On the limits of Brans-Dicke spacetimes: a coordinate-free approach

We investigate the limit of Brans-Dicke spacetimes as the scalar field coupling constant omega tends to infinity applying a coordinate-free technique. We obtain the limits of some known exact solutions. It is shown that these limits may not correspond to similar solutions in the general relativity theory.Comment: LaTeX, 16 pp, report DF/UFPB/02-9

Dispersion of carbon nanotubes in poly(lactic acid)

Carbon nanotubes (CNT) present excellent mechanical, electrical and thermal properties, and are expected to impart these properties into their composites. However, the CNT are grown as entangled bundles that are difficult to disperse in polymer matrices, or even in solvents. Several approaches have been tried for the efficient dispersion of CNT in polymer matrices, ranging from the CNT chemical modification to the use of different mixing methods. Previous studies have shown that the resulting filler dispersion is strongly dependent on the characteristics of the melt mixing equipment [1]. It is also known that the dispersion level of nanofillers strongly affects the final nanocomposite properties [2]. The present work reports the optimization of the CNT dispersion in poly (lactic acid) (PLA) using a small-scale twin-screw extruder. The CNT were chemically modified for improved interaction with PLA. The effect of varying the mixing parameters on the dispersion level of the CNT and functionalized CNT was evaluated using optical and electron microscopy. The electrical resistivity and mechanical properties of the composites were measured. It was observed that the incorporation of 1% (weight) of CNT reduced the electrical resistivity of the composite to 400 Ohm.m, and that 3% CNT rendered the composite conductive, with an electrical resistivity of 0.6 Ohm.m.Fundação para a Ciência e a Tecnologia (FCT) - POCI/QUI/59835/2004, bolsa de doutoramento SFRH/BD/32189/2006

A comparative study of the dispersion of carbon nanofibres in polymer melts

The dispersion of carbon nanofibres (CNF) in a polymer matrix using two melt mixing methods is studied. Distributive and dispersive mixing were evaluated by optical and electron microscopy. The CNF were chemically modified to improve the interface with the matrix. The results showed that the two methods produced good distribution of the filler, but extensional stresses induced higher dispersion. The latter correlated well with a decrease in electrical resistivity. Also, the chemical modification largely improved the CNF/polymer interfaceFundação para a Ciência e a Tecnologia (FCT

Composites with polymer-grafted carbon nanotubes

Carbon nanotube (CNT)/polymer composites exhibit the processability advantages of plastics, while conveying electrical conductivity characteristics suitable for electric transport, or for sensing functionalities. The success of their application depends on the ability to homogeneously disperse the CNT in the polymer matrices to form a stable conductive network. The structural strength of the nanocomposite is also desirable, and may be a requirement. The chemical functionalization of the CNT is known to improve the mechanical properties of the nanocomposites, although it is observed to have a negative influence on the electrical conductivity. The present work reports the chemical modification of CNT to graft polycarbonate (PC) molecules to their surface, and the analysis of the CNT (as-received and PC-grafted) dispersion in PC by melt mixing using prototype equipment. The modified CNT were analyzed by thermogravimetry, X-ray photoelectron spectroscopy and scanning tunnelling microscopy, showing evidence for the formation of a polymer coating over the CNT. The nanocomposites were processed and reprocessed twice. The CNT dispersion was analyzed by optical microscopy using a similar analysis as described for PLA-grafted CNT [1], displaying considerable differences for CNT dispersion depending on their surface treatment. The electrical resistivity of the composites was measured, showing typically lower resistivity values for the composites with pristine CNT. The results also showed that resistivity of the composites with PC-grafted CNT may be considerably reduced depending on the processing steps.Fundação para a Ciência e a Tecnologia (FCT

Estimating the sustainability of carbon nanotube composites: reprocessing studies

Polylactide (PLA) has attracted particular attention in the area of environmentally degradable polymer materials. Some applications require the incorporation of a reinforcement material to produce a composite with specific properties. It was observed that PLA /carbon nanotube (CNT) composites present adequate properties for liquid sensing. The health risks involved in the use of carbon nanotubes are still under study, and thus the application of these materials must be planned with caution. From the point of view of environmental protection, recycling the CNT composites is the way to reduce these problems to the lower level possible, by maximizing they life cycle. The work presented reports the processing of PLA/CNT composite monofilaments, produced for sensing applications, and their reprocessing to form new monofilament composites. The monofilaments were reprocessed three times, and the tensile properties and electrical resistivity was measured. The melt flow index of the nanocomposites after each reprocessing step was measured. The thermal stability was evaluated by thermogravimetric analysis. It was observed that, although the melt flow index increased with reprocessing cycles, the tensile and electrical properties of the monofilaments produced after reprocessing were not affected, even after the fourth processing cycle. The thermal stability of the filaments was comparable after each processing cycle.Project Inteltex, FP

