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|Other Titles: ||Preparation and Dispersion of Ultra-Fine Ba-Ti-O Dielectric Powders by Chemical Methods|
在Ba2Ti9O20微波介電材料粉體製作與其特性研究中，採用三種化學合成方法得到Ba2Ti9O20微波介電材料粉體，分別為改良式共沉澱法、逆微乳膠法以及水熱法。在改良式共沉澱法(Modified co-precipitation，MCP)方面，運用酸鹼度值控制與超音波霧化設備，進行自動化的共沉澱反應。經由此一反應所得之改良式共沉法前驅物粉體，可有效改善粉體凝團及降低粉體平均粒徑(48.8 nm)。在不同的氧分壓氣氛環境下(O2, air, N2 and N2/H2)進行粉體熱處理，所得到材料相變化過程與結果，亦有所不同。在富氧氣之燒結環境下製作的Ba2Ti9O20，在燒結體特性上明顯優於其他氣氛環境製作。於純氧環境下，可於相對低燒結溫度下(1200℃/4 h)，得到相當優越的微波介電特性(K=38.7 and Q×f=30,502)。
逆微乳膠法(Inverse-microemulsion，IME)合成Ba2Ti9O20微波介電材料方面，藉由界面活性劑於溶劑中所形成之乳膠微胞做為反應單元，進行微小化合成製作。採用PE(petroleum ether)/ NP-5(poly (oxyethylene)5 nonyl phenol ether) /Water之三相比例為1.5：1：0.1，進行雙逆微乳膠反應，反應溶液pH值控制在9.5。反應所得之Ba2Ti9O20前驅物粉體，其粉體平均粒徑為21.5 nm，亦為三種化學合成法中所得之最細微粉體。逆微乳膠法合成粉體在氧氣環境下熱處理，可於相對低溫燒結條件下(1200℃/4 h)反應形成Ba2Ti9O20，並達到高微波介電特性。但在更高溫氧氣燒結環境下(＞1250℃)，Ba2Ti9O20亦容易較早產生崩解與二次相(TiO2)形成。此一結果與改良式共沉法粉體在氧氣環境下熱處理所得結果相同。
利用水熱合成法製作Ba2Ti9O20前驅物粉體中，使用不同大小奈米級的TiO2粉末(40 nm，100 nm與200 nm)與Ba(OH)2水溶液作為起始原物料，控制適當添加劑(H2O2)及強鹼性製作環境(pH>12)，可使得Ba離子在水熱反應過程中於TiO2表面形成微小的BaTiO3晶體，並吸附於TiO2顆粒上，即經由短距離的溶解-析出機制(short-range dissolution-precipitation)，而形成TiO2-BaTiO3之core-shell結構奈米粉體。經實驗得知，以40nm TiO2粉末與適當的水熱反應製作條件，可較早得到符合鋇/鈦離子比(Ba/Ti=2/9)的Ba2Ti9O20前驅物粉體。再經由粉末煆燒處理，可在相當低的熱處理溫度下(1100℃/4 h)，生成Ba2Ti9O20相。將水熱反應製得的粉末，經由煆燒(1050℃/4 h)後壓片燒結(1300℃/4 h)，可得具有優越微波介電特性的Ba2Ti9O20陶瓷材料(K=37.2, Q×f=28835)。
另一重要研究方向，是朝向開發奈米級粉體的應用性，即有機溶劑系奈米級鈦酸鋇粉體漿料之開發與應用。將針對奈米級BaTiO3微粉(50 nm)於有機溶劑中之的分散特性進行研究。除了對於奈米級BaTiO3粉體選擇適當的溶劑(1-methoxy-2-propanol)之外，於粉體分散性研究過程中，採用自行合成之高純度磷酸酯類界面活性劑，在其分子結構大小與粉體吸附關係對粉體漿料分散性質影響，亦同時進行探討。由界面電位分析、分散劑吸附行為與吸附量分析、漿料沉降時間與黏度量測等試驗得知，2-ethoxyethyl dihydrogen phosphate作為分散劑，添加於奈米BaTiO3 (50 nm)粉體有機溶劑漿料(1-methoxy-2-propanol)中，可得到最佳抗沉降特性。亦將200 nm的BaTiO3粉體導入分散研究比較發現，較大顆粒粉體漿料需較長鏈段的磷酸酯結構物之分散劑，藉以維持長時間的漿料分散穩定狀態。並利用此兩種高固含量粉體漿料(40 wt%)，製作出超薄的陶瓷薄帶(2.6~2.8 micrometer)。同時亦導入積層陶瓷電容元件的製作與元件特性比較，發現較細微粉體漿料所製作出的元件，可在介電特性(K-value)與抗電壓特性(BD voltage)上得到較佳的表現。|
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.
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