Hydrogen diffusion in liquid and boiling alumina by laser ablation towards a carbon-free aluminum production

The Hall-Héroult process is the main production process of aluminum, which consumes carbon electrodes to reduce alumina (Al₂O₃) to aluminum, while releasing the by-product carbon-dioxide (CO₂). To overcome this issue and work towards a green transformation, laser reduction of Al₂O₃ has been proposed as a carbon-free aluminum production method. By irradiating a continuous-wave laser onto the Al₂O₃ surface, micro-sized aluminum particles are precipitated on the surface. Furthermore, the use of hydrogen as an ambient gas has shown the appearance of pores beneath to the surface, which furthermore contain aluminum particles. In this study, the correlation between the internal pore structure through hydrogen diffusion and the aluminum production on laser-irradiated Al₂O₃ is investigated. The pore structure is furthermore maximized in terms of volume by rotating the Al₂O₃ target during laser irradiation. Using a hydrogen atmosphere at 0.9 bar, the diffusion of hydrogen depending on the laser fluence and its effect on the formation of pores is examined. The rotating speed of a 20 mm diameter target size is varied between 2 rpm and 4 rpm. The laser power is constant at 1.5 kW with a spot radius of 1.22 mm to 3 mm. Results show a successful increase of the pore volume by a factor of 20 considering the change from an argon to a hydrogen atmosphere under otherwise equal conditions. The abundance of Al particles in the pores is validated through scanning electron microscopy (SEM). The aluminum particle precipitation inside of the pores shows a directional behavior, as the particles are exclusively found at the bottom of the pores. The existence of aluminum particles in the pores concludes an increase of precipitated particles by increasing the pore volume as performed in this study.