Pore Boundary Research Articles

The Role of Intragranular Nanopores in Capacity Fade of Nickel-Rich Layered Li(Ni1-x-yCoxMny)O2 Cathode Materials.

Ni-rich layered LiNi1-x-yCoxMnyO2 (NCM, x + y ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li1+x(Ni0.85Co0.1Mn0.05)1-xO2 (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.

In situ scanning electron microscopy analysis of effect of temperature on small fatigue crack growth behavior of nickel-based single-crystal superalloy

In situ observation and measurement of the high-temperature deformation of advanced hot-section materials using scanning electron microscopy (SEM) is beneficial for understanding their microscopic failure mechanisms. Through in-situ measurement and subsequent analysis, crack nucleation site, growth rate and propagation behavior can be determined as well as the interaction between crack and microstructure, such as inclusion, pore or grain boundary. However, these measurements are challenging above 700 °C as the SEM images are severely degraded by the thermal electron emission. In this work, an in situ mechanical testing system for extremely high temperatures (up to 1000 °C) inside an SEM was developed and used to investigate the small fatigue crack growth behavior of a nickel-based single-crystal superalloy from room temperature to 980 °C. The fatigue crack propagation modes and growth rates were investigated at different temperatures. The mechanism of the propagation mode transition was discussed in detail. Conventional Paris law was found no longer capable of characterizing the growth behavior of small crack. Subsequently, with the help of the digital image correlation technique, plastic zone size of the crack tip was obtained, and it was found that it could characterize the growth behavior of the small crack.

Drained pore modulus and Biot coefficient from pore-scale digital rock simulations

We present a digital rock workflow to determine poroelastic parameters which are difficult to extract from well-log or laboratory measurements. The drained pore modulus is determinant in the compaction problem. This modulus represents the ratio of pore volume change to confining pressure when the fluid pressure is constant. In laboratory experiments, bulk volume changes are accurately measured by sensors attached to the outer surface of the rock sample. In contrast, pore volume changes are notoriously difficult to measure because these changes need to quantify the pore boundary deformation. Hence, accurate measures of the drained pore modulus are challenging. We simulate static deformation experiments at the pore-scale utilizing a digital rock image. We model an Ottawa F-42 sand pack obtained from an X-ray micro-tomographic image. This image is segmented into a network of grains and pore space. The network of grains is taken as an elastic, isotropic and homogeneous continuum. We then compute the linear momentum balance for the network of grains. We calculate the change in pore volume using a post-processing algorithm, which allows us to compute the local changes in pore volume due to the applied load. This process yields an accurate drained pore modulus. We then use an alternative estimate of the drained pore modulus. We exploit its relation to the drained bulk modulus and the solid phase bulk modulus (i.e., Biot coefficient) using the digital rock work- flow. Finally, we compare the drained pore modulus values obtained from these two independent analyses and find reasonable agreement.

Assembly of Helical Structures in Systems with Competing Interactions under Cylindrical Confinement.

The behavior under confinement of nanoparticles interacting with the short-range attraction and long-range repulsion potential is studied by means of Monte Carlo simulations in the grand canonical ensemble. The study is performed at thermodynamic conditions at which a hexagonal cylindrical phase is the most stable phase in bulk. In these conditions, cylindrical confinement promotes the formation of helical structures whose morphology depends upon both the pore radius and boundary conditions. As the pore radius increases, the fluid undergoes a series of structural transitions going from single to multiple intertwined helices to concentric helical structures. When the pore ends are closed by planar walls, ring and toroidal clusters are formed next to these walls. Dependent upon the cylinder length, molecules away from the pore edges can either keep growing into ring and toroidal aggregates or arrange into helical structures. It is demonstrated that the system behaves in cylindrical confinement in the same way as the block copolymer systems. Such behavior has not been observed for the colloidal systems in cylindrical confinement with only repulsive interactions.

