Search

Ice templating, also known as freeze casting, is a popular shaping route for macroporous materials. Over the past 15 years, it has been widely applied to various classes of materials, and in particular ceramics. Many formulation and process parameters, often interdependent, affect the outcome. It is thus difficult to understand the various relationships between these parameters from isolated studies where only a few of these parameters have been investigated. We report here the results of a meta analysis of the structural and mechanical properties of ice templated materials from an exhaustive collection of records. We use these results to identify which parameters are the most critical to control the structure and properties, and to derive guidelines to optimize the mechanical response of ice templated materials. We hope these results will be a helpful guide to anyone interested in such materials., Comment: 31 pages, 16 figures, 172 references. Datasets and Jupyter (IPython) Notebook available from Figshare (http://dx.doi.org/10.6084/m9.figshare.1412626) as soon as the paper is published

This paper presents a study on thermal stability of alkali activated slag (AAS) prepared from electric arc furnace slag (EAFS) using a mixture of alkaline sodium hydroxide and sodium silicate solutions. The samples were investigated by means of XRD, SEM, TG/DTA and porosity analysis. Compressive strengths of MS samples before and after exposure to elevated temperatures ranging from 600 degrees C to 1000 degrees C were determined. The significant microstructural modifications highlighted by changes in porosity due to the sintering process are responsible for the strengthening of MS sample after heating at 600,800 and 1000 degrees C. Moreover, in MS sample after heating to and above 600 degrees C wustite transforms to spinel.

The behaviour of porous materials in comminution processes differs substantially from that of non-porous materials. It is strongly affected by the type of porosity, which may be characterized by different void shapes and interconnection degrees. This paper discusses the comminution of porous materials having voids of compact shape (like spheres) and not interconnected. For such materials different behaviours can be expected for the two comminution ranges above and below the mean voids interspace. In fact, for the coarser sizes the mechanical behaviour of the material is that of the porous material defined by its bulk mechanical properties; below the mean voids interspace down to very fine sizes the mechanical behaviour of the material is that of the non-porous material constituting the matrix. Therefore, a transitional region should exist, with the result of bi- or trimodal size distributions for comminuted porous materials. To verify the above mentioned theory, tests were performed on two types of materials, artificial and natural: (a) perforated bricks with 20 mm voids; (b) natural pumice stone. The resulting size distributions differ in slope above and below the mean voids interspace, and the two parts of the distribution are connected by a transition curve. The results may be of interest in studying the comminution of different types of materials, particularly secondary materials and scraps.

The paper compares two theoretical approaches for modeling gas flow through brush seals. In the first approach, the bristle pack is represented as a porous medium. The resistance to the flow through the bristle pack is described by viscous and inertial resistance coefficients defined in a bristle streamwise direction and directions normal to bristles. In the second approach, several tens of individual bristles representing the bristle matrix segment are modeled directly. In contrast to the model based on the porous medium approach, the model with the resolved bristle matrix provides more accurate local flow characteristics but at the expense of complexity and high computational costs. Two brush seals are studied numerically to compare two theoretical approaches in terms of leakage, pressure and velocity distributions. In addition, the predicted results are compared with available experimental data on leakage and the axial pressure drops for various operating conditions. The capability of both approaches to predict brush seal performance is discussed in detail.

In this paper we present laboratory experiments where we address the Ground Penetrating Radar (GPR) capability of monitoring oil displacement processes in porous media. This has significant applications in near-subsurface environments where remediation of contaminant such as oil-derived liquids is considered one of the major technical, economical and environmental challenges. Potential applications extend to oil reservoir fields, where technique capable of detecting the water advance towards production well is strongly required. An oil saturated sand box is flooded by water and the process is continuously monitored by a GPR placed at the sand surface and by impedance measurements taken along the sand box height. Strong radar reflections in conjunction with sharp impedance changes occur in the higher oil saturated sand layers. The displacement process is poorly described when the water goes through the lower oil saturated layers due to the reduced electromagnetic contrast. In addition, we present water flooding experiment when the sand initially presents residual water saturation. Overall, the GPR results give a good understanding of the flow dynamics in the sand box.

