The effect of detonation polycrystalline diamond modification on the thermal decomposition of RDX

Abstract

A well-dispersed micron-scale detonation polycrystalline diamond (DPD) was prepared and modified on RDX particles, preparing four DPD-modified RDX (DMR) composites with the modification amount increasing from 1/9 to 1/3. The effect of modification on the thermal decomposition and kinetics of RDX was studied using dynamic pressure measuring thermal analysis (DPTA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) techniques. As the modification amount increased, the gas emission, reaction rate constant, decomposition temperature, and kinetic and thermodynamic parameters of the composites increased firstly and decreased afterwards. The DPD had a catalytic effect on the thermal decomposition of RDX, but this effect was not in linear correlation with the modification amount. The DPD-modification amount of 1/7 had the optimal catalytic effect. For DPD modification of less than 1/7, the thermal decomposition of RDX was accelerated by DPD. As the modification amount exceeded 1/7, the excessive DPD modification conversely hindered the decomposition of RDX. The thermal decomposition kinetics demonstrated that the thermal decomposition of the DMRs conformed to a multi-step reaction mechanism involving a catalytic reaction and a secondary reaction, while they had the same rate-determining step, which is the scission of the N–NO₂ bonds of RDX. The DPD modification changes the reaction pathway and reaction rate to affect the decomposition mechanism and kinetics.

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Introduction

Detonation polycrystalline diamond (DPD) is a micron-sized synthetic diamond powder produced from graphite and an explosive under detonation conditions.¹ Naturally-formed diamond is the hardest and most abrasion resistant of all materials. The synthetic DPD inherits the superior properties of bulk diamond and delivers them at a small scale. DPD has extreme hardness, wear resistance and thermal conductivity like diamond, additionally it has large surface effects and small scale effects like micron-scale materials.^2–4 Due to the above-mentioned advantages, DPD is a great material for use in composite preparation, precision polishing, head processing for computer hard disks, and connecting optical fibers.^5,6 With the development of micro- and nano technology, composite modification using ultrafine particles, such as graphene,^7,8 carbon nanotubes^9,10 and detonation nanodiamonds,¹¹ is one of the most important techniques for the preparation of novel materials. Therefore, ultrafine DPD can be applied to the research and development of modified composites.

Hexahydro-1,3,5-trinitro-l,3,5-triazine (RDX) is an important explosive widely used in the aviation, ordnance and mining industries.^12,13 The development of high-energy and low-sensitivity explosives has become a hot topic in recent years.¹⁴ One of the most effective attempts is to design composite explosives through adding combustible, oxidizing, or energetic components.^15,16 Ultrafine metal powders, such as Al, Cu and Ni, have shown great potential due to a high enthalpy release, high surface reactivity and a density increase.^17–19 However, metal powders tend to oxidize, at worst to self-ignition, and they can cause sizeable disasters as well as metal pollution, and they have poor cost efficiency. When ultrafine DPD is modified with RDX, the composite promises to exhibit some exceptional performances. However, so far research is sparse due to potential explosion hazards and the difficulties which lie in the fabrication process. In this work, four kinds of DPD-modified RDX (DMR) composites with different modification amounts were prepared, and the thermal decomposition and kinetics were studied using DPTA, DSC and TG techniques.

Experimental

Preparation of the DPD micropowder

DPD was prepared using high-purity graphite powder and an explosive as the starting materials, through the direct conversion of the carbon source at an ultra-high pressure of more than 10 GPa, and a temperature higher than 2000 °C. The high pressure and temperature were generated by the detonation of a powerful explosive, RDX or HMX, in a non-oxidizing cooling medium. The chemical purification was performed using perchloric acid and sulfuric acid at an elevated temperature of ∼300 °C, where the condensed carbon was oxidized in the liquid phase and the non-diamond structures decomposed gradually. The primary product was washed using deionized water, then filtrated and dried to obtain a gray-black powder. The powder was screened to obtain the refined DPD micropowder. The preparation process is shown in Fig. 1.

