Assembly of composites into a core–shell structure using ultrasonic spray drying and catalytic application in the thermal decomposition of ammonium perchlorate

Abstract

Nano-sized composite combustion catalysts show extensive potential application in the catalytic thermal decomposition of ammonium perchlorate (AP) because of their synergetic catalytic effect. Facile and effective technology for large-scale preparation of nano-sized composite combustion catalysts is essential for their practical application. Here, a ultrasonic spray drying method is introduced to produce composite micro–nanospheres of 3,5-dinitrobenzoate {(3,5-DNB)M·M′, M = Fe(III), Co(II) or Cu(II)} with a core–shell structure, aimed at providing a facile, large-scale and cost-effective method for manufacturing composite combustion catalysts. The formation mechanism of the core–shell structure is proposed. The results of TEM and SEM illustrate that the heating temperature, carrier gas velocity (pressure), and concentration of the precursor solution have an obvious impact on the droplet-to-particle process. The DSC data indicate that (3,5-DNB)FeCo and (3,5-DNB)FeCu micro–nanospheres are highly effective catalysts for the thermal decomposition of AP, and substantially increase the apparent heat. The (3,5-DNB)FeCo and (3,5-DNB)FeCu micro–nanospheres show good synergetic catalysis which is changed with the component ratio.

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Introduction

To enhance material performance, such as catalytic activity and drug delivery, developing nanostructures and nanoparticles is a direct and effective way.^1–4 With rapid advances in nanotechnology and daily rising requirements for nanomaterial applications, many technologies have been used to assembly novel nanoparticles with variable size, morphology,^5,6 and composition in biology⁷ and not in biology.⁸ In particular, the composite materials (or hybrid materials) with self-assembled nanostructures exhibit unexpected physical and chemical properties due to synergy among composite components, and allow the development of new applications. Inspired by these works, many studies focus on emerging strategies in the design and synthesis of nano-sized composite catalysts to promote the catalytic performance (i.e. activity, stability and selectivity).^7,12–14 Industrial scale spray drying technology is routinely used in pharmaceutical and food industries due to its easy operation, great variety of liquid precursors, and high product yield. For instance, to develop a formula of pulmonary delivery of siRNA suited for treating viral respiratory infectious diseases in clinical applications, complexes of histidine, 2,3-diaminopropionic acid and siRNA were prepared into dry powders by spray drying with mannitol. The spray-dried powders were found to be suitable for inhalation with good stability, preserving the integrity of the siRNA as well as the biological and antiviral activities.⁹ Bayberry juice was spray-dried with maltodextrin, which suggested that spray drying was a satisfactory technique for drying heat-sensitive polyphenols.¹⁰ In particular, the spray drying method, as an effective strategy to tailor nanostructures and control morphology, has shown potential to enable the design and preparation of various types of nanoparticles or nanocomposites in recent years.^11–13 The technology has many advantages such as controllable size distribution, novel self-assembled structures, various compositions, and large-scale production. Hence, spray drying technology is employed for large-scale preparation of nanocomposite catalysts with the new designed apparatus in this work.

Nanocomposites of transition metal oxides present good catalytic performance in a wide range of technology.^14–16 For example, the CoAl₂O₄/γ-Al₂O₃ nanocomposite has a high catalytic activity in the decomposition of H₂O₂ at room temperature.¹⁷ Co₃O₄(NPs)/graphene oxide efficiently reduced the decomposition temperature of ammonium perchlorate (AP) which releases more exothermic heat.¹⁸ CuO/Fe₂O₃ composite nanoparticles can accelerate the decomposition of ammonium perchlorate (AP),¹⁹ and porous ZnO–NiO composite nanostructures show a remarkable catalytic effect for the thermal decomposition of AP.²⁰ On the other hand, nitrobenzoates of transition metals are important and popular compounds due to their excellent performance in supramolecular catalysis,²¹ biological systems,^22,23 crystal engineering,²⁴ and combustion catalysts.^25,26 However, there is little research on composite nitrobenzoates of transition metals as combustion catalysts for AP thermal decomposition, as well as on composite preparations with well-defined nanostructures.

Therefore, the ultrasonic spray drying method is employed to prepare 3,5-dinitrobenzoate composite {(3,5-DNB)M·M′, M = Fe(III), N=Co(II) or Cu(II)} micro–nanospheres with the aim of having low cost and scalable manufacturing. The structures of the sample particles are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of spray drying conditions on the morphology of the sample particles are explored, of which the formation mechanism is carefully investigated. The catalysis of (3,5-DNB)M·M′ micro–nanospheres with different component ratios on AP thermal decomposition is investigated by differential scanning calorimetry (DSC). Compared with sole 3,5-dinitrobenzoate micro–nanospheres, the (3,5-DNB)M·M′ micro–nanospheres exhibit novel synergetic catalysis for AP decomposition.

