氧化鋁單晶強化機制及在其表面生長奈微米鎂鋁尖晶石之研究

The research of strengthening mechanisms of sapphire crystal and the growth of Mg-Al spinel nano-micro crystals on sapphire surface

劉哲銘(Che-Ming Liu)

指導教授 ： 陳志臣

國立中央大學/工學院/機械工程學系/博士(2006年)

摘要

氧化鋁單晶（Sapphire）為一高機械強度，高化學穩定性及高穿透率之氧化物晶體，因此目前經常被使用在紅外線光窗上，為了增加光窗的強度，使其有更寬廣的應用範圍，因此Sapphire的強化工作將變的相當重要。在本研究中利用了多項強化方式來針對Sapphire進行強化的工作，包括了有固溶強化（dopant）、析出強化（precipitation hardening）、熱處理（heat treatment）、預壓應力膜（compressive coatings），其中固溶強化的部分是利用雷射加熱提拉法（laser-heated pedestal growth, LHPG)來進行不同摻雜量的Mg：Sapphire晶纖生長，以瞭解在不同雜質濃度時對晶體強度之影響，同時也對Sapphire晶體內Mg雜質的偏析現象進行深入的探討。在其他三項強化機制上則是利用外購之一英吋Sapphire單晶片來進行研究，並利用Ring-on-ring的雙軸向破壞測試，獲得晶體之破壞模數（modulus of rupture, MOR）強度。其中在析出強化的實驗上，我們是藉由在Sapphire晶片上Mg金屬膜的鍍製、高溫擴散、時效處理等程序後，在Sapphire單晶中產生Spinel析出物來強化Sapphire強度，並能有效降低晶體的強度穩定性至10%以下，並探討強化的機制。同時也藉由不同的熱處理條件，瞭解熱處理對Sapphire晶體強度之影響，更成功的增加晶體強度達90%。最後配合氮化矽之預壓應力薄膜之鍍製，能更有效的增加Sapphire晶體的強度達一倍以上。
另外本研究的另一個重點是提出一種新的微奈米晶體生長法，主要是利用異質成核的觀念及固溶析出的方法在Sapphire 表面生長Mg-Al Spinel晶體。所生長之Spinel型態包括了微米晶體、跟奈米晶體兩種尺寸。其中Spinel微晶是利用鍍有Mg金屬膜之C軸Sapphire單晶，經過高溫擴散處理後在Sapphire表面之蝕刻坑所形成的。同時透過Sapphire基板平面上方向(in–plane orientation)的量測與表面析出結構的觀察，可以獲得Spine微晶與Sapphire基板間離軸 (off-axis)方向上的關係。另外本研究也在Sapphire表面上之階梯結構上成功的生長出奈米尺寸之Spinel晶體。並探討階梯(terrace-and-step)結構對析出晶體之影響。同時利用所觀察到Sapphire表面之階梯結構受到Spinel析出物之Pining 現象，來進一步證實晶體表面階梯移動之現象。

關鍵字

固溶析出 ； 微米晶體 ； 奈米晶體 ； 強化 ； 鎂鋁尖晶石 ； 氧化鋁單晶

並列摘要

In this study, annealing, precipitation-strengthening, compressive coating and dopant processes were used to enhance the strength of sapphire crystal. In terms of the annealing process, the biaxial strength of the specimen was larger for a higher annealing temperature. After an Mg-sputtered sapphire crystal substrate went through a precipitation-strengthening process, Mg-Al spinel precipitation would occur. Sapphire that had undergone a precipitation-strengthening process had the best strength reliability in comparison with other strengthening processes. After the compressive coating process, the crystallized silicon nitride layer made the sapphire approximately twice as strong as an uncoated sample. The results show that a crystallized silicon nitride layer produces the greatest strength, with regard to the other strengthening processes. Beside, we grew Mg doped sapphire crystal fibers doped with various Mg concentrations from 0.5 mol% to 4 mol%, by the laser-heated pedestal growth method. It was easy to grow defect-free 0.5 mol% Mg-doped sapphire crystal fibers with lengths longer than 50mm, however, the length of the defect-free region decreased as the amount of MgO doping increased. The growth rates also influenced the quality of the fibers. We discuss the influence of the thermocapillary convection and the growth rate on the Mg distribution, and the reason for the formation of defects by constitutional supercooling is described. By means of thermomechanical analyzer measurements, we suggest that the strength of the sapphire can be enhanced by MgO doping, due to the increasing Young’s Modulus.

The Mg-Al spinel microcrystals and nanocrystals were successfully grown by the solution-precipitation process at the c-axial sapphire single crystal surface. The proposed innovative growth concept used the etch pits or the atomic steps as heterogeneous nucleation points for the growth process. Once Mg ions diffused into the sapphire crystal, the spinel crystals could be precipitated by quenching and aging treatment at etch pits or atomic steps. We found that the precipitated crystals were (111) Mg-Al spinels with a triangular pyramidal shape on etch pits. The results of X-Ray Diffraction analysis indicate that the in–plane orientation of the spinel crystal has particular crystallographic directions. And the precipitated crystals were Mg-Al spinels with a circular pyramidal shape on (along) atomic step. And the spinel which would pin of the moving step could obviously be observed by atomic force microscopy (AFM).

並列關鍵字

Sapphire ； Spinel ； Microcrystal ； solution-precipitation ； Nanocrystal ； Strengthening

參考文獻

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