Piezoelectric properties of KNN–NTK composite lead-free piezoelectric ceramic and of MT-18K (Navy Type I PZT, NGK Spark Plug Co., Ltd.).
We developed a (K,Na)NbO3-based lead-free piezoelectric ceramic with a KTiNbO5 system, (K1−xNax)0.86Ca0.04Li0.02Nb0.85O3−δ–KTiNbO5–BaZrO3–Co3O4–Fe2O3–ZnO (KNN–NTK composite). This KNN–NTK composite exhibits a very dense microstructure, and kp = 0.52, ε33T/ε0 = 1600, and d33 = 252 pC/N. We found that a portion of the KTiNbO5 converted into K2(Ti,Nb,Co,Zn)6O13 and/or CoZnTiO4. We were able to reproducibly prepare granulated powder of KNN–NTK in batches of 100 kg using a spray-dryer. In addition, we performed a detailed investigation of the microstructure of KNN–NTK composite. The results show that a tetragonal and an orthorhombic phase coexist in a main KNN phase over a wide range of 0.56 ≤ x ≤ 0.75. The granular nanodomains of the orthorhombic phase dispersed within the tetragonal matrix in the KNN phase. A maximum value of kp = 0.56 occurred for x = 0.56. The Na fraction x corresponding to maximum kp was also the minimum x required to generate the orthorhombic phase. We conclude that the KNN–NTK composite exhibits excellent piezoelectric properties because of the two-phase coexisting state. This gentle phase transition of KNN–NTK composite seems to be a relaxor, but the diffuseness degree γ = 1.07 suggests otherwise.
- two-phase coexisting
- coupling coefficient
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 (
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
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  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
1.2. Tetragonal and orthorhombic two-phase coexisting state in KNN–NTK composite lead-free 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 , Ahtee and Hewat , and Baker et al. [22, 23] proposed phase diagrams for undoped KNN for various values of Na fraction
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.  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
Optimizing the morphotropic phase boundary (MPB) composition is widely thought to improve the properties of piezoelectric materials. Dai et al.  reported the dependence of the crystal system and piezoelectric properties of an undoped K1–
In this work, we investigate the crystal structure, texture, and piezoelectric properties of a series of KNN-based composite systems (K1–
2. Experimental method
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–
The calcined powders were again weighed assuming the chemical formula 0.92K1–
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 into discs under a uniaxial pressure of 200 kg/cm2. The samples were sintered in air at 1150°C 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
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 1607.
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 CeO2 specimen. The crystal structure was analyzed by Rietveld refinement with the help of the RIETAN-2000 code . The profile parameters were refined using the split-type Pearson VII function , and partial profile relaxation was applied to the diffraction peaks from the domain-wall planes. The values reported by Waasmaier and Kirfel  were used to correct for dispersion.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) data were acquired using a PHI TRIFT V nano TOF with a 30 keV Bi3++ primary ion source in pulsed mode. For each spectrum, the area analyzed is 50 × 50 μm. The mass resolution (m/Δm) typically exceeds 4000 for the (m/z) 27 peaks in the positive ion spectra. Positive ion spectra were mass calibrated using CH3+, C2H5+, and C3H7+ fragments.
For TEM analysis, the samples were cut into 3 mm in diameter discs, polished to a thickness of approximately 50 μm and dimpled to approximately 10 μm thick at the disc center. The specimens were prepared by ion milling with 2–4 keV Ar ions incident at an angle of 4° with respect to the normal to the sample surface. High-resolution TEM observations were made using a TOPCOM EM-002B TEM equipped with an energy dispersive X-ray spectrometer (EDS) with a 200 keV accelerating voltage. To acquire the SAD patterns, we used a 200 nm diameter aperture.
3. Results and discussions
3.1. KNN–NTK composite lead-free piezoelectric ceramic
3.1.1. Improvement of microstructure of KNN piezoelectric ceramic with NTK phase
Figure 2 shows SEM images of KNN–NTK composite lead-free piezoelectric ceramic and a Li-doped KNN single-phase ceramic  for comparison. As shown in Figure 2a, many voids approximately 10 μm in size appear in the Li-doped KNN ceramic. In contrast, such voids are rare in the image of the KNN–NTK composite ceramic in Figure 2b. By comparing these images, the effect of the NTK phase becomes clear; namely, the KNN–NTK composite lead-free piezoelectric ceramic forms a very dense surface with few voids.
