Nitrocellulose Catalyzed With Manganese Oxide Nanoparticles: Advanced Energetic Nanocomposite With Superior Decomposition Kinetics

Nanostructured energetic materials can fit with advanced energetic first-fire, and electric bridges (microchips). Manganese oxide, with active surface sites (negatively charged surface oxygen, and hydroxyl groups) can experience superior catalytic activity. Manganese oxide could boost decomposition enthalpy, ignitability, and propagation rate. Furthermore manganese oxide could induce vigorous thermite reaction with aluminium particles. Hot solid or liquid particles are desirable for first-fire compositions. This study reports on the facile fabrication of MnO2 nanoparticles of 10 nm average particle size; aluminium nanoplates of 100 nm average particle size were employed. Nitrocellulose (NC) was adopted as energetic polymeric binder. MnO2/Al particles were integrated into NC matrix via co-precipitation technique. Nanothermite particles offered an increase in NC decomposition enthalpy by 150 % using DSC; ignition temperature was decreased by 8 C. Nanothemrite particles offered enhanced propagation index by 261 %. Kinetic study demonstrated that nanothermite particles experienced drastic decrease in NC activation energy by 42, and 40 KJ mol using Kissinger and KAS models respectively. This study shaded the light on novel nanostructured energetic composition, with superior combustion enthalpy, propagation rate, and activation energy.


Introduction
Nanostructured energetic materials can offer interesting characteristics in terms of high heat output, high reaction rate, as well as controlled sensitivity [1][2][3]. Nanothermite (metal oxide/metal) can act as efficient high energy dense material, with high volumetric energy density compared with common explosives. Nanothermites can experience reaction rate 100 times compared with conventional counterparts [4][5][6]. Much research has been directed to the development of nanothermites for initiation means, propulsion, micro-actuator, and electric match compositions [4,[7][8]. Electric match includes a thin metal wire (bridge wire) coated with a dab of heat-sensitive pyrotechnic composition [9]. Hot solid or liquid particles are desirable for first-fire compositions. Reactive fuel particles such as aluminium can produce good heat output and hot solid particulates. Nanothermite particles (metal oxide/metal) are emerging class of high energy density materials. Nanothermites can enhance not only the heat output, but also the ignitability, thermal conductivity, and propagation rate [10]. Manganese oxide is one of the most effective oxidizers for nanothermite reactions. Furthermore MnO2 has the potentials for catalytic decomposition of energetic materials. Low co-ordination surface oxygen atoms are negatively charged; they have the potential to act as electron donor to electron deficient explosive material [11]. Hydroxyl surface groups could be evolved at low temperature and could attack the NC cyclic ring with the release of active NO2 [4,12]. MnO2 particles could induce condensed phase reaction of gaseous products with the increase in decomposition enthalpy ( Figure 1) [13][14][15]. On the other hand aluminium can experience decomposition enthalpy of 32000 J/g; it is of interest as high energy density material [16]. Aluminium can induce thermite reaction with MnO2 nanoparticles; this binary mixture can secure solid particles at high reaction temperature [6]. Nitrocellulose (NC) is the most common energetic polymeric binder; NC can offer gaseous products of 871 l/kg and explosion heat of 4312 kJ/kg [17]. NC can be employed as energetic binder for advanced electric match compositions [18]. These features can offer enhanced propagation index (PI). PI is a simple method to assess mixture's tendency to sustain burning upon initial ignition by an external stimulus (Equation 1) [9].

PI = ΔHreaction / Tignition (1)
Compositions with high reaction enthalpy and low ignition temperature can secure self sustained propagation at high rate. NC has recently been employed to modify some energetic

Synthesis of MnO2 nanoparticles
MnO2 particles were manufactured using hydrothermal processing. Nanoparticles were fabricated in continuous manner via direct mixing of metal salt with supercritical fluid.
Schematic for continuous hydrothermal synthesis is provided ( Figure S1). Further details about hydrothermal synthesis can be found in the following references [5,22,24]. Nano-oxide particles were flocculated from their synthesis medium and re-dispersed in organic solvent with aluminium nanoparticles.

