Shrinkage and mechanical properties of unsaturated polyester reinforced with clay and core–shell rubber

Glassy unsaturated polyester (UP) resin was reinforced using an organically modified montmorillonite (OMMT) and toughened with core?Cshell rubber (CSR) particles. The nanostructure, morphology, and deformation mechanism of composite specimens were studied by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and reflected optical microscopy (ROM). An intercalated nanostructure with partial exfoliation was observed in the UP reinforced by various amounts of OMMT. Locally clustered but globally good CSR particle dispersion in the UP matrix was evident in UP toughened with 5 and 10 wt% CSR particles. Simultaneous presence of OMMT and CSR particles in UP/OMMT/CSR hybrid composites was found to cause partial phase separation with bigger rubber particle agglomerates and lower clay-intergallery height increase. The effects of OMMT and CSR contents on volume shrinkage, impact fracture energy, fracture toughness, and compressive yield strength of UP were investigated. The introduction of OMMT of up to 3?wt% into the UP matrix lowered volume shrinkage to some extent, while further addition increased the shrinkage slightly. In the hybrid nanocomposites, the volume shrinkage decreased to a minimum level of 5.2?% with increases in OMMT level. The impact fracture energy of UP improved with increasing the OMMT level of up to 3?wt%, whereas its further addition decreased the impact fracture energy slightly due to the clay particle agglomeration. The hybrid composites with OMMT level below 3?wt% showed higher impact fracture energy compared to the reinforced UP specimens with the same OMMT levels. Interestingly, a synergism in the fracture toughness (K _IC) was observed in the hybrid composite containing 1?wt% OMMT and 10?wt% CSR particles. The presence of OMMT as reinforcement in the hybrid composites could compensate the lowering of the compressive yield strength caused by low-modulus CSR particles. The clay?Crubber particle interaction in the hybrid systems seems to increase the threshold of shear deformation of the UP matrix to some extent.     

Study on Shrinkage Deformation of Food in Microwave–Vacuum Drying

Drying shrinkage is an important problem in the food industry. Focusing on microwave–vacuum drying, we study the mechanism of deformation due to shrinkage of the food structure. A relationship between the strain and the water content is introduced for a finite element analysis. The temperature and water distributions are obtained by a finite difference method with the use of a variable permeability and diffusion coefficient depending on the water content. Comparisons with experimental data on radishes, carrots, and tofu indicate that the present model can express the deformation as well as the water content inside the materials.     

Oxidation of HfB2–SiC ceramics under static and dynamic conditions

The kinetics and the mechanism of oxidation of ceramics based on HfB₂ and SiC, manufactured by elemental self-propagating high-temperature synthesis followed by hot pressing were investigated. The synthesis product contained HfC_(x) and HfO₂ as impurity phases. Depending on the ratio between the main components, the samples were characterized by high structural and chemical homogeneity, porosity of 3–6 vol%, hardness up to 29 GPa, bending strength of 500–600 MPa, fracture toughness of 5.6–8.9 MPa × m^1/2, and thermal conductivity of 86.0–89.7 W/(m × K). The oxidation was performed under static conditions at 1650 °C and upon exposure to a high-enthalpy gas flow. A dense layer consisting of HfO₂/HfSiO₄ grains formed on the surface of the ceramics during both oxidation conditions; the space between the grains was filled with amorphous SiO₂–B₂O₃. The best heat resistance was observed for the ceramics with 16 wt% SiC for static conditions and 8 wt% SiC for gas-dynamic conditions.     

