Silane-modified MWCNT/PMMA composites – Preparation, electrical resistivity, thermal conductivity and thermal stability

Multiwalled carbon nanotubes (MWCNT) were modified using 3-isocyanato- propyltriethoxysilane (IPTES). Crosslinkable PMMA was prepared from MMA monomer and Vinyltriethoxysilane (VTES) (PMMA–VTES). The IPTES-modified MWCNT (Si-MWCNT) was mixed with the PMMA–VTES copolymer and crosslinked with catalyst to form Si-MWCNT/PMMA–VTES composites. The degree of condensation of tri-distribution structure of the Si-MWCNT/PMMA–VTES composites decreases as the Si-MWCNT content increases. The morphology of the Si-MWCNT/PMMA–VTES composites was analyzed by SEM and TEM. The MWCNTs were well dispersed in the PMMA–VTES matrix. Surface and volume electrical resistivity decreased as the MWCNT content increased. The thermal conductivity of the PMMA–VTES composites increased by 87.5% when 0.99 wt% Si-MWCNT content was added to neat PMMA–VTES. The thermal stability of the PMMA–VTES in nitrogen and air increased significantly even when a small quantity (0.5 wt%) of Si-MWCNT was added.     

Enhanced electrical conductivity and mechanical properties of ABS/EPDM composites filled with graphene

Acrylonitrile–butadiene–styrene (ABS)/ethylene–propylene–diene monomer (EPDM) composites reinforced with graphene nanoplatelets (GN) were fabricated by the direct melt blending, dried premixing and wet premixing process, respectively. The electrical resistivity, tensile strength, impact strength, microstructure, thermal stability, glass transition temperature and morphology of fracture surface of composites were investigated. In case of direct melt blending process, the maximum tensile strength with minimum impact strength is obtained. But this result is reversed while the fabrication of composites by wet premixing process. SEM results show that GN is prior to distributing in the continuous ABS phase. The percolation threshold could be significantly decreased from 11.8 wt% to 6.6 wt% when prepare composites by wet/dried premixing process instead of melt blending.     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

Improving the through-thickness thermal and electrical conductivity of carbon fibre/epoxy laminates by exploiting synergy between graphene and silver nano-inclusions

Fibre-reinforced polymer composites typically feature low functional (e.g., electric and thermal conductivity) and structural (e.g. mechanical strength and fracture toughness) properties in the laminate’s thickness direction. In the event of lightning strikes, overheating, and impact by foreign objects, composite laminates may suffer wide spread structural damage. This research explores the synergistic physical interaction between two-dimensional nanostructured (graphene nano-platelets) and, zero- or one-dimensional conductive fillers (silver nanoparticles or silver nanowires, respectively) when both are dispersed in fibre–polymer laminates. The results reveal a synergistic improvement in the through-thickness thermal conductivity that is more than the additive improvements by each constituent. Specifically, the simultaneous inclusion of graphene nano-platelets and silver nanoparticles/nanowires at a combined loading of 1 vol% resulted in approximately 40% enhancement in the through-thickness thermal conductivity while the inclusion of graphene nano-platelets alone at the same loading resulted only in 9% improvement. Similarly, the through-thickness electrical conductivity of carbon fibre/epoxy laminates incorporating graphene nano-platelets together with silver nanoparticles/nanowires was notably higher (⩾70%) than can be achieved by graphene nano-platelets alone (∼55%). These results demonstrate that the presence of nano-reinforcements exhibiting varied phonon transport and electron transfer pathways, and geometric aspect ratios promote synergistic physical interactions. Small improvements were found in the mechanical properties, including tensile, flexural or compressive properties of the carbon fibre-reinforced laminates, due to the relatively low concentrations of the nano-fillers.     

Effects of aluminium nitride inclusions on thermal and electrical properties of epoxy and polypropylene: An experimental investigation

Thermal and dielectric properties of polymers reinforced with micro-sized aluminium nitride (AlN) particles have been studied. A set of epoxy–AlN composites, with filler content ranging from 0 to 25 vol% is prepared by hand lay-up technique. With similar filler loading, polypropylene -AlN composites are fabricated by compression molding technique. Density (ρ_c), effective thermal conductivity (k_eff), glass transition temperature (T_g), coefficient of thermal expansion (CTE) and dielectric constant (ε_c) of these composites are measured experimentally. The various experimental data were interpreted using appropriate theoretical models. Incorporation of AlN in both the resin increases the k_eff and T_g whereas CTE of composite decreases favourably. The dielectric constant of the composite also found to get modified with filler content. With improved thermal and modified dielectric characteristics, these AlN filled polymer composites can possibly be used for microelectronics applications.     

Microstructural image analysis and structure–electrical conductivity relationship of single- and multiple-filler conductive composites

The electrical conductivity of polypropylene/graphite (PP/G) composites and polypropylene/graphite/carbon black (PP/G/CB) was investigated in this paper. The conductivity experimental data of PP/G composites was correlated to theoretical models, which exist in the literature, and the results showed higher values of the exponent t compared to the expected typical values. Moreover, these analytical models were unable to describe the electrical behaviour for multiple-filler conductive composites such as PP/G/CB composites. A 2D computer simulation to numerically compute the electrical conductivity based on digital image analysis was found to be somewhat useful to describe the mechanism of conduction in PP/G/CB composites and to determine the critical factors in developing high electrically conductive composites.     

