The structure and catalytic aspects of tungsten bronzes have been considered. A series of potassium tungsten bronzes, KxW03, 0.05 =< x =< 0.8, and the corresponding series of sodium tungsten bronzes, NaxW03, 0.05 =< x =< 0.8 were prepared by a thermal method. The thermal stability of the prepared samples was studied in the presence of both an oxidising and a reducing gas. The number and strength of acid sites present on the bronzes was studied by temperature-programmed desorption of ammonia. Xps, xrd and electrical resistivity measurements gave information about the bulk and surface properties of the bronzes. The activity of the tungsten bronzes for isopropanol decomposition and propene metathesis was investigated. In addition, a silica-supported tungsten bronze and a copper-potassium tungsten bronze, CUyKxW03, were prepared. Their structures and catalytic values were considered.

Studies of semiconducting glasses have become of great interest in recent years because of their importance in the theory of solid state physics and in their applicability to electronic devices. Of these classes of materials the transition metal oxide glasses are much the most thoroughly studied. In these glasses the transition metal oxide e. g. V205-TiO2-MoO39 WO3, Cu0 is a major constituent i.e > 50 mol%. We start this work with a discussion about a critical review of the history, formation and modern theory of non-crystalline semiconductors in general and glassy state in particular. The aim of the experimental work in present study is to check the validity of the theories and models proposed so far to explain the origin and the nature of the charge carriers, structure, electrical and optical properties of some transition metal oxide glasses based on vanadium pentoxide. For this purpose series of binary V205-P205 glass samples containing 50 to 90 mol% V205 as well as ternary V2015-P205-Te02 glasses containing 60 mol% V2051 (40-x) mol% P205 and x mol% Te02 in which x varies from 5 to 35 were prepared by normal cooling from the melt. It was found that the glass forming region of the system under consideration is fairly large and in binary V205-P205 systems, glass with up to 95 moll V205 could be prepared. Density measurements indicate that in binary V205 - P205 glasses the density increases linearly with increasing V205 content and in ternary V205 - P205 - TeO2 systems density increases with increasing Te02 content. It was also found that the density of both systems are affected by annealing temperature. Electron spin resonance (E.S.R.) studies show that in vanadate glasses, the vanadium exists in more than one valency state mainly V5+ and V4+ of which V4+ is paramagnetic and detectable by E. S. R. This could be taken as some evidence of a hopping conduction mechanism in vanadate glasses and conduction is due to transfer of charge form V4+ to V5+ ions and this is discussed in the terms of small polaron. It is found that the conduction in these glasses is Ohmic up to a field of the order of 4x 105 Vcm-1 with an activation energy range from 0.31 to 0.48 e.V depending on composition and independent of temperatures in our range of temperature (above room temperature). Above this field conduction becomes non-Ohmic which is found to be due to lowering the potential barrier of the carriers at high electric field as was predicted by Poole and Frenkel. Memory switching is observed in thin blown film samples of both binary and ternary glass systems, which is associated with field-induced crystallization of a localized region and formation of conductive channel in the switched area due to self heating effect. In other words the conducting zone consists of VO2 crystals which possess more metal-like conductivity. Infra-red absorption spectra of these glasses revealed that some of the absorption bands of glasses and crystalline V205 are similar which is some evidence that the vanadium ions exists in six-fold co-ordination in disordered glassy systems as well as ordered crystalline V205. The fundamental absorption edge of these glasses occurs in the short wave length region of the visible and is dependent on composition, and the fundamental absorption arises form direct forbidden transitions and occurs at a photon energy of about 1.9-2.6 e.V depending on composition. The absorption edge in these glasses is found to be of the same order of magnitude as that in crystalline V205.

Separating the study of kinematic geometry of the human knee from the study of its behaviour under load provides insight into the complex relationship between form and function at the joint. The development of a three-dimensional mathematical model which examines the mobility and stability of the joint in sequence is described in this thesis. A previously proposed model of knee mobility, in which the ligaments and ar- ticular surfaces act as rigid constraints between the bones in a single degree-of- freedom spatial mechanism, was re-examined and its limitations addressed. A new geometric-numerical approach to solving the model kinematics, capable of handling both idealised and more anatomical representations of the articular surfaces, was developed. A database of specimen-specific motion and geometry was established, based on cadaver studies. Articular contact kinematics and ligament length patterns were also quantified. In experiment, all components of passive knee movement were found to be coupled to the flexion angle, providing justification for the underlying concept of the model of knee mobility. Specimen-specific models of mobility were successful in predicting the main fea- tures of passive knee motion through a full range of flexion. Incorporation of second order tibial articular surfaces permitted the prediction of physiological motion com- patible with more realistic contact point movement. Through incorporation of continuous three-dimensional arrays of extensible lig- ament fibres, a preliminary model of knee stability was formulated. Although in need of further refinement, sample predictions of joint behaviour during a/p drawer and axial rotation have demonstrated the potential of the model in highlighting the subtleties of ligament mechanics. It was concluded that the sequential approach is appropriate for the study of joint behaviour in three dimensions and that, based on the success of the analogous two-dimensional theory, it provides an invaluable tool in the study of joint mechanics in activity and in the design and assessment of surgical procedures for treating knee injury and disease.

