How to made Aluninum Powder
Theory of Aluminium Powder
Because of its extreme high affinity for oxygen, aluminium is always covered by an oxide. The thickness of the oxide is dependent on the temperature at which it formed and the atmosphere in which it is stored, particularly the humidity. Aluminium powders produced by air-atomisation have typical oxygen contents of about 0.3 wt.% or 0.3–0.6 wt.%. Fresh oxide on bulk aluminium at room temperature is widely reported as being 10–20 nm thick. The thickness on atomised powder can vary from 50 to 150 nm. The variation in the thickness is partially attributed to the method used for the measurement. For example, the oxide thickness was reported to be about 40–60 nm according to the depth profiles of Al and O collected from an Auger analysis of air-atomised Al-4Fe-4Ni- 1.1Si-(0–1.0)Mg powders. The thickness determined by other means was reported to be in the range of 5–15 nm Binary aluminium powders produced by inert gas-atomisation, such as argon, however, have a much thinner oxide film, 0.7–2.04 nm.
The oxide on aluminium is usually amorphous and hydrated with an adsorbed water layer. The crystalline structure of the oxide film is dependent on the crystallographic face on which the oxide has nucleated and also on the thickness of the oxide film. Thermodynamic analyses showed that the energy of the Al-Al2O3 interface stabilizes the amorphous Al2O3 only up to a few nanometres, e.g., 4 nm on {110} Al and 2 nm on {100} Al at room temperature; and 7 nm on {110} Al and 3 nm on {100} Al at 600°C. As a consequence, inert gas-atomised aluminium powder is thus more likely than air-atomised powder to be covered by an amorphous oxide layer.
Because of hygroscopic characteristics of the oxide film, the surface of air atomised powder normally consists of a system of the Al-Al2O3·nH2O type, which includes both hydrated aluminium oxide, Al(OH)3, and physically adsorbed water. Air-atomised aluminium powders pre-alloyed with magnesium, lithium and zinc are more hygroscopic than pure aluminium powders. Hydration can accelerate further oxidation of the powder, as the remaining oxide layer becomes thinner. On heating, dehydration usually starts from approximately 150°C and is complete at a temperature slightly above 500°C, leaving crystalline γ-Al2O3.
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