Evolution Of Pore Size Distribution Research Articles

Evolution of microstructure and performance in magnesium potassium phosphate ceramics: Role of sintering temperature of MgO powder

The reactivity of the MgO powder employed in the formulation of Mg–K phosphate ceramics can be modulated through the calcination temperature of MgCO3 source material, which has a direct impact on production costs. Upon annealing, MgO undergoes sintering, and in order to optimize the design of products for applications, it is of primary importance to disclose the link between the sintering temperature, reaction mechanisms, microstructure and performance in this class of ceramics. Small angle neutron scattering was used to measure the specific surface area of pores in samples aged 30 days produced with 5 different MgO powders, and to follow the evolution of pore size distribution during the setting reaction, in a time-resolved experiment. Quantification of amorphous and crystalline fraction up to 28 days was accomplished in synchronous with flexural strength tests. Results indicate that mechanical properties improve thanks to the progressive buildup of a pervasive network of tabular crystals filling the entire volume. Increasing the sintering temperature above 1500°C yields a more compact ceramic, with less, but larger, pores, containing more crystalline fraction and less amorphous. This is consistent with the recently proposed mechanisms describing the ceramic setting reaction. The analysis of the fractured surface suggests that strength might be effectively improved modifying the density and orientation of crystals in the ceramic volume, a way for engineering new tailor-made ceramics.

Evolution of pore size distribution during sintering of oxide nuclear fuel

Uranium dioxide pellets were sintered at various temperature routes and atmospheres with different oxygen content. Statistically calculated pore size distribution of the sintered pellets and distribution function was obtained. It is shown that the average pore size is almost unchanged at intermediate stage of sintering while the total number of pores reduced.

Effects of Coupled Chemo-Mechanical Processes on the Evolution of Pore-Size Distributions in Geological Media

The pore space in rocks, sediments, and soils can change significantly as a result of weathering (see Navarre-Sitchler et al. 2015, this volume), diagenetic, metamorphic, tectonic, and even anthropogenic processes. As sediments undergo compaction during burial, grains are rearranged leading to an overall reduction in porosity and pore size (Athy 1930; Hedberg 1936; Neuzil 1994; Dewhurst et al. 1999; Anovitz et al. 2013). In addition, geochemical reactions can induce the precipitation and dissolution of minerals, which can either enhance or reduce pore space (e.g., Navarre-Sitchler et al. 2009; Emmanuel et al. 2010; Stack et al. 2014; Anovitz et al. 2015). During metamorphism too, mineral assemblages can change, altering rock fabrics and porosity (Manning and Bird 1995; Manning and Ingebritsen 1999; Neuhoff et al. 1999; Anovitz et al. 2009; Wang et al. 2013). As the pore space in geological media strongly affects permeability, evolving textures can influence the migration of water, contaminants, gases, and hydrocarbons in the subsurface. Although models—including the Kozeny–Carman equation (Kozeny 1927; Bear 1988)— exist to predict the relationship between porosity and permeability, they are often severely limited, in part because little is known about how pore size, pore geometry, and pore networks evolve in response to chemical and physical processes (Lukasiewicz and Reed 1988; Costa 2006; Xu and Yu 2008). In the case of geochemical reactions, calculating the change in total porosity due to the precipitation of a given mass of mineral is straightforward. However, predicting the way in which the precipitated minerals are distributed throughout the pores remains a non-trivial challenge (Fig. 1; Emmanuel and Ague 2009; Emmanuel et al. 2010, Hedges and Whitlam. 2013; Wang et al. 2013; Stack et al. 2014; Anovitz et …

Experimental investigation of the transport mechanism of several gases during the CVD post-treatment of nanoporous membranes

Abstract The current study stresses the importance of combining gas transport theories with experimental gas permeability results on the effort to elucidate nanopore structure evolution of nanofiltration membranes during their modification through the chemical vapour deposition technique. To this end, silica membranes of 1 nm nominal pore size were post-treated by applying a sequential cyclic CVD method at 573 K and the Tetraethyl-orthosilicate/Ozone reaction system. Alteration of the gas transport characteristics was investigated by conducting single gas permeance measurements of Helium and Nitrogen at selected temperatures following the completion of several CVD cycles. The experimental results were interpreted on the basis of gas transport theories combined with a model for the evolution of pore size distribution of the membrane’s separation layer during silica deposition. By monitoring permeation properties of the treated membranes with the progress of deposition using Helium and Nitrogen as probe gases, optimized treatment conditions can be established in order to fabricate selective and highly permeable – with respect to Hydrogen gas – ceramic membranes. Furthermore, the dependence of the permeation properties on the pore size of the studied membranes before and after their CVD treatment was investigated by performing single-gas permeance measurements of several probe gases within a wide temperature range.

