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Homogeneous Dispersion (homogeneous + dispersion)
Selected AbstractsInfluence of Surface Chemistry on Dehydrogenation in Carbon Cryogel Ammonia Borane NanocompositesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2009Saghar Sepehri Abstract This paper reports the synthesis and characterization of boron- and nitrogen-modified carbon cryogel (CC) ammonia borane (AB) nanocomposites (BNCC-AB) for hydrogen storage. Resorcinol,formaldehyde (RF) derived CCs were modified by homogeneous dispersion of AB in RF hydrogel prior to pyrolysis. Nanocomposites were fabricated by immersing CC in the AB solution. Nitrogen sorption analysis, X-ray photoelectron spectroscopy, and differential scanning calorimetry at multiple heating rates were used to study the structure and dehydrogenation properties of the nanocomposites. The results demonstrated lower dehydrogenation temperatures and reduced activation energies for AB when confined inside pores of B- and N-modified CCs relative to AB when confined in the unmodified CC with the same pore size.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Synthesis and Characterization of Hydroxyapatite/Poly(Vinyl Alcohol Phosphate) Nanocomposite BiomaterialsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 1 2008Nabakumar Pramanik A hydroxyapatite (HAp)/poly(vinyl alcohol phosphate) (PVAP) nanocomposite has been prepared by a chemical method by varying the HAp content by 10,60% (w/w). The bonding between HAp and PVAP has been investigated through Fourier transform infrared absorption spectra, X-ray diffraction, and thermogravimetric analyses. Transmission electron microscopy study shows a homogeneous dispersion of nanoparticles in the polymer matrix. Scanning electron microscopy study shows enhancement of the surface roughness of the composite with an increase in the nanoparticle content. The mechanical properties of the composites improve significantly with an increase in the HAp content. The HAp/PVAP nanocomposite prepared may have bone,implant applications. [source] Mechanical and morphological properties of organic,inorganic, hybrid, clay-filled, and cyanate ester/siloxane toughened epoxy nanocompositesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2007S. Nagendiran Abstract Organic,inorganic hybrids involving cyanate ester and hydroxyl-terminated polydimethylsiloxane (HTPDMS) modified diglycidyl ether of bisphenol A (DGEBA; epoxy resin) filled with organomodified clay [montmorillonite (MMT)] nanocomposites were prepared via in situ polymerization and compared with unfilled-clay macrocomposites. The epoxy-organomodified MMT clay nanocomposites were prepared by the homogeneous dispersion of various percentages (1,5%), and the resulting homogeneous epoxy/clay hybrids were modified with 10% HTPDMS and ,-aminopropyltriethoxysilane as a coupling agent in the presence of a tin catalyst. The siliconized epoxy/clay prepolymer was further modified separately with 10% of three different types of cyanate esters, namely, 4,4,-dicyanato-2,2,-diphenylpropane, 1,1,-bis(3-methyl-4-cyanatophenyl) cyclohexane, and 1,3-dicyanato benzene, and cured with diaminodiphenylmethane as a curing agent. The reactions during the curing process between the epoxy, siloxane, and cyanate were confirmed by Fourier transform infrared analysis. The results of dynamic mechanical analysis showed that the glass-transition temperatures of the clay-filled hybrid epoxy systems were lower than that of neat epoxy. The data obtained from mechanical studies implied that there was a significant improvement in the strength and modulus by the nanoscale reinforcement of organomodified MMT clay with the matrix resin. The morphologies of the siloxane-containing, hybrid epoxy/clay systems showed heterogeneous character due to the partial incompatibility of HTPDMS. The exfoliation of the organoclay was ascertained from X-ray diffraction patterns. The increase in the percentage of organomodified MMT clay up to 5 wt % led to a significant improvement in the mechanical properties and an insignificant decrease in the glass-transition temperature versus the unfilled-clay systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source] Water-resistant conducting hybrids from electrostatic interactionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 8 2007Jing Luo Abstract Conductive hybrids were prepared in a water/ethanol solution via the sol,gel process from an inorganic sol containing carboxyl groups and water-borne conductive polyaniline (cPANI). The inorganic sol was prepared by the hydrolysis and condensation of methyltriethoxysilane with the condensed product of maleic anhydride and aminopropyltriethoxysilane as a catalyst, for which the carboxyl counterion along the cPANI backbone acted as an electrostatic-interaction moiety. The existence of this electrostatic interaction could improve the compatibility of the two components and contribute to the homogeneous dispersion of cPANI in the silica phase. The electrostatic-interaction hybrids displayed a conductivity percolation threshold as low as 1.1 wt % polyaniline in an emeraldine base, showing 2 orders of magnitude higher electrical conductivity than that without electrostatic interactions. The electrostatic-interaction hybrids also showed good water resistance; the electrical conductivity with a cPANI loading of 16 wt % underwent a slight change after 14 days of soaking in water. