From nonwetting to prewetting: The asymptotic behavior of 4He drops on alkali substrates

We investigate the spreading of 4He droplets on alkali-metal surfaces at zero temperature, within the frame of finite range density-functional theory. The equilibrium configurations of several 4HeN clusters and their asymptotic trend with increasing particle number N, which can be traced to the wetting behavior of the quantum fluid, are examined for nanoscopic droplets. We discuss the size effects inferring that the asymptotic properties of large droplets correspond to those of the prewetting film.

From nonwetting to prewetting: The asymptotic behavior of 4He drops on alkali substrates

We investigate the spreading of 4He droplets on alkali-metal surfaces at zero temperature, within the frame of finite range density-functional theory. The equilibrium configurations of several 4HeN clusters and their asymptotic trend with increasing particle number N, which can be traced to the wetting behavior of the quantum fluid, are examined for nanoscopic droplets. We discuss the size effects inferring that the asymptotic properties of large droplets correspond to those of the prewetting film.

Spontaneous infiltrations of compound systems of Y(2)O(3), Sm(2)O(3), RE(2)O(3), Al(2)O(3) and AIN in SiC ceramics

Silicon carbide ceramics (SiC) are used in different applications in the engineering area due to the excellent properties, mainly in high temperatures. They are usually obtained by liquid-phase sintering enabling to form volatile products and, consequently, defects. The present work aims at studying the obtention of SiC ceramics by spontaneous infiltration using a eutectic composition of the Al(2)O(3)/Y(2)O(3), AIN/Y(2)O(3), Al(2)O(3)/Sm(2)O(3), AlN/Sm(2)O(3), Al(2)O(3)/RE(2)O(3) and AlN/RE(2)O(3) Systems. RE(2)O(3) is the concentrate of the rare-earth oxide obtained from Xenotime ore. Infiltration tests were carried out in argon atmosphere, graphite crucibles, in several temperatures near the melting point of each system, varying from 2.5 to 60 min. It was observed that Al(2)O(3)/Y(2)O(3), Al(2)O(3)/SM(2)O(3), AlN/SM(2)O(3) and Al(2)O(3)/RE(2)O(3) systems do not infiltrate appropriately and the AlN/Y(2)O(3) and AlN/RE(2)O(3) systems infiltrated spontaneously more than 20 mm; however, the first one presented a higher degree of infiltration, approximately 97%. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 degrees C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al2O3/Y2O3 liquid was done at 1850 degrees C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS. (c) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Vegetable Oil Quenchants: Calculation and Comparison of The Cooling Properties of a Series of Vegetable Oils

The compositions of canola, soybean, corn, cottonseed and sunflower oils suggest that they exhibit substantially different propensity for oxidation following the order of Canola < corn < cottonseed < sunflower approximate to soybean. These data suggest that any of the vegetable oils evaluated could be blended with minimal impact on viscosity although compositional differences would surely affect oxidative stability. Cooling curve analysis showed that similar cooling profiles were obtained for different vegetable oils. Interestingly, no film boiling or transition nucleate boiling was observed with any of the vegetable oils and heat transfer occurs only by pure nucleate boiling and convection. High-temperature cooling properties of vegetable oils are considerable faster than those observed for petroleum oil-based quenchants. (C)2010 Journal of Mechanical Engineering. All rights reserved.

Characterization of physical, mechanical and chemical properties of quiscal fibres : the influence of atmospheric DBD plasma treatment

