Multiphase Composite Materials Research Articles

Influence of different sizes of concrete and roller compacted concrete on double-K fracture parameters

Affected by physical properties of various components, characteristics and stress states of junction surface and other multiple factors, concrete, as a kind of multi-phase composite material, has complicated failure mechanism, thus making its fracture mechanism research difficult. But concrete has been widely used in engineering construction, so research on concrete fracture theory is of important realistic significance and construction value. This study discusses influence rule of specimen size and crack-depth ratio (?0/h) on doubleK fracture parameters (initial fracture toughness ini ICK and unstable fracture toughness un ICK ) and its size effects by using fracture test. Different sizes of concretes and roller compacted concrete (RCC) specimens are adopted to explore influence of specimen size on concrete double-K fracture parameters. Results reveal that initial fracture toughness ini ICK and unstable fracture toughness un ICK increase as specimen size enlarges, showing a size effect; besides, subcritical crack extends with the increase of specimen size. With regard to specimens with different crack-depth ratios, its unstable fracture toughness un ICK is unrelated to initial crack-depth ratio when crack-depth ratio is more than or equal to 0.4, while initial fracture toughness ini ICK is correlated with initial crack-depth ratio, which indicate that double-K fracture parameters can be considered as material constants describing concrete initial fracture, stable expansion and whole process of stability failure.

Designing and Harnessing the Metastable States of a Modular Metastructure for Programmable Mechanical Properties Adaptation

Recent studies on periodic metamaterial systems have shown that remarkable properties adaptivity and versatility are often the products of exploiting internal, coexisting metastable states. Motivated by this concept, this research develops and explores a local-global design framework wherein macroscopic system-level properties are sought according to a strategic periodic constituent composition and assembly. To this end and taking inspiration from recent insights in studies of multiphase composite materials and cytoskeletal actin networks, this study develops adaptable metastable modules that are assembled into modular metastructures, such that the latter are invested with synergistic features due to the strategic module development and integration. Using this approach, it is seen that modularity creates an accessible pathway to exploit metastable states for programmable metastructure adaptivity, including a near-continuous variation of mechanical properties or stable topologies and adjustable hysteresis. A model is developed to understand the source of the synergistic characteristics, and theoretical findings are found to be in good agreement with experimental results. Important design-based questions are raised regarding the modular metastructure concept, and a genetic algorithm (GA) routine is developed to elucidate the sensitivities of the properties variation with respect to the statistics amongst assembled module design variables. To obtain target multifunctionality and adaptivity, the routine discovers that particular degrees and types of modular heterogeneity are required. Future realizations of modular metastructures are discussed to illustrate the extensibility of the design concept and broad application base.

A fully coupled thermal–electrical–mechanical micromodel for multi-phase periodic thermoelectrical composite materials and devices

Abstract This paper introduces a new fully coupled micro thermal–electrical–mechanical (TEM) formulation for periodic multi-phase thermoelectric (TE) material systems and devices. The high fidelity generalized method of cells (HFGMC) micromechanical method is extended to account for the fully (three-way) coupled TEM effects including Seebeck, Peltier, and Joule heat. A special attention is directed towards the induced mechanical field which has often neglected in the literature. The three-dimensional HFGMC formulation is performed by analyzing an isolated periodic volume or a repeated unit-cell (RUC) and by subdividing it into sub-volumes (subcells). Interfacial continuity between adjacent subcells along with periodicity conditions are expressed in terms of average TEM fields and fluxes. The three TEM conservation laws are also expressed in a volume average over the subcell. A three-way coupled set of constitutive equations are used to express the thermoelectric, thermomechanical and electro–mechanical relations for each subcell. Applications are presented for the steady state solutions of the TEM field distributions in two cases. The first for a TE generator device that consists of an array of repeating RUC. The second is for multi-layered laminated composite with a repeating stack of layers including two layers reinforced with p-type and n-type inclusions. The steady-state stress distribution is shown to be a nonlinear solution to the system of coupled TEM governing equations. The new HFGMC-TEM micromodel is effective in establishing the overall (average) TEM constitutive laws for the equivalent homogenized medium along with the spatial distributions of different local fields within the RUC.

Predicting Mechanical and Thermal Behavior of Multiphase Composite Containing Phase Change Material (PCM) by Micromechanical Approaches

This work is devoted to the study of thermal and mechanical properties (stiffness and thermal conductivity tensors) of multiphase composite materials (mortar with PCMs). A micro-macro modeling is presented in order to predict these properties by using some classical homogenization schemes. The effective thermo-mechanical behavior is then analyzed by considering various PCM particle forms, concentrations, and orientations.