Experimental

Materials and methods

All reagents and solvents (analytical grade) are purchased from Sinopharm Chemical reagent Beijing Co., Ltd, and used without any further purification. The thermal decomposition of AP in the presence of the (3,5-DNB)M·M′ micro-nanospheres is investigated by differential scanning calorimetry (DSC, CDR-4, power compensation type, Shanghai Precision & Scientific Instrument Co., Ltd.) at a heating rate of 10 °C min⁻¹, with a sample mass of 1.5 mg. The surface morphology and microstructure of the particles are determined by scanning electron microscopy (SEM, S4800, Hitachi, Japan, operating at 15.0 kV) and transmission electron microscopy (TEM, Tecnai G2 F30, USA). The elemental mappings of the composite particles were evaluated by scanning transmission electron microscopy (STEM, Tecnai G2 F30, USA).

Synthetic procedures

All 3,5-dinitrobenzoates {(3,5-DNB)_nM, M = Co, Fe, Cu, n = 2 or 3} are synthesized by an improved method.²⁶ The synthesis process, taking cobalt 3,5-dinitrobenzoate {(3,5-DNB)₂Co} as an example, is demonstrated as follows (Scheme 1):

Scheme 1 The synthesis progress of (3,5-DNB)₂Co.

(1) 3,5-Dinitrobenzoic acid (3,5-DNBA, 2.12 g, 0.01 mol) was added into 50 mL of deionized water. Then, NaOH (0.4 g, 0.01 mol) was added into the mixture at 60 °C. The yellow transparent solution of sodium 3,5-dinitrobenzoate was obtained after reacting for 30 min. (2) Co(NO₃)₂ (1.5 g, 0.005 mol) was added to the solution at 60 °C and stirred for 30 min. After the reaction finished, the solution was evaporated by reduced pressure distillation to obtain a (3,5-DNB)₂Co solid powder. (3) (3,5-DNB)₂Co powders were purified with acetone. The synthesis process of copper 3,5-dinitrobenzoate {(3,5-DNB)₂Cu} and ferric 3,5-dinitrobenzoate {(3,5-DNB)₃Fe} was almost identical to that of (3,5-DNB)₂Co.

Preparations of composite 3,5-dinitrobenzoate micro–nanospheres

The composite 3,5-dinitrobenzoate {(3,5-DNB)M·M′, M = Fe(III), Co(II) or Cu(II)} micro–nanospheres were spray-dried from a composite precursor solution (CPS). The CPS, with a concentration of 0.5 wt%, was prepared by diluting the as-prepared (3,5-DNB)₃Fe and (3,5-DNB)₂Co(Cu) into acetone. The CPS was mixed with different compositions of 2 : 8, 4 : 6, 5 : 5, 6 : 4 and 8 : 2 mass ratio of (3,5-DNB)₃Fe/(3,5-DNB)₂Co(Cu).

Firstly, the CPS was atomized into droplets by an ultrasonic atomizer. Then, the droplets were introduced into the spray drying equipment by N₂ gas and evaporated into micro–nanoparticles. Finally, the particles were collected by a collector (Fig. 1).

Fig. 1 Schematic illustration of the ultrasonic spray drying equipment.

The ultrasonic spray drying apparatus consists of a ultrasonic atomizer (ultrasonic frequency F = 1.7–2.4 MHz), a heating chamber (T = 20–200 °C), a nitrogen cylinder (output pressure of carrier gas P_{carrier gas} = 0–1.6 MPa), a particle collector (filter < 0.2 μm), a vacuum pump (the top vacuum = 0.098 MPa) and a solvent condenser. The specific operating conditions are given in the ESI, Table 1.†

Results and discussion

The formation mechanism of the core–shell structure

The formation of the (3,5-DNB)M·M′ micro–nanospheres is interpreted with a core–shell mechanism, as shown in Fig. 2. The CPS was firstly atomized into droplets of 4–10 μm by the ultrasonic atomizer.^27,28 The droplets are introduced into the heating chamber with the carrier gas resulting in rapid solvent evaporation from the droplets and transformation into core–shell particles. A hypothesis is introduced to explain the core–shell formation in the drying process, and more detail is shown in the ESI.†^29,30 When the droplet is rapidly evaporated, two types of solutes are deposited into particles with different sizes, and capillary flow is formed in the meniscus region. The precipitate particles with various sizes self-assemble into core–shell spheres driven by capillary forces and Brownian movement.^28,31–33 The diffusivity of the precipitate particles in the droplet can be described by the Stokes–Einstein equation, as shown in the ESI, eqn (1).†^28,32,34

Fig. 2 Schematic illustration of the formation mechanism for (3,5-DNB)M·M′ micro–nanospheres.