This paper reports the first attempt of characterizing various physical, mechanical and chemical properties of Quiscal fibres, used by the native communities in Chile and investigating the influence of atmospheric dielectric barrier discharge plasma treatment on various properties such as diameter and linear density, fat, wax and impurity%, moisture regain, chemical elements and groups, thermal degradation, surface morphology, etc. According to the experimental observations, Quiscal fibre has lower tenacity than most of the technical grade natural fibres such as sisal, hemp, flax, etc., and plasma treatment at optimum dose improved its tenacity to the level of sisal fibres. Plasma treatment also reduced the amount of fat, wax and other foreign impurities present in Quiscal fibres as well as removed lignin and hemicellulose partially from the fibre structure. Plasma treatment led to functionalization of Quiscal fibre surface with chemical groups, as revealed from attenuated total reflection spectroscopy and also confirmed from the elemental analysis using energy dispersive Xray technique and pH and conductivity measurements of fibre aqueous extract. The wetting behavior of Quiscal fibre also improved considerably through plasma treatment. However, untreated and plasma treated Quiscal fibres showed similar thermal degradation behavior, except the final degradation stage, in which plasma treated fibres showed higher stability and incomplete degradation unlike the untreated fibres. The experimental results suggested that the plasma treated Quiscal fibres, like other technical grade natural fibres, can find potential application as reinforcement of composite materials for various industrial applications.

Melt topology and chemistry during partial melting in contact aureoles : an experimental study and field example

