Strain rate dependence of tension and compression behavior in nano-polycrystalline vanadium nitride

We performed molecular dynamics (MD) uniaxial tension and compression simulations of nano-polycrystalline (npc) vanadium nitride (VN) with different strain rates to investigate strain rate effects and tension-compression asymmetry in npc VN. The Zener's familiar anisotropy index A of the polycrystalline VN model is 0.957, very closed to 1.0, demonstrating its elastic isotropy. The Young's modulus increases with the increase of strain rate. The yield strain, yield stress and flow stress increase in power form with the increase of strain rate. As the strain rate varies in the range between 5E8 and 1E9 s⁻¹, strain rate effect becomes insignificant, and the corresponding loading can be regarded as quasistatic or low strain rate loading. Tension-compression asymmetry was observed and analyzed systematically. The elastic tension-compression asymmetry should be ascribed to the higher friction in compression, the asymmetry of interatomic potential, and the effect of cutoff distance in a MD simulation. While the plastic tension-compression asymmetry should mainly result from the different kinds of atomic interaction, intergranular fracture and dislocation glide. No distinct grain boundary sliding was found.     

Characteristic time of strain induced crystallization of crosslinked natural rubber

Real time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 s^?1–280 s^?1) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value λ_c. λ_c is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value.     

Thermostability,oxidation, and high-temperature tribological properties of nano-multilayered AlCrSiN/VN coatings

In this study, monolithic AlCrSiN, VN, and nano-multilayered AlCrSiN/VN coatings were deposited using a hybrid deposition system combining arc ion plating and pulsed direct current magnetron sputtering. The microstructure, thermostability, mechanical, oxidation and tribological properties of the coatings were comparably investigated. The multilayered AlCrSiN/VN coating exhibited a face-centered cubic (fcc) structure with (200) preferred orientation and showed the highest hardness (30.7 ± 0.5 GPa) among these three coatings due to the multilayer interface enhancement mechanism and higher compressive stress. The AlCrSiN sublayers effectively prevented the V element from rapid outward diffusion to the surface of AlCrSiN/VN coating at elevated temperatures, which improved the oxidation resistance of the coating. Decomposition of V (Cr)–N bonds occurred at annealing temperatures from 800 °C to 1000 °C and V₂N phase appeared at 1100 °C. The AlCrSiN/VN coating showed excellent tribological performance at high temperatures by combining the merits of VN layers for low friction coefficient and AlCrSiN layers for superior oxidation resistance. Compared to VN and AlCrSiN coatings, AlCrSiN/VN coating showed the lowest wear rate of 2.6×10^-15 m³/N·m at 600 °C and lowest friction coefficient of 0.26 at 800 °C with a relativity low wear rate of 39.4×10^-15 m³/N·m.     

Poly(sulfonated phenylene)-block-poly(arylene ether sulfone) copolymer for polymer electrolyte fuel cell application

Novel poly(2-(3-sulfo)benzoyl-1,4-phenylene)-block-poly(arylene ether sulfone) copolymers (PSP-b-PAESs) were successfully synthesized by Ni(0)-catalyzed copolymerization of 2,5-dichloro-3′-sulfo-benzophenone (DCSB) and chlorobenzophenone-endcapped oligo(arylene ether sulfone). Their physical property, morphology and polymer electrolyte fuel cell (PEFC) performance were investigated compared to those of the DCSB-based random copolymers and Nafion112. PSP-b-PAES with a measured ion exchange capacity (IEC) of 1.82 meq g^?1, of which the hydrophilic/hydrophobic block lengths were evaluated as 17/8.4, showed the relatively small number of water molecules sorbed per sulfonic acid group (λ = 15) in water and the anisotropic membrane swelling with 2.4 times larger through-plane swelling than in-plane one, whereas it showed the almost isotropic proton conductivity. The PSP-b-PAES exhibited a microphase-separated structure composed of hydrophobic and hydrophilic domains, whereas the random copolymers exhibited a homogenous morphology. The PSP-b-PAES had the larger proton conductivity than the random copolymer with an IEC of 2.01 meq g^?1, especially under the low relative humidities. Even at a low humidification of 17% RH at 90 °C and 0.2 MPa, the PSP-b-PAES exhibited the high PEFC performance; namely, cell voltage of 0.69 V at load current density of 0.5 A cm^?2 and maximum output of 0.73 W cm^?2, which were much higher than those at 30% RH for the random copolymer (0.65 V and 0.51 W cm^?2) and Nafion112 (0.70 V and 0.61 W cm^?2). The PSP-b-PAES showed the fairly high durability of 750 h under PEFC operation at 90 °C in spite of the relatively low molecular weight. PSP-b-PAESs have the high potential as polymer electrolyte membrane for PEFC applications.     

