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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.
NGK Spark Plug Co., Ltd., Iwasaki, Komaki, Aichi, Japan
*Address all correspondence to: k-obayashi@mg.ngkntk.co.jp
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 (Tc). 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 materials. 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 d33 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 d33 = 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 kp = 0.52, which is close to the highest value previously reported for KNN-based composite lead-free piezoelectric ceramics [8, 12–15].
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 [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 P4mm tetragonal structure of KNN is stabilized in MO-doped KNN ceramics at room temperature. They also suggest that the tetragonality aspect ratio c/a correlates with the piezoelectric 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–xNaxNbO3 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 d33 = 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–xNax)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.
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–xNaxN, NTK, and BZ refer to (K1–xNax)0.86Ca0.04Li0.02Nb0.85O3–δ, K0.85Ti0.85Nb1.15O5, and BaZrO3, respectively. At the outset, hygroscopic 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–xNaxN, NTK, and BZ corresponding to their respective composition formulas were weighted out, after which the materials, were calcined at 930°C in air for 4 h.
The calcined powders were again weighed assuming the chemical formula 0.92K1–xNaxN–0.047NTK–0.023BZ–0.0017Co3O4–0.002Fe2O3–0.005ZnO (x = 0.51) and 0.910K1–xNaxN–0.042NTK–0.036BZ–0.0016Co3O4–0.0025Fe2O3–0.0069ZnO (x = 0.33, 0.42, 0.50, 0.56, 0.58, 0.61, 0.67, 0.71, and 0.75).
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 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 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 [30]. The profile parameters were refined using the split-type Pearson VII function [31], and partial profile relaxation was applied to the diffraction peaks from the domain-wall planes. The values reported by Waasmaier and Kirfel [32] 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.
3.1.1. 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 [10] 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 2.
SEM images of polished surface of (a) Li-doped KNN prepared under common conditions and (b) KNN–NTK composite lead-free piezoelectric ceramic. Scale bar = 10 μm.
Figure 3.
TEM-EDS elemental mapping, indicating the spatial distribution of K, Na, Nb, Ti, Co, and Zn in KNN–NTK composite lead-free piezoelectric ceramic. Scale bar = 1 μm.
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 [33]. However, KTi1−xNb1+ xO5, which contains oxygen defects, is reported to exhibit semiconducting behavior [34]. The NTK used in the present work is not a simple material; it was complexed with KNN and sintered under ambient atmosphere. Therefore, it must represent the settled ratio that forms at thermal equilibrium during sintering. K2(Ti,Nb,Co,Zn)6O13 has a layered monoclinic structure C2/m, and CoZnTiO4 has an inverse spinel-type structure [35]. Thus, a portion of the NTK phase must have transformed into K2(Ti,Nb,Co,Zn)6O13 and/or CoZnTiO4 by a reaction with Co and/or Zn solutes in the phase. However, tungsten bronze-type Ba2KNb5O15 appeared in the specimens sintered under unsuitable conditions.
Figure 4.
(a) XRD pattern from KNN–NTK composite ceramic and (b) enlarged view of panel (a).
Figure 5.
Positive ion images of KNN–NTK composite lead-free piezoelectric ceramic obtained by ToF-SIMS. Scale bar = 10 μm.
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.
Figure 6.
(a) STEM image of NTK phase, scale bar = 1 nm. (b) TEM image of NTK/KNN interface of KNN–NTK composite ceramic. Scale bar = 5 nm.
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 [10], 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 kp = 0.52 and the dielectric constant ε33T/ε0 = 1600. The value of ε33T/ε0 is equivalent to that of PZT, implying that the KNN–NTK composite ceramic is a suitable substitute for PZT. The piezoelectric properties of KNN–NTK composite ceramic are summarized in Table 1.
KNN–NTK
MT-18K
Dielectric constant
ε33T/ε0
1600
1450
Coupling coefficient
kp
0.52
0.60
kt
0.41
0.41
k33
0.57
0.72
k31
0.29
0.34
k15
0.48
0.54
Piezoelectric constant (pC/N)
d33
240
340
d31
104
142
d15
312
300
Frequency constant (Hz m)
Np
3170
2200
Nt
2940
2150
N33
2210
1500
N31
2220
1650
N15
1420
1300
Elastic compliance coefficient (pm2/N)
s33E
12.0
15.7
Dielectric loss (%)
tan δ
1.9
0.4
Mechanical quality factor
Qm
88
1800
Density (g/cm3)
ρ
4.54
7.60
Curie temperature (°C)
Tc
290
300
Table 1.