Fig. 1 Preparation process for DPD.

Preparation of DMR composites

1 g of RDX was dissolved in 100 mL of acetone in a 60 °C water bath under stirring. 0.5 g of DPD was dispersed in 100 mL of anhydrous ethanol with the aid of 0.10 mL of the surfactant sorbitan monooleate (Span-80). The solution was treated using ultrasonic oscillation for more than 1 hour until the DPD was completely dispersed. The dispersion was put onto a magnetic stirring device and heated to 60 °C under 60 rpm stirring. Meanwhile, the pre-warmed acetone solution in which RDX had been dissolved was added dropwise into the DPD dispersion. After dropping, the mixed solution was kept stirring for 30 minutes, then quickly transferred into a vacuum distillation flask and distilled using a rotary evaporator with a rotation speed of 45 rpm at 80 °C. After 80% of the solvent had evaporated, the remaining thick slurry was filtrated rapidly and washed using anhydrous ethanol and deionized water. The filter cake was dried at 40 °C over 24 hours, obtaining a gray-black powder. To determine the DPD-modification effect, four DMR composites with different ratios of ingredients were prepared under the same experimental conditions. The ingredient ratios of DPD to RDX were designated as 1 : 8 (DMR1), 1 : 6 (DMR2), 1 : 4 (DMR3), and 1 : 2 (DMR4) which correspond to increasing DPD-modification amounts of 1/9, 1/7, 1/5 and 1/3.

Apparatus and methods

Dynamic pressure measuring thermal analysis (DPTA) was used to study the initial thermal decomposition at a constant temperature.²⁰ A sample was weighed (1.0000 ± 0.0010) g and loaded in an explosion-proof glass test tube. The tube was sealed and evacuated, then put in to the thermostat, and held at 100 °C for 48 hours.

Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were applied to study the complete thermal decomposition under linear heating. A Pyris-1 TGA (PerkinElmer, USA) unit was employed with an unsealed platinum pan. Less than 0.5 mg of sample was heated from 50 °C to 500 °C at rates of 5, 10, 15 and 20 °C min⁻¹ respectively. The atmosphere was high-purity nitrogen with a flow rate of 20 mL min⁻¹ under a pressure of 0.2 MPa. A Pyris-1 DSC (PerkinElmer, USA) unit was used with an uncovered aluminum crucible. Less than 0.5 mg of sample was heated from 50 °C to 500 °C at a rate of 10 °C min⁻¹ under a dynamic nitrogen atmosphere.

Results and discussion

Morphology characterization

The as-prepared DPD is polyhedral with an average particle size of 2 μm, as shown in Fig. 2a and c. The surface is slightly rough. Fig. 2b shows that one particle is an aggregate of many nanodiamonds, and open pores and defects are obviously observed on the surface. The specific surface area is 15.403 m² g⁻¹, tested by a multipoint BET method. The X-ray diffraction spectrum (see Fig. 2d) contains wide diffraction maxima at 2θ = 43.9°, 75.3° and 91.5° which correspond to the (111), (220) and (311) reflections of the diamond-like lattice. The strong peak of d = 2.06763 and the weak peak of d = 2.17821 represent the cubic crystal system and the hexagonal crystal system, respectively. Thus, DPD is composed of a large quantity of cubic diamonds and a small quantity of hexagonal diamonds. No graphite peak is detected in the region 2θ = 20–30°, which indicates that the formed powder is well-purified DPD.

Fig. 2 Characteristics of DPD : SEM images of (a) DPD powder and (b) one particle; (c) particle size analysis; (d) a powder-XRD spectrum.

