The Evolution of Aluminum Hydroxides During the AlN Powder Hydrolysis

author: Andraž Kocjan, Odsek za inženirsko keramiko, Institut "Jožef Stefan"
published: Feb. 12, 2008,   recorded: October 2007,   views: 1672
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Description

The reaction of aluminum nitride (AlN) powder with water has been known for a long time. In the presence of water AlN will decompose, forming aluminum hydroxide and ammonia: AlN + 3H2O → Al(OH)3 + NH3

Bowen et al.1 proposed more detailed reaction scheme for the reaction of AlN powder with water at room temperature (RT): AlN + 2H2O → AlOOH(amorph) + NH3 NH3 + H2O ↔ NH4+ + OH- AlOOH(amorph) + H2O → Al(OH)3(xstal) According to Bowen et al.1, AlN powder first reacts with water to form amorphous aluminum hydroxide (pseudoboehmite, AlOOH) recryctallizing to bayerite (Al(OH)3) with time. The kinetics of AlN hydrolysis was described using an unreacted-core model and the chemical reaction at the product-layer/unreacted-core interface was proposed to be the rate-controlling step. It is anticipated that the dissolution-recrystallization process during AlN hydrolysis is very similar to that of the crystallization of aluminum hydroxide gels, where pseudoboehmite forms from fresh, highly hydrated, amorphous hydroxide. In the present work the influence of hydration temperature, from RT up to 90 °C, and ageing time, from 10 minutes to 24 hours, on the formation of crystalline products found after the AlN powder hydrolysis was investigated. The AlN hydrolysis behaviour was observed by measuring the pH of the suspension, whereas for the characterization of the reaction products XRD, SEM and TEM analyses were employed. After a short incubation time (< 0.27 to 17 minutes), which was found to be temperature dependant, the hydrolysis reaction started accompanied with the increase in pH and temperature. Higher starting temperatures also increase the reaction rate. The starting temperature and especially the ageing time (time of hydrolysis) strongly influence the reaction products and their morphology. It was confirmed that at RT the main crystalline reaction product is bayerite (Al(OH)3), regardless on ageing time in the mother liquor. It appears in the form of large somatoids. At elevated temperatures the first crystalline product is nanostructured boehmite (AlOOH), also exhibiting a high specific surface area. With prolonged ageing the bayerite conversion takes place with dissolution of pseudoboehmite and recrystallization of bayerite. After 24 hours of ageing in the temperature range from 40 °C to 70 °C bayerite became the predominant phase. At higher temperatures, e.g., at 80 °C and at 90 °C, both phases are present, but after 24 hours of ageing boehmite remains the predominant phase (Fig 1). Based on these results, an extension of Bowen et al’s1 model, for the AlN powder hydrolysis at RT to elevated temperatures is proposed. It is anticipated that at any temperature the very first solid reaction product formed on the surface of the AlN particles is nanostructured pseudoboehmite. Once formed, part of the pseudoboehmite will be transformed to crystalline boehmite, while the other part will further react with water to form bayerite, according to the reaction scheme: AlOOH(amorph) → AlOOH(xstal) AlOOH(amorph) + H2O → Al(OH)3(xstal)

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