Vanadium nitride as a novel thin film anode material for rechargeable lithium batteries

Vanadium mononitride (VN) thin films have been successfully fabricated by magnetron sputtering. Its electrochemical behaviour with lithium was examined by galvanostatic cell cycling and cyclic voltammetry. The capacity of VN was found to be stable above 800 mAh g⁻¹ after 50 cycles. By using ex situ X-ray diffraction, high-resolution transmission electron microscopy and selected area electron diffraction as well as in situ spectroelectrochemical measurements, the electrochemical reaction mechanism of VN with lithium was investigated. The reversible conversion reaction of VN into metal V and Li₃N was revealed. The high reversible capacity and good stable cycle of VN thin film electrode made it a new promising lithium-ion storage material for future rechargeable lithium batteries.     

Antireflective coatings prepared by sol–gel processing: Principles and applications

Sol–gel processing is a powerful tool to prepare antireflective (AR) coatings on optical surfaces. In this paper the different strategies to obtain antireflective properties are reviewed: porous λ/4 layers, multilayer interference-type films and index-gradient materials such as “moth eye” structures. The processing of the respective films is described and evaluated; references to respective commercial products on glass substrates are given.AR coatings may have a particularly high importance for transparent ceramics as their index of refraction is significantly higher than that of common glass types. Reflective losses therefore are higher which is especially unpleasant for materials with a yet improvable intrinsic transparency.Recent studies indicate that specific porous λ/4 layers may exhibit pronounced anti-soiling features. Laboratory experiments as well as outdoor exposure tests were used to demonstrate the dust-repellant properties.     

Effect of surface heat transfer coefficient gradient on thermal shock failure of ceramic materials under rapid cooling condition

A rapid thermal processor (RTP) device as well as quenching technique is employed to systematically investigate the effect of surface heat transfer coefficient (h) gradient on thermal shock failure of a hot-pressed ZrB₂-based ceramic. Two typical kinds of quenchant with different surface h gradients during quenching tests, water and boiling water, are used for this study. When water as the cooling medium, two different cooling modes of indirect contact cooling by RTP device and direct contact cooling by quenching are also studied. The experimental results and related numerical simulations illustrate that surface h gradient plays an important role in thermal shock failure. This study confirms the previous presumption that the combination of body temperature gradient and surface h gradient leads to thermal stress damage and thermal shock failure. Under water quenching condition, water phase changes form bubbles randomly and produce great surface h gradient. Accordingly, critical body temperature gradient (V(max)_c) is small (~ 270?°C?s^?1). Under aqueous polymer quenching condition, the introduction of polymer chains into water lowers the random formation of steam bubble and mediates the surface h gradient. The corresponding V(max)_c hence become larger (~ 500?°C?s^?1). Under boiling water quenching condition, there is no surface h gradient and V(max)_c is even larger (> 600?°C?s^?1). This study provides useful complementary information for understanding the thermal shock behavior and gives suggests for predicting materials performance in actual service.     

A method of determining the elastic properties of diamond-like carbon films

The elastic properties of diamond-like carbon (DLC) films were measured by a simple method using DLC bridges which are free from the mechanical constraints of the substrate. The DLC films were deposited on a Si wafer by radio frequency (RF) glow discharge at a deposition pressure of 1.33 Pa. Because of the high residual compressive stress of the film, the bridge exhibited a sinusoidal displacement on removing the substrate constraint. By measuring the amplitude with a known bridge length, we could determine the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allows the calculation of the biaxial elastic modulus, E/(1−ν), where E is the elastic modulus and ν is Poisson's ratio of the DLC film. The biaxial elastic modulus increased from 10 to 150 GPa with increasing negative bias voltage from 100 to 550 V. By comparing the biaxial elastic modulus with the plane–strain modulus, E/(1−ν²), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. The elastic modulus, E, ranged from 16 to 133 GPa in this range of the negative bias voltage. However, large errors were incorporated in the calculation of Poisson's ratio due to the pile up of errors in the measurements of the elastic properties and the residual compressive stress.     

