Characterization of Pyrogenic Powders with Conventional Particle Sizing Technique: I. Prediction of Measured Size Distributions

Pyrogenic powders consist of fractal like aggregates with nanosized primary particles. The formation of such aggregates, their hydrodynamic behavior and their optical properties are in principle well understood. Even so, there is only little experience in interpreting results from particle sizing of such materials. Dramatic differences in size distribution obtained from different measurement techniques give frequently rise to confusion on the “true” aggregate size. However, such differences can be attributed to the different particle properties used for size measurement and to the different types of quantities, by which the frequency of the individual size fractions are weighted. For two conventional sizing techniques, Dynamic Light Scattering and Optical Centrifugation Analysis, the influence of the structural properties on the relevant optical and hydrodynamic aggregate properties is discussed on the basis of virtual aggregates as well as of empirical data for pyrogenic powders. Finally measurable size distributions are predicted in a case study.     

Xenon porometry: a novel method for the derivation of pore size distributions

Xenon porometry is a novel method used for characterizing porous materials by the (129)Xe nuclear magnetic resonance of xenon gas. With the method, the diffusion of gas is slowed down by immersing the material in a medium, which can be in liquid or solid state during measurements. Because of slow diffusion, the signal of a xenon atom is characteristic of the properties of only one pore, and the composite signal of all atoms represents the distribution of properties. The method is especially applicable for determining pore size distribution because the chemical shifts of two different xenon signals (one from liquid and the other from gas pockets in solid) are dependent on pore size. Therefore, the shapes of these signals represent pore size distribution function. In addition, the porosity of the material can be determined by comparing the intensities of two signals. This article focuses on describing xenon signals observed from gas pockets in a solid medium, which has turned out to be most convenient for pore size determination.     

Apparent Electrical Conductivity of Porous Titanium Prepared by the Powder Metallurgy Method

Porous titanium is produced by thepowder metallurgy method. Dependence of the electrical conductivity on the porosity and pore size is investigated and the experimental results are compared with a number of models. It is found that the minimum solid area model could be successfully applied to describe the relationship between the electrical conductivity and the porosity of porous titanium. This kind of conductivity increases with increasing pore sizes.     

Porosity depth profiling of spin-coated silica thin films produced by different precursors sols

The structural evolution of nanoporous silica thin films was studied by Doppler broadening spectroscopy (DBS), 2-3 gamma ratio of positronium (3γ-PAS) and Fourier transform infrared spectroscopy (FT-IR). Four series of silica films with thickness in the 300-600 nm range were deposited by spin coating on Si substrate changing the content of sacrificial porogen in the sol precursors. The effect on the porosity of different amount of porogen and of the thermal treatments in the 400-900 °C temperature range have been highlighted. The evolution of the porosity is discussed considering the removal of porogen and of the silanol Si-OH groups by thermal treatments as pointed out by FT-IR. Pores with size from less than 1 nm up to sizes larger than 2.0 nm have been detected. In samples with maximum porogen load oPs escaping was observed indicating onset of connected porosity. At temperatures higher than 700 °C a decrease of the porosity due to a progressive pore collapsing was evidenced. A strong correlation was found between the shift of the Si-O-Si transversal optical (TO₃) mode in the FT-IR spectra and the pore size in the porous silica films as revealed by DBS and 3γ-PAS.     

Influence of mesoporous substrate morphology on the structural, optical and electrical properties of RF sputtered ZnO layer deposited over porous silicon nanostructure

Morphological, optical and transport characteristics of the RF sputtered zinc oxide (ZnO) thin films over the mesoporous silicon (PS) substrates have been studied. Effect of substrate porosity on the grain growth and transport properties of ZnO has been analyzed. Physical and optical properties of ZnO-PS structures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. Our experimental results indicate that on changing porosity of the PS substrates, regularity of the spatial distribution of the ZnO nanocrystallites can be controlled. While the morphology and grain size of ZnO depended strongly on the morphology and pore size of the PS substrates, the rectifying factors of the metal semiconductor junction were found to be different by a factor of 3. The deposition of semiconducting oxides on such mesoporous substrates/templates offers the possibility to control their properties and amplify their sensing response.     

