Investigation of a novel nanothermite colloid based on CuO coated CNTs on the thermo-analytical characteristics of 1,3,5-trinitro-1,3,5-triazinane


 Nanothermite colloid is a promising field of research used to enhance the characteristics of energetic materials. In this study, pre-treated surfaces of multiwall-carbon nanotubes (MWCNTs) were catalysed to help the deposition of a metal and the coating process by copper (Cu) nanoscale layer through electroless deposition. The formed hybrids coated by Cu were treated at 250°C to form MWCNTs coated by CuO. Isopropyl alcohol was used to suspend the coated MWCNTs with aluminium nanoparticles (120 nm) in order to form nano thermite colloid by ultrasonic technique. The presence of CuO layer plays the role of an active oxidizer for the Aluminum nanoparticles. The obtained colloid was incorporated and dispersed in 1,3,5-trinitro-1,3,5-triazinane (RDX). The influence of colloid on RDX decomposition kinetic was evaluated using the isoconversional methods (modified Kissinger-Akahira-Sunose (KAS) methods). The mean value of apparent activation energy was reduced by 37.5 %. This dramatic change in RDX decomposition ascribed trait to the nano-thermite colloid reactivity and the facile integration of colloidal thermite particles with the RDX.


Introduction:
The energy output of high explosives can be increased by raising the contribution of the heat of explosion [1,2]. The reactive metal particles might raise the energy density with increasing the total impulse of explosives [3,4]. Aluminum with its variable shapes and sizes has always been an area of interest when used with metal oxides and incorporated in energetic formulations to increase the overall energy and performance [5,6].
Thermite reactions (metal oxide/metal) have high exothermicity and propagate based on their sustained oxygen content; these reactions find wide applications in high energy systems. On the other side, nano-thermites have other various applications especially in the devices used to generate energy in the application of modern energetic materials (MEMs) [3]. Since multiwall carbon nanotubes (MWCNTs) offer relatively large surface areas, they are effective carriers for nanothermite particles.
For pure ordinary organic explosives; the explosion heat depends on the combustion heat produced from the constituent fuel elements on the material (carbon and hydrogen) [7].
Simply the combustion heat could be increased by adding fuels (with high heat of combustion) to the explosive to enhance the explosive overall energy [8], [9]. Most traditional energetic materials might produce free oxygen in the plasma phase of the explosion. The presence of oxidizer (such as thermites or hybrid materials containing sufficient oxidizer) could enhance the performance of the energetic materials due to the availability of oxygen inside the matrix itself [10,11]. Thermites are composed of fuel and oxidizer which are able to undergo reduction-oxidation (Redox) reaction and caused the oxidation reaction to produce a stable oxide and free metal from the reduction process of the metallic oxide. The reaction has high exothermicity and propagates by the self-sustaining oxygen content. Consequence of the released heat and the produced temperature from the thermite reactions, thermites have several applications in welding, reactive fragments and ordinance disposal [12]. Due to the redox reaction, the formed metal might tolerate further combustion in the availability of more oxygen under high temperature and generate more heat [9].
A comparison of the volumetric and gravimetric heats of reaction of the thermite/redox reaction with that of highly energetic explosives proved that the thermite reactions produce higher energies [13] as shown in Fig.1. Comparing the produced energy from thermites composition with common explosives [13] One of the advantages of nanotechnology is the great decreasing of the material particle size and increasing the ratio of surface/volume as shown in Fig. 2. Consequently, the highly increase in the surface energy causes improvement of the contact area and mixing homogeneity [14].
Fig. 2 Effect of reactants particle sizes on the reaction rate and released energy [15] All these properties resulted in significant enhancement of the reactivity and increasing the propagation of the reaction rate. The enhancement of nano-composite thermite reactivity is resulted from the high connection of Al with the oxidizer (metal oxide) [16].
Multi-walled-carbon nanotubes (MWCNTs) contain a structure with rich pore and large surface area (> 700 m 2 /g), which might be used as catalyst [17,18]. If MWCNTs were functionalized with energetic groups, compounds or metal fuels, they become more energetic and might find applications in highly energetic systems, fine welding, aircraft ejection seats and nanothermite synthesis [19] This superior surface area of multiwall-carbon nanotubes (MWCNTs) can be employed in synthesis of hybrid materials by plating the nanotubes with different metals to form a nanocomposite. The metallization process depends on the surface catalyzation of MWCNTs, which increases the activity of the surface to enhance the bonding of the nanotubes with the deposited metal [20][21][22].
The metallization process of MWCNTs could be carried out by an electroless deposition process at room temperature. Electroless deposition technique [17] is the preferred technique when it is compared with other techniques such as molecular-level mixing [23], and chemical/physical vapour deposition [24] due to its simplicity and high efficiency where it is used for the deposition of different elements such as copper [21] , nickel [25], silver, gold [26], and cobalt onto MWCNTs [27].
MWCNTs should be pretreated by acid to purify and catalyze the surface of MWCNTs in order to be ready for the covering process, as shown in schematic representation in Fig. 3. Nano-composite manufacture is based on the dispersion of CuO-MWCNTs in acetone with aluminum nanoparticles. The acetone acts as dispersion medium and solvent for the energetic system to enhance the nano-particle distribution in the medium.
The synthesized energetic hybrid material (MWCNTs/CuO/Al) produces high amount of gaseous where the produced Cu metal might be converted to gas as a result of its low boiling point (1000 o C) [29]. While the conversion process takes place, MWCNTs/CuO/Al reactions will proceed with high rate. RDX (1,3,5-trinitro-1,3,5-triazinane) is one of the most usable explosives all over the world [30], it has several applications such as production of plastic explosives (composition C-4 and semtex-1H) [31,32], highly pressed explosive for shaped charges and formation of demolition charges [33,34]. In this paper, hybrid material based on CuO plated MWCNTs was synthesized by electroless plating process and employed with nano Al(100 nm) to form a hybrid thermite material. The influence of the prepared nano-thermite mixture on the thermoanalytical characteristics of RDX was discussed. This study presents a novel process for the development of a hybrid thermite materials and their incorporation with the explosives for several applications in the field of energetic materials.