Drained pore modulus determination using digital rock technology

Geomechanics helps us understand the life-cycle of a hydrocarbon reservoir and, in turn, impacts geophysical monitoring programs. A common geomechanical problem is to predict reservoir compaction or zones of abnormal pore pressure. These predictions mostly use simple empirical relations, but recently, the use of rock deformation models based on static poroelasticity are becoming the norm. These models require accurate values for the poroelastic parameters. We present a digital rock workflow to determine these poroelastic parameters which are difficult to extract from well-log or laboratory measurements. The drained pore modulus is determinant in the compaction problem. This modulus represents the ratio of pore volume change to confining pressure when the fluid pressure is constant. In laboratory experiments, bulk volume changes are accurately measured by sensors attached to the outer surface of the rock sample. In contrast, pore volume changes are notoriously difficult to measure because these changes need to quantify the pore boundary deformation. Hence, accurate measures of the drained pore modulus are challenging. We simulate static deformation experiments at the pore-scale utilizing digital rock images. We model an Ottawa F-42 sand pack obtained from X-ray micro-tomographic images. We stack two-dimensional micro-CT images to generate a three-dimensional F-42 sand pack sample. We extract a sub-volume from this sample for numerical simulation. We first segment the cropped sample consists of grains and pore spaces and then use the segmentation to generate a volumetric mesh. We compute the elastic, linear momentum balance in the structural domain (grains) and solve the system using the commercial software package ABAQUS. The network of grains (solid phase) is assumed elastic, isotropic, and homogeneous. We calculate the change in pore volume using a new post-processing algorithm, which allows us to compute the local changes in pore volume due to the applied load. This process yields an accurate drained pore modulus. We then use an alternative estimate of the drained pore modulus. We exploit its relation to the drained bulk modulus and the solid phase bulk modulus (i.e., Biot’s coefficient) using the digital rock workflow. Finally, we compare the drained pore modulus values obtained from these two independent analyses and find reasonable agreement.

Modeling of Methane/Shale Excess Adsorption Under Reservoir Conditions

Summary Micropores and mesopores are the main storage volumes in shale matrix. Because of their small pore sizes, the force between pore boundary and gas molecules is significant. A larger amount of adsorbed gas is in a shale gas reservoir than in a conventional gas reservoir. People usually measure adsorption through volumetric methods under an isothermal condition. Because of a limitation of volumetric methods, only excess adsorption data are directly measured; then, a chosen model is applied to calculate an absolute adsorption through fitting the measured data. An adsorption process induces changes in free-gas volume. However, the changes in absorbent volume and methane absorption into organic matter also alter the measured gas volume, which is widely neglected in previous studies. In this study, one volume term, which accounts for the unexpected changes in gas volumes caused by the other mechanisms except adsorption, is added to the Dubinin-Astakhov (DA) model (pore-filling theory). The in-situ methane is in a supercritical condition under reservoir conditions. Because of the lack of a saturation pressure of a supercritical fluid, an adsorbed-phase gas density is used to replace the saturation pressure in the DA model. The modified model is validated by the isothermal adsorption data from four different shale plays. The calculated data by the proposed model have a better match with the measured data than those by the DA model. All shale samples demonstrate a nonmonotonic deformation of adsorbent (volume shrinkage in the low-pressure region, then swelling as pressure increases), which coincides with the results of previous molecular simulation. The key parameters of the proposed model such as a maximum adsorption capacity are more accurate and reasonable than the ones of the DA model. The proposed model provides a good approach to quantify absolute adsorption through experimental data, especially under reservoir conditions, and to emphasize the important effects of volume on methane/shale adsorption.

Characterization of a macro porous polymer membrane at micron-scale by Confocal-Laser-Scanning Microscopy and 3D image analysis

Due to the structural complexity of phase inversion macroporous polymer membranes there is a general need for characterization methods on the pore size scale of such membranes. Structure characteristics like pore size distribution, porosity, pore size gradient and the specific surface area often have an impact on the requested performance of such membranes in their field of usage. Here, a commercially available, symmetric, phase inversion nitrocellulose membrane, typically applied in lateral flow immunoassays in the field of in-vitro diagnostics, is investigated. However, commonly used methods for the determination of these characteristics are either cumulative methods, not being able to resolve a 3D pore structure on a local level, or are limited in their ability to reproducibly quantify pore size characteristics. The proposed membrane characterization method uses confocal laser scanning microscopy (CLSM) images combined with computer based image analysis to derive a complete 3D representation of the membrane structure with sufficient resolution and a quantitative characterization thereof. In order to extract quantifiable structural information, the CLSM derived images are binarized using a suitable threshold to match experimental values for porosity. Afterwards a signed distance function is calculated in the 3D structure measuring the smallest distances between pore boundaries followed by a medial axis marking the center of the signed distance function. Structure characteristics like pore size distribution, porosity and specific surface area of the membrane can be determined on a local level. Furthermore, using Stokes equation, the permeability of the membrane is calculated to support the 3D representation obtained by CLSM and the structure characteristics determined by computer based image analysis. Pore size distribution and permeability derived by image analysis show very good agreement with experimental values for the membrane.

