Materials Science Forum Vols. 587-588

Abstract: Cork is a natural cellular material which has been used for centuries, in natural and agglomerate forms, mainly for applications related to the wine, the automotive and the construction industries. It is a very durable and ecological material, used for thermal, acoustic and vibrating insulation as well as packaging, among others. This paper highlights some of the aspects of a topic of great interest, not much explored yet, which consists of the study of the dynamic mechanical behaviour of innovative structures incorporating cork, dedicated to energy-absorption. Experimental and numerical tests, using the finite element method software LS-DYNA™, were performed in order to evaluate the effects of filling agglomerate cork inside thin-walled metallic tubes, with variable geometries and thicknesses, impacted uniaxially at quasi-static and high strain rates. Some relevant comparisons were carried out and the results obtained allowed concluding that cork might be a viable energy-absorbing material for application in some metallic structures subjected to impact loadings.

Abstract: In an attempt to improve the mechanical properties of an all-cellulose based composite we made solid flexible films containing microcrystalline cellulose fibres produced by a shearing casting technique. To an anisotropic (liquid crystalline) solution of 2-hydroxypropylcellulose (HPC) in N,N – dimethylacetamide (60 w/w %) were added AVICEL fibres (0, 2, 4, 6, 10, 12 w/wHPC %). After homogenization these solutions were poured into a Teflon plate and sheared at a constant rate of 5 mm/s. The cellulosic microfibres in the liquid crystalline polymer medium were partially aligned by shear flow. Mechanical and morphological properties of the films were investigated using tensile tests, polarized optical microscopy and scanning electron microscopy. The mechanical properties of these films, as expected, are higher than those of the films produced from an isotropic solution of HPC. The results show that the orientation of the microfibres, and the mesoscopic properties, can be tuned by adjusting the topography of the nematic matrix in the micro-nano scale.

Abstract: Geometry optimization of free alizarin, purpurin and luteolin and coordinated Fe(II) complexes was performed at DFT/B3LYP level. TD-DFT spectra were also calculated for free and coordinated alizarin and luteolin. For the Fe(II) complexes several spin multiplicities have been calculated and quintuplet spin structures were found to be the most stable. In the luteolin-Fe(II) complex, the coordination of the chromophore with the iron leads to a decrease in the lower energy band. In the case of luteolin complex, a new band emerges due to interactions between the delocalized π electrons of the luteolin molecule with the d metal orbitals.

Abstract: Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight, but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork, TetraPak® containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork, TetraPak® and paraffin, different granules size, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate, which makes it a better material for charge storage.

Abstract: Activated carbons have been prepared by physical activation in CO2 of commercial byproduct kraft lignin (as received, after de-ashing and after impregnation with NaCl) and natural cork. The results obtained show that the presence of natural inorganic impurities increases the reactivity of cork and lignin significantly and also results in limiting values for the micropore volume and in widening of the micropore size. Pure de-ashed lignin is exceptionally non-reactive but allows microporous activated carbons to be obtained which have very narrow micropore widths of ~0.5-0.6nm. When the de-ashed lignin is impregnated with NaCl similar micropore volumes and widths can be obtained but in a considerably shorter time (~30min instead of ~8h) which would result in a considerable energy saving and is therefore a promising procedure for the production of microporous activated carbons from low-cost kraft lignin.

Abstract: This study reports the use of X-ray diffraction quantitative phase analyses in NiTi alloys produced by MARES (Mechanically Activated Reactive Extrusion Synthesis). These analyses were performed with the PowderCell 2.4 software. The mechanically activated powders heated in a DTA furnace at 500 °C had as main phases Ni (27 wt %) and Ti (30 wt %) and the major intermetallic phase was Ni3Ti (20 wt %). Above 500 °C the intermetallic phases were predominant. At 600 °C the major phase was Ni3Ti (29 wt %) and at 700 °C was NiTi2 (32 wt %). In this temperature range the NiTi was a minor intermetallic phase (14-20 wt %). No changes in the constitution or in the amount of the phases were detected between the degassed powder samples and the extruded materials. The intermetallic phases were always predominant and the major was Ni3Ti (27-32 wt %). The NiTi phase content was in a range of 15-22 wt %. The weighted residual error, Rwp, of the fittings ranged between 17 and 27. Using the Williamson and Hall plot, crystallite sizes within the range of 26-53 nm and of 12-25 nm were evaluated for the metallic and intermetallic phases, respectively. Vickers micro-hardness measurements were virtually unchanged with the extrusion parameters but increased relatively to the mechanically activated powders.

