Microstructure and Piezoelectricity of (Na,K,Li)(Nb,Sb)O 3 –(Bi,Na)(Sr)ZrO 3 –BaZrO 3 Ceramics

: In this paper, (Na,K) 1 − x Li .x (Nb,Sb)O 3 –(Bi,Na)(Sr)ZrO 3 –BaZrO 3 ceramics were fabricated with x ( = Li) substitution by two-step sintering method, and their physical characteristics were investigated. When Li substitution was added to the ceramics, piezoelectric constant (d 33 ) and electromechanical coupling factor (k p ) were rapidly reduced. However, mechanical quality factor (Q m ) was enhanced. For the KNN-BNZ((K,Na)(Nb)O 3 –(Bi,Na)(Sr)ZrO 3 ) ceramics with Li(x) = 0 substitution , the best physical properties of d 33 = 300 [pC / N], k p = 0.40, Q m = 33 and dialectic constant ( ε r ) = 2430 were shown, respectively. Additionally, the KNN-BNZ ceramics with Li(x) = 0.02 , the d 33 of 246[pC / N], the k p of 0.37, the Q m of 42 and the ε r of 2090 appeared, which were suitable for the low-loss piezoelectric actuator. of grain size. The KNN-BNZ ceramics with Li(x) = 0 , and the best physical properties of d 33 = 300 [pC / N], k p = 0.40, Q m = 33 and ε r = 2430 were shown, respectively. In case of using low-loss piezoelectric actuator, because of low dielectric constant ε r of 2090 at the KNN-BNZ ceramics with Li(x) = 0.02, low power consumption is anticipated.

Sato et al. have presented (Na,K)NbO 3 -system ceramics with excellent piezoelectric properties as Pb-free materials capable of being substituted for PZT ceramics. Nevertheless, it is not very easy to manufacture KNN(K,Na)(Nb)O 3 ceramics with the excellent piezoelectric properties owing to the volatilization of alkali lements such as Na and K. Recently, Juhyun Yoo et al. [12] have reported composition ceramics with an excellent piezoelectric constant (d 33 ) of~269 (pC/N) and a high Curie temperature (Tc) of~275 ( • C) in the (Bi,Na)ZrO 3 -substituted NKN ceramics with rhombohedral-tetragonal (R-T) phase boundary regions. In general, PZT system ceramics show excellent piezoelectric properties in R-T phase regions [13,14]. Accordingly, we added (Bi 0.5 Na 0.5 )ZrO 3 and BaZrO 3 to NKN system ceramics in order to form these R-T phase regions. (Bi 0.5 Na 0.5 )ZrO 3 and BaZrO 3 can increase rhombohedral-orthorhombic transition temperature (T R-O ) and decrease orthorhombic-tetragonal transition temperature (T O-T ) of (Na,K)NbO 3 ceramics near room temperature. As a result, by forming the R-T phase regions, superior d 33 can be induced. It is necessary that the low-loss piezoelectric actuators have low dielectric constant, high d 33 , and temperature stability of piezoelectric properties.
