Thermal oxidative cutting of multi-walled carbon nanotubes

Commercially available, multi-walled carbon nanotubes grown by CVD are usually inherently entangled, but can be separated by cutting. However, most cutting methods both cause damage to the nanotubes and involve a lengthy work-up procedure. The use of abrupt, repeated exposure to oxidising conditions in air proved to be an efficient (68% yield) means of producing material with open ends, moderate functionalisation, and enhanced solvent dispersibility; the average lengths were reduced from over 5 μm to approximately 650 nm. Additionally, the character of the surface oxides can be tuned to have either an acidic or basic character by using a simple thermal treatment. These approaches could be deliberately integrated into conventional CVD processes, but also have implications for the products of standard nanotube syntheses. Raman spectroscopy and electron microscopy were used to study the impact of cutting on the intrinsic graphitic structure and the length distribution. X-ray photoelectron spectroscopy was used to determine the extent of functionalisation. The cut carbon nanotubes were dispersed in dimethylformamide (DMF), a Lewis basic solvent, and chloroform, a Lewis acidic solvent, using mild sonication. Through the use of an experimentally determined extinction coefficient (ε = 35.10 ml mg⁻¹ cm⁻¹), the relative dispersibility of the cut and functionalised carbon nanotubes in DMF and chloroform was determined.     

Passivity and Corrosion of Cu–<Emphasis Type="BoldItalic">x</Emphasis>Zn (<Emphasis Type="BoldItalic">x</Emphasis> = 10–40 wt%) Alloys in Borate Buffer Containing Chloride Ions

The electrochemical behaviour of Cu–xZn alloys and of Cu and Zn metals was studied by cyclic voltammetry and chronopotentiometry in borate buffer, pH = 9.2, with and without the addition of chloride ions in the range from 0.01 m to 1 m. In general, the shape of voltammograms of four Cu–xZn alloys with 10 – 40 wt.% of zinc resembles that of copper more than that of zinc. With increasing zinc content several characteristics of zinc are observed. In borate buffer containing chloride anions, Cu–xZn alloys are susceptible to pitting corrosion. The breakdown potential, E_b, at which the current density in the passive region starts to increase abruptly, becomes more negative with increasing zinc content in the alloy. The general relationship E_b = a + b log c_NaCl held in all cases, with constants a and b, however, being dependent on the zinc content of the alloy and on the chloride concentration. The corrosion resistance of Cu–xZn alloys was less than that of copper metal but significantly greater than that of zinc.     

Effect of artificial weathering and temperature cycling on the performance of coating systems used for wooden windows

The effects of ultraviolet light irradiation and water spray and temperature cycling on the color stability of waterborne coating systems on spruce were investigated. The test samples were treated with three coating systems (white and brown pigmented acrylate waterborne coatings systems with different layering) from six producers. The artificial weathering was carried out based on standardized (504 h) and increased weathering parameters (504 h). Thirty cycles of temperature changes were performed. Discoloration suggesting the rate of degradation process and color stability was measured by a spectrophotometer in L*a*b* color space. The results showed greater color stability of white coating systems during both experiments. The producer of acrylate coatings with iodopropynyl butyl carbamate and permethrin as biocides and benzotriazole, silicon dioxide and methylsiloxane as additives was characterized by the best coatings performance. The most sensitive color parameter to describe the surface quality as a result of irradiation and water spray has proven to be a change of lightness. A tendency towards degradation with increasing lightness was observed during weathering. The effect of temperature itself on color change was minimal. Scanning electron microscopy and energy dispersive spectroscopy revealed microscopic changes of coatings caused by artificial weathering.     

The impact of natural and artificial weathering on the visual,colour and structural changes of seven tropical woods

This study investigates the effects of wood weathering on changes in its macroscopic and colour characteristics in connection with changes in its molecular and anatomical structure. Seven hardwoods suitable for outdoor architecture—bangkirai, cumaru, cumaru rosa, ipé, jatobá, kusia, and massaranduba—were exposed to the exterior out of ground contact for 1–36 months according to EN 927-3, and for 1–12 weeks in Xenotest with water spraying according to partly modified EN 927-6. With prolonged weathering, the following changes occurred in the top surfaces of all tropical woods: (1) visual—creation of longitudinal macro-cracks, (2) spectrophotometry and CIE-L ^? a ^? b ^? —darkening in exterior exposure mainly due to pollutants, except for ipé, and vice versa lightening in Xenotest, as well as greening and blueing in both modes of exposure, (3) FTIR—faster decrease of guaiacyl than syringyl lignin, absolute decrease of conjugated and unconjugated carbonyl groups in the newly formed lignin-polysaccharide-extractive substrate in the photo-oxidized and washed-out cell walls, and decrease of cellulose crystallinity, (4) SEM—damaging of cell-walls by micro-cracks, and their degradation by thinning. Connections between changes of the individual characteristics of weathered woods, for example, between the colour (ΔE*, etc.) and the molecular structure (carbonyls, etc.), were also determined.     

