Table of contents

The Langmuir-Blodgett technique is a way of making ultrathin organic films with a controlled layer structure, which have many possible applications in technology for signal processing and transformation. Recent research has shown that their characteristics are strongly influenced by the highly anisotropic, essentially two-dimensional organisation of their constituent molecules. Novel effects in charge transport at the molecular scale ( approximately 1 nm) appear to be connected with very strong electron-phonon interactions, but are still not understood.

The statistical accuracy of measurements of the structure function of a Gaussian random fractal is investigated. It is shown that, for a given delay length, the relative error depends only on the number of data points within the sample and not on the actual sample length. The error is found to decrease more slowly with increasing numbers of data points for smooth fractals than for rougher ones. A procedure for estimation of the fractal dimension is also analysed and it is concluded that there is little to be gained by model fitting of structure function measurements spanning a large range of delays because the errors at different delays are correlated and increase with delay length. Analysis of experimental surface profile data supports these conclusions.

The frictional characteristics of keratin fibres are anisotropic. The most obvious characteristic feature is that the frictional work required to slide one fibre over another is greater in the direction tip-to-root than the converse; this is the so-called differential friction effect (DFE). The authors describe a series of experiments where fine polymer (polyethylene terephthalate) and carbon fibres are slid in the two directions with a range of normal loads over cleaned human, camel, guanaco and llama hairs in order to quantify the DFE. These hair fibres exhibit a range of asymmetrical cuticular structures which have been characterised using electron microscopy. It is suggested, in keeping with established wisdom, that it is this topographical asymmetry which is responsible for the DFE. An analytical model is developed and evaluated in order to rationalise the origins of the DFE. The model combines various precedents from contact mechanics, the adhesion theory of friction and earlier studies of friction. The predictions of the model are consistent with the experimental data.

Electron swarm behaviour in SF₆ and nitrogen mixtures is analysed over the E/N range from 141 to 707 Td by a three-term Boltzmann equation method. A set of electron collision cross sections, which was determined consistently with measurements for the respective pure gases by the authors ((SF₆, Itoh et al., 1988); N₂(Ohmori et al., 1988)), is used. The calculation is carried out for the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) experiments. In particular the TOF parameters of these mixtures are deduced for the first time as far as the authors are aware. It is found that the calculated values of the effective ionisation coefficient alpha shows a monotonic variation with percentage of SF₆ at a fixed E/N, the same as previous measurements. The E/N value at which alpha =0, i.e. the limiting E/N, is also deduced and in close agreement with measurements. The variations of the electron energy distribution and other transport coefficients with the partial SF₆ pressure are calculated and discussed in detail.

In the growth of a system of droplets (breath figures), the droplets increase in size by a combination of direct condensation of vapour at their surface, referred to as intrinsic growth, and by their coalescence. An analytical model is presented which predicts the transient response of this 2D constrained system of droplets to intrinsic growth that follows a simple scaling law, and, if required, to other growth laws as well. The model predicts the mean diameter of the droplets, the fraction of the substrate covered by the droplets, and the number of droplets per unit area, as functions of time. It is applicable from the onset of the condensation, during which the influence of coalescence on the growth process is small, up to and including the intermediate, self-similar regime, during which the influence of coalescence on the transient features of the growth system is significant. The model is validated by comparing its solution with results from computer simulations.

High electrical conductivity has been induced in silicate glasses containing Li₂O after subjecting them to a lithium/silver ion exchange treatment followed by a suitable combination of temperature and electric field. The room-temperature resistivities of these switched samples range from 50 Omega cm to 1000 Omega cm. The activation energy for conduction has values in the range 0.01 to 0.16 eV. The critical electric field for switching is found to be of the order of a few volts per centimetre. The field-induced switching is explained on the basis of growth of a silver-rich phase to the percolation configuration. Wagner's asymmetric polarisation cell measurements show that the charge carriers are ionic in the glasses in their highly conducting states.

It is shown that overtone properties of thickness-shear vibrations in contoured, AT-cut quartz resonators can be represented by solutions in terms of the degenerate hypergeometric function. Expressions are derived for the resonant frequencies, the dynamic inductance and the equivalent resonant radius. The effect of the geometry of the quartz piezoelement on the distribution of acoustic wave energy trapping is studied by X-ray transmission topography.

A theoretical relationship between the magnetic viscosity parameter S_v and coercive force H_c has been obtained, which gives a very good explanation of the Barbier plot. Magnetic viscosity coefficients S of several magnetic recording media have been measured. The results show that S- chi _irrS_v and chi _irr may be represented approximately by a Gaussian function.

Magnetic Barkhausen noise (MBN) measurements were made on a semicircular section of pipe applying the Barkhausen excitation field in different directions through 360 degrees . A DC bias field was then applied in the axial and circumferential directions and MBN measurements were made. A circumferential tensile stress of 129 MPa was then applied and MBN measurements were made with and without the DC bias field. The MBN data were analysed in the light of changes in the domain configuration due to the application of tension and/or the DC bias field. The relative orientations of preferred easy axis, Barkhausen excitation field, bias and residual fields and stress were all found to influence MBN generation.

The chemical composition and structure of silicon on insulator (SOI) formed by the combination of O⁺ (200 keV, 1.8*10¹⁸ cm^-2) and N⁺ (180 keV, 4*10¹⁷ cm^-2) implanted into silicon has been investigated by Auger electron, IR absorption and reflection spectroscopic measurements. After annealing at 1200 degrees C for 2 h, the in-depth composition profiles of the SOI structure show that the nitrogen segregates to the wings of the buried silicon oxide layer where it forms an oxynitride. The characteristic Auger spectrum for the buried oxide layer identified a chemical state of silicon whose major transition at 85 eV is different from that of bulk silicon or silicon in SiO₂ and the ratio of the amplitude of the Si peak at 85 eV to the Si peak at 76 eV remains unchanged in the buried oxide layer. Infrared (IR) absorption and reflection spectra in the wavenumber range 800-5000 cm^-1 were measured for the SOI structure. Interference fringes were observed in the IR spectra. By detailed theoretical analysis and computer simulation of the IR reflection interference spectra, refractive index profiles of SOI structure were obtained.

The photoelectron spectra of NEA GaAs photocathodes have been studied by the Monte Carlo simulation technique for both reflective and transmissive modes. It is concluded that the photoelectron spectra of the transmissive mode differ from those of the reflective one, and that the photoelectron transit time plays a dominant role in the electron energy relaxation mechanism.

A computational technique is proposed for the analysis of optical absorption spectra for a variety of materials in terms of the well established power law for the optical absorption coefficient. Values of the power index n and optical energy gap E₀ can be obtained from a linear plot using the standard method of numerical differences for a set of experimental data.

Fabrication and characterisation of photoelectrochemical solar cells employing flux-grown copper tungstate single crystals have been accomplished. Mott-Schottky plots have been drawn to provide a useful estimation of the properties of CuWO₄ from the measurements, and thus the nature and the values of the parameters N_D, E_C, E_F and E_V of the semiconductor have been determined. The J-V characteristics of the crystal in contact with KCl/H₂SO₄ electrolyte are discussed with reference to the charge transfer mechanism operative at the interface.

The finite size of a fractal cluster of dimension D causes a reduction in the dimension of its projection to D* on any finite observation scale. A theoretical argument is presented which relates the apparent dimension D* to D and to the observation scale as a fraction of the outer fractal scale. The theory is tested for computer-generated fractal clusters. When applied to a sample of electron micrographs of diesel soot particles whose apparent dimension is 1.83+or-0.06, the theory gives 1.90+or-0.07 as the true fractal dimension of the soot.