Effect of water saturation on gas slippage in circular and angular pores

A unified model for gas slip flow through circular and angular pores in both single phase flow and two‐phase flow conditions is developed, and the effect of water saturation on gas slippage factors in different pore shapes are revealed. For circular pores, the water saturation retains as thin film binding on pore surfaces without changing the shape of the cross section, and the hydraulic diameters continuously reduce as water saturation increases, directly leading to an increase in the slippage factor. However, for angular pores, the water saturation retains as both films at boundaries and condensations at corners, and the film‐water and corner‐water gradually change their cross‐section shape (from sharp corners to round corners), which further affects the gas slip behavior. Due to the presence of round corners, the ratio of the cross‐sectional area and perimeter, which can also be regarded as the reciprocal of a specific surface area, can even increase at a low water saturation condition. Thus the collisions between gas molecules and pore boundaries weaken, resulting in a slight reduction in the gas slippage factor. This interesting finding in the angular pore case directly explains the contradiction of the published experimental results with the general knowledge (i.e., the gas slip factor always increases as water saturation increases). Thus, the validity of the common assumption regarding actual porous media as capillaries with a circular cross‐section must be considered more carefully. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3529–3541, 2018

Pore characteristics and formation mechanism of high-maturity organic-rich shale in Lower Cambrian Jiumenchong Formation, southern Guizhou

In order to investigate pore characteristics and formation mechanism in the high-maturity organic-rich shale of Lower Cambrian Jiumenchong Formation in southern Guizhou, the pore structure, pore type and storage properties are well studied through the rock thin section, total rock X-ray diffraction, low-temperature nitrogen adsorption, high pressure mercury injection-adsorption test, helium porosity test, argon ion polishing-scanning electron microscope, thermal evolution and pore evolution history reconstruction, and based on the diagenesis and compaction as well as thermal evolution process, a microscopic pore formation and evolution model of high-maturity organic-rich shale are established. The result shows that the high-maturity organic-rich shale of Jiumenchong Formation has the average total specific surface area of 12.66 m2/g and the total pore volume of 11.54 × 10−3 cm3/g, and the total specific surface area have a positive correlation with total pore volume; the total specific surface area and the total pore volume are slightly lower compared with the Lower Silurian shale. The pores are dominated by micropores and mesopores, while macropores are very rare. The pores of the organic-rich shale mainly are organic pores with small diameter usually less than 30 nm, and the pore boundary form is irregular, the inorganic mineral pores are not developed. Compared with the Silurian shale, the shale has poorer reservoir property, the average porosity is only 2.80%; the horizontal permeability is 1–3 times of the vertical permeability, indicating the horizontal lamellations are not developed. The formation and evolution of pores in high-maturity organic-rich shale is jointly influenced by the evolutionary process of intergranular pores of inorganic mineral under the control of the diagenesis and compaction, the organic pore formation process in the hydrocarbon generation-oil formation-oil and gas transformation sequence under the control of thermal evolution, and the natural gas loss-supply equilibration process under the condition of later pore preservation.

Dynamic modulation of inter-particle correlation during colloidal assembly in a confined medium: revealed by real time SAXS

Using real time small-angle X-ray scattering, we ellucidate a hitherto unobserved non-monotonic evolution of inter-particle correlation while colloidal particles assemble across pore boundary in a confined medium under influence of solvent evaporation. Time variation of local volume fraction of the particles passes through distinct modulation prior to reaching equilibrium. It has been demonstrated that the amplitude of oscillation depends strongly on size of the assembling particles. We comprehend such non-linear temporal evolution of particle correlation through density functional theory and molecular dynamics simulation.