Ablation resistance and mechanism of SiC–LaB6 and SiC–LaB6–ZrB2 ceramics under plasma flame

In this study, silicon carbide-lanthanum hexaboride (SiC–LaB₆) and silicon carbide–lanthanum hexaboride–zirconium boride (SiC–LaB₆–ZrB₂) ceramics were fabricated by spark plasma sintering at 1900 °C, and their ablation resistance was tested under plasma flames over 2300 °C. The results indicate that the SiC–LaB₆–ZrB₂ ceramic exhibits better ablation resistance than the SiC–LaB₆ ceramic. After ablation under the plasma flame for 60 s, the mass and linear ablation rates of the SiC–LaB₆ ceramic were 15.83 μg/s and 1.08 μm/s, respectively, while those of SiC–LaB₆–ZrB₂ were -8.42 μg/s and -0.27 μm/s. With the addition of ZrB₂, SiC–LaB₆–ZrB₂ ceramic attained a high density and fewer inner oxygen diffusion channels. Moreover, the ZrO₂–La₂O₃–SiO₂ oxide scale with good self-healing ability and excellent stability was formed in the ablation centre, which can retard the further oxidation during ablation.     

Simulation of Ignition and Combustion of a Homogeneous Methane–Air Mixture under Local Thermal and Photochemical Impacts

Combustion, Explosion, and Shock Waves - Ignition and combustion of a homogeneous stoichiometric methane–air mixture under simultaneous local thermal and photochemical impacts, resulting in...     

Diverse reactions of S–S function under metal-mediated and metal-free thermolysis conditions

Parallel comparative studies on reactivities of S–S function in 2-ppds (2-ppds = bis(4-(pyridin-2-yl)pyrimidin-2-yl)disulfane) have been carried out under the same thermolysis conditions. In the presence of FeCl₂, thermolysis reaction of 2-ppds in the MeOH/CH₂Cl₂ mixture results in a mononuclear compound [FeCl₂(2-pps)] with newly generated organic ligand 2-pps (2-pps = bis(4-(pyridin-2-yl)pyrimidin-2-yl)sulfane). In the absence of FeCl₂, the result of solvolysis depends upon the conditions. Thus, in pure MeOH, 2-ppds partially converts into a mixture of S-(4-pyridin-2-yl)pyrimidin-2-yl (4-pyridin-2-yl)pyrimidine-2-sulfonothioate (2-ppst) and 2-pps, while in a MeOH/CH₂Cl₂ mixture, 2-(chloromethylthio)-4-(pyridin-2-yl)pyrimidine (2-ppct) is the main product.     

Hydrodynamics and simulation of air–water homogeneous bubble column under elevated pressure

A gas–liquid Eulerian computational fluid dynamics (CFD) model coupled with a population balance equation (PBE) was presented to investigate hydrodynamics of an air–water bubble column (1.8 m in height and 0.1 m in inner diameter) under elevated pressure in terms of pressure drop, gas holdup, mean bubble size, and bubble surface area. The CFD-PBE model was modified with three pressure correction factors to predict both the total gas holdup and the mean bubble size in the homogeneous bubbly flow regime. The three correction factors were optimized compared to experimental data. Increasing the pressure led to increasing the density, reducing the bubble size, and increasing the gas holdup. The bubble size distribution moved toward a smaller bubble size, as the pressure increased. The modified CFD-PBE model validated with experimental data and empirical models represented well hydrodynamics of the bubble column at P = 0.1, 1.5, and 3.5 MPa.     

Effects of thiourea and urea on zinc–cobalt electrodeposition under continuous current

The effect of thiourea and urea on zinc-cobalt alloys obtained from chloride baths under continuous current deposition are described and discussed. The deposit morphology was analyzed using Scanning Electron Microscopy (SEM) and an X-Ray Diffraction (XRD) was used to determine the preferred crystallographic orientations of the deposits. The use of additives does not refine the grain size of the Zn–Co alloy and an especially porous alloy was produced in the presence of urea. The preferred crystallographic orientations of zinc–cobalt alloys do not change in the presence additives. Zinc–cobalt alloys were without texture in the presence and absence of additives. Also, in the absence of additive and in the presence of urea, the XRD lines of the Zn–Co alloys are slightly shifted with respect to the pure zinc XRD lines, whereas, in the presence of thiourea, the XRD lines are not shifted. The alloy composition was examined using Energy Dispersive X-ray Fluorescence Spectroscopy (EDXRF). The percentage of cobalt in the alloy decreases slightly from 1.04 to 0.91 wt.% in the presence of urea and in the presence of thiourea it increases from 1.04 to 7.70 wt.%. Voltammetric studies show that thiourea increases the reduction rate of cobalt. This explains the increase in cobalt percentage in the alloy in the presence of thiourea.     

Dry Fractionation of Cupuassu (Theobroma grandiflorum) Fat: Physical–Chemical Properties and Polymorphic Behavior

Physical chemical properties of cupuassu fat were modified by dry fractionation. Stearin and olein fractions were obtained at 29, 26, and 24 °C. Polymorphic behavior of unfractionated cupuassu fat (UCF) and its fractions were studied in situ by small-angle (SAXS) and wide-angle (WAXS) X-ray scattering using synchrotron light. Polymorphic transitions were followed in real time tempering samples with a thermal cycle. For UCF, the main polymorphic form crystallized under selected conditions was the β’₂. α and β’₁-forms appeared in trace amounts. β₂-form was obtained after storage at 25 °C for 3 months. Stearins obtained at 26 (S-26) and 24 °C (S-24) showed a similar polymorphic behavior. However, S-26 with improved physical properties might be more suitable for chocolate production or as a trans-fat alternative than UCF. Stearin fraction obtained at 29 °C (S-29) had a complex polymorphic behavior. The α-form was the first polymorphic form detected followed by β’₂-form. There was a polymorphic transition from α to β’₁-form but no transition between β’-forms. They were independent to each other showing fractionation in two different solid solutions. Increased contents of the triacylglycerols (TAG) SOA and SOB together with lower contents of SOO compared to UCF led to co-crystallization because there was no complete compatibility among all TAG present in S-29. β₁-form crystallized after storage forming crystals with a double-layer arrangement and a characteristic morphology. This form could be useful for accelerating crystallization process in melted liquid systems.     

Ni–Nb-Based Mixed Oxides Precursors for the Dry Reforming of Methane

Perovskite-related mixed-oxides based on La Ni Nb and La Sr Ni Nb were synthesized by the auto combustion method to use as precursors materials for the catalytic reforming of methane at 700 ºC, atmospheric pressure, CH₄:CO₂ = 1:1. LaNiO₃ and LaNbO₄ were used as reference. XRD analysis show that the synthesis method produce a new series of precursor family formed by a mixture of oxides where Ni crystallized as part of a perovskite and Ruddlesden–Popper structure while Nb formed lanthanum orthoniobate LaNbO₄, a scheelite-type structure alternating with oxide layers, with phase distribution depending on niobium content. For Nb (x ≤ 0.3) Ni crystallizes as LaNiO₃ perovskite-type oxide while for Nb (x ≥ 0.7) it forms mainly the orthoniobate phase LaNbO₄ a scheelite-type structure. At higher calcined temperatures (~1100 °C) La₂Ni_0.8Nb_0.2O₄ was formed with a Ruddlesden–Popper structure consisting of three perovskite type layers along the c-axis alternating with a layer of the rock salt type phase. TEM analysis showed the presence of cubic particles with sizes varying between 5 and 60 nm depending on the extent of substitution of Ni by Nb. Reduction of the perovskite-related precursor oxides produced a series of Ni⁰/La₂O₃–NbOx oxides with high metallic dispersion which favors the activity and stability of the catalysts. Introduction of doping quantities of Sr into LaNi_0.8Nb_0.2O_3±λ structure produced a mixture of oxides with Sr dissolved in the lanthanum orthoniobate LaNbO₄ scheelite-type structure due to the similarity of ionic radii of La and Sr. Under the reaction conditions conversions near the thermodynamic equilibrium were attained which remains for long periods of time assessing the stability of the synthesized catalysts.     