Piezoelectric properties of KNN–NTK composite lead-free piezoelectric ceramic and of MT-18K (Navy Type I PZT, NGK Spark Plug Co., Ltd.).
Figure 7 shows the planar-mode resonance characteristics of a KNN–NTK composite ceramic disc. The maximum phase angle θ is 86°, and sufficient phase inversion is observed. The elastic compliance coefficient s33E of 12 pm2/N is much smaller than that for conventional PZT. This small elastic compliance coefficient causes the piezoelectric constant d33 of KNN–NTK composite ceramic to be less than that of the conventional PZT.
However, the mechanical quality factor for KNN–NTK composite ceramic is Qm = 88, which is almost the same as that of the conventional Navy Type II PZT. Other characteristics of KNN–NTK composite ceramic include the frequency constant Np of 3170 Hz m, which is about 50% greater than that of conventional PZT, and the density, which is fairly less than that of conventional PZT. These characteristics of KNN-based piezoelectric ceramic deserve attention.
Figure 7.
Planar-mode resonance characteristics of KNN–NTK lead-free piezoelectric ceramic disc of 35 mm in diameter and 2 mm in thick. Impedance magnitude is drawn as a solid line, and phase angle is drawn as a dotted line.
Figure 8 shows the dielectric constant ε33T/ε0 and the coupling coefficient kp as a function of temperature between −50 and 350°C. The Curie temperature Tc of the KNN–NTK composite ceramic is 290°C, which is equivalent to that of conventional PZT. For comparison, Figure 8 also shows the dielectric constant of MT-18K (Navy Type I PZT, NGK Spark Plug Co., Ltd.) near room temperature. In the practical temperature range from 0 to 150°C, the rate of change of the dielectric constant of MT-18K exceeds 80%, whereas that of the KNN–NTK composite ceramic is less than 10%. The temperature dependence of the dielectric constant of KNN–NTK composite ceramic is thus much weaker than that of Navy Type I PZT MT-18K. Consequently, the thermal stability of KNN–NTK composite ceramic is confirmed, and its Tc is sufficiently high to satisfy the requirements for in-vehicle applications. After 1000 temperature cycles between −40 and 150°C, the rate of the piezoelectric constant d33 of KNN–NTK composite ceramic decreased by less than 2%, which compares favorably with that of 10% for MT-18K. Therefore, this KNN–NTK composite ceramic offers an advantage for sensor applications. Furthermore, the dielectric constant of this KNN–NTK composite ceramic does not significantly vary within the practical temperature range that is common in conventional KNN piezoelectric ceramics.
Figure 9 shows the aging properties of the coupling coefficient kp and the frequency constant Np of KNN–NTK composite ceramic. The same parameters of Navy Type I PZT MT-18K are also shown. To facilitate comparison, the initial values are normalized to unity. The rate of deterioration in the coupling coefficient kp of MT-18K was approximately 7% after the sample was aged by polarization for 1000 days, whereas kp of the KNN–NTK composite ceramic decreases by approximately 4% under the same conditions. As found for these aging characteristics, kp of KNN–NTK composite ceramic ages better than that of MT-18K. Similarly, the frequency constant Np of MT-18K increases by approximately 2% upon similar aging, whereas that of KNN–NTK composite ceramic remains unchanged, indicating that Np for KNN–NTK composite ceramic is extremely stable.
Figure 8.
Temperature dependence of (a) dielectric constant ε33T/ε0 and (b) planar-mode electromechanical coupling coefficient kp of KNN–NTK composite piezoelectric ceramic.
Figure 9.
Aging characteristics of (a) planar-mode electromechanical coupling coefficient kp, and (b) frequency constant Np of KNN–NTK composite lead-free piezoelectric ceramic, comparing with Navy Type I PZT MT-18K.
