Condensation and morphology of magnesium particles in vapour plumes

A plume chamber study of condensation in magnesium vapour was performed in flowing argon at atmospheric pressure and source temperatures (T ₀) up to 1213 K. The wall temperature (T ) was at 300 K. Sampled particles closely resembled those from zinc and cadmium aerosols and included spheres and prisms, indicative of vapour-liquid and vapour-solid nucleation. The spheres solidify from single rafts, and in addition to the hexagonal prisms that grow from dendrites, flatter and elongated forms occur as twins. The presence or absence of either spheres or prisms was found to depend on the setting ofT ₀, in agreement with theory, which predicts thatT ₀ will determine the position of the nucleation threshold temperature (T _n) relative to the melting point (T _f). The occurrence of particles condensed as solid as well as liquid nuclei whenT _n was higher thanT _f showed that supersaturated vapour states can persist as the vapour cools belowT _n. The twinned particles observed with magnesium do not occur with zinc or cadmium at atmospheric pressure. It is suggested that the presence of hydrodynamic stresses causes twinning in magnesium whereas in zinc and cadmium it results in malformed hexagonal prisms.     

Condensation of cadmium aerosols

Cadmium aerosols were prepared by the heat-pulse method and fall-out collected at temperatures T from ambient to just above the melting point T _f. Deposits were examined by electron microscopy after coating with Au-Pd. Representative particles were removed for electron and X-ray diffraction studies.The particles are crystalline. The types, and their proportions and size distributions vary with temperature. At low temperatures particles are monocrystals of high specific surface, chiefly prisms with deep fissures and cavities but also rough spheres and stellate dendrites. At higher temperatures the crystal forms are perfected, the proportion of prisms falls, and polycrystalline as well as monocrystalline spheres are found. Spheres can exceed 50m in diameter but the largest prisms are 2 to 3 m and disappear at 0.8 to 0.9 T _f. Up to this point spheres have one or more circular {0001} depressions, or dishes, depending on the number of crystallites they contain. At higher temperatures they are quite smooth.It is concluded that smooth spheres are droplets which have supercooled and frozen on the collector, and as such are not aerosol particles. Droplets that freeze in the cloud become dished spheres, and their subsequent growth involves condensation on areas between the dishes. All particles are nucleated from the vapour close to the source, the prisms apparently at T<T _f.     

Particle condensation in metallic vapour plumes

An apparatus is described for the study of aerosol particle condensation in a plume of vapour rising from a heated source. The principal advantages of this plume chamber over the heat-pulse cloud chamber previously used include the attainment of steady-state conditions at a controlled source temperature and the suppression of recirculation. The coagulation of particles and the further growth or re-evaporation that tends to occur in the outer zone of the heat-pulse equipment is discouraged, and samples taken on substrates positioned in the plume are expected to be more representative of the original nucleation and growth processes.Aerosol particles were condensed in zinc vapour emitted at various source temperatures (T ₀) with the walls at room temperature (T ). Trends occur in particle size and morphology when T ₀ is varied at fixed T which broadly resemble those in the cloud chamber work, where T is varied. Thus, in addition to the dendrites and prisms typical of low T ₀ there is an increasing proportion of spheres when T ₀ is raised above the melting point, until they eventually become the only species present. The particles condensing in the invisible fume generated at T ₀ settings below the melting point, were found to consist only of prisms and dendrites. This confirms that these particles condense directly in the solid state, unlike the spheres, which are originally liquid. The trends observed when changing T ₀ at constant T are shown to be in agreement with theoretical predictions for nucleation in a boundary layer.     