SANS investigation on evolution of pore morphology for varying sintering time in porous ceria

Precipitates of ceria were synthesized by homogeneous precipitation method using cerium nitrate and hexamethylenetetramine at 80°C. The precipitates were ground to fine particles of average size ∼0.7 μm. Circular disks with 10 mm diameter, 2 and 3 mm thickness were prepared from the green compacts by sintering at 1300° C for three different sintering times. Evolution of the pore structures in these specimens with sintering time was investigated by small-angle neutron scattering (SANS). The results show that the peak of the pore size distribution shifts towards the larger size with increasing sintering time although the extent of porosity decreases. This indicates that finer pores are eliminated from the system at a faster rate than the coarser ones as sintering proceeds and some of the finer pores coalesce to form bigger ones.     

Comparison of two modeling approaches for highly heterogeneous porous media

The purpose of this paper is to study the acoustic behavior of highly heterogeneous, low density porous structures having a complex pore size distribution using two distinct theoretical approaches. The first approach requires the direct numerical integration of the Biot viscosity correction function. The main requirement here is a knowledge of the probability density function of the pore size, which can be achieved by an optical pore-counting technique. The fact that the observed pore size distribution in these materials could be distinctively split into two parts suggested the use of the second approach based upon the double-porosity theory by Olny and Boutin [J. Acoust. Soc. Am. 114(1), 73-89 (2003)]. The latter approach assumes a low permeability contrast between the two porous scales so that the acoustic properties could be estimated using the semi-phenomenological models of Johnson and Lafarge for the viscous and thermal dynamic permeabilities. Numerical results predicted by the two models are then compared with impedance tube experimental data showing good accuracy of the selected prediction methods.     

Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type porous silicon (PS) are mainly attributed to some morphological changes related to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of the PS layers. It has been found that when PS is formed in pure aqueous HF solution, it can exhibit a non-conventional and reproducible “porosity – PL peak relationship”. By correlating the PL spectral behaviour and PS morphology throughout a quantum-confinement model, we explain both conventional and non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibit the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throughout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.     

气孔对透明陶瓷激光输出性能的影响

定量分析气孔大小和气孔率对陶瓷激光输出性能的影响,建立了气孔分布模型并结合Mie散射和固体激光技术相关理论,讨论气孔散射对陶瓷激光透过率和输出性能的影响。结果表明,气孔率对激光陶瓷的透过率和斜率效率有着决定性的影响。在给定的气孔尺寸分布下,气孔率越高,陶瓷透过率和激光斜率效率越低;在给定的气孔率下,气孔中心尺寸越大,陶瓷样品的透过率和激光斜率效率越低。减小气孔尺寸至0.3 m以下能有效提高激光斜率效率。气孔越大,激光阈值随气孔率增大而上升越快,越不利于实现激光输出。在实际工作中,改进和控制工艺减小气孔尺寸对提高陶瓷的输出性能同样有着重要意义。     

Computer simulation to the porosity

Abstract

Computer simulations to the porosity of starting materials for the synthesis of polycrystalline diamond (PCD) bodies are described. The logarithmic normal distributed spheres were deposited in three dimensions by means of random coordinates. Conditions for distances between the particles have to be satisfied. Thus one can realize a defined overlapping, contact or distance. Results were obtained for the porosity dependence as a function of standard deviation of the size distribution and particle overlapping. The comparison of calculated and measured porosity under normal pressure showed good agreement. The filling of pores yielded the estimation of pore size distribution and a clear reduction of the pore volume.     

Study of the microporosity in nanoporous carbon films by small-angle X-ray scattering

The structural difference in the microporous structures of nanoporous carbon films is revealed by small-angle X-ray scattering; it consists in a higher porosity of the layers formed from the titanium carbide. The pore shape is shown to be equiaxed. Pores 20 Å in diameter mainly contribute to the porosity of the nanoporous carbon films. The characteristics of the porous structure of the nanoporous carbon layers synthesized from the titanium or silicon carbide are found using small-angle X-ray scattering. The porous structure is shown to consist of two size fractions of equiaxed pores 10 and 40 Å in radius. The porosity of the films is mainly contributed by the pores of the small size fraction; their fraction is 46 or 10% for the layers synthesized from the titanium or silicon carbide, respectively.     

