Research On The Seed-free Solid-state Growth Process,Structure And Electrical Properties Of Mn-doped KNN-based Piezoelectric Single Crystals

The lead-based piezoelectric materials,represented by lead zirconium titanate ceramics,are widely used in various fields of industry and life due to their excellent piezoelectric properties.But lead is toxic and becomes potentially dangerous to environmental safety and human health.Therefore,it is urgent to develop lead-free piezoelectric materials to replace the lead-based piezoelectric materials.Among many lead-free piezoelectric systems,the potassium-sodium niobate(K_0.5Na_0.5Nb O₃,KNN)lead-free piezoelectric materials are considered that hopes to replace lead-based piezoelectric materials due to its excellent electrical properties and environmentally friendly characteristics.In this paper,（1-x）(99.6K_0.5Na_0.5Nb O₃-0.4Li Bi O₃)-x Mn O₂single crystals were prepared by seed-free solid-state crystal growth（SFSSCG）method.The effects of doping,growth process,crystal structure,microstructure,electrical properties,conductive mechanism and ferroelectric domain have been studied to obtain large-size,high-performance KNN-based piezoelectric single crystals.The major research contents and experimental results are as follows:The effect of growth process on the growth and microstructure of99.5（KNN-LB）-0.5Mn lead-free piezoelectric single crystal were studied.The experimental results show that the doping amount x=0.5 at%is beneficial to obtain large-size KNN-based single crystals with dimensions of 4 mm×7 mm×1.7 mm,and appropriately increasing the sintering temperature,cooling rate and calcination temperature are useful for the growth of KNN-based single crystals.It is more useful for the growth of KNN-based single crystals when the sintering temperature is 1105℃and the cooling rate is 2℃/min.Increasing the calcination temperature is beneficial to synthesize of pure orthorhombic-phase the pre-synthesized powder.However,the phase structure of the single crystals is not significantly affected by the calcined temperature and which is the orthogonal phase.The average size of the pre-synthesized powder varies slightly with the calcined temperature.The variation range of the average size is not more than 3 nm for the powder calcined at different calcination temperatures.The area fraction of the largest crystal accounts for more than 81%from 750 to 850℃,In particular,the largest dimension of the single crystals obtained was 17 mm×15 mm×2 mm at 800℃.These results show that it is beneficial to obtain larger KNN-based crystals when the calcined temperature is in the range of 700～850℃.The effects of calcined temperature on the electrical properties,domain structure and conductive mechanism of 99.5（KNN-LB）-0.5Mn lead-free piezoelectric single crystals are studied.The experimental results show that the changes of calcined temperature can effectively improve the ferroelectric properties of KNN-based single crystals.The maximum P_r is 49.1μC/cm² at 800℃,and the P_r are more than 37μC/cm² in the pre-sintering temperature range of 750～850℃,which shows that the excellent ferroelectric properties.With rising of the calcined temperature the width of the ferroelectric domains of 99.5（KNN-LB）-0.5Mn single crystal first decreases and then increases.The minimum domain width of 260 nm and the maximum piezoelectric constant value of 209 p C/N are obtained for the crystals with the calcined temperature of 850℃.It shows that choosing a proper calcined temperature can reduce the size of the ferroelectric domain,and decreasing the domain size can remarkably enhance the piezoelectric properties of the crystals.In addition,the different calcined temperature does not change the conduction mechanism of KNN-based crystals,which is space charge limited conduction mechanism.The effect of annealing temperature on the crystal structure and electrical properties of 99.5（KNN-LB）-0.5Mn single crystals were studied.The experimental results show that the phase structure of the KNN-based single crystal is changed by annealing treatment,which is changed from a single orthogonal phase to the coexistence of the orthogonal phase and the tetragonal phase.The orthogonal phase accounted for 78%and the tetragonal phase accounted for 22%.In addition,the proportions of the orthogonal phase versus the tetragonal phase hardly changes with the annealing temperature increases.The content of oxygen increases with the increase of annealing temperature,indicating that the oxygen vacancies is significantly reduced by annealing treatment.Furthermore,the d₃₃ of the single crystals are increased by 2 to 3 times,ε,Q_mand K_t are also improved.After annealing,the tanδis slightly reduced and the minimum tanδis about 0.014.KNN-based single crystals are prepared at calcined temperature of 700℃,are obtained the best electrical properties at annealing temperature of 700℃,its d₃₃=411 p C/N,ε=865,tanδ=0.017.And the best electrical properties,d₃₃=311 p C/N,ε=542,tan=0.014 are obtained for the crystal at annealing temperature of 800℃,which are prepared at calcined temperature of 800℃.The results show that the annealing treatment in air atmosphere can effectively improve the piezoelectric,ferroelectric and dielectric properties of KNN-based crystals,and different calcined temperatures can affect the annealing temperature.