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1424,1431, 2007 [source] Kinetic Studies of Mullite Synthesis from Alumina Nanoparticles and a Preceramic PolymerJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008Flavio Griggio The crystallization kinetics of mullite formation in a diphasic precursor consisting of a silicone resin filled with commercial ,-alumina nanoparticles (15 nm mean particle size, specific surface area of 100 m2/g), heated in air from 1250° to 1350°C, was studied by X-ray diffraction. Transitional ,-alumina and amorphous silica from the pyrolysis of the preceramic polymer exhibited a remarkable reactivity, as demonstrated by a very low incubation time (from 500 s at 1250°C to 20 s at 1350°C), a high mullite yield (about 80 vol%, after 100 s at 1350°C), and a low activation energy for nucleation (677±60 kJ/mol). The activation energy values found were lower than those reported previously for other diphasic systems, including sol,gel precursors. Besides the high specific surface of nanosized ,-alumina particles, the low energy barrier could be attributed to the highly reactive silica deriving from the oxidation of Si,CH3 bonds in the silicone and to the homogeneous dispersion of the nanosized filler inside the preceramic polymer. Furthermore, the possibility of applying plastic shaping processing methods to the mixture of a preceramic polymer and nanosized filler makes this approach particularly valuable, in comparison, for instance, with sol,gel based alternatives. [source] Sol-Gel-Derived Silicon-Boron Oxycarbide Glasses Containing Mixed Silicon Oxycarbide (SiCxO4,x) and Boron Oxycarbide (BCyO3,y) UnitsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2001Christel Gervais The introduction of B atoms in SiOC glass networks has been achieved through the pyrolysis of sol-gel-derived polyborosiloxanes under an inert atmosphere. The starting gels were obtained from hydrolysis-condensation reactions of triethylborate (B(OEt)3) and an organically modified trialkoxysilane (EtSi(OEt)3). The resulting hybrid EtSiO1.5 -B2O3 gels showed a homogeneous dispersion of the B atoms in the siloxane network via ,Si,O,B, bonds. The presence of such borosiloxane bridges prevents the formation of cyclic or cage siloxane entities and leads to relatively high ceramic yields (,80%). The transformation of the polyborosiloxanes into amorphous SiBOC glasses was followed using Fourier transform infrared spectroscopy and multinuclear magic-angle spinning-nuclear magnetic resonance (MAS-NMR) (11B, 13C, and 29Si). An important change in the carbon, silicon, and boron environments occurs during pyrolysis. Interestingly, the 11B MAS-NMR spectra suggest a progressive replacement of the B,O bonds by B,C bonds, which leads to a distribution of trigonal BCxO3,x sites in the glass that was pyrolyzed at 1000°C, with a residual amount of B(OSi)3 sites. The resulting glasses can thus be described as silicon-boron oxycarbide networks that are based on SiCxO4,x and BCyO3,y mixed environments. [source] Effect of Functionalized Carbon Nanotubes on Molecular Interaction and Properties of Polyurethane CompositesMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 19 2006Nanda Gopal Sahoo Abstract Summary: Functionalized MWNTs were incorporated into PU by solution mixing to improve the mechanical and thermal properties of composites. A homogeneous dispersion of MWNTs was successfully achieved in PU matrix as evidenced by scanning electron microscopy. It may be attributed to the hydrogen bonds existing between CO groups of hard segments of PU chains and COOH groups of the MWNT-COOH. The incorporation of the MWNTs effectively enhanced the crystallization of the PU matrix through heterogeneous nucleation, and the nucleation effect was more evident at 10 wt.