Doctoral dissertation
Na0.5Bi0.5TiO3-based materials with mechanical-stress-dependent dielectric properties
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Abstract
The stress dependence of the electrical permittivity offers a convenient way of measuring pressure. This effect is suitable for static and quasi-static stress applications, where the conventional method based on the piezoelectric effect becomes inadequate. Although the principle of applying this effect for pressure sensing has been known for almost thirty years, the literature reports on the stress dependence of the permittivity are scarce. The change of the permittivity under an applied pressure is connected with the converse electrostrictive effect. This, in principle, linear effect, is of practical use in materials with a high permittivity and the associated pressure dependence. Therefore, it is of interest to study the stress dependence of high-permittivity materials like ferroelectrics and relaxor ferroelectrics.
This dissertation presents a systematic study of the characteristic features of the uniaxial stress dependence of the permittivity in lead-free Na0.5Bi0.5TiO3-based materials that influence the applicability of the materials for pressure sensing. In the study, the properties of the relaxor ferroelectric Na0.5Bi0.5TiO3 were modified by tailoring the samples’ composition using the incipient ferroelectrics NaTaO3 and KTaO3, and a ferroelectric, K0.5Bi0.5TiO3.
In the first part of the work, the mechanisms of the solid-state reaction synthesis of the materials from the Na0.5Bi0.5TiO3–NaTaO3, Na0.5Bi0.5TiO3–KTaO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems were investigated. It was found that in all three systems solid solutions were formed across the whole concentration range. However, in systems that contain potassium oxide, volatilization was observed and small quantities of potassium poly-titanate secondary phases were formed. The electrical properties of the prepared samples were measured and related to their structural characteristics. With the addition of NaTaO3 and KTaO3, shifting of the Na0.5Bi0.5TiO3 phase transitions toward lower temperatures and broadening of the dielectric maximum were observed. These systems are characterized by a strong dielectric dispersion, indicating relaxor properties. Materials from the Na0.5Bi0.5TiO3–NaTaO3 system were found to be ferroelastic as well, while the materials from the Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 system showed ferroelectric properties at room temperature.
The uniaxial stress dependence of the permittivity was measured in materials with different polar and elastic order. The highest influence of the stress was observed in samples with a morphotropic phase composition and samples with relaxor properties. These compounds also showed the highest values of the permittivity. Materials in the ferroelectric and/or ferroelastic phase showed strong dynamic and irreversible changes of the permittivity on the application of stress and release. Such behaviour was connected with the extrinsic contributions of the macroscopic domain structure existing in these materials. The reversibility of the permittivity under cycling the uniaxial pressure was observed in the relaxor compositions of the Na0.5Bi0.5TiO3–KTaO3 system. The reversibility was related to the relaxor structure of the material, in which the polar nanoregions are dispersed in the cubic matrix and can re-orient back into the equilibrium position after the removal of the stress. Among the investigated systems, the most applicable properties were found for the compositions with the addition of 10 to 30 mol% of KTaO3.
A comprehensive examination of the behaviour of the permittivity in Na0.5Bi0.5TiO3-based materials under an applied uniaxial pressure has been carried out. It was found that materials with a macroscopic domain structure are less appropriate for pressure-sensing applications, and relaxors are preferred in order to use the converse electrostrictive effect for pressure sensing.