Dandan Yang

Temperature dependence of electric and dielectric behaviors of Ni/polyvinylidene fluoride composites

Composites of polyvinylidene fluoride (PVDF) filled with metallic particles of nickel (Ni) were prepared via a blending and hot-molding technique. Rescaled temperature dependence of electric and dielectric behaviors of Ni/PVDF composites were studied at wide content ranges. Results show that there is significant positive temperature coefficient effect and giant dielectric constant as the concentration of Ni is near the percolation threshold. The nonuniform distribution of filler particles in PVDF host is observed from the scanning electron microscope micrograph since they are aggregated in amorphous regions of PVDF and form a network of conducting chains. Two relaxation peak regions of dielectric constant are observed from −10 to 40 °C and from 100 to 150 °C, which can be attributed to the contribution of polar effect of PVDF. The percolation theory, the thermal expansion model, and the simple concept of polarization in the capacitors are employed to explain these experimental results.

Synergistic flame retardant effect of α-zirconium phosphate in low-density polyethylene/ethylene–vinyl acetate/aluminum hydroxide hybrids

Organophilic α-zirconium phosphate (α-ZrP, OZrP) with a lamellar structure is used as a synergistic agent with aluminum hydroxide (ATH) to improve the flame retardancy of low-density polyethylene and ethylene–vinyl acetate (LDPE/EVA) hybrid. The structures of LDPE/EVA/(ATH, OZrP) hybrids are characterized by X-ray diffraction, transmission electron microscopy, and high resolution transmission electronic microscopy. The results indicate that the intercalated and exfoliated nanocomposites are formed. The flammability characterization and synergistic effects of LDPE/EVA/(ATH, OZrP) hybrids have been investigated by cone calorimeter test, limiting oxygen index, and UL-94 tests. The analyses show that hybrids with a given amount of OZrP have apparently reduced the heat release rates and can pass the V-0 rating in the UL-94 test. Based on these findings, a possible and reasonable synergistic flame retardant mechanism of OZrP and ATH is given in the discussion.

Preparation and properties of alkaline functionalized carbon nanotubes reinforced polyurethane composites

Composites consisting of polyurethane (PU)/carbon nanotubes (CNTs) have been successfully prepared by solution mixing method. CNTs were modified through mechano-chemical reaction to increase the compatibility with PU via hydrogen bondings. SEM microphotographs proved that modified CNTs (M-CNTs) became shorter and FTIR spectra showed that hydroxyl groups had been introduced to the surface of M-CNTs. SEM images of PU/M-CNTs composites also proved that M-CNTs were effectively dispersed in PU matrix. Mechanical property tests showed that addition of M-CNTs could significantly improve the tensile properties of PU/M-CNTs composite (breaking strength enhancement ratio for composite with 5.0 wt% M-CNTs was 103.81 %). The thermal stability of composites with M-CNTs was also improved. The initial degradation temperature enhancement was 19.9 oC for the composite with 0.5 wt% M-CNTs. Electrical property tests showed that the electrical properties were improved by adding M-CNTs. The volume conductivities increased 3 and 5 orders of magnitude for the composites with 5.0 wt% and 10 wt% M-CNTs, respectively. The addition of M-CNTs had little effect on the elastic properties of the composites.

Significant Ultra-low Percolation Threshold, Enhanced Positive Temperature Coefficient Effect and Dielectric Permittivity in Polymer-based Composites with Immiscible Polymers

Significant conductive polymer-based composites consisting of immiscible semi-crystalline polymers PP and PVDF, at different volume ratios loaded with a certain concentration of CB were prepared by blending and sequent hot-pressing technology. The distributing status of CB in the polymers was evaluated through the micrographs of the composite. The percolation threshold of this kind of composite is much lower than those of the individual polymers. Even more, the composite at 1/1 volume ratio of PP and PVDF displays the best conductivity among different ratios at a certain concentration of CB, and it displays an outstanding PTC effect more than five orders of magnitude, and synchronously an enhanced dielectric permittivity about 24.9 at 100 Hz. These inimitable properties may owe to the formation of PP/PVDF co-continuous phases and a double-percolation effect in the composite. The novel polymer-based composite with ultra-low percolation threshold, enhanced PTC effect, as well as the significant dielectric permittivity is promising a potential application.

Comparative study on the dielectric properties of three polyvinylidene fluoride nanocomposites incorporated with carbon filler

Poly(vinylidene fluoride) (PVDF)-carbon nanotube (CNT) composites with three different CNTs were prepared by a solution blending and hot-press method. The morphologies of nanocomposites were studied by scanning electron microscopy. The X-ray diffraction and differential scanning calorimeter data indicated that the addition of CNTs can promote the formation of β-phase of PVDF. The dielectric constant values of three PVDF/CNTs are much greater than that of neat PVDF. It has been found that the dielectric loss of the short hydroxylated CNTs filled PVDF nanocomposite is lower than those of other two long pristine CNTs filled PVDF nanocomposites.

Novel Positive Temperature Coefficient Effect of Electrical Resistivity in Ni Particles Filled Two Semi-Crystalline Polymer Intermixture Nanocomposite.

A novel positive temperature coefficient (PTC) effect nanocomposite by employing both of polyvinylidene fluoride (PP) and polypropylene (PVDF) intermixture in 1-1 volume ratio as host materials and Ni nanoparticles as conductive filler was prepared via a simple hot compaction method. This study focused on the effect of Ni content on percolation threshold and temperature dependence of direct current (DC) resistivity, as well as frequency dependence of alternating current (AC) resistivity of nanocomposites. This kind of material showed a very small percolation threshold value (about 6.0 vol% of Ni) and a novel strong PTC effect of electrical resistivity between temperatures ranging 150-167 °C, which coincided with the both melting temperatures of PP and PVDF. Close to the percolation threshold, abnormal electrical behavior was observed and interpreted based on the tunnel conductivity between Ni clusters. The curves of the AC resistivity versus the frequency, at different loadings of Ni and at different temperatures, offered a further image of the arrangement of conducting clusters within the host.