Mechanical synthesis and characterization of Bi₄Ti₃O₁₂ nanopowders

Abstract

Nanocrystalline bismuth titanate (BiT) powders with the particle size distribution in the range of 30–45 nm were synthesized from their oxides Bi₂O₃ and TiO₂ using mechanical ball milling process. The ball milling synthesis was performed in air for 5 h. The synthesized BiT ceramic was calcined at 900 °C in air atmosphere for 2 h without a pre-calcination step. The X-ray diffraction pattern was used to analyze the formations of phase and crystal structure. The morphology and the microstructure of the nanosized bismuth titanate powders were studied using scanning electron microscope. The Raman spectrum measurement was used to investigate the structural changes observed in the formation of BiT nanopowders. The photoluminescence measurement showed that the material exhibited maximum emissions at 454 nm and 469 nm in the visible emission band around blue–green colour.

Introduction

Bismuth titanate (Bi₄Ti₃O₁₂) belongs to Aurivillius family with a general formula of (Bi₂O₂)[A_m−1(B)_mO_3m+1], which consists of (Bi₂Ti₃O₁₀)²⁻ layers sandwiched between bismuth oxide (Bi₂O₂)²⁺ layers [1]. BiT has relatively high dielectric permittivity (∼200), very high Curie temperature [T_C = 675 °C], and breakdown strength [2]. It is an interesting lead-free ferroelectric material with layered structure and environmental friendly ceramic with potential applications in electronic industry as capacitors, sensors, memory storage devices, optical display and other electro-optical devices [3], [4]. Studies on BiT shows that it has a high leakage current and domain pinning due to defects such as Bi vacancies accompanied by oxygen vacancies [5], [6]. Furthermore, it suffers from a low durability against a repeated switching of polarization states [7], which is probably ascribed to lattice defects [8], [9]. These electrical properties prevent it from practical ferroelectric random access memory (FRAM) and piezoelectric applications.

Recently, efforts have been devoted to study Bi₄Ti₃O₁₂ ceramics and thin films fabricated by various methods. The bismuth titanate powder is prepared using the conventional ceramic route such as solid state reaction method [10]. This conventional method produces non-stoichiometric composition due to the undesirable loss of bismuth content through volatilization at elevated temperature. In addition, it also produces crystallites coarsening and particle aggregation due to calcination at high temperature.

Hence, many chemical methods such as hydrothermal method [11], sol–gel processes [12], [13], coprecipitation method [14], the microemulsion method [15], the metal–organic polymeric precursor process [16] and so on have been investigated as alternatives. The advantages of the chemical methods over the other methods are the controlled morphology, narrow particle size distribution, high purity, high crystallinity, and possible reduction in sintering temperatures. However, the chemistry based methods use moisture or light sensitive chemicals, such as bismuth salts, that make them difficult to deal with.

Among the various techniques available for the synthesis of BiT powders, the mechanical alloying has certain advantages such as low cost and it uses widely available oxides as the starting materials and skips the calcination steps at intermediate temperatures, leading to a simplified process [17]. Recently, the literature has reported few studies on the mechanical synthesis of the layered structures such as Bi₄Ti₃O₁₂ [18], [19]. Therefore, in this study, BiT powders were prepared by mechanical ball milling method and heat treated at 900 °C for 2 h. The lattice parameters, ac impedance, Raman spectrum, and photoluminescence of BiT nanopowders were studied using different characterization techniques.