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Jinn P. Chu Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan, Republic of China (Received 9 April 2002; accepted 28 October 2002)

Preparation of dense and phase-pure Ba2Ti9O20 is generally difficult to achieve using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition. This study investigated the effects of B2O3 on the densification, microstructural evolution, and phase stability of Ba2Ti9O20. Samples from the host material (2BaO ⭈ 9TiO2) with and without the addition of 5 wt% B2O3 were prepared through different processing routes. For the pure host material sintered at temperatures ranging from 800 to 1100 °C, the reaction products followed the sequence of BaTi2O5 → BaTi4O9 → BaTi5O11 → Ba2Ti9O20. The phase transformation proceeded faster in the bulk compared to the free surface of the sample. BaTi5O11 and BaTi4O9 with a minor amount of Ba2Ti9O20 were found in the ground powder of ceramics sintered at 1100 °C. For the samples prepared from host material with the addition of 5 wt% B2O3, Ba2Ti9O20 started to form at temperatures as low as 800 °C. The sequence of reaction products followed Ba4Ti13O30 → BaTi4O9 → BaTi5O11 → Ba2Ti9O20. Sintering at above 1000 °C yielded pure Ba2Ti9O20 phase, suggesting the effective role of B2O3 on the phase stability of Ba2Ti9O20. It was found that precalcination of host material before the addition of B2O3 gives an additional benefit to the Ba2Ti9O20 formation. Crystallization of pure Ba2Ti9O20 phase was completed at a sintering temperature as low as 900 °C without any solid solution additive such as SnO2 or ZrO2, due to the fact that the phase transformation of the samples began with BaTi4O9 and BaTi5O11 during sintering. Also, B2O3 was found to be unstable during the high-temperature sintering at 1200 °C, and the results are discussed.

I. INTRODUCTION

Ba2Ti9O20, which contains 81.8 mol% TiO 2 and 18.2 mol% BaO, has received much attention owing to its good microwave properties.1,2 It has a good quality factor (8000 at a frequency of 4 GHz), a high dielectric constant (39.8), and a low-temperature coefficient of capacitance (␶f ⳱ 2 ppm/°C).3,4 For the preparation of Ba2Ti9O20 from BaCO3 and TiO2 by solid-state reaction, the stoichiometry and fabrication parameters must be precisely controlled. This is because various thermodynamically stable compounds exist in the vicinity of the desired composition of TiO2-rich BaO–TiO2 system: Ba6Ti17O40, BaTi3O7, BaTi4O9, BaTi5O11, and Ba4Ti13O30.5 a)

Address all correspondence to this author. Present address: Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei, Taiwan, Republic of China. J. Mater. Res., Vol. 18, No. 1, Jan 2003

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Furthermore, studies revealed the existence of a curvature of Ba2Ti9O20 equilibrium phase boundary in the BaO–TiO2 system at high temperatures, which causes a further co