硫酸鈣的溶解對冷燒結製程之影響研究

Abstract

冷燒結製程(CSP)在近年來已經成為陶瓷方面的熱門議題，這個製程不僅可以降低製程溫度，同時也為陶瓷與高分子複合材料的製備方法提供一個契機。然而，這個技術尚未被應用在含有結晶水的材料系統，像是硫酸鈣-水的系統(CaSO4-H2O)，而這種系統的緻密化機制可能會有別於其他陶瓷系統，因此，這篇論文的主要目的就是探討硫酸鈣系統在室溫下進行冷燒結製程的細節。 實驗的起始材料為半水硫酸鈣(CaSO4．1/2 H2O)粉末，並運用不同的熱處理溫度來製備帶不同結晶水的硫酸鈣相。在這篇論文中，冷燒結製程的進行是在室溫下將粉末經由一單軸壓力加壓，並添加特定比例的液體，此外，探討的製程變因包含壓力、液體的種類與粉末的起始相。最後，實驗結果顯示離子的過飽和度是此製程的關鍵因素，並輔以微結構觀察與機械性質測量證明。 實驗結果說明半水硫酸鈣能經由此論文中的冷燒結製程在室溫下被有效緻密化，緻密化後的相主要為二水硫酸鈣；如果製程中沒有施加壓力，樣品的撓曲強度較低、晶粒尺寸也較大；將液體的種類從去離子水換成磷酸緩衝溶液說明粉末的溶解比析出物的形成種類重要。而在無水硫酸鈣二(CaSO4 II)作為起始相的情況下，冷燒結製程並無法將之緻密化，藉由溶解度測試說明，不僅過飽和度沒有達到析出物生成的條件，相較於半水硫酸鈣為起始相的情況，微結構觀察也沒有顯示出明顯的形貌變化，僅有一些微小孔洞生成於粉末表面。然而，由80 wt%的無水硫酸鈣二的20 wt%半水硫酸鈣組成的混合物卻能經由冷燒結製程有效燒結，最主要的關鍵在於半水硫酸鈣的溶解能析出二水硫酸鈣在無水硫酸鈣二的顆粒表面作為一保護層，此保護層能以較快速率形成，因此能使半水硫酸鈣能完全轉換為二水硫酸鈣，形成更均勻且細緻的結構，也讓混合物製成的冷燒結產品與純粹由半水硫酸鈣製成的冷燒結產品擁有相似的撓曲強度。

Cold Sintering Process (CSP) has become one of hot issues in recent years. The process could not only lower the process temperature but also exhibit potential for the preparation of polymer-ceramics composites. However, this technique has not been applied to crystalline-water containing materials such as CaSO4-H2O system, whose densification mechanism may differ from other ceramics. Hence, the primary purpose of this study is to explore the process details on CSP of CaSO4-H2O system at room temperature. The starting powder is CaSO4．1/2 H2O. Various heat treatments were used to prepare different phases for calcium sulfate. In the present study, CSP is proceeded through exerting uniaxial pressure on powders with the addition of a small amount of transient liquid at room temperature. The pressure, transient liquid and starting phase of powder are taken into consideration in this study. After all, the results show that the supersaturation of ions plays a determining role in this process. Microstructure observation and mechanical properties measurement give evidence to prove it. Experimental results show that CaSO4．1/2 H2O is able to effectively densify through CSP at room temperature. The resulting phase after densification is mainly CaSO4．2 H2O. Flexural strength shows a lower value; and a larger grain size was observed when pressure is not applied. To replace deionized water with phosphate buffer solution shows an enhancement on dissolution of powder than the formation of precipitate. While in the case of CaSO4 II, it is not densified through CSP in this study. The supersaturation from dissolution test did not attain the boundary condition of precipitation. Microstructure observation detects no obvious morphology change compared to CaSO4．1/2 H2O dissolution. Only some micro-pores form on the powder surface of CaSO4 II. However, a mixture of 80 wt% CaSO4 II and 20 wt% CaSO4．1/2 H2O could be effectively sintered in this study. The major key parameter is the dissolution of CaSO4．1/2 H2O and the precipitation of CaSO4．2 H2O on the surface of CaSO4 II phase as a coating. The precipitation would take place fast to protect CaSO4 II phase from contacting to liquid. Hence, it makes CaSO4．1/2 H2O completely transformed to CaSO4．2 H2O. As a result, a more uniform and finer structure is formed. The flexural strength of CSP products made from powder mixture is about the same as those from all CaSO4．1/2 H2O.