Effect of mechanical stretching on electrical conductivity and positive temperature coefficient characteristics of poly(vinylidene fluoride)/carbon nanofiber composites prepared by non-solvent precipitation

Poly(vinylidene fluoride) (PVDF)/carbon nanofiber (CNF) composites with filler content ranging from 0.047 to 4.7 vol.% were prepared with non-solvent precipitation followed by melt compression. The morphology and electrical conductivity of the composites before and after mechanical stretching were examined. The results showed that CNFs were dispersed homogeneously in the PVDF matrix and a low electrical percolation threshold of 0.90 vol.% CNFs was obtained. Mechanical stretching led to a sharp decrease in the electrical conductivity of a composite containing 0.94 vol.% CNF. This was caused by the destruction of a conducting network structure when the fillers aligned along the stretching direction. This did not happen when the filler content was increased to 1.88 vol.%. The percolating composites displayed a positive temperature coefficient (PTC) effect with the effect being larger in stretched composites. This can be attributed to the presence of PVDF β-phase in stretched composites as revealed by X-ray diffraction and Fourier transform infrared spectroscopy.     

Electrical conductivity of carbon nanotube/poly(vinylidene fluoride) composites prepared by high-speed mechanical mixing

Electrically conductive multi-walled nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites with a segregated structure were prepared by high-speed mechanical mixing method. It was found that MWCNTs were uniformly dispersed on polymer particle surfaces. At the MWCNTs composition of 0.1 vol.%, the composites exhibited a dramatic enhancement in electrical conductivity by 11 orders of magnitude. A low percolation threshold was achieved at the CNT concentration of 0.078 vol.%. The mechanical mixing method presented can be adapted to other CNT/polymer composites with a segregated structure.     

Effects of hybrid fillers on the electrical conductivity,EMI shielding effectiveness,and flame retardancy of PBT and PolyASA composites with carbon fiber and MWCNT

The effects of hybrid fillers of carbon fiber (CF) and multiwall carbon nanotube (MWCNT) on the electrical conductivity, electromagnetic interference shielding effectiveness (EMI SE), flame retardancy, and mechanical properties of poly(butylene terephthalate) (PBT)/poly(acrylonitrile-co-styrene-co-acrylate) (PolyASA) (70/30, wt %) with conductive filler composites were investigated. The CF was used as the main filler, and MWCNT was used as the secondary filler to investigate the hybrid filler effect. For the PBT/PolyASA/CF (8 vol %)/MWCNT (2 vol %) composite, a higher electrical conductivity (1.4 × 10⁰ S cm⁻¹) and EMI SE (33.7 dB) were observed than that of the composite prepared with the single filler of CF (10 vol %), which were 9.0 × 10⁻² S cm⁻¹ and 23.7 dB, respectively. This increase in the electrical properties might be due to the longer CF length and hybrid filler effect in the composites. From the results of aging test at 85 °C, 120 h, the electrical conductivity and EMI SE of the composites decreased slightly compared to that of the composite without aging. The results of electrical conductivity, EMI SE, and flame retardancy suggested that the composite with the hybrid fillers of CF and MWCNT showed a synergetic effect in the PBT/PolyASA/CF (8 vol %)/MWCNT (2 vol %) composite. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48162.     

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Large-size graphene microsheets as a protective layer for transparent conductive silver nanowire film heaters

A new strategy is reported for the fabrication of silver nanowire (AgNW) film heaters using reduced small/or large-size graphene oxide (rSGO or rLGO) sheets as an over-coating protective layer. The results show that ultrathin rLGO microsheets provide the best combination of protective effect and electrical properties on AgNW networks and thus could enable the design of high-performance transparent film heaters. As a consequence, good optical transparency and electrical conductivity, good oxidation resistance and thermal stability, and good heating performances are achieved with as-made rLGO/AgNW film heaters. Specifically, the rLGO/AgNW hybrid film annealed at 700 °C shows a low sheet resistance of 27 Ω sq⁻¹ and a good optical transparency of 80%. Furthermore, it exhibits good heating characteristics and defrosting performance at low voltages. The results presented here may pave the way for a new promising application of rLGO/AgNW hybrid film in transparent film heaters and other electrical devices.     

Low percolation threshold of graphene/polymer composites prepared by solvothermal reduction of graphene oxide in the polymer solution

Graphene/polyvinylidene fluoride (PVDF) composites were prepared using in-situ solvothermal reduction of graphene oxide in the PVDF solution. The electrical conductivity of the composites was greatly improved by doping with graphene sheets. The percolation threshold of such composite was determined to be 0.31 vol.%, being much smaller than that of the composites prepared via blending reduced graphene sheets with polymer matrix. This is attributed to the large aspect ratio of the SRG sheets and their uniform dispersion in the polymer matrix. The dielectric constant of PVDF showed a marked increase from 7 to about 105 with only 0.5 vol.% loading of SRG content. Like the other conductor-insulator systems, the AC conductivity of the system also obeyed the universal dynamic response. In addition, the SRG/PVDF composite shows a much stronger nonlinear conduction behavior than carbon nanotube/nanofiber based polymer composite, owing to intense Zener tunneling between the SRG sheets. The strong electrical nonlinearity provides further support for a homogeneous dispersion of SRG sheets in the polymer matrix.     

