## 3 Results and Discussion
### Catalytic Activities
In our experiment, open alumina crucibles without lids were used, so the HTD process is endothermic for pure AP. After adding NiCo\({}_{2}\)O\({}_{4}\) nanostructures as catalytic additives, the HTD process of AP were catalyzed by NiCo\({}_{2}\)O\({}_{4}\) additives, which made the reaction intensify to release reaction heat and thus the HTD process became exothermic. Figure 7b shows the HTD temperatures of all samples. In comparison with the HTD temperature of pure AP, those of the mixtures were greatly decreased. This indicates that all mesoporous NiCo\({}_{2}\)O\({}_{4}\) nanostructures showed obvious catalytic activity towards the thermal decomposition of AP. In particular, the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires exhibited the lowest HTD temperature of 312.3 \({}^{\circ}\)C, which decreased the HTD temperature of AP by 99.3 \({}^{\circ}\)C. The catalytic activity toward AP thermal decomposition of the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires is much better than other reported metal oxides, such as CuO nanocomposites [25], Co\({}_{3}\)O\({}_{4}\) nanowires [26], NiO nanostructures [27], and NiCo\({}_{2}\)O  
Figure 7: **(a)** Differential scanning calorimeter (DSC) curves of ammonium perchlorate (AP) mixed with as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures and pure AP at a heating rate of 10 K/min; (**b**) histogram of the corresponding high-temperature decomposition (HTD) temperatures. superstructures [28]. Therefore, the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires can be an effective additive to catalyze the HTD process of AP thermal decomposition. The activation energy of AP mixing with as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires was further studied by the thermal behavior of the mixture at different heating rates. For each given heating rate, a 5 mg mixture of AP and as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires was used separately. The results are shown in Figure 8. As can be seen, the crystal transformation temperature of AP is independent on heating rate, which remained as 242.2 \({}^{\circ}\)C (Figure 8a). Comparatively, the HTD temperature of the AP shifted toward higher temperature with the increasing heating rates from 10 to 30 K/min. According to the Kissinger correlation [29], the activation energy (E\({}_{\text{a}}\)) can be calculated by fitting the slope of the plot of \(\ln(\beta/\text{T}_{\text{P}}{}^{2})\) as a function of \(1/\text{T}_{\text{P}}\), where \(\beta\) is the heating rate and T\({}_{\text{P}}\) is the HTD temperature. Figure 8b shows the plot of \(\ln(\beta/\text{T}_{\text{P}}{}^{2})\) as a function of \(1/\text{T}_{\text{P}}\) based on the data in Figure 8a. The E\({}_{\text{a}}\) was calculated to be 317 kJ mol\({}^{-1}\), which is larger than reported activation energy (100-130 kJ mol\({}^{-1}\)) of pure AP [30]. The increased activation energy for AP decomposition has been observed in several metal oxide additives such as CoO [24], CuO [31], and MgAl\({}_{2}\)O\({}_{4}\)[32], which is ascribed to the kinetic compensation effect [31]. Though it appears that adding as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires increases rather than decreases the activation energy (energy barrier) of the AP decomposition reaction, we may conclude that the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires is an effective additive which decreased the HTD temperature of AP by 99.3 \({}^{\circ}\)C. Though the reaction mechanism of AP thermal decomposition is complicated, it is anticipated that the excellent catalytic activity of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires is due to their large surface and unique mesoporous nanostructures, which facilitates the gas diffusion and provides more active sites for AP thermal decomposition [30]. In addition, the multiple convertible valence state of Ni and Co atom could promote ammonia oxidation and the dissociation of ClO\({}_{4}\)\({}^{-}\) species [28].