Abstract: The aim of the study was to develop an innovative processing method of magnetorheological elastomers (MRE). This method comprises optimization of the MRE structure in the context of their performance in the magnetic field. The influence of the amount of ferromagnetic particles and their arrangement in relation to the external magnetic field was investigated. Urethane magnetorheological elastomers were manufactured using polyurethane gels, supplied by Dow Chemical Company. As the ferromagnetic carbonyl–iron powder with particles size from 6-9)m produced by Fluka was used. The amount of the carbonyl iron particles was varied from 1.5 to 33.0 vol. %. Magnetic field strengths used during the fabrication of MRE were 0.1 and 0.3 T. The samples with particle chains aligned or slopped at 45 degree to the long sample axis were produced. To evaluate the external magnetic field effect on the magnetorheological properties a deflection in the magnetic field was measured. Samples were placed parallel to the magnetic field lines and deflected prior to the application of a magnetic field. After the application of the magnetic field the sample tended to straighten which was measured by displacement sensor. Magnetic field in a range of 0-0.9 T has been applied. Also the compression tests were carried out without and within external magnetic field with the strength of 0.3 T. The experiment showed that application of the magnetic field increases stiffness of the material. The amount of iron particles and their arrangement have influence on the stress-strain curves course.

Abstract: Shape memory effect (SME) in Nickel-Titanium (Ni-Ti) alloys is ascribed to the thermoelastic reversible martensite phase transformation. Phase transformation is established to be affected by the pre- thermal and mechanical history of the alloy. The present work deals with the effect of mechanical working, known as ‘marforming’ and ‘ausforming’, on the phase transformation characteristics and mechanical behaviour of Ti-rich Ni-Ti alloy. Tensile study and measurement of the hardness data were carried out at room temperature. Mere heat treatment or heat treatment at 773 K after the marforming shows similar characteristics, whereas, the as-received and the ausformed samples exhibit different behaviours. Hardness numbers of the heat treated samples are found to be smaller than those of the as-received and mechanically worked samples.

Abstract: A numerical model was developed which enables the calculation of the optical constants (refractive index, n and extinction coefficient, k) of thermochromic coatings based in undoped and doped vanadium dioxide thin coatings deposited on glass for use as an intelligent window - a window that can change the optical properties in response to the temperature. From experimental results it can be seen that the vanadium dioxide coating prepared by Atmospheric Chemical Vapour Deposition shows a switching efficiency of about 30% at 2500 nm. In the visible range the transmittance and the reflectance does not change with the temperature both for the undoped and Nb doped VO2. For the Nb doped vanadium dioxide coating the switching efficiency is about 20% at 2500 nm. From the numerical simulations a n=2.89 and k=1.33 above Tc and n=2.39 and k=0.52 below Tc (at wavelength of 2500 nm) were determined for the undoped vanadium dioxide coating. The Nb doped vanadium dioxide coating calculations results on n= 2.45 and k=1.56 above Tc and n=1.92 and k=0.88 below Tc.

Abstract: In this study the embedding of piezoelectric ceramics in carbon-fibre/epoxy laminates is studied with the purpose to be used for structural health monitoring from vibration measurements. Piezoelectric elements were embedded in two laminate types made of two weaved prepregs and with four additional unidirectional prepregs, respectively. The efficiency of the embedding process was analysed from the capability of the piezoelectric ceramic to transmit vibrations to the composite plate. The sensing element was successfully used to monitor the composite plate when submitted to three-point bending dynamic tests at different frequencies.