## 3 Results and Discussion
### Catalytic Role of NiO Additives  
Ammonium perchlorate (AP) is an important oxidant commonly used in rocket solid composite propellant. Its catalyzed thermal decomposition plays a key role in satellite launching and space exploration. The core part of catalyzed thermal decomposition of AP is the additives that can promote AP to achieve highly concentrated thermal decomposition at lower temperatures. The thermal decomposition of AP can be divided into two stages. One stage is usually in the range of 290-330 \({}^{\circ}\)C, which is called the low temperature decomposition (LTD) stage. The other stage occurs in the range of 350-450 \({}^{\circ}\)C and is called the high temperature decomposition (HTD) stage [28]. In the LTD stage, AP decomposes into NH\({}_{3}\) and HClO\({}_{4}\). The excess NH\({}_{3}\) continuously adsorbs on the AP surface to stop the LTD stage. When the temperature increases further, NH\({}_{3}\) adsorbed on the AP surface will desorb from the AP surface, and then the HTD of AP begins.  
The catalytic role of as-prepared mesoporous NiO nanomaterials towards AP thermal decomposition was evaluated by the DTA technique. Figure 7a shows the DTA curves of pure AP and AP mixtures with NiO nanomaterials as additives. For pure AP, there is an endothermic peak at 248.8 \({}^{\circ}\)C and two exothermic peaks at 337.8 and 461.4 \({}^{\circ}\)C. Previous studies have shown that the endothermic peak at 248.8 \({}^{\circ}\)C corresponds to the transition temperature of AP from the rhombic phase to the cubic phase; the exothermic peak at 337.8 \({}^{\circ}\)C is associated with the LTD temperature (T\({}_{\rm a}\)) of AP, and the exothermic peak at 461.4 \({}^{\circ}\)C is ascribed to the HTD temperature (T\({}_{\rm b}\)) of AP. After adding NiO nanomaterials as additives, the DTA curve of the AP mixture has changed obviously. The phase transition temperature of AP did not change after adding NiO additives, which indicates that NiO additives have no effect on the phase transition temperature of AP [29]. Additionally, the intensity and area of the exothermic peak are obviously increased though the LTD temperatures of AP are slightly decreased. The HTD temperatures of AP are obviously decreased, and the exothermic peaks at a higher  
Figure 6: The synthesis route of hydrothermal reaction and subsequent calcination for mesoporous NiO hierarchical nanostructures.  
temperature are weakened. It indicates that the NiO additives have an obvious catalytic role towards the thermal decomposition of AP.  
In order to further study the catalytic effects, the LTD temperatures and HTD temperatures of different samples are compared, as shown in Figure 7b. It can be seen that NiO sub-microspheres exhibit the best catalytic role among all samples, which can reduce the HTD temperature of AP from 461.4 to 386.2 \({}^{\circ}\)C and the LTD temperate from 337.8 to 318.7 \({}^{\circ}\)C, respectively. It indicates that the HTD and LTD temperature of AP can be decreased by 75.2 and 19.1 \({}^{\circ}\)C. The catalytic activity of NiO sub-microsphere toward AP thermal decomposition is much better than the commercial spectrograde NiO reported by [PERSON] et al. [29] with an HTD temperature of 412 \({}^{\circ}\)C. Although the mechanism of AP thermal decomposition is complex, we believe that the superior catalytic role of NiO sub-microsphere should be related to their large specific surface area, pore volume, and small grain size, which can facilitate gas diffusion and provide more reactive sites.