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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Lead-free ferro- and piezoelectric K0.5Na0.5NbO3-based materials

Awarded: NOK 9.4 mill.

A piezoelectric material will generate an electrical voltage across it if you mechanically compress or bend it and on the other hand if you place the material in an electrical field it will be deformed. These kinds of materials can therefore be used as energy harvesters transforming mechanical work into electricity or as actuators (motors) if an electrical field is applied. We are in our daily life very dependent on these kinds of materials in electronics, speakers, sensors, switches, capacitors as well as ferroelectric thin-film memories. For several decades lead based ceramic materials (PZT) are used for these applications, however the lead-containing materials are to be phased out by alternative lead-free materials due to legislations to remove the toxic electronic waste from PZT. At the moment there are no lead-free piezoelectric materials that have the same performance as the lead-containing ones and we are therefore developing new lead-free materials which in the future will substitute the lead-based ones. We have prepared textured ceramic materials of KNN-based materials by a novel tape casting procedure. Characterization of the sintered, textured ceramics revealed strong texture parallel to the tape cast direction and a weaker texture normal to the tape cast plane. Synchrotron X-ray diffraction using a 2D detector confirmed the texture as described above and allowed in situ measurement of domain switching during electric field loading. Our results provide valuable information on mechanisms of improved piezoelectric performance in these textured ceramics. In addition, we use a combination of an aqueous chemical solution deposition method and texturing by using a molten salt route to prepare KNN thick films on various oriented SrTiO3 substrates. Clearly textured materials are achieved by the preparation of these films on the selected substrate. The effect of the substrate orientation and the presence of molten salt on the thin film crystallization, microstructure and piezoelectric properties have been investigated. Our results also clearly show that an epitaxial film is starting to grow from the interface between the substrate and the film. These KNN films show the best piezoelectric and dielectric properties for similar kinds of films reported. Since there are challenges with the commercialization of KNN-based materials, the project also covers the searching for new compound systems. Ceramics with compositions in the (1-x)Bi0.5K0.5TiO3 - xBiFeO3 (BKT-BFO) and Bi0.5K0.5TiO3 - Bi0.5Na0.5ZrO3 (BKT-BNZ) systems has been prepared as lead-free prototype. The material system BKT-BFO has been studied with respect to its conductivity, dielectric properties and ferroelastic properties. A defect model has been developed based on measurements of the DC conductivity, confirming that the dominating charge carriers are electrons and holes, and the presence of oxygen vacancies. The presence of A-site vacancies was assumed as part of the model. The dielectric properties of BKT-BFO materials with Ti-substitution for Fe have also been investigated and the conductivity is significantly decreased with increasing Ti substitution which has been one of the major obstacles for using BFO piezoelectric materials. The ferroelastic properties of these materials have been investigated during a research stay of one of the PhD candidates at Darmstadt Technical University, Germany. The results show that a comprehensive understanding of the point defect chemistry is necessary in the further development of lead free piezoceramics based on BFO. In the BKT-BNZ system it has been established that only a small substitution of BNZ into BKT, there is a significant change of crystal structure from tetragonal towards cubic or pseudo-cubic. This is promising with respect to finding one or several morphotropic phase boundaries as BNZ-substitution is increased towards pure BNZ which might give very interesting high performance piezoelectric compositions. BaTiO3-based piezoelectrics have recently also received much attention due to large piezoelectric properties when doped with Ca and Zr, this doping does however reduce the thermal stability. In this work, we have used grain texture to improve the piezoelectric response. Strongly textured lead-free Ba0.92Ca0.08TiO3 piezoelectric ceramics were fabricated by tape casting and templated grain growth. Enhanced piezoelectric performance was demonstrated for ceramics with <100> texture, in line with the predictions based on reported piezoelectric coefficients of tetragonal BaTiO3.

Application of polycrystalline piezo- and ferroelectric materials requires fundamental understanding of the relationship between grain-/domainstructure and the functional properties. The present proposal aims to extend the applicants research groups compe tence in ceramics engineering by domain texturing and enhancement of piezoelectricity and ferroelectricity of the most promising lead-free family of materials, K0.5Na0.5NbO3-based ceramics. The development of these materials both in the form of bulk mate rials and as well as thin films on a substrate will enable the substitution of the current lead-based materials in electronic and electromechanical applications with lead-free and hence environmental friendly alternatives. The research activities are both fundamental and technological in nature and will result in new understanding of the fabrication methods, the piezo- and ferroelectric properties and thereby utilize the use of these materials in the near future. The project will educate two PhD candidate s and one post doc and 10 publications in international peer review journals will be published.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek