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At least 343 records · Page 19Linked to original sources
PubMed2001

Expanded Prussian blue analogues incorporating [Re6Se8(CN)6](3-/4-) clusters: adjusting porosity via charge balance.

Face-capped octahedral [Re(6)Se(8)(CN)(6)](3-/4-) clusters are used in place of octahedral [M(CN)(6)](3-/4-) complexes for the synthesis of microporous Prussian blue type solids with adjustable porosity. The reaction between [Fe(H(2)O)(6)](3+) and [Re(6)Se(8)(CN)(6)](4-) in aqueous solution yields, upon heating, Fe(4)[Re(6)Se(8)(CN)(6)](3).36H(2)O (4). A single-crystal X-ray analysis confirms the structure of 4 to be a direct expansion of Prussian blue (Fe(4)[Fe(CN)(6)](3).14H(2)O), with [Re(6)Se(8)(CN)(6)](4-) clusters connected through octahedral Fe(3+) ions in a cubic three-dimensional framework. As in Prussian blue, one out of every four hexacyanide units is missing from the structure, creating sizable, water-filled cavities within the neutral framework. Oxidation of (Bu(4)N)(4)[Re(6)Se(8)(CN)(6)] (1) with iodine in methanol produces (Bu(4)N)(3)[Re(6)Se(8)(CN)(6)] (2), which is then metathesized to give the water-soluble salt Na(3)[Re(6)Se(8)(CN)(6)] (3). Reaction of [Co(H(2)O)(6)](2+) or [Ni(H(2)O)(6)](2+) with 3 in aqueous solution affords Co(3)[Re(6)Se(8)(CN)(6)](2).25H(2)O (5) or Ni(3)[Re(6)Se(8)(CN)(6)](2).33H(2)O (6). Powder X-ray diffraction data show these compounds to adopt structures based on the same cubic framework present in 4, but with one out of every three cluster hexacyanide units missing as a consequence of charge balance. In contrast, reaction of [Ga(H(2)O)(6)](3+) with 3 gives Ga[Re(6)Se(8)(CN)(6)].6H(2)O (7), wherein charge balance dictates a fully occupied cubic framework enclosing much smaller cavities. The expanded Prussian blue analogues 4-7 can be fully dehydrated, and retain their crystallinity with extended heating at 250 degrees C. Consistent with the trend in the frequency of framework vacancies, dinitrogen sorption isotherms show porosity to increase along the series of representative compounds 7, Ga(4)[Re(6)Se(8)(CN)(6)](3).38H(2)O, and 6. Furthermore, all of these phases display a significantly higher sorption capacity and surface area than observed in dehydrated Prussian blue. Despite incorporating paramagnetic [Re(6)Se(8)(CN)(6)](3-) clusters, no evidence for magnetic ordering in compound 6 is apparent at temperatures down to 5 K. Reactions related to those employed in preparing compounds 4-6, but carried out at lower pH, produce the isostructural phases H[cis-M(H(2)O)(2)][Re(6)Se(8)(CN)(6)].2H(2)O (M = Fe (8), Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-dimensional framework in which [Re(6)Se(8)(CN)(6)](4-) clusters are interlinked through a combination of protons and Fe(3+) ions.

Journal Article↗
PubMed2005

Using paramagnetic particles as repulsive templates for the preparation of membranes of controlled porosity.

Using mixtures of repulsive superparamagnetic polystyrene particles and a photopolymerizable organic liquid (trimethylolpropane trimethacrylate) that are applied to a water surface, it is possible to prepare porous membranes with controlled porosity. The particles were polarized by applying a magnetic field H perpendicular to the interface and spread out over the interface making use of the induced repulsive magnetic dipole interactions. As a consequence, the organic liquid in which the particles were embedded covered the water surface uniformly. Subsequent photo cross linking of the organic liquid and dissolution of the embedded particles gave rise to membranes whose porosities were controlled mainly by the chosen areas per particle. The spatial distribution of the pores and the deviation from a crystalline arrangement were characterized in terms of the 2D pair-correlation function and the mean nearest-neighbor interpore distance.

Journal Article↗
PubMed2005

Controlling porosity within colloidal heteroaggregates.

Heteroaggregates of cationic poly(2-vinylpyridine) microgels and anionic polystyrene latex particles have been made by mixing dilute, aqueous suspensions. The growth of the heteroaggregates was arrested by the addition of anionic silica particles that adsorbed to the free surface of the cationic microgel particles. The resulting heteroaggregates were then concentrated by vacuum filtration, freeze-dried, and characterized by mercury porosimetry and electron microscopy. The inclusion of soft, deformable microgels resulted in heteroaggregates with higher porosity than obtained with heteroaggregates of anionic and cationic latex particles. Control of the pore volumes within the freeze-dried filter cakes was demonstrated by two approaches. In the first approach, heteroaggregation at a constant KCl concentration of 0.01 mM was arrested at different times after mixing the latex and microgel particles, thereby limiting the size of the aggregates. The porosity of the resulting filter cake increased from 61 to 65 vol % as the aggregation time increased from 15 to 120 s. In the second technique, the aggregation time prior to arrest was maintained at 120 s while the KCl concentration was varied between 0.01 and 10 mM. The pore volume of the aggregates decreased from 65 to 57 vol % as the electrolyte concentration increased, a trend explained in terms of the effect of the Debye length on the aggregation process.

