Strain energy-based homogenization of nonlinear elastic particulate composites

The macroscopic constitutive law for a heterogeneous solid containing two dissimilar nonlinear elastic phases undergoing finite deformation is obtained. Attention is restricted to the case of spherical symmetry such that only the materials consisting of an irregular suspension of perfectly spherical particles experiencing all-round uniform loading are considered which leads to a one-dimensional modeling. For the homogenization procedure, a strain-energy based scheme which utilizes Hashin’s composite sphere is employed to obtain the macroscopic stress-deformation relation added by the initial volume fraction of the particles. As applications of the procedure, the closed-form macroscopic stress expression for a generalized Carroll composite material is derived. Then, by choosing carbon black-filled rubbers, unknown bulk modulus of the carbon black particles is calculated. Finally, the particle-reinforced flexible polyurethane foam is studied using the Ritz method. It is shown that the analytical outcome for composites filled by compressible inclusions is applicable for porous materials with the same matrix.     

Indomethacin/Cu/LDPE porous composite for medicated copper intrauterine devices with controlled release performances

A novel functional material, indomethacin/copper/low density polyethylene (IDM/Cu/LDPE) porous composite, has been developed for medicated copper intrauterine devices (Cu-IUDs). The aim of this study was to report its preparation, structure and release performances. Samples of the IDM/Cu/LDPE porous composite were prepared by a combined technique involved in injection molding, particulate leaching, IDM solution incubating, and solvent vacuum drawing. Results of structure characterizations of these prepared samples show that the IDM/Cu/LDPE porous composite is obtained successfully and it is only a simple hybrid of IDM, copper particles and LDPE, and both the IDM and the copper particles are generally distributed homogeneously in its porous LDPE matrix. Results of IDM-loaded and release properties characterizations of these prepared samples show that the amount of loaded-IDM increases with the increasing of introduced porosity, and the release rates of both IDM and cupric ions increase with the increasing of porosity for these prepared samples with the same 25 wt.% of copper particles. It indicates that this novel porous composite not only can control its release of IDM, but also can control its release of cupric ions, its release properties can be regulated easily by controlling its amount of introduced porosity.     

Use of Nanoindentation,Finite Element Simulations,and a Combined Experimental/Numerical Approach to Characterize Elastic Moduli of Individual Porous Silica Particles

This article describes the use of a combination of experimental nanoindentation and finite element numerical simulations to indirectly determine the elastic modulus of individual porous, micron-sized silica (SiO₂) particles. Two independent nanoindentation experiments on individual silica particles were employed, one with a Berkovich pyramidal nanoindenter tip, the other with a flat punch nanoindenter tip. In both cases, 3D finite element simulations were used to generate nanoindenter load–displacement curves for comparison with the corresponding experimental data, using the elastic modulus of the particle as a curve-fitting parameter. The resulting indirectly determined modulus values from the two independent experiments were found to be in good agreement, and were considerably lower than the published values for bulk or particulate solid silica. The results are also consistent with previously reported modulus values for nanoindentation of porous thin film SiO₂. Based on a review of the literature, the authors believe that this is the first article to report on the use of nanoindentation and numerical simulations in a combined experimental/numerical approach to determine the elastic modulus of individual porous silica particles.     

Derivation of strain gradient length via homogenization of heterogeneous elastic materials

We present explicit upper bound estimates of the microstructural length used in simple gradient elasticity. Our model is a two dimensional composite made of circular hard inclusions randomly dispersed in a soft matrix. Both inclusions and matrix are described by isotropic linear elastic constitutive laws. The composite, however, is described by an isotropic gradient elastic law. The elastic modulus and the Poisson’s ratio are given by the exact classic analysis of Christensen. The in-plane microstructural length is estimated by energy optimization, based on solutions of the gradient elastic hollow cylinder. It was shown that the microstructural length decreases with the composition of the particles, taking high values at low particle composition. Naturally, the microstructural length is proportional to the particle diameter and increases with the stiffness of the particles. It was shown that there can be no microstructural prediction for particles that are softer than the matrix. This interesting result seems to be complementary to the result of Bigoni and Drugan who found that, for the couple-stress composite model, there can be no prediction for the microstructural length when the particles are stiffer than the matrix.     

