Synthesis of porous hollow silica spheres using functionalized polystyrene latex spheres as templates

Uniform sized and integrated hollow silica spheres with porous shells were prepared by using sulfuric and carboxylic acid-functionalized polystyrene latex spheres as templates, sodium silicate as a precursor, and cetyltrimethylammonium bromide as a shell structure-directing agent. When polystyrene-methyl acrylic acid latex spheres were used as the templates, the pores in the shells of the resultant hollow silica spheres were composed of both micropores and mesopores. The pores in the shells of the hollow silica spheres were mainly composed of mesopores when sulfonated polystyrene-methyl acrylic acid latex spheres were used as the templates. The shell thickness and the specific surface area of the hollow silica spheres increased with the increase in the surface acidity of the latex spheres.     

Electrophoresis in concentrated dispersions of charged porous spheres

Electrophoresis in a concentrated suspension of charged porous spheres is investigated theoretically, taking into account of the polarization effect of double layer in particular. The double layer thickness and fixed charge density of the porous spheres are arbitrary and double layer overlapping is considered as well. A pseudo-spectral method based on Chebyshev polynomial is used to solve the resulted general electrokinetic equations. Local extrema are observed in the mobility profile as the double layer thickness varies, which is absent in previous theoretical studies neglecting polarization effect but agrees well with the experimental observations in the literature, indicating the importance of polarization effect in correctly interpreting the electrophoretic experimental results. A simple mutual transform of mobility predictions between Levine-Neale and Shilov-Zharkikh boundary conditions is established and confirmed by direct calculations, which facilitates the dialogue between theoretical and experimental researches. In general, the higher the fixed charge density, the more significant the polarization effect is.     

Measurement of high-temperature effective thermal conductivity of porous spheres

A new unsteady-state method for measuring the high temperature effective thermal conductivity of porous spheres is described. The method is based on moving a spherical body from an enclosure at a low temperature into one at a higher temperature and measuring the surface temperature with a recording radiation pyrometer. The thermal conductivity is evaluated using the known relations between the dimensionless surface temperature and time. The apparatus requirements are discussed in detail. The thermal conductivities of calcined dolomite/silicon metal mixtures, of the type used in the production of magnesium, were measured at 750 to 1150°C with compaction pressures of 67 to 185 MPa both before and after reaction.     

Measurement of band emissivity of porous spheres at high temperatures

A relatively simple technique has been developed to measure the band emissivity of spherical solids at high temperatures. The technique involves measuring the surface temperature of a sphere when in thermal equilibrium with a furnace enclosure, followed by isolating it from the enclosure by means of a shield. The emissivity is evaluated from the drop in energy flux from the pellet consequent upon changing the enclosure from an effectively black-body to a freely radiating one. The value of the band emissivity is validated by simulating the experimental cooling curve using independently measured values of the effective thermal conductivity and total emissivity of the solid to obtain internal consistency. In this way highly accurate values of band emissivity are obtained. The technique was applied to pellets of both reacted and unreacted mixtures of calcined-dolomite and silicon, as used in the commercial production of magnesium.     

Continuous preparation of porous organofunctionalized (Hetero-)polysiloxane spheres

When in contact with water, organoalkoxysilanes and other alkoxy metal compounds react to organofunctionalized polysiloxanes or heteropolysiloxanes. A new method for the continuous preparation of porous organopolysiloxane spheres was developed. Droplets of the precursor mixture are injected into the vertical reactor column. Movement of the gelating droplets is controlled by flow of the aqueous reaction medium. Advantages are adjustable residence time, minimized coalescence and low reactor height. An example shows typical product characteristics like narrow particle size distribution and macroporosity.     