Effects of concentration on morphology of the composite particles

The concentration of the CPS is a dominant factor in controlling the particle size distribution (PSD), as well as influencing the final morphology of the products.^35,36 To obtain nano-sized particles, a reduced concentration of the CPS can effectively decrease the particle size. However, it is not clear whether there exists a limit concentration (smallest concentration) and what morphology forms below this concentration. Here, taking (3,5-DNB)FeCo as an example, three CPS concentrations of 0.1 g/100 mL, 0.3 g/100 mL and 0.5 g/100 mL (T = 100 °C, P_{carrier gas} = 0.02 MPa, F = 2.4 MHz) are selected to study the effects on the particle morphology. The (3,5-DNB)FeCo micro–nanospheres were investigated by TEM and SEM, and those obtained images are shown in Fig. 3.

Fig. 3 Morphologies of (3,5-DNB)FeCo with different concentrations of CPS: (A) formation mechanisms; (B) and (E) the core–shell spheres produced with 0.5 g/100 mL (C1); (C) and (F) the cratered spheres formed with 0.3 g/100 mL (C2); (D) and (G) the mushroom particles developed with 0.1 g/100 mL (C3).

On adjusting the concentration of the CPS to 0.5 g/100 mL, the number of (3,5-DNB)₃Fe and (3,5-DNB)₂Co precipitate particles caused by droplet evaporation is enough to assemble a well-packed spherical structure, which can defend against shocks of carrier gas (Fig. 3A-C1). The result of SEM and TEM, shown in Fig. 3B and E, also proved that the particles formed with a core–shell spherical structure. On continuing to decrease the concentration of the CPS to 0.3 g/100 mL, the acetone volume increases while the number of (3,5-DNB)₃Fe and (3,5-DNB)₂Co precipitates decreases. Therefore, acetone cannot be removed completely and remains on the surface of the final spheres under constant drying conditions. This sphere easily forms craters on its soft surface by the carrier gas impact on the filter (red frames, Fig. 3C and F). When the concentration decreased to 0.1 g/100 mL, this deformation phenomenon became more obvious (Fig. 3D and G). The number of precipitate particles is insufficient to build a rigid spherical structure and a high volume of acetone remains in the final sphere leading to particle deformation under carrier gas shock (Fig. 3A-C2). Therefore, the optimized concentration of CPS is 0.5 g/100 mL for preparing the smallest particles with a well-defined morphology.

Influences of carrier gas pressure on particle morphology

The droplet produced by the ultrasonic atomizer needs an inert carrier gas to introduce it into the hot chamber. Therefore, the operating parameters of the carrier gas are an important factor in the droplet-to-particle process. The carrier gas employed to transport droplets into the chamber popularly is neat air,³⁷ hot gas,³⁸ inert gas (e.g. nitrogen, argon),³⁹ or solvent steam, which is based on the chemical characteristics of the components and solvent.⁴⁰ Because (3,5-DNB)_nM is easily soluble in acetone, the CPS employs acetone as solvent. Nitrogen gas (N₂) as a carrier gas is better for safety in this work.

The evaporation time of the droplets depends on the residence time in the heating chamber. It has been found in experiments that the residential time is very critical to the morphology of the final particles. An effective way to adjust the droplet residence time is to change the flow rate of the carrier gas which is representative of the carrier gas pressure in the work. Taking (3,5-DNB)FeCo as an example, a N₂ gas output pressure of 0.02 MPa, 0.025 MPa and 0.03 MPa (T = 100 °C, C_CPS = 0.5 g/100 mL, F = 2.4 MHz) is selected to explore the effects of N₂ gas on the morphology of the final particles. The result is shown in Fig. 4. When the pressure is adjusted to a proper pressure of 0.02 MPa, the droplet has enough residence time (τ₃) to completely remove the solvent and self-assemble into core–shell particles (Fig. 4A-G1). Therefore, the collected particle with a hard surface receives a small impact of N₂ gas and does not deform. SEM and TEM illustrate that the particles are well-defined spheres with a smooth surface (Fig. 4B and E).

