Sr/Ti ratio in strontium titanate ceramics: designing the microstructure, envisaging the properties


Among the lead-free titanate-based materials, strontium titanate (SrTiO3), pure or modified, is of particular importance. SrTiO3 is an incipient ferroelectric with high dielectric permittivity and low losses, which finds application in tunable microwave devices due to a dependence of the permittivity on the electric field, or in grain-boundary barrier layer capacitors, taking advantage of highly resistive grain boundaries. The grain boundary characteristics and, therefore, electrical properties of polycrystalline SrTiO3 are dependent on the defect chemistry and microstructure, which are thus fundamental in establishing the utility and value of the material. In their turn, such factors as Sr/Ti ratio, second phases or sintering conditions strongly affect the microstructure and alter the defect chemistry. Therefore, if properly understood these factors may be intentionally used to tailor the electrical response of electroceramics. Starting from discovery of the grain growth anomaly in Ti-rich strontium titanate ceramics, impedance spectroscopy became a very useful tool in the investigation of this phenomenon, in addition to traditional defect chemistry and grain growth analysis. Three discontinuities in the Arrhenius-type temperature dependence of grain growth, which take place at sintering temperatures around 1500, 1550 and 1605°C, are accompanied by discontinuities in the dependence of the activation energy for grain boundary conductivity, which can be eliminated by annealing in oxygen. These results will be discussed briefly in the introductory section. This section of the chapter will be completed by a summary of an impedance spectroscopy study of SrTiO3 ceramics with different Sr/Ti ratios (from 0.995 to 1.02). Bulk and grain boundary conductivity, activation energy and capacitance reveal strong systematic variations with the degree of nonstoichiometry. In the experimental results section of the chapter the effect of nonstoichiometry on the grain growth of SrTiO3 ceramics is discussed. The grain boundary mobility assessed from grain growth data at 1450°C for Ti-rich, stoichiometric and Sr-rich compositions increases with decrease of the Sr/Ti ratio. An exponential increase of the grain boundary mobility with the grain size is also found independently of the Sr/Ti ratio, being more pronounced for the Ti-rich composition.


Amaral, L; Tkach, A; Vilarinho, PM; Senos, AMR

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