In this experiment, for the application to the low-loss piezoelectric actuator, the (Na,K)NbO 3 systems substituted with (Bi, Na)(Sr) ZrO 3 and BaZrO 3 were manufactured with lithium substitution by the two-step sintering method and their physical properties were analyzed. Figure 1 presents the X-ray diffraction (XRD) pattern according to x (= Li). All the specimens exhibit pure perovskite phase, and no second phases are observed. As can be seen in Figure 1b, all the ceramic specimens from x = 0 to x = 0.05 according to the increase in lithium substitution include R-T phase regions, which are characterized by the tetragonal (002) and (200) peaks along with the rhombohedral (200) peak between 42 • and 50 • . R-T coexistence has a large number of polarization directions that can be formed into eight directions (rhombohedral phases) and six directions (tetragonal phases) in the equivalent orientation of spontaneous polarization. Accordingly, the polarization of specimens can be done easily, and strong piezoelectricity can be induced.      The density of the specimen with x is shown in Figure 3. The density was enhanced owing to the lithium substitution. In this experiment, because the eutectic point of Li2CO3 and Na2CO3 is 514 °C, the liquid phase can be performed during the sintering process. In this experiment, the two-step sintering method was used for the purpose of increasing the piezoelectricity of the KNN-BNZ ceramics. For the sintering process, the temperature was increased suddenly to 1180 °C, maintained for 5 min, then cooled down for 5 min to 1070 °C and kept for 20 h to cool down further. Through these methods, the densification of the ceramics can be performed [15]. The density of the specimen with x is shown in Figure 3. The density was enhanced owing to the lithium substitution. In this experiment, because the eutectic point of Li 2 CO 3 and Na 2 CO 3 is 514 • C, the liquid phase can be performed during the sintering process. In this experiment, the two-step sintering method was used for the purpose of increasing the piezoelectricity of the KNN-BNZ ceramics. For the sintering process, the temperature was increased suddenly to 1180 • C, maintained for 5 min, then cooled down for 5 min to 1070 • C and kept for 20 h to cool down further. Through these methods, the densification of the ceramics can be performed [15].  Figure 3. Density of specimens with x. Figure 4 presents the kp of the specimen with x. Electro mechanical coupling factor kp ensures the efficient conversion of electrical energy into mechanical energy. Here, a maximum value of 0.400 was obtained when the lithium substitution was 0. Thereafter, the kp was continuously decreased. Here, in spite of the increase of density, the reason why electro mechanical coupling factor kp is continuously decreased can be analyzed by the fact that the average grain size of the specimens is reduced according to the increase of lithium substitution, and lithium substitution also acts as acceptor dopant. The mechanical quality factor (Qm) with x is presented in Figure 5. When x was 0.05, the maximum value of 54 appeared. This is because kp and dielectric constant were decreased at the  Figure 4 presents the kp of the specimen with x. Electro mechanical coupling factor kp ensures the efficient conversion of electrical energy into mechanical energy. Here, a maximum value of 0.400 was obtained when the lithium substitution was 0. Thereafter, the kp was continuously decreased. Here, in spite of the increase of density, the reason why electro mechanical coupling factor kp is continuously decreased can be analyzed by the fact that the average grain size of the specimens is reduced according to the increase of lithium substitution, and lithium substitution also acts as acceptor dopant.  Figure 3. Density of specimens with x. Figure 4 presents the kp of the specimen with x. Electro mechanical coupling factor kp ensures the efficient conversion of electrical energy into mechanical energy. Here, a maximum value of 0.400 was obtained when the lithium substitution was 0. Thereafter, the kp was continuously decreased. Here, in spite of the increase of density, the reason why electro mechanical coupling factor kp is continuously decreased can be analyzed by the fact that the average grain size of the specimens is reduced according to the increase of lithium substitution, and lithium substitution also acts as acceptor dopant. The mechanical quality factor (Qm) with x is presented in Figure 5. When x was 0.05, the maximum value of 54 appeared. This is because kp and dielectric constant were decreased at the The mechanical quality factor (Qm) with x is presented in Figure 5. When x was 0.05, the maximum value of 54 appeared. This is because kp and dielectric constant were decreased at the same time.
Here, the oxygen vacancies were performed, causing in the enhancement of Qm through prohibiting the domain wall motion. The ε r according to lithium substitution is shown in Figure 6. The maximum value of ε r was 2014 at the lithium = 0. After x of lithium substitution increased to more than 0.1, ε r was abruptly decreased because lithium substitution also acts as acceptor dopant.
The maximum value of εr was 2014 at the lithium = 0. After x of lithium substitution increased to more than 0.1, εr was abruptly decreased because lithium substitution also acts as acceptor dopant.
