Configurational effects of templating on the adsorption isotherms of templated porous materials

The porosity and pore geometry of disordered materials can be influenced by employing a removable template during synthesis. A theoretical and simulation study is reported of the configurational effects of template size and density on the adsorption isotherms of templated porous materials. To isolate the configurational (entropic) contributions, the adsorbate, matrix and template components are modelled as hard spheres. The replica Ornstein-Zernike equations proposed by Zhang, L. and Van Tassel, P. R. (2000) J. chem Phys., 112, 3006 are used within the Percus-Yevick approximation to calculate adsorption isotherms for differently sized adsorbate and template components. These theoretical results are compared with results from Monte Carlo simulation. It is found that adsorption is most enhanced whenever the size of the template is equal to or slightly larger than that of the adsorbate. Also, for systems of constant matrix density or constant matrix plus template density, increasing the density of template enhances the adsorption.     

A molecular model of adsorption in a dilute semiflexible porous network

A Monte Carlo simulation study has been made of adsorption and desorption of a gas in a semiflexible porous network with a porosity of over 95%, which is comparable with that of silica aerogels. Although dilute, the network has substantial effects on the properties of confined fluids. Network flexibility is shown to make a significant difference to both the adsorption/desorption isotherms and the fluid phase coexistence when compared with the case where the network is rigid. For one of the systems studied the solid volume changes by as much as 30% during desorption. This is qualitatively similar to an effect seen in nitrogen adsorption/desorption in some silica aerogels.     

Acoustic enhancement of diffusion in a porous material

Lattice Boltzmann simulations are used to model the enhancement of diffusion which results from Eckart (attenuation driven) acoustic streaming in model porous material. Comparisons are made to Fickian diffusion where no flow is present and the diffusion when a fluid jet is used, which represents a more conventional method of enhancement. We show that streaming can produce a higher diffusion rate for the same average flow velocity and propose that this is the result of the continuation within the material of the driving force that produces the acoustic streaming.     

Analysis of ignition of a porous energetic material

A theory of ignition is presented to analyse the effect of porosity on the time to ignition of a semi-infinite porous energetic solid subjected to a constant energy flux. An asymptotic perturbation analysis, based on the smallness of the gas-to-solid density ratio and the largeness of the activation energy, is utilized to describe the inert and transition stages leading to thermal runaway. As in the classical study of a nonporous solid, the transition stage consists of three spatial regions in the limit of large activation energy: a thin reactive–diffusive layer adjacent to the exposed surface of the material where chemical effects are first felt, a somewhat thicker transient–diffusive zone and, finally, an inert region where the temperature field is still governed solely by conductive heat transfer. Solutions in each region are constructed at each order with respect to the density-ratio parameter and matched to one another using asymptotic matching principles. It is found that the effects of porosity provide a leading-order reduction in the time to ignition relative to that for the nonporous problem, arising from the reduced amount of solid material that must be heated and the difference in thermal conductivities of the solid and gaseous phases. A positive correction to the leading-order ignition-delay time, however, is provided by the convective flow of gas out of the solid, which stems from the effects of thermal expansion and removes energy from the system. The latter phenomenon is absent from the corresponding calculation for the nonporous problem and produces a number of modifications at the next order in the analysis arising from the relative transport effects associated with the gas flow.     

Theoretical simulation of electroacoustic borehole logging in a fluid-saturated porous formation

Electroacoustic (E-A) logging describes the acoustic response to an electromagnetic (EM) source in a fluid-filled borehole surrounded by a porous medium. The E-A response is simulated by two different methods in this paper. In the coupled method, the EM field and the acoustic field are modeled using Pride's model, which couples Maxwell's equations and Biot's equations. In the uncoupled method, the EM field is uninfluenced by the converted acoustic field, resulting in separate acoustic formulation with an electrokinetic source term derived from the primary EM field. The difference of the transient full waveforms between the above two methods is remarkably small for all examples, thus confirming the validity of using the computationally simpler uncoupled method. It is shown from the simulated waveforms that an EM-accompanying acoustic field is coupled to the EM field and appears with an apparent phase velocity of the EM wave in the formation. Acoustic waves with the conventional acoustic velocities are also seen in the converted full waveforms. For the sandstone models used in this paper, when permeability is less than 1 Darcy, the E-A Stoneley wave amplitude increases with porosity, which is different from that in conventional acoustic-to-acoustic logging.     

Sound absorption characteristics of a high-temperature sintering porous ceramic material

This article is dedicated to sound absorption properties of porous zeolite with macropores, a ceramic material fabricated by high-temperature sintering. Acoustical properties of this ceramic material are studied by two analytical models, Delany–Bazley model and Johnson–Allard model, where the latter one shows a better fit to the experimental results. Moreover increasing the thickness of samples would improve the sound absorption in the low frequency ranges. Raising the porosity could increase the highest sound absorption coefficient. The resonance frequencies of the materials with 3–5 mm particles are more obvious. Comparing with glass wool, porous zeolite has a better sound absorption.     

Determination of absolute adsorption in highly ordered porous media

Recently developed Metal Organic Frameworks (MOFs) are the materials with the highest intrinsic surface areas to date and their discovery increased the research activity in the field of microporous adsorption materials significantly. In this contribution, a generic method of analysis for volumetrically measured adsorption isotherms is presented that separates absolute adsorption from excess adsorption to the best possible degree by representing the absolute adsorption isotherm by a superposition of in respect to pressure strictly monotonously increasing fitting function. The procedure allows to determine the heat of adsorption at constant gas uptake via implicitly defined quantities. The method was applied to adsorption data of hydrogen on MOF-5 ranging from 40 K to 200 K. Methane adsorption on MOF-5 was used to demonstrate that the common practice of neglecting the difference between excess and absolute adsorption leads to erroneously increased heat of adsorption values at high coverages and temperatures.     

