Improving the Characteristics of Ba2Ti9O20 Materials by Using Pre-reacted Ba-Ti-O Compounds

I-Nan Lin, Chi-Ben Chang, Keh-Chyang Leou
2007 PIERS Online  
The effect of starting materials Ba-Ti-O compound mixture, 2BaTiO 3 +7TiO 2 (type A) or BaTi 4 O 9 +BaTi 5 O 11 (type B), on the characteristics of the Ba 2 Ti 9 O 2 0 materials was investigated. Both materials can be sintered to the same high density (>96% T. D.) with the same large dielectric constant (K = 35 ∼ 38). While the microwave dielectric constant (K) of the materials are not sensitive to detailed microstructure of the samples, the Q × f -value of the materials correlates with the
more » ... elates with the microstructure of the samples closely and therefore varies appreciably with the processing details. Comparing with the "two-step densification processes", the "one-step densification processes" improved pronouncedly the characteristics of the type B materials, but degraded markedly those of the type A materials. The possible explanation for such a phenomenon is that direct sintering of type A mixture requires complicated reaction steps to form the Hollandite-like phase and thus leads to non-uniformed microstructure, which results in inferior microwave properties. Ba 2 Ti 9 O 20 phase was first reported by Jonker and Kwestroo in BaO-TiO 2 -SnO 2 ternary system [1] and was observed to possess marvelous microwave dielectric properties, including high dielectric constant and large quality factor, by O'Bryan et al. [2] . Although increasing the calcinations temperature can result in pure Hollandite-like Ba 2 Ti 9 O 20 phased powders and circumvent the deleterious effect of the preferentially formed BaTi 4 O 9 or BaTi 5 O 11 phases on the microstructure uniformigy of the Hollandite-like structured Ba 2 Ti 9 O 20 materials. Low activity and anisotropic growth of the Ba 2 Ti 9 O 20 particulates along c-axis will hinder the densification of the samples. Therefore, in conventional mixed oxide process for preparing the Ba 2 Ti 9 O 20 materials [3-7], the powders were always calcined at a temperature lower than that is needed to completely transform the powders into Hollandite-like Ba 2 Ti 9 O 20 phase, resulting in multiple phased powders. Therefore, the occurrence of reactions among the phases during the sintering process is inevitable and the processing reliability in the preparation of Ba 2 Ti 9 O 20 materials is thus not satisfactory. In this paper, the reactions among the constituents in the sintering process was carefully controlled by using pre-reacted Ba-Ti-O compounds, viz. either 2BaTiO 3 + 7TiO 2 or BaTi 4 O 9 + BaTi 5 O 11 mixtures. The characteristics of the Ba2Ti9O20 materials thus obtained will be described and the possible mechanism will be discussed. The Ba 2 Ti 9 O 20 materials were synthesized via the conventional mixed oxide process, using nanosized BaTiO 3 (∼50 nm) and anatase TiO 2 (∼50 nm) as starting materials. Two types of mixture were used for preparing the Ba 2 Ti 9 O 20 samples. In type A materials, 2BaTiO 3 and 7TiO 2 powders were mixed thoroughly, whereas, in type B materials, the BaTi 4 O 9 and BaTi 5 O 11 powders were first prepared by calcining the BaTiO 3 +3TiO 2 and BaTiO 3 +4TiO 2 mixture at 1000 • C/4 h, pulverized, and then thoroughly mixed. Two densification processes were adopted for synthesizing the Ba 2 Ti 9 O 20 materials. In the "two-step densification" process, both of the type A and type B of mixtures were first calcined at 1000 • C for 4 h (in air), followed by a 3-dimensional milling process to disintegrate the agglomerates. These powders were then pressed into pellets and were sintered at 1250∼1400 • C for 4 h (in air). In the "one-step densification" process, the 2BaTiO 3 + 7TiO 2 (type A) and BaTi 4 O 9 + BaTi 5 O 11 (type B) powder mixtures were pulverized, pelletized and then sintered directly. The temperature was increased to 1000 • C, held for 6 h and then was directly increased to 1300∼1410 • C, soaked for 4 h. The microstructure of the sintered samples was examined using scanning electron microscopy (Jeol 6700F). The crystal structure of the calcined powders and sintered samples was examined using x-ray diffractometry (Rigaku D/max-II). The density of the sintered materials was measured using Archimedes method. The microwave dielectric
doi:10.2529/piers060903001851 fatcat:qexne46akrewfezyk3upxpf3um