Synthesis of nickel–rubber nanocomposites and evaluation of their dielectric properties

Nanocomposites based on natural rubber and nano-sized nickel were synthesized by incorporating nickel nanoparticles in a natural rubber matrix for various loadings of the filler. Structural, morphological, magnetic and mechanical properties of the composites were evaluated along with a detailed study of dielectric properties. It was found that nickel particles were uniformly distributed in the matrix without agglomeration resulting in a magnetic nanocomposite. The elastic properties showed an improvement with increase in filler content but breaking stress and breaking strain were found to decrease. Dielectric permittivity was found to decrease with increase in frequency, and found to increase with increase in nickel loading. The decrease in permittivity with temperature is attributed to the high volume expansivity of rubber at elevated temperatures. Dielectric loss of blank rubber as well as the composites was found to increase with temperature.     

Wear rate and fracture toughness of porous particle-filled phenol composites

We experimentally investigated the effects of filler volume fraction of the phenol composites filled with porous particles on the fracture toughness and the wear rate against a smooth metal surface under multi-pass condition. Porous particles, made from rice husks, of various volume fractions from 0 to 0.5 were added to phenol resin as carbon filler. For the reported results of adhesive wear under multi-pass condition, we correlated the bulk parameters associated with the fracture toughness to the wear rate. We found an empirical power–law relation between the reciprocal of the product of stress and strain at rupture in bending test and the wear rate with various filler volume fractions. We also proposed the modified mixture law of the wear rate by taking account of the area fraction of transfer layer, which can provide a good prediction of the filler volume fraction dependence.     

Effect of morphology on the behaviour of ternary composites of polypropylene with inorganic fillers and elastomer inclusions

The effects of phase morphology, interfacial adhesion, rigid filler particle shape and elastomer volume fraction on the tensile yield strength of polypropylene (PP) filled with inorganic filler (CaCO₃ or Mg(OH)₂) and ethylene-propylene elastomer (EPR) were investigated. Separation of the filler and elastomer particles was achieved using maleic-anhydride-grafted PP (MPP) to enhance the filler-matrix adhesion. Encapsulation of the rigid filler by the elastomer was achieved using maleic-anhydride-grafted EPR (MEPR) to increase the filler-elastomer adhesion. The two limiting morphologies differ significantly in mechanical properties under tensile loading at the same material composition. Elastomer particles separately dispersed in the matrix enhance the shear banding in the bulk matrix which prevents the crazes growing from the filler surface from becoming unstable and, thus, increases the ductility of the material. Encapsulation by an elastomer layer on the filler surface relieves triaxial stresses at the filler surface, changing the major local failure mechanism from crazing to shear yielding and, hence, increasing the ductility of the material. Increase of the elastomer volume fraction also causes, in both cases, an increase in matrix ductility. Composite models are used to predict upper and lower limits of yield strength (_y) for the two limiting morphologies over an interval of elastomer volume fractions (V _e) from 0 to 0.2 at a constant filler loading of 30 vol.% and over a filler volume fraction from 0 to 0.4 at a constant EPR content in the matrix. Satisfactory agreement was found between the experimental data and theoretical predictions.     

Rubber composites based on graphene nanoplatelets,expanded graphite,carbon nanotubes and their combination: A comparative study

Solution styrene butadiene rubber (S-SBR) composites reinforced with graphene nanoplatelets (GnPs), expanded graphite (EG), and multiwalled carbon nanotubes (MWCNTs) were prepared and the electrical and various mechanical properties were compared to understand the specific dispersion and reinforcement behaviours of these nanostructured fillers. The electrical resistivity of the rubber composite gradually decreased with the increase of filler amount in the composite. The electrical percolation behaviour was found to be started at 15 phr (parts per hundred rubber) for GnP and 20 phr for EG filled systems, whereas a sharp drop was found at 5 phr for MWCNT based composites. At a particular filler loading, dynamic mechanical analysis and tensile test showed a significant improvement of the mechanical properties of the composites comprised of MWCNT followed by GnP and then EG. The high aspect ratio of MWCNT enabled to form a network at low filler loading and, consequently, a good reinforcement effect was observed. To investigate the effect of hybrid fillers, MWCNT (up to 5 phr) were added in a selected composition of EG based compounds. The formation of a mixed filler network showed a synergistic effect on the improvement of electrical as well as various mechanical properties.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

The effect of volume fraction of primary α phase on fracture toughness behaviour of Timetal 834 titanium alloy under mode I and mixed mode I/III loading

The effect of volume fraction of primary α phase on mode I and mixed mode I/III fracture toughness of Timetal 834 titanium alloy was investigated. The mode I and mixed mode I/III fracture toughness values for loading angle of 30° were found to initially decrease and subsequently increase with increase in volume fraction of primary α phase. On the other hand, mixed mode I/III fracture toughness for loading angle of 45° was found to monotonically decrease with increasing volume fraction of α phase. The fracture toughness was also found to marginally increase with increasing loading angle for the two lower primary α volume fractions, i.e. 6% and 15% whereas it marginally decreases with increasing loading angle for primary α volume fraction of 30%. The results were explained on the basis of the nature of stress field ahead of the crack tip under mixed mode I/III loading as well as the fracture mechanisms operative in this alloy for different α volume fractions.     

