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Projekt Druckansicht

Erhöhte Gitterpolarisierbarkeit durch Abschreckung in bleifreien Ferroelektrika

Antragsteller Professor Dr. Hans-Joachim Kleebe, seit 1/2022 (†)
Fachliche Zuordnung Herstellung und Eigenschaften von Funktionsmaterialien
Förderung Förderung von 2019 bis 2023
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 414311761
 
Erstellungsjahr 2023

Zusammenfassung der Projektergebnisse

Within the scope of this project, the structural and electrical characteristics of quenched Bi-based ceramics were studied. It was found that quenching such lead-free materials can act as a viable processing technique in order to tailor their functional properties. The main focus was on understanding the structural mechanisms that govern the material properties upon quenching and on establishing structure-microstructure-processing-property correlations. A detailed and comparative study on the NBT-BT solid solution in the furnace cooled and quenched states was performed, while likewise quenching-induced effects on the piezoelectric properties and structure of the BF-BT and BF-BT-NBT systems were explored. NBT- and BF-based ceramics were prepared via solid state synthesis. Quenching in air did not lead to any degradation of the mechanical properties. Quenched NBT-BT compositions exhibit an increase in TF-R by 23-44 °C, thus increasing the temperature-dependent stability of the dielectric and piezoelectric properties for the possible range of applications. A two-zone quenching process also proved to be applicable, in order to further reduce residual thermal stresses. The hypothesized increase in ferroelectric order upon quenching NBT-BT reflects in altered piezoelectric properties, such as a higher strain, coercive field, as well as maximum and remanent polarization. While the important aspect of quenching NBT-BT is the significant increase in the depolarization temperature, quenching of BF-based ceramics is explored under the aspect of improving the piezoelectric properties. While changes for the quenched 67BF-33BT composition are rather negligible, the ternary composition of 67BF-33BT+5NBT exhibits a significant improvement of piezoelectric properties upon quenching, such as an increase in d33 and in total strain. High resolution powder and synchrotron X-ray diffraction depicted the decrease of the pseudocubic phase fraction and increase in the non-cubic phases with more pronounced tetragonal/rhombohedral distortions upon quenching both NBT- and BF-based ceramics. In the NBT-BT system, global structural changes are most pronounced for the relaxor NBT-6BT. For NBT-9BT, quenching induces a spontaneous relaxor-to-ferroelectric transformation. The strengthening of the ferroelectric order is also corroborated by the delayed (NBT-6BT Q) or absent (NBT-9BT Q) relaxor-to-ferroelectric transition upon poling, as evidenced in the in situ X-ray synchrotron diffraction study. Measurements of the Young´s modulus and subsequent calculations showed an increase of the PNR volume fraction at elevated temperature (300 °C), which is most pronounced for the quenched NBT-6BT composition, possibly facilitating the formation of a more ferroelectric order at room temperature. The presence of a long-range lamellar domain contrast upon quenching could be verified by TEM investigations. Opposed to NBT-6BT FC, the quenched sample exhibits a significant increase in both rhombohedral (R3c) and tetragonal (P4mm) lamellar domains. These domains, which are not only ferroelectric, but also ferroelastic, can accommodate the enhanced lattice strain arising upon quenching. The likewise reduction in volume fraction of the relaxor P4bm phase further accounts for a more ferroelectric behavior upon quenching. In situ temperature-dependent TEM microscopy on quenched NBT-6BT revealed a higher temperature-dependent stability of tetragonal P4mm domains in unpoled and also poled samples. The later onset of tetragonal domain disintegration shifts the stability of piezoelectric properties to higher temperatures and thus accounts for the increase in Td and TF-R upon quenching. (S)TEM investigations on NBT-BT illustrated the complexity of the relaxor ferroelectric nanostructure and revealed a hierarchical configuration of P4bm and R3c nanodomains embedded within the long-range polar structure of lamellar P4mm domains. Quenching BF-BT(-NBT) resulted in slightly more pronounced lamellar rhombohedral domains, coexisting with a pseudocubic phase where domains are absent. The consolidated ferroelectric structure observed via TEM thus aligns with the improvement of piezoelectric properties upon quenching BF-based ceramics. Regarding underlying mechanisms, which lead to the increase in depolarization temperature and enhanced ferroelectric order of Bi-based ceramics, the off-centering characteristic of Bi3+ is believed to strongly influence the local structure upon quenching, enhancing structural deviations from the pseudocubic symmetry. The results obtained in this project can be regarded as a valuable insight for understanding the mechanisms of quenching Bi-based materials and facilitating the procedure of quenching as a tool to modify the properties of ferroelectrics.

Projektbezogene Publikationen (Auswahl)

 
 

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