Anisotropic Porosity Research Articles

Sintering behavior, microstructure and thermoelectric properties of calcium cobaltite thick films for transversal thermoelectric multilayer generators

The sintering behavior and the thermoelectric performance of Ca3Co4O9 multilayer laminates were studied, and a multilayer thermoelectric generator was fabricated. Compacts and multilayer samples with anisotropic microstructure and residual porosity were obtained after conventional sintering at 920°C, whereas dense and isotropic multilayer samples were prepared by firing at 1200°C and reoxidation at 900°C. A hot-pressed sample has a dense and anisotropic microstructure. Samples sintered at 920°C exhibit low electrical conductivity due to the low density, whereas the Seebeck coefficient is not sensitive to preparation conditions. However, thermal conductivity of multilayers is very low, and, hence acceptable ZT values are obtained. A transversal multilayer thermoelectric generator (TMLTEG) was fabricated by stacking layers of Ca3Co4O9 green tapes, AgPd conductor printing, and co-firing at 920°C. The TMLTEG has a power output of 3mW at ΔT=200 K in the temperature interval of 25°C to 300°C.

Wave propagation speeds and source term influences in single and integral porosity shallow water equations

In urban flood modeling, so-called porosity shallow water equations (PSWEs), which conceptually account for unresolved structures, e.g., buildings, are a promising approach to addressing high CPU times associated with state-of-the-art explicit numerical methods. The PSWE can be formulated with a single porosity term, referred to as the single porosity shallow water model (SP model), which accounts for both the reduced storage in the cell and the reduced conveyance, or with two porosity terms: one accounting for the reduced storage in the cell and another accounting for the reduced conveyance. The latter form is referred to as an integral or anisotropic porosity shallow water model (AP model). The aim of this study was to analyze the differences in wave propagation speeds of the SP model and the AP model and the implications of numerical model results. First, augmented Roe-type solutions were used to assess the influence of the source terms appearing in both models. It is shown that different source terms have different influences on the stability of the models. Second, four computational test cases were presented and the numerical models were compared. It is observed in the eigenvalue-based analysis as well as in the computational test cases that the models converge if the conveyance porosity in the AP model is close to the storage porosity. If the porosity values differ significantly, the AP model yields different wave propagation speeds and numerical fluxes from those of the BP model. In this study, the ratio between the conveyance and storage porosities was determined to be the most significant parameter.

Surface-oxidized, freeze-cast cobalt foams: Microstructure, mechanical properties and electrochemical performance

Cobalt with anisotropic open porosity is fabricated by directional solidification of aqueous slurries of nanometric Co3O4 powder where ice dendrites push powders into aligned interdendritic spaces, followed by ice sublimation, reduction of the oxide to metallic Co powders, and sintering of these Co powders into parallel lamellae. As the Co3O4 powder slurry fraction decreases (from 10 to 4 vol%), Co lamellae width in the final foam also decreases (from 93 to 8 μm) while foam porosity increases (from 66 to 85%). A drop in solidification temperature (from −10 to −50 °C) decreases porosity (from 77 to 63%) and lamellae width (from 11 to 5 μm) at a constant 8 vol% slurry fraction. Finally, with increasing sintering time (for −10 °C solidification temperature and 8% slurry fraction), Co foam porosity decreases (from 77 to 68%) and lamella width strongly increases (from 10 to 59 μm), consistent with sintering-induced coalescence of lamellae. The Co foams exhibit high strength but relatively low stiffness as compared to simple theoretical models, consistent with internal Co lamella buckling. A uniform Co oxide layer is grown by oxidation to create an active coating on the Co lamellae useful for lithium-ion storage. A coin-cell test carried out on the oxidized Co foam demonstrates a capacity (1283 mAhg−1) almost twice that of a control oxidized Co foil anode, owing to its considerably larger surface area. Finite-element analysis is used to compute stresses and plastic strain evolutions during the lithiation process to understand the effect of oxide layer thickness and roughness, and micropores within the Co lamellae.

