Restricted diffusion and exchange of water in porous media: average structure determination and size distribution resolved from the effect of local field gradients on the proton NMR spectrum

NMR Pulsed field gradient measurements of the restrained diffusion of confined fluids constitute an efficient method to probe the local geometry in porous media. In most practical cases, the diffusion decay, when limited to its principal part, can be considered as Gaussian leading to an apparent diffusion coefficient. The evolution of the latter as a function of the diffusion interval yields average information on the surface/volume ratio of porosities and on the tortuosity of the network. In this paper, we investigate porous model systems of packed spheres (polystyrene and glass) with known mean diameter and polydispersity, and, in addition, a real porous polystyrene material. Applying an Inverse Laplace Transformation in the second dimension reveals an evolution of the apparent diffusion coefficient as a function of the resonance frequency. This evolution is related to a similar evolution of the transverse relaxation time T2. These results clearly show that each resonance frequency in the water proton spectrum corresponds to a particular magnetic environment produced by a given pore geometry in the porous media. This is due to the presence of local field gradients induced by magnetic susceptibility differences at the liquid/solid interface and to slow exchange rates between different pores as compared to the frequency differences in the spectrum. This interpretation is nicely confirmed by a series of two-dimensional exchange experiments.     

X波段脉冲压缩装置的数值模拟及优化设计

 利用有限元方法，通过调节X波段脉冲压缩装置的腔体长度、输入膜片宽度、输出耦合口位置等参量，研究了这些参量对谐振频率和品质因数的影响关系。研究表明：通过微调腔体长度并优化结构参数，可以将该装置的工作频率限制在规定的范围内；输入膜片缝隙由宽变窄的过程中，驻波比由大变小，达到临界耦合后，驻波比又由小变大，且谐振频率略有升高；品质因数与H-T分支的T形输出口的微波泄露密切相关，而该输出口泄露的大小随短路面到H-T分支中线的距离周期性变化。模拟得到的膜片距输出口中线长度为1 133.75 mm，分支中线到短路面的长度为95.25 mm，输入膜片宽9 mm，本征频率为9.387 2 GHz，输入耦合系数为1.15，品质因数为8 000。根据数值模拟结果，优化设计了一套脉冲压缩装置，实测得到的工作频率为9.388 4 GHz，输入耦合系数为1.03，品质因数为5 500。由于该装置输出口微波泄露较大，因此实测的品质因数比模拟的要小一些，这一点可以通过改进终端滑动短路面的调节方式来改善。     

Dielectric constant of porous ultra low-k dielectrics by fractal-Monte Carlo simulations

The effective dielectric constant of porous ultra low-k dielectrics is simulated by applying the fractal geometry and Monte Carlo technique in this work. Based on the fractal character of pore size distribution in porous media, the probability models for pore diameter and for effective dielectric constant are derived. The proposed model for the effective dielectric constant is expressed as a function of the dielectric coefficient of base medium and the volume fractions of pores and base medium, fractal dimension for pores, the pore size, as well as random number. The Monte Carlo simulations combined with the fractal geometry are performed. The predictions by the present simulations are shown in good accord with the available experimental data. The proposed technique may have the potential in analyzing other properties such as electrical conductivity and thermal conductivity in porous ultra low-k dielectrics.     

The internal magnetic field distribution, and single exponential magnetic resonance free induction decay, in rocks

When fluid saturated porous media are subjected to an applied uniform magnetic field, an internal magnetic field, inside the pore space, is induced due to magnetic susceptibility differences between the pore-filling fluid and the solid matrix. The microscopic distribution of the internal magnetic field, and its gradients, was simulated based on the thin-section pore structure of a sedimentary rock. The simulation results were verified experimentally. We show that the 'decay due to diffusion in internal field' magnetic resonance technique may be applied to measure the pore size distribution in partially saturated porous media. For the first time, we have observed that the internal magnetic field and its gradients in porous rocks have a Lorentzian distribution, with an average gradient value of zero. The Lorentzian distribution of internal magnetic field arises from the large susceptibility contrast and an intrinsic disordered pore structure in these porous media. We confirm that the single exponential magnetic resonance free induction decay commonly observed in fluid saturated porous media arises from a Lorentzian internal field distribution. A linear relationship between the magnetic resonance linewidth, and the product of the susceptibility difference in the porous media and the applied magnetic field, is observed through simulation and experiment.     