This study was initiated to investigate partial melting within the high-grade metamorphic rocks beneath the Little Cottonwood contact aureole (Utah, USA), in order to understand the melt generation, melt migration, and geometry of initial melt distribution on grain scale during crustal anatexis. The emplacement of the Little Cottonwood stock produced a contact aureole in the pelitic host rocks of the Big Cottonwood formation (BC). Metamorphic isogrades in pelitic rocks range form biotite to 2nd sillimanite grade as a function of distance from the contact. Migmatites are restricted to the highest grade and resulted form partial melting of the BC formation rocks. First melt was produced by a combined muscovite/biotite dehydration reaction in the sillimanite + k-feldspar stability field. Melt extraction from the pelites resulted in restites (magnetite + cordierite + alumosilicate ± biotite) surrounded by feldspar enriched quartzite zones. This texture is the result of gradual infiltration of partial melts into the quartzite. Larger, discrete melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins, and ductile shear zones. Melt composition are Si02- rich, crystallized as pegmatites, and apparently were very mobile. They were able to infiltrate the quartzite pervaisivly. These melts are similar in composition to first melts produced in the hydrothermal partial melt experiments at 2kbar between 700 - 800°C on fine grained high metamorphic rocks (andalusite-cordierited-biotite-zone) of the BC formation. The experimental melts are water rich and in disequilibrium with the melting rock. Initial melt composition is heterogeneous for short run duration, reflective a lack of chemical equilibrium between individual melt pools. Rock core scale heterogeneity decreased with time indicating partial homogenization of melt compositions. A simultaneous shift of melt composition to higher silica content with time was observed. The silica content of the melt increased due to local melt/mineral reactions. Melt textures indicate that reactive melt transport is most efficient along grain boundaries rimmed by dissimilar grains. Melt heterogeneity resulted in chemical potential gradients which are major driving forces for initial melt migration and govern melt distribution during initial melting. An additional subject of the thesis is the crystal size distributions of opaque minerals in a fine-grained, high-grade meta-pelite of the Big Cottonwood which were obtained from 3D X-ray tomography (uCT) and 2D thin section analysis. µCT delivers accurate size distributions within a restricted range (~ a factor of 20 in size in a single 3D image), while the absolute number of crystals is difficult to obtain from these sparsely distributed, small crystals on the basis of 2D images. Crystal size distributions obtained from both methods are otherwise similar. - Ce travail de recherche a été entrepris dans le but d'étudier les processus de fusion partielle dans les roches fortement métamorphiques de l'auréole de contact de Little Cottonwood (Utah, USA) et ceci afin de comprendre la génération de liquide de fusion, la migration de ces liquides et la géométrie de la distribution initiale des liquides de fusion à l'échelle du grain durant l'anatexie de la croûte. L'emplacement du petit massif intrusif de Little Cottonwood a produit une auréole de contact dans les roches pélitiques encaissantes appartenant à la Foimation du Big Cottonwood (BC). Les isogrades métamorphiques dans les roches pélitiques varient de l'isograde de la biotite à la deuxième isograde de la sillimanite en fonction de la distance par rapport au massif intrusif. Les migmatites sont restreintes aux zones montrant le plus haut degré métamorphique et résultent de la fusion partielle des roches de la Formation de BC. Le premier liquide de fusion a été produit par la réaction de déshydratation combinée de la muscovite et de la biotite dans le champ de stabilité du feldspath potassique Pt de la sillimanite. L'extraction du liquide de fusion des pélites forme des restites (magnétites + cordiérite + aluminosilicate ± biotite) entourées par des zones de quartzites enrichies en feldspath. Cette texture est le résultat de l'infiltration graduelle du liquide de fusion partielle dans les quartzites. Des accumulations distinctes et plus larges de liquide de fusion ont lieu dans des domaines d'extension ou de transpression tels que les boudins, les veines, et les zones de cisaillement ductile. La composition des liquides de fusion est similaire à celle des liquides pegmatoïdes, et ces liquides sont apparemment très mobiles et capables d'infiltrer les quartzites. Ces liquides de fusion ont la même composition que les premiers liquides produits dans les expériences hydrotheunales de fusion partielle à 2kbar et entre 700-800° C sur les roches finement grenues et hautement métamorphiques (andalousite-cordiérite-biotite zone) de la Formation de BC. Les liquides de fusion obtenus expérimentalement sont riches en eau et sont en déséquilibre avec la roche en fusion. La composition initiale des liquides de fusion est hétérogène pour les expériences de courte durée et reflète l'absence d'équilibre chimique entre les différentes zones d'accumulation des liquides de fusion. L'hétérogénéité à l'échelle du noyau s'estompe avec le temps et témoigne de l'homogénéisation de la composition des liquides de fusion. Par ailleurs, on observe parallèlement un décalage de la composition des liquides vers des compositions plus riches en silice au cours du temps. Le contenu en silice des liquides de fusion évolue vers un liquide pegmatitique en raison des réactions liquides/minéraux. Les textures des liquides de fusion indiquent que le transport des liquides est plus efficace le long des bordures de grains bordés par des grains différents. Aucun changement apparent du volume total n'est visible. L'hétérogénéité des liquides s'accompagne d'un gradient de potentiel chimique qui sert de moteur principal à la migration des liquides et à la distribution des liquides durant la fusion. Un sujet complémentaire de ce travail de thèse réside dans l'étude de la distribution de la taille des cristaux opaques dans les pélites finement grenues et fortement métamorphiques de la Formation de Big Cottonwood. Les distributions de taille ont été obtenues suite à l'analyse d'images 3D acquise par tomographie ainsi que par analyse de lames minces. La microtomographie par rayon X fournit une distribution de taille précise sur une marge restreinte (- un facteur de taille 20 dans une seule image 3D), alors que le nombre absolu de cristaux est difficile à obtenir sur la base d'image 2D en raison de la petite taille et de la