Influence of Nb₂O₅ doped amount on the property of BCTZ lead-free piezoelectric ceramics doped with Li₂CO₃

Abstract

Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCTZ) lead-free piezoelectric ceramics doped with Nb₂O₅ (0.1, 0.3, 0.5, 0.7, 0.9 wt%) and Li₂CO₃ (0.6 wt%) were prepared by conventional solid-state reaction method. Influence of Nb₂O₅ doping amount on the piezoelectric property, dielectric property, phase composition and microstructure of prepared BCTZ lead-free piezoelectric ceramics doped with Li₂CO₃ were investigated by X-ray diffraction and scanning electron microscopy and other analytical methods. The results showed that the sintered temperature decreased greatly when the BCTZ lead-free piezoelectric ceramics were co-doped with Nb₂O₅ and Li₂CO₃; a pure perovskite structure of BCTZ lead-free piezoelectric ceramics co-doped with Nb₂O₅ and Li₂CO₃ sintered at 1,020 °C could be also obtained. The grain size decreased when Nb₂O₅ doping amount increased. The piezoelectric constant (d₃₃), the planar electromechanical coupling factor (k_p), the relative dielectric constant (ε_r) of BCTZ ceramics doped with Li₂CO₃ increased firstly and then decreased, the dielectric loss (tanδ) decreased firstly and then increased when Nb₂O₅ doping amount increased, indicating that Nb₂O₅ was “soft” additive. When Nb₂O₅ doping amount (z) was 0.7 wt% and Li₂CO₃ doping amount was 0.6 wt%, the BCTZ ceramics sintered at 1,020 °C possessed the best piezoelectric property and dielectric property, which d₃₃ was 238 pC/N, k_p was 29.33 %, ε_r was 4,691, tanδ was 2.07 %.

1 Introduction

Piezoelectric materials is a kind of important functional materials of high technology, widely used in medical, aerospace, military field and etc. Now lead-based lead zirconate titanate (PZT) piezoelectric ceramics with lead content more than 60 % still takes up the dominant position of piezoelectric materials. Due to the poison effect of lead element, it will inevitably cause serious harm to the human body and environment in the process of the lead based piezoelectric ceramic preparation, piezoelectric device preparation and aging device recovery. Therefore, the searching for new type lead-free piezoelectric ceramic materials is the urgent problem [1]. Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCTZ) system piezoelectric ceramics with high piezoelectric constant was synthesized for the first time by Liu et al. [2] and was very expected to replace lead-based piezoelectric ceramic materials [3–7]. But the BCTZ lead-free piezoelectric ceramics has a big weakness: the sintering temperature is very high and is more than 1,500 °C and is very difficult co-fired with cheap conductive metal electrode material (such as silver, copper, nickel) and is seriously restricts its application. How to reduce the sintering temperature of BCTZ ceramics and realizing low temperature co-fired with cheap electrode materials are very important for the widely used BCTZ implementation. Li₂CO₃ is a commonly used sintering aids and have low melting point and higher sintering activity and has a notable effect to promote the green body density in sintering process and can effectively decrease the sintering temperature of ceramics, but Li₂CO₃ doping will reduce the piezoelectric property of piezoelectric ceramics in a certain extent [8]. In lead zirconate titanate piezoelectric ceramics and other lead based piezoelectric ceramic doped modification, Nb₂O₅ was a soft piezoelectric additive and Nb₂O₅ doping could produce lead vacancy and make ceramic coercivity decrease and increase piezoelectric property of piezoelectric ceramics [9–13]. In lead-free Bi_0.5(Na_0.82K_0.18)TiO₃ piezoelectric ceramics studying, the electric field-induced strain was markedly enhanced and the piezoelectric property was increased by doping of Nb₂O₅ and Nb₂O₅ also showed “soft effect” piezoelectric additive [14]. So, the author attempted to increase piezoelectric property of BCTZ lead-free piezoelectric ceramics and decrease sintering temperature through Li₂CO₃, Nb₂O₅ co-doping and the effect of Nb₂O₅ doping amount on the piezoelectric property, dielectric property, phase composition and microstructure of prepared BCTZ lead-free piezoelectric ceramics were studied and the related mechanism was discussed.

