Morphological characterization and modelling of electrical conductivity of multi-walled carbon nanotube/poly(p-phenylene sulfide) nanocomposites obtained by twin screw extrusion

In this study, poly(p-phenylene sulfide) based nanocomposites containing multi-walled carbon nanotubes (MWNTs) were produced by dilution of a 15 wt.% MWNT/PPS masterbatch via twin screw extrusion process. The electrical conductivities of the nanocomposites were measured and percolation threshold was observed below 0.77 vol.% MWNTs. The state of dispersion and distribution quality of MWNTs was analyzed on macro- and nanoscale through transmission light and scanning electron microscopy (SEM). A good deagglomeration of primary macroagglomerates and a homogenous MWNT distribution on nanoscale was found. The dependence of conductivity on MWNT concentration was estimated using statistical percolation theory which matches the experimental data quite well. A new empirical equation was set up to fit the electrical conductivity using quantitative values of visible percolating MWNTs which were detected by charge contrast imaging in SEM.     

Synthesis and characterization of well-dispersed multi-walled carbon nanotube/low-bandgap poly(3,4-alkoxythiophene) nanocomposites

In this study, we prepared nanocomposites of multi-walled carbon nanotubes (MWCNTs) and low-energy-bandgap conjugated polymers incorporating 3,4-alkoxythiophene monomers. Poly(3,4-dihexyloxythiophene) (PDHOT) and poly(3,4-dimethoxythiophene-co-3,4-dihexyloxythiophene) [P(DMOT-co-DHOT)] have relatively low-energy-bandgaps (ca. 1.38 and 1.34 eV, respectively), determined from the onsets of absorbances in their UV–Vis spectra, because of the electron-donating effects of their alkoxy groups. MWCNTs have poor solubility in common organic solvents; after surface modification with alkyl side chains using the Tour reaction, however, the p-hexylaniline modified MWCNT derivative (MWCNT-HA) was readily dispersed in CHCl₃ and could be mixed with the low bandgap polymers. Scanning electron microscopy images revealed that MWCNT-HA was dispersed well in each polythiophene derivative; only a few MWCNT-HA bundles could be observed at a high MWCNT-HA content (≧20 wt.%). The electrical conductivities of the MWCNTs/PDHOT composites were dependent on their MWCNT content, reaching 16 S/cm at 30 wt.% MWCNT-HA. We suspect that the two hexyloxy chains of PDHOT enhanced its solubility and allowed it to wrap around the surfaces of the MWCNTs more readily.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

A review and analysis of electrical percolation in carbon nanotube polymer composites

We review experimental and theoretical work on electrical percolation of carbon nanotubes (CNT) in polymer composites. We give a comprehensive survey of published data together with an attempt of systematization. Parameters like CNT type, synthesis method, treatment and dimensionality as well as polymer type and dispersion method are evaluated with respect to their impact on percolation threshold, scaling law exponent and maximum conductivity of the composite. Validity as well as limitations of commonly used statistical percolation theories are discussed, in particular with respect to the recently reported existence of a lower kinetic (allowing for re-aggregation) and a higher statistical percolation threshold.     

Hydrothermally synthesized RuO2/Carbon nanofibers composites for use in high-rate supercapacitor electrodes

A conventional hydrothermal deposition process is used to graft ruthenium oxide (RuO₂) nanoparticles onto carbon nanofibers (CNFs). The obtained RuO₂ nanoparticles have an average diameter of 2 nm and are homogenously distributed on the CNF surfaces. Supercapacitors are fabricated using the resulting RuO₂ grafted CNFs nanocomposite as the electrodes. The existence of CNFs leads to reduced contact resistance among the RuO₂ nanoparticles and provides a network for fast electron transport, which then contributes to enhanced electrochemical performance. The enhancement is proportional to the RuO₂ content and can be as high as 638% at a high sweep rate of 200 mV s⁻¹, at which a capacitance is 155 F g⁻¹. Stability of the RuO₂-grafted CNF capacitor is also demonstrated by subjecting the capacitor to a potential sweep at 500 mV s⁻¹ for 1000 cycles. Furthermore, the RuO₂ grafted CNF capacitor exhibits a very short relaxation time of 0.17 s, which is desirable for high rate charge and discharge.     

