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Some dielectric, ferroelectric, piezoelectric properties of (Na0.48K0.48Li0.04)NbO3
lead-free ceramics sintered by two-step method Phan Dinh Gio*,Nguyen Thi Muoi, Nguyen Van Quynh Department of Physics, University of Sciences, Hue University
*) Corresponding author's e-mail: [email protected]
Abstract
Two-step sintering method has been successfully applied to prepare (Na0.48K0.48Li0.04)NbO3-0.25 wt% CuO (KNLN-Cu) lead-free piezoelectric ceramics. The effect of the sintering temperature on the microstructure and electrical properties of LNKN-Cu ceramics were investigated in detail. The experimental results showed that all samples have pure perovskite phase with orthorhombic structure. Under the optimal sintering condition „„1000/5/925/7‟‟, the obtained ceramics shows the much improved microstructure with a density value of 4.31 g/cm3 and comparatively uniform grain size distribution, which is different from the corresponding microstructure by conventional sintering and show the maximum values of the piezoelectric coefficient (d33), electromechanical coupling coefficient (kp), dielectric constant (), and remnant polarizations (Pr), which are 261 pC/N, 48%, 456, and 14.0C/cm2, respectively. Two-step sintering is considered as an effective means for fabricating the dense KNN based lead-free piezoelectric ceramics with good dielectric and piezoelectric properties.
1. Introduction
Ferroelectric ceramics are one of the advanced new materials, they play a very important role in many technical fields. Over the last half century, the piezoelectric ceramic systems have been manufactured and mainly used are the lead zirconate titanate (PZT) based piezoelectric ceramics [1_3], they contain a large amount of lead, the toxicity of lead oxide and its high vapor pressure during processing leading to environmental pollution and affect human health [4_5].
Therefore, it is necessary to develop lead-free piezoelectric ceramics with excellent ferroelectric, piezoelectric properties for replacing the lead-based ceramics in different devices [6].
During recent years, intensive efforts have been made to develop lead-free piezoelectric ceramics such as BaTiO3, Na0.5Bi0.5TiO3, (Bi0.5Ka0.5)TiO3, (K, Na)NbO3, etc. [7_12]. Among them, (K, Na)NbO3 (KNN) based piezoelectric ceramics was the most interested because of its strong ferroelectricity and high Curie temperature (about 420 ℃) [7,13,14], thereby it has become one of the most promising candidates for replacing Pb-based ceramics [15-18]. However, it is very difficult to obtain dense KNN ceramics and good electrical properties using ordinary sintering process due to the high volatility and hygroscopic of alkaline elements [19].
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To improve the sinterability and properties of free-lead piezoelectric ceramics, on the basis of the conventional solid phase sintering method, various advanced manufacturing techniques have been applied to fabrication of free-lead ceramic materials, such as hot-pressed sintering [20], spark–plasma sintering [21], microwave sintering [22] two-step sintering [23], texturing engineering [24], etc..
Among them, the two-step sintering technique is a simple and effective method of improving the properties of KNN based free-lead ceramics, which is currently being interested by many scientists [25_31].
Sintering is the most important step of the ceramic material fabrication technology, which has a significant effect on the quality of ceramic samples.
Sintering reduces pores and enhances properties such as durability and conductivity effectively. During sintering, the molecular diffusion causes the surface of the powder particles to dissipate, starting with the formation of the bottleneck between the particles and ending with the disappearance of small pores at the end of the process [32].The diffusion of grain boundaries and diffusion capacity depends on temperature. Therefore temperature control is very important for sintering. A dominant temperature control technique used to control microstructures during the sintering is two-step sintering technique (TS) [33]. This is an improved sintering technique used to improve some properties of ceramic materials compared to conventional sintering methods (CS).
