Surface effects on mechanical behavior of elastic nanoporous materials under high strain

This paper studies surface effects on the mechanical behavior of nanoporous materials under high strains with an improved anisotropic Kelvin model. The stress-strain relations are derived by the theories of Euler-Bernoulli beam and surface elasticity. Meanwhile, the influence of strut (or ligament) size on the mechanical properties of nanoporous materials is discussed, which becomes a key factor with consideration of the residual surface stress and the surface elasticity. The results show that the decrease in the strut diameter and the increase in the residual surface stress or the surface elasticity can both lead to an increase in the carrying capacity of nanoporous materials. Furthermore, mechanical behaviors of anisotropic nanoporous materials in different directions (the rise direction and the transverse direction) are investigated. The results indicate that the surface effects in the transverse direction are more obvious than those in the rise direction for anisotropic nanoporous materials. In addition, the present results can be reduced to the cases of conventional foams as the strut size increases to micron-scale, which confirms validity of the model to a certain extent.     

Scaling laws of nanoporous gold under uniaxial compression: Effects of structural disorder on the solid fraction,elastic Poisson's ratio,Young's modulus and yield strength

In this work the relationship between the structural disorder and the macroscopic mechanical behavior of nanoporous gold under uniaxial compression was investigated, using the finite element method. A recently proposed model based on a microstructure consisting of four-coordinated spherical nodes interconnected by cylindrical struts, whose node positions are randomly displaced from the lattice points of a diamond cubic lattice, was extended. This was done by including the increased density as result of the introduced structural disorder. Scaling equations for the elastic Poisson's ratio, the Young's modulus and the yield strength were determined as functions of the structural disorder and the solid fraction. The extended model was applied to identify the elastic–plastic behavior of the solid phase of nanoporous gold. It was found, that the elastic Poisson's ratio provides a robust basis for the calibration of the structural disorder. Based on this approach, a systematic study of the size effect on the yield strength was performed and the results were compared to experimental data provided in literature. An excellent agreement with recently published results for polymer infiltrated samples of nanoporous gold with varying ligament size was found.     

Effect of surface/interface stress on the plastic deformation of nanoporous materials and nanocomposites

Surface and interface play an important role on the overall mechanical behaviors of nanostructured materials. We investigate the effect of surface/interface stress on the macroscopic plastic behaviors of nanoporous materials and nanocomposites, where both the surface/interface residual stress and surface/interface elasticity are taken into account. A new second-order moment nonlinear micromechanics theory is developed and then reduced to macroscopically isotropic materials. It is found that the effect of surface/interface residual stress is much more prominent than that of the surface/interface elasticity, causing strong size effect as well as asymmetric plastic deformation for tension and compression. The variation of yield strength is more prominent with smaller pore/inclusion size or higher pore/inclusion volume fraction. For a representative nanoporous aluminum, the surface effect becomes significant when the pore radius is smaller than about 50 nm. When hard inclusions are embedded in a ductile metal matrix, the interface effect and resulting size effect are much smaller than that of nanoporous materials. The results may be useful for evaluating the mechanical integrity of nanostructured materials.     

Fast Spontaneous Transport of a Non-wetting Fluid in a Disordered Nanoporous Medium

Transport in Porous Media - The experimental study of cooperative fast transport of non-wetting fluid in a disordered nanoporous medium is carried out in this work. New experimental data for...     

含有序孔洞的多孔氧化铝薄膜的弹性模量测量

高纯铝箔在特定的溶液下经过电化学阳极氧化腐蚀，可在其表面生成一层多孔的非晶氧化铝层，孔大致呈六方密排，孔径分布均匀。此类薄膜具有规则的纳米级孔径，大的比表面积，可用在微纳滤方面和纳米材料组装方面。然而，对于此类薄膜力学性能的研究较少，在一定程度上限制其功能的开发和应用。为了获得此类多孔膜的弹性常数，本文用鼓膜法结合散斑干涉实验方法、单轴拉伸结合双光束干涉法和多普勒测振仪三种方法测量氧化铝多孔膜的弹性模量，得到的宏观弹性模量基本相同，并对三种方法的优缺点进行了比较，分析了多孔氧化铝膜与块状氧化铝材料或致密氧化铝膜力学性能的差并。     

Closed-form formulas for the effective properties of random particulate nanocomposites with complete Gurtin–Murdoch model of material surfaces

