| The research and development of piezoelectric ceramics has become one of the hottest topics in the material field. At present, traditional Pb(Zr,Ti)O3 (PZT)-based ceramics are widely used as piezoelectric materials, which casue environment pollution during processing, preparing, using, and discarding. For this reason, it would be desirable if the lead-based ceramics could be substituted by lead-free piezoelectric ceramics. Lead-free piezoelectric ceramics are usually produced by conventional mixed-oxide method. However, it is difficult to fulfill the request for practical application due to the innate shortcoming of solid reaction. As a new synthesis technology, soft chemical synthesis method leads to accurate chemical stoichiometry, high composition homogeneity and lower crystallization temperature due to the mixing of liquid precursors at the molecular level and thus improves the properties of materials. Therefore, it is of significance to develop the stable chemical processing of lead-free piezoelectric ceramics.In the study, BNT based and NKN based lead-free piezoelectric ceramics with perovskite structure were synthesized and prepared by the citrate method. In addition, the formation mechanism of perovskite powders was discussed. The synthetic technological conditions and microstrures and electrical properties of the ceramics were investigated systemtically.On the one hand, BNT-BT6 and BNT-BT6-x%Nb ceramics were prepared by the citrate method. In addition, the sintering densification behavior, microstructure, electrical properties of the BNT-BT6 and BNT-BT6-x%Nb ceramics were investigated. The results indicated that BNT based ceramics prepared by the citrate method can be well densified at temperatures 50℃lower than those by mixed-oxide method and conventional sol-gel method. The precipitation of barium nitrate in traditional sol-gel method was solved successfully by the citrate method, achieving a real molecular-level complete mixing, which leads to enhanced piezoelectric properties of d33～180pC/N and kp～30%. Furthermore, the results indicated that the addition of a small amount of Nb5+ has no remarkable effect on the crystal structure but can improve the densification. The addition of Nb5+ lowers the temperature Td and thus makes the anti-ferroelectric phase zone broader. On the other hand, the dielectric maxima were obviously suppressed due to the Nb5+ addition. The piezoelectric properties could be slightly improved as a small amount of Nb5+ doped, acting as a soft additive. The BNT-BT6～0.5%Nb ceramics exhibit improved electrical properties of d33～205 pC/N and kp～33%.On the other hand, based on our own experience derived from the preparation of BNT based ceramics, we extended this method to the preparation of NKN based ceramics. NKN-Cu and NKNS-3.75LT ceramics were prepared by the citrate method using Nb2O5 as niobium source and Ta2O5 as tantalum source. In this work, Nb2O5 and Ta2O5 can be changed into stable and soluble chelate complexes through a series of chemical reactions. The thermal decomposition of the gel was characterized by the methods of TG-DSC and FT-IR spectra. The crystallization behaviors of powders were investigated by XRD and Raman spectrum. The results showed that the calcination temperature of NKN-Cu and NKNS-3.75LT powders is approximately 300℃lower than those prepared by conventional mixed oxide method and the sintering properties and the properties of materials are improved to some extent. The properties of the NKN-Cu materials get hardened due to the addition of CuO. The ceramics sample sintered at 1060℃shows an improved densification behavior and a quite good performance:RD～97.1%, Pr～24.6μC/cm2, d33～117pC/N, kp～0.38, Qm～725,εr～605. The NKNS-3.75LT ceramics prepared by the citrate method display excellent piezoelectric properties of d33～418 pC/N and kp～0.54, which are almost the best piezoelectric properties reported previously for NKN based systems, demonstrating that citrate sol-gel method is an advantageous alternative to conventional solid-state method.
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