Study on Two-Step Sintering of α-Al2O3

摘要 本研究主要是利用兩批不同粒徑的α-Al2O3 (slurry A-0.2 μm 及slurry B-0.1μm)，進行不同的燒結熱處理方式，並對其燒結結果進行研究。所嘗試的燒結方式包括兩階段燒結，升溫速率的改變，以及傳統升溫步驟。藉由不同參數的設計，包含改變第一階段溫度，第二階段持溫溫度，升溫速率...等，來進行微結構，燒結密度，晶粒大小的觀察與分析。此外，TMA 被用來分析在不同升溫速率下（2 到 90 oC/min），上述 α-Al2O3 的緻密化行為。而紅外線快速加熱爐亦被用來研究氧化鋁對於紅外線之吸收效應。 就兩階段燒結的結果來說，比起傳統單一步驟燒結，slurry A 可獲得較高的燒結密度以及小的粒徑(0.86μm)，此燒結條件10oC/min，升溫到 1500 oC，爐冷後，溫度降到1300oC，持溫20 個小時。另外一個樣品，升溫到1450 oC 並於1300 持溫20 小時，所得到的燒結密度約98％TD，晶粒尺寸約0.57μm。 不同升溫速率確實對燒結行為有所影響，slurry B 的樣品隨著加熱速率增加，其密度跟著變小，但是晶粒卻無明顯差異。結果顯示，當加熱速度增加，初始燒結溫度增加; 最大收縮速率所對應的溫度也是隨之增加。利用Young 與Cutler [36]等人的分析法快速升溫的燒結機制為體擴散，活化能值為688kJ/mole，非常接近鋁離子在多晶的擴散。Abstract In this research, sintering behavior of two α-Al2O3 powders with average particle size 0.1μm (Slurry B) and 0.2 μm (Slurry A) were investigated. Several heating processes were tried. One followed a two-step sintering method given by Chen and Wang [1], The effects of temperature, holding time, initial grain size and porosity were controlled. Various thermal treatments were also done comparing to the conventional (one-step) process. Secondly, TMA was used to analyze the sintering behaviors of the α-Al2O3 at various heating rates, from 2 to 90 oC/min. Finally, an infra-red fast sintering furnace was designed and used to study the heating effect on the fine α- Al2O3. A high density (>99% T.D.) and small average grain size (0.86 μm) were achieved by the two-step sintering of Al2O3 and better than the results by the conventional sintering (1500 oC for 1 h). The other case, sintered Al2O3 – B samples, in a smaller grain size (0.57μm) and a good density (>98% T.D.) were achieved by heating to 1450 oC, then holding at 1350 oC for 20 h. Lower first-step temperature and second-step temperature than the temperatures of sample – A were noted. Different sintering rates indeed influence the sintered densities, e.g. the results of Al2O3 – B, but no influence on the average grain size. The results showed, while the sintering rate increased from 2 oC /min to 90 oC /min, the sintered density decreased from full density to 97%TD if heating to 1500oC then holding for 1 h. For both cases, the initial sintering temperature increased with the heating rate, due to the limitation of heat transfer in Al2O3 samples. Maximum shrinkage rate and the corresponding temperature increased with the heating rate, too. The sintering mechanism of rapid-heating rate has been determined by Young and Cutler [36]. The mechanism of initial sintering belongs to a lattice diffusion, and the activation energy is 688 kJ/mole, which is interpreted as a volume diffusion of Al3+ ions in polycrystalline matrix.Content Chapter 1 Introduction…………………………………………….1 Chapter 2 Literature Review…………………………………….4 2.1 Introduction to alumina……………………………………..4 2.1.1 Characteristics and applications ..…………4 2.1.2 Process-Microstructure-Properties…………..5 2.2 Densification of alumina ……………..…………...9 2.2.1 Methods of fast sintering……………………..9 2.2.2 Fast sintering……………………………………14 2.2.3 Two-step sintering………………………………16 2.3 Sintering of Al2O3…………………………………….18 2.3.1 Sintering mechanism……………………..…….18 2.3.2 Gain growth………………………………………………….23 2.3.3 Effect of doping…………………………….….24 2.3.4 Effect of thermal transferring………………25 Chapter 3 Experimental Procedure…………………………….28 3.1 Experimental planning…………………………………28 3.2 Materials…………………………………………………28 3.3 Preparation of specimen…………………….……….28 3.4 Thermal treatment………………………............30 3.5 Characterization………………………..…………….30 Chapter 4 Results and Discussion………………………..…..39 4.1 Two-step sintering………………………………………....39 4.1.1 Green density and thermal analysis……………………39 4.1.2 sintering density and grain size………………………42 4.2 Fast sintering…………………………………………………51 4.2.1 Comparison between die pressed and pressure casting samples….........................................51 4.2.2 Heating rate…………………………………………………59 4.2.3 Infra-red heating………………………………………….65 4.3 Sintering kinetics…………………………………….70 4.3.1 Diffusion path…………………………………..70 4.3.2 Activation energy of densification or grain growth…………………......................73 4.3.3 Thickness effect…………………………………76 Chapter 5 Conclusions……………………………………………….........78 Reference…………………………………………………………….80 Appendix多孔性氧化鋁陶瓷濾材之製程與特性之研究…………..8