MWCNTs Catalyzation process
Catalyzation of MWCNTs is a surface pre-treatment procedure which can be employed to deposit different metals on the surface. Catalyzation process encompasses two main stages including sensitization and activation as demonstrated in Fig. 4 [35][36][37].
Sensitization is the initial process to create active sites on the structure of MWCNTs using

Metallizing the MWCNTs by Copper
Electroless plating of MWCNTs was performed by the deposition of copper nano-crystals to get 16 fold of Cu to 1 fold nano-carbon materials [35,38,39].

Characterization of MWCNT.
The crystal morphology of MWCNTs and CuO/MWCNTs was studied by TEM (JEM-2010F by Joel Corporation). The crystallinity of the samples was determined by using X-ray diffraction (XRD) based on Bruker D8 Discover instrument. Also, SEM (ZEISS SEM EVO 10 MA) was used to study the crystal sizes and shapes of the studied samples.

Dispersion of CuO-MWCNTs with Alex into energetic matrix
CuO-MWCNTs/Al/RDX was developed by solvent blending approach. The following procedure was applied: The calculated amounts of 94% RDX, 1% CuO-MWCNTs and 5% Al were weighed accurately. CuO-MWCNTs was added with aluminium nanoparticles (Alex) in acetone placed on ultrasonic path in order to dissipate the oxidizer and fuel particles and eliminate any aggregation. The prepared nanothermite was added to the completely dissolved RDX in acetone under sonication followed by gradual heating to guarantee the evaporation of acetone and complete dispersion of nanothermite particles with the energetic matrix (RDX).

Thermal analysis study
The RDX/MWCNT/Al/CuO and the pure RDX were studied by Thermogravimetric analysis (TGA55, TA Instruments, USA , open high temperature platinum crucible) at heating rates of 3, 5, 7 and 10 0 C min -1 . The samples were tested in a range of temperature of 40-350 o C, and the mass of the sample was 1-2 mg tested in dynamic nitrogen atmosphere (40 ml min -1 ).