Development of NC nanocomposite
Nanoparticles have natural tendency to aggregate with decrease in their surface area [25].
Enhanced dispersion characteristics could be accomplished via integration of dispersed particles into energetic matrix [5,25]. Stoichiometric binary mixture of MnO2/Al nanoparticles was re-dispersed into acetone; consequently NC was dissolved in acetone colloid. The hybrid nanocomposite material was developed via co-precipitation technique ( Figure 2).

Characterization of NC nanocomposite
Size and shape of synthesized nanothermite particles were investigated using TEM (JEM-2100F

Thermal behaviour of NC nanocomposite
Nanothermite particles can act as efficient high energy density material. Thermal behaviour of MnO2/Al/NC nanocomposite was investigated using DSC Q200 by TA. The tested sample was heated up to 500 0 C, at 5 0 C min -1 . MnO2/Al/NC weight loss was with temperature was further assessed using TGA 55 by TA.

Kinetics analysis
Kinetic analysis is vital to assess the main decomposition parameters including: pre-exponential Kissinger-Akahira-Sunose (KAS) were adopted for kinetic analysis.

Activation energy calculation
Where β, and Tp are the heating rate, and peak temperature at that heating rate respectively [28][29]. Furthermore, precise evaluation of activation energy (Ea) can be accomplished by

Characterization of nanothermite particles
TEM micrographs revealed mono-dispersed MnO2 particles of 10 nm average particle size

Characterization of NC nanocomposites
Incorporation of colloidal particles into energetic matrix can secure enhanced levels of particle dispersion [35-37]. Enhanced particle (metal oxide/metal) dispersion is mandatory for high reaction rate, and heat release rate. Morphology of dry NC nanocomposite was investigated with SEM. SEM

Thermal behaviour of NC nanocomposite
Nanothermite particles offered an increase in NC total heat release by 150 %, with decrease in ignition temperature by 8 0 C using DSC (Figure 9). The surge increase in total heat release can offer enhanced ignitability and self-sustained reaction at high propagation rate.

Kinetic parameters via Kissinger model
The impact of nanothermite particles on NC kinetic decomposition was investigated using nonisothermal technique. MnO2/Al/NC nancomposite was investigated using TGA; weight loss was investigated at four different heating rates of 2, 4, 6, 8 and 10 °C·min -1 ( Figure 11).

Kinetic parameters via KAS model
The activation energy at the different fractional conversion was determined by using modified Kissinger-Akahira-Sunose (KAS) method. The kinetics parameters of NC and MnO2/Al/NC are tabulated in Table 1. The mean value of the activation energies of NC and MnO2/Al/NC nanocomposite was 420 kJ·mol -1 and 380 kJ·mol -1 respectively. These findings confirmed results from Kissinger's model.

Conclusion and future work
MnO2/Al can secure one of the most vigorous nanothermite reactions in terms of heat output, gaseous products. Mono-dispersed MnO2 particles of 10 nm particle size were developed by hydrothermal synthesis. Aluminium nanoplates of 100 nm were adopted. Integration of colloidal nanothermite particles into nitrocellulose offered enhanced dispersion characteristics.
Consequently high interfacial surface area, high heat release rate can be achieved.
Nanothermite particles offer an increase in NC heat output by 150 %; additionally the ignition temperature was decreased by 8 0 C. Nanothermite particles offered an enhanced propagation index of NC by 261. Additionally, nanothermite particles offered decrease in NC activation energy by 10 %. It can be concluded that nanothermite particles can act as catalyst and high energy density material due to synergism between aluminium and MnO2 nanoparticles.  Figure 1 Schematic for active surface sites of manganese oxide catalyst (i. e. negative surface oxygen, and hydroxyl groups).

Figure 2
Schematic for integration of colloidal nanothermite particles into NC via co-precipitation technique.         Kissinger method to determine the activation energy of NC (a), MnO2/Al/NC (b).