Fig. 4 Morphology of (3,5-DNB)FeCo at different pressures of carrier gas: (A) formation mechanism; (B) and (E) the spheres produced at 0.02 MPa (G3); (C) and (F) the cratered spheres formed at 0.025 MPa (G2); (D) and (G) the mushroom-like particles developed at 0.03 MPa (G1).

When the N₂ gas pressure increases to 0.025 MPa, the droplet residence time (τ₂) in the heating camber is reduced resulting in residual solvent in the final particles (Fig. 4A-G2). The N₂ gas pressure shocks on the moist particle cause craters on the surface (Fig. 4C and F). When the N₂ gas pressure increases to 0.03 MPa, the droplet residence time (τ₃) is further reduced, causing much solvent residue (Fig. 4A-G3). The moist particle gets many pits on the surface under great impact of the N₂ gas at the collector (Fig. 4D and G). In particular, the large droplet is greatly influenced under the same gas pressure, and the particle distortion is serious (mushroom). Therefore, the high gas pressure could shorten the droplet residence time resulting in solvent residual and particle deformation. The optimal pressure of N₂ carrier gas is 0.02 MPa, which makes sure that the droplets are completely dry and the particles are without deformation.

Influences of heating temperature on the particle morphology

The droplet-to-particle process involves solvent evaporation, phase transformation and precipitate self-assembly. The solvent evaporation is an important step in the droplet-to-particle process.³ The evaporation rate of the solvent directly affects the final particle morphology and productivity. An effective way to improve the solvent evaporation rate is to increase the chamber temperature based on the solvent properties. However, the droplet morphology is more stable for the generation of spherical particles under low temperature.²⁷

Here, taking (3,5-DNB)FeCo as the example, temperatures of 100 °C, 110 °C, 120 °C, 130 °C, 140 °C and 150 °C (C_CPS = 0.5 g/100 mL, P_{carrier gas} = 0.02 MPa, F = 2.4 MHz) are selected to explore the optimal heating temperature (Fig. 5). A high temperature (T ≥ 110 °C) leads to a large surface tension gradient along the droplet surface and forms cratered spheres, bowl-shaped particles and mushroom-like particles. Especially, the droplet using acetone as a solvent is more sensitive to high temperature. Hence, the heating temperature needs to be considered when using low boiling point organic solvents. SEM images show that 100 °C is the optimal temperature for developing core–shell spherical particles which become misshapen with increasing temperature.

Fig. 5 Morphologies of (3,5-DNB)FeCo at different heating temperatures.

The catalysis of (3,5-DNB)FeCo and (3,5-DNB)FeCu on the thermal decomposition of AP

The spray drying method is good at producing composite particles because the method promotes homogeneous precipitation during droplet evaporation. In particular, preparing various composite catalysts has achieved very good results.⁴¹ Hence, the method is employed to synthesize (3,5-DNB)M·M′ micro–nanospheres with an elemental homogeneous distribution. They were characterized by TEM and elemental mapping (ESI, Fig. 1†).

AP is the main oxidizer in propellants and many composite energetic materials because it notably influences the combustion behaviors.^42–44 Therefore, improving the decomposition performance of AP can optimize the mechanical and ballistic properties of propellants. Fig. 6 shows the DSC curves for AP loaded with 2 wt% (3,5-DNB)M·M′ micro–nanospheres, compared with the AP loaded with 2 wt% (3,5-DNB)_nM micro–nanospheres (heating rate of 10 °C min⁻¹).

Fig. 6 DSC curves of (A) AP + 2 wt% (3,5-DNB)FeCo composites, (B) AP + 2 wt% (3,5-DNB)FeCu composites with different mass ratios, and (C) AP.

For pure AP, the thermal decomposition includes three steps. The first endothermic peak appears at 244 °C due to a solid state phase transition from orthorhombic to cubic;⁴² the second exothermic peak at 318 °C is ascribed to AP decomposition into intermediate products, known as low temperature decomposition (LTD); and the third exothermic peak around 419 °C is ascribed to complete decomposition on the solid phase and the gas phase, called high temperature decomposition (HTD) (Fig. 6C). It is quite well established that the LTD is a heterogeneous process which starts proton migration from the cation NH⁴⁺ to the anion ClO₄⁻ until it is captured by a proton trap (ClO₄⁻ or ClO₃⁻) in the defect-bearing site of the lattice.⁴² The formed HClO₄ and HClO₃ are decomposed rapidly into ClO₂ which later interacts with NH₃ and NH⁴⁺ in the dislocation crossover points and the boundaries of blocks. The HTD includes the simultaneous dissociation of HClO₄ and NH₃ either on the surface of AP or in the gas phase above the surface.¹

