Porosity Of Composites Research Articles

The potential of harakeke fibre as reinforcement in polymer matrix composites including modelling of long harakeke fibre composite strength

Mechanical properties of aligned long harakeke fibre reinforced epoxy with different fibre contents were evaluated. Addition of fibre was found to enhance tensile properties of epoxy; tensile strength and Young’s modulus increased with increasing content of harakeke fibre up to 223MPa at a fibre content of 55wt% and 17GPa at a fibre content of 63wt%, respectively. The flexural strength and flexural modulus increased to a maximum of 223MPa and 14GPa, respectively, as the fibre content increased up to 49wt% with no further increase with increased fibre content. The Rule of Mixtures based model for estimating tensile strength of aligned long fibre composites was also developed assuming composite failure occurred as a consequence of the fracture of the lowest failure strain fibres taking account porosity of composites. The model was shown to have good accuracy for predicting the strength of aligned long natural fibre composites.

Effect of B2O3 on physical properties of LZAS vitrified bond and mechanical properties of diamond composites

The properties of vitrified bond abrasive tools, which are composed of diamond grains and Li2O–ZnO–Al2O3–SiO2 (LZAS) vitrified bond with and without B2O3 were comprehensively investigated. The results show that the vitrified bond incorporating 10mol% B2O3 exhibits a lower refractoriness and a higher fluidity compared with that of vitrified bond without B2O3. Moreover, the thermal expansion coefficient (TEC) of vitrified bond containing 10mol% B2O3 is very close to the TEC of diamond grains. Meanwhile, the open porosity of diamond composite is decreased from 41% to 29% by incorporating 10mol% B2O3, which was 28.5% lower than that of vitrified bond without B2O3. The bending strength and Rockwell hardness of the diamond composite are significantly improved when the content of B2O3 reached 10mol%. SEM observation revealed that the diamond grains were closely overlaid by vitrified bond and diamond composite containing 10mol% B2O3 exhibited a relatively denser structure. The relation between the structural change and compositional variation by introducing B2O3 was analyzed by Fourier transform infrared spectroscopy (FTIR).

Electrospun polyimide-composite separator for lithium-ion batteries

Non-woven mats of thermally stable polyimide (PI) composites were fabricated as a separator of lithium-ion batteries (LIBs) by first electrospinning a mixture of the pre-polymer, poly(amic acid) ammonium salt (PAAS), and inorganic nanoparticles of SiO2 or Al2O3 and then imidizing the electrospun nanofibers of the PAAS composites at 350°C. The microstructures of the electrospun PI nanofibers, electrospun PI–SiO2-composite nanofibers, electrospun PI–Al2O3-composite nanofibers, and the commercial separator SV718 were examined using field-emission scanning electron microscopy and transmission electron microscopy. Test results of the thermal properties of the PI nanofibers, PI-composite nanofibers, and SV718, obtained with a thermal gravimetric analyzer and a differential scanning calorimeter, indicate the superior thermal stability of PI and PI composites, which showed no melting peak and no decomposition at 600°C, while SV718 had a melting peak at 137°C and decomposed at 300°C. The thermal stability of the separators, evaluated in a hot-oven test, showed no shrinkage of PI and PI composites at 200°C, while SV718 started to shrink at above 100°C. Using a drop of liquid electrolyte on the surface of each separator, the electrolyte contact angle on PI and PI composites was around 10° and that on SV718 was 54°, indicating that PI and PI composites had better wettability than SV718. The porosity and liquid-electrolyte uptake of the PI composites were over 90% and 790%, respectively, while the corresponding values for SV718 were 40% and 101%, respectively, implying that the separators consisting of the non-woven mats of PI–SiO2-composite nanofibers and PI–Al2O3-composite nanofibers had lower interfacial resistance than the commercial SV718 separator. The electrochemical performance of the PI-composite separator assembled between the LiCoO2 cathode and the Li anode of an LIB exhibited more stable cycle performance, higher discharge capacity, and better capacity retention compared with those using the SV718 separator. Hot oven test for the charge cells at 150°C, the PI and PI-composite separator showed better thermal stability than SV718.

