The accumulation of undesired matter at heat transfer surfaces (fouling) is a severe problem to industry. The growth of calcium sulphate dihydrate on heated metal surfaces has been examined and a mechanism for the effect of surface roughness on the amount of deposition has been proposed. A novel piece of equipment was designed and constructed in which the growth of crystals on heat transfer surfaces with different surface roughnesses could be observed under controlled solution conditions. The test section was a transparent rectangular channel into Rhich three 25 mm diameter test pieces could be inserted such that the polished faces were planar with the rear face of the channel. The back faces of these test pieces were heated by contact with hot Mater. Using Reynolds numbers of 300 to 13,500 and calcium sulphate 2+ solutions with bulk concentrations from 20 to 50 mH Ca , no gross effect due to surface roughness was seen. However a limited effect, which distinguished grit-blasted surfaces from polished surfaces, was 2+ found in experiments with a bulk concentration from 28 to 33 mH Ca . In all of the experiments it was observed that the presence of bubbles enhanced crystal growth. It was also found that the amount of deposition formed on any surface decreased with decreasing dissolved oxygen content of the bulk solution. It is suggested that a bulk concentration of approximately 33 mH 2+ Ca is a critical level of supersaturation, which corresponds with the so-called metastable limit of supersaturation. The surface roughness effect may be associated with two factors. Firstly, as the critical supersaturation is approached crystal growth is enhanced at certain sites. in particular the edges of bubbles. Secondly, very rough surfaces. such as grit-blasted surfaces. more readily support and initiate bubble formation and consequently the grit-blasted surface shows greater growth. Additional work with a different test rig. using a stagnant solution, indicated that suppression of bubble formation during an experiment, either by initially degassing the surface or by pressurising the system, decreased the amount of crystal growth. This work suggests that a surface roughness effect exists which is related to the presence of bubbles at the surface. Consequently the amount of deposition can be decreased either by controlling the formation of the bubbles (e.g. by completely degassing both the solution and the surface. and pressurising the system). or by polishing the surface. However. the surface roughness effect was most apparent between the very coarse, grit-blasted surfaces (R ~ 2.90 um) and the a medium finish 180 or 240 Grit surfaces (R ~ 0.23 um). a Little or no further improvement was obtained in polishing to a mirror finish. i. e. 0.25 or 0.1 um (R ~ 0.03 um). a further polishing beyond removing gross roughness would not appear to Therefore. for industrial purposes, yield sufficient benefit to justify the large cost involved. It is recommended that further work be undertaken to determine whether the deposition which occurs in the absence of bubbles is influenced by the surface roughness, and whether other chemical system and surfaces behave similar to the calcium sulphate - stainless steel system studied in this work. Moreover, additional work should be undertaken to examine the effect of surface physical properties on crystal and bubble nucleation and growth.

No description available.

Recent high-pressure experiments on iridium show a transition to a 14 atomic layer superlattice structure. Since iridium has a high bulk modulus, it is used in many high-pressure applications, for instance as a gasket for high-temperature, high-pressure diamond anvil cell experiments. The effects of pressure on this material are hence of interest. Of the transition metals, the iridium surface has been one of the most extensively studied surfaces experimentally. The field ion microscope has made it possible to observe in detail the behaviour of adatoms on the surface, and has led to interesting discoveries of the nature of atomic adsorption on the lr(111) surface. A number of theoretical and semi-empirical studies have been made on this topic. However, none of these studies take atomic relaxations into account in a satisfactory manner, and therefore do not give a complete understanding of the process of incorporation of adatoms onto the surface. In the present work, first-principles total energy calculations based on the plane wave pseudopotential method within the framework of the density functional theory are employed in the study of the bulk properties of iridium, and the crystal phases and defect structures of iridium under pressure. The bond-orientation model is extended to include the effects of pressure, and used to compute all of the ~2n defect structures of iridium as a function of atomic volume. Allowance for full atomic relaxations is made in computing the ideal and relaxed surface formation energies of the three low-index surfaces of iridium, and in investigating the nature of adsorption of single adatoms on the lr(111) surface. The formation energy of a vacancy on the Ir(111) surface is also computed. This is the first time such a calculation has been made. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2003.

