First Principles Study On The Crystal Structure And Properties Of Lead-free Potassium Sodium Niobate Ferroelectric Ceramic

Piezoelectric ceramics are widely used in military and civil products because of their excellent electromechanical conversion performance.It is of great strategic significance to the national economic development and national defense construction all over the world.Lead-based piezoelectric ceramics are widely used because of their excellent performance,low price and easy preparation.However,due to the toxicity of heavy metal Pb and its damage to the environment and human beings in the process of production and waste treatment,many countries have successively issued regulations prohibiting or restricting the use of lead.Potassium sodium niobate（KNN）lead-free piezoelectric ceramics have become candidate materials to replace lead-based ceramics because of their high Curie temperature and adjustable excellent electrical properties.It is one of the research hotspots in the field of piezoelectric ceramics.The first-principles theory is based on the interaction principle and basic motion law between the nucleus and the electrons outside the nucleus.After a series of approximate treatments,the correlation between the microstructure and macro properties of the material can be obtained,which provides certain theoretical guidance for the preparation and doping modification of ceramic materials.Compared with other perovskite lead-free piezoelectric ceramics,the first-principles calculation of potassium sodium niobate（KNN）is less mentioned.Therefore,this paper mainly takes KNN as the research object.Using Materials Studio software,combined with density functional theory and ultra-soft pseudopotential method,the energy band structure,partial density of states and phonon spectrum of KNN ceramics are calculated.The relationship between the change of lattice constant of KNN ceramics and associated electrical as well as mechanical properties are studied from different point of view,that is,the reasons and methods of ceramic phase structure evolution and performance improvement are systematically studied using calculation and experiment.It lays a foundation for the research on doping modification of KNN ceramics.The specific research contents are as follows:1.Firstly,the band structure and partial density of states of lead-free system KNN and lead-containing system PZT（lead zirconate titanate）,their ferroelectrics and antiferroelectrics（NN,KN,PZ,PT）are studied by first principles calculation.The results show that the band structures of KNN and PZT are similar to their corresponding antiferroelectrics,and have smaller band gaps than their antiferroelectrics.From the point of view of the partial wave density of States,for KNN,the hybridization is mainly caused by Nb-O atoms,and the K and Na atoms at a site hardly participate in the hybridization;In PZT,Zr and Ti at B site are relatively weak hybrid with O atom,and Pb at a site is involved in the hybrid.By comparing the electron density diagrams of KNN and PZT,it is found that the strong hybridization of Pb at the A site in PZT is the key to the formation of the tetragonal phase of PZT which is different from the tetragonal phase of KNN.By analyzing the relationship between the change of lattice constants,atomic hybridization and electronegativity of the two ceramics,the reason why KNN and PZT with the same perovskite structure have different phase structures is explained.2.It is easy to form oxygen vacancies in ceramics by sintering in different atmospheres and doping of non equivalent elements.In order to deeply study the effect of oxygen vacancy on perovskite ceramics,BT ceramics with perovskite structure were taken as the research object,and different sintering methods were used to explore the effect of oxygen vacancy defects on the structure and properties of BT ceramics.The results show that the lattice constant of BT is greatly changed and the crystal grows directionally due to the introduction of oxygen vacancy.At the same time,the dielectric constant and loss increase obviously.By analyzing the energy band structure and equivalent resistance,it is found that oxygen vacancy makes BT tend to metal state,showing better conductivity.According to the phonon calculation,the structure of BT with oxygen vacancy becomes more stable than that of BT.It is found that all of them are brittle materials and meet the mechanical stability standard.As the anti deformation ability of BT with oxygen vacancy in z-axis direction increases,the piezoelectric properties of ceramics decrease.Therefore,oxygen vacancy leads to the decrease of electrical properties and the improvement of mechanical properties of ceramics.3.On the basis of the above research,four doped elements Li,Ag,Sb,Ta with different coupling ability were selected and equivalently doped into KNN ceramic lattice by solid-state method to explore the influence of different coupling ability of A-site or B-site doped atoms on the structure and properties of KNN crystal.The results show that the ion coupling ability is not determined by the position where the doped ions enter the lattice（i.e.A and B sites）,but by the inherent electronic orbital configuration of the atom,which will have different effects on Nb O₆.It is concluded that the elements with weak coupling ability（Li and Sb）vibrate Nb O₆more violently,which makes the temperature of O-T phase transition lower and easier to occur.The elements with strong coupling ability（Ag and Ta）make the distortion of Nb O₆more serious,increase the displacement of the central atom Nb in Nb O₆,thus improving the spontaneous polarization intensity,and then improving the piezoelectric properties of ceramics.4.As a lead-free ferroelectric energy storage ceramic system,KNN ceramics have attracted more and more attention in recent years because of their small grain size and strong puncture resistance.In order to study the component design principle of KNN energy storage ceramics,based on the existing statistical results of KNN energy storage ceramics,two groups of KNN based energy storage ceramic systems are designed by combining the first principle calculation,and the KNN based energy storage ceramics with relaxor ferroelectric phase with small delay are obtained.The feasibility of material component design from the perspective of atomic track coupling strength is verified,which provides a valuable reference for the research of KNN energy storage ceramics.