RDX, with a particle diameter of 100–150 μm, is used as the substrate material for modification as shown in Fig. 3a. Its surface is comparatively smooth aside from a minute amount of fragments. DPD has a much smaller particle size and a large surface energy, and therefore has a stronger surface attraction. It deposits and attaches on the RDX surface via intermolecular forces under the effect of the surfactant Span-80. As shown in Fig. 3b–e, the much rougher surfaces of the DMRs indicate that DPD is successfully modified on RDX. Because no residue was detected after the experiment, the effective modification amounts for the DMRs were reckoned by the ratios of DPD to RDX, which were as follows: DMR1, 1/9; DMR2, 1/7; DMR3, 1/5; DMR4, 1/3.

Fig. 3 SEM images of RDX and the four DMRs: (a) RDX; (b) DMR1 with the DPD modification amount of 1/9; (c) DMR2 with the amount of 1/7; (d) DMR3 with the amount of 1/5; (e) DMR4 with the amount of 1/3.

DPTA analysis

The gas emission of DPTA is used to evaluate the thermal stability of a material; a lower gas emission signifies better stability.^21,22 DPTA was used to study the initial thermal decomposition of the DMRs, recording the gas pressure of the thermal decomposition in real time, normalized to the standard conditions of a 1.0 g quantity, a 25 mL volume and a 273.15 K temperature, as shown in Fig. 4.

Fig. 4 Time dependencies of the decomposing gas pressures of the DMRs at 100 °C, recorded using DPTA.

The gas emissions of all the DMRs generally increase with the increase in heating time. The initial 500 minutes is a fast decomposition period. The pressures increase rapidly and the increasing rates are arranged in the increasing order of DMR4 < DMR3 < DMR1 < DMR2. Subsequently, the decomposition turns into a slow process. The pressures continue to increase but the increasing rates decrease gradually to different degrees. The DMRs decompose and release gas slowly for a long duration. The thermal decompositions of the DMRs do not reach equilibrium by the end of DPTA test (during 48 h). This implies that under actual storage conditions, the DMRs keep up a slow steady decomposition for a long period of time until equilibrium is established. The gas pressures are recalculated into the volumes and listed in Table 1.

Table 1 Gas emissions and reaction rate constants for the DMRs from the DPTA data^a

Samples	Gas emission		Reaction rate constant
	p/kPa g^−1	V/mL g^−1	k/10^−7 s^−1	b	r
a b: intercept of the fitting plot of the solid phase reaction kinetic equation G(α) = kα + b; r; linear correlation coefficient; the highest probability of k is selected according to the smallest value of b and the largest value of r.
DMR1	0.7696 ± 0.0385	0.1899 ± 0.0095	4.28	−0.01679	0.9952
DMR2	0.8854 ± 0.0443	0.2185 ± 0.0109	4.94	−0.07962	0.9934
DMR3	0.6097 ± 0.0305	0.1505 ± 0.0075	4.13	−0.00682	0.9955
DMR4	0.4694 ± 0.0235	0.1158 ± 0.0058	3.88	−0.00010	0.9982

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Good thermal stability is required where the gas emission is less than 2.00 mL g⁻¹ at 100 °C during 48 h.^23,24 The gas emissions of the DMRs are all less than 2.00 mL g⁻¹, indicating that they have good thermal stability. A previous report on the DPTA gas emission of RDX gave a value of about 0.10 mL g⁻¹.²⁵ The DMRs release more gas than RDX, thus DPD modification is conducive to the decomposition of RDX. The total gas emission in increasing order is DMR4 < DMR3 < DMR1 < DMR2. DPD modification obviously affects the gas emission, but there is a non-linear correlation between the gas emission and the modification amount. The dependence of gas emission on the modification amount shows a maximum value that corresponds to DMR2, with the modification amount of 1/7. The reaction rate constants (k) were calculated using the solid-phase reaction kinetic equation²⁶ and are listed in Table 1. The k values ranked in an increasing order gives DMR4 < DMR3 < DMR1 < DMR2, which shows the same order as for gas emission. The reaction kinetics theory holds that k means the decrease in the concentration of a reactant or the increase of the concentration of a product per unit time. Due to interactions among DPD, RDX and the gaseous products, a moderate DPD modification could activate the reactant RDX to the greatest extent. On the microscopic level, the concentration of the activated reaction molecules and the chance of effective collision increase, and thus the reaction rate is accelerated. DMR2 with a 1/7 modification amount has the fastest reaction rate and the highest gas emission from DPTA within the specified time. Therefore, a moderate amount of DPD modification has the most efficient catalytic action, providing the greatest accelerating effect on the thermal decomposition of RDX.