Bending strength (MPa)
117
Vickers hardness (N/mm2)
518
Young’s modulus (GPa)
100
Poisson’s ratio
0.36
Thermal conductivity (W/m K)
2.5
Table 2.
Mechanical properties of KNN–NTK composite lead-free piezoelectric ceramic.
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.
Figure 10.
(a) Granulated powder for KNN–NTK composite lead-free piezoelectric ceramic. (b) KNN–NTK composite lead-free piezoelectric ceramic element for knocking sensor.
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 2θ from 14° to 22° for pulverized samples of KNN–NTK composite lead-free piezoelectric ceramic and with a magnified intensity scale. The strong peaks at 2θ = 14.2°, 17.4°, and 20.2° correspond to the Miller indices of the KNN phase (110pc, 111pc, and 200pc, respectively) with perovskite-type structure. Here, the subscript “pc” refers to the pseudo-cubic cell. Weak peaks marked by solid circles in Figure 11 are assigned to CoZnTiO4, which has an inverse spinel-type structure. The Miller indices for these peaks are 311, 222, and 400, respectively. Throughout the range 0.33 ≤ x ≤ 0.75, the intensities of the weak peaks are almost unchanged. We suggest that the formation of CoZnTiO4 depends on the element and the amount of additives but is independent of the Na fraction.
Figure 11.
XRD patterns of K1–xNaxN–NTK composite lead-free piezoelectric ceramic in 2θ range from 14° to 22°, diffraction peaks of orthorhombic phase are can be seen only at x = 0.75.
All main diffraction peaks in the XRD patterns are attributed to the perovskite-type structure. These peaks appear for 0.33 ≤ x ≤ 0.67 and are attributed to the tetragonal KNN system, which is known as the high temperatures stable structure of undoped KNN [20]. Assuming P4mm symmetry, the Rietveld refinement fits significantly better compared with the results obtained upon assuming the ideal cubic perovskite-type structure. The R-values of the Rietveld refinements are RI = 3.6–4.8% and RF = 1.8–2.2% for the P4mm tetragonal model, whereas RI = 6.1–12.8% and RF = 3.6–7.0% for the Pm–3m cubic model. The results of the XRD analysis show that KNN for 0.33 ≤ x ≤ 0.67 is a single-phase tetragonal system and likely belongs to the P4mm symmetry. Assuming that P4mm symmetry restricts the displacement of the atoms to be along the c axis, the NbO6 octahedra permit no tilting, so the tilt system should be expressed by the Glazer notation a0a0c0 [36].
The main features of the XRD pattern for x = 0.75 do not significantly change compared with those for x ≤ 0.67. However, the XRD pattern for x = 0.75 shows weak peaks that cannot be assigned to the tetragonal system with a0a0c0. Ahtee and Glazer suggested that the crystallographic symmetry of undoped Na-rich (ca. 0.75 < x < 0.9) KNN ceramic at temperatures ranging from 200 to 400°C belongs to the Imm2 space group (a+b+c0 system) [20, 21], whereas Baker et al. [23] suggested that the symmetry belongs to the Amm2 space group (a+b0c0 system). We hypothesize that Na-rich KNN has Imm2 symmetry with an a+b+c0 tilt system on the structure refinement because the optimized lattice constants of the pseudo-cubic cell indicate that this assignment is more appropriate. The weak peaks are attributed to the Imm2 orthorhombic phase, which has the tilting of the NbO6 octahedra. The Miller indices for these peaks are {310}, {321}, and {330}. This orthorhombic structure has double lattice constants, which are represented by the 2 × 2 × 2 superlattice setting in the pseudo-cubic cell.
Assuming the combination of P4mm tetragonal and Imm2 orthorhombic structures, the XRD patterns for x = 0.75 are fit by two-phase Rietveld refinement. The overall R-factor is estimated to be RP = 5.87% with the two-phase model, whereas at best RP = 6.96% with the single-phase P4mm model. The lattice constants of the orthorhombic structure are estimated to be a = 7.88875 Å, b = 7.93082 Å, and c = 7.96895 Å.
Figure 12.
Structure model of Imm2 orthorhombic phase projected along the [100] direction for x = 0.75.