Anodized aluminum oxide membranes of tunable porosity with platinum nanoscale-coating for photonic application

We fabricate anodized aluminum oxide (AAO) membranes with platinum (Pt) coating of various thickness ranging from 0 nm to 12 nm. We also demonstrate the tunability of membrane porosity, to add a degree of freedom in addition to the Pt coating, for optical modulation that can be used in photonic application. The reflectance measurement and its analysis are provided based on Fabry–Perot etalon optics that takes into account spectral dependence of optical absorption and loss in the AAO membrane. It is revealed that the frequency-dependent optical finesse (color purity) enhances nonlinearly with the Pt thickness, by the quantitative estimation, while the effective refractive index is substantially governed by the membrane porosity. These features can allow for the AAO membrane to be used as a dye-free color displaying film with efficient tunability of both displaying color and its color purity. In particular, the strong dependence of an effective refractive index on porosity is demonstrated such that the AAO membrane can have the effective refractive index down to about 1.2, making it a possible candidate for a single layer anti-reflection coating material deposited on optical components including low-index glass. We also fabricate, by facile dipping method, an AAO membrane whose pore size changes gradually along a direction on its surface. This intra-membrane modulation of porosity allows it to find application in an optical grating or band-pass filter whose transmission wavelength varies with position on the device surface.     

Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging.

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.     

Characterization of multi-scale porous structure of fly ash/phosphate geopolymer hollow sphere structures: From submillimeter to nano-scale

In the present work, the porous structure of fly ash/phosphate geopolymer hollow sphere structures (FPGHSS), prepared by pre-bonding and curing technology, has been characterized by multi-resolution methods from sub-millimeter to nano-scale. Micro-CT and confocal microscopy could provide the macroscopic distribution of porous structure on sub-millimeter scale, and hollow fly ashes with sphere shape and several sub-millimeter open cells with irregular shape were identified. SEM is more suitable to illustrate the distribution of micro-sized open and closed cells, and it was found that the open cells of FPGHSS were mainly formed in the interstitial porosity between fly ashes. Mercury porosimeter measurement showed that the micro-sized open cell of FPGHSS demonstrated a normal/bimodal distribution, and the peaks of pore size distribution were mainly around 100 and 10 μm. TEM observation revealed that the phosphate geopolymer was mainly composed of the porous area with nano-pores and dense areas, which were amorphous Al–O–P phase and α-Al₂O₃ respectively. The pore size of nano-pores demonstrated a quasi-normal distribution from about 10 to 100 nm. Therefore, detailed information of the porous structure of FPGHSS could be revealed using multiple methods.     

Resistive Pulse Sensing as a High‐Resolution Nanoparticle Sizing Method: A Comparative Study

One of the main challenges of sizing methods for nanoparticle (NP) suspensions is to distinguish between particles and particle populations with very small size differences. This would be especially important to follow various surface functionalization processes of nanoparticles resulting in small alterations of their size. In this respect, methods involving the detection of single particles, such as resistive pulse sensing (RPS) or nanoparticle tracking analysis, are generally considered superior to ensemble measuring methods such as dynamic light scattering. However, to compare the exact capabilities of these methodologies require systematic investigations in optimized conditions for each method. Here, such a study is presented for a narrow size range of spherical latex nanoparticles (60–200 nm). It is concluded that the RPS methodology based on quartz nanopipets as single nanopore counters, is the only sizing method among those studied capable to fully resolve a ternary mixture of 70, 110, and 140 nm average diameter NPs. The practical usefulness of this size resolution is demonstrated by following the increase in diameter of latex nanoparticles after their surface modification with antibodies.     

Pore matrices prepared at supercritical temperature by computer simulations: matrix characterization and studies of diffusion coefficients of adsorbed fluids

We present series of molecular dynamics simulations to study the structure of different porous matrix configurations. The present simulations are an extension of recently reported data at a temperature below the critical. Here, we show how temperature modifies the structure and porosity of pore matrices during their preparation in comparison with the previous work. Moreover, in these studies at a higher temperature, we studied in more detail the structure of the porous matrix. Matrices were prepared by two different processes. The first method consisted of a single Lennard-Jones fluid simulated at a fixed density and at a supercritical temperature. Then, the matrix configuration was taken from the last configuration of the fluid. The second method was prepared from a binary mixture, where one of the components served as a template material. The final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at two different densities and at different matrix particle interactions. The volume distribution, the cluster formation and the connectivity between the particles in the pore matrix were investigated. The importance of using template particles was clear since they produced larger voids and higher porosities. On the other hand, the temperature of preparation seems to modify the size of the voids and the porosity in the matrices. For instance, at this high temperature, the matrix porosity is higher when template particles are present in the system. These results point in the opposite direction compared to those found in a previous paper at a lower temperature. The diffusion of fluids immersed in the different matrices was also analysed by calculating the mean square displacements of their particles. It was observed that this quantity was higher when matrices were prepared with template particles. These results also point to different directions in contrast with pore matrices prepared at a lower temperature. Finally, the results show that temperature plays an important role in the pore matrix formation.     

