Dielectric response and thermistor behavior of lead-free x NaNbO3 - (1-x) BiFeO3 electroceramics

• Published in: Ceramics international 2018-10, Vol.44 (15), p.18560-18570
• Main Authors: Saad, Y., Álvarez-Serrano, I., López, M.L., Hidouri, M.
• Format: Article
• Language: English
• Subjects: Electrical properties; Impedance; Perovskites; Thermistors
• ISSN: 0272-8842; 1873-3956
• DOI: 10.1016/j.ceramint.2018.07.078

The electroceramics of the x NaNbO3 - (1-x) BiFeO3 (x = 0.15, 0.25, 0.50, 0.75, 0.85, 1) system (NBNFO) have been prepared at moderate temperatures (600 °C). Microstructural analysis from high resolution electron microscopy reveals the existence of small crystals (10–150 nm), which agglomerate leading to micro-sized crystals. All the NBNFO compounds show perovskite-type structure and a transition from rhombohedral (S.G. R3c) to orthorhombic (S. G. P21ma) mean macroscopic symmetry takes place for x = 0.50, as analyzed by refinement of XRD data. Nevertheless, at the microstructural scale, the system can be described by a compositional-segregation picture in which structural heterogeneity is present. The materials show a positive temperature coefficient of resistivity (PTCR)-thermistor behavior above room temperature for x ≥ 0.50, up to maxima temperature values of 410, 393, 405 and 463 K for x values of 0.50, 0.75, 0.85 and 1, respectively. Above these temperatures, in the NTCR temperatures-region, conduction and polarization phenomena are analyzed from impedance and modulus formalisms in terms of two mechanisms implying different charge carriers: polaron hopping and/or diffusion of cation and oxygen vacancies. The activation energy values, Ea, corresponding to these processes are of about 0.50 and 1 eV, respectively. Finally, in the dielectric response displayed by the NBNFO ceramics, the development of a FE contribution as x increases is detected, which can be interpreted considering microstructural features.