(Carl Wagner Memorial Award -and- Battery Division Research Award) Anodes for Lithium Ion Batteries Revisited: From Graphite to High-Capacity Alloying- and Conversion-Type Materials and Back Again

Currently, graphitic carbon is still the most commonly used anode material for commercial lithium ion batteries (LIBs) due to its relatively high specific capacity, high structural stability and low operation potential.(1) For graphitic materials, it is widely accepted that the surface chemistry and morphology of the prismatic surfaces have a significant impact on the reactivity in LIBs. Current strategies for optimization of this type of anode material, e.g. graphite surface modifications, focus on the improvement of the irreversible capacity in the first cycle and rate performance.(2)Regarding the need for high capacity anode materials in order to further increase the specific energy (Wh kg-1) and energy density (Wh L-1) of LIBs, silicon is one of the most promising candidates. Among different alloying elements, such as tin or germanium, silicon displays the highest reversible capacity of ca. 3,500 mAh g-1 and a relatively low lithiation potential of ca. 0.3 V vs. Li/Li+. However, drawbacks arise with the use of silicon, which are mainly related to the severe volume changes (up to 300%) during the lithium uptake/release process and the low intrinsic electronic conductivity.(3) The large stress and strain during lithiation/de-lithiation may lead to particle cracking and even particle pulverization resulting in a contact loss between the electrode components as well as into an ongoing formation of the solid electrolyte interphase (SEI) and an increase in the cell impedance.(3, 4)Several strategies have been developed in order to reduce the detrimental effects of the large volume changes of silicon and to suppress the side reactions with the electrolyte and thus to improve the long-term cycling stability (Figure 1).(3) One main approach is to reduce the silicon particle size to the nanometer range. Another strategy is to design porous structures, e.g. macroporous (pore width ˃ 50 nm) or mesoporous materials (2 to 50 nm), that can offer sufficient local void space to absorb the volume expansion.(5) A further strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, i.e. by introducing components, which display no or only less volume changes as well as preferentially a high electronic conductivity. The idea of this approach is to use the host matrix to buffer the volume changes of silicon and thus to maintain the electrode integrity and electronic network. Examples are silicon-carbon composite materials and silicon-based alloys or composite heterostructures, such as Si/SiOx/carbon composites, or intermetallic compounds like Si/TiSi2 or NiSi alloys.(5, 6) Figure 1. Strategies to improve high-capacity anode materials for LIBs.Besides alloying-type anode materials, also conversion-type materials, such as transition metal oxides like Fe3O4 and MnO, which store charge via a conversion reaction (MOx + 2xLi ⇌ M + xLi2O), are of growing interest for next generation LIBs.(7) This is related to their high specific capacity of ca. 800 to 1200 mAh g-1 and the broad range of material combinations (oxides, sulfides, etc.). However, these materials typically suffer from a large irreversible capacity in the first charge/discharge cycle and from a relatively poor cycling stability.(7)Here, we report on the improvement of graphite-based anode materials as well as high-capacity materials based on alloying and conversion mechanisms for application in LIBs. The latter ones include the optimization of group IV elements such as silicon and germanium (porous and core-shell structures) and conversion materials like ZnFe2O4, with a special focus on the metal-ion dissolution mechanism.

Effective elastic properties of a particulate composite with transversely-isotropic matrix

The paper focuses on calculation of the overall elastic properties of a multiphase composite material with transversely-isotropic matrix. For this goal we use Maxwell homogenization scheme and explicit expressions for compliance and stiffness contribution tensors obtained by Sevostianov et al. (2005). The approach is illustrated by example of cortical bone tissue. The expressions for effective elastic constants of cortical bone are obtained in closed form and compared with experimental data available in literature.

Facile Synthesis Procedure and Lithium Storage Characteristics of a Porous NiSi2/Si/Carbon Composite Anode Material for Lithium-Ion Batteries

