Smooth Pore Research Articles

Mechanical and dielectric properties of porous Si 3N 4–SiO 2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si 3N 4–SiO 2 composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si 3N 4–SiO 2 composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m 1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10 −3 at a resonant frequency of 14 GHz.

Normal and anomalous diffusion of non-interacting particles in linear nanopores

The diffusion of gas molecules in pores is determined by the collisions between the molecules as well as by the collisions of the molecules with the pore walls. In many applications the so-called Knudsen regime is of particular interest. In this regime the collisions of the molecules with the pore walls play the crucial role, while the inter-molecular collisions can be neglected. Here we study the influence of surface roughness on the coefficients of self (or tracer) diffusion and transport diffusion. Considering the first four iterations of a generalised fractal Koch surface, we construct pore models of different roughness. For these model pores we have performed detailed simulations of both diffusion coefficients using a cube-based algorithm. The molecular trajectories can be mapped onto L´evy walks to determine the diffusion properties. In linear two-dimensional (2d) channels we observe anomalous diffusion, which can also be induced in smooth and rough three-dimensional (3d) pores by anomalous reflection laws. Normal diffusion is found in convoluted 2d pores and in all 3d pores when a diffuse reflection law is applied.

熱間等方加圧（ＨＩＰ）法による開気孔質多孔体の作製とその焼結メカニズム

Open-pore porous materials can be produced by the capsule-free Hot Isostatic Pressing (HIPing) process. In this paper, sintering mechanisms, features and applications of capsule-free HIPed open-porous materials are discussed. During sintering powder compacts by capsule-free HIPing process, high pressure gas penetrates inside of the green bodies, and forms pores. There are two mechanisms to explain this phenomenon. Surface-diffusion is enhanced during capsule-free HIPing, because high frequency gas-collision with pore surfaces, forms grown necks between raw powder particles, and enhances pore formation. Another possible mechanism is the matter of spatial compatibility of gas atoms and sintered material. For the shrinkage of pores, i. e., for the densification of the green bodies, the high pressure gas, or the high density gas must be removed from inside of pores. The higher density requires the higher energy to remove high density gas. At the end of the sintering process, the gas must be removed from pores to ambient pressure to make those pores to be open pores. The above mentioned sintering mechanisms provide higher open porosity, more grown necks, smoother pore surfaces and narrower pore size distribution in capsule-free HIPed materials compared with those in conventionally sintered porous materials. As a consequence, high Young's modulus, high fracture strength, high fluid permeability are obtained by the capsule-free HIP process. The porous materials fabricated by the capsule-free HIPing can be applied for filters, grinding wheels, electrochemical electrodes and others with better performance than conventional products.

Fast Mass Transport Through Carbon Nanotube Membranes

The May 19, 2006 issue of Science included a paper by Holt et al. on "Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes". The paper was also featured on the cover, showing methane molecules translating inside a carbon nanotube (CNT). The authors explained how they prepared 2-6-mum thin membranes consisting of double-walled carbon nanotubes (DWNTs) all aligned perpendicular to the apparent membrane surface. These tubes are open at both ends and the space between the tubes is filled with dense Si(3)N(4). Pure gas and water fluxes were measured at room temperature with the application of a small pressure difference. Interpretation of the results led to the conclusion that the membranes showed much higher fluxes than what was estimated from Knudsen gas diffusion and Poiseuille viscous flow models. The membranes have a straight-channel morphology with a narrow pore-size distribution and exceptionally smooth pore walls. The unusual geometry and surface properties make it difficult to compare the membrane's properties with common membranes but there is no question that the mass transport in the aligned DWNTs is fast indeed. To appreciate how fast, we will consider their transport properties starting from the perspective of "conventional" porous membrane technology. Recent molecular dynamics simulations suggest that none of the classic models for gas (Knudsen) and water (Poiseuille) permeation work in a meaningful way for these nanotube membranes, and new models are needed.

Modeling of adsorption in pores with strongly heterogeneous walls: parametric lattice-site wall model

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by a parametric lattice-site approach. The sites are randomly distributed over the pore walls. They could be attractive, neutral or repulsive with respect to the smooth pore model. Each site is characterized by two amplitudes (structural and energetic) which modify locally the structure and energetic properties of the surface. The results presented here show how different parameters of the model affect the mechanism of adsorption and, consequently, the form of the isotherm.

Smoothening the Pores Walls in Macroporous n-Si

Our work reports on the etching of very smooth pores in n-Si with backside illumination. From the point of view of the so- called current burst model we explain the origin of the roughness on macropore walls. By trying different types of electrolytes at very low or moderate pH, electrolytes with very small dissolution rates of SiO2, or etching at very low temperatures, we show that the roughness of the walls can be controlled. Finally we propose to use a viscous electrolyte which lead to a roughness of only 9 nm on a 800 x 800 nm2 area which is a factor 5 smaller than the roughness observed for aqueous electrolytes.