In previous work [34], we found that with the addition of 0.25 wt% CuO, the 0.96(K0.5Na0.5)NbO3-0.04LiNbO3 ceramics have been well sintered by conventional sintering methods at a low temperature (950 oC) and density has improved. In this paper, we present some research results on the effect of sintering temperature on the microstructure and electrical properties of (Na0,48K0,48Li0,04)NbO3 -0.25 wt% CuO ceramics fabricated by two-step sintering method. The two-step sintering technique plays an important role in sintering behavior and enhances the electrical properties of the ceramics at room temperature [25, 26, 31].
2. Experimental Procedure
The (Na0,48K0,48Li0,04)NbO3-0.25 wt% CuO
(KNLN-Cu)
piezoelectric ceramics were synthesized by two-step sintering method. The carbonates K2CO3, Na2CO3, Li2CO3, and oxides CuO, Nb2O5 (purity ≥ 99%) were used as starting materials. Before being weighed, the K2CO3 and Na2CO3 powders were dried in an oven at 150 ℃ for 2 hour to minimize the effect of moisture. Mixed powder was milled for 10 hour with the ZrO2 balls in ethanol. Two calcinations at temperature 850 oC for 2 hour were performed to obtain the homogeneity of the composition.Thereafter the calcined powders were ball milled again for 16 hour. The ground materials were pressed into disk 12 mm in diameter and 1.5 mm in thick under 1.5 T/cm2 and then were sintered by conventional sintering (CS) and two-step sintering (TS). For conventional sintering, the heating rate was 5 oC/min, the samples were sintered at temperatures of 950 oC for 2 hours [34] and then naturally cooled down.
By contrast, in the case of two-step sintering, the temperature was raised at a rate of
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5 °C/min to the first-step temperature T1 of 1000 oC, after holding at T1 for a short socking time t1 (5 min), it was rapidly cooled to a lower second-step temperature T2 at a fast rate of 10 oC/min, and then held for a long time t2 (7 h). After that, it followed the furnace cooling. The symbol “1000/5/T2/7” is utilized to represent for TS.
The crystal structure of the ceramic samples was examined by X-ray diffraction (XRD, D8 ADVANCE) with CuK radiation of wavelength 1.5405 Å at room temperature. The microstructure of the samples was examined by using a scanning electron microscope (SEM) (Hitachi S_ 4800). The density of samples was measured by Archimedes method. To measure electrical properties, the ceramic samples were coated with silver paint on two surfaces and heated at 600°C for 15 min. Temperature dependence of dielectric constant and dielectric loss were determined using RLC HIOKI 3532 with automatic programming. The samples were poled in a silicone oil bath at 60 oC by applying electric field of 40 kV/cm for 30 min. They were aged for 24 h prior to testing. The d33 piezoelectric constant was determined using a d33 meter (YE2730A, SINOCERA, China) and electromechanical coupling factor was determined from the resonance and antiresonance frequency using an impedance analyzer (HP 4193A and RLC HIOKI 3532). The ferroelectric properties were measured by Sawyer-Tower method.
3. Results and Discussion
3.1. Effect of sintering conditions on the Structure and Microstructure of KNLN- Cu Ceramics
Figure 1 shows the XRD patterns measured at room temperature over the 2θ range of 20–80 o of the KNLN-Cu ceramics sintered by conventional sintering and two-step sintering with parameters: 1000/5/900/7, 1000/5/925/7, 1000/5/950/7.
20 30 40 50 60 70 80
CS (042) (311)
(231)
(202)
(221) (112)
T2 = 950oC T2 = 925oC
2 (Degrees)
Intensity, I (a.u)
T2 = 900oC
(100) (111)
(110) (220) (020) (131) (311)
900 925 950
3.8 3.9 4.0 4.1 4.2 4.3 4.4
Density(g/cm3 )
Sintering temperature T
2(0C)
Figure 1. X-ray diffraction patterns of the KNLN_ Cu ceramics sintered by CS and TS with parameters:
1000/5/900/7, 1000/5/925/7, 1000/5/950/7
Figure 2. The density of KNLN_ Cu ceramics as a function of the sintering temperature T2
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It can be seen that for both methods, all the ceramic samples exhibit the coexistence of perovskite phase, no secondary phase was detected. This means that Li+ has diffused into the KNN ceramics and formed a homogeneous solid solution.