Proceeding of: Electrostatics 2011 The electroperforation distribution in thin porous materials is investigated using the quadrat counts method (QCM), a classical statistical technique aimed to evaluate the deviation from complete spatial randomness (CSR). Perforations are created by means of electrical discharges generated by needle-like tungsten electrodes. The objective of perforating a thin porous material is to enhance its air permeability, a critical issue in many industrial applications involving paper, plastics, textiles, etc. Using image analysis techniques and specialized statistical software it is shown that the perforation locations follow, beyond a certain length scale, a homogeneous 2D Poisson distribution. The authors acknowledge the TRACE project from Ministerio de Ciencia e Innovación, Spain. Publicado

We show that the so-called hidden potential symmetries considered in a recent paper [M.L. Gandarias, New potential symmetries for some evolution equations, Physica A 387 (2008) 2234-2242] are ordinary potential symmetries that can be obtained using the method introduced by Bluman and collaborators [G.W. Bluman, S. Kumei, Symmetries and Differential Equations, Springer, New York, 1989 G.W. Bluman, G.J. Reid, S. Kumei, New classes of symmetries for partial differential equations, J. Math. Phys. 29 (1988) 806-811]. In fact, these are simplest potential symmetries associated with potential systems which are constructed with single conservation laws having no constant characteristics. Furthermore we classify the conservation laws for classes of porous medium equations, and then using the corresponding conserved (potential) systems we search for potential symmetries. This is the approach one needs to adopt in order to determine the complete list of potential symmetries. The provenance of potential symmetries is explained for the porous medium equations by using potential equivalence transformations. Point and potential equivalence transformations are also applied to deriving new results on potential symmetries and corresponding invariant solutions from known ones. In particular, in this way the potential systems, potential conservation laws and potential symmetries of linearizable equations from the classes of differential equations under consideration are exhaustively described. Infinite series of infinite-dimensional algebras of potential symmetries are constructed for such equations. © 2008 Elsevier B.V. All rights reserved. 388 4 343 356 Cited By :15

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE12 pages; International audience; We have performed extensive experimental and numerical studies of spontaneous percolation of small beads through an unconsolidated porous media made with large glass beads. In this paper, an experimental setup and a fast "discrete element method" algorithm are presented to deal with large numbers of particles during our interparticle percolation phenomenon studies. In all the experimental and numerical analyses, the size ratio between the moving beads and the stable packing was chosen larger than the geometrical trapping threshold: ξc=((2/(√3−1))^−1=6.464.... We measure the longitudinal and transverse dispersion coefficients versus the height of the porous medium or the number of falling small beads. The influence of bead properties such as density, diameter, or restitution coefficients was investigated by using either steel or glass beads. The individual description of these effects and their explanations were made possible by confrontation and coupling between experimental and numerical results. Indeed, with our numerical model, individual analysis of the effects of these mechanical or geometrical parameters were made possible and carried out.

PurposeThe purpose of this paper is to introduce a robust mathematical model and finite element‐based numerical approach to solve solid oxide fuel cell (SOFC) problems.Design/methodology/approachA robust mathematical model is constructed by studying pros and cons of different SOFC and other fuel cell models. The finite element‐based numerical approach presented is a unified approach to solve multi‐disciplinary aspects arising from SOFC problems. The characteristic‐based split approach employed here is an efficient way of solving various flow, heat and mass transfer regimes in SOFCs.FindingsThe results presented show that both the model and numerical algorithm proposed are robust. Furthermore, the approaches proposed are general and can be easily extended to other similar problems of practical interest.Originality/valueThe model proposed is the first of this kind and the unified approach for solving flow, heat and mass transfer within a fuel cell is also novel.

Chemical waves induce density gradients in fluids which may lead to convection. This paper studies the convective effects on chemical waves propagating in porous media or in fluids confined between two parallel vertical walls. Chemical waves in the iodate–arsenous acid system are modeled with a one variable reaction‐diffusion equation. The fluid flow is modeled using Darcy’s law. A linear stability analysis on convectionless fronts shows a transition to convection. The full nonlinear equations describing the convective front are solved numerically on a vertical slab. Convective fronts propagate faster than convectionless fronts. Near the onset of convection, the fronts are raised on one side of the slab and lowered on the opposite side. Away from the onset, the fronts are raised at the middle of the slab with the opposing sides at the same height.

