Dielectric and Pyroelectric Properties of Lead-Free Sodium Bismuth Titanate Thin Films Due to Excess Sodium and Bismuth Addition

Abstract

Pb-free ferroelectric (Na 0.5 Bi 0.5 )TiO 3 (NBT) thin films were prepared by a modified sol-gel process. Their structural, dielectric and pyroelectric properties were investigated as a function of the excess Na/Bi ratio and the annealing temperature. In the case of thin films containing no excess Na and Bi, only partial amounts of the perovskite NBT were crystallized, where the films consisted mainly of the pyrochlore phase of Bi 2 Ti 2 O 7 for annealing conditions of 600~800 o C. With increasing excess Na/Bi ratio, the proportion of the perovskite phase effectively increased due to the compensation of the volatile Na and Bi components. For a Na/Bi ratio of 2.0, the thin film with single NBT perovskite phase was obtained within XRD detection limit after annealing at 700 o C for 10 min and it showed the excellent dielectric properties, εr of ~550 and tan δ of 0.03. While these properties were degraded for Na/Bi ratio of 2.5 despite the existence of pure perovskite phase. The NBT thin film with Na/Bi ratio of 2.0 are also promising candidates for applications requiring pyroelectric devices because it was found to have pyroelectric coefficients of 1.3~7 nC/cm 2 K in the temperature range of 30~100 o C.

Influence of Sintering Temperature on Phase Formation and Optical Properties of Lead-Free Ferroelectric BI0.5NA0.5TIO3 Materials

“…In case of amount of Na less than 40 %, the gelforming vaporization results in the lack of a necessary amount of Na, hence the new phase of Bi 2 Ti 2 O 7 is formed to. This observation is in a good agreement with the report of Kang et al [14]. In addition, when the Na amount in range of 10 -30 mol%, other imputity phases are observed and difficult to identify the origin of these phases.…”

“…Kang et al found that the phase formation, ferroelectronic and insulating properties of Bi 0.5 Na 0.5 TiO 3 film synthesized by sol-gel method strongly depended on the primary ratio of Bi/Na because these elements are easy to evaporate during gelization and annealing processes [14]. However, these reports about the evaporating amount of Na during gelization,the influence of sintering temperture on the phase information and optical properties of this material have not been performed in a systematic manner.…”

Microstructural and ferroelectric properties of rare earth (Ce, Pr, and Tb)-doped Na0.5Bi4.5Ti3O15 thin films

“…However, the PZT often showed poor fatigue endurance with a Pt electrode and contains the highly toxic lead ion as part of its chemical composition, which acts as a major obstacle for use in practical devices [2][3][4]. Particularly, the volatile PbO in PZT is considered to pose a serious threat to human health because it is highly toxic [4].…”

“…The formation of secondary phases and oxygen vacancies lead to poor electrical and ferroelectric properties in the NaBTi thin film. Various methods, such as compositional compensation by the addition of excessive Na and Bi ions and cationic doping, have been established to obtain single-phase NaBTi and to control oxygen vacancies, respectively [2,4,16]. In fact, for Bi-based ferroelectric compounds, doping with RE metal ions has been strongly recommended as a way of controlling the leakage current and to improve the ferroelectric properties [2,9,17].…”

“…In the crystal structure of NaBTi, some Bi-sites in the pseudo-perovskite block have been occupied by Na-atoms [14]. Though NaBTi exhibits many superior properties, the preparation of a single-phase NaBTi compound is difficult due to the high volatility of Na and Bi ions, which often introduces secondary phases in this material [4]. The formation of secondary phases and oxygen vacancies lead to poor electrical and ferroelectric properties in the NaBTi thin film.…”