The electrical behavior of laminated conductive polymer composite at low temperatures

The study deals with the electrical characteristics of laminated conductive polymer composites consisting of epoxy and carbon fibers with different concentration. The composites contain 7, 17.5, and 25.2 wt% carbon fibers (10, 25, and 36 layers of carbon fibers), respectively. The DC electrical conductivity was studied as a function of filler concentration in low temperature range 25–275 K. It was found that the composites exhibit negative temperature coefficient of resistivity (TCR) and electrical conductivity enhancement with temperature and carbon fibers concentration. The semiconducting behavior of the observed electrical conductivity is characterized by two different regions: high temperature range where the conductivity increases gradually (thermal process) and low temperature range where the conductivity increases with a less rate (Motts hopping process) with increasing of temperature.     

Thermoelectric power and AC electrical properties of PAN-based carbon fiber composites

The thermoelectric power and AC electrical properties of conductive polymer composites made of polycarbonate filled with randomly distributed PAN-based carbon fibers of different concentration: 0, 5, 10, 18, and 30 wt% were studied. The thermoelectric power was measured as a function of temperature in the frequency range from 200 kHz to 12 MHz. It was found that the observed Seebeck coefficient and thermoelectric activation energy depend on temperature, frequency and fiber concentration. The Seebeck coefficient calculated using the electrical transport theory of semiconductors decreases with both increasing temperature and carbon fiber content. The thermoelectric power results revealed that the composites function electrically as semiconductors. Dielectric constants and AC conductivity were calculated from impedance and phase angle measurements. It was found that both increase with increasing temperature. The activation energy and relaxation time decrease with increasing temperature measured and applied frequency. The thermoelectric power results indicated that electrical conduction in bulk composites is produced from a combination of transport processes involving: electrons, holes, ions and charged impurity motion in addition to protonic migration.     

Low temperature electrical conductivity of low-density polyethylene/carbon black composites

The study deals with the electrical characteristics of carbon black/low-density polyethylene (CB/LDPE) composites of various CB filler concentrations (10, 15, and 20 wt.%). The DC electrical conductivity was studied as a function of filler concentration in low temperature range 25–285 K. It was found that the composites exhibit negative temperature coefficient of resistivity (TCR) at low temperatures and high enhancement in the electrical conductivity with both temperature and carbon black concentration. The observed increase of conductivity with the filler concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the given composites on temperature (25–285 K) was analyzed in terms of a formula in consistence with Mott hopping mechanism.Visiting scientist from the Jordan University of Science and Technology, Irbid-Jordan.     

Electrical properties of polyvinylidene fluoride (PVDF)/multi-walled carbon nanotube (MWCNT) semi-transparent composites: Modelling of DC conductivity

Transparent conductive composites can be achieved from PVDF–MWCNT at very low concentration of MWCNT. These composites show different degree of UV–Visible radiation absorption depending on MWCNT concentration in composites. The composition dependent dielectric properties and AC conductivity were also measured for these composites. Properties like AC conductivity, dielectric constant and loss are increasing with filler concentration. The variations of DC conductivity against composition and temperature are also reported. The electrical hysteresis and electrical set are observed for PVDF–MWCNT composites when subjected to heating–cooling cycle. The validity of different theoretical models depicting percolation threshold with respect to DC conductivity was tested for these composites.     

Electrical properties of epoxy resin filled with carbon fibers

The electrical properties of an epoxy resin filled with carbon fibers were studied. By discharging a high voltage through the composite it was found that the resistivity of the composite decreased. This effect was attributed to local dielectric breakdown of polymer layer between carbon fibers. The conduction mechanism of common and breakdown composites was studied by means of exploring of current-voltage characteristics and the frequency dependence of resistivity. A positive temperature coefficient (PTC) effect of resistivity behavior was observed both for common and breakdown samples.     

A study on the DC electrical properties of PAN-based carbon fiber/polycarbonate composites

The DC conductivity and Hall effect studies were used to investigate the nature, type, and development of the charge carriers in conductive polymer composite containing PAN-based carbon fibers of different concentration. The dependence of the electrical conductivity on temperature is characterized by a two-stage electrical conduction process with a semiconducting type of behavior and two activation energies. It was found the measured Hall voltage varies linearly with Hall current with two different signs of slopes. This suggests that a composite of low fiber content is functioning as p-type material, and then changes to n-type with increasing the carbon fiber content more than 15 wt.%. The density of the charge carriers increases with carbon fiber content in a behavior similar to the electrical conductivity for all given composites, showing a percolation phenomenon. The calculated charge carriers density includes both the magnetostatic arising from the polycarbonate matrix and from the free charge carriers themselves. Considering the filled carbon fibers as a random semiconducting material, the results obtained for various composites were described in terms of the band structure model. Other approach of results analysis was based on the composite bulk morphology observed by the SEM microscopy.     