In this paper, we describe nuclear magnetic resonance measurements of water diffusion in highly confined and heterogeneous colloidal systems using an anomalous diffusion model. For the first time, temporal and spatial fractional exponents, alpha and mu, introduced within the framework of continuous time random walk, are simultaneously measured by pulsed gradient spin-echo NMR technique in samples of micro-beads dispersed in aqueous solution. In order to mimic media with low and high level of disorder, mono-dispersed and poly-dispersed samples are used. We find that the exponent alpha depends on the disorder degree of the system. Conversely, the exponent mu depends on both bead sizes and magnetic susceptibility differences within samples. The new procedure proposed here may be a useful tool to probe porous materials and microstructural features of biological tissue. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3610367]

In this research work, a modified water displacement method (MWDM) was designed and used in addition to geometry method (GM) to measure the bulk volume and then determine the bulk density values of asbestos ceiling board, cardboard paper, chalk, clay (compacted) and gypsum board that have been sun-dried to constant weight. The mean bulk densities determined by both methods were compared with the reference bulk density values of the same porous materials obtained in this work using standard test procedure in accordance with ASTM D6683-14. It was observed that, for all the tested porous materials, the percentage error in the mean bulk density values ranged from 2.3% to 49.6% when using GM and 0.9% to 5.7% by using the MWDM. Also, at 0.05 level of significance with a degree of freedom of 3, correlation coefficients of 0.7430 and 0.9955 were obtained in the cases of GM and the MWDM respectively. Again, all other analyses performed similarly revealed that the mean bulk densities obtained by the MWDM only were in close agreement with their corresponding reference values, thereby implying that apart from being cost-effective, the MWDM is better than GM in terms of accuracy, reliability, and validity. More importantly, it is noteworthy that even if the glass cylinder available for use is ungraduated, this MWDM can be employed to obtain accurate, reliable and valid bulk density values of porous materials in order to enhance thorough physical characterization, proper selection and suitable applications of such materials.

The quality of a cast product depends on its macro- and microstructures and the type and amount of defects present in it. Grain size and porosity are two important features that determine the casting quality. Various methods have been used to determine the grain size and the porosity content of a casting. For example, measurement of ultrasonic parameters, viz attenuation and velocity are used to evaluate the casting quality non-destructively. In this paper, the influence of porosity, pore diameter and grain size on ultrasonic longitudinal wave velocity in Al-4.5% Cit alloy castings is presented. Cross-correlation method has been employed in this study to determine the ultrasonic velocity. The ultrasonic velocity has been found to vary marginally with an increase in the grain size which is attributed to the anisotropy in thin test specimens. The velocity also decreases when the porosity content of the casting increases. The pore diameter appears to have no influence on ultrasonic velocity at a given level of percentage porosity for the levels of porosities investigated in this study.

The rate and extent of transport of macromolecules and other solutes into cellulosic materials and fibers have important applications in such fields as papermaking, textiles, medicine, and chromatography. This review considers how diffusion and flow affect permeation into wood, paper, and other lignocellulosic materials. Because pore sizes within such materials can range from nanometers to millimeters, a broad perspective will be used, also considering some publications related to other porous materials. Factors that limit the rate or extent of polymer or other solute transport into pores can involve thermodynamics (affecting the driving motivation for permeation), kinetics (if there is insufficient time for the system to come to equilibrium), and physical barriers. Molecular flow is also affected by the attributes of the solute, such as molecular mass and charge, as well as those of the substrate, such as the pore size, interconnectedness, restricted areas, and surface characteristics. Published articles have helped to clarify which of these factors may have a controlling influence on molecular transport in different situations.

In this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 ¿m. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain.

International audience; The wide use of porous materials in vibro-acoustics led up to study a novel material that can exhibit interesting behaviours for vibro-acoustics applications, and possibly improve the efficiency of performances. This paper is focused on the analysis of absorbing foams, which are rendered auxetic (Negative Poisson’s ratio) thanks to a specific forming process. It first illustrates the efficiency of auxetic foams compared to melamine samples using experimental results. Then a study is conducted in order to improve the identification of mechanical and coupling modelling parameters for the considered auxetic transverse isotropic foam. The method associates a preliminary parameters sensitivity analysis with an optimization study. A global sensitivity analysis of the outputs of interest is performed using the Fast technique in order to estimate the first-order and total effects of the numerous parameters of the model. The results of the analysis are then used to perform the optimal identification of the parameters by readjusting finite elements analyses results over experimental data. The results and benefits of the preliminary use of parameters sensitivity analysis associated with optimization are finally presented.