faible abondance de ces cristaux. Les distributions de taille obtenues par les deux méthodes sont sinon similaire. Abstact: Chemical differentiation of the primitive Earth was due to melting and separation of melts. Today, melt generation and emplacement is still the dominant process for the growth of the crust. Most granite formation is due to partial melting of the lower crust, followed by transport of magma through the crust to the shallow crust where it is emplaced. Partial melting and melt segregation are essential steps before such a granitic magma can ascent through the crust. The chemistry and physics of partial melting and segregation is complex. Hence detailed studies, in which field observations yield critical information that can be compared to experimental observations, are crucial to the understanding of these fundamental processes that lead and are leading to the chemical stratification of the Earth. The research presented in this thesis is a combined field and experimental study of partial melting of high-grade meta-pelitic rocks of the Little Cottonwood contact aureole (Utah, USA). Contact metamorphic rocks are ideal for textural studies of melt generation, since the relatively short times of the metamorphic event prevents much of the recrystallization which plagues textural studies of lower crustal rocks. The purpose of the study is to characterize melt generation, identify melting reactions, and to constrain melt formation, segregation and migration mechanisms. In parallel an experimental study was undertaken to investigate melt in the high grade meta pelitic rocks, to confirm melt composition, and to compare textures of the partial molten rock cores in the absence of deformation. Results show that a pegmatoidal melt is produced by partial melting of the pelitic rocks. This melt is highly mobile. It is capable of pervasive infiltration of the adjacent quartzite. Infiltration results in rounded quartz grains bordered by a thin feldspar rim. Using computed micro X-ray tomography these melt networks can be imaged. The infiltrated melt leads to rheological weakening and to a decompaction of the solid quartzite. Such decompaction can explain the recent discovery of abundant xenocrysts in many magmas, since it favors the isolation of mineral grains. Pervasive infiltration is apparently strongly influenced by melt viscosity and melt-crystal wetting behavior, both of which depend on the water content of melt and the temperature. In all experiments the first melt is produced on grain boundaries, dominantly by the local minerals. Grain scale heterogeneity of a melting rock leads thus to chemical concentration gradients in the melt, which are the driving force for initial melt migration. Pervasive melt films along grain boundaries leading to an interconnected network are immediately established. The initial chemical heterogeneities in the melt diminish with time. Résumé large public: La différenciation chimique de la Terre primitive est la conséquence de la fusion des roches et de la séparation des liquides qui en résultent. Aujourd'hui, la production de liquide magmatique est toujours le mécanisme dominant pour la croissance de la croûte terrestre. Ainsi la formation de la plupart des granites est un processus qui implique la production de magma par fusion partielle de la croûte inférieure, la migration de ces magmas à travers la croûte et finalement son emplacement dans les niveaux superficielle de la croûte terrestre. Au cours de cette évolution, les processus de fusion partielle et de ségrégation sont des étapes indispensables à l'ascension des granites à travers la croûte. Les conditions physico-chimiques nécessaires à la fusion partielle et à l'extraction de ces liquides sont complexes. C'est pourquoi des études détaillées des processus de fusion partielle sont cruciales pour la compréhension de ces mécanismes fondamentaux responsables de la stratification chimique de la Terre. Parmi ces études, les observations de terrain apportent notamment des informations déterminantes qui peuvent être comparées aux données expérimentales. Le travail de recherche présenté dans ce mémoire de thèse associe études de terrain et données expérimentales sur la fusion partielle des roches pélitiques de haut degré métamorphiques provenant de l'auréole de contact de Little Cottonwood (Utah, USA). Les roches du métamorphisme de contact sont idéales pour l'étude de la folination de liquide de fusion. En effet, la durée relativement courte de ce type d'événement métamorphique prévient en grande partie la recristallisation qui perturbe les études de texture des roches dans la croûte inférieure. Le but de cette étude est de caractériser la génération des liquides de fusion, d'identifier les réactions responsables de la fusion de ces roches et de contraindre la formation de ces liquides et leur mécanisme de ségrégation et de migration. Parallèlement, des travaux expérimentaux ont été entrepris pour reproduire la fusion partielle de ces roches en laboratoire. Cette étude a été effectuée dans le but de confirmer la composition chimique des liquides, et de comparer les textures obtenues en l'absence de déformation. Les résultats montrent qu'un liquide de fusion pegmatoïde est produit par fusion partielle des roches pélitiques. La grande mobilité de ce liquide permet une infiltration pénétrative dans les quarzites. Ces infiltrations se manifestent par des grains de quartz arrondis entourés par une fine bordure de feldspath. L'utilisation de la tomography à rayons X a permis d'obtenir des images de ce réseau de liquide de fusion. L'infiltration de liquide de fusion entraîne un affaiblissement de la rhéologie de la roche ainsi qu'une décompaction des quartzites massifs. Une telle décompaction peut expliquer la découverte récente d'abondants xénocristaux dans beaucoup de magmas, puisque elle favorise l'isolation des minéraux. L'infiltration pénétrative est apparemment fortement influencée par la viscosité du fluide de fusion et le comportement de la tension superficielle entre les cristaux et le liquide, les deux étant dépendant du contenu en eau dans le liquide de fusion et de la température. Dans toutes les expériences, le premier liquide est produit sur les bordures de grains, principalement par les minéraux locaux. L'hétérogénéité à l'échelle des grains d'une roche en fusion conduit donc à un gradient de concentration chimique dans le liquide, qui sert de moteur à l'initiation de la migration du liquide. Des fines couches de liquide de fusion le long de bordures de grains formant un réseau enchevêtré s'établit immédiatement. Les hétérogénéités chimiques initiales dans le liquide s'estompent avec le temps.