Thermal conductivity and dielectric properties of Al/PVDF composites

Polymeric composites with relatively high thermal conductivity, high dielectric permittivity, and a low dissipation factor are obtained in the present study. Three types of core-shell-structured aluminum (Al) particles are incorporated in poly(vinylidene fluoride) (PVDF) by melt-mixing and hot-pressing processes. The morphological, thermal, and dielectric properties of the composites are characterized using thermal analysis, a scanning electron microscope, and a dielectric analyzer. The results indicate that the Al particles decrease the degree of crystallinity of PVDF, and that the particle size and shape of the filler affect the thermal conductivity and dielectric properties of Al/PVDF. No variation in the dissipation factor is observed up to 60 wt.% Al. Thermal conductivity and dielectric permittivity values as high as 1.65 W/m K and 230, respectively, as well as a low dissipation factor of 0.25 at 0.1 Hz, are realized for the composites with 80 wt.% spherical Al.     

Mechanical, thermal and electrical properties of aluminum nitride/polyetherimide composites

The mechanical, thermal and electrical properties of modified AlN/polyetherimide (PEI) composites were investigated. It revealed that the surface of AlN modified by silane could effectively increase the adhesion with matrix, which was beneficial for AlN to reinforce the polyetherimide matrix. After silane modification, the AlN showed good dispersion and wetibility in the polyetherimide matrix and imparted excellent mechanical, electrical and thermal properties. The tensile strength, modulus, electrical and thermal stability were improved with the increasing of AlN content. The tensile strength of AlN/PEI composites increased by 27% when 12.6 vol.% AlN was added to neat polyetherimide. The thermal conductivity of the 57.4 vol.% AlN/PEI composites increased three times compared with neat polyetherimide. Test results indicate that the silane grafted AlN incorporated into the polyetehetimide matrix effectively enhance the thermal stability, thermal conductivity and mechanical properties of the polyetherimide composites.     

Effect of silica coating thickness on the thermal conductivity of polyurethane/SiO2 coated multiwalled carbon nanotube composites

Silica coated multiwalled carbon nanotubes (SiO₂@MWCNTs) with different coating thicknesses of ∼4 nm, 30–50 nm, and 70–90 nm were synthesized by a sol–gel method and compounded with polyurethane (PU). The effects of SiO₂@MWCNTs on the electrical properties and thermal conductivity of the resulting PU/SiO₂@MWCNT composites were investigated. The SiO₂ coating maintained the high electrical resistivity of pure PU. Meanwhile, incorporating 0.5, 0.75 and 1.0 wt% SiO₂@MWCNT (70–90 nm) into PU, produced thermal conductivity values of 0.287, 0.289 and 0.310 W/mK, respectively, representing increases of 62.1%, 63.3% and 75.1%. The thermal conductivity of PU/SiO₂@MWCNT composites was also increased by increasing the thickness of the SiO₂ coating.     

Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity (k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m⁻¹ K⁻¹. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m⁻¹ K⁻¹, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m⁻¹ K⁻¹). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.     

Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.     

Dielectric dispersion and ac conductivity in—Iron particles loaded—polymer composites

Polymer composites of an epoxy resin matrix with randomly dispersed Iron micro-particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140 °C. Obtained data were analysed by means of electric modulus formalism.

Interfacial or Maxwell-Wagner-Sillars relaxation process was revealed in the frequency range and temperature interval of the measurements, which was found to follow the Cole-Davidson approach for the distribution of relaxation times.

The examined systems exhibit strong dispersion with frequency. At low frequencies ac conductivity tends to be constant, while at higher becomes frequency dependent varying as a power of frequency. Conductivity increases with temperature in the low frequency regime, remaining almost unaffected at higher frequencies.     

Rheology,electrical conductivity and crystallinity of a polyurethane/graphene composite: Implications for its use as a hot-melt adhesive

Homemade graphene based polyurethane composites elaborated using a facile melt mixing method were studied. The polyurethane (PUR) was a hot-melt adhesive and the research was carried out considering the perspective of electrically conductive adhesive composites. The crystallization of PUR was significantly modified by graphene; for instance the crystallization temperature was shifted to higher temperatures. Avrami equation results indicated that the composites constituted nucleated axialites, instead of espherulites. These results were used to explain the initial decrease of conductivity, followed by a recovery, observed during the crystallization process.The observed huge conductivity enhancement, together with the increase of the crystallization temperature, opens interesting perspectives in the development of electrically conductivity adhesives which offer a rapid welding.     

