Papers by Author: A. Fonseca

Abstract: In this work, nanocomposites of ultrahigh molecular weight polyethylene (UHMWPE) reinforced with multiwalled carbon nanotubes (MWCNTs) were evaluated for their ability to produce nanocomposites with superior mechanical properties. As homogeneity of the nanocomposite plays an important role into final mechanical properties, mechanical ball-milling is used to prepare homogeneous UHMWPE/MWCNTs powders, where special emphasis is given to milling-time optimization. Mechanical ball-milling seems to be a suitable and rather simple technique for preparing nanocomposites even outside laboratory conditions and it is presented here as an interesting technique for nanoscience industrial applications. A fact that is worth noting since the great majority of research breakthroughs fail due to lack of industrial accomplishment. The powder mixture was further processed through compression moulding in a hot plate press. The impact of milling time on mechanical properties of the nanocomposites was evaluated. Nanocomposites with different volume fractions of MWCNTs were prepared using the optimized milling time, processed via compression moulding and their mechanical properties were evaluated. It was observed an enhancement of the Young’s modulus of about 80%, for higher volume fractions of MWCNTs (1.0%), as compared with the pure UHMWPE.

Abstract: Ultrahigh molecular weight polyethylene (UHMWPE) is a unique polymer with outstanding physical and mechanical properties that makes it particularly attractive to fabricate the bearing surface for artificial joints. Despite the requirement of visco-elastic properties of the UHWMPE and its composites, the characterization of them has received relatively little attention. The objective of this work is concerned with the studies on visco-elastic behaviour of UHMWPE and nanocomposites, which were prepared at optimized ball milling time with different cooling techniques. It is observed that stiffness of the materials increases appreciably at 0.2wt.% CNTs with an increase of frequency till 30Hz which confirms the reinforcing effect of CNTs in composites. The loss modulus of the sample is observed to be converged at higher temperature irrespective of frequency. The damping effect of the sample could be kept within the limit of polymer at any frequency range when the temperature is low and it is also possible at any temperatures at higher frequencies except LN2 cooled sample. The relaxation fraction increases with an increase of temperature and decreases with an increase of frequency. It is concluded that air cooled sample could be used wherever modulus is the main criteria irrespective of temperature and frequency, LN2 cooled sample can be used where more damping is required and water cooled samples may be used where more strength and toughness are required.

Abstract: The research work presented here intends to contribute to the overall research effort towards nanofluids engineering and characterization. To accomplish the latter, multiwalled carbon nanotubes (MWCNTs) are added to an ethylene glycol (EG) based fluid. Different aspects concerning the nanofluids preparation and its thermal characterization will be addressed. The study considers and exploits the relative influence of CNTs concentration on EG based fluids, on the suspension effective thermal conductivity and viscosity. In order to guarantee a high-quality dispersion it was performed a chemical treatment on the MWCNTs followed by ultrasonication mixing. Furthermore, the ultrasonication mixing-time is optimized through the UV-vis spectrophotometer to ensure proper colloidal stability. The thermal conductivity is measured via transient hot-wire within a specified temperature range. Viscosity is assessed through a controlled stress rheometer. The results obtained clearly indicate an enhancement in thermal conductivity consistent with carbon nanotube loading. The same trend is observed for the viscosity, which decreases with temperature rise and its effect is nullified at higher shear rates.