All the components have a double peak (220)/(020) with an relative intensity ratio of I220/I002 = 2/1 around 2
= 45.5 o, according to work of Fang-Zhou Yao [35], with an orthorhombic symmetry, the ideal ratio of I220/I002 equals to 2:1, while it evolves to 1:2 for a tetragonal phase. So, all samples exhibit a single perovskite structure with an orthorhombic symmetry over the whole sintering temperature range, indicating that the changing the sintering conditions have an insignificant effect on the crystalline structure of the all ceramic samples. This result is consistent with the work of Ting Zheng [25]. Figure 2 shows the bulk density of KNLN_Cu ceramics prepared by two-step sintering process with various T2 sintering temperatures. As shown, the bulk density increased with increasing T2 sintering temperature and reached the highest value (4.31 g/cm3) at T2 = 925 oC, then decreased, indicating that with the „„1000/5/925/7‟‟ sintering conditions the density of KNLN_Cu ceramics isthe
best. Besides, the density of KNLN_Cu ceramics prepared by two- step sintering process was higher than that of conventional sintered ceramics (4.14 g/cm3). These results are consistent with the microstructure of fracture surfaces of the KNLN_Cu ceramic samples as shown in figures 3. According to the work of Mohammad Reza Bafandeh et al. [36], the density of the ceramics sintered by the second step of sintering will be promoted without grain growth. Thus, it can be seen that the „„1000/5/925/7‟‟ sintering conditions are suitable for manufacturing KNLN_ Cu ceramics with high density.
Figure 3 shows the SEM images of ceramic samples sintered by CS and TS with different T2 sintering temperatures. As shown in Figure 3(a), with CS method, the microstructure of the ceramics consisted of rectangular shaped large grains and discrete distributions, porous, the average grain size of about 3,7m. However, with
„„1000/5/T2/7‟‟ two-step sintering method, the microstructure of samples becomes denser and average grain size significantly reduced (2 m) (Fig. 3(b)). A homogeneous microstructure with comparatively small grains (1 m), less pores developed for the “1000/5/925/7” samples (Fig. 3(c)). Figure 3(d) shows that further increasing T2 sintering temperature to 950 oC, the average grain size of the ceramics increases (3,4 m), porous microstructure. This result is consistent with the works of Jae-Hoon Ji [37] and Jiagang Wu [29]. Such by optimizing the T2
sintering temperature as 925 oC in the two-step sintering process, the obtained ceramics shows the much improved microstructure with a density value of 4.31 g/cm3 and comparatively uniform grain size distribution, which is different from the corresponding microstructure by conventional sintering. According to the work of U. Sutharsini et al. [38], the fine‐grained microstructure enhances the mechanical, electrical, as well as piezoelectric properties of ceramics.
Figure 4 shows the room temperature dielectric constant ε and the dielectric loss was measured at 10 kHz frequency of KNLN_Cu ceramics prepared by TS process with various T2 sintering temperatures: 900, 925, 950 oC. The dielectric
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constant ε increases with the T2 sintering temperature increases and reaches the highest value (ε = 456) at T2 = 925 oC. However, when T2 > 925 oC, the dielectric constant ε decreased. Conversely, when the T2 sintering temperature increases, the value of the dielectric loss tanδ decreases to the smallest value (tanδ = 0.025) at T2
= 925 oC, then increases. These may be related to the density and microstructure of ceramics.
