ABSTRACT
Many inorganic ceramics have large permanent dipole moment, due to their chemical compositions and crystal structures. They typically exhibit very high dielectric permittivity up to a few thousands, in contrast to the low dielectric permittivity of polymer materials. However, with increasing demand of flexible, embedded, and miniaturized electronic devices, these ceramic materials alone fail to achieve similar success as in traditional electronics. Some reasons behind this include high brittleness, high mass density, low dielectric breakdown strength as well as poor processability of ceramic materials, and so on, compared with polymers. Thus, incorporating dielectric ceramics into polymers to fabricate polymer composites/
nanocomposites has been adopted to combine the advantages of both types of materials [13, 34, 94].Metal oxides with perovskite crystal structures have been drawing tremendous attentions from both academia and industry for dielectric polymer nanocomposite applications, because of their high dielectric permittivity, ferroelectricity, piezoelectricity, pyroelectricity, and so on. Popular perovskite type of metal oxides for dielectric polymer nanocomposites include barium titanate (BaTiO3), lead zirconate titanate, Pb[ZrxTi1-x]O3 (PZT, 0 < x < 1), and their doped ceramic structures [26, 34, 35, 41, 52, 53, 60, 61]. These perovskite-type metal oxides show multiple crystal phases altered by temperature and particle size, leading to different dielectric properties. In this chapter, perovskite ceramic materials will be introduced, with a special focus on BaTiO3. In particular, besides the discussion on the factors such as concentrations, particle size, and the morphologies of nanoparticles, special attention will be paid to the interfacial structures, namely, the surface modification of the ceramic nanomaterials, and their effects on dielectric performances of the polymer nanocomposites. 3.1 Crystal Structures of Barium TitanateBarium titanate (BaTiO3) is one of the most important perovskite-type ceramic materials with various crystal structures. The strong permanent dipole moments in some BaTiO3 crystals result in high dielectric permittivity and ferroelectric behaviors. Meanwhile, BaTiO3 exhibits remarkable piezoelectricity and pyroelectricity as well. The outstanding dielectric performances of BaTiO3 have popularized it in polymer composite dielectric films. The dielectric performances of BaTiO3 are directly related to its crystal structures. BaTiO3 is a type of perovskite oxide with a chemical formula of ABO3 (A = Ba, Sr . . .; B = Ti, Zr . . .), the crystal structure of which consists of AO12 cuboctahedra and BO6 octahedra (Fig. 3.1). BaTiO3 shows five crystal phases, and they are rhombohedral, orthorhombic, tetragonal, cubic, and hexagonal perovskite crystal structures [19, 36, 52, 61]. The phase transition among different crystal structures is a function of both temperature and particle size. Typically, the rhombohedral to orthorhombic transition occurs
at 183 K, the orthorhombic phase to tetragonal transition occurs at 278 K, and the tetragonal to cubic transition occurs at 403 K [36, 68]. At a high temperature of 1705 K, the cubic-to-hexagonal phase transition will happen [36].
