Role of manufacturing process on structure and properties of NBT-based solid solutions (2020.-2022.)

Project leader Marija Dunce

Agreement No 1.1.1.2/16/I/001

Research application No 1.1.1.2/VIAA/3/19/558

The aim of the project is to study influence of the producing process on structure and properties in pure Na0.5Bi0.5TiO3 (NBT) and in NBT-based solid solutions, as a result optimizing producing process and improving properties of these materials, using innovative producing technologies. In its framework, it is planned to study influence of producing parameter on pure NBT considering stoichiometric and non-stoichiometric NBT, doped NBT and NBT-BT solid solutions, as well as evaluate the possibility to obtain pure and modified NBT with reduced porosity. The studied materials will be prepared, using different technologies, including innovative solutions. The research will  include studies of structure, microstructure, chemical composition and homogeneity, domain structure, dielectric and other properties of these materials, using such modern material characterization methods as SEM, EDX, XRD, PFM, dielectric spectroscopy and other. The obtained results will show, how to optimize physical properties for potential applications, by varying the producing process and composition of the material, and will allow improvement of industrial manufacturing technologies.

Total duration of the project is 36 months, the total costs are 133,805.88 EUR.

 

Project Progress

30.12.2020

Studies of NBT composition family, produced by the traditional ceramics sintering method with different sintering temperatures, are finalized, summarized and interpreted. Based on the quantitative and qualitative analysis the matrix grains, pores and inclusions of the obtained NBT ceramics, significant conclusions are done concerning the processes occurring during production of these compositions. By means of EDX analysis, there is stated that the predicted unintentional non-stoichiometry does not exists in the matrix grains of NBT even in the case of high sintering temperatures. There is shown that more intensive volatilization of Na and Bi, which obviously occurs during production of NBT, most probably influences formation of pores and inclusions with the basic composition of the material remaining unchanged. It is also inferred that the optimal sintering temperature of NBT at this production method is 1130-1160ºC, when the maximal density – 98% of the theoretical – is reached. At lower sintering temperatures, densification of the ceramics is incomplete, whereas, at higher temperatures, expressed macroscopic pores appear and abnormal grain growth begins.

The obtained results are summarized and described in a manuscript submitted for the Journal of the European Ceramic Society and presented in an international conference FM&NT-2020 (obtaining the Best Poster Award) and in a seminar in the framework of the doctoral school “Functional materials and nanotechnologies”. There is submitted and accepted an abstract for the conference PIEZO2021. Thus, the first milestone of the project is reached.

Besides, there is published one manuscript related to the topic of the project, devoted to luminescence in NBT ceramics doped with Eu, partly basing on the results obtained in the framework of the project, which was not previously planned:

M. Dunce, G. Krieke, E. Birks, L. Bikse, M. Antonova, and A. Sarakovskis. The role of structural disorder on luminescence of Eu-doped Na0.5Bi0.5TiO3. J. Appl. Phys. 128, 244104 (2020); doi.org/10.1063/5.0031305

There is started work on the second work pachage of the project – devoted to NBT compositions with and without Bi overstoichiometry. There are prepared samples for two NBT composition families with Bi overstoichiometry in the range from 0% to 6%: 1) adding overstoichiometric Bi at initial weighing of constituents; 2) adding overstoichiometric Bi after the second annealing – exactly before the sintering. SEM micrographs are obtained for all non-etched samples from the NBT composition family where overstoichiometric Bi is added at the initial weighing of the constituents. Based on the obtained micrographs, concentration and average size of the inclusions is determined as a function of Bi overstoichiometry. Quantitative analysis of porosity is started. For a large part of the samples, XRD, polarization and dielectric permittivity measurements are started.


30.09.2020

Studies of NBT composition family, produced by the traditional ceramics sintering method with different sintering temperatures, are continued. For all compositions, XRD measurements are performed and the Rietveld analysis is done. There is established that all of the studied compositions can by characterized by pseudocubic structure with a constant unit cell parameter – which practically does not change upon changing the sintering temperature.