Dry reforming of methane over palladium–platinum on carbon nanotube catalyst

A dry reforming (DR) catalyst based on bimetallic Pd–Pt supported on carbon nanotubes is presented. The catalyst was prepared using a microwave-induced synthesis. It showed enhanced DR activity in the 773–923?K temperature range at 3 atm. Observed carbon balances between the reactant and product gases imply minimal carbon deposition. A global three-reaction (reversible) kinetic model—consisting of DR, reverse water gas shift, and CH₄ decomposition (MD)—adequately simulates the observed concentrations, product H₂/CO ratios, and reactant conversions. Analysis shows that, under the conditions of this study, the DR and MD reactions are net forward and far from equilibrium, while the RWGS is near equilibrium.     

Peculiarities of interaction in the Zn–C system under high pressures and temperatures

The wettability of graphite by a melt of zinc at 8.0 GPa in the temperature range 1900–3100 K has been investigated experimentally, and the temperature dependence of the wetting angle of graphite by the zinc melt determined. A polycrystalline diamond layer has been revealed at the interface in the zinc–graphite system in the diamond stability region. Metallographic and X-ray studies of zinc–carbon alloys obtained under high pressure have been carried out. The existence of the carbide ZnC₂ was found, which participates in two three-phase, non-variant equilibria in the system. It is determined that octahedral diamond crystals form in high-carbon alloys of zinc at the pressure of 8.0 GPa between 2100 and 2500 K.     

Wet film application techniques and their effects on the stability of RuO2–TiO2 coated titanium anodes

The effect of wet film application techniques on the physical and electrochemical properties and operational stability of RuO₂–TiO₂ coated titanium anodes was evaluated. Four compositions of RuO₂–TiO₂ coatings were applied to Ti substrates by three different wet coating methods—brush, dip and spin. Changing the coating technique resulted in different morphologies. Electrochemically active surface area of the coatings was related to the morphology. A shift in Ru(III)/Ru(IV) oxidation potential occurred upon changing the application technique. For lower ruthenium content coatings, this shift was related to coating lifetime. Anode stability in accelerated lifetesting showed that dip coated samples lasted up to three times longer than brush coated samples for lower ruthenium content.     

Structure and catalytic performance of Pt-promoted alumina-supported cobalt catalysts under realistic conditions of Fischer–Tropsch synthesis

The structure of alumina-supported cobalt catalysts promoted with platinum and their catalytic performance in Fischer–Tropsch synthesis were investigated under realistic reaction conditions (P = 20 bar, T = 493 K) using in situ time-resolved X-ray diffraction with simultaneous analysis of reaction products. The catalysts were prepared via incipient wetness impregnation and characterized by a wide range of ex situ techniques. Direct in situ measurements were indicative of considerable versatility of alumina-supported cobalt catalysts during Fischer–Tropsch synthesis. Cobalt sintering occurred at the first hours of the reaction and resulted in a significant drop of the catalytic activity. In addition to sintering, partially oxidized catalysts containing smaller cobalt particles (mean particle size <5 nm) were slowly reducing during Fischer–Tropsch reaction. Treatment of cobalt catalysts in pure carbon monoxide led to selective transformation of cobalt metallic phases to Co₂C cobalt carbide. Cobalt carbidization followed by hydrogenation selectively led to cobalt hcp metallic phase, which seems to be more active in Fischer–Tropsch synthesis than cobalt fcc phase. Cobalt oxidation by water was not significant in the catalysts with metal particles larger than 5 nm even at high water concentrations.     