Electrophoresis in suspensions of charged porous spheres in salt-free media

Electrophoresis in either dilute or concentrated suspensions of charged porous spheres in salt-free media is investigated theoretically in this study. The Brinkman model and the Kuwabara’s unit cell model are adopted to simulate the porous structure and the suspensions, respectively.We found, among other things, that the polarization effect due to the convection flow within the porous sphere is a crucial factor in determining its electrophoretic behavior. An induced electric field opposite to the applied electric field is generated, which deters the particle motion significantly when the particle is highly permeable. Approximate analytical prediction for dilute suspensions neglecting convection flow can overestimate the mobility severely in this situation. The approximate analytical prediction is satisfactory when the permeability of particle is low, though. Counterion condensation happens at high fixed charge density which decreases the mobility drastically and the mobility approaches a constant value asymptotically. The mobility profile of the particles with increasing volume fraction can exhibit local minimum if the corresponding dimensionless parameter Q_fix/(λa)² is high, where Q_fix and λa are, respectively, the fixed charge density and the friction coefficient of the porous particles in dimensionless form. This is due to the overlapping of counterion clouds surrounding particles, which offsets the polarization effect, becomes significant as the suspension gets concentrated. No such phenomenon for low Q_fix/(λa)², where the mobility profile decreases monotonously with increasing volume fraction. Comparison with experimental data available in the literature for polyelectrolyte suspensions is excellent, indicating the reliability of this analysis, as well as the success of using charged porous sphere to model a polyelectrolyte system.     

Synthesis of porous carbon and silica spheres using PEO-PF polymer blends

In this paper, we performed a physical mixture of PEO and PF polymers (i.e. a polymer blend) as an organic template for synthesizing PF-PEO-silica homogeneous composites in a dilute silicate solution at pH = 4.0–5.0. The PF-PEO-silica composites exhibit spherical morphology, in micrometer dimension, and the sphere size is dependent on the pH value of the solution. After undergoing calcination to remove the organic part of the PF-PEO-silica composites with and without the hydrothermal treatment, porous silica spheres of different pore sizes were obtained. Due to the existence of the carbonizing PF polymer in the PF-PEO-silica composite, porous carbon spheres can be conveniently obtained from pyrolysis of the PF-PEO-silica composites under a N₂ atmosphere and HF-etching procedures. TEM images demonstrate that the mesostructures of the mesoporous silica and porous carbons are disordered.     

Synthesis of porous carbon and silica spheres using PEO-PF polymer blends

In this paper, we performed a physical mixture of PEO and PF polymers (i.e. a polymer blend) as an organic template for synthesizing PF-PEO-silica homogeneous composites in a dilute silicate solution at pH = 4.0–5.0. The PF-PEO-silica composites exhibit spherical morphology, in micrometer dimension, and the sphere size is dependent on the pH value of the solution. After undergoing calcination to remove the organic part of the PF-PEO-silica composites with and without the hydrothermal treatment, porous silica spheres of different pore sizes were obtained. Due to the existence of the carbonizing PF polymer in the PF-PEO-silica composite, porous carbon spheres can be conveniently obtained from pyrolysis of the PF-PEO-silica composites under a N₂ atmosphere and HF-etching procedures. TEM images demonstrate that the mesostructures of the mesoporous silica and porous carbons are disordered.     

Hydrodynamic drag forces on two porous spheres moving along their centerline

This paper numerically evaluates the hydrodynamic drag force exerted on two highly porous spheres moving steadily along their centerline (sphere #1 and sphere #2) through a quiescent Newtonian fluid over a Reynolds number ranging from 0.1 to 40. At creeping flow limit, the drag forces exerted on both spheres were identical. At higher Reynolds numbers the drag force on sphere #1 was higher than sphere #2, revealing the shading effects produced by sphere #1 on sphere #2. At dimensionless diameter (β, =d_f/2k^0.5, d_f and k are floc diameter and interior permeability, respectively) >20, the spheres can be regarded nonporous. At β<20, the drag forces dropped. At β<2, the drag forces approached “no-spheres” limit. An increased size ratio of two spheres (d_f1/d_f2) would increase the drag force on sphere #1 and reduce that on sphere #2. At increasing β for both spheres, the drag force on sphere #2 was increased because of the more difficult advective flow through its interior, and at the same time the drag was reduced owing to the stronger wake flow produced by the denser sphere #1. The competition between these two effects leads to complicated dependence of drag force on sphere #2 on β value. These effects were minimal when β became low. Two identical spheres could move steadily along their centerline. At higher Reynolds number, the two spheres would move closer because of the incorporation of inertia force. For spheres of different diameters, the sphere # 2 would move faster than sphere #1 regardless of their size ratio and β value. This occurrence yielded efficient coagulation when two porous spheres were moving in-line.     

