The dissertation is devoted to the establishment of the influence of size effects on the dielectric, pyroelectric (PE) and electrocaloric (EC) properties of spherical ferroelectric (FE) nanoparticles in micro- and nanoelectronics devices, such as PE sensors and solid-state coolers.
FE materials are essential elements of PE sensors and EC coolers, which are relevant for existing and innovative solutions in the field of safety and energy. However, all existing FEs require significant improvements in their performance to meet modern requirements for advanced multifunction devices. The study of nanosized FEs is promising in terms of the ability to control the properties of the composite by selecting the size of nanoparticles in order to achieve maximum electrosaloric response and PE transformation
Using the Landau-Ginzburg-Devonshire theory in the effective medium approximation, typical dependences of PE and EC transformation parameters on the external electric field, temperature and radius of spherical monodomain FE nanoparticles of the core-shell type are calculated analytically. The considered physical model corresponds to the nanocomposites with low concentration of FE nanoparticles. Within the analytical model, it was found out how the change in nanoparticle size affects the temperature and behavior of the parameters of PE and EC conversion in an electric field on the example of BaTiO3 nanoparticles coated with a semiconductor shell and placed in a dielectric polymer.
It is shown that by changing the particle size it is possible to obtain the maxima of the PE coefficient and changes in the EC temperature, as well as to change their width, magnitude and sign. The revealed size effect opens the possibility of controlling the PE and EC properties of FE nanocomposites, which can be important for their advanced applications in energy converters and cooling systems.
The influence of an ensemble of non-interacting spherical single-domain FE nanoparticles of the "core-shell" type of different sizes, embedded into the dielectric matrix, is considered. The size distribution function of these nanoparticles is chosen in the form of Gaussian distribution truncated from the minimum to the maximum radius. For such nanocomposites, the dependences of electric polarization, changes in EC temperature, PE and EC coefficients and dielectric permittivity on the external electric field and temperature. The dependence of the listed characteristics on the parameters of the particle size distribution function is analyzed.
The dependences of PE quality factors on the average radius of nanoparticles in the composite are calculated and analyzed. The dependences confirm the presence of a size-induced phase transition in nanoparticles. The obtained results open a new possibility of controlling PE and EC parameters of FE nanocomposites through nanoparticle size distribution parameters.
The influence of elastic anisotropy, electrostriction, flexoelectricity and mismatch strain on the morphology of the domain structure in spherical FE nanoparticles of the "core-shell" type has been studied. Finite element modeling (FEM) has been performed for multiaxial FE nuclei of nanoparticles coated with an elastically-isotropic soft or elastically-anisotropic rigid paraelectric shell with mismatch strains. The latter are induced by the difference between the constant lattices of the core and the shell.
We also studied the influence of the radius of the nucleus on the temperature behavior of the morphology of the domain structure, the polarization value and the phase transition temperature, and obtained approximate analytical expressions for analyzing the influence of elastic properties of the shell and mismatch strain on the phase diagrams. The phase diagram of a core coated with an elastically-isotropic soft shell shows a relatively small but noticeable increase in the temperature of the paraelectric-ferroelectric phase transition caused by flexoelectric coupling, while the phase diagram for a core covered with an elastically-anisotropic rigid shell shows a relatively small changes in the transition temperature. The influence of the flexoelectric effect is negligible for rigid shells.
Obtained analytical results allow to select the optimal parameters to achieve high values of PE coefficient or EC temperature change of the ensemble of non-interacting core-shell nanoparticles, which is important for energy converters and cooling systems. FE core-shell nanoparticles, the polarization of which is located in a vortex-like structure with different types of Bloch points and / or dipolar nuclei, are promising candidates for nanosized field-effect transistors and logic blocks.