The objective of this work is to present an approach allowing for inclusion of the complete Gurtin–Murdoch material surface equations in methods leading to closed-form formulas defining effective properties of particle-reinforced nanocomposites. Considering that all previous developments of the closed-form formulas for effective properties employ only some parts of the Gurtin–Murdoch model, its complete inclusion constitutes the main focus of this work. To this end, the recently introduced new notion of the energy-equivalent inhomogeneity is generalized to precisely include all terms of the model. The crucial aspect of that generalization is the identification of the energy associated with the last term of the Gurtin–Murdoch equation, i.e., with the surface gradient of displacements. With the help of that definition, the real nanoparticle and its surface possessing its own distinct elastic properties and residual stresses are replaced by an energy-equivalent inhomogeneity with properties incorporating all surface effects. Such equivalent inhomogeneity can then be used in combination with any existing homogenization method. In this work, the method of conditional moments is used to analyze composites with randomly dispersed spherical nanoparticles. Closed-form expressions for effective moduli are derived for both bulk and shear moduli. As numerical examples, nanoporous aluminum is investigated. The normalized bulk and shear moduli of nanoporous aluminum as a function of residual stresses are analyzed and evaluated in the context of other theoretical predictions.     

MULTIAXIAL BEHAVIOR OF NANOPOROUS SINGLE CRYSTAL COPPER： A MOLECULAR DYNAMICS STUDY

The stress-strain behavior and copper are studied by the molecular dynamics incipient yield surface of nanoporous single crystal （MD） method. The problem is modeled by a periodic unit cell subject to multi-axial loading. The loading induced defect evolution is explored. The incipient yield surfaces are found to be tension-compression asymmetric. For a given void volume fraction, apparent size effects in the yield surface are predicted： the smaller behaves stronger. The evolution pattern of defects （i.e., dislocation and stacking faults） is insensitive to the model size and void volume fraction. However, it is loading path dependent. Squared prismatic dislocation loops dominate the incipient yielding under hydrostatic tension while stacking-faults are the primary defects for hydrostatic compression and uniaxial tension/compression.     

Modern Methods of Drying Nanomaterials

This paper presents the types of nanomaterials whose technology requires drying during processing. The methods of drying of three groups of nanomaterials are discussed: nanoparticles, nanolayers (nanofilms), and nanoporous materials. Principally the method of spray drying using ultrasonic nebulizers and electrospraying is presented for the first group. The Langmuir–Blogget technique is presented for the second group. The method of supercritical drying and the solvent replacement technique is described for the third group. Technological aspects of the presented methods are also briefly described.     

Theory of sorption hysteresis in nanoporous solids: Part I: Snap-through instabilities

The sorption–desorption hysteresis observed in many nanoporous solids, at vapor pressures low enough for the liquid (capillary) phase of the adsorbate to be absent, has long been vaguely attributed to some sort of ‘pore collapse’. However, the pore collapse has never been documented experimentally and explained mathematically. The present work takes an analytical approach to account for discrete molecular forces in the nanopore fluid and proposes two related mechanisms that can explain the hysteresis at low vapor pressure without assuming any pore collapse nor partial damage to the nanopore structure. The first mechanism, presented in Part I, consists of a series of snap-through instabilities during the filling or emptying of non-uniform nanopores or nanoscale asperities. The instabilities are caused by non-uniqueness in the misfit disjoining pressures engendered by a difference between the nanopore width and an integer multiple of the thickness of a monomolecular adsorption layer. The wider the pore, the weaker the mechanism, and it ceases to operate for pores wider than about 3 nm. The second mechanism, presented in Part II, consists of molecular coalescence, or capillary condensation, within a partially filled surface, nanopore or nanopore network. This general thermodynamic instability is driven by attractive intermolecular forces within the adsorbate and forms the basis for developing a unified theory of both mechanisms. The ultimate goals of the theory are to predict the fluid transport in nanoporous solids from microscopic first principles, determine the pore size distribution and internal surface area from sorption tests, and provide a way to calculate the disjoining pressures in filled nanopores, which play an important role in the theory of creep and shrinkage.     