Laboratory Determination of Water Retention Characteristics and Pore size Distribution in Simulated MSW Landfill Under Settlement

To examine how water retention characteristics in a landfill environment change as a result of solid waste settlement and to derive the evolution of pore size distribution, a lab scale long column experiment was carried out at two different time periods while keeping all the other characteristics constant. Two columns, Column 1 and Column 2, were used in the study. The log normal distribution model was applied to model the experiment data. The Column 1 was used to obtain the water retention curve at time 0 day and the Column 2 at time 180 day. During the experiment period, landfill settlement was mainly in the primary stage of settlement. Pore size distribution was obtained by assuming that the capillary theory is applicable in the landfill environment. The results showed that water retention characteristics in solid waste landfill environment is varied with time and influenced by landfill settlement due to the changes in pore spaces. The obtained results will be taken into consideration when simulating unsaturated leachate and gas flow in municipal solid waste landfill environment.

Evolution of pore characteristics in the 3D numerical direct shear test

The quantitative analysis of the pore characteristics of granular materials has been often challenging due to arbitrarily shaped geometry of pores despite its significant implications. In this study, we investigate the size distribution and orientation of pores in dilative and contractive assemblies in the direct shear test by performing 3D discrete element simulations in conjunction with image processing of pore geometry. We quantitatively define unit pores by the Delaunay Tessellation followed by pore mergence and fitting them with ellipsoids. It is observed that the evolution of pore size distribution depends on the dilatancy of assemblies. Results also show that the direction of principal stresses governs the orientations of pores during shearing, with respect to the size of pores. This study highlights that the dominant factors of the pore characteristics upon shearing are stress anisotropy and particle mobilization to make the internal structure stable.

A framework for quantifying size dependent deformation of nano-scale pores in mudrocks

The evolution of pore size distributions during sediment consolidation controls critical parameters such as porosity and permeability. Two phenomenological models are developed that describe the evolution of pore size distributions during stress induced consolidation. The first model predicts the evolution of pores subjected to an applied stress for systems in which all pores deform equally irrespective of size; in the second model, the rate of pore deformation decreases with size (i.e., smaller pores deform less readily than larger ones). To determine which model best describes the behavior of clay-rich rocks during consolidation, cumulative void volume curves from consolidation experiments carried out on Boston Blue Clay are compared with results from numerical simulations. While the uniform deformation model is able produce a good fit during the initial stage of the consolidation (0.1–1 MPa), it is unable to capture system behavior at elevated stresses (1–10 MPa). By contrast, the size dependent deformation model produces excellent fits with the data at both initial and later stages of consolidation. Furthermore, the model shows that size dependent behavior is restricted to pores with radii of < 100 nm; significantly, small pores may be up to 47% less compressible than large pores. Crucially, by comparing sediments from different burial depths but possessing similar mineralogical compositions, the framework can be used to assess the behavior of natural sediments under geological conditions. ► A new model for pore size evolution during consolidation is presented. ► The model is used to quantify the effect of stress on pore consolidation. ► In mudrocks , pores < 100 nm deform more readily than larger pores by up to 47%.

Dual Drug Release of Triamterene and Aminophylline from Poly (N-Isopropylacrylamide) Hydrogels

We used temperature-sensitive poly(N-isopropylacrylamide) hydrogels as drug delivery systems, so changes in body temperature induced by pathogens could act like external stimuli to activate controlled release of the drugs incorporated in the hydrogel. In the distilled water combined release studies, we chose two model drugs: aminophylline and triamterene. The amount of drug released was measured by UV-Vis spectroscopy following the evolution of the absorption peaks of aminophylline (271 nm) and triamterene (365 nm). The maximum release time was greater for triamterene than for aminophylline at 37 °C, so these time-release profiles enabled the active ingredients to work over different periods of time. By increasing molar mass or solubility of the drug, we observed that the diffusion coefficient decreased. On the contrary, increasing hydrophobicity of the drug leads to a diffusion coefficient increase. The evolution of pore size distribution of hydrogels during loading and releasing was measured by quasi-elastic light scattering and by environmental electronic scanning microscope. When loading and releasing the drugs, the pore size of the hydrogel decreased and increased again without reaching the initial pore size of the hydrogel, respectively. We observed that the greater the concentration of drug loaded into the hydrogel, the greater the reduction in pore size.