Modeling of DC conductivity for ethylene vinyl acetate (EVA)/polyaniline conductive composites prepared through insitu polymerization of aniline in EVA matrix

Ethylene vinyl acetate (EVA)/polyaniline (Pani) composites were prepared by insitu polymerization technique. DC and AC conductivity of the composites have been investigated. Different theoretical models like Voet, Scarisbrick, Bueche, and McCullough have been applied to predict DC conductivity of the composite systems. Scarisbrick model exhibits somewhat similarity between experimentally observed and theoretically predicted conductivity. The limitations of the models as well as the deviations between the theoretically predicted and experimentally observed results have been discussed. A new model for conductivity has been proposed which fit well with the experimentally observed results.     

Tuning the localization of functionalized MWCNTs in SAN/PC blends by a reactive component

The influence of the reactive component (N-phenylmaleimide styrene maleic anhydride) on the blend morphology, the localization of functionalized multiwalled carbon nanotubes (MWCNTs), and the electrical resistivity of MWCNT filled blend systems of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) was investigated. SAN, PC, amino-functionalized MWCNTs (Nanocyl™ NC3152) and the reactive component (RC) were melt mixed in a DSM Xplore microcompounder using different mixing sequences. The RC containing maleic anhydride (MA) groups is miscible with SAN and is assumed to act as linking agent to the functionalized MWCNTs.The morphology of the SAN/PC blends was studied depending on the concentration of the RC. Thereby co-continuous morphologies were found for all blends with 40 wt.% SAN and 60 wt.% PC. In all nonmodified blends the MWCNTs were localized within the PC phase. After the addition of RC the MWCNTs migrated completely into the miscible SAN–RC phase. Consequently, the electrical resistivities of the blends changed in dependence on the localization. Whereas the SAN/PC/MWCNT blends showed low electrical resistivity values, much higher values were found for SAN–RC/PC/MWCNT blends. This was assigned to a coupling or strong interaction of MA groups to the nanotubes disturbing electrical contacts and percolation between them. The occurrence of the MWCNT migration from PC towards SAN was found to be dependent on the concentrations of RC and MWCNTs. By adapting that ratio and the mixing strategy, the localization of the carbon nanotubes in the blend phases can be tuned. The investigations indicated that MWCNTs once coupled with the RC remain in the SAN–RC phase. Thus, a chemical reaction or strong interactions seem to be the driving forces for localization of the MWCNTs in the SAN–RC blend phase.     

Effects of carboxyl radical on electrical resistance of multi-walled carbon nanotube filled silicone rubber composite under pressure

The piezoresistance of a multi-walled carbon nanotube filled silicone rubber composite under uniaxial pressure was studied. The experimental results show that the active carboxyl radical on multi-walled carbon nanotubes can effectively improve the homogeneous distribution and alignment of conductive paths in the composite. As a result, the composite presented positive piezoresistance with improved sensitivity and sensing linearity for pressure, both of which are key parameters for sensor applications.     

Synthesis and electrochemical performance of LiV3O8/carbon nanosheet composite as cathode material for lithium-ion batteries

To improve the rate capability and cyclability of LiV₃O₈ cathode for Li-ion batteries, LiV₃O₈ was modified by forming LiV₃O₈/carbon nanosheet composite. The LiV₃O₈/carbon nanosheet composite was successfully achieved via a hydrothermal route followed by a carbon coating process. The morphology and structural properties of the samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM observations demonstrated that LiV₃O₈/carbon composite has a very flat sheet-like morphology, with each nanosheet having a smooth surface and a typical length of 400-700 nm, width of 200-350 nm, and thickness of 10-50 nm. Each sheet was surrounded by a thick layer of amorphous carbon. Electrochemical tests showed that the LiV₃O₈/carbon composite cathode features long-term cycling stability (194 mAh g⁻¹ at 0.2 C after 100 cycles) and excellent rate capability (110 mAh g⁻¹ at 5 C, 104 mAh g⁻¹ at 10 C, and 82 mAh g⁻¹ at 20 C after 250 cycles). Electrochemical impedance spectra (EIS) indicated that the LiV₃O₈/carbon composite electrode has very low charge-transfer resistance compared with pristine LiV₃O₈, indicating the enhanced ionic conductivity of the LiV₃O₈/carbon composite. The enhanced cycling stability is attributed to the fact that the LiV₃O₈/carbon composite can prevent the aggregation of active materials, accommodate the large volume variation, and maintain good electronic contact.     

Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes and their dispersion and percolation behaviour in melt mixed polycarbonate composites

Ball milling of carbon nanotubes (CNTs) in the dry state is a common way to produce tailored CNT materials for composite applications, especially to adjust nanotube lengths. For Nanocyl^TM NC7000 nanotube material before and after milling for 5 and 10 h the length distributions were quantified using TEM analysis, showing decreases of the mean length to 54% and 35%, respectively. With increasing ball milling time in addition a decrease of agglomerate size and an increase of packing density took place resulting in a worse dispersability in aqueous surfactant solutions. In melt mixed CNT/polycarbonate composites produced using masterbatch dilution step, the electrical properties, the nanotube length distribution after processing, and the nano- and macrodispersion of the nanotubes were studied. The slight increase in the electrical percolation threshold in the melt mixed composites with ball milling time of CNTs can be assigned to lower nanotube lengths as well as the worse dispersability of the ball milled nanotubes. After melt compounding, the mean CNT lengths were shortened to 31%, 50%, and 66% of the initial lengths of NC7000, NC7000-5 h, and NC7000-10 h, respectively.     

Enhanced field emission characteristics of nitrogen-doped carbon nanotube films grown by microwave plasma enhanced chemical vapor deposition process

Nitrogen-doped carbon nanotube (CNT) films have been synthesized by simple microwave plasma enhanced chemical vapor deposition technique. The morphology and structures were investigated by scanning electron microscopy and high resolution transmission electron microscopy. Morphology of the films was found to be greatly affected by the nature of the substrates. Vertically aligned CNTs were observed on mirror polished Si substrates. On the other hand, randomly oriented flower like morphology of CNTs was found on mechanically polished ones. All the CNTs were found to have bamboo structure with very sharp tips. These films showed very good field emission characteristics with threshold field in the range of 2.65-3.55 V/μm. CNT film with flower like morphology showed lower threshold field as compared to vertically aligned structures. Open graphite edges on the side surface of the bamboo-shaped CNT are suggested to enhance the field emission characteristics which may act as additional emission sites.     

A novel carbon-based nanocomposite plate as a counter electrode for dye-sensitized solar cells

This study develops cost effective and high performance composite conductive plates for use in dye-sensitized solar cells (DSSCs). Composite plates with various graphite contents at a constant carbon nanotube (CNT) loading were prepared by bulk molding compound (BMC) process. Results show that the bulk electrical resistance of the composite plate gradually decreases from 6.7 mΩ cm to 1.7 mΩ cm as the graphite content increases, which is due to the formation of efficient electronic conducting networks. For DSSCs, the composite plates may be suitable substitutes for the conductive glass plates in the counter electrodes substrates of DSSCs. Results reveal that composite plates at the optimum level (80 wt.% graphite loading) provide lower cell resistance, lower preparation cost and higher cell performance than common conductive glass plates. Therefore, in order to decrease the cost of the cells and to maintain good cell performance, this graphite-like composite plate prepared by the BMC process is a promising substitute component for DSSCs.     