Superior mechanical and tribological properties governed by optimized modulation ratio in WC/a-C nano-multilayers

WC/a-C nano-multilayers with different modulation ratio (WC:a-C) ranging from 1:10 to 1.5:1 were deposited by fixing a-C individual layer thickness and tailoring WC individual layer thickness. The effect of modulation ratio on mechanical and tribological performance of WC/a-C nano-multilayers were investigated. Superior mechanical and tribological properties were simultaneously achieved at modulation ratio of 1:1.2. In addition to the improvement of mechanical properties, the improved tribological properties should also be attributed to the friction-induced formation of a WO₃-rich transfer film under an appropriate WC individual layer thickness, which combing the graphitized worn film surface constructed an intrinsically weak-interacting sliding interface (WO₃/C interface). Also, graphitized carbon is an essential coadjutant for the formation of WO₃-rich transfer film.     

Improvement in performance of indium gallium oxide thin film transistor via oxygen mediated crystallization at a low temperature of 200 °C

We report the fabrication of high-performance polycrystalline indium gallium oxide (IGO) thin film transistors (TFTs) at a low temperature of 200 °C. Growth of a highly aligned cubic phase with a bixbyite structure was accelerated at a certain proportion of oxygen plasma density during deposition of the IGO thin film, which leads to outstanding electrical characteristics. The resulting polycrystalline IGO TFT exhibited a high field-effect mobility of 56.0 cm²/V, a threshold voltage (V_TH) of 0.10 V, a low subthreshold gate swing of 0.10 V/decade, and a current modulation ratio of >10⁸. Moreover, the crystalline IGO TFTs have highly stable behaviors with a small V_TH shift of +0.8 and ?1.0 V against a positive bias stress (V_GS,ST ?V_TH = 20 V) and negative bias illumination stress (V_GS,ST ?V_TH = ?20 V) for 3,600 s, which is attributed to the high quality of the bixbyite crystalline structure.     

Elasticity at large deformations and high strain rates in injection molded polypropylene

The deformation behavior of isotactic polypropylene (PP) as a function of strain rate was investigated at 50°C in uniaxial tension. Injection molded dogbone specimens were tested at high strain rates, ε = 10^?1 – 10² s^?1, and the local deformation in the neck was studied using fast tensile videometry. A strong elastic recoil was observed after fracture in this strain rate range with local elastic strains as high as ?_e = 2.0 – 3.2. The recoil is very fast and takes place within 1 ms. The elastic fraction of the strain at break was found to increase with the local strain rate. The elasticity further depends on strain and temperature. The elastic deformation behavior is part of the known transition from ductile cold drawing behavior to brittle fracture that occurs with strain rate or temperature. The elasticity in PP is thought to be due to a decrease in crystallinity, resulting in a discontinuous crystalline structure comparable to that of thermoplastic elastomers.     

Investigations into capacity fading as a result of a Jahn-Teller distortion in 4 V LiMn2O4 thin film electrodes

This study reports the onset of the Jahn-Teller distortion in 4 V LiMn₂O₄ thin film electrodes that was investigated using an in situ bending beam method (BBM). The phase transformation during lithium insertion/extraction could be detected using the BBM technique. The phase transformation between the cubic and tetragonal phases was depicted by the larger value of the compressive or tensile differential strain, which is consistent with a well-known phase transformation between those phases in 3 V LiMn₂O₄. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously. The potential ranges responsible for the Jahn-Teller distortion in 4 V range, which takes place at the electrode surface, was determined by the charge versus. differential strain (dε/dQ) curve. The onset of the Jahn-Teller distortion was observed at the end of the cathodic scan, and the relaxation of the Jahn-Teller distortion was observed at the beginning of anodic scan. Furthermore, the onset of the Jahn-Teller distortion was found to be dependent on the lithium ion insertion rate, which was controlled by the scan rate.     

Morphology of PEO/PDMS blends during shear: Coexistence of two droplet/matrix structures and additive effects

The morphologies of blends of polyethyleneoxide (PEO 37) and poly(dimethylsiloxane)s (PDSM), with viscosity ratios, λ, of approximately one (PDMS 230) or 2.8 (PDMS 314, being the component of higher viscosity) and interfacial tensions on the order of 10 mN/m, were investigated at 70 °C as a function of shear rate (up to 10 s⁻¹) and of time. For the system PEO 37/PDMS 230 we have also studied the influence of the compatibilizer dimethyl-ethyleneoxide-copolymer (PDMS-co-PEO), which is only reasonably soluble in PEO. To investigate the morphologies we have used an optical shear cell in combination with a light microscope. The most important observation consists in the formation of two coexisting droplet/matrix structures for volume fractions of PDMS ranging from 0.4 to 0.6 for both λ values; the presence of the copolymer extends this region to 0.7. In the case of λ≈1 the average droplet radii are within experimental error independent of composition and morphology; for λ=2.8 they depend on the matrix phase in which they are contained and do again not vary with composition. The reduction in drop size caused by the copolymer is markedly larger if PEO forms the matrix. The present morphological observations suggest that the two coexisting droplet/matrix phases develop out of a single droplet/matrix structure via coalescence processes.     

