液态Al氧化的环境透射电镜观察

OBSERVATION ON THE OXIDATION OF LIQUID Al BY ENVIRONMENTAL TRANSMISSION ELECTRON MICROSCOPE

为研究液态Al初期氧化产物的相组成及其氧化机理,采用环境透射电镜(E TEM)对纳米Al粉末在720℃时的氧化进行了原位观察.差热分析(DTA)和TEM形貌观察结果表明,纳米Al粉末表面存在5—6nm厚的非晶Al_2O_3膜,该非晶膜在350—550℃间转变为γ-Al_2O_3.纳米Al粉末在10^(-4) Pa高真空下熔化后冷却,可以得到无氧化膜的固态Al.再次熔化后的Al在720℃,10^(-2) Pa氧分压下氧化,直接形成棒状的纳米αAl_2O_3.同时,XRD检测结果表明,熔融的块状金属Al在720℃,10^(-2)Pa氧分压下氧化,也得到αAl_2O_3.液态Al的这种特殊氧化结果与其液态结构以及液态Al蒸发成气体再氧化有关.

The study of liquid metal oxidation is of particular importance since some liquid metals are highly reactive with oxygen even under ultrahigh vacuum. The investigation of the oxidation behavior of liquid aluminum is strongly demanded due to its wide applications but relatively low melting point. However, it is still ambiguous about the products at the initial oxidation stage of liquid aluminum through the researches done in the past several decades. In order to investigate the initial oxidation products together with the oxidation mechanism of liquid aluminum, in-situ oxidation of nano aluminum powder was carried out at 720℃ in the environmental TEM （E-TEM）. Prior to the oxidation, the nano aluminum powder was visualized and analyzed by E TEM and DTA respectively. The results show that there is an amorphous A1203 film of 5-6 nm thick on the surface of the aluminum powder, which transforms to γ-Al2O3 at a temperature range of 350-550℃. By melting the nano powder to liquid followed by cooling under a total pressure of 10^-4 Pa in E-TEM, oxide free solid aluminum is obtained. Re-melting of the solid aluminum at 720℃ yields oxide free liquid aluminum. After oxidation of the oxide-free liquid aluminum under an oxygen partial pressure of 10-2 Pa in E-TEM, α-Al2O3 nano wires appear on the liquid aluminum surface, demonstrating that the initialoxidation product of liquid aluminum is α-Al2O3. Moreover, the oxidation of liquid aluminum pre- pared from oxide-free bulk aluminum at 720℃ under an oxygen partial pressure of 10^-2 Pa was also studied. The XRD analysis of the products confirms that the oxide is constituted of single α-Al2O3. Additionally, this particular oxidation result of liquid aluminum is attributed to its liquid structure and vapor oxidation process.