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  • 學位論文

瞬閃燒結機制探討及其於牙瓷製備之應用

Flash Sintering Mechanism Exploration and application to Dental Utilization

指導教授 : 陳洵毅

摘要


陶瓷工業發展至今,除了早期的陶瓷藝品,近年精密陶瓷根據其不同功能性更是廣為運用在諸多領域之中。因此,相較於傳統燒結製備陶瓷的方式,許多快速燒結陶瓷方式被提出,像是微波燒結法以及電漿燒結法。但這些方法受限於材料種類或是設備結構過於複雜,未獲廣泛應用;直到2010瞬閃燒結首度被提出後,透過單純外部電場輔助,可大幅降低燒結溫度與時間,被視為快速燒結中具備量產陶瓷之潛能。 瞬閃燒結大致上可分為主要的三階段,第一階段為固定壓時期,此時只有非常微小的電流通過樣品,同時也被稱為孵育期,用以引發瞬閃燒結的時期。接著依照不同材料,直到數秒鐘至數分鐘不等,電流突然急遽增加達到預設值的上限,此瞬間亦會放出亮光,稱為第二階段。在這短時間內,樣品也會瞬間近乎燒結完成達到緻密化。而後電源輸出轉為定電流模式,維持電源的穩定輸出,完成瞬閃燒結。 由於瞬閃燒結可以大幅降低燒結溫度與時間,能避免相較於傳統燒結中高溫使擴散加速的現象,而導致晶粒成長顯著。因此,瞬閃燒結可製備出具有較小粒徑尺寸且緻密化的陶瓷。在牙瓷材料中目前大多採用氧化鋯,但人體口腔內潮濕環境使得氧化鋯低溫裂化成為主要問題。本研究使用瞬閃燒結法製備摻有氧化釔穩定之氧化鋯(YSZ)用於牙瓷,抑制形成過大的晶粒,同時增強其機械強度。另外,本研究利用瞬閃燒結製備之YSZ成品進行老化檢測,確認相變化未發生,有效避免低溫裂化情形,預期增長使用壽命。 瞬閃燒結提出至今,諸多研究希望藉由釐清其燒結機制,以利此技術後續推廣成熟。目前主要分為三大解釋,第一是來自電場輸出的焦耳熱使得樣品本身溫度較高,得以達到快速擴散結果,但主要發生在二、三階段。而第二種理論是在第一階段時期電場主要用以提高材料缺的濃度,以誘發瞬閃暫態。第三種機制是根據實驗結果發現兩側晶粒尺寸分佈不對稱,認為與電化學還原現象相關,導致還原後的陽離子具有更高的擴散能力,同時氧空位在陰極累積形成孔洞。本研究利用真空環境中進行瞬閃燒結實驗,驗證外部氧氣並非瞬閃燒結必要條件,同時觀察到微小晶粒析出,且確認樣品表面變黑區域為部份還原之氧化鋯。因此,提出瞬閃燒結過程應有電化學氧化還原反應牽涉其中,第一階段為孵育部份還原氧化鋯之時期,待貫穿內部材料後,其高導電性可提供瞬閃燒結進入二三階段之定電流模式。透過部分還原氧化鋯提供瞬閃燒結樣品的導電性後,增強的電場輸出能量得以加速擴散現象導致樣品燒結緻密化。

並列摘要


Apart from artistic ceramics in early stage, advanced ceramics has been widely used in different applications according to their functions along with development of ceramics industry. Thus, comparing to conventional sintering method of ceramics, lots of rapid sintering methods had been proposed like microwave sintering and spark plasma sintering. However, some development of these rapid sintering methods were limited by categories of materials or complicated sintering structure. Until flash sintering was first reported in 2012, it had potential for commercial manufacturing ceramics among these rapid sintering with lowering sintering temperature and sintering time by only assisting external electric field. There are three stages during flash sintering. Stage I is for constant voltage period with only tiny current through specimen, and also called as incubation period to induce flash event. Then depending on different materials, after seconds to several minutes, current would abruptly surge to the pre-setting upper limit with flash event and these called stage II. In this short period, specimen was almost densified instantaneously. Then, output of electric field would turn to constant current mode and maintain the stable dissipation, and finally finish the flash sintering. Due to lower sintering temperature and shorter sintering time, specimen would be densified with smaller grain size, comparing to conventional sintering in higher sintering temperature resulted in higher diffusion rate and grain growth rate. Most dental material use zirconia. However, low temperature degradation is the main issue for zirconia in the human mouth of humid environment. In this work, we manufacture YSZ for dental application by flash sintering to prevent larger grain size occurred and increase mechanical strength simultaneously. In addition, the specimen under flash sintering method would be conducted with aging test to prevent from low temperature degradation without phase transformation and this specimen would be expected using for longer life. There were lots of literatures worked on mechanism of flash sintering from it was first reported and expect to commercialize flash sintering method in the future. So far, there were three main explanation in flash sintering. One is Joule heating from external electric field output and higher diffusion rate was resulted from higher temperature of specimen. But Joule heating was only attributed during stage II and III. Another explanation is higher concentration of extrinsic defect was nucleated by electric field and then it could induce transient state of flash event. The other explanation is according to experience results of asymmetric grain size distribution between two electrodes. This phenomenon was considered to electrochemical reduction related and resulted in higher diffusion rate of reduced cations and pores produced by accumulation of oxygen vacancies. In this work, we conducted flash sintering in vacuum environment and showed external oxygen is not necessary during flash sintering. In addition, we find some small grains were precipitated and blackening phenomenon on the sample surface of partial reduction zirconia. Therefore, we consider electrochemical redox reaction was involved in flash sintering. Stage one is for incubation of partial reduction zirconia. Until partial reduction zirconia penetrated the core of specimen, its high conductivity could offer a path for steady current during stage II and III in flash sintering. Through high conductivity of specimen, larger output energy of electric field could densified specimen rapidly with higher diffusion rate.

參考文獻


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ASTM. Standard Specification for High PurityDense Yttria Tetragonal Zirconium Oxide polycrystal(Y-TZP) for surgical Application. Annual Book forASTM Standard, F1873-98 ASTM
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