Preparation And Characterization Of The KNN-based Lead-free Piezoelectric Ceramics

Among various kinds of lead-free piezoelectric ceramics, sodium potassium niobate (K_0.5Na_0.5)NbO₃(KNN)-based ceramics have been received much attention and considered to be a promising environmental-friendly material replacing for lead-containing piezoelectric ceramics due to its high Curie temperature and large electromechanical coupling factors. In this paper, the modifications of KNN-based ceramics were carried out by ion substitution, and with the addition of new compositions and sintering acids. Main results are listed below:1. Li⁺, Sr²⁺ and Sb⁵⁺ ions were introduced into KNN ceramics, and the compositional dependence of phase structure and the electrical properties of the ceramics were studied. Results showed that the phase structure of the ceramics changed from orthorhombic to tetragonal phases, and at the coexistence of the two phases, the properties were effectively enhanced, e.g. (K_0.5Na_0.5)_0.90Li_0.06Sr_0.02Nb_1-xSb_xO₃(KNLSN-Sb_x;at x = 0.01,P_r = 20.5μC/cm2,d₃₃ = 145 pC/N,k_p = 38%);(K_0.5Na_0.5)_0.96-xLi_xSr_0.02Nb_0.98Sb_0.02O₃ (KNSNS-Li_x;atx = 0.04 ,P_r = 20.4μC/cm2,d₃₃ = 142 pC/N,k_p = 38.2%).In addition, Sb-doping has great effects on the dielectric properties of KNN-based ceramics, and the ceramics with proper amount Sb addition exhibited typical relax-behavior with the broadεr peaks.2. The effects of the sintering acids such as MnO₂, K₄CuNb₈O₂₃ (KCN) and Bi₂O₃ on the phase structure, microstructure and electrical properties of the (K_0.5Na_0.5)₀.94Li_0.06NbO₃ (KNLN6) ceramics were investigated. It was found that both the addition of MnO₂ and KCN induced a perovskite structure transition from coexistence of orthorhombic and tetragonal phases to orthorhombic symmetry. A small amount of MnO₂ or Bi₂O₃ doped in KNLN6 ceramic, as grain growth inhibitors, have evident effects on grain size reduction. While KCN addition promoted the grain growth of KNLN6 ceramics, and abnormal grain growth was observed at higher doping content. Moreover, the addition of sintering acids weakened the ferroelectricity of KNLN6 ceramics and changed the KNLN6 to"hard"ceramics. The piezoelectric contant d₃₃, planar electromechanical coefficient k_p and dielectric loss tangent tanδdecreased gradually with the increasing doping content, while the mechanical quality factor Qm was greatly enhanced due to the"hardening effect".3. The effects of (Bi_0.5Na_0.5)TiO₃ (BNT) on the microstructure, electrical properties and temperature stability of KNLN6 ceramics were studied. Results showed that BNT-doping induced the crystal structure changing from coexistence of orthorhombic and tetragonal phases to single tetragonal symmetry. BNT had an evident effect on the microstructure of KNLN6 ceramics, which brought about an obvious grain size reduction. The addition of BNT in the ceramics shifted the polymorphic phase transition (PPT) below room temperature, and greatly enhanced the temperature stability of KNLN6 ceramics. Proper amouts of BNT modified ceramics exhibited a flat, temperature-stable behavior over the range 25–180 oC with slightly variations of P_r and k_p, together with the smaller temperature coefficient.