PVDF/CB composites prepared by novel solvent casting method for low voltage PTC temperature regulation applications

A novel solvent casting preparation technique utilizing three variants of poly(vinylidene fluoride) (PVDF) to achieve a thermal cut off and a self regulation effect at a low applied voltage is reported in this study. The positive temperature coefficient (PTC) composites were prepared by dissolving PVDF in 1‐methyl‐2‐pyrrolidone (NMP) solvent, blending with Vulcan® XC72 carbon black (CB) filler, crosslinking with vinyl trimethoxysilane (VTMOS) and quenching in water. All composites displayed a highly macrovoidal structure that promoted a PTC effect when subjected to a thermal expansion effect via an electrical current. Subsequently the current was cut off and self regulation behavior was exhibited. Kynar® 761A PVDF resulted in the strongest PTC effect, and displayed temperature regulation at around 100°C which may be attributed to the highly semi crystalline nature and the larger molecular weight of this polymer in comparison with the other PVDF composites studied. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Interfacial polarization and layer thickness effect on electrical insulation in multilayered polysulfone/poly(vinylidene fluoride) films

In this study, we report layer thickness effect on the electrical insulation property of polysulfone (PSF)/poly(vinylidene fluoride) (PVDF) multilayer films having a fixed composition of PSF/PVDF = 30/70 (vol./vol.). Breakdown strength, dielectric lifetime, and electrical conductivity were studied for 32- and 256-layer films having various total film thicknesses. Among these films, those having thinner PVDF and PSF layers exhibited lower breakdown strength, shorter lifetime, and higher electrical conductivity than those having thicker layers. These experimental results were explained by Maxwell–Wagner–Sillars interfacial polarization due to contrasts in dielectric constant and electronic conductivity for PVDF and PSF, respectively. When both PVDF and PSF layers were thick (ca. > 100–200 nm), more space charges were available in PVDF and no electronic conduction was allowed for PSF. These accumulated interfacial charges could serve as effective traps for injected electrons from metal electrodes under high electric fields. As a result, reduced electrical conductivity and enhanced breakdown strength/dielectric lifetime properties were obtained. When both layers were thin (ca. < 100 nm), fewer space charges were available in PVDF and significant electronic conduction through PSF resulted in low interfacial polarization. Consequently, higher electrical conductivity, lower breakdown strength, and shorter lifetime were observed. These results provide us insights into potential physics to enhance electrical insulation property of polymer films using a multilayered structure having large dielectric constant contrast.     

Mixed-ionic and electronic conduction and stability of YSZ-graphene composites

Composites of 8 mol% Yttria-stabilized Zirconia (YSZ) containing 0, 7, 10 and 14 vol.% of graphene nano-platelets (GNP) were fully densified by Spark Plasma Sintering. The effect of GNP on the electrical performance of the composites was analyzed by impedance spectroscopy as a function of temperature (150–800 °C) and oxygen partial pressure (0.21–10⁻²⁰ atm). Results show that below GNP percolation threshold (7.1 vol.%), the electrical behavior is dominated by the matrix oxygen-ion conductivity. Above the threshold, the conductivity is predominantly electronic provided by the GNP network. The total conductivity of composites was used as an indicator of GNP stability in different atmospheres. YSZ/GNP composites remain stable in inert conditions up to 600 °C, and in reducing conditions up to 800 °C, making them good alternatives to perovskite-based materials used for electrochemical applications.     

Improvement of the thermal transport performance of a poly(vinylidene fluoride) composite film including silver nanowire

The miniaturization trend of electronic devices requires that components have a high heat dissipation in industrial applications and in daily life. In this context, a highly thermally conductive film was fabricated with silver nanowire (AgNW) and poly(vinylidene fluoride) (PVDF) with a bar‐coating method. The thermal transport performance and mechanism of the AgNW/PVDF composite film were investigated. The through‐plane and in‐plane thermal conductivity of the AgNW/PVDF composite film reached 0.31 and 1.61 W m^?1 K^?1, respectively; these values far exceeded those of the pristine PVDF film. The experiment illustrated that the thermally conductive pathways formed successfully in the PVDF substrate with the addition of AgNW, and the heat tended to transfer along the thermally conductive pathway rather than along the PVDF substrate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43554.     

Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites

Novel fly ash cenosphere (FAC)/metakaolin (MK)-based geopolymeric composites were prepared by adding FAC to the MK-based geopolymeric slurry. Microstructure, mechanical property, thermal conductivity, and bulk density of the FAC/MK-based geopolymeric composites were investigated. It was confirmed by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) that the FAC did not dissolve in alkaline condition, but element diffusion took place around the interface between geopolymeric matrix and FAC. The compressive strength, thermal conductivity and bulk density of FAC/MK-based geopolymeric composites decreased monotonically with the increase of the FAC content from 15 vol.% to 40 vol.%, and the minimum values for the 40 vol.% FAC/MK-based geopolymeric composite reached 36.5 MPa, 0.173 W m⁻¹ K⁻¹ and 0.82 g cm⁻³, respectively, in the range of FAC content from 15 vol.% to 40 vol.%. The results showed that the FAC could lower thermal conductivity effectively and bulk density of FAC/MK-based geopolymeric composites at a cost of slight decrease of mechanical properties. The 40 vol.% FAC/MK-based geopolymeric composite was a promising candidate material for intermediate-temperature thermal insulation applications due to its low thermal conductivity and low density.     

Preparation and electrical properties of graphene nanosheet/Al2O3 composites

Fully dense graphene nanosheet(GNS)/Al₂O₃ composites with homogeneously distributed GNSs of thicknesses ranging from 2.5 to 20 nm have been fabricated from ball milled expanded graphite and Al₂O₃ by spark plasma sintering. The percolation threshold of electrical conductivity of the as-prepared GNS/Al₂O₃ composites is around 3 vol.%, and this new composite outperforms most of carbon nanotube/Al₂O₃ composites in electrical conductivity. The temperature dependence of electrical conductivity indicated that the as-prepared composites behaved as a semimetal in a temperature range from 2 to 300 K.     

One-Dimensional Nanostructures of Polypyrrole for Shielding of Electromagnetic Interference in the Microwave Region

Polypyrrole one-dimensional nanostructures (nanotubes, nanobelts and nanofibers) were prepared using three various dyes (Methyl Orange, Methylene Blue and Eriochrome Black T). Their high electrical conductivity (from 17.1 to 60.9 S cm⁻¹), good thermal stability (in the range from 25 to 150 °C) and resistivity against ageing (half-time of electrical conductivity around 80 days and better) were used in preparation of lightweight and flexible composites with silicone for electromagnetic interference shielding in the C-band region (5.85–8.2 GHz). The nanostructures’ morphology and chemical structure were characterized by scanning electron microscopy, Brunauer–Emmett–Teller specific surface measurement and attenuated total reflection Fourier-transform infrared spectroscopy. DC electrical conductivity was measured using the Van der Pauw method. Complex permittivity and AC electrical conductivity of respective silicone composites were calculated from the measured scattering parameters. The relationships between structure, electrical properties and shielding efficiency were studied. It was found that 2 mm-thick silicone composites of polypyrrole nanotubes and nanobelts shield almost 80% of incident radiation in the C-band at very low loading of conductive filler in the silicone (5% w/w). Resulting lightweight and flexible polypyrrole composites exhibit promising properties for shielding of electromagnetic interference in sensitive biological and electronic systems.     

Effect of Nano-Sized Poly(Butyl Acrylate) Layer Grafted from Graphene Oxide Sheets on the Compatibility and Beta-Phase Development of Poly(Vinylidene Fluoride) and Their Vibration Sensing Performance

In this work, graphene oxide (GO) particles were modified with a nano-sized poly(butyl acrylate) (PBA) layer to improve the hydrophobicity of the GO and improve compatibility with PVDF. The improved hydrophobicity was elucidated using contact angle investigations, and exhibit nearly 0° for neat GO and 102° for GO-PBA. Then, the neat GO and GO-PBA particles were mixed with PVDF using a twin screw laboratory extruder. It was clearly shown that nano-sized PBA layer acts as plasticizer and shifts glass transition temperature from −38.7 °C for neat PVDF to 45.2 °C for PVDF/GO-PBA. Finally, the sensitivity to the vibrations of various frequencies was performed and the piezoelectric constant in the thickness mode, d₃₃, was calculated and its electrical load independency were confirmed. Received values of the d₃₃ were for neat PVDF 14.7 pC/N, for PVDF/GO 20.6 pC/N and for PVDF/GO-PBA 26.2 pC/N showing significant improvement of the vibration sensing and thus providing very promising systems for structural health monitoring and data harvesting.     

