Mechanical and dielectric properties of epoxy–clay nanocomposites

Epoxy–clay nanocomposites were prepared using two types of surface-treated montmorillonite (Closite 30B and Nanomer I28E). Wide angle X-ray scattering showed that all the nanocomposites had an intercalated structure. Improvements in tensile and fracture properties were found. The pure epoxy polymer was very brittle with a fracture energy, G _c, of 131 J m^?2. The addition of the nanoclays significantly increased the value of G _c, up to 240 J m^?2 for 5 wt% C30B. The toughening mechanisms acting in the nanocomposites were identified using scanning electron microscopy as crack deflection and plastic deformation of the epoxy matrix around the clay platelets following debonding. From electrical testing, the permittivity and loss angle of the nanocomposites decreased, and their breakdown strength increased as desired for insulation applications. The breakdown strength of the pure epoxy was found to be 11.7 kV mm^?1, while for a 2 wt% C30B nanocomposite, it increased to 14.7 kV mm^?1. It was concluded that the restriction of chain mobility inhibited electrical polarisation and thus decreased the permittivity and loss angle. The electrical damage zone was analysed using scanning electron microscopy. It was found that the higher resistance-to-surface degradation by partial discharges and the creation of a tortuous electrical path, which delayed the propagation of the electrical tree, were the main factors which improved the breakdown strengths of the nanocomposites.     

The activation energies of crystallization in the amorphous alloy METGLAS® 2826A

Nucleation and growth rates of the MSI metastable phase crystals of the amorphous alloy Fe₃₂Ni₃₆Cr₁₄P₁₂B₆ have been studied in detail using quantitative transmission electron microscopy. Three different types of activation energies are involved in the crystallization:E _n for nucleation,E _g for growth of the crystals, andE _c for the whole process of crystallization. All three were determined experimentally:E _n andE _g in the present paper andE _c in a former work. A general formula was developed which combines the three and was found to be in good agreement with the experimental values. Furthermore, attention has been drawn to the experimental value of the Avrami exponent which is in very good agreement with a detailed theory developed in 1955 by Ilschner, and differing with common literature values. Also, the diffusivity of metalloid in this alloy has been reported using the growth rate of MSI crystals.     

Synthesis of nickel–rubber nanocomposites and evaluation of their dielectric properties

Nanocomposites based on natural rubber and nano-sized nickel were synthesized by incorporating nickel nanoparticles in a natural rubber matrix for various loadings of the filler. Structural, morphological, magnetic and mechanical properties of the composites were evaluated along with a detailed study of dielectric properties. It was found that nickel particles were uniformly distributed in the matrix without agglomeration resulting in a magnetic nanocomposite. The elastic properties showed an improvement with increase in filler content but breaking stress and breaking strain were found to decrease. Dielectric permittivity was found to decrease with increase in frequency, and found to increase with increase in nickel loading. The decrease in permittivity with temperature is attributed to the high volume expansivity of rubber at elevated temperatures. Dielectric loss of blank rubber as well as the composites was found to increase with temperature.     

The metastable crystallization phases in the amorphous alloy METGLAS® 2826A

It is proved experimentally by transmission electron microscopy that the first crystallization of METGLAS® 2826A is a two-stage process, called MSI and MSII, rather than one-stage as described by Chang and Sastri. The details of MSI and MSII were investigated (structure, composition, morphology, activation energies) and discussed in terms of kinetics. MSI is controlled by small-scale diffusion involving metalloid depletion and, therefore, decelerated (t-type) reaction kinetics; MSII grows linearly witht because it is entirely interface-controlled. Certain differences in the results in comparison with Chang and Sastri's have been explained.     

