A New ZnO-SiO₂ Low Dielectric Constant High Frequency Microwave Dielectric Ceramics

With the rapid development of communication businesses, the demands for miniature and functional microwave components used in mobile communication devices have been greatly increased. At the same time, the research and utilization of microwave dielectric ceramics have also achieved remarkable progresses. In order to meet with the requirement for the miniaturization of mobile communication terminal devices, Low Temperature Co-fired Ceramics (LTCC for short) technology and multilayer microwave components have been the research focus. In the middle & High dielectric constant ceramic field, the research on additives and LTCC materials have been developed rapidly and furthermore, have been put into application. Simultaneously, with the communication devices of mobile communication, wireless LAN and military communication developing to higher frequencies, a large market demand emerged for low dielectric constant high frequency microwave ceramics.Recent years, low dielectric constant microwave ceramics have received much interests. New kinds of low dielectric constant microwave ceramics were developed continuously. However, these materials have some problems and disadvantages, such as (1) difficult to sintered at lower temperature; (2) large Ï„_f or low Q×f values; (3)the research on cofiring with Ag or Cu was limited. Therefore, it is of much importance to develop new kind of low dielectric constant microwave ceramics used in LTCC technology.The research by author on ZnO-SiO₂ ceramic system shows: ZnO-SiO₂ ceramic have low dielectric constant of 6 and excellent dielectric properties with lower expenses. But this ceramic system have some disadvantages such as high sintering temperature, narrow sintering range and large negative Ï„_f. In this study, the sintering characteristics and dielectric properties of ZnO-SiO2 ceramics were improved by adding additives. The sintering temperature was decreased to 900°C with composite sintering aids. The low-fired ceramics can prepare stable slurry and have good compatibility with Ag electrode. Thus, it can be a promising low dielectric constant microwave ceramics working at higher frequency. Based on these results, nano ZnO-SiO₂ ceramic powders with excellent dispersity and properties were obtained by sol-gel method. This study established foundation to prepare miniature multilayer components. The main research results are as follows:1. The sintering characteristics and microwave dielectric properties of ZnO-SiO₂ ceramics were investigated systematically at the first time. The sintering characteristics and microwave dielectric properties of ZnO-SiO₂ ceramics were improved effectively by MgO and TiO₂ addition. A low dielectric constant microwave ceramics with wide sintering rangeand excellent dielectric properties especially near zero Ï„_f- was obtained. This results provide a potential material for microwave electronic components working at higher frequency. (1) Itis difficult for ZnO-ySiO₂ to obtain a dense structure at high temperature. When the sintering temperature was higher than 1440℃, the ceramic began to melt. Excess SiO₂ content can reduce the grain size and inhibit abnormal grain growth. The ceramics with y=0.6 obtained best dielectric properties of ε_r = 6.23, Q×f= 52500GHz, Ï„_f= -55.2ppm/℃. (2) Al₂O₃ can improve the sintering process by improving the formation of main phase. The sintering temperature was decreased to 1300℃ and the ZnO-0.5SiO₂ ceramic with 1wt% Al₂O₃ addition obtained properties of ε_r=6.43, Q×f =43500GHz , Ï„_f = -48.4ppm/℃. (3) Mg²⁺ can substitute Zn²⁺(<50mol%) and form a solid solution in Zn₂SiO₄ crystalline and thus improved the sintering process, the sintering range is more than 75℃. Zn_0.8Mg_0.2O-0.5SiO₂ ceramic sintered at 1275℃ obtained dielectric properties of ε_r=6.19, Q×f=49000 GHz , Ï„_f= -54.1ppm/℃. (4) TiO₂ did not react with host material, thus it can