2 Experimental

Li₂CO₃ (0.6 wt%) and Nb₂O₅ (z = 0.1 wt%, z = 0.3 wt%, z = 0.5 wt%, z = 0.7 wt%, z = 0.9 wt%) co-doped BCTZ ceramic samples were prepared by conventional solid-state reaction method. The powder materials of BaCO₃, CaCO₃, ZrO₂ and Li₂CO₃ and Nb₂O₅ with a purity of 99.0 % were produced by Sinopharm Chemical Reagent Co. China and TiO₂ with a purity of 99.8 % was produced by Xiantao Zhongxing Electronic Material Co. China were used as raw materials. The raw materials were weighed according to the chemical formula Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ and then mixed for 8 h in a planetary ball mill with zirconia balls using alcohol as the media. Then the mixed powders were dried and calcined at 1,250 °C for 4 h in air and in a crucible. Different amounts of Nb₂O₅ doping were added into the calcined powders, respectively. The resulting mixtures were ball milled again in anhydrous ethanol for 8 h, the dried powders were mixed with 5–6 wt% polyvinyl alcohol (PVA) and pressed into disks with a diameter of 15 mm and a thickness of 1.0–1.5 mm by uniaxial pressing at 30 MPa. After excluding PVA binder at 500 °C for 1 h, the pressed pellets were sintered at 1,020 °C for 4 h in air. For measuring electrical property, silver paste was coated on both polished surfaces of the sintered samples and was fired at 600 °C for 30 min to form Ag electrodes. Specimens for piezoelectric measurements were polarized at 40–60 °C in a silicone oil bath by applying a dc electric field of 4 kV/mm for 20 min. Piezoelectric and dielectric properties were measured after laying the polarized specimens for 24 h to release the remnant stress and charge.

The crystal phase structure was examined using a Rigaku D/max2500PC X-ray diffraction meter (XRD) and its diffraction patterns was obtained. Its natural surface microstructure was observed by JXA-840A scanning electron microscope (SEM) and corresponding pictures were obtained. The capacitance (C) and dielectric loss (tanδ) of poled samples were measured at 1 kHz and room temperature by an YY2814 LCR meter, and then dielectric constant (ε_r) value of poled samples were worked out. Piezoelectric coefficient (d₃₃) value were measured by a quasi-static d₃₃ meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China). The planar electromechanical coupling factor (k_p) was determined by a resonance antiresonance method with an Agilent 4294A impedance analyzer.

3 Results and discussion

3.1 Influence of Nb₂O₅ doping amount on the material phase and microstructure of BCTZ ceramics doped with Li₂CO₃

Figure 1 is the XRD patterns of BCTZ ceramics doped with Li₂CO₃ and various Nb₂O₅ doping amount and sintered at 1,020 °C. As can be seen from Fig. 1, a single perovskite structure was observed in the XRD patterns of BCTZ ceramics doped with various amount of Nb₂O₅ and there is no trace of second phase in the ceramics. The diffraction peak profiles offset to high angle and shows that the lattice parameters decreases when Nb₂O₅ doping amount increases. This is because Nb⁵⁺ ion radius (0.069 nm) is less than Ti⁴⁺ ion radius(0.086 nm) and the lattice parameters decrease when Nb₂O₅ took the place of Ti⁴⁺ according to brag equation of 2d sinθ = λ [13, 15].

Fig. 1

XRD patterns of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents

Figure 2 is the SEM micrographs of BCTZ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents and sintered at 1,020 °C. It shows that ceramic grain size decreased rapidly and surface structure of ceramics are relatively dense and there is only a small number of porosity when Nb₂O₅ doping amount increased. When Nb₂O₅ doping amount is 0.7 wt%, crystal arrangement of ceramics is the most compact and there is a tiny particles and shows that Nb₂O₅ doping content slightly exceed the solid solubility of Nb₂O₅ in BCTZ at this time. When the Nb₂O₅ doping amount is 0.9 wt%, there are many tiny particles in ceramics and shows that this doping amount has been completely beyond the solid solubility and grain uniformity density decreases and the piezoelectric property of ceramics decreases. The suitable amount of Nb⁵⁺ is conductive to increase the density of the ceramics, due to an increased concentration of barium vacancies which in turn allow for better atomic diffusion and thus supports the densification process [13, 16]. Excess amount of the Nb₂O₅ beyond solid solubility limit will possibly segregate at the grain boundary, resulting in inhibiting grain growth to form a smaller grain size [17, 18], when the excess amount of the Nb₂O₅ is more and the domain developing is not enough and the piezoelectric property of the ceramics decreases [9].