Electrochemical Pore Etching in n- and p-Type Ge

Little was known about porous Ge until recently; here some substantial progress in producing porous Ge will be reported, mostly for the first time. i) n-type Ge in aqueous solution: Pore geometries, morphologies and growth peculiarities were found to be quite different from other semiconductors, such as Si and III-V. Nucleation is generally difficult, the preferred growth direction is <100> (and <111>), major stop planes are of the {110} type, but others are also found. In addition, there is always a strong electropolishing component compromising pore geometry and stability. ii) n- and p-type Ge in organic solution: In DMSO solution, the growth direction is <111>, and the stopping planes are still mainly {110}; somewhat unexpected, because this has never been observed in Si, and because no pores have been found in other p-type semiconductors so far, with the exception of Si. Nucleation seems to be difficult too, and new domain-forming phenomena are observed. Smooth or rough pore walls can be obtained, dependent on the experimental conditions.

Adsorption in cylindrical pores: Mixed lattice-site/off-site Monte Carlo simulations in pores with heterogeneous wall structure

We present results of grand canonical Monte Carlo simulations of adsorption in cylindrical pores with rough surface modeled by lattice-site approach. Each site is characterized by two parameters: structural and energetic, which locally modify the structure and energy properties of the surface. There are three types of sites, randomly distributed over the wall: attractive, neutral and repulsive with respect to the smooth pore model. The results presented here show how this model affects the mechanism of adsorption and how it changes the forms of adsorption isotherm. We compare our numerical results with the experimental data of adsorption of a simple fluid (CH 4, T = 77 K) in cylindrical silica pore of diameter d = 4 nm (MCM-41 material).

Pressurized 4He in Cylindrical and in Hexagonal Pores

We give a variational Monte Carlo description at T = 0 K of 4He filling under pressure a porous glass within the shadow wave function technique. We have considered as confining media two different smooth pores, one with a circular cross-section of radius R = 13 A resembling a Gelsil pore, and the other with a hexagonal cross–section of side S = 14 A resembling a FSM-16 pore. In all the studied cases the density profiles show a strong layering of the 4He atoms. As the density is increased, solidification takes place layer by layer, starting from the pore wall. Computing the one-body density matrix we are able to estimate the Bose–Einstein condensate fraction, which is still non–zero even when the whole system is in the solid phase.

Thick etch-through macroporous Si membrane from p- and n-Si, and fast pore etching and tuning the pore size from n-Si

A thick macroporous silicon (macro PS) membrane has been fabricated by electrolytic etching through either n- or p-type silicon wafers. The microscopic structure of the membrane has been investigated by scanning electron microscopy. Three different electrolytes were investigated: (i) an aqueous solution of hydro fluoric acid (HF) with ethanol (EtOH); (ii) an organic solution of HF with dimethyl sulfoxide (DMSO); (iii) an organic solution of HF with dimethyl formamide (DMF). The concentration and the applied current in the electrolytic cell during etching have been varied and been optimized with respect to etching rate and suitability of the resulting structure for different applications. For the p-type case etched with the organic electrolytes, smooth and straight pores were realized but only when the conditions are near optimized. Outside that parameter space, one can get too thin pore walls on one side of the wafer, for example by increasing the HF concentration, etching for excessive times or omitting to refresh the HF during etching. For the n-type case, a fast pore etching up to 10 μm/min was achieved for the first time for high HF concentration and a high current density. The pore size has been tuned over a large range by varying the Si doping concentration; the lower the doping, the larger the pore size and the faster the etch rate. The macropores in the n-type wafers were highly branched and the morphology could be varied for the electrolytes with different oxidizing agents. Suitability of the different membrane structures for specific applications is briefly discussed.

Small-angle neutron scattering investigation of the effect of a heat-treatment on the microstructure of porous basalt rocks

AbstractSmall-angle neutron scattering has been used to study the microstructure of natural porous basalt rocks. The effect of temperature on the rock microstructure has been investigated on ‘as received’ and heat-treated basalts. The magnitudes of α, the power-law scattering exponent were between 3 and 4 for the majority of the rocks, indicating a surface fractal structure between the basalt matrix and the pore space. Heat-treated basalts show higher α values, and therefore a smoother pore surface. Internal surface areas were determined for all basalts depending on the thermal history.

