Sintering and Characterization of (Li, Sb, Ta)-Modified (Na, K)NbO₃ Lead-Free Ceramics

Abstract

Lead-free alkaline niobate-based piezoceramics, (Na_0.52K_0.435Li_0.045)Nb_0.87Sb_0.08Ta_0.05O₃ (abbreviated KNLNT-S8), were prepared by conventional solid-state sintering method. The effects of sintering temperature on microstructure and piezoelectric properties of the (Li, Sb, Ta)-modified (Na, K) NbO₃ were investigated. Microstructure of the samples sintered at different temperatures was observed by scanning electron microscopy (SEM) and optical microscopy. The KNLNT-S8 sample sintered at 1100°C possessed highest piezoelectric constant 𝑑₃₃ and high-field piezoelectric strain coefficient 𝑑^∗₃₃ of 332 pC/N and 530 pm/V, respectively, with electromechanical coupling factors 𝑘_𝑝 of 0.52 and 𝑘_𝑡 of 0.48.

1. Introduction

Lead zirconium-titanate solid solution, PZT, has been in the leading position in piezoceramics for half a century because of its excellent piezoelectric properties. although, the content of PbO in PZT is higher than 60 wt%, the Pb volatilization during sintering process and the discarded PZT products pollute the environment and do harm to human health. Therefore, it is an urgent task developing environment-friendly industrial piezoceramics products. Among several families of lead-free piezoelectric materials, (K, Na)NbO₃ (KNN) system has been considered a good candidate for PZT alternative material owing to its strong piezoelectricity. However, it is difficult to prepare well-densified KNN ceramics because of the volatilization of potassium and its reactivity with moisture [1–3]. KNN modified by other compound has been studied in order to improve this piezoelectric properties and sintering performance [4–7].

In our previous research, the morphotropic phase boundary (MPB) of (Na_0.52K_0.48-xLi_x)Nb_1-x-ySb_xTa_yO₃ was identified and (Na_0.52K_0.435Li_0.045)Nb_0.905Sb_0.045Ta_0.05O₃ was found to have a high piezoelectric constant 𝑑₃₃ of 308 pC/N [8]. In this paper, more Sb was introduced near the MPB composition, to form (Na_0.52K_0.435Li_0.045)Nb_0.87Sb_0.08Ta_0.05O₃ (KNLNT-S8) piezoceramics with a further improved piezoelectric constant 𝑑₃₃ (332 pC/N). The sintering effects on properties of the (Li, Sb, Ta)-modified (Na, K)NbO₃ were investigated.

2. Experimental

Chemical composition of our samples in this study was (Na_0.52K_0.435Li_0.045)Nb_0.87Sb_0.08 Ta_0.05O₃. Analytical-grade Na₂CO₃ (99.8%), K₂CO₃ (99.5%), Li₂CO₃ (99.9%), Nb₂O₅ (99.8%), Ta₂O₅ (99.8%), and Sb₂O₅ (99.9%) were used as starting materials. With a ratio to alcohol of 1 : 1.3, the chemicals were wet-milled in polyethylene bottles with ZrO₂ balls as milling media for 12 h. The milled slurry was dried and pressed into discs of 30 mm in diameter. The discs were calcined at 880–920°C for 3–5 h. The calcined discs were crushed and ball-milled again for 12 h. The dried powder was mixed with 0.5% weight PVA binder and pressed into discs of 15 mm in diameter and 1.2 mm in thickness at 160 MPa. After burning out the PVA at 650°C, the discs were put into Al₂O₃ crucibles and sintered at 1070–1120°C for 3–6 h. Silver paste was applied on faces of the sintered discs to form electrodes by firing at 550°C, and then the samples were poled in silicon oil at 30°C for 15 min under a DC field of 4.0 kV/mm. Piezoelectric constants 𝑑₃₃ were determined by using Berlincourt-type quasistatic meter at 50 Hz. High-field piezoelectric strain coefficient, 𝑑^∗₃₃, was calculated from the slope of the field-induced strain curves. Electromechanical coupling coefficients, 𝑘_𝑝 and 𝑘_𝑡, were determined by a resonance and antiresonance method according to IEEE standards by using an impedance analyzer (Agilent 4294A). Sample surface microstructure was observed by using a scanning electron microscopy (SEM), and the polished surface morphology was observed by using an optical microscopy. Polarization hysteresis and strain-electric field curves were measured by using a modified Sawyer-Tower circuit and linear variable differential transducer (LVDT) driven by a lock-in amplifier (Stanford Research Systems, Model SR 830).

