Porosity Shape Research Articles

Preparation and polymeric encapsulation of powder mineral pellets for self-healing cement based materials

Using mineral additives and admixtures as self-healing agents in cement-based composites has been extensively researched. However, if the minerals are added directly to the cementitious matrix without any protection, they could immediately react, leading to a decrease in self-healing efficiency with further associated side-effects on the mechanical properties of cementitious composites. Thus, this paper describes the development of coated pellets as a self-healing system in cement based materials. Pan pelletisation was utilised for producing pellets from three different powder minerals as potential healing agents: reactive magnesium oxide (MgO), silica fume and bentonite. Of these materials, two types of developed prototype pellets in addition to another two commercial types of MgO pellets with different pellet sizes were then encapsulated in a polyvinyl alcohol (PVA) based film coating. The PVA coating was evaluated for the apparent solubility in water and in alkaline solution, swelling property, water permeability, dynamic mechanical properties, and shell thickness on pellets. Although PVA coating exhibited a decrease in its mechanical properties in water or in the simulated concrete environment, it retained integrity and stability in both environments. The PVA shell thickness varied from 10 to 50 µm. As the coated pellets partially replaced the natural sand in mortar mixtures, they were characterised in comparison with sand for the particle size distribution, density, porosity, crushing strength and particle shape. Experimental results indicated that the different types of pellets showed higher porosity and lower crushing strength compared to sand. In mortar mixtures, the pellets showed excellent compatibility with minimal influence on the fresh mixture properties and the compressive strength of the hardened specimens. This was accompanied by good distribution inside the concrete matrix. Further investigations on the self-healing performance of these pellets are under way.

Fabrication of Porous TiO2 Ceramics Using Corn Starch and Graphite as Pore Forming Agents

Two sources of pore former have been used; namely, starch and graphite to control porosity, pore sizes and pore shapes. The three batches were fabricated using 10 mass-% graphite, 5 mass-% graphite plus 5 mass-% starch and 10 mass-% starch, respectively. The dried green batches were subjected to fire at different temperatures, e.g. 1100, 1200 and 1300 °C for 1 h. The results revealed that the specimens containing 5 and 10 mass-% starch attained porous structure at different firing temperatures with micro pore sizes. The batch prepared using starch exhibited very high and sharply increased strength in comparison with those containing graphite and graphite plus starch. This is due to the formation of homogeneous foaming structure with smaller pore size and larger number of pores. Moreover, the strength of the batches fired at 1100 and 1200 °C was lower than that fired at 1300 °C .

A mechanism of failure in shear bands

We have carried out dilatant plasticity simulations to investigate the process of void-mediated failure inside a shear band. The constitutive model accounts for possibly inhomogeneous flow within the band, void rotation and void elongation. We found that the material in the band may soften with no increase in the void volume fraction. For a given matrix hardening capacity, the rate of softening was found to depend strongly on the ratio of shear band width to in-plane void spacing. The emergent softening led to complete loss of load bearing capacity thereby providing a physical mechanism of failure in shear bands. The mechanism is consistent with essential features of shear-fractured specimens in terms of surface roughness, porosity and dimple shape.

Study on design and performance of metal-bonded diamond grinding wheels fabricated by selective laser melting (SLM)

Porous metal-bonded grinding wheels with high porosity and controllable pore shape, size, quantity and distribution are in great demand but hard to realize for traditional process. In this study, the novel method is proposed to fabricate porous metal-bonded grinding wheels based on selective laser melting technology. Three kinds of grinding wheels, including octahedron structure wheel, honeycomb structure wheel and solid structure wheel, are designed and fabricated using this novel method. Additionally, morphological characterization is conducted in terms of porous structure, microstructure and bonding condition. Furthermore, grinding performance is evaluated in terms of grinding force, material removal rate, ground surface roughness and hardness. Experimental results indicate that the SLM-fabricated grinding wheels possess the excellent dressing and self-sharpening ability, good bonding strength, controllable pore structure, high porosity and good comprehensive grinding performance. Compared with electroplated wheel, better machined surface roughness and slighter work hardening can be achieved by the SLM-fabricated wheels based on the similar material removal capacity. Among the SLM-fabricated wheels, octahedron structure wheel possesses the smallest extrusion and friction force, the highest instantaneous cutting force and the slightest work hardening, and solid structure wheel possesses the largest material removal rate.

