Sintering and Characterization of ( Li , Sb , Ta )-Modified ( Na , K ) NbO 3 Lead-Free Ceramics

Lead-free alkaline niobate-based piezoceramics, (Na0.52K0.435Li0.045)Nb0.87Sb0.08Ta0.05O3 (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 kp of 0.52 and kt of 0.48.


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 environmentfriendly industrial piezoceramics products.Among several families of lead-free piezoelectric materials, (K, Na)NbO 3 (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][2][3].KNN modified by other compound has been studied in order to improve this piezoelectric properties and sintering performance [4][5][6][7].
In our previous research, the morphotropic phase boundary (MPB) of (Na 0.52 K 0.48−x Li x )Nb 1−x−y Sb x Ta y O 3 was identified and (Na 0.52 K 0.435 Li 0.045 )Nb 0.905 Sb 0.045 Ta 0.05 O 3 was found to have a high piezoelectric constant d 33 of 308 pC/N [8].In this paper, more Sb was introduced near the MPB composition, to form (Na 0.52 K 0.435 Li 0.045 ) Nb 0.87 Sb 0.08 Ta 0.05 O 3 (KNLNT-S8) piezoceramics with a further improved piezoelectric constant d 33 (332 pC/N).The sintering effects on properties of the (Li, Sb, Ta)-modified (Na, K)NbO 3 were investigated.

Experimental
Chemical composition of our samples in this study was (Na 0.52 K 0.435 Li 0.045 )Nb 0.87 Sb 0.08 Ta 0.05 O 3 .Analytical-grade Na 2 CO 3 (99.8%),K 2 CO 3 (99.5%),Li 2 CO 3 (99.9%),Nb 2 O 5 (99.8%),Ta 2 O 5 (99.8%), and Sb 2 O 5 (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 ZrO 2 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 Al 2 O 3 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  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).

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 Al 2 O 3 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 sinter-ing temperature are shown in Figure 1(b).The piezoelectric constant, d 33 , 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.
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.
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 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/cm 2 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.
Table 1 shows piezoelectric and dielectric properties of the KNLNT-S8 ceramics sintered at 1100 • C and some KNNbased ceramics reported previously.It is found that our KNLNT-S8 ceramics have excellent piezoelectric and electromechanical properties, such as high piezoelectric constant d 33 of 332 pC/N, low dielectric loss tan δ 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 0.52 K 0.435 Li 0.045 )Nb 0.905 Sb 0.045 Ta 0.05 O 3 ceramics [8], high Sb content led to improved piezoelectric properties.

Conclusions
In conclusion, high-performance lead-free (Na 0.52 K 0.435 Li 0.045 )Nb 0.87 Sb 0.08 Ta 0.05 O 3 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)NbO 3 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 d 33 of 332 pC/N, and a dielectric loss tan δ 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.

Table 1 :
Dielectric and piezoelectric properties of the KNLNT-S8 ceramics, compared to some previously reported KNN-based ceramics.