The Oxidation Mechanism of Pure Magnesium Powder Particles: A Mathematical Approach
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INTRODUCTION
MAGNESIUM powder is widely used in propellants,[1–6] pyrotechnics,[2–4,7] and explosives.[3,4] Although combustion enthalpy (DHc) of the magnesium powder is lower than that of the aluminum powder,[8] magnesium powder has a higher burning rate[9] and lower ignition temperature,[8] which have attracted the attention of many researchers.[3,7,10,11] Chunmiao et al.[3] investigated the nonisothermal oxidation of magnesium powder under air atmosphere and showed that this process occurs in three stages. The results obtained by others[7,12–14] revealed that two main products, magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2), are produced during the oxidation process. As presented in Eqs. [1] and [2], MgO and Mg(OH)2 can be formed by the reaction of magnesium with oxygen and water vapor that it is dependent on the temperature and pressure of oxygen and water vapor. Moreover, the pressure of oxygen and water vapor can lead to formation of Mg(OH)2 from MgO (Eq. [3]).[3,15,16]
M. KARIMPOUR, S.R. EATEZADI, and S. HASANI are with the Department of Mining and Metallurgical Engineering, Yazd University, Yazd 89195-741, Iran. Contact email: [email protected] A. GHAEI is with the Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. Manuscript submitted November 19, 2018. Article published online May 3, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
1 MgðSÞ þ O2 ðgÞ ! MgOðsÞ 2
½1
MgðSÞ þ 2H2 OðgÞ ! MgðOHÞ2 ðsÞ þ H2ðgÞ
½2
MgOðSÞ þ H2 OðgÞ ! MgðOHÞ2 ðsÞ
½3
Nie et al.[7] showed that the rate of the oxidation of magnesium powder is controlled by diffusion of oxygen through the oxide or hydroxide layers. Furthermore, it is reported that the formation of Mg(OH)2 on the surface of powder particles can lead to a decrease in their pyrotechnic properties.[16] However, Mg(OH)2 is decomposed to MgO in the temperatures higher than 200 C;[3] thereby, the crystalline defects, including pores[12,17] and microcracks,[12,14,15] form through the oxide layer due to the shrinkage of the crust (oxide layer). On the other hand, different thermal expansion coefficients of the core and the crust can lead to the generation of thermal stresses in the oxide layer,[12,18–20] which is similar to what has been reported about aluminum powder.[21–23] In our previous publication,[23] the effect of stress on the rupture of the oxide film of aluminum powder particles during the oxidation process under nonisothermal condition was investigated, and it has been shown that these stresses play the main role in oxidation behavior. Also, Rosenband et al.[19,20] showed that different thermal expansion coefficients of the core and the crust and also the difference between their densities can be effective on the isothermal oxidation of magnesium, aluminum, zirconium, and titanium powders. Therefore, it seems that a more comprehensive study is required
VOLUME 50B, AUGUST 2019—1597
to determine the effect of tension on the oxidation behavior of magnesium powder particles. In addition, no mathematical model has
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