Figure 12 shows a structural model of Imm2 projected along the [010] direction. This orthorhombic structure has tilt ordering of the NbO6 octahedra, where 2 × 2 × 2 Immm symmetry is predicted without deformation of the NbO6 octahedra. The NbO6 octahedra are likely to be simultaneously deformed and tilted in the Imm2 phase of this composite system. Note that because Imm2 is a noncentrosymmetric space group, it allows polarization; in contrast, because Immm is a centrosymmetric space group, it forbids polarization. The structural details optimized by the Rietveld refinement will be discussed in another presentation.
Figure 13 shows the cell volume and tetragonality ratio c/a of the primary tetragonal phase calculated from the dimensions of the crystal unit cell. It also shows the dielectric polarization P estimated from the point-charge model with the formal charges of the ions located at positions optimized by the Rietveld refinements. The cell volume monotonically decreases with increasing Na fraction x, which is caused by the decrease in effective ionic radius upon replacing K+ with Na+. However, the rate of decline increases in the range x > 0.56. At x = 0.75, the cell volume 62.01 Å3 of the primary tetragonal phase approaches that of the pseudo-cubic cell of the secondary orthorhombic phase 62.32 Å3. The tetragonality ratio of the tetragonal phase is estimated to lie between 1.010 and 1.012 for 0.33 ≤ x ≤ 0.67, and to be 1.006 for x = 0.75. The value defined by 2c/(a + b) for the secondary orthorhombic phase is estimated to be 1.007 for x = 0.75, which is close to the tetragonality ratio c/a of the primary phase of this composition. We hypothesize that the maximum value of the tetragonality ratio occurs around x = 0.50. However, the estimated dielectric polarization P increases gradually for 0.33 ≤ x ≤ 0.50 and drops sharply for 0.67 ≤ x ≤ 0.75.
Figure 13.
Comparison of cell volume, tetragonality ratio c/a, and dielectric polarization P of K1–xNaxN–NTK composite lead-free piezoelectric ceramic as a function of Na fraction x. Sample for x = 0.75 gives the values of the primary tetragonal phase.
The maximum value of 363 μC/cm2 is about an order of magnitude larger than that of undoped KNN [37]. These results suggest that the dielectric polarization P cannot be correlated with the tetragonality ratio. Note that the discrepancy between P and the tetragonality ratio has also been reported, for a PZT system [38].
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 [100]pc, and the bottom row shows [210]pc zone-axis SAD patterns. From left to right, the panels correspond to x = 0.33, 0.56, 0.58, 0.67, and 0.75, respectively. The 001pc, 011pc, and 1–20pc reflections appear in all SAD patterns for 0.33 ≤ x ≤ 0.75. The SAD patterns for x = 0.33 consist only of these spots, which conforms to the single-phase model that we derive from the Rietveld refinement. However, superlattice reflections are observed for x = 0.56, 0.58, 0.67, and 0.75. The slanted and vertical arrows in Figure 14c, e–j indicate the directions indexed by 011 and 1–21 based on the 2 × 2 × 2 superlattice unit cell. However, the SAD patterns in Figure 14h, j exhibit very weak spots (sideways arrows) that cannot be assigned to the 2 × 2 × 2 superlattice structure.
Figure 14.
SAD patterns of (a) [100]pc and (b) [210]pc zone-axis for x = 0.33, (c) [100]pc and (d) [210]pc zone-axis for x = 0.56, (e) [100]pc and (f) [210]pc zone-axis, (g) [100]pc and (h) [210]pc zone-axis for x = 0.67, (i) [100]pc and (j) [210]pc zone-axis for x = 0.75. The spots marked by slanted and vertical arrows are superlattice reflections based on the 2 × 2 × 2 superlattice unit cell.