Figure 3: The SEM micrographs of KNLN_Cu ceramics prepared by conventional sintering process at 950 °C (a) and two-step sintering process with various T2 sintering
temperatures (b) 900 oC, (c) 925 oC, (d) 950 oC
3.2. Effect of sintering conditions on Electrical Properties of KNLN-Cu Ceramics
Figure 5 shows the temperature dependence of dielectric constant ε and dielectric loss tanδ were measured at 10 kHz frequency of KNLN-Cu ceramics sintered by CS and TS process at various T2 sintering temperatures of 900, 925, 950
oC. As seen, all the
(T) curves of the ceramic samples have two obvious peaks: a peak at low temperature, which is the peak corresponding to the orthorhombic- tetragonal ferroelectric phase transition temperature (TO-T), the second peak at higher temperatures, corresponding to the ferroelectric-paraelectric phase transition temperature (TC) [39]. From figure 5 shows that the sintering conditions have a negligible influence on the TO-T and TC phase transition temperatures of KNLN-Cu ceramics. For example, with “1000/5/925/7” two-step sintered ceramics, the TO-T(b)
(c) (d
) (a)
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and TC values are 124 and 404 oC, respectively, while that of conventionally sintered ceramics are 126 and 410 oC. This result is consistent with the research results on the effect of sintering conditions on the structure of ceramics and and the same with the result of Ting Zheng [25]. Figure 5 also shows that the peak of the sharp dielectric constant, indicating that the ceramics is a normal ferroelectric.
Besides, dielectric loss of ceramics is relatively low over a wide temperature ranges from room temperature to 250 oC.
To determine piezoelectric properties of ceramics, resonant vibration spectrum of samples were measured at room temperature as shown in Figure 6. As seen, impedance Z reaches the minimum value at 211.5 kHz frequency and maximum value at 233.5 kHz frequency.
0 100 200 300 400 500
-1000 0 1000 2000 3000 4000 5000 6000
900oC
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
925oC 950oC CS
TO-T
TC
Dielectric constant,
Temperature, T (oC)
Dielectric loss, tan
900 925 950
200 250 300 350 400 450 500
Sintering temperature T2 (oC)
Dielectric constant,
1000/5/T2/7
0.02 0.04 0.06 0.08 0.10
Dielectric loss, tg
Figure 5. Temperature-dependent dielectric constant and dielectric loss tan (10 kHz) of the KNLN_Cu ceramics prepared by CS and TS process with various T2 temperatures: 900, 925 and 950 oC
Figure 4. Room-temperature dielectric constant ε and the dielectric loss tan of
KNLN_Cu ceramics with various sintering temperatures of T2
200 210 220 230 240 250
0 1x104 2x104 3x104 4x104 5x104
Frequency (kHz)
Impedance Z()
-100 -80 -60 -40 -20 0 20 40 60 80 100
1000/5/925/7
Z
(degree)
Figure 6. Spectrum of radial resonance of 1000/5/925/7 sample
0.32 0.36 0.40 0.44 0.48 0.52
900 925 950
kp
d33
50 100 150 200 250
Piezoelectric factor, d33(pC/N)
Sintering temperature, T2 (oC) Electromechanical coefficients, kp, kt
kt
Figure 7. The values kp, kt and d33, as a function of the T2 temperatures
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From these resonant spectra, the piezoelectric parameters of ceramics were determined. Figure 7 shows the electromechanical coupling factors (kp, kt), the piezoelectric constant (d33) of KNLN-Cu ceramics change as a function of T2
sintering temperature. As seen, the two-step sintering method has significantly improved piezoelectric properties of ceramics. When the T2 sintering temperature increases from 900 to 950 oC, the values of kp, kt, and d33 rapidly increase and reach largest values for kp of 0.48, kt of 0.50, d33 of 261 pC/N at T2 = 925 oC, then decrease. While that of conventionally sintered ceramics are only 0.33, 0.43 and 130 pC/N, respectively. These results may be related to the density and fine‐grained microstructure of ceramics with optimizing the T2 sintering temperature as 925 oC in the two-step sintering process [30].