Shallow-water models with anisotropic porosity and merging for flood modelling on Cartesian grids

Shallow-water models with porosity are used to compute floods at a relatively coarse resolution while accounting indirectly for detailed topographic data through porosity parameters. In many practical applications, these models enable a significant reduction of the computational time while maintaining an acceptable level of accuracy. In this paper, we improve the use of porosity models on Cartesian grids by three original contributions. First, a merging technique is used to handle cells with low porosity values which tend otherwise to seriously hamper computational efficiency. Next, we show that the optimal method for the determination of the porosity parameters depends on the modelling scale, i.e. the grid resolution compared to the characteristic size of obstacles and flow ways. Finally, we investigate the potential benefit of using a different porosity parameter in each term of the shallow-water equations. Five test cases, two of them being original, are used to validate the model and assess each contribution. In particular, we obtained speedup values between 10 and 100 while the errors on water depths remain around few percent.

Ionothermal Carbon Nanochemistry and Its Use in Energy Conversion

Nanoporous carbon materials are known for their applicability in important areas such as sorption, catalysis and electrochemistry (e.g. fuel cell catalysts, supercapacitor or battery electrodes) It was shown that mesoporous nitrogen doped carbons (NDCs) act as an inexpensive and highly active non-metal catalyst in the oxygen reduction reaction (ORR), with the potential to reach performances of practical need one day.1More importantly nitrogen doped carbons are acting as solid-state ligand and support material for non-noble catalysts using low cost transition metals like iron or cobalt. This class of materials has the potential for a disruptive technology in fuel cells e.g. for automotive applications. A key strategy towards improvement for this aim comprises the generation of carbons with advantageous porosity, which typically means high surface area and mass transport pores. My group is developing a novel nanochemistry strategy that is using inorganic salt melts as unconventional reaction medium for the porogenesis, thereby revisiting classic activation techniques.2 Carbon materials with extra high surface area and pore volumes, four times as high as in commercial activated carbons, are obtained.3 In our most recent work we introduce an in-situ template transformation as a new tool to develop anisotropic tubular porosity towards highly porous NDCs. While we previously observed that residual solid salt can introduce additional macroporosity, we herein purposely in-situ crystallize nanoscopic fibrous salt crystals acting as anisotropic templates for mass transport nanopores.4 The utilization of magnesium chloride in contrast to the more commonly used zinc chloride supports the scalability and shows the more general applicability of the salt templating strategy. The obtained NDCs show very high specific surface areas up to 2780 m2 g 1 and outstanding total pore volumes up to 3.86 cm3 g-1, because of the present unique nanochannel pores. In a next step, we exploit the unique porosity for the development of a mild low temperature formation of non-noble oxygen reduction catalysts with the conservation of the doped carbon chemistry. Very active catalysts with a half wave potential of up to 0.76 V vs. RHE in 0.05 M H2SO4 are obtained after metalation with iron. The catalyst shows four electron selectivity and exceptional stability with only a very low performance degradation of down to ∆E1/2 = 5 mV after 1000 cycles between 0.4-1.0 V at 50 mV s-1 in O2-saturated electrolyte. The Fe K-edge fourier transform EXAFS profile of the most active catalyst is almost completely matching that of the iron(II) phthalocyanine, indicating the possibility to create active and stable FeN4 sites for advanced non-noble oxygen reduction electrocatalysts at chemically non-destructive low temperatures.

Consistency and bicharacteristic analysis of integral porosity shallow water models. Explaining model oversensitivity to mesh design

The Integral Porosity and Dual Integral Porosity two-dimensional shallow water models have been proposed recently as efficient upscaled models for urban floods. Very little is known so far about their consistency and wave propagation properties. Simple numerical experiments show that both models are unusually sensitive to the computational grid. In the present paper, a two-dimensional consistency and characteristic analysis is carried out for these two models. The following results are obtained: (i) the models are almost insensitive to grid design when the porosity is isotropic, (ii) anisotropic porosity fields induce an artificial polarization of the mass/momentum fluxes along preferential directions when triangular meshes are used and (iii) extra first-order derivatives appear in the governing equations when regular, quadrangular cells are used. The hyperbolic system is thus mesh-dependent, and with it the wave propagation properties of the model solutions. Criteria are derived to make the solution less mesh-dependent, but it is not certain that these criteria can be satisfied at all computational points when real-world situations are dealt with.

