Reaction Sintering Process for Improving the Microwave Dielectric Properties of Ba2Ti9O20 Materials

Abstract

The efiect of processing parameters on the characteristics of the reaction-sintered Ba2Ti9O20 materials was investigated. The characteristics of the A-series samples, which were prepared from 2BaTiO3+7TiO2 mixture, vary markedly with the pre-reacting process, whereas those of the B-series samples, which were prepared from BaTi4O9+BaTi5O11 mixture, are in sen- sitive to this process. The possible explanation is that the B-series mixture forms the Ba2Ti9O20 Hollandite-like phase in one step, which is much simpler than the direct sintering of A-series mixture. The best microwave dielectric properties obtained are K = 38:2 and Q£f = 36;000 for B-series Ba2Ti9O20 materials, which were pre-reacted at 1000 - C for 6h and sintered at 1410 - C for 4h by \one-step densiflcation processes. DOI: 10.2529/PIERS060903001620 Ba2Ti9O20 phase was flrst reported by Jonker and Kwestroo (1) in BaO-TiO2-SnO2 ternary system and was observed to possess marvelous microwave dielectric properties, including high dielectric constant and large quality factor, by O'Bryan etal. (2). However, the reported results are quite controversial, which is mainly due to the di-culty in forming single phase Hollandite-like structured Ba2Ti9O20 material. Secondary phases, such as BaTi4O9 or BaTi5O11, are observed to form preferentially in the calcinations of BaCO3-TiO2 mixture (3{6) that hinder the formation of Hollandite-like phase for the Ba2Ti9O20 phase. In this paper, nano-sized starting powders were used to enhance the reaction kinetics and the reaction-sintering technique was adopted to simplify the densiflcation process. How such a non-conventional synthesizing process in∞uences the characteristics of the Ba2Ti9O20 materials will be described and the possible mechanism will be discussed. The Ba2Ti9O20 materials were synthesized via the conventional mixed oxide process, using nano-sized BaTiO3 (» 50nm) and anatase TiO2 (» 50nm) as starting materials. Two types of mixture were used for preparing the Ba2Ti9O20 samples. In A-series materials, 2BaTiO3 and 7TiO2 powders were mixed thoroughly, using ball milling technique. The B-series materials used the BaTi4O9 and BaTi5O11 mixture as starting materials, which were pulverized using 3-dimensional milling (3DM, Model 2C, Willy A Bachofen AG, Switzerland) technique, where the BaTi4O9 and BaTi5O11 powders were flrst prepared by calcining the BaTiO3+3TiO2 and BaTiO3+4TiO2 mix- ture at 1000 - C/4h. The 2BaTiO3+7TiO2 (A-series) or BaTi4O9+BaTi5O11 (B-series) powder mixtures were pelletized and then sintered directly, that is, the temperature was increased slowly (3 - C/min) to 1000 - C, held for 0{12h, which is designated as pre-reaction process. The temperature was then directly increased again (3 - C/min) to 1300 » 1410 - C, soaked for 4h and then cooled slowly (2 - C/min), viz. the samples were directly sintered without experiencing the calcinations process. The microstructure of the sintered samples was examined using scanning electron microscopy (Jeol 6700F). The crystal structure of the samples was examined using x-ray difiractometry (Rigaku D/max-II). The density of the sintered materials was measured using Archimedes method. The microwave dielectric constant (K) and quality factor (Q£f) of the Ba2Ti9O20 samples were mea- sured using a cavity method at 7{8GHz (7). Figures 1(a) and 1(b) show the variation of microwave dielectric constant (K) and quality factor (Q£f), respectively, for the A-series materials, indicating that these characteristics of the samples vary markedly with the processing parameters. Generally, the microwave dielectric constand (K) increases monotonically with sintering temperature and reached a K-value higher than 38 for those sintered at a temperature higher than 1300 - C/4h, provided that the samples were pre-reacted at 1000 - C for su-cient long (tr > 6h) during the heating process. The K-value of the samples pre- reacted at 1000 - C for too short period (tr < 3h) is appreciably smaller (K < 36). The microwave qualify factor (Q £ f) of the samples vary with the pre-reaction process even more markedly. The best properties achieved are dielectric constant K = 38 and qualify factor Q £ f = 29;200GHz,