Modelling of degradation i n black copper photothermal collector coatings

The degradation mechanism in black copper photothermal collector coatings was investigated through the use of kinetic analysis, microstructural determinations and optical modelling. The initial structure of black copper was identified using reflection electron diffraction, scanning electron microscopy and Auger electron spectroscopy sputter profiles. These results were used to develop an optical model of the as-deposited coatings. In this model, the coatings was best described as a two-layer film. The layer next to the substrate consists of dense copper oxide with metallic copper inclusions, while the rough outer layer is modelled as copper oxide dispersed in air. A substantial decrease in the solar absorptance (from 0.96 to 0.80) of coatings exposed to temperatures above 150°C in air was found to occur within 30 min and was explained by a decrease in the surface roughness of the coatings. After longer exposures, an increase in the thickness of the oxide layer near the substrate occurs at the expense of the surface layer. Incorporating this change in the model, the optical properties after thermal ageing were predicted.     

Ion-plated aluminium/aluminium oxide coatings using a pulsed oxygen process

Pulsed oxygen was utilized in a thermionically assisted triode ion plating process to produce dense, hard and highly adherent aluminium/aluminium oxide coatings on steel substrates. The coatings were deposited in a 0.1 Pa argon discharge while oxygen gas was injected at various pulse rates into the chamber. The influence of the oxygen flow rates, the pulse rates and the substrate bias on the structure and crystal phase of the coating were investigated using scanning and transmission electron microscopy, X-ray diffraction and Rutherford backscattering techniques. The analysis showed that the resultant coating structure consist of a mixed state of crystalline and non-crystalline phases depending on the oxygen flow rate and on the substrate bias at a constant oxygen pulse rate. Analysis showed that the non-crystalline phase approached stoichiometry for Al₂O₃ at oxygen flow rates of 7.5 Torr l s^-1 and a substrate bias of 2 kV and that there was a substantial increase in hardness.     

RF magnetron sputtered aluminium oxide coatings on iridium

The effects of process parameters on the microstructural morphology of aluminium oxide (Al₂O₃) coatings on Ir have been studied. Al₂O₃ coatings were deposited on Ir-coated isotropic graphite (IG) substrates at substrate temperatures of room temperature (RT)-1073 K, RF power of 200–600 W in an Ar, or Ar+1–10% O₂, sputtering gas atmosphere by RF magnetron sputtering. Al₂O₃ coatings which were deposited at high substrate temperatures and high RF powers in an Ar, or an Ar+O₂, sputtering gas atmosphere were found to contain a dense, fine columnar structure with a -Al₂O₃ phase, low Ar content and a relatively high hardness value of ca. 1050 H _v. Furthermore, high resolution transmission electron microscopy (HRTEM) results revealed the epitaxial growth of Al₂O₃ coatings on Ir-coated IG substrate. It was found that the interface between Al₂O₃ and Ir coatings was sharp and Al₂O₃ coatings remained intact with the Ir-coated IG substrate.     

Modelling of the scratch resistance of particle filled sol-gel coatings on aluminium

Sol-gel derived coatings with 12 nm and 0.5-10 μm silica fillers were prepared on aluminium to evaluate the effect of particle size and filler content on coating properties The measured maximum crack-free thickness and hardness strongly depended on the type of particles used and the filler volume fraction. The scratch resistance primarily depended on coating thickness and much less on the mechanical properties of the coating. This finding was interpreted via modelling of the stresses under the scratching stylus. It was established that the initial yield occurred in the substrate for most of the coatings and the load needed for this initial yield had a relatively small dependence on the coating properties but it was strongly influenced by the coating thickness.     

Characterization of optical coatings by photothermal deflection

An overview of photothermal deflection principles and applications is given. The modeling of temperature distribution and the calculation of deflection that is due to both the refractive-index gradient and the thermal deformation of the sample are presented. Three configurations usually employed are compared, and their respective advantages are discussed in relation to their application. The calibration for absolute measurement of absorption is detailed, showing that calibration limits the accuracy of measurement. Some examples of specific information obtained by photothermal mapping of absorption are given.     

A reflection electron diffraction study on some metal oxide solar collector coatings: Part I

The capability of reflection electron diffraction (RED) in the structural analysis of thin films is shown with studies made on copper oxide, zinc oxide, black chrome and cobalt oxide solar-selective coatings. The growth of copper oxide layers could be observed as changing from an initial epitaxial growth to random orientation followed by growth of oxide needles. The resulting copper oxide coating was found to consist of a needle structure of cupric oxide formed on a smooth cuprous oxide layer. In the zinc oxide overgrowths, epitaxial growth could not be observed, and the coating consisted of relatively small zinc oxide crystallites throughout the film. No crystalline zinc inclusions could be found in this layer.Black chrome deposited onto nickel showed an increase in chromium oxide with distance from the substrate, although no change in the orientation of the microcrystalline chrome could be observed. The formation of black cobalt was also investigated using scanning electron microscopy (SEM) and reflection diffraction analysis. The plated cobalt overgrowths were found to change from an initially smooth nodular coating to a plate-like oxide layer with heat treatment time. This latter layer could be indexed to cobalt(II, III) oxide.The study conclusively shows that RED, combined with SEM and ion milling procedures, can indeed lead to structural characterization of thin films without the need for their removal from their substrates and the concominant introduction of artifacts.     

