Low Filler Volume Fraction Research Articles

Numerical homogenization of thermal conductivity of particle-filled thermal interface material by fast Fourier transform method

Thermal interface material (TIM) is pivotal for the heat dissipation between layers of high-density electronic packaging. The most widely used TIMs are particle-filled composite materials, in which highly conductive particulate fillers are added into the polymer matrix to promote heat conduction. The numerical simulation of heat transfer in the composites is essential for the design of TIMs; however, the widely used finite element method (FEM) requires large memory and presents limited computational time for the composites with dense particles. In this work, a numerical homogenization algorithm based on fast Fourier transform was adopted to estimate the thermal conductivity of composites with randomly dispersed particles in 3D space. The unit cell problem is solved by means of a polarization-based iterative scheme, which can accelerate the convergence procedure regardless of the contrast between various components. The algorithm shows good precision and requires dramatically reduced computation time and cost compared with FEM. Moreover, the effect of the particle volume fraction, interface thermal resistance between particles (R-PP), interface thermal resistance between particle and matrix (R-PM), and particle size have been estimated. It turns out that the effective conductivity of the particulate composites increases sharply at a critical filler volume fraction, after which it is sensitive to the variation of filler loading. We can observe that the effective thermal conductivity of the composites with low filler volume fraction is sensitive to R-PM, whereas the it is governed by R-PP for the composites with high filler content. The algorithm presents excellent efficiency and accuracy, showing potential for the future design of highly thermally conductive TIMs.

Analytical formulation for electrical conductivity and percolation threshold of epoxy multiscale nanocomposites reinforced with chopped carbon fibers and wavy carbon nanotubes considering tunneling resistivity

Electrical conductivity (EC) and percolation threshold of chopped carbon fiber (CF)-carbon nanotube (CNT)-reinforced epoxy multiscale nanocomposites considering tunneling resistivity are studied based on micromechanics. The percolation theory is applied to determine transition from low to high conductivity. Influences of CNT/polymer interphase and CNT waviness are considered to evaluate percolation threshold of CNT-enriched epoxy nanocomposite prior to CF percolation threshold. Multiscale fillers provide benefit of enhanced EC with different percolation thresholds for different fillers. These characteristics of multiscale filler with high aspect ratio of filler facilitate formation of conductive network even at low multiscale filler volume fraction. Variation of EC as a function of tunneling distance, CNT and CF aspect ratios and intrinsic EC for different CNT volume fractions is investigated. The nanocomposite exhibits a percolation threshold at less than 0.2 vol% of CNTs. The percolation threshold is predicted to be reduced when higher volume fractions of CFs are considered.

Assessment of existing micro-mechanical models for asphalt mastic considering inter-particle and physico-chemical interaction

Micromechanical models have been used since the 1990s to predict the properties of asphalt mastic. However, most of these models are found to be unsatisfactory in predicting the mastic’s properties because the models were derived from the research of particle-filled composites that did not take into account the asphalt-filler physico-chemical interactions and particle interactions. In this paper, three micro-mechanical models for predicting the complex shear modulus master curve of asphalt mastic are evaluated: the generalized self-consistent scheme model, the four-phase micro-mechanical model, and the particle interaction model. All three micro-mechanical models are based on the mechanical properties of their constituent materials as well as the filler-asphalt physiochemical interactions and the particle interactions. Two virgin asphalt binders and two polymer modified asphalt binders were selected to fabricate 16 mastics of four different filler volume fractions. The accuracy of prediction was evaluated by comparing the relative differences between the experimental complex shear modulus master curves and that predicted by the models. The results suggest that (1) the generalized self-consistent scheme model have a satisfactory prediction at a low filler volume fraction, but their accuracy is significantly affected by frequency; (2) the particle interaction model cannot calculate the complex shear modulus of mastic with a filler volume fraction of more than 0.53; (3) the four-phase model shows a precise forecast of complex shear modulus for mastic with moderate and high filler volume fractions; and (4) for all of the three models, the deviations increase severely with the increase of the filler volume fraction.

Exfoliation of graphite and expanded graphite by melt compounding to prepare reinforced, thermally and electrically conducting polyamide composites

High performance composites based on a thermoplastic polyamide matrix containing well dispersed graphene nanoplatelets were prepared, by delaminating expanded graphite (EG) into its constituent graphene nanoplatelet (GNP) layers, by melt compounding. Results from scanning electron microscopy, transmission electron microscopy, and 3D tomography showed good dispersion of the GNP, while contact angle measurements revealed that the polyamide and graphene-based constituents had excellent interfacial interactions. Compared to flake graphite, exfoliation of EG into graphene nanoplatelets substantially improved the electrical, thermal and mechanical properties at low filler volume fractions: the electrical percolation threshold was attained at 1.9 vol%, the thermal conductivity increased by 1300% at 8 vol%, and the flexural modulus by 270% at 15 vol%. Substantial improvements in the impact properties of the composites were imparted by adding a maleated elastomeric impact modifier, which was well-dispersed within the PA matrix, resulting in excellent toughening performance. This research achieves composites with superior properties, using an industrially relevant, easy to implement melt compounding method.

