Heterogeneous Pore Distribution Research Articles

Pore surface fractal analysis of palladium-alumina ceramic membrane using Frenkel–Halsey–Hill (FHH) model

The alumina ceramic membrane has been modified by the addition of palladium in order to improve the H 2 permeability and selectivity. Palladium-alumina ceramic membrane was prepared via a sol–gel method and subjected to thermal treatment in the temperature range 500–1100 °C. Fractal analysis from nitrogen adsorption isotherm is used to study the pore surface roughness of palladium-alumina ceramic membrane with different chemical composition (nitric acid, PVA and palladium) and calcinations process in terms of surface fractal dimension, D. Frenkel–Halsey–Hill (FHH) model was used to determine the D value of palladium-alumina membrane. Following FHH model, the D value of palladium-alumina membrane increased as the calcinations temperature increased from 500 to 700 °C but decreased after calcined at 900 and 1100 °C. With increasing palladium concentration from 0.5 g Pd/100 ml H 2O to 2 g Pd/100 ml H 2O, D value of membrane decreased, indicating to the smoother surface. Addition of higher amount of PVA and palladium reduced the surface fractal of the membrane due to the heterogeneous distribution of pores. However, the D value increased when nitric acid concentration was increased from 1 to 15 M. The effect of calcinations temperature, PVA ratio, palladium and acid concentration on membrane surface area, pore size and pore distribution also studied.

Three‐dimensional culture of human osteoblastic cells in chitosan sponges: The effect of the degree of acetylation

In this investigation, the effect of the degree of acetylation (DA) of chitosan on the behavior of human osteoblastic MG-63 cells cultured in three-dimensional chitosan matrices was assessed. Chitosan sponges with DAs in the range of 4 to 49% were prepared and characterized in terms of microstructure, porosity, and pore size. Collagen sponges were used as 3D control. Cell proliferation was determined using the MTT assay while the retention of the osteoblastic phenotype was monitored by assaying alkaline phosphatase activity. Cell morphology, cytoskeletal organization, and viability were assessed using different microscopy techniques. Chitosan sponges showed a similar microstructure regardless the DA, except for the highest DA used, where a more heterogeneous pore distribution was observed. In terms of cell proliferation, alkaline phosphatase activity and cell viability, cells cultured in chitosan scaffolds performed as well as in the 3D control regardless the DA, except for the highest DA used, where an inhibitory effect on cell proliferation was found. However, while in sponges with DAs < or = 13% cells attached and spread displaying long cell filopodia and numerous cell-to-cell contacts, in sponges with higher DAs cells tended to remain spherical and grow into spheroid-like cellular aggregates. In the present study, the DA played a key role in determining the affinity of osteoblastic cells towards the substrates, possibly by influencing the nature of the initial adsorbed protein layer.

Seasonal Changes in the Structure of RBC Heterotrophic Biofilms

Heterotrophic Biofilms, growing in the successive three treatment stages of the RBC reactor in a full-scale wastewater treatment plant in the summer of 2002, were investigated and compared to those growing in the fall of 2001. The seasonal changes in biofilm structure were studied in terms of biofilm thickness, dry solid density, internal pore distribution and patterns, and surface roughness. The comparison results indicated that due to the higher wastewater flow rate in the summer of 2002, these biofilms had thinner, denser and less rugged surface structure than the biofilms sampled in the fall of 2001; furthermore, they showed less heterogeneous internal pore distribution. However, biofilms from both seasons showed some similarities: biofilms in the treatment stages 2 and 3 had similar structure; and their internal pore distribution and pore surface roughness didn't show any correlation with the biofilm depth.

Scaling of fracture strength in ZnO: Effects of pore/grain-size interaction and porosity

Recent experiments on the fracture strength of zinc oxide (ZnO) ceramics showed that the size of specimens has almost no influence on their mean strength although strength data of a set of nominally identical specimens are still found to scatter. It is suspected that the complex nature of defect–microstructure interaction and the high density of porosity in ZnO ceramics are possible reasons for this insensitivity to strength scaling. In the paper, numerical results obtained by finite element analysis show that the fracture strength of ZnO is more influenced by the pore/grain-size interaction than only by the size of a pore or its shape. As a consequence, the pore/grain-size interaction will increase the fracture probability of small pores and lead to a homogenisation of critical flaw sizes. Furthermore, the high degree of porosity, especially the heterogeneous distribution and clustering of pores, could favour further homogenisation of critical crack sizes. This implies that the fracture strength of ZnO is insensitive to the size of specimens as corroborated by experiments. Finally a simple statistical explanation is given.

The microstructure of porous building materials: Study of a cement and lime mortar

Building materials such as cement mortars and concrets present a very broad distribution of pore sizes, from some tenths of angstroms to several micra. This distribution is very important in establishing their macroscopic properties, e.g., vapor adsorption and desorption and moisture transfer. It is, thus, important to develop procedures to analyze the microstructure of these materials in the full range of pore sizes. In the present work, two complementary methods are used for obtaining the pore sizes distribution of a cement and lime mortar, often used as a building coating material. Electron scanning microscopy is used for pore sizes greater than 1250 a, from a sequence of pictures taken with magnifications from 25x to 12500x, for highly polished surfaces. The heterogeneous spatial distribution of pores is discussed, related to the problem of the geometrical reconstitution of porous structure. For pore sizes smaller than 1250 a, adsorption isotherms obtained at 30 ‡C are used. Molecular physical adsorption is supposed to be the dominant adsorption mechanism in a wide range of relative humidities and modeled using the De Boer and Zwikker theory. This is confirmed by a very high correlation coefficient equal to 0.994 for the present case, for values of RH smaller than 80%. Capillary condensation is supposed to become significant at the point where the adsorption curve deviates from the linear behavior as predicted by the De Boer and Zwikker theory, and the Broekhoff and De Boer theory is used for predicting the pore size distribution from the adsorption isotherm,starting from the deviation point andincreasing RH. The results show the pore size distribution between 200 a and 13Μm.