氧化鋁陶瓷由於具備優異的機械性質，是工業應用上不可或缺的重要材料，當其晶粒逐漸縮減時，陶瓷體之硬度、強度、抗磨耗性、可見光穿透率等性質皆可獲得提升，並拓展其應用面；然而，在燒製微晶粒氧化鋁陶瓷體的過程中，大部分成功的例子均採用成本高昂的加壓燒結方式。故本研究的目的在藉由對陶瓷製程中數個重要的階段加以控制(依照製程順序分別為添加劑的使用、注漿成形技術的改善及二階段燒結參數的設計)，探討其對氧化鋁微粉之燒結、微結構演變和機械性質上的影響，並期望在常壓燒結的前提下製備出微晶粒之緻密陶瓷體。
在含5 wt% 氧化鋯之α-氧化鋁微粉作為起始原料的研究中發現，TiO2的添加、固溶可降低氧化鋁的燒結溫度約100oC，並促進其晶粒的成長。緻密陶瓷體的硬度隨著TiO2-ZrO2-Al2O3三者間於燒結過程中反應生成之第二相的種類和含量而改變，TiO2添加量較多且經較高溫度燒結之樣品表現出較低的硬度值；破壞韌性則在TiO2添加後獲得提升，主要導因於較大的氧化鋁晶粒加上破裂模式轉變為完全延晶破裂所引發的裂縫偏折韌化。
在另一以高純度α-氧化鋁微粉作為起始原料、並以TiO2和MgO作為添加劑的研究中則發現，MgO添加於一含TiO2之氧化鋁系統時，兩種離子同時固溶於氧化鋁中可能比其分別單一添加固溶較為容易。在1 wt% TiO2-0.4 wt% MgO的添加量配比下，其燒結機制相對於純氧化鋁的晶界擴散/體擴散共存，將轉變為體擴散；而晶粒成長速率相對於單一添加TiO2者則表現出減緩的趨勢。燒結末期過程中，TiO2造成的燒結速率增加結合MgO造成的晶粒成長抑制效應，可促成氧化鋁坯體得以在較短的時間內達燒結緻密且晶粒大小僅為0.7 μm。
針對氧化鋁陶瓷體微結構-機械強度的探討，本研究以注漿成形法製備出幾無缺陷的生坯，並燒製出四點抗折強度在750 MPa以上之陶瓷體。注漿成形用漿料之特性是獲得高強度氧化鋁陶瓷體的關鍵，在採用聚電解質PAA-NH4作為分散劑的前提下，其添加量可控制在達飽和吸附以上但還未開始大量產生分子間架橋作用的濃度範圍，此狀態之漿料 可能有助於增加粉體粒子間的黏結，因而有效減少了生坯乾燥過程中產生裂縫、缺陷的機會，促使陶瓷體強度的明顯提升。
除了利用添加劑作為控制微結構發展的方法外，本研究亦評估以二階段燒結法製備微晶粒陶瓷體的可行性。在對含5 wt%氧化鋯之α-氧化鋁微粉進行初始的燒結測試後，本研究提出燒結溫度T1的適當範圍應選取等升溫速率燒結過程中緻密化速率最大的區間；燒結溫度T2的範圍則可依據等溫燒結過程中晶粒成長速率的差異及低溫燒結過程中粉體粒子的外形變化、孔洞演變而界定出上下限。依此設計概念進行二階段燒結試驗後，可製備出相對密度高於99%，晶粒大小介於0.62~ 0.88 μm的氧化鋁陶瓷體；此外，本研究與文獻中純氧化鋁之二階段燒結結果比較後，可發現粉體系統中既存的ZrO2粒子具有減緩第二階段燒結過程中晶粒成長的作用。
藉由對氧化鋁陶瓷製作過程中數種技術的嘗試，此研究達到的成果包含 (1)使用添加劑降低燒結溫度、縮短陶瓷體緻密時間，對高純度、粒徑約200 nm之α-氧化鋁微粉而言，可於1250oC/4 h燒結至相對密度高於99%以上，(2)利用添加劑或燒結參數設計概念，可改變氧化鋁之燒結微結構演變，並燒製出晶粒介於0.5~ 1 μm之緻密陶瓷體，(3)探討添加劑或成形技術運用下氧化鋁陶瓷體的微結構-機械性質間之關係，並提出韌化或強化氧化鋁陶瓷體之方法。

Alumina ceramics is an important and essential material for the industrial applications owing to its excellent mechanical properties. The reduction in the grain size for the alumina ceramics has been recognized to improve not only their mechanical properties further, such as hardness, strength, wear resistance and toughness, but also the transmittance of visible light. However, most of the efforts are undergone by using a simultaneous pressure application during sintering, which always correspond to a high process cost. The goal of this study is to control several important stages in the whole ceramic processes to investigate their influences on the sintering behaviors, microstructure evolutions and some mechanical properties. The controlling fields include the using of additives, the improvement on the forming technologies and the designing of heating parameters for the two-step sintering. Moreover, the object is to prepare the high density ceramic with submicron grain by using pressureless sintering method.
