Effects of LiF on the Structure and Electrical Properties of (Na0.52K0.435Li0.045)Nb0.87Sb0.08Ta0.05O3 Lead-Free Piezoelectric Ceramics Sintered at Low Temperatures ()
1. Introduction
Although the piezoelectric ceramic materials on the basis of PZT have the perfect physical properties and have many important applications in science and technology such as in sensors, actuators and the other electronic devices [1] - [6] , they contain large amounts of toxic lead more than 60 wt%, their high volatilization and toxicity of PbO during firing process can contaminate the environment and damage human health. So, the replacement of lead by lead-free materials for piezoceramics is essential.
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 piezoelectric properties and high Curie temperature [7] [13] - [16] . However, the sintering temperatures of KNN based ceramics fabricated by conventional methods are quite high (>1000˚C), this leads to evaporation of alkaline elements during sintering process, resulting in the reduced properties of ceramics. Therefore, lowering sintering temperature of KNN based ceramics is very important.
Many researchers have successfully decreased the sintering temperature of lead-free ceramics by using the low-temperature melting additives such as Bi2O3, Li2CO3, CuO, LiF, etc. [17] - [22] . In some cases, these additives can reduce easily the sintering temperature, but also reduce the piezoelectric properties of ceramics due to the appearances of second phase, or variation of structure phase [17] . Currently, the research and manufacture of ceramics material sintered at a low temperature, which leads to improvement or does not reduce the piezoelectric properties of ceramics system, is very interesting.
In this work, we present some research results on the effect of LiF on the sinterability, structure and electrical properties of (Na0.52K0.435Li0.045)Nb0.87Sb0.08Ta0.05O3 ceramics. The LiF dopant significantly reduced the sintering temperature and enhanced the electrical properties of the ceramics at room temperature.
2. Experimental Procedure
The (Na0.52K0.435Li0.045)Nb0.87Sb0.08Ta0.05O3 (KNNLST) + x wt% LiF ceramics, where x = 0, 2, 4 and 6, were synthesized by a conventional mixed-oxide method. Potassium carbonate (K2CO3), sodium carbonate (Na2CO3), lithium carbonate (Li2CO3), and oxides Sb2O5, Nb2O5, Ta2O5 (purity ≥99%) were used as starting materials. Mixed powder KNNLST was milled for 20 h with the ZrO2 balls in ethanol. Two calcinations at temperature 850˚C for 2 h were then performed to obtain the single phase formation. Thereafter lithium fluoride (LiF) was mixed with the calcined KNNLST powder, and powders milled for 16 h. The ground materials were pressed into disk 12 mm in diameter and 1.5 mm in thick under 100 MPa. To prevent the evaporation of alkaline elements, these pellets were covered by the powders with the same composition and then were sintered in a sealed alumina crucible at the temperature of 900˚C, 930˚C, 950˚C, and 1000˚C for 2 h.
The crystal structure of the sintered samples was examined by X-ray diffraction (XRD, D8 ADVANCE). The microstructure of the samples was examined by using a scanning electron microscope (SEM) (Hitachi S-4800). The densities of samples were measured by Archimedes method. The ceramic samples were polarized in silicone oil at 120˚C by applying dc field of 30 kV/cm for 20 min, then cooled down to room temperature under the same electric field. They were aged for 24 h prior to testing. The piezoelectric properties were determined by the resonance and antiresonance frequencies using an impedance analyzer (HP 4193A and RLC HIOKI 3532). Temperature dependence of dielectric constant was determined using RLC HIOKI 3532 with automatic programming; temperature of the samples was measured using Digital Multimeter 7562. The ferroelectric property was measured by Sawyer-Tower method.
3. Results and Discussion
3.1. Sintering Behavior
Figure 1 shows the densities of KNNLST ceramics with 4 wt% LiF content as a function of sintering temperature. With increasing of sintering temperature, the density increases and reaches the maximum value (4.26 g/cm3) at 930˚C sintering temperature, then decreases. Meanwhile, the sintering temperature of undoped KNNLST ceramics was as high as 1100˚C, the density of 4.24 g/cm3 (Table 1). Thus, the addition of LiF improved the sinterability, reduced the sintering temperature of 170˚C compared with pure samples and increasing density of the ceramic samples. This is probable due to the formation of liquid phase of LiF. As is well known, LiF has a low melting point of 845˚C, therefore it may form liquid phase during sintering process, which could probably promote the densification behavior of KNN-based piezoceramics at reduced temperatures [17] .
