Graphene-iron oxide nanocomposite (GINC): an efficient catalyst for ammonium perchlorate (AP) decomposition and burn rate enhancer for AP based composite propellant

Abstract

A facile and ecofriendly method for the synthesis of nano-sized iron oxide (Fe₂O₃) decorated graphene (GINC) hybrid by ultrasonication via microwave irradiation has been developed. During this process, nano-sized Fe₂O₃ particles with a size of approximately 20–30 nm were uniformly decorated over a graphene sheet. The nanohybrid was characterized by XRD, HRTEM, Raman spectroscopy and Raman mapping. To study the enhancement of catalytic activity of iron oxide by preparing GINC, several AP based compositions containing 1–5 weight% GINC were made and characterized through simultaneous thermal analysis (STA). Along with this, formulations with other catalysts with 1–5 weight% concentrations were also prepared and evaluated. Experimental results showed that GINC with 5 weight% concentration was considerably more effective as compared to other compositions. To further extend this application as a burn rate enhancer in composite propellants, several formulations of composite propellants containing 1 part of different burn rate enhancers, such as Fe₂O₃, nano-sized Fe₂O₃ and GINC, were prepared and evaluated using theoretical prediction, viscosity, ballistic properties, sensitivity parameter and thermophysical properties. To quantify the burn rate enhancement in the presence of GINC, burn rate measurement, STA, DSC and activation energy calculation were performed. The results show that the burn rate of propellant increases from micron-sized Fe₂O₃ (30% increases) to nano-sized Fe₂O₃ (37% increase). In the presence of GINC, a significant increase (52%) in burn rate is achieved. In GINC, effective iron content is about 50% as compared to nano- and micron-sized Fe₂O₃. Hence, GINC was found to be an excellent burn rate modifier for an advanced AP based propellant system.

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1. Introduction

Graphene is a special type of carbon having a flat sheet, which is just one atom thick. It has attracted great attention in recent years due to its unique features such as electrical, optical, catalytic and mechanical properties. Graphene nanosheet shows very high electrical conductivity at room temperature due to the high mobility of electrons. Graphene was discovered in 2004 by Andre Geim and Konstantin Novoselov. They obtained a graphene sheet by splitting graphite crystals into increasingly thinner particles until individual atomic planes were obtained. This remarkable contribution was felicitated by the Nobel Prize in Physics in 2010 and led to a sudden increase in research interest on graphene.^1,2

Graphene is the mother form of all the graphitic forms. It is a building unit for carbon nanostructures of all other dimensionalities such as 0D Bucky balls, 1D nanotubes and 3D graphite. It has a lot of similarities to carbon nanotube (CNT) such as structure, properties with high aspect ratio (i.e. lateral size/thickness), a large surface area, good mechanical properties and rich electronic states. It has lots of scope in different areas where CNTs have already been exploited. Graphene is a better electrode material compared to CNTs. The 2D planar geometry of graphene sheet assists electron transport. Table S1 (see ESI†) depicts some properties of graphene as compared with CNTs.

Due to excellent properties with large surface area, graphene nanosheets (GNSs) with a 2D geometry have been considered to be a new class of promising materials for prospective applications in solar cells, actuators, sensors, field-effect transistors, field-emission devices, batteries and super capacitors.^1–9 In addition to this, the production cost of graphene is considerably lower compared to CNTs^10,11 and hence large scale production of graphene is feasible. Li et al.¹² have mentioned a facile approach to synthesize graphene materials. It opens up tremendous opportunities for various technological applications. Graphene is composed of sp² hybridized carbon atoms and has a honeycomb structure.^13–16 A thorough review by Noorden¹⁷ on different carbon nanostructures concluded that research interests on fullerenes and carbon nanotubes become saturated but increased research interest may be expected for graphene in the next decade.

Recently, graphene sheets have been successfully used as a substrate that helps to stabilize and disperse nanoparticles (CdS, CdSe, Fe₃O₄, TiO₂, SnO₂, Co₃O₄).^18–24 This dispersion along with the stabilization of nanoparticles helps to enhance catalytic activity; hence, it needs to be synthesized by facile and environmentally benign techniques.

