Z.-Y. Cheng

Metal-polymer nanocomposites with high percolation threshold and high dielectric constant

By combining a solution cast and a hot-press process, a process to prepare uniform metal-polymer nanocomposites is introduced. It is confirmed using two composite systems: nanosized Ni particles embedded into P(VDF-TrFE) 70/30 mol. % and P(VDF-CTFE) 88/12 mol. % copolymer, respectively. Composites with 0 vol. %–60 vol. % of Ni nanoparticles are studied. The dielectric property of each composite is characterized over a frequency range from 100 Hz to 1 MHz. The results show that two nanocomposite systems show very similar percolation behavior with a high percolation threshold (>55 vol. %) and exhibit a high dielectric constant (∼1000 at 100 Hz).

Process and Microstructure to Achieve Ultra-high Dielectric Constant in Ceramic-Polymer Composites

Influences of process conditions on microstructure and dielectric properties of ceramic-polymer composites are systematically studied using CaCu3Ti4O12 (CCTO) as filler and P(VDF-TrFE) 55/45 mol.% copolymer as the matrix by combining solution-cast and hot-pressing processes. It is found that the dielectric constant of the composites can be significantly enhanced–up to about 10 times – by using proper processing conditions. The dielectric constant of the composites can reach more than 1,000 over a wide temperature range with a low loss (tan δ ~ 10−1). It is concluded that besides the dense structure of composites, the uniform distribution of the CCTO particles in the matrix plays a key role on the dielectric enhancement. Due to the influence of the CCTO on the microstructure of the polymer matrix, the composites exhibit a weaker temperature dependence of the dielectric constant than the polymer matrix. Based on the results, it is also found that the loss of the composites at low temperatures, including room temperature, is determined by the real dielectric relaxation processes including the relaxation process induced by the mixing.

High-permittivity polymer nanocomposites: influence of interface on dielectric properties

Flexible dielectric composites with high permittivity have been extensively studied due to their potential applications in highdensity energy capacitors. In this review, effects of interface characteristics on the dielectric properties in the polymer-based nanocomposites with high permittivity are analyzed. The polymer-based dielectric composites are classified into two types: dielectric–dielectric (DD, ceramic particle-polymer) composites and conductor–dielectric (CD, conductive particle-polymer) composites. It is highly desirable for the dielectric–dielectric composites to exhibit high permittivity at low content of ceramic particles, which requires a remarkable interface interaction existing in the composite. For conductor–dielectric composites, a high permittivity can be achieved in composite with a small amount of conductor particle, but associated with a high loss. In this case, the interface between conductor and polymer with a good insulating characteristic is very important. Different methods can be used to modify the surface of ceramic/conductor particles before these particles are dispersed into polymers. The experimental results are summarized on how to design and make the desirable interface, and recent achievements in the development of these nanocomposites are presented. The challenges facing the fundamental understanding on the role of interface in high-permittivity polymer nanocomposites should be paid a more attention.

Physical aspects of 0-3 dielectric composites

0-3 dielectric composites with high dielectric constants have received great interest for various technological applications. Great achievements have been made in the development of high performance of 0-3 composites, which can be classified into dielectric–dielectric (DDCs) and conductor–dielectric composites (CDCs). However, predicting the dielectric properties of a composite is still a challenging problem of both theoretical and practical importance. Here, the physical aspects of 0-3 dielectric composites are reviewed. The limitation of current understanding and new developments in the physics of dielectric properties for dielectric composites are discussed. It is indicated that the current models cannot explain well the physical aspects for the dielectric properties of 0-3 dielectric composites. For the CDCs, experimental results show that there is a need to find new equations/models to predict the percolative behavior incorporating more parameters to describe the behavior of these materials. For the DDCs, it is indicated that the dielectric loss of each constituent has to be considered, and that it plays a critical role in the determination of the dielectric response of these types of composites. The differences in the loss of the constituents can result in a higher dielectric constant than both of the constituents combined, which breaks the Wiener limits.

