielectric dispersion measurements as a function of frequency (100 Hz⩽f⩽10 MHz) and temperature (27 °C⩽T⩽150 °C), we determined the dielectric relaxation and defect quantity of the films and propose an equivalent circuit on the basis of complex capacitance, admittance and impedance spectral studies.

The effect of slot antenna structures on plasma production was investigated in the large-area planar surface wave plasmas (SWPs) excited with 2.45 GHz microwave energy. Plasma production characteristics were measured for various types of slot antennae (inclined, transverse, longitudinal and diverging slots) in Ar discharges. In all the slot antennae, we clearly observed the density jumps when the incident power and pressure were varied. These density jumps correspond to the mode jumps between transverse magnetic (TM) eigenmodes in the SWP. In spite of different slot antenna structures, the same optical emission patterns of the TM₆₂ mode were observed at the almost same electron density, except for the diverging slots, in which the TM₉₂ mode was selectively excited under the same operating conditions, where the incident power was 0.8-1.2 kW at a pressure of 100-280 mTorr. At lower pressures, say 10 mTorr, the plasmas entirely broadened over the chamber cross section for all the slot antennae. Among the four slot antennae, the transverse slot antenna was more efficient for plasma production under the same incident microwave power at a fixed pressure.

Using the transfer matrix method and Floquet's theorem we have derived the dispersion relation for acoustic wave propagation in a periodic layered structure in which each unit cell contains several sub-layers of different materials. Structures with unit cells containing two, four and six sublayers of different thickness and two types of materials were used as examples for our study. It was found that narrow passbands and broad stopbands could be obtained when the unit cell has more than two sublayers. The calculated results were verified experimentally using glass-water structures containing only one to three cells. Good agreement was obtained between the experimental results and the transfer matrix calculations. Desired bandgap structures can be produced in less than three cells, revealing application potential for the bandgap materials in vibration control devices and acoustic filters.

Electrostatic powder impact deposition (EPID) is a novel method to deposit thin films by electrostatic acceleration of powder material between charged plates in vacuum. The EPID method has been used to deposit Ge films on Si and Cu substrates at room temperature. Surface morphology and microstructure as studied by SEM showed a very rough surface. XRD and RBS measurements revealed that the films were mostly nanocrystalline or amorphous Ge oxide. The grain size distribution of the Ge powder was measured before and after deposition. Initial distribution showed a median grain size of 32 µm and distribution width of 80 µm. After the deposition the median grain size had decreased to 16 µm and the width decreased to 55 µm. The grain size of the deposited film was less than 1 µm.

The calculation of the residual stress tensor components in glass-coated amorphous ferromagnetic microwire is carried out on the basis of the theory of viscoelasticity. The approach takes into account the relaxation of the stresses both in a metallic core and a glass shell of the wire within a certain temperature interval near the point of the wire's vitrification. The distribution of the residual stresses is investigated as function of mechanical characteristics of metallic core and glass shell at different ratios of the metallic core radius to the total wire radius. The magnetic behaviour of a glass-coated amorphous microwire with small negative magnetostriction is analysed and is shown to be consistent with the experimental data.

An investigation has been made into plasma kinetics mechanisms of an optimized copper vapour laser by applying a genetic algorithm: (1) an impedance resonance of the thyratron takes place when the pulsed laser appears, and the matching between the laser tube and the discharging circuit is well improved by the optimization; (2) both the rapidly rising plasma electron temperature and the increasing electron density pump more particles so as to populate more quickly the upper laser levels during the discharging pulsed period after the optimization; (3) maintenance of a high wall temperature can increase the lasing population.

The relationship between beam focus position and penetration depth in CW laser welding was studied numerically and experimentally for different welding conditions. Calculations were performed using a transient hydrodynamic model that incorporates the effect of evaporation recoil pressure and the associated melt expulsion. The simulation results are compared with measurements made on a series of test welds obtained using a 1650 W CO₂ laser. The simulations predict, and the experiments confirm, that maximum penetration occurs with a specific location of the beam focus, with respect to the original sample surface, and that this relationship depends on the processing conditions. In particular, beam absorption in the plasma has a significant effect on the relationship between penetration and focus position. When the process parameters result in strong beam absorption in the keyhole plasma, the maximum penetration will occur when the laser focus is at or above the sample surface. In a case of weak absorption however, the penetration depth reaches its maximum value when the beam focus is located below the sample surface. In all cases, the numerical results are in good agreement with the experimental measurements.