Condensation of helium in nanoscopic alkali wedges at zero temperature

We present a complete calculation of the structure of liquid 4He confined to a concave nanoscopic wedge, as a function of the opening angle of the walls. This is achieved within a finite-range density functional formalism. The results here presented, restricted to alkali metal substrates, illustrate the change in meniscus shape from rather broad to narrow wedges on weak and strong alkali adsorbers, and we relate this change to the wetting behavior of helium on the corresponding planar substrate. As the wedge angle is varied, we find a sequence of stable states that, in the case of cesium, undergo one filling and one emptying transition at large and small openings, respectively. A computationally unambiguous criterion to determine the contact angle of 4He on cesium is also proposed.

The effect of cationic-anionic polyelectrolyte multilayer surface treatment on inkjet ink spreading and print quality

The focus of the work reported in this thesis was to study and to clarify the effect of polyelectrolyte multilayer surface treatment on inkjet ink spreading, absorption and print quality. Surface sizing with a size press, film press with a pilot scale coater, and spray coating, have been used to surface treat uncoated wood-free, experimental wood-free and pigmentcoated substrates. The role of the deposited cationic (polydiallydimethylammonium chloride, PDADMAC) and anionic (sodium carboxymethyl cellulose, NaCMC) polyelectrolyte layers with and without nanosilica, on liquid absorption and spreading was studied in terms of their interaction with water-based pigmented and dye-based inkjet inks. Contact angle measurements were made in attempt to explain the ink spreading and wetting behavior on the substrate. First, it was noticed that multilayer surface treatment decreased the contact angle of water, giving a hydrophilic character to the surface. The results showed that the number of cationic-anionic polyelectrolyte layers or the order of deposition of the polyelectrolytes had a significant effect on the print quality. This was seen for example as a higher print density on layers with a cationic polyelectrolyte in the outermost layer. The number of layers had an influence on the print quality; the print density increased with increasing number of layers, although the increase was strongly dependent on ink formulation and chemistry. The use of nanosilica clearly affected the rate of absorption of polar liquids, which also was seen as a higher density of the black dye-based print. Slightly unexpected, the use of nanosilica increased the tendency for lateral spreading of both the pigmented and dye-based inks. It was shown that the wetting behavior and wicking of the inks on the polyelectrolyte coatings was strongly affected by the hydrophobicity of the substrate, as well as by the composition or structure of the polyelectrolyte layers. Coating only with a cationic polyelectrolyte was not sufficient to improve dye fixation, but it was demonstrated that a cationic-anionic-complex structure led to good water fastness. A threelayered structure gave the same water fastness values as a five-layered structure. Interestingly, the water fastness values were strongly dependent not only on the formed cation-anion polyelectrolyte complexes but also on the tendency of the coating to dissolve during immersion in water. Results showed that by optimizing the chemistry of the layers, the ink-substrate interaction can be optimized.

