片狀氧化鋁因為具有高熔點、絕緣性、耐磨性、機械強度以及優異的化學穩定性。且其具有二維的外型，在工業界上已被廣泛的添加到其他材料中當作補強物，形成多功能的複合材料。本研究藉由添加不同莫耳比例的硫酸鉀(K2SO4)於含5mol%Fe3+離子的水鋁石(boehmite)中，經由1000oC熱處理後，可獲得六角片狀α-Al2O3單晶粉末，其長度約為200nm。若再共添加1mol% Ti4+離子則可增加其形狀異方性，片狀晶粒直徑可增加到400nm。隨著添加硫酸鉀莫耳比例的增加，其相轉換路徑由未添加硫酸鉀前的γ→θ→α-Al2O3，改變成γ→α-Al2O3，且其抑制γ-Al2O3晶粒成長的效果也隨之提昇；當硫酸鉀的添加量和boehmite的莫耳比是4:1時，其α-Al2O3的相轉換量只達到83%，顯示硫酸鉀可抑制γ-Al2O3之晶粒成長使其無法達到α-Al2O3相轉換的臨界粒徑。又由添加1mol% Ti4+離子之樣品中發現Ti4+離子傾向偏析在α-Al2O3的(004)面上，使得α-Al2O3產生異向性的成長，進而增加片狀氧化鋁的異方性(anisotropy)。研究中藉由DTA、XRD、TEM、EDS來分析添加硫酸鉀及Ti4+離子對Al2O3相轉換及其外型上的影響，結果發現添加共添加硫酸鉀、Fe3+離子及Ti4+離子不但可以提前γ→α-Al2O3之相轉換，也可以使α-氧化鋁之外型由原本的蠕蟲狀改變成薄片狀；而片狀氧化鋁的成長機構主要是由針狀α-Al2O3以硫酸鉀為模板，藉由粒子的重排而生成片狀氧化鋁。

Alumina exhibits superior properties, such as high melting point, wear resistance, thermal as well as electrical insulation, perfect mechanical strength and chemical stability. Therefore, flaky alumina powders are often used as reinforcements in various materials to form multi-functional composites because of its two-dimensional shape. In this study different amount of potassium sulfate were added into boehmite with 5mol%Fe3+ ions to prepare a single-crystal α- Al2O3 hexagonal flake with a diameter of about 200 nm after calcining at 1000℃. Codoping 1mol% Ti4+ ions could promote the shape anisotropy and the diameter growth to about 400 nm. Without the addition of potassium sulfate, transformation of boehmite to α-Al2O3 is via the γ→θ→δ→α-Al2O3 path. However the transformation path was changed to γ→α-Al2O3 with the addition of potassium sulfate. Only achieved 83% of α-Al2O3 phase transformation was obtained as the molar ratio of potassium sulfate to boehmite achieved four. Potassium sulfate suppressed γ-Al2O3 coarsening which hence inhibited γ→α-Al2O3 phase transformation and inhibited the γ-Al2O3 to reach the critical nuclei size of α-Al2O3. In addition, it was found that Ti4+ ions had a strong tendency to segregate onto the (004) phase of α-Al2O3,which enhanced α-Al2O3 anisotropic growth and increased the shape anisotropy of flaky α-Al2O3. The effect of the addition of potassium sulfate and Ti4+ ions on the Al2O3 phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), Energy Dispersive Spectrometer (EDS). It was found that the addition of potassium sulfate, Fe3+ ions and Ti4+ ions could effectively prevent from the occurrence of α-Al2O3 vermicular growth and promoted phase transformation of intermediate γ-Al2O3 to α-Al2O3, which helped the formation of flaky single-crystal α-Al2O3 growth mechanism of flaky α-Al2O3 was via the oriented attachment of needle-like α-Al2O3 using of potassium sulfate as a template.

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