Preparation And Application Of Microwave Ceramic Nanopowder With Low Dielectric Constant And Low Sintering Temperature By Sol-gel Method

With the rapid progress in information technology, it has been strongly required that the related electronic components become small-sized, light-weighted, integrated, multifunctional, low-cost, and usable at high frequency range. Especially, small-size and using at high frequency have become the basic character of advanced electronic components. In order to meet the requirement of small-size and high frequency, it is important to prepare nanopowder that should have the following characters: could be co-fired with low cost and high conductivity inner electrode such as Ag and Cu, have good microwave dielectric properties in high frequency after sintered, and should be sufficiently to meet the requirement for preparation technology of multilayer components.Nanopowder with good microwave dielectric properties after sintered has been investigated for a long time, and the study mainly focuses on improving the pure of materials and dielectric properties of ceramic. However, little investigation was carried on the preparation of nanopowder which could be sintered at low temperature, and so did the preparation of subminiature component with these nanopowders. So far, the preparation of nanopowder which could be co-fired with Ag or Cu used at microwave frequency range has not been investigated yet. There are many problems as follows: (1) It's difficult to control the component and phase of nanopowder because the composition is complex, especially when a small amount of sintering aids were added in. (2) Dielectric properties critically deteriorated though the sintering temperature of nanopowder was below the melting point of Ag electrode. (3) Many materials have bad adaptability to the fabrication process of components. For example, it's difficult to get dense green tape by tape casting process use nanopowder, and there may be reaction between ceramic and electrode. (4) It's difficult to control the grain size of ceramic. If the grain grows larger, there will be only two or three grains between the two electrode layers, which deteriorate the dielectric properties.In order to overcome the above problems and prepare dielectric nanopowder used at microwave frequency which could be co-fired with Ag, the low temperature co-fired (Ca, Mg)SiO₃-CaTiO₃ nanopowder was investigated in this work by sol-gel method. Dissolving the precursor of sintering aids into host materials sol, the nanopowder was prepared with uniform component, controllable phase and particle size, and the sintering temperature was below 900℃. The dielectric properties of ceramic were optimized through controlling the phase and particle size of nanopowder. The grain size of ceramic was minimized through enwrapping with gel, doping with rare earth elements, adjusting the phase composition and sintering process. Based on above researches, the applied technology of nanopowder was studied and green tape with a thickness of thinner than 10μm was prepared, which can be co-fired with Ag electrode and was adaptive to fabricate subminiature multilayer ceramic components.1. The sol-gel process of (Ca, Mg)SiO₃ and CaTiO₃ was studied, respectively. The mechanism of gelatin was discussed. Based on phase composition, micro-morphology and calcination temperature, the model of (Ca, Mg)SiO₃ phase formation and grain growth was founded. The nanopowder with excellent dispersion was prepared, and then the sintering process and dielectric properties of nanopowder were investigated. (1) In the sol of (Ca, Mg)SiO₃ and CaTiO₃, the network of gel was formed through the hydrolysis and aggregation of Si(OC₂H₅)₄ and Ti(OC₄H₉)₄, respectively, and the ions of Ca²⁺ and Mg²⁺ were embedded in the gel network. (2) Controlling the factors of (Ca, Mg)SiO₃ (CaTiO₃) sol under the condition of c(precursors) = 0.8mol/l, T(bath temperature) = 60℃, pH = 4.5 (2), [H₂O]/[Si] = 4/1 (none), and added with 2wt% oil acid (PEG400) as dispersant, the gel process was appropriate and the nanoparticles with excellent dispersion were obtained by calcining gel. (3) The model of (Ca, Mg)SiO₃ phase formation and grain growth was founded. When the calcination temperature of the gel was below 800℃, a small amount of crystal phases were formed, and the grain size reduced gradually with increasing the calcination temperature due to the break of Si-O-Si bond in the network. When the calcination temperature was over 900℃, a large amount of crystal phases were emerged