Research On The Seed-free Solid-state Growth And Electrical Property Modification Of Potassium-sodium Niobate-based Lead-free Piezoelectric Single Crystal

Piezoelectric material is an important functional material widely used in the field of electronic information.Due to the toxicity of lead-based piezoelectric materials,lead-free piezoelectric materials have been extensively studied in recent years.Among them,the potassium sodium niobate(K_0.5Na_0.5Nb O₃,KNN)piezoelectric single crystal has attracted much attention due to its high piezoelectric properties,high Curie temperature,and environmentally friendly advantages.Compared with the high-temperature melt crystal growth method,the seed-free solid-state growth?SFSSCG?method has the advantages of simple process equipment,low energy consumption and cost,reduced element volatility and high piezoelectric ferroelectric performance of the grown KNN crystal.Although the SFSSCG method has been able to produce larger and better performing KNN-based piezoelectric single crystals,the overall performance of the crystal still needs to be improved,and the crystal growth mechanism is not yet clear.Therefore,?1-x?(99.6K_0.5Na_0.5Nb O₃-0.4Mn Bi O₃)-x Li₂O and 99.7(99.6K_0.5Na_0.5Nb O₃-0.4Mn Bi O₃)-0.3B₂O₃piezoelectric single crystals were prepared by the SFSSCG method.The relationship between its composition,growth process,crystal structure,electrical domain structure,and electrical properties has been systematically studied,with a view to improving the overall performance of crystals such as piezoelectric,dielectric and ferroelectric.The main research contents and innovations are as follows:The effects of a small amount of Li doping on the structure and electrical properties of KNN-based piezoelectric single crystals were investigated.The research results show that the addition of a small amount of Li₂CO₃has the effect of a flux,which makes the growing crystal composition more uniform and larger;at the same time,the addition of a certain amount of Li can alleviate the degree of lattice distortion of KNN crystals;with the gradual increase of the Li doping content,the tetragonal phase content in the crystal increases first and then decreases,reaching the maximum when the Li doping content is 0.3at%,which is 25.83%;the reason for the phase transition may be due to the different content of Li doping makes the K and Na elements volatilize differently during the crystal growth process,resulting in a phase transition.The piezoelectric constant d₃₃,inverse piezoelectric constant d^*₃₃,and residual polarization P_rof the Li-doped KNN-based crystals first increase and then decrease with increasing the Li addition.When the doping amount is 0.2at%-0.3at%,the electrical performance of the crystal is the best:tan?=1%and P_r=26.1?C/cm².Li doping has little effect on the transition temperature(T_O-T)of the crystal from the orthogonal phase to the tetragonal phase,but it significantly increase the Curie temperature?T_C?of the single crystal.The effects of B doping on the structure and piezoelectric properties of KNN-based piezoelectric single crystals were studied.The results show that B doped99.6K_0.5Na_0.5Nb O₃-0.4Mn Bi O₃single crystal has better fluxing effect and can promote crystal growth and can effectively improve the piezoelectric performance of the single crystal.The effects of air annealing on oxygen vacancy defects,domain structure and electrical properties of 99.7(99.6K_0.5Na_0.5Nb O₃-0.4Mn Bi O₃)-0.3B₂O₃single crystal were studied.Studies have shown that different annealing time in the air has little effect on the crystal structure of KNN-based crystals.The crystals are perovskite structures in which the orthogonal phase and the tetragonal phase coexist.The fraction ratio of the orthogonal phase versus the tetragonal phase in the crystal is78:22.Piezoelectric and ferroelectric properties of the crystal increase first and then decrease with the extension of annealing time,and reach the maximum when the annealing time is 8 h.The Curie temperature T_Cis about 410 ^oC.At the same time,optimizing the annealing time can effectively reduce the leakage current and dielectric loss of the single crystal.When the annealing time is 8 h,the concentration of oxygen vacancies in the crystal is significantly reduced;the size and structure of the electrical domains in the crystal can be effectively improved by adjusting the annealing time.As a result,the piezoelectric ferroelectric performance of the crystal is improved.By observing the interface between the single crystal region and the ceramic region,it can be seen that the SFSSCG method can not only promote the rearrangement of ceramic particles to form single crystals,but also promote the rearrangement of ferroelectric domains and improve domain structure.The reason of improvement of the electrical properties of the KNN single crystal by the annealing treatment may be attributed to that the annealing reduces the concentration of oxygen vacancies in the crystal and modifies the domain structure in the crystal.The effects of the cooling rate during the temperature-decreasing stage of crystal growth on the structure and properties of 99.7(99.6K_0.5Na_0.5Nb O₃-0.4Mn Bi O₃)-0.3B₂O₃single crystal were systematically studied.The results show that optimizing the cooling rate is beneficial to the growth of single crystals,and to grow single crystals with larger sizes.When the cooling rate is lower,the crystal surface is flatter and the quality of the crystal is higher.The effect of the cooling rate on the crystal structure of the single crystal is small.When the cooling rate is 2°C/min,the KNN-based piezoelectric single crystal exhibits excellent piezoelectric properties:its piezoelectric coefficient d₃₃reaches 510 p C/N,the inverse piezoelectric coefficient d^*₃₃is 1425 pm/V,and the single crystal also has a lower dielectric loss tan?=0.02,and a higher Curie temperature T_C=416°C,and a residual polarization intensity P_r=20?C/cm².