Although the peaks of the 2 × 2 × 2 superlattice phase do not appear in the XRD patterns for x = 0.58 and 0.67, TEM analysis indicates that the superlattice phase of KNN does exist at these Na fractions, in the same way as they do for x = 0.75. The SAD pattern for x = 0.56 shows broad and dim superlattice reflections, which suggest that a short coherent length for the structural modulation. We believe that the KNN phase of the KNN–NTK composite lead-free piezoelectric ceramic around x = 0.56 consists of a two-phase coexisting state. According to the Rietveld refinement discussed above, the weight fraction of the superlattice phase is estimated to be 44.2 wt% for x = 0.75. In the K1–xNaxN–NTK system, the tetragonal and orthorhombic phases of KNN coexist for x ≥ 0.56, with the volume fraction of the orthorhombic phase gradually increases with increasing Na fraction x.
If the secondary superlattice phase of KNN that exists for x = 0.56, 0.58, 0.67, and 0.75 has the tilt-ordered structure, the weak spots can naturally be assigned to the 1/2{hh0}pc (h: odd) planes, whereas such spots are not observed for the primary P4mm tetragonal phase. We calculated the Fourier transforms (FT) of the HR-TEM images (i.e., extracted the 1/2{110}pc spots) and synthesized the dark-field images using the inverse FT of the extracted peaks.
Figure 15a–c show the results of the inverse FT treatment of the HR-TEM images of samples for x = 0.58, 0.67, and 0.75. In the images, the brighter areas correspond to the superlattice phase. We also applied EDS to the dark and bright areas to confirm that the contrast is not caused by the compositional segregation within the local area. The probe has a diameter of approximately 1.0 nm. The contrast shown in the inverse FT-treated images suggests that the tilt ordering of the superlattice phase is confined within the granular nanodomains dispersed in the tetragonal matrix. The granular nanodomains gradually increase with x for 0.58 ≤ x ≤ 0.67, and an abrupt increase and agglomeration is observed at x = 0.75. The formation of the superlattice structure with the tilting of the NbO6 octahedra is probably caused by the reduction in cell volume with increasing of the smaller Na+ radius in the large x region. Considering the XRD, SAD, and FT-treated HR-TEM results, the primary phase of the KNN belongs to 1 × 1 × 1 tetragonal structure, whereas the secondary phase belongs to a 2 × 2 × 2 orthorhombic structure with the tilt ordering of the NbO6 octahedra.
Figure 15.
Inverse FT-treated HR-TEM images for (a) x = 0.58, (b) x = 0.67, and (c) x = 0.75, where the brighter area corresponds to the 2 × 2 × 2 Imm2 orthorhombic phase and the darkness areas correspond to the tetragonal phase matrix. Scale bar = 5 nm.
3.2.2. Phase transition and piezoelectric properties of KNN–NTK composite lead-free piezoelectric ceramic
Figure 16 shows the dielectric constant ε33T/ε0 and the coupling coefficient kp as a function of the Na fraction x. The ε33T/ε0 is almost constant for 0.33 ≤ x ≤ 0.56, then increases slightly for 0.56 < x ≤ 0.67, and finally drops sharply to lower values for 0.67 < x ≤ 0.75. The behavior of ε33T/ε0 is similar to that of the dielectric polarization P (see Figure 13).
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 [28]. 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 ≤ x ≤ 0.67. This result suggests a reduction in the energy barrier when the structure transforms from tetragonal to orthorhombic, and vice versa, and easier rotation of the polarization from [001] to [010], which may be assisted by the formation of the intermediate orthorhombic structure with small polarization in this compositional range.
Figure 16.
Phase transition and piezoelectric properties of K1–xNaxN–NTK composite lead-free piezoelectric ceramic as a function of Na fraction x.
The dielectric polarization P, calculated from the atomic positions optimized for the orthorhombic phase at x = 0.75, is 1.28 μC/cm2. The decrease in the dielectric constant ε33T/ε0 around x = 0.75 is partly related to the decrease in the tetragonal phase that results from the increase in the orthorhombic phase. The P of the orthorhombic phase is more than two orders of magnitude lower than that of the tetragonal phase.
In contrast, the coupling coefficient kp gradually increases with increasing x for 0.33 ≤ x ≤ 0.56, reaches a maximum of 0.56 near x = 0.56, and then decreases with increasing x for x ≥ 0.67. This behavior differs from that of the dielectric constant ε33T/ε0 or the dielectric polarization P, but resembles the behavior of the tetragonality ratio. The deterioration of kp for x > 0.56 is naturally related to the smaller tetragonality ratio in this region.