Enhanced dielectric properties of ferroelectric polymer with perflurooctanoic acid doped reduced polyaniline/reduced graphene oxide fillers

In this report, we provided a method to fabricate perfluorooctanoic acid doped reduced polyaniline/reduced graphene oxide/poly (vinylidene fluoride) (PFOA-rPANI/rGO/PVDF) nanocomposite with enhanced dielectric properties. The PFOA-rPANI modification not only prevented the agglomeration of rGO nanosheets, but also improved the affinity of rGO nanosheets to PVDF matrix. The as-fabricated PFOA-rPANI/rGO/PVDF nanocomposites exhibited high dielectric constant while they maintained low dielectric loss. The highest dielectric constant of nanocomposites could reach 1754 at a low filler volume fraction of 0.5%.

Frequency electrical conductivity dependence and dielectric relaxation of ZnO–P2O5/Co composites

This work focuses on the experimental study of the conductivity and the permittivity of ZnO–P2O5/Co (ZP/Co) composites as function of frequency in the range of 100–106 Hz at room temperature. The conductivity evolution shows a high percolating jump from low to high loaded ZP matrix with Co filler, indicating an insulator–semiconductor phase transition. Above the percolation threshold, the obtained conductivity is between 10−1 and 10° S/cm and is frequency independent. While, for low filler volume fraction, it is frequency dependent and it is about 10−8 S/cm at 100 Hz. Moreover, it should be noticed that around the percolation threshold, a significant value of dielectric constant about 105 was observed at room temperature and low frequency 100 Hz. Furthermore, the distribution of the relaxation processes was also investigated below the percolation threshold. Indeed, a relaxation peak about 106 rad/s in the curves of electric modulus M″(ω) was observed. It is successively described by Davidson–Cole model and an account of the interpretation of obtained results is given.

A review of porous lightweight composite materials for electromagnetic interference shielding

Abstract Lightweight porous materials for electromagnetic interference (EMI) shielding applications are reviewed. EMI shielding refers to the capability of a material to protect from electromagnetic fields (EMFs) generated by electronic devices. Traditionally conducting metals are used in EMI shielding applications, which are slowly being replaced by conducting polymer based shields. This review is narrowly focused on understanding the approaches related to porous high EMI shielding composite materials that have very low density values. While metallic fillers can increase the EMI shielding capabilities of polymers, they also increase the weight, which can be offset by inducing the porosity in the matrix. Porosity is found to be effective in providing higher shielding effectiveness at low filler volume fraction due to concentrating the filler in the solid polymers. However, use of gas porosity results in composites with low mechanical properties. This problem can be alleviated to some extent by reinforcing polymer foams with lightweight conductivefillers such as carbon nanofibers (CNFs), carbon nanotubes (CNTs) and graphene. But the properties of pores such aspore size and distribution cannot be effectively controlled in such cases. Syntactic foams containing hollow particle fillers seem to be the best combination of EMI shielding capabilities and mechanical properties. These composites can be either filled with a second phase conducting filler, or hollow particles can be coated with a conducting layer, or hollow particles made of conducting materials can be used as fillers. The hollow particle wall thickness and volume fractions can be optimized to obtain the desired combination of properties in syntactic foams to enable their multifunctional applications.

The fabrication and thermal conductivity of epoxy composites with 3D nanofillers of AgNWs@SiO2&GNPs

The development of electronic makes increasing attentions focus on the high thermally conductive polymer composites. In this work, outstanding epoxy nanocomposites embedded with 3D structure fillers of silver nanowires@SiO2 and graphite nanoplatelets are prepared successfully. Insulative SiO2 layers coating on the surface of silver nanowires can not only provide abundant silicon hydroxyl groups to react and form strong covalent bonding with epoxy matrix and graphite nanoplatelets, minimizing the scattering of phonon and contributing to construction of thermally conductive network, but also effectively protect electrical insulation from the forming of electrical conductive pathways. An ultrahigh thermal conductivity of nanocomposites 1.092 W/m K, an enhancement of 474% to pristine epoxy, is achieved only with low filler volume fraction of 0.8 vol% silver nanowires@SiO2 and 2.7 vol% graphite nanoplatelets, at the same time, an ultralow volume conductivity less than 1 × 10−5 S/cm is available at the frequency range 1 × 102Hz to 1 × 106Hz, which indicates strong potential in the field of electronic packaging.

