Kinetics of Thermal Decomposition Of Aluminum Hydride In Argon

Thermogravimetric analysis was utilized to investigate the decomposition kinetics of alane (AlH3) in argon atmosphere and to shed light on the mechanism of alane decomposition. Two kinetic models have been successfully developed and used to propose a mechanism for the complete decomposition of alane and to predict its shelf-life during storage. Under non-isothermal heating, alane decomposes in two steps; the slowest is solely controlled by solid state nucleation of aluminum crystals; the fastest is due to growth of the crystals. Thus, during decomposition, hydrogen gas is liberated and the initial polyhedra AlH3 crystals yield final amorphous aluminum particles. Nucleation of aluminum atoms is the rate determining step. After establishing the kinetic model, prediction calculations indicated that alane can be stored in inert atmosphere at temperatures below 10°C for long periods of time (e.g. 15 years) without significant decomposition. After 15 years storage, the kinetic model predicts ~ 0.1% decomposition. Storage at higher temperatures (e.g. 30°C) is not recommended. INTRODUCTION Recently, the interest in alane (AlH3) as a solid rocket fuel has been renewed presumably after the development of new methods of preparations and the discovery of new stabilizers that can slow the rate of alane decomposition and thus increase its shelf-life. With the concern on alane thermal stability, extensive physical and chemical characterization of newly produced alane is needed, especially in the area of thermal stability and hydrogen generation during long term storage. Alane can be produced in at least six different crystal forms [1]. The most stable form is α-alane which has been used in the current investigation. The crystals of the α-phase were mostly hexagonal and cubical. Traditionally, the stability of alane is determined by performing a vacuum thermal stability (VTS) experiments [1,2], utilizing the Taliani method [3]. The heat of decomposition of aluminum hydride samples have been measured experimentally in nitrogen atmosphere using a modified bomb calorimeter accommodating a small suspended heating oven containing the sample [4]. At 298 K, the calculated average enthalpy of formation was -11.4 ± 0.8 kJ/mol, absolute entropy was 30.0 ± 0.4 kJ/mol °C and Gibbs energy of formation was 45.4 ± 1.0 kJ/mol. These values indicate that alane is an unstable compound with respect to its forming elements. Thus, thermodynamically, alane should naturally decompose to yield aluminum metal and hydrogen gas. The kinetics and mechanism of the thermal decomposition of several solvated aluminum hydride compounds have been reported by Zakharov and Tskhai [5]. The volume of decomposition products was traced as a function of time at several isothermal temperatures between 50 and 100°C. For dry alane samples (that is, after the removal of the solvents), an s-shaped type plot was obtained during the liberation of hydrogen. The kinetics of hydrogen liberation was described by a first order autocatalytic equation. The activation energy of this step was 72.2 ± 2.5 kJ/mol. The thermal decomposition of AlH3 and AlD3 was investigated using NMR [6]. Samples of alane were decomposed isothermally (at 86127°C) and typical s-shaped plots, correlating percent decomposition (or amount of aluminum 1 Approved for public release; distribution unlimited. Report Documentation Page Form Approved