Quartz crystal impedance response of nonhomogenous composite electrodes in contact with liquids

A new model of quartz-crystal impedance (QCI) of nonuniform layers composed of bumps of carbon particles (either porous or nonporous) and a polymeric binder layer has been proposed. The solid particles are modeled by semispherical and oblate semispheroid bumps embedded into the "sea" of a polymeric binder layer. On the basis of this model and elaborating on the principles of hydrodynamic spectroscopy of composite electrode materials, the geometric and porous structure parameters of nanoporous carbon and nonporous graphite composite electrodes in contact with liquids have been reliably determined. This work is believed to create a solid theoretical background for both advanced studies and optimized formulations of the composite electrodes suited to practical electrochemical devices and for the interpretation of the processes of ions and solvent insertion into nanoporous carbon electrodes uniquely probed by the QCI method (supercapacitive cells, desalination membranes).     

Physical properties of cement composites designed for aerostatic bearings

This paper investigates the physical properties of cement composites based on ordinary Portland cement (OPC) and silica particles as a potential material for porous aerostatic bearings for precision engineering applications. A full factorial design (2²4¹) was carried out to study the effects of silica properties (size and geometry) and uniaxial pressure (10 and 30 MPa) on the composite properties, namely bulk density, apparent porosity and intrinsic permeability of the ceramic composites. Scatter graphs were plotted to identify the existence of significant correlations between parameters. The cementitious composite manufactured with small silica particles, non-spherical shape and low level of compaction pressure exhibited the most appropriate properties for the proposed application. In addition, mathematical models obtained from the response-correlation plots are potentially important tools for the development and design of new composites for porous bearing applications.     

Identification of effective properties of particle reinforced composite materials

For the determination of effective elastic properties an energy averaging procedure has been used for particle reinforced composite materials. This procedure is based on finite element calculations of the deformation energy of a characteristic volume element. The proposed approach allows the determination of effective properties of particle reinforced composite with acceptable precision. The calculated effective properties of the composite are found in range between upper and lower Hashin-Shtrikman bounds. The averaging elastic properties of the composite depend on the properties of the particles, matrix volume fraction of the particles and some parameters taking into account the influence of the interphase between matrix and particles. These dependencies can be presented by simple analytical functions approximatically. An identification procedure basing on numerical experiments allows the estimation of the unknown approximation parameters. The obtained functions describe precisely the numerical data for any relationship between material constituents.     

Complete determination of elastic moduli of interpenetrating metal/ceramic composites using ultrasonic techniques and micromechanical modelling

The complete stiffness matrices of several metal/ceramic composites were analysed using the complementary ultrasonic spectroscopic techniques ultrasound phase spectroscopy (UPS) and resonant ultrasound spectroscopy (RUS). Three different aluminum/alumina composites having complex interpenetrating architectures were studied: a composite based on freeze-cast ceramic preform, a composite based on open porous ceramic preform obtained by pyrolysis of cellulose fibres, and a composite based on discontinuous fibre preform. Six of the nine independent elastic constants describing orthotropic elastic anisotropy were pre-determined by ultrasound phase spectroscopy and used as initial guess input for resonant ultrasound spectroscopy analysis, making the final results of all nine elastic constants more reliable. In all cases, consistent and reproducible results are obtained. Finally the experimental results were compared with effective elastic constants calculated using micromechanical modelling and a good correspondence between both is obtained.     