Fig. 2

SEM micrographs of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents

3.2 The influence of Nb₂O₅ doping amount on the dielectric and piezoelectric properties of BCTZ ceramics doped with Li₂CO₃

Figure 3 is the curves of piezoelectric constant (d₃₃) and planar electromechanical coupling factor (k_p) of BCTZ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents. As can be seen from Fig. 3, when Nb₂O₅ doping amount increases, the piezoelectric constant (d₃₃) and the planar electromechanical coupling factor (k_p) of BCTZ ceramics doped with Li₂CO₃ increases firstly and then decreases. When Nb₂O₅ doping amount is 0.7 wt%, the BCTZ ceramics doped with Li₂CO₃ and sintered at 1,020 °C possess the best piezoelectric property of d₃₃ = 238pC/N, k_p = 29.33 %. A small amount of Nb₂O₅ makes the ceramic density and grain uniformity increase, the piezoelectric property of the ceramics is increased. An excess of Nb₂O₅ over the solid solubility of ceramics is gathered at the grain boundary so that the piezoelectric performance decreases. When Nb₂O₅ doping amount is small amount, Nb⁵⁺ take the place of Ti⁴⁺ and produced Ba vacancy due to the price balance, electric domain easily orientation along the electric field when polarization, the piezoelectric property increases. When Nb₂O₅ doping amount is more amount, Nb⁵⁺ take the place of Ti⁴⁺ and produced the bigger lattice distortion due to the different of ion radius and electric domain orientation along the electric field is difficult when polarization, the piezoelectric property decreases. In the Nb⁵⁺ instead of Ti⁴⁺ process, when the Nb⁵⁺ instead of Ti⁴⁺ is less amount, produced Ba vacancy effect dominates, the piezoelectric property increases when Nb₂O₅ doped amount increases. When the Nb⁵⁺ instead of Ti⁴⁺ is more amount, produced the bigger lattice distortion effect dominates, the piezoelectric property decreases when Nb₂O₅ doped amount increases.

Fig. 3

The piezoelectric constant (d₃₃) and planar electromechanical coupling factor (k_p) of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents

Figure 4 is the dielectric constant (ε_r) and dielectric loss (tanδ) curves of BCTZ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents. As can be seen from Fig. 4, the dielectric constant (ε_r) of BCTZ ceramics doped with Li₂CO₃ increases firstly and then decreases, the dielectric loss decreases firstly and then increases when Nb₂O₅ doping amount increases. When Nb₂O₅ doping amount is 0.7 wt%, the BCTZ ceramics doped with Li₂CO₃ and sintered at 1,020 °C possess the best dielectric property of ε_r = 4,691 and tanδ = 2.07 %. A small amount of Nb₂O₅ inhibited the increase of ceramic grain and ceramic grain is the uniform and compact, the dielectric constant increases and the dielectric loss decreases. The more excessive Nb₂O₅ doping amount gathered in the grain boundary and resulting in inhibiting grain growth to form a smaller grain size and makes the dielectric constant of the ceramics decrease and dielectric loss of the ceramics increase.

Fig. 4

The dielectric constant (ε_r) and dielectric loss (tanδ) of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ ceramics doped with Li₂CO₃ and various Nb₂O₅ contents

4 Conclusions

Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ lead-free piezoelectric ceramics doped with Nb₂O₅ (0.1, 0.3, 0.5, 0.7, 0.9 wt%) and Li₂CO₃ (0.6 wt%) were prepared by solid state reaction method. The sintered temperature decreased greatly when the BCTZ lead-free piezoelectric ceramics were co-doped with Nb₂O₅ and Li₂CO₃; a pure perovskite structure of BCTZ lead-free piezoelectric ceramics co-doped with Nb₂O₅ and Li₂CO₃ sintered at 1,020 °C could be also obtained. The piezoelectric constant (d₃₃), the planar electromechanical coupling factor (k_p), the relative dielectric constant (ε_r) of BCTZ ceramics doped with Li₂CO₃ increased firstly and then decreased, the dielectric loss (tanδ) decreased firstly and then increased when Nb₂O₅ doping amount increased, indicating that Nb₂O₅ was “soft” additive. The grain size decreased when Nb₂O₅ doping amount increased. When Nb₂O₅ doping amount (z) was 0.7 wt% and Li₂CO₃ doping amount was 0.6 wt%, the BCTZ ceramics sintered at 1,020 °C possessed the best piezoelectric property and dielectric property, which d₃₃ was 238 pC/N, k_p was 29.33 %, ε_r was 4,691, tanδ was 2.07 %.