Microstructure And Properties Of High-Q Low-ε Microwave Dielectric Ceramics

The development of high- Q microwave dielectric ceramics with various ε has been required with the advanced wireless communication systems, which include mobile and satellite communication systems, have developed. Thus, a number of these materials have been investigated and developed. Among which, α-Al₂O₃ ceramics with corundum structure show a very high Q-value (Q·f=300000 GHz), an appropriate dielectric constant (ε about 10) and widely used. However, the high sintering temperature and the temperature coefficient of resonant frequency (Ï„_f) limited its farther development as low-temperature sintered substrate ceramics. On the other hand, the Mg₄Nb₂O₉ compound which has the ordered corundum structure shows optimum microwave dielectric properties which are comparable to that of α - A1₂O₃ and can be sintered at a relatively low temperature. Thus, it can be expected to be developed as a new substrate ceramics.In this study, the synthesis of Mg4Nb₂O₉, by solid state reaction and its serials ceramics were performed and microstructure and microwave dielectric properties were investigated in detail. The principal experimental results can be concluded as following:(1) We didn't gain pure Mg4Nb₂O₉ compound when synthesized between 600℃¹500℃ for 5h, using MgO and Nb2O5 as raw materials. MgO, Nb2O5 and MgNb2O6 three phases coexisted as the synthesis temperature was from 600℃ to 700℃; whereas MgO, Nb2O5,MgNb2O6 and Mg4Nb₂O₉ phases coexisted when the synthesis temperature was between 700℃-950℃; Nb2O5 phase disappeared from the compounds as the synthesis temperature higher than 950℃; Mg₅Nb₄O₁5appeared at 1250℃ and MgNb₂O6 disappeared at 1300℃; The optimum synthetic temperature for Mg₄Nb₂O₉ compound is 1400℃ and near pure Mg4Nb₂O₉ phase gained, minor MgO and Mg5Nb4O₁5 coexisted.(2) The sintered Mg4Nb₂O₉ ceramics had been made of Mg4Nb₂O₉ andMg5Nb₄O₁5 phases. The synthesis condition for Mg₄Nb₂O₉ powder had no evident effect to the phases of ceramics. The MgO and MgNb2O6 phases, which existed in the prepared powder, had transformed to Mg₄Nb₂O₉ and Mg5Nb₄O₁5, and these reaction baffled the grain growth. Typically, Mg4Nb2O9 prepared at 900℃ for 2h and sintered at 1350℃ for 10h show optimum microwave dielectric properties: e = 12.00, Qf =194 820 GHz (at 8.400 GHz).(3) Mg4(Nb2_xSbx)O9 (0≤X≤2) ceramics were prepared by conventional solid reaction method. The compounds were continuous solid solutions as composition x ranged between 0-1.6 and had the ordered corundum structure of Mg4Nb20, and the lattice parameters, a, c and V, decreased as x increasing, no secondary phase was detected, whereas only Mg4Sb209 existed , which structure changed, at x - 2.0. At the composition x ranging from 1.7 to 1.9, two phase, i.e. MgjNbjO, and MgjSb4O21, coexisted. The Sb substitutions for Nb is effective to lower the sintering temperature of Mg4Nb2O9from 1400"C to 1300*C without degenerated the Qf values when composition x ranged from 0.4 to 0.8. However, the density, p, and Q ? / value of Mg4(Nb2_xSbx)O9 both deteriorated as sintering temperature increasing and dwell time prolonging because of abnormal grain growth. It's difficult to densify Mg4(Nb2_xSbx)O9 ceramics because of the microstructure packed by plat-like grain and the presence of many pores between grains when composition x % 1.2. Typically, Mg4(Nb16Sb04)O9 and Mg4(Nbj 2Sba8)O9 ceramics sintered at 1300*0 for 5h show optimum properties: *r = 12.26, Qf = 168 450 GHz (at 8.714 GHz) ande,* 11.62, Q ?/ = 169 820 GHz (at 9.275 GHz), respectively.(4) Two compounds, Mg4Nb209 and Mg5Nb4Ou , coexisted in the Mg4Nb209 ceramics when added 0 s; x s 5.0wt.% TiO2 , and no pure TiO2 detected. The Ti4+ replaced Mg2+ and Nb5+ in Mg5Nb40i3 phase and had not effected Mg4Nb209 phase. The addition of TiO2 can densify the ceramics at relatively low sintering temperature and increased the microwave properties of ceramics. The Mg4Nb209 ceramics with 2.5wt.% TiO2 addition sintered at 1300*C for 5 h had good microwave dielectric properties: s, =13.61, Q ? f =196 620 GHz? rf =-50.4ppm/°C.(5) The effect of V2OS additions on the sintering temperature and dielectric properties of Mg4Nb209has been investigated. It was found that the addition of 1% and 4% V2O5 to Mg4Nb209 lowered the sintering temperature to 1050'Cand...