Model for dynamics of inhomogeneous and bulk fluids

An accurate model for the density of states (DOS) for strongly inhomogeneous and bulk fluids has been proposed based on gamma distributions. The contribution to the density of states from the collective dynamics is modeled as an incomplete gamma distribution and the high frequency region is obtained from the solution of the memory equation using a sech memory kernel. Using only the frequency moments as input, the model parameters for the collective dynamics are obtained by matching moments of the resulting distribution. The model results in an analytical expression for the self-diffusivity of the fluid. We present results for soft sphere fluids confined in slit-shaped pores as well as bulk soft sphere liquids. Comparisons of the DOS, velocity autocorrelation functions, and memory kernels with molecular dynamics simulations reveal that the model predicts features in the DOS over the entire frequency range and is able to capture changes in the DOS as a function of fluid density and temperature. As a result the predicted VACFs, memory kernels, and self-diffusivities are accurately predicted over a wide range of conditions. Since the frequency moments for bulk liquids can be obtained from pair correlation functions, our method provides a direct route from fluid structure to dynamics. For fluids confined in slit-shaped pores, where the frequency moments are obtained from molecular dynamics simulations, the predicted self-diffusivities capture the resulting oscillations due to variations in the solvation pressure, and in the case of smooth walled pores, the predictions are superior to those obtained using kinetic theory.

Influence of Adsorbate Interaction on Transport in Confined Spaces

This article provides a review of the recent theory of transport in nanopores developed in the author's laboratory. In particular the influence of fluid-solid interactions on the transport coefficient is examined, showing that such interactions reduce the value of the coefficient by almost an order of magnitude in comparison to the Knudsen theory for non-interacting systems. The activation energy and potential energy barriers for diffusion in smooth pores with a one-dimensional potential energy profile are also discussed, indicating the inadequacy of the commonly used assumption of proportionality between the activation energy and heat of adsorption or the minimum pore potential energy. A further feature affected by fluid-solid interactions is the nature of the reflection of fluid molecules colliding with a pore wall surface, varying from being nearly specular - such as in carbon nanotubes - to nearly diffuse for amorphous solids. Diffuse reflection leads to momentum loss and reduced transport coefficients. However, fluid-solid interactions do not affect the transport coefficient in the single-file diffusion regime when the surface reflection is diffuse, and the transport coefficient in this case is largely independent of the adsorbed density.

A Model for the Structure of MCM-41 Incorporating Surface Roughness

The surface area of MCM-41 mesoporous silica, estimated by several models in the literature, is significantly less than the value derived from BET analysis of nitrogen adsorption at 77.4 K. In the past, the difference has been attributed to several reasons including the errors involved in the BET analysis of the multilayer-capillary condensation region and the heterogeneity of the walls. In the present work, we present an alternate model of MCM-41 based on molecular simulations that gives surface area values that are in closer agreement to those determined by experiment. The model incorporates bulk heterogeneity of the material, surface hydroxyls, and most importantly, physical deformations or indentations of the pore surface. The model predicts small-angle X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) results that are consistent with experimental data as well as surface areas and pore volumes that compare favorably with published experimental results. The simulation results are consistent with the hypothesis that the interstitial space in MCM-41 is relatively amorphous despite the regular arrangement of the mesopores. The surface roughness associated with the amorphous structure increases the surface area beyond the nominal value produced by assuming smooth cylindrical pores.

Adsorbate Transport in Nanopores

We present a tractable theory of transport of simple fluids in cylindrical nanopores, considering trajectories of molecules between diffuse wall collisions at low-density, and including viscous flow contributions at higher densities. The model is validated through molecular dynamics simulations of supercritical methane transport, over a wide range of conditions. We find excellent agreement between model and simulation at low to medium densities. However, at high densities the model tends to over-predict the transport behaviour, due to a large decrease in surface slip that is not well represented by the model. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores.

Improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts

The application of microwave technology to a diverse range of materials and processes has resulted in a wide spectrum of materials that are commercially processed using microwaves, from the heating of food to the vulcanisation of rubber to the sintering of specialty ceramics. Microwave sintering of elemental or alloy metal powders has gained significance in recent times as a novel processing method since it offers many advantages over the conventional sintering method. Despite substantial R&D investment in this area in the past two decades, no competitive microwave technology has yet emerged for powder metallurgy (PM) sintering. In sharp contrast, because it is 'obvious' that microwaves are reflected by metals, it is not uncommon to be unable to locate many journal papers or literature, wherein metal powders have been sintered in a microwave field. This paper reports the improved mechanical properties and microstructural development of microwave sintered copper and nickel steel PM parts as compared with that obtained using conventional sintering technique.The paper describes the fabrication details of the FC-0208 and FN-0208 composition steel PM parts, and the in house modified commercial microwave oven used for sintering. Microwave sintering resulted in higher sintered density and improved mechanical properties for both Cu and Ni steel PM parts as compared with that processed using conventional sintering under identical conditions. The improved mechanical properties can be attributed primarily to more uniform distribution of the alloying elements, which resulted in greater material homogeneity at the nano- and microlevels as revealed by the Cu and Fe X-ray maps using high spatial resolution scanning transmission electron microscopy (STEM). The optical micrographs of both the etched and unetched samples clearly showed development of novel sintered microstructures having distinct characteristics for the porosity distributions: smooth and rounded pores with low stress concentration regions for microwave sintering as against sharp, triangular and wedge shaped pores with high stress concentration regions for conventional sintering.