Processing,microstructure, and physical properties of interpenetrating Al2O3/Ni composites

Abstract

Alumina/nickel composites have been fabricated by hot pressing powder blends of various volume fractions of nickel and alumina. The electrical resistivities and Young's moduli of these composites have been measured and their dependence on the volume fraction of reinforcement has been investigated. Microstructural parameters such as contiguity were measured to quantify the distribution of the phases in these composites, and existing property models based on these data were used to predict the properties of the composites. The percolation threshold of nickel was found to occur at between 7.5 and 15 vol.-%Ni. The Young's modulus decreases as the volume fraction of nickel increases and is dependent on the contiguity of alumina. Composites containing 25, 35, 50, and 65 vol.-%Ni display microstructures with interpenetrating networks of alumina and nickel. The property models were found to fit both the resistivity and modulus data well, although the percolation threshold was predicted at a lower volume fraction than measured experimentally.     

Evaluation of a.c. conductivity of rubber ferrite composites from dielectric measurements

The effect of frequency, composition and temperature on the a.c. electrical conductivity were studied for the ceramic, Ni_1−xZn_xFe₂O₄, as well as the filler (Ni_1−xZn_xFe₂O₄) incorporated rubber ferrite composites (RFCs). Ni_1−xZn_xFe₂O₄ (where) (bix)varies from 0 to 1 in steps of 0.2 were prepared by usual ceramic techniques. They were then incorporated into a butyl rubber matrix according to a specific recipe. The a.c. electrical conductivity (σ_a.c) calculations were carried out by using the data available from dielectric measurements and by employing a simple relationship. The a.c. conductivity values were found to be of the order of 10⁻³ S/m. Analysis of the results shows that σ_a.c. increases with increase of frequency and the change is same for both ceramic Ni_1−xZn_xFe₂O₄ and RFCs. σ_a.c increases initially with the increase of zinc content and then decreases with increase of zinc. Same behaviour is observed for RFCs too. The dependence of σ_a.c on the volume fraction of the magnetic filler was also studied and it was found that the a.c. conductivity of RFCs increases with increase of volume fraction of the magnetic filler. Temperature dependence of conductivity was studied for both ceramic and rubber ferrite composites. Conductivity shows a linear dependence with temperature in the case of ceramic samples.     

Modelling percolation in fibre and sphere mixtures: Routes to more efficient network formation

Mixtures of perfectly conducting fibres and spheres, as well as mixtures of fibres of different aspect ratios, were simulated using a Dissipative Particle Dynamics (DPD) method, and the connectivity of the resulting assemblies was analysed using a Monte Carlo algorithm to predict the threshold volume fraction of filler material required for electrical percolation. For both isotropic and uniaxially oriented fibre–sphere mixtures, it was found that gradually replacing fibres with an equivalent volume of spheres increased the percolation threshold. By contrast, in aligned mixtures of fibres of two different aspect ratios, replacing a small fraction of higher aspect ratio fibres with shorter fibres led to a reduction in the percolation threshold, since the shorter fibres orient less well and provide bridging links between the highly oriented longer fibres. These theoretical results suggest that mixtures of fibres of different aspect ratio may be helpful in reducing the volume fraction of high aspect ratio filler particles (such as multi-wall carbon nanotubes) required to achieve significant electrical conductivity in composite materials.     

Dynamic mechanical,electrical and magnetic properties of ferrite filled styrene-isoprene-styrene

The dynamic mechanical, electrical and magnetic properties of highly filled magnetic polymeric composites containing 75 to 85 wt % barium ferrite in a thermoplastic elastomer matrix styrene-isoprene-styrene (SIS), are reported. The dependence of the properties on the volume fraction of the filler has been investigated. It is shown that the toughness and shore hardness of the composite may be correlated to its dynamic mechanical parameters. The use of coupling agents for surface treatment of ferrites has been shown to improve the magnetic properties of the composite due to better filler dispersion.     

