4.5. Thermal decomposition studies
TG curves of various samples of AP and NTO are depicted in Figure 5. Thermal decomposition of N0, N1, and N2 samples takes place between 200-300 oC in a single step. The incorporation of additives in pure NTO causes the TG curve of NTO to shift towards the left side (i.e., lower temperature region). The order of thermal decomposition temperature was: N0>N2>N1. The TG curve reveals that the mass loss of N2 was 4 % higher and that in the case of N1 was 5 % lower compared to the pure N0 sample. As the mass ratio of the additive: pure NTO was 95:5, both the samples containing additives (i.e., N1 and N2) must have decomposed to yield a total mass loss that corresponds to 5 % lower than pure N0. However, the plausible explanation for higher mass loss in N1 could be that N1 must have catalyzed the residue that may have remained un-reacted when pure N0 was decomposed leading to greater mass loss than expected. The mass loss of N2 was 5 % lower that was because of the presence of the 5 % additive.
A0 decomposes in two steps; (i) between 250-330oC leading to a mass loss of 18 % called low-temperature decomposition (LTD) and (ii) between 331-450 oC with a mass loss of 82 % called high-temperature decomposition (HTD). In the presence of the additives (A1 and A2), the TG curve of AP shifts to lower temperatures compared to A0. The TG curve of AP was changed drastically when the additives were incorporated. A2 decomposes in two steps at a lower temperature range, but the LTD step of A2 becomes more influential than LTD in A0. The mass loss in LTD of A2 was 13 % higher than in LTD of A0. The LTD step in A1 becomes the major decomposition step of AP with a mass loss of 90 % and other small decomposition takes place with only 7 % mass loss.
Figure 5. TG curve of NTO (a), and AP (b) with and without BZC, and BZC/rGO additiveDTA curves of A0-A2 and N0-N2 are depicted in Figure 6. N0, N1, and N2 decompose in a single exothermic event with a maximum curve temperature (T­m) of 276, 230, and 239 oC, respectively. DTA curve of N0 in the presence of additive was shifted to the left side, and the corresponding temperature value of DTA curves are depicted in Table 2. DTA curve of A0 exhibit three peaks, one endothermic peak ~242 oC corresponding to the phase transformation of AP from orthorhombic to cubic, and two exothermic peaks belonging to the decomposition of AP. In the presence of additives, the endothermic peak of AP was varied by only 1-2oC, but LTD and HTD peaks of AP were affected drastically. LTD and HTD peaks of A2 were decreased by 21 and 71oC, respectively. In A1, two exothermic peaks of AP merge into a single exothermic peak at 292 oC. The difference between onset temperature (To) and maximum temperature (Tm) also plays an important factor in determining the thermal performance of energetic materials (Table 2). ∆T of A1 and A2 was lower than A0, indicating the faster decomposition of AP in the presence of additives. N1 and N2 increase the ∆T of N0 by 3, and 26 oC, respectively. The TG and DTA results suggest that BZC was a more suitable catalyst for influencing the thermal performance of both NTO and AP compared to BZC/rGO. The comparisons of previously reported additives (3 % by mass) on the thermal decomposition of AP is given in Table 3.27–30From Table 3, it was evident that BZC containing AP composition decomposes at low temperature and hence, it can influence the burning rate properties of AP based propellants and formulations to compare to other catalysts with same content reported in Table 3.Figure 6. DTA curve of (a) NTO, and (b) AP with and without BZC, and BZC/rGO additive
Table 2. DTA and TG data of AP, and NTO with and without catalyst ( β=10oC min-1)