Photoelectrocatalytic properties and bactericidal activities of silver-treated carbon nanotube/titania composites

Two types of carbon nanotube/TiO₂ and silver-treated carbon nanotube/TiO₂ electrodes were prepared and characterized by X-ray diffraction, transmission and scanning electron microscopy, energy dispersive X-ray analysis, photoelectrocatalytic activity and antibacterial activity. The photoelectrocatalytic degradation of a methylene blue solution was attributed to the combined effects of the photo-degradation of TiO₂, the electron assistance of carbon nanotube network and the enhancement of silver and was a function of the applied potential. The composites treated with silver showed enhanced photo-degradation of methylene blue, and the photoelectrocatalytic activity increased with increasing amount of silver. The silver-treated carbon nanotube/TiO₂ prepared as bactericides have stronger antibacterial activity against Escherichia coli K-12 than standard ampicillin, tetracycline and carbon nanotube/TiO₂ under sunlight or dark conditions. The presence of silver in the silver-treated carbon nanotube/TiO₂ composites enhanced the inactivation of the E. coli K-12.     

Preparation and properties of poly (p-phenylene sulfide)/multiwall carbon nanotube composites obtained by melt compounding

In this paper, electrical and mechanical properties of Poly (p-phenylene sulfide) (PPS)/multi-wall carbon nanotubes (MWNTs) nanocomposites were reported. The composites were obtained just by simply melt mixing PPS with raw MWNTs without any pre-treatment. The dispersion of MWNTs and interfacial interaction were investigated through SEM &TEM and Raman spectra. The rheological test and crystallization behavior were also investigated to study the effects of MWNTs concentration on the structure and chain mobility of the prepared composites. Though raw MWNTs without any pre-treatment were used, a good dispersion and interaction between PPS and MWNTs have been evidenced, resulting in a great improvement of electrical properties and mechanical properties of the composites. Raman spectra showed a remarkable decrease of G band intensity and a shift of D bond, demonstrating a strong filler–matrix interaction, which was considered as due to π–π stacking between PPS and MWNTs. The storage modulus (G′) versus frequency curve presented a plateau above the percolation threshold of about 2–3 wt% with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behavior. Meanwhile, a conductive percolation threshold of 5 wt% was achieved and the conductivity of nanocomposites increased sharply by several orders of magnitude. The difference between electrical and rheological percolation threshold, and the effect of critical percolation on the chain mobility, especially on crystallization behavior of PPS, were discussed. In summary, our work provides a simple and fast way to prepare PPS/MWNTs nanocomposites with good dispersion and improved properties.     

TiO2-nanorod decorated carbon nanotubes for high-permittivity and low-dielectric-loss polystyrene composites

Polymer composites with high permittivity and low dielectric loss are highly desirable in electronic and electrical industry. Adding conductive fillers could significantly increase the permittivity of a polymer. However, polymer composites containing conductive fillers often exhibit very high dielectric loss due to their large electrical conduction or leakage currents. In this work, by engineering TiO₂-nanorod-decorated multi-walled carbon nanotubes (TD-CNTs), polystyrene (PS) composite with high permittivity and low dielectric loss have been successfully prepared. The composite containing of 17.2 vol.% TD-CNTs has a permittivity of 37 at 1 kHz, which is 13.7 times higher than that of the pure PS (2.7), while the dielectric loss still remains at a low value below 0.11. The dielectric properties of the composites are closely related to the length of CNTs and the loading level of TiO₂-nanorods on the CNT surfaces.     

Preparation and properties of hollow glass microsphere-filled epoxy-matrix composites

Hollow glass microsphere (HGM)–filled epoxy composites, with filler content ranging from 0 to 51.3 vol.%, were prepared in order to modify the dielectric properties of the epoxy. The results showed that the dielectric constant (D_k) and dielectric loss (D_f) of the composites decreased simultaneously with increasing HGM content, which was critical for the provision of superior high-frequency device performance. Other properties of the composite, such as the coefficient of thermal expansion (CTE) and the glass transition temperature (T_g), were also improved. The improvement in these properties was related to strong interaction between the HGM and epoxy, which was indicated by the formation of an interphase between the HGM and epoxy-matrix. It was unsatisfactory in this study that the thermal conductivity of the composites also decreased with HGM content. In order to obtain relatively high thermal conductivity and a low dielectric constant simultaneously, this paper suggests further adding other filler.     