Figure 8 shows the shapes of P-E feroelectric hysteresis loops measured at room temperature of the KNLN-Cu ceramic samples sintered under different conditions. As shown in figure 8, sintering conditions have an obvious influence on the ferroelectric properties of the ceramics. A round-shaped P–E hysteresis loop is obtained for the conventional sintered ceramics, showing
a large leakage current [40]. M
eanwhile, for the ceramics sintered by two-step sintering, well-saturated P–E hysteresis loops are observed, showing good ferroelectric property. From the shape of these loops, the remanent polarization Pr and the coercive field EC were determined, as shown in Figure 9. With increasing of T2 sintering temperature, the value of Pr increases and reaches the highest value (14.0 µC/cm2) at T2 = 925 oC, then decreases. The coercive field EC decreases slightly with increasing of T2 and reaches smallest value (4.54 kV/cm) at T2 = 925 oC. With the conventional sintered ceramics, the values of Pr and EC are 9.33 µC/cm2 and 9.82 kV/cm, respectively.
These results are in good agreement with the studied piezoelectric properties of the ceramics.
900 925 950
8 10 12 14
Pr
Ec
Coercive field, EC(kV/cm) Sintering temperature,T2 (oC)
Remanent polarization, Pr(C/cm2 )
4.0 4.5 5.0 5.5 6.0
Figure 8. The P–E hysteresis loops of ceramic samples at the different
sintering conditions
Figure 7. The values Pr and EC, as a function of the T2 temperatures
-20 -15 -10 -5 0 5 10 15 20
-20 -15 -10 -5 0 5 10 15 20
E (kV/cm) P (C/cm2)
CS 1000/5/900/7 1000/5/925/7 1000/5/950/7
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4. ConclusionsThe (Na0.48K0.48Li0.04)NbO3-0.25 wt% CuO lead-free piezoelectric ceramics were fabricated by conventional sintering and two-step sintering. The effect of second-step sintering temperature on the microstructure and electrical properties of ceramics were investigated in detail. All samples have pure perovskite phase with an orthorhombic symmetry. Research results have shown that ceramic samples sintered by two-step method have better electrical properties than those of conventional sintered ceramic samples. With the „„1000/5/925/7‟‟ sintering conditions, physical properties of ceramics are best: the density of 4.31 g/cm3; the electromechanical coupling factor, kp = 0.48 and kt = 0.50; the dielectric constant, ε
= 456; the dielectric loss (tanδ) of 0.025; the piezoelectric constant (d33) of 261 pC/N; the remanent polarization (Pr) of 14 C/cm2. The two-step sintering is an effective method for fabricating the dense KNN based lead-free piezoelectric ceramics.
Acknowledgement
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.08 References
[1] K. Uchino, Piezoelectric Actuators and Ultrasonic Motors.
Kluwer Academic Publishers, Boston, (1997).
[2] B. Jaffe, W. R. Cook, and H. Jaffe (Eds.), Piezoelectric Ceramics, Academic, New York (1971).
[3] Y. Xu (Eds.), Ferroelectric Materials and Their Applications, Elsevier Science, Amsterdam-London-New York-Tokyo (1991).
[4] Saito Y, Takao H, Tani T, Nonoyama T, Takatori K, Homma, Nagoya, T, Nakamura M., Lead-free piezoceramics,Nature 432, 84-87 (2004). DOI:
10.1038/nature03028
[5] In-Young Kang, In-Tae Seo, Yu-Joung Cha, Jae-Hong Choi, Sahn Nahm,Tae-Hyun Sung, Jong-Hoo Paik, Low temperature sintering of ZnO and MnO2-added (Na0.5K0.5)NbO3ceramics. Journal of the European Ceramic Society 32, 2381–2387 (2012). DOI: 10.1016/j.jeurceramsoc.2012.01.030
[6] Wenxin Ma, Xinghua Fu, Wenhong Tao, Lei Yang, Guoyuan Cheng and Liping Zhao, KNN-Sb lead-free piezoelectric ceramics synthesized by hydrothermal method, Materials Science Forum, Vol. 859, pp 3-7 (2016).