Journal Article↗
PubMed2001

Hydroxyapatite spheres with controlled porosity for eye ball prosthesis: processing and characterization.

Porous hydroxyapatite spheres were prepared by a modified gelcasting method producing a ceramic prosthesis with controled porosity. The spheres are approximately 2.2 cm in diameter with a relatively homogeneous pore size distribution from 10 to 40 microm in diameter. The samples were characterized by X-ray powder diffractometry (XRD) and Fourier transform infra-red spectroscopy (FTIR) to identify the phases both prior to and after the gelcasting process. Surface morphology analysis and porosity evaluations were performed with scanning electron microscopy (SEM), while surface area measurements were carried out by the BET technique.

Journal Article↗
PubMed2006

Crystal porosity and the burden of proof.

The study of porosity in the context of crystal engineering is rapidly growing in intensity. However, claims of porosity are often highly subjective and use of the term "porous" is susceptible to abuse. This contribution discusses some of the criteria to be considered when stating that a particular crystal structure is porous.

Journal Article↗
PubMed2001

Grain reconstruction of porous media: application to a low-porosity Fontainebleau sandstone.

The fundamental issue of reconstructing a porous medium is examined anew in this paper, thanks to a sample of low-porosity Fontainebleau sandstone that has been analyzed by computed microtomography. Various geometric properties are determined on the experimental sample. A statistical property, namely, the probability density of the covering radius, is determined. This is used in order to reconstruct a porous medium by means of a Poissonian generation of polydisperse spheres. In a second part, the properties of the real experimental sample and of the reconstructed one are compared. The most important success of the present reconstruction technique is the fact that the numerical sample percolates despite its low porosity. Moreover, other geometrical features and conductivity are found to be in good agreement.

Journal Article↗
PubMed2003

Linear dynamics of double-porosity dual-permeability materials. II. Fluid transport equations.

For the purpose of understanding the acoustic attenuation of double-porosity composites, the key macroscopic equations are those controlling the fluid transport. Two types of fluid transport are present in double-porosity dual-permeability materials: (1) a scalar transport that occurs entirely within each averaging volume and that accounts for the rate at which fluid is exchanged between porous phase 1 and porous phase 2 when there is a difference in the average fluid pressure between the two phases and (2) a vector transport that accounts for fluid flux across an averaging region when there are macroscopic fluid-pressure gradients present. The scalar transport that occurs between the two phases can produce large amounts of wave-induced attenuation. The scalar transport equation is derived using volume-averaging arguments and the frequency dependence of the transport coefficient is obtained. The dual-permeability vector Darcy law that is obtained allows for fluid flux across each phase individually and is shown to have a symmetric permeability matrix. The nature of the cross coupling between the flow in each phase is also discussed.

Journal Article↗
PubMed2004

Simultaneous measurement of rock permeability and effective porosity using laser-polarized noble gas NMR.

We report simultaneous measurements of the permeability and effective porosity of oil-reservoir rock cores using one-dimensional NMR imaging of the penetrating flow of laser-polarized xenon gas. The permeability result agrees well with industry standard techniques, whereas effective porosity is not easily determined by other methods. This NMR technique may have applications to the characterization of fluid flow in a wide variety of porous and granular media.

Journal Article↗
PubMed2004

A1 and A2 transitions in superfluid 3He in 98% porosity aerogel.

Superfluid 3He in high porosity aerogel is the system in which the effects of static impurities on a p-wave superfluid can be investigated in a systematic manner. We performed shear acoustic impedance measurements on this system (98% porosity aerogel) in the presence of magnetic fields up to 15 T at the sample pressures of 28.4 and 33.5 bars. We observed the splitting of the superfluid transition into two transitions in high fields in both bulk and liquid in aerogel. The field dependence of the splitting in aerogel resembles that of the bulk superfluid 3He caused by the presence and growth of the A1 phase. Our results provide the first evidence of the A1 phase in superfluid (3)He/aerogel.

Journal Article↗
PubMed2004

Stochastic reaction-diffusion phenomena in porous media with nonlinear kinetics: effects of quenched porosity fluctuations.

We study the effects of quenched porosity fluctuations in the presence of nonlinear kinetics in a reaction-diffusion porous system. Adopting a weak-noise approximation and a mean-field assumption, a renormalized equation of motion for the average concentration of a chemical species is obtained. It is characterized by a smaller effective diffusion coefficient and by the presence of supplementary nonlinear reactive terms. Explicit results are given for a Gaussian porosity distribution and for a long-ranged one. Comparisons with simulations in one dimension are presented.

