Materials Science Forum Vols. 730-732

Abstract: This study evaluates the influence of graphite and multi-wall carbon nanotubes on the mechanical and electric properties of cast epoxy resin. The epoxy resin based composites were prepared with various graphite and MWNCT content up to 5.0%. Specimens were characterized by DMA, SEM and electric resistivity tests. The observation of fracture surfaces showed a reasonable dispersion of graphite and MWCNT into the epoxy matrix. The graphite and MWCNT have almost the same effect in the electric conductivity of the epoxy composites at low content (0.2 and 0.5 %). The MWCNT composites seem to reach percolation at concentrations near 0.5 % whereas graphite composites reach it at 2%. Higher concentration of graphite and MWCNT have limited effect in the electric conductivity but reduces mechanical properties.

Abstract: Metal nanoparticles inside glass present useful properties to photonic applications and have been object of several research works. In particular, laser beams have shown their potential in its creation and both ultraviolet beams with nanosecond pulses, and near-infrared beams with femtosecond pulses, have been used. In this paper, the authors add new possibilities by experimentally demonstrating that it is possible to achieve the same results by using near-infrared laser beams and nanosecond pulses. Copper and gold nanoparticles are created in silica-doped glass using nanosecond laser pulses in the near-infrared. Recorded absorption spectra of the glass samples before, and after laser irradiation and further annealing allowed measuring absorption peaks located at 537 nm for copper and 563 nm for gold, which are in accordance with the expected values. Based on Mie theory and using the full-width half maximum for those peaks, the average particle radii of the embedded nanoparticles was estimated to be about 7 nm for copper and 3 nm for gold nanoparticles, respectively.

Abstract: In the past decade, new materials have been developed based on the physical and chemical properties of carbon nanotubes. The combination of polyaniline with multiwall carbon nanotubes results in a new functional material with advantageous electromagnetic properties. The objective of this study was to produce a radar absorbing structure consisting of glass fiber woven fabric impregnated with a formulation containing carbon nanotubes, polyurethane resin, with or without polyaniline. A different formulation was used for each woven sheet (multilayer structure). The electromagnetic properties of these nanocomposite materials were characterized by reflectivity measurements using Naval Research Laboratory arch method (frequency range, 8 to 12 GHz). The attenuation of both sides of each nanocomposite material was also measured and compared. The attenuation of electromagnetic energy was as high as 70 %, approximately, indicating that these materials can be used as microwave absorbers.

Abstract: Nanostructured copper-diamond composites can be tailored for thermal management applications at high temperature. A novel approach based on multiscale diamond dispersions is proposed for the production of this type of materials: a Cu-nDiamond composite produced by high-energy milling is used as a nanostructured matrix for further dispersion of micrometer sized diamond. The former offers strength and microstructural thermal stability while the latter provides high thermal conductivity. A series of Cu-nDiamond mixtures have been milled to define the minimum nanodiamond fraction suitable for matrix refinement and thermal stabilization. A refined matrix with homogenously dispersed nanoparticles could be obtained with 4 at.% nanodiamond for posterior mixture with mDiamond and subsequent consolidation. In order to define optimal processing parameters, consolidation by hot extrusion has been carried out for a Cu-nDiamond composite and, in parallel, for a mixture of pure copper and mDiamond. The materials produced were characterized by X-ray diffraction, scanning and transmission electron microscopy and microhardness measurements.

Abstract: Ferrihydrite is natural ferric oxyhydroxide occurring exclusively nanocrystalline. With ideal formula 5 Fe₂ O₃ . 9 H₂ O, ferrihydrite is quite abundant in sediments, weathering crusts and mine wastes, being characteristic of red pre-soils formed by loose weathered rock plus mineral debris (regoliths) and commonly designated as “2-line” or “6-line” on the basis of the broadened maxima observed in the X-ray diffraction pattern. Synthetic nanocrystalline “6-line” ferrihydrite was recently studied through methods based on atomic-pair distribution functions disclosing the possible occurrence of icosahedral clusters formed by twelve octahedra centred by an inner tetrahedron, all filled by Fe ³⁺ ions. However, Mössbauer studies were inconclusive about the existence of 4-coordinated iron, thus suggesting that the tetrahedral cation may well be Si⁴⁺. In view of such structural uncertainty, a XANES study at the Fe K-edge was undertaken on ferrihydrite from a regolith to ascertain the occurrence of tetrahedral iron. Comparison with data collected from well crystallized iron oxide and hydroxide minerals where Fe ^3+/2+ ions occur in octahedral and tetrahedral coordination is described and the results so far obtained are discussed, showing that supplementary study is needed on the elusive structure of ferrihydrite.