Condensation of helium in nanoscopic alkali wedges at zero temperature

We present a complete calculation of the structure of liquid 4He confined to a concave nanoscopic wedge, as a function of the opening angle of the walls. This is achieved within a finite-range density functional formalism. The results here presented, restricted to alkali metal substrates, illustrate the change in meniscus shape from rather broad to narrow wedges on weak and strong alkali adsorbers, and we relate this change to the wetting behavior of helium on the corresponding planar substrate. As the wedge angle is varied, we find a sequence of stable states that, in the case of cesium, undergo one filling and one emptying transition at large and small openings, respectively. A computationally unambiguous criterion to determine the contact angle of 4He on cesium is also proposed.

Wettability of Aqueous Rhamnolipids Solutions Produced by Pseudomonas aeruginosa LBI

The wetting behavior of rhamnolipids produced by Pseudomonas aeruginosa LBI strain grown on waste oil substrate and sodium dodecyl sulfate (SDS) on glass, polyethylene terephthalate (PET), poly(vinyl chloride) (PVC), poly(epsilon-caprolactone) (PCL) and polymer blend (PVC-PCL) was investigated by the measuring contact angle of sessile drops, to determine the wetting characteristics of rhamnolipids. The comparison of the wetting profiles showed that at low SDS and rhamnolipid concentrations, the contact angle increased and when the concentration of the surfactant increased further, the contact angle decreased. The blend surface (PVC-PCL) showed better wettability than the homopolymers themselves and the blend changed the surface hydrophobicity of the polymer, making it more hydrophilic. The rhamnolipids produced by the LBI strain exhibited superior wetting abilities than the chemical surfactant SDS one. This is the first work that evaluates the wetting properties of rhamnolipids on polymer blends.

Wettability of Aqueous Rhamnolipids Solutions Produced by Pseudomonas aeruginosa LBI

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Two-Phase Flow In Microchannels: Morphology And Interface Phenomena

The existence and morphology, as well as the dynamics of micro-scale gas-liquid interfaces is investigated numerically and experimentally. These studies can be used to assess liquid management issues in microsystems such as PEMFC gas flow channels, and are meant to open new research perspectives in two-phase flow, particularly in film deposition on non-wetting surfaces. For example the critical plug volume data can be used to deliver desired length plugs, or to determine the plug formation frequency. The dynamics of gas-liquid interfaces, of interest for applications involving small passages (e.g. heat exchangers, phase separators and filtration systems), was investigated using high-speed microscopy - a method that also proved useful for the study of film deposition processes. The existence limit for a liquid plug forming in a mixed wetting channel is determined by numerical simulations using Surface Evolver. The plug model simulate actual conditions in the gas flow channels of PEM fuel cells, the wetting of the gas diffusion layer (GDL) side of the channel being different from the wetting of the bipolar plate walls. The minimum plug volume, denoted as critical volume is computed for a series of GDL and bipolar plate wetting properties. Critical volume data is meant to assist in the water management of PEMFC, when corroborated with experimental data. The effect of cross section geometry is assessed by computing the critical volume in square and trapezoidal channels. Droplet simulations show that water can be passively removed from the GDL surface towards the bipolar plate if we take advantage on differing wetting properties between the two surfaces, to possibly avoid the gas transport blockage through the GDL. High speed microscopy was employed in two-phase and film deposition experiments with water in round and square capillary tubes. Periodic interface destabilization was observed and the existence of compression waves in the gas phase is discussed by taking into consideration a naturally occurring convergent-divergent nozzle formed by the flowing liquid phase. The effect of channel geometry and wetting properties was investigated through two-phase water-air flow in square and round microchannels, having three static contact angles of 20, 80 and 105 degrees. Four different flow regimes are observed for a fixed flow rate, this being thought to be caused by the wetting behavior of liquid flowing in the corners as well as the liquid film stability. Film deposition experiments in wetting and non-wetting round microchannels show that a thicker film is deposited for wetting conditions departing from the ideal 0 degrees contact angle. A film thickness dependence with the contact angle theta as well as the Capillary number, in the form h_R ~ Ca^(2/3)/ cos(theta) is inferred from scaling arguments, for contact angles smaller than 36 degrees. Non-wetting film deposition experiments reveal that a film significantly thicker than the wetting Bretherton film is deposited. A hydraulic jump occurs if critical conditions are met, as given by a proposed nondimensional parameter similar to the Froude number. Film thickness correlations are also found by matching the measured and the proposed velocity derived in the shock theory. The surface wetting as well as the presence of the shock cause morphological changes in the Taylor bubble flow.