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Figure 1. The density of KNNLST + 4 wt% LiF ceramics sintered at different temperatures: 900˚C, 930˚C, 950˚C, and 1000˚C.
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Table 1. Some properties of undoped KNNLST (sintered at 1100˚C) and KNNLST + 4 wt% LiF (sintered at 930˚C) ceramics.
Figure 2 shows the planar electromechanical coupling factor kp, kt and dielectric constant ε of KNNLST + 4 w% LiF ceramics as a function of sintering temperature. The kp, kt and ε show similar tendency, initially they increase with the sintering temperature, reach the maximum values at 930˚C sintering temperature, then decrease when sintering temperature exceeds 930˚C. As indicated in Table 1, for the KNNLST + 4 w% LiF ceramics sintered at 930˚C showed the good electrical properties, the electromechanical coupling factor kp, kt are 0.27, 0.40, respectively, which were relatively higher than that pure KNNLTS ceramics. These results may be related to the suppressed volatilization of alkali elements at the low temperature sintering and the increase of density as discussed above.
From the results above, the sintering temperature for KNNLST doped LiF ceramics was selected as 930˚C.
3.2. Structure, Microstructure and Electrical Properties
Figure 3(a) shows the XRD patterns at room temperature with the 2q range from 20˚ to 70˚ of KNNLST + x wt% LiF ceramic samples sintered at 930˚C. It can be seen that all samples exhibit dominant phase structure of perovskite ABO3 type, beside a small amount of second phase LiSbO3 (PDF card # 024-0598) and K6Nb10.8O30 (PDF card # 070-505) have been found in pure KNNLST and KNNLST doped 2 w% LiF samples as shown in Figure 3(a). The appearance of second phase may be due to the composition inhomogeneity of ceramics [17] . Crystal structure of the samples is modified significantly by LiF additions as shown in Figure 3(b).
Figure 3(b) shows the enlarged XRD patterns of the ceramics in the ranges of 2θ from 45˚ to 47˚. It is well known that KNN-based ceramics usually show orthorhombic symmetry at room temperature [14] . According to research results of the works [17] [22] [23] , when the ceramics is of orthorhombic phase, the relative intensity of (002)/(020) peaks is approximately 2 and the (002) peak appears at a smaller Bragg angle, but the I(002)/I(020) decreases to 0.5 for a tetragonal phase. Therefore, from X-ray diffraction diagram in Figure 3(b) shows that likely the dominate phase in the pure KNNLST ceramics is orthorhombic, whereas the addition of LiF changes it to tetragonal symmetry. With x equals 4, the ceramics shows the coexistence of two phases, and then with x = 6, the tetragonal phase became dominant. This result suggests that may be Li+ ions are substituted for A-site of perovskite structure ABO3 which leads to the change of crystal structure from orthorhombic to tetragonal. So it could be said that the composition with x = 4 is near to the morphotropic phase boundary of KNNLST doped LiF ceramics [16] .
Figure 4 shows the SEM images of the KNNLST + x wt% LiF ceramics sintered at 930˚C: (a) x = 0, (b) x = 2, (c) x = 4 and (d) x = 6. It can be seen that the microstructure of the pure KNNLST ceramics with square or
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Figure 2. Electromechanical coupling factor kp, kt and dielectric constant ε of KNNLTS + 4 w% LiF ceramics as a function of sintering temperature.
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Figure 3. XRD patterns of the KNNLTS + x wt% LiF ceramics (a) and enlarged region for (002)/(020) peaks of the diffraction patterns (b).
rectangular shaped grains discrete distributions, porous, not sintered materials as shown in Figure 4(a). However, the microstructure of samples becomes denser and average grain size increases from 0.4 to 2 mm (shown in Table 2) as the LiF content is increased (Figures 4(b)-(d)). A homogeneous microstructure developed for the sample with 4 wt% LiF added (Figure 2(c)). Figure 4(d) also shows that further increasing LiF content to 6 wt% gives rise to some large abnormal grains, porous. Such with the 4 wt% LiF added sample, the highly dense and homogeneous microstructure was obtained, which may expect improved properties of ceramics. The grain growth with LiF addition can be explained by liquid phase sintering.