Geometric analysis of enhanced thermal conductivity in epoxy composites: A comparison of graphite and carbon nanofiber fillers

We analyze the geometric effects of two different carbon fillers on the enhancement of the thermal conductivity of carbon‐epoxy composites. This study compares the thermal properties of composites containing graphite powder (2‐dimensional) and carbon nanofibers (1‐dimensional) incorporated in an industrial epoxy. Calculations using the generalized effective medium model were also used to examine the effect of the geometry and aspect ratio of the carbon filler. Experiments show that at a filler volume fraction loading of 0.10, the effective thermal conductivity of the composites was improved up to eightfold for carbon nanofiber and threefold for graphite in comparison to the neat epoxy. The superior performance of the carbon nanofiber composite is due to the larger aspect ratio of nanofiber which allows greater overlap between neighboring particles. However, this greater overlap also results in the composite becoming prohibitively viscous at low filler volume fractions. In graphite composite at the maximum filler volume fraction of 0.3, the resulting thermal conductivity improvement was 14‐fold over the neat epoxy. Calculations indicated that the improved thermal conductivity was primarily due to the filler particle geometry. Additionally, calculations suggest the wider distribution of graphite particle aspect ratio could have a positive influence on enhancing composite thermal conductivity.

Ac-conductivity and dielectric response of new zinc-phosphate glass/metal composites

The ac-conductivity and dielectric response of new composites based on zinc-phosphate glass with composition 45mol%ZnO–55mol%P2O5, filled with metallic powder of nickel (ZP/Ni) were investigated by impedance spectroscopy in the frequency range from 100Hz to 1MHz at room temperature. A high percolating jump of seven times has been observed in the conductivity behavior from low volume fraction of filler to the higher fractions, indicating an insulator – semiconductor phase transition. The measured conductivity at higher filler volume fraction is about 10−1S/cm and is frequency independent, while, the obtained conductivity for low filler volume fraction is around 10−8S/cm and is frequency dependent. Moreover, the elaborated composites are characterized by high dielectric constants in the range of 105 for conductive composites at low frequencies (100Hz). In addition, the distribution of the relaxation processes was also evaluated. The Debye, Cole-Cole, Davidson–Cole and Havriliak–Negami models in electric modulus formalism were used to model the observed relaxation phenomena in ZP/Ni composites. The observed relaxation phenomena are fairly simulated by Davidson–Cole model, and an account of the interpretation of results is given.

Micromechanics prediction of effective modulus for asphalt mastic considering inter-particle interaction

This paper presents the development and validation of a new micromechanical model, Ju–Chen (J–C) model, to predict the effective viscoelastic modulus of asphalt mastic by considering the inter-particle interaction. Based on the approximate solutions for two-particle interaction problem, the radial distribution function is integrated into the ensemble-volume averaged eigenstrain tensor so as to consider the inter-particle interaction, and the solution could be extended and simplified to predict the effective complex modulus of asphalt mastic with different filler volume fractions. It is found that the inter-particle interaction increases with the filler volume fraction and decreases with the test frequency, and the predictions agree well with the experiment data at low and moderate filler volume fractions, not exceeding 50%. Compared with two commonly used micromechanical models (M–T model and DSEM model), the J–C model relatively gives the best estimation of the effective modulus of mastics, and the applicability of J–C model could be further improved.

Multifunctional and specific interactions of CNTs in NBR and HNBR

There are different techniques in practice to disperse multiwalled carbon nanotubes (MWCNTs) in elastomers. In the present work, commercially available MWCNTs NANOCYL NC 7000™ are used as filler. A synthetic Nitrile Butadiene Rubber (NBR) and Hydrogenated Nitrile Butadiene Rubber (HNBR) were used as polymer matrix for the composites prepared by melt blending. The filler dispersion in HNBR was studied through Transmission Electron Microscopy (TEM). The mechanical properties are investigated through strain sweep and tensile tests. Dielectric measurements are carried out to study electrical conductivity of the nanocomposites. Dynamic mechanical measurements showed an enhanced Payne effect and improvement in stiffness with increase in CNTs content into rubber matrix An improvement in electrical and mechanical properties in both NBR and HNBR system resulted by an increase in filler loading. The mechanical and electric percolation threshold of CNTs was found at very low filler volume fraction. Equilibrium swelling experiments were used to study polymer-filler interaction with the help of Kraus plot and diffusion coefficient. NBR showed higher polymer-filler interaction compared to HNBR. In NBR nanocomposites, CNTs showed higher interaction than carbon black. Good dispersion and effective interaction of the CNTs with the polymer led to significant mechanical reinforcing effects.