-% functionalized MWNTs as compared to other composite systems. Mechanical properties of the PU-MWNTs composites were assessed as a function of MWNT concentration and dispersion of MWNT in PU matrix. The most significant improvement in mechanical properties was obtained, e.g., 740% increase in modulus and 180% increase in tensile strength over pure PU with 20% MWNT content. The thermal stability of composites due to thermal gravimetric measurements was significantly improved. A possible interaction of H-bonding existed between PU chain and MWNT-COOH. [source] Deformation mechanism of polystyrene toughened with sub-micrometer monodisperse rubber particlesPOLYMER INTERNATIONAL, Issue 11 2006Guanghui Gao Abstract Core,shell polybutadiene- graft -polystyrene (PB- g -PS) rubber particles with different ratios of polybutadiene to polystyrene were prepared by emulsion polymerization through grafting styrene onto polybutadiene latex. The weight ratio of polybutadiene to polystyrene ranged from 50/50 to 90/10. These core-shell rubber particles were then blended with polystyrene to prepare PS/PB- g -PS blends with a constant rubber content of 20 wt%. PB- g -PS particles with a lower PB/PS ratio (,70/30) form a homogeneous dispersion in the polystyrene matrix, and the Izod notched impact strength of these blends is higher than that of commercial high-impact polystyrene (HIPS). It is generally accepted that polystyrene can only be toughened effectively by 1,3 µm rubber particles through a toughening mechanism of multiple crazings. However, the experimental results show that polystyrene can actually be toughened by monodisperse sub-micrometer rubber particles. Scanning electron micrographs of the fracture surface and stress-whitening zone of blends with a PB/PS ratio of 70/30 in PB- g -PS copolymer reveal a novel toughening mechanism of modified polystyrene, which may be shear yielding of the matrix, promoted by cavitation. Subsequently, a compression-induced activation method was explored to compare the PS/PB- g -PS blends with commercial HIPS, and the result show that the toughening mechanisms of the two samples are different. Copyright © 2006 Society of Chemical Industry [source] Destabilisation of homogeneous bubbly flow in an annular gap bubble columnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010Fahd M. Al-Oufi Abstract Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102,m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014,0.200,m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift-flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity. Des résultats expérimentaux sont présentés pour montrer qu'il existe de très grandes différences dans les fractions de vide gazeux moyennes mesurées dans un tube ouvert et une colonne à bulles à espace annulaire, lorsqu'ils sont utilisés à la même vitesse superficielle de gaz, au moyen d'un aérateur poreux. On a effectué des mesures dans une colonne verticale avec un diamètre interne de 0.102,m, avec une portée de tubes internes concentriques pour former un espace annulaire, procurant des rapports de diamètre de 0.25 à 0.69; des vitesses superficielles de gaz de 0.014 à 0.200,m/s ont été étudiées. La fraction de vide gazeux moyenne diminue avec le rapport croissant du diamètre interne à externe de la colonne à espace annulaire et la transition à la circulation hétérogène se produit à des vitesses superficielles de gaz et fractions de vide plus basses. Deux raisons sont proposées et validés par les vérifications expérimentales: (1) la présence du tube interne provoque la formation de grandes bulles près de l'aérateur, ce qui déstabilize l'écoulement à bulles homogène et réduit la fraction de vide moyenne; cet état a été confirmé en injectant délibérément de grandes bulles dans une dispersion homogène de plus petites bulles et, (2) la forme des profils de fraction de vide change avec la géométrie de l'espace qui les sépare, ce qui a des conséquences sur le paramètre de distribution du modèle à flux de dérive. Ces deux effets servent à réduire la fraction de vide gazeux moyenne dans une colonne à bulles à espace annulaire, en comparaison avec un tube ouvert à la même vitesse superficielle de gaz. [source] Carbon Nanotube,Acridine Nanohybrids: Spectroscopic Characterization of Photoinduced Electron TransferCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2009Nicolas Mackiewicz Dr. Abstract Change happens: Acridine,carbon nanotube nanohybrids were built (see figure) and their photoinduced electron-transfer properties investigated, showing variable behaviour depending on the acridine partner. Single-walled carbon nanotubes (NT) were covalently functionalized with either 9-phenyl acridine (PhA) or 10-methyl-9-phenyl acridinium (PhMeA+). Absorption and fluorescence properties of acridine derivatives tethered to the nanotubes were studied in homogeneous dispersions. Exciplex emission was observed for NT functionalized with 9-phenylacridine. This phenomenon was attributed to an "intramolecular" interaction between excited phenyl acridine and carbon nanotubes. Interestingly, reverse photoinduced electron transfer from the nanotube to 10-methyl-9-phenylacridinium was detected for the NT-PhMeA+ nanohybrid. This electron transfer led to a strong quenching of the acridinium fluorescence and to the formation of a metastable acridine radical. Evidence for the formation of this radical was obtained by ESR studies. [source] | |||||||||||||||||||||||||