Crystallography of metallic aerosol precipitates

In a previous paper it was concluded from microscopical evidence that the radius of a basal raft nucleated on a freezing droplet of Zn or Cd expands to a fixed fraction of the droplet radius before thickening into a grain. Further studies on polycrystalline spheres from condensation aerosols, together with observations by other investigators working with much larger, sessile drops, confirm that the raft remains quite thin while the radius is expanding. Additional surface features developed by epitaxial growth from the vapour on monocrystalline aerosol spheres show that in the following stage, in which the raft thickens but its upper surface no longer expands, the growth front propagates into the melt by the build-up of layers parallel to c. Evidence of a growth helix is found opposite the basal flat. Its formation is attributed to growth on a screw dislocation generated by the stress that accumulates at the perimeter of the expanding raft. The onset of rapid helical thickening coincides with termination of raft expansion. Differences in evaporation behaviour of particles are considered to depend on whether the dislocation remains in the solidified droplet or is expelled by thermal stresses. Glide of the same dislocations may be involved in the slip previously observed in polycrystals.     

The formation of microsplits and damage rafts in proton-bombarded GaAs

The technique of cross-sectional transmission electron microscopy has been used to study the nature and formation of damage rafts, which act as dislocation sources, in proton-bombarded (5×10¹⁵, 10¹⁶ and 10¹⁷ H⁺ cm^–2) and annealed GaAs. The results show that the rafts consist of a planar array of voids lying on the {110} cleavage planes of GaAs. The dislocations generated at these rafts are glissile, of the (a/2) 110 type, and glide on the {111} planes intersecting the rafts. Models are presented to show that these damage rafts originated at microsplits on the {110} cleavage planes of GaAs following the cracking open of small hydrogen-filled platelets on {110} planes when the internal gas pressure exceeds that which is necessary for crack propagation. From the analysis of the results an average diffusion length of 1 m was estimated for vacancies in proton-bombarded GaAs at 900° C.     

Crystallography of metallic aerosol precipitates: dendritic origin of hexagonal prisms in Cd and Zn

The aspect ratio was measured on simple hexagonal prisms in the sediment from condensation aerosols of Cd and Zn. For Cd the ratio R=p3/p, where p and q are the respective lengths of the prism edges parallel and perpendicular to the basal pinacoid, varies symmetrically above and below a most frequent value of 1.2 to 1.3. The corresponding R-distribution for Zn was broader, less symmetrical and peaked at 1.5 to 1.6. Similar results had been found with hexagonal bubbles of Ar in annealed foils of these metals and interpreted as denoting the equilibrium shape. In the condensation of aerosols, however, crystals grow from the vapour under highly non-uniform conditions, and the constancy of aspect ratio is attributed to the operation of a dendritic growth mechanism as put forward by Buckle and Pointon.     

Millimetre-sized hollow spheres of lead zirconate titanate by a sol-gel method

Hollow spheres of lead zirconate titanate (PZT) [chemical formula Pb(Zr_0.52Ti_0.48)O₃)], with outer diameter of 1–2 mm and a wall thickness of about 100 m, were fabricated by gellation of a PZT sol inside solid polymer spheres and then burning the polymer out. Monomodally sized polyacrylamide spheres, with diameter 1.40–1.90 mm, were soaked in a PZT sol, prepared by dissolving Pb(NO₃)₂, zirconiumn-butoxide and titanium isopropoxide inN, N-dimethylformamide. The absorbed sol was then gelled beneath the surface of the polymer sphere by the action of NH₃. Upon calcination of the spheres at 850 °C for 4 h in air, hollow spheres of pure PZT perovskite phase (as identified by X-ray diffraction patterns) were obtained. The density of the hollow spheres was 1.13gcm^–3, while that of the wall of the spheres was 3.10g cm^–3. The scanning electron microscopic examination of the broken spheres showed that the inner surface of the spheres contained rib-like structures, which provided strength to the hollow spheres. The planar coupling factor,k _p, of six hollow spheres, placed at a close-packed arrangement in a plane, was 0.22, indicating the possibility of fabrication of low-density transducer arrays.     