Metal/metal oxide particles have received considerable attention due to their distinctive optical, electronic and catalytic properties.^25,26 Transition metal particles (Fe, Co and Ni) and metal oxides (such as Fe₃O₄, TiO₂, SnO₂, and Co₃O₄) have a distinctive catalytic nature. They have been widely used to form a new class of nanocomposites by decorating CNTs. Note that these nanocomposites show enhancement in catalytic behavior. On the other hand, relatively less attention has been given to metal/metal oxide nanoparticles supported on graphene sheets.^27–29 Iron oxide is a very good magnetic material that has been broadly used in magnetic storage, microwave absorbing materials, catalysts and new devices. Raw iron oxide is a good burn rate enhancer in ammonium perchlorate based composite propellant. When nano-sized iron oxide was used in the composition, catalytic activity increased as per a previously reported study.³⁰ We hope that Fe₂O₃/graphene nanocomposite (GINC) can have a possible synergistic effect or a combination of the properties of the two components (graphene sheet and Fe₂O₃) in new composite materials that impart unique features for catalysis and nanotechnology.

To evaluate the effect of GINC in AP based composite propellants, several compositions were processed. Composite propellant mainly consists of two parts, i.e. HTPB based binder and oxidizer. AP is generally used as a potential oxidizer. Burn rate is one of the vital properties for a composite propellant. For strategic reasons, propellants with different burn rates were required to be processed for different missiles. From ultra low burn rate (r ≤ 3 mm s⁻¹ at 70 ksc) to very high burn rate, propellants (r ≥ 50 mm s⁻¹ at 70 ksc) were processed and used in different missile systems. As shown in many studies, the reaction rate, activation energy and temperature of the thermal decomposition of propellant are closely related to the burn rate of the composite propellants. The lower the decomposition temperature, the higher will be the burning rate of the propellant.^31–33 The burn rate enhancement effects of some transition metal oxides in different composite propellants have been widely used. The result shows that the final decomposition temperature of composite propellant directly depends on the amount of burn rate catalyst added, as well as the particle size of the burn rate enhancer, both of which are important factors that affect the decomposition temperature of the propellant. It is well proven that a nano-sized catalyst is more effective as compared to a micron-sized catalyst.^34,35 It is also known that nanoparticles are able to agglomerate due to their small size, large surface area, and high surface activity, which greatly affects their catalytic behavior. However, agglomeration causes an inhomogeneous dispersion of the nanocatalyst; hence, reduced catalytic efficiency was observed, which leads to an increase in the practical cost of the catalyst. Graphene has a very large surface area of nanoparticle to disperse and distribute on; moreover, it has many more important properties such as low mass density, good thermal stability, chemical inertness and high electrical and thermal conductivity (see Table S1†).

Herein, we have highlighted (see Scheme 1) a simple, facile method for the decoration of iron oxide nanoparticle over a graphene substrate. Presently, the catalytic behavior of transition metal oxide decorated graphene nanocomposite in the propellant burning rate, as well as thermal decomposition of AP, have rarely been reported. Therefore, in this work, graphene and nano-sized iron oxide were separately prepared and characterized. GINC was prepared by ultrasonication through microwave irradiation and characterized. The comparative study of GINC's catalytic effect within the AP decomposition reaction was investigated. Along with this, catalytic performance with a variety of concentrations were also evaluated and optimized. Several propellant compositions with different burn rate modifiers (1 part), i.e. normal Fe₂O₃, nano-sized Fe₂O₃, GINC and a blank sample were processed and evaluated by strand burn rate data, simultaneous thermal analysis (STA), differential scanning calorimetry (DSC) and activation energy calculation.

Scheme 1 Synthesis, processing and application of GINC as burn rate modifier in missile system.

2. Experimental section

2.1 Synthesis procedures

All the chemicals used were of analytical grade and used without further purification. Graphite flakes used in this work were purchased from Reinste nano ventures, India. Graphene was prepared in three steps as reported in the literature.³⁶ In the first step, graphite oxide was prepared from graphitic flakes by the Hummers method.³⁷ In the second step, thermally expanded graphene oxide (TEGO) was prepared by thermal expansion/exfoliation at 1050 °C (Ar, 30 s). Finally, the graphene nanosheet (GNS) was obtained by hydrogen reduction of TEGO at 400 °C for 2 hours.

