Wang, Shuai (2019)
Phase-Field Modeling of Relaxor Ferroelectrics and Related Composites.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In the 1950s, relaxor ferroelectrics or relaxors were discovered and have lately received a renewed interest in condensed matter physics. Despite the fact that the physical interpretation of the relaxors remains the subject of controversy, their extraordinary electromechanical properties and the resultant potential applications in industries have motivated a vast amount of theoretical studies on different aspects.

In this thesis, based on the random field theory, an electromechanically fully coupled phase-field model is proposed to simulate the peculiar behavior of the relaxor ferroelectrics. The model introduces a quenched local random field to characterize the effect of the chemical disorder. By treating the spontaneous polarization as an independent order parameter and the random field as an internal microforce, a thermodynamic analysis is performed. The deduced nonlinear constitutive and evolution equations are further discretized by the finite element method. Numerical examples show that the model can reproduce typical relaxor features, such as the miniaturization of domain size, the reduction of remanent polarization, and the enhancement of large-signal piezoresponse. The influence of the random field strength on the domain structure and the hysteresis loops is also revealed and validated with the related experimental results.

Subsequently, the phase-field model of relaxors, in combination with the conventional ferroelectric model is applied to analyze the large-signal piezoresponse for relaxor-based composites. More specifically, a series of simulations are presented for the relaxor/ferroelectric layer composites with different types of interfaces. The results confirm that the lateral strain coupling, in addition to the polarization coupling, contributes considerably to the large-signal piezoelectric coefficient. The lateral strain mismatch lowers the remanent strain in the ferroelectric layer and thus increases the macroscopic piezoelectric response. It is worth to be highlighted that the composition ratio of the relaxor constituent is optimized for different electric loadings. The composites with higher relaxor content are inclined to obtain higher large-signal piezoresponse with the increase of the applied electric field. These results can be referred in the future design of high-performance relaxor-based composites.

Finally, the core-shell structures in relaxors, as well as the associated microscale features and mechanisms are explored by the combined ferroelectric-relaxor-flexoelectric phase-field simulations. For BNT-25ST at room temperature, it is found that the increased electric potential beneath the core is responsible for the in-plane domain evolution. The resultant field-induced domains at the coherent core-shell interface play an important role in enhancing the polarization in the non-polar shell region and thus promoting the giant strain.Moreover, at an extreme temperature of 800 degree Celsius, the domain-like nanoregions found in BNT-25ST core-shell nanoparticle are attributed to the flexoelectric effect, where the strain gradient is believed to originate from the Vegard effect. This hypothesis is verified by the comparison between the simulation and experimental results on the electric field mediated redistribution of the polarization.

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