3. Results and Discussion

In order to obtain high-density KNN-based ceramics, it is necessary to consider the volatility of sodium and potassium [9]. Figure 1(a) shows density and dielectric loss of the KNLNT-S8 ceramics as a function of sintering temperature. The density of the KNLNT-S8 ceramics increases with increasing sintering temperature from 1070 to 1100°C and reaches its maximum of 4.49 g/cm (relative density over 96%) at 1110°C. As all samples were sintered in covered Al₂O₃ crucibles and embedded in powder with the same composition, the volatility of sodium and potassium was suppressed effectively at sintering temperatures of 1110°C. As the sintering temperature exceeds 1120°C, density of the KNLNT-S8 sample decreases owing to the excessive volatility of sodium and potassium. Dielectric loss of the KNLNT-S8 ceramics keeps to be about 2.0% when the sintering temperature varies from 1080 to 1110°C, much lower than that (~13%) of the sample sintered at 1070°C. Piezoelectric properties of the KNLNT-S8 ceramics as a function of sintering temperature are shown in Figure 1(b). The piezoelectric constant, 𝑑₃₃, and electromechanical coupling factors, 𝑘_𝑝 and 𝑘_𝑡, reach their maximum values of 332 pC/N, 0.52 and 0.48, respectively, at 1100°C. However, the high-field piezoelectric strain coefficient keeps its maximum value in a very wide temperature range from 1080 to 1100°C. This property makes KNLNT-S8 system a good candidate in terms of industrial production.

(a)

(b)

Figure 1 

Sintering temperature dependence of (a) density and dielectric loss, (b) piezoelectric properties for NKLNT-S8 ceramics.

Figure 2 shows surface SEM images of the KNLNT-S8 ceramics sintered at 1080°C, 1100°C, and 1120°C. The sample sintered at 1080°C has grains with a wide size distribution. As the sintering temperature is raised to 1100°C, it can be seen that brick-like shaped grains congregate closely and become homogeneous. Uniform grain microstructure is beneficial to enhancing the mechanical strength of piezoelectric ceramics [10]. However, as the sintering temperature exceeds 1120°C, pores and abnormal grains appear in the ceramics, which may be attributed to the volatilization and segregation of the alkaline elements.

Figure 2 

Surface SEM images of the KNLNT-S8 ceramics.

Optical microscopy images of the polished samples are shown in Figure 3. Many abnormally huge pores are observed in the samples sintered at 1080°C and 1120°C, indicating that the sample was not fully identified at low temperature while much alkaline elements volatilized at high temperature. For the sample sintered at 1100°C, the pores are smaller in size and distributed regularly, showing good compactness. This result is in a good agreement with the density curve (Figure 1(a)).

Figure 3 

Optical microscopy images of the KNLNT-S8 ceramics.

Figure 4 shows 𝑃-𝐸 hysteresis loops of the KNLNT-S8 ceramics sintered at 1080, 1100, and 1110°C. The samples sintered between 1080°C and 1110°C have good ferroelectric properties with a high remanent polarization 𝑃_𝑟 of 22 μC/cm² and a coercive field 𝐸_c of 21 kV/cm. For samples sintered below 1070°C or over 1120°C, it is difficult to obtain closed 𝑃-𝐸 hysteresis loops because of their high conductivity and defects. It is worth noting that coercive field of the KNLNT-S8 ceramics (~21 kV/cm) is higher than the value of pure KNN (~8 kV/cm) [14], demonstrating a hardening‌ effect. Unipolar field-induced strain of the KNLNT-S8 ceramics sintered at 1100°C is shown in Figure 5. The sample exhibits very high electric-field-induced strain; the value of high-field piezoelectric strain coefficient 𝑑^∗₃₃ is as high as 530 pm/V for the 10–20 kV/cm induced strain curve. The high 𝑑^∗₃₃ makes this system a promising lead-free piezoceramics for high-power actuator devices applications.

Figure 4 

𝑃-𝐸 hysteresis loops of the KNLNT-S8 ceramics sintered at different temperatures.

Figure 5 

Unipolar field-induced strain curves of the KNLNT-S8 ceramics sintered at 1100°C.

Table 1 shows piezoelectric and dielectric properties of the KNLNT-S8 ceramics sintered at 1100°C and some KNN-based ceramics reported previously. It is found that our KNLNT-S8 ceramics have excellent piezoelectric and electromechanical properties, such as high piezoelectric constant 𝑑₃₃ of 332 pC/N, low dielectric loss tan𝛿 of about 2.0%, high electromechanical planar coupling coefficient 𝑘_𝑝 of 0.52, and thickness coupling coefficient 𝑘_𝑡 of 0.48. Compared to (Na_0.52K_0.435Li_0.045)Nb_0.905Sb_0.045Ta_0.05O₃ ceramics [8], high Sb content led to improved piezoelectric properties.

Table 1 

Dielectric and piezoelectric properties of the KNLNT-S8 ceramics, compared to some previously reported KNN-based ceramics.

4. Conclusions

In conclusion, high-performance lead-free (Na_0.52K_0.435Li_0.045)Nb_0.87Sb_0.08Ta_0.05O₃ piezoceramics have been synthesized by the conventional mixed oxide route. The sintering effects on piezoelectric properties and microstructure of (Li, Sb, Ta)-modified (Na, K)NbO₃ ceramics have been studied systematically. The KNLNT-S8 ceramics sintered at 1100°C had the lowest porosity and the highest density of 4.49 g/cm, piezoelectric constant 𝑑₃₃ of 332 pC/N, and a dielectric loss tan𝛿 of about 2.0%. In particular, the sample possessed good high-field piezoelectric strain effects, with a 𝑑^∗₃₃ value of 530 pm/V, making it promising candidate for practical application.

Acknowledgment

The authors deeply acknowledge the financial support of the National Natural Science Foundation of China (no. 50802038).