The synthesis of nano-sized iron oxide consists of two steps: the preparation of polymeric gel citrate, followed by gel calcination. Gel preparation was carried out in a gel preparation unit and calcination was carried out in a calcination unit, i.e. muffle furnace. A typical batch operation was as follows: solid citric acid and ferric nitrate nonahydrate were dissolved in ethylene glycol in the dissolution vessel to get a brown color solution. The solution was slowly heated to 90 °C by circulating hot thermic fluid (silicone oil) in the reactor jacket. Initial exothermicity was observed at a reaction temperature of ∼100 °C with evolution of NOx fumes, which was controlled by stopping the circulation of hot thermic fluid in the reactor jacket. Exothermicity of the reaction subsides during the rest of the reaction period. Then, the hot oil circulation was started in the reactor jacket and temperature of reaction mass was increased from 100 °C to 140 °C. Water molecules evolved during the reaction were condensed by a condenser using chilling water circulation in the tubes, and uncondensed gases were scrubbed by venturi scrubber. After the collection of ∼3.5 L condensate, the polymeric gel was prepared, and then, it was placed in the muffle furnace. The gel was first dried at 300 °C for 2 h under oxygen rich atmosphere by purging compressed air. Evolved gas was exhausted through a chimney by an ID fan. Then, the dried gel was calcined at 600 °C for 6 hours to obtain a pure nano-sized α-Fe₂O₃.

For preparing GINC, first, 50 mg graphene was dispersed in absolute ethanol by ultrasonication for 40 min. After ultrasonication, nano-sized iron oxide was added into the graphene dispersion and further ultrasonicated for 120 min. After ultrasonication, the dispersed composite solution was placed in ambient condition for drying. After the evaporation of ethanol, the material was deposited on a Petri dish. The deposited material was placed into the microwave reactor for 2 min for better exfoliation. After cooling, the sample was collected in a sample vial (see Scheme 1).

To study the efficiency of catalytic behavior of GINC for AP decomposition, four different compositions with different catalysts such as graphene, iron oxide (micron-sized and nano-sized) and GINC with 1 wt% concentration were prepared and characterized.

2.1.1 Composite propellant processing. To evaluate the burn rate enhancing capability of GINC with respect to other burn rate enhancers for the composite propellant, several formulations (4 nos) were processed. Composite propellant consists of two parts, i.e. binder and solid ingredient. The binder, which consist of hydroxy terminated polybutadiene (HTPB: OH value 40–50 mg g⁻¹, moisture 0.15%, from Orion Chemicals), was cured with toluene di-isocyanate (TDI: purity 99%, RI 1.565-1.567 at 30 °C; from Bayer). Dioctyl adipate (DOA: saponification value 303 ± 3, moisture 0.5%, from Subhash Chemicals) was used as a plasticizer to increase the processability. Indigenously prepared GINC, nano-sized iron oxide, graphene and micron-sized iron oxide (from S. D. Fine Chemicals) were used as a burning rate enhancer. The mixture (1 : 2) of trimethylolpropane (OH value 1220, moisture 0.5%, purchased from Celanese) and butane-1,4-diol (OH value 1220, RI 1.444 ± 0.002 at 30 °C, moisture 0.5%, purchased from Biaf) was used as an adduct in the composition. Two different sizes of ammonium perchlorate were used in the propellant formulations. The first consisted of pure, research grade ammonium perchlorate (purity 99%, density 1.95, from Tamil Nadu Chlorates) with an average particle size of 300 μm. The other size of ammonium perchlorate was prepared by grinding ammonium perchlorate (>99% pure) in a fluid energy mill to an average particle size of 60 μm. Aluminum metal powder (from MEPCO) of an average particle size of 15 μm was used as a metal fuel. The propellant formulation is given in Table S3 (see ESI†). The propellant formulations were mixed in 10 kg batches using a vertical planetary mixer of 15 L capacity. During mixing, vacuum (2–3 Torr) was applied at 55 °C, in order to remove air bubbles from the formulation prior to casting. The propellant mixture was cast under vacuum by slurry cast techniques.³⁸ The propellant was cured at 60 °C for 10–12 days in a water-jacketed oven. The base composition, without burning rate enhancer, was also processed in the same manner. The propellant formulations were subjected to various performance tests. The detailed process flow chart is given in Fig. 1.

Fig. 1 Flow chart for the processing of composite propellant.

