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Heterogeneous Nucleation (heterogeneous + nucleation)
Selected AbstractsThe Combination of Colloid-Controlled Heterogeneous Nucleation and Polymer-Controlled Crystallization: Facile Synthesis of Separated, Uniform High-Aspect-Ratio Single-Crystalline BaCrO4 NanofibersADVANCED MATERIALS, Issue 2 2003S.-H. Yu Uniform, separated BaCrO4 single-crystalline nanofibers with high aspect ratio (>,5000, see Figure) can be fabricated at room temperature in aqueous solution using a double hydrophilic block copolymer as structure-directing agent and introducing colloidal nucleation agents. Such fibers represent a model case for advanced polymer fillers and the exploration of quasi-1D nanostructures with interesting electrical, optical, or catalytic properties. [source] Fabrication of Fine Mullite Powders by Heterogeneous Nucleation and Growth ProcessingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004YueFeng Tang Heterogeneous nucleation and growth was used to prepare composite particles with homogeneous component distribution. Composite particles consisting of ,-Al2O3 cores with an outer amorphous homogeneous silica layer were prepared by heterogeneous nucleation and growth processing using an ethanol suspension containing ammonia, tetraethylorthosilicate, and ,-Al2O3. Fine mullite powders of average particle size 0.53 ,m were fabricated by calcinating the composite particles at 1500°C for 2 h. [source] Co3O4 Nanoboxes: Surfactant-Templated Fabrication and Microstructure Characterization,ADVANCED MATERIALS, Issue 8 2006T. He Co3O4 nanoboxes (see figure), prepared by a template-synthesis process and composed of compactly assembled cuboid Co3O4 nanocrystals, are reported. Heterogeneous nucleation and interparticle coarsening growth of the nanocrystals is proposed as the formation mechanism. These nanoboxes are characterized by their unexpected magnetic behavior, and may lead to the possibility of novel metal oxide hierarchical structures. [source] Fabrication of Fine Mullite Powders by Heterogeneous Nucleation and Growth ProcessingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004YueFeng Tang Heterogeneous nucleation and growth was used to prepare composite particles with homogeneous component distribution. Composite particles consisting of ,-Al2O3 cores with an outer amorphous homogeneous silica layer were prepared by heterogeneous nucleation and growth processing using an ethanol suspension containing ammonia, tetraethylorthosilicate, and ,-Al2O3. Fine mullite powders of average particle size 0.53 ,m were fabricated by calcinating the composite particles at 1500°C for 2 h. [source] Encapsulation of magnetic self-assembled systems in thermoreversible gelsMACROMOLECULAR SYMPOSIA, Issue 1 2003Jean-Michel Guenet Abstract We describe two different ways of encapsulating within the fibrils of thermoreversible polymer gels the filaments of a supermolecular polymer formed by self-assembly of a bicopper complex. Heterogeneous nucleation is brought about with gels made from isotactic poly(styrene) while compound formation occurs with gels made from poly(hexyl isocyanate). These ways depend upon the interaction between the wings of the supermolecular polymer and the side groups of the polymer. In all cases, the filaments retain their 1-D structure. Preliminary results from magnetic susceptibility measurements show a striking difference between the pure and the encapsulated supermolecular polymer. [source] Heterogeneous nucleation of three-dimensional protein nanocrystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2007Dilyana G. Georgieva Nucleation is the rate-limiting step in protein crystallization. Introducing heterogeneous substrates may in some cases lower the energy barrier for nucleation and thereby facilitate crystal growth. To date, the mechanism of heterogeneous protein nucleation remains poorly understood. In this study, the nucleating properties of fragments of human hair in crystallization experiments have been investigated. The four proteins that were tested, lysozyme, glucose isomerase, a polysaccharide-specific Fab fragment and potato serine protease inhibitor, nucleated preferentially on the hair surface. Macrocrystals and showers of tiny crystals of a few hundred nanometres thickness were obtained also under conditions that did not produce crystals in the absence of the nucleating agent. Cryo-electron diffraction showed that the nanocrystals diffracted to at least 4,Å resolution. The mechanism of heterogeneous nucleation was studied using confocal fluorescent microscopy which demonstrated that the protein is concentrated on the nucleating surface. A substantial accumulation of protein was observed on the sharp edges