Fabrication And Physical Properties Of Self-poled Ceramics In Na₂O-Bi₂O₃-TiO₂System

Our group originally found a special nonferroelectric piezoelectric polycrystalline material that contains macroscale interface in recent research. We tentatively described these composite ceramics as self-poled ceramics because they have permanent, macroscopic polarization and exhibit strong nonferroelectric piezoelectricity without undergoing an electrical poling process. Appearance of self-poled ceramics breaks our traditional understanding of piezoelectric ceramics, therefore, it is worth further exploring the origin of the special piezoelectricity on aspect of theoretical research. In addition, self-poled ceramics, whose piezoelectricity is not limited to Curie temperature, possess potentiality to apply in high-temperature environment. Given the relevant experimental data of self-poled ceramics is limited, it is necessary to study systematically to explore the physical mechanisms and further tap the potential value of these new polar materials.In this study, we focused on fabrication and properties of self-poled ceramics in the Na2O-Bi2O3-TiO2system.1. we investigated a new system of bismuth layer-structured self-poled ceramics using the nominal composition formulas:Nao.5Bi45Ti4O15+xNa2CO3(x=0.2,0.25,0.33,0.5)、Na0.5Bi4.5Ti4O15+xBi2O3(x=0.5,1,2.25,2.5,3,4,5,6)、 Na0.5Bi4.5Ti4O15+xTiO2(x=1,4,10,20)、Na0.5Bi4.5Ti4O15+2.25Bi2O3+xNa2CO3(x=0.15,0.25,0.35,0.5)�'�Na0.5Bi4.5Ti4O15+2.25Bi2O3+xTi02(x=0.5,1,2,3). In particular, we focused on the effect of composition on the structure and piezoelectricity of these composite ceramics. It was found that adding an appropriate amount of excess Bi2O3to Na0.5Bi4.5TiO15, self-poled ceramics could be synthesized, whereas only adding excess Na2CO3or excess TiO2to Na0.5Bi4.5TiO15did not have this effect. The obtained Na0.5Bi4.5O15-based self-poled ceramics all contain Na0.5Bi4.5Ti4O15and Bi12TiO20phases, the latter of which seems to be a more critical composition because it did not appear in the nonpolar ceramics. Raman spectroscopy revealed that the local self polarization of the Na0.5Bi4.5Ti4O15-based polar ceramics should originate from the partial alignment of distorted BiOs polyhedra and the distorted TiO6octahedra related to Bi1TiO20and Na0.5Bi4.5Ti4O15, respectively. For the Na0.5Bi4.5Ti4O15+2.25Bi2O3self-poled ceramic, adding additional Na2CO3could further enhance the piezoelectric activity, and a maximum d33piezoelectric-strain-coefficient value of approximately8pC/N was obtained. The higher content of Bi12TiO20and the relatively high density may partially explain the higher d33value in this system. Because the Na0.5Bi4.5Ti4O15-based self-poled ceramics possess depoling temperatures higher than700℃, they are thought to have potential piezoelectric applications in the high-temperature region.2. The existence range of self-poled ceramics in the Na2O-Bi2O3-TiO2system was identified through experiments, located roughly in the area with Na2O content from2%to10%, Bi2O3content from30%to70%, TiO2content from30%to55%. However, when Bi2O3content in the raw materials exceed50%, ceramics were easy to volatilize and exhibited high dielectric losses, which affecting their piezoelectric applications. By XRD analysis, it was found that existence of Bi12TiO20is necessary condition for possessing self polarization in the Na2O-Bi2O3-TiO2system. Most of self-poled ceramics in the experiment were composite ceramics containing Na0.5Bi0.5TiO3and Bi12TiO20phase, Na0.5Bi4.5TiO15and Bi12TiO20phase, or Na0.5Bi0.5TiO3, Nao.sBi4.5Ti4O15and Bi12TiO20phase.3. A series of ceramic samples with the formulation of Na0.5Bi4.5Ti2O11were fabricated by ordinary sintering technique and gradient temperature heat treatment technique separately. By ordinary sintering technique, samples with sintering temperature varied between820and920℃possessed self polarization, while samples with sintering temperature exceeded940℃did not have this effect, and a maximum d33value of6pC/N was found in samples sintered under870℃. Enhanced piezoelectricity was obtained in the samples fabricated by gradient temperature heat treatment technique, possessing more uniform d33value distribution and a maximum d33value of11pC/N. By XRD analysis, Bi12TiO20phase were found in all self-poled ceramics and the self polarization disappeared with disappearance of Bi12TiO20; By combining Raman spectroscopy and x-ray photoelectron spectroscopy techniques, we have identified existence of amorphous phased and local polarization as distorted BiO5polyhedra and TiO6octahedra, and we proposed the macroscopic polarization originated from the partial alignment of distorted units located within the grain boundary amorphous phases. There was no big difference in XRD, Raman and XPS spectra between samples fabricated by ordinary sintering technique and that fabricated by gradient temperature heat treatment technique, and no obvious preferential orientation were found in samples fabricated by gradient temperature heat treatment technique. Temperature gradient may be induction factor of aligment of distorted units, and the enhanced piezoelectricity possibly caused by the enhanced alignment or increase of Bi12TiO20content.