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A study of experimental techniques for the determination of diffusion coefficients for binary mixtures and a study of the existing relationships for these coefficients were carried out. A new three-compartment diffusion cell was developed capable of measuring diffusion coefficients at temperatures up to the normal boiling point (24). By means of this cell, diffusion coefficients were measured for the systems ethanol-water, acetone-water and acetone- chloroform for a range of temperatures up to the normal boiling points. Thus diffusion coefficients for the above mixtures including those at boiling points and at infinite dilution are presented. A relationship was developed to relate diffusion coefficients with temperature and concentration (equation 3-1.21) in binary systems. It agrees better with the experimental data for the associated systems than some literature correlations. By application of parachors a new equation (3-2.4) was developed for the prediction of diffusion coefficients at infinite dilution , (201). This equation, because of the ease of calculating parachors, is more convenient to use than other equations based on the Stokes-Einstein equation. An additive method for the prediction of self-diffusion coefficients was introduced and a correlating equation (3-3.4) was developed. The bond and structural contributions to the constant of the equation were calculated on the basis of a limited amount of experimental data. Despite this the correlation gives reasonable predictions for the temperature range between melting point and boiling point. Another correlation for the prediction of self-diffusion coefficients was developed (203) (equation 3-3.6) by modifying an existing equation. This was possible by applying the relationship between the molal volume at the boiling point and the critical molal volume developed in this work (202). The new equation is more convenient to use. The correlating property of the critical temperature was used to devise a relationship between diffusion coefficients, critical temperature and the working temperature. The two correlating equations (3-4.6) and (3-4.7) can predict diffusion coefficients at various temperatures if one value of the diffusion coefficient at a single temperature is known.

The Reactive Atom Plasma® (RAP) process is a plasma chemical etching process. RAP was developed at RAPT Industries as a process for removing subsurface damage from silicon carbide optics. The process is being investigated at Cranfield University as a novel method for the fine surface correction of large optics, with the aim of shortening the manufacturing period of the next generation of large telescopes. RAP offers material removal rates that are up to 10 times higher than those of ion beam figuring, the current state-of-the-art technique and the convenience in that it can be operated at atmospheric pressure. Cont/d.

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This thesis is concerned with developing suitable models for the investigation of instabilities in rigid and deformable roll coating flows. Consideration is restricted to Newtonian, incompressible fluids in the absence of inertia. In each coating flow analysed the steady two dimensional base state solutions of the system axe explored before the stability of the system is considered. The particular rigid roll coating flow that is studied in this thesis is the flow formed between an intermediate pair of contra-rotating rolls within a multiple roll coater. The base state is modelled using lubrication theory and the flow domain is terminated with simple boundary conditions. It is found that the form of the meniscus location solutions differ from those found in similax flows by previous workers and this motivates a detailed re-examination of the boundaxy conditions. A matched asymptotic analysis applicable for small capillaxy numbers Ca and small inter-roll gaps gives the leading order correction to the boundary conditions and it is found that the inclusion of the correction terms alter the results significantly. It is found that a critical roll speed ratio, Sc exists beyond which no steady two dimensional solutions exist. A simple stability hypothesis predicts the upstream meniscus to be neutrally stable at Sc and this point is associated with bead break. Under certain operating conditions the stability hypothesis predicts multiple steady states. The lineax stability of this multiple roll coating flow is investigated. Special attention is given to the relationship between the wavelength of the disturbance and the form of the boundary conditions. The main predictions of the stability hypothesis axe confirmed and an additional ribbing instability on the downstream meniscus is predicted. The base flow of a deformable roll coater is investigated using lubrication theory for the fluid and a linear elastic plain strain model for the compliant layer. The boundary conditions developed for the multiple roll problem are extended to higher values of Ca. A finite element method is developed to numerically solve the governing equations. The effects of Youngs modulus E and layer thickness L on the steady state operation of the coater are investigated. Previous workers have used spring models to describe the compliant layer with the implicit assumption that E and L-1 have the same affect. Here it is shown that E and L-1 have a different effect upon the meniscus location in the negative gap regime and hence the underlying assumption of all spring models is shown to be incorrect. The plain strain model is extended to include viscoelastic terms and it is shown that these terms can account for the discrepancy between experimental results and previous steady state elastic theories. The lineax stability of the deformable roll coater is investigated with the plain strain model being extended to account for lateral disturbances. The effect of E and L-1 on the stability of the system is investigated and it is found that in keeping with the steady state results, they have a different effect on the stability of the system in the negative gap regime. It is demonstrated that perturbations to the compliant layer play a negligible role in the stability analysis and it is shown how the viscoelastic extension to the base state can be incorporated.

This thesis contains literature reviews relating to inductively coupled plasmas and their use in etching gallium nitride with chlorine based plasmas. The properties of gallium nitride, how these properties make gallium nitride a suitable material for high power microwave transistors and how such transistors will help improve the systems in which they might be used are also reviewed. In this thesis, a novel, non-destructive method of measurement of the conductivity of a semiconductor through measurement of the increase in the bandwidth of the resonant peak of a microwave dielectric resonator when it is brought near a semiconductor wafer is presented. Using this method the conductivity of a thin gallium nitride film is obtained and found to be within the expected range it was found to be very difficult to measure the conductivity of this gallium nitride wafer using a four-point probe, as the film was too thin. Also presented in this thesis are studies of the etch characteristics of gallium nitride and photoresist in mixed boron trichloride and chlorine plasmas generated in two Oxford Instruments inductively coupled plasma etchers (ICP 180 and 380). The ICP 380 was used to etch the mesa isolation of gallium nitride based heteroj unction field effect transistors that were fabricated at Cardiff University. A method of making the angle of the mesa sidewall acute by melting of the photoresist is presented. An acute mesa-sidewall angle facilitated the easy traverse of the mesa edge by the gate metal. Characterisations of ohmic and Schottky contacts that were fabricated as part of the effort to produce a working gallium nitride based heteroj unction field effect transistor are presented and reasons given for the failure of some of the ohmic contacts. The dc characteristics of the best transistor fabricated during the project are presented.

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