of the hair's cuticles, explaining the strong nucleating activity of the surface. [source] Protein crystal nucleation: Recent notionsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2007Christo N. Nanev Abstract The nucleation of protein crystals is reconsidered taking into account the specificity of the protein molecules. In contrast to the homogeneous surface properties of small molecules, the protein molecule surface is highly inhomogeneous. Over their surfaces proteins exhibit high anisotropic distribution of patches, which are able to form crystalline bonds, the crystallization patch representing only a small fraction of the total surface of the protein molecule. Therefore, an appropriate spatial orientation of the colliding protein molecules is required in order to create a crystalline cluster. This scenario decreases considerably the success ratio of the attempt frequency for crystal nucleation. On the other hand a heterogeneous nucleation of (protein) crystals may be accelerated due to the arrival on some support of under-critical clusters that are formed in bulk solution; when arriving there they may acquire the property of critical nuclei. Thus, a plausible explanation of important peculiarities of protein crystal nucleation, as inferred from the experimental data, is suggested. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Isothermal crystallization of high density polyethylene and nanoscale calcium carbonate compositesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2008Jiann-Wen HuangArticle first published online: 26 NOV 200 Abstract High density polyethylene (HDPE) and calcium carbonate (CaCO3) nanocomposites with maleic anhydride grafted HDPE (manPE) as a compatibilizer were prepared via compounding in a twin-screw extruder. The CaCO3 are well dispersed in the HDPE matrix from the observation of transmission electron microscope. The isothermal crystallization kinetics was studied by differential scanning calorimetry and simulated by Avrami and Tobin models. The nucleation constants and fold surface free energy were estimated from Lauritzen,Hoffman relation. The results indicate that both manPE and well-dispersed CaCO3 particles would act as nuclei to induce heterogeneous nucleation and enhance crystallization rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Removal of cupric ions from acidic sulfate solution using reticulated vitreous carbon rotating cylinder electrodesJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 9 2004Gavin W Reade Abstract The potentiostatic deposition of copper from acid sulfate solutions (0.50 mol dm,3 Na2SO4 at pH 2 and 298 K) was studied at four porosity grades (10, 30, 60 and 100 pores per linear inch, ppi) of reticulated vitreous carbon (RVC) rotating cylinder electrode (RCE). The rate of removal of cupric ions from a 200 cm3 volume of electrolyte was examined as a function of the grade of RVC foam, the electrode potential and the initial cupric ion concentration. For the 100 ppi material, the product of the mass transport coefficient and the electroactive area per unit volume of electrode (kmAe) was equal to 0.28 s,1 at a potential of ,500 mV vs SCE for an initial cupric ion concentration of 0.85 mmol dm,3 and a constant rotation speed of 1500 rev min,1. Under the experimental conditions, an initial dissolved copper concentration of 63.5 ppm could be reduced to <0.1 ppm in approximately 60 min using a 100 ppi RVC RCE. SEM studies showed some non-uniform deposition of metal due to heterogeneous nucleation of copper together with the development of rough deposits. Copyright © 2004 Society of Chemical Industry [source] Contrasting textural record of two distinct metamorphic events of similar P,T conditions and different durationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2005O. LEXA Abstract A structural, metamorphic and geochronological study of the Staré M,sto belt implies the existence of two distinct metamorphic events of similar peak P,T conditions (700,800 °C, 8,10 kbar) during the Cambro-Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro-Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The parameter N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro-Ordovician banded amphibolite complex. The spatial distribution of amphibole and plagioclase shows intense mechanical mixing for lower-temperature mylonitic metagabbros. In high-temperature mylonites a strong aggregate distribution is developed. Cambro-Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation of individual phases. This pattern, together with CSD, was subsequently modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism. The differences between the observed textures of the amphibolites are interpreted to be a