Journal Article↗
PubMed1976

Characterization of porosity of isostatically pressed and sintered nickel-base powdered metal.

Characterization of the pore structure of compacted and sintered parts made from a nickel-base powder was accomplished using the mercury porosimetry method. The theoretical density values for the sintered specimens varied from 56.3 to 96.7% which corresponds to a porosity of 43.7 to 3.3%. A maximum interconnecting median pore diameter of 21 mum resulted from a -80/+200 mesh powder compacted at 138 MN/m2 and sintered for 2 h at 1250 degrees C. Photomicrographs of the same sample showed that it had a maximum pore diameter of 200 mum. The interconnected pore volume decreased with decreasing particle size of the powder, increasing compaction pressure, and increasing sintering temperature. Mechanical properties of tensile strength, yield strength, elastic modulus and percentage elongation were correlated with the pore structure. Proper selection of particle size, compaction pressure, sintering times and sintering temperatures should permit parts with controlled porosity characteristics to be produced that possess adequate mechanical properties for application as implants.

Chemical Phenomena↗
PubMed1981

The porosity and roughness of four direct filling resins.

Four direct filling resins were examined in order to compare the roughness and porosity of different materials, mixed by different methods. From each material six specimens were prepared. Each specimen was formed against a Mylar sheet and the Mylar-formed surface was examined. Subsequently it was trimmed and polished and re-examined. Each specimen was examined with a microscope using incident light and employing an automatic point-counting attachment in order to assess the percentage surface (apparent) porosity. Each specimen was also examined with a stylus-type profile meter. Rt (total roughness height) was recorded for all specimens and Ra (roughness average) was recorded for all polished specimens. The findings support the conclusion that a capsulated resin without an inorganic filler yields a better surface than other resins.

Acrylic Resins↗
PubMed1983

Porosity of restorative resins.

Polyethylene tubes were filled with different types of composite resins using either the bulk-packing technique or the syringe injection technique. The porosity of the specimens was studied. Specimens made by light-activated systems were less porous compared to specimens made by two-paste systems. Use of the injection technique was followed by a reduction in porosity in both groups compared to use of the bulk-packing technique.

Chemical Phenomena↗
PubMed1977

Porosity in base metal partial denture casting alloys, related industrial alloys, and pure metals.

Two types of specimens were cast from a variety of pure metals and alloys. A unidirectional solidification system was used for the production of the first, whilst a deliberate feeding problem was incorporated in the production of the second. The specimens were subjected to radiographic and metallographic evaluation in conjunction with porosity determinations, in order to ascertain how the amount and pattern of porosity varied with alloy complexity and alloy composition.

Absorptiometry, Photon↗
PubMed1991

Effects of compaction variables on porosity and material tensile strength of convex-faced aspirin tablets.

The porosity and tensile strength of convex-faced aspirin tablets formed under a compaction pressure in the range 40-320 MPa and at punch velocities in the range 0.008 to 500 mm s-1 have been determined. The material tensile strength, sigma f, was calculated from the observed fracture load, Ps, using the equation of Pitt et al (1988): sigma f = 10 Ps/pi D2(2.84 t/D - 0.126 t/W + 3.15 W/D + 0.01)-1 where D is the tablet diameter, t is the overall tablet thickness and W is the central cylinder thickness. Tablets formed at lower compaction pressures had a higher porosity and lower tensile strength than those formed at higher compaction pressures. Tablets of face curvature ratio (D/R) in the range 0.25-0.67 and a normalized cylinder length (W/D) of 0.2 had the optimum tensile strength. (R is the radius of curvature of the tablet face.) Tablets formed at high compaction rates were significantly weaker than those formed at lower compaction rates.

Aspirin↗
PubMed2003

Acoustic wave propagation in double porosity media.

A study of the propagation of waves in porous media with an interconnected network of pores and micropores of very different characteristic sizes, saturated by a compressible Newtonian fluid, is proposed. With this aim, the homogenization technique for periodic separated scales media, is applied to realistic double porosity materials with motionless skeleton. From preliminary explicit estimations of wavelengths in the two fluid networks, it is shown that the macroscopic descriptions depend on the contrast of static permeability between pores and micropores and on frequency. The local equations are solved in the cases of low and high contrasts of permeability, and two main macroscopic behaviors are obtained. In the low contrast situation, the macroscopic flow is given by a kind of generalized Darcy's law involving both pores and micropores, and their respective characteristic frequencies. Regarding compressibility effects, both pore networks act in parallel. The high permeability contrast reveals that the macroscopic flow law is governed by the pores. The microporous part of the material is submitted to pressure diffusion effects, bringing dissipation, and modifying the dynamic bulk modulus of the material. The two situations of coupling are illustrated for simple geometry of double porosity materials, including perforated--and slits--microporous materials.

Journal Article↗