An indicator of urban morphology for landscape planning in Lombardy (Italy)

PurposeThe spatial development of urban areas affects the characteristics of landscape as well as people’s aesthetic perception of it. Specifically, sprawl results in an urban morphology which is diametrically opposed to the compact city model and which assumes several kinds of patterns: for example “striped”, “ribbon” or “leapfrogged” urban development. Assessing urban morphology in spatial terms is crucial to urban policy, while landscape metrics are the key to a comprehensive understanding of different urban development patterns. The purpose of this paper to design and test an urban morphology indicator (UMI) for the Lombardy Regional Landscape Plan.Design/methodology/approachThe paper describes an UMI that can be used to identify the heterogeneity of built-up patterns according to urban porosity, fragmentation and patch shape. This UMI is a result of Esri ArcGIS 10.3 “grouping analysis” which works by applying a spatial statistical metric for clustering geometries in a given geographical area.FindingsMorphological analysis was used in regional urban development policies with a view to minimising impact on surrounding ecosystems and preserving the natural environment and landscape. It defines 28 different urban morphology patterns in the region, which are divided into systems, polarities and urbanised units.Originality/valueThe proposed methodology differs from those traditionally used in qualitative/descriptive landscape planning and supports the identification of morphological features with quantitative statistical and spatial data, allowing a fine-scale assessment of complex metrics.

Clusters of cataclastic deformation bands in porous sandstones

Clusters of cataclastic deformation bands represent potential barriers or baffles to reservoir fluid flow, and their processes of formation remain debated. In this work, we rely on an integrated field study at seven sites to describe the extent of clusters, their morphology and their density of deformation as a function of several parameters: the tectonic loading, the burial depth of deformation, the Andersonian stress regime and the lithology of the sandstone. We perform porosity, sorting and grain shape analyses of the deformed material to improve the understanding of microscopic process of cluster development. In agreement with previous works on cataclastic deformation bands, our results reveal that the tectonic loading constrains the extent and the morphology of the clusters. Extensional tectonics favors the formation in normal-fault Andersonian regime of series of hundreds of meter long, rather thin and dense clusters, forming kilometer long networks associated with faults. We find that the formation of major slip-surfaces (faulting) can occur at any stage of cluster thickening. Contractional tectonics favors the formation of strike-slip clusters of medium-thickness, and more rarely the formation of tens of meter long, low-density, thick thrust clusters. The clusters formed in contractional tectonic settings are sparse. Since 15%–79% of the bands/slip-surface of the clusters have permeabilities close to or lower than 1 × 101 mD, and because clusters can have significant dimensions, they may be significant baffles, particularly in extensional tectonics. Our analysis of the porosity and shape evolution of the clasts with increasing deformation corroborates the hypothesis of strain hardening of the band by combined increase of cohesion and friction induced by the cataclasis. We propose that the variability of hardening associated with the variable packing and related cementation in specific parts of the deformed material controls the development and the morphology of the clusters as a function of the tectonic loading.

Experimental study of mean overtopping discharge at perforated caissons for non-impulsive waves

In this study, hydraulic model tests are carried out to investigate the mean overtopping discharge at perforated caisson breakwaters for non-impulsive waves. Based on the experimental data, the mean overtopping discharges of perforated and non-perforated caissons are compared. It is found that when the relative crest freeboard is smaller than 1.6, the mean overtopping discharge of a breakwater can be reduced by at least half by using perforated caissons with 35% porosity instead of non-perforated caissons. The effects of the relative crest freeboard, the caisson porosity and perforation shape, the relative wave chamber width and the relative water depth on the mean overtopping discharge at perforated caissons are clarified. Then, predictive formulas for the mean overtopping discharge at perforated caissons are developed. The predictive formulas based on the experimental data are valid in a wide range of the relative crest freeboard and involve the effects of the caisson porosity and the relative water depth. The predictive formulas developed in this study are of significance for the hydraulic design of perforated caissons.

