Author

Yuting Peng

ORCID Identifier(s)

0000-0003-2360-5638

Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Qiming Zhang

Abstract

Nowadays, with the increasing environmental concerns, K1-xNaxNbO3 (KNN)-based materials have gained enormous interest and attention as one of the most promising lead-free piezoelectric materials because of their excellent piezoelectric performance and high Curie temperatures. There are lots of experimental efforts focusing on improving the piezoelectricity of KNN ceramics. However, it is lack of theoretical explanations for the intrinsic physical mechanism. In this dissertation, density functional theory (DFT) based first-principles calculations implemented by the Vienna ab initio simulation package (VASP) are performed to investigate the electronic structure, ferroelectric and piezoelectric properties of KNN related materials, including the pure KNbO3 (KN) and NaNbO3 (NN) crystals, KNN systems with the particular K/Na ratio as 1:1 and various Na concentrations, as well as the KN materials with variety of monodoping and codoping. The structural properties, electronic band structures and density of states are calculated for the KN and NN crystals of four phases, including cubic, tetragonal, orthorhombic and rhombohedral phase. All these KN and NN crystals show indirect bandgap except for the rhombohedral NN, which has a direct bandgap at the Γ point. Then, systematical studies are performed on the ferroelectric and piezoelectric properties of KNN systems with various Na concentrations. The introduction of Na atoms causes the distortion of lattice structure and the enhanced relaxation of Na and adjacent O atoms in orthorhombic KNN crystals because of the smaller atomic size of Na than that of K. The large internal atomic coordinate response of Na and O atoms to the macroscopic strain leads to the elastic softening and further gives rise to the enhanced intrinsic piezoelectric constants. Then, the spontaneous polarization of KNN single crystals is found to increase monotonically with Na concentrations, indicating an enhanced ferroelectricity in KNN. Moreover, the averaged shear, transverse and longitudinal piezoelectric coefficient "d" ̅_15^"*" , "d" ̅_31^"*" and "d" ̅_33^"*" of KNN piezoceramics positively correlates with Na concentrations and exhibits the best performance at x = 0.5625, which agrees well with the available experimental data. Finally, the effect of various monodoping and codoping at A-site and B-site on the structural, ferroelectric and piezoelectric properties are studied for the orthorhombic KN piezoceramics. The incorporation of foreign atoms causes the distortion of lattice structure and results in spontaneous polarizations. Results clearly show that the substitution of Na, Rb, Cs, Ca, and Bi at A-site, and the incorporation of V, Ta, Ti and Sb to substitute Nb atoms induces better piezoelectric performance. Moreover, the codoping of (Na, Sb)- and (Ba, Zr)- doped KN crystals significantly enhances the piezoelectricity compared with the pure KN and those monodoping cases. In addition, the formation energies for all these considered doping cases are calculated to predict the possible stability of each compound in experiments.

Keywords

first-principles, piezoelectricity, ferroelectricity, lead-freeceramics

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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