consequence of the different durations of the Carboniferous and Cambro-Ordovician thermal events. [source] Surface Devitrification and the Growth of Cristobalite in Borofloat® (Borosilicate 8330) GlassJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2010Berker Mo, ulkoç Borofloat® (borosilicate 8330) glass is an important type of inorganic glass, both scientifically and commercially. During prolonged heat treatment of this glass above its glass transition temperature of 525°C, heterogeneous nucleation, and growth of cristobalite crystals occur. The kinetics and morphology of this crystal growth have been studied for heat treatment temperatures at and above 660°C. The activation energy for crystal growth is estimated to be 185±10 kJ/mol. This is attributed to the diffusion of boron, rather than sodium, being the rate-limiting step within the borosilicate framework. Contact with the atmosphere is shown to initiate the nucleation of cristobalite crystals, while deposition of a thin silicon nitride surface coating on the glass helps to prevent this nucleation. [source] Sc3+,Lu3+ -Doped ,-SiAlONsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Feng Ye Sc3+ and dual Sc3+,Lu3+ -doped ,-SiAlON compositions were sintered by hot pressing, and the formation behavior, microstructure, and mechanical properties were assessed. It was found that the small cation Sc3+ could not be accommodated into the ,-SiAlON structure alone. The addition of Lu2O3 in the composition induces the Sc3+ cation to enter the ,-SiAlON structure, and leads to the production of ,-SiAlON with an elongated-grain microstructure. Transmission electron microscopy analysis shows that ,-SiAlON grains always contain an ,-Si3N4 core, implicating heterogeneous nucleation to be in present in a mixed lutetium/scandium-doped ,-SiAlON system. [source] Fabrication of Fine Mullite Powders by Heterogeneous Nucleation and Growth ProcessingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2004YueFeng Tang Heterogeneous nucleation and growth was used to prepare composite particles with homogeneous component distribution. Composite particles consisting of ,-Al2O3 cores with an outer amorphous homogeneous silica layer were prepared by heterogeneous nucleation and growth processing using an ethanol suspension containing ammonia, tetraethylorthosilicate, and ,-Al2O3. Fine mullite powders of average particle size 0.53 ,m were fabricated by calcinating the composite particles at 1500°C for 2 h. [source] Nanocrystalline Tin Oxide Thin Films via Liquid Flow DepositionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2003Sitthisuntorn Supothina Nanocrystalline films of SnO2 were deposited by liquid flow deposition (LFD), i.e., by flowing aqueous solutions of SnCl4·5H2O and HCl over single-crystalline silicon substrates at 80°C. The substrates were either oxidized and fully hydrolyzed (bare silicon) or oxidized, hydrolyzed, and then coated with siloxy-anchored organic self-assembled monolayers (SAMs). Continuous, adherent films formed on sulfonate- and thioacetate-functionalized SAMs; adherent but sometimes discontinuous films formed on bare silicon and methyl-functionalized SAMs. The films contained equiaxed cassiterite crystals, ,4,10 nm in size. The film thickness increased linearly with deposition time. The maximum growth rate observed was 85 nm·h,1 on sulfonate SAM, and the maximum film thickness obtained was 1 ,m. A new dimensionless parameter, the normalized residence time, ,, was introduced for the purpose of interpreting the influence of solution conditions (i.e., degree of supersaturation, as controlled via pH, and tin concentration) and flow characteristics (flow rate and the configuration of the deposition chamber) on the growth rate in LFD processes. The results were consistent with a particle attachment mechanism for film growth and inconsistent with heterogeneous nucleation on the substrate. [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] Impact of Nanoscale Confinement on Crystal Orientation of Poly(ethylene oxide)MACROMOLECULAR RAPID COMMUNICATIONS, Issue 4 2010Haopeng Wang Abstract Using a layer-multiplying coextrusion process to fabricate films with thousands of alternating polymer nanolayers, we report here a new crystalline morphology in confined polymer nanolayers and an abrupt transition in the crystallization habit. At higher temperatures, poly(ethylene oxide) crystallizes as large, in-plane lamellae. A 5,°C change in the crystallization temperature produces an on-edge lamellar orientation. The results point to a transition from heterogeneous nucleation to substrate-assisted nucleation. This may be a general phenomenon that accounts for previously unexplained differences in the preferred chain alignment of