2.2 Characterizations

2.2.1 GINC characterizations. Sonication was conducted using an ultrasonication bath (35 kHz, Kudos). Microwave irradiation was carried out by a microwave reactor (make: Raga). Environmental scanning electron microscopy (ESEM) was carried out with a Quanta 200, FEI. High resolution transmission microscopy (HRTEM images, SAED pattern) characterization was carried out by Tecnai F-30, FEI with a 300 kV field emission gun (FEG). Nano-sized Fe₂O₃, graphene and graphene-iron oxide nanocomposite were dispersed in methanol with ultrasonication. The dispersion was placed on a TEM grid, and the solvent was evaporated by oven drying. Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 5700, Thermoscientific. Raman traces and mapping were measured by inVia reflex micro Raman, Renishaw. X-ray diffraction traces were collected by X-ray diffractometer D8 Avance, Bruker, with Cu Kα source in the measurement angle range of 2θ = 2–90° with a scan rate of 2° min⁻¹.

To evaluate the catalytic effect of GINC on AP decomposition reaction, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out by Mettler Toledo, Model-TGA/SDTA851 at a heating rate of 20 °C min⁻¹ in a static nitrogen atmosphere with alumina reference material. Comparison with different burn rate enhancers, such as micron-sized iron oxide, nano-sized iron oxide and graphene with concentration variation were also carried out.

2.2.2 Propellant characterization. Before making an actual composition, some important ballistic properties, such as characteristic velocity (C*), flame temperature (T_f) and specific impulse (I_sp), were calculated by NASA CEC-71 software.

To study the effect of GINC in AP based composite propellant, several important properties have been evaluated during the course of propellant processing. During processing, viscosity is one of the important parameter, and viscosity of the propellant mixture was characterized by Brookfield viscometer at 40 °C. These measurements were carried out for 4–5 h at an interval of one hour.

After the completion of curing reaction, the strand burning rate of the propellants was determined in the pressure range of 5–9 MPa by employing an acoustic emission technique.^39,40 The methodology involved the combustion of the strand (ignited using a Nichrome wire) of dimensions 100 × 6 × 6 mm in the nitrogen pressurized steel bomb. The acoustic signal generated and the perturbations caused by the propellant deflagration were transmitted through the water medium to a piezoelectric transducer (200 kHz) connected to an oscilloscope. The burning rates were computed from the time that was recorded in the trial conducted at each pressure for each sample. The standard deviation was of the order of 0.2%.

Calorimetric values were obtained using a bomb calorimeter (make: Parr instruments, Germany). The densities of propellant compositions were determined by a Mettler density kit (density 1.432 g cm⁻³, heat of formation −565.8 kJ mol⁻¹). Note that toluene was used as a liquid.

Density = (weight of the sample/weight of the sample in solvent) × specific gravity of the solvent (toluene).

The sensitivity to impact stimuli of the propellant compositions was determined with a fall hammer apparatus (2 kg drop weight) using the Bruceton Staircase method⁴¹ and the results are given in terms of the statically obtained 50% probability of explosion (H50). The friction sensitivity was measured with a Julius Peter apparatus by incrementally increasing the load from 0.2 to 36 kg, until there was an ignition/explosion in five consecutive test samples.

The ignition temperature was measured by a Julius Peters apparatus. The sample was uniformly heated at a constant rate (5 °C min⁻¹) in a Wood's metal bath until it exploded or ignited at the ignition temperature.

Thermophysical properties were also evaluated by Flashline-3000, Anter Corporation. Thermal analysis of the propellants was carried out using STA (Q-600, USA), Perkin Elmer Pyris Diamond DSC apparatus at a heating rate of 20 °C min⁻¹ under a N₂ atmosphere (sample mass ∼10 mg). The activation energy for propellant combustion was also determined with the help of DSC at different heating rates by following the Kissinger Ozawa equation.

The mechanical properties were obtained with an Instron device (Model TIC-1185, UK). The operating instrumental parameters were always maintained constant: gauge length, 25 mm; crosshead speed, 50 mm min⁻¹. The stress and strain properties were determined using a dumbbell-shaped specimen as per the ASTM-D-638 specification.

3. Results and discussion

High resolution transmission electron microscopy (HRTEM) and a selected area electron diffraction (SAED) were carried out to inspect the quality of nano-sized iron oxide decoration on the graphene layer. Fig. 2 shows the HRTEM micrographs with an SAED pattern of graphene, nano-sized iron oxide and GINC. The results confirmed that the size of nano-sized iron oxide and graphene are in the nano range. In fact, graphene sheets were clearly visible, and both nano-sized iron oxide and graphene were found to be crystalline in nature as confirmed by the SAED pattern. According to Fig. 2c, iron oxide nanoparticles were placed over the graphene sheet to form a nanocomposite; moreover, it nicely shows arranged lattice fringes in GINC.

