Degradation of AlN Powder in Aqueous Environments
- PDF / 2,752,648 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 54 Downloads / 213 Views
reactivity of AlN powder in an aqueous environment was studied by measuring the pH and the temperature during the hydrolysis of the powder at room and elevated temperatures. The influences of the powder concentration and the starting pH of the slurry were also investigated. The results of the measurements at room temperature show that there is an incubation time before the start of the AlN hydrolysis reactions. Once this incubation time is over, the pH and the temperature of the slurry start to increase, indicating the onset of the reactions. A higher starting temperature not only speeds up the reaction of the AlN powder with water, but it also shortens the incubation time. In addition, the starting temperature influences the morphology of the reaction product: at temperatures below 60 °C, the final product of the hydrolysis is crystalline Al(OH)3, whereas at higher temperatures (above 60 °C), crystalline AlOOH is formed. At very low pH values (pH ⳱ 1), the reaction of AlN powder with water is prevented (i.e., the incubation time is very long), whereas in an alkaline environment, the incubation time is approximately the same as in distilled water, but the reaction is accelerated.
I. INTRODUCTION
The reactivity of aluminum nitride powder with water is well known. Until the invention of the cheaper, Haber– Bosch process,1 the reaction was used as the basis for ammonia production. The reaction of AlN powder with water follows the reaction AlN + 3H2O → Al(OH)3 + NH3
.
(1)
The degradation of AlN in water is very similar to the degradation (corrosion) of metal aluminum in water. Several aluminum hydroxides and/or oxides can form on the surface of the metal that is exposed to the water2 Al + 2H2O → AlOOH + 1.5H2
,
(2)
Al + 3H2O → Al(OH)3 + 1.5H2
,
(3)
2Al + 3H2O → Al2O3 + 3H2
.
(4)
The first reaction product is usually amorphous aluminum hydroxide, which then ages to form a variety of crystalline products, depending on the time, the temperature, and the pH.3 The typical aging sequence can be written as:4 AlOOH → Al(OH)3(amorph) → Al(OH)3(xstal) → Al2O3(xstal) . (5)
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0150 J. Mater. Res., Vol. 19, No. 4, Apr 2004
http://journals.cambridge.org
Downloaded: 18 Mar 2015
During the reaction of AlN and water, similar aluminum hydroxides are formed. Bowen et al.5 proposed the following reaction scheme AlN + 2H2O → AlOOH(amorph) + NH3 , NH3 + H2O ↔ NH4 + OH +
−
(6)
,
AlOOH(amorph) + H2O → Al(OH)3(xstal)
(7) .
(8)
AlN powder first reacts with water to form amorphous aluminum oxide hydroxide (pseudoboehmite phase, AlOOH), which later crystallizes as aluminum 3-hydroxide [bayerite or gibbsite, Al(OH)3], according to reaction (8).2,5,6 This reaction is slow if the concentration of OH− ions is low and is relatively quick if the concentration of OH− ions is more than three times higher than the concentration of Al3+ ions, which is the molar ratio needed for the formation of Al(OH)3. When AlN reacts with water, initiall
Data Loading...