Kinetics of combined noncatalytic and catalytic hydrolysis of jute fiber under ultrasonic–far infrared energy synergy

Hydrolysis of pretreated waste jute fiber was intensified for maximizing reducing sugar (RS) yield deploying a novel reactor equipped with ultrasonic–far-infrared-waves (US–FIRW). At optimal 70°C temperature, 2.5 wt% Amberlyst-15 catalyst concentration, 15 min hydrolysis time and 10 (wt/wt) water loading; US–FIRW rendered significantly greater RS yield (74.82 mol%) compared to other reactors provided with far-infrared-wave (69.63 mol%), ultrasonication (50.34 mol%), and conventional thermal system (48.16 mol%). Kinetic models were developed considering noncatalytic-pseudo-homogenous (NCPH) in addition to the combined catalytic-pseudo-homogeneous (CPH) and catalytic heterogeneous (CHE) hydrolysis pathways. The results revealed that pseudo-homogenous–heterogeneous Eley–Rideal (PHHER) model could represent the hydrolysis kinetics most accurately. Remarkably, the lowest activation energy [16.75 kJ mol⁻¹ (NCPH), 13.82 kJ mol⁻¹ (CPH), 40.01 kJ mol⁻¹ (CHE)] required in US–FIRW evidently established its greater energy-efficiency among investigated reactors. The novel reactor and the simulated kinetic models can be applicable to other lignocellulosic biomass conversion for sustainable biorefinery.     

In situ synthesis of PcBN composites from Sialon–TiN–TiB2 binder and cBN under high temperature and pressure

Polycrystalline cubic boron nitride (PcBN) composites were synthesized in situ with TiB₂, TiN, and Sialon as binders under ultrahigh temperature (1550°C) and high pressure (5.5 GPa). X-ray diffractometry, universal testing machine, field emission scanning electron microscopy, and transmission electron microscopy were used to study the effect of Sialon content (0%, 20%, 40%, 60%, and 80% by mass of binder) on the phase composition, microstructure, and mechanical properties of PcBN composites. The results show that the main phases in the system are lamellar TiN, needle-rodlike TiB₂, and irregular long-rodlike Sialon. Some TiB₂ grows along the (1 0 −1 0) face, and Sialon grows along the (0 0 0 1) face. The density and mechanical properties of the sintered product are significantly improved due to the formation of Sialon. When the content of Sialon is 60%, the binder is uniformly distributed, the cBN interface is well bonded, and the density is the highest. At this time, the strength reaches the maximum 34.57 GPa, the fracture toughness is 6.82 MPa m^1/2, and the flexural strength reaches the maximum 870.79 MPa. On the whole, cBN composites have excellent prospects for future applications in cutting inserts due to their excellent mechanical properties.     

Reactive processing of polyethylene and polyethylene–ethylene/propylene/dicyclopentadiene blends under static and dynamic conditions

Abstract

Dicumyl peroxide induced reactive melt processing of polyethylene (PE) in a shear mix at 170°C in the absence or presence of selected acrylic monomers (acrylic acid, ethyl acrylate, and butyl acrylate) has been studied. The acrylic graft copolymers of PE showed development of higher shear stress compared with the control PE when studied rheologically in a plate and cone viscometer at 160–190°C. All the modified PE products retained the pseudoplastic flow behaviour of PE. Measure of rupture shear parameters and of thixotropic and relaxation behaviour of the different modified PEs and of the control PE were also evaluated and compared. The observed effects and unexpected trends were analysed and interpreted.

The comparative effects of sulphur vulcanisation of polyethylene–ethylene/propylene/dicyclopentadiene terpolymer (PE–EPDM) blends by static and dynamic techniques were also studied using both a conventional curative system and a silane curative system. Rheometric studies indicated development of a co-continuous phase morphology for the 30/70 PE–EPDM blend. For a given blend, cured under given conditions, tensile strength and elongation at break at 25°C were higher for vulcanisates obtained statically than for those obtained dynamically, while the corresponding modulus values followed the opposite trend. The conventional curative usually cured at a higher rate. The property differences from static and dynamic vulcanisation are explained in the light of the differences in the developed morphology.     