Influence of surface modified TiO2 nanoparticles on dielectric properties of PVdF–HFP nanocomposites

Polyvinylidene fluoride-co-hexaflouropropylene (PVdF–HFP)/TiO₂ hybrid nanocomposites membranes for electrical applications have been prepared using a solvent casting technique. The interface between PVdF–HFP and TiO₂ was modified using aminopropyltrimethoxysilane (APS) coupling agent. The silane linkages on the TiO₂ surface have been confirmed using Fourier transform infra red spectroscopy. WAXD and DSC analysis has been employed to estimate the variation in crystallinity within the membrane as a function of the incorporation of both untreated and APS treated TiO₂. The dispersion of both nanoparticles in the PVdF–HFP matrix were characterized by atomic force microscopy and differences were observed in the images of APS treated and untreated. Variation in electrical properties such as conductivity, dielectric constant, dielectric loss and electric modulus of the hybrid composite films were studied employing AC impedance spectroscopy over a range of frequency from 1 kHz to 1 MHz at room temperature. Theoretical models like Maxwell, Faruka, Rayleigh and Lichtenecker were employed to calculate the effective dielectric constant of hybrid nanocomposite membranes and the estimated values were compared with the experimental data. Further, the variation in thermal stability of PVdF–HFP membrane as a function of untreated and silane treated TiO₂ reinforcement has been estimated using thermogravimetric analysis.     

Photocatalytic decomposition of VOCs by AC–TiO2 and EG–TiO2 nanocomposites

Clean Technologies and Environmental Policy - In this study, TiO2 nanoparticles (NPs)-based catalysts were prepared for the photocatalytic removal of toluene as a model VOC from air under UV light....     

Discrimination of matrix–fibre interactions in all-cellulose nanocomposites

All-cellulose nanocomposites are produced using dissolved microcrystalline cellulose (MCC) as the matrix and cellulose nanowhiskers (CNWs), produced by acid hydrolysis, as the reinforcement. These nanocomposites are then characterised using X-ray diffraction to determine their crystallinity, and Raman spectroscopy to discriminate the reinforcing phase (cellulose I) from the CNWs and the matrix phase (cellulose II) from the dissolved MCC. Mechanical testing of the composites shows that there is a significant systematic reinforcement of the matrix material with the addition of CNWs. Furthermore, Raman spectroscopy is used to show that distinct spectroscopic bands for each phase within the composite spectrum can be used to discriminate the effects of both reinforcement and matrix. It is shown that a Raman band located approximately at 1095 cm⁻¹ can be used to follow the micromechanical deformation of the CNWs and matrix, whereas another band located at 895 cm⁻¹ arises purely from the matrix. This spectroscopic fingerprint is used to gain insights into the complex interactions occurring in these potentially recyclable composite materials, and offers a way forward to optimising their properties.     

The real analytic Feigenbaum–Coullet–Tresser attractor in the disc

We consider a real analytic diffeomorphism ψ₀ on an n-dimensional disc 𝒟, n ≥ 2, exhibiting a Feigenbaum–Coullet–Tresser (FCT) attractor. We assume that in the C ^ω(𝒟) topology it is far from the standard FCT map φ₀ fixed by the double renormalization. We prove that ψ₀ persists along a codimension-one manifold ? ? C ^ω(𝒟), and that it is the bifurcating map along any one-parameter family in C ^ω(𝒟) transversal to ?, from diffeomorphisms exhibiting sinks to those which exhibit chaos, filling a gap in the usually accepted proof of this assertion. The main tool in the proofs is a theorem of functional analysis, which we state and prove in this article, characterizing the existence of codimension-one submanifolds in any abstract functional Banach space.     

Deformation behavior of Al–Si alloy based nanocomposites reinforced with carbon nanotubes

Nanocrystalline Al–Si alloy-based composites containing carbon nanotubes (CNTs) were produced by hot rolling ball-milled powders. During the milling process, the grain size was effectively reduced and the Si element was dissolved in the Al matrix. Furthermore, CNTs were gradually dispersed into the aluminum powders, providing an easy consolidation route using a thermo-mechanical process. The composite sheet containing 3 vol.% of CNTs shows ～520 MPa of yield strength with a 5% plastic elongation to failure.     

Energetic ion irradiation induced crystallization of Ni–Mn–Sn ferromagnetic shape memory alloy thin film

The ion irradiation induced crystallization of Ni–Mn–Sn ferromagnetic shape memory alloy (FSMA) thin film is investigated. Thin films of Ni–Mn–Sn FSMA synthesized by DC magnetron sputtering on Si substrate at 200 °C are irradiated by a beam of 120 MeV Ag ions at different fluence varying from 1 × 10¹² to 6 × 10¹² ions/cm². X-ray diffraction pattern reveals that the pristine film grows in L2₁ cubic austenite phase with poor crystallinity and crystallinity of the film improves with increasing ion fluence, which is attributed to the strain relaxation by the energy deposited by incoming ions and promotes the grain growth. Grain growth is further confirmed by Atomic force microscopy. The temperature dependent magnetization measurements show improvement in the magnetic and shape memory properties of the films with increasing fluence, which is ascribed to the ordering of austenite phase. Nanoindentation measurements show that with increasing fluence of 120 MeV Ag ions, films exhibit a greater stiffness and smaller tendency towards plastic deformation.     

