Ab initio modelling of basal plane oxidation of graphenes and implications for modelling char combustion

Ab initio calculations have been performed to determine the energetics of oxygen atoms adsorbed onto graphene planes and the possible reaction path extracting carbon atoms in the form of carbon monoxide. From the energetics it is confirmed that this reaction path will not significantly contribute to the gasification of well ordered carbonaceous chars. Modelling results which explore this limit are presented.     

Stepwise-Graded Si3N4–SiC Ceramics with Improved Wear Properties

The processing of stepwise graded Si₃N₄/SiC ceramics by pressureless co-sintering is described. Here, SiC (high elastic modulus, high thermal expansion coefficient) forms the substrate and Si₃N₄ (low elastic modulus, low thermal expansion coefficient) forms the top contact surface, with a stepwise gradient in composition existing between the two over a depth of ∼1.7 mm. The resulting Si₃N₄ contact surface is fine-grained and dense, and it contains only 2 vol% yttrium aluminum garnet (YAG) additive. This graded ceramic shows resistance to cone-crack formation under Hertzian indentation, which is attributed to a combined effect of the elastic-modulus gradient and the compressive thermal-expansion-mismatch residual stress present at the contact surface. The presence of the residual stress is corroborated and quantified using Vickers indentation tests. The graded ceramic also possesses wear properties that are significantly improved compared with dense, monolithic Si₃N₄ containing 2 vol% YAG additive. The improved wear resistance is attributed solely to the large compressive stress present at the contact surface. A modification of the simple wear model by Lawn and co-workers is used to rationalize the wear results. Results from this work clearly show that the introduction of surface compressive residual stresses can significantly improve the wear resistance of polycrystalline ceramics, which may have important implications for the design of contact-damage-resistant ceramics.     

Substrate Flexibility of the Flavin-Dependent Dihydropyrrole Oxidases PigB and HapB Involved in Antibiotic Prodigiosin Biosynthesis

In the biosynthesis of the tripyrrolic pigment prodigiosin, PigB is a predicted flavin-dependent oxidase responsible for the formation of 2-methyl-3-amylpyrrole (MAP) from a dihydropyrrole. To prove which dihydropyrrole is the true intermediate, both possibilities, 5-methyl-4-pentyl-3,4-dihydro-2H-pyrrole ( 5 a , resulting from transamination of the aldehyde of 3-acetyloctanal) and 2-methyl-3-pentyl-3,4-dihydro-2H-pyrrole ( 6 , resulting from transamination of the ketone), were synthesised. Only 5 a restored pigment production in a strain of Serratia sp. ATCC 39006 blocked earlier in MAP biosynthesis. PigB is membrane-associated and inactive when its transmembrane domain was deleted, but HapB, its homologue in Hahella chejuensis, lacks the transmembrane domain and is active in solution. Two colourimetric assays for PigB and HapB were developed, and the HapB-catalysed reaction was kinetically characterised. Ten analogues of 5 a were synthesised, varying in the C2 and C3 side chains, and tested as substrates of HapB in vitro and for restoration of pigment production in Serratia ΔpigD in vivo. All lengths of side chain tested at C3 were accepted, but only short side chains at C2 were accepted. The knowledge that 5 a is an intermediate in prodigiosin biosynthesis and the ease of synthesis of analogues of 5 a makes a range of prodigiosin analogues readily available by mutasynthesis.     

Electrical,mechanical, structural,and thermal behaviors of polymeric gel electrolyte membranes of poly(vinylidene fluoride‐co‐hexafluoropropylene) with the ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate plus lithium tetrafluoroborate

Polymeric gel electrolyte membranes based on the polymer poly(vinylidene fluoride‐co‐hexafluoropropylene) [P(VdF–HFP)] with different weight percentages of the ionic liquid (IL) 1‐butyl‐3‐methylimidazolium tetrafluoroborate plus 0.3M lithium tetrafluoroborate (LiBF₄) salt were prepared and characterized by scanning electron microscopy, X‐ray diffraction, differential scanning calorimetry, thermogravimetric analysis, Fourier transform infrared (FTIR) spectroscopy, complex impedance spectroscopy, pulse echo techniques, and Vickers hardness (H) testing. After the incorporation of the IL plus the salt solution in the P(VdF–HFP) polymer, the melting temperature, glass‐transition temperature (T_g), degree of crystallinity, thermal stability, elastic modulus (E), and hardness (H) gradually decreased with increasing content of the IL–salt solution as a result of complexation between the P(VdF–HFP) and IL. This was confirmed by FTIR spectroscopy. A part of the IL and LiBF₄ were found to remain uncomplexed as well. The ionic conductivity (σ) of the polymeric gel membranes was found to increase with increasing concentration of the IL–salt solution. The temperature‐dependent σs of these polymeric gel membranes followed an Arrhenius‐type thermally activated behavior. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41456.     

Preparation,Characterization, and Dielectric Properties of PP/CaTiO3 Composites for Microwave Substrate Applications

Calcium titanate (CaTiO₃) filled polypropylene (PP) composites have been fabricated through compression molding method. The phase purity of the PP/CaTiO₃ composites was studied using X‐ray diffraction studies. Scanning electron microscopy technique has been employed to study the dispersion of the particulate filler in the PP matrix. The dielectric constant and loss tangent of the composites were measured at X‐band frequency region using waveguide cavity perturbation technique. PP/CaTiO₃ composite has an effective dielectric constant of 11.74 and loss tangent 0.007 at optimum filler loading. The experimental dielectric constant of filled composites was compared with theoretically predicted dielectric constant values obtained using different modeling approaches. The linear coefficient of thermal expansion of PP/CaTiO₃ composites was studied using thermomechanical analyzer.     

