Qingguo Chen

High dielectric permittivity and low loss in PVDF filled by core-shell Zn@ZnO particles

In this study, metal-semiconductor Zn@ZnO core-shell particles were prepared by the heat treatment of raw Zn powder under air atmosphere, and the prepared Zn@ZnO particles were incorporated into poly(vinylidene fluoride) (PVDF) to obtain high dielectric permittivity polymer. The results indicate that the Zn@ZnO particles remarkably increased the dielectric constant of the PVDF composites compared with the raw Zn/PVDF due to the duplex interfacial polarizations induced by ZnO-Zn interface and ZnO-PVDF interface. Moreover, the dielectric permittivity of the Zn@ZnO/PVDF composites can be further optimized by adjusting the thickness of ZnO shell. The dielectric loss and conductivity were still remained at low acceptable level owing to the presence of ZnO shell between Zn core and PVDF matrix which serves as an interlayer between the Zn cores preventing them from contacting with each other. The developed Zn@ZnO/PVDF polymer composites with high dielectric constant and low loss are potential for embedded capacitor applications.

Temperature-Dependent Dielectric Properties of Al/Epoxy Nanocomposites

Broadband dielectric spectroscopy was carried out to study the transition in electrical properties of Al/epoxy nanocomposites over the frequency range of 1–107xa0Hz and the temperature range of −20°C to 200°C. The dielectric permittivity, dissipation factor, and electrical conductivity of the nanocomposites increased with temperature and showed an abrupt increase around the glass transition temperature (Tg). The results clearly reveal an interesting transition of the electrical properties with increasing temperature: insulator below 70°C, conductor at about 70°C. The behavior of the transition in electrical properties of the nanocomposites was explored at different temperatures. The presence of relaxation peaks in the loss tangent and electric modulus spectra of the nanocomposites confirms that the chain segmental dynamics of the polymer is accompanied by the absorption of energy given to the system. It is suggested that the temperature-dependent transition of the electric properties in the nanocomposite is closely associated with the α-relaxation. The large increase in the dissipation factor and electric conductivity depends on the direct current conduction of thermally activated charge carriers resulting from the epoxy matrix above Tg.

Dynamic thermal-dielectric behavior of core-shell–structured aluminum particle-reinforced epoxy composites:

The broadband dielectric spectroscopy was carried out in the frequency range of 1–107 Hz at the −20–200°C range to investigate the effect of temperature on the dynamic thermal–dielectric behavior of the aluminum (Al)/epoxy composite. The epoxy composites with core-shell–structured Al particles were prepared by solution method. The results show that the dielectric permittivity of the composites increased smoothly with a rise of filler content and reduced with an increase in frequency at room temperature. While the dielectric loss and conductivity still remained at low level owing to the nanoscale alumina insulating shell serving as a barrier layer to control the dielectric loss. The dielectric permittivity, dissipation factor, and conductivity of the composites increased with temperature and exhibited an abrupt rise around the glass transition temperature (Tg). A large increase in the dissipation factor and conductivity with temperature is attributed to the direct current conduction of thermal-activated charge carriers resulting from pure epoxy above Tg. The observed temperature-dependent dielectric relaxations of the composites indicated a thermally activated behavior of the relaxation time of epoxy chain segments.

Mechanical and dielectric properties of epoxy composites filled with hybrid aluminum particles with binary size distribution

Epoxy composites incorporated with three kinds of hybrid aluminum (Al) particles with binary size distribution, that is, [1 μm/45 μm], [1 μm/18 μm], and [18 μm/45 μm], respectively, were prepared, and the mechanical and dielectric properties of the hybrid Al/epoxy composites were investigated as a function of relative weight fraction of smaller-size Al (Ws) of hybrid Al particles at a total filler content of 50 wt%. The mechanical and electrical properties of the hybrid Al/epoxy composites are found to mainly depend on the type of hybrid filler and the Ws and can be tuned by changing the Ws. The maximum tensile strength and elongation at break of the composites appear at an optimal Ws. Furthermore, the dielectric permittivity, dielectric breakdown strength, and volume resistivity of the hybrid Al/epoxy composites also exhibit the similar variations as the mechanical properties with the Ws. The obvious enhancements in the physical properties can be ascribed to the synergistic effect of hybrid particles in the matrix at the optimal Ws, which endows the composites with better mechanical and dielectric properties. So, the results give a facile strategy to enhance the dielectric and mechanical properties of the composites by choosing a proper Ws at a fixed total filler loading.