Study On Low-temperature Co-fired Microwave Ceramics With Middle Dielectric Constant And The Component

With the rapid progress in mobile telecommunication, wireless local area network (LAN) and military radar technologies, it has been strongly required that the related components become small-sized, light-weighted, multifunctional, low-cost, and usable at higher frequency range. In order to fulfill these requirements, design and fabrication of multilayer chip microwave components using low temperature co-fired ceramics (LTCC) technology has become a research hotspot nowadays.The key technology of LTCC is preparation of microwave dielectric ceramics which can co-fired with low cost and high conductivity inner electrode such as Ag and Cu.Ca[(Li_1/3Nb_2/3),Ti]O_3-δ (CLNT) and Magnesium titanate (MgTiO₃) ceramics exhibit moderate dielectric constant, low dissipation factor and adjustable temperature coefficients of resonant frequency. These two material systems could be promising LTCC materials if they can be sintered at low temperature and compatible with LTCC technics. Recently, some research work on low temperature sintering ceramic based on these two material systems have been reported, but there still exist many problems as follows: (1) It's difficult to decrease the sintering temperature and the study on liquid phase sintering mechanism is limited. (2) Though the sintering temperature of dielectrics was decreased lower than the melting point of Ag electrode, dielectric properties critically deteriorated. For example, CLNT ceramic with 2wt%B₂O₃-6wt%Bi₂O₃ compound additives can be densified at 920℃, whereas the quality factor values (Q×f) rapidly decreased to 10600GHz. (3) Research and development of new materials disjoint with design and preparation process of components. Many materials have bad adaptability with the fabrication process. For example, it's difficult to get dense green tape by tape casting process use ceramic powder with B₂O₃ additive, because B₂O₃ is easy to react with binder such as PVB.From the view points of combination of material properties, components design and preparation technics, ceramic systems of CLNT and MgTiO₃ were selected as the research objects in this dissertation. Based on the effect of different sintering aids to reduce the sintering temperature of microwave ceramic systems, the low temperature sintering mechanisms were revealed. The effects of different sintering aids on sintering characteristics, phase compositions, microstructures and dielectric properties of microwave ceramics were studied systematically. The relation among phase compositions, microstructures and dielectric properties of low temperature sintered ceramics was explained. The sintering aids and preparation processes were optimized. Engineering practice problems of LTCC materials (CLNT and MgTiO₃) including slurry character and co-firing behavior between ceramic and silver electrode were also studied. Based on above researches, a multilayer component model was built. By component simulation and LTCC technics,a kind of multilayer band-pass filter with demand property was prepared.1. Based on the effect of different sintering aids to reduce the sintering temperature of microwave ceramic systems, combined with sinterability, phase compositions and microstructures, the liquid-phase sintering mechanisms of LTCC ceramics were revealed. It can provide references for choosing sintering additives. (1) The reaction between sintering addition and host material can increase the substance activity, liquid phase formed simultaneously, so the sintering process gets accelerated. During the sintering process, Bi₂O₃ reacted with CLNTto form liquid phase of Li₂O-Bi₂O₃, Bi₂O₃-TiO₂ at low temperature; LiF reacted with CLNT with the production of CaF₂, LiF and CaF₂ transferred into liquid phase at the eutectic point of 766℃; Bi₂O₃ and V₂O₅ reacted with MgTiO₃ host material at low temperature to form new phases like Bi₄V_1.5Ti_0.5O_10.85 and BiVO₄. The new phases melted into liquid phase at 815～850℃. (2) Borosilicate glass exhibits low softening temperature. At low temperature, it formed into molten glass with low viscosity, so can effectively decrease the sintering temperature of ceramics. The softening temperature of ZnO-B₂O₃-SiO₂ (ZBS) and Li₂O-B₂O₃-SiO₂ (LBS) glass are 638℃ and 402℃ respectively, with which the sintering temperature of CLNT and MgTiO₃ ceramics can be decreased to 930℃ and 900℃, respectively. (3) The combined additives are more effective to decrease the sintering temperature of microwave ceramics than single additive. Bi₂O₃ and ZBS can decrease the sintering temperature of CLNT to 1020℃ and 930℃ independently. CLNT ceramic with Bi₂O₃-ZBS compound additive can densified at 900 ℃ by the effect of ZnO-Bi₂O₃-B₂O₃-SiO₂ molten glass with low viscosity. As for LiF-ZBS compound additive, the replacement of the bridging oxygen of ZBS glass network by F^- can disaggregate the anion conglomeration, which can decrease the viscosity of molten glass and accelerate the liquid-phase sintering process.2. The relationship among phase