高体积含量颗粒增强复合材料的一个细观力学模型Ⅰ: 弹性分析与等效模量

提出了一种高体积含量颗粒增强复合材料的细观力学模型。该模型将颗粒简化为同质、同尺寸的弹性圆球, 两颗粒之间的粘接材料(基体) 简化为连接颗粒的一段圆柱体, 假设了圆柱形基体中的细观位移分布形式, 在此基础上分析了一对颗粒之间弹性的细观应力场和细观弹性系数, 将颗粒对的细观弹性系数在空间各个方向上平均, 得到材料的宏观弹性常数, 并建立了宏、细观分析之间的联系。最后用本模型分析了一种实际材料(两种体积含量) , 弹性常数的预测与实验吻合良好, 研究还发现颗粒的空间分布方式对材料宏观弹性常数的影响不大, 而对细观应力的影响显著。      

Synthesis of micron-sized porous CeO2SiO2 composite particles for ultraviolet absorption

Micron-sized porous composite particles composed of CeO₂ and SiO₂ nanoparticles were prepared for a UV absorption application by an aerosol spray-drying process from as-prepared CeO₂ nanoparticles, commercial SiO₂, and a polystyrene latex template. The morphology, structure crystallinity and pore size distribution of the as-prepared porous CeO₂SiO₂ composite particles were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Barrett-Joyner-Halenda (BJH) method, respectively. The porous CeO₂SiO₂ composite particles, with diameters of approximately 10 μm, showed a spherical morphology. As the contents of CeO₂ in the precursor was increased from 0.25 wt% to 1.5 wt%, we observed a change in the morphology of the composite particles from compactly packed porous particles to loosely packed porous particles. The as-prepared CeO₂SiO₂ composite particles were composed of meso- and macropores in the range of 3–200 nm. The effect of the CeO₂ content on the porous composite particles in terms of the UV absorption properties was also investigated by UV-visible spectroscopy. When the content of CeO₂ exceeded 0.75 wt% in the precursor, the particles showed higher UV absorption values compared to those of commercial TiO₂ nanoparticles. The as-prepared porous CeO₂SiO₂ composite particles can therefore be promising materials given their high UV absorption value.     

Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers

Carbonaceous porous matrices were prepared from a tannin-based resin by physical foaming, having improved thermal properties by addition of various kinds and various amounts of graphite fillers. The resultant composite carbon foams presented much higher thermal conductivity, making them suitable for hosting phase-change materials with the aim of using them in seasonal storage applications. These materials were investigated in terms of porous structure, thermal and mechanical properties. It was shown that, unlike what was a priori expected, smaller particles were far more suitable for getting conductive, strong and porous matrices. The smaller were the particles, the better were the results. These findings were explained and justified, making such biomass-based composite carbon foams interesting and cheap candidates for thermal storage applications.     

TC4负泊松比复合结构的人工骨力学性能调控研究

目的 基于不同变形机制的负泊松比结构优化设计新型复合多孔结构样件,增加力学性能的调控维度,以满足人体骨低弹性模量的匹配要求。方法 用内凹多边形替代手性结构的圆环,以获得新型的复合胞元结构。利用选区激光熔化成形技术制备负泊松比多孔人工骨样件,通过压缩实验揭示胞元结构类型、结构参数、孔隙率对屈服强度、弹性模量的影响规律,评测不同结构样件与人体骨间的力学性能匹配程度。结果 当孔隙率为65%～85%时,复合结构样件的成形质量、力学性能基本介于手性结构的和内凹结构的之间,且与孔隙率密切相关。手性结构、内凹结构和复合结构的弹性模量分别为2.39～4.64、1.12～3.77、1.01～3.47 GPa,屈服强度分别为65.19～223.06、45.25～195.81、26.54～143.58MPa。复合结构的弹性模量随环径和内凹角度的增大而减小。当孔隙率为75%时,环径由2.4 mm变至2.0 mm,弹性模量由2.651 GPa降低至2.082 GPa。当内凹角度由85°变至65°时,弹性模量则由3.566GPa降低至1.982GPa。结论 复合胞元结构可以融合材料特性,增加调控维度,进而匹配人工...     