Molecular simulation of phase coexistence in adsorption in porous solids

In this work a recently proposed method, the gauge-cell Gibbs ensemble Monte Carlo, is extended to deal with polar substances. The behaviour of water, a hydrogen bonding, weakly adsorbing fluid, is compared with that of methane, a strongly adsorbing, non-polar fluid, in the vicinity of the phase transition. The mechanisms of condensation for the two species are seen to be significantly different in nature. A systematic study of the effect of the pore width on the phase equilibrium of water is also performed. Our results show that the narrowing of the pore shifts the equilibrium transition pressure to lower values and reduces the extent of vapour metastability, but exerts little influence on the stability of the liquid phase.     

含三聚氰胺多孔材料分层复合介质吸声特性*

三聚氰胺泡沫材料是一种具有高开孔率的多孔材料,具备优良的吸音、防火隔热及环保性能,可以作为吸声材料与弹性板、空腔介质形成复合结构,在建筑、航空、交通工具等工程领域有广泛的应用。该文基于Biot理论和分层介质在交界面处的不同边界条件,建立非均匀复合介质背衬刚性壁面结构的理论声学模型,详细分析了多孔材料布局对复合结构吸声特性的影响。该文理论模型计算的结果与阻抗实验得到的垂直入射吸声系数基本一致,验证了理论模型的正确性。结果表明:在多孔材料前面增加空气层可以改善高频吸声特性;在多孔材料后面增加空气层可以改善复合结构低频吸声特性。通过合理配置多孔材料,可以在应用需求频段上达到满意的吸声效果。     

Stochastic resonance in a realistic model for surface adsorption

We study a model for a monolayer single adsorbate system used to describe pattern formation on adsorbates with lateral interactions, when it is submitted to pressure oscillations. Through numerical and analytical (based on a two-state approximation) methods to analyze the existence of stochastic resonance in such a bistable system. This is a first step toward the study of resonant phenomena in adsorbate systems with moving fronts and/or with presence of micro-reactors or spots.     

Imaging of multiphase fluid saturation within a porous material via sodium NMR

We present in this paper a method to monitor multiphase fluid core saturation through measurement of the sodium NMR signal. In a rock core saturated with water and oil, sodium will be present only in the water phase, and therefore can be used to separate the two fluids. Two dimensional sodium images were taken to monitor the movement of brine into oil saturated rock cores. The measured fluid exchange agrees well with expected behavior from traditional core analysis methods. Indications of damage to the rock structure can be seen from the patterns of fluid imbibition.     

An overview of engineered porous material for energy applications: a mini-review

The ordered porous materials, developed using various templating materials, have generated huge interest among the electrochemist community due to their plenty of unique properties and functionalities that can be effectively applied in optoelectronic devices. Mesoporous materials possess excellent opportunities in energy storage and energy conversion applications due to their extraordinarily high surface area and large pore size. These properties may enhance the performance of porous materials in terms of lifetime and stability, energy and power density. In this review, we have tried to club the fields of optoelectronics and mesoporous materials. Also, we have summarised the primary methods for preparing mesoporous materials using various templates and described their applications as electrodes and catalysts in fuel cells, solar fuel production, dye-sensitised solar cells, perovskite, supercapacitors and rechargeable batteries. Finally, we have highlighted the research and development challenges of mesoporous materials those need to be overcome to enhance their contribution in renewable energy applications.     

On the sensitivity analysis of porous material models

Porous materials are used in many vibroacoustic applications. Different available models describe their behaviors according to materials' intrinsic characteristics. For instance, in the case of porous material with rigid frame, and according to the Champoux–Allard model, five parameters are employed. In this paper, an investigation about this model sensitivity to parameters according to frequency is conducted. Sobol and FAST algorithms are used for sensitivity analysis. A strong parametric frequency dependent hierarchy is shown. Sensitivity investigations confirm that resistivity is the most influent parameter when acoustic absorption and surface impedance of porous materials with rigid frame are considered. The analysis is first performed on a wide category of porous materials, and then restricted to a polyurethane foam analysis in order to illustrate the impact of the reduction of the design space. In a second part, a sensitivity analysis is performed using the Biot–Allard model with nine parameters including mechanical effects of the frame and conclusions are drawn through numerical simulations.     

Mathematical modelling of damp porous material drying in diffusion approximation

The problem of drying a porous permeable material is considered. Self-similar solutions are derived for the process of the moisture diffusion transport. The dependence of drying intensity on the porous medium initial state as well as on the parameters of external effect is investigated.     

Current-voltage characteristics of structures with a porous silicon layer at adsorption of carbon monoxide

The current-voltage characteristics of structures with a layer of porous silicon of 73% porosity were measured at adsorption of gas (carbon monoxide) at room temperature. Estimations are performed of the height of potential heterobarrier at the interface between porous silicon and p ⁺-type single-crystal silicon, of the perfectness factor and the resistance of a layer of porous silicon in air, in air with 0.4% CO, and in air with 2% CO. Physical causes explaining the experimental data are discussed.