Fabrication and performance of PNN-PZT piezoelectric ceramics obtained by low-temperature sintering

Abstract The Pb(Ni1/3Nb2/3)O3-Pb(ZrxTi1−x)O3 (PNN-PZT) piezoelectric ceramics with CuO and LiBiO2 doping were successfully fabricated by the low-temperature solid-state reaction to effectively restrain the PbO volatilization. The microstructure and electrical properties of the PNN-PZT ceramics were characterized. The experimental results reveal that the PNN-PZT ceramics are composed of a pure perovskite structure in which the rhombohedral and tetragonal phases coexist. Meanwhile, the good electric properties, including low dielectric loss, outstanding diffusion phase transition and palpable dielectric relaxation, are exhibited in PNN-PZT ceramics with 0.2 wt.% CuO and 1 wt.% LiBiO2 addition. This piezoceramic composition possibly provides a reference for the application of multi-layer piezoelectric actuators.


Introduction
With the rapid development of the electronic information technology, the electronic components widely applied in various devices, such as transformers, reproducers and fil-ters, trend to be smaller, lighter and cheaper [1][2][3]. As the most popular lead-based piezoelectric ceramic, The PNN-PZT system with excellent piezoelectric constant, high electromechanical coupling factor and eminent permittivity are widely used both in military and civilian applications [4,5]. As is known, the sintering temperature of PNN-PZT ceramic is about 1200~1300 ∘ C [6]. The strong volatilization of lead oxide beyond 960 ∘ C in the conventional solid-state reaction can deteriorate the electrical properties of samples [7]. Moreover, the lead is one of heavy metals which are extremely harmful for human health [8]. Additionally, the PbO volatilization in the cofire of PNN-PZT ceramic and Ag-Pd electrode can substantially improve the crack formation in the multi-layer piezoelectric actuators. Currently, the most common approach to slow the volatilization of PbO is sintering in a hermetic atmosphere and adding an excessive amount of PbO, however this problem is not solved efficiently. Thereby, the adjustable sintering temperature (below 960 ∘ C), which can postpone availably the volatilization of PbO and promote the energy saving, has aroused the extensive concerns [9][10][11].
The research on the low-temperature sintering of piezoelectric ceramics has been developed for decades [12]. Generally, there are two methods to reduce the sintering temperature. One is refining effectively the particle size of powders. The ceramic powders prepared by sol-gel methods or hydrothermal process possess superfine particle sizes and high surface energy, which can indeed reduce the sintering temperature [13]. Besides, the another one is adding tiny amounts of efficacious sintering aids, such as V 2 O 5 , SiO 2 , etc, which is an economic and efficient approach to reducing the sintering temperature. The sintering aids are conducive to forming low-melting-point glass phases during the solid reaction. The glass phase formed in the grain boundary accelerates the rate of atomic migration, which is contribute to fast growth of grains. The studies showed that moderate B 2 O 3 and CuO doped (Ba, Ca)(Ti, Sn)O 3 can reduce the sintering temperature from 1250 ∘ C to 1200 ∘ C. Furthermore, the introduction of a tiny amount of low-melting frit (B 2 O 3 -Bi 2 O 3 -CdO) can reduce the sintering temperature even to 960 ∘ C.
Commonly, the electrical properties of piezoelectric ceramics could be deteriorated when the sintering temperature reduced improperly [14,15]. Therefore, on the basis of ensuring that Ag layer is not melted (~960 ∘ C), obtaining high-performance piezoelectric ceramic materials is a key scientific problem to be solved urgently in the development of piezoelectric actuators. As is known, the PNN-PZT ceramic is a typical "soft material", which exhibits a relatively low sintering-temperature and excellent electrical properties. Introduction of CuO and Bi 2 O 3 into the PNN-PZT ceramics as the sintering aids can reduce effectively the sintering temperature [16][17][18]. Therefore in this notion, a series of PNN-PZT ceramics doped by various amounts of CuO and LiBiO 2 as sintering aids to achieve a novel electric properties via low-temperature sintering. Thereinto, the LiBiO 2 can be synthesized by Li 2 CO 3 + Bi 2 O 3 = 2LiBiO 2 + CO 2 . The effect of varying contents of sintering aids on the microstructure and electrical performances were carefully investigated. The related research results will provide a potential candidate for the multi-layer piezoelectric actuators. powders with a purity of more than 99.9% were used as the raw materials. The powders were weighed by an electronic balance of 0.1mg (AL204), and then blended with alcohol by ball-milling in a nylon jar for 12 h at 300 rpm. Subsequently, the mixed powders were dried and calcined at 900 ∘ C for 2h. After addition of CuO, Li 2 CO 3 and Bi 2 O 3 , the pre-sintered powders were ball-milled again with alcohol and dried. Hereafter, these powders mixed with 5 wt.% PVA as binder were pressed under 12 MPa into a planchette whose size is Φ10 mm×1 mm. Besides, the ceramics were sintered at 960 ∘ C in air for 2 h. And then the sintered samples were printed on both sides with sliver electrodes at 850 ∘ C. Finally, the samples for piezoelectric measurements were poled at 70 ∘ C in silicone oil by applying a DC electric field of 3 kV·mm −1 for 20 min.