Regarding the current need for high capacity anode materials in order to further increase the specific energy (Wh kg-1) and energy density (Wh L-1) of LIBs, silicon is one of the most promising candidates. Among different alloying elements, such as tin or aluminum, silicon displays the highest theoretical capacity of 4,200 mAh g-1 (practical: ca. 3,500 mAh g-1) and a relatively low lithiation potential of 0.2 V vs. Li/Li+, compared to the conventional carbonaceous anode materials, displaying a relatively low specific capacity (372 mAh g-1), which cannot meet the growing demand for the next-generation high performance lithium-ion batteries. However, there are some critical drawbacks arising with the use of silicon, which are mainly related to the severe volume changes (up to 300%) during the lithium insertion/extraction process and the low intrinsic electronic conductivity.[1,2] The large stress and strain during lithiation/de-lithiation may lead to particle cracking and even particle pulverization. The volume changes of alloying materials in general also lead to a contact loss between the active material particles and the conductive carbon and/or current collector as well as to an ongoing formation of the solid electrolyte interphase (SEI; dynamic process of breaking off and reforming) and an increase in the overall resistance, thus resulting in an enhanced capacity fading during charge/discharge cycling.[3,4] Several strategies have been developed in order to reduce the detrimental effects of the large volume changes of silicon and to suppress the side reactions with the electrolyte and thus to improve the long-term cycling stability.[3] One main approach is to reduce the silicon particle size to the nanometer range, while several material structures, including e.g. nanowires[5], nanospheres[6] and nanotubes[7], have been proposed. In addition, another strategy is to design porous structures, e.g. macroporous (pore width ˃ 50 nm) or mesoporous materials (pore width of 2 to 50 nm), that can offer sufficient local void space to absorb the volume expansion of silicon and thus to enhance the cycling stability. A further main strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, i.e. by introducing a second or more components, which display no or only less volume changes as well as preferentially a high electronic conductivity. The idea of this approach is to use the host matrix to buffer the volume changes of silicon and thus to maintain the electrode integrity and electronic network. Examples are silicon-carbon composite materials, e.g. silicon particles dispersed in a carbon composite matrix[8], and silicon-based alloys or composite heterostructures, such as Si/SiOx/carbon composites[9], or intermetallic compounds like Si/TiSi2[10] or NiSi alloys[11]. In this work, we report a facile synthesis route for a porous core-shell NiSi2/Si/carbon composite material and its electrochemical characterization as anode material for lithium-ion batteries. The as-prepared composite material consists of NiSi2, silicon and carbon phases, in which the NiSi2 phase is embedded in a silicon matrix having homogeneously distributed pores, while the surface of this composite is coated with a carbon layer (Figure 1). The electrochemical characterization shows that the porous and core-shell structure of the composite anode material can effectively absorb and buffer the immense volume changes of silicon during the lithiation/de-lithiation process. The obtained NiSi2/Si/carbon composite anode material displays an outstanding electrochemical performance, which gives a stable capacity of 1,272 mAh g-1for 200 cycles at a charge/discharge rate of 1C. The simple and high-yield synthesis process, using cheap and abundant raw materials, makes it more competitive for industrial applications.

An effective medium approach for predicting the intrinsic permeability of heated cement paste

Determination of the intrinsic permeability of heated cement paste is essential to fire spalling analysis of high-performance concrete. The purpose of this paper is to develop a three-scale scheme for predicting the intrinsic permeability of heated cement paste. In this scheme, cement paste is modelled as a multi-phase composite material and further represented by three two-phase composite constituents. By analysing the thermal decomposition, the variation of various constituents in cement paste is expressed as a function of the heating history. Based on effective medium theory, the intrinsic permeability of heated cement paste is obtained in terms of the volume fractions and permeabilities of various constituents of the cement paste. Finally, the validity of the three-scale scheme is preliminarily verified with experimental data obtained from the literature.

Effective multi-field properties of electro-magneto-thermoelastic composites estimated by finite element method approach

A finite element approach based on the micromechanics was performed to estimate the multi-field properties of electro-magneto-thermoelastic composites. The thermal field and the involved pyroelectric and pyromagnetic effect of the multi-phase composite materials were taken into account in the investigation and implemented in the finite element modeling. The multi-fields related to the electric field, magnetic field, deformation and temperature field, as well as their coupling effects of the smart composites under periodic boundary conditions were obtained numerically. Especially, by means of the homogenization approximation, the effective thermal expansion coefficients, pyroelectric coefficients, pyromagnetic coefficients and other elastic, electric, and magnetic properties for the piezoelectric material, piezomagnetic material and magnetoelectric material were calculated, respectively. Some results are compared to the theoretical predictions by the well-known Mori-Tanaka method to show good agreements.

Conductivity of periodic composites made of matrix and polydispersed aggregates.

We present closed-form expressions of the effective conductivity of composites made of matrix and impenetrable polydispersed particles. The approach developed in a recent work has been generalized to treat periodic multiphase composite materials. Particular cases including orthorhombic arrangements of spherical or spheroidal inclusions and random distributions of spheres are considered where simple closed-form expressions are obtained. The case of random distributions allows us to recover Mori-Tanaka estimation after introducing partial structure factors as a statistical information on the microstructure.