Low-temperature synthesis of calcium–hexaluminate/magnesium–aluminum spinel composite ceramics

Calcium–hexaluminate/magnesium–aluminum spinel ceramic composites were synthesized at 1450°C for 3h using a precursor from dolomite and industrial aluminium hydroxide through a 200°C hydrothermal treatment, which was 100°C lower than the traditional process. The main reason for this treatment was to initiate a phase transformation from gibbsite to boehmite. The morphology of calcium hexaluminate was determined by the crystal structure of aluminium hydroxide. Loosely stacked platelets were caused by tabular gibbsite, and cross-distribution flakes were due to cross-sheet boehmite. The porosity and compressive strength of composites synthesized at 1500°C for 3h increased with increasing hydrothermal temperature. CA6 of composites from hydrothermal treatment at 200°C had a more regular hexagonal plate-like morphology, total porosity of 72.2%, and compressive strength of 142.7MPa. However, composites from hydrothermal treatment at 25°C exhibited a weaker diffraction peak of calcium hexaluminate and an uncompleted crystal appearance.

Microstructure, Mechanical and Dielectric Properties of Si<sub>3</sub>N<sub>4</sub>-BN Composites with Different Porosities

Si3N4-BN composites were prepared by die-pressing and precursor infiltration and pyrolysis (PIP) route using borazine as the precursor. The composition, microstructure, mechanical, and dielectric properties of the composites with different porosities were investigated. With the adoption of starch as pore forming substance, drawn the Si3N4preform from the liquid precursor borazine and decrease the pressure during curing, the porosity of the Si3N4-BN composites were effectively increased. Along with the increase of the porosity of the composites, the mechanical properties were decreased and the dielectric properties were improved. With 20 wt.% starch and drawn Si3N4preform from borazine before curing, the density, porosity, flexural strength and elastic modulus of the composites were 1.70 g·cm-3, 29.78%, 48.05MPa and 32.45GPa, respectively. The dielectric constants and loss tangents were 4.20~4.44 and 0.48~3.42×10-3at the frequency 7~ 18GHz. Composites with various dielectric and mechanical properties can be designed and prepared according to the application needs.

Fabrication and characterization of composite materials based on porous ceramic preform infiltrated by elastomer

Abstract The paper presents the experimental results of fabrication and characterization of ceramic- elastomer composites. They were obtained using pressure infiltration of porous ceramics by elastomer As a result the composites in which two phases are interpenetrating three-dimensionally and topologically throughout the microstructure were obtained. In order to enhance mechanical properties of preforms a high isostatic pressure method was utilized. The obtained ceramic preforms with porosity gradient within the range of 20-40% as well as composites were characterized by X-ray tomography. The effect of volume fraction of pores on residual porosity of composites was examined. These results are in accordance with SEM images which show the microstructure of composites without any delaminations and voids. Such composites exhibit a high initial strength with the ability to sustain large deformations due to combining the ceramic stiffness and rubbery elasticity of elastomer. Static compression tests for the obtained composites were carried out and the energy dissipated during compression was calculated as the area under the stress-strain curve. The dynamic behavior of the composite was investigated using the split Hopkinson pressure bar technique. It was found that ceramic-elastomer composites effectively dissipate the energy. Moreover, a ballistic test was carried out using armor piercing bullets.

