Abstract
The purposes of this thesis include two main researching fields. Firstly, the preparation of nano-sized Ba2Ti9O20 microwave dielectric materials via chemical synthesized processes was studied, which consisted of modified co-precipitation (MCP), inverse-microemulsion (IME) and hydrothermal process. After the precursors of Ba2Ti9O20 obtained, these precursors were processed by the thermal treatments (clacination & sintering) in different atmospheres (O2, air, N2 and N2/H2), sintered ceramics were obtained with various microwave dielectric properties and phase transformation. According to the above experimental results, the novel composition of Ba2Ti9O20 precursor was developed to decrease the temperature of Ba2Ti9O20 formation effectively. Secondly, a valuable research was carried out on the application of nano-powders by using a well-dispersed nano-BaTiO3 nonaqueous suspension with high solid content. Different kinds of synthesized phosphate esters were used as dispersants and added respectively in the nano-BaTiO3 nonaqueous suspension to identify the optimal molecular structure and proper amount of phosphate ester. Moreover, this powder slurry of nano-BaTiO3 was used to manufacture an ultra-thin ceramic film for MLCC. Summaries of all the topics in this thesis, are described as follows: (1) Modified co-precipitation process (MCP), which combines the automatic pH -value control system and supersonic spraying of co-precipitants, was used to synthesize nano-sized Ba2Ti9O20 precursors (~48.8 nm). Kinetics for crystallization process is markedly enhanced when the O2 atmosphere was replaced with air in the calcinations and sintering processes. When O2-processed, high density Ba2Ti9O20 materials (>94%T.D.), possessing good microwave properties (K=38.7 and Q×F=30,502), were obtained by sintering the sample at 1200℃/4 h, which was lower than the sintering temperature when processed in air (i.e., 1250℃/4 h) and nitrogen. (2) Double inverse-microemulsion (IME) process is also used for synthesizing nano-sized Ba2Ti9O20 powders. The crystallization of thus obtained powders and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders were of nano-size (~21.5 nm) and possessed high activity. The BaTi5O11, intermediate phase had resulted when the IME-derived powders were calcined at 800℃(4 h) in air. However, high density Ba2Ti9O20 materials with pure triclinic phase (Hollandite-like) can still be obtained by sintering such a BaTi5O11 pre-dominated powders at 1250℃/4 h. The phase transformation kinetics for the IME-derived powders was markedly enhanced when the air replaced with O2 during calcination and sintering processes. Both the calcination and densification temperatures were lowered by around 50℃ compared to the processes undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in large dielectric constant (K＝39) and high quality factor (Q×f ＝28,000 GHz) for the samples possessing a density higher than 95%T.D., regardless of sintering atmosphere. Over firing dissociates Ba2Ti9O20 materials and results in poor quality factor. (3) A hydrothermal process has also been successfully utilized for the preparation of Hollandite-like Ba2Ti9O20 precursors. TEM investigation, in conjunction with chemical analyses on reacted powders, indicates that Ti4+-species were first dissolved in the solution and then reacted with Ba2+-species to form perovskite phased BaTiO3 in the hydrothermal process. Excessively large particle size for the starting TiO2 (anatase powders) results in insufficient Ti-ions in the solvent and incomplete reaction with Ba2+-species, which leads to Ba2+-deficit powder mixture. A residual TiO2 phase thus results after calcination. Only small TiO2 particles (40 nm) can result in sufficient Ti4+-species in the solution, which fully react with Ba2+-species and lead to TiO2/BaTiO3-ratio of the correct stoichiometry to form Ba2Ti9O20. TiO2/BaTiO3 powder mixtures, prepared in this way possess high activity and can be converted into pure Ba2Ti9O20 materials. After calcination and sintering processes (1300℃/ 4 h), such materials possess high sintered density (93.3% T.D) and good microwave dielectric properties (K = 36 and Q×f = 28000). (4) Evolution of the phases during the calcination in mixed oxide processing of BaTiO3 and TiO2 mixture were investigated. Based on the observations, a reaction sequence of the constituent phases was proposed and a modified process has suggested viz. to utilize BaTi4O9 and BaTi5O11 mixture instead of BaTiO3 and TiO2 mixture as starting materials. The phase transformation kinetics is thus greatly enhanced. While the reaction of 2BaTiO3/7TiO2 mixture (type I, ~138 nm) requires 1100oC/4 h to completely transfer the mixture into Ba2Ti9O20 Hollandite-like phase, it needs only 1000 oC /4 h to fulfill the phase transformation process for the BaTi5O11/BaTi4O9 mixture (type III, ~88 nm). When directly sintering the materials prepared from these powder mixture, type III mixture leads to higher siterability and more uniform granular structure, as compared with the type I mixture. Furthermore, local microwave dielectric properties measured by evanescent microwave probe (EMP) reveals that the grains of type III Ba2Ti9O20 materials possess larger dielectric constant than that of type I, indicating that the type III powders not only of higher sinterability but also react much better in forming Hollandite-like phase. (5) The stability of nano-sized barium titanate (BaTiO3) nonaqueous suspension with different solvents and phosphate esters has been investigated by means of zeta potential, adsorption, sedimentation and viscosity measurements. Without dispersant addition, the viscosity of the solvent is an essential dispersing factor for stabilizing the nano-sized BaTiO3 suspensions. Three typical phosphate esters were synthesized, including mono-alkyl, di-alkyl, and ethoxy type. The results of phosphate ester adsorption and suspension sedimentation corroborate that the specific amount of 2-ethoxy ethyl dihydrogen phosphate as a dispersant can provide the longest stable status for BaTiO3 (50 nm) contained (40 wt%) in 1-methoxy-2-propanol. Analyzing the influence of suspension dispersion for powder particle size, we found out that longer molecular chain of dispersant is required for larger powder (200 nm), but the additional amount of dispersant was less than the smaller one (50 nm). The nano-sized BaTiO3 non-aqueous suspension was successfully used to manufacture the ultra-thin ceramic film (2.6~2.8 micrometer) by tape casting and applied to multi-layer ceramic device.