氢化多孔石墨烯反渗透特性及机理分析

单层石墨烯凭借超薄的厚度和优异的力学化学防污性能,成为新一代纳滤膜材料的最佳选择之一.本文采用经典分子动力学方法,研究了氢化多孔石墨烯反渗透膜对盐水的反渗透特性.结果表明,水渗透量会随着驱动力、孔径和温度的增加而增加;而孔径大于水合半径的条件下,盐离子截留率会随驱动力和温度的增加而降低.当反渗透膜和盐水存在切向运动时,随着切向速度的增加可以有效提高盐离子截留率和减弱浓差极化现象,但也在一定程度上牺牲了水通量.通过分析水流沿渗透方向的能障分布、水分子的氢键分布和离子水合状态,解释了各参数变化对盐水在氢化多孔石墨烯中反渗透特性的影响机理.研究结果将提供基于单层多孔石墨烯反渗透特性的理论认识,并将为纳米级反渗透膜的设计提供帮助.     

Damping caused by microplasticity in porous 316L steels

Damping in porous structures is enhanced by localized stresses in comparison with corresponding dense materials. An analytical model for mechanical damping in porous materials is suggested for the ‘microplastic’ range of deformation on the basis of the statistical mechanics of micro-heterogeneous materials. Two types of heterogeneity are considered: one-level porosity (directly sintered powder steels) and two-level porosity (larger pores in a carcass with smaller pores) on one hand, and fatigue-induced cracks on the other. Special emphasis is placed on the crack-related contribution to damping using a Dugdale approach to crack-induced microplasticity.     

Porous media characterization by PFG and IMFG NMR

Fully and partially filled with tridecane quartz sand was studied by different NMR techniques. The set of NMR experiments was carried out to obtain information about porous media geometry and fluid localization in it in case of partially filled porous space. The study was done using three NMR approaches: pulse field gradient NMR (PFG NMR), DDif experiment and tau-scanning experiment. The possibility to use all three approaches to study porous media properties even at the high resonance frequency is shown together with complementarity of the given by them information. Thus, first two approaches give information about porous sizes and geometry, at the same time tau-scanning experiment allows us to obtain information about distribution of internal magnetic field gradients in the porous space and draw conclusions about fluid localization in it.     

Influence of elastic strains on the adsorption process in porous materials. Thermodynamics and experiment

If we disregard the shape of the boundary hysteresis loop, H1 for SBA-15, MCM-41 and KIT-6, H2 for p⁺-type porous silicon and porous glass, the hysteretic features inside the loop are qualitatively the same for all these systems and show that none of them are composed of independent pores whether the pores are interconnected or not. We hence believe that the physical parameter which couples the pores is not the interconnectivity but the elastic deformation of the porous matrix. The thermodynamic approach we develop includes the elastic energy of the solid. We show that the variation of the surface free energy, which is proportional to the deformation of the porous matrix, is an important component of the total free energy. With porous silicon, we experimentally show that a stress external to the porous matrix exerted by the substrate on which it is supported significantly increases the total free energy and the adsorbed amount and decreases the condensation pressure compared to that of the same porous matrix detached from its substrate which is the relaxed state of the supported layer. This stress can be partly relaxed by making thicker porous layers due to the breaking of Si-Si bonds. This results in the shift of the isotherms towards that of the membrane. We propose a new interaction mechanism occurring through the pore wall elastic deformation in which the external mechanical stress is imposed on a given pore by its neighbours.     

Characterization of PSD of activated carbons by using slit and triangular pore geometries

A mixed geometry model for activated carbons, representing the porous space as a collection of an undetermined proportion of slit and triangular pores, is developed, evaluated theoretically and applied to the characterization of a controlled series of samples of activated carbon obtained from the same precursor material. A method is proposed for the determination of the Pore Size Distribution (PSD) for such a mixed geometry model, leading to the unique determination of the proportion of pores of the two geometries fitting adsorption data. By using the Grand Canonical Monte Carlo (GCMC) simulation method in the continuum space, families of N2 adsorption isotherms are generated both for slit and triangular geometry corresponding to different pore sizes. The problem of the uniqueness in the determination of the PSD by fitting an adsorption isotherm using the mixed geometry model is then discussed and the effects of the addition of triangular pores on the PSD are analyzed by performing a test where the adsorption isotherm corresponding to the known PSD is generated and used as the “experimental” isotherm. It is found that a pure slit geometry model would widen the PSD and shift it to smaller sizes, whereas a pure triangular geometry model would produce the opposite effect. The slit and triangular geometry families of isotherms are finally used to the fit experimental N₂ adsorption data corresponding to a family of activated carbons obtained from coconut shells through a one-step chemical activation process with phosphoric acid in air, allowing for the determination of the micropore volume, the proportion of slit and triangular pores and the PSD corresponding to the mixed geometry. The same experimental data were fit using both the conventional slit pore model and the mixed geometry model. From the analysis of the effect of different preparation procedures on the resulting PSDs, it is concluded that the proposed mixed geometry model may probably better capture the morphology and energetics of activated carbons prepared by chemical activation under mild temperatures.     

