(753f) Thermal Reaction Characteristics of Nano and Micro-Scale Aluminum Powders in Carbon Dioxide

Thermal Reaction Characteristics of Nano and Micro-scale Aluminum Powders in Carbon Dioxide

Yunlan Sun^*, Baozhong Zhu, Rong Sun, Fan Li

School of Energy and Environment, Anhui University of Technology, Maanshan, Anhui 243002, China

Extended Abstract

    The idea of metal-CO₂ propulsion for Mars missions has been known for a long time. Martian atmosphere consists of approximately 95% CO₂. Using metals as fuel and CO₂ as an oxidizer can reduce the cost of Mars exploration in CO₂-breathing jet [1] or rocket [2] engines. This approach is based on the ability of some metals to burn in CO₂. Aluminum powder, as a very common energetic material, is widely used as a fuel additive in propulsion systems, explosives, and pyrotechnics due to its relatively low cost and high combustion enthalpy. However, the thermal reaction characteristics in CO₂ atmosphereare less understood due to the addition of carbon element.

    In this work, a major part of this investigation will focus on thermal reaction characteristics of nano and micro-scale aluminum powders with a mean particle diameter of 50 nm and 1-2 µm in CO₂ atmosphere. The thermal reaction of nano and micro-scale aluminum powders were investigated by a thermogravimetric analysis coupled with a differential scanning calorimetry (TG-DSC). Evolution of the samples of nano and micro-scale aluminum powders was determined by collecting the products at different reaction stages of the process. The morphology of reaction products was examined using scanning electron microscopy (SEM). The corresponding chemical changes were analyzed by X-ray diffraction spectrometry (XRD). It is hoped that this study would be useful in providing some helps of metal-CO₂propulsion studies.

    TG-DSC experiments were carried out with a Netzsch STA449C simultaneous TG-DSC thermal analyzer at heating rate of 10°C/min from 20°C to 1300°C. The experimental gas flow rate was controlled at 40 mL/min. The results showed that thermal reaction characteristics of nano-scale aluminum powder differed significantly from micro-scale aluminum powders. The main reaction of nano-scale aluminum powders takes place at 470-690°C. The melting temperature of nano-scale aluminum powder is 652.6°C. The mass gains of nano-scale aluminum powder are 39.94% before the melting temperature and are 26.37% after the melting temperature. The melting temperature of micro-scale aluminum powder is 654.0°C. The mass gains of micro-scale aluminum powder are 4.42% before the melting temperature and are 75.84% after the melting temperature. The main reaction of micro-scale aluminum powders takes place at 697-88°C. This showed that the key reaction of nano-scale aluminum powders took place before their melting temperature. While for micro-scale aluminum powder, the key reaction took place after their melting temperature.

    SEM techniques have been widely used today to characterize products morphology. In this study, the morphologies of thermal reaction products of nano and micro-scale aluminum powder at different stages were examined by SEM. The results showed that there was a striking difference for the thermal reaction products between nano-scale aluminum powder and micro-scale aluminum powder. The morphology of thermal reaction products of micro-scale aluminum powder changed evidently with the increase of the temperature. For micro-scale aluminum powder, after being heated to 600 °C, the powder was composed of discrete spherical particles. The diameters of these spherical particles are 1–6 μm. After being heated to 1000 °C, many holes were formed on the spherical particles. Also, there are a small amount of fragments, but the exterior surfaces of the particles are rougher at 1000 °C than that at 600 °C. After being heated 1400 °C, cracked oxide shells and fragments were apparent, and irregular globular body had also been found. The morphology of thermal reaction products of nano-scale aluminum powder did not change obviously with the increase of the temperature.

    In order to trace the thermal reaction products at different stage, XRD was used to identify the alumina polymorphs formed for the reaction of aluminum with CO₂ at different temperatures. XRD pattern of micro-scale aluminum particles at ambient temperature contained only aluminum peaks and peaks of any oxide phases were not detected. After being heated to 600 °C, the formed alumina phases are referred to as amorphous. The alumina phases are also amorphous for the sample recovered from 800 °C. After aluminum powder was heated 1000 °C, in addition to strong peaks of metallic Al, the peaks of θ-Al₂O₃ and γ-Al₂O₃ were observed, but the amorphous alumina was not detected. After the temperature increased to 1400 °C, X-ray diffraction showed that only α-Al₂O₃ was detected and no other oxide peak was observed. Amorphous alumina was not detected for the reaction of nano-scale aluminum powder and CO₂.

    Based on the above experiment, the thermal reaction mechanism was discussed. It can be concluded that the thermal diffusion dominates the reaction of nano-scale aluminum powder and CO₂. Micro-scale aluminum powder has a more complicated thermal reaction mechanism due to the disruption of oxide shell. Disruption of oxide shell dominates the reaction of micro-scale aluminum powder and CO₂.

    The authors are grateful to the National Natural Science Foundation of China for financial support (No. 51376007 and No. 51206001).

References

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[2] Shafirovich, E. Y. and Goldshleger, U. I. (1997) J. Propul Power., 13, 395.