Highly porous carbon spheres for electrochemical capacitors and capacitive flowable suspension electrodes

In flowable and conventional electrochemical capacitors, the energy capacity is largely determined by the electrode material. Spherical active material, with high specific surface area (SSA) represents a promising material candidate for film and flow capacitors. In this study, we synthesized highly porous carbon spheres (CSs) of submicrometer size to investigate their performance in film and suspension electrodes. In particular, we studied the effects of carbonization and activation temperatures on the electrochemical performance of the CSs. The CSs activated at optimum conditions demonstrated narrow pore size distribution (<3 nm) with high SSA (2900 m²/g) and high pore volume (1.3 cc/g), which represent significant improvement as compared to similar materials reported in literature. Electrochemical tests of CSs in 1 M H₂SO₄ solution showed a specific capacitance of 154 F/g for suspension electrode and 168 F/g for film electrode with excellent rate performance (capacitive behaviors up to 100 mV/s) and cycling performance (95% of initial capacitance after 5000 cycles). Moreover, in the film electrode configuration, CSs exhibited high rate performance (78 F/g at 1000 mV/s) and volumetric power density (9000 W/L) in organic electrolytes, along with high energy density (21.4 Wh/L) in ionic liquids.     

Terminal velocity of dense particles in the multisolid pneumatic transport bed

The terminal velocity of dense particles in an air-fine particle heterogeneous medium defines the upper operating limit of the gas velocity in the multisolid pneumatic transport bed reactor. The terminal velocity of dense particles in an air-fine particle medium was experimentally determined from the turbulent fluidized bed voidage vs air velocity relationship in this study. It was found that the terminal velocity of dense particles decreases with the increase of the fine particle flow rate. The fine particle and dense particle interaction coefficient was defined based on the momentum balance equation for a dense particle. The interaction coefficient was empirically correlated and utilized to account for the terminal velocity of dense particles in the air-fine particle heterogeneous medium.     

Terminal velocity of non-spherical particles falling through a Carreau model liquid

A procedure has been proposed for the estimation of the terminal falling velocity of non-spherical particles moving in a Carreau model fluid in the transition flow region. The procedure is based on a modification of the relationship formerly developed for the fall of spherical particles including the particle dynamic shape factor. The suitability of the proposed procedure has been confirmed by good agreement between experimental and calculated terminal falling velocity data. In the experiments, the terminal falling velocity of short cylinders and rectangular prisms in polymer solutions of different measure of shear thinning and elasticity has been measured.     

Facile fabrication of porous hollow CeO2 microspheres using polystyrene spheres as templates

Porous hollow CeO₂ microspheres were fabricated using negative-charged PS microspheres as templates by a facile method. The hollow CeO₂ microspheres were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and N₂ adsorption?Cdesorption. The results showed that the as-synthesized hollow CeO₂ microspheres are well monodisperse and uniform in size. The porous shells of hollow microspheres are relatively rough and composed of tiny nanoparticles. The external diameter, internal diameter, and shell thickness of hollow CeO₂ microspheres are about 190, 160, and 15?nm, respectively. A possible mechanism for the formation of hollow CeO₂ spheres was also discussed.     

Using polystyrene spheres to produce thin,porous interlayers in alumina laminates

Porous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (K_IC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and K_IC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.     

Template-free fabrication of porous zinc oxide hollow spheres and their enhanced photocatalytic performance

Porous zinc oxide (ZnO) hollow spheres have been fabricated by calcination of a precursor complex in a furnace. The precursor was precipitated in a chemical solution at 80 °C. The field-emission scanning electron microscope and transmission electron microscope reveal the porous and hollow structure of the samples. The spherical hollow precursor is self-assembled in the solution under the coordination effect of citrate ions and the Kirkendall effect working together. The precursors can be converted to pure ZnO crystals by heating in a furnace above 300 °C. The Brunauer–Emmett–Teller (BET) specific surface area of this sample is 95.4 m²/g. The photocatalytic degradation of methyl blue solution test shows the ZnO hollow spheres have superior photocatalytic activity.     

