Nanocrystalline BaTiO 3 from freeze-dried nitrate solutions
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Nanocrystalline BaTiO3 from freeze-dried nitrate solutions J. M. McHale,a) P. C. McIntyre, K. E. Sickafus, and N. V. Coppa Los Alamos National Laboratory, Los Alamos, New Mexico 85745 (Received 12 April 1995; accepted 16 January 1996)
An aqueous, all nitrate, solution-based preparation of BaTiO3 is reported here. Rapid freezing of a barium and titanyl nitrate solution, followed by low temperature sublimitation of the solvent, yielded a freeze-dried nitrate precursor which was thermally processed to produce BaTiO3 . XRD revealed that 10 min at temperatures >600 ±C resulted in the formation of phase pure nanocrystalline BaTiO3 . TEM revealed that the material was uniform and nanocrystalline (10 –15 nm). The high surface to volume ratio inherent in these small particles stabilized the cubic phase of BaTiO3 at room temperature. It was also found that the average particle size of the BaTiO3 produced was highly dependent upon calcination temperature and only slightly dependent upon annealing time. This result suggests a means of selection of particle size of the product through judicious choice of calcination temperature. The experimental details of the freeze-dried precursor preparation, thermal processing of the precursor, product formation, and product morphology are discussed.
I. INTRODUCTION
The wide range of electronic applications utilizing the high dielectric constant and ferroelectricity exhibited by BaTiO3 , in conjunction with the difficulty in preparing morphologically homogeneous samples of the material by conventional ceramic processing, have resulted in the development of several solution-based methods for BaTiO3 preparation. Solution-based routes to barium titanate include coprecipitation of oxalates,1,2 the Pechini process,3 sol gel methods,4–6 and the hydrothermal method.7,8 These processes have the common goal of achieving product formation under mild reaction conditions (low temperatures and short reaction times) in order to limit the extent of grain growth and control particle size. Small and uniform particle size BaTiO3 powders allow for thinner layers of the ceramic to be used in multilayer capacitors without loss of dielectric properties. In addition, small and uniform particle morphology offers the advantage of lower sintering temperatures for such devices, which may allow for the use of less expensive electrode materials.9 The permittivity and Curie temperature of BaTiO3 have also been shown to be dependent upon sample morphology.10,11 However, control of grain size is virtually impossible using conventional ceramic processing due to the high temperatures needed to facilitate ionic diffusion and reaction in the solid state. The low temperature synthesis of BaTiO3 has been demonstrated using alkoxide precursors.7,8,12
a)
Present address: Princeton University, Department of Geological and Geophysical Sciences, Princeton, New Jersey 08544. J. Mater. Res., Vol. 11, No. 5, May 1996
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