DSC analysis

The nonisothermal decomposition of the DMRs was recorded using DSC at a heating rate of 10 °C min⁻¹. The DSC curves are shown in Fig. 5.

Fig. 5 DSC curves for the DMRs at 10 °C min⁻¹.

The DSC curves firstly show one endothermic peak caused by melting, immediately followed by one exothermic peak from intense thermal decomposition. The DMRs decompose in the molten state, like RDX.²⁷ This indicates that DPD does not decompose but affects the decomposition of RDX. The detailed DSC data are summarized in Table 2.

Table 2 DSC parameters of the DMRs at a heating rate of 10 °C min^−1a

Samples	Melting endothermic peaks				Decomposition exothermic peaks
	To/°C	Tp/°C	Te/°C	ΔH1/J g^−1	To/°C	Tp/°C	Te/°C	ΔH2/J g^−1
a To – onset temperature; Tp – peak temperature; Te – end temperature; ΔH1 and ΔH2 – enthalpy changes.
DMR1	180.40	201.78	213.14	−86.73	213.56	235.19	253.23	992.56
DMR2	180.06	198.64	206.33	−76.46	212.35	234.68	252.67	897.73
DMR3	184.66	204.69	207.62	−75.69	215.78	238.83	258.36	638.05
DMR4	186.35	208.48	215.01	−68.48	218.98	240.82	267.12	293.77

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Pure RDX has a melting temperature of 205–208 °C and a decomposition temperature of 238–241 °C.²⁸ After being modified with DPD, both the melting and decomposition temperatures of the DMRs are lower than those of RDX. The DPD modification improves the decomposition of the composites. As shown in Table 2, all the characteristic temperatures of thermal decomposition increase in the order of DMR2 < DMR1 < DMR3 < DMR4. DMR2 with a 1/7 modification amount has the lowest decomposition temperatures and thus the highest reaction activity. However, the enthalpy changes for the endothermic and exothermic decompositions (ΔH₁ and ΔH₂) conform to the order of DMR4 < DMR3 < DMR2 < DMR1. The heat change is caused by the decomposition of RDX, therefore ΔH₁ and ΔH₂ are in direct proportion to the RDX content.

TG/DTG analysis

The thermal decomposition of the DMRs at different heating rates were studied using TG, and the mass losses and their differential curves are shown in Fig. 6. The characteristic parameters of the TG/DTG curves are listed in Table 3.

Fig. 6 TG/DTG curves of the DMRs: (a) DMR1; (b) DMR2; (c) DMR3; (d) DMR4.

Table 3 Characteristic parameters of the DMRs from non-isothermal TG/DTG data^a

Sample	Curve parameters			Characteristic temperatures
	β/°C min^−1	Tp/°C	Δmmax/% min^−1	Tb/°C	TSADT/°C
a β – heating rate; Tp – peak temperature of mass loss rate; Δmmax – maximum mass loss rate; Tb – critical temperature of thermal explosion; Tp = Tp0 + aβ + bβ^2, Tb = [Ea − (Ea^2 − 4EaRTp0)^1/2]/2R; TSADT – self-accelerating decomposition temperature, TSADT = Tb − (RTb^2/Ea).
DMR1	5.0	220.21	18.84	227.75	210.12
	10.0	225.03	34.38
	15.0	237.55	47.05
	20.0	240.48	62.39
DMR2	5.0	220.44	17.32	219.27	205.68
	10.0	230.05	28.99
	15.0	236.84	44.85
	20.0	237.70	58.25
DMR3	5.0	215.41	16.24	222.23	203.55
	10.0	222.91	28.48
	15.0	234.43	41.96
	20.0	238.79	55.88
DMR4	5.0	218.14	14.39	224.63	205.27
	10.0	222.64	27.58
	15.0	238.04	39.93
	20.0	238.65	55.18