Sound absorption of porous cement composites: effects of the porosity and the pore size

We prepared sound absorbing cement–hydrogel composites using a hydrogel slurry templating technique. We air-dried the wet cement composites containing a varying percentage and size of entrapped hydrogel microbeads to produce a porous cement with a controlled porosity and pore size matching the hydrogel bead distribution. The composites porosity, mass density, compressional strength and sound absorption properties were analysed. SEM analysis showed residual domains from the dried hydrogels beads within the voids created by the hydrogel bead evaporation in the cement samples. The sound absorption coefficient of the composite varied with the templated hydrogel bead size and the overall porosity. The composite samples made with hydrogel beads of average size 0.7 mm showed high absorption coefficients between 0.5 and 0.80 for 500–800 Hz for 50 vol% porosity. Samples produced by templating hydrogels of 1 mm bead size and 70 vol% porosity showed an increased absorption over the sound frequency range 200–2000 Hz. Templating a mixture of the 1.6 and 1.0 mm hydrogel beads slurries with cement slurry did not appear to yield synergistic effect in the sound absorption of the produced porous composites compared to samples made from the separate slurries. The mechanical strength of the obtained porous cement composites decreased with the increase of porosity. Such low fabrication-cost and environmentally friendly composites have a potential to be used as passive sound absorbers by the building and transport industries.

On Porosity Formation in Metal Matrix Composites Made with Dual-Scale Fiber Reinforcements Using Pressure Infiltration Process

This is the first such study on porosity formation phenomena observed in dual-scale fiber preforms during the synthesis of metal matrix composites (MMCs) using the gas pressure infiltration process. In this paper, different mechanisms of porosity formation during pressure infiltration of Al-Si alloys into Nextel™ 3D-woven ceramic fabric reinforcements (a dual-porosity or dual-scale porous medium) are studied. The effect of processing conditions on porosity content of the ceramic fabric infiltrated by the alloys through the gas PIP (PIP stands for “Pressure Infiltration Process” in which liquid metal is injected under pressure into a mold packed with reinforcing fibers.) is investigated. Relative density (RD), defined as the ratio of the actual MMC density and the density obtained at ideal 100 pct saturation of the preform, was used to quantify the overall porosity. Increasing the infiltration temperature led to an increase in RD due to reduced viscosity of liquid metal and enhanced wettability leading to improved feedability of the liquid metal. Similarly, increasing the infiltration pressure led to enhanced penetration of fiber tows and resulted in higher RD and reduced porosity. For the first time, the modified Capillary number (Ca*), which is found to predict formation of porosity in polymer matrix composites quite well, is employed to study porosity in MMCs made using PIP. It is observed that in the high Ca* regime which is common in PIP, the overall porosity shows a strong downward trend with increasing Ca*. In addition, the effect of matrix shrinkage on porosity content of the samples is studied through using a zero-shrinkage Al-Si alloy as the matrix; usage of this alloy as the matrix led to a reduction in porosity content.

Structure and Property Formation of Composite Materials on the Basis of Polytetrafluoroethylene Under the Explosive Processing

The paper considers the peculiarities of structure forming in composite materials on the basis of polytetrafluoroethylene (PTFE) containing metals or polyimide while explosive processing of powder compositions by the ring shock front. The stepwise change of the structure in the center of pressing, accompanied by occurrence of nanodimensional phases, change of fine structure and increase of microhardness is determined. It is achieved under a certain combination of the explosive's parameters and powder composition's porosity.

Progress in Research on Carbon Nanotubes Reinforced Cementitious Composites

As one-dimensional (1D) nanofiber, carbon nanotubes (CNTs) have been widely used to improve the performance of nanocomposites due to their high strength, small dimensions, and remarkable physical properties. Progress in the field of CNTs presents a potential opportunity to enhance cementitious composites at the nanoscale. In this review, current research activities and key advances on multiwalled carbon nanotubes (MWCNTs) reinforced cementitious composites are summarized, including the effect of MWCNTs on modulus of elasticity, porosity, fracture, and mechanical and microstructure properties of cement-based composites. The issues about the improvement mechanisms, MWCNTs dispersion methods, and the major factors affecting the mechanical properties of composites are discussed. In addition, large-scale production methods of MWCNTs and the effects of CNTs on environment and health are also summarized.

