All-Ceramic Percolative Composites with a Colossal Dielectric Response

Abstract

Dielectric materials, which are used to control and store charges and electric energy, play a key role in modern electronics and electric power systems. As commercial and consumer requirements for compact and low cost electronic and electrical power systems as well as for very high energy capacitive storage systems grow substantially, the development of high dielectric constant materials has become one of the major scientific and technology issues (Reynolds & Buchanan, 2004; Scott, 2007). High dielectric constant materials are highly desirable for use, not only as capacitor dielectrics, but also in a broad range of advanced electromechanical applications, such as actuators, sonars, and, particularly, as highfrequency transducers (Zhang et al., 2002). The input electric energy that can be converted into the strain energy is namely directly proportional to the square of the electric field and to the dielectric constant of the electroactive material. Thus, by increasing the dielectric constant the required electromechanical response, i.e., strain can be induced under a much reduced electric field. Extremely large dielectric constants are expected only for ferroelectrics in a very narrow temperature range close to the paraelectric-to-ferroelectric phase transition or for systems with hopping charge carriers yielding dielectric constant that diverges towards low frequencies. High-capacitance ceramic capacitors are therefore mostly made of very thin layers of ceramic material (usually a ferroelectric) placed between conductive plates. The most important part of the market in passive devices is, at present, made up of multilayer ceramic capacitors (MLCCs), comprising alternating thin layers of conductor (inner electrodes) and ceramic (Takeshima et al., 1997), which turns out to be the most efficient geometry for attaining high-density charge storage. A similar geometrical approach can also intuitively explain the dielectric response of a percolative composite − a composite comprising a conductive filler embedded in a dielectric matrix. The fact that the effective dielectric constant of the mixture is much larger than the dielectric constants of the individual constituents is due to the fact that close to the percolation point (the volume fraction when the conductive admixture forms a continuous network and, consequently, the system begins to conduct electricity) there are many conducting particles which are isolated by very thin dielectric/ferroelectric layers. A comparison between configurations of a MLCC and a percolative composite is presented in Fig. 1. Unfortunately, the percolative