Fig. 6A shows the DSC curves of pure AP and the mixtures of AP and (3,5-DNB)FeCo micro–nanospheres with different component ratios. The DSC curves indicate that the peak temperatures of the LTD of AP in the presence of 2 wt% (3,5-DNB)FeCo micro–nanospheres decrease and the HTD disappeared completely. The state phase transition temperature has no change (about 245 °C). Moreover, the AP thermal decomposition has a close relationship with the component ratio of (3,5-DNB)FeCo. With an increasing proportion of (3,5-DNB)₂Co in (3,5-DNB)FeCo from 2 : 10 to 8 : 10, the LTD temperature reduced from 358.1 °C to 324 °C. The decomposition temperature of the AP loaded with 2 wt% (3,5-DNB)FeCo {(3,5-DNB)₃Fe : (3,5-DNB)₂Co = 4 : 6} sharply decreased to 324.3 °C. The catalysis of (3,5-DNB)FeCo strongly contrasts with that of (3,5-DNB)₃Fe (red dotted curve) and (3,5-DNB)₂Co (purple dotted curve). When AP is mixed with (3,5-DNB)₂Co, the AP decomposition has only the LTD process (a peak at 318 °C). In contrast, (3,5-DNB)₃Fe only just decreases the HTD temperature of AP from 419 °C to 384 °C. The comparison illustrates that (3,5-DNB)FeCo has a synergetic effect on the AP thermal decomposition. The (3,5-DNB)FeCo micro–nanospheres accelerate the intermediate compound dissociation on the surface of AP at low temperature.

As shown in Fig. 6B, the decomposition temperature of AP adding 2 wt% (3,5-DNB)FeCu micro–nanospheres with different component ratios is significantly reduced (LTD 328.6–302.6 °C, HTD 377.9–344.5 °C). The state phase transition temperature is unchanged (about 246 °C). The component ratio of (3,5-DNB)FeCu affected its catalysis on the thermal decomposition of AP. Particularly, when the mixed ratio of (3,5-DNB)₃Fe and (3,5-DNB)₂Cu was 2 : 8, (3,5-DNB)FeCu has the best catalytic effect (HTD 344.5 °C, LTD 302.6 °C). It is clearly observed that the catalysis of (3,5-DNB)₃Fe (red dotted curve) and (3,5-DNB)₂Cu (blue dotted curve) is significantly different from that of (3,5-DNB)FeCu. The thermal decomposition of AP loaded with 2 wt% (3,5-DNB)₂Cu has three decomposition steps (peaks at 298.9 °C, 313.4 °C and 327.6 °C) below pure AP. Compared with sole (3,5-DNB)₃Fe and (3,5-DNB)₂Cu, the (3,5-DNB)FeCu micro–nanospheres notably improve the catalytic effects and have a synergetic catalytic effect.

The decomposition heat of AP and its mixtures with the (3,5-DNB)M·M′ micro–nanospheres is determined by integration of the exothermic peaks areas, and detailed data are shown in Table 1. The decomposition heat of AP + (3,5-DNB)₂Co, AP + (3,5-DNB)₂Cu, and AP + (3,5-DNB)₃Fe increases from 298.4 J g⁻¹ to 942 J g⁻¹, 1118 J g⁻¹ and 631.1 J g⁻¹, respectively. Strikingly, the decomposition heat of AP loaded with 2 wt% (3,5-DNB)FeCo (5 : 5) increases to 1102 J g⁻¹ which is higher than that with (3,5-DNB)₂Co and (3,5-DNB)₃Fe. The decomposition heat of AP in the presence of 2 wt% (3,5-DNB)FeCu (2 : 8) increases to 1064 J g⁻¹ between (3,5-DNB)₂Cu and (3,5-DNB)₃Fe. Compared with the sole components, both (3,5-DNB)FeCo and (3,5-DNB)FeCu improve catalysis of the AP thermal decomposition and increase the AP decomposition heat. In addition, the decomposition temperature and heat of AP are varied with different component ratios.

