Masato Yamazaki

Improvement of (K,Na)NbO3-based lead-free piezoelectric ceramics by asymmetric octahedra

Due to their excellent piezoelectric properties, including a coupling factor kp = 0.55, dielectric constant eT33/e0 = 2725, piezoelectric constant d33 = 375 pC N−1, Qm = 60, and dielectric tanδ = 1.87%, 0.891(K0.37Na0.63)0.86Ca0.04Li0.02Nb0.85O3−δ–0.041K0.85Ti0.85Nb1.15O5–0.055BaZrO3–0.0016Co3O4–0.0025Fe2O3–0.0069ZnO ceramic without Ta, Sb, Bi, or Hf is promising as a lead-free piezoelectric ceramic. In this system, a solid solution of BaZrO3 into (K0.37Na0.63)0.86Ca0.04Li0.02Nb0.85O3−δ effectively contributes to both the piezoelectric properties and the stability around room temperature. The solid solution of BaZrO3 induces asymmetry in the BO6 octahedra in the c-axis direction, improving kp, steeply increasing eT33/e0 at room temperature, while reducing the temperature dependence of eT33/e0.

Phase transition behavior of (K,Na)NbO3-based high-performance lead-free piezoelectric ceramic composite with different phase compositions depending on Na fraction

The structures of the main (K1− x Na x )NbO3 perovskite in a high-performance lead-free piezoelectric ceramic composite (K1− x Na x )0.86Ca0.04Li0.02Nb0.85O3−δ–K0.85Ti0.85Nb1.15O5–BaZrO3–MgO–Fe2O3 (x = 0.52 and 0.70) with trace amounts of LiMgFeTiO4 inverse spinel and (Li,K)2(Mg,Fe,Ti,Nb)6O13 layered structure have been investigated by transmission electron microscopy (TEM) and synchrotron powder X-ray diffractometry (XRD) with varying temperatures. The bright-field TEM images have shown tetragonal 90°-domain contrasts at 80 and 40 °C, and the XRD profile has been simulated by adding an average structure of two differently oriented tetragonal structures bound by a 90°-domain wall for the x = 0.52 sample. Aggregates of tilted NbO6 nanodomains have been observed in a high-resolution TEM image, and the crossover of P4mm–Amm2 features from 60 to 20 °C and diffuse 2 × 2 × 2 superlattice reflections of the tilted NbO6 Imm2 structure have been observed in XRD data for the x = 0.70 sample.

Characteristic Design of Microwave Dielectric Materials by Using Self-Flux Compositions

In order to improve the sinterability and controllability of dielectric properties, we focused on self-flux composition. In the case of Ba(Mg1/3Nb2/3)O3, we selected Ba(1-b)NbbOd as self-flux system and investigated correlations between Q-factor and the b value. Interestingly, high Q-value was obtained only at the b=0.45. Moreover, the dielectric constant and tf change linearly by the quantity of Ba0.55Nb0.45Od under keeping up high-Q value. As a result, it was indicated that the dielectric properties could be controlled by the assumption of stoichiometric composition and liquid phase consisted by self-flux.