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Composites have been extensively studied for dielectric and related applications. This is a review of polymer based 0–3 composites that exhibit a high dielectric constant. These composites are classified into two types: Dielectric–dielectric composite and conductor–dielectric composite. The physical principles and related models are presented with associated assumptions and approximations. In general, a dielectric–dielectric composite needs a higher concentration of the fillers to reach a high dielectric constant than a conductor–dielectric composite. The high dielectric constant observed in the conductor–dielectric composites is usually associated with a high dielectric loss and a low electric breakdown field. The experimental results are summarized to illustrate the principles for, and the achievements in, the development of these composites. The challenges facing the fundamental understanding and the further development of these composites for different applications are discussed.

Composite thin film is highly desirable for the dielectric applications. In order to develop composite thin film, a nanocomposite, in which nanosized CaCu 3 Ti 4 O 12 (CCTO) particles are used as filler and P(VDF-TrFE) 55/45 mol% copolymer is used as polymer matrix, is investigated. The contents of CCTO in the nanocomposites range from 0% to 50 vol%. The dielectric property of these nanocomposites was characterized at frequencies ranging from 100 Hz to 1 MHz and at temperatures ranging from 200 K to 370 K. A dielectric constant of 62 with a loss of 0.05 was obtained in nanocomposite with 50 vol% CCTO at room temperature at 1 kHz. At the phase transition temperature (∼340 K) of the copolymer, a dielectric constant of 150 with a loss less than 0.1 was obtained in this nanocomposite. It is found that the dielectric loss of the nanocomposites is dominated by the polymer which has a relaxation process. Comparing to composites made using microsized CCTO, the nanocomposites exhibit a much lower dielectric loss and a lower dielectric constant. This indicates that the nanosized CCTO particles have a lower dielectric constant than the microsized CCTO particles.

Although extensive studies have been done on lead-free dielectric ceramics to achieve excellent dielectric behaviors and good energy storage performance, the major problem of low energy density has not been solved so far. Here, we report on designing the crossover relaxor ferroelectrics (CRFE), a crossover region between the normal ferroelectrics and relaxor ferroelectrics, as a solution to overcome the low energy density. CRFE exhibits smaller free energy and lower defect density in the modified Landau theory, which helps to obtain ultrahigh energy density and efficiency. The (1–x)­Ba0.65Sr0.35TiO3–xBi­(Mg2/3Nb1/3)­O3 ((1–x)­BST–xBMN) (x = 0, 0.08, 0.1, 0.18, 0.2) ceramic was synthesized by a solid-state reaction method. The solid solutions exhibit dielectric frequency dispersion, which suggests typical relaxor characteristics with the increasing BMN content. The crossover ferroelectrics of 0.9BST–0.1BMN ceramic possesses a high energy storage efficiency (η) of 85.71%, a high energy storage density (W) of 3.90 J/cm3, and an ultrahigh recoverable energy storage density (W rec) of 3.34 J/cm3 under a dielectric breakdown strength of 400 kV/cm and is superior to other lead-free BaTiO3 (BT)-based energy storage ceramics. It also exhibits strong thermal stability in the temperature range from 25 to 150 °C under an electric field of 300 kV/cm, with the fluctuations below 3% and with the energy storage density and energy efficiency at about 2.8 J/cm3 and 82.93%, respectively. The enhanced recoverable energy density and breakdown strength of BT-based materials with significantly high energy efficiency make it a promising candidate to meet the wide requirements for high power applications.

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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).

An all-organic composite system using newly developed PPy nanoclips is developed. The composites have a uniform microstructure due to the unique preparation process. The composites have a very low percolation threshold (<8 wt.%) and exhibit a high dielectric constant. At room temperature, the composites exhibit a dielectric constant of more than 1,000. At temperatures higher than 98 o C, the composites exhibit a dielectric constant of about 2,000. More interestingly, the high dielectric constant reported here is associated with a loss much smaller than the loss reported for other CDCs using 1-D fillers. It is indicated that a new dielectric relaxation process is induced due to the mixture of PPy with P(VDF-TrFE), whose relaxation time decreases with increasing PPy content. The loss observed in the composites at low temperature including room temperature is mainly determined by this relaxation process rather than the conductivity. If this relaxation process has a strong contribution to the dielectric constant, the widely used percolation formula describing relationship between the dielectric constant and the composite cannot be used.

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.