Fig. 2 HR-TEM image (I, II and III) and SAED pattern (IV) of (a) nano-sized iron oxide, (b) graphene, (c) graphene-iron oxide nanocomposite (GINC).

Fig. 3(a) shows the FTIR spectrum, which confirms the absence of a functional group and presence of low defect on the graphene surface. Along with this, the Raman traces of Fe₂O₃ were observed. Raman spectroscopy was used to examine the quality of the graphene sheet before and after the formation of nanocomposite using the abovementioned techniques. The most pronounced Raman traces were a D band at 1310 cm⁻¹, corresponding to defects, and a G band at 1575 cm⁻¹, which corresponds to the in-plane vibration of sp² carbon. A 2D band at 2627 cm⁻¹, which is generated due to a two phonon double resonance process, has also been observed. According to Fig. 3b, the D band of graphite was weak, whereas graphene shows a small D band. The lower intensity D band indicates the presence of a lower defect on graphene flakes. Coleman and coworkers suggested⁴² that the defects are mainly present at the edges of the flakes, and the basal plane is found to be defect free. The I(D)/I(G) of the GINC is increased by 2 times (0.993) with respect to pure graphene (0.497). Several defects with sp² domain were formed during the formation of nanocomposite. Fig. 3(b) shows a characteristic Raman signature; moreover, the decoration of nano-sized iron oxide over graphene substrate was confirmed by Raman mapping (see Fig. 3(b)).

Fig. 3 (a) shows FTIR traces, confirming the presence of low defect content in graphene. (b) shows a characteristic Raman signature. The decoration of nano-sized iron oxide over graphene substrate is confirmed by Raman mapping.

Fig. 4 XRD traces of (a) iron oxide, (b) graphene (c) graphene iron oxide nanocomposite (GINC).

Table S2 (see ESI†) depicts the thermal analysis results of AP and AP with 1 wt% of different burn rate enhancers, i.e. graphene, GINC, nano-sized Fe₂O₃, and micron-sized Fe₂O₃. The results show that AP decomposes at lower temperature and gives higher ΔH value in the presence of 1 weight% GINC as compared to other burn rate enhancers. Hence, GINC was found to be a more efficient catalyst for AP decomposition reaction. This was also supported by thermogravimetric analysis data in which 73.81% weight loss was observed in a temperature range of 224.7–384.5 °C. These temperature regions were relatively low when compared to other compositions. Furthermore, the catalytic efficiencies of GINC, graphene, nano-sized iron oxide and normal iron oxide with concentration variation (from 1, 3, 5 wt%) were also studied.

Fig. 4a–c depicts the typical XRD pattern of nano-sized iron oxide, graphene and GINC from 0° to 100°. All the peaks were assigned to crystallographic phases. The XRD pattern of GINC (Fig. 4c) showed graphene peaks, as well as nano-sized Fe₂O₃ peaks. It was revealed that during processing, the crystallographic phases of graphene and iron oxide remains intact. According to the Scherrer equation, the calculated crystallite sizes of graphene nanosheet (GNS) and nano-sized Fe₂O₃ were 28 nm and 38 nm, respectively. The catalytic efficiencies of all the burn rate enhancers were found to increase with increase in concentration (see Fig. 5a–l).

Fig. 5 DTA traces of (a) AP; (b) AP with 1%, 3%, 5% graphene; (c) AP with 1%, 3%, 5% iron oxide; (d) AP with 1%, 3%, 5% nano-sized iron oxide; and (e) AP with 1%, 3%, 5% graphene iron oxide nanocomposite (GINC). TGA traces of (f) AP with 1%, 3%, 5% graphene; (g) AP with 1%, 3%, 5% iron oxide; (h) AP with 1%, 3%, 5% nano-sized iron oxide, and (i) AP with 1%, 3%, 5% graphene iron oxide nanocomposite (GINC). Comparative bar diagrams of (j) HTD with different compositions in DSC analysis; (k) ΔH; and (l) final temperature of different compositions in DTA analysis.

The DTA results also support the DSC result. The final temperature, i.e. T_f, was found to be lowest in case of GINC (see Fig. 5l).