https://doi.org/10.4028/www.scientific.net/MSF.859.3
[7] Saito, Y., Takao, H., Tani, T., Nonoyama, T., Takatori, K., Homma, T., Nagaya, T. and Nakamura, M. Lead Free Piezoceramics. Nature, 432, 84-87 (2004).
http://dx.doi.org/10.1038/nature03028
[8] Yeo, H.G., Sung, Y.S., Song, T.K., Cho, J.H., Kim, M.H. and Park, T.G. Donor Doping Effects on the Ferroelectric and the Piezoelectric Properties of Pb-Free
137
(Bi0.5Na0.5)TiO3 Ceramics. Journal of Korean Physical Society, 54, 896-900 (2009). http://dx.doi.org/10.3938/jkps.54.896
[9] Abe, J., Kobune, M., Kitada, K., Yazawa, T., Masumoto, H. and Goto, T. Effects of Spark-Plasma Sintering on the Piezoelectric Properties of High-Density (1- x)(Na0.5K0.5)NbO3-xLiTaO3 Ceramics. Journal of Korean Physical Society, 51, 810-814 (2007). http://dx.doi.org/10.3938/jkps.51.810
[10] Song, T.K., Kim, M.-H., Sung, Y.-S., Yeo, H.-G., Lee, S.H., Jeong, S.-J. and Song, J.-S. Depolarization Temperatures in Pb-Free Piezoelectric Materials.
Journal of Korean Physical Society, 51, 697-700 (2007).
http://dx.doi.org/10.3938/jkps.51.697
[11] Karaki, T., Yan, K. and Adachi, M. Barium Titanate Piezoelectric Ceramics Manufactured by Two-Step Sintering. Japanese Journal of Applied Physics, 46, 7035 (2007). http://dx.doi.org/10.1143/JJAP.46.7035
[12] Karaki, T., Adachi, M. and Yan, K. High-Performance Lead-Free Barium Titanate Piezoelectric Ceramics. Advances in Science and Technology, 54, 7-12 (2008). http://dx.doi.org/10.4028/www.scientific.net/AST.54.7
[13] Wang, K., Li, J.-F. and Liu, N. Piezoelectric Properties of Low-Temperature Sintered Li-Modified (Na, K)NbO3 Lead-Free Ceramics. Applied Physics Letters, 93, Article ID: 092904 (2008). http://dx.doi.org/10.1063/1.2977551
[14] Wongsaenmai, S., Ananta, S. and Yimnirun, R. Effect of Li Addition on Phase Formation Behavior and Electrical Properties of (K0.5Na0.5)NbO3 Lead Free Ceramics. Ceramics International, 38, 147-152 (2012).
http://dx.doi.org/10.1016/j.ceramint.2011.06.049
[15] M. Matsubara, K. Kikuta, and S. Hirano, Piezoelectric properties of (K0.5 Na0.5)(Nb 1-xTax)O3-K5.4CuTa10O29 ceramics. J. Appl. Phys. 97, 114105 (2005).
https://doi.org/10.1063/1.1926396
[16] S. J. Zhang, R. Xia, and T. R. Shrout, et al., Piezoelectric properties in perovskite 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 lead-free ceramics. J. Appl. Phys.
100, 104108 (2006). https://doi.org/10.1063/1.2382348
[17] Q. Chen, L. Chen, Q. S. Li, X. Yue, D. Q. Xiao, J. G. Zhu, X. L. Shi, and Z. Q.
