Fabrication of an Electrically-Resistive,Varistor-Polymer Composite

This study focuses on the fabrication and electrical characterization of a polymer composite based on nano-sized varistor powder. The polymer composite was fabricated by the melt-blending method. The developed nano-composite was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FeSEM), and energy-dispersive X-ray spectroscopy (EDAX). The XRD pattern revealed the crystallinity of the composite. The XRD study also showed the presence of secondary phases due to the substitution of zinc by other cations, such as bismuth and manganese. The TEM picture of the sample revealed the distribution of the spherical, nano-sized, filler particles throughout the matrix, which were in the 10–50 nm range with an average of approximately 11 nm. The presence of a bismuth-rich phase and a ZnO matrix phase in the ZnO-based varistor powder was confirmed by FeSEM images and EDX spectra. From the current-voltage curves, the non-linear coefficient of the varistor polymer composite with 70 wt% of nano filler was 3.57, and its electrical resistivity after the onset point was 861 KΩ. The non-linear coefficient was 1.11 in the sample with 100 wt% polymer content. Thus, it was concluded that the composites established a better electrical non-linearity at higher filler amounts due to the nano-metric structure and closer particle linkages.     

Effects of chicken eggshell filler size on the processing,mechanical and thermal properties of PVC matrix composite

The effects of filler particle size of poly(vinyl chloride)/chicken eggshell powder (PVC/ESP) composites on the processing, tensile properties, morphology and thermal degradation were investigated. The mixing of composites was done using Rheomix internal mixer. The processing torque of PVC/ESP composite at a particle of 0.2 μm exhibits lower processing torque compared to that at a particle size of 7 μm due to the dispersive resistance from larger ESP filler particles. Good interfacial adhesion exists between the filler and matrix in composites prepared via a filler particle size of 0.2 μm, which has improved the tensile strength and modulus of PVC/ESP composite compared to a filler particle size of 7 μm as justified from FESEM images on the tensile fracture surface of the composites. Thermogravimetric analysis results show that the filler particle size of 0.2 μm composite exhibits higher thermal stability compared to the filler particle size of 7 μm composite.     

Effect of temperature,pressure, and composition on DC resistivity and AC conductivity of conductive styrene–butadiene rubber–particulate metal alloy nanocomposites

Conductive nanocomposites were prepared using styrene butadiene rubber as the polymer matrix and nanosized powder of copper–nickel (Cu–Ni) alloy as the filler. The filler loading was varied from 0 to 40 phr. The electrical conductivity of filled polymer composites is due to the formation of some continuous conductive networks in the polymer matrix. Atomic force microscopy was used to determine the particle size of the nanofiller and its nature of dispersion in the rubber matrix. The DC volume resistivity was measured against the loading of the nanofiller to check the percolation limit. The effect of temperature, applied pressure, time duration under constant compressive stress on the DC resistivity and AC conductivity of the composites with different filler loading were investigated. The change in DC resistivity and AC conductivity against temperature of these composites exhibited positive coefficient of temperature. With the change in applied pressure and time duration under constant compressive stress the DC resistivity undergoes an exponential decrease. The effect of AC field frequency on the AC conductivity was investigated. POLYM. COMPOS. 28:696–704, 2007. © 2007 Society of Plastics Engineers     

Tailoring the nonlinear conducting behavior of silicone composites by ZnO microvaristor fillers

Silicone composite filled with zinc oxide microvaristors possesses excellent nonlinear conducting behavior as ZnO varistor does. For better adjusting the composite's electrical behavior to satisfy the practical field‐grading requirement, this article studied the influence of ZnO filler's property on the nonlinearity of the composite. Several groups of ZnO‐silicone composite samples in different filler volume fraction and filler diameter were prepared, the measured J‐E characteristics show that the percolation threshold of ZnO‐silicone composite is around 35%, above which the composites present reliable nonlinear behavior. The switching voltage of the composite exhibits a considerable decrease as filler's diameter increases or filler's volume fraction increases, while the nonlinear coefficient remains stable. Moreover, filler's size also has a little influence on composite's percolation limit. The conclusion above fits very well with the theory of the conducting composites and percolation process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42645.     

Visualisation of conductive filler distributions in polymer composites using voltage and energy contrast imaging in SEM

Semiconductive composites have been examined using advanced scanning electron microscopy (SEM). For the first time, voltage contrast and energy contrast between the conductive filler and the polymer matrix have been revealed using a secondary electron detector placed inside the lens system and an energy selective backscattering detector respectively. Critical parameters, including loading level, distribution, dimension and shape of conductive fillers, correlating to the electrical conductivities of the composites, have been investigated and quantitatively determined. These parameters are essential for performance predictions, product quality control and new product development. The volume fractions of the conductive fillers in the two investigated composites were determined as 20.9% and 14.2% respectively. Higher frequency of distribution distance between the conductive filler aggregates within the ranges of 20–100 nm was revealed for the composite with volume fraction of 20.9%. The aggregates of conductive fillers showed mainly branched shapes. The information obtained provides further insight into the conductivity mechanism of conductive filler loaded polymer composite.     

