## 3 Results and Discussion
### Catalytic Activities  
AP is a common oxidant for composite solid propellants and its thermal decomposition can be divided into two stages: low-temperature decomposition (LTD) stage and high-temperature decomposition (HTD) stage [19]. The LTD stage usually occurs around 290-330 \({}^{\circ}\)C and the HTD stage usually occurs around 350-450 \({}^{\circ}\)C. The LTD stage involves the reaction process taking place at the local sites on the surface of AP crystal. During the reaction process, AP is dissociated by proton transfer to form NH\({}_{3}\) and HClO\({}_{4}\), which will adsorb on the surface of AP and react with each other. Because NH\({}_{3}\) adsorbed at low temperature cannot be completely oxidized by the decomposition products of HClO\({}_{4}\), NH\({}_{3}\) continuously covers the AP crystal surface with the decomposition process. If NH\({}_{3}\) covers all the activation centers (reaction centers) on the surface of AP, the decomposition process will stop. With the increase of temperatures, the HTD stage begins. The adsorbed NH\({}_{3}\) would release from the surface of AP and thus potential reaction centers are reactivated. At the same time, partial liquefaction of AP produces more NH\({}_{3}\) and HClO\({}_{4}\), which makes the reaction take place  
Figure 5: X-ray photoelectron spectroscopy (XPS) spectrum of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires: (a) survey spectrum; and binding energy spectrum of (b) Ni 2p; (c) Co 2p; (d) O 1s. Figure 6: (a) N\({}_{2}\) adsorption-desorption isotherm curves and (b) pore size distribution curves of the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures. in the whole condensate phase, and the reaction becomes more intense. The catalytic activities of as-prepared mesoporous NiCo\({}_{2}\)O\({}_{4}\) nanostructures towards AP thermal decomposition were analyzed by DSC, and the results are shown in Figure 7. As shown in Figure 7a, pure AP sample shows two endothermal peaks at about 242.2 \({}^{\circ}\)C and 411.6 \({}^{\circ}\)C. The first endothermal peak at 242.2 \({}^{\circ}\)C corresponds to the phase transformation of AP and the second endothermal peak corresponds to the HTD stage of AP. It is noted that no apparent peak corresponding to the LTD stage is observed, which may be ascribed to the particle size effect of AP crystal. Previous studies have reported that the LTD stage is closely related to the particle size of AP [20; 21]. For AP crystals with large particle size, there is a large internal stress in the crystal itself. During LTD stage, the accumulation of adsorbed reaction products, such as NH\({}_{3}\) and HClO\({}_{4}\), increases the internal stress and promotes the breakdown of large crystals to form smaller crystals. The newly formed crystals can provide larger specific surface area to react, which makes LTD intensely. However, the internal stress of fine AP particles is small, and the accumulation of adsorbed reaction products is not enough to make AP particles break down to form smaller particles. Therefore, the initial decomposition of NH\({}_{3}\) quickly covers the surface of small particles, so that no obvious first stage decomposition of fine AP particles can be seen. When 2 wt% as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures were used as additives, all curves exhibited similar crystal transformation temperature at around 242.2 \({}^{\circ}\)C. It indicates that NiCo\({}_{2}\)O\({}_{4}\) additives did not affect the crystal transformation of AP, consistent with previous reports [22]. However, the HTD peaks shifted towards lower temperature and became exothermic type instead of endothermic, as one observed in pure AP. It has been well documented that the HTD process of pure AP can be either exothermic or endothermic, depending on the use of lids [23; 24]. When the sample container with lid was used, the HTD process was the exothermic type; while the sample container without lid was used, the HTD process would be the endothermic type.