## 1 Introduction  
Solid propellant charges are prone to burning or exploding under unexpected thermal stimulation, leading to catastrophic accidents [1; 2]. Ammonium perchlorate (AP) is the most abundant ingredient in solid composite propellants and is widely used in solid propellant engines and aerospace boosters, where the AP content is over 70% [3; 4]. AP has many advantages, including an ideal density, a favorable oxygen balance, and tailorable burn rates. While AP delivers excellent performance in propellants, it is an unstable compound when heated and has the potential to react violently and explode [5; 6; 7]. Its safety and thermal decomposition performance directly affect the safety and burning rate of solid propellants.  
In general, the thermal decomposition of AP can be divided into two stages. Thermal degradation of AP occurring below the orthorhombic-to-cubic phase transition temperature of 240 \({}^{\circ}\)C is known as low-temperature decomposition [8; 9; 10; 11]. There is a cessation of decomposition after an approximately 30 wt.% loss. By increasing the temperature, decomposition continues until complete decomposition [12; 13; 14]. During low-temperature decomposition, it is known that the AP particle surface becomes porous, which can be observed using microscopy. The remaining material contains pores up to a few microns in size; however, the chemical composition of the particle remains unchanged [15]. The reason for this has been described in the literature; pore growth is often attributed to crystalline defects, and the drastic drop in decomposition rate is often attributed to the exhaustion of defect nuclei [16; 17; 18; 19].  
Although there is an abundance of literature that examines AP decomposition, most of it has focused on bulk material decomposition rather than single particle behaviour [20; 21]. However, due to the particularity of the low-temperature decomposition of AP, the formation and evolution of the aperture at low-temperature decomposition is necessary to understand the AP decomposition mechanism. Research that considered the structure of the residue of AP decomposition reported that only the structure of the surface or cross-section can be observed; the specific morphology inside the particles could not be observed [22]. However, only by understanding the evolution of pores during the low-temperature decomposition of AP particles can we further analyse the effect of the low-temperature decomposition of AP on the thermal safety of propellants.  
The burning rate of the propellant can obviously be accelerated by using porous AP instead of ordinary AP in the propellant [23; 24; 25]. However, the pore structure may lead to convective combustion, cause combustion instability, and even increase the reaction intensity of the propellant when subjected to unexpected thermal stimulation, resulting in explosion. The above problems are closely related to the thermal decomposition characteristics of AP particles at low temperature.  
Herein, to better explore the decomposition of AP particles at low temperature and its influence on the thermal safety of propellant, it is necessary to quantitatively study the microstructure evolution of AP particles and the effect of the pore structure on combustion properties during low-temperature thermal decomposition. First, the decomposition characteristics of AP at slow cook-off temperatures were quantitatively analysed. Then, the microstructure and its evolution were analysed by using scanning electron microscopy and 3D nano-computed tomography, which can identify and quantify size and number density of the pore location in the particle. Finally, the effect of porous AP on combustion performance was studied by using a closed bomb, and the response mechanism of the composite propellant under slow cook-off conditions was analysed.