**3. Results and discussions**

≤ 0.75. The results clarify that the granular nanodomains of the orthorhombic phase dispersed within the tetragonal matrix are in a KNN phase. Furthermore, we identify a relationship between the piezoelectric properties and the two-phase coexisting state, which reads to the conclusion that the KNN–NTK composite lead-free piezoelectric ceramic exhibits excellent

The samples were prepared by a conventional solid-state reaction method. The raw materials were powders of K2CO3, Na2CO3, Li2CO3, Nb2O5, CaCO3, TiO2, BaCO3, ZrO2, Co3O4, Fe2O3, and ZnO with a purity of more than 99%. Here, K1–*x*Na*x*N, NTK, and BZ refer to (K1– *<sup>x</sup>*Na*x*)0.86Ca0.04Li0.02Nb0.85O3–δ, K0.85Ti0.85Nb1.15O5, and BaZrO3, respectively. At the outset, hygro‐ scopic powders of K2CO3, Na2CO3, and Li2CO3 were dried in an oven at 150°C for 2 h to ensure accurate weighing. The masses of K1–*x*Na*x*N, NTK, and BZ corresponding to their respective composition formulas were weighted out, after which the materials, were calcined at 930°C in

The calcined powders were again weighed assuming the chemical formula 0.92K1–*x*Na*x*N– 0.047NTK–0.023BZ–0.0017Co3O4–0.002Fe2O3–0.005ZnO (*x* = 0.51) and 0.910K1–*x*Na*x*N– 0.042NTK–0.036BZ–0.0016Co3O4–0.0025Fe2O3–0.0069ZnO (*x* = 0.33, 0.42, 0.50, 0.56, 0.58, 0.61,

The weighed powders were mixed with a ball mill for 15 h and then re-calcined at 930°C in air for 4 h. The dispersant and binder were added to the calcined powder, and the mixture was ball-milled for 15 h. The slurry was filtered through a 25 μm mesh sieve and dried, and the dried powder was classified with a 250 μm mesh sieve. The classified powders were pressed

for 4 h, following which they were polished and silver electrodes were painted onto both surfaces of the samples. The samples used for electrical properties measurements were 35 mm in diameter and 2 mm in thick for *x* = 0.51, and 15 mm in diameter and 1 mm in thick for *x* = 0.33, 0.42, 0.50, 0.56, 0.58, 0.61, 0.67, 0.71, and 0.75 for the planar and thickness vibration modes. For the transverse-, longitudinal-, and shear-vibration mode, the samples dimensions were, 23 × 3 × 0.15 mm, 2.2 mm in diameter and 8 mm in thick, and 7 × 2 × 0.2 mm, respectively. The samples were polarized for 30 min in silicone oil under an electric field of 6 kV/mm at 80°C.

The piezoelectric properties of 1-day-old samples were measured using the resonance– antiresonance method with Hewlett-Packard 4194A impedance analyzer. The mechanical characteristics of 3 × 4 × 40 mm were evaluated according to the Japan Industrial Standard R

XRD samples were prepared by grinding particles with a 10 μm initial diameter in a Si3N4 mortar. The resulting fine powder was sealed in a 0.3 mm diameter Lindemann glass capillary. XRD measurements were done at the BL19B2 beam line of SPring-8 synchrotron, which is equipped with a Debye–Scherrer camera. The incident X-ray wavelength was estimated to be 0.69948 Å by calibration with a standard CeO<sup>2</sup> specimen. The crystal structure was analyzed

. The samples were sintered in air at 1150°C

piezoelectric properties because of the two-phase coexisting state.

**2. Experimental method**

air for 4 h.

6 Piezoelectric Materials

1607.

0.67, 0.71, and 0.75).

into discs under a uniaxial pressure of 200 kg/cm2
