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Effect of polymorphic phase transformations in Al_2O_3 film on oxidation kinetics of aluminum powders

机译：Al_2O_3薄膜中多相相变对铝粉氧化动力学的影响

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Thermogravimetry was used to study the oxidation of aluminum powders at elevated temperatures. Aluminum powders of various particle sizes and surface morphologies were heated in oxygen up to 1500℃ at different heating rates. Partially oxidized samples were recovered from selected intermediate temperatures and the oxide phases present were analyzed by X-ray diffraction. The experimental data were related to current information on stabilities and phase changes of Al_2O_3 polymorphs. Aluminum powders were observed to oxidize in four distinct stages in the temperature range from 300 to 1500℃. During stage Ⅰ, from 300 to about 550℃, the thickness of the natural amorphous alumina layer on the particle surface increases. The rate of this process is controlled by the outward diffusion of Al cations. At about 550℃, when the oxide layer thickness exceeds the critical thickness of amorphous alumina of about 4 nm, the oxide transforms into γ-Al_2O_3. The specific volume of γ-Al_2O_3 is less than that of amorphous alumina; therefore, the newly formed γ-Al_2O_3 only partially covers the aluminum surface. The oxidation rate increases rapidly at the onset of stage Ⅱ, but it decreases when the γ-Al_2O_3 layer becomes continuous. During stage Ⅲ oxidation, the γ-Al_2O_3 layer grows and partially transforms into the structurally similar θ-Al_2O_3 polymorph. Finally, oxidation stage Ⅳ is observed after the transition to stable α-Al_2O_3 results in an abrupt reduction of oxidation rate. Qualitative analysis of the rates of oxidation at the different stages enables one to understand the wide range of aluminum ignition temperatures observed for particles of different sizes.
机译：热重法用于研究高温下铝粉的氧化。将不同粒径和表面形貌的铝粉在氧气中以不同的加热速率加热到1500℃。从选定的中间温度中回收部分氧化的样品，并通过X射线衍射分析存在的氧化物相。实验数据与有关Al_2O_3多晶型物的稳定性和相变的最新信息有关。观察到铝粉在300至1500℃的温度范围内四个不同阶段发生氧化。在阶段Ⅰ中，从300℃到约550℃，颗粒表面天然无定形氧化铝层的厚度增加。该过程的速率由Al阳离子的向外扩散控制。在约550℃下，当氧化物层的厚度超过非晶态氧化铝的临界厚度约4nm时，氧化物转变为γ-Al_2O_3。 γ-Al_2O_3的比容小于非晶态氧化铝的比容。因此，新形成的γ-Al_2O_3仅部分覆盖铝表面。氧化速率在阶段Ⅱ开始时迅速增加，但在γ-Al_2O_3层连续时降低。在Ⅲ期氧化过程中，γ-Al_2O_3层生长并部分转变为结构相似的θ-Al_2O_3多晶型物。最后，过渡到稳定的α-Al_2O_3后，氧化阶段Ⅳ突然降低。对不同阶段氧化速率的定性分析使人们能够了解针对不同尺寸颗粒观察到的铝着火温度的范围很广。

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