Evaluation and Examination of Volume Fraction, Porosity, Microstructure and Computational Modeling of Hybrid Metal Matrix Composites to Reveal the Hea

Evaluation and Examination of Volume Fraction, Porosity, Microstructure and Computational Modeling of Hybrid Metal Matrix Composites to Reveal the Hea

A Binder Viscosity Effect on the Wet-Wounded Composite Porosity in the Impregnating Bath

A Binder Viscosity Effect on the Wet-Wounded Composite Porosity in the Impregnating Bath

An Analytical Model of Thermal Conductivity for Carbon/Carbon Composites with Pitch-Based Matrix

The carbon/carbon (C/C) composites are composed of carbon fibers, carbon matrix, and pores and cracks, which have been successfully used in various aerospace applications. In this paper, nanoscale submodel is proposed to describe the thermal conductivity of the matrix based on its microscopic structure, and then the submodel is incorporated into a microscale model to analytically predict the equivalent thermal conductivities of the composites by equivalent circuit approach. The results predicted by the present model agree well with those from the experimental measurements. Based on the model, the effects of the composite porosity as well as the thickness and porosity of the interface phase on the thermal performance of five composites are studied. It is found that the thermal conductivities show decreasing trends in responding to an increase in each of the three parameters. The composite porosity has a significant effect on the thermal conductivities both parallel and transverse to the fiber axis, while the thickness and the porosity of the interface phase remarkably affect the thermal conductivity only transverse to the fiber axis.

The effect of curing time on the ASR expansion of different HPC composites

HPC composites have been an interesting topic in the field of concrete research over the last few decades. Many fresh and hardened concrete properties have been investigated. However, investigation of early alkali–silica reactions (ASR) in HPC is reported in only a few studies that correlate HPC and ASR expansion. The presence of a different combination of mineral admixtures in HPC is a must. Mineral admixtures such as pulverized fly ash (PFA) and silica fume (SF) have different pozzolanic activities. Therefore, the effect of different early curing regimes on the ASR expansion in HPC composites containing alkali reactive aggregates requires investigation. A total of 25% glass bottle aggregate was found to provide an expansion near that given by most reactive natural aggregates. The effect of curing regime was noticeably different in the control HPC mixture, as cement hydration is highly affected by time. However, in the binary and ternary cementitious HPC composites the performance was found to depend on the constituents forming this composite. The addition of superplasticizer plays an important role in mitigating the ASR expansion. Within the ternary composites, an optimum combination of PFA and SF was found. For the optimum combination, the attractive force among the constituents in the cementitious matrix is maximized. This significant reduction in porosity and total permeability of HPC composites can be utilized to lower the ASR expansion.

Fracture toughness enhancement of cement paste with multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) have been incorporated into cement pastes to investigate the effect on compressive strength and fracture toughness. Cement-based composites have been prepared from Portland cement with various amounts of common MWCNTs or MWCNTs with a carboxyl group (MWCNTs-COOH), ranging from 0% to 0.1% by weight. Nanotubes were uniformly dispersed into cement paste by applying ultrasonic energy in combination with the use of surfactants. Results indicated that fracture toughness of composites increased by the addition of common MWCNTs, while compressive strengths showed no significant increase. However, MWCNTs-COOH significantly improved fracture energy, fracture toughness and compressive strengths of cement pastes. The porosity and pore size distribution of composites were measured by Mercury Intrusion Porosimetry, and the microstructure of samples was analyzed using a Scanning Electron Microscope. The number of micropores with diameters of 25–50nm was found to reduce significantly with the addition of MWCNTs, especially MWCNTs-COOH, and the nanotubes bridged the cement particles.

Mesomechanical characterization of porosity in cementitious composites by means of a voxel-based finite element model

At the mesoscale, concrete is regarded as a heterogeneous material with two main phases, namely mortar and coarse aggregates. However, the presence of pores at this scale may play an important role on the macromechanical properties of the material. Due to their complex geometry, these mesostructures usually require a large number of discretization elements. In this paper, a voxel-based (volumetric pixel) model is used in order to account for porosity while reducing the number of elements on the characterization of the mesomechanical properties of concrete. In order to validate the voxel model, a comparison to an equivalent tetrahedralized mesh is carried out, showing an important reduction in computational times but with similar results. For such validation, the effect of the voxel size and the consideration of the interfacial transition zone are also accounted for. Finally, uniaxial tension tests are carried out in order to characterize the elastic (i.e. elastic modulus and Poisson’s ratio) and fracture (i.e. tensile strength) properties in concrete mesostructures of different sizes (25, 35 and 50mm). The effect of porosity is analyzed by considering different pore fractions and validated with analytical and experimental results.