Liu, Piezoelectric properties of K4CuNb8O23 modified (Na0.5K0.5)NbO3 lead-free piezoceramics. J. Appl. Phys. 102 (10), 104109 (2007).
https://doi.org/10.1063/1.2815498
[18] J. G. Wu, Y. Y. Wang, D. Q. Xiao, J. G. Zhu, and Z. H. Pu, Effects of Ag content on the phase structure and piezoelectric properties of (K0.44-
xNa0.52Li0.04Agx)(Nb0.91Ta0.05Sb0.04)O3 lead-free ceramics. Appl. Phys. Lett. 91, 132914 (2007). https://doi.org/10.1063/1.2793507
[19] Egerton L., Dillon D. M., Piezoelectric and dielectric properties of ceramics in the system potassium-sodium niobate. J. Am. Ceram. Soc. 42: 438–442 (1959).
https://doi.org/10.1111/j.1151-2916.1959.tb12971.x
[20] Li K., Li F.L., Wang Y., et al. Hot-pressed K0.48Na0.52Nb1-xBixO3 (x = 0.05–
0.15) lead-free ceramics for electro-optic applications, Mater. Chem. Phys., 131:
320–324, (2011). https://doi.org/10.1016/j.matchemphys.2011.09.048
138
[21] Jiro Abe, Masafumi Kobune, Kazuya Kitada and Tetsuo Yazawa, E
ff
ects of Spark-Plasma Sintering on the Piezoelectric Properties of High-Density (1 − x)(Na0.5K0.5)NbO3-xLiTaO3 Ceramics, Journal of the Korean Physical Society, Vol.51, No. 2, pp.810-814, (2007). DOI: 10.3938/jkps.51.810
[22] Xie Z, Gui Z, Li L, et al. Microwave sintering of lead-based relaxor ferroelectric ceramics. Mater. Lett., 36: 191–194, (1998).
https://doi.org/10.1016/S0167-577X(98)00024-X
[23] Kensuke KATO, Ken-ichi KAKIMOTO, Keiichi HATANO, Keisuke KOBAYASHI and Yutaka DOSHIDA, Lead-free Li-modified (Na,K)NbO3 piezoelectric ceramics fabricated by two-step sintering method, Journal of the Ceramic Society of Japan 122 [6], 460-463, (2014). DOI: 10.2109/jcersj2.122.460 [24] Li, P., Zhai, J., Shen, B., Zhang, S., Li, X., Zhu, F., & Zhang, X.. Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3 -Based Lead-Free Ceramics.
Advanced Materials, 30(8), 1705171 (2018) https://doi.org/10.1002/adma.201705171
[25] Ting Zheng and Jiagang Wu, Enhanced piezoelectricity over a wide sintering temperature (400–1050 oC) range in potassium sodium niobate-based ceramics by two step sintering, J. Mater. Chem. A, 3, 6772, (2015). DOI: 10.1039/C5TA00732A [26] Gunhyun Lee, Jae-Hoon Ji, Jung-Hyuk Koh. Enhanced piezoelectric properties of (Bi,Na)TiO3 -(Bi,K)TiO3 ceramics prepared by two-step sintering process.
International Journal of Applied Ceramic Technology, 15(2), 531–537 (2017).
https://doi.org/10.1111/ijac.12798
[27] Jialiang Zhang, Yalin Qin, Yong Gao, Weizeng Yao, and Minglei Zhao, Improvement of Physical Properties for KNN-based Ceramics by Modified Two- Step Sintering, J. Am. Ceram. Soc., 97 [3] 759–764 (2014).