Conducting aluminum‐filled nylon 6 composites

This work is concerned with the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder. The electrical conductivity, density, hardness and morphology of composites were investigated. The electrical conductivity of the composites is < 10⁻¹¹ S/cm unless the metal content reached the percolation threshold, beyond which the conductivity increased markedly by as much as 10¹¹. The volume fraction of conductive filler at the percolation threshold was calculated from experimental data, by fits to functions predicted by the percolation theory. Decreasing the average particle diameter of filler leads to increased percolation threshold (it varies from 23 to 34 vol% for the three different fillers studied) and decreased maximal conductivity of composites. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. Furthermore, it is shown that for certain sizes of particle filler, the hardness decreases initially with the increase of metal concentration, possibly because of poor surface contact with the nylon matrix, but, starting from a certain value, there is a hardness increase. For the smallest particle filler, the hardness of samples is not influenced by the presence of the filler.     

BN填充PA6基导热绝缘复合材料导热性能研究

以聚酰胺6（PA6）为基体, 氮化硼（BN）作为导热填料,经双螺杆挤出机熔融共混,模压成型制得导热绝缘复合材料。研究了BN含量、粒径、形状和不同BN粒径复配对复合材料导热性能的影响,并研究了BN含量和粒径对复合材料绝缘性能的影响。结果表明,在各种粒径下,复合材料热导率均随BN填充量的增加而增大;在BN粒径为5 μm、填充量为25 %（体积分数,下同）时,复合材料热导率达到1.2187 W/(m·K);在BN填充量相同时,填料粒径对复合材料热导率的影响不是简单的单调规律,呈现50、100 μm时较小,1、5、15 μm时较大,150 μm时最大的规律;片状BN填料比球状BN填料更有利于提高复合材料的热导率;2种不同粒径填料复配所填充的复合材料的热导率大于单一粒径填充的复合材料;5 μm与150 μm粒径BN复配,在填充量为20 %,配比为1:3时,复合材料的热导率最大,达到1.3753 W/(m·K),为纯PA6的4.9倍;在不同BN含量和粒径下,复合材料体积电阻率均能达到10000000000000 Ω·cm以上,满足绝缘性能。     

Thermal,electrical, and mechanical properties of Si3N4 filled LLDPE composite

Silicon nitride (Si₃N₄) filled linear low-density polyethylene (LLDPE) composite was prepared. The effects of Si₃N₄ filler content, dispersion, and LLDPE particle size on the thermal conductivity, and Si₃N₄ filled content on the mechanical and electrical properties of Si₃N₄ reinforced LLDPE composites prepared using powder mixing were investigated. The results indicate that there existed a unique dispersion state of Si₃N₄ particles in LLDPE, shell-kernel structure, in which Si₃N₄ particles surrounded LLDPE matrix particles. With increasing filler content and LLDPE particles size, thermal conductivity increased, and reached 1.42 W/m K at 30 vol% of filler, seven times as that of unfilled LLDPE. Furthermore, the examinations of Agari model demonstrate that larger size LLDPE particles form thermal conductive networks easily compared with smaller ones. The values predicted by theoretical model underestimate the thermal conductivity of Si₃N₄/LLDPE composites. In addition, the composites still possessed rather higher electrical resistivity and dielectric properties, but the mechanical properties decreased. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Structure and electrical properties of polymer composites based on tungsten oxide varistor ceramics

Composites based on nonlinear tungsten oxide ceramics and a polyethylene matrix with a volume fraction of ceramic filler from 10 vol % to 43 vol % were studied. It was shown that composites are an isotropic mixture of WO₃ grains in a polymer matrix. The current-voltage characteristics of the composites were nonlinear. Composites have a high positive temperature coefficient of resistance up to 15.8 K^-1 in the temperature range of 40–75 °C. It is shown that the temperature coefficient of resistance is positive and increases with increasing electric field strength. The decrease in the electrical conductivity of composites with increasing temperature is explained by the expansion of the polymer matrix and the rupture of the current-conducting channels between the conducting grains of the varistor ceramics. The dependence of the electrical conductivity of the composite on the volume fraction of varistor ceramics is well described within the framework of a three-dimensional percolation model for a two-phase system.     

