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.