Abstract: Aluminium powder was milled under vacuum and/or ammonia atmospheres, in order to evaluate the effect of the order of the atmospheres on the amount of nitrides appearing in the powder after a subsequent heat treatment. All milling experiences were carried out at room temperature for 5 h. The XRD study of sintered powders showed that important amounts of AlN appeared after heating. The use of vacuum and ammonia flow allows controlling the percentage of N rich phases formed. Moreover, the capacity of incorporating nitrogen to the aluminium lattice is very influenced by the vacuum and ammonia flow atmospheres order during the milling process.

Abstract: This work is inserted in a wider project aiming at the conservation and durability of historical renders, through compatibles techniques and materials; in particular the restitution of cohesion of historical renders, turned friable by the loss of binder due to physical or chemical actions, is studied. Surface consolidation, directed to restore cohesion and stability, is based on the use of materials with aggregating properties. This operation is reached usually through the application of organic or mineral consolidants, but inorganic consolidants (such as calcium hydroxide or ethyl silicate) are preferred due to better compatibility and durability. Based on the results of previous studies, two mineral compatible products were selected: a commercial suspension of nanoparticles of calcium hydroxide in propanol (Nanorestore); a silicate product, consisting on a limewater dispersion of ethyl silicate. Consolidation products were than applied on different mortars samples previously prepared, in order to assess their efficacy by determining their physical, mechanical and microstructural properties before and after the consolidation treatment. Mechanical and physical analyses were performed, such as compression and flexural strenght and superficial hardness. Microstructural and chemical analyses of the consolidation product and of the consolidated samples are also reported.

Abstract: Wear is one of the most worrying problems in industry; it affects many production sectors. Therefore, the wear resistance of materials must be assessed in order to predict their response and anticipate possible failures. Maintenance could then be scheduled accordingly. Due to the large number of situations where wear is important, one of the main complications in tribology is that it is difficult to replicate in a laboratory the precise conditions of service. Thus, there is a need to choose between different kinds of tests to simulate actual conditions. However, this aim is difficult to achieve, as variables and conditions of service are numerous. In this situation, it is neither practical nor possible to have as many test devices in the laboratory as real possibilities. It is necessary to find a test that can be extrapolated to many possible situations. An important question is if the results obtained with different configurations simulated in the laboratory are good equivalents or, on the contrary, the choice of method has an influence and to what extent the latter case is true. In addition, it should be noted that wear test standards mention how difficult it is to reproduce results and how they are influenced by operating conditions In this paper, three wear tests methods are studied- the pin-on-disk, dry sand/rubber wheel test and wet sand/rubber wheel- in order to find a relationship between the results obtained by them. Furthermore, different techniques are analysed to establish, if possible, which ones are more likely to achieve more reliable results.

Abstract: Warpage and poor dimensional stability of rotomoulded products are two of the main obstacles to the use of this technique in the production of engineering parts. The knowledge of the effect of the processing conditions on the shrinkage of rotomoulded parts will allow overcoming some of the restrictions of this process. In the present work the influence of the processing conditions on the development of shrinkage and warpage of rotomoulded parts was studied. The moulding of the parts was performed using a rotational moulding machine build at the University of Minho. The shrinkage and the warpage of the moulded parts were assessed using 3D MMC (3D measuring Machine Control) equipment, and understanding the microstructural development.

Abstract: The concept of hybrid mould combines the conventional techniques of mould manufacturing and Rapid Prototyping and Rapid Tooling, resorting to non-conventional materials for producing moulding blocks, e. g., epoxy resin composites. Composites based on an epoxy system with 15% weight fraction of short steel fibres (SSF) were considered adequate for improving the performance of moulding blocks. The epoxy/short steel fibre composite moulding blocks were produced by vacuum casting in silicone moulds. Polypropylene (PP) was mixed with a commercial PP masterbatch with 50% of nanoclay and injected in a hybrid mould under various processing conditions. These were chosen from a central composite design with 15 experiments. The moulding microstructure was assessed by polarized light microscopy and differential scanning calorimetry. The skin-core morphology was observed and suggested that the low thermal conductivity of the epoxy composite produces a thinner skin when compared to all-steel moulds. The nanoclay concentration was the variable with the most significant effect on skin thickness and crystallinity. The addition of 1 wt% nanoclay under certain processing conditions favours the formation of β-form spherulites and the increase of crystallinity.