Figure 5 shows temperature dependence of dielectric constant ε and dielectric loss tand measured at 1 kHz of KNNLST + x wt% LiF ceramics sintered at 930˚C. As seen in Figure 5, KNNLST doped LiF ceramics undergo two phase transition, that are the orthorhombic-tetragonal phase transition (TO-T) and tetragonal-cubic phase transition (TC) during the measured temperature range [16] [17] [22] [23] . Figure 6 shows the orthorhombic-tetragonal phase transition temperature (TO-T) and Curie temperature (TC) of KNNLST ceramics sintered at 930˚C change as a function of LiF content. The pure KNNLST ceramics possess TO-T and TC temperature of 67˚C and 193˚C, respectively. With the addition of LiF, the TO-T temperature of KNNLST ceramics decreases consistent with the analysis of XRD patterns that the phase structure of ceramics changes toward tetragonal symmetry when LiF content increases, while TC temperature increases due to diffusion of Li+ into A-site of KNN lattice, showing similar tendency to the works [17] [23] . Figure 5 also shows that the dielectric loss little changes as the temperature rises from room temperature to 150˚C and increases rapidly in the vicinity Tc.
Some physical properties of KNNLST+ x wt% LiF ceramics sintered at 930˚C measured at room temperature
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Figure 4. SEM images of KNNLST + x wt% LiF ceramics sintered at 930˚C: (a) x = 0; (b) x = 2; (c) x = 4; (d) x = 6.
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Figure 5. Temperature dependence of dielectric constant ε and dielectric loss tand of the KNNLST + x wt% LiF ceramics sintered at 930˚C.
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Figure 6. The orthorhombic-tetragonal phase transition temperature (TO-T) and Curie temperature (TC) of KNNLST ceramics as a function of LiF content.
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Table 2. Physical properties of KNNLST + x wt% LiF ceramics sintered at 930˚C.
also listed in Table 2. It is seen that with increasing of LiF content, the density, dielectric constant (e), planar electromechanical coupling factor (kp), (kt), piezoelectric constant (d31) are increased. The largest values for density of 4.26 g/cm3, kp of 0.27, kt of 0.40, d31 of 41 pC/N, e of 744 were obtained at 4 wt% LiF content, then decrease. These are probably related to characteristics of the density, the increasing grain size and morphotropic phase boundary effect [16] .
Figure 7 shows the shape of ferroelectric hysteresis loops of the samples KNNLST+ x wt% LiF measured at room temperature. From the shape of these loops, the remanent polarization Pr and the coercive field EC of ceramics were determined, as shown in Figure 8. With increasing of LiF content, a sharp increases in Pr was observed for samples until x = 4, reaches the highest value (11.96 µC/cm2) at this content, and then decreases; while the coercive field EC strong decreases from 8.06 to 5.38 kV/cm during the measured LiF content range. These results are in good agreement with the studied dielectric and piezoelectric properties of the ceramic samples.
4. Conclusion
The effect of LiF addition on the sintering behavior and physical properties of
(Na0.52K0.435Li0.045)Nb0.87Sb0.08Ta0.05O3(KNNLST) + x wt% LiF (x = 0 ÷ 6) ceramics was investigated. The addition of LiF improved the sinterability of the ceramics and caused an increase in the density and grain size at low sintering temperature (930˚C). All samples had perovskite phase structure with a change from orthorhombic to tetragonal by LiF additions and a possible morphotropic phase boundary near composition x = 4. With increasing LiF content, the TO-T temperature of the ceramics was shifted to lower temperature, while the Curie TC temperature was shifted to higher temperature. At the LiF content of 4 wt%, physical properties of ceramics sintered
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Figure 7. Hysteresis loops of KNNLTS + x wt% LiF ceramic samples measured at room temperature.
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Figure 8. The remnant polarization (Pr) and coercive field (EC) of KNNLST ceramics as a function of the LiF content.
at 930˚C are the best.
NOTES
*Corresponding author.