Advancement in the pressureless sintering of CP titanium using high-frequency induction heating

High-frequency induction heating is applied as an alternative heating source for pressureless sintering of commercially pure Ti powders, aiming to intensify the sintering process. The effects of the process parameters on the properties of the sintered material are systematically studied. The initial powder compact density is the most influential parameter permitting sintered structures with highly porous to almost fully dense appearance. Short heating time combined with sintering to temperatures just above the β-transus resulted in a strong diffusion bonding between the Ti powder particles, and grain growth is observed at the former boundaries of the neighboring powder particles. The dimpled appearance of the fracture surface at those regions confirmed the strong metallic interparticle bonding. Tensile properties comparable to those of Ti-Grade 3 and Ti-Grade 4 are achieved, which also demonstrates the efficiency of the induction sintering process. A mechanism explaining the fast and efficient sintering is proposed. The process has the added advantage of minimizing the oxygen pickup

Influence of Humidity on Microtribology of Vertically Aligned Carbon Nanotube Film

The aim of this study is to probe the influence of water vapor environment on the microtribological properties of a forestlike vertically aligned carbon nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor deposition. Tribological experiments were performed using a gold tip under relative humidity varying from 0 to 100%. Very low adhesion forces and high friction coefficients of 0.6 to 1.3 resulted. The adhesion and friction forces were independent of humidity, due probably to the high hydrophobicity of VACNT. These tribological characteristics were compared to those of a diamond like carbon (DLC) sample

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 40 hours) with a maximum optical fluence of 2.3 mJ·cm−2. 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

Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/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 – COMPETE" and by national funds by FCT - Fundação para a Ciência 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 ”European Scientific Network for Artificial Muscles” and the COST action MP0902 “Composites of Inorganic Nanotubes and Polymers (COINAPO)

Thermal Properties of Carbon Nanotube–Copper Composites for Thermal Management Applications

Carbon nanotube–copper (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

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 enhancement of UHMWPE fibers by coating with carbon nanoparticles

Fiber-reinforced plastic (FRP) is composed of reinforced fibers and matrix resin, and has high specific strength and low-density materials. Because of the orientation of the fibers within them, FRPs are prone to buckling damage when under compression along the axial direction of the fiber, especially flexible organic ones. The compressive performance of FRP is largely dependent on fiber properties. the buckling load of FRP will increase with the increasing of fiber's. In this study, we developed a way to improve the compressive and bending strength of ultra-high molecular weight polyethylene (UHMWPE) fibers. Carbon nanotubes (CNTs) and vapor-grown carbon fibers (VGCFs) were coated on the surface of UHMWPE fibers by pyrrole vapor deposition. The transverse compressive strength and bending strength of single UHMWPE fibers were determined by microcompression and single fiber bending measurements, respectively. The experiment result showed that coating UHMWPE fibers with CNTs and VGCFs increased both their transverse compressive strength and bending strength. It is excepted that the improved fiber would applied in FRP for better compressive performance.ArticleFIBERS AND POLYMERS. 15(4):723-728 (2014)journal articl

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 China’s 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