Nitriding of Co–Cr–Mo alloy in nitrogen

Using the results of a thermodynamic analysis, a Co–Cr–Mo alloy was successfully nitrided in nitrogen at temperatures of 1073–1473 K. The near-surface microstructure of the treated Co–Cr–Mo alloy was characterized using X-ray diffraction, field-emission scanning electron microscopy, electron probe micro-analyzer, and transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy. The results indicated that the highest nitriding efficiency was achieved at the treatment temperature of 1273 K, with the size and coverage of the nitride particles on sample's surface increasing with an increase in the treatment duration. After nitriding at 1273 K for 2 h, numerous nitride particles, consisting of an outer Cr₂N layer and an inner π phase layer, were formed on top of the nitrogen-containing γ phase, and some π phase also precipitated in the alloy matrix at the sub-surface level.     

Mechanochemical preparation of BaTiO3–Ni nanocomposites with high dielectric constant

A mechanochemical procedure is proposed for an easy preparation of a BaTiO₃–Ni composite in a single step. BaTiO₃ and Ni powders available in the market are mixed by dry ball milling producing a decrease of particle size and an evenly distribution of both phases. In the sintered pellets the nickel particles are homogeneously distributed into the BaTiO₃ matrix and isolated from others Ni particles. The dielectric constant of the composite is considerably higher than that of the barium titanate. Moreover, the temperature of the ferroelectric ↔ paraelectric transition of the BaTiO₃–Ni composite here prepared is much lower than the one of the pure BaTiO₃ single phase.     

Raman spectroscopy and AC conductivity of polyaniline montmorillonite (PANI–MMT) nanocomposites

A series of polyaniline/montmorillonite clay (PANI–MMT) nanocomposites were synthesised by in situ polymerisation of aniline in acidic medium in the presence of varying amounts (from 1 to 30 wt%) of MMT and a substoichiometric amount of ammonium persulfate (APS). For a preferred molar ratio of monomer to oxidant of 2:1, the aniline was polymerised and largely incorporated into the MMT. The PANI–MMT nanocomposites were characterised and compared by wide-angle powder X-ray diffraction and UV–Vis spectroscopy. Raman spectroscopy was used to investigate the interaction between clay layers and the intercalated polymer chains. Room temperature AC conductivity was measured in the frequency range 30 Hz to 1 MHz. Pure PANI and all the composites exhibited a low frequency region of frequency independent AC conductivity followed by a high frequency dispersive region where the AC conductivity obeyed a fractional power law of frequency dependence. The fractional exponent n for all the nanocomposites is significantly high as compared to that of pure PANI; n = 0.9 for all the composites whereas for pure PANI n = 0.2. In pristine PANI the onset of the dispersive region ω _c is at a much higher frequency (at 300 kHz) as compared to the nanocomposites in which ω _c is about 10 kHz.     

The effect of Ca on the in vitro corrosion performance of biodegradable Mg–Nd–Y–Zr alloy

The effect of 0.4% Ca on the in vitro corrosion behavior of Mg–1.2% Nd–0.5% Y–0.5% Zr was evaluated in a simulated physiological environment in the form of 0.9% NaCl solution saturated with Mg(OH)₂ at ambient temperature and at 37 °C. The microstructure examination was carried out using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. The corrosion behavior was evaluated by immersion test, salt spray testing, and potentiodynamic polarization analysis. The stress corrosion behavior was examined using slow strain rate testing analysis in different strain rates. The results obtained have shown that the addition of 0.4% Ca has a beneficial effect on the corrosion resistance of the tested alloy. This was mainly attributed to the effect of calcium, which reduces oxidation in the molten condition and consequently improves the soundness of the obtained casting. Hence, it is believed that the reduction in the extent of inherent casting defects in the alloy containing calcium overcomes the detrimental micro-galvanic effect of the Mg₂Ca phase that was formed in the tested alloy. Contrary to the beneficial effect of calcium on the corrosion performance, the addition of calcium has a damaging effect on the stress corrosion behavior in terms of reduced ultimate tensile strength and ductility. This was mainly due to the embrittlement effect of calcium that was generated by the formation and distribution of Mg₂Ca phase at grain boundaries.     