An NMR protocol for determining ice crystal size distributions during freezing and pore size distributions during freeze-drying

Nuclear magnetic resonance water proton relaxometry is widely used to investigate pore size distributions and pore connectivity in brine-saturated porous rocks and construction materials. In this paper we show that, by replacing water with acetone, a similar method can be used to probe the porous structure of freeze-dried starch gels and therefore the ice crystal size distribution in frozen starch gels. The method relies on the observation that the starch surface acts as a powerful relaxation sink for acetone proton transverse magnetization so that Brownstein-Tarr theory can be used to extract the pore size distribution from the relaxation data. In addition the relaxation time distribution is found to depend on the spectrometer frequency and the Carr-Purcell-Meiboom-Gill pulse spacing, consistent with the existence of large susceptibility-induced field gradients within the pores. The potential of this approach for noninvasively measuring ice crystal size distributions during freezing and pore size distributions during freeze-drying in other food systems is discussed.     

A facile method to determine pore size distribution in porous scaffold by using image processing

Image processing permits scientists to investigate morphological properties of three-dimensional structures starting from their bi-dimensional gray-scale representation. In many cases porous structure with complex architecture has to be designed in order to attempt specific properties such in the case of scaffold for tissue engineering.Traditional morphological characterization, like scanning electron microscopy, should be coupled with quantitative information such as pore size distribution (PSD) in order to get a deeper understanding of the influence of the porous structure on tissue regeneration processes and on other related applications, it is remarkable to study a quantitative analysis of porosity and of pores dimension.In this work it was developed as a software able to accomplish the segmentation of images containing pores of any geometry in a semi-automatic way with the aim to measure the PSD. Case study constituted by PLA porous scaffolds with different pore size was adopted. Results indicate that image processing methods well fit the pore size features of PLA scaffolds, overcoming the limits of the more invasive porosimetry techniques.     

Microscopic damping mechanism of micro-porous metal films

Metal thin films are used widely to solve the vibration problem. However, damping mechanism is still not clear, which limits the further improvement of the damping properties for film and the development of multi-functional damping coating. In this paper, Damping microscopic mechanism of porous metal films was investigated at both macroscopically and microscopically mixed levels. Molecular dynamics simulation method was used to model and simulate the loading-unloading numerical experiment on the micro-pore and vacancy model to get the stress-strain curve and the microstructure diagram of different defects. And damping factor was calculated by the stress-strain curve. The results show that dislocations and new vacancies appear in the micro-pores when metal film is stretched. The energetic consumption from the motion of dislocation is the main reason for the damping properties of materials. Micro-mechanism of damping properties is discussed with the results of in-situ experiment.     

Magnetic resonance imaging by synergistic diffusion-diffraction patterns

Inferring on the geometry of an object from its frequency spectrum is highly appealing since the object could then be imaged noninvasively or from a distance (as famously put by Kac, "can one hear the shape of a drum?"). In nuclear magnetic resonance of porous systems, the shape of the drum is represented by the pore density function that bears all the information on the collective pore microstructure. So far, conventional magnetic resonance imaging (MRI) could only detect the pore autocorrelation function, which inherently obscures fine details on the pore structure. Here, for the first time, we report on a unique imaging mechanism arising from synergistic diffusion-diffractions that directly yields the pore density function. This mechanism offers substantially higher spatial resolution compared to conventional MRI while retaining all fine details on the collective pore morphology. Thus, using these unique synergistic diffusion-diffractions, the "shape of the drum" can be inferred.     

