Phase formation and characterization of BaBi2Ta2O9 obtained by mixed oxide procedure
Introduction
The layered structure perovskites have recently received considerable interest in view of their potential application as low-voltage, high-speed nonvolatile random access memories (NvRAM) [1]. On account of their read–write speed, nonvolatility and low operating power and radiation hardness, NVFRAM are promising candidates for substituting silicon based, electrically erasable, programmable read-only memories (EEPROMs) and flash EEPROMs. Despite the fact that conventional ferroelectric materials have great potential for high-density FeRAMS (typically 1 Mbit), their commercial utility has seen little success due to factors such as fatigue, imprint, retention and aging, which reduce their lifetime. BaBi2Ta2O9 (BBT) is one material which seems to have excellent fatigue endurance; it has been demonstrated that there is practically no polarization fatigue up to about 1012 switching cycles [2].
These materials were first synthesized by Aurivillius [3], [4], [5] and were named Aurivillius compounds. The chemical formula of these compounds is expressed as (Bi2O2)2+ (Ax−1BxO3x+1)2−, where x indicates the number of perovskite building blocks between two (Bi2O2)2+ layers and A and B represent the different cations with low and high valences in the structure [6]. This group of compounds exhibits a perovskite-like structure in the way that the B cations form chemical bonding with the oxygen ions to build BO6 octahedrons, whereas the A cations are located in the space between BO6 octahedrons. The BO6 octahedrons compose continuous layers perpendicular to the c-axis direction, but are interrupted by (Bi2O2)2+ layers along the c-axis direction. The BO6 octahedrons exhibit spontaneous polarization, thereby resulting in ferroelectric properties.
Mixed bismuth oxide layer compounds were first reported by Bengt Aurivillius in 1949, and the ferroelectric nature of the Aurivillius phase was investigated by Smolenskii and Agronovskaya [7] a decade later. In the 1990s, there has been a resurgence of interest in some of the bismuth oxide layered compounds for their interesting ferroelectric properties. Barium bismuth tantalate (BaBi2Ta2O9, BBT) has been identified for potential applications in ferroelectric nonvolatile random access memories because of its excellent resistance to polarization fatigue [8]. In ferroelectric memory applications improved performance is observed with BaBi2Ta2O9 due to its specific domain structure, which is two-dimensional. It does not induce high internal stress and makes it highly suitable for switching applications. Besides improvement in the fatigue characteristics, a longer polarization retention time, less tendency to imprint, and lower leakage currents have also been observed [9].
It is well known that the properties of materials depend on their synthesis processes. Besides the conventional method basing on the solid-state reaction between Bi2O3, Ta2O5 and Ba5Ta4O15 at high temperatures (about 1000 °C), other methods such as sol–gel [10], polymeric gel [11], sputtering [12] or metal-organic chemical vapour deposition [13] have been studied to synthesize BaBi2Ta2O9. The solid-state method has the disadvantage of high reaction temperatures, but it is simple to operate and uses the cheap and easily available oxides as starting materials. The chemical solution methods can provide products of fine and homogeneous particles with large specific surface areas, but the processes are generally complicated and the raw materials are very expensive.
In view of this, our paper reports on phase formation of BaBi2Ta2O9 using a mixed oxide procedure with attempt to obtain dense ceramics with controlled microstructure.
Section snippets
Experimental
BaCO3 (Mallinckrodt), Bi2O3 (Baker Analyzed) and Ta2O5 (Alfa Aesar) were used as raw materials. Appropriate quantities of BaCO3, Bi2O3 and Ta2O5 were mixed and homogenized in an atritor (Szeguari Atritor 01HD) with zirconia balls at 400 rpm during 1 h in acetone medium. After drying and sieving, the powders were placed in a zirconium oxide crucible and calcinated in a conventional furnace at various temperatures ranging from 800 to 1000 °C for 2 h. The resulting products were crashed in a
Results and discussions
In order to determine the best condition of annealing to obtain BBT ceramics, a thermal analyses was performed. Fig. 1 shows the TG and DTA curves of the BBT powder obtained from room temperature up to 1200 °C using a heating rate of 3 °C/min. The DTA curve shows a broad endothermic peak in the range of 570–670 °C due to the formation of Ba5Ta4O15 resulting from the reaction between BaCO3 and Ta2O5 as shown in Eq. (1). During the endothermic reaction, it is noted a weight loss around 3% (DTA
Conclusions
BaBi2Ta2O9 powder was obtained by solid-state reaction at 950 °C free of secondary phases. The BBT phase purity during the sintering process is strongly influenced not only by the sintering temperature, but also by the compaction conditions of the pellets. Good preparation conditions to obtain ceramics of high density are a very high compaction of the pellets and a sintering temperature of 1000 °C. Sintering at 1050 °C leads to abnormal grain growth and lower density due to the volatilization
Acknowledgements
The authors gratefully acknowledge the financial support of the Brazilian agencies FAPESP, CNPq, and CAPES.
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