The growth of oriented ceramic eutectics

Controlled microstructures of the two eutectics in the alumina-titania system have been grown using a special electron beam heating technique. In the aluminium titanate-titania system, the eutectic interlamallar spacing varies with the freezing rate R as =AR ^–n where n=0.5 and the value of the constant A is 8.5×10^–6 cm^3/2sec^–1/2. Primary plate-like dendrites of aluminium titanate in a matrix of discontinuous aluminium titanate-titania eutectic are formed on solidifying a composition TiO₂-20 wt % Al₂O₃. These dendrites appear to deflect cracks in this ceramic. In the alumina-aluminium titanate system, primary rod-like dendrites of alumina were grown in a ribbon-like eutectic of alumina and aluminium titanate on solidifying a composition Al₂O₃–38.5% TiO₂.     

Stacking-faults in tellurium-doped gallium arsenide

Samples of bulk-grown gallium arsenide single crystals, taken from both static freeze, and Czochralski ingots doped to a high level with tellurium, have been examined using transmission electron microscopy. Observation of single and multiple stacking-fault layers which have fault vectors of the kindR=a/3111 is reported. It is shown by diffraction contrast experiments that the stacking-fault defects are extrinsic. They are thought to be rafts of tellurium substituting for arsenic in {111} planes. The prevalence of the observed layer defects correlates well with the increase in carrier concentration in certain regions of the crystals.     

Microstructure and corrosion resistance of Ni-based alloy laser coatings with nanosize CeO2 addition

Abstract

Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^{- 1}) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃ C₆ and Ni₃ B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale.     

Treadmilling and dynamic protrusions in fire ant rafts

Fire ants (Solenopsis invicta) are exemplary for their formation of cohered, buoyant and dynamic structures composed entirely of their own bodies when exposed to flooded environments. Here, we observe tether-like protrusions that emerge from aggregated fire ant rafts when docked to stationary, vertical rods. Ant rafts comprise a floating, structural network of interconnected ants on which a layer of freely active ants walk. We show here that sustained shape evolution is permitted by the competing mechanisms of perpetual raft contraction aided by the transition of bulk structural ants to the free active layer and outward raft expansion owing to the deposition of free ants into the structural network at the edges, culminating in global treadmilling. Furthermore, we see that protrusions emerge as a result of asymmetries in the edge deposition rate of free ants. Employing both experimental characterization and a model for self-propelled particles in strong confinement, we interpret that these asymmetries are likely to occur stochastically owing to wall accumulation effects and directional motion of active ants when strongly confined by the protrusions'' relatively narrow boundaries. Together, these effects may realize the cooperative, yet spontaneous formation of protrusions that fire ants sometimes use for functional exploration and to escape flooded environments.     

Evolution of microstructure and local thermal conditions during directional solidification of A356-SiC particle composites

Solidification microstructures of aluminium silicon alloy (A-356) containing 0, 10, 15 and 20 vol % silicon carbide particles formed during directional solidification from a chill have been studied and compared with the structures obtained during solidification of the base alloy under similar mould and chill conditions. Columnar dendritic structure was observed during solidification of the base alloy at all distances from the chill. In the case of composites, the presence of silicon carbide particles disturbs the orderly aligned arrangement of dendrites observed in the base alloy, under similar solidification conditions, except near the chill surface where a particle-free zone is observed due to probable pushing of particles by the macroscopic solidification front with cell spacings finer than the particle size. During the entire range of solidification conditions studied in this work, the silicon carbide particles are pushed by growing dendrites of -aluminium into the last freezing eutectic liquid. The observations on pushing of silicon carbide particles have been examined in relation to existing models on particle pushing by planar solidification fronts. Even in the regions away from the chill, where silicon carbide particles are present, there are large regions covering several dendrite arm spacings where there are no particles representing another form of macrosegregation of particles. It is observed that the secondary dendrite arm spacings (DAS)of -aluminium are related to cooling rate by an equation DAS =b (T) ⁿ for the base alloy as well as for the composite. The coefficientb is generally higher for composites than for base alloy, and it is found to be a function of particle content. The value ofn for the composite is close to the value of the base alloy and is not significantly influenced by the presence of particles. Cooling rate, temperature gradients and the rate of advancement of the solidification front have been experimentally measured for the base alloy as well as for the composites during unidirectional solidification. The study indicates that the presence of particles themselves alters the cooling rates, temperature gradients and growth rate of the macroscopic solidification front under identical thermal surroundings during solidification. The possible influences of these alterations in growth condition on the solidification microstructure due to the presence of particles are discussed together with the other possible direct influences of particles on dendritic growth of aluminium-silicon alloys.     