Homogenized conductivity tensor and absorption function of a locally periodic porous medium

We study a process of stationary diffusion in locally-periodic porous media with nonlinear absorption at the pore boundary. This process is described by a boundary-value problem for an elliptic equation considered in a complex perforated domain, with a nonlinear third boundary condition on the perforation boundary. In view of the smallness of the local scale of porosity of the media and the complexity of the perforated domain, the direct solution of such boundary-value problems is almost impossible. Therefore, a natural approach in this situation is to study the asymptotic behavior of the solution when the microstructure scale tends to 0, and the transition to the homogenized macroscopic model of the process. Our earlier papers were devoted to homogenization the diffusion equation in a wide class of non-periodically perforated domains: strongly-connected domains, which includes locally-periodically perforated domains. In these works, an homogenized model was obtained, the coefficients of which are expressed in terms of “mesoscopic” (local energy) characteristics of the media, which are determined in small cubes, the size of which, however, are much larger than the microstructure scale. In these papers, convergence theorems were proved under the conditions of the existence of limiting densities of "mesoscopic" characteristics, the fulfillment of which is generally difficult to show, but in a number of specific situations this can be done. In this paper, we show the fulfillment of these conditions and, by studing them, we obtain explicit formulas for the effective characteristics of the locally-periodic porous medium: a conductivity tensor and a function of absorption.

Microscope image based fully automated stomata detection and pore measurement method for grapevines

BackgroundStomatal behavior in grapevines has been identified as a good indicator of the water stress level and overall health of the plant. Microscope images are often used to analyze stomatal behavior in plants. However, most of the current approaches involve manual measurement of stomatal features. The main aim of this research is to develop a fully automated stomata detection and pore measurement method for grapevines, taking microscope images as the input. The proposed approach, which employs machine learning and image processing techniques, can outperform available manual and semi-automatic methods used to identify and estimate stomatal morphological features.ResultsFirst, a cascade object detection learning algorithm is developed to correctly identify multiple stomata in a large microscopic image. Once the regions of interest which contain stomata are identified and extracted, a combination of image processing techniques are applied to estimate the pore dimensions of the stomata. The stomata detection approach was compared with an existing fully automated template matching technique and a semi-automatic maximum stable extremal regions approach, with the proposed method clearly surpassing the performance of the existing techniques with a precision of 91.68% and an F1-score of 0.85. Next, the morphological features of the detected stomata were measured. Contrary to existing approaches, the proposed image segmentation and skeletonization method allows us to estimate the pore dimensions even in cases where the stomatal pore boundary is only partially visible in the microscope image. A test conducted using 1267 images of stomata showed that the segmentation and skeletonization approach was able to correctly identify the stoma opening 86.27% of the time. Further comparisons made with manually traced stoma openings indicated that the proposed method is able to estimate stomata morphological features with accuracies of 89.03% for area, 94.06% for major axis length, 93.31% for minor axis length and 99.43% for eccentricity.ConclusionsThe proposed fully automated solution for stomata detection and measurement is able to produce results far superior to existing automatic and semi-automatic methods. This method not only produces a low number of false positives in the stomata detection stage, it can also accurately estimate the pore dimensions of partially incomplete stomata images. In addition, it can process thousands of stomata in minutes, eliminating the need for researchers to manually measure stomata, thereby accelerating the process of analysing plant health.

Molecular simulation of the pore size distribution effect on phase behavior of methane confined in nanopores

An understanding of the phase behavior of hydrocarbons is important in the petroleum reservoir simulation. However, fluid phase behavior in a shale reservoir is substantially different from conventional behavior. Since fluids are stored inside nanopores of shale rocks, there is a strong interaction between the pore boundary and fluid molecules. Due to this interaction, the fluid molecules are distributed heterogeneously inside the nanopores and the phase diagram is shifted under confinement. Advanced theoretical procedures such as molecular simulations are needed to properly model the heterogeneous molecular distribution inside the shale nanopores. Previous molecular simulation studies of nanoconfined hydrocarbon phase behavior have been limited to single pore size models. However, shale rocks usually have a wide pore size distribution (PSD) and single pore-size models are not accurate enough to represent a real shale system. In this work, to understand the PSD effect on the phase behavior, a recently proposed molecular simulation method, gauge-GCMC, is used to generate phase diagrams based on two types of cylindrical models (single pore and multiple pores, including one based on Eagle Ford shale rock). In single pore tests, the pore diameter is changed from 4 to 10 nm. Our results for multi-pore systems show that with an increasing pore size, the phase equilibrium properties approach the bulk values. Also, smaller pores cause a more significant shift in the phase diagram. Our results show that the small pores are filled before the large ones, which means that liquid will first be condensed in the small pores. In the Eagle Ford case, the pore model is designed by discretizing PSD data from experiments. The results show that it is possible to use a single pore model with a 10 nm diameter to represent the pore system of this shale sample.

