Dielectric And Energy Storage Properties Of Ferroelectric Thin Films In The K-Na-Nb-O System

Functional materials are important carriers for human beings to obtain,transmit and process information.In recent years,many devices based on functional materials have played an important role in various fields such as national defense,military,aerospace,transportation,communication,and people’s daily production and life.With the progress and development of society,people begin to demand intelligent,miniaturized,lightweight,and integrated electronic devices with richer functions,better performance,smaller dimensions,lower power consumption and cost.Ferroelectric/piezoelectric materials have great development potential in the fields of Piezo-MEMS,nonvolatile storage,dielectric capacitors,tuning devices and so on.With the enhancement of people’s awareness of environmental protection,the development of lead-free ferroelectric/piezoelectric materials has become an inevitable trend.(K,Na)NbO3-based lead-free piezoelectric materials are considered as the most promising candidate materials to replace lead-based materials due to their high Curie temperature(Tc～410℃)and good piezoelectric properties.In 2004,Saito et al.reported that(K,Na)NbO3based ceramics with(00l)textured have a d33 of 416 pC/N,(K,Na)NbO3-based lead-free piezoelectric materials ushered in a research upsurge.At present,the research on(K,Na)NbO3-based lead-free materials is mainly focused on piezoelectric properties,the researches on dielectric tunability and energy storage are less.Based on the above factors,we have systematically studied the preparation and dielectric properties of potassium sodium niobate thin films of perovskite phase in the K-Na-Nb-O(KNN)system,as well as the preparation and dielectric energy storage properties of tetragonal tungsten bronze(TTB)KNN-TTB thin filmsIn this paper,using a multi-target magnetron sputtering system and using K-Na-Nb-O(KNN)based lead-free materials as the research object,perovskite and tetragonal tungsten bronze phase KNN films were deposited on(100)STO and(100)Si/SiO2 substrates to explore the structure-activity relationship between the chemical composition,crystal orientation,microstructure,and residual strain of the films and their dielectric and energy storage properties.The research contents are as follows:(1)The relationship between the phase structure and in-plane strain of perovskite KNN thin films was calculated to guide experimental design Using the modified Landau Ginsberg Devonshire(LGD)thermodynamic theory.The dielectric tunability factor λf and dielectric tunability η of tetragonal perovskite phase epitaxial ferroelectric thin film have also been derived using this thermodynamic theory.The formula can be used to predict the dielectric tunability of tetragonal perovskite phase epitaxial ferroelectric thin films.The thermodynamic calculation results show that when the in-plane compressive strain is-1.0%to-2.0%,the perovskite KNN film exhibits a tetragonal phase.It can be seen that the dielectric tunability factor λf of a tetragonal perovskite phase epitaxial ferroelectric film of a given material is determined by remnant polarization P0f and the zero field dielectric constant χ0f the film.Under the guidance of theoretical calculation results,tetragonal perovskite heteroepitaxial KNN thin films(KNN/SRO/(100)SrTiO3)with an in-plane compressive strain of-1.8%were successfully prepared by magnetron sputtering.The thin film has a large remnant polarization P0f(～21μC/cm2)and a high dielectric constant χ0f(830～860).Experimental results show that the dielectric tunability of the thin film is as high as 81%,which is highly consistent with the theoretical calculated dielectric tunability(80%).(2)High dielectric tunability perovskite KNN thin films with different crystal plane orientations were synthesized at low temperature.KNN films were grown in situ on single crystal Si substrate at 350℃ to 500℃.The ferroelectricity of KNN films grown in situ at 350℃was weak.When the temperature was above 400℃,the ferroelectricity of KNN films was significantly enhanced.The as grown perovskite KNN films at 400℃ and 450℃ were polycrystalline orthorhombic phase with mixed orientations of(00l)and(110).The as grown film grown at 500℃ is a(110)oriented film,also known as an orthorhombic crystal phase.Perovskite KNN thin films with high dielectric tunability were prepared by high-temperature rapid annealing process.The dielectric tunability of perovskite KNN films grown at 400℃,450℃,and 500℃ increased from 49.8%,65.0%,and 68.9%of as-grown films to 70.4%,77.9%,and 81.5%of annealed films,respectively.(3)High energy storage density tetragonal tungsten bronze KNN(KNN-TTB)ferroelectric thin films with complex domain structures were prepared by controlling the element ratio of the target.On the one hand,by adjusting the composition of the target material,the domain structure of KNN-TTB thin film was controlled and its electrical performance is optimized.Thin films with corresponding P-E loops were designed for different applications.On the other hand,by reducing the thickness of the buffer layer SRO,the horizontal nanorod shaped domains in KNN-TTB thin film are reduced,further improving its energy storage characteristics.The XRD results show that both KNN50(3,17)/SRO60nm/STO100 film and KNN50(7,17)/SRO60nm/STO100 film are tetragonal tungsten bronze phase.The former is a chemical ratio of 3%excess potassium element,while the latter is a chemical ratio of 7%excess potassium element.When the potassium element is excessive by 7%,the KNN-TTB film mainly grows in the form of vertical nanorods,and the film exhibits a typical ferroelectric hysteresis loop.When the potassium element is excessive by 3%,the KNN-TTB film grows in the form of horizontal nanorods perpendicular to each other in the plane,and the film exhibits a " thin and long" relaxation ferroelectric hysteresis loop.The breakdown field strength of KNN50(3,17)/SRO60nm/STO100 thin film is 5.27 MV/cm,the maximum effective energy storage density Wrec is 87 J/cm3,and the energy storage efficiency is 79.2%.By using strain engineering methods,the thickness of the buffer layer SRO was reduced to 20 nm,and the length of horizontal nanorods in the film was reduced by half.The KNN-TTB film with the best energy storage performance in the K-Na-Nb-O system was prepared,with a breakdown field strength of 5.625 MV/cm,an effective energy storage density Wrec of up to 120.6 J/cm3,and an energy storage efficiency 73.7%.The KNN-TTB thin film has excellent temperature(20℃～140℃),cycle(109 times),and frequency(500 Hz～10 kHz)stability.Within the wide temperature range of 20 ℃～400 ℃,the variation of its capacitance temperature coefficient(TCC)is within ± 15%,indicating that the film has a wide temperature application space.