Piezoceramic characterization
The phase composition of samples was measured by a X'pert-PRO X-ray diffractometer (XRD). The microstructure was observed by a Supra-40 scanning electron microscopy (SEM, Germany). The piezoelectric constant (d 33 ) was tested at room temperature using a quasi-static piezod 33 meter (ZJ-3A, China), in which at least 10 times were conducted to obtain the average value. The dielectric properties were characterized using an LCR meter (Agilent, E4980A), connected to a computer-controlled temperature chamber, by measuring at 10 kHz. cated in morphotropic phase boundary (MPB), possessing the excellent piezoelectric properties [19,20]. It is suggested that the grains are relatively small and many pores exist while the sintering aids short supplied. However, with the increase of sintering aids, the grains grow gradually and the density and homogeneity of ceramics get better, particularly in the PNN-PZT-0.1% CuO-2% LiBiO 2 ceramic. It demonstrates that moderate CuO and LiBiO 2 additions could enhance the grain growth and increase the densification. Nevertheless, as the dopant successively increases, the CuO and LiBiO 2 will aggregate at the grain boundary and cannot dissolve completely into crystal lattices as shown in Figure 2 (c), causing that the overall electric performance gets worse. Table 1 shows the electrical properties of PNN-PZT+xCuO +yLiBiO 2 ceramics. As is seen from Table 1, the changes of kp and d 33 possess the same tendency and reach the maximum values when x = 0.2, y = 1. They are 608 pC·N −1 and 0.65 respectively. Meanwhile, the relative permittivity (ϵr) reaches 3843 and the loss (tan δ) is approximately equal to 2.19%. That means, the upper density, good homogeneity and few second phases (in the sample 3 # ) are significantly helpful for improving the electric properties of ceramics.  Figure 3 shows the temperature dependence of dielectric constant and loss of PNN-PZT ceramics with different contents of CuO and LiBiO 2 measured at 100 kHz. As can be seen from Figure 3 (a), the phase-transition peaks of ferroelectric and paraelectric are observed in all ceramics, which corresponds to the Curie temperature (Tc). Obviously, the Tc reduced with the increasing of LiBiO 2 contents. However, the content of CuO is helpful for the increasing of Tc and the reduction of tan δ. The ceramic with 0.2 wt.% CuO and 1wt.% LiBiO 2 possesses the highest Tc of 191.6 ∘ C, and the lowest loss of 2.19%, respectively. Figure 4 shows the relationship between inverse dielectric constant and temperature of PNN-PZT+x CuO+y LiBiO 2 ceramics at 10 kHz. The permittivity of the ferroelectric materials calculated according to the Curie-Weiss law when the temperature is beyond Tc, as shown in Eq. (1) [12]:

Relaxation behavior of PNN-PZT ceramics
where Tcw represents the Curie-Weiss temperature and C is the Curie-Weiss constant. The deviation degree of Curie transition can be described in Eq. (2) [12]: where ∆Tm is the deviation, T B is the initial temperature that the dielectric materials obey the Curie-Weiss law, and Tm refers to the temperature corresponding to the maximum of permittivity. It can be seen from Figure 4 that PNN-PZT ceramic belongs to a typical relaxor ferroelectric. The ceramics with fewer LiBiO 2 contents exhibits a higher ∆Tm than that of ceramic with 2 wt.% LiBiO 2 additions. Hence, the appropriate addition of LiBiO 2 can lower the sintering temperature, thus enhancing the relaxation behavior of piezoelectric ceramics. The plots of ln(1/ϵr −1/ϵm)/ ln(T − Tm) for PNN-PZT+x CuO+y LiBiO 2 ceramics measured at 100 kHz are shown in Figure 5. The dispersion coefficient can be obtained by linear fitting of Eq. 2. The ceramic will be a normal ferroelectric when is equal to 1, but will transform into imperfect relaxor ferroelectric when is ranged from 1 to 2.   When = 2, the ceramic will become a relaxor ferroelectric with diffuse phase transition. It is observed from Figure 5 that the increases firstly with the increase of LiBiO 2 concentration, especially for the PNN-PZT ceramic modi-fied with 1wt.%CuO and 2wt.%LiBiO 2 , the is approach to 2.036 at 100 Hz, which means that the PNN-PZT ceramic modified with 1wt.%CuO and 2wt.%LiBiO 2 is an excellent relaxor ferroelectric.

Discussion
Universally, the improvement of dielectric constant (d 33 ) is closely related to the formation of Pb-ion vacancies to effectively promote the domain rotation. The extensive additions of Bi3 + introduced into the ceramics substitute for the Pb-sites, also result in many Pb 2+ vacancies to keep the electrical neutrality. In this study, the adjustable amounts of sintering aids were dissolved into the perovskite solid solution, further leading to the increase of d33. However, the excessive additions of sintering aids could segregate in the grain boundaries to pin the domain rotation, further decreasing the value of d33. Besides, the LiBiO2 and CuO added to the PNN-PZT ceramics are much beneficial for achieving a dense and uniform microstructure, which was also helpful to obtain a prominent d 33 improvement.
To comprehensively evaluate the piezoelectric properties of the PNN-PZT ceramics with CuO and LiBiO2 additions, the d 33 and sintering temperature of conventional lead-based ceramics have been listed in Figure 6. As is seen, the PHT-PNN ceramics with various sintering aids possess excellent piezoelectric properties while sintered at 950, 1000 and 1050 ∘ C. However, a large amount of precious Hf elements were contained in the PHT-PNN ceramics, seriously limiting their industrial applications. Hereinto, the d33 of the ceramic achieved in this work shows the high value among the low-temperature sintering piezoceramics ever reported in the literature. 19% and 191.6 ∘ C respectively. 3. The diffusion phase transition and the dielectric relaxation characteristics associated with Curie temperature can be found in this series, especially when x = 0.2 and y = 2, the sample shows the lowest Tc, the maximum of diffusion phase transition and dielectric relaxation behavior.