The Preparation And Properties Of KNN-based And BNT-based Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics are widely used in military,communications,medical and other fields due to their unique performance.However,most piezoelectric ceramics devices contain a amount of lead.Considering the health and the environment,developing high-performance lead-free piezoelectric materials has become an urgent problem in current researches.In our work,perovskite-type KNN-based piezoelectric ceramics and BNT-based piezoelectric ceramics were selected as the research matrix for doping modification,and the conventional solid oxide route was used to fabricate ceramics.The phase structure,dielectric properties,microstructure,ferroelectric,electric field-induced strain and other properties of the samples were studied systematically.The defect dipoles play a significant influence on the piezoelectric properties of the acceptor-doped KNN-based ceramics.In this work,K0.5N0.5NbO3 is selected as the research matrix,while different valence states of iron ions,copper ions and tetravalent tin ions are chosed as dopants.Then the structure,electrical properties and piezoelectric ferroelectric properties of samples are studied.The XRD diffraction pattern of the ceramic samples shows that the doping ions don't affect the phase structure,and all samples are still a perovskite structure;the dielectric temperature spectrum indicate that the iron ions,copper ions have little influence on TF-R and Tc,and the doping tin ions lead to decrease of Tc.The S-E curves show that the non-equivalent ion doping has a great influence on the electro-strain properties of ceramics,among which the strain of metastable ions(Fe²⁺and Cu⁺)doped ceramic shows a sharp increase,and the maximum strain can reach 0.4%and 0.5%,respectively,due to the influence of internal defect dipoles.The strain properties of tin ions doped samples also increase.According to the morphotropic phase boundary?MPB?,Bi0.5?Na0.78K0.22?0.5TiO3was selected as the research matrix,and the complex?Fe0.5Nb0.5?⁴⁺ions were used as dopants,and ceramic samples were fabricated through conventional solid oxide route with perovskite structure.With the increase of dopants,the size of ceramic grains increases gradually and the grain size's distribution is relatively uniform,while the grains form well.The dielectric temperature spectrum shows that the TF-R drops to room temperature while it has almost no effect on Tc with the doping increasing;after?Fe0.5Nb0.5?⁴⁺doped into ceramics,the strain changes greatly.It is attributed to that the sample gradually turns from ferroelectric phase to ergodic relaxor phase,and then the negative strain disappears.The maximum strain obtained at x=0.04 is about 0.43%?d33*=860 pm/V?;the fatigue test shows that all samples have good fatigue resistance,and the ferroelectric properties hardly change under the test conditions;It is affected greatly by the frequency,because the frequency could affect the internal domains of the piezoelectric ceramic during electric field-induced strain.