and the grains grew with increasing the calcination temperature. (4) The soluble limitation of Mg²⁺ in CaSiO₃ was below 0.2 and CaSiO₃ phase was transformed into CaMgSi₂O₆ phase with the substitution of Ca²⁺ by Mg²⁺. When x is 0.3, the growth of grains was restrained and pores decreased due to the coexistence of CaSiO₃ and CaMgSi₂O₆, and then the density of ceramic was enhanced. The dielectric constant and quality factor of (Ca_0.7Mg_0.3)SiO₃ ceramic sintered at 1320℃ is 6.62 and 36962 GHz, respectively. (5) CaTiO₃ nanoparticles with an average grain size of 60-70nm were obtained by calcining the CaTiO₃ gel at 800℃. CaTiO₃ ceramic sintered at 1250℃ prepared from nanopowders had good dielectric properties: ε_r= 171, Q×f = 4239 GHz, Ï„_f= +768 ppm/℃.2. Through mingling LiNO₃-NH₄VO₃ liquid phase sintering aids and precursors of (Ca, Mg)SiO₃ in sol, the (Ca, Mg)SiO₃-LiV nanopowder with lower sintering temperature and good dielectric properties was prepared, which initiated a novel route to prepare nanopowder could be sintered at low temperature. (1) The crystallization temperature decreased enormously (>300℃) by mingling precursors of Li-V liquid phase sintering aids and precursors of (Ca, Mg)SiO₃ in sol. Calcining the (Ca, Mg)SiO₃-LiV gel at 700℃, the nanopowder with grain size of 60⁸0nm and phases of CaSiO₃ CaMgSi₂O₆ Ca₂MgSi₂O₇ was obtained. (2) The sintering properties of (Ca, Mg)SiO₃-LiV powder deteriorated if the grain size was smaller or larger than one dimension. Using the powder with grain size of 80-100nm as raw material, the ceramic sintered at 890℃ possessed excellent dielectric properties: ε_r = 6.96, Q×f= 23645GHz, Ï„_f=-75.10ppm/¤. (3) Ca₃LiMgV₃O₁₂ and Li₂SiO₃ were obtained due to the reaction between sintering additions and host materials, which increased the substance activity and resulted in the acceleration of the sintering process. (4) The Ï„_f value was adjusted by the addition of CaTiO₃ nanopowder into (Ca, Mg)SiO₃-LiV nanopowder. Doping 12wt% CaTiO₃ nanopowder into (Ca,Mg)SiO₃-LiV nanopowder, the ceramic sintered at 890℃ had good dielectric properties: ε_r = 9.42, Q×f= 15767GHz, T_f=+2.3ppm/℃.3. The factors influenced the grain size were investigated, which including the means of (Ca, Mg)SiO₃-LiV nanopowders enwrapped with CaTiO₃ gel, the addition of rare earth elements, and the adjustment of component and sintering process. The average grain size of low temperature co-fired (Ca, Mg)SiO₃-CaTiO₃ ceramic was refined to 0.6μm, which offered new approaches to refine the grain size of ceramic. (1) It's hard to play a role in refining the grain size of (Ca, Mg)SiO₃ through enwrapping with CaTiO₃ gel, though the dielectric properties were improved. (2) The dielectric properties deteriorated enormously with doping rare earth elements, which had a little effects on refining the grain size of (Ca, Mg)SiO₃-CaTiO₃ ceramic. (3) The grain size of low temperature co-fired (Ca, Mg)SiO₃-CaTiO₃ ceramic was refined by adjusting component. Adjusting the component of (Ca_0.7Mg_0.3SiO₃ to (Ca_0.5Mg_0.5)SiO₃, the average grain size of (Ca_0.5Mg_0.5)SiO₃-LiV-12wt%CaTiO₃ ceramic sintered at 890℃ was refined to 0.6μm with the main phases of CaMgSi₂O₆ and CaTiO₃, which had good dielectric properties: ε_r = 9.74, Q×f= 16433GHz, Ï„_f = -2.78ppm/℃. (4) It was effective to refine the grain size and improve the dielectric properties of (Ca, Mg)SiO₃-CaTiO₃ ceramic with appropriate sintering process (first sintered for 0.5h at 890℃, and then sintered at 850℃ for a long time).4. Based on the study about dispersion of nanopowders, preparation of slurry, co-firing of ceramic and Ag electrode, the green tape with the thickness of 9.8μm was gained by the tape casting process, which was appropriate to prepare subminiature multilayer component. (1) It was limited to disperse low temperature co-fired (Ca, Mg)SiO₃-CaTiO₃ nanopowders by adding non-ionic surfactant or anionic organic acid. Owing to steric hindrance and electrostatic rejection, the agglomeration of nanopowders could be reduced effectively and a homogeneous mixture suspension could be obtained with the ionic surfactant AB001 added in. (2) Green tape with glabrous surface was gained by the tape casting process with the content ratio of M_nanopowder : M_dispersant:M_solvent:M_bond:M_plasticizer = 100:1.5: 70: 65: 4. (3) When co-firing the green tape and Ag slurry, there was a good conjoint status and chemical compatibility between ceramic and Ag electrode, which indicated that the nanopowder is appropriate to prepare subminiature multilayer component.