Fabrication of undoped near-monophase Ba 2 Ti 9 O 20 via rapid thermal processing
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Fabrication of undoped near-monophase Ba2 Ti9 O20 via rapid thermal processing Wen-yi Lin and Robert F. Speyer School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Received 19 November 1996; accepted 22 January 1999)
Rapid thermal processing of BaTiO3 and TiO2 pressed powders at 500 ±Cymin to 1250 ±C for 2 h in an infrared furnace resulted in a mixture of Ba2 Ti9 O20 , BaTi4 O9 , and TiO2 . Further heat treatment at 1390 ±C led to 96 vol% phase-pure Ba2 Ti9 O20 from an initial mixture devoid of any dopant. Heat treatment at rates decreasing to 5 ±Cymin facilitated agglomeration of TiO2 . This, in turn, increased the diffusion distance required for reaction of BaTi4 O9 and TiO2 to form Ba2 Ti9 O20 .
I. INTRODUCTION
The shorter wavelengths associated with microwave frequencies facilitate wireless communication with a more focused signal from the emitter. Microwave communication is thus commonplace for satellite and cellular systems. Efforts to miniaturize microwave circuitry have stimulated the development of smaller, highly stable filters and compatible oscillators. Dielectric resonators with large values of permittivity offer a means for miniaturizing these functions. The temperature coefficient must be near zero; that is, the resonance frequency should not drift with temperature. This coefficient is dependent on the compensating effects of thermal lattice dilation and changes in permittivity with temperature. The quality factor can be approximated as the inverse of the dielectric loss factor stan dd; thus an increasing quality factor is an indicator of decreasing energy loss through third harmonic distortions. The high quality factor of contemporary dielectric resonators minimizes drain on battery reserves. Two important materials for dielectric resonators are BaTi4 O9 and Ba2 Ti9 O20 , since they both have high dielectric constants and quality factors (38 and .10, 000, respectively, for BaTi4 O9 , and 39.8 and 10,000 for Ba2 Ti9 O20 , respectively, at 4 GHz).1 However, Ba2 Ti9 O20 has a reported temperature coefficient of resonance frequency of 2 ppmy±C which is closer to zero than BaTi4 O9 s12 ppmy±Cd.1 This makes Ba2 Ti9 O20 a more favorable choice. However, the difficulty in fabricating undoped monophase Ba2 Ti9 O20 (e.g., without residual TiO2 and BaTi4 O9 ) has daunted its development for years.2 The most recent BaTiO3 –TiO2 phase diagram3 shows Ba2 Ti9 O20 decomposing into BaTi4 O9 and TiO2 at 1393 ±C, and BaTi4 O9 incongruently melting at 1446 ±C. There is disagreement about the ability to form phase-pure Ba2 Ti9 O20 .4–12 In our previous attempts J. Mater. Res., Vol. 14, No. 5, May 1999
(using BaCO3 and TiO2 ), formation of Ba2 Ti9 O20 through conventional sintering at 10 ±Cymin to 1360y1390 ±C, holding for 5 h, was not possible (average particle sizes were 1 mm for BaCO3 and 0.6 mm for TiO2 ).13 Yoon et al. found that a mixture of Ba2 Ti9 O20 and BaTi4 O9 formed after sintering at 1400 ±C for 6 h.11 O’B
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