Linear rheology of compressible soft nanocomposites

The influences of interfacial tension and compressibility to the linear viscoelastic properties of nanocomposite and nanoporous materials are considered theoretically. The effective bulk and shear moduli of the systems are calculated within the generalized composite sphere model which takes into account the effect of interfacial tension. It is found that frequency dependence of the effective dynamic shear and bulk moduli of nanocomposites with the compressible elastic matrix and viscous inclusions may be represented in terms of the Zener model comprising of the viscoelastic Kelvin element in series with the elastic spring. The relations of the Zener model parameters with the material characteristics are revealed. The physical interpretation of the frequency behavior of the dynamic shear and bulk moduli against the interfacial tension, component compressibility, viscosity, and inclusion volume fraction is discussed. Victor G. Oshmyan deceased.     

Nanoporous metal organic framework materials for hydrogen storage

Hydrogen is expected to play an important role in future transportation as a promising alternative clean energy source to carbon-based fuels.One of the key challenges to commercialize hydrogen energy is to develop appropriate onboard hydrogen storage systems,capable of charging and discharging large quantities of hydrogen with fast enough kinetics to meet commercial requirements.Metal organic framework (MOF) is a new type of inorganic and organic hybrid nanoporous particulate materials.Its diverse networks can enhance hydrogen storage through tuning the structure and property of MOFs.The MOF materials so far developed adsorb hydrogen through weak disperston interactions,which allow significant quantity of hydrogen to be stored at cryogenic temperatures with fast kinetics.Novel MOFs are being developed to strengthen the interactions between hydrogen and MOFs in order to store hydrogen under ambient conditions.This review surveys the development of such candidate materials,their performance and future research needs.     

Void-shape effects on strength properties of nanoporous materials

In this paper, strength properties of nanoporous materials with spheroidal nanocavities are investigated via a Molecular Dynamics approach applied to a nanovoided aluminium single crystal, in the case of a fixed porosity level, and for prolate, oblate and spherical void shapes. Estimates of the effective strength domain are provided, by considering several mechanical loadings including axisymmetric and shear strain-rate states. Void-shape effects are quantified for different values of the void aspect ratio, mainly resulting in an overall weakening of the sample as the spheroidal nanovoid assumes either an oblate or a prolate shape, in comparison to the case of a spherical void. Finally, it is observed that the computed strength profiles exhibit the following specific features: (i) a strong dependence on the hydrostatic, second-order and third-order deviatoric stress invariants, (ii) more significant void-shape effects for triaxial-expansion stress states with a small hydrostatic component, and (iii) a more pronounced influence of the spheroid shape, as the aspect ratio is varied, in the presence of an oblate nanovoid rather than of a prolate one.     

NANOSCALE PROCESS ENGINEERING

The research of nanoscale process engineering (NPE) is based on the interdisciplinary nature of nanoscale science and technology. It mainly deals with transformation of materials and energy into nanostructured materials and nanodevices, and synergizes the multidisciplinary convergence between materials science and technology, biotechnology, and information technology. The core technologies of NPE concern all aspects of nanodevice construction and operation, such as manufacture of nanomaterials “by design“, concepts and design of nanoarchitectures, and manufacture and control of customizable nanodevices. Two main targets of NPE at present are focused on nanoscale manufacture and concept design of nanodevices. The research progress of nanoscale manufacturing processes focused on creating nanostructures and assembling them into nanosystems and larger scale architectures has built the interdiscipline of NPE. The concepts and design of smart, multi-functional, environmentally compatible and customizable nanodevice prototypes built from the nanostructured systems of nanocrystalline, nanoporous and microemulsion systems are most challenging tasks of NPE. The development of NPE may also impel us to consider the curriculum and educational reform of chemical engineering in universities.     

Endurance Tests on a Colloidal Damper Destined to Vehicle Suspension

Endurance tests on a colloidal damper destined to vehicle suspension are performed. Such absorber represents an ecological application of nano-damping; it employs the hysteresis which occurs when water is forced to penetrate and then naturally exudes from a nanoporous silica gel matrix, modified to become liquid-repellent. Damping performances decrease at the increasing of the number of working cycles, partially since the silica gel grains that undergo gradual fatigue fracture are able to escape at the packing used to seal the test chamber, and partially due to the fatigue fracture alone, which is accompanied by an enhancement of the hydrophilic silanol groups on the silica gel surface and a pore size redistribution. In order to augment damper’s life, silica gel is introduced inside of a tank that is separated by a filter from the main cylinder, in which only water is supplied. One discusses the influence of filtration on the colloidal damper performances and the variation of damper’s life versus the ratio of filter pore’s diameter to the mean size of the silica gel particles.     

