Research On The Preparation, Structure And Properties Of KNN-based Lead-free Piezoelectric Single Crystals And Thin Films

In recent years,researchers in various countries are actively exploring new materials and their applications.Piezoelectric material,as a kind of material with special physical properties,has been widely studied.Due to the pollution of lead-based piezoelectric materials to the environment,the research and application of lead-free piezoelectric materials are being accelerated.The advantages of lead-free piezoelectric materials include:environmental protection,good stability,high efficiency,versatility and easy processing.Among them,potassium niobate(K_0.5Na_0.5NbO₃,KNN)based piezoelectric single crystal is attracting more and more attention due to its excellent electrical properties and high stability.Seedless solid phase crystal growth method（SFSSCG）has the advantages of relatively low cost of equipment and raw materials required by solid phase method,reducing element volatilization,easy operation,and can prepare high-performance KNN crystals.Therefore,（1-x）[0.997(0.996K_0.5Na_0.5NbO₃-0.004MnBiO₃)-0.003B₂O₃]-x Li₂CO₃single crystal was prepared by SFSSCG.In this work.The effects on the morphology,structure and properties of single crystal by different doping amounts of Li were mainly studied.The effects of annealing temperature and cooling rate on single crystal were investigated.To explore the miniaturization of KNN,KNN-based thin films were prepared by pulsed laser deposition（PLD）.The primary focus of this work and its unique contributions are outlined as follows:In this work,the effects of Li doping on the morphology,structure and properties of KNN based single crystal were investigated.The results show that Li doping is beneficial to the growth of KNN-based single crystal and induces the formation of two phases of orthogonal-tetraval-phase perovskite structure,which is most obvious when the doping amount of Li is 0.3at%（atomic percentage）.At the same time,the piezoelectric coefficient of the single crystal reaches the maximum,d₃₃=422 pC/N.In addition,the crystal has the following excellent electrical properties:T_C=429^oC,tanδ=0.034,P_r=22.8μC/cm²,E_c=11.9 k V/cm and Q_m=166.Secondly,We focused on the effect of morphology,structure and properties by different annealing temperatures on 0.997[0.997(0.996K_0.5Na_0.5NbO₃-0.004MnBiO₃)-0.003B₂O₃]-0.003Li₂CO₃single crystal.The results show that 600^oC is the optimum annealing temperature for the single crystal in terms of electrical properties.Proper annealing temperature can reduce the T_O-Tof crystal,but has relatively little effect on T_Cof crystal.In order to further change the growth process of single crystal and improve the electrical properties of the single crystal,In this paper,the effects of different cooling rates on the morphology,structure and electrical properties of0.997[0.997(0.996K_0.5Na_0.5NbO₃-0.004MnBiO₃)-0.003B₂O₃]-0.003Li₂CO₃ piezoelectric single crystal are studied.The results show that piezoelectric crystals with smoother surfaces can be grown at an appropriate cooling rate,and the growth of grains can be promoted,especially when the cooling rate is 1^oC/min.At 1^oC/min,the piezoelectric properties of the samples are the best,d₃₃=443 pC/N,d₃₃^*=369 pm/V,the cooling rate has a limited effect on the ferroelectric properties of the crystals,especially the residual polarization strength P_r.In addition,the faster cooling rate is conducive to obtain better mechanical quality factor Q_mand electromechanical coupling coefficient k_t.For example,when the cooling rate is 2^oC/min,Qm reaches the maximum value 166,and when the cooling rate is 4^oC/min,k_treaches the maximum value 0.47.In this work,we took 0.997[0.997(0.996K_0.5Na_0.5NbO₃-0.004MnBiO₃)-0.003B₂O₃]-0.003Li₂CO₃single crystal as substrate by pulsed laser coating method and plated homogeneous film on it.Combined with literature research,The effect of different annealing time on KNN films is studied systematically.The results show that in situ annealing stage,annealing temperature is 650^oC,annealing oxygen pressure is100 Pa,annealing time is 90 min,the KNN-based film is the most smooth,smooth,film roughness is 8.1 nm.