Direct measurement of the wetting behavior of individual carbon nanotubes by polymer melts: the key to carbon nanotube-polymer composites

Carbon nanotube wetting is the critical parameter for the development of nanocomposites yet, due to the lack of suitable methods of qualifying and, more importantly, quantifying nanoscale wetting and adhesion phenomena, it is often overlooked. Here, we discuss a qualitative approach that provides wetting/nonwetting information and present a microfluidics method to produce "nanodroplets" of polymers on individual carbon nanotubes which enable the direct quantification of contact angles.     

Selective digital filters with quadratic phase

A method for design of a new class of digital infinite impulse response filters realized as parallel connection of two all‐pass filters is presented in this paper. A new approach to approximation of quadratic phase of all‐pass filter at all frequencies is given. Chosen parallel structure offers opportunity for realization of filters with arbitrary shape phase. The presented algorithm is based on all‐pass filter phase approximation. Phases of both all‐pass filters approximate ideal quadratic phase in minimax sense at all frequencies. Such filters can be applied for chirp signal compression or expansion. Magnitude characteristic of described filters is very selective and elliptic‐like. Obtained filters are compared with elliptic filter and group delay corrector in cascade. For the same specifications, much better results are achieved by the proposed filters. Parallel connection of all‐pass filters introduces lower signal delay, and for a given maximal phase, approximation error demands less complex network. Examples to illustrate the proposed method are given. Copyright © 2016 John Wiley & Sons, Ltd.     

Microstructure and dislocation density evolutions in MgAlZn alloy processed by severe plastic deformation

Commercial MgAlZn alloy AZ31 was processed by hot extrusion and equal channel angular pressing (ECAP) known as EX-ECAP. Microstructure and defect structure evolution with strain due to ECAP were investigated by TEM, positron annihilation spectroscopy (PAS), and X-ray diffraction. Significant grain refinement was obtained by EX-ECAP. In the extruded condition relatively low density of dislocations was determined by PAS. Sharp increase of dislocation density occurred during the first two passes of ECAP, followed by the saturation and even a decline manifesting the dynamic recovery at higher strains. XRD line profile analysis confirmed the results of PAS with slightly higher values of dislocation densities in individual conditions. Detailed analysis of contrast factors allows to determine the type of dislocations and to draw conclusions about slip activation and its variations with strain. The influence of microstructure evolution on mechanical properties is discussed.     

The corrosion resistance of Nitinol alloy in simulated physiological solutions: Part 1: The effect of surface preparation

The corrosion behaviour of Nitinol alloy containing nearly equi-atomic composition of nickel and titanium and its constituent metals (nickel and titanium) was investigated in simulated Hanks physiological solution (pH value 7.5) and pH modified simulated Hanks physiological solution (pH values 4.5 and 6.5) and by electrochemical method of anodic potentiodynamic polarization at 37 °C. In this chloride-rich medium the corrosion stability of Nitinol is limited by the susceptibility to localized corrosion and is in that sense more similar to nickel than to titanium. The corrosion stability of Nitinol is strongly dependent on the surface preparation—grinding, polishing or chemical etching. Whereas a ground surface is not resistant to localized corrosion, polished and chemically etched surfaces are resistant to this type of corrosion attack. The reasons for this behaviour were investigated through metallurgical, topographical and chemical properties of the surface as a function of surface preparation. For that purpose, scanning electron microscopy combined with chemical analysis, confocal microscopy and X-ray photoelectron spectroscopy were used. The surface roughness decreased in the following order: chemically etched > ground > polished surface. Besides differences in topography, distinct differences in the chemical composition of the outermost surface are observed. Ground, rough surfaces comprised mainly titanium oxides and small amounts of nickel metal. Chemically etched and, especially, polished surfaces are composed of a mixture of titanium, nickel and titanium oxides, as studied by angle resolved X-ray photoelectron spectroscopy. These results emphasize the importance of detailed investigation of the metal surface since small differences in surface preparation may induce large differences in corrosion stability of material when exposed to corrosive environments.     

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Introduction

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