Characterization And Application Of Li₂O-Nb₂O₅-TiO₂ Microwave Ceramic Tapes Prepared By Aqueous Tape Casting

With the rapid development of mobile telecommunication, wireless local area network (LAN), it has been strongly requied that the related components become small-sized, light-weighted, multifunctional and low-cost. In order to fulfill these requirements, design and fabrication of multilayer chip microwave components using Low Temperature Co-Fired Ceramics (LTCC) technology have been the research focus. The key technology of LTCC is preparation of ceramic tape which can be co-fired with inner electrode such as Ag or Cu.Tape casting is the most efficient and reliable method to produce thin ceramic tapes that are necessary for manufacturing multilayer microwave ceramics. Traditionally, a variety of non-aqueous organic solvents, such as alcohols, ketones and hydrocarbons were commonly used in tape casting process. In recent years, the environmental and health aspects of tape casting process have received comprehensive attentions. There is now a trend to move away from organic solvents and an expected transition towards water-based systems.Li_1.075Nb_0.625Ti_0.45O₃ (LNT) exhibit moderate dielectric constant, low dissipation factor and adjustable temperature coefficient of resonant frequency. This ceramic could be a promising LTCC material if it can be sintered at low temperature. However, many studies have reported that the sintering aids such as V₂O₅ and B₂O₃ had bad adaptability with tape casting process. For example, B₂O₃ can react readily with PVB or PVA, which were major binders in tape casting process.This dissertation focuses on two main fields to meet the challenges in LTCC processing. One is tape casting of LNT microwave dielectric ceramic tapes using different water-based binder systems to find the optimum conditions in each system. The other is investigates the influence of V₂O₅ and B₂O₃ on tape casting process, which is known to be the drawback in tape casting process. It also offers a potential application in LTCC technics. The main parts are as followings:1. Study on the Dispersion of LNT Powders in WaterThe dispersion process of LNT powders in aqueous suspension was studied and characterized in terms of zeta potential, sedimentation test and apparent viscosity. Without dispersant, the isoelectric point (IEP) of LNT particles was found at about pH 3.7. The absolute value of zeta potential was about 90 mV at pH 9-10. With the addition of dispersant (Ammonium polycarboxylate, APC), the IEP was moved to smaller pH value of about 2.6. Meantime, the absolute value of zeta potential increased to about 95 mV at pH 9-10. The specific energy value of interaction between LNT powders and APC was 8.931RT, which indicated a strong adsorption of APC on LNT powder surface. The optimal LNT aqueous suspension can be obtained at pH 9-10 with 0.8 wt% APC addition2. Study on Preparation of LNT Aqueous Tape Casting SlurriesAll slurries exhibited the desired shear thinning behavior. The viscosity and yield stress revealed an increasing trend with increasing amounts of binder and solid loading. Generalized Casson model was used to fit the experimental datas. The slurries with PVA as binder were shown to follow Bingham patten. As PVAc was increased, the slurries with PVA/PVAc as binder were shown a transition from Bingham patten to original form of Casson patten, and the yield stress decreased. The slurries with WB4101 as binder were followed the original form of Casson patten. The rheological tests indicated that the slurries exhibited no time-dependent behavior.3. Study on Drying Kinetics and Tape FormationThe drying process of the tape casting slurry was shown to follow a two stages mechanism (initial stage: the rate of drying was constant; and second stage: the rate of drying was falling). The activation energy in two stages was calculated. At first stage, E_PVA=41.22KJ/mol, E_WB4101=42.78KJ/mol, which agreed well with the value of pure water. It can be concluded that the initial stage was controlled by water evaporation process. At second stage, E_PVA=36.80K.J/mol,E_WB4101=34.06KJ/mol, which was slightly less than the value of first stage. The second stage was an internal diffudion controlled process. During drying process, two binder systems have different film forming mechanisms, PVA moleculars form a network structure under Van de Waals force and hydrogen bond to obtain tape; WB4101 latex particles form a cross-linking structure under compressive force to obtain tape.4. Study on Properties of the LNT TapeThe effects of slurry composition on mechanical properties, pore size distribution and relative density were studied. The tensile strength and strain were strongly dependent on the binder and plasticizer contents. When there were excessive binder amounts, the resulting green tape tends to develop cracks, and resulted in decresing of tensile strength. The plasticizer increased the elongation of the green tape while reduced the strength. The pore size distribution in green tapes was narrow with the most frequent pore radius was between 0.5-0.6μm. The relative density of the green tapes increased with increasing solid loadings. The sintering schedule was established by DTA-TG analysis. The sintered tapes have a fairy uniform, dense microstructure.5. Study on Co-firing of LNT/Ag Multilayer SheetsThe effects of V₂O₅ and B₂O₃ on slurry and tape properties were investigated. With the increase of V₂O₅, viscosity of the slurry increased slightly. The addition of B₂O₃ has increasing slurry viscosity, and decreasing mechanical property of the tape. To correlate these phenomena, polyhydroxy compounds were added into the slurry. The polyhydroxy compounds were effective in enhancing the properties of LNT/B₂O₃ tape. Sliver inner-electrode was pasted on green tapes and the sheets were stacked, pressed, and sintered at 900℃. SEM analysis showed that the samples were fully dense with fairly uniform microstructure and no reaction been observed between ceramic and sliver layers. Therefore, aqueous tape casting can be use to fabricate LNT/Ag multilayer sheets for LTCC application.