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The thermal decompositions of all the DMRs show only one intense mass loss process. They decompose completely because their lost masses are in good agreement with the amount of RDX in the DMR and the residues are almost equal to the unreacted DPD. Both the peak temperature of thermal decomposition (T_p) and the maximum mass loss rate (Δm_max) increase with an increase in the heating rate. The critical temperature of thermal explosion (T_b) and the self-accelerating decomposition temperature (T_SADT)^29,30 are calculated (see Table 3) to predict the thermal safety of the DMRs.

Higher characteristic temperatures denote better thermal safety. T_b is ranked in the increasing order of DMR2 < DMR3 < DMR4 < DMR1 while T_SADT is ranked in the order DMR3 < DMR4 < DMR2 < DMR1. No identical variation is shown in the temperatures due to the complex interaction between RDX and DPD. The best thermal safety belongs to the composite with the modification amount of ca. 1/7.

The thermodynamic parameters of the thermal decomposition of the DMRs were calculated based on transition state theory and the kinetic parameters were calculated using Kissinger and Ozawa methods.²⁶ All the parameters are summarized in Table 4.

Table 4 Thermodynamic and kinetic parameters of the DMRs from non-isothermal TG^a

Sample	Thermodynamic parameters			Kinetic parameters
	ΔH^≠/kJ mol^−1	ΔS^≠/J K^−1 mol^−1	ΔG^≠/kJ mol^−1	Kissinger method			Ozawa method
				EaK/kJ mol^−1	lg(AK/s^−1)	−rK	EaO/kJ mol^−1	−rO
a ΔH^≠ – enthalpy of activation, ΔH^≠ = Ea − RT; ΔS^≠ – entropy of activation, ΔS^≠ = R[ln A − ln(kBT/h)]; ΔG^≠ – free energy of activation, ΔG^≠ = ΔH^≠ − TΔS^≠; Ea – apparent activation energy; A – pre-exponential factor; r – linear correlation coefficient; the subscripts K and O indicate that the parameters were calculated using Kissinger and Ozawa methods, respectively.
DMR1	112.02	−65.10	143.48	116.04	10.04	0.9505	118.31	0.9565
DMR2	143.68	−0.60	143.94	147.64	13.40	0.9866	148.33	0.9879
DMR3	102.60	−83.15	142.24	106.56	9.09	0.9778	109.24	0.9808
DMR4	99.60	−90.05	142.68	103.57	8.73	0.9257	106.43	0.9354

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The thermodynamic and kinetic parameters are ranked in the same order of DMR4 < DMR3 < DMR1 < DMR2. Transition state theory and collision theory hold that A closely relates to S^≠, because both parameters represent the confusion degree and collision probability of the reaction molecules.^31,32 Due to the higher chemical activity, micro- and nano materials have a greater confusion degree and collision probability than their larger-sized counterparts. Therefore, for micro- and nano materials, A plays a dominant role in determining the feasibility and activity of the reaction involving catalysis. This conclusion has also been confirmed by our earlier research.³³ DMR2, corresponding to the modification amount of 1/7, has the largest value of A and therefore the highest reactivity.

The direct relation between E_a and A indicates that the kinetic compensation effect lies in the thermal decomposition of the DMRs.³⁴ This suggests that their decompositions have the same rate-determining reaction step.³⁵ The different quantities of DPD modification could change some reaction pathways and affect the thermal decomposition kinetics, but not change the rate-determining step. Therefore, all the DMRs have the same rate-determining step as RDX. Previous research has concluded that the rate-determining step in the thermal decomposition of RDX is the scission of one of the N–NO₂ bonds (see Scheme 1).^36–39 Likewise, this step dominates the decomposition of the DMRs.

