The Impact of Metastable Intermolrecular Nanocomposite Particles on Kinetic Decomposition of Heterocyclic Nitramines Using Advanced Solid‐Phase Decomposition Models

Surface oxygen of oxide catalyst has low coordination number; they are negatively charged. Surface oxygen can act active site for decomposition of energetic nitramines (i.e. HMX). Additionally hydrous catalyst surface can release active OH radicals. Colloidal oxide particles can fulfil these requirements. Furthermore oxide particles can induce thermite reaction with aluminium particles. This study reports on the facile fabrication of colloidal ferric oxide particles of 5 nm average particle size. Aluminium nanoplates of 100 nm particle size were dispersed in ferric oxide colloid. Colloidal Fe2O3/Al binary mixture was integrated into HMX matrix via co-precipitation technique. SEM micrographs demonstrated uniform dispersion of nanothermite particles into energetic matrix. Naonothermite particles experienced dramatic change in HMX thermal behaviour with increase in total heat release by 63% using DSC. The impact of thermite particles on HMX kinetic decomposition was evaluated via an integral isoconversional method using KAS, and Kissinger models. The mean value of apparent activation was reduced by 23.5 and 24.3% using Kissinger and KAS models respectively. This dramatic change in HMX decomposition could be ascribed to ferric oxide reactivity. Facile integration of colloidal thermite particles into HMX can secure high interfacial surface area.

HMX is one of the most vigorous energetic nitramines; however the performance of energetic materials is limited to hydrocarbon combustion [2,3]. Energetic nanocomposite materials are emerging class with controlled performance in terms of sensitivity, decomposition enthalpy, kinetic decomposition (i.e. activation energy) [4][5][6]. Optimization between performance and sensitivity is a crucial issue [7][8][9].
Ferric oxide can act as efficient catalyst. Oxygen atoms on the surface of ferric oxide can experience low coordination; and have exceptional electron donor properties [10]. These electron donor sites can induce catalytic decomposition of electron deficient material [11]. Additionally ferric oxide can experience hydrous surface; free OH radical could be evolved and attack nitramine heterocyclic ring [5,11,12]. Colloidal ferric oxide particles can secure hydrous surface, and high catalyzing ability. There is a potential for fabrication technology that could offer synthesis of ferric oxide particles in dispersion. Hydrothermal processing can fulfil such requirements. Hydrothermal processing has an edge over all other classical synthesis techniques such as ball milling, sintering, and firing (Fig. 2).
Hydrothermal processing can secure mono-dispersed particles with controlled geometry and high crystalline structure [16,17]. Further details about hydrothermal processing can be found in the following literatures [18,19]. Ferric oxide particles can experience superior catalytic activity for different energetic systems including nitric 1 3 esters, and nitramines [20][21][22][23]. Ferric oxide particles can expose hydrous surface (hydroxyl groups); these hydroxyl groups can be evolved at low temperature as active OH radicals. These active species would attack HMX heterocyclic ring and alter the decomposition mechanism from H-C cleavage to hydrogen atom abstraction [5,11]. Additionally ferric oxide particles can induce vigorous thermite reaction with aluminium particles. Nanothermite particles can offer low critical diameter, enhanced ignitability, low activation energy, and high reaction rate [24][25][26][27][28]. Furthermore nanothermite particles can experience dramatic change in decomposition kinetics. Different thermal analysis techniques can be adopted for thermal decomposition study i.e. TGA, DTA, and DSC [29,30].
Kinetic parameters i.e. activation energy, pre-exponential factor, and the reaction model can be evaluated (Eq. 1).
Where d dt , is the reaction rate, k(T) is the absolute temperature, and f ( ) is the solid state reaction model. Isconversional model is based on the principle that the reaction rate is function of temperature at constant fraction reacted (Eq. 2).   [13][14][15] Second term of the Eq. (2) would be zero by applying first derivative. Finally, activation energy could be determined from the slope of the plot (Eq. 3).
Where α is the extent of conversion, E α is the apparent activation energy, and R is the universal gas constant. The current study reports on the facile development of colloidal nanothermite particles and effective integration of nanoparticles (NPs) into energetic matrix (HMX). The impact of nanothermite particles on HMX thermal behaviour was evaluated using DSC. The impact of nanothemrite particles on HMX kinetic decomposition was evaluated via an integral isoconversional method using KAS, and Kissinger models. The mean value of apparent activation was reduced by 23.5 and 24.3% using Kissinger and KAS models respectively. This dramatic change in HMX decomposition could be ascribed to the high reactivity of Fe 2 O 3 NPs and the facile integration of colloidal nanothermite particles.