Wear and friction performance under lubricated reciprocating tests of steel powder mixtures sintered by Spark Plasma Sintering

Abstract Literature indicates that porosity in sintered materials decreases mechanical properties and fatigue resistance. However, given porosity shape and distribution on a surface, it is possible that these small cavities could improve surface lubrication conditions. This work aims to evaluate the effects of porosity, surface finishing, oil viscosity and heat treatment on wear and friction of two sintered carbon steels during lubricated high contact pressure tests. Samples were characterized prior and after tests by SEM, optical profilometry and hardness measurements. Main results showed that surface hardness and porosity controlled wear behavior of sintered material in high load conditions. Rougher material seemed to activate anti-wear additives contained on the lubricant, promoting less worn volume than polished samples.

Digital porosity

A flexible and interactive method to design porosity patterns should give designers the ability to control the porosity gradient (i.e., the ratio of solid to void at different locations in space), pore radii, cell topology, directionality, and shape including variations in thickness, curvature and texture. In this paper, we present a method for combining volumetric texture coordinates with halftoning typographic techniques to enable an arbitrary "flow" of a micro-pattern defined by the potential fields of the texture coordinates. We call this method modulated pattern binarisation. Using this approach, we can leverage and even extend existing techniques developed for 2D surface reparameterisation, image analysis and typography. Furthermore, we examine a set of methods which can be used to analyse the geometric properties of both the pattern cell and its mapping. We conclude by discussing some of the possible techniques available for materialising porous structures using additive manufacturing (AM).

Three-Dimensional Morphological and Chemical Evolution of Nanoporous Stainless Steel by Liquid Metal Dealloying.

Nanoporous materials, especially those fabricated by liquid metal dealloying processes, possess great potential in a wide range of applications due to their high surface area, bicontinuous structure with both open pores for transport and solid phase for conductivity or support, and low material cost. Here, we used X-ray nanotomography and X-ray fluorescence microscopy to reveal the three-dimensional (3D) morphology and elemental distribution within materials. Focusing on nanoporous stainless steel, we evaluated the 3D morphology of the dealloying front and established a quantitative processing-structure-property relationship at a later stage of dealloying. The morphological differences of samples created by liquid metal dealloying and aqueous dealloying methods were also discussed. We concluded that it is particularly important to consider the dealloying, coarsening, and densification mechanisms in influencing the performance-determining, critical 3D parameters, such as tortuosity, pore size, porosity, curvature, and interfacial shape.

Study on thermal resistance performance of 8YSZ thermal barrier coatings

The application of ceramic thermal barrier coatings (TBCs) can significantly increase gas turbine performance. The microstructure of TBCs fabricated by APS and EB-PVD is remarkably different. The thermal and mechanical properties of TBCs with the same material are determined by its microstructures. In order to establish structure-property relationships of TBCs with complex inhomogeneous geometry, in this work, various 3D-microstructures of TBCs were reconstructed using the geometric reconstruction software developed by our team. A three-dimensional model based on Lattice Bolzmann method (LBM) was developed to investigate the heat transfer behaviors in TBCs. The effect of microstructure parameters such as porosity, pore size and pore shape on the effective thermal conductivity of TBCs was studied in detail. The relationship between microstructures and effective thermal conductivity of TBCs was obtained. The calculated values of the effective thermal conductivity of 3D-TBCs agree well with the experimental values, and all relative errors are less than 4%. It can be found that the layered coatings should have larger size pores, and the columnar coatings should have smaller size pores. This can make the internal temperature distribution of TBCs more uniform. The effect of pores size on the thermal conductivity of two types of TBCs is different, but their thermal conductivity increase with the increase of porosity. The effective thermal conductivity of the columnar TBCs with the same porosity decreases with the decrease of pores size, but that of the layered TBCs increases with the decrease of pores size. The pores shape has a great influence on the effective thermal conductivity of the coating, especially for high porosity columnar coating. The effective thermal conductivity of the columnar coatings logarithmically increases with the length of the elongated pores, and it is increased by 21.6% when the porosity is 20%. However, the effective thermal conductivity of the layered structure coatings logarithmically decreases with the length of the elongated pores. The results will provide us a theoretical guide to design TBCs with a high thermal insulation property.