Experimental and theoretical characterization of the engineering behavior of bitumen mixed with mineral filler

The dynamic shear rheometer (DSR) and the bending beam rheometer (BBR) were used to characterize the rheological properties of bitumen mixed with mineral filler that is smaller than 75 μm in size. The study focuses on using a rheology-based model to assess the effect of two distinctly different fillers, quartz and calcite, on the engineering behavior of the bitumen-mineral filler mastic. Four conventionally different bitumens were selected to assess the filler effect. By mathematically modeling the rheological response, predicting the rheological behavior of mastics becomes simpler and more efficient in approach. The rheological properties of bitumen-mineral filler mastics are characterized using the time–temperature superposition principle after data obtained from DSR and BBR are converted to the same unit. The stiffening effects of the filler are relatively small at short loading times or low temperatures, but are larger at higher temperatures or long loading times. This stiffening effect is found to be bitumen dependent as well as filler dependent. The validity of a micromechanical model is confirmed in this study. The Nielsen model was selected since it employs rheological parameters that could explain the filler effect. The micromechanical model shows good agreement with testing data at the filler volume fraction up to 22%.     

Electrical and mechanical properties of electrically conductive polyethersulfone composites

Electrically conductive polyethersulphone (pes) composites containing carbon fibres, nickel fibres, stainless steel fibres or aluminium flakes at various volume fractions up to 40% were fabricated and tested. For electromagnetic interference (emi) shielding effectiveness > 50 dB, the minimum filler volume fraction was 40% for carbon fibres of length 200 or 400 μm, 20% for nickel or stainless steel fibres, and 30% for aluminium flakes. The tensile strength first increased and then decreased with increasing filler content, such that the highest tensile strength occurred at 30 volume% (vol%) for carbon fibres (of length 200 or 400 μm) as the filler and at 10 vol% for nickel or stainless steel fibres. However, for carbon fibres (of length 100 μm) and aluminium flakes, the tensile strength increases up to at least 40 vol%. The best overall performance was provided by aluminium flakes at 40 vol%; the resistivity was 7 × 10⁻⁵ Ω cm, the emi shielding effectiveness was > 50 dB and tensile strength was 67 MPa. The resistivity of the aluminium flake composites was not affected by heating in air at 140°C for up to at least 144 h.     

Failure mechanics in ternary composites of polypropylene with inorganic fillers and elastomer inclusions

The effects of phase morphology, interfacial adhesion and filler particle shape and volume fraction on the fracture toughness of polypropylene (PP) filled with CaCO₃ or Mg(OH)₂ and ethylene-propylene elastomer (EPR) were investigated. Separation of the inorganic filler and elastomer particles was achieved using maleic-anhydride-grafted PP (MPP) to enhance the inorganic filler-matrix adhesion. Encapsulation of the rigid filler by the elastomer was achieved by using maleic-anhydride-grafted EPR (MEPR) to increase the inorganic filler-elastomer adhesion. The two limiting morphologies differed significantly in fracture toughness under impact loading at the same material composition. A model for a mixed mode of failure, accounting for the plane strain and plane stress contributions to the strain energy release rate,G _c, was used to predict the upper and lower limits forG _c for the two limiting morphologies over an interval of elastomer volume fractions,v _e, from 0–0.2 at a constant filler volume fraction,V _f = 0.3, and over the filler volume fraction from 0–0.4 at constant EPR content. The role of material yield strength in controlling fracture toughness has been described successfully using Irwin's analysis of plastic zone size. The presence of elastomer enhances both the critical strain energy release rate for crack initiation,G _c, and the resistance to crack propagation as expressed by Charpy notched impact strength for the two limiting morphologies. Satisfactory agreement was found between the experimental data and predictions of upper and lowerG _c limits.     

Magnetic and processability studies on rubber ferrite composites based on natural rubber and mixed ferrite

Polycrystalline single phasic mixed ferrites belonging to the series Ni_1–x Zn _x Fe₂O₄ for various values of _x have been prepared by conventional ceramic techniques. Pre-characterized nickel zinc ferrites were then incorporated into a natural rubber matrix according to a specific recipe for various loadings. The processability and cure parameters were then determined. The magnetic properties of the ceramic filler as well as the ferrite loaded rubber ferrite composites (RFC) were evaluated and compared. A general equation for predicting the magnetic properties was also formulated. The validity of these equations were then checked and correlated with the experimental data. The coercivity of the RFCs almost resemble that of the ceramic component in the RFC. Percolation threshold is not reached for a maximum loading of 120 phr (parts per hundred rubber by weight) of the filler. These studies indicate that flexible magnets can be made with appropriate magnetic properties namely saturation magnetisation (M _s) and magnetic field strength (H _c) by a judicious choice of x and a corresponding loading. These studies also suggest that there is no possible interaction between the filler and the matrix at least at the macroscopic level. The formulated equation will aid in synthesizing RFCs with predetermined magnetic properties.     