adjust the Ï„_f of host material. However, the ceramics had narrow sintering range with TiO₂ addition. (5) When MgO and TiO₂ were added, ZnO-0.5SiO₂ ceramics can be sintered from 1250 to 1325℃. With 10wt% TiO₂ addition, Zn_0.8Mg_0.2O-0.5SiO₂ ceramics had an excellent dielectric properties of ε_r= 8.16, Q×f=43200GHz, Ï„_f= -13.7ppm/℃.2. The sintering temperature of ZnO-SiO₂ and ZnO-SiO₂ ceramics with TiO₂&MgO addition (ZMST for short) were decreased to 900℃ by adding composite sintering aids. The low-fired ceramics can prepare stable slurry and have good compatibility with Ag electrode. The low-fired ceramics with low dielectric constant was suitable for LTCC technology and was promising to prepare LTCC microwave components working at millimeter wave. (1) Li₂CO₃-Bi₂O₃ can form liquid phases at 598℃ and 720℃. During the sintering process, it can react with host material to form low-melting-point phase Bi₄Si₃O₁₂. Multiple liquid phases can effectively reduce the sintering temperature of ZnO-ySiO₂ from 1400 to 910℃. Bi₂O₃-SiO₂ aids was calcined to form Bi₄Si₃O₁₂ phase, which can melt at 833℃. The single liquid phase can reduce the sintering temperature of ZnO-0.6SiO₂ ceramics form 1380 to 960-990℃. (2) The densities and ε_r of ZnO-ySiO₂ decreased as y increased. Q × f values were closely related with phase composition and microstructure. ZnO-0.6SiO₂ ceramics with 5wt°/oLi₂CO₃-4wt%Bi₂O₃ obtained properties of ε_r = 6.65, Q×f= 33000 GHz, Ï„_f= -70ppm/℃. (3) As Bi₂O₃-SiO₂ addition increased, densities and ε_r increased and Q×f decreased slightly. With 3wt% Bi₂O₃-SiO₂ addition, the ZnO-0.6SiO₂ ceramics have properties of ε_r=6.19, Q×f=41800GHz, Ï„_f=-52.9ppm/℃. (4) Li₂O-B₂O₃-SiO₂ glass have a low soften point(402℃). The mixture from Li₂CO₃-B₂O₃ calcined at 600℃ can form a liquid phase at 660℃. Both additives can form a high activity unknown phase with TiO₂. These results can decreased the sintering temperature of ZMST to 840-900℃. (5) Thedielectric properties of low-fired ZMST ceramics show a complex variety influenced by phase composition and microstructure. With 3wt% Li₂CO₃-B₂O₃ addition, ZMST ceramics obtained optimum properties of ε_r = 8.84, Q×f = 15500GHz, Ï„_f = +17.8ppm/℃. (5)Stable and well dispersive slurry of the low-fired ceramics can be prepared and the green tapes have a glabrous surface. There were no aggregations of the particles. The low-fired ceramics also have good compatibility with Ag electrode. It can be a promising low dielectric constant microwave ceramics for LTCC technology.3. The ZnO-SiO₂ nanopowders with particle size of 100-200nm and well dispersity were prepared by sol-gel method at a relatively low temperature. Compared to solid reaction method, the nanopowders prepared by sol-gel method have higher sintering activity, and can be sintered well at 1250℃ and excellent dielectric properties were obtained. These results were useful to further application in thinner dielectric layers.(1) Using Zn(NO₃)₂·6H₂O and Si(OC₂H₅)₄ as precursors, ethanol as solvent, transparent and homogeneous ZnO-SiO₂ gel was prepared by controlling gelation process. The gel of ZnO-SiO₂ system was formed though the hydrolysis and polymerization of Si(OC₂H₅)₄, and the Zn²⁺ ion were embedded in the gel network. (2) The gelation of ZnO-SiO₂ system and the characteristics of powders calcined from dried gel were influenced by process parameters. The transparent gel and suitable gelation time were obtained under the condition of c(precursors) = 0.5-1mol/l, T(gelation temperature) = 10-40℃, pH = 1.5-3, [H₂O]/[Si]=0-8/1. At the same process condition, the nanopowders with particle size of 100-200nm and well dispersity were obtained when the Si(OC₂H₅)₄ were hydrolyzed and polymerized completely. (3)The ZnO-SiO₂ nanopowders prepared by sol-gel method have higher sintering activity, and compared to solid reaction method, it decreased the sintering temperature more than 150℃ and increased the Q×f values from 52500 to 67500GHz.