There are finalized the microstructure studies, considering polished, etched surface of all studied compositions. Besides qualitative comparison of microstructure of the samples, detailed quantitative analysis of the obtained micrographs is done. Grain size distribution, average grain size, porosity and inclusion concentration (inhomogeneity) as functions of the sample sintering temperatures are determined. It is confirmed that the average grain size increases and the grain size distribution broadens upon increasing of the sintering temperature. It is established that porosity increases at high sintering temperatures and it occurs due to macroscopic pores, while contribution from the three-grain-boundary pores is comparatively small. Evaluation of porosity indicates that, in case of sintering temperatures in a range from 1100 to 1200oC, high quality, dense ceramics is obtained with density above 97%, which is confirmed by density measurements using the Archimedes’ method.

Special attention is paid to the local analysis of composition of the inclusions, using EDX, for the NBT samples, which were sintered at different temperatures. It is established that, at low sintering temperatures, inclusions with chemical composition Bi0.5Na0.5Ti3O7 appear. Whereas, upon increasing of the sintering temperature, the character of the inclusions changes at approximately 1130oC – TiO2 inclusions appear and grow in size in the whole high temperature range of sintering temperatures till 1240oC.

Such a dependence of the pore concentration and inclusion character on the sintering temperature allows developing recommendations for improvement of ceramics producing technology.


30.06.2020

There is supplemented the family of pure NBT solid solutions, which are obtained using the traditional ceramics synthesis method, varying the sintering temperature in the range from 1020oC to 1240oC.

For most samples from the composition family, there are done x-ray diffraction measurements, structure of the studied materials and corresponding unit cell parameters are determined. At the moment, the obtained results indicate that all compositions have cubic structure, the unit cell parameter of which increases upon increasing of the sintering temperature.

For most of the samples, SEM micrographs are obtained. The micrographs are mutually compared both qualitatively and quantitatively, analyzing and comparing grain sizes, concentration of inclusions and porosity. This work is in progress. However, the already obtained results indicate, for example, that the average grain size increases upon increasing of the sintering temperature and the grain size distribution becomes wider, whereas the porosity reaches minimal values at the sintering temperature around 1160oC.

Using EDX, concentrations of constituents of the basic substance and inclusions are locally analyzed for the obtained pure NBT samples and compared for different compositions in the family (the work is in progress). The already obtained results indicate that concentrations of Bi and other chemical elements do not significantly change due to changes of the sintering temperature, which could be an important conclusion about the valence compensation mechanisms for the case of nonstoichiometric compositions.

For most of the studied pure NBT samples there are done measurements of polarization hysteresis loops and dielectric permittivity. For examples, there is shown that remnant polarization decreases upon increasing of sinterinf temperature of the samples.


26.03.2020

In the framework of the project, there is started work on studies of manufacturing technology of pure NBT. Review of literature was done. Compositions of pure NBT were synthesized using conventional ceramics synthesis from oxides and carbonates at different sintering temperatures: 1100oC, 1130oC, 1160oC, 1180oC, 1200oC and 1220oC. For the obtained compositions, microstructure studies, using scanning electron microscopy, structural studies, using x-ray diffractometry, as well as measurements of dielectric spectroscopy and polarization hysteresis loops are started.

 

Before beginning the experimental work, the leader of the project went through training and obtained knowledge for work with x-ray diffractometer Rigaku, scanning electron microscope Phenom Pro and scanning electron microscope Tescan Lyra, as well as obtained license for work in the clean-room, after undergoing the appropriate training. The work is done in close cooperation with the Team of Chemical Synthesis of the Laboratory of Ferroelectrics.

 

On February 6, the project author participated in Latvian Radio 1 radio popular science broadcast “The known in the unknown” with a conversation of her work area in science, background and topicality of the project, as well as its aims and tasks: https://lr1.lsm.lv/lv/raksts/zinamais-nezinamaja/jauni-materiali-svina-aizstasanai.-latvijas-zinatnieku-petijumi.a126217/