(Mg_1-xZn_x)Al₂O₄ Low Dielectric Constant Microwave Dielectric Ceramics

Accompanied with the rapid development of wireless communication, microwave communication has gradually become the most important part of the modern communication technology. Moreover, the fast increase of information carried by microwave technology is forcing the working frequency toward higher band, and subsequently attract the increasing attention on the materials with high Q and low dielectric constant(ε_r＜10). However, materials with low ε_r usually have a large negative resonant frequency temperature coefficient Ï„f, which confines their wide application, so more research is necessary for a breakthrough in this field. In this thesis, the dielectric loss and structure of (Mg_1-xZn_x)Al₂O₄ ceramics were researched, and their infrared spectra were also analyzed. The relationship of the ceramics' dielectric property and their compositions, microstructures and etc were explored, and the theoretic losses of the ceramics were calculated, in order to infer the influence of the extrinsic factors.Firstly, MgAl₂O₄ microwave dielectric ceramics were modified by Zn-substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg_1-xZn_x)Al₂O₄ ceramics were obtained by sintering at 1550 ℃-1650 ℃ in air for 3 h, and the (Mg_1-xZn_x)Al₂O₄ solid solution was determined in the entire composition range. With Zn-substitution for Mg, the dielectric constant sof MgAl₂O₄ just varied from 7.90 to 8.56, while the Qf value was significantly improved up toward the maximal value of 106,000 GHz at x=1.0. Moreover, the Ï„f of MgAl₂O₄ ceramics was suppressed from -73ppm/℃ to -63ppm/℃. As for (Mg_1-xZn_x)Al₂O₄ ceramics, the high sintering temperature and large negative Ï„f are still the main problems to be solved.Moreover, infrared reflection spectra of (Mg_1-xZn_x)Al₂O₄ ceramics were analyzed by Kramers-Kroning analysis and classical oscillator model simulation. The dielectric properties were extrapolated down to the microwave range using the classical oscillator model for fitting the dielectric function. According to structure analysis, the losses originating from bend vibration and stretch vibration of the bond between A-site cation and oxygen anion dominated the whole dielectric losses of the spinel ceramics. Coexistence of Mg and Zn deteriorated the intrinsic dielectric properties due to the bond asymmetry thus introduced. The calculated Qf (～10⁵GHz) was much higher than the measured ones (～10⁴GHz), suggesting that the extrinsic loss was significant and concealed the excellent dielectric property. Therefore, the microwave dielectric properties of MgAl₂O₄ and ZnAl₂O₄ could be improved much by microstructure modification, and the little superiority in their solution compared with the end-members was due to microstructure improvement.