Advective solute transport through porous media along streamlines

Transforming the standard solute transport models into one-dimensional ones along streamlines by excluding from them diffusive and dispersive components and direct including in simulation procedures the mechanical dispersion, the predominate factor defining solute transport in day-to-day applications, permit reducing these models to ordinary differential equations of the first order, making simulations less cumbersome and more flexible and permitting analytical solutions under about any practical conditions. The models of solute transport along flux tubes and streamlines are discussed in numerous publications. Most of those models, if not all, are one-dimensional copies of the mass conservation law and do not take in consideration streamline specificity: the absence of sinks and source within flux tubes to which the streamlines belong. This makes the obtained equations either inadequate or unnecessary oversimplified by assuming the pore water velocity constant along the streamlines. The development of the solute transport equations along streamlines taking in account the above mentioned specificity is presented in Appendix 1. Appendix 2 contains development of analytical solutions to the corresponding models for spatially varying actual pore water velocities, arbitrary initial and boundary conditions, linear and non-linear (Freundlich and Langmuir isotherms) interaction of solutes with the surroundings. Calculating concentration at given points and instants, the concentrations for the involved streamlines must be summarized accordingly to mechanical dispersion. The method of weighted summarizing is suggested. The approach is illustrated by field examples.

Tunable thermal conductivity in mesoporous silicon by slight porosity change

We report the thermal conductivity of photoelectrochemically synthesized mesoporous silicon (MPS), with ∼20-nm diameter pores and 52%–58% porosity. The thermal conductivity of MPS samples with a thickness of a few microns was measured using the three omega (3ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7 W/m K at room temperature and is dependent on the photoelectrochemical etching times used during the MPS synthesis, which induces a slight change in the MPS porosity. Calculations were conducted using the Boltzmann transport equation in the relaxation time approximation, with the results suggesting that the large thermal conductivity reduction in the MPSs was not entirely explained by the pore boundary scattering. Our findings indicate that elastic softening in the mesoporous structure may be responsible for the reduction in the thermal conductivity.

A detailed study on phonon transport in thin silicon membranes with phononic crystal nanostructures

A common method to improve thermoelectric performance is to reduce thermal conductivity by enhancing phonon scattering. In this paper, a frequency-dependent phonon radiative transport equation (PRTE) solver, based on the discrete ordinates method, is developed to simulate phonon transport in thin silicon membranes with phononic crystal nanostructures. The influence of geometric parameters on phonon transport is discussed in detail. Besides, a nonlinear regression model is attained for predicting the thermal conductivity of thin silicon membranes with phononic crystal nanostructures using the non-linear least-squares method. The results indicate that thermal conductivity is reduced by phononic crystal nanostructures mainly due to the back scattering of phonons with pore boundaries, and phonons with larger mean free path have stronger back scattering. When the pore placement is fixed, pore configuration affects phonon transport in thin silicon membranes with phononic crystal nanostructures. In addition, thermal conductivity is primarily controlled by three geometric parameters, including r⊥, r||, and Au. Moreover, the obtained regression model reveals the relationship between thermal conductivity and geometric parameters well, which can offer useful suggestions for fabricating thin silicon membranes with low thermal conductivity.

Investigating the effect of internal gradients on static gradient nuclear magnetic resonance diffusion measurements

Nuclear magnetic resonance (NMR) methods can be used to measure the diffusion coefficient [Formula: see text] of fluids. In porous materials, diffusion of the pore fluid is restricted by pore boundaries, such that [Formula: see text] may be smaller than the diffusion coefficient of the bulk fluid. This reduction in [Formula: see text] provides information about the geometry of the pore space. Significant overestimates of [Formula: see text] can, however, occur due to internal gradients caused by magnetic susceptibility contrasts between the pore fluid and the solid phase. We have investigated the way in which internal gradients can impact the measured diffusion coefficient and obscure the link to pore geometry in unconsolidated sediments. We focus on measurements of [Formula: see text] obtained with a static gradient diffusion-editing sequence, which can be used with NMR logging tools to measure [Formula: see text] in subsurface sediments. Laboratory measurements of [Formula: see text] measured with a diffusion-editing [Formula: see text]-[Formula: see text] sequence indicate significant impacts from internal gradients, including [Formula: see text] values several orders of magnitude larger than [Formula: see text] of bulk water. The log-mean [Formula: see text] values were found to be highly correlated with estimated internal gradient magnitudes and indicate no clear relationship to pore size. Samples with heterogeneous magnetic susceptibility of the solid phase indicate [Formula: see text] distributions with multiple peaks, reflecting the nonuniform distribution of internal gradients in the sediment. We found evidence of high internal gradients impacting a majority of our samples, resulting in increased [Formula: see text] values that do not reflect pore size even in samples with low magnetic susceptibility.