The point x = 0.56 at which the maximum coupling coefficient kp occurs corresponds to the minimum value of x at which the orthorhombic phase is generated. However, the highest dielectric constant occurs near x = 0.60, where the two-phase coexists progressed state. We thus conclude that this KNN–NTK composite lead-free piezoelectric ceramic exhibits excellent piezoelectric properties because of the two-phase coexisting state.
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 x > 0.56. In this way, this phase transition differs from the drastic phase transition at the MPB in PZT. This gentle transition is similar to the behavior of a relaxor. To verify the relaxation degree, we estimated the relaxor ferroelectricity of the KNN–NTK composite lead-free piezoelectric ceramic in the two-phase coexisting state.
Figure 17.
(a) Temperature dependences of dielectric constant of K1–xNaxN–NTK composite lead-free piezoelectric ceramic at 1 kHz (solid line), 10 kHz (dashed line), and 100 kHz (dotted line) for x = 0.67. (b) Behavior of inverse dielectric constant (1/ε − 1/εm) as a function of (T − Tm)γ.
Figure 17a shows the dielectric constant of K1–xNaxN–NTK composite lead-free piezoelectric ceramic as a function of temperature around Tc at frequency of 1, 10, and 100 kHz for x = 0.67. The dielectric constant hardly decreases with increasing frequency. Relatively sharp peaks corresponding to Tc appear around 280°C, but Tc does not shift as a function of frequency.
The diffuseness can be described by a modified Curie–Weiss law [39],
1ε−1εm=(T−Tm)γCE1
where γ is the diffusivity of dielectric relaxation, ranging from 1 for a normal ferroelectric to 2 for a relaxor ferroelectric. C is Curie constant, and Tm is the temperature at which the dielectric constant reaches its maximum εm. Figure 17b shows the inverse dielectric constant as a function of temperature at 100 kHz using K1–xNaxN–NTK composite ceramic for x = 0.67, at which the highest dielectric constant is obtained. The diffusivity constant γ estimated by a linear fit is 1.07. As γ approaches unity, the KNN–NTK composite ceramic exhibits normal ferroelectricity. These results indicate that the KNN–NTK composite lead-free piezoelectric ceramic is not a relaxor.
We developed K1–xNaxN–NTK composite lead-free piezoelectric ceramic composed of the two primary phases KNN and NTK. The NTK phase is a dielectric substance with a layered structure and is not piezoelectric. This phase fills the voids between KNN crystalline particles, and a portion of this phase transforms into K2(Ti,Nb,Co,Zn)6O13 and/or CoZnTiO4. Consequently, a densely sintered ceramic is obtained. This KNN–NTK composite lead-free piezoelectric ceramic exhibits enhanced piezoelectric characteristics, such as a planar-mode electromechanical coupling coefficient kp = 0.52, a dielectric constant ε33T/ε0 = 1600, Np = 3170 Hz m, and a high thermal durability.
In this system, KNN forms the single tetragonal phase for x < 0.56. However, near x = 0.56 appears a two-phase coexisting state containing both the P4mm tetragonal phase and the Imm2 orthorhombic phase, and the fraction of the orthorhombic phase increases with x. No XRD peaks corresponding to the Imm2 orthorhombic phase are observed for the initial coexisting state; however, superlattice reflections appear in the SAD patterns. This K1–xNaxN–NTK composite lead-free piezoelectric ceramic, with granular Imm2 orthorhombic nanodomains dispersed in a P4mm tetragonal matrix, has excellent piezoelectric properties, with a planar-mode electromechanical coupling coefficient kp = 0.56 at the onset of the two-phase coexisting state. The phase transition of K1–xNaxN–NTK composite lead-free piezoelectric ceramic occurs gently, which distinguishes it from the drastic phase transition of the MPB of PZT. However, the diffusivity γ of K1–xNaxN–NTK shows that K1–xNaxN–NTK composite lead-free piezoelectric ceramic is close to a ferroelectric but is not a relaxor.
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).
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Written By
Kazushige Ohbayashi
Submitted: 08 December 2015Reviewed: 06 March 2016Published: 24 August 2016
Open access peer-reviewed chapter
