Synthesis, densification and structural characterization of high Tc La₂Ti₂O₇ ceramics
摘要
Next-generation systems such as aircraft engines, nuclear power plants, aerospace vehicles etc. are expected to work at temperatures exceeding 500 °C for at least 10,000 h. Considering the criticality of these applications, the structural health monitoring of these systems is essential. Piezoelectric sensors capable of performing at these temperatures can serve the purpose. As a thumb rule, the Curie temperature (Tc) of ferroelectric piezo sensors should be twice the usage temperature. As a result, ferroelectric ceramics with Tc greater than 1000 °C are needed for such applications. Perovskite layer structure (PLS) ceramics belonging to a series AnBnO3n+2 (n representing the number of BO6 octahedra) exhibit Tc in the range of 1073 to 1500 °C are significant for applications under consideration. La2Ti2O7 with Tc of about 1460 °C is the most cost-effective ceramic in the PLS family. In the present study, La2Ti2O7 was synthesized by a solid-state reaction between La2O3 and TiO2 at 1150 °C. The formation of single-phase monoclinic structure was confirmed by X-Ray diffraction (XRD) study. Subsequently, synthesized powders were compacted by uniaxial pressing. Dense polycrystalline ceramics were prepared by pressureless sintering between 1350 and 1425 °C. To the best of knowledge of authors, there is no report in the literature studying the effect of sintering temperature on the structure of ceramics. Such a study stands important from the fact that if the sintering conditions cause any reversible structural change in these ceramics, it is likely to impact their piezoelectric behaviour. Hence, sintered specimens were subjected to Raman and X-ray photoelectron spectroscopy (XPS) studies. Raman spectroscopic study was useful to study if the sintering temperatures had any effect on the modes of vibration. Wavenumbers corresponding to various vibrations remained same in all sintered samples indicating no structural change as a function of sintering temperature. To study if the sintering temperature caused any change in the oxidation state of constituent elements of La2Ti2O7, X-Ray photoelectron spectroscopy (XPS) of sintered samples was carried out. However, irrespective of sintering temperatures, La, Ti and O showed their regular oxidation states of + 3, +4 and − 2, respectively. Moreover, the binding energy for La and Ti was uniform across sintering temperatures, suggesting no structural change in these ceramics. Ferroelectric to paraelectric transition in these ceramics corresponding to orthorhombic to cubic transformation was indirectly established by high temperature differential scanning calorimetry (DSC) and dilatometric studies. Tc determined by thermal methods was above 1400 °C, indicating the possible usage temperature of about 730 °C for La2Ti2O7 ceramics. Additionally, the study is of technological significance as it opens up the avenues to use an inexpensive solid-state route to prepare dense La2Ti2O7 ceramics versus single stage and two stage spark plasma sintering, hot pressing, magnetic field assisted sintering etc.