Modeling and Simulation of Cu Diffusion in Porous Low-k Dielectrics

Integrating nanometer sized pores into low-k ILD films is one of the approaches to lower the RC signal delay and thus help sustain the continued scaling of micro-electronic devices. While increasing porosity of porous dielectrics achieves the desirable lowering of the dielectric constant (k), it also creates a myriad of reliability and implementation issues. One of the problems is the little understood metal diffusion in porous media. Here, we present a rigorous simulation method of Cu diffusion based on master equation with elementary jump probabilities within the contiguous dielectric film, along the pore boundary, from the dielectric matrix to the pore boundary, and from the pore boundary to the matrix material. In view of the diffusional jump distance being as large as 2 nm, the nano-pores being on a similar length scale, and the film thickness being only a few tens of nanometers, the conventional diffusion equation in differential equation form is grossly inadequate and elementary jump frequencies are required for a proper description of the Cu diffusion in porous dielectric. The present atomistic approach allows a consistent implementation of Cu ion drift in electric field by lowering and raising of the diffusion barriers along the field direction. Two numerical methods are being used to evaluate the atomistic Cu transport: 1) Diffusion tensor theory (4th rank tensor in 2D, 9th rank in 3D), 2) Sequential lattice point evaluation of elementary diffusion jumps. The 2nd method is very efficient in 3D, where the simulation time increases only linearly with the dimension of the matrix, while the 1st method allows an easy implementation of jump frequencies at the boundaries of the film and pore boundaries.

Plasma flows and magnetic field interplay during the formation of a pore

We studied the formation of a pore in AR NOAA 11462. We analysed data obtained with the IBIS at the DST on April 17, 2012, consisting of full Stokes measurements of the Fe I 617.3 nm lines. Furthermore, we analysed SDO/HMI observations in the continuum and vector magnetograms derived from the Fe I 617.3 nm line data taken from April 15 to 19, 2012. We estimated the magnetic field strength and vector components and the LOS and horizontal motions in the photospheric region hosting the pore formation. We discuss our results in light of other observational studies and recent advances of numerical simulations. The pore formation occurs in less than 1 hour in the leading region of the AR. The evolution of the flux patch in the leading part of the AR is faster (< 12 hour) than the evolution (20-30 hour) of the more diffuse and smaller scale flux patches in the trailing region. During the pore formation, the ratio between magnetic and dark area decreases from 5 to 2. We observe strong downflows at the forming pore boundary and diverging proper motions of plasma in the vicinity of the evolving feature that are directed towards the forming pore. The average values and trends of the various quantities estimated in the AR are in agreement with results of former observational studies of steady pores and with their modelled counterparts, as seen in recent numerical simulations of a rising-tube process. The agreement with the outcomes of the numerical studies holds for both the signatures of the flux emergence process (e.g. appearance of small-scale mixed polarity patterns and elongated granules) and the evolution of the region. The processes driving the formation of the pore are identified with the emergence of a magnetic flux concentration and the subsequent reorganization of the emerged flux, by the combined effect of velocity and magnetic field, in and around the evolving structure.

Anisotropic Heat Conduction in Two-Dimensional Periodic Silicon Nanoporous Films

Research on heat conduction in periodic nanoporous silicon films has drawn much attention due to its importance for developing highly efficient thermoelectric devices. Here, the thermal transport in two-dimensional (2D) periodic silicon nanoporous films is studied by a phonon Monte Carlo (MC) method and the theoretical analyses based on the Boltzmann transport equation (BTE). It is found that both the cross-plane and the in-plane effective thermal conductivities are significantly reduced when compared to those in the diffusive limit, and decrease with the increasing porosity or the decreasing period length. Importantly, our work reveals a strong anisotropy of the effective thermal conductivity of the 2D periodic nanoporous films; that is, the effective thermal conductivity in the in-plane direction is significantly less than that in the cross-plane direction, due to the anisotropic effects of material removal and pore boundary scattering. Interestingly, even the influence of the specular parameter that de...