Potassium-Doped Ni–MgO–ZrO2 Catalysts for Dry Reforming of Methane to Synthesis Gas

Ni/K–MgO–ZrO₂ catalysts for dry reforming of methane, with a range of Mg/Zr ratios and each containing about 10 wt% Ni, were prepared via Ni nitrate impregnation on MgO–ZrO₂ supports synthesized by co-precipitation using K₂CO₃. It was found that a proportion of the potassium of the precipitant remained in the samples and improved the stability of the catalysts in the reaction. It was also shown that reduction of the catalysts at 1,023 K without calcination in air is necessary for stable and high activity; calcination in air at 1,073 K gives a deterioration of the catalytic properties, leading to rapid deactivation during the reaction. The order of the CH₄ conversions of the reduced catalysts after 14 h on stream was as follows: Ni/K–Mg₅Zr₂ ~ Ni/K–Mg ≥ Ni/K–Mg₂Zr₅ ? Ni/K–Zr. A catalyst with 0.95 wt% K on MgO–ZrO₂ with a Mg:Zr mole ratio of 5:2 showed the best resistance to deactivation. Experiments in a microbalance system showed that there was only negligible coke deposition on the surface of this sample. This behaviour was attributed to the presence of Ni nanoparticles with a diameter of less than 10 nm located on a MgO/NiO solid solution shell doped by K ions; this in turn covers a core of tetragonal ZrO₂ and/or a MgO/ZrO₂ solid solution. This conclusion was supported by EDS/TEM, XPS, XRD and H₂ chemisorption measurements.     

Investigation of structural transformation and residual stress under single femtosecond laser pulse irradiation of 4H–SiC

Silicon carbide (SiC) is an attractive semiconductor material for devices that operate under extreme conditions. However, its high hardness and chemical stability hinder micromachining. Femtosecond laser has been proposed extensively as an effective micromachining tool for SiC. However, the fundamental mechanisms of laser-material interaction during femtosecond laser irradiation process remain elusive. This paper presents a comprehensive study of the structural transformation and residual stress induced by irradiating a 4H–SiC target with a single-pulse femtosecond laser. The energy dependence of the structural characteristics at the spot center and the spatial distribution across the laser spot were determined using optical microscopy and micro-Raman spectroscopy. The effect of the laser fluence on the residual stress was discussed. The obtained results showed that no structural changes occurred at low energy, whereas bond breaking occurred among crystalline SiC (c-SiC) at high energy. A central disk with no structural transformation and no residual stress was formed for the fluence of 22.2 J/cm², owing to phase explosion-induced spluttering. Meanwhile, an inhomogeneous distribution of the structural transformation was generated from the spot centre to the edge. Based on this, threshold fluences for modification and structural transformation were proposed and calculated. The fundamental mechanisms for different laser-fluence regimes are discussed, and some suggestions for improving the surface quality are put forward. This study provides deep insights into the laser-material interaction mechanisms and is beneficial for optimising the utilisation of femtosecond laser for 4H–SiC micromachining.     

On the nature of simultaneous formation of nano- and micron-size diamond fractions under pressure–temperature-induced transformations of binary mixtures of hydrocarbon and fluorocarbon compounds

Based on comparative studies of pressure–temperature-induced transformations of naphthalene, octafluoronaphthalene, and their binary mixtures, the nature of formation of nano- and micron-size diamond fractions in the products of transformations of hydrocarbons and fluorocarbons has been revealed. It was found that the main reason for the massive formation of nano-size diamonds is the specifics of carbonization of fluorocarbon compounds under pressure. In this particular process, micron-size particles of graphite are formed simultaneously with a significant amount of closed two- to five-layered carbon nanoparticles of 5–15-nm size, acting as precursors for the formation of nano-size diamond fractions. The obtained results open up a new avenue for the metal catalyst-free synthesis of nano/micron-size fractions of pure and doped diamonds.