Determining the representative volume element size for three-dimensional microstructural material characterization. Part 1: Predictive models

Recent advances in microstructural imaging and characterization tools have led to an increase in the direct examination of 3-D microstructures of composite materials in devices such as solid oxide fuel cells, battery electrodes, and composite gas separation membranes. In particular, focused ion beam-scanning electron microscopy and x-ray nanotomography methods have opened the door for materials characterization at the tens of nanometer scale. These experimental methods have enabled the quantitative evaluation of microstructures for a variety of important parameters. However, there is often uncertainty in the literature regarding the necessary size of a representative volume element (RVE) in order to capture an accurate description of the structure. In an effort to provide guidance for the field, in this work simplified geometric models are presented to provide basis for the a priori estimation of RVE size for three common microstructural parameters: volume fraction, particle size, and network contiguity. In a second and complementary paper (Part 2), the results of the model will be compared to experimental data obtained from the extensive imaging of a multiphase composite membrane material using x-ray nanotomography.

3D Printing of Fractal Deterministic Shapes into Polymer Matrix with Respect to Final Composite Mechanical Properties

This paper deals with mechanical properties research of innovative polymer multiphase composite materials consisting of matrix and isotropic or anisotropic oriented deterministic fractal shapes made by 3D printing. Standard polymer composite materials consisting for example of polypropylene matrix and glass fibres have mechanical properties which depend mainly on matrix-fibre interface strength, fibre length, fibre strength and fibres orientation. [1] In case of under critical fibre length is fibres orientation stochastic in for example injection moulded composites. So is possible to say that mechanical properties of these (standard) composites with short fibres are isotropic and thus all direction limited. However by creating of reinforcing internal structure consisting of deterministic fractal connected shapes is possible to gain unlimited mechanical properties directing. These fractal shapes – placed in multiphase system matrix – are significantly influencing whole material system mechanical properties mainly in case of stress on the limit of strength, proportional elongation on the limit of strength or tensile/ flexural modulus. Fractal shapes are also possible to properly locate, orient or shape modify according to potential material using with goal to gain maximal efficiency of fractal shapes occurrence. Producing of this multiphase system is realized by the help of two component 3D printing technology. So is possible to create any fractal shapes placed in polymer matrix which are by another technology unmanufacturable. Mechanical properties analyse is performed with respect to fractal shape type, fractal dimension, fractal shape orientation and shear and tensile strength of matrix.

New approach to predict the micromechanical behavior of a multiphase composite material

New approach to predict the micromechanical behavior of a multiphase composite material

Experimental study of effects of water–cement ratio and curing time on nonlinear resonance of concrete

Experimental techniques based on the theory of nonlinear acoustic resonance have been proved highly sensitive to the development of damage in concrete, particularly during the early age. As a multiphase composite material, the microstructure of concrete has a significant influence on damage development of concrete when subjected to external loading. Hence, it is important to understand the effect of main parameters associated with material microstructure such as water–cement (w/c) ratio and curing period on the nonlinear resonance response of concrete. In this research, theoretical background is first introduced for the implemented nonlinear impact resonance acoustic spectroscopy (NIRAS) technique, in which a parameter representing material nonlinearity is designated as the overall indicator for discontinuous material features of concrete samples. Applying the NIRAS technique, the nonlinearity parameter is analyzed for two sets of concrete samples mixed with different w/c ratios and cured for 7 and 28 days, respectively. The results show that under the increased loading, the longer curing time generally reduces the nonlinear response of concrete because the late filling effect of hydrated products in voids of interfacial transition zone could limit the development of microcracks. The higher w/c ratio will accelerate the nonlinear response of concrete due to the interconnection effect of material porosity on the propagation of microcracks. It is also noteworthy that the nonlinear response of samples in intact state corresponding to a lower w/c ratio is, however, enhanced. This effect is explained by the fact that less capillary water existing in the samples mixed with a lower w/c ratio work as lubricating fluid in voids to mitigate the opening–closing motion of initial material defects.

Influence of interfacial zone between asphalt mastic and aggregate on the elastic behavior of asphalt concrete

The interfacial zone between asphalt mastic and aggregate has a significant influence on the macro-mechanical properties and failure mechanisms of asphalt concrete (AC). To investigate the effect of interfacial zone, a 3D two layer built-in micromechanical model is established first, and the interface spring layer is considered to simulate the interfacial zone. Then, the effective elastic modulus and shear modulus of single-phase composite materials with the specific aggregate size are derived. A stepping-scheme is adopted to further obtain the effective elastic properties of multi-phase composite materials. The developed closed-form equations are used to predict the elastic behavior of asphalt concrete, and are able to take into account material characteristic of asphalt mastic and aggregate, the aggregate size and its volume fraction, the interface property, and air voids. Tests on four types of AC are conducted to verify the proposed approach. Finally, the influence of interfacial zone on the elastic behavior of AC is discussed. The developed method can also be used to research other multi-phase particle reinforced composites.

