## 2 Experiment and Measurement  
In this study, AP was used with a particle size of 150 to 220 \(\upmu\)m obtained from the Dalian potassium chlorate factory. A synchronous thermal analyser (TG-DSC, STA449F3 Jupiter(r), NETZSCH) by NETCH in Selb, Germany and a nanoVoxel-5000 series X-ray three-dimensional microscope by Sanying Precision Instruments in Tianjin, China with a minimum resolution of 300 nm were also used. The mass of each AP sample was approximately 10 mg heated in an inert environment in the DSC-TG thermal analyser. First, the temperature was raised to 230 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min and then maintained at 230 \({}^{\circ}\)C constant. By changing the heating time, AP residues with different mass loss were prepared, as shown in Table 1. Figure 1 shows the mass loss process of AP under different heating conditions (where \(\bigstar\) represents the decomposition termination point at different heating times). The repeatability of the experiments was good. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Time/Min** & **Mass Loss/\%** \\ \hline a & 120.0 & 29.9 \\ b & 38.9 & 20.2 \\ c & 26.1 & 9.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The decomposition test conditions of AP. The apparent micromorphological characteristics of the AP residue are shown in Figure 2. These images correspond to the experimental conditions listed in Table 1. Figure 2d shows the initial AP where a smooth surface has no pores. When the mass loss was 29.9% (Figure 2a), numerous pores were spread across the surface of the entire particle. As the mass loss was reduced, the pore intensity was reduced, and when the loss rate was low, the pore distribution became uneven, although the pore size did not change, with a pore diameter between 2 and 5 \(\upmu\)m.  
However, scanning electron microscopy could only observe the topography of the surface of the AP particles, and the internal distribution and decomposition of the particles could not be obtained. To further study the micromorphological evolution of AP under low-temperature decomposition, the \"Digital Core\" technique in rock physics was used to analyse porosity, specific surface area, and pore distribution uniformity by using high-precision 3D nano-computed tomography (CT). According to differences in the internal density of the sample, the attenuation degree of X-rays differed. When the density was high, the X-ray penetration was low, and the X-ray intensity received by the detector was weak, with the image reconstruction algorithm showing this as a lighter colour. The X-rays penetrated areas with low density, such as internal cracks and pores, and the X-ray intensity received by the detector was strong, with the image reconstruction algorithm showing these areas in a darker colour. Based on this, areas with similar density can be distinguished by improving the image resolution. Figure 1: The thermal mass loss of AP at 230 \({}^{\circ}\)C.  
Figure 2: Scanning electron microscopy images of AP low-temperature decomposition residue. (**a**) AP with 29.9% mass loss; (**b**) AP with 20.2% mass loss; (**c**) AP with 9.5% mass loss; (**d**) initial AP without mass loss. First, the decomposed AP particles were scanned using nano-CT, and the density distribution inside the AP particles was found to be uneven because of the pores. Second, the CT results were reconstructed using Avizo (v2019.1) software to establish a 3D structure model, and an individual AP particle was extracted from this model. Finally, the microstructure characteristics of porous AP particles were analysed by filtering, image enhancement and threshold segmentation. Porous AP, with mass loss of 29.9%, 20.2% and 9.5%, was prepared by heating to 230 \({}^{\circ}\)C while varying the heating time. The microstructure of AP particles for different mass losses and how this changed with the degree of decomposition were studied. A sample with a mass loss of 29.9% is described as an example.