Effect of N–Ni coordination bond on the electrical and thermal conductivity of epoxy resin/nickel-coated graphite

The agglomeration of nickel-coated graphite (NCG) in epoxy resin (EP) composites leads to low electrical conductivity of EP composites, which limits their development in electronic devices and multilayer circuits. In order to improve the electrical and thermal conductivity of NCG/EP composites, ethylenediamine (EDA) was used to modify NCG and compared with pure NCG-filled EP composites. It was found that the conductive effect of modified composites with 20 wt% filler is better than that of unmodified composites with 40 wt% filler. The results of Fourier transform infrared spectroscopy and thermogravimetric analysis of EDA-modified NCG (ENCG) showed that a coordination adsorption reaction occurred between EDA and NCG, forming N–Ni coordination bonds. When the filling amount of ENCG was 40 wt%, the conductivity and thermal conductivity of the composite are improved most significantly. The volume resistivity was reduced from 2.636 to 0.109 Ω cm, a decrease of 95.85%, and the thermal conductivity was improved from 0.517 to 0.968 W/(m K), an increase of 87.23%, respectively. Meanwhile, ENCG has better dispersion in the EP matrix than NCG.     

Effect of Microstructure on Strength of Si3N4-SiC Composite System

The Si₃N₄-SiC composite system was investigated to better understand the effect of microstructure on the strength-controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot-pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9- and 32-μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5-μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high-temperature structural materials.     

A novel method to prepare high-resistivity polymer composites

A novel method was developed for the preparation of high-resistivity conductive polymer composites reproducibly. The conventional method which involves the simple mixing of a conductive filler and a polymer usually produces a conductive polymer composite having a loading curve with a region in which the resistivity changes rapidly as the filler concentration changes. Hence, it is very difficult to obtain reproducible materials in that region. This newly developed method involves the preparation of a conventional conductive polymer composite, which is prepared by the simple mixing of carbon black and a polymer as the first step. The resulting compound, which is crosslinked by either electron-beam radiation or a chemical-crosslinking agent, is ground into a fine powder (composite filler) with the particle size less than 250 microns. The composite filler is mixed with another polymer to form a conductive particulate composite which has a loading curve showing a more gradual change of resistivity as a function of filler concentration. The modification of the loading curve is controlled by the resistivity, the shape, and the size of the composite filler.     

Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure

Multi-walled carbon nanotube (MWCNT)/high density polyethylene (HDPE) and graphene nanosheets (GNS)/HDPE composites with a segregated network structure were prepared by alcohol-assisted dispersion and hot-pressing. Instead of uniform dispersion in polymer matrix, MWCNTs and GNSs distributed along specific paths and formed a segregated conductive network, which results in a low electrical percolation threshold of the composites. The electrical properties of the GNS/HDPE and MWCNT/HDPE composites were comparatively studied, it was found that the percolation threshold of the GNS/HDPE composites (1 vol.%) was much higher than that of the MWCNT/HDPE composites (0.15 vol.%), and the MWCNT/HDPE composite shows higher electrical conductivity than GNS/HDPE composite at the same filler content. According to the values of critical exponent, t, the two composites may have different electrical conduction mechanisms: MWCNT/HDPE composite represents a three-dimensional conductive system, while the GNS/HDPE composite represents a two-dimensional conductive system. The improving effect of GNSs as conducting fillers on the electrical conductivity of their composites is far lower than theoretically expected.     

Piezoresistivity of silicone‐rubber/carbon black composites excited by Ac electrical field

Flexible conductive composites were prepared using liquid silicone rubber as a matrix and conductive carbon black (CCB) as a filler, and the filler loading was varied from 1 to 15 phr in mass ratio. The surface conductivity was studied as a function of CCB concentration (1, 5, 10, 15 wt %), frequency in the range from DC to 1 MHz. The AC resistivity of the composites with low CCB concentration was found to be frequency dependent, whereas the composites with high CCB concentration was almost frequency independent. The resistance/impedance drift of the composites with time decreases sharply with the increase of frequency of applied electrical field. The piezoresistivity of the composite with 5 wt % CCB concentration (the upper percolation limit) was studied. It is found that the composite exhibits prominent positive piezoresistivity coefficient effect through the measurement frequency, and the sensitivity becomes steeper with the increase of exciting frequency. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical behavior and dilatation of particulate-filled thermosets in the glassy state