Characterization of Nanothermite Particles
Ferric oxide NPs were developed using established hydrothermal processing. Further details can be found in our previous work [6,9,24]. Aluminium nanoplates of 100 nm were employed. Morphology of ferric oxide and aluminium NPs was investigated using TEM (JEM-2100 F by Joel Corporation). The dry powder size and shape was investigated with SEM ZEISS SEM EVO 10 MA.

Formulation of HMX Nanocomposite
It is widely accepted that the integration of colloidal particles into energetic matrix can secure high dispersion levels [31]. Colloidal Fe 2 O 3 particles were harvested from their synthesis medium and re-dispersed in acetone. Aluminium NPs were dispersed in acetone colloid; consequently HMX was dissolved in Fe 2 O 3 /Al colloid. HMX nanocomposite was developed via co-precipitation technique (Fig. 3). Morphology of developed HMX nanocomposite was investigated using SEM. Dispersion of nanothermite particles into energetic matrix was investigated using EDAX detector.

Thermal Behaviour of HMX Nanocomposite
Thermal behaviour of HMX nanocomposite was investigated by DSC Q20 by TA, USA. The tested sample was heated at 5 °C min − 1 up to 500 °C, under N 2 gas flow of 50 ml min − 1 .

Decomposition Kinetics of HMX Nanocomposite
Isoconversional (model free) and model fitting are the two main models for decomposition kinetic study. Decomposition kinetic parameters were evaluated by isoconvertional method using KAS, and Kissinger models. Activation energy of HMX nanocomposite was evaluated to pure HMX. DSC experiments were conducted at three different heating rates 2, 3, and 5 °C·min − 1 .

Integral Isconversional Model
Integral methods are driven from the integration of Eq. 4.
Where g(α) is the integral form of the reaction model. There is a number of integral isoconversional methods that have different approximation of the temperature integral (Eq. 5) [32].
Where β is the heating rate; A is the pre-exponential factor. In this manuscript the integral isoconvertional method using Kissinger-Akahira-Sunose (KAS) Eq. (6) has been adopted for activation energy calculation.

Kissinger Model
Kissinger model is a straight forward method; the condition was the maximum reaction rate and at this point The fraction reacted at maximum rate should be constant at the three different heating rates. Activation energy can be evaluated from the slope of the straight line of ln β Tm,i 2 versus( 1 Tm ).
SEM micrographs of dry Fe 2 O 3 particles demonstrated high affinity to decrease their number and surface area (Fig. 5a, b). Aluminium flakes were reported from SEM micrographs (Fig. 5c, d).
There a great potential to integrate colloidal particles into different energetic matrix can eliminate integration of dry aggregates. Therefore superior particle dispersion could be accomplished [13,34].

Characterization HMX Nanocomposite
Size and shape of HMX nanocomposite was investigated with SEM. Whereas starting HMX demonstrated cubic structure of 100 μm; HMX nanocomposite demonstrated cubic crystals with average particle size of 5 μm (Fig. 6).
Elemental mapping using EDAX detector confirmed enhanced dispersion of nanothermite particles. Uniform dispersion of main element Al, Fe, O, N is obvious (Fig. 7).
Elemental analysis confirmed the existence of main component in proper percentage (Fig. 8). Elemental analysis confirmed the absence of interfering impurities.