Preparation of magnesium oxide and magnesium silicate replicas retaining the hierarchical structure of pine wood

Abstract Replicas retaining the structural characteristics of softwood (Pinus sylvestris) were obtained by infiltrating pretreated templates with a methanolic methoxymagnesium methyl carbonate (MeOMgOCO2Me) solution as a precursor which then hydrolyzed into MgCO3 nanoparticles. Subsequent calcination at temperatures ranging from 500 to 1450°C yielded annealed MgO replicas on levels of hierarchy from the macroscopic to the submicron scale. The mechanical stability of the replicas could be improved through calcination at 1450°C. However, this treatment leads to considerable shrinkage (Δax=56%). Even more stable MgO replicas were obtained by infiltrating the pine template first with MeOMgOCO2Me, followed by a second infiltration step with an ethanolic tetraethyl orthosilicate (TEOS) solution and subsequent calcination at 1350°C. The resulting replicas constitute an MgO framework overgrown with Mg2SiO4 (forsterite) and exhibit compression strengths of 31±8 MPa, as well as hierarchical structures combined with an anisotropic porosity.

Microstructural imaging and characterization of oil shale before and after pyrolysis

The microstructural evaluation of oil shale is challenging which demands the use of several complementary methods. In particular, an improved insight into the pore network structure and connectivity before, during, and after oil shale pyrolysis is critical to understanding hydrocarbon flow behavior and enhancing recovery. In this experimental study, bulk analyses are combined with traditional and advanced imaging methods to comprehensively characterize the internal microstructure and chemical composition of the world’s richest oil shale deposit, the Green River Formation (Mahogany Zone). Image analysis in two dimensions (2-D) using optical and scanning electron microscopy (SEM), and in three dimensions (3-D) using X-ray microtomography (µCT) reveals a complex and variable fine-grained microstructure dominated by organic-rich parallel laminations of the order of 10µm thick which are tightly bound in a highly calcareous and heterogeneous mineral matrix. We also report the results of a detailed µCT study of the Mahogany oil shale with increasing pyrolysis temperature (300–500°C) at 12µm and 2µm voxel sizes. The physical transformation of the internal microstructure and evolution of pore space during the thermal conversion of kerogen in oil shale to produce hydrocarbon products was characterized. The 3-D volumes of pyrolyzed oil shale were reconstructed and image processed to visualize and quantify the volume and connectivity of the pore space. The results show a significant increase in anisotropic porosity associated with pyrolysis between 400 and 500°C with the formation of micro-scale connected pore channels developing principally along the kerogen-rich lamellar structures. Given the complexity and heterogeneity of oil shale, we also characterize the representative size at which porosity remains constant. Our results provide a direct observation of pore and microfracture development during oil shale pyrolysis and the petrophysical measurements from this study serve as valuable input parameters to modeling oil shale pyrolysis processes.

Steady-flow experiments in urban areas and anisotropic porosity model

ABSTRACTModelling flow through an urban area is challenging because of the significant difference between the scale of individual streets and the whole district scale. This paper presents idealized experiments where buildings are represented by square blocks. The experiments were designed to highlight the effect of the street width and orientation to the main flow direction. In addition, a porosity model is developed for representing the urban district as a whole without considering the detailed geometry of the streets, but accounting for the main effects on the flow. The model introduces a tensor of drag coefficients aimed at representing directional effects. The drag coefficients are calibrated and validated using the experimental results.

The influence of substrate temperature and spraying distance on the properties of plasma sprayed tungsten and steel coatings deposited in a shrouding chamber

Plasma spraying is among the alternative technologies for manufacturing tungsten-based coatings or graded interlayers for plasma facing components of fusion devices. The main limitation lies in the relatively low thermal conductivity, stemming from the anisotropic porosity and imperfect bonding between the splats. For several materials, it has been shown that increased substrate temperature leads to improved intersplat bonding and increased coating density. However, spraying of metals at elevated temperatures in ambient atmosphere is accompanied by enhanced oxidation.This work is focused on the effects of substrate temperature on the properties of plasma sprayed tungsten and steel coatings. A range of substrate temperatures was achieved by varying the preheating temperature, pauses between torch passes, spraying distance and intensity of cooling. To suppress oxidation, the substrates were placed in a special shrouding chamber flushed with argon-hydrogen mixture. The coatings were characterized for their structure, porosity, hardness, thermal conductivity and oxygen content. A marked improvement of these properties with substrate temperature was observed and correlated to the spraying parameters.

