Study On Composition Design And Phase Boundary Construction Of Sodium Lead Silicate Based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics which achieve the transformation between electrical and mechanical energy,are widely used in machinery,electronics,communications and other fields.The lead-based piezoelectric ceramics,represented by Pb（Zr,Ti）O₃（referred to as PZT）ceramics,are widely used in a variety of piezoelectric devices because of their excellent dielectric and piezoelectric properties.Nevertheless,Lead-based ceramics huve a hig lead content（around 70%in weight）,which is harmful to the human health and the environment.The research and development of lead-free piezoelectric ceramics has become an urgent and significant social and economic significance.Potassium sodium niobate（KNN）lead-free piezoelectric ceramics is one of the widely research system of lead-free,because of its good electrical properties and relatively high Curie temperature.However,compared with PZT based ceramics,there are larger gap of comprehensive performance between KNN based ceramic.To promote the KNN ceramics of practical,doping modification is necessary,as did in the past of PZT ceramic,especially to construct new phase boundary system in KNN based ceramic.Firstly,we constructed and prepared the（1-x）(K_0.48Na_0.535)Nb O₃-x Ca Zr O₃（KNN-x CZ）lead-free piezoelectric ceramics.The effects of the dopants（Ca Zr O₃）on the crystalline phase,microstructure and electrical properties were mainly studied.The results of XRD analysis showed that doped ions has completely into the KNN lattice,formed a single stable solid solution in the studied composition range.From the XRD and the temperature dependence of the dielectric constant,we found that with the increase of CZ content,rhombohedral-orthorhombic phase coexistence appears in the composition of 0.03￡x￡0.05.From the SEM photos,one can easily found that the grain size first rises and then declines with the increase of CZ content.At the same time,the piezoelectric properties of the ceramics also got certain improvement(x=0.05,d₃₃=181p C/N,k_p=0.37,ε_r=1814).In addition,the T_C and the T_O–T of KNN-x CZ ceramics both shift to lower temperatures with increasing the CZ content which is beneficial to improving piezoelectric properties.Then,lead-free KNLNS_x-CZ piezoelectric ceramics were prepared by conventional solid-state reaction method,and effects of the dopants（Li,Sb）on the crystalline phase,microstructure and electrical properties were mainly studied.The results of XRD analysis showed that the ceramics possess a pure perovskite structure and no secondary phase can be detected.Furthermore,the crystal structure of the ceramics gradually transforms from an orthorhombic phase to a rhombohedral phase with the increase of Sb content,and the rhombohedral-orthorhombic phase coexistence appears in the composition range of 0.03￡x￡0.05.In addition,the T_C and the T_O–T of KNLNS_x-CZ ceramics both shift to lower temperatures.From the SEM photos,the grain size first rises and then declines with the increase of Sb content which has maximum grain size x=0.05.A relatively high piezoelectric activity is achieved for the KNLNS_x-CZ ceramics in the compositions near the rhombohedral-orthorhombic phase boundary,especially for the composition x=0.03,which exhibits the optimum piezoelectric properties:d₃₃=213p C/N and k_p=0.37.Finally,the（0.98-x）(K_0.4Na_0.6)Nb O₃-0.02Ca Zr O₃-x(Bi_0.5Na_0.5Hf O₃)lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method,and effects of the dopants（BNH）on the crystalline phase,microstructure and electrical properties,were mainly studied.The results of XRD analysis showed that doped ions has completely into the KNN lattice,formed a single stable solid solution,no secondary phase can be detected.The crystal structure of the ceramics gradually transforms from an rhombohedral phase to a tetragonal phase near room temperature.From the XRD and the temperature dependence of the dielectric constant,the rhombohedral-tetragonal phase coexistence appears in the composition of 0.04￡x￡0.45.From the SEM photos,the grain size first rises and then declines with the increase of BNH content which has（0.98-x）KNN-0.02CZ-x BNH ceramics both shift to lower temperatures.Owing to the polymorphic phase boundary with R and T phase as the boundaries,the piezoelectric properties of（0.98-x）KNN-0.02CZ-x BNH have been improved greatly(x=0.045,d₃₃=320p C/N,k_p=0.40,ε_r=1896).