Additive controlled crystallization of barium titanate powders and their application for thin-film ceramic production: P
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G. Miehe Technische Universita¨t Darmstadt, Fachbereich Materialwissenschaft, Petersenstraße 23, D-64287 Darmstadt, FRG
G. Wegnera) Max Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, FRG (Received 11 February 2000; accepted 5 April 2001)
Stoichiometric barium titanate (BaTiO3) was synthesized in aqueous solution with acetone and/or methanol as additives to control the crystallization process. Adjusted nano-sized particles and narrow particle size distributions were achieved at 60 °C with additive concentrations up to 243 ml/l. The growth kinetics showed that the additives influence the nucleation of the BaTiO3 particles and tend to suppress Ostwald ripening.
I. INTRODUCTION
Barium titanate (BaTiO3) and BaTiO3-based ferroelectric materials of high dielectric constant are the main constituents of passive electroceramic components such as multilayer capacitors and nonlinear resistors. The ceramics are formed by compacting powder particles and sintering the green compacts.1 Continuous demands for miniaturization and enhanced quality have led to greater sophistication in the processing of these materials, both at the stage of powder synthesis and subsequent densification to solid components or thin-layered dielectrics.2–5 Enhancement of properties can be achieved by starting with stoichiometric BaTiO3 powders consisting of nearly spherical, well-crystallized, submicrometer (1 m.1,5 With the aim to overcome these deficiencies different solution-based routes to barium titanate powders have been developed.6 One of the best processes to obtain BaTiO3 powders of precise stoichiometry is the oxalate process. The starting materials, TiCl4 and BaCl2, are reacted with water and oxalic acid (H2C2O4) to precipitate a double oxalate [BaTiO(C2O4)2 ⭈ 4H2O] at 20–60 °C, which is then decomposed and calcined at 960–1200 °C to BaTiO3.3,7 Similar methods are the Pechini process8 (also known as the citrate process9–11) and a route reported by Ali and Milne12 that is based on the calcination of a freeze-dried catechol complex Ba[Ti(C6H4O2)3] ⭈ 4H2O. As in the case of the conventional method, powders with a broad particle size distribution are obtained, although the mean particle diameter can be reduced to 0.5 m.1 Fine powders of aggregated, submicron BaTiO3 particles have been reported to result when a hydrothermal synthesis route was employed. 13,14 Kutty and coworkers15 –17 demonstrated the hydrothermal synthesis of ultrafine BaTiO3 powders at temperatures as low as 85 °C and pressures of 1500 to 6500 kPa using titania gels (TiO2 ⭈ xH2O) and Ba(OH)2. Unfortunately, BaCO3 contamination will occur unless the reaction is performed in a CO2-free atmosphere. In addition, the stoichiometry is affected by the type and amount of solvent and is © 2001 Materials Research Society
1901 IP address: 134.117.10.200
B. Grohe et al.: Additive controlled crystallization of barium titanate powders and their application for thin-film ceramic production: Part I.
difficult to control.3 The presence of aggregates due to small p
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