Urban flood modeling using shallow water equations with depth-dependent anisotropic porosity

The shallow water model with anisotropic porosity conceptually takes into account the unresolved subgrid-scale features, e.g. microtopography or buildings. This enables computationally efficient simulations that can be run on coarser grids, whereas reasonable accuracy is maintained via the introduction of porosity. This article presents a novel numerical model for the depth-averaged equations with anisotropic porosity. The porosity is calculated using the probability mass function of the subgrid-scale features in each cell and updated in each time step. The model is tested in a one-dimensional theoretical benchmark before being evaluated against measurements and high-resolution predictions in three case studies: a dam-break over a triangular bottom sill, a dam-break through an idealized city and a rainfall-runoff event in an idealized urban catchment. The physical processes could be approximated relatively well with the anisotropic porosity shallow water model. The computational resolution influences the porosities calculated at the cell edges and therefore has a large influence on the quality of the solution. The computational time decreased significantly, on average three orders of magnitude, in comparison to the classical high-resolution shallow water model simulation.

Cryostructured hierarchical scaffolds with zonal biochemistry and perpendicular anisotropic porosity promote tissue-mimetic matrix remodeling of human mesenchymal stem cells

Event Abstract Back to Event No Records Found Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Google Google Scholar PubMed Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Shallow water equations with depth-dependent anisotropic porosity for subgrid-scale topography

This paper derives a novel formulation of the depth-averaged shallow water equations with anisotropic porosity for computational efficiency reasons. The aim is to run simulations on coarser grids while maintaining an acceptable accuracy through the introduction of porosity terms, which account for subgrid-scale effects. The porosity is divided into volumetric and areal porosities, which are assigned to the cell volume and the cell edges, respectively. The former represents the volume in the cell available to flow and the latter represents the area available to flow over an edge, hence introducing anisotropy. The porosity terms are variable in time in dependence of the water elevation in the cell and the cumulative distribution function of the unresolved bottom elevation. The main novelty of the equations is the formulation of the porosities which enables full inundation of the cell. The applicability of the equations is verified in five computational examples, dealing with dam break and rainfall-runoff simulations. Overall, good agreement between the model results and a high-resolution reference simulation has been achieved. The computational time decreased significantly: on average three orders of magnitude.

Alginate–Hydroxyapatite Bone Scaffolds with Isotropic or Anisotropic Pore Structure: Material Properties and Biological Behavior

AbstractAlginate/hydroxyapatite composites, with isotropic or anisotropic porosity, were developed using two different freezing methods. Morphological analysis highlighted the differences in their three‐dimensional network. Swelling and in vitro degradation studies revealed that both structures are highly hydrophilic and stable. Static compression tests showed that isotropic scaffolds possess higher modulus and toughness than anisotropic ones. Cyclic compression tests revealed that wet scaffolds are able to withstand high deformations with a peculiar shape‐recovery behaviour. In vitro biological tests showed that osteoblast‐like cells proliferate on both types of structures in a comparable manner.

The relationships among stress, effective porosity and permeability of coal considering the distribution of natural fractures: theoretical and experimental analyses

Although the relationships among stress, effective porosity and permeability of coal are a fundamental research topic that has been studied for decades and are widely used in analyzing the mechanical behavior of coal seams and predicting coalbed methane production, most relevant studies are based on idealized models and do not consider the influence of natural fracture distributions. To obtain a comprehensive understanding of the interrelationships among stress, effective porosity and permeability of coal, a series of effective porosity and permeability determinations have been conducted under different overburden stresses using an automated permeameter–porosimeter considering the directional distribution of natural fractures in coal. The experimental results show that the directional distribution of natural fractures provides a substantial contribution to the anisotropy of the effective porosity and absolute permeability of coal, which exponentially decrease with increasing overburden stress. An existing permeability model was modified to reflect the influence of the natural fracture distribution on the power law relationship between effective porosity and permeability, i.e., the exponent is not constant, but a variable related to the natural fracture distribution. The anisotropic effective porosity sensitivity and stress sensitivity of coal are also discussed, and the coal mass is shown to have the highest effective porosity sensitivity and lowest stress sensitivity in the direction perpendicular to the bedding planes compared to those in other directions.