DC electrical conductivity of poly(methyl methacrylate)/carbon black composites at low temperatures

The study deals with the dc electrical conduction of poly(methyl methacrylate)/carbon black composites of different carbon black (CB) filler concentrations (2, 6, 12 wt%). The dc electrical conductivity was studied as a function of filler concentration, and temperature in the range (20–290 K). It was found that the composites exhibit negative temperature coefficient of resistivity (NTCR) at low temperatures and enhancement in the dc electrical conductivity with both temperature and CB concentration. The observed increase of conductivity with CB concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the composites in low temperatures was analyzed in term of a formula in consistence with Mott variable rang hopping (VRH) mechanism. The observed overall mechanism of electrical conduction has been related to the transfer of electrons through the carbon black aggregations distributed in the polymer matrix.     

Dielectric,thermal and mechanical properties of Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> based polymer/ceramic composites

Polymer/Sr₂ZnSi₂O₇ (SZS) ceramic composites suitable for substrate applications have been developed using the polymers polystyrene (PS), high density polyethylene (HDPE) and Di-Glycidyl Ether of Bisphenol A (DGEBA). The dielectric, thermal and mechanical properties of the composites are investigated as a function of various concentrations of the ceramic filler. The obtained values of relative permittivity, dielectric loss tangent, thermal conductivity and coefficient of thermal expansion of the composites are compared with the corresponding theoretical predictions. The relative permittivity of the polymer/ceramic composites increases with filler loading. The dielectric loss tangent also shows the same trend except for DGEBA/SZS composites. The major advantages of the ceramic loading are improvement in thermal conductivity and a decrease in the coefficient of thermal expansion. The tensile strength of the composites decreases with increase in filler content, whereas an improvement is observed in microhardness. The variation of relative permittivity (at 1 MHz) of the composites is also studied as a function of temperature.     

A comparative study on the electrical,thermal and mechanical properties of ethylene–octene copolymer based composites with carbon fillers

Conducting polymer composites (CPC) were prepared with an ethylene–octene copolymer (EOC) matrix and with either carbon fibers (CFs) or multiwall carbon nanotubes (MWCNTs) as fillers. Their electrical and thermal conductivities, mechanical properties and thermal stabilities were evaluated and compared. CF/EOC composites showed percolation behavior at a lower filler level (5 wt.%) than the MWCNT/EOC composites (10 wt.%) did. Alternating current (AC) conductivity and real part of permittivity (dielectric constant) of these composites were found to be frequency-dependent. Dimensions and electrical conductivities of individual fillers have a great influence on the conductivities of the composites. CF/EOC composites possessed higher conductivity than the MWCNT-composites at all concentrations, due to the higher length and diameter of the CF filler. Both electrical and thermal conductivities were observed to increase with increasing filler level. Tensile moduli and thermal stabilities of both (CF/EOC and MWCNT/EOC) composites increase with rising filler content. Improvements in conductivities and mechanical properties were achieved without any significant increase in the hardness of the composites; therefore, they can be potentially used in pressure/strain sensors. Thermoelectric behavior of the composites was also studied. Accordingly, CF and MWCNT fillers are versatile and playing also other roles in their composites than just being conducting fillers.     

Electrical and thermal properties of polyimide/silica nanocomposite

The electrical and thermal properties of thin films of polyimide/silica nanocomposites prepared via sol–gel process were studied as a function of nanosilica particles content, temperature and applied field frequency. It was found that the dielectric constant and dielectric loss of the nanocomposites decrease with both the frequency and the nanosilica content, while increase with temperature. The AC-conductivity measured in frequency range 200?kHz–1.5?MHz decreases with the filler concentration and increases with increasing temperature. For the (25?wt%) nanocomposite, it was found that the AC-conductivity increases with temperature, and the Cole–Cole plots showed that the calculated activation energy and relaxation time decrease with temperature. The observed thermal conductivity increases gently with temperature. The empirical universal law was used to fit the observed electrical data under the measuring conditions.     

Epoxy Composites Filled with Micro-Sized AlN Particles for Microelectronic Applications

With the rapid development of the electronic information industry, better properties are required for substrate and packaging materials such as high thermal conductivity, low coefficient of thermal expansion, and low dielectric constant. Polymers are ordinarily being used for this purpose due to their high electrical resistivity and low density, but unfortunately they suffer from a disadvantage like low thermal conductivity. To offset this deficiency, adding inorganic conductive particles to polymer is a versatile method. In view of this, the present work aims at developing a class of particulate filled polymer composites with micro-sized aluminum nitride (AlN) particles having an average particle size of 60–80 µm reinforced in epoxy matrix. A set of composites, with filler content ranging from 0 to 25 vol%, have been prepared by the hand-layup technique. Effects of filler percentage on various properties like effective thermal conductivity (k_eff), coefficient of thermal expansion (CTE), glass transition temperature (T_g), and dielectric constant (ε_c) are studied. It is found that the incorporation of AlN in resin increases the k_eff and T_g, whereas CTE of the composite decreases favorably. Though dielectric constant of the matrix increases with filler content yet it remains well within the desirable limit. With modified thermal and dielectric characteristics, these composites can possibly be used for microelectronics applications.     