Theoretical investigation
The decomposition kinetics are based on determining the activation energy, Ea, with the preexponetial factor, A, in addition to the model of the reaction, f(α), which are known as kinetic triplet. The evaluation of the solid phase reactions could be investigated by various techniques [40]. Different isothermal or nonisothermal methods could be applied to obtain the decomposition kinetics.
Isoconversional method could be investigated depending on the change of the rate of process In this case, the reaction model is f(α) and k(T) is a rate constant (depend on the temperature) and could be determined by applying Arrhenius equation By adding eq. 2 to eq.1, the kinetic differentiation could be: By the integration of eq.3, the following equation was obtained Here; g(α) represents the integrated order for the model of the reaction.
In the iso-conversional method, the model of the reaction is based on Eq. 1 and does not depend on the temperature. In this study, the modified Kissinger-Akahira-Sunose (KAS) methods were investigated: The symbol Tp represents the exothermic peak temperature at the selected rates and  is the selected heating rate. By plotting of ln (

CuO-MWCNTs characteristics
TEM images of hybrid nancomposite CuO-MWCNTs reveal the CuO nano-particles (14 nm particle size) deposition on the CNT walls. Figure 5 confirms the success of the electroless plating methodology to enhance effectively the surface area occupied by the copper oxide.
These surface areas with the localized oxidizer sites could support rapid reactions in the presence of Alex metallic fuel.

Non isothermal kinetic parameters studies
For the advanced modified energetic compositions, it is important to determine the thermal characteristics due to their intrinsic connection with the performance of detonation and combustion of the used composition [42][43][44][45][46]. Also the thermal stability and the activation energy (Ea) are essential to be determined in addition to the decomposition heat [47][48][49][50][51].
Several isoconversional methods can be applied to study the decompositions characteristics of the energetic colloids such as the modified Kissinger-Akahira-Sunose (KAS) which is recommended according to ref. [40]. In this study, KSA is used to determine the Ea at conversion rates ranging from 5 to 90%. In this study, the mean values of α were selected in the interval of 0.25 to 0.85 due to the inaccuracy of the results from the tail peaks. The obtained data are reported in Table 2 decrease their decomposition peak temperature [19]. In case of RDX/MWCNT/Al/CuO, the decomposition reaction is complex and nonlinear in comparison with the pure RDX.
It was stated the decomposition mechanism of RDX depends on the grain formation, nucleation and growth of reactive sites [52]. The addition of nano-materials could change the decomposition mechanism of the RDX, causing the modification of the gaseous products with different percentages. In addition, the used MWCNTs have high conductivity heat, which caused the enhancement of the conduction heat on the surface of the colloid and could decrease the decomposition temperature. Also the large SSA of MWCNTs could absorb some of the reductive gaseous products and prevent their getting-away from the surface. So, the reaction was continued in the condensed phase and resulted in the improvement of the catalytic effect.

Influence of the conversion extend on the kinetic parameters
In figure 9, the dependence of the reaction conversion (on the temperature of While the RDX/MWCNT/Al/CuO activation energy decreased along with the extent of conversion of 0.55 then started to be on the same value. As mentioned, decreasing the particle size leads to increasing both the specific surface area and the number of surface atoms of the particles which caused strong molecular vibrations of RDX molecules on the surface and increase the molecular energy. It was mentioned that the addition of nanoscale metals or metal oxides, accelerates the decomposition of some explosives due to the improvement of the molecular energy level [52,53]. In this study, the RDX molecules on the crystal surface have high energy levels resulted in decreasing the decomposition temperature. Also it was stated by Joseph et al that the presence of nanoscale metal might promote the decomposition of HMX explosive as a result of the physical adsorption applied by the nano-scale metal and the formation of hotspots [54]. This explanation might be suitable to explain the results of RDX (nitramine explosive such as HMX) integrated with MWCNT/Al/CuO arrays. The improvement of the thermal conductivity is due to the presence of nano MWCNT with metal/metal oxide resulted in more pyrolysis products which activated the molecules and promoted the RDX decomposition, as a result the activation energy was decreased. However, the kinetics of decomposition of nano-thermite composite was changed and decreased the activation energy. The activation energy of the nano-thermite is 135 kJ mol -1 and is lower than the RDX (181.6 kJ.mol -1 ). The activation energy of the studied nanothermite composite, decreased along with the extent of conversion of 0.55 then started to be on the same value till the end of decomposition. The thermal stability considered constant after the addition of the nano-thermite to the composite. The nano-thermite composite seems to enhance the performance due to presence of the Alex and CuO with high stability after their addition to the RDX and could be used for practical military and civilian applications.