Well-designed rectangular cavity resonator for FMR experiment

The unloaded quality factor of the cavity resonator is the ratio between the stored energy of the cavity resonator to the power loses in the cavity resonator. The homemade rectangular cavity resonator in X-band shows higher unloaded quality factor compare with standard cavity resonator in the TE₁₀₂ mode. Because the inner walls of rectangular cavity resonator are treated through high quality polishing and high purity Au plating. Also the inner walls are made by printed circuit board which has thin Cu foil, two problems such as mechanical vibration and thermal expansion can be solved by minimizing unwanted eddy current. Through the ferromagnetic resonance measurement by using our rectangular cavity resonator, we can be obtained reasonable values of resonance frequency and linewidth by using NiFe thin film. As a result, the Gilbert damping constant from the experimental result is in good agreement with the typical value of damping parameter of the NiFe thin film.     

A second,low-frequency mode of vibration in the intact mammalian cochlea

The mammalian cochlea is a structure comprising a number of components connected by elastic elements. A mechanical system of this kind is expected to have multiple normal modes of oscillation and associated resonances. The guinea pig cochlear mechanics was probed using distortion components generated in the cochlea close to the place of overlap between two tones presented simultaneously. Otoacoustic emissions at frequencies of the distortion components were recorded in the ear canal. The phase behavior of the emissions reveals the presence of a nonlinear resonance at a frequency about a half octave below that of the high-frequency primary tone. The location of the resonance is level dependent and the resonance shifts to lower frequencies with increasing stimulus intensity. This resonance is thought to be associated with the tectorial membrane. The resonance tends to minimize input to the cochlear receptor cells at frequencies below the high-frequency primary and increases the dynamic load to the stereocilia of the receptor cells at the primary frequency when the tectorial membrane and reticular lamina move in counterphase.     

Molecular mobility and transport in polymer membranes and polyelectrolyte multilayers

Polyelectrolyte multilayers prepared by the layer-by-layer technique provide an efficient way to generate planar structures of tailored surface charge and hydrophobicity, which are used as membranes for pervaporation. The use of polyelectrolyte multilayers to form the membrane permits tailoring the surface charge of the membrane and, thus, selectivity; at the same time, it reduces fouling of the membrane by adsorption of organic matter. Pulsed field gradient (PFG) nuclear magnetic resonance has been used to investigate the diffusion of probe molecules into polymer systems. Evaluation of the apparent diffusion coefficient in porous poly(amide) results in a pore size of 4 microm, as found in electron micrographs. For the pore size obtained for polyelectrolyte multilayers, no equivalent pores could be found in microscopy. Propagators for the diffusion of propanol and propanol-water mixture into multilayers reveal that there might be selective interaction of probe molecules with the polyelectrolyte system.     

Analysis of the alterations in porosity features of some natural stones due to thermal effect

Three types of commercially available carbonate rocks were used in the study to determine the effect of thermal treatment in the range from 100 °C to 500 °C on porosity features in terms of two different approaches such as pore shape factor and quality index values. The ratio of the ultrasonic velocity measurements before and after water saturation was used to differentiate porosity of pores from porosity of cracks under varying temperatures. It was found that, pores in Burdur Beige and Usak White are in the form of cracks, which are situated through inner structure. On the other hand, pores in Patara Limestone are in the form of porosity with lower pore shape factor values. Quality index calculation is another approach based on the comparison of the measured and theoretical ultrasonic velocity values. When the rocks were subjected to higher temperatures, internal stress was developed, crack lengths and numbers were increased and finally the higher pore shape factor and lower quality index values were obtained. This situation was proven by the higher water absorption values for all the stone types with the higher pore shape factor and lower quality index values depend on the noticeable increase in effective porosity values.     

Ferromagnetic resonance and magnetization in permalloy films with nanostructured antidot arrays of variable size

We present the results of structural and magnetic characterization of Permalloy (Fe₂₀Ni₈₀) films deposited by sputtering on self-organized nanoporous alumina previously treated with phosphoric acid to vary the pore diameters. SEM and AFM images of the top film surfaces show a triangular array of pores with diameters similar to the untreated porous alumina. However, the underlying pore enlargement is evidenced by the magnetic study. Indeed magnetization measurements reveal a decrease in the easy-plane anisotropy energy with elapsing time of chemical etching. Consistent with these results, ferromagnetic resonance measurements perpendicular to the film indicate a systematic reduction of the resonance field which can be directly related to an increase in the pore diameter. The effect of lateral confinement (due to the pores) in the spin-wave resonance is evidenced in multiple absorption lines when the applied field is in the film plane and perpendicular to it. This contrasts with the results for the continuous test film.