Pore Orientation Research Articles

The effect of porosity on the hot corrosion failure of thermal barrier coatings

It has been found that the use of low-grade fuels in land-based turbines results in the hot corrosion (Type-I) degradation of turbine blades in Saudi Arabia. Due to the diffusion of a molten salt, V2O5, into the top coat of thermal barrier coatings, volumetric expansion of the coating occurs as a result of the tetragonal-to-monoclinic transformation of zirconia. The top coat material is usually made porous due to the need for higher thermal resistance in the coatings. In the present study, a phase field model that estimates the kinetics of microstructure evolution during the corrosion process is estimated at 900 °C. The transformation-induced stresses are predicted by coupling the phase transformation with elasticity. The governing equations are implemented numerically using the finite element method. The effect of pore size, shape, orientation, and dispersion is also investigated. The result shows that very high compressive stresses are developed within the coating cross-section, which eventually causes the spallation failure of the coating.

Pore shapes, volume distribution and orientations in monodisperse granular assemblies

The complex mechanical behaviour of granular materials is commonly studied by considering the evolving particle contact network. An often overlooked feature is the influence of micro-scale geometric configuration of pores on the macroscopic response. This paper presents a series of tools to quantify the shape, volume distribution and orientation characteristics of the pore space. The proposed approach is compared against data extracted from physical and numerical experiments with monodisperse assemblies of spheres. Individual pores are represented by polyhedral pore unit cells obtained by combining the Delaunay tessellation with an algorithm for merging Delaunay cells based on the concept of maximal inscribed sphere, after Al-Raoush et al. (Soil Sci Soc Am J 67(6):1687–1700, 2003). A pore shape parameter is proposed that considers pore volume and surface, and is analytically related to the void ratio and the number of edges forming the polyhedral pore unit cell. The pore volume distribution is shown to be uniquely described by the analytical k-gamma distribution proposed by Aste and Di Matteo (Phys Rev E 77(2):021309, 2008). A pore orientation tensor is introduced to define the principal orientation of individual pore units. This is subsequently used to define a global orientation tensor that reveals an isotropic pore network for the reference monodisperse assemblies. The global orientation tensor is analytically expressed in terms of the parameters defining the pore volume distribution.

PETROGRAPHY FOR ASSESSMENT OF MOULDING COMPOUND OF ANCIENT POTTERY

Ancient ceramics are the valuable artifacts which saves the information about ancient people and their traditions. The process of making pottery is begun with the choosing and preparation of raw clay and temper material and further applying of different techniques (moulding of vessel, decoration etc.) and finally firing. Petrographical analysis of ancient pottery allows to determine the mineralogical composition of ceramic matrix, the features of their technology and to identify the raw mineral sources (Feliu et al. 2004, Papadopoulou et al. 2006, Bastie et al. 2006). Such kind investigations expand the frames of our knowledge about ancient technological traditions in pottery making. The following characteristics could be determined in thin-sections of ceramic shards: nature and features of plastic and aplastic inclusions; textural and optical characteristics of clay matrix; shape, amount and orientation of pores; features of surface treatment, decoration. This information is animportant for understanding of principles of ancient technologies. The different temper materials inside clay matrix such as sand, crushed rocks, organic materials, shells, grog can be identified accurately using petrography. The ceramic structure and characteristics of mineral changes during the firing are indicators of firing temperatures and atmosphere. A distribution of clay particles and porous inside of ceramic matrix is useful for reconstruction of moulding methods. The study of ceramic collection of the same cultural tradition and from the same region using petrography gives the possibility for differentiation of the natural and cultural factors influenced on the choose of raw materials, variations in the ceramic composition and techniques. This method is one the most precise for identification of an import pottery.

Effect of reaction time on structure of ordered mesoporous carbon microspheres prepared from carboxymethyl cellulose by soft-template method

Cellulose is the most abundant renewable organic material which constitutes about over 50% of plants’ matter. Till now, many researchers have successfully produced CMC from agricultural waste cellulose sources, Hence, using CMC as raw materials prepared high value-added materials will provide theoretical support for direct utilization of rough crops.In this paper, carbon microspheres with tunable, ordered, mesoporous structures and properties were prepared via soft-template/hydrothermal method using carboxymethyl cellulose (CMC) as the carbon precursor and the triblock copolymer Pluronic F127 as the soft template by varying the reaction time. The mesoporous structure was formed by cooperative assembly of inorganic species with the surfactants; different space groups and pore orientations were formed by varying the reaction time, leading to the generation of a diverse intermediate liquid crystal phase. As a result, a controllable mesoporous structure was obtained that varied from stripe-like hexagonal, mixed hexagonal and cubic, to cubic when reaction time increased from 10 to 30h. Cage-like spheres with perfectly ordered cubic mesoporous structure were obtained when the reaction time was 30h. Components of the F127 molecules participated more effectively in the reaction with CMC and its intermediate products, creating a more uniform structure as reaction time increased, and producing carbon microspheres with favorable thermal stability. The diverse, ordered mesoporous structure resulted in a high tetracycline hydrochloride (TCH) adsorption capacity.