Room temperature synthesized spherical V-MCM-41: a catalyst for vapour phase oxidation of diphenylmethane

Mesoporous spherical Si-MCM-41 and V-MCM-41 materials with a variable Si/V ratio of 25, 50, 75 and 100 were prepared by an economical route at room temperature and characterized by various techniques for qualitative and quantitative understanding of vanadium species. The morphology and mesoporosity of the spherical particles were confirmed by SEM and TEM micrographs. The catalytic activity of the synthesized catalysts was tested for vapour phase oxidation of diphenylmethane to benzophenone using CO₂ free air as oxidant at atmospheric pressure. The conversion of diphenylmethane and selectivity for benzophenone were measured in the temperature range of 593?C713?K over V-MCM-41 catalysts synthesized by adopting room temperature method as well as by direct hydrothermal method. The catalytic performance was related to vanadium content on the surface of the catalysts.     

Synthesis of Chlorinated and Non-chlorinated Polyols from Model Cross-Metathesis Modified Triacylglycerols

Three pure triacylglycerols (TAG) containing decenoic acid (D), and stearic acid (S), were hydroxylated into chlorinated and non-chlorinated polyols and studied in detail. D is a fatty moiety that can result from the cross metathesis of small olefins with common vegetable oils such as soybean oil. A fundamental understanding relating chemical composition and derived structure, particularly the number and position of the hydroxyl groups, to physical properties was established allowing us to add some perspective to the growing body of knowledge on industrially relevant polyol and polyurethane systems produced with metathesis-modified TAG (MTAG). The hydroxyl value, crystallization and melting behaviors, thermal degradation behavior of the polyols were directly related to their peculiar shortened and primary functionality inherited from the parent MTAG. The effect of regiochemistry on the physical properties of the polyols was investigated with the chromatography fractions of the trichlorinated polyol of propane-1,2,3-triyl tris(dec-9-enoate), giving an unhindered insight into the role of short and terminal functionality of MTAG polyols that will help select the optimal isomer composition for designer polyurethane materials.     

The intramolecular capture of thermally generated merocyanine dyes derived from naphthopyrans: Photochromism of 5-(diarylhydroxymethyl)-2H-naphtho[1,2-b]pyrans

A series of 2H-naphtho[1,2-b]pyrans bearing a diarylmethanol unit at C-5 have been synthesised by the addition of an excess of an aryllithium reagent to alkyl 2H-naphtho[1,2-b]pyran-5-carboxylate precursors. These naphthopyrans show photochromism when exposed to ultraviolet irradiation and also generate intense colours at low pH through triarylmethine cation generation. However, photochromism of the triarylmethine cation derived from the naphthopyran unit could not be detected. An irreversible cascade process initiated by the thermally-induced ring-opening of the diarylmethanol substituted 2H-naphtho[1,2-b]pyrans in the presence of an acid catalyst afforded novel benzopentalenonaphthalenone dyes.     

Cationic Micelles Modulated in the Presence of α,ω-Alkanediols: A SANS,NMR and Conductometric Study

The solution behavior of a typical cationic surfactant, tetradecyltrimethylammonium bromide, in mixed solvent systems composed of water and varying concentrations of α,ω-alkanediols; 1,2-ethanediol (ED), 1,4-butanediol (BD), 1,6-hexanediol (HD) and 1,8-octanediol (OD) was examined via electrical conductance measurements, ¹³C-NMR spectroscopy and small angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) values and degree of counterion dissociation (α) indicate that both ED and BD oppose micellization, whereas HD and OD enhance micelle formation. Changes in the ¹³C-NMR chemical shifts (∆δ values) reveal that the short chain diols reside almost exclusively in the bulk phase and hence, affect the formation of micelles by altering the solvent properties in the bulk of the solution, whereas HD and OD partition between the pseudomicellar phase and the bulk phase. SANS studies indicated that both the micellar size and aggregation number (N _agg) decrease in the presence of all diols. ED and BD behave like cosolvents and increase the α and CMC values and decrease N _agg. We note that the effect of HD and OD on the properties of the micelles is concentration dependent; at low concentrations, these diols interact with the micelles and behave as cosurfactants (as evidenced by the trends in the micellar properties), while at higher concentrations, they enhance the surfactant solubility and behave as a cosolvent.     

Effect of a Hydrophilic PEO–PPO–PEO Copolymer on Cetyltrimethyl Ammonium Tosylate Solutions in Water

Cetyltrimethyl ammonium tosylate (CTAT) in water forms long flexible wormlike micelles at concentrations above 10 mM, leading to highly viscous solutions and viscoelastic stiff gels above 100 mM. In the presence of a nonmicellar hydrophilic PEO–PPO–PEO triblock copolymer F87 (TBC-F87, Total mol.wt. = 7,700, EO = 70%) these wormlike micelles RE transformed into smaller structures, as evident from a sharp decrease in viscosity and increase in specific conductance. These results are quantified by small angle neutron scattering (SANS) measurements. The PPO middle block of TBC-F87 gets inserted in the CTAT micelle, the size and total aggregation number of CTAT/TBC-F87 mixed micelles decreased but the number of TBC-F87 molecules in the mixed micelles increased with an increase in [TBC-F87]. Two break points in the typical specific conductance versus CTAT concentration plot at various [TBC-F87] amounts represent interactions between CTAT and TBC-F87. The penetration of PPO of TBC-F87 inside CTAT micelles decreases hydrophobicity of the core while the presence of PEO end blocks enhances hydrophilicity each favoring smaller micelles.