Atmospheric-pressure plasma oxidation of aluminum – mechanism and elementary film properties – “AppoxAl”

铝及其合金在日常产品中发挥着巨大且不断增加的作用，这是由于它们对氧化和腐蚀的动力学阻力--尽管在环境条件下形成氧化物时存在热力学上的不稳定性。铝(合金)的动力学稳定性是由于自然形成的几纳米厚的氧化膜，在环境氧分压和低温下自发形成的。然而，对于一系列光学、电气、电化学或汽车应用，使用环保的低成本工艺进一步提高氧化层厚度将是有意义的。大气压等离子体工艺的出现可能为实现这一目标提供了一种可行的解决方案。在拟议的项目中，关于介质阻挡放电的常压等离子体增强氧化铝的知识状态将得到实质性的扩展，这些知识到目前为止开发得很差。应阐明DBD中形成的氧化物种(如氧原子、臭氧或一氧化二氮)所起的作用，以及物理因素(如VUV区域的等离子体发射或与丝状DBD中流光微放电的接触)的影响。为了实现这些目标，将使用实验方法，并结合等离子体或光化学气相动力学的简化模型，允许估计潜在相关物种的平均密度或通量，并将模型结果与观察到的氧化物生长关联起来。为了验证模型，还将使用Xe作为辐射测量仪，将结果与放电流出物中的臭氧或一氧化二氮浓度以及光发射测量结果进行比较。氧化实验将使用三种类型的反应器进行：(1)2D-梯度DBD反应器，允许在单个实验中使用多种温度和氧气浓度组合策略；(2)单丝DBD反应器，允许在铝表面局部定位流光微放电；以及(3)VUV反应器，允许在存在受控气相的情况下照射氧化物表面，其光子能量低于或高于氧化物带隙。实验将在电学、光学和分析放电表征之前或伴随进行。由此产生的氧化膜将常规地根据厚度和成分进行表征。单丝实验将根据厚度分布和氧化物组成进行评估。由于铝是钝化金属的原型，本项目的科学成果对其他钝化金属也具有一定的参考价值。由于DBD工艺的经济和生态优势，与其他制造方法相比，项目结果对铝的应用也具有很大的实用价值。因此，等离子体产生的氧化膜的技术可用性的第一个迹象将通过电气和电化学测试来获得。

The large and ever-increasing role played by aluminum and its alloys in everyday products is made possible by their kinetic resistance against oxidation and corrosion - in spite of a thermodynamic instability with respect to oxide formation under ambient conditions. The kinetic stability of aluminum (alloys) is due to a native, a few nanometer thick oxide film, formed spontaneously at ambient oxygen partial pressures and low temperatures. For a range of optical, electrical, electrochemical or automotive applications, however, it would be of interest to further enhance the oxide thickness using an eco-friendly low-cost process. The advent of atmospheric-pressure plasma processes may offer a viable solution to achieve this goal. In the proposed project the state of knowledge regarding atmospheric-pressure plasma-enhanced oxidation of aluminum by dielectric barrier discharges, poorly developed so far, shall be expanded substantially. Roles played by oxidizing species formed in DBDs such as oxygen atoms, ozone, or nitrous oxide as well as effects of physical factors like the plasma emission in the VUV region or the contact with streamer microdischarges in filamented DBDs shall be elucidated.In order to achieve these objectives an experimental approach will be used, along with a simplified modeling of plasma- or photochemical gas-phase kinetics, allowing to estimate the average densities or fluxes of potentially relevant species and to correlate model results with observed oxide growth. To validate the models, the results will also be compared with ozone or nitrous oxide concentrations in effluents from the discharges and with optical emission measurements, using Xe as an actinometer.Oxidation experiments will be conducted with three types of reactors: (1) a 2D-gradient DBD reactor allowing a combinatorial strategy using multiple combinations of temperature and oxygen concentrations in a single experiment, (2) a single-filament DBD reactor allowing to localize streamer microdischarges on the Al surface, and (3) a VUV reactor enabling irradiation of the oxide surface in the presence of controlled gas phases with photon energies below or above the oxide band gap. The experiments will be preceded or accompanied by electrical, optical, and analytical discharge characterization. The resulting oxide films will routinely be characterized with respect to thickness and composition. Single-filament experiments will be evaluated with respect to thickness distribution and oxide composition. As aluminum is a prototype passivating metal, scientific results obtained in this project are of interest for other passivating metals, too. Due to economical and ecological advantages of DBD processes, compared with other manufacturing methods, project results are also of substantial practical interest for applications of Al. Therefore, first indications of the technical usability of plasma-generated oxide films will be obtained by electrical and electrochemical tests.