**1. Introduction**

## **1.1. Improvement of microstructure of (K,Na)NbO3-based lead-free piezoelectric ceramic with KTiNbO5 phase**

Recently, the development of lead-free piezoelectric ceramics as substitutes for lead zirconate titanate (PZT) has become an important objective. Alkaline niobate ceramics (K,Na)NbO3 exhibit particularly high piezoelectric characteristics and a relatively high Curie temperature (*T*c). However, the crystalline particles of these ceramics spontaneously form dice-like particles, which tend to generate voids between particles. The presence of an excessive number of voids in a sintered ceramic decreases its chemical stability and mechanical strength and facilitates dielectric breakdown during polarization because the electric field concentrates at the voids. Our research shows that these voids may degrade the piezoelectric properties of these materi‐ als. If this problem can be resolved, alkaline niobate ceramics could see use in practical applications; for example, in ultrasonic motors [1, 2], actuators [3], inkjet heads [4, 5], and transducers [6].

The preparation of alkaline niobate ceramics with high piezoelectric properties has been reported, the hot-press sintering method which decreases the crystal grain size, increases the density of the ceramics from 4.25 to 4.46 g/cm3 , and doubles the piezoelectric constant *d*33 from 80 to 160 pC/N [7]. For KNN prepared using the reactive-templated grain growth method, Saito et al. [8] reported a high piezoelectric constant of *d*<sup>33</sup> = 416 pC/N, which is equivalent to that of PZT.

**Figure 1.** Crystal structure of KTiNbO5 has a layered structure and is not piezoelectric material.

Although KNN has been reported to exhibit attractive piezoelectric characteristics, problems such as stability and productivity remain. Consequently, alkaline niobate ceramics are still under development. To fill these voids, we focus on combining KNN with a dielectric material. An example of such an approach was reported [9] that a glass phase (e.g., K3Nb3O6Si2O7) was added to KNN to improve the insulating characteristics of KNN by decreasing the particle diameter and the number of voids.

In our present study [10, 11], after due consideration of the dielectric constant, we combine KNN with the KTiNbO5 (NTK) phase, which has a layered structure as shown in **Figure 1** and is not piezoelectric material. With this approach, we prepared and densified a KNN–NTK composite ceramic that exhibits enhanced piezoelectric properties; notably, a planar-mode electromechanical coupling coefficient *k*<sup>p</sup> = 0.52, which is close to the highest value previously reported for KNN-based composite lead-free piezoelectric ceramics [8, 12–15].

**1. Introduction**

4 Piezoelectric Materials

**with KTiNbO5 phase**

transducers [6].

that of PZT.

density of the ceramics from 4.25 to 4.46 g/cm3

diameter and the number of voids.

**1.1. Improvement of microstructure of (K,Na)NbO3-based lead-free piezoelectric ceramic**

Recently, the development of lead-free piezoelectric ceramics as substitutes for lead zirconate titanate (PZT) has become an important objective. Alkaline niobate ceramics (K,Na)NbO3 exhibit particularly high piezoelectric characteristics and a relatively high Curie temperature (*T*c). However, the crystalline particles of these ceramics spontaneously form dice-like particles, which tend to generate voids between particles. The presence of an excessive number of voids in a sintered ceramic decreases its chemical stability and mechanical strength and facilitates dielectric breakdown during polarization because the electric field concentrates at the voids. Our research shows that these voids may degrade the piezoelectric properties of these materi‐ als. If this problem can be resolved, alkaline niobate ceramics could see use in practical applications; for example, in ultrasonic motors [1, 2], actuators [3], inkjet heads [4, 5], and

The preparation of alkaline niobate ceramics with high piezoelectric properties has been reported, the hot-press sintering method which decreases the crystal grain size, increases the

80 to 160 pC/N [7]. For KNN prepared using the reactive-templated grain growth method, Saito et al. [8] reported a high piezoelectric constant of *d*<sup>33</sup> = 416 pC/N, which is equivalent to

**Figure 1.** Crystal structure of KTiNbO5 has a layered structure and is not piezoelectric material.

Although KNN has been reported to exhibit attractive piezoelectric characteristics, problems such as stability and productivity remain. Consequently, alkaline niobate ceramics are still under development. To fill these voids, we focus on combining KNN with a dielectric material. An example of such an approach was reported [9] that a glass phase (e.g., K3Nb3O6Si2O7) was added to KNN to improve the insulating characteristics of KNN by decreasing the particle

, and doubles the piezoelectric constant *d*33 from

### **1.2. Tetragonal and orthorhombic two-phase coexisting state in KNN–NTK composite leadfree piezoelectric ceramic**

As described above, the KNN–NTK composite lead-free piezoelectric ceramic exhibits excellent piezoelectric properties. However, the crystal structure of the main phase of this KNN composite system has yet to be fully determined. Thus, the crystal structure must be elucidated before the piezoelectric properties of this material can be exploited. The crystal structure of KNN-based piezoelectric ceramics has been investigated by many groups [16–19]. For example, Ahtee and Glazer [20], Ahtee and Hewat [21], and Baker et al. [22, 23] proposed phase diagrams for undoped KNN for various values of Na fraction *x*, where *x* = Na/(K + Na). With increasing temperature and for *x* ≤ 0.5, the crystal system of an undoped KNN ceramic with a perovskite-type structure is suggested to change from orthorhombic to tetragonal and then to cubic.

Many reports exist stating that the crystal system of KNN can be controlled using additives. The orthorhombic–tetragonal polymorphic phase transition temperature may even be lowered below room temperature [24, 25]. Guo et al. [25] reported that the main phase of LiNbO3-doped KNN ceramic is a tetragonal system at room temperature. Rubio-Marcos et al. [26, 27] reported how KNN is affected by doping with the fourth-period transition metal oxides, MO (M = Ni, Cu, Co, and Mn). Based on powder X-ray diffraction (XRD) studies, they concluded that the *P*4*mm* tetragonal structure of KNN is stabilized in MO-doped KNN ceramics at room tem‐ perature. They also suggest that the tetragonality aspect ratio *c*/*a* correlates with the piezo‐ electric properties of the doped KNN system.

Optimizing the morphotropic phase boundary (MPB) composition is widely thought to improve the properties of piezoelectric materials. Dai et al. [28] reported the dependence of the crystal system and piezoelectric properties of an undoped K1–*x*Na*x*NbO3 system in the composition range for 0.48 ≤ *x* ≤ 0.54. They assumed that the MPB composition of undoped KNN lies within this range of Na fraction and suggested that the MPB exists in the range *x* = 0.520–0.525 at room temperature, which separates monoclinic and orthorhombic phases. The maximum piezoelectric constant *d*33 = 160 pC/N occurs at *x* = 0.52. Recently, Karaki et al. reported that the slope of the MPB in the BaZrO3–KNN binary system is adjustable. Upon increasing the (Bi,Na)TiO3 content, the slope of the tetragonal–rhombohedral MPB slope of BaZrO3–KNN changes from negative to positive [29].

In this work, we investigate the crystal structure, texture, and piezoelectric properties of a series of KNN-based composite systems (K1–*x*Na*x*)CaLiNb–NbTiK–BaZr–CoFeZn, using synchrotron powder XRD, high-resolution transmission electron microscopy (HR-TEM), selected-area electron diffraction (SAD), while varying the K/Na ratio over the range 0.33 ≤ *x* ≤ 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 piezoelectric properties because of the two-phase coexisting state.
