Regulating magnesium combustion using surface chemistry and heating rate

摘要

The magnesium (Mg) particle surface can be used to regulate fluorination or oxidation reactions depending on the applied heating rate and Mg particle size. Magnesium particles are surrounded by a complex hydroxide shell composed of an inner layer of magnesium oxide (MgO) and outer layer of magnesium hydroxide (Mg(OH)(2)). As particles approach the nanoscale, the thick oxide shell (e.g., 22 nm) becomes an appreciable portion of the overall powder and can be exploited to regulate reactivity. In this study, the reactivity of 800 nm Mg particles (nMg) was compared to 44 mu m Mg particles (mu Mg) when combined with Perfluoropolyethyer (PFPE), providing both fluorine and oxygen for Mg reactions. Experiments were performed at slow heating rates (10 degrees C/min) and separate experiments were performed at fast heating rates (6.0 x 10(5) degrees C/min). The slow heating rate studies used a differential scanning calorimeter (DSC) and thermogravimetric (TG) analyzer to examine reaction kinetics. The faster heating rate experiments used a hot wire to ignite a thermal run-away reaction. Powder X-ray diffraction (XRD) analysis of recovered residue at various temperatures corresponding to exothermic events in the DSC revealed reaction pathways for nMg + PFPE favoring oxidation reactions. For nMg powders, the outer Mg(OH)(2) surface layer dehydrates at low temperatures (313 degrees C) creating highly reactive sites for surface oxidation reactions in the condensed phase leading to a higher conversion of Mg(OH)(2) to MgO and greater consumption of Mg through oxidation reactions. For mu Mg, higher Mg(OH)(2) dehydration temperatures (498 degrees C) stabilize mu Mg particles and the bulk of reactions occur at elevated temperatures and in the gas phase producing higher MgF2 concentrations. Under high heating rate conditions, MgF2 formation is favored over MgO formation for both particle sizes owing to the high reaction temperatures that promote gas phase reactions favoring MgF2 formation. Theoretical analysis using density functional theory (DFT) through cluster models for Mg(OH)(2) and MgO surfaces further show that the Mg(OH)(2) surface is more reactive with fluorine species than MgO, especially at elevated temperatures. The DFT results help explain the high heating rate reaction pathway that favors fluorination reactions independent of Mg particle size. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.