Figure 7 The diagram of different combustion phenomenon, (a) Al/MnO2 nanothermite, (b) Al/MnO2/10wt%-terpolymer nanothermite, (c) Al/MnO2/20wt%-terpolymer nanothermite (d) Al/MnO2/30wt%-terpolymer nanothermite
As for Al/MnO2nanothermite (Figure 7(a)), without any addition of terpolymer, the ignition point should be near the heating wire and between the Al NPs and MnO2 NRs, and then theoretically the flame travels equally fast in each direction.
In Figure 7(b), the 10wt% content of P(VDF-ter-HFP-ter-TFE) terpolymer exists in the gaps among components, decreasing the heat spread and direct contact of nanothermite components. It can be the reason for the delay of flame growth. But the content of terpolymer is not high, so the negative effect on flame growth is not terrible, either. Each coin has two sides. Based on ignition test, the addition of terpolymer can reduce the ignition energy threshold from the current value at ignition moment. The terpolymer is very sensitive to the temperature increment. On one hand, high temperature can lead the process of thermal decomposition with heat release. On the other hand, the decomposition of terpolymer can release the fluoride. The fluoride can directly react with nanothermite components at early stage. Besides, since fluorine is the most electronegative element, it can deal with the alumina shell covered on the Al NPs, leading to the increasing active aluminum directly react with oxidizer.
Figure 7(c) and (d) shows the combustion diagram of Al/MnO2/20wt%-terpolymer and Al/MnO2/30wt%-terpolymer nanothermite, respectively. Comparatively, the negative effect on flame growth become increasing great with the increasing content of terpolymer. Terpolymers even totally cover the nanothermite components in some parts, which leads to the clear ignition delay phenomenon. At the same time, much more heat will release at an early stage due to the more content of terpolymer decomposition as well as reaction with components from TG-DSC results, resulting in a lower ignition energy threshold accordingly.