No description available.

The interaction between high-n Rydberg states of molecular hydrogen and metal surfaces has been investigated for the first time. Rydberg states of hydrogen possessing either 0 or 2 units of rotational angular momentum, defined by the quantum number N⁺ , and principal quantum numbers in the range n= 17 22 (for the N⁺= 2 states) and n=41-45 (for the N⁺= 0 states) are directed at a grazing angle onto a metal surface (gold or aluminium). At a sufficiently close distance ionisation may occur via tunnelling of the Rydberg electron into the vacant metal conduction band. Any ions formed in the vicinity of the metal are extracted by the application of an electric field and information about the distance at which the ions are formed can be inferred from the magnitude of the applied field required for detection. Two novel effects are observed. Firstly, it appears that the rotation of the H2⁺ core has a significant effect on the ionisation properties of the Rydberg states in a manner akin to rotational autoionisation, such that the rotational energy of the core is given up to the Rydberg electron. Secondly, the surface ionisation profiles do not vary smoothly with applied field suggesting that at certain fields the feasibility of ionisation is either enhanced or reduced. A preliminary discussion of the origin of the structure is presented in terms of the crossings in the Stark map between the N⁺= 0 and N⁺= 2 Stark manifolds. The development of a theoretical model, and an associated Fortran program, involving the technique of complex scaling is also reported. The hydrogen molecules are modeled using an atomic hydrogen system which provides a good first approximation to the behaviour of the Rydberg electron for states with n > 5. Energies and linewidths, for states with principal quantum number n= 6 9 interacting with a model surface, are explicitly calculated at a range of surface separations. From this information, predictions of the ionisation behaviour expected for states of higher principal quantum number are presented.

The influence of modal interactions on the response of harmonically excited flexible L-shaped metallic and composite structures has been investigated analytically and experimentally. Each metallic structure possesses a two-to-one internal resonance, while each composite structure possesses a three-to-one internal resonance and either a two-to-one or a one-to-·one internal resonance. For the metallic structures, a weakly nonlinear analysis is used to derive the autonomous system of equations which describe the evolution of the amplitudes and phases of the internally resonant modes. These equations are obtained for primary- and secondary-resonant excitations. The excitation frequency or amplitude is used as a control parameter and the resulting bifurcations (saddle-node, pitchfork, and Hopf bifurcations) are studied. Theoretical analyses for internally resonant systems are used to predict and explain the responses of the composite structures. / Ph. D.

Performance of air operated valves is a major maintenance concern in process industries. Anecdotal information indicates that reliability of some high maintenance valves has been improved by using an ion deposition process to achieve engineered surfaces on selected components. This project compared friction for various surface treatments of selected valve components. Results indicate valve performance may be slightly more consistent when an engineered surface is applied in the valve packing area; however surface treatment in this area does not appear to have a dominant affect on reducing valve friction. Results indicate a linear relation between stem friction and torque applied to packing flange nuts, and even after a valve is in service, controlled packing adjustments can be made without significantly changing valve stroke time.

[by] W.B. Nowak. / "May 27, 1949." / Bibliography: p. [146]. / Army Signal Corps Contract No. W-36-039 sc-32037, Project no. 102B. Dept. of the Army Project No. 3-99-10-022.

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This dissertation addresses atomic-level structure and related properties of the (100) surfaces of Vanadium and Niobium. The primary motivation of the surface structure experiments is based on an observation by P.J. Feibelman [[superscript 3,4]] that ab-initio calculations of surface relaxation (interplanar separation of near-surface planes) exhibit significant and consistent deviation from corresponding experimental relaxation values obtained through LEED crystallography. The deviation is much larger than the accepted accuracy of both the theoretical and experimental techniques. Resolving this dilemma is the primary objective for this systematic investigation of multilayer relaxation at transition metal surfaces. This research also addresses important secondary issues. Closely related to the surface relaxation issue for Nb(100) are experiments that provide an experimentallydetermined structural basis for the "sub-surface valve" model that has been proposed to explain the novel hydrogen uptake kinetics in Niobium. Definitive experiments that address the longstanding search for ferromagnetic order at the surface of V(100) is another important issue.