Figure 3 shows TEM-EDS elemental mapping of the KNN–NTK composite ceramic. The Na map shows dice-like particles; these correspond to the KNN phase. The low-intensity area of the Na map seems to be voids. However, this area corresponds to the high-intensity area of the Ti map, that is, the low-intensity area of the Na map is not voids but correspond to the NTK phase. These results indicate that the voids are filled with the NTK phase. Furthermore, the high-intensity areas of the Co and Zn maps correspond to those of the Ti map, and the concentrations of the infinitesimal additives (e.g., Co and Zn) in the KNN phase are low.
Figure 4 shows an XRD pattern of the KNN–NTK composite ceramic. The small peaks marked with triangles, open circles, and closed circles in the enlarged view shown in Figure 4b are attributed to KTiNbO5 (PDF#04-010-2961), K2(Ti,Nb,Co,Zn)6O13 (PDF#00-039-0822), and CoZnTiO4 (PDF#04-006-7279), respectively. The stoichiometric KTiNbO5 is reported to be a dielectric material . However, KTi1−
Li appears frequently in the KNN system. Figure 5 shows positive ion images of the KNN–NTK composite ceramic obtained by ToF-SIMS. The high-intensity area corresponds to high element concentration. These images show that K and Nb have similar distributions, so the high-intensity areas in these images must correspond to the KNN phase. However, the image of Li is not the consistent with those of K and Nb, whereas the image of Li is similar to that of Ti. In other words, Li probably exists in KTiNbO5, K2(Ti,Nb,Co,Zn)6O13 and CoZnTiO4. Therefore, at least for our materials, Li diffused out from the KNN phase, so less Li remains in the KNN phase than was put in when we blended it to make the KNN phases.
Figure 6a shows an annular bright-field STEM image of the NTK phase. The NTK phase has a layered structure; the K layer and the layer composed of Ti and Nb fall on a line. This elemental alignment corresponds to that of the KTiNbO5 structure (see Figure 1). Therefore, we conclude that the NTK phase remains intact in KNN–NTK composite. Figure 6b shows a Cs-STEM image of a KNN/NTK interface. In general, in a material that consists of two or more phases, diffusion at the interface of the different phases directly deteriorates the electrical properties of the materials and must therefore be avoided. However, no intermediate phase is observed in the KNN/NTK interface region. Therefore, because of the difference between the formation temperatures of the phases, the NTK phase must have crystallized via epitaxial-like growth on the KNN crystal grain during sintering, so both phases are assumed to have adhered. The plane direction of a KNN/NTK interface is (001) or (100) and (001); that is, the NTK (001) plane grows on the KNN (001) or (100) plane.
As previously mentioned, the NTK phase contains the additives. Thus, the absorption of these additives must have reacted with a portion of the NTK phase. The single phases of KTiNbO5 and CoZnTiO4 were sintered at 1100 and 1050°C, respectively [33, 35]. Therefore, they crystallized during cooling after the KNN phase was crystallized. This sintering reaction must have proceeded through the liquid-phase sintering.
The resistivity of the Li-doped KNN ceramic is 6.0 × 107 Ω cm , whereas that of the KNN–NTK composite ceramic is 3.6 × 1010 Ω cm. This KNN–NTK composite ceramic was polarized under a high voltage of 6 kV/mm. Because the voids are filled with the NTK phase, the electric field does not concentrate at the voids, resulting in improved polarizability.