Solvation force, structure and thermodynamics of fluids confined in geometrically rough pores.

The effect of periodic surface roughness on the behavior of confined soft sphere fluids is investigated using grand canonical Monte Carlo simulations. Rough pores are constructed by taking the prototypical slit-shaped pore and introducing unidirectional sinusoidal undulations on one wall. For the above geometry our study reveals that the solvation force response can be phase shifted in a controlled manner by varying the amplitude of roughness. At a fixed amplitude of roughness, a, the solvation force for pores with structured walls was relatively insensitive to the wavelength of the undulation, lambda for 2.3<lambda/sigma(ff)<7, where sigma(ff) is the Lennard-Jones diameter of the confined fluid. This was not the case for smooth walled pores, where the solvation force response was found to be sensitive to the wavelength, for lambda/sigma(ff)<7.0 and amplitudes of roughness, a/sigma(ff)>/=0.5. The predictions of the superposition approximation, where the solvation force response for the rough pores is deduced from the solvation force response of the slit-shaped pores, was in excellent agreement with simulation results for the structured pores and for lambda/sigma(ff)>/=7 in the case of smooth walled pores. Grand potential computations illustrate that interactions between the walls of the pore can alter the pore width corresponding to the thermodynamically stable state, with wall-wall interactions playing an important role at smaller pore widths and higher amplitudes of roughness.

Tractable molecular theory of transport of Lennard-Jones fluids in nanopores.

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed.

Grand canonical Monte Carlo simulation of argon adsorption at the surface of silica nanopores: effect of pore size, pore morphology, and surface roughness.

Argon adsorption (77 K) in atomistic silica nanopores of various sizes and shapes has been studied by means of grand canonical Monte Carlo simulations (GCMC). We discuss the effects of confinement (pore size), pore morphology (ellipsoidal, hexagonal, constricted pore), and surface texture (rough/smooth) on the thickness variation of the adsorbed film with pressure onto the disordered inner surface of porous materials (usually called t-plot or t-curve). We show that no confinement effect occurs when the diameter of the regular cylindrical pore is larger than 10 nm. For pores smaller than 6 nm, we find that the film thickness increases as the pore size decreases. We show that the adsorption isotherm in the rough pore can be described as the sum of an adsorbed amount similar to that found for a smooth pore (of the same radius) and a constant contribution due to atoms "trapped" in the infractuosities of the rough surface which act as a microporous texture. Simulation snapshots for Ar adsorption in hexagonal and ellipsoidal smooth pores indicate that at low pressures the gas/adsorbate interface retains memory of the pore shape and becomes cylindrical prior to the capillary condensation of the fluid in the pore. The film thickness in the hexagonal pore is close to that obtained for a cylindrical pore having a similar dimension. By contrast, we find that the film thickness for an ellipsoidal pore is always larger than that for an equivalent cylindrical pore (having the same length and volume but a circular section). We show that this effect strengthens as the pore size decreases and/or the pore asymmetry increases. Ar adsorption in a cylindrical constricted pore shows that the presence of the narrower part considerably modifies the adsorption mechanism. Finally, we report GCMC simulations of Ar adsorption (77 K) on a plane silica reference substrate for different intermolecular potentials. We discuss the effect of the interaction on the shape of the adsorption isotherm and compare our results with experiments.

Solid–solid transitions in slit nanopores

The structure of Lennard–Jones (LJ) fluids confined in smooth slit-like pores have been investigated using grand canonical Monte Carlo simulations for a bulk state point lying close to the liquid-solid freezing line. The structure of the fluid is examined by computing the in-plane pair correlation function and bond orientational order parameters. Simulations reveal that the fluid not only freezes into the pore but also undergoes a solid–solid transition from a square to a triangular lattice for small changes in pore widths. This transition occurs at pore widths that accommodate two and three fluid layers. Classification of solid structures based on the 3D counterparts, indicate that the transition occurs between solids that correspond to a body centred tetragonal structure to the hexagonal close packed structure. In the two layer structures the pore fluid consists of the first two basal planes of the bct and hcp structures and in the three layer system a complete unit cell of bct and hcp forms in the direction of confinement.