Effect of spheronizer plate design on the spheronization of ketoprofen

Spheronization is a rapid process for production of microspheres. The process involves the dry mixing of drug with microcrystalline cellulose (MCC), which on subsequent wetting with water forms a plastic mass suitable for extrusion and spheronization. As in any pharmaceutical operation, large number of factors may affect the production of pellets, which can be related to formulation or processing machinery. Among the factors related to spheronizers is the spheronization plate design. This work is aimed at evaluation of the effect of using the radial design and the cross-hatch design on product quality through a factorial experiment using ketoprofen as a model drug. The factors studied were MCC level, spheronizer speed and spheronization time. The evaluation methods included sieving analysis, density and porosity measurements, shape analysis, and dissolution testing. Preliminary experiments revealed that MCC level is of great significance on pellets yield. Also, all the produced pellets were of acceptable sphericity score. The factorial experiments showed that an increase of pellets yield of desired size can be obtained when using the radial design of friction plate, while no significant changes were found regarding density, porosity and dissolution rate.

A finite-strain homogenization model for viscoplastic porous single crystals: II – Applications

In part I of this work (Song and Ponte Castañeda, 2017a), a new homogenization-based constitutive model was developed for the finite-strain, macroscopic response of porous viscoplastic single crystals. In this second part, the new model is first used to investigate the instantaneous response and the evolution of the microstructure for porous FCC single crystals for a wide range of loading conditions. The loading orientation, Lode angle and stress triaxiality are found to have significant effects on the evolution of porosity and average void shape, which play crucial roles in determining the overall hardening/softening behavior of porous single crystals. The predictions of the model are found to be in fairly good agreement with numerical simulations available from the literature for all loadings considered, especially for low triaxiality conditions. The model is then used to investigate the strong effect of crystal anisotropy on the instantaneous response and the evolution of the microstructure for porous HCP single crystals. For uniaxial tension and compression, the overall hardening/softening behavior of porous HCP crystals is found to be controlled mostly by the evolution of void shape, and not so much by the evolution of porosity. In particular, porous HCP crystals exhibit overall hardening behavior with increasing porosity, while they exhibit overall softening behavior with decreasing porosity. This interesting behavior is consistent with corresponding results for porous FCC crystals, but is found to be more significant for porous HCP crystals with large anisotropy, such as porous ice, where the non-basal slip systems are much harder than the basal systems.

Mechanical properties of rock materials with related to mineralogical characteristics and grain size through experimental investigation: a comprehensive review

Mechanical properties of rock materials are related to textural characteristics. The relationships between mechanical properties and textural characteristics have been extensively investigated for differently types of rocks through experimental tests. Based on the experimental test data, single- and multiple- variant regression analyses are conducted among mechanical properties and textural characteristics. Textural characteristics of rock materials are influenced by the following factors: mineral composition, size, shape, and spatial distribution of mineral grains, porosity, and inherent microcracks. This study focuses on the first two: mineral composition and grain size. This study comprehensively summarizes the regression equations between mechanical properties and mineral content and the regression equations between mechanical properties and grain size. Further research directions are suggested at the end of this study.