Dielectric properties of Poly(vinylidene fluoride)/CaCu3Ti4O12 composites

The possibility of obtaining relatively high dielectric constant polymer–ceramic composite by incorporating the giant dielectric constant material, CaCu₃Ti₄O₁₂ (CCTO) in a Poly(vinylidene fluoride) (PVDF) polymer matrix by melt mixing and hot pressing process was demonstrated. The structure, morphology and dielectric properties of the composites were characterized using X-ray diffraction, Thermal analysis, scanning electron microscope, and impedance analyzer. The effective dielectric constant (ε_eff) of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz–1 MHz) under study. The dielectric loss did not show any variation up to 40% loading of CCTO, but showed an increasing trend beyond 40%. The room temperature dielectric constant as high as 95 at 100 Hz has been realized for the composite with 55 vol.% of CCTO, which has increased to about 190 at 150 °C. Theoretical models like Maxwell’s, Clausius–Mossotti, Effective medium theory, logarithmic law and Yamada were employed to rationalize the dielectric behaviour of the composite and discussed.     

Selective location and conductive network formation of multiwalled carbon nanotubes in polycarbonate/poly(vinylidene fluoride) blends

Multiwalled carbon nanotubes (MWCNTs)-filled polycarbonate (PC), poly(vinylidene fluoride) (PVDF) and PC/PVDF conductive composites were fabricated using melt mixing, respectively. The dynamic process of MWCNTs conductive network formation in the composites was in situ traced by recording the variation of electrical resistivity with time during annealing treatments. As a result, the percolation threshold for the MWCNTs-filled PC/PVDF system was much lower than those of MWCNTs-filled individual polymers and the MWCNTs were selectively located in the PC phase of PC/PVDF composite, which had been verified by scanning electron microscopy measurements. The activation energy of conductive network formation for PC/PVDF/MWCNTs composite was close to that of the PC/MWCNTs system, which further confirmed that MWCNTs were dispersed mainly in the PC phase. Furthermore, the assembly velocity of MWCNTs in the polymer melt increased with annealing temperature.     

Comparison of nanotubes produced by fixed bed and aerosol-CVD methods and their electrical percolation behaviour in melt mixed polyamide 6.6 composites

The electrical percolation behaviour of five different kinds of carbon nanotubes (CNTs) synthesised by two CVD techniques was investigated on melt mixed composites based on an insulating polyamide 6.6 matrix. The electrical percolation behaviour was found to be strongly dependent on the properties of CNTs which varied with the synthesis conditions. The lowest electrical percolation threshold (0.04 wt.%) was determined for as grown multi-walled carbon nanotubes without any purification or chemical treatment. Such carbon nanotubes were synthesised by the aerosol method using acetonitrile as ferrocene containing solvent and show relatively low oxygen content near the surface, high aspect ratio, and good dispersability. Similar properties could be found for nanotubes produced by the aerosol method using cyclohexane, whereas CNTs produced by the fixed bed method using different iron contents in the catalyst material showed much higher electrical percolation thresholds between 0.35 and 1.02 wt.%.     

Preparation and characterization of permanently anti-static packaging composites composed of high impact polystyrene and ion-conductive polyamide elastomer

Antistatic composites composed of high impact polystyrene (HIPS) and NaSCN doped poly(ether-block-amide) (PEBA) solid-polymer-electrolyte (SPE) were successfully prepared in a Haake torque rheometer. The influence of PEBA component on the surface resistivity of PEBA-based SPE was extensively studied by surface resistivity test. The dependence of the surface resistivity of HIPS/SPE composites on both temperature and relative humidity (RH) was also investigated. Thermal decomposition behaviors, mechanical properties, and surface morphology of the HIPS/SPE composites were evaluated by thermal gravimetric analysis (TGA), tensile test, and scanning electron microscopy (SEM), respectively. The results show that the surface resistivity of the HIPS/SPE composites can be effectively reduced to 10⁹ Ω cm⁻² orders of magnitude when the SPE content reaches 20 phr. TGA result reveals that the antistatic HIPS/SPE (70/30 by wt.) composite can be fabricated by traditional thermoplastic processing method without any decomposition once the processing temperature is lower than 320 °C. SEM results show that the antistatic ability of HIPS/SPE composites is originated from the formation of continuous PEBA-based ion-conductive channels or networks.