Dilatation of specimens is measured during tensile tests to investigate the mechanical response of particulate-filled amorphous networks in the glassy state. The effects of particle size, volume fraction of filler, coupling agents, and crosslink density of the matrix on the mechanical-dilatational behavior are studied on model composites of glass-bead-filled polyurethanes. It is found that the stress-strain response of composites with untreated glass beads shows nonlinearity and subsequent yielding due to dewetting of particles from the matrix. In contrast, composites containing particles coated with a comupling agent fracture in a brittle manner, showing no significant nonlinearity and dewetting. Coated particles provide a higher tensile strength, but a lower strain at fracture, than uncoated particles. The volume fraction of the filler has an effect on Young's modulus, which is independent of the degree of coupling between the matrix and the filler. Tensile strength and strain at break decrease with increasing filler content for coated and uncoated particles. No strong effect of particle size is observed on either the tensile modulus or the dilatational behavior in the 25 μm to 160 μm diameter range. However, strain at break increases with decreasing particle size. When the accompanying yield phenomena shift to smaller strains, and a transition to brittle fracture takes place at high crosslink densities.     

Comparison on the Properties of Nickel-Coated Graphite (NCG) and Graphite Particles as Conductive Fillers in Polypropylene (PP) Composites

The present study was carried out to evaluate the performance of nickel-coated graphite (NCG) in comparison with graphite as conductive fillers in polypropylene (PP) matrix. Graphite exhibits smaller particle size and higher aspect ratio (length/thickness) than NCG particles. The results showed that the additions of graphite filler in PP exhibits higher tensile properties and electrical conductivity compared to NCG filled PP composites. The electrical results showed that the percolation threshold of graphite and NCG filled PP composites occurred in the range of 10 to 20 vol.% and 15 to 25 vol.%, respectively.     

Inclusion-Size-Independent Strength of Glass/Particulate-Metal Composites

Dispersed-phase metallic inclusions limit brittie-matrix defect size and contribute to toughness via plastic deformation. In this work, it is shown that composite strength depends on the dispersed-phase volume fraction and not on the diameter of uniformly sized inclusions if the plastic deformation thereof is fully utilized. Experimental results for silicate glass/aluminum-particle systems substantiate this finding. Particulate composites of 40 vol% Al and 40 vol% Al-alloy soda–lime–silica glass exhibit 1.5- and 2-fold strength increases, respectively. Both 20 vol% Al composites with 57- and 20-μm particle size showed 1- to 3-fold increases in strength, respectively. Model calculations of strength are overestimates and it is proposed that particle/matrix interfacial separation limits full utilization of the potential plastic work of deformation.     

Electrical conductivity and EMI shielding effectiveness of polyurethane foam–conductive filler composites

The morphological, electrical, and thermal properties of polyurethane foam (PUF)/single conductive filler composites and PUF/hybrid conductive filler composites were investigated. For the PUF/single conductive filler composites, the PUF/nickel‐coated carbon fiber (NCCF) composite showed higher electrical conductivity and electromagnetic interference shielding effectiveness (EMI SE) than did the PUF/multiwall carbon nanotube (MWCNT) and PUF/graphite composites; therefore, NCCF is the most effective filler among those tested in this study. For the PUF/hybrid conductive fillers PUF/NCCF (3.0 php)/MWCNT (3.0 php) composites, the values of electrical conductivity and EMI SE were determined to be 0.171 S/cm and 24.7 dB (decibel), respectively, which were the highest among the fillers investigated in this study. NCCF and MWCNT were the most effective primary and secondary fillers, and they had a synergistic effect on the electrical conductivity and EMI SE of the PUF/NCCF/MWCNT composites. From the results of thermal conductivity and cell size of the PUF/conductive filler composites, it is suggested that a reduction in cell size lowers the thermal conductivity of the PUF/conductive filler composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44373.     

Electrical properties of composites of hard metal carbides in a polymer matrix

The electrical conductivity of composites containing hard metal carbide powder in a nonconducting polymer matrix has been studied. In composites with low and medium filler contents, the expansion of the nonconducting matrix with increasing temperature caused interruption of conducting paths and a rapid drop in conductivity (switching effect). In contrast, at high filler loadings (Φ_P > 50 vol%), a continuous and slight decrease in conductivity with temperaute inducates a specific composite structure with a dense particle arrangement and a great number of conducting contants, which change only little during heating.     

LDPE/PEG插层剥离改性氮化硼导热复合材料的制备及其性能

以聚乙二醇(PEG)为插层剂,通过机械球磨法制备了PEG插层剥离改性氮化硼.以低密度聚乙烯(LDPE)为基体,PEG插层剥离改性氮化硼为导热填料,采用双辊开炼、压片成型制备LDPE/PEG插层剥离改性氮化硼导热复合材料,研究了改性氮化硼用量及粒径对复合材料导热性能、力学性能和电绝缘性能的影响.结果表明:随着PEG插层剥...