Additionally, the decreases of average grain size reduced the dielectric constant of the specimens because of the increase of grain boundary layer containing lower dielectric constant. The dependence of the d33 piezoelectric constant with x is presented in Figure 7. The maximum value of d33 was 300 [pC/N] when x was 0. In this composition, rhombohedral and tetragonal (R-T) coexistence phases appeared to be weak. Additionally, the decrease of d33 according to lithium substitution is considered because lithium ion largely acted as the cause of decreasing grain size. In general, in the case of grain size decrement, Qm was increased, and kp and d33 were decreased through prohibiting the domain wall motion, respectively. Figure 8 presents the P-E hysteresis loop of the ceramics with x = 0, 0.02, 0.03 0.04 and 0.8, sintered at 1070 °C. The remnant polarization (Pr) decreased from 5.47 μC/cm 2 to 4.98 μC/cm 2 , and coercive field (Ec) gradually showed the trend of The dependence of the d33 piezoelectric constant with x is presented in Figure 7. The maximum value of d33 was 300 [pC/N] when x was 0. In this composition, rhombohedral and tetragonal (R-T) coexistence phases appeared to be weak. Additionally, the decrease of d33 according to lithium substitution is considered because lithium ion largely acted as the cause of decreasing grain size. In general, in the case of grain size decrement, Qm was increased, and kp and d33 were decreased through prohibiting the domain wall motion, respectively. Figure 8 presents the P-E hysteresis loop of the ceramics with x = 0, 0.02, 0.03 0.04 and 0.8, sintered at 1070 °C. The remnant polarization (Pr) decreased from 5.47 μC/cm 2 to 4.98 μC/cm 2 , and coercive field (Ec) gradually showed the trend of Additionally, the decreases of average grain size reduced the dielectric constant of the specimens because of the increase of grain boundary layer containing lower dielectric constant.
The dependence of the d 33 piezoelectric constant with x is presented in Figure 7. The maximum value of d 33 was 300 [pC/N] when x was 0. In this composition, rhombohedral and tetragonal (R-T) coexistence phases appeared to be weak. Additionally, the decrease of d 33 according to lithium substitution is considered because lithium ion largely acted as the cause of decreasing grain size. In general, in the case of grain size decrement, Qm was increased, and kp and d 33 were decreased through prohibiting the domain wall motion, respectively. Figure 8 presents the P-E hysteresis loop of the ceramics with x = 0, 0.02, 0.03 0.04 and 0.8, sintered at 1070 • C. The remnant polarization (P r ) decreased from 5.47 µC/cm 2 to 4.98 µC/cm 2 , and coercive field (Ec) gradually showed the trend of increment from 6.27 kV/Cm to 7.3, 7.32, 7.02 and 7.17 kV/Cm, respectively, as a function of x. These results can be also analyzed in light of the fact that the lithium substitution acts as acceptor dopant. increase in lithium substitution, because of the decrease of grain size. The KNN-BNZ ceramics with Li(x) = 0, and the best physical properties of d33 = 300 [pC/N], kp = 0.40, Qm = 33 and εr = 2430 were shown, respectively. In case of using low-loss piezoelectric actuator, because of low dielectric constant εr of 2090 at the KNN-BNZ ceramics with Li(x) = 0.02, low power consumption is anticipated.     The variation of the dielectric constant with temperature is shown in Figure 9. The T c was constantly maintained from 230 • C (x = 0) to 240 • C (x = 0.05) according to the increase in lithium substitution. The peak value of dielectric constant at T c was also largely decreased according to the increase in lithium substitution, because of the decrease of grain size. The KNN-BNZ ceramics with Li(x) = 0 , and the best physical properties of d 33 = 300 [pC/N], k p = 0.40, Q m = 33 and ε r = 2430 were shown, respectively. In case of using low-loss piezoelectric actuator, because of low dielectric constant ε r of 2090 at the KNN-BNZ ceramics with Li(x) = 0.02, low power consumption is anticipated.