confined polymer crystals. [source] Rheological Investigations in Understanding Shear-Enhanced Crystallization of Isotactic Poly(propylene)/Multi-Walled Carbon Nanotube CompositesMACROMOLECULAR RAPID COMMUNICATIONS, Issue 11 2007Ke Wang Abstract A novel experimental technique to follow the crystallization processes of poly(propylene)/MWCNT composites that experience a steady shear deformation using dynamic melt rheometry is described. The effects of heterogeneous nucleation, temperature, and preshear on the crystallization behaviors were determined. A quantitative evaluation of crystallization kinetics difference between quiescent and preshear conditions could be achieved. By combining rheology with POM, we demonstrate that two different crystallization processes account for the shear-enhanced crystallization at low and high temperatures, respectively. [source] Isothermal crystallization behavior of polyamide 6,6/multiwalled carbon nanotube nanocompositesPOLYMER ENGINEERING & SCIENCE, Issue 12 2009Shi-Ya Lin The good dispersion of functionalized multiwalled carbon nanotube (f-MWCNT) in polyamide 6,6 (PA 6,6) matrix was prepared by solution mixing techniques. The crystalline structure and crystallization behaviors of PA 6,6 and PA 6,6/f-MWCNT nanocomposites were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM) analysis. DSC isothermal results revealed that the overall isothermal crystallization rates of PA 6,6 increased as well as the activation energy of PA 6,6 extensively decreased by adding f-MWCNT into PA 6,6, suggesting that the addition of f-MWCNT probably induces the heterogeneous nucleation. The effect of f-MWCNT on the chain arrangement for the crystallization of PA 6,6/f-MWCNT nanocomposites was also discussed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers [source] Variation of non-isothermal crystallization behavior of isotactic polypropylene with varying ,-nucleating agent contentPOLYMER INTERNATIONAL, Issue 10 2010Linli Xu Abstract The non-isothermal crystallization behavior, the crystallization kinetics, the crystallization activation energy and the morphology of isotactic polypropylene (iPP) with varying content of ,-nucleating agent were investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC results showed that the Avrami equation modified by Jeziorny and a method developed by Mo and co-workers could be successfully used to describe the non-isothermal crystallization process of the nucleated iPPs. The values of n showed that the non-isothermal crystallization of ,- and ,-nucleated iPPs corresponded to a tridimensional growth with homogeneous and heterogeneous nucleation, respectively. The values of crystallization rate constant showed that the rate of crystallization decreased for iPPs with the addition of ,-nucleating agent. The crystallization activation energy increased with a small amount (less than 0.1 wt%) of ,-nucleating agent and decreased with higher concentration (more than 0.1 wt%). The changes of crystallization rate, crystallization time and crystallization activation energy of iPPs with varying contents of ,-nucleating agent were mainly determined by the ratio of the content of ,- and ,-phase in iPP (,-PP and ,-PP) from the DSC investigation, and the large size and many intercrossing lamellae between boundaries of ,-spherulites for iPPs with small amounts of ,-nucleating agent and the small size and few intercrossing bands among the boundaries of ,-spherulites for iPPs with large amounts of ,-nucleating agent from the SEM examination. Copyright © 2010 Society of Chemical Industry [source] Preparation and characterization of polyurethane,gold nanocomposites prepared using encapsulated gold nanoparticlesPOLYMER INTERNATIONAL, Issue 7 2010Chao-Ching Chang Abstract Gold nanoparticles (GNPs) have been widely studied due to their unique properties. Although many research groups have developed the synthesis of GNPs using various polymers as stabilizing or reducing agents, the effects of GNPs on the structures and properties of polymer matrices have been less reported. We propose a new design for the preparation of polyurethane,gold (PU,Au) nanocomposites. 