composition, microstructure and dielectric properties of low temperature sintered ceramic was disclosed. (1) Second phases, including additives introduced and impurity formed during reaction, affect dielectric properties of ceramic greatly. When ZBS and LBS, which all have low dielectric constant and high dissipation factor, added into CLNT and MgTiO₃ ceramic respectively. ε_r and Q×f value of ceramic decreased, Ï„_f shift to negative value; As for MgTiO₃ specimen with Bi₂O₃-V₂O₅ combined additives, when V₂O₅ content was 1～2mol%, Bi₂O₃ and V₂O₅ could react with host material to generate lots of Bi₂Ti₂O₇ and Bi₄V_1.5Ti_0.5O_10.85 phases with high dissipation factor, which caused the low Q×f value of ceramic. When the V₂O₅ content increased, these phases decreased gradually and then disappeared, the Q×f value increases sharply. (2) Microstructure, such as pore and crystalline size, is also important factor that can affect dielectric properties. The volatilization of Bi₂O₃ led to the increasing of pores in CLNT ceramic, which caused the decrease of bulk density (ρ), ε_r and Q×f value; The volatilization of LiF result in the porous structure of CLNT ceramic, ρ and ε_r decreased quickly with the increase of LiF content; BiVO₄ induced the abnormal grain growth of MgTiO₃, which caused the inhomogeneity of microstructure and led to the great decrease of Q×f value. (3) Lattice defect affect greatly on dielectric properties. For CLNT ceramic with Bi₂O₃-ZBS combined additives, Bi³⁺ ions substituted Ca²⁺ ions in the A-site of perovskite structure that resulted in formation of calcium vacancy and relaxation of crystalline lattice. As a result, ε_r increased and Q×f value greatly decreased, Ï„_f shift to negative value; LiF and ZBS glass could coordinated and accelerated the phase transformation from Ca[(Li_1/3Nb(2/3)),Ti]O_3-δ phase to stoichiometric Ca[(Li_1/4Nb_3/4),Ti]O₃ phase. Oxygen vacancy in the ceramic body decreased during the transformation, which was helpful to keep the high Q×f value.3. In order to lay the foundation for the industrialization of LTCC materials, engineering techniques including slurry character and co-firing behavior of ceramic and silver electrode were studied systematically. (1) Gelation between V₂O₅ and binder such as PVB or PVA could greatly increase the viscosity of MgTiO₃ slurry system with Bi₂O₃-V₂O₅ combined additives. The obtained slurry with high viscosity is unsuitable for tape casting. Preliminary reaction of Bi₂O₃ with V₂O₅ to form BiVO₄ could resolve the tape casting problem of the systemwith free V₂O₅. (2) The co-fired interface between CLNT ceramic with LiF addition and Ag electrode is distinct. The delamination phenomenon was observed for the nonmatched shrinkage of ceramic and Ag. The Ag electrode also had anomalous porous structure. All these problems limited the application of CLNT ceramic with LiF addition. The substitution of LiF-ZBS combined addition for LiF can resolve these problems. (3) CLNT ceramic with Bi₂O₃-ZBS or LiF-ZBS and MgTiO₃-CaTiO₃ ceramic with LBS glass can avoid the disadvantageous effect of free B₂O₃ on slurry character. Green tapes with glabrous surface were gained by the tape casting process. It has a good conjoint status and chemical compatibility between ceramic and Ag electrode. No evident diffusion was happened.4. On the premise of harmonization and optimization of cooling effect, dielectric properties, slurry character and co-firing behavior of ceramic and silver electrode, two kinds of low temperature firing microwave dielectric ceramics which had excellent properties and application values were acquired. (1) Low temperature sintering ceramic CLNT was prepared with LiF-ZBS compound additives. It acquired microwave dielectric properties of ε_r=34.28, Q×f=17400GHz, Ï„_f= - 4.6×10^-6/℃ sintered at 900℃. The Q×f value of low temperature sintered CLNT ceramic increased greatly than reported before. (2) CaTiO₃ was used to adjust Ï„_f of MgTiO₃, home-made LBS glass with low softening temperature was adopted as the sintering additive, low temperature sintering microwave dielectric ceramic with dielectric properties of ε_r= 16.38. Q×f=11640GHz, Ï„_f=-1.45ppm/℃ was acquired when sintered at 890℃. Compared with dielectric properties of low temperature sintered MgTiO₃ ceramic reported before, dielectric constant increased evidently and Q×f also got improved.5. According to the structure and design principle of multilayer filter, a multilayer laminated bandpass filter model was constructed based on broadside coupled strip line. Ansoft HFSS software was adopted for electric properties simulation. Low temperature sintering ceramic CLNT with LiF-ZBS addition was used as the dielectric layer. By using LTCC fabrication technics, a mulilayer laminated bandpass filter with dimension of 3.2mm×1.6mm ×1.4mm was acquired. The electric properties were as following: center frequency is 1.907GHz, bandwidth >100MHz, VSWR is 17.9dB, insertion loss is 2.07dB, stopband loss at f₀-450MHz is 53.2dB. Its synthesized performance reached the similar level of the same kind of products of Murata company.