Thermal Behaviour of HMX Nanocomposite
Integration of thermite nanoparticles into HMX matrix demonstrated superior change in the thermal behaviour (Fig. 9).
Nanothermites offered an increase in the total heat release by 63%. Furthermore the maximum decomposition temperature has been decreased by 10 °C.

Kinetic Decomposition of HMX Nanocomposite
Main kinetic decomposition parameters and activation energy were evaluated using KAS and Kissinger models respectively. HMX nanocomposite were heated at different heating rates of 2, 3, 5 °C min − 1 using DSC (Fig. 10). It is widely accepted that optimum operation conditions include low heating rate, to minimize heat losses [35].
It is obvious that maximum decomposition peak temperature shifts to high value with heating rate increase [36]. The fraction reacted with temperature for different heating rates were calculated (Fig. 11).
Fraction reacted with temperature offered the capability to calculate the decomposition kinetics.

Kinetic Study using KAS Model
The kinetic parameters, obtained using integral isoconversional method of KAS model, were tabulated at Table 1. These parameters were evaluated at different reacted fractions.

3
The apparent activation energy of pure HMX was reported to be 376.8 KJ mol − 1 [37]. HMX nanocomposite experienced activation energy of 281.4 kJ mol − 1 . It can be conclude that nanothermite particles experienced dramatic decrease in HMX activation energy by 23.5 %using KAs model (Fig. 12).
In addition, the different values of the calculated activation energy at different fraction reacted are consistent. This gives confidential result of activation energy calculations.

Kinetic Study using Kissinger Model
Apparent activation energy from the Kissinger model was calculated from the slope of the straight line in (Fig. 13) between,ln β Tm,i 2 versus( 1 Tmi ). While activation energy of pure HMX was reported to be 360.4 KJ mol − 1 [37]; apparent activation energy of developed HMX nanocomposite was found to be 272.82 kJ mol − 1 (Fig. 13).
It can be concluded that nanothermite particles demonstrated dramatic change in HMX activation energy by 24.3%. The solid-state kinetic model has been determined and it was found to fit the two-dimensional diffusion model (Fig. 14). Good agreement with diffusion model could be ascribed to different crystal structure of molecules presented in the HMX nanocomposite and the diffusion process that occurs between these different crystalline structures.
For solid state reactants, Thadani reported that reduction in onset temperature would result a lower activation energy and a higher reaction rate [38]. It can be concluded that heterogeneous solid phase reaction was accurately modelled; the apparent activation energy calculations were found to be in good accord with KAS and Kissinger kinetic models. The integration of nanothermite particles into HMX demonstrated dramatic decrease in HMX activation energy by 23.5 and 24.3% using KAs and Kissinger models respectively. The catalytic effect of nanothermite particles can be correlated to the hydrous surface of ferric oxide particles.

3
The surface hydroxyl groups could be released at low temperature; active OH radicals would attack HMX heterocyclic ring and abstract hydrogen atom from HMX heterocyclic ring (Fig. 15) [12].
As a result of hydrogen abstraction, energy of the N-NO 2 bond would decrease and the nitro group would be released easily [40]. Furthermore, the released nitro group could attack another HMX molecule or absorbed on the surface of the nanoparticles with an increase in reaction exothermicity [41].

Conclusions
Colloidal Fe 2 O 3 NPs, of 5 nm average particle size, were fabricated using established hydrothermal synthesis technique. Colloidal Fe 2 O 3 /Al nanothermite particles were dispersed in acetone and integrated into HMX via co-precipitation technique. Nanothermite particles demonstrated dramatic change in HMX decomposition with an increase in decomposition enthalpy by 63%. Nanothermite particles could act as high energy dense material with catalytic activity. Nanothermite particles demonstrated dramatic change in HMX decomposition kinetics. The apparent activation energy was reduced by 23.5% using isoconversional KAS model and by 24.3% using Kissinger model.