Bismuth based ferroelectric compound ABi2M2O9 and ABi4Ti4O15 (where A=Sr, Ba, Ca, and M=Ta, Nb) that belongs to the Aurivillius family, have attracted great attention as potential ferroelectric materials for non-volatile memory device applications due to their relatively high curie temperatures high dielectric breakdown strength, low dielectric dissipation factor, high anisotropy and are because of their low fatigue of remnant polarization under cycling. The properties of these ceramic materials are greatly effected by their powder characteristics and thus by the processing conditions and techniques. High temperature and prolonged sintering schedules, often associated with their processing, lead to evaporation of bismuth from the system and causes serious degradation in their properties and lowers their efficiency.
In this abstract we report the preparation of nanocrystalline Ba-based layered perovskite Powders [BaBi2Ta2O9 (BBT), BaBi2Nb2O9 (BBN) and BaBi4Ti4O15 (BBTi) powders through thermolysis of a novel, aqueous based precursor solution. The precursor solution have been composed of stoichiometric amounts of EDTA complexes of Ba+2, Tartarate complex of Ta5+/ Nb5+/ Ti+4 and Triethanolamine complex of Bi+3 along with optimum amounts of Triethanolamine (TEA; around 6M with respect to the total moles of the metal ions). The metal ions remained in solution through the formation of coordinated complexes with readily available, inexpensive compounds (such as: tartaric acid, EDTA and TEA) while the presence of TEA in excess to the stoichiometric requirement helped in avoiding any chance of precipitation during the process. The thermolysis of the precursor solution generated a voluminous, fluffy, mesoporous black organic based precursor material. Single step calcination of the precursor material at 600°C/2h for BBTi, 700°C/2h for BBN and 725°C/2h for BBT resulted in the desired perovskite phase.
Calcination of the precursors generated heat through combustion of the carbonaceous remains of the decomposed metal-complexes and TEA. The entire thermal effect was accompanied by the evolution of various gases, such as CO, CO2, NH3, water vapor etc. The virgin precursors were X-ray amorphous. The bright field TEM micrographs of the calcined (at their phase formation temperature) powders showed a highly mesoporous structure with average pore diameters ranging between 30-40nm. The smallest visible isolated spot in the bright field TEM micrographs has been identified with the particle/ crystallite agglomerate and their average diameters have been found to lie between 15-20nm. The corresponding SAED pattern of the sample showed distinct rings, characteristic of an assembly of nanocrystallites. Studies on the variations of the dielectric constant (ε') with temperature of the compacted and sintered (at 900°C-1000°C/4h) pellets showed the relaxor behavior with ε'max= 846 at the Curie temperature (Tc) of 1°C at 1KHz for BBT, ε'max=1117 at the Curie temperature (Tc) of 103°C at 1KHz for BBN.
The realization of the mesoporous structured of the bi-based layer perovskite phase on calcination of their precursors at relatively low external heat-treatment temperatures, indicated the presence of small atomic clusters of appropriate chemical homogeneity in the amorphous precursors, which facilitated the crystallization process.
Prof. Panchanan Pramanik
Department of Chemistry, Indian Institute of Technology,
Kharagpur, West-Bengal 721 302 INDIA
Phone: +91-3222-83322 Fax: +91-3222-755303