Colossal dielectric constant of poly- and single-crystalline CaCu 3Ti 4O 12 fibres grown by the laser floating zone technique

Abstract

The laser floating zone technique (LFZ) was used to produce CaCu 3Ti 4O 12 (CCTO) samples with different pulling rates ( R p ). Polycrystalline samples were obtained for higher R p , and single crystals for low R p . Morphologic characterization was done by scanning electron microscopy (SEM), while the structural analysis was accomplished using X-ray diffraction and Raman spectroscopy. The best crystallinity was obtained for the fibre grown at the lowest pulling rate, resulting in a CCTO single crystal. Dielectric spectroscopy was performed between 40 Hz and 50 MHz in the temperature range of 90–350 K. The dielectric strength varies from 3 × 10 4 for the fibres grown at higher R p (80 mm h −1), to 5 × 10 4 for the ones grown at lower R p (5 mm h −1). The Cole–Cole model was used to fit the dielectric data in order to calculate the relaxation parameters. The dielectric properties measured at microwave frequency (2.7 GHz) by the resonant cavity method confirm the high dielectric constant (56.7) and relatively low values of tan δ (0.04). This is a clear indication that this material is potentially interesting for microwave device applications. The similarities between the AC activation energies of LFZ fibres and bulk CCTO, calculated from relaxation times, suggest that an additional polarization mechanism common to single and polycrystalline CCTO fibres must also be present and should contribute for the further increase of the dielectric constant.