212 research outputs found
Electric-field-induced alignment of electrically neutral disk-like particles: modelling and calculation
This work reveals a torque from electric field to electrically neutral flakes that are suspended in a higher electrical conductive matrix. The torque tends to rotate the particles toward an orientation with its long axis parallel to the electric current flow. The alignment enables the anisotropic properties of tiny particles to integrate together and generate desirable macroscale anisotropic properties. The torque was obtained from thermodynamic calculation of electric current free energy at various microstructure configurations. It is significant even when the electrical potential gradient becomes as low as 100 v/m. The changes of electrical, electroplastic and thermal properties during particles alignment were discussed
Electrical resistance of CNT-PEEK composites under compression at different temperatures
Electrically conductive polymers reinforced with carbon nanotubes (CNTs) have generated a great deal of scientific and industrial interest in the last few years. Advanced thermoplastic composites made of three different weight percentages (8%, 9%, and 10%) of multiwalled CNTs and polyether ether ketone (PEEK) were prepared by shear mixing process. The temperature- and pressure-dependent electrical resistance of these CNT-PEEK composites have been studied and presented in this paper. It has been found that electrical resistance decreases significantly with the application of heat and pressure
Effect of the carbon nanotube surface characteristics on the conductivity and dielectric constant of carbon nanotube/poly(vinylidene fluoride) composites
Commercial multi-walled carbon nanotubes (CNT) were functionalized by oxidation with HNO3, to introduce oxygen-containing surface groups, and by thermal treatments at different temperatures for their selective removal. The obtained samples were characterized by adsorption of N2 at -196°C, temperature-programmed desorption and determination of pH at the point of zero charge. CNT/poly(vinylidene fluoride) composites were prepared using the above CNT samples, with different filler fractions up to 1 wt%. It was found that oxidation reduced composite conductivity for a given concentration, shifted the percolation threshold to higher concentrations, and had no significant effect in the dielectric response
Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styreneâbutadieneâstyrene/multiwall carbon nanotubes composites
Composites of styrene-butadiene-styrene (SBS) block copolymer with multiwall carbon
nanotubes (MWCNT) were processed by solution casting in order to investigate the
influence of filler content, the different ratio of styrene/butadiene in the copolymer and
the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical
properties of the composites. It was found that filler content and elastomer matrix
architecture influence the percolation threshold and consequently the overall composite
electrical conductivity. The mechanical properties are mainly affected by the styrene
and filler content. Hopping between nearest fillers is proposed as the main mechanism
for the composite conduction. The variation of the electrical resistivity is linear with the
deformation. This fact, together with the gauge factor values in the range of 2 to 18,
results in appropriate composites to be used as (large) deformation sensors.This work was funded by FEDER funds through the "Programa Operacional Factores de
Competitividade â COMPETE" and by national funds by FCT - Fundação para a
CiĂȘncia e a Tecnologia, through project references PTDC/CTM/69316/2006,
PTDC/CTM/73465/2006, PTDC/CTM-NAN/112574/2009, and NANO/NMed-
SD/0156/2007. PC, JS and VS also thank FCT for the SFRH/BD/64267/2009,
SFRH/BD/60623/2009 and SFRH/BPD/63148/2009 grants, respectively. The authors
also thank support from the COST Action MP1003 âEuropean Scientific Network for
Artificial Musclesâ and the COST action MP0902 âComposites of Inorganic Nanotubes
and Polymers (COINAPO)
A new approach to cure and reinforce cold-cured acrylics
Purpose: The low degree of polymerization of cold-cured acrylics has resulted in inferior mechanical properties and fracture vulnerability in orthodontics removable appliances. Methods: In this study, the effect of reinforcement by various concentrations of chopped E-glass fibers (0%, 1%, 2%, 3% and 5% by weight of resin powder) and post-curing microwave irradiation (800 W for 3 min) on the flexural strength of cold-cured acrylics was evaluated at various storage conditions (at room temperature for 1 day and 7 days; at water storage for 7, 14 and 30 days). Results: The data was analyzed by using 1-way and 2-way ANOVA, and a Tukey post hoc test (α = .05). The specimens with chopped E-glass fibers treated with post-curing microwave irradiation significantly increased the flexural strength of cold-cured PMMA. The optimal concentration might be 2% fibers under irradiation. Conclusions: The exhibited reinforcement effect lasted in a consistent trend for 14 days in water storage. A new fiber-acrylic mixing method was also developed. © 2012 The Author(s).published_or_final_versio
Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation
Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40âhours) with a maximum optical fluence of 2.3 mJ·cmâ2. We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers
Thermal Properties of Carbon NanotubeâCopper Composites for Thermal Management Applications
Carbon nanotubeâcopper (CNT/Cu) composites have been successfully synthesized by means of a novel particles-compositing process followed by spark plasma sintering (SPS) technique. The thermal conductivity of the composites was measured by a laser flash technique and theoretical analyzed using an effective medium approach. The experimental results showed that the thermal conductivity unusually decreased after the incorporation of CNTs. Theoretical analyses revealed that the interfacial thermal resistance between the CNTs and the Cu matrix plays a crucial role in determining the thermal conductivity of bulk composites, and only small interfacial thermal resistance can induce a significant degradation in thermal conductivity for CNT/Cu composites. The influence of sintering condition on the thermal conductivity depended on the combined effects of multiple factors, i.e. porosity, CNTs distribution and CNT kinks or twists. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. CNT/Cu composites are considered to be a promising material for thermal management applications
Tooling design and microwave curing technologies for the manufacturing of fiber-reinforced polymer composites in aerospace applications
The increasing demand for high-performance and quality polymer composite materials has led to international research effort on pursuing advanced tooling design and new processing technologies to satisfy the highly specialized requirements of composite components used in the aerospace industry. This paper reports the problems in the fabrication of advanced composite materials identified through literature survey, and an investigation carried out by the authors about the composite manufacturing status in Chinaâs aerospace industry. Current tooling design technologies use tooling materials which cannot match the thermal expansion coefficient of composite parts, and hardly consider the calibration of tooling surface. Current autoclave curing technologies cannot ensure high accuracy of large composite materials because of the wide range of temperature gradients and long curing cycles. It has been identified that microwave curing has the potential to solve those problems. The proposed technologies for the manufacturing of fiber-reinforced polymer composite materials include the design of tooling using anisotropy composite materials with characteristics for compensating part deformation during forming process, and vacuum-pressure microwave curing technology. Those technologies are mainly for ensuring the high accuracy of anisotropic composite parts in aerospace applications with large size (both in length and thickness) and complex shapes. Experiments have been carried out in this on-going research project and the results have been verified with engineering applications in one of the project collaborating companies
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