Mechanisms of particulate filled polypropylene finite plastic deformation and fracture

The plastic deformation and fracture of aluminium hydroxide filled polypropylene has been investigated. A transition between two mechanisms with an increase of the filler volume fraction has been observed. Below a critical filler volume content φcr ≈ 20 vol% (designated region 1) adhesive failure processes and polymer deformation in the neighbourhoods of different particles occur in an uncorrelated manner. Above this critical value (designated region 2) exfoliation along the surface of the initial portion of inclusions causes the formation of craze-like deformation zones transverse to the direction of the loading. The concentration of craze-like zones is essentially determined by the filler content and the level of interphase interaction which in turn depends on the particle size. In region 1 deformation occurs in a macro heterogeneous way with the formation and growth of a neck. The elongation to break decreases with an increase in the mean diameter of the filler phase. At φ>φcr composites, filled with small particles, fail in quasi brittle manner with the formation of a short and narrow neck. In contrast to the case for a small filler concentration, an increase of the inclusion size leads to an increase in the ultimate elongation and a tendency to macro homogeneous yielding. An explanation of the observed behaviour is proposed based on a change in adhesive failure conditions with filler content and size. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of nickel coated mica as a conductive filler

Electroless plating was used to prepare nickel coated mica fillers. To optimise the conductivity of the filler the nickel coating needed to exceed a certain weight percentage, depending upon the particle size of the mica, and cover the surface of the mica particles. Treatment of the filler in hydrogen improved its conductivity considerably. The fillers were incorporated into an ABS resin to prepare composites as potential electromagnetic interference shielding materials. Increasing the particle size of the mica reduced the critical filler loading required to produce electrically conductive composites. Reducing sample thickness caused a decrease in resistance, due to changes in filler orientation.     

Dielectric characteristics of sisal–oil palm hybrid biofibre reinforced natural rubber biocomposites

Natural rubber was reinforced with sisal and oil palm fibers. Biocomposites were prepared by varying the weight fraction of the fibers. The dielectric properties such as dielectric constant, volume resisitivity and dielectric loss factor of the biocomposites were evaluated as a function of fiber loading, frequency and chemical modification of fibers. The dielectric constant values were found to be higher for fiber reinforced system than the gum due to polarization exerted by the incorporation of lignocellulosic fibers. Chemical modification of fibers resulted in decrease of dielectric constant values and volume resisitivity values. The volume resisitivity of the composites was found to decrease with fiber loading due to increase of hydrophilicity imparted by the lignocellulosic fibers. The dissipation factor was found to increase with fiber content.     

Acoustic properties of particle/polymer composites for ultrasonic transducer backing applications

The acoustic impedance and attenuation in composites made of particle fillers loaded in polymer matrices for transducer backing applications is investigated. The acoustic impedance of tungsten/vinyl composites was modeled, and an experimental matrix identifying variables that contribute to composite attenuation was established. The variable included the particle type, the particle size and volume fraction of a filler, the physical characteristics of the polymer matrix, and the processing route that determined the composite connectivity. Experimental results showed that with an increase in filler particle size or a decrease in volume fraction of filler, there is an increase in composite attenuation. Overall, the various types of filler, the polymer matrix, and the interface between the two contribute to attenuation in the composite, as confirmed by the acoustic properties and the microstructural analysis.     

Size-scale effects of silica on bis-GMA/TEGDMA based nanohybrid dental restorative composites

The present study focuses on the effect of size-scale combination of silica on the mechanical and dynamic mechanical properties of acrylate based (50% Bis-GMA and 50% TEGDMA by weight) composites with an aim to overcome the conventional problem of high-volume fraction filling of acrylate based composites, typically used in restorative dentistry. Two classes of light-cured composites based on the size-scale combination of silica (7 nm + 2 μm; 14 nm + 2 μm) as the filler were prepared. FTIR spectroscopy revealed functionality and interactions whereas morphological investigations concerning the state of distribution and dispersion of nano- and micro-silica has been carried out by SEM–EDX Si-dot mapping. The dynamic mechanical properties, compressive, flexural and diametral tensile strengths were characterized. Micromechanical analysis of viscoelastic storage moduli following Kerner composite model has revealed an enhancement in the reinforcement efficiency of the nanohybrid composites based on the filler size-scale combination of 14 nm + 2 μm with 10 wt.% nanofiller loading. The compressive strength of the micro-filled composite (with 2 μm silica only) was found to remain comparable to that of the nanohybrid with 5 wt.% of 7 nm silica and 10 wt.% of 14 nm silica filled composites. Diametral tensile strength has been observed to be influenced by the size-scale combination and extent of nanofiller loading. The effective volume fractions in the composites validating the experimentally determined DTS were calculated following Nicolais–Narkis model. Our study demonstrates the conceptual feasibility of exploring the optimization of size-scale combinations of filler for enhancement in reinforcement efficiency by manipulating the volume fraction of filler induced immobilized polymer chains by resorting to the principle of micromechanics.