Synthesis and characterization of iron-impregnated porous carbon spheres prepared by ultrasonic spray pyrolysis

Porous carbon microspheres impregnated with iron-based nanoparticles are prepared in a single step, continuous process using ultrasonic spray pyrolysis (USP). Precursor solutions containing a carbon source, an inorganic salt, and an iron salt are ultrasonically aerosolized and pyrolyzed. Solutions containing nitrate or chloride salts are examined. During pyrolysis, sucrose is dehydrated to carbon, and the metal salt is converted to crystalline or non-crystalline iron species, depending on processing conditions. The product’s porosity is generated from: (1) aromatization of carbon around an in situ template, (2) in situ gasification of isolated carbon, or (3) in situ chemical activation of the carbon precursor. Porous carbon spheres (0.5–3 μm diameter) containing well-dispersed iron oxide nanoparticles (4–90 nm diameter), referred to here as Fe–C, are prepared. Iron loadings between 1 and 35 wt.% are achieved while maintaining well-dispersed Fe nanoparticles with as-produced surface areas up to 800 m²/g. Post-pyrolysis heat and hydrogen treatments increase the surface area of the materials while reducing iron species. USP Fe–C materials may have useful catalytic applications due to their potential for high-loading of well-dispersed metal nanoparticles. Despite negligible surface Fe content, chromium reduction tests indicate that internal Fe sites are catalytically active.     

Collisions of spheres with wet and dry porous layers on a solid wall

A theory that combines Darcy's law for flow in porous media with inelastic solid mechanics, to model collisions of solid spheres with wet or dry porous layers placed on a solid wall, is found to closely describe the trends in data collected from particle-collision experiments. An exponential-hardening, stress-strain model is used for the porous layer, validated with dynamic mechanical analyzer measurements. Low-velocity collisions were performed in the low-gravity environment afforded during parabolic flight of a KC-135 aircraft, and also under normal gravity with a pendulum-based setup. Both theory and experiments show a decrease in the dry restitution coefficient with an increase in impact velocity, mainly due to increased inelastic losses in the porous material. The wet restitution coefficient is also found to decrease with an increase in the impact velocity, in contrast to the wet restitution coefficient for collisions of a solid sphere with a wet wall without a porous layer. Moreover, a critical impact velocity (below which no rebound occurs) is observed for wet collisions without a porous layer but not with a porous layer. The wet restitution coefficient is always found to be lower than the dry restitution coefficient, due to the viscous losses associated with fluid flow in addition to the inelastic losses associated with the porous layer.     

Wall factor for acceleration and terminal velocity of falling spheres at high reynolds numbers

The wall factor for spheres in the acceleration and terminal velocity ranges was determined experimentally for very high Reynolds numbers (13 500 < Re < 70 000). Experiments were performed with 12, 15, 18, 21, 25 and 31.75 mm spheres, falling in water inside cylinders 3.4, 4.9, 7, 10, 14 and 19 cm in diameter. A published empirical equation was found to yield good results for the terminal velocity wall factor in the range of studied Reynolds numbers.     

An explicit equation for the terminal velocity of solid spheres falling in pseudoplastic liquids

An explicit equation is proposed which predicts directly the terminal velocity of solid spheres falling through stagnant pseudoplastic liquids from the knowledge of the physical properties of the spheres and of the surrounding liquid. The equation is a generalization of the equation proposed for Newtonian liquids. By properly defining the dimensionless diameter, d_*, a function of the Archimedes number, Ar, and the dimensionless velocity, U_*, a function of the generalized Reynolds number, Re, to account for the non-Newtonian characteristics of the liquid, the final equation relating these two variables has similar form to the Newtonian equation. The predictions are very good when they are compared to 55 pairs of Re—C_D for non-Newtonian data and 37 pairs for Newtonian data published previously. The root mean square error on the dimensionless velocity is 0.081 and much better than the only other equation previously proposed.     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.