A high-speed camera was used to record the change of flame shape during the extinction of methane-air flame. The structures of the flame in different time nodes during the fire-extinguishing process were shown inFig.S6 , in which the time nodes were defined as 10s after the last time changing the concentration of the agent as the starting point (to wait the new ratio mixture of extinguishant and air getting into the cup-burner), and the flame being completely extinguished as the end point. As the time went by, it can be found that the flame lost stability and rapidly shocked, then the flame was suspended upward, finally the suspended flame completely detached from the cup, and the flame extinguished quickly. This process is similar to that of HFC-236fa, which also suggests that CF₃I has potential as Halon replacement。

Extinguishing mechanism of fire extinguishing agent actually includes physical and chemical extinguishing mechanism. Physical extinguishing mechanism is realized by reducing temperature and isolating oxygen, etc., while chemical extinguishing mechanism refers to the capture and depletion of active radicals of hydroxyl and hydrogen by reactants required for combustion reaction. Highly active free radicals and molecular fragments generated by the pyrolysis of fire extinguishing agents can capture a large number of essential free radicals OH· and H· in the combustion reaction to block the combustion reaction and achieve the purpose of fire extinguishing¹⁹. Based on the low boiling point of CF₃I at 1 atmosphere, it can be inferred that CF₃I will vaporize rapidly at flame temperature and absorb heat at the same time, which indicates that CF₃I has the possibility of physical fire extinguishing. In addition, all of the above decomposition reactions are endothermic reactions, which can also reduce the temperature of the fire site. Based on the geometry of CF₃I, the most likely fire-extinguishing free radical produced by CF₃I is CF₃·. At present, a large number of literatures have reported its reaction mechanism of capturing flame radicals such as OH· and H·^20-23, which confirmed its fire extinguishing effect. Therefore, in this section, the research on the mechanism of fire suppression in CF₃I is mainly devoted to exploring the formation path of CF₃· in CF₃I. The research method is the same as the study of thermal decomposition mechanism. The calculated fire-extinguishing mechanism is shown inFig.6 .