Time-dependence of the electromechanical bending actuation observed in ionic-electroactive polymers

The characteristics of the electromechanical response observed in an ionic-electroactive polymer (i-EAP) are represented by the time (t) dependence of its bending actuation (y). The electromechanical response of a typical i-EAP — poly(ethylene oxide) (PEO) doped with lithium perchlorate (LP) — is studied. The shortcomings of all existing models describing the electromechanical response obtained in i-EAPs are discussed. A more reasonable model: y=ymaxe−τ∕t is introduced to characterize this time dependence for all i-EAPs. The advantages and correctness of this model are confirmed using results obtained in PEO-LP actuators with different LP contents and at different temperatures. The applicability and universality of this model are validated using the reported results obtained from two different i-EAPs: one is Flemion and the other is polypyrrole actuators.

Characterization of percolation behavior in conductor–dielectric 0-3 composites

The equation eeff ∝ (ϕc - ϕ)-s which shows the relationship between effective dielectric constant (eeff) and the filler concentration (φ), is widely used to determine the percolation behavior and obtain parameters, such as percolation threshold φc and the power constant s in conductor–dielectric composites (CDCs). Six different systems of CDCs were used to check the expression by fitting experimental results. It is found that the equation can fit the experimental results at any frequency. However, it is found that the fitting constants do not reflect the real percolation behavior of the composites. It is found that the dielectric constant is strongly dependent on the frequency, which is mainly due to the fact that the frequency dependence of the dielectric constant for the composites close to φc is almost independent of the matrix.

Dielectric response and percolation behavior of Ni–P(VDF–TrFE) nanocomposites

Conductor–dielectric 0–3 nanocomposites using spherical nickel nanoparticles as filler and poly(vinylidene fluoride–trifluoroethylene) 70/30mol.% as matrix are prepared using a newly developed process that combines a solution cast and a hot-pressing method with a unique configuration and creates a uniform microstructure in the composites. The uniform microstructure results in a high percolation threshold φc (>55 vol.%). The dielectric properties of the nanocomposites at different frequencies over a temperature range from −70∘C to 135∘C are studied. The results indicate that the composites exhibit a lower electrical conductivity than the polymer matrix. It is found that the nanocomposites can exhibit an ultra-high dielectric constant, more than 1500 with a loss of about 1.0 at 1kHz, when the Ni content (53 vol.%) is close to percolation threshold. For the nanocomposites with 50 vol.% Ni particles, a dielectric constant more than 600 with a loss less than 0.2 is achieved. It is concluded that the loss including high loss is dominated by polarization process rather than the electrical conductivity. It is also found that the appearance of Ni particles has a strong influence on the crystallization process in the polymer matrix so that the polymer is converted from a typical ferroelectric to a relaxor ferroelectric. It is also demonstrated that the widely used relationship between the dielectric constant and the composition of the composites may not be valid.

Dielectric properties of polystyrene based composites filled with core-shell BaTiO 3 /polystyrene hybrid nanoparticles

In this work, core-shell structured BaTiO3/polystyrene nanoparticles (BT-PS) with different thickness of PS shell were synthesized through atom transfer radical polymerization and the influence of their shell thickness on dielectric properties of BT-PS/PS composites was studied. Two types of BT-PS with the PS shell of 3 nm or 12 nm were obtained by controlling the polymerization time. The structure of BT-PS was carefully characterized by infrared spectroscopy, X-ray diffraction, thermal gravimetric analysis, and transmission electron microscope. The results confirmed the successful preparation of core-shell BT-PS nanoparticles. Compared to pristine BT, the core-shell particles can be more homogeneously dispersed into PS matrix. As a result, higher dielectric constant, higher breakdown strength, and lower dielectric loss were achieved in BT-PS/PS composites. Moreover, the dielectric constant of BT-PS/PS composites displayed frequency independent behavior. In addition, the composites filled by BT-PS with 3 nm of PS shell showed better dielectric properties than those filled by BT-PS with 12 nm of PS shell. A maximum energy density as large as 4.24 J/cm3 was obtained in BT-PS/PS films.

Enhanced thermal and pyroelectric properties in 0–3 TGS:PVDF composites doped with graphene for infrared application

Pyroelectric composites of triglycine sulfate (TGS)-polyvinylidene difluoride (PVDF) doped with graphene are studied. It is found that the graphene can effectively improve the polling efficiency and thermal property of the composites so that the infrared detective performance can be significantly improved. For example, by adding about 0.83 wt.% of graphene, the infrared detective property can be improved by more than 30%. It is also found that the size of the graphene plays a critical role on the property improvement. For example, the small-sized graphene prepared by ultrasonic exfoliation (UE) method is more effective than the big-sized graphene prepared by electrochemical exfoliation (EE) method.