Figure 4 TG-DSC results nanothermite samples (a) Al/MnO2 nanothermite, (b) Al/MnO2/10wt%-terpolymer nanothermite, (c) Al/MnO2/20wt%-terpolymer nanothermite, (d) Al/MnO2/30wt%-terpolymer nanothermite
Figure 4(b) shows the TG-DSC results of Al/MnO2/10wt%-P(VDF-ter-HFP-ter-TFE) terpolymer nanothermite sample. At about 472 oC, the minimum of total mass appears, about 89.1%. Considering the evaporation of solvent, more than 80% of terpolymer decomposes to the gaseous products, leading to mass loss. However, D’Orazio and his co-authors [29] have reported the thermal decomposition property of P(VDF-ter-HFP-ter-TFE) terpolymer, and the results shows that all of terpolymer will convert the gaseous products before 500oC. Meanwhile, combined with previous reports [24,30], a few parts of terpolymer react with nanothermite components directly, which can explain those exothermic DSC signals (signal b-1, b-2 and b-3), implying reactions among Al NPs, MnO2 NRs, terpolymer degradation products and/or the terpolymer matrix. Then, as the temperature increases gradually, the exothermic DSC signal (b-4) occurs, indicating the thermite reaction between remaining Al NPs and MnO2 NRs, similar to the exothermic signal in Figure 4(a). By contrast, the peak temperature point (b-4) is delayed by 26 oC since some sections of thermite components react with P(VDF-ter-HFP-ter-TFE) terpolymer before.
Next, the content of P(VDF-ter-HFP-ter-TFE) terpolymer reaches 20wt% in Figure 4(c). At about 483 oC, the minimum of total mass appears, about 86.2%, implying that there is 13.8% mass loss. Similarly, it is caused by evaporation of solvent and thermal decomposition of terpolymer. It is inferred that near 50% of terpolymer directly react with the Al NPs and MnO2 NRs rather than decomposition with gaseous products release, leading to the three exothermic DSC signals (c-1, c-2 and c-3) accordingly[24, 29,30]. Comparing to Figure 4(b), those three exothermic DSC signals (c-1, c-2 and c-3) are bigger and more obvious, especially signal c-2. However, the more proportion of terpolymer reacts with nanothermite components, the less amount of nanothermite components remains for thermite reaction, which results in the corresponding smaller exothermic DSC signal (c-4) for thermite reaction with less heat release.
Finally, the formulation with 30wt% P(VDF-ter-HFP-ter-TFE) terpolymer content is shown in Figure 4(d). At about 491 oC, the minimum of total mass appears, about 83.9%. Same to above reason for mass loss, after estimating, more than 50% of terpolymer directly react with Al NPs and/or MnO2 NRs, which leads to the big exothermic DSC signal (d-2). Similarly, much proportion terpolymer reaction with nanothermite components will affect the further thermite reaction and the corresponding exothermic DSC signal (d-4).
Besides, from Figure 4(b), (c) and (d), it can be found that not only the terpolymer itself will influence the thermal reaction process of Al/MnO2 nanothermite system, but the mass fraction of P(VDF-ter-HFP-ter-TFE) terpolymer also affect the thermal properties and processes. Specifically, from above TG-DSC results, as the content of terpolymer is no more than 10wt%, the main exothermic reaction is still thermite reaction between Al NPs and MnO2 NRs. In contrast, if the mass fraction of terpolymer is more than 20wt%, the thermite reaction will be greatly decreased and the main exothermic signal will be replaced by the reaction between terpolymer and nanothermite components.
3.3 Residues XRD analysis
From TG-DSC results, it can be clearly found that the addition and content of P(VDF-ter-HFP-ter-TFE) terpolymer can significantly change the thermal processes with the rise of temperature. To further understand the reaction products from different nanothermite samples, we collected the residues in crucible after TG-DSC tests, and used XRD analysis to study the phase of residues, as shown in Figure 5.
Without the addition of terpolymer, the reaction products are mainly Mn3O4, MnO and Al2O3 caused by thermite reaction between Al NPs and MnO2 NRs (green curve in Figure 5, AMT-0-residues). Then, the blue curve presents the XRD results of Al/MnO2/10wt%-terpolymer nanothermite reaction products. The reaction products have mainly changed to galaxite (MnAl2O4) with some sections of MnO and aluminum manganese (Al2Mn3). The red and black curves show the residues of Al/MnO2/20wt%-terpolymer and Al/MnO2/30wt%-terpolymer nanothermite samples, respectively, and their reaction products are almost same, containing Al2O3, manganese carbide (Mn7C3), Al2Mn3, MnO, AlF3, C and remaining Al, which are greatly different from Al/MnO2/10wt%-terpolymer nanothermite sample as well as Al/MnO2 nanothermite sample. We guess that the different content of P(VDF-ter-HFP-ter-TFE) terpolymer influences the detailed thermal processes during the TG-DSC tests, and then the different thermal processes lead to the different reaction products finally.
The different content of terpolymer can distinctly change the reaction products. More than 80% terpolymer decompose before the thermite reaction. Namely, few terpolymer and/or terpolymer degradation products involves into thermite reaction. So, the phases of reaction products are mainly MnAl2O4, MnO and Al2Mn3 rather than AlF3. In contrast, when the content of terpolymer increase, more and more terpolymer react with components, leading to the AlF3occurrence from residues analysis.