Pyrolysis-induced polymetallosiloxane coatings for aluminium substrates

Inorganic amorphous polymetallosiloxane (PMS) coating films on aluminium substrate were produced through the polycondensation-pyrolysis reaction mechanisms of a sol-precursor solution. The precursor solution was formed by HCl-catalysed hydrolysis of a mixture ofN-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPI) and M(OC₃H₇) _n , (M=Zr, Ti and Al,n=3 or 4). The TSPI/Zr(OC₃H₇)₄ or Ti(OC₃H₇)₄ precursor systems formed higher quality thin coating films, compared to the /Al(OC₃H₇) system. This was because of the critical formation of the polyorganosiloxane terminated by end groups containing zirconium and titanium oxides. These end groups were derived by a dechlorinating reaction between the Cl, bonded to the propyl C in organosilane, and the hydroxylated Zr or Ti compounds in the sintering stages of the film production. Good film-forming performance resulted from moderate degrees of cross-linking of metal oxides to polysiloxane chains and of the densification of metal -O-Si linkages in the pyrolysis-induced PMS network. In addition to these factors, the formation of an oxane bond at the interface between PMS and aluminium provided corrosion protection for the aluminium substrate.     

Surface characteristics and wear performance of laser surface engineered iron oxide coatings on cast aluminium alloy

Abstract

Surface characteristics of laser surface engineered iron oxide coatings on a cast aluminium alloy developed for improved wear performance have been studied by atomic force microscopy (AFM) in association with microhardness measurements. An attempt is made to relate the grain size and surface roughness of FeO and Fe₃O₄ based coatings to wear performance determined by block-on-disk dry sliding wear testing. Fe₃O₄ based coating exhibits finer grain and smoother surface in comparison to FeO coating. However, FeO displays superior wear performance even though the Knoop microhardness of both coatings was comparable.     

Cerium-based oxide coatings

Cerium-based oxide compounds are known for their wide range of applications in catalysis, corrosion prevention, electrochemical cells, photocatalysis, UV absorbers, biomaterials, microelectronics, optical devices, thermal coatings, and glass abrasives. The technological applications of these materials are possible due to a combination of the electronic structure of Ce and the size effects at the nanoscale. In particular, reversible transformation between the Ce(III) and Ce(IV) oxidation states on the surface of cerium oxides is critical to the functionality and potential uses of the materials. In this paper, the main technological applications of cerium-based oxide coatings are reviewed based on the work done to date. Special interest is placed on the emerging trends.     

Modelling of continuous recrystallization in aluminium alloys

Microstructure and microtexture analyses have been made of three aluminium alloys after annealing alone and after concurrent straining and annealing, and simulative models of microstructure/microtexture evolution processes have been formulated. Both experimental and modelling results are presented as boundary misorientation distributions. For each alloy, the results show that annealing alone does not significantly alter the boundary misorientation distribution, while concurrent straining and annealing (up to a strain of 0.5) decreases the fraction of low-angle boundaries. To understand the mechanisms by which concurrent straining and annealing alter the boundary misorientation distribution, three simulative models of microstructure/microtexture evolution during concurrent straining and annealing have been formulated. Application of the models to experimentally determined initial microstructure/microtexture states shows that the boundary sliding (sub)grain rotation model decreases the fraction of low-angle boundaries, the dislocation glide (sub)grain rotation model increases the fraction of low-angle boundaries, and the (sub)grain neighbour switching model has a modest effect on the boundary misorientation distribution. A combination of the boundary sliding (sub)grain rotation model and the (sub)grain neighbour switching model most closely reproduces the boundary misorientation distributions found experimentally.     

Modelling of oxide surfaces

Ab initio methods have led to very rapid recent progress in the modelling of oxide surfaces. They have made important contributions to knowledge of three areas: equilibrium surface structure; the energetics and structures associated with oxidation and reduction; and the energetics of adsorption and dissociation of simple molecules such as H₂O and HCOOH.     