Urban flood modeling with porous shallow-water equations: A case study of model errors in the presence of anisotropic porosity

Porous shallow-water models (porosity models) simulate urban flood flows orders of magnitude faster than classical shallow-water models due to a relatively coarse grid and large time step, enabling flood hazard mapping over far greater spatial extents than is possible with classical shallow-water models. Here the errors of both isotropic and anisotropic porosity models are examined in the presence of anisotropic porosity, i.e., unevenly spaced obstacles in the cross-flow and along-flow directions, which is common in practical applications. We show that porosity models are affected by three types of errors: (a) structural model error associated with limitations of the shallow-water equations, (b) scale errors associated with use of a relatively coarse grid, and (c) porosity model errors associated with the formulation of the porosity equations to account for sub-grid scale obstructions. Results from a unique laboratory test case with strong anisotropy indicate that porosity model errors are smaller than structural model errors, and that porosity model errors in both depth and velocity are substantially smaller for anisotropic versus isotropic porosity models. Test case results also show that the anisotropic porosity model is equally accurate as classical shallow-water models when compared directly to gage measurements, while the isotropic model is less accurate. Further, results show the anisotropic porosity model resolves flow variability at smaller spatial scales than the isotropic model because the latter is restricted by the assumption of a Representative Elemental Volume (REV) which is considerably larger than the size of obstructions. These results point to anisotropic porosity models as being well-suited to whole-city urban flood prediction, but also reveal that point-scale flow attributes relevant to flood risk such as localized wakes and wave reflections from flow obstructions may not be resolved.

Nanocellulose-assisted formation of porous hematite nanostructures.

We report the formation of porous iron oxide (hematite) nanostructures via sol-gel transformations of molecular precursors in the confined space of self-organized nanocrystalline cellulose (NCC) used as a shape-persistent template. The obtained structures are highly porous α-Fe(2)O(3) (hematite) morphologies with a well-defined anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as the precursor and the heat treatment. Moreover, a postsynthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The nanocellulose templating technique established here enables the straightforward fabrication of a variety of mesoporous crystalline iron oxide scaffolds with defined porous structure and is particularly attractive for the processing of porous hematite films on different substrates.

Coarse‐Grid‐CFD for a Wire Wrapped Fuel Assembly

AbstractComputational fluid dynamics (CFD) simulations of complete nuclear reactor core geometries requires exceedingly large computational resources. However, in most cases there are repetitive geometry‐ and flow patterns allowing the general approach of creating a parameterized model for one segment and composing many of these reduced models to obtain the entire reactor simulation. Traditionally, this approach lead to so‐called subchannel analysis codes that are relying heavily on transport models based on experimental and empirical correlations. With our method, the Coarse‐Grid‐CFD (CGCFD), we intend to replace the experimental or empirical input with CFD data. Our method is based on detailed and well‐resolved CFD simulations of representative segments. From these simulations we extract and tabulate volumetric source terms. Parameterized data is used to close an otherwise strongly under resolved, coarsely meshed model of a complete reactor setup. In the previous formulation only forces created internally in the fluid are accounted for. The Anisotropic Porosity (AP) formulation wich is subject of the present investigation adresses other influences, like obstruction and flow guidance through spacers and in particular geometric details which are under resolved or ignored by the coarse mesh. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal conductivity of wood-derived graphite and copper–graphite composites produced via electrodeposition

The thermal conductivity of wood-derived graphite and graphite/copper composites was studied both experimentally and using finite element analysis. The unique, naturally-derived, anisotropic porosity inherent to wood-derived carbon makes standard porosity-based approximations for thermal conductivity poor estimators. For this reason, a finite element technique which uses sample microstructure as model input was utilized to determine the conductivity of the carbon phase independent of porosity. Similar modeling techniques were also applied to carbon/copper composite microstructures and predicted conductivities compared well to those determined via experiment.

Injectable Materials with Magnetically Controlled Anisotropic Porosity

We propose a method to create aligned porosity in injectable materials by using magnetically responsive microrods as pore forming sacrificial templates. Rod alignment occurs through the application of an external magnetic field after injecting the material into the desired end location. Removal of the sacrificial templates through dissolution or resorption generates porosity in deliberately tuned orientations after injection, offering a powerful method to design the porous architecture of injectable materials.