I have studied two magnetic properties of superconductors using different neutron scattering techniques. Due to their magnetic moment and lack of electrical charge, neutrons can provide an extremely sensitive probe into magnetism on a small scale. Unusual magnetic structures have previously been observed in the non-superconducting PrBa₂Cu₃O₆+_δ, in particular showing a coupling between the rare-earth and the Cu-O₂ planes. For comparison, I have used elastic neutron scattering to measure the temperature dependent magnetic structure of the high-T_c superconductor NdBa₂Cu₃O₇, and its nonsuperconducting reduced form NdBa₂Cu₃O₆. The magnetic structures of both the Nd and Cu sub-lattices are presented. Measurements are shown for single crystals of both the oxygenated and reduced form, and also for a non-stoichiometric single crystal with Nd substituted on the Ba sites. I present a quantitative analysis of the magnetic moments, with account taken of the instrumental resolution including sample mosaicity and intrinsic peak shapes. No evidence was found for any coupling of the type seen in PrBa₂Cu₃O₆+_δ. Many of the underlying properties of superconductors are evidenced in the way magnetic fields are excluded in the different states. I have used polarized neutron reflectometry to measure the magnetic field profile in thin film superconductors. I present measurements for pure niobium and preliminary measurements for YBa₂Cu₃O₇. The samples studied are much thicker than those previously measured, to provide a closer match to the bulk superconductor properties and allow meaningful measurements of the higher field states. For niobium, the applied field dependence is measured for fields covering the Meissner and mixed states, and models are compared for these states and the surface superconductivity state.

A complex simulation model, MAESTRO, has been developed and validated against field measurements in plantation in both Scotland and Australia. It has been shown that MAESTRO can reasonably predict the daily course of PAR (photosynetically active radiation) transmittance at points below the canopies of radiata pine and Sitka spruce plantations. 1. Four structural properties of the Sitka spruce tree crown have been identified and evaluation in relation to PAR absorption, photosynthesis and transpiration. They are : the total amount leaves within the tree crown, leaf inclination angle distribution and crown shape. The total area of leaves and their spatial distribution within the tree crown are ranked as the two most significant properties. 2. Among the four different structural properties studied, crown shape is least important for PAR absorption, photosynthesis and transpiration. For the Sitka spruce stand studied, transpiration was most sensitive to the total area of leaves within the tree crown, whereas photosynthesis can be significantly affected by the leaf angle distribution. The influence of crown structure also depends on the beam fraction, radiance of diffuse radiation, incident angle of beam radiation. 3. Application of MAESTRO has shown the importance of the spatial distribution of leaf area density distribution and the age structure of the leaf population within a dense Sitka spruce canopy for the radiation regime and for photosynthesis. It has been found that the non-uniform canopy structure. The actual leaf age structure within a dense Sitka spruce canopy has been compared with the leaf age structure that is optimal for maximising the PAR use efficiency of the canopy and it has been found that PAR use efficiencies of both canopies are very close. 4. The non-uniform structure of a tree crown has been described using a two-dimensional leaf area density distribution function within the tree crown. It was found that the spatial distribution of leaf area density and the age structure of the leaf population within the crowns of radiata pine trees were significantly affected by water stress and nutrient deficiency during the growing season. 5. The radiance distribution of diffuse radiation from the sky and the beam fraction of the incident radiation also affect PAR absorption, photosynthesis and transpiration by altering the uniformity of the radiation regime within the tree crown. The light use efficiency of the canopy decreases with the increase in beam fraction. 6. The stomatal and photosynthetic properties of the shoots of Sitka spruce were measure in an open path gas exchange system to parameterize a biochemical photosynthesis model. It was found that co-limitation of photosynthesis plays an important role in the PAR responses of both an isolated shoot and of a canopy. It was shown that co-limitation of canopy photosynthesis enables the canopy to use a high flux density of incident PAR efficiently. 7. The dry matter production of the above-ground biomass of a Sitka spruce stand was studied in relation to PAR absorption to test the Monteith hypothesis. It was found the relationship between the above-ground dry matter production and PAR absorption is approximately linear. The slope of this linear relationship is affected by nitrogen fertilization, but not by thinning for the Sitka spruce stands studied. Crown structure plays an important role in the processes or radiation transfer, photosynthesis and transpiration within the tree crown. Detailed descriptions of the important structural properties of the tree crown are necessary for adequate understanding of canopy processes, such as photosynthesis. Co-limitation of crown photosynthesis, determined by the structure of all the tree crowns in the stand, results in an efficient use of the incident solar radiation by the trees in the stand. Photosynthesis of the forest canopy is roughly proportional to the incident PAR. PAR absorption is the primary determinant of dry matter production of the above-ground biomass for the Sitka spruce stand studied.