Thermal conductivity of flat-pressed wood plastic composites at different temperatures and filler content

AbstractThe thermal conductivity of wood flour (WF) filled high-density polyethylene composites (wood plastic composite, WPC) is investigated experimentally as a function of filler content and temperature. Samples are prepared by compression molding process of previously blended and extruded WPC pellets, up to 50% weight content of WF. The thermal conductivity is measured by the heat flow meter technique in a temperature range from -15°C to 80°C. Experimental results show that the WPC thermal conductivity decreases with temperature and WF content, with the last effect due to the increase in porosity with the filler content, as confirmed by density measurements. Using the thermal conductivity of bare WF, the thermal conductivity of the wood material in WPC is estimated. This value successfully predicts the upper and lower bounds of the WPC thermal conductivity by means of the parallel and series conduction model of a multiphase composite material.

Dielectric and magnetic response of Sr–Zn ferrite composite

Sr–Zn ferrite composites were synthesized by chemical coprecipitation with different sintering process. The composites were characterized by XRD, XPS, IR, VSM, and N2-adsorption. The results revealed that the composites were composed of S-type ZnFe2O4 and M-type SrFe12O19. Zn2+ ions inserted and occupied the 12k or 2a sites for Fe3+ ions in M-type SrFe12O19. The Zn2+ ions were therefore not on the external surface of the composites. The microwave absorption properties at a frequency range of 0–18 GHz were estimated using the electromagnetic parameters obtained through the agency of microwave vector network analyzer. The microwave absorbing properties of composites were excellently superior to that of the pure SrFe12O19 or ZnFe2O4 due mainly to the exchange coupling between the hard- and soft-magnetic phases. The synthesis process was found to facilitate mass production for various composite materials, especially the two phases or multiphase composite materials.

Wear of multiphase composites in the friction area: mathematical model

The wear of multiphase composite materials is modeled as accumulation of microdamage in each component. The probability of damage in the composite components is assessed by solving a statistical boundary-value problem of the micromechanics of structurally inhomogeneous media subject to coupled processes of deformation and microdamage. The accumulation of microdamage is described by random functions whose statistical characteristics for each component are determined experimentally. The approach is validated by comparing the calculated and experimental wear of 15 multiphase composite materials. It is established that the content of fiber reinforcement, thermally expanded graphite, and boron nitride influences the wear of composite materials.

On Maxwell's concept of equivalent inhomogeneity: When do the interactions matter?

The concept of Maxwell's equivalent inhomogeneity is re-evaluated in the context of the effective elastic properties of unidirectional multi-phase composite materials with the fibers of circular cross-section. The following key questions are addressed in this paper. Is the concept of equivalent inhomogeneity valid only for the materials with low volume fractions, as originally suggested by Maxwell? When do the interactions among the fibers matter? Could Maxwell's concept be generalized to allow for accurate estimates of the effective properties? The approach advocated in the paper provides an unique opportunity to study, in the framework of one model, the effects of the interactions among fibers on the effective transversely isotropic elastic properties of multi-phase composite materials.

Integration design of membrane electrode assemblies in low temperature solid oxide fuel cell

In this paper, an integration design of membrane electrode assemblies in low temperature solid oxide fuel cells (LTSOFCs) is accomplished by using a mixed ionic–electronic conductor. The mixed ionic–electronic conductor is a composite material, LiNiCuZn oxides, Gd 2 O 3 and Sm-doped CeO 2 composited with Na 2 CO 3 (LiNiCuZn oxides-NGSDC), which consists of ionic conductor, n-type and p-type semiconductors. The multi-phase composite material can also be used in single layer fuel cell (SLFC) to replace single-phase materials. A SLFC using the LiNiCuZn oxides-NSGDC composite exhibits an OCV of 1.05 V and maximum power density of 800 mW cm −2 , which is comparable to the cell performance of conventional LTSOFCs and much higher than that of SLFC reported before. The reasons leading to the good performance are porous structure of electrode and the matching of ionic conductor and semiconductor. ► An integration design of membrane electrode assemblies is realized. ► The membrane electrode assemble is made by a mixed ionic–electronic conductor. ► The multi-phase composite material can be used in single layer fuel cell. ► A power density of 800 mW cm −2 is achieved by single layer fuel cell at 550 °C.