Effects of pore-size and shape distributions on diffusion pore imaging by nuclear magnetic resonance.

In medical imaging and porous media research, NMR diffusion measurements are extensively used to investigate the structure of diffusion restrictions such as cell membranes. Recently, several methods have been proposed to unambiguously determine the shape of arbitrary closed pores or cells filled with an NMR-visible medium by diffusion experiments. The first approach uses a combination of a long and a short diffusion-weighting gradient pulse, while the other techniques employ short gradient pulses only. While the eventual aim of these methods is to determine pore-size and shape distributions, the focus has been so far on identical pores. Thus, the aim of this work is to investigate the ability of these different methods to resolve pore-size and orientation distributions. Simulations were performed comparing the various pore imaging techniques employing different distributions of pore size and orientation and varying timing parameters. The long-narrow gradient profile is most advantageous to investigate pore distributions, because average pore images can be directly obtained. The short-gradient methods suppress larger pores or induce a considerable blurring. Moreover, pore-shape-specific artifacts occur; for example, the central part of a distribution of cylinders may be largely underestimated. Depending on the actual pore distribution, short-gradient methods may nonetheless yield good approximations of the average pore shape. Furthermore, the application of short-gradient methods can be advantageous to differentiate whether pore-size distributions or intensity distributions, e.g., due to surface relaxation, are predominant.

Pore orientation mediated control of mechanical behavior of scaffolds and its application in cartilage-mimetic scaffold design

Scaffolds with aligned pores are being explored in musculoskeletal tissue engineering due to their inherent structural anisotropy. However, influence of their structure on mechanical behavior remains poorly understood. In this work, we elucidate this dependence using chitosan–gelatin based random and aligned scaffolds. For this, scaffolds with horizontally or vertically aligned pores were fabricated using unidirectional freezing technique. Random, horizontal and vertical scaffolds were characterized for their mechanical behavior under compressive, tensile and shear loading regimes. The results revealed conserved trends in compressive, tensile and shear moduli, with horizontal scaffolds showing the least moduli, vertical showing the highest and random showing intermediate. Further, these scaffolds demonstrated a highly viscoelastic behavior under cyclic compressive loading, with a pore orientation dependent relative energy dissipation. These results established that mechanical behavior of porous scaffolds can be modulated by varying pore orientation alone. This finding paved the way to recreate the structural and consequent mechanical anisotropy of articular cartilage tissue using zonally varied pore orientation in scaffolds. To this end, monolithic multizonal scaffolds were fabricated using a novel sequential unidirectional freezing technique. The superficial zone of this scaffold had horizontally aligned pores while the deep zone consisted of vertically aligned pores, with a transition zone between the two having randomly oriented pores. This depth-dependent pore architecture closely mimicked the collagen alignment of native articular cartilage which translated into similar depth-dependent mechanical anisotropy as well. A facile fabrication technique, biomimetic pore architecture and associated mechanical anisotropy make this multizonal scaffold a promising candidate for cartilage tissue engineering.

Porosity Control and Transfer in Silicon Nanostructures through Electrochemical & Chemical Etching