The Pearson VII function, which is more general than the Gaussian, Lorentzian and other profiles, is used to fit the x-ray spectral lines produced in a laser-generated plasma, instead of the more usual, but computationally expensive, Voigt function. The mean full-width half-maximum of the fitted lines is 0.102±0.014 nm, entirely consistent with the value expected from geometrical considerations, and the fitted line profiles are generally inconsistent with being either Lorentzian or Gaussian.

We calculated the drift velocity vector W of electron swarms in crossed electric and magnetic fields using a Monte Carlo method. Values of W in F₂, CF₄ and constant-collision-frequency (CCF) model gases at various values of the reduced electric and magnetic fields, E/N and B/N (N is the gas molecule number density), were illustrated as vector loci in velocity space. Under a CCF condition, W draws right semicircular loci with varying B and N at a constant E when the number of electrons is conservative. These semicircular loci can be a reference to characterize the W behaviour in real gases. We presented an analytical derivation of W in the CCF condition, extending its expression to that in the presence of ionization and attachment. Features of the vector loci in F₂ and CF₄ were discussed.

In order to check our physical model for partial discharges (PDs) in a tree channel, the light emission from a PD in an actual tree was observed. The experimental results agreed with the model. By using the model, conditions for the formation of typical phase-amplitude-number (ϕ-q-n) pattern of PDs from treeing was discussed.

The model for partial discharges (PDs) in tree channels indicates that PDs in an electrical tree are inclined to be induced and concentrated near the needle tip, and the total field distribution is dynamically influenced by the PDs. In view of the effects of PDs on tree growth, a new model for tree growth is put forward. Tree growth is related to both the damage caused by PDs and by the local field. In the early stage of tree growth, the damage by PDs is concentrated near the needle tip, but the damage by the local field is inclined to be higher at tree tips. By using this model, various experimental phenomena can be explained.

When the electrode separation is long or the applied voltage is low, the tree cannot directly grow to the plane electrode, and extinction of partial discharge (PD) with the re-growth of new channels can be observed after a long time. Before PD extinction, the light emission of the PD changes from the whole tree structure to only the region near the needle tip, and the PD pattern also changes from wing-like to turtle-like. These phenomena are considered to be related to the increase of channel conductivity by PDs. Assuming the channel conductivity is higher in the region at tree tips or in the tree channels than near the needle tip, the simulation results coincide with the experimental results.

The relaxation spectrum is an important tool for studying the behaviour of viscoelastic materials. The most popular procedure is to use data from a small-amplitude oscillatory shear experiment to determine the parameters in a multi-mode Maxwell model. However, the discrete relaxation times appear nonlinearly in the mathematical model for the relaxation modulus. The indirect calculation of the relaxation times is an ill-posed problem and its numerical solution is fraught with difficulties. The ill-posedness of the linear regression approach, in which the relaxation times are specified a priori and the minimization is performed with respect to the elastic moduli, is well documented. A nonlinear regression technique is described in this paper in which the minimization is performed with respect to both the discrete relaxation times and the elastic moduli. In this technique the number of discrete modes is increased dynamically and the procedure is terminated when the calculated values of the model parameters are dominated by a measure of their expected values. The sequence of nonlinear least-squares problems, solved using the Marquardt-Levenberg procedure, is shown to be robust and efficient. Numerical calculations on model and experimental data are presented and discussed.

Synchrotron white beam with a wavefront limited by a 5 µm slit was used for obtaining the Bragg-case section patterns in silicon substrates and epitaxial layers. The section images contained various interference fringes, such as the Uragami fringes and fringes caused by crystal curvature. The system of fringes connected with individual defects was also observed.

The experimental images were compared with simulated theoretical images obtained by numerical integration of the Takagi-Taupin equations. A reasonably good correspondence was obtained for dislocations inclined to the surface and misfit dislocations. The elements of the image were analysed using the visualization of |D_h|² and |D_o|² intensities in the plane of diffraction, where an additional amount of transmitted wave intensity indicated the decomposition of wavefields or the reflection of the redirected waves from the surfaces. Comparative studies of simulated precipitation images and modification of dislocation images caused by curvature and by the diffusion of an epitaxial junction were also performed.

A TGS ((NH₂CH₂COOH)₃·H₂SO₄)/silicone oil suspension, which is the simplest system, is employed to investigate the complex permittivity dependence of the electrorheological (ER) effect under ac electric fields. By measuring the temperature dependence of the shear stress across the Curie temperature of TGS particles, the dependence of the ER effect on dielectric constant has been reliably obtained under ac fields. Under high-frequency fields in which range the dielectric mismatch dominates, the shear stress increases with the particle dielectric constant. The experimental data are in agreement qualitatively with the previous mismatch models although deviation appears. The shear stress of the ER fluid was also measured at intermediate frequencies.