Anisotropic surface energetics and wettability of macroscopic form I paracetamol crystals

Advancing (θA) and receding (θR) contact angles were measured with several probe liquids on the external facets (201), (001), (011), and (110) of macroscopic form I paracetamol crystals as well as the cleaved (internal) facet (010). For the external crystal facets, dispersive surface energies γd calculated from the contact angles were found to be similar (34 ± 1 mJ/m2), while the polar components varied significantly. Cleaving the crystals exposed a more apolar (010) surface with very different surface properties, including γd = 45 ± 1 mJ/m2. The relative surface polarity (γp/γ) of the facets in decreasing order was (001) > (011) > (201) > (110) > (010), which agreed with the fraction of exposed polar hydroxyl groups as determined from C and O 1s X-ray photoelectron spectroscopy (XPS) spectra, and could be correlated with the number of non-hydrogen-bonded hydroxyl groups per unit area present for each crystal facet, based on the known crystal structures. In conclusion, all facets of form I paracetamol crystals examined exhibited anisotropic wetting behavior and surface energetics that correlated to the presence of surface hydroxyl groups.  © 2006 American Chemical Society.

Role of carbon nanotube dispersion in fracture toughening of plasma sprayed aluminum oxide-carbon nanotube nanocomposite coating

Aluminum oxide (Al2O3, or alumina) is a conventional ceramic known for applications such as wear resistant coatings, thermal liners, heaters, crucibles, dielectric systems, etc. However applications of Al 2O3 are limited owing to its inherent brittleness. Due to its excellent mechanical properties and bending strength, carbon nanotubes (CNT) is an ideal reinforcement for Al2O3 matrix to improve its fracture toughness. The role of CNT dispersion in the fracture toughening of the plasma sprayed Al2O3-CNT nanocomposite coating is discussed in the current work. Pretreatment of powder feedstock is required for dispersing CNTs in the matrix. Four coatings namely spray dried Al2O 3 (A-SD), Al2O3 blended with 4wt.% CNT (A4C-B), composite spray dried Al2O3-4wt.% CNT (A4C-SD) and composite spray dried A1203-8wt.% CNT (A8C-SD), are synthesized by plasma spraying. Owing to extreme temperatures and velocities involved in the plasma spraying of ceramics, retention of CNTs in the resulting coatings necessitates optimizing plasma processing parameters using an inflight particle diagnostic sensor. A bimodal microstructure was obtained in the matrix that consists of fully melted and resolidified structure and solid state sintered structure. CNTs are retained both in the fully melted region and solid-state sintered regions of processed coatings. Fracture toughness of A-SD, A4C-B, A4C-SD and A8C-SD coatings was 3.22, 3.86, 4.60 and 5.04 MPa m1/2 respectively. This affirms the improvement of fracture toughness from 20% (in A4C-B coating) to 43% (in A4C-SD coating) when compared to the A-SD coating because of the CNT dispersion. Fracture toughness improvement from 43% (in A4C-SD) to 57% (in A8C-SD) coating is evinced because of the CNT content. Reinforcement by CNTs is described by its bridging, anchoring, hook formation, impact alignment, fusion with splat, and mesh formation. The Al2O3/CNT interface is critical in assisting the stress transfer and utilizing excellent mechanical properties of CNTs. Mathematical and computational modeling using ab-initio principle is applied to understand the wetting behavior at the Al2O 3/CNT interface. Contrasting storage modulus was obtained by nanoindentation (∼210, 250, 250-350 and 325-420 GPa in A-SD, A4C-B, A4C-SD, and A8C-SD coatings respectively) depicting the toughening associated with CNT content and dispersion.