Functionalized Graphene–PVDF Foam Composites for EMI Shielding

Novel foam composites comprising functionalized graphene (f‐G) and polyvinylidene fluoride (PVDF) were prepared and electrical conductivity and electromagnetic interference (EMI) shielding efficiency of the composites with different mass fractions of f‐G have been investigated. The electrical conductivity increases with the increase in concentration of f‐G in insulating PVDF matrix. A dramatic change in the conductivity is observed from 10^?16 S · m^?1 for insulating PVDF to 10^?4 S · m^?1 for 0.5 wt.% f‐G reinforced PVDF composite, which can be attributed to high‐aspect‐ratio and highly conducting nature of f‐G nanofiller, which forms a conductive network in the polymer. An EMI shielding effectiveness of ≈20 dB is obtained in X‐band (8–12 GHz) region and 18 dB in broadband (1–8 GHz) region for 5 wt.% of f‐G in foam composite. The application of conductive graphene foam composites as lightweight EMI shielding materials for X‐band and broadband shielding has been demonstrated and the mechanism of EMI shielding in f‐G/PVDF foam composites has been discussed.

     

New potassium sodium niobate/poly(vinylidene fluoride) functional composite films with high dielectric permittivity

New and homogeneous KNN/PVDF functional hybrid films loaded with 2 vol.%, 4 vol.%, 9 vol.%, 21 vol.%, 30 vol.%, and 40 vol.% Na_0.5K_0.5NbO₃ (KNN) have been prepared by a solution casting process. The effects of the doping KNN particles on the structure, morphology, and dielectric properties of KNN/PVDF hybrid films were investigated. The introduction of KNN fillers has a remarkable influence on the α-, β-, γ-phase structure and the crystallinity of the polymer matrix. And it is also effective in improving the dielectric performance. A dielectric permittivity as high as 250 is obtained at 10 Hz when the concentration of the KNN filler reaches 30 vol.%, which is 28 times higher than that of the pure PVDF matrix. The conductivity of the hybrid film with 40 vol.% KNN concentrations is lower than 8 × 10^?10 (S cm^?1) at 10² Hz and at room temperature, which shows its excellent insulativity and the potential to be applied into the electronic industry.     

Positive-temperature-coefficient effect of electrical resistivity below melting point of poly(vinylidene fluoride) (PVDF) in Ni particle-dispersed PVDF composites

This paper discusses the positive-temperature-coefficient effects of resistivity in Ni particle-dispersed poly(vinylidene fluoride) (PVDF) composites based on experiment results from SEM, DSC, and pressure-volume-temperature (PVT) measurements. The melting points of composites with Ni content of 20, 30, 40, and 50vol.% were equal to that of pure PVDF. The PTC effects in composites with Ni content of 40 and 50vol.% occurred at temperatures near the melting point of the PVDF matrix, whereas those in composites with Ni content of 20 and 30vol.% occurred at temperatures below the melting point of the PVDF matrix. We found that the PTC effect occurs even without melting of the matrix polymer. Moreover, we determined that a slight increase in specific volume at temperatures below the melting point of the matrix polymer acts fully as a driving force for forming a gap between fillers. This suggestion was backed up by theoretical analyses using percolation theory and a thermal-fluctuation-induced tunneling model.     

Novel electrically conductive α-β SiAlON/TiCN composites

Electrically conductive α-β SiAlON/TiCN composites were produced in the form of a segregated network in a ceramic matrix structure. A continuous 3D network of conductive TiCN particles was successfully achieved by mechanically coating spray-dried SiAlON granules with varying amounts of nano sized TiCN (0-10 vol.%) particles. For comparison, the same SiAlON matrix was incorporated with 25 vol.% micron sized TiCN particles to give a particle reinforced composite. Densification, together with mechanical and electrical properties of the composites produced were discussed in terms of conventional and novel approach. Fully dense composites were obtained by gas pressure sintering (GPS) under a nitrogen pressure of 100 bar at a peak temperature of 1990 °C. The electrical resistivity of the SiAlON (1 × 10¹³ Ω m) matrix material was drastically reduced with the addition of only 5 vol.% TiCN (18 × 10⁻⁴ Ω m) to the composites prepared by the coating method.     

Electrical conductivity modeling of carbon‐filled liquid‐crystalline polymer composites

Electrically conductive resins are needed for bipolar plates used in fuel cells. Currently, the materials for these bipolar plates often contain a single type of graphite powder in a thermosetting resin. In this study, various amounts of two different types of carbon, carbon black and synthetic graphite, were added to a thermoplastic matrix. The resulting single‐filler composites were tested for electrical conductivity, and electrical conductivity models were developed. Two different models, the Mamunya and additive electrical conductivity models, were used for both material systems. It was determined how to modify these models to reduce the number of adjustable parameters. The models agreed very well with experimental data covering a large range of filler volume fractions (from 0 to 12 vol % for the carbon black filled composites and from 0 to 65 vol % for the synthetic graphite filled composites) and electrical conductivities (from 4.6 × 10^?17 S/cm for the pure polymer to 0.5 S/cm for the carbon black filled composites and to 12 S/cm for the synthetic graphite filled composites). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3293–3300, 2006