Scheme 1 Rate-determining step in the thermal decomposition of RDX: the scission of N–NO₂ bond.

Isoconversional kinetic analysis

The isoconversional principle states that the reaction rate at the constant extent of conversion is only a function of temperature. The temperature dependence of the isoconversional rate can be used to evaluate the isoconversional values of the activation energy (E_α) without assuming or determining any particular form of reaction model.⁴⁰ The E_α dependence is important for detecting and treating multistep kinetics. A significant variation of E_α with the extent of conversion (α) indicates that a reaction is a kinetically complex multi-step process.^41,42 Although the isoconversional principle holds strictly for a single-step process, the principle continues to work as a reasonable approximation because isoconversional methods describe the process kinetics by using multiple single step kinetic equations. The International Confederation for Thermal Analysis and Calorimetry (ICTAC) Kinetics Committee recommends the Kissinger–Akahira–Sunose (KAS) equation to calculate the isoconversional activation energy E_α.⁴⁰

KAS equation:

where the subscript α indicates an isoconversional value, β_i is the heating rate of TG, and T_α,i is the temperature corresponding to a given conversion value at a different β_i value. Firstly, the α vs. T curves for the four DMRs are plotted in Fig. 7.

Fig. 7 α vs. T curves for the DMRs under different heating rates: (a) DMR1; (b) DMR2; (c) DMR3; (d) DMR4.

All of the curves obey a similar sigmoidal trend. This shows that the thermal decomposition process for the DMRs is subdivided into three stages i.e. the lag, acceleration and deceleration stages. The value of E_α is determined from the slope of the plot of ln(β_i/T_α,i) vs. 1/T_α,i as shown in eqn (1). The values of E_α at different α are listed in Table 5.

Table 5 Isoconversional kinetic parameters of DMRs by KAS method^a

α	DMR1		DMR2		DMR3		DMR4
	Eα/kJ mol^−1	−r	Eα/kJ mol^−1	−r	Eα/kJ mol^−1	−r	Eα/kJ mol^−1	−r
a SD – standard deviation.
0.1	110.58 ± 15.59	0.9807	152.01 ± 18.77	0.9851	118.41 ± 14.02	0.9863	104.97 ± 25.36	0.9463
0.2	109.37 ± 15.27	0.9811	149.06 ± 16.09	0.9886	115.25 ± 14.25	0.9849	104.83 ± 23.02	0.9550
0.3	108.84 ± 16.03	0.9790	142.51 ± 13.40	0.9913	112.56 ± 14.79	0.9832	102.95 ± 24.04	0.9496
0.4	106.94 ± 16.81	0.9762	139.69 ± 13.13	0.9913	110.13 ± 15.63	0.9805	100.60 ± 25.22	0.9425
0.5	108.62 ± 16.57	0.9775	136.35 ± 13.08	0.9910	108.04 ± 15.57	0.9799	96.93 ± 26.07	0.9347
0.6	109.78 ± 17.08	0.9766	133.51 ± 11.38	0.9928	106.08 ± 15.12	0.9803	97.29 ± 26.09	0.9350
0.7	109.24 ± 17.54	0.9752	132.39 ± 11.38	0.9927	103.62 ± 15.63	0.9780	97.08 ± 25.66	0.9367
0.8	108.99 ± 17.61	0.9749	131.47 ± 12.26	0.9914	103.34 ± 15.62	0.9779	98.17 ± 24.63	0.9424
0.9	108.19 ± 16.86	0.9766	130.68 ± 11.26	0.9927	101.40 ± 15.62	0.9771	100.21 ± 22.79	0.9520
Mean	108.95 ± 16.60		138.63 ± 13.42		108.76 ± 15.15		100.34 ± 24.77
SD^a	0.9631		7.3704		5.4694		3.0693

