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  • 學位論文

主導曲線模型運用在奈米氧化鋁和奈米二氧化鈦陶瓷粉末燒結之研究

Using Master Curve on the Sintering of Nanocrystalline Alumina and Titania Ceramic Powders

指導教授 : 鄧茂華

摘要


「主導曲線模型」為本人所屬研究團隊從一般動力學反應方程式所推導而得的一個數學模型,運用在數個領域,包括結晶動力學和化學反應動力學上,均能成它a描述反應中的過程變化。已完成之初期研究成果亦證實此模型能夠用來描述與預測微米級及次微米級陶瓷粉末之燒結緻密化過程。但是否能用來描述奈米陶瓷粉末之燒結,則尚未經過實驗工作檢驗。此外,由於奈米陶瓷粉末之燒結所牽涉到的機制遠較傳統陶瓷粉末(指微米級及次微米級陶瓷粉末)燒結複雜,釵h修改自傳統陶瓷燒結的模型均不能普適於奈米陶瓷燒結,因此若本研究工作能以實驗證明主導曲線模型可以用來定量描述奈米陶瓷燒結緻密化的過程,則不僅在奈米陶瓷之燒結實務上有所貢獻,更在基礎科學上有重要的突破。 本實驗共採用五種奈米陶瓷粉末作為測試材料,分別為:平均粒徑約50 nm的氧化鋁(ㄛ菗陞D)、約15 nm的氧化鋁(γ相)、約35 nm的二氧化鈦(rutile相為主)、約10 nm的二氧化鈦(anatase相)和約45 nm的二氧化鈦(anatase相為主)粉末。實驗設計以等升溫速率進行常壓燒結,燒結體則以阿基米得法量測其密度,並用XRD和SEM分析晶相變化與觀測其微構造。 由於奈米粒徑效應,欲燒結之奈米級初始粉末常常包含有亞穩定相的成分在內,則燒結過程中常會發生相變化。本實驗的分析結果可分為兩部分:(一)燒結過程中僅發生小部分相轉變,此種奈米陶瓷燒結緻密化行為具有清楚的主導曲線關係,其緻密化行為和升溫速率無關;(二)燒結過程中發生大量地相轉變,此種奈米陶瓷燒結會出現二階段的緻密化行為:第一階段應為相變所造成的體積收縮和擴散作用所造成的孔隙移除之緻密化結果,此階段的實驗數據無法擬合出一條唯一曲線,至於相變後正常燒結的第二階段則具有明確的主導曲線關係。 模型分析擬合所得的視燒結活化能,不論是奈米氧化鋁或奈米二氧化鈦,均高於傳統陶瓷燒結分析所得活化能值,尤其是當所燒結之初始奈米陶瓷粉末粒徑愈小時,燒結所得之活化能值就愈大。此結果不僅表示奈米粒徑陶瓷粉末燒結緻密化之不容易,也表示無法單純以傳統陶瓷燒結機制(如:晶界擴散或體擴散機制等)來解釋其燒結緻密化行為。至於此偏高之燒結視活化能值,推測是因為奈米陶瓷在燒結初期表面擴散作用耗掉了其燒結驅動能,使晶粒變形,黏成棒狀或長柱狀,導致不利於後續燒結之緻密化,因而提高了視活化能值。

並列摘要


“Master Curve Model” was originally derived from the general equation of kinetic reactions by our group. It can be used in several fields, including crystallization kinetics and chemical kinetics, to adequately describe the variations of each reaction systems. It has also been proved that the model can describe and predict the densification behaviors of micron- and submicron-sized ceramic sintering in our preliminary study earlier. But there are no any experimental evidence yet whether it can be used in the sintering of nanocrystalline ceramic powders. Furthermore, the sintering of nanocrystalline ceramic powders involves more complicated mechanisms than the sintering of conventional ceramic powders, so the conventional sintering models cannot be applied to the sintering of nanocrystalline ceramic powders. To date no practical model has been developed to describe the sintering of nanoceramics. This study explores the feasibility of using the master curve model to quantitatively describe the densification behaviors of nanocrystalline ceramic powders in sintering. We believe the results will contribute not only to the sintering practice in industry but also to the understanding of the basic science of sintering. Five powders were used in this study. They are ?Al2O3 (average diameter~50 nm)、γ-Al2O3 (~15 nm)、rutile-TiO2 (~35 nm)、anatase-TiO2 (~10 nm) and Degussa P25-TiO2 (~45 nm). The sintering is conducted in air at designed heating paths. The densities of the sintered specimens are measured by the Archimedes method. The phases and microstructure of the samples are determined by XRD and SEM, respectively. Because the particle size effect, the nanocrystalline ceramic powders usually contain metastable phases. Therefore, phase transformation may occur during sintering. I summarize the experimental results into two parts: (1) Only a small fraction of the powders experiences phase transformation during the sintering. In this case, the densification behaviors of the powders show a clear master curve relationship, which is independent of heating path. (2) A large fraction of the powders experiences phase transformation during the sintering. That is when an unmistakable two-stage densification behavior is observed. The 1st stage is the results of both the volume shrinkage due to the phase transformation and the elimination of porosity during the early period of the sintering. The data of this stage cannot be merged into a single curve. The 2nd stage is the results of the sintering after the phase transformation has been completed, and again we observe a clear master curve relationship. The values of the apparent sintering activation energy, which were derived from both the alumina and the titania, are greater than that of the conventional ceramic powders. In particular, we find that the starting powders with a smaller size will give a relatively greater value. The relationship indicates not only the complexity in nanosintering, but also that we cannon use the conventional sintering mechanisms to explain the densification behaviors of nanoceramics. The higher values of the apparent sintering activation energy might due to the predominant surface diffusion during the early stage sintering of nanoceramics. The surface diffusion quickly consumes the sintering driving forces by the shape-change of the grains (into the shape of rod or column), which in term makes the densification process harder in the later sintering stage and effectively makes the sintering apparent activation energies higher than their conventional counterparts.

參考文獻


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被引用紀錄


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