The effect of TiO2 addition (0 ~ 4.0 wt%) on the sintering behavior and mechanical properties of an ultrafine (150 nm) alumina-5 wt% zirconia powder has been investigated. TiO2 is a beneficial additive, resulting in lower sintering temperature and higher sintered density. The grain growth is shown to be enhanced simultaneously after TiO2 addition. At higher amounts of TiO2 addition and at higher sintering temperature, the formation of secondary phases of ZrTiO4 or Al2TiO5, which has lower elastic moduli compared with alumina, reduces the hardness of the sintered bulk from 19.3 to 17.9 GPa. The lager grain size and the transition of fracture mode to fully intergranular due to the TiO2 addition seem to improve the toughness from 4.4 to 5.2 MPa×m1/2.
For an ultrafine alumina powder (200 nm) with high purity, the codoping of MgO and TiO2 seems to present a higher solubility than single doping by individual one. Under the codoping levels of 1 wt% TiO2 plus 0.4 wt% MgO, the sintering mechanism would transfer from the coexisting of grain boundary/volume diffusion to complete volume diffusion as comparing with pure alumina, and the grain growth rate would be suppressed evidently as comparing with TiO2 single doped alumina. The combination effect of the enhanced densification and the inhibited grain growth contributing from TiO2 and MgO doping, respectively, could promote a short required time to get a high density alumina ceramic with grain size of 0.7 μm.
The third investigation aims to prepare an alumina ceramic with high mechanical strength through adjusting the characterization of alumina slurry and forming by slip casting. It was found that the dispersant PAA-NH4 has to been controlled at a critical addition content, which corresponding to higher than the level of saturated adsorption but lower than the level that causes the bridging effect between polymer molecules. The non-adsorption polymer can be regarded as the binder that provides the sufficient binging force between alumina particles and thus reduces the formation of flaws in the green compact during the drying process. An alumina ceramic with high density (~99%), flaws free and homogeneous microstructure is obtained after sintering at 1500oC/8 h, and yields a high four-point bending strength higher than 750 MPa.
Except for the using of sintering additives as mentioned above, this study also investigates the feasibility of two-step sintering process on the fabrication of high density, fine grained alumina-5 wt% zirconia ceramics. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450oC is effective for the first step sintering (T1) due to its highest densification rate. The isothermal sintering is then carried out at 1350 ~ 1400oC (T2) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62 ~ 0.88 μm) is achieved.
This study attains three achievements through improving some technologies for ceramic process, it could be summarized as: (1) Reducing the sintering temperature or shortening the sintering time via applying suitable additives. For the high purity alumina powder with particle size of 200 nm, a full dense (> 99% relative density) ceramic bulk can be got under 1250oC/4h. (2) The microstructure evolutions during sintering are modified via the effort of TiO2/MgO codoping or two-step sintering. Hence, the high density ceramics with average grain size between 0.5~ 1 μm are obtained successfully. (3) Some relationships between mechanical properties and microstructure are investigated after using additives or taking the different forming technologies/parameters. The possible toughening or strengthening mechanisms for the alumina ceramics are also proposed.

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