Double crystal x-ray topography using a synchrotron radiation source has been used to measure the lattice distortions present in 50mm diameter samples of (001) semi-insulating LEG gallium arsenide. Lattice strains and tilts have been mapped in In-doped and undoped samples as well as annealed and unannealed samples taken from the seed and tail ends of boules. The properties of the x-ray source which are necessary for these measurements are discussed and it is concluded that a synchrotron source is the only practical choice. Lattice strains of 90ppm and tilts greater than 100 arc seconds were measured in In-doped material both of which appear to be due to a combination of In concentration variations and the inhomogeneous dislocation distribution. Undoped samples were found to be more uniform with lattice strains of typically +20ppm towards the samples edges where the dislocation density is largest. The lattice tilt distribution in seed and undoped samples invariably exhibited a four-fold symmetry which was enhanced by the presence of lineage features lying along the <110> directions. Tail end samples were generally less uniform in lattice strain and showed a lower symmetry in their lattice tilts. These results are discussed in the light of current ideas concerning the origin of variations in lattice strain and EL2 concentration. An x-ray diffraction method involving integrated intensity measurements of the quasi-forbidden 200 reflection, which is highly stoichiometry sensitive, is investigated. The results, however, show no conclusive stoichiometry variations but do highlight important experimental conditions which must be satisfied if such measurements are to be meaningful. The images of dislocations in double crystal x-ray topographs are investigated and compared with theoretical simulations in order to assess the effects of point defect environment on the dislocation strain field. The results suggest that the EL2-dislocation interaction is not significantly strain driven.

In this study reliable film type NTCR thermistors based on NiMn(_2)O(_4)+δ were produced and their electrical properties were studied in detail. Electron-beam evaporation procedures have been applied to produce thin film NTCR thermistors. Phase pure NiMn(_2)O(_4)+δ target material was produced via a traditional ceramic precursor oxide route and thin films were deposited in an optimised procedure. The thickness distribution of evaporated films showed good agreement with a theoretical model, derived from evaporation theory and the sticking coefficient of the vapour on the substrates was approximately 80% ± 1.5%. The composition of electron-beam evaporated films was found to be not controllable in terms of the phase purity and the Ni : Mn ratio. In order to avoid these problems thick film NiMn(_2)O(_4)+δ NTCR thermistors were developed using direct screen-printing techniques. Detailed Rietveld refinement analysis was carried out for the source powder used for screen-printing. The main focus of the work was the measurement of resistance-temperature (R-T) characteristics of thin and thick films and pellets. In the temperature range of concern (77 K -550 K) conduction was found to be by variable-range hopping (VRH) and nearest-neighbour hopping (NNH); R ~ exp (TʆT)(^p), where the index p depends on the mode of hopping. Detailed analysis of R-T data showed that screen-printed films and pellets exhibited a p-value of 0.5, which was identified with VRH with a parabolic density of states (DOS) with an exponential dependence of resistance: R ~ exp (TʆT)(^0.5). For electron-beam evaporated films the mechanisms detected were NNH: R ~ exp (TʆT); and VRH with a constant DOS {p = 0.25) following: R ~ exp (TʆT)(_0.25). For screen-printed films with incorporated glass phase the electrical conduction mechanism was analysed using a.c. impedance spectroscopy and at low frequencies the hopping conduction was in agreement with the d.c. behaviour. The time constant of this mechanism could be described by an equivalent circuit containing a RC element. For higher frequencies a second mechanism was found, best described by a CRL element.

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No description available.

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