To evaluate the efficiency of different burn rate enhancers (iron oxide, nano-sized iron oxide, GINC), four different propellant compositions based on AP/HTPB/Al were processed (see Table S4 in ESI†). Burn rate enhancers were added at a concentration level at 1 weight% over the batch in three formulations. The theoretical performance of the formulations containing graphene, iron oxide, nano-sized iron oxide and GINC was computed using NASA CEC-71 (see Table S4 in ESI†). From Table S4 (see ESI†), it was observed that the flame temperature (T_f), C*, and specific impulse (I_sp) were decreased due to the incorporation of burn rate enhancers. The overall percentage of inert material increased, which was reflected by the predicted theoretical data.

During processing, marginal increase in mixture viscosity of CP-2 propellant composition (containing 1 wt% GINC over the batch) was observed with respect to base composition. For Fe₂O₃ and nano-sized Fe₂O₃, the viscosity of propellant mix significantly increases, which creates a processing problem (see Fig. 6a).

Fig. 6 Comparative bar diagram of (a) viscosity, (b) UTS, (c) peak temperature in DTA, and (d) burn rate of propellant.

The strand burning rate experiments were conducted in a pressure range of 5–10 MPa, and the base composition exhibited a burning rate of 6.7–9.5 mm s⁻¹ (see Table S6 in ESI†). The addition of GINC leads to a 30–40% enhancement in the burning rate of the propellant composition. Other burn rate enhancers, such as micron size Fe₂O₃ and nano-sized Fe₂O₃, also increased the burning rate. However, the rate of increase with GINC was found to be maximum (52% increase) compared to other compositions. Though the burning rate of GINC containing composition was considerably higher compared to other compositions, the pressure exponent was found to be relatively low, which is the most desirable⁴³ condition for composite propellants (see Fig. 6d). Indeed, for missiles, composite propellant burns at pressure ranges of 5–10 MPa. At this pressure range, shifting of n value was not observed. Beyond these pressure ranges, i.e. at 60–70 MPa, marginal variation in the pressure exponent can be observed. Along with this, a propellant composition containing soot-based iron oxide has been processed, and the burn rate was measured at a pressure of 70 MPa. In this experiment, we have maintained the moisture level less than 0.01%, and further enhancement of burn rate was not observed due to the presence of soot. The results are also supported by one of the works reported in contemporary literature.⁴⁴

All of the propellant formulations containing different burn rate modifiers were marginally more sensitive to impact and friction as compared to the base composition (see Table S4 in ESI†). The sensitiveness may be attributed to the presence of the burning rate enhancer in contrast to AP.

CP-1: binder (15%) + Al (17%) + AP (68%) + burn rate enhancer (Fe₂O₃) − Nil; CP-2: binder (15%) + Al (17%) + AP (68%) + burn rate enhancer (GINC) − 1 wt% over the batch; CP-3: binder (15%) + Al (17%) + AP (68%) + burn rate enhancer (nano-sized Fe₂O₃) − 1 wt% over the batch; and CP-4: binder (15%) + Al (17%) + AP (68%) + burn rate enhancer (micron-sized Fe₂O₃) − 1 wt% over the batch.

Table S5 (see ESI†) shows the DSC, STA results of four compositions containing burn rate enhancer. In the presence of GINC, peak temperature decreases up to 44 °C, which mainly governs the catalytic efficiency. For Fe₂O₃ and nano-sized Fe₂O₃, peak temperature decreases up to 34 °C. Note that graphene as a substrate increases the electron transfer process; hence, catalytically enhanced heat is released by two fold. In the presence of graphene, catalytic activity of nano-sized Fe₂O₃ increases tremendously.

These results are also supported by the activation energy calculation through Kissinger Ozawa equation. DTA results also agree with the above finding. TGA results showed that with the GINC based composition (CP-2), weight loss started at 230 °C, which was approximately 20 °C lower compared to other compositions (see Fig. 6c). TGA data were measured between 50 °C and 400 °C. At this temperature range, 25% residual materials were present. With increases in temperature, i.e. beyond 400 °C, residual material also gets burnt and forms a gaseous product.

Thermophysical properties were also highlighted in Table S5 (see ESI†). CP-2 showed the lowest thermal expansion coefficient (81.4 × 10⁻⁶/C at 30 °C), indicating better structural integrity of propellant grain with temperature. Thermal conductivity of CP-2 was found to be higher in comparison to other compositions. Due to the presence of GINC, thermal conductivity was increased with decrease in curing time.