Theory of sorption hysteresis in nanoporous solids: Part II Molecular condensation

Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple “hierarchical wetting” model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.     

Influence of interfaces on effective properties of nanomaterials with stochastically distributed spherical inclusions

The method of conditional moments is generalized to include evaluation of the effective elastic properties of particulate nanomaterials and to investigate the size effect in those materials. Determining the effective constants necessitates finding a stochastically averaged solution to the fundamental equations of linear elasticity coupled with surface/interface conditions (Gurtin–Murdoch model). To obtain such a solution the system of governing stochastic differential equations is first transformed to an equivalent system of stochastic integral equations. Using statistical averaging, the boundary-value problem is then converted to an infinite system of linear algebraic equations. A two-point approximation is considered and the stress fluctuations within the inclusions are neglected in order to obtain a finite system of algebraic equations in terms of component-average strains. Closed-form expressions are derived for the effective moduli of a composite consisting of a matrix and randomly distributed spherical inhomogeneities. As a numerical example a nanoporous material is investigated assuming a model in which the interface effects influence only the bulk modulus of the material. In that model the resulting shear modulus is the same as for the material without surface effects. Dependence of the bulk moduli on the radius of nanopores and on the pore volume fraction is analyzed. The results are compared to, and discussed in the context of other theoretical predictions.     

Multi-scale Analysis of Gas Transport Mechanisms in Kerogen

Quantification of natural gas transport in organic-rich shale is important in predicting natural gas production. However, laboratory measurements are challenging due to tight nature of the rock and include large uncertainties. The emphasis of this work is to understand mass transport mechanisms inside the organic nanoporous material known as kerogen under subsurface conditions and describe its permeability. This requires a multi-scale theoretical approach that includes flow measurements in model nanocapillaries and within their network. Molecular dynamics simulation results of steady-state supercritical methane flow in single-wall carbon nanotube are presented in this article. A transition from convection to molecular diffusion is observed. The simulation results show that the adsorbed methane molecules are mobile and contribute a significant portion to the total mass flux in nanocapillaries with diameter \({<}\)10 nm. They experience cluster diffusion that is dependent on the applied pressure drop across the capillary. A modified Hagen–Poiseuille equation is proposed considering the convective–diffusive nature of the overall transport in nanocapillary. The molecular-level study of steady-state transport is extended to a simple network of interconnected nanocapillaries representing kerogen. The modified Hagen–Poiseuille equation leads to a representative elementary volume of the model kerogen. The estimated permeability of the volume is sensitive to compressed and adsorbed fluids density ratio and to surface properties of the nanocapillary walls, indicating that fluid–wall interactions driven by molecular forces could be significant during the large-scale transport within shale. A modified Kozeny–Carman correlation is proposed, relating kerogen porosity and tortuosity to the permeability.     

Ultrasonic measurement of the bending of a bolt in a shear joint

An ultrasonic pulse/echo technique is used to measure preload in bolts used in structural joints. In this paper, the same instrument is used in a different way to measure change in the ultrasonic measurements due to bending in the bolts. A theory that explains the ultrasonic measurements is developed. The bending loads result in a rotation and a translation of the ultrasonic pulse reflecting face. It also creates a stress gradient in the bolt. This results in a phase variation (or gradient) in the received ultrasonic beam across the face of the transducer. It also results in a physical shift in the received beam relative to the ultrasonic transducer. The phase gradient and the shift in the beam results in change in the pulse travel time. A number of experiments were performed on the bolt to study the effect of the bending on the ultrasonic measurements. The experiments and the theory validate a sensitive new method for measuring the bending loads in the bolts.     

双柱单锥型液-液旋流管内流场的激光诊断

应用激光测速仪，对一种双柱单锥型液 液旋流管内的流动结构，进行了全场范围内的多工况流动诊断研究．揭示出其切向速度由内旋流区和外旋流区构成，其中内旋流区中的速度分布符合准强制涡关系，外旋流区中的速度分布符合准自由涡关系；轴向速度由上行流动区和下行流动区构成，两者之间在直管段以零速点作分界，在锥体段则以零速区作过渡并伴随有一定的回流出现，且该过渡区或回流区的大小随锥体截面的收缩而减小，直到进入直管段后消失；各湍流量的分布以管芯处最大向外逐渐减小，沿轴向是直管段中的湍流度大于锥体段中的湍流度，而且湍流度在旋流管内具有各向异性的特性．