Surface chemistry of intermetallic AlSb-anodes for Li-ion batteries

The solid electrolyte interphase (SEI) layer on AlSb electrodes has been studied in Li/AlSb cells containing a LiPF₆ EC/DEC electrolyte using X-ray photoelectron spectroscopy (XPS). Data were collected before SEI-formation, during formation, and after formation at 0.01 V versus Li⁰/Li⁺, and at full delithiation in cycled cells at 1.20 V. The thickness of the SEI layer increases during lithiation and decreases during delithiation. This dynamic behaviour occurs continuously on cycling the cells. The growth of the SEI layer can be attributed predominantly to the deposition of carbonaceous species below 0.50 V versus Li⁰/Li⁺; these species disappear almost completely during delithiation. The extra surface-layer formation is a consequence of the additional charge that is needed to lithiate the remaining Sb component of the micrometer-sized AlSb particles at low potentials as seen by synchrotron-based X-ray diffraction. Aluminium is not reactive to lithium alloying in this electrolyte. Relatively small amounts of LiF were detected in the AlSb SEI layers compared to that commonly found in the SEI layers on graphite electrodes.     

Photoactive polymers containing vanadium: 1. Preparation,characterization and photoreactions

Photoactive polymers have been prepared by reaction of VOQ₂OH (Q = 8-quinolyloxo) with polymers of types A and B carrying pendant hydroxyl groups. Type A was represented by copolymers of methyl methacrylate and 2-hydroxyethyl methacrylate and type B by a copolymer of styrene and p-vinylbenzyl alcohol. The final copolymers are designated VA, VB, respectively. The copolymers were characterized by near u.v.—visible spectra. On irradiation (λ = 365 nm) the vanadium (IV) chelate VOQ₂ is formed; this is indicated by the u.v.—visible spectral changes, and also by e.s.r. spectroscopy. The infra-red spectra of the copolymers VA, VB show important differences, notably the presence of a strong band near 960 cm^?1 (attributable to V=O stretching vibrations) in the former and its absence from the latter. From this and other evidence it is concluded that the vanadium residues have different structures in the two types of copolymer, viz. O=VQ₂OR and (HO)₂VQ₂OR in VA and VB respectively, where R is the remainder of the copolymer molecule. Irradiation (λ = 365 nm) is considered to bring about scission of VOR bonds in copolymer VA with formation of a macroalkyloxy radical OR. This is consistent with spin-trapping experiments with benzylidene nitrone. On the other hand, photolysis of copolymer VB appears to give radicals identical with those from VOQ₂OH which are most probably OH.The formation of different types of radical on irradiation of copolymers VA, VB is also indicated by spin-trapping observations with nitrosodurene in benzene solution and also the occurrence of photografting when VA (but not VB) is irradiated in a monomer. The photolysis of VOQ₂OH to which we have referred above may provide a convenient source of hydroxyl radicals in non-aqueous solution.     

Electrochemical properties of anodic gold oxide layers—I: Potentiostatic oxide growth and double layer capacity

The potentiostatic growth of gold oxide layers was investigated at pH = 0, 2, and 6.2 in the potential range 1.4 V ? ? ? 2.3 V (vs hess) and for polarization times between 10^?4 and 10³ s. At ? < 2.0 V the growth of monomolecular oxide layers (oxide 1) with a thickness up to 10 Å was observed. This process takes place according to a high field mechanism. In this range, the double layer capacity decreases with increasing thickness d. With increasing ? there is a continuous change of oxide growth.At ? > 2.0 V a potential dependent linear growth law is observed with the formation of thick layers (oxide 2) up to 600 Å or more. In this range, further peaks arise in the cathodic charging curves, and the double layer capacity becomes independent of d.It follows that on the monomolecular boundary layer (oxide 1) the bulk oxide 2 is formed, which has a good ionic and electronic conductivity. Hence, the potential drop is small in oxide 2, but large in oxide 1. Even at high potentials the rate-determining step is the field dependent ionic migration in oxide 1.     