The current–voltage characteristics of high-voltage spark discharge over a dielectric surface in vacuum

Investigation of temporal dependences of the voltage and current of high-voltage spark discharge over a dielectric surface in vacuum has shown that, in the presence of a longitudinal magnetic field, delay of the leading edge of the pulse-breakdown voltage takes place and the current, in turn, is converted from a well-defined impulse into alternating oscillations. Deceleration of the breakdown process in the presence of a low-current discharge that precedes the spark has been observed.     

Liquid phase hydrogenation of 2-ethylanthraquinone over La-doped Ni–B amorphous alloy catalysts

A series of La-doped Ni–B amorphous alloy catalysts were prepared by a KBH₄ reduction method. Ni powders were prepared by hydrazine reduction. These samples were characterized by inductively coupled plasma (ICP), BET, XRD, X-ray photoelectron spectroscopy (XPS), H₂-chemisorption, H₂-TPD, etc., and studied in the hydrogenation of 2-ethylanthraquinone (EAQ). The Ni–B amorphous alloy catalysts showed higher specific surface area, higher extents of H₂-chemisorption and higher H₂-TPD areas than Ni powders. In Ni–B amorphous alloys, with the addition of La, the extents of H₂-chemisorption and H₂-TPD areas first increased markedly and then decreased, with the maximum appeared at the atomic ratio of La to Ni of 0.034. The hydrogenation activity also showed the same trend. The effects of La were attributed to its dispersion of Ni particles, resulting in the formation of more surface Ni centers. However, much higher La contents may result the coverage of surface Ni centers. After heat treatment at higher temperatures, the amorphous structure of Ni–B was destroyed.     

The improvement of the amorphous environment of the germanate–tellurate glasses in the presence of the gadolinium ions

Glasses in the system xGd₂O₃·(100 − x)[TeO₂·GeO₂] with 0 ≤ x ≤ 50 mol% have been prepared from melt quenching method. In this paper, we investigated changes of the coordination numbers of germanium, tellurium, and gadolinium ions by investigations of FTIR, EPR, and UV–VIS spectroscopy. By analyzing the structural changes resulted from the IR spectra we found that the bending modes of [GeO₄] structural units and the deformed modes of the Te–O–Te linkages produce intercalation of the [GdO _n ] entities in the germanate–tellurate chain network and densification of the glasses by increasing the number of [GeO₆] structural units. EPR spectra of the studied samples reveal that the gadolinium ions play a role of network former. The UV–VIS spectra show broad UV absorption bands located in the 250–350 nm region. Their intensity increase with the increasing of Gd₂O₃ content showing that these stronger transitions can be due to the presence of the O=Ge bonds (n–π* excitations) of [GeO₅] structural units. The [GeO₅] structural units are more stable thermodynamically than their analogues and the [GeO₆] structural units produce the improvement of the amorphous character of these glasses.     

The role of microstructure of nickel–aluminium–bronze alloy on its cavitation corrosion behavior in natural seawater

An investigation was carried out in our laboratory to study the effect of the microstructure of nickel–aluminum–bronze (NAB) alloy on its cavitation corrosion behaviour in seawater using a 20-kHz ultrasonic induced cavitation facility. Cavitation tests were made under free corrosion conditions as a function of exposure time in natural seawater. Optical and scanning electron microscopy showed NAB immersed in stagnant seawater suffered from selective corrosion of the copper-rich α phase at boundaries with intermetallic κ precipitates. The κ precipitates and precipitate-free zones did not suffer corrosion. Cavitation made the surface of this alloy very rough, with large cavities or pits, ductile tearing and corrosion of the boundaries of the α columnar grains. In addition, the number of cavities and their size increased with exposure time. Microcracks 5–10 μm long were observed in the α phase adjacent to κ precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were considered to be the cause of the cracks observed.