Porous silicon particles and nanostructures are receiving growing interest from the scientific community. Two important parameters usually used to define a porous material are porosity and pore size. However, internal surface area and pore morphology are also of high importance, as they determine the material performances in many applicative domains such as Li-ion batteries (1), catalysis, separation, drug delivery (2) etc. High surface area porous silicon structures were prepared through silicon anodization in acidic HF electrolyte under galvanostatic control. The layers were then either taken off mechanically, forming powders, or subjected to a metal-assisted chemical etching (MaCE) post-treatment to produce high aspect ratio silicon nanowires through the depth of the porous film. In the first scenario, careful attention was paid to the electrolyte composition in order to increase the internal surface area of the powders. In acidic solutions (i.e. HF mixed with strong acids such as HCl or H2SO4), powders with surface area exceeding 850 m²/g were achieved by lowering the average pore size to around 3 nm, whilst maintaining moderate porosity (~ 65%) (3). In the latter scenario, MaCE (4) on porous silicon thin films facilitated the transfer of the parameters defining the mesoporous framework (porosity, pore diameter, surface area, pore orientation) from the thin film to high aspect ratio nanowires. This consisted of periodic modulation of the porosity with depth and applying MaCE post-treatment as illustrated by electron microscopy images, below, the nanowires clearly displayed the porosity gradient of the original porous silicon thin film. The control of both anodization conditions, through an accurate choice of the silicon wafer doping type and level and electrolyte composition, to achieve ultra-high surface area porous powder, and porous layer post-treatment such as MaCE, to convert the porous framework from thin film to nanowires, provide new and promising opportunities in many application fields. X. Li, M. Gu, S. Hu, R. Kennard, P. Yan, X. Chen, C. Wang, M.J. Sailor, J.G. Zhang and J. Liu Nat. Comm., 5, 4105 (2014).C. Chiappini, X. Liu, J.R. Fakhoury and M. Ferrari, Adv. Funct. Mater., 20, 2231 (2010).A. Loni, T. Defforge, E. Caffull, G. Gautier and L.T. Canham, Micro. Meso. Mater., (2015): DOI:10.1016/j.micromeso.2015.03.006Z.Huang, N.Geyer, P.Werner, J.de Boor, U.Gosele. Adv.Mater23 : 285 (2011) Figure 1

Image Analysis and Modeling of the Orientation of Pores in a Constrained Film on a Rigid Substrate

The nature of porosity in functional materials is often a critical parameter in determining their functionality, for example, in structure materials and fuel cell electrodes. Here, we study the development of the anisotropy of porous yttria‐stabilized zirconia (YSZ), focusing particularly upon the contribution of pore orientation to this anisotropy. Simulation from when the ink is deposited on the surface of a rigid substrate shows that platelike pores are found tend to align along the transverse direction of the substrate. Cross‐sectional image analysis of the pores from the attendant pores of YSZ particles or pore‐forming agent (PFA) matches with the simulated modeling when materials transport is insignificant in determining the shape of pores. The anisotropy created in the densification stage is separated from that in the green body by analyzing the image of porous structures formed with spherical glassy carbon which is unable to contribute to anisotropy during green‐state processing.

EFFECTS OF COMPACTION DUE TO MACHINERY TRAFFIC ON SOIL PORE CONFIGURATION

Soil compaction has been recognized as a severe problem in mechanized agriculture and has an influence on many soil properties and processes. Yet, there are few studies on the long-term effects of soil compaction, and the development of soil compaction has been shown through a limited number of soil parameters. The objectives of this study were to evaluate the persistence of soil compaction effects (three traffic treatments: T0, without traffic; T3, three tractor passes; and T5, five tractor passes) on pore system configuration, through static and dynamic determinations; and to determine changes in soil pore orientation due to soil compaction through measurement of hydraulic conductivity of saturated soil in samples taken vertically and horizontally. Traffic led to persistent changes in all the dynamic indicators studied (saturated hydraulic conductivity, K0; effective macro- and mesoporosity, εma and εme), with significantly lower values of K0, εma, and εme in the T5 treatment. The static indicators of bulk density (BD), derived total porosity (TP), and total macroporosity (θma) did not vary significantly among the treatments. This means that machine traffic did not produce persistent changes on these variables after two years. However, the orientation of the soil pore system was modified by traffic. Even in T0, there were greater changes in K0 measured in the samples taken vertically than horizontally, which was more related to the presence of vertical biopores, and to isotropy of K0 in the treatments with machine traffic. Overall, the results showed that dynamic indicators are more sensitive to the effects of compaction and that, in the future, static indicators should not be used as compaction indicators without being complemented by dynamic indicators.

Correlation Between Anisotropic Green Microstructure of Spherical‐Shaped Alumina Particles and Their Shrinkage Behavior

Dimensional control is one of the basic problems in ceramic processing, especially for tape cast ceramic sheets which are used to build up multilayer structures for high‐integrated components. Uncontrolled anisotropic shrinkage can cause geometrical distortion and circuit failure of multilayer ceramics during sintering. The understanding of the relationship between green tape microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer devices. In this study, alumina powders with spherical particle shape were used to cast green tapes. The microstructure as well as the pore orientation and the shrinkage behavior were analyzed. According to the sintering theory, grain growth and pore elimination are the two most important mechanisms to describe sintering shrinkage. In this work, three‐dimensional shrinkage behavior of tape cast alumina powders with spherical particle shape was investigated and correlated with pore orientation in the microstructure. Specifically, the reason for the different shrinkage in z‐direction compared to the lateral shrinkage is in focus. The study is based on experiments as well as on mathematical visualization.

