| Summary: | 博士 === 國立成功大學 === 電機工程學系 === 102 === With the rapid progress of microwave wireless communication such as mobile telephone systems, global positioning systems (GPS) and wireless local area networks (WLAN), the demand for high-performance dielectric ceramics has been ever-increasing. Because the working frequency of communication systems will be extended to a higher frequency regime in the future, the research of microwave dielectric ceramics has focused on the development of extremely low-loss (high quality factor, Q) dielectrics. Moreover, the reduction of sintering temperatures has also become an important issue in the fabrication of miniaturized multi-layer devices. As mentioned above, this dissertation is divided three parts which includes the study of high quality factor and low sintering temperature microwave dielectrics, and their applications on microstrip bandpass filters at the microwave frequency.
1. Investigation of High-Q Mg4Nb2O9 Ceramics
[a] The corundum-structured Mg4Nb2O9 has an excellent microwave dielectric properties, but it also required a sintering temperature of as high as 1350–1400°C /10 h. In this dissertation, Fe2O3 was chosen as a sintering aid and was added to Mg4Nb2O9 ceramics to lower its sintering temperature. A sintering temperature reduction (~130°C) and an improvement of Q×f value could be achieved by adding Fe2O3 to the Mg4Nb2O9 ceramics.
[b] The (Mg1–xZnx)4Nb2O9 ceramic with a smaller increment in x were prepared by solid-state method and its microwave dielectric properties and microstructures were also investigated in this topic. The forming of (Mg1–xZnx)4Nb2O9 (x = 0.02–0.08) solid solution was confirmed by the measured lattice parameters and the EDS analysis. The (Mg0.95Zn0.05)4Nb2O9 ceramics possesses a good combination of dielectric properties with εr ~ 13, Q×f ~ 247,000 GHz at 11.9 GHz, and τf ~ –67 ppm/°C.
[c] In order to produce a temperature-stable material, SrTiO3 was added to (Mg0.95Zn0.05)4Nb2O9 ceramic. Doping with 1 wt% Fe2O3 effectively achieve dense sintered ceramics at low sintering temperatures. A two-phase system was confirmed by the XRD patterns, BEI and EDS analysis. By appropriately adjusting the x value in the (1–x)(Mg0.95Zn0.05)4Nb2O9–xSrTiO3 ceramic with 1w% Fe2O3 additive, a near zero f value can be achieved. A dielectric constant εr of 20.81, a Q×f value of 108,000 GHz at 9.63 GHz and a τf value of –4.3 ppm/°C were obtained for 0.6(Mg0.95Zn0.05)4Nb2O9–0.4SrTiO3 ceramics with 1w% Fe2O3 additive sintered at 1150°C for 4 h, which makes it a very promising candidate material for applications in microwave planar filters.
2. Study of Low-temperature Sintered Zn3Nb2O8 Ceramics
Recently, monoclinic-structured Zn3Nb2O8 ceramics have attracted more attention because of its relatively low dielectric loss and low sintering temperature (1150°C). The effects of M (M = Mg, Co and Ni) substitution for Zn on the phase relation, crystal structure and microwave dielectric properties of Zn3Nb2O8 ceramics were investigated in this dissertation. Results indicate that the partial substitution boosted the Q×f to an even higher value. In order to reduce the sintering temperature for application in LTCC devices, small amounts of B2O3 and CuO were added to the ceramics. Furthermore, the microwave dielectric properties, the presence of the second phase, and the chemical compatibility with the silver electrodes were also investigated.
3. Design and Fabrication of Planar Filters
A miniaturized bandpass filters using stepped impedance ring resonators with zero-degree feed structure are presented in this section. In addition, the bandpass filter using high-permittivity ceramic substrate provide good miniaturization ability. The selectivity of passband can be improved by utilizing the zero-degree feed structure. The miniaturized bandpass filters are designed at a center frequency of 2.4 GHz with bandwidth of about 9%. The frequency responses of the filters using FR4, Al2O3 and 1wt% Fe2O3 doped 0.6(Mg0.95Zn0.05)4Nb2O9–0.4SrTiO3 ceramic substrates are investigated.
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