11-Mercapto-1-undecanol-coated GNPs acted as the chain extenders and reacted with isocyanates to form covalent bonds between PU and GNPs. PU,Au nanocomposites were successfully synthesized, and the effects of multifunctional GNPs on the structures, morphology and properties of poly(ester urethane) were investigated. Scanning electron microscopy images suggested the GNPs can be dispersed uniformly in the PU matrix. Maltese-cross of spherical crystals was observed in the PU,Au nanocomposites, and the size of the crystals decreased with an increase in gold content. As the gold content increased, the thermal decomposition temperature and the temperature of the maximum decomposition rate increased. The glass transition temperature, crystal melting temperature and melting enthalpy of the soft segment also increased progressively. The results showed that multifunctional GNPs concentrated hard segments and resulted in an increase of heterogeneous nucleation, phase separation and elasticity. Copyright © 2010 Society of Chemical Industry [source] The Fouling Behaviour of Sodium Aluminosilicate Polytypes in High Ionic Strength Caustic Media Heat ExchangersASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2006J. Addai-Mensah Sodium aluminosilicate (SAS) scale formation, simulating high ionic strength caustic media process heat exchanger fouling, has been investigated for a temperature range of 30,240°C. Findings from several studies performed with a variety of liquor, substrate-type and agitation conditions show that SAS polytypes; i.e. amorphous, zeolite A, sodalite and cancrinite phases, differing in thermodynamic stability and kinetic behaviour may deposit. The polytypes formed and subsequent transformations to more thermodynamically stable phases are strongly dependent on liquor composition, temperature and time. The scale deposition process is substrate- mediated heterogeneous nucleation and particle growth (precipitation foulin&, which may be accompanied by bulk liquor-nucleated solids adsorption (particulate fouling) at suficiently high supersaturations. Liquor seeding with stable SAS solid phases is found to be efective in mitigating fouling. The precipitation fouling behaviour is principally determined by the clystallo-chemical nature of the SAS polytype and the solution conditions. The substrate (mild steel, 316 stainless steel, nickel and tef2on) surface physico-chemical structure has a signlficant impact on the scale particle morphology and layer structure, particularly at high temperatures. [source] Heterogeneous nucleation of three-dimensional protein nanocrystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2007Dilyana G. Georgieva Nucleation is the rate-limiting step in protein crystallization. Introducing heterogeneous substrates may in some cases lower the energy barrier for nucleation and thereby facilitate crystal growth. To date, the mechanism of heterogeneous protein nucleation remains poorly understood. In this study, the nucleating properties of fragments of human hair in crystallization experiments have been investigated. The four proteins that were tested, lysozyme, glucose isomerase, a polysaccharide-specific Fab fragment and potato serine protease inhibitor, nucleated preferentially on the hair surface. Macrocrystals and showers of tiny crystals of a few hundred nanometres thickness were obtained also under conditions that did not produce crystals in the absence of the nucleating agent. Cryo-electron diffraction showed that the nanocrystals diffracted to at least 4,Å resolution. The mechanism of heterogeneous nucleation was studied using confocal fluorescent microscopy which demonstrated that the protein is concentrated on the nucleating surface. A substantial accumulation of protein was observed on the sharp edges of the hair's cuticles, explaining the strong nucleating activity of the surface. [source] Induction and Inhibition of Preferential Enrichment by Controlling the Mode of the Polymorphic Transition with Seed CrystalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2006Rui Tamura Prof. Abstract Both induction and inhibition of "preferential enrichment", an unusual symmetry-breaking enantiomeric-resolution phenomenon observed upon simple recrystallization of a certain kind of racemic crystals from organic solvents, have been successfully achieved by controlling the mode of the polymorphic transition during crystallization with appropriate seed crystals. Such control of the polymorphic transition can be interpreted in terms of a novel phenomenon consisting of 1) the adsorption of prenucleation aggregates, 2) the heterogeneous nucleation and crystal growth of a metastable crystalline form, and 3) the subsequent polymorphic transition into the more stable form; these three processes occur on the same surface of a seed crystal. We refer to this phenomenon as an "epitaxial transition", which has been confirmed by means of in situ attenuated total reflection (ATR) FTIR spectroscopy in solution and the solid state, differential scanning calorimetry (DSC) measurements of the deposited crystals, and X-ray crystallographic analysis of the single crystals or the direct-space approach employing the Monte Carlo method with the Rietveld refinement for the structure solution from the powder X-ray diffraction data. [source] | ||||||||||||||||