Moisture Retention Properties of Municipal Solid Waste in Relation to Compression

Original laboratory setups are used to study the moisture retention properties of municipal solid waste taking into account the porous medium’s structural evolution from compression. A controlled suction oedometer allowed the moisture retention curves (MRCs) of compacted samples to be determined for both wetting and drainage with a matric suction range of 0 to 10 kPa. Another setup utilizing an extraction plate was used to determine a drainage MRC for a noncompacted sample with matric suction varying from 0 to 450 kPa. The experimental results demonstrated the complexity of municipal solid waste (MSW) porous medium compared to soil. The MRC of lightly and uncompacted samples did not exhibit a measurable air-entry suction. Moreover, significant hysteresis between the wetting and drainage MRCs was observed. The experimental MRCs were interpreted with two different models, and a pore size distribution evolution with compression was proposed. Finally, the concept of field capacity in relation to the moisture retention properties is discussed.

Interfacial energy effects and the evolution of pore size distributions during quartz precipitation in sandstone

Pore size is usually thought to control the rate of crystal growth in porous geological media by determining the ratio of mineral surface area to fluid volume. However, theory suggests that in micron-scale to nanometer scale pores, interfacial energy (surface energy) effects can also become important. Interfacial energy typically increases the solubility of very small crystals growing in tiny pores, and when the fluid is close to equilibrium – as is often the case in geological systems – mineral precipitation could occur in relatively large pores, while in very small adjacent pores crystal growth might be suppressed. Such a mechanism would effectively restrict the reactive surface area of the porous medium, thereby reducing the bulk reaction rate. We investigated the pore size distributions in naturally cemented sandstone adjacent to an isolated stylolite and found that quartz precipitation was inhibited in pores smaller than 10 μm in diameter. Furthermore, we demonstrate that kinetic formulations which assume constant solubility cannot reproduce the observed pore size patterns in mineralized samples; by contrast, excellent fits with the data are obtained when interfacial energy effects are taken into account. Reaction rates in geological media determined in field studies can be orders of magnitude lower than those measured in laboratory experiments, and we propose that reduced reaction rates in porous media with micron and submicron-scale porosity could account for much of the apparent paradox.

A theoretical model on pore size distribution in low dielectric constant nanoporous silica films

A pore growth mechanism is proposed for the description of the pore size distribution and the porosity of the low dielectric constant materials. A theoretical model is constructed to portray the evolution of pore size distribution in nanoporous silica. The applicability of the model is justified by fitting it to the experimental data reported in the literature, and the effects of the essential parameters of the model on both the pore size distribution and on the porosity of a silica film are examined. The results of numerical simulation reveal that the quality of a film can be improved by increasing the rate of production of pore seeds. In particular, the total number of pores increases with the increase in the rate of production of pore seeds, and the faster the growth rate of pore the greater the number of large pores and the larger the total number of pores produced.

Analytical Models for Soil Pore‐Size Distribution After Tillage

Tillage causes soil fragmentation thereby increasing the proportion of interaggregate (structural) pore space. The resulting tilled layer tends to be structurally unstable as manifested by a gradual decrease in interaggregate porosity until a new equilibrium has been reached between external loads and internal capillary forces at a rate governed by the soil rheological properties. The soil pore‐size distribution (PSD) will change accordingly with time. We have previously applied the Fokker‐Planck equation (FPE) to describe the evolution of the PSD as the result of drift, dispersion, and degradation processes that affect the pore space in unstable soils. In this study, we provide closed‐form solutions for PSD evolution, which can be used to predict temporal behavior of unsaturated soil hydraulic properties. Solutions and moments of the PSD were obtained in case: (i) drift and degradation coefficients depend on time and the dispersivity is constant and (ii) drift and dispersivity are also linearly related to pore size. Both solutions can model the reduction in pore size during the growing season while the second solution can account for a reduction in the dispersion of the PSD. The solutions for PSD were plotted for a mathematically convenient expression for the drift and degradation coefficients and for an expression derived from a model for soil aggregate coalescence. Experimental data on the settlement of a Millville (coarse‐silty, carbonatic, mesic Typic Haploxeroll) silt loam during wetting and drying cycles were used to determine time‐dependent drift and degradation coefficients according to this coalescence model. The solution for the PSD was used to independently predict the water retention curve, which exhibited a satisfactory agreement with experimental retention data at the end of two drying cycles.