A comparative study on the raft chemical properties of various alginate antacid raft-forming products

Objective: Research to measure the chemical characterization of alginate rafts for good raft performance and ascertain how formulation can affect chemical parameters.

Significance: A selection of alginate formulations was investigated all claiming to be proficient raft formers with significance between products established and ranked.

Methods: Procedures were selected which demonstrated the chemical characterization allowing rafts to effectively impede the reflux into the esophagus or in severe cases to be refluxed preferentially into the esophagus and exert a demulcent effect, with focus of current research on methods which complement previous studies centered on physical properties. The alginate content was analyzed by a newly developed HPLC method. Methods were used to determine the neutralization profile and the acid neutralization within the raft determined along with how raft structure affects neutralization.

Results: Alginate content of Gaviscon Double Action (GDA) within the raft was significantly superior (p?Conclusion: Alginate formulations require three chemical reactions to take place simultaneously: transformation to alginic acid, sodium carbonate reacting to form carbon dioxide, calcium releasing free calcium ions to bind with alginic acid providing strength to raft formation. GDA was significantly superior (p?<.0001) to all other comparators.     

Laser induced rotation of trapped chiral and achiral nematic droplets

We study the response of optically trapped achiral and chiralized nematic liquid crystal droplets to linear as well as circular polarized light. We find that there is internal dissipation in rotating achiral nematic droplets trapped in glycerine. We also demonstrate that some chiralized droplets rotate under linearly polarized light. The best fit to our data on chiralized droplets indicates that rotational frequency of these droplets with radius R is approximately proportional to 1/R ², rather than to 1/R ³.     

Evolution and analysis of γ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.     

Steric stabilization of Stöber silica dispersions using organosilanes

Organosilanes of the general formula R _x Si(OR)_4–x (where R is an alkyl group and R = CH₃ or CH₂CH₃) were used to sterically stabilize hexane dispersions of submicrometre silica spheres. The dispersions were characterized according to sediment volume results. For 0.5m silica particles, the sediment density increased by more than a factor of three up to 50 to 55% of theoretical in the presence of organosilanes with 12 or more carbons in the R group. Solid-state¹³C nuclear magnetic resonance was used to characterize the powder-dispersant interaction; this technique can distinguish between carbons in the R group of the organosilane and residual organic groups in the silica. Scanning electron micrographs of filter compacts were used to further characterize the dispersions and indicated the presence of primary particles as well as small agglomerates.     

Monodisperse micron-sized polymethylmethacrylate particles having a crosslinked network structure. V

Highly monodisperse micron-sized polymethylmethacrylate (PMMA) particles crosslinked with carboxylic group-containing urethane acrylates (CUA) were produced by simple dispersion polymerization in methanol. In proper condition, CUA employed as a crosslinker had an excellent ability to achieve the monodisperse PMMA particles to the concentration of about 10 wt%. This arose from the fact that CUA formed monomer-swellable primary particles due to its structurally long tetramethylene oxide groups in the molecule. The influence of the concentrations of the initiator and CUA on the particle diameter (D _n), particle number density (N _p), and polymerization rate (R _p) was found to obey the following approximate relationship, D _n [initiator]^0.41[CUA]^–0.06, N _p [initiator]^–1.22 [CUA]^0.21, and R _p [initiator]^0.34±0.03, respectively. The power law dependence of D _n and N _p on the initiator concentration coincided well with that of linear polymers in the literature. Especially, in this study, it was found that CUA did not have a serious influence on the nucleation. However, the dependence of R _p on the initiator concentration was observed to be higher than that of linear polymers, suggesting that uniquely, the solution polymerization process competed with the heterogeneous polymerization during polymerization, because of the crosslinked network structure of the primary particles.     

