Study On Modifying And Properties Of KNN-based Lead-free Piezoceramics

Piezoelectric ceramics,a functional material that can convert mechanical energy and electrical energy to each other,have been widely used in force,heat,light,electricity,sound,medicine and other fields.Due to the high content of lead in traditional piezoelectric materials,it will pollute the environment in production,utilization and waste treatment.Therefore,the research and development of lead-free piezoelectric ceramics have become an inevitable trend.Potassium-sodium niobate-based lead-free piezoelectric ceramics is one of the systems that are more extensively studied in environmentally friendly piezoelectric materials.To obtain Potassium niobite lead-free piezoelectric ceramics with good performance,choosing(K0.5Na0.5)NbO3-based lead-free piezoelectric ceramics as the research object,this paper first discussed the impact of A,B-site ions doping on the electronic structure of KNN-based ceramics system based on the first principles,and analyzed its doping mechanism.Secondly,Ba(Zr0.05 Ti0.95)O3 and[Bi0.5(Na0.7La0.3)0.5]0.9Zn0.1ZrO3 were used as the second phase doped with modified KNN-based lead-free piezoelectric ceramics,and the impact of doping ions on the polycrystalline phase boundary and properties of KNN ceramics was studied systematically.Finally,(K0.5Na0.5)NbO3-Ba(Zn1/3Nb2/3)O3 transparent ceramics were prepared by the solid phase method,and the impact of components on the phase structure,grain size,light transmittance and electrical properties was researched.The main contents and conclusions of the paper are as follows:When KNN system is doped with the A-site ions,Ca and Bi are more inclined to replace Na atom at the A-site,while Ba is more inclined to replace K atom at the A-site,and from the stability of the KNN after replacement:Ca>Bi>Ba.The contribution of Ca and Bi to the conduction band in the A-site ion is greater than Ba,and the low energy level and conduction band of the KNN system after doping move to a direction of low energy,and the band gap becomes narrow.When a smaller ion or ion group replaces the A-site ion in KNN lattice,the Nb-O octahedron in KNN ceramics collapses towards the center.After the B-site ion is doped with KNN,the entire energy band is shifted toward the high energy direction.The band gap of KNN is widened by Zr and Ti ions,while the band gap of KNN is narrowed by Ta ions,and the Fermi surface is more likely to move to the valence band.The stability of KNN ceramic after B-site ions replacement is:Ti>Zr>Ta.For KNN system,Ti ion is easier to replace Nb ion than the other two ions.When the B-site ions in KNN lattice are replaced by larger ions or ion groups,the Nb-O octahedron in KNN ceramics will expand to the surroundings.There is a strong orbital hybridization at the top of the valence band between the 3d state of Nb and the 2p state of O.The suppression of short-range repulsion forces stabilizes the piezoelectricity of the KNN system.When the CaZrO3 doping concentration is 0.05,A-site atoms have a greater impact on the band gap than B-site atoms do,and the band gap becomes narrower,Eg=2.164eV.Because after doping,p and d electrons of Ca and Zr contribute to the valence band and the conduction band near the Fermi surface,the orbital hybridization between Nb-O is strengthened,the Nb-O bond length is changed,and then the oxygen octahedron is distorted.The crystal structure of KNN ceramics is transformed from a quadrature phase to a tetragonal phase,making the electric domain wall motion easier and enhancing the piezoelectric properties of KNN.0.95KNN-0.05BZT piezoelectric ceramics were prepared by two-step sintering method and adding the sintering aid.The optimal process for the two-step sintering is 1190?/10min/1050?/15h.The performance parameters obtained under the process are:d33=106 pC/N,kp=50%,?r=885.8,tan?=2.3%.After the sintering aid CuO was added,the comprehensive properties of the obtained ceramics were d33=116 pC/N,kp=52%,?r=1116.8 and tan?=2.08%.As for the preparation of KNN-BZT lead-free piezoelectric ceramics,the sintering process where sintering aids are added is more suitable.(1-x)K0.5Na0.5NbO3-xBa(Zr0.05Ti0.95)O3 lead-free piezoelectric ceramics were prepared by the solid phase method with the addition of sintering aid CuO,the impact of ion doping on the phase structure of(1-x)KNN-xBZT ceramics was studied,and the influence of doping on the microstructure,and dielectric and piezoelectric properties of the ceramics were investigated.With the increasing content of x,the phase structure changes from a quadrature phase to a trigonal phase,indicating that an ion with high electronegativity is selected for the A-site substitution in the KNN lattice,whereas an ion with a smaller radius is selected for the B-site substitution.The formation of the R phase of KNN can be induced near the room temperature.Moreover,the dielectric,piezoelectric and ferroelectric properties first increase and then decrease as BZT content increases.Upon the comprehensive comparison,when the content of Ba(Zr0.05Ti0.05)O3 is 0.06,the properties of KNN-BZT ceramics are optimal,d33=130pC/N,kp=0.5,?r=495.6,tan?=0.0257,Tc=379.4?,Pr=15.18 ?C/cm2,Ec=11.68 KV/cm.(1-x)K0.5Na0.5NbO3-x[Bi0.5(Na0.7La0.3)0.5]0.9Zn0.1ZrO3{(1-x)KNN-xBNLZZ} ceramics were prepared by the solid phase sintering method,and the influence of ion doping on its microstructure,phase composition,the dielectric and piezoelectric properties were studied.Under the circumstance of BNLZZ doping,ceramics forms the poly crystalline phase boundary of O-T-R at 0.06?x?0.07.Co-existence of multiple phases can help to reduce the anisotropy of the system,indicating that the domain wall motion is a major factor for determination of ferroelectric and piezoelectric ceramic properties.Therefore,the piezoelectric properties of ceramics are improved substantially,and compared with the R-O phase boundary,the potentiation of R-T polycrystalline phase boundary on ceramic properties is more significant.In addition,the closer the radius of A-site doping ion is to Na ion,the better the performance of KNN ceramics is.When the BNLZZ content is 0.06,d33=328 pC/N,kp=0.52,Pr=25.2 ?C/cm2.(1-x)(K0.5Na0.5)NbO3-xBa(Zn1/3Nb2/3)O3 transparent ceramics was prepared by solid phase sintering.The influence of BZN content on the phase structure and grain size,transparency and electrical properties of ceramics were studied.The results show that the grain boundaries of KNN-BZN ceramics are clear and the grains grow in the stack.With the increase of BZN content,pores are generated in the ceramics,the compactness is reduced,and the grain boundaries become blurred.When the content of Ba(Zn1/3Nb2/3)O3 is 0.07,the lattice caused by the B-site ion is stretched along the c-axis,making the phase composition of ceramics turn into a pseudo-cubic phase.Under this composition,KNN-based ceramics reach the maximum linear transmittance in the visible light regions(43%),with its electrical properties ?r=975.9,tan?=0.02,Pr=2.231?C/cm2,Ec=9.82 KV/cm.