Preparation and cell infiltration of lotus-type porous nano-hydroxyapatite/polyurethane scaffold for bone tissue regeneration

The pore size, porosity and pore connectivity are important structural parameters that affect the mechanical strength and biological performance of a scaffold for bone tissue regeneration. A lotus-type porous nano-hydroxyapatite/polyurethane (n-HA/PU) scaffold was designed and fabricated, and its mechanical property and cell behavior were evaluated. The results showed that the scaffold had an anisotropic pore structure, the porosity was about 84% and the macropore size on cross-section was uniform, close to 600µm. The mechanical properties also showed an anisotropic feature according to the oriented pore structure. When stress applied parallel to the pore orientation, the longitudinal compressive strength and elastic modulus were 3.52MPa and 44.25MPa respectively, much higher than the lateral 2.59MPa and 10.42MPa. Cell culture indicated that the lotus-type n-HA/PU scaffold exhibited good cell affinity and cytocompatibility, the proper pore size and high pore’s connectivity were favourable for cell infiltration into the inner part of the scaffold. The lotus-type n-HA/PU scaffold has promising prospect in bone tissue regeneration field.

Three-dimensional shrinkage behavior of green tapes derived from spherical-shaped powders: Experimental studies and numerical simulations

In tape-cast products a higher shrinkage is observed in the thickness direction compared to the in-plane shrinkage, which is attributed to an anisotropic green tape microstructure caused by shearing and drying during manufacturing. In the present study, cast tapes composed of spherical particles were investigated experimentally and numerically. The shrinkage behavior was analyzed after binder removal and sintering at different temperatures in all three spatial directions. The correlation between anisotropic shrinkage and microstructure concerning pore orientation and coordination number was discussed. Furthermore, it is shown that the anisotropic shrinkage during binder removal contributes strongly to the total shrinkage anisotropy.

The evolution of pore shape and orientation in plastically deforming metals: Implications for macroscopic response and shear localization

A constitutive model is proposed for the macroscopic response of porous plastic metals at finite strains. Besides taking into account the porosity evolution, which leads to pressure sensitivity and dilatant response, the model can also account for changes in the average shape and orientation of the pores by means of suitable microstructural variables which play the role of internal variables and serve to characterize the evolving anisotropy of the material. In particular, the model is used to determine the evolution of the average shape and orientation of the voids under simple shear loading, as well as to explore the concomitant implications for the macroscopic response and shear localization. The intrinsic effect of the void rotations is deduced from comparisons with corresponding results for pure shear loading (where the voids change shape, but undergo no rotation on average), and found to be significant. In addition, more general loading conditions, involving combined tension and shear, are considered, and the effect of the stress triaxiality is investigated. For such more general plane strain conditions, it is found that there is an abrupt transition in the localization strain at a certain value of the triaxiality of about 0.3–0.4, with the localization strain dropping sharply both as the triaxiality increases, or decreases from this value. Furthermore, the results suggest that void rotations can dramatically enhance the susceptibility of the material to shear localization for a certain range of triaxiality values (between, approximately, 0.3 and 0.8).

Gas sensing using porous materials for automotive applications.

Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and 'real world' scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material - zeolites and metal-organic frameworks (MOFs) - are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOx, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.

Electrophoretic deposition and constrained sintering of strontium titanate thick films

Thick films of functional oxides are currently substituting counterparts bulk ceramics, as in the case of low loss dielectrics. For SrTiO3 (ST) based compositions it is demonstrated that electrophoretic deposition (EPD), using acetone as a suspension media with iodine addition, is a suitable technology to fabricate 12 μm thick films. The microstructural analysis of the films sintered at 1500 °C shows that highly densified microstructures can be obtained and, by slightly varying the Sr/Ti stoichiometry in the powder composition, increased densification and grain size and enlargement of the distribution with decreasing Sr/Ti ratio can be observed. In spite of the high densification of the films, it is also demonstrated that due to the constraint imposed by the substrate a smaller grain size is observed in thick films as compared to equivalent bulk ceramics. In addition, a preferential vertical pore orientation is observed in ST thick films. These results may have broad implications if one considers that the dielectric losses and dielectric tunability is affected by pore orientation, since it affects the electric field distribution.