Effect of high aspect ratio filler on dielectric properties of polymer composites: a study on barium titanate fibers and graphene platelets

High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed.

Direct Measurement of Polymer Chain Conformation in Well-Controlled Model Nanocomposites by Combining SANS and SAXS

We studied by small-angle neutron scattering (SANS) the polymer chain conformation in model silica/polystyrene (PS) nanocomposites. Using the zero average contrast method, we can properly match silica signal to directly measure the form factor of a single PS chain. An important effort has been put to eliminate the silica scattering using two distinct approaches (called respectively “three” and “four” components methods) leading to analogue results. By combining SANS data with small-angle X-ray scattering (SAXS) measurements and transmission electronic microscopy (TEM) images on the same samples, we obtain a very clear result about the effect on polymer chain conformation of the filler dispersion, either at low filler volume fraction, where silica arrange in small aggregates or at higher concentration where it forms a connected network. From SANS the chain conformation in nanocomposites is identical to the one without silica in a large q range illustrating that the chain conformation remains independent of the filler dispersion whatever the silica connectivity or the polymer molecular weight. This result opens the way to a better overview of the polymer chain conformation contribution, especially adsorption or chain mobility modification effects, in the complex mechanical reinforcement mechanisms of nanocomposites. However, at low q, an unexpected shoulder appears in the SANS curves: this effect does not vary in a systematic way neither with chain molecular weight nor with silica concentration. The non reproducibility of these observations could be related to unusual phase separation between normal and deuterated chains.

Molecular-dynamics simulation of model polymer nanocomposite rheology and comparison with experiment

The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results; shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction.

A frictional molecular model for the viscoelasticity of entangled polymer nanocomposites

The dynamics of polymer melts and concentrated solutions reinforced with nanoscale rigid spherical particles is analyzed. Nanocomposites with low filler volume fraction and strong polymer-filler interactions are considered. Entanglement effects are represented by requiring the diffusion in the chain contour direction to be more pronounced than in the direction transverse to the chain primitive path. Filler particles are treated as material points. They reduce the polymer mobility in both longitudinal and transverse tube directions due to short-range energetic filler-polymer interactions. Hence, the contribution to chain dynamics and stress production of both filler-polymer and polymer-polymer interactions is considered to be purely frictional in nature. In the model, the strain rate sensitivity is associated with the thermal motion of chains, with the convective relaxation of entanglement constrains and with the polymer-filler attachment/detachment process. The effect of model parameters is discussed and the predictions are compared with experimental data.

Network model for the viscoelastic behavior of polymer nanocomposites

A theoretical network model reproducing some significant features of the viscoelastic behavior of unentangled polymer melts reinforced with well dispersed non-agglomerated nanoparticles is presented. Nanocomposites with low filler volume fraction (∼10%) and strong polymer–filler interactions are considered. The model is calibrated based on results obtained from discrete simulations of the equilibrium molecular structure of the material. This analysis provides the statistics of the network of chains connecting fillers, of dangling strands having one end adsorbed onto fillers, and that of the population of loops surrounding each nanoparticle. The network kinetics depends on the attachment–detachment dynamics of grafted chains of various types and is modeled by using a set of convection equations for the probability distribution functions. The overall viscoelastic response depends strongly on the lifetime of the polymer–filler junctions. The largest reinforcement is observed at low strain rates and low frequency oscillations. A solid like behavior is predicted for systems in which the polymer molecules interact strongly with the nanoparticles, effect which is associated with the behavior of the network of bridging segments.

Predictive modeling of elastic properties of particulate-reinforced composites

The Young's modulus and Poisson's ratio of isotropic composites made of particulate glass fillers in a BISGMA/TEGDMA resin matrix are determined using an axisymmetric spherical cell finite-element model developed by Guild and co-authors for modeling the elastic properties of rubber particles/epoxy composites with volume fractions of less than 0.5. Here, we have used this method to calculate the elastic properties of particulate-filled composites with a range of filler volume fractions, including those with filler fractions above 0.5, which are of primary interest for dental clinical applications. We have also conducted modeling studies on the influence of imperfect interfaces and thin interlayers on the elastic properties of composites with low and high filler volume fractions. Finally, we have conducted experimental tensile tests in order to compare theoretical predictions with experimental data. The predictions from the current model agree well with the test results for a range of volume fraction up to 0.53.