Preparation And Electrical Properties Of KNN-based Lead-free Piezoelectric Ceramics

(K,Na)NbO3-based ceramics have been considered as one of promising candidate for lead-free piezo ceramics due to its high Curie temperature and excellent piezoelectric properties. In this thesis, the effects of powder calcination temperature, borax addition and acceptor doping on the sintering and piezoelectric properties of KNN ceramics were investigated in detail.The effects of calcination temperature of (K0.5Na0.5)NbO3 (KNN) and (Na0.4825K0.4825Li0.035)(Nb0.8Ta0.2)O3 (KNLNT) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. It is found that the incomplete reaction at 700℃ and 750℃ calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after 800℃ calcination. During the sintering at 1070℃, the presence of the liquid K-rich phase due to the lower melting point has significant impact on densification, abnormal grain growth and the deteriorated piezoelectric properties. In the viewpoint of piezoelectric properties, the optimal calcination temperature is 800℃ for KNN and 800-850℃ for KNLNT, respectively. It is also found that piezlectirc properties of KNN can be improved by poling at high temperature, annealing and two-step sintering of KNN ceramics.Borax was found to effectively improve the sinterability and piezoelectric properties of KNN ceramics. The KNN with 0.45 wt% borax showed optimal properties as follows:piezoelectric constant d33=131.6 pC/N, planar electromechanical coupling coefficient kp= 34.8%.The effects of acceptor dopants on the synthesis, sintering, microstructure and electrical properties of KNN ceramics were studied for (K0.5Na0.5)(Nb0.994A0.006)O3-δ (A=Ge4+, Ga3+, Zn2+, Mn2+, Ni2+, and Cu2+) ceramics. The results reveal that the eutectic phase(s) is (are) formed between the acceptor oxide and alkali metal carbonate, thereby significantly improving the synthesis and sintering of the materials. Compared to the trivalent and tetravalent acceptor dopants, divalent dopant can lead to higher Qm values, most likely due to the formation of defect complexes.