Searching for the superior solution to the population-based optimization problem: Processing of the wear resistant commercial AA6061 AMCs

In this present work, stir casting technique was used to fabricate aluminum matrix composites with varying weight percentages of SiC (5, 10 and 15) reinforcements. The proper selection of process parameter such as pouring temperature, stirring speed, stirring time and pre-heat temperature of reinforcement can all influence the quality of the fabricated composites. The porosity level of composite should be minimized and the chemical reaction between reinforcement and matrix should be avoided. Optimization is an applied science which explores the best values of the parameters of a problem that may take under specified conditions. The execution of an optimized stir casting technique yields relatively homogenous and fine microstructure which improves the addition of reinforcement material in the molten metal. The influence of SiC content, SiC size and secondary mechanical processing with different rolling reductions on the dry sliding wear characteristics of Al matrix composites has been assessed using a pin-on-disc wear test. The porosity and hardness of the resultant composites were also examined.

Flash Thermography to Evaluate Porosity in Carbon Fiber Reinforced Polymer (CFRPs).

It is a fact that the presence of porosity in composites has detrimental effects on their mechanical properties. Then, due to the high probability of void formation during manufacturing processes, it is necessary to have the availability of non-destructive evaluation techniques, which may be able to discover the presence and the distribution of porosity in the final parts. In recent years, flash thermography has emerged as the most valuable method, but it is still not adequately enclosed in the industrial enterprise. The main reason of this is the lack of sufficient quantitative data for a full validation of such a technique. The intention of the present work is to supply an overview on the current state-of-the-art regarding the use of flash thermography to evaluate the porosity percentage in fiber reinforced composite materials and to present the latest results, which are gathered by the authors, on porous carbon fiber reinforced polymer laminates. To this end, several coupons of two different stacking sequences and including a different amount of porosity are fabricated and inspected with both non-destructive and destructive testing techniques. Data coming from non-destructive testing with either flash thermography or ultrasonics are plotted against the porosity percentage, which was previously estimated with the volumetric method. The new obtained results are a witness to the efficacy of flash thermography. Some key points that need further consideration are also highlighted.

Investigations on mechanical properties of aluminum hybrid composites

A double stir casting process was used to fabricate aluminum composites reinforced with various volume fractions of 2, 4, 6, and 8wt% RHA and SiC particulates in equal proportions. Properties such as hardness, density, porosity and mechanical behavior of the unreinforced and Al/x%RHA/x%SiC (x=2, 4, 6, and 8wt%) reinforced hybrid composites were examined. Scanning electron microscope (model JSM-6610LV) was used to study the microstructural characterization of the composites. It was observed that the hardness and porosity of the hybrid composite increased with increasing reinforcement volume fraction and density decreased with increasing particle content. It was also observed that the UTS and yield strength increase with an increase in the percent weight fraction of the reinforcement particles, whereas elongation decreases with the increase in reinforcement. The increase in strength of the hybrid composites is probably due to the increase in dislocation density. A systematic study of the base alloy and composites was done using the Brinell hardness measurement and the corresponding age hardening curves were obtained. It was observed that in comparison to that of the base aluminum alloy, the precipitation kinetics of the composites were accelerated by adding the reinforcement. This effectively reduced the time for obtaining the maximum hardness by the aging heat treatment.

Tailoring porosities and electrochemical properties of composites composed of microfibrillated cellulose and polypyrrole

The porosities of composites of polypyrrole and nanocellulose can be tailored from 30 to 98% with ∼10% increments enabling the electrochemical behavior of the materials to be readily controlled.