Beach sand from Cancun Mexico: a natural macro- and mesoporous material

Sand particles from Cancun, Mexico were studied using a number of advanced spectroscopic and microscopic techniques. The main chemical composition of sand particles was confirmed to be calcium carbonate by X-ray photoelectron spectroscopy and IR spectroscopic analysis. X-ray diffraction analysis revealed that the sand particles are aragonite, which has an Orthorhombic—Dipyramidal crystal structure. The morphological study of the sand particles by scanning electron microscopy and transmission electron microscopy revealed the presence of a highly porous channel-like structure in the sand particles. The sorption isotherm indicates that Cancun sand is a mesoporous material. The specific surface area of Cancun sand was determined to be 2.259 m²/g by BET measurement, which is significantly higher than that of Florida sand and other forms of natural aragonite and calcite. Furthermore, it was found that the porous sand particles can adsorb gold nanoparticles of the size of a few nanometers very efficiently. The distribution of gold nanoparticles demonstrated a channel-like porous inner structure of the sand particles. We also prepared a polymer composite material by mixing the sand particles with a poly(methyl methacrylate) matrix. SEM analysis of the composite materials showed a good interfacial adhesion between sand particles and polymer matrix. These results suggest that Cancun sand, as a natural macro- and mesoporous material, may find promising applications in filtration, pollution control, composite materials and biomaterials development. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Thermal expansion responses of pressure infiltrated SiC/Al metal-matrix composites

Aluminium-matrix composites containing thermally oxidized and unoxidized SiC particles featuring four average particle diameters ranging from 3 to 40 μm were produced by vacuum assisted high pressure infiltration. Their thermal expansion coefficient (CTE) was measured between 25 and 500°C. Oxidation of the SiC particles in air produces the formation at their surface of silicon oxide in quantities sufficient to bond the particles together, and confer strength to preforms. After infiltration with pure aluminium, the composites produced showed no sign of significant interfacial reaction. The CTE of the composite reinforced with unoxidized SiC particles featured an abrupt upward deviation upon heat-up near 200°C, and a second abrupt decrease near 400°C. The first transition is attributed to an inversion of stress across particle contact points. When composites are produced with oxidized SiC particles, these two transitions were removed, their CTE varying smoothly and gradually from the lower elastic bound to the upper elastic bound as temperature increases. With both composite types, the CTE decreased as the average particle size decreased. This work illustrates the benefits of three-dimensional reinforcement continuity for the production of low-CTE metal matrix composites, and shows a simple method for producing such composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Shear waves in a piezoceramic layered structure

The propagation of SH-type wave is studied in a composite structure consisting of alternating polymeric layers and porous piezoelectric layers. The porous piezoelectric materials of the composite structure are assumed to have 6mm symmetry and their poling direction is along z-axis. Layers of the polymer are considered as isotropic dielectric elastic material. Solutions of the field equations for the porous piezoelectric material and for the polymeric material are obtained. Two cases, first when the direction of propagation of the SH-type wave is taken along the direction normal to the layering of the composite structure, and second when the propagation direction is taken along the layering, are considered for the derivation of the phase velocity. The dispersion and the stop-pass band behavior of the Floquet wave is also discussed. Numerical results for phase velocity and stop band effect are presented for a periodic system of alternating PZT-5H and polythene layers. The influence of volume fraction on phase velocity and stop band effect is discussed.     

OTS-modified HA and its toughening effect on PLLA/HA porous composite

In this paper, hydroxyapatite (HA) particles was modified with long-chain organic silane-Octadecyltrichlorosilane (OTS), and the modified particles were further used for preparing Poly(l-lactic acid) PLLA/HA porous composite. The modified particles were characterized by means of XRD, FTIR, and XPS techniques. Both XPS and FTIR results showed that OTS had been combined with HA, and the formation of P–O–Si bond, a covalent bond, on the HA particle surface was confirmed by XPS. OTS-modified HA particles were used to prepare porous composites by thermally induced phase separation method. The results showed that the composite had an interconnected pore structure with 100–300 μm macropores. With OTS dosage increasing during modification, the mechanical properties of PLLA/OTS-modified HA porous composites increased obviously. These results showed that OTS modification can effectively improve the interface compatibility between HA surface and PLLA.     