DOI: 10.1111/jace.12794
[28] Kensuke KATO, Ken-ichi KAKIMOTO, Keiichi HATANO, Keisuke KOBAYASHI and Yutaka DOSHIDA, Lead-free Li-modified (Na,K)NbO3 piezoelectric ceramics fabricated by two-step sintering method, Journal of the Ceramic Society of Japan 122 [6], 460-463, (2014). DOI: 10.2109/jcersj2.122.460 [29] Jiagang Wu and Yumei Wang, Two-step sintering of new potassium sodium niobate ceramics: a high d33 and wide sintering temperature range, Dalton Trans., 43, 12836 (2014). DOI:10.1039/C4DT01712A
[30] Ubenthiran Sutharsini, Murugathas Thanihaichelvan and Ramesh Singh, Sintering of Functional Materials, Chapter 1: Two-Step Sintering of Ceramics, Licensee InTech. (2018) (http://dx.doi.org/10.5772/68083)
[31] Jae-Hoon Ji, Jinhwan Kim, Jung-Hyuk Koh, Improved dielectric and piezoelectric properties of K/Na excessed (Na,K)NbO3 lead-free ceramics by the two step sintering process, Journal of Alloys and Compounds, Vol. 698, 938-943 (2017).https://doi.org/10.1016/j.jallcom.2016.12.267
[32] Turriff D. M. R. (2007). Process Kinetics of Transient Liquid Phase Sintering in a Binary-Isomorphous Alloy System. A thesis presented to the University of Waterloo, Ontario, Canada. ISBN: 978-0-494-35164-2
139
[33] N.J. Lóh; L. Simão; C.A. Faller; A. De Noni Jr.; O.R.K. Montedo, A review of two-step sintering for ceramics, Ceramics International, Volume 42, Issue 11, Pages 12556-12572, (2016). https://doi.org/10.1016/j.ceramint.2016.05.065
[34] Phan Dinh Gio, Huynh Quang Viet, Le Dai Vuong, Low-temperature sintering of 0.96(K0.5Na0.5)NbO3-0.04LiNbO3 lead-free piezoelectric ceramics modified with CuO, International journal of Materials Reseach, Vol. 09, No. 11, 1071-1076, (2018). DOI: 10.3139/146.111706
[35] Fang-Zhou Yao, Eric A. Patterson, Ke Wang,Wook Jo, Jurgen Rodel, and Jing-Feng Li, Enhanced bipolar fatigue resistance in CaZrO3-modified (K,Na)NbO3 lead-free piezoceramics, Applied Physics letters 104, 242912, (2014).
https://doi.org/10.1063/1.4884826
[36] Mohammad Reza Bafandeh, Raziyeh Gharahkhani, Jae-Shin Lee, Sintering behavior, dielectric and piezoelectric properties of sodium potassium niobate-based ceramics prepared by single step and two-step sintering, Ceramics International 41, 163–170, (2015). https://doi.org/10.1016/j.ceramint.2014.08.054
[37] Ji, J.-H., Moon, U.-C., Kwon, H.-I., & Koh, J.-H.. The two-step sintering effect on the dielectric and piezoelectric properties of (Na,K)NbO 3 -BiScO3 lead-free ceramics. Ceramics International, 43, S97–S101 (2017).
doi:10.1016/j.ceramint.2017.05.237
[38] Ubenthiran Sutharsini ,Murugathas Thanihaichelvan and Ramesh Singh (2018). Two-Step Sintering of Ceramics. In book Sintering of functional materials.
http://dx.doi.org/10.5772/68083]
[39] Youliang Wang, Yiqing Lu, Mengjia Wu, Dong Wang, and Yongxiang Li, Phase Structure and Enhanced Piezoelectric Properties of Lead-Free Ceramics (1x)(K0.48Na0.52)NbO3–(x/5.15) K2.9Li1.95Nb5.15O15.3 with High Curie Temperature, Int. J. Appl. Ceram. Technol., 9 [1] 221–227 (2012) DOI:10.1111/j.1744-7402.2011.02650.x
[40] Jian Fang, Xiaohui Wang,w Zhibin Tian, Caifu Zhong, and Longtu Li, Two- Step Sintering: An Approach to Broaden the Sintering Temperature Range of Alkaline Niobate-Based Lead-Free Piezoceramics, J. Am. Ceram. Soc., 93 [11]
3552–3555 (2010). https://doi.org/10.1111/j.1551-2916.2010.04085.x