Kinetics of Solid State Reactions With Fractal Reagent

In the present research we theoretically studied the kinetics of nucleation-limited solid state reactions as influenced by the fractal properties of solid reagent. We consider the model of equal-sized primary particles assembled in fractal cluster. The geometry of such an object is assumed to be described solely by its fractal dimension D and by upper (R _max) and lower (R _min) cutoffs of fractality further identified with the overall size of the object and the size of the primary particle correspondingly. Depending on the ratio between R _max, R _min and the radius of the critical nucleus R _nucl the following cases are considered: (1) R _max R _nucl. In this case the reaction kinetics is described by the equation: = 1 – B{ln(k + 1)}^D/(D–3), where B, k are constants. Numerical solution of this equation gives rise to n-order reaction kinetics with n > 1. (2) R _min R _nucl R _max. In this case under certain conditions there can exist non-trivial critical density _crit 0, 1 that favors the formation of the critical nuclei of the new phase. The asymptotic kinetic equation for large times corresponds to n-order reaction with n = (D + 3)/(D + 1). (3) R _min R _nucl R _max. In this case the reaction follows the first-order kinetics with D-dependent rate constant.     

Correlation and prediction of dense fluid transport coefficients. I. n-alkanes

Recent accurate measurements of the self-diffusion coefficient for n-hexadecane and n-octane and of the viscosity coefficient for n-heptane, n-nonane, and n-undecane over wide pressure ranges have been used to provide a critical test of a previously described method, based on consideration of hard-sphere theory, for the correlation of transport coefficient data. It is found that changes are required to the universal curve for the reduced viscosity coefficient as a function of reduced volume and, also, to the parameters R _D, R , and R which were introduced to account for effects of nonspherical molecular shape. The scheme now accounts most satisfactorily for the self-diffusion, viscosity, and thermal conductivity coefficient data for all n-alkanes from methane to hexadecane at densities greater than the critical density.     

Cell response of cultured macrophages, fibroblasts, and co-cultures of Kupffer cells and hepatocytes to particles of short-chain poly[(R)-3-hydroxybutyric acid]

The known biodegradability of poly[(R)-3-hydroxybutyric acid] (PHB) in certain biological environments has lead to its proposed use as biodegradable, biocompatible polymer. Recently, a new, rapidly biodegradable blockcopolymer has been synthesized that contains crystalline domains of PHB blocks. During degradation of these polymers, the PHB-domains are transformed in a first step into small crystalline particles of short-chain PHB. Therefore, particles of short-chain poly[(R)-3-hydroxybutyric acid] (M_n2300) (PHB-P), as possible degradation products, are investigated here for their effects on the viability and activation of macrophages, fibroblasts, and co-cultures of rat Kupffer cells and rat hepatocytes. Results obtained in the present study indicate that phagocytosis of particles of short-chain poly[(R)-3-hydroxybutyric acid] at high concentrations (higher than 10 g/ml) is dosedependent and associated with cell damage in macrophages but not in fibroblasts. At low concentrations, particles of PHB-P also failed to activate macrophages and are biocompatible. Besides the PHB phagocytosis by Kupffer cells, treatment of co-cultures of Kupffer cells and hepatocytes with 1 g PHB/ml showed neither cytotoxic (lactate dehydrogenase activity) effects nor any change in albumin secretion by hepatocytes.