Tight chalk: Characterization of the 3D pore network by FIB-SEM, towards the understanding of fluid transport

Tight chalk intervals play a major role in North-Sea hydrocarbon fields, by controlling fluid flow pathways of hydrocarbon and water. Recent studies reveal that low-permeability chalk properties are dependent on clay content and cementation. Therefore, in this research, three characteristic samples were selected: (A) a porous micritic chalk, (B) a cemented chalk and (C) an argillaceous chalk. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) analyses were performed and 3D pore networks were reconstructed for each sample. By using MATLAB® and Avizo® softwares, relevant pore data were extracted, including pore volumes, lengths and network tortuosity. Results show that the pore length is reduced in tight chalks, with 140 nm on average in argillaceous chalk and 533 nm in cemented chalk, compared to 1091 nm in micritic reservoir chalk. Pore shape analysis demonstrates that, when present, clay flakes are predominant. Argillaceous chalk displays 35% of flattened pores, while these represent only 15 and 18% of pores in micritic and cemented chalk respectively. Virtual rock porosity calculated from FIB-SEM is consistent with helium porosity lab-measurements. MICP pore-throat diameters also match calculated pore widths. These preliminary findings confirm the potential of FIB-SEM analyses in characterizing chalks porous media. In order to understand fluid transport, fluid flow was simulated on each sample using the Lattice Boltzmann Method (LBM), which allows visualising fluid pathways and calculating permeabilities. Simple phase LBM-modelled permeabilities are on the same order of magnitude as gas permeabilities measured on centimetric plugs with Klinkenberg correction. This means that the pore network characterized by FIB-SEM has a significant contribution to macroscopic fluid transport. The impact of clay content and cementation degree on total porosity and pore shapes in chalk is a key finding, which requires to be accounted for, e.g. in mesoscale fluid flow modelling.

3D hierarchical multiscale analysis of heterogeneous SMA based materials

A 3D hierarchical multiscale computational homogenization framework with periodic Representative Volume Element (RVE) is proposed in the present study for analysis of shape memory alloy (SMA) fiber reinforced composite and porous SMA specimen. The shape memory alloy, which exhibits forward and reverse transformations, is wired through a matrix, while a dense SMA matrix encircles voids as inclusions in the porous model. The asymptotic mathematical homogenization approach is used to express the microscopic and macroscopic governing equations. An iterative solution is used at each point in order to derive the tangential constitutive response of the corresponding periodic RVE. Several numerical problems are solved and the results are compared with those obtained from a fully meshed direct numerical model. Detailed discussions are presented regarding the distribution of field variables, in particular the martensitic volume fraction and the stress state inside RVEs located at critical points of the macro models, subjected to cyclic stress loads in the form of tension, flexure and torsion. Moreover, a micromechanical analysis is conducted to investigate the effect of the shape of ellipsoid porosities on the overall response of porous SMA.

Porosity measurements by X-ray computed tomography: Accuracy evaluation using a calibrated object

Accurate identification and measurement of internal voids and porosity is an important step towards improvement of production processes to obtain high quality materials and products. Recently, the importance of knowing the exact size, shape, volume and location of defects has become even higher as tighter requirements and new standards have been introduced in industry. There are several well-established methods for defects evaluation based on various principles (both destructive and non-destructive). However, all conventional methods have various deficiencies and the information about internal voids/porosity that can be extracted is limited. Most of these drawbacks can be overcome by using X-ray computed tomography (CT). Unlike other methods, CT provides full three-dimensional information about shape, size and distribution of internal voids and porosity; however, the accuracy of measurements is still under investigation. Hence, further evaluations on CT porosity measurements must be performed in order to consider X-ray computed tomography a reliable instrument for the assessment and detection of internal defects.A reference object with artificial defects was used in this research work in order to evaluate the accuracy of porosity measurements by CT. The reference object was manufactured by ultra-precision micro-milling. The object contains dismountable components with embedded internal hemispherical features that simulate internal porosity. The artificial porosity was micro-milled on top surfaces of dismountable cylindrical inserts. The hemispherical calottes were thereafter calibrated by traceable coordinate measuring systems and calibrated values were compared to actual values measured by a CT system. The accuracy of CT porosity measurements was then evaluated based on results obtained on various measurands, using different software tools and measuring procedures, comparing real scans to numerical simulations and investigating the influence of CT system parameters settings on measurement results.