3.1.2. Piezoelectric properties and productivity of KNN–NTK composite lead-free ceramic
KNN–NTK composite lead-free piezoelectric ceramic exhibits excellent piezoelectric properties, with the planar-mode electromechanical coupling coefficient
|Piezoelectric constant (pC/N)||240||340|
|Frequency constant (Hz m)||3170||2200|
|Elastic compliance coefficient (pm2/N)||12.0||15.7|
|Dielectric loss (%)||tan ||1.9||0.4|
|Mechanical quality factor||88||1800|
|Curie temperature (°C)||290||300|
Figure 7 shows the planar-mode resonance characteristics of a KNN–NTK composite ceramic disc. The maximum phase angle
However, the mechanical quality factor for KNN–NTK composite ceramic is
Figure 8 shows the dielectric constant
Figure 9 shows the aging properties of the coupling coefficient
|Bending strength (MPa)||117|
|Vickers hardness (N/mm2)||518|
|Young’s modulus (GPa)||100|
|Thermal conductivity (W/m K)||2.5|
The mechanical properties of KNN–NTK composite ceramic are summarized in Table 2. The bending strength of KNN–NTK composite ceramic is 117 MPa, which exceeds that of MT-18K of 100 MPa. All mechanical properties of KNN–NTK composite lead-free piezoelectric ceramic are equal or exceed those of conventional PZT.
Mass production is also an important factor for commercialization. We scaled the manufacturing process to 100 kg per batch for granulated ceramic powder using a spray-drying technique (Figure 10a). The calcination process is very important for obtaining high-quality spray-drying powder. Piezoelectric elements in the form of 70 mm in diameter, 10 mm in thick discs were prepared from these powders. Furthermore, we conducted durability tests of a knocking sensor fabricated with this KNN–NTK composite lead-free piezoelectric ceramic (Figure 10b). The results showed that the durability of the sensor fabricated with the KNN–NTK composite was equal or superior to that of the sensor fabricated with PZT. Moreover, the output level of KNN–NTK composite-based sensor almost approaches that the PZT-based sensor. We confirmed that the resulting KNN–NTK composite lead-free piezoelectric ceramic still had attractive piezoelectric properties.
3.2. Improvement of KNN–NTK composite lead-free piezoelectric ceramic with two-phase coexisting state
3.2.1. Tetragonal and orthorhombic two-phase coexisting state in the KNN–NTK composite lead-free piezoelectric ceramic
To improve the piezoelectric properties, we analyze in detail the crystal structure and phase transition. Figure 11 shows XRD patterns as a function of 2
All main diffraction peaks in the XRD patterns are attributed to the perovskite-type structure. These peaks appear for 0.33 ≤
The main features of the XRD pattern for
Assuming the combination of
Figure 12 shows a structural model of
Figure 13 shows the cell volume and tetragonality ratio
The maximum value of 363 μC/cm2 is about an order of magnitude larger than that of undoped KNN . These results suggest that the dielectric polarization
The structural information obtained from XRD is dominated by the structure averaged over the macroscopic volume. In other words, it is not sensitive to identify the microstructure of ceramic. In this study, TEM was used to investigate the KNN–NTK composite ceramic microstructure.
Figure 14 shows SAD patterns obtained from a single grain of KNN in the KNN–NTK composite ceramic. The top row shows pc, and the bottom row shows pc zone-axis SAD patterns. From left to right, the panels correspond to
Although the peaks of the 2 × 2 × 2 superlattice phase do not appear in the XRD patterns for
If the secondary superlattice phase of KNN that exists for
Figure 15a–c show the results of the inverse FT treatment of the HR-TEM images of samples for
3.2.2. Phase transition and piezoelectric properties of KNN–NTK composite lead-free piezoelectric ceramic
Figure 16 shows the dielectric constant
The enhanced piezoelectric properties of PZT near the MPB composition are suggested to mainly originate from the polarization rotation rather than from the formation of nanodomains . However, the coexistence in a PZT system of the tetragonal structure with <001> polarization and the rhombohedral structure with <111> polarization can still be correlated with easier rotation of the polarization direction, because it indicates the similar free energies of the two phases and a lower energy barrier for polarization rotation. In our KNN–NTK composite lead-free piezoelectric ceramic, we observe the coexistence of orthorhombic nanodomains dispersed in the tetragonal matrix over a wide range of Na fraction for 0.56 ≤
The dielectric polarization
In contrast, the coupling coefficient
The phase transition of the KNN–NTK composite piezoelectric ceramic occurs gently, and the orthorhombic and tetragonal phases coexist in the KNN for a wide range of
Figure 17a shows the dielectric constant of K1–
The diffuseness can be described by a modified Curie–Weiss law ,
We developed K1–
In this system, KNN forms the single tetragonal phase for
The synchrotron radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI). Part of this work was supported by Japan Fine Ceramics Center (JFCC).