Preparation and ferroelectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 thin films deposited on Pt electrodes using LaNiO3 as buffer layer
Introduction
Lead-based ferroelectric materials represented by PbTiO3 and Pb(Zr, Ti)O3 are widely used due to their excellent ferroelectric and piezoelectric properties. However, lead toxicity is a tremendous disadvantage to the environment. Inevitably, developing lead-free materials is of great importance. Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) is considered to be one of the promising materials since it was discovered by Smolenski et al. [1]. Bulk ceramics of NBT have shown strong ferroelectricity at room temperature (remnant polarization Pr = 38 μC/cm2 and coercive field Ec = 73 kV/cm) with a relative high Curie temperature of 320 °C. NBT-based bulk ceramics modified with BaTiO3 (BT) show improved properties due to the existence of rhombohedral–tetragonal morphotropic phase boundary (MPB) [2], [3]. The piezoelectric constant d33 of the NBT-BT ceramics with the MPB composition can reach up to 125 pC/N [4], and the (1 0 0)-oriented crystals grown by spontaneous nucleation method showed a piezoelectric constant of d33 = 450 pC/N and electric field induced strain up to 0.25% [5]. Also, for MPB composition, some researchers got bulk ceramics with pyroelectric coefficient Pi = 3.9 × 10−4 cm−2 K−1 [6], which possibly resulted from the large spontaneous and remnant polarization (Pr = 40 μC/cm2) and lower depolarization temperature (Td = 100 °C) [4]. However, NBT-BT in the form of thin films has rarely been studied because the existence of precipitation when adding BT to NBT. Recently, we use ammonia solution to stabilize the solution. Meanwhile, buffer layers are widely used to improve the properties of the thin films, such as LaNiO3 and NaNbO3 [7], [8], [9]. LaNiO3 (LNO) has a pseudo cubic perovskite structure (a = 3.84 Å), which is not only a good candidate for the electrode material due to its good metallic conductivity, but also a promising buffer layer material for oriented growth.
Section snippets
Experimental
Two solutions were fabricated with nominal compositions of Na0.5Bi0.5TiO3 (NBT) and BaTiO3 (BT), respectively.
The NBT sol was synthesized first. Sodium acetate, CH3COONa·3H2O, bismuth nitrate, Bi(NO3)3·5H2O, were mixed and stirred in acetic acid at 70 °C for 120 min, which was denoted as sol 1. On the other hand, acetylacetone (CH3COCH2COCH3) was mixed with 2-methoxyethanol, stirred for 30 min, and we added titanium tetra-n-butyl into the mixture. The mixture was stirred constantly until a
Results and discussion
Fig. 2 shows the XRD patterns of the NBT-BT thin films deposited on Pt/Ti/SiO2/Si (1 1 1) and LNO/Pt/Ti/SiO2/Si (1 1 1) substrates. The XRD patterns indicated that the thin film deposited on Pt/Ti/SiO2/Si substrates is polycrystalline structure. But, films deposited on LNO/Pt/Ti/SiO2/Si substrates showed a highly preferred orientation of (1 0 0). The degree of grain orientation can be calculated by Lotgering factor f, which is defined as [10]where
Conclusions
(Na0.5Bi0.5)0.94Ba0.06TiO3 thin films were deposited on Pt/Ti/SiO2/Si (1 1 1) and LNO/Pt/Ti/SiO2/Si (1 1 1) substrates by a sol–gel process. The X-ray diffraction pattern indicated that the NBT-BT thin film deposited on Pt/Ti/SiO2/Si (1 1 1) substrates is polycrystalline structure without any preferred orientation. But the NBT-BT thin film deposited on LNO/Pt/Ti/SiO2/Si substrates shows highly (1 0 0) orientation. NBT-BT thin film deposited on LNO/Pt/Ti/SiO2/Si substrates possessed a much lower leakage
Acknowledgement
The authors would like to acknowledge the support from the National Natural Science Foundation of China under grant Nos. 50972108 and 50932007.
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2021, Journal of Alloys and CompoundsCitation Excerpt :However, researchers have reported that there is a problem of high leakage current that affects the piezoelectric and ferroelectric applications of NBT. This leakage problem is mainly because of the oxygen vacancies which generates mobile charge defects because of the A-site element volatilization [14–17]. He and Mo [18] explained a proper model for the doping strategy of NBT materials which states that by partial substitution of dopants at A-site NBT-based materials properties can be enhanced.
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2018, Ceramics InternationalCitation Excerpt :Bismuth sodium titanate Na0.5Bi0.5TiO3 (NBT), one of the A-site complex perovskite structure ferroelectrics, has attracted great attentions as a possible alternative to lead-based ferroelectric materials due to its superior electrical properties [1–3]. However, the properties of the pure NBT itself are not sufficiently good for practical applications because of its high leakage current associated with the weak Bi‒O bonds and oxygen vacancies generated through A-site element vaporization during sample processing [4–7]. Therefore, its electrical properties need additional improvement and intensive investigations were carried out on the modification in NBT by changing the A-site cation stoichiometry.
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2016, Ceramics InternationalCitation Excerpt :Like Pb2+ ions in PbTiO3, Bi3+ have stereochemically active lone pairs which contributes to the lager ferroelectricity in NBT-based materials [5,6]. NBT ceramic possesses a relatively large remnant polarization (Pr) of 38 μC/cm2 and high Curie temperature of 320 °C [7]. For the pure NBT thin film, the intrinsic properties can be obtained difficultly at room temperature [8].