Thermoelectric measurements of PEDOT:PSS/expanded graphite composites

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/expanded graphite films were cast as thin films with different expanded graphite contents at room temperature. The thermoelectric properties of the composites were investigated as a function of the graphite concentration. The electrical conductivity and Seebeck coefficient were measured as a function of the graphite concentration. The electrical conductivity and power factor show similar trends with a sharp increase at around 55 wt% of expanded graphite content. The Seebeck coefficient does not show a significant dependence with the graphite content. SEM and TEM images indicate a nearly homogenous distribution of the filler in the matrix. The initial thermal stability is not modified with the filler.     

Electrical,mechanical and thermal properties of polyvinyl chloride composites filled with aluminum powder

In this work, the electrical, mechanical and thermal properties of polyvinyl chloride (PVC) composites filled with different content of aluminum powder varying from 0 to 40 wt.% have been prepared. The dielectric properties of these composites were investigated in the frequency range 100 Hz–100 kHz at temperature range from 30 to 98 °C. The percolation threshold concentration, which is the concentration after which the conductivity increases many orders of magnitude with very little increase in the filler content for PVC/Al composites depends upon the measuring temperature, whether it is below or above the glass transition of the polymer matrix. The highest value of the electrical conductivity, σ, of the composites was found to be in the order of 10⁻⁸ S cm⁻¹, this value recommend such composites to be used in electrostatic dissipation applications as the range of conductivity for such application should be in the range of 10⁻⁵–10⁻⁹ S cm⁻¹.     

Enhanced electrical conductivity of polypyrrole/polypyrrole coated short nylon fiber/natural rubber composites prepared by in situ polymerization in latex

In this paper we report the characterization and properties of conductive elastomeric composites of polypyrrole (PPy) and PPy coated short Nylon-6 fiber (F-PPy) based on natural rubber (NR) prepared by in situ polymerization method. PPy/NR blends were prepared by polymerizing pyrrole in NR latex using anhydrous ferric chloride as oxidizing agent, p-toluene sulphonic acid as dopant and vulcastab VL as stabilizer. PPy/F-PPy/NR composites were prepared as above in presence of short nylon fiber. The products were coagulated out, dried, compounded on a two roll mill and moulded. The cure pattern, DC electrical conductivity, morphology, mechanical properties, thermal degradation parameters and microwave characteristics of the resulting composites were studied. Incorporation of PPy to elastomer retards the cure reaction whereas addition of fiber accelerates the cure reaction. DC conductivity up to 6.25 × 10⁻² was attained for NR/PPy/F-PPy system. The composites containing F-PPy exhibited better mechanical properties compared to NR/PPy systems. The absolute value of the dielectric permittivity, AC conductivity and absorption coefficient of the conducting composites prepared were found to be much greater than the gum vulcanizate. PPy and F-PPy were found to decrease the dielectric heating coefficient and skin depth significantly.     

MWCNTs改善WS2/三元乙丙橡胶复合材料的非线性电导特性与热导性能

通过在一定量的纳米WS₂中添加极少量的多壁碳纳米管（MWCNTs）,形成MWCNTs-WS₂复配填料,采用双辊开炼机将三元乙丙橡胶（EPDM）与不同配比的复配填料混合制备了不同MWCNTs含量的MWCNTs-WS₂/EPDM复合材料。并研究了极少量的MWCNTs添加对MWCNTs-WS₂/EPDM复合材料非线性电导性能、直流击穿性能和导热性能的影响。结果表明,极少量的MWCNTs对MWCNTs-WS₂/EPDM复合材料在25℃时的非线性电导特性起到明显的增强作用,且随着MWCNTs含量的增加,复合材料非线性电导特征有明显的规律性变化;由于MWCNTs自身的高电导率和电导正温度系数效应,MWCNTs-WS₂/EPDM复合材料电导率随电场强度的变化趋势在80℃时不再表现非线性特征。另外,极少量的MWCNTs对MWCNTs-WS₂/EPDM复合材料的热导率有明显地改善。      

Preparation and properties of polyamide 6 thermal conductive composites reinforced with fibers