Population balance model for solid state sintering I. Pore shrinkage and densification

A general population balance model incorporating the rate of shrinkage of individual pores during solid state sintering of ceramics is described. The kinetics of pore shrinkage is assumed to follow a simple power law with floating value of the exponent. The resulting equation of continuity is solved for the evolution of pore size distribution in time by a transformation technique. The simulated pore size distributions for a number of ceramic oxide systems are in good agreement with experimental data generated in-house as well as from published sources. The model can be embedded in a coupled phenomenon of densification and grain growth, as shown in Part II.

Evolution of pore size distribution and average pore size of porous ceramic membranes during modification by counter-diffusion chemical vapor deposition

The modification of porous ceramic membranes by counter-diffusion chemical vapor deposition (CVD) is studied theoretically and experimentally. Numerical simulations of the evolution of the membrane permeance, average pore size and pore size distribution as a function of extent of modification are presented and compared with experimetal data. It is found that the change of the average pore size of the membranes after modification strongly depends on the initial pore size distribution of the membrane, CVD reaction kinetics and characterization method. Experimental data suggest that CVD of zirconia (and yttria) inside porous ceramic membranes by reaction of zirconium (and yttrium) chlorides with steam/air at elevated temperatures proceeds by quasi-zero reaction kinetics with respect to the oxidant, typical of non-stoichiometric supply of the reactants from opposite sides of the membrane. Under such conditions, CVD modification may result in a modest increase of the average pore size of coarse-pore ceramic membranes as suggested by numerical calculations and experimental data.

Integral formulation to simulate the viscous sintering of a two-dimensional lattice of periodic unit cells

In this paper a mathematical formulation is presented which is used to calculate the flow field of a two-dimensional Stokes fluid that is represented by a lattice of unit cells with pores inside. The formulation is described in terms of an integral equation based on Lorentz's formulation, whereby the fundamental solution is used that represents the flow due to a periodic lattice of point forces. The derived integral equation is applied to model the viscous sintering phenomenon, viz. the process that occurs (for example) during the densification of a porous glass heated to such a high temperature that it becomes a viscous fluid. The numerical simulation is carried out by solving the governing Stokes flow equations for a fixed domain through a Boundary Element Method (BEM). The resulting velocity field then determines an approximate geometry at a next time point which is obtained by an implicit integration method. From this formulation quite a few theoretical insights can be obtained of the viscous sintering process with respect to both pore size and pore distribution of the porous glass. In particular, this model is able to examine the consequences of microstructure on the evolution of pore-size distribution, as will be demonstrated for several example problems.

A model for predicting evolution of pore size distributions in UO 2 fuel

The model generalizes the description of Dollins and Nichols by including pore size distributions. A thermal densification formulation is also derived. The results show that at T< 900° C, densification is independent of temperature and is mainly due to interstitial migration. In the range 1000–1350°C, vacancy diffusion and fission spike/pore interaction also interfere. Here, the densification rate decreases as the temperature rises and a slight porosity growth appears around 1300°C. Above 1500°C, pore coalescence is stimulated and the densification rates are enhanced. The predicted evolutions of pore size distributions, under irradiation at low temperatures and during resintering tests, are in reasonable agreement with experimental data.

Characterization of HWR fuel pellets fabricated using UO 2 powders from different conversion processes

Heavy-water-reactor (HWR) fuel manufacturing involves the fabrication of high density natural UO 2 pellets with controlled microstructure. It is necessary to investigate the correlation between powder characteristics, pellet properties and process parameters, especially when the UO 2 powders are from different conversion processes. To establish the UO 2 pelletizing technology better to obtain such tightly quality controlled pellets, it is found necessary, apart from the quality requirements described in various specifications, to determine the particle morphology, the pore size distribution in particular, with regard to the compaction behaviour, which strongly affects the densification during sintering. Pore structures of different UO 2 powders converted via the AUC and ADU processes and the evolution of pore size distribution in the compacted pellets from these different UO 2 powders are analysed and compared by mercury porosimetry and microstructure observations of compacted pellets by scanning electron microscopy. Further, the connection of the evolution of pores in compacted pellets to the attainable densities of sintered pellets is discussed for the optimization of process parameters and as a proposal for quality control requirements and methods.

Pore size distribution of portland cement slurries at very early stages of hydration (influence of curing temperature and pressure)

Pore size distribution of Portland cement pastes has been studied using helium picnometry and mercury porosimetry. Cement samples were hydrated under varying conditions of temperature and pressure and were investigated at very early stages of hydration : thickening, setting, and early hardening. The evolution of pore size distribution with time has been related to physical and chemical properties (compressive strength, shrinkage, and combined water). The interpretation has been based on the repartition between free pores of tubular shape and trapped pores of rounded shape, and a model is proposed for describing cement pore size distribution.