Figure 5 XRD results of residues after TG-DSC tests
3.4 Ignition and combustion
Ignition and combustion performance represent the practical characteristics for energetic materials future application. In this work, we carried out the ignition and combustion by using fast heating wire test as well as high-speed camera. The detailed ignition and combustion performance for each sample is shown in Figure 6. The first picture of a firelight is defined as the beginning time, set as 0 us. The DC power supplier is selected, and the current value for ignition moment is recorded as the ignition energy threshold.
In Figure 6(a), when the current rise to 1.192 A, the Al/MnO2 nanothermite sample without any additive successfully ignites. The speed of flame grow is very rapid, and at about 350 us, the size of flame reaches the maximum. Then, the flame fades away gradually with clear flying sparks (Supplementary movie 1).
In Figure 6(b), the sample with 10wt%- P(VDF-ter-HFP-ter-TFE) terpolymer additive ignites at about 1.137A current value. The speed of flame growth is still relatively fast, and at about 1 ms, the size of flame reaches the maximum. However, comparing to the Al/MnO2 nanothermite sample in Figure 6(a), the speed of flame grow and the size of maximum flame are decreased clearly. The addition of terpolymer can help reduce the number of flying sparks to concentrate the flame. Besides, the duration of combustion is longer than that of Al/MnO2 nanothermite sample (Supplementary movie 2).
Furthermore, when the contents of terpolymer reach 20wt% and 30wt%, the current value at ignition moment are 1.083A and 1.075A, respectively. As for Al/MnO2/20wt%-terpolymer sample, in Figure 6(c), (Supplementary movie 3) the flame grows pretty slow, and the maximum flame appears at about 7 ms. After the maximum flame moment, the flame seems not to fade away gradually but further forms a certain burning flame shape, which might be caused by the unevenly distributed terpolymer, leading to the ignition delay phenomenon [31-32]. Such phenomenon is also found in the last sample, in Figure 6(d) (Supplementary movie 4). After the maximum flame moment at about 8 ms ~9 ms, a small but clear bulge appears at the top of flame.
When the contents of terpolymer are below or equal to 10wt%, the phenomenon of ignition delay is not obvious, and the shapes of flame are mainly concentrated sphere. In contrast, once the terpolymer content is over 20wt%, the ignition delay phenomenon and different shapes of flame can be captured. Besides, the addition of P(VDF-ter-HFP-ter-TFE) terpolymer can reduce the ignition energy threshold from the current value at ignition moment, but it has a greatly negative influence on flame growth, leading to the long combustion duration time.