The Effect Of A-site Composition Design On The Structure And Properties Of SNN Tungsten Bronze Niobate Ferroelectric Ceramics

As a derivative of the classical perovskite structure,tungsten bronze lead-free piezoelectric ceramics are the second largest category of ferroelectrics next to perovskite structure compounds.They get the advantages of large spontaneous polarization,high Curie temperature,excellent dielectric properties and low loss.They have broad application prospects in the fileds of dielectric,piezoelectric,ferroelectric,thermoelectric and nonlinear optics.The general formula of tungsten bronze structure compound is(A1)4(A2)2C4(B1)2(B2)8O30.Metal cations with different valence and radius are selectively filled in A1,A2,C,B1,and B2 sites,which can induce a series of peculiar dielectric,pyroelectric,piezoelectric,and optical properties.In this paper,Aiming at the problem that it is difficult to obtain pure phase in the SNN system,this work starts from the binary solid solution phase diagram of SrNb2O6-NaNbO3,explores the existence and content of the second phase and change of composition and temperature,determines the existence range of the pure phase;at the same time,combined with the research results of the solid solution system,we selected the component with the second phase on the basis of SNN,and introduced the alkali metal Li+with smaller radius to replace the Na+ with larger radius,in order to obtain the effect of A-site composition design on the structure and performance of SNN tungsten bronze niobate ceramics,enrich the basic and theoretical research of the system.The main results are as follows:1.According to SrNb2O6-NaNbO3 binary solid solution phase diagram,a series of SNN based ceramics were synthesized by conventional mixed-oxide method.The results show that when the mole percentage of NaNbO3 is 20.0%-24.0%,SrNb2O6-NaNbO3 can be solid-dissolved at higher temperature to obtain a pure phase;when the mole percentage of NaNbO3 is 25.0%-27.5%,SrNb2O6-NaNbO3 can be dissolved at 1200-1340? to obtain a stable pure phase;When the mole percentage of NaNbO3 increases to 30.0%,SrNb2O6-NaNbO3 can be dissolved at 1220 ?,but when the temperature rises to 1340?,the second phase will be formed;when the mole percentage of NaNbO3 is 33.3%-35.0%,it can not be dissolved in the temperature range of 1200-1340?.When the mole percentage of NaNbO3 is 25.0%-30.0%,the dielectric and ferroelectric properties of the obtained ceramic samples are relatively better.2.Sr2Na1-xLixNb5O15(x=0.0?0.1?0.2?0.3?0.4)ceramic samples were prepared by two-step sintering method using SrNb2O6,NaNbO3 and LiNbO3 as raw materials.The study found that with the increases of Li+content,the original second phase NaNbO3 decreases until it disappears,which indicates that the introduction of Li+can inhibit the formation of the second phase,but also changes the phase structure of the ceramics.At x=0.4,the pure orthorhombic tungsten bronze structure strontium sodium niobate lithium phase(mm2)is obtained;SEM images show that the introduction of Li+deteriorates the compactness of the ceramics and increases the pores between grains,which leads to the decrease of the electrical properties of the ceramic.In general,Sr2Na1-xLixNb5O15(x=0.0?0.1?0.2?0.3?0.4)ceramic samples have good dielectric and ferroelectric properties at x=0.1,Tc=317?,?m=1760,Pr=7.12 ?C/cm2,Ec=15.9 kV/cm.3.In order to further solve the problem of the difficult densification of the Sr2Na1-xLixNb5O15 system,we optimized the preparation process of the Sr2Na1-xLixNb5O15 system by changing the starting materials,and studied the sintering behavior,microstructure,dielectric and ferroelectric properties of Sr2Na1-xLixNb5O15 ceramics prepared with SrCO3,Na2CO3,Li2CO3 and Nb2O5 as raw materials and prepared with SrNb2O6,NaNbO3 and LiNbO3 as raw materials under the same sintering conditions.The results show that when x=0.4,the pure phase of strontium sodium niobate lithium(mm2)is obtained,which is consistent with the research results using niobate as the raw material;the SEM image shows that its microstructure is compact,there is no obvious cracks.In summary,Sr2Na1-xLixNb5O15 ceramic samples prepared from carbonate have excellent dielectric and ferroelectric properties at x=0.1,Tc=332?,?m=2129,Pr=7.71 ?C/cm2,Ec-18.96 kV/cm.Compared with Sr2Na1-xLixNb5O15 ceramics prepared with niobate,the ceramics prepared from carbonate have better micro morphology,dielectric and ferroelectric properties.The above results show that the introduction of a small amount Li+instead of Na+ at A-sites can indeed reduce the second phase.At the same time,using carbonate as the starting reactant on the basis of the introduction of Li+and combining with the two-step sintering process can obviously promote the densification of ceramics and greatly reduce the sintering temperature of SNN-based tungsten bronze structural ceramics,which provides an important basis and reference for the composition design and densification process research of tungsten bronze niobate ferroelectric ceramics.