Thermal degradation of silica fibre-reinforced aluminium

At elevated temperatures, the aluminium matrix reduces the reinforcing silica fibres to silicon, severely lowering the mechanical properties. The kinetics of the degradation process have been studied and results of previous work, in which silica rods were attacked by molten aluminium, have been confirmed. Fibres corrode at a constant rate (of penetration) at a fixed temperature and the activation energy for the silica reduction/ alumina formation process was found to be 280 kJ mol^–1, in close agreement with previous work. The alumina is formed within the boundaries of the original silica fibres. An activation energy of 170 kJ mol^–1 has been determined for the commencement of the reaction and it appears that a higher activation energy is required for initiation if solid, rather than liquid aluminium is participating in the reaction. A technique is proposed for the production of alumina fibres in situ in composite materials, based on the deliberate reduction of silica and other oxides in aluminium matrices. It is suggested that the effectiveness of the method may depend upon the relative volumes of the products and the reactants. Shrinkage may cause fibre-matrix separation and porosity formation, as in the Al/SiO₂ composite.     

Wetting of aluminium oxide by molten aluminium and other metals

The sessile drop technique has been used to measure the contact angle of molten aluminium, aluminium-nickel and aluminium-copper alloys, copper and gold, with sapphire, ruby and recrystallised alumina. Measurements were madein vacuo, and as a function of time and temperature over the range 800 to 1500° C. Cinematography and time-lapse photography were used. At temperatures below 950° C, sessile drops of aluminium reached equilibrium only after a period of time which increased with decrease in temperature and could be in excess of one hour. A rapid increase in contact area occurred around 900° C. Above 1150° C drops of aluminium and of the aluminium alloys were observed to spread and contract repeatedly. Contractions were observed with both polycrystalline and single-crystal alumina, although they were much more pronounced with the latter, and were associated with the formation of a series of reaction rings on the plaque. Ruby and sapphire behaved similarly. The shape of the rings depended on the crystallographic orientation of the plaque: the reaction profile tended to terminate in certain low index directions. Neither contractions nor reaction was observed with copper or gold. The observations are discussed in terms of the combined effects of evaporation, chemical reactivity and interfacial tensions in the system.     

Modelling of dealumination of aluminium silicate molecular sieves

Abstract

A simplified treatment of the dealumination of aluminium silicate (zeolite like) molecular sieves has been developed that considers both the diffusion of aluminium atoms and the formation and migration of vacancy sites. The instantaneous concentration of aluminium atoms as a function of time and location has been derived according to Fick's second law. Critical parameters affecting the dealumination process are discussed. The calculated results agree with the ratios of silicon to aluminium reported in the literature from infrared and ²⁷Al NMR spectroscopy studies on the commercial aluminium silicate molecular sieves materials Mordenite and ZSM-5, following exposure to air and water vapour.     

Oxidized potato-starch films as primer coatings of aluminium

Potato-starch (PS) films for use as primer coatings of aluminium substrates were prepared in two steps, chemical-thermal-catalysed oxidation routes. The PS was modified with cerium (IV) ammonium nitrate (CAN) as a chemical oxidizer, followed by thermal oxidation at 150°C in the presence of atmospheric oxygen; this led to the formation of a functional carbonyl derivative caused by cleavage of the glycol C-C bonds in glycosidic rings, thereby resulting in the ring openings. Increasing oxidation by raising the temperature to 200 and 250°C promoted the conversion of carbonyl into carboxylate derivatives, while facilitating the breakage of C-O-C linkages in the open rings. The latter phenomenon reflected the formation of another carboxylate. The intermediate carboxylate derivatives favourably reacted with Ce4+ ions released from CAN to form cerium-bridged carboxylate complexes. Cerium-complexed carboxylate films used as primer coatings not only afforded some protection of aluminium substrates against corrosion, but also displayed excellent adhesion to both the polyurethane (PU) top-coating and aluminium sites. The latter demonstrated that the loss of adhesion at PU/primer/aluminium joints occurs in the PU layers, representing the mode of cohesive failure. This revised version was published online in November 2006 with corrections to the Cover Date.     

Modelling oxide scale fracture

Abstract

In the present paper the approaches to modelling scale failure described in EPRI report FP 686 are compared to a more recent approach based on micromechanical considerations. The latter requires a detailed analysis of the stress situation in the oxide/metal system and of the “quality” of the oxide scale and scale/metal interface, respectively. It turns out that a considerable number of model equations exists describing the role of oxide stresses, including such from cooling and surface or component geometries, and the role of physical defects, e.g. pores, voids, microcracks, etc. Additionally, these equations need material property data, such as fracture toughness or energy and elastic moduli, which sometimes can be found in the literature. In the final step it is proposed to combine the EPRI approach with the more recent micromechanics based models leading to a powerful tool for a detailed assessment of failure susceptibility of oxide scales. The general “EPRI diagram” can serve as a foreground diagram behind which the more sophisticated equations of the micromechanics approach are hidden. In order to enable wide use of this extended model, however, a number of additional input data are needed.