The mechanical properties (see Table S4 in ESI†) were evaluated for all the four compositions. Only marginal changes in the properties (TS, %E and E modulus) were observed between base composition (TS: 10.5 ksc, %E: 43%, E modulus: 45 ksc) and CP-2 (TS: 10.0 ksc, %E: 36.1%, E modulus: 52 ksc). In the case of GINC formulation (CP-2), elongation is reduced by 17% with respect to CP-4. If we look at two other mechanical parameters, i.e. tensile strength and modulus, it is found that GINC formulation (CP-2) has higher TS and modulus compared to CP-4. Elongation can be maintained in CP-2 with respect to CP-4 by two ways: either by tailoring NCO/OH ratio with decrease in NCO/OH ratio or by increasing the quantity of chain extender (n-butane diol).

For other two compositions (CP-3 and CP-4), mechanical properties (TS and modulus) significantly degraded. This was due to the addition of non-reinforcing filler, such as Fe₂O₃, which reduced the mechanical properties (see Fig. 6b).

3.1 Mechanism of thermal decomposition of AP

A number of studies were carried out to realize the decomposition mechanism of AP^45–50 and they provide good accounts of the probable decomposition mechanism of AP; moreover, numerous mechanisms have been proposed, but the decomposition mechanism of AP is still a debated matter. Two probable mechanisms have been highlighted here.

3.1.1 Electron transfer mechanism. In this mechanism, AP decomposition follows a three step mechanism as proposed by Song and Zhang:⁵¹

(1) In the first step, the endothermic transformation happens from the orthorhombic phase (low temperature) to the cubic phase (high temperature).

(2) In the second step, the exothermic low temperature decomposition process (LTD) of AP (300–330 °C) takes place as follows:

NH₄ClO₄ ↔ NH₄⁺ + ClO₄⁻ ↔ NH₃ (g) + HClO₄ (g) ↔ NH₃ (s) + HClO₄ (s)

(3) In the third step, the exothermic high temperature decomposition process (HTD) of AP would take place (450–480 °C), and the heterogeneous decomposition of deprotonized HClO₄ gas on the solid surface occurs.

According to the above experimental findings, it can be assumed that GINC accelerates both LTD and HTD. A probable mechanism has been shown in Scheme 2.

Scheme 2 Decomposition mechanism of AP in the presence of GINC.

For LTD, the gas and solid phase reactions occur simultaneously. During this process, dissociation and sublimation take place^52,53 as highlighted in the second step. For LTD, the transfer of an electron from ClO₄⁻ to NH₄⁺ should be the controlling step, while for HTD, the controlling step is the transformation from oxygen (O₂) to superoxide (O₂⁻). It is well known that graphene exhibits several unique properties, such as good conductivity, distinct electric field effect with a charge concentration as high as 10¹³ cm⁻³ and mobility⁵⁴ as high as 1.5 × 10⁴ cm² V⁻¹ s⁻¹. The movement of an electron in graphene is considerably faster than in metal atoms and can reach an effective speed of 300 times less than the speed of light in vacuum and travel a large distance without deflection.¹ Hence, the synthesized GINC used here helps to accelerate the electrons to speed up the abovementioned controlling steps. In other words, due to the accelerated electron flow provided by graphene, NH₄⁺ and ClO₄⁻ are transformed to NH₃ and HClO₄. Next, HClO₄ generates O₂, which subsequently forms superoxide (O₂⁻) more rapidly with the help of graphene as a perfect bed for accelerated electron flow. These superoxide ions help to decompose NH₃ with other side products generated from HClO₄ and ensure complete decomposition of AP.

In case of bare nano-sized iron oxide, no supporting substrate is available; hence, it is more likely to form an aggregate and render less active sites to adsorb a gas molecule (NH₃, HClO₄) to accelerate the reaction. However, when decorated on graphene, which is the best catalyst substrate with the theoretical surface area¹⁰ of 2600 m² g⁻¹, the decorated particles are able to unfold on the graphene substrate to generate more active sites and react with NH₃ of HClO₄, and hence accelerate the catalysis process. However, a marginal difference was observed for the catalytic performance of 3% and 5% GINC.

Similar to AP decomposition reaction, GINC also enhances the burn rate of propellant. The enhancement was found to be considerably higher when compared to the other burn rate enhancers such as graphene, Fe₂O₃ (micron-sized), and Fe₂O₃ (nano-sized). During the combustion of propellant, AP decomposes as per the abovementioned mechanism. Along with this, the combustion of propellant binder (HTPB based) also occurs at a faster rate due to rapid electron flow to the combustion step from the graphene substrate. Hence, a higher burn rate of propellant was observed.

