Study On Preparation And Nonstoichiometric Composition Of KNN-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are functional materials with great potential and they are widely used in electronic components in medical engineering,sensor devices,electronic communications and other areas of daily life.Lead-based piezoelectric materials rely on excellent electrical properties and good temperature/fatigue stability to occupy the main market of piezoelectric materials for a long time.In latest years,as the society pays close attention to human health and people’s awareness of environmental protection gradually increases,the main research topics in the field of piezoelectric ceramics are inclined to lead-free.Due to the higher phase transition temperature of ferroelectric-paraelectric（T-C）and higher piezoelectric coefficient,potassium sodium niobite-based（KNN）lead-free piezoelectric ceramics stand out among a host of lead-free piezoelectric ceramic materials.However,the presence of volatile alkali metal（potassium,sodium,etc.）elements in KNN ceramics leads to an extreme imbalance in the development of electrical properties,making their experimental processes less reproducible.Based on the above-mentioned research background and technology challenges,this thesis carries out a research on the preparation of KNN-based lead-free piezoelectric ceramics and the composition of nonstoichiometric ratio.The main research contents are as follows:（1）The stoichiometric ratio of potassium and sodium elements is regulated to compensate for elemental volatilization during the sintering process.This study is based on 0.96(K_0.48Na_0.52)_1+xNb_0.96Sb_0.04O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃ceramics with Rhombohedral-Tetragonal（R-T）phase coexistence,focusing on the effect of the nonstoichiometric ratio of potassium and sodium elements in the matrix on the microstructure,phase structure and electrical properties of the ceramics.When the stoichiometric ratio of potassium and sodium elements is 1.005,the ceramic obtains the best polarization state,and the coexistence of R-T phase boundary make the ceramic polarization with a significantly lower energy potential barrier,thereby the best overall performance(d₃₃=532 p C/N,k_p=58%,T_C=238°C)was obtained for the components with x=0.005.The results demonstrated that the compensation of potassium and sodium elements for KNN ceramics is a facile and effective way to modify the piezoelectric properties.（2）In the classical KNN system,the Bi_0.5Na_0.5ZrO₃（BNZ）is a powerful dopant that can substantially improve the piezoelectric properties by building the R-T phase boundary with Sb³⁺near room temperature.The boiling point of raw material bismuth oxide（Bi₂O₃）is lower than that of potassium and sodium elements,and Bi is more likely to deviate from its stoichiometric ratio during the sintering process,therefore,the compensation for Bi cannot be neglected in the preparation process.In this section,0.96K_0.48Na_0.52Nb_0.96Sb_0.04O₃-0.04Bi_{0.5*（1+x）}Na_0.5Zr O₃ ceramics were manufactured by solid-state sintering,and the effects of stoichiometric ratio of Bi on the ceramic properties were investigated.The results show that an appropriate excess of Bi can enhance the content of the tetragonal phase and reduce the percentage of oxygen vacancies in ceramic sample,and the external contribution of the piezoelectric effect is enhanced.The piezoelectric properties were gradually improved with increasing Bi content,and finally reached the optimal piezoelectric properties(d₃₃=522 p C/N,k_p=58.9%,T_C=235℃)when Bi stoichiometry ratio of 1.05.（3）Sintering aids not only can considerably reduce the sintering temperature of ceramic,improving ceramic denseness,but also enable certain properties of ceramics to be significantly improved.Proper amount of sintering aids can improve the electrical properties,while a higher content of sintering aids may"harden"the KNN ceramics and weaken the piezoelectricity of the KNN ceramics.In addition,the changes of electrical properties are different because different mechanisms of action are generated between sintering aid and KNN ceramics.In this chapter,the0.96K_0.48Na_0.52Nb_0.96Sb_0.04O₃-0.04Bi_0.5Na_0.5Zr O₃-0.05wt%Fe/Cu/Mn ceramics were fabricated by the solid-state sintering method,the effects of trace amounts of different sintering aids on the phase structure,electrical properties,and microstructure of KNN ceramics are investigated in detail.It is shown that the KNN ceramics with trace amounts of Fe and Mn doping exhibit soft doping characteristics(KNNS-Fe:d₃₃=520p C/N,k_p=56%,Q_m=28;KNNS-Mn:d₃₃=483 p C/N,k_p=57%,Q_m=31),while the KNN ceramics with trace amounts of Cu doping exhibit hard doping characteristics(KNNS-Cu:d₃₃=400 pC/N,k_p=52%,Q_m=35).