The majority of inorganic particles-filled thermal conductive composites highlight thermal conductivity in detriment of mechanical properties. In this work, magnesium hydroxide (Mg(OH)₂), alumina (Al₂O₃) and flake graphite-filed polyamide 6 (PA6) composites prepared by twin-screw extruder, were reinforced with carbon and glass fibers separately. Effects of fiber type and content on thermal conductivity, mechanical properties and heat deflection temperature (HDT) of the PA6-based composites were investigated. The results showed that the thermal conductivity of the composites improved with increasing carbon fiber content, while decreased slightly with glass fiber loading. Furthermore, strength, modulus and HDT of the PA6-based composites increased with the increase of fiber content. The reinforcing effects of the two fibers on the thermal and mechanical properties of the composites were compared and interpreted in this paper. By incorporating simultaneously high thermal conductive fillers and high-strength fibers, the combined composites hold a good potential in heat dissipation applications.     

Fabrication, thermal, and dielectric properties of self-passivated Al/epoxy nanocomposites

The current paper reports the effects of an epoxide-functionalized, silane surface-treated, self-passivated aluminum (Al) nanoparticles on the glass transition, morphology, thermal conductivity, dielectric properties of an epoxy composite. The surface modification of the Al nanoparticles improved the dispersion of the filler, as well as the glass transition temperature, thermal conductivity, and dielectric properties of the epoxy composites. The epoxy/Al nanocomposites showed a dielectric constant transition concentration. The dielectric constant and dissipation factor increased when the Al particle loading exceeded the critical content but gradually decreased with the frequency. The epoxy nanocomposites containing 15 % by weight Al nanoparticles have a high thermal conductivity and a high dielectric constant but a low dissipation factor. The enhancements in the thermal and dielectric properties of the epoxy nanocomposites show potential for future engineering applications.     

Dielectric properties of epoxy/short carbon fiber composites

The dielectric properties of epoxy/short carbon fiber composites at different concentrations 0, 5, 10 and 15% by weight, different thicknesses 2 and 4 mm, and frequency in the range from 20 Hz to 1 MHz were characterized. Scanning electron microscopy and differential scanning calorimetry were utilized. The alternating current (ac) electrical properties (complex impedance, dielectric constant, dielectric loss, real part of electric modulus, imaginary part of electric modulus, electrical conductivity, and relaxation time) were determined. It was found that the applied frequency, filler concentrations, and composite thickness affected the ac electrical properties of the epoxy/carbon fiber composites. The dielectric behaviors of the interfacial polarization between epoxy matrix and carbon fibers could be described by the Maxwell–Wagner–Sillars relaxation. The analysis of the complex electric modulus in the frequency range from 20 Hz to 1 MHz revealed that the interfacial relaxation followed the Cole–Davidson distribution of relaxation times. The universal power-law of ac conductivity was observed in the epoxy/carbon fiber composites. The calculated power exponent (near unity) is physically acceptable within this applied model.     

Thermal diffusivity, thermal conductivity, and specific heat of flax fiber–HDPE biocomposites at processing temperatures

There is increasing work on the use of flax fibers as reinforcement for manufacturing composites because of their lower cost and environmental benefit. During manufacturing of such natural fiber–plastic composites, heat transfer is involved, but information about the thermal conductivity and thermal diffusivity at the processing temperatures is not available. In this study, the thermal conductivity, thermal diffusivity, and specific heat of flax fiber–high density polyethylene (HDPE) biocomposites were determined in the temperature range of 170–200 °C. The fiber contents in biocomposites were 10%, 20%, and 30% by mass. Using the line-source technique, the instrumental setup was developed to measure the thermal conductivity of biocomposites. It was found that the thermal conductivity, thermal diffusivity, and specific heat decreased with increasing fiber content, but thermal conductivity and thermal diffusivity did not change significantly with temperature in the range studied. The specific heat of the biocomposites increased gradually with temperature.     

Effect of particles size on the AC electrical behavior of iron/polystyrene composites

The AC electrical characteristics of polystyrene/Iron composites filled with iron particles of average sizes: 5, 40, 110 and 250 μm, have been investigated. The AC electrical properties were studied in frequency range (50 kHz–1 MHz), and temperature range (30–110 °C) using the impedance method. The AC-conductivity and dielectric constants were determined from the measured impedance data. It was found that the applied frequency, temperature, and iron particles size affect the electrical and dielectric properties of the composites. The AC-electrical conductivity is increasing with temperature. The dielectric constant and the dielectric loss of the composites increase with decreasing the iron particles size. The universal power-law of the electrical conductivity gives exponent with 0 < m < 1 characterizing hopping conduction. The small values of the activation energy indicate that the composite of smallest iron particle size, electrons can tunnel or hop more easily from the valence band to conduction energy band due to the reduction of interparticles separation.