The influence of grain shape and volume fraction of sheet silicates on elastic properties of aggregates: Biotite platelets in an isotropic matrix

Elastic anisotropy of sheet-silicate-rich rocks such as shales and slates strongly depends on the orientation distribution of platelet-shaped minerals, as well as shape and orientation of pores. Bulk elastic anisotropy of the rock results in the anisotropy with respect to the propagation of elastic waves, and consequently, the fastest P-waves can travel with velocities exceeding the slowest velocities by a factor of two or even greater. An important factor is the sheet-silicate’s grain shapes. We approached a model system of biotite platelets in an isotropic matrix with different methods: A mean-field self-consistent method that considered ellipsoidal particles in an effective anisotropic matrix, and a full-field method based on fast Fourier transforms that considered the microstructure, the topology of the polycrystal, and local interactions. Both methods provided numerically very close results. Using these results, we predicted that the aggregate with more oblate grain shape (thinner platelets) was elastically more anisotropic than the material with grains of less oblate shape, but only for small volume fractions of oriented platelets. For large fractions of platelets, the opposite was true. This switchover in the elastic anisotropy depended on texture strength, platelet shape, and elastic properties of the isotropic matrix.

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.

X-ray CT를 이용한 베레아 사암의 공극크기분포 산정

다공질 암석의 공극구조는 그 암석의 수리-역학적 거동에 매우 큰 영향을 미친다. 다공질 암석의 공극구조 특성을 표현하는 대표적인 방법은 공극률, 공극크기분포, 방향성 등이 있다. 암석의 유효공극률은 실험적으로 비교적 쉽게 구할 수 있지만, 공극의 크기분포 특성을 파악하는 것은 매우 어려운 실정이다. 본 연구에서는 X-ray CT를 이용하여 대표적 다공질 암석인 베레아 사암의 공극구조 중 공극크기분포를 이미지 기반으로 평가하였다. 공극크기분포 평가를 위해 본 연구에서는 이미지 필터링, 이진화, 골격화 등의 이미지 기반 처리 작업을 수행하였으며, 국부 두께평가 및 star chord를 이용하여 베레아 사암의 공극크기분포를 구하였다. 본 연구에서 제시된 X-ray CT 기반의 공극크기분포 평가방법을 이용하여 향후 다공질 재료의 공극구조 평가에 새롭고 효율적인 방안을 제시하고자 하였다. Pore structures in porous rock play an important role in hydraulic & mechanical behaviour of rock. Porosity, size distribution and orientation of pores represent the characteristics of pore structures of porous rock. While effective porosity can be measured easily by conventional experiment, pore size distribution is hard to be quantified due to the lack of corresponding experiment. We assessed pore size distribution of Berea sandstone using X-ray CT image based analysis combined with associated images processing, i.e., image filtering, binarization and skeletonization subsequently followed by the assessment of local thickness and star chord length. The aim of this study is to propose a new and effective way to evaluate pore structures of porous rock using X-ray CT based analysis for pore size distribution.

The structural evolution of multi-layer graphene stacks in carbon fibers under load at high temperature – A synchrotron radiation study

Single carbon fibers were directly heated and tensile tested in vacuum in a specially designed in-situ X-ray testing device. We followed the structural evolution by diffraction of synchrotron radiation X-rays and determined the effect of time, temperature and applied stress on the growth of nanocrystallites as well as their orientational change with respect to the fiber axis. As the nanocrystallites are stacks of graphene sheets, this is a direct measurement of the development of multi-layer graphene with temperature. In the same experiment, size and orientation of pores within the material were obtained from simultaneous in-situ small-angle X-ray scattering. It turned out that the final temperature is decisive for structural evolution, significantly more than time or load. The multi-layer graphene network structure in fibers after heat treatment without stress is stable, whereas this is not the case in fibers without heat treatment, no matter if the carbon fibers were produced from polyacrylonitrile or from mesophase pitch precursor.

Predicting Long Term Storage of CO2 in Abandoned Jharia Coalfield-A Numerical Approach

The increasing anthropogenic CO2 emissions have challenged the world to find ways to reduce the atmospheric CO2 concentration. Whatever may be the consequences, it nearly impossible to move the energy infrastructure away from fossil fuel in the near future, and the only other way left out is to curb the CO2 concentration in atmosphere through sequestration and storage in geological media. This paper studies the sequestration of the CO2 in coal bearing sandstone typical of Jharia Coal field (mostly produce bituminous coal) and presents the effect of the porosity and pore orientation on the rate and extent of the CO2 that could be stored in the sandstone. Sandstone with higher porosities was found to be more efficient in terms of the rate and extent of sequestration. With Finite Element Method, different parameters like gas concentration, gradient and porosity variation with distance and time have been analyzed. The concentration of gas at the point of injection is calculated to be 1780kg/m 3 after 120 days. The concentration of CO2 and its gradient decreases as the porosity increases and vice versa for gas flux. The concentration has been measured around 100Kg/m 4 for all different porosity. KeywordsCO2 Sequestration; Sandstone; FEM; Porosity