Table 1 The decomposition heat of AP and AP + (3,5-DNB)M·M′ with various proportions

Sample	Morphology	Mass ratio	Additive ratio (wt%)	Peak temperature (°C)		ΔH (J g^−1)
		M(Fe) : N(Co, Cu)		LTD	HTD
AP	—	—	Pure	318.4	419.3	298.4
AP + (3,5-DNB)2Co	Micro–nanosphere	—	2	318.5	—	942
AP + (3,5-DNB)2Cu	Micro–nanosphere	—		298.9	—	1118
AP + (3,5-DNB)3Fe	Micro–nanosphere	—		—	384	631.3
	Micro–nanosphere	2 : 8		338.8	—	1042
	Micro–nanosphere	4 : 6		324.3	—	1026
AP + (3,5-DNB)FeCo	Micro–nanosphere	5 : 5		335.5	—	1102
	Micro–nanosphere	6 : 4		340.4	—	969.3
	Micro–nanosphere	8 : 2		358.1	—	989.6
	Micro–nanosphere	2 : 8		298.9	344.5	1064
	Micro–nanosphere	4 : 6		311.1	346	914
AP + (3,5-DNB)FeCu	Micro–nanosphere	5 : 5		306	348.6	786
	Micro–nanosphere	6 : 4		311	350	695.6
	Micro–nanosphere	8 : 2		328.6	377.9	723.8
CoO–Co3O4 (ref. 1)	Nanofilm	—	10	285	—	1162
Copper b-resorcylate^45	Nano semi-spherical	—	2	342	—	1639
γ-Fe2O3 (ref. 46)	Nanocrystalline	—	1	314	380	1063
γ-Fe2O3 (ref. 46)	Nanocrystalline	—	1	320	353	1284
AP + GINC (graphene iron oxide)^47	Nanocomposite	—	1	—	372.3	4071
AP + GTNC (graphene titanium oxide)^48	Nanocomposite	—	5	—	372.5	3903

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In other studies, the decomposition heat of AP loaded with 10 wt% CoO–Co₃O₄ nanofilms¹ increases to 1162 J g⁻¹, which is close to that of AP loaded with 2 wt% (3,5-DNB)M·M′ micro–nanospheres. The decomposition heat of AP loaded with 2 wt% copper β-resorcylate nanoparticles (1639 J g⁻¹)⁴⁵ (or nanocrystalline 1 wt% γ-Fe₂O₃, 1063 J g⁻¹ and 1284 J g⁻¹,⁴⁶ 1 wt% GINC nanocomposite, 4071 J g⁻¹,⁴⁷ and 5 wt% GTNC nanocomposite, 3903 J g^−1 48) is higher than that of AP loaded with 2 wt% (3,5-DNB)M·M′, accompanied by an increasing temperature of the LTD and HTD (342 °C, 380 °C, 353 °C, 372.3 °C, and 372.5 °C). These differences could be attributed to the differences in the morphologies. The spherical particles secure a uniform mixture with AP which obviously facilitates the catalytic activity.

As a consequence, (3,5-DNB)FeCo and (3,5-DNB)FeCu micro–nanospheres exhibit an excellent catalytic effect on the decomposition temperature and heat of AP. It is more interesting that the thermal decomposition rate and the decomposition heat of AP could be controlled by changing the component ratio of (3,5-DNB)M·M′ based on special needs.

Conclusions

(3,5-DNB)M·M′ micro–nanospheres with a core–shell structure are successfully produced by ultrasonic spray drying technology. The operational parameters (heating temperature, concentration and carrier gas pressure) have a significant influence on the particle morphology. (3,5-DNB)M·M′ exhibits synergistic catalysis for the thermal decomposition of AP. The (3,5-DNB)FeCu (2 : 8) micro–nanospheres with 2 wt% have the best catalytic activity for AP thermal decomposition, the LTD decreases from 318 °C to 302 °C, and ΔH increases from 298.4 J g⁻¹ to 1064 J g⁻¹. The catalytic effects on the thermal decomposition of AP closely relate to the component ratio of (3,5-DNB)M·M′. It can be imagined that the thermal decomposition temperature and heat of AP could be adjusted by changing the component ratio as needed.

Caution: Although no problems occurred during the synthesis and handling of the materials studied in this work, AP is an energetic material and the drying process evaporates the acetone, which tends to explode under certain conditions. All compounds should be treated with respect and appropriate safety precautions should be taken in all spray drying procedures. Safety equipment such as an explosion-proof box and face mask is necessary, and experimenters should stay away from the spray drying system and maintain good ventilation.

Acknowledgements

We gratefully acknowledge the financial support from the State Key Laboratory of Explosion Science and Technology (No. QNKT12-02).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08150a

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