Effect of the internal microstructure in rapid-prototyped polycaprolactone scaffolds on physical and cellular properties for bone tissue regeneration

Biomedical scaffolds should be designed to optimize their inter-microstructure to enable cell infiltration and nutrient/waste transport. To acquire these properties, several structural parameters, such as pore size, pore shape, porosity, pore interconnectivity, permeability, and tortuosity are required. In this study, we explored the effect of tortuosity on the viable cell proliferation and mineralization of osteoblast-like-cells (MG63) in polycaprolactone scaffolds. For analysis, we designed four different scaffolds of various tortuosities ranging from 1.0 to 1.3 under the same porosity (56?%) and 100?% pore interconnectivity. The pore size of the scaffolds was set as 150 and 300?μm, and a mixture of these sizes. We found that despite the porosity being same, the elastic modulus was dependent on the pore size of the scaffolds due to the distributed stress concentration. In addition, the relative water movement within scaffolds was also related to the internal microstructure. Cell viability and Ca²⁺ deposition of the cell-seeded scaffolds showed that the proliferation of viable cells and mineralization in the scaffolds with appropriate tortuosity (1.2) was relatively high compared to those of the scaffolds displaying low (1.05 and 1.1) or high (1.3) tortuosity. Our findings indicated that the internal microstructure of the scaffolds may influence not only the physical properties, but in addition the cellular behavior.     

Effects of sediment physical properties on the phosphorus release in aquatic environment

Particle size, porosity, and the initial phosphorus concentration in sediments are the main factors affecting phosphorus release flux through the sediment-water interface. Sediments can be physically divided to muddy and sandy matters, and the adsorption-desorption capacity of sediment with phosphorus depends on particle size. According to phosphorus adsorption-desorption experiments, phosphorus sorption capacity of the sediment decreases with the increase of particle dimension. But among the size-similar particles, sediment with a bigger particle size has the larger initial phosphorus release rate. In terms of muddy and sandy sediments, there are inversely proportional relationships between the release rate and the flux. Due to the contact of surface sediment and the overlying water, the release flux from the sediment is either from direct desorption of surface sediment layer or from the diffusion of pore water in the sediment layer, which is mainly determined by sediment particle size and porosity. Generally, static phosphorus release process may include two stages: the first is the initial release. As for coarse particles, phosphorus is desorbed from surface sediment. And for fine particles, phosphorus concentration in water often decreases, mainly from pore water by the molecular diffusion. During the second stage, pore water flows faster in coarse sediment, and phosphorus is easy to desorb from the surface of the particles as diffusion dominates. For the smaller liquid-solid ratio of fine particles and the larger amount of phosphorus adsorption, the release flux from pore water due to diffusion is very small with longer sorption duration.     

Mechanical resonance properties of porous graphene membrane; simulation study and proof of concept experiment

Mechanical resonance properties of porous graphene resonators were investigated by simulation studies. The finite element method was utilized to design the porous graphene membrane pattern and to calculate the mechanical resonance frequency and quality factor. The changes in the resonance frequency and quality factor were systematically studied by changing the size, number, and relative location of pores on the graphene membrane. Mass loss and carbon-carbon bond break were found to be the main competing parameters for determining its mechanical resonance properties. The correlation between the geometry and the damping effect on the mechanical resonance of graphene was considered by suggesting a model on the damping factor and by calculating the membrane deflections according to the pore location. Based on the simulation results, an optimal porosity and porous geometry were found that gives the maximum resonance frequency and quality factor. Suspended graphene with various number pore structures was experimentally realized, and their mechanical resonance behaviors were measured. The trend of changes in resonance frequency and quality factor according to the number of pores in the experiment was qualitatively agreed with simulation results.