Abstract
Abstracts This work is to study the effect of processing parameters on the characteristics of Ba2Ti9O20 microwave dielectric materials and to use these materials for fabricating two-dimensional Ba2Ti9O20 electromagnetic bandgap structured（EBG） slabs. The Ba2Ti9O20 materials were synthesized via different milling techniques and sintering processes, using 2BaTiO3+7TiO2 （type A） or （BaTi4O9+BaTi5O11）（type B）mixtures as starting materials. The effect of process parameters on the crystal structure and the related microwave dielectric properties of Ba2Ti9O20 materials was systematically investigated. Moreover, the two-dimensional Ba2Ti9O20 EBG slabs with square or triangular lattices were designed using HFSS simulation package and were than fabricated. The effect of designing parameters, such as hole-size & lattice parameters, on bandgap of the two-dimensional Ba2Ti9O20 EBG slabs was studied. The bandgap of the fabricated EBG slab is in agree with the HFSS simulation results. Si3N4-HeM process can efficiently disintegrate the Ba2Ti9O20 powders, enhancing the reactivity and sinterability of the materials. However, this process will induce SiO2-contamination and therefore, pronouncedly degrade the Qxf-value of the materials. The SiO2-contamination was effectively inhibited when the ZrO2-(0.25h)-HeM process was used to replace for the Si3N4-(0.25h)-HeM one. The characteristics of the Ba2Ti9O20 materials were markedly improved, as compared with the ZrO2-HeM or BM processes. Utilization of the 3DM process in place of the HeM process can produce the powders of the same high activity but will not induce SiO2-contamination and other side effects and, therefore, markedly improve the microwave dielectric behavior for the Ba2Ti9O20 materials. The 3DM materials possess the same value of tne dielectric constant but have markedly better quality factor (more than 10% higher), as compared with the BM samples.] Addition of SnO2 decreases the length-to-diameter aspect ratio of the grains. The bridging effect due to anisotropic growth of the high-aspect-ratio grains is thus pronoucedly suppressed such that the densification process is facilitated. All the SnO2-doped materials possessed very high density (>98%T. D.), when they were sintered at 1350℃/4h. On the other hand, the lower polarizability of Sn+4-ions in the SnO6-octahedrons, as compared with the Ti4+-ions in the TiO6-octahedrons, will lowers the K-value the Ba2Ti9O20 materials. Moreover, the presence of Sn4+-ions in the TiO6-octahedrons will degrade the coherency of the intrinsic lattice vibration modes, which will impose detrimental effect on the quality factor for the Ba2Ti9O20 materials. Such a phenomenon accounted clearly the unusauall effect of SnO2-addition on modifying the Qxf-value of the materials. While the addition of small amount of SnO2-species facilitates the densification process by suppressing the anisotropic growth of the grains, incorporation of larger amount of SnO2-doping than necessary will not further improve the granular structure for the Ba2(Ti9-xSnx)O20 materials, but will degrade the coherency in lattice vibrational modes for the materials. Therefore, the Qxf-value of the Ba2(Ti9-xSnx)O20 materials increased firstly for lightly SnO2-doped samples and then it decreased with the proportion of SnO2-species when the SnO2-content is abundant. The Ba2Ti9O20 materials prepared from the （BaTi4O9+BaTi5O11）（type B）mixtures possess superior microstructres and Qxf-value to the materials prepared from 2BaTiO3+7TiO2 （type A） mixtures. The microwave dielectric constant (K) is insensitive to processing details, as the K-value of the materials is only closely related with density of the samples and is insensitive to the detailed microstructure of the samples. In contrast, the Qxf-value of the samples is very sensitive to the microstructure of the materials. Materials containing large grains of short-rod-geometry with small-aspect-ratio possess superior Qxf-value to the those which contain small-grains of long-rods-geometry with large- aspect-ratio. Among the 4 materials processed, type A & type B powder mixtures and 1-step & 2-step densification processes, the materials prepared from type B mixture and processed by “1-step densification” technique show most uniform microstructure and exhibit the highest Qxf-value （(Qxf) =34,000 GHz）. It is ascribed to the simplicity in reaction routes for the formation of the Ba2Ti9O20 Hollandite-like phase from BaTi4O9 + BaTi5O11 mixture. The characteristics of the A-series samples, which were prepared from 2BaTiO3+7TiO2 mixture, vary markedly with the pre-reacting process in the “1-step & 2-step densification routes “, whereas those of the B-series samples, which were prepared from BaTi4O9+ BaTi5O11 mixture, are insensitive to such a process. The B-series materials, which were prepared from B-series mixture and pre-reacted at 1000℃ for 6 h or longer, show most uniform microstructure and exhibit the highest Qxf-value. It is ascribed to the simplicity in reaction routes for the formation of the Ba2Ti9O20 Hollandite-like phase from BaTi4O9 + BaTi5O11 mixture, which results in better granular structure for the materials. We then used the Ba2Ti9O20 materials for fabricating the 2-dimensional (2D) electromagnetic band gap crystals (EBG). The square or triangular lattice two-dimensional Ba2Ti9O20 EBG slab showed the measured bandgap, in the range of 30~40 GHz, was close to the bandgap calculated using HFSS. For square or triangular-type 2D EBG structures with fixed lattice constant and slab thickness, the effective dielectric constant of the lattice decreases with increasing diameter of air hole such that the mid-frequencies in the bandgap shifted toward a higher frequency. Moreover, increasing the filling factor will result in the wider bandgap. For square -type 2D EBG structures with fixed diameter of air hole and slab thickness, the effective dielectric constant of the lattice decreases with increasing lattice constant, but the -value increases such that the mid-frequencies in the bandgap move toward a lower frequency. Furthermore, decreasing the filling factor will result in narrower bandgap and less attenuation in the bandgap.