Electrochromism of dinitrobenzoyl-derivatised polypyrrole films deposited on ITO/glass electrodes

Poly[(R)-(-)-3-(1-pyrrolyl)propyl-N-(3,5-dinitrobenzoyl)-α-phenylglycinate] films of various thicknesses were galvanostatically deposited on ITO/glass electrodes using different deposition charges (Q_dep), and their electrochromic properties were investigated. Depending on the Q_dep employed, the polymeric films presented green-yellow or green-brown colours in the reduced state (λ_max at 350 nm and shoulder at 390 nm) and a blue-grey colour in the oxidised state (λ_max at 460 nm) with high absorption in the near infrared region (λ_max > 800 nm). Whilst films deposited with a Q_dep of 40 mC cm⁻² presented the highest chromatic contrast (20%), films with a Q_dep of 50 mC cm⁻² exhibited greater stability to redox cycling (ca. 350 cycles), high coulombic efficiency (>73%) and good optical memory in the reduced state (E = 0.0 V).     

Characterization of blends of naphthalene-containing polymers with poly(alkyl methacrylates) by combined steady-state fluorescence spectroscopy and fluorescence decay measurements

The extent of mixing in two-component solution-cast films of poly(2-vinyl naphthalene) (P2VN) and poly(2-iso-propenyl naphthalene) (P2IPN) with poly(methyl methacrylate) and poly(n-butyl methacrylate) was studied by steady-state and time-resolved fluorescence spectroscopy. The steady-state fluorescence spectrum of 1% P2IPN in the two poly(alkyl methacrylates) evolves from 100% monomer emission (λ_max = 340 nm) to >70% excimer emission (λ_max = 390 nm) as its molecular weight increases from 1900 to 278000. Despite such clear-cut changes in the steady-state spectra on phase separation, the fluorescence decays were nonexponential for all mixed films. Triple-exponential decay functions were necessary to describe excimer decays at 430 nm and monomer decays at 340 nm. Moreover, the fluorescence decays varied slightly across the excimer emission band, and changed significantly when the polymer films were annealed. Studies on blends containing from 1 to 100% of P2VN and P2IPN established that this multicomppnent fluorescence decay behavior is intrinsicto the naphthalene-containing polymer phase, it is proposed that excimers formed between chromophores in meso and racemic dyads have different mean decay times and that imperfectly aligned chromophore pairs lead to an additional short-lived excimer decay component.     

Micromechanical properties of glassy and rubbery polymer brush layers as probed by atomic force microscopy

Carboxylic acid terminated polystyrene and polybutylacrylate were grafted from melt onto a silicon substrate modified with the epoxysilane monolayer. The tethered layers fabricated from polymers of different molecular weights are smooth, uniform, mechanically stable, and cover homogeneously the modified silicon surface. Micromechanical properties of the dry glassy and rubbery brush layers were measured with atomic force microscope. We observed that for the PS layers with the thickness higher than 7 nm, the average value of the elastic moduli reached 1.1 GPa, which is close, but still lower than the expected for bulk polymer. The elastic modulus of PS polymer brush layers dramatically depends upon molecular weight and follows the inverse law with segment molecular weight, M_c of 18,000 known for bulk PS. This result indicates that the process of the formation of the physical network within polymer melt of chains tethered to a solid substrate is similar to that occurring in unconstrained polymer melt. Under these conditions, three PS brush layers studied in this work represent different cases of chains without stable entanglements for M<M_c as well as chains with stable entanglements for brushes with M∼M_c. This transition shows itself in significant reduction of the compliance reflected in twofold increase in elastic modulus. Our estimation predicts that modest lowering of ‘limiting’ elastic modulus of 1.4 GPa can be expected for thicker polymer brushes.     

Tensile behavior of polypropylene blended with bimodal distribution of styrene‐ethylene‐butadiene‐styrene particle size

The objective is to characterize the effects of the bimodal distribution of rubber particles and its blend ratio on the mechanical properties of the thermoplastic polypropylene blended with two different styrene‐ethylene‐butadiene‐styrene triblock copolymer at the intermediate and high strain rates. Tensile tests are conducted at the nominal strain rates from 3 × 10^?1 to 10² (1/s). Phase morphology is investigated to estimate the bimodal rubber particle size distribution. In addition, the in situ observation is conducted during uniaxially stretching within transmission electron microscopy step by step to investigate the deformation events depending on the elongation of samples. The elastic modulus increased gradually as the blend ratio of large rubber particle increased. An increase in the rupture strain and the strain energy up to failure was found for the bimodal rubber particle distributed blend system where the blend ratios of small rubber particle and large rubber particle were same. This is because the smaller particles dominant blend systems show the bandlike craze deformation while the localized plastic deformation is taken place in the larger particles dominated blend systems. The synergistic effect of these rubber particles gives rise to a strong increase in the ductility of these bimodal rubber particle distributed polypropylene systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008