In situ thermal reduction of graphene oxide for high electrical conductivity and low percolation threshold in polyamide 6 nanocomposites

Electrically conductive and thermally stable polyamide 6 (PA 6) nanocomposites were prepared through one-step in situ polymerization of ε-caprolactam monomer in the presence of electrically insulating and thermally unstable graphene oxide (GO) nanosheets. These nanocomposites show a low percolation threshold of ∼0.41 vol.% and high electrical conductivity of ∼0.028 S/m with only ∼1.64 vol.% of GO. Thermogravimetric analysis and X-ray photoelectron spectroscopy results of GO before and after thermal treatment at the polymerization temperature indicate that GO was reduced in situ during the polymerization process. X-ray diffraction patterns and scanning electron microscopy observation confirm the exfoliation of the reduced graphene oxide (RGO) in the PA 6 matrix. The low percolation threshold and high electrical conductivity are attributed to the large aspect ratio, high specific surface area and uniform dispersion of the RGO nanosheets in the matrix. In addition, although GO has a poor thermal stability, its PA 6 nanocomposite is thermally stable with a satisfactory thermal stability similar to those of neat PA 6 and PA 6/graphene nanocomposite. Such a one-step in situ polymerization and thermal reduction method shows significant potential for the mass production of electrically conductive polymer/RGO nanocomposites.     

Micromechanically-based effective thermal conductivity estimates for polymer nanocomposites

The Effective Continuum Micromechanics Analysis Code was modified to predict the effective thermal conductivities of composites containing multiple distinct nanoheterogeneities (fibers, spheres, platelets, voids, etc.) each with an arbitrary number of coating layers based upon either the modified Mori–Tanaka or modified self-consistent methods for steady state heat conduction. A parametric study was performed to investigate the effect of nanoreinforcement morphology, volume fraction, orientation, and nanoreinforcement–resin interphase properties on calculated effective thermal conductivities. Predicted thermal conductivities matched experimentally measured values for vapor-grown carbon nanofiber/polypropylene, exfoliated graphite flake/epoxy, glass microsphere/polystyrene, cupric oxide sphere/epoxy, and aluminum sphere/epoxy composites.     

Modeling of the thermal conductivity and thermomechanical behavior of diamond reinforced composites

Semi-analytical Mori-Tanaka methods and numerical models for studying the overall thermal conduction behavior of metal matrix composites reinforced by diamond particles are presented, special emphasis being put on the effects of finite interfacial conductances. Good agreement between the simulation approaches is obtained and the influence of particle shapes and homogeneous vs. inhomogeneous interfacial conductances on local and global responses is studied. Analogous methods are applied to modeling the elastic and thermoelastoplastic behavior of diamond reinforced metals.     

Comparison of the thermal,dynamic mechanical and morphological properties of PLA-Lignin & PLA-Tannin particulate green composites

The study of Lignin and Tannin as filler materials in PLA-based polymeric systems has been uncommon in literature. Composites of PLA-Lignin with 5, 10, 15 wt% Lignin and PLA-Tannin with 5, 10, 15 wt% Tannin were fabricated using injection moulding. SEM morphology reveals Lignin forms droplet like dispersions within the PLA matrix in contrast to Tannin. The particle size of Lignin within the matrix is also 10–150 times smaller than Tannin. Isothermal frequency sweeps on the composites show that storage modulus of PLA-Tannin composites starts to degrade at 15 wt% filler concentration and damping rises. PLA-Lignin composites do not show such degradation in storage modulus. The tensile strength of both PLA-Lignin and PLA-Tannin composites falls with increase in filler content. Lignin has a more inhibitory effect on PLA crystallization than Tannin. The onset of thermal degradation of PLA-Lignin and PLA-Tannin composites occurs at slightly lower temperatures than pure PLA.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

Investigation of the electrical conductivity of HDPE composites filled with bundle-like MWNTs

Bundle-like multi-walled carbon nanotubes (MWNTs) were melting-mixed with high-density polyethylene (HDPE). MWNTs are distributed in the matrix mostly isolated. Agglomerates are observed at higher concentrations. The electric conductivity of the composites follows the theory of a percolation system, and the charge transport shows different models. The temperature dependence of the electrical conductivity for the composites shows that both positive (PTC) and negative (NTC) temperature coefficients appear near the melting temperature of matrix. The NTC behaviors within the temperature range below the melting point depend on the MWNTs concentration and the NTC intensity increases with the increasing MWNTs concentration.     

Enhancement of dielectric and electrical properties in BT/SiC/PVDF three-phase composite through microstructure tailoring

A polymer composite with high dielectric permittivity was prepared by embedding silicon carbide (SiC) whisker with an average diameter of 500 nm–1 μm in poly(vinylidene fluoride) (PVDF). However, the high dielectric loss and electrical conductivity of the two-phase composite prohibits its potential applications. Barium titanate (BT) particles with average diameter of 100 nm and 1 μm were incorporated as a third phase to fabricate a three-phase composite. The morphology structure, dielectric and electrical properties before and after the addition of BT particles were investigated. The three-phase composite exhibits largely suppressed dielectric loss and electrical conductivity without sacrificing the high dielectric permittivity, which was extremely hard to be realized for two-phase composite. It is also found that the nano-size BT is more favorable in achieving high dielectric permittivity than the micro-size BT, where their dielectric loss and electrical conductivity are similar. Furthermore, electric modulus analysis confirms the largely suppressed electron conduction process which results in the enhanced dielectric and electrical properties in three-phase composite.