Study On Ultra-Low Dielectric Loss And Fluorescence Properties Of The Potassium Sodium Niobate Based Lead-Free Piezoelectric Ceramics

Lead-free piezoelectric ceramics possess a wide application as one of the inorganic piezoelectric materials with stable property and simple preparation.This study is aimed to prepare KNLNST-Cu O ceramics based on the traditional solid-state method and to investigate the effects of different sintering temperatures on the properties of ceramics by combining with experimental data and theoretical analysis methods.Besides,the electrical and optical properties of Cu O modified KNLNST lead-free piezoelectric ceramics are explored in two chapters.（1）[(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃-x mol%Cu O ceramics are synthesized at different temperatures,abbreviated as KNLNST-x Cu O（x=0.0 mol%,0.1 mol%,0.2 mol%,0.3 mol%,0.5 mol%,0.8 mol%,1.2 mol%,1.5 mol%,2.0 mol%,3.0 mol%）.The results suggest that the Cu O is beneficial to the decrease of sintering temperatures,which can be reduced to 1000°C.All ceramics can obtain the relatively best electrical properties after optimizing the sintering conditions.For example,ceramics without Cu O addition acquire the best piezoelectric constant d₃₃=278 p C/N.The Q_m of ceramic with x=2.0 mol%is up to 317.（2）The impact about the properties of[(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃ceramics with different Cu O contents is studied.It can be found that with the increase of Cu O contents,the tetragonal phase characteristics of lattice disappear gradually,corresponding to the increment of orthogonal phase contents.The morphology on the particle of ceramics also shows a significant change.The grains of ceramics have no obvious edges or corners,and small particles,irregular melts exist on the ceramics’surface after the addition of Cu O.Moreover,the grain size also changes remarkably.The KNLNST ceramics exhibit both“hard”and“soft”doping characteristics due to the introduction of Cu O: reducing the piezoelectric constant d₃₃,planar electromechanical coupling coefficient kp,relative dielectric constant and coercive field E_c;enhancing the mechanical quality factor Q_m.Importantly,the dielectric loss tan δ decreases first and then increases.The phase transition temperature in T_O-T increases after Cu O doping while T_C keeps unchanged.Besides,it can be seen that ceramics are obviously relaxed.The EDS on the natural surface of ceramics shows obvious element segregation.The study about the defect form and concentration in ceramics shows that the content of oxygen vacancy in the lattice increases first and then decreases.The specific occupation mode in lattice after Cu²⁺ introduction can be obtained by the defect equation.In addition,when x = 0.3 mol%,the piezoelectric ceramics show the lowest dielectric loss,tan δ = 0.0031.（3）Photoluminescence and photochromism are found in Cu O-doped [(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃ ceramics.The color of the ceramics changed significantly under visible light irradiation.Based on the photochromic reaction,the photoluminescence intensity of ceramics can be effectively modulated,which is manifested by the weakening of the luminescence intensity,and the ratio is 46.9%.By alternating light irradiation and thermal stimulation,the ceramic can achieve reversible photochromic reaction,and fluorescence modulation behavior which also shows a great cycle-stability.In addition,the mechanism about the photoluminescence and photochromism in [(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃-Cu O ceramics is also explained in this study.In the research of lead-free piezoelectric ceramics without rare elements,defect luminescence and its intensity modulation,as a new mechanism,are of great significance for broadening the single characteristics of KNN-based piezoelectric materials and developing towards the multifunctionalization of photoelectricity.