Dispersion and re-agglomeration of graphite nanoplates in polypropylene melts under controlled flow conditions

The kinetics of GnP dispersion in polypropylene melt was studied using a prototype small scale modular extensional mixer. Its modular nature enabled the sequential application of a mixing step, melt relaxation, and a second mixing step. The latter could reproduce the flow conditions on the first mixing step, or generate milder flow conditions. The effect of these sequences of flow constraints upon GnP dispersion along the mixer length was studied for composites with 2 and 10 wt.% GnP. The samples collected along the first mixing zone showed a gradual decrease of number and size of GnP agglomerates, at a rate that was independent of the flow conditions imposed to the melt, but dependent on composition. The relaxation zone induced GnP re-agglomeration, and the application of a second mixing step caused variable dispersion results that were largely dependent on the hydrodynamic stresses generated.Project Matepro – Optimizing Materials and Processes (NORTE-07-0124-FEDER-000037) by Programam Operacional Regional do Norte (ON.2

Functionalized carbon nanotubes-polyamide composites produced by microinjection moulding

Polymer composites containing carbon nanotubes (CNT) have attracted much attention due to the possibility to obtain electrically conductive and reinforcing materials at relatively low CNT concentrations, and for their potential applications in electronics, and chemical and biological sensing. Microinjection molding (μIM) is an emerging efficient and cost-effective process for the large-scale production of thermoplastic nanocomposite microparts. The present work reports the dispersion of CNT in polyamide 6 (PA 6) for the production of nanocomposites with different amounts of functionalized and non-functionalized CNT. The nanocomposites were microinjection molded under specific conditions and the electrical and mechanical properties of the specimens obtained were measured.Fundação para a Ciência e a Tecnologia (FCT) - POCI/QUI/59835/2004, bolsa de doutoramento T. Ferreira (SFRH/BD/39119/2007)

Conical refraction in generalized biaxial media: A geometric algebra approach

It is well-know that conical refraction occurs for electric anisotropic biaxial crystals when the wave vector has the direction of the medium optic axes. In this paper, we show that conical refraction occurs in an analogous away for a more general type of biaxial media that have simultaneously electric and magnetic anisotropies. Furthermore, the new coordinate-free approach based on geometric algebra, developed by the authors in previous papers to address anisotropy, is shown to shed new light on this classic topic of optics that is conical refraction.info:eu-repo/semantics/acceptedVersio

Dispersion and re-aggregation phenomena in carbon nanotube polymer composites

The preparation of polymer nanocomposites is usually carried out in twin-screw extruders or internal batch mixers. Previous studies have extensively shown that the resulting filler dispersion is strongly dependent on the characteristics of the melt mixing equipment. It is also known that the dispersion level of nanofillers strongly affects the final nanocomposite properties. The present work focuses on the study of the dispersion of carbon nanotubes (CNT) in polypropylene using a prototype mixer that develop high thermomechanical stress. The distribution and dispersion of the CNT was evaluated by optical and electron microscopy. The level of dispersion achieved was analysed, and the electrical resistivity of the composite, along the mixing cycles, was measured. The enhancement of the CNT dispersion along the mixing process correlated with a large decrease in electrical resistivity of the composite. A “percolation time”, defined as the time, or number of mixing cycles, required to achieve electrical conductivity, was measured, and correlated with mixing conditions. The composite formed was re-heated and reprocessed using the same mixing system. Re-agglomeration and loss of electrical conductivity was observed, and the final dispersion level varied with the mixing conditions imposed. Agglomerate size and electrical resistivity increased when reprocessing was performed at lower shear rate, as compared to the first processing cycle

Produção de matéria seca e teores de nitrogênio em milho para silagem adubado com uréia misturada a zeólita.

bitstream/CPPSE/17524/1/ComuTecnico77.pd