3.1.2 Proton transfer mechanism. Jackobs et al. have proposed a scheme for the thermal decomposition of AP by proton transfer mechanism (Boldyrev;⁵⁵ Jacobs and Russell-Jones^56,57):

NH₄ClO₄↔NH₄⁺ (a) + ClO₄⁻ (a) ↔NH₃ (g) + HClO₄ (g)

This mechanism also consists of a three step mechanism. Step I involves pairs of ions in a perchlorate ammonium lattice. In Step II, the decomposition step starts with proton transfer from the cation NH₄⁺ to the anion ClO₄⁻ via a molecular complex. This molecular complex decomposes into ammonia and perchloric acid in Step III.

In this mechanism, NH₃ and HClO₄ molecules either react in the absorbed layer over the surface of perchlorate or interact in gas phase by desorption and sublimation. Several reactions quickly occur in the gas phase between NH₃ and HClO₄, forming O₂, N₂O, Cl₂, NO, and H₂O as side products at a low temperature (<350 °C).

A crucial feature of the AP decomposition mechanism is that it occurs in pores beneath the surface at a distance of a few microns. This is the basic difference between sublimation and decomposition of AP.

In this mechanism, perchloric acid is desorbed more quickly compared to ammonia;^56,57 hence, it causes incomplete oxidation of ammonia, creating a saturated atmosphere^56,57 of NH₃. As a result, HTD decelerates and undergoes incomplete transformation; moreover, during HTD, NO, O₂, Cl₂ and H₂O products are produced in the gas phase reaction.

Both HTD and LTD have a common start, i.e. transfer of proton from NH₄⁺ to ClO₄⁻ anion. The difference between LTD and HTD is only in the fact that a low temperature stage occurs on crystal defects (topochemistry), whereas a high temperature slow reaction proceeds in the lattice of the remaining normal crystal. During LTD, orthorhombic modification proceeds in the pores below the surface in the sites where secondary dissociated product gets accumulated, and the sites where active centre regeneration exists. This occurs due to the generation of pressure caused by gases formed in the pores or due to perchloric acid decomposition and interaction of decomposition product with ammonia. This accumulation of reaction product and the formation of new active centres are the main reasons for the decomposition of the orthorhombic modification.

The scenario changes when high temperature is employed. At this stage, the process takes place at the surface of the crystal. Adsorption and desorption of ammonia and chloric acid take place. Thus, the role of the primary process is connected with proton transfer. By addition of dopant, changes the concentration of the protons in the lattice during decomposition of the cubic modification and stronger effect on ammonia. The assumption formulated by Kaidymov and Gavazova^58,59 is quite relevant to mention.

It refers to the catalytic activity of the orthorhombic modification with respect to perchloric acid decomposition, which is an important stage through which thermal decomposition proceeds. Note that thermal decomposition is quite high compared to the cubic modification. Hence, despite the improvement in investigating the mechanism of AP decomposition, the primary mechanism is still uncertain.

4. Conclusions

Graphene and nano-sized iron oxide were separately prepared and characterized. GINC was successfully synthesized by a one-pot, green synthesis method by using graphene and nano-sized iron oxide as raw materials. It showed excellent catalytic activity on the thermal decomposition of AP and significantly increased the burn rate of propellant. The presence of nano-sized Fe₂O₃ reduces the activation energy of propellant combustion, and graphene helps to improve the flow of electrons, which can further increase heterogeneous decomposition rate of deprotonized HClO₄ gas on the outer surface of the catalyst particle. Lower catalyst concentration favours further decomposition of AP. When GINC catalyst was used in lower concentration (1 weight%), the catalysis effect became cheaper. These processes are convenient for further scale up. The method of preparation of GINC highlighted in this paper can be extended to other metal/graphene nanocomposites used for AP and AP based composite propellant. Moreover, it can be foreseen that graphene based metal/metal oxides will be potential catalysts for AP based composite propellants.

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Footnote

† Electronic supplementary information (ESI) available: Table S1: comparison of important properties of graphene with CNT, Table S2: thermal analysis results with different burn rate enhancer with AP, Table S3: approximate propellant compositions, Table S4: physico-chemical properties with different burn rate suppressant in propellant composition, Table S5: thermal analysis results with different burn rate suppressant in propellant composition, Table S6: physico-chemical properties with different burn rate suppressant in propellant composition. See DOI: 10.1039/c4ra10812d

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