Equilibrium and kinetic adsorption studies of reactive orange onto resorcinol-formaldehyde carbon gel

Adsorbents in granular form with small particle size are not suitable for continuous adsorption process. Resorcinol-formaldehyde carbon gel is a synthetic polymeric adsorbent that can be moulded to desired size and shape of high surface area and porosity, and controllable porous structures. This work aims to evaluate the feasibility of carbon gel as the adsorbent in dye wastewater treatment. The carbon gels were prepared by mixing resorcinol and formaldehyde in a presence of sodium carbonate catalyst at resorcinol/catalyst (R/C) ratios of 100, 200, 1,000 and 2,000. The carbonisation of carbon gels was fixed at 600 °C for 3 h, whereby part of the carbonised carbon gels was oxidised with nitric acid (HNO3). The performance of carbon gels was investigated by the adsorption of reactive orange 16 (RO16). Carbon gel with R/C ratio of 1,000 displays a specific surface area of 638.9 m2/g and RO16 maximum capacity of 6.8 mg/g. The oxidised carbon gels exhibit poorer adsorption performance when compared to the non-oxidised ones. The equilibrium data of RO16 by the non-oxidised carbon gels fitted well with Langmuir model, suggesting a monolayer adsorption process. The kinetics data indicates that physisorption plays a dominating role in the adsorption of RO16.

Effect of environmental exposure on charcoal density and porosity in a boreal forest

Although charcoal's density and porosity shape its environmental roles (e.g. controlling its landscape movement and determining the internal pore space available as microbial habitat), the rate of change of these properties in the environment remains largely unknown. With time, charcoal pores may fill or charcoal particles may shatter, altering the ecosystem services delivered. In this study we examined the effects of environmental exposure on the density and porosity of charcoals pyrolyzed at two different temperatures (350 and 520°C). Fresh charcoal made from the dominant local tree (Pinus sylvestris) was returned in litter incubations to Norwegian boreal forest soils for 20months in three different placements: (i) aboveground, (ii) in the humus layer, and (iii) under the humus layer in contact with the mineral subsoil. By varying soil horizon placement we were able to separate the effects of infill from the effects of environmental disturbance on charcoal density and porosity.Environmental exposure changed charcoal density and porosity, and the response varied with environmental placement. Charcoal placed in soil layers increased in porosity by ~1–2% after 20months' incubation. This is likely because open indentations on the charcoal surface became partially occluded, creating more detectable pore space. In contrast, the porosity of charcoal incubated aboveground decreased slightly (~1–2% over 20months). Because there were no minerals or humic substances to infill the aboveground charcoal samples, this porosity reduction was likely caused by breakage of particles induced by weathering. When charcoal particles cleave through pores, internal pore space is destroyed.The small changes observed here indicate that environmental exposure did not trigger rapid shifts in charcoal density and porosity. In addition, these physical properties appear not to have reached equilibrium after 20months incubation, suggesting that the effect of environmental exposure on charcoal's physical properties occurs on the timescale of years to decades.

Direct numerical method for studying heat transfer behaviors in an enclosure with dispersed thermal conductive particles

Numerical simulations in a two-dimensional (2-D) thermal energy storage (TES) unit were carried out to investigate the thermal behaviors of phase change materials (PCMs) - sodium nitrate (NaNO3) inside porous media. The porous media are composed of randomly distributed particles with high thermal conductivity. The effects of heat conduction through particles, natural convection of liquid PCM, and the geometric parameters such as porosity and particle shape were numerically examined. The results show that for the TES dispersed with square particles, the heat transfer coefficient of the system can be increased up to 27–310% for the porosity ranging from 60% to 90%; while for the particles of expanded graphite (EG)-like structure, the heat transfer coefficient of the TES at 60% porosity can be raised as much as 93.3 times. The high weight fraction of particles in the TES could depress the happening of natural convection, especially for EG-like particles. The Rayleigh number considering the permeability of material (RaK) was proposed to evaluate the roles of heat conduction and natural convection. When RaK is lower than 20.6, the heat conduction starts to dominate the heat transfer inside the composite that leads to the total heat performance of the composite with square particles is lower than that of the system with pure liquid PCM.