Effect of MnO2 morphology on the thermal properties and combustion behavior of nano-Al/MnO2 thermite

Abstract

Al/granular MnO2 and Al/rod MnO2 thermite samples were prepared to investigate the effects of different morphologies of MnO2 on the thermal properties and combustion behavior of nano-Al/MnO2 thermite. The morphology and thermal properties of the thermite were characterized by field emission scanning electron microscopy (FE-SEM), x-ray diffractometry (XRD), and differential scanning calorimetry (DSC). DSC results show that the Al/rod MnO2 releases 1274.39 J·mol−1 heat, which is 292.58 J·mol−1 more than the Al/granular MnO2. The initial reaction temperature of Al/rod MnO2 is 567.39 °C, which is delayed by 17.39 °C versus 550 °C of Al/granular MnO2. Non-isothermal thermodynamic analysis was used to measure the activation energy of Al/rod MnO2 to be 234.36 kJ·mol−1, which is 46.53 kJ·mol−1 higher than that of Al/granular MnO2. This corresponds to an increase in the ignition temperature in the DSC curve and indicating a higher safety profile. In the open burning experiment, the burning time of Al/granular MnO2 was longer and sparks sputtered around the flame. The Al/rod MnO2 has a large combustion flame and a fast combustion rate. The light intensity peaks of the two groups of samples are close in the light intensity test. The light intensity existence time of Al/rod MnO2 is 0.0146 s, which is 0.094 s shorter than Al/granular MnO2. This shows that the combustion rate of Al/rod MnO2 is much faster than that of Al/granular MnO2. The closed-tube combustion experiment shows that the combustion wave velocity of Al/rod MnO2 increases first and then decreases; the maximum wave velocity reaches 339.6 m·s−1, and Al/granular MnO2 cannot self-propagate combustion in the microporous environment. In the constant volume combustion experiment, the peak combustion pressure of Al/rod MnO2 is 0.938 Mpa, and the peak value of Al/granular MnO2 combustion pressure is 0.581 Mpa; the difference is obvious. This shows that the rod MnO2 gas production performance is better. According to the duration of the pressure peak, the burning speed of Al/rod MnO2 in the light intensity test is confirmed again to be much faster than that of Al/granular MnO2. The Al/rod MnO2 is better than Al/granular MnO2 in thermal and combustion performance and is also safer. This provides a basis for future performance and safety research on aluminothermic materials.