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The E_α vary with the α, thus the decomposition of the DMRs can be described as a multi-step process. For energetic explosives, the thermal decomposition process includes one slow initial decomposition step and one rapid autocatalytic decomposition step, the latter is caused by secondary reactions between the reactants and products. Therefore, the decomposition of the DMRs includes at least two kinds of accelerating reactions; they are the autocatalytic reaction caused by their own gaseous products and the catalytic reaction caused by the DPD modification. As shown in Fig. 8, the trends in the change of E_α vs. α for the DMRs are different from each other. This shows that the effect of DPD modification on the reaction mechanism varies with the modification amount. According to the standard deviation (SD), DMR2 has the greatest change in E_α vs. α, so the modification amount of 1/7 has the greatest effect on the thermal decomposition of RDX. Although all the DMRs have the same rate-determining step, DPD modification changes the reaction pathway and reaction rate and then affects the reaction mechanism and kinetics.

Fig. 8 Dependency of E_α vs. α using the KAS method.

DPD-modification effect

The characteristic parameters of the thermal decomposition of the DMRs are not in direct proportion to the DPD modification amount. The DPD-modification effect on the thermal decomposition of RDX is illustrated in Fig. 9.

Fig. 9 Effect of DPD modification on the thermal decomposition of RDX: (a) DPD particle is aggregated by nanodiamonds. The moderate DPD modification is conductive to the gas release and accelerates the decomposition; (b) the excess modification prevents the gas release and is unfavorable to the decomposition.

DPD has high activity due to its small size, large surface and fast heat & mass transfer. It also has many open pores and defects on its surface (see Fig. 2b) which are available to act as active sites for reaction and as potential attachment sites for active hydrogen. The gaps between the aggregated nanoparticles are the diffusion path for the gaseous products. These factors increase the catalysis of DPD upon thermal decomposition. As the modification amount increases from 0 to 1/7, the concentration of activated molecules increases, and the probability of reactive collisions between DPD, RDX and the gaseous products increases, thus the catalytic activity becomes higher. Consequently, the DPD modification leads to a lower decomposition temperature, a faster reaction rate and more gas emission within a given time. However, when the modification amount exceeds 1/7, excess DPD modification hinders the diffusion of the gaseous products and decreases the activity of the reaction interface of RDX, and thus has a negative effect on the thermal decomposition of RDX.

Conclusions

DPD with a particle size of 2 μm was prepared from graphite through direct detonation preparation. DPD was modified on micron-sized RDX particles, preparing four DMR composites. The effect of DPD modification on the thermal decomposition of RDX was studied using DSC, TG and DPTA techniques. The thermal stability was ranked in the order of DMR4 < DMR3 < DMR1 < DMR2. Similar trends were found for gas emission, the decomposition temperature, and the kinetic and thermodynamic parameters with increasing modification amount. DMR2, with a modification amount of 1/7, reached the extreme values of the decomposition characteristic parameters and improved the thermal properties of RDX to the greatest extent. The catalytic effect of the DPD modification was not linearly proportional to the modification amount. A moderate amount of DPD modification as a catalyst accelerated the decomposition, while excess modification conversely obstructed the decomposition. The thermal decomposition kinetics indicated that the thermal decomposition of the DMRs had the same rate-determining step, i.e. the scission of one of the N–NO₂ bonds of RDX, and conformed to a multi-step reaction mechanism involving the catalytic reaction and a secondary reaction.

Acknowledgements

This work was financially supported by the Science and Technology Fund of the Applied Physical Chemistry Laboratory (No. 9140C3703051105 and 9140C370303120C37142), and the Key Support Foundation of the State Key Laboratory of Explosion Science and Technology (No. QNKT12-02 and YBKT 10-05).

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