Erhöhung der Thermoschockbeständigkeit von Sinterglas und Keramik über das Verbundwerkstoffkonzept

Increasing the Thermal Shock Resistance of Sintered Glass and Ceramics by the Composite Materials Concept The thermal shock resistance of brittle materials such as glass and ceramics is one of their weaknesses. Pores and above all incorporated second phases in these materials alter these properties which are decisive for thermal shock behavior, and may therefore increase this behavior in a precalculable manner. The present paper will first theoretically demonstrate when and why porosity leads to an improvement in thermal shock resistance. The thermal shock resistance for porous borosilicate sintered glass and porous eutectic calcium titanate ceramic are calculated and compared to experimental values. They confirm

• that low porosities lead to an improvement in thermal shock resistance
• that the thermal shock resistance has a maximum at a certain porosity and
• that above certain porosities the presence of pores deteriorates the thermal shock resistance.

If one considers porous materials as a special case of composite materials then relations valid for composite materials can be transferred to porous materials (“composite material concept”) and viceversa. This is investigated using the examples of borosilicate sintered glass with incorporated antimony particles and eutectic calcium titanate ceramic with incorporated paladium particles. In the case of the glass-antimony composite material, improvements in thermal shock resistance of about 15% with 10 vol% antimony incorporation were calculated and confirmed experimentally, while for calcium titanate-paladium composite materials a 15% improvement in thermal shock resistance was already achieved with about 5 vol% of the metallic phase.     

The structure of porous nickel titanium reinforced by monolithic nickel titanium

The structure and elemental composition of a composite representing porous nickel titanium reinforced by a monolithic nickel titanium rod (TN-10 alloy) have been studied. The structures of the initial TiNi powder (PN55T45S grade), the porous and monolithic parts of this composite, and the transition regions between these parts have been studied using a scanning electron microscope (SEM 515). Elemental compositions of the TiNi matrix phase and secondary phase inclusions were determined using an electron probe microanalyzer (JEOL JSM 840). It is established that there is no clear interface between the sintered porous and monolithic parts of the composite. Instead, the TiNi particles and monolithic TN-10 alloy rod exhibit complete mutual dissolution. The elemental composition of the TiNi matrix region exhibits rather insignificant variations between monolithic and porous parts.     

Calendering-Compatible Macroporous Architecture for Silicon–Graphite Composite toward High-Energy Lithium-Ion Batteries

Porous strategies based on nanoengineering successfully mitigate several problems related to volume expansion of alloying anodes. However, practical application of porous alloying anodes is challenging because of limitations such as calendering incompatibility, low mass loading, and excessive usage of nonactive materials, all of which cause a lower volumetric energy density in comparison with conventional graphite anodes. In particular, during calendering, porous structures in alloying-based composites easily collapse under high pressure, attenuating the porous characteristics. Herein, this work proposes a calendering-compatible macroporous architecture for a Si–graphite anode to maximize the volumetric energy density. The anode is composed of an elastic outermost carbon covering, a nonfilling porous structure, and a graphite core. Owing to the lubricative properties of the elastic carbon covering, the macroporous structure coated by the brittle Si nanolayer can withstand high pressure and maintain its porous architecture during electrode calendering. Scalable methods using mechanical agitation and chemical vapor deposition are adopted. The as-prepared composite exhibits excellent electrochemical stability of > 3.6 mAh cm⁻², with mitigated electrode expansion. Furthermore, full-cell evaluation shows that the composite achieves higher energy density (932 Wh L⁻¹) and higher specific energy (333 Wh kg⁻¹) with stable cycling than has been reported in previous studies.     

Approaches to the fabrication of calcium phosphate-based porous materials for bone tissue regeneration

This paper reviews advances in the fabrication of calcium phosphate materials for injured bone tissue regeneration. We examine the key features of rapid prototyping for the fabrication of porous ceramic scaffolds with tailored architectures, the technology of biopolymer-based composite materials reinforced with calcium phosphate particles, and the fabrication of porous scaffolds via cement route.