Jun Tao, Xiao-Feng Wang, Kun Zhang, Xue-song Feng

The Second Department, Xi’an Modern Chemistry Research Institute, Xi’an 710065, China

Keywords:5,5′-bistetrazole-1,1′-diolate Ammonium perchlorate Density-functional theory Molecular dynamics Hydrogen bond

ABSTRACT The combination of 5,5′-bistetrazole-1,1′-diolate (TKX-50) and ammonium perchlorate (AP) can make greater use of the chemical energy of TKX-50 based energetic materials. The research on the interaction mechanism between TKX-50 and AP is very important for designing TKX-50-AP compounds and judging the formation feasibility of composite particles, which can lay a theoretical foundation for the preparation of TKX-50-AP mixed crystals and the application of TKX-50 in propellant,propellant and explosive.Herein, in order to research the interaction mechanism between TKX-50 and AP, density-functional theory calculation was applied to optimize three configurations of TKX-50-AP compounds. The geometry structure, electrostatic potential and binding energy of the compounds were predicted, and the electronic density topological analysis was also carried out. Then TKX-50-AP mixed crystals structures were constructed,and the radial distribution function of H-O and H-N in mixed crystals was calculated.Finally, solvent/non-solvent method was applied to prepare TKX-50-AP composites, and the infrared spectroscopy and the non-isothermal decomposition performance of the composites were characterized.Results show that the superposition of positive charges in TKX-50 molecule and negative charges in AP makes the electrostatic potential distributions of TKX-50-AP compounds different from that of TKX-50 and AP. The interaction energies of TKX-50-AP 1, TKX-50-AP 2 and TKX-50-AP 3 are 39.743 kJ/mol,61.206 kJ/mol and 27.702 kJ/mol, respectively. The interaction between TKX-50 molecules and AP molecules in TKX-50-AP mixed crystals both depends on hydrogen bonds and van der Waals force, and the number and strength of hydrogen bonds are significantly greater than that of van der Waals force. The composition of AP and TKX-50 makes the absorption peak of the five-membered rings and NH3OH+ of TKX-50 shift to low wavenumber in the infrared spectroscopy. In general, TKX-50 interacts with AP via hydrogen bonds and van der Waals force, and the calculated results are in good agreement with the experimental results. The composition of TKX-50 and AP can also prolong the decomposition process.

1. Introduction

With the change of war mode,the requirement of new weapons for explosives is not only high energy density, but also safety and environmental friendliness. Insensitive energetic materials have gradually become a research hotspot for scientific researchers[1,2].Nitrogen rich compounds, as a kind of energetic materials, have been widely concerned in recent years. The compounds own advantages of high energy density,low sensitivity,good safety and environmental friendliness [3,4]. Tetrazole, especially bi-tetrazole compounds, has abundant N-N single bond and N=N double bonds,whose nitrogen content is high and chemical stability is also good. The energy release comes from the breaking and combination of N and N bonds,which can produce high energy and produce N2without pollution to the environment [5-7]. In 2012, Niko Fischer [8] of Munich University designed and synthesized a new type of nitrogen rich energetic ion salt called 5,5′-bistetrazole-1,1′-diolate (TKX-50), which belongs to tetrazole. The theoretical density of TKX-50 is 1.918 g/cm, detonation velocity is 9679 m/s,detonation pressure is 42.4 GPa, and standard enthalpy of formation is 446.6 kJ/mol. Compared with the traditional energetic materials RDX,HMX,CL-20,the friction and impact sensitivity of TKX-50 is lower, and the gas yield is higher. Therefore, TXK-50 is an energetic ionic salts with high-energy and low sensitivity [9,10].

At present, most of the studies on TKX-50 are on synthesis,energy performance, safety and other properties [11,12]. In 2015,V.P. Sinditskii [13] of Russian conducted an experimental study on the combustion of TKX-50, and found that the combustion heat of TKX-50 was 2054±6 kJ/mol.The calculated results showed that the formation enthalpy of TKX-50 was only 111±16 kJ/mol,which was not as high as that reported by Germans. TKX-50 is a serious negative oxygen energetic material(oxygen balance is-27.1%).It is necessary to adjust the energy output structure of TKX-50 by combining TKX-50 with oxygen-rich oxidant. AP is a commonly used oxidant, which is often used to regulate negative oxygen energetic materials (such as CL-20 [14], HMX [15], RDX [16], etc.). At present, there are few reports on the preparation of energetic composite formed by TKX-50 and other substances. Only Z X Ge et al. prepared a CL-20/TKX-50 composite sample via in-situ crystallization, whose friction sensitivity, impact sensitivity and characteristic drop height are 76%, 52% and 44.7 cm, respectively [17].However,there is no report on the combination of AP and TKX-50 to adjust the energy performance of TKX-50. This is because that different from CL-20-AP composites,both TKX-50 and AP are ionic compounds,the interaction mechanism between TKX-50 and AP is not clear,and the formation feasibility of their energetic complexes is also unknown.The study on the binding mode and mechanism of TKX-50 and AP is the precondition for the formation of TKX-50-AP energetic composites. Therefore, the interaction mechanism of TKX-50 and AP has certain reference significance for the study of the composition of TKX-50-oxidant and energy release mechanism of TKX-50.It will lay a theoretical foundation for the design of TKX-50-AP mixed crystals and the application of TKX-50 in the field of propellant, propellant and explosive, and will have far-reaching academic significance and broad application value.

In this study,the binding energy and mechanism between TKX-50 and AP were studied via density functional theory (DFT) calculation, molecular dynamics simulation and experiments, which provided a basis for the feasibility of TKX-50-AP mixed crystals.The results can provide guidance for the design and preparation of TKX-50-AP mixed crystals and TKX-50-AP co-crystals.

2. Experimental and computational methods

2.1. Density functional theory calculation

Based on density functional theory (DFT) [18,19], the possible geometric configurations of TKX-50-AP compounds were optimized and calculated by using 6-311++G(d,p)basis set in B3LYP method. The calculation was completed by using the Gaussian 09 program[20]. When there is no imaginary frequency in the vibration analysis of the obtained structure, the structure is stable.‘Multiwfn’program[21]was applied to analyze the wave function,and the electronic density topology was graphically analyzed.

2.2. Molecular dynamics simulation

2.2.1. Force field selection

COMPASS force field[22,23] is the first force field derived from ab initio calculation of quantum mechanics. It can simulate the structure and properties of condensate more accurately and has good applicability to covalent bond compounds. However, TKX-50 and AP are ionic compounds, and COMPASS force field has poor applicability to TKX-50 and AP. ReaxFF force field is a chemical reaction force field described by van Duin in 2001, which can describe the process of bond forming and bond breaking[24].The force field describes the covalent interaction between the bonding atoms based on bond level.However,the force field is less suitable for the new chemical structure system and the binding energy between ionic compounds. Therefore, COMPASS force field and ReaxFF force field are not appropriate to be applied to carry out MD simulation for TKX-50 and AP. DREDING molecular force field can describe a large number of organic compounds, biological molecules and all inorganic molecules [25]. Especially for organic compounds containing C,H,O,N and other major group elements,good simulation results can be obtained, which breaks through the limitation of describing a few molecules consisting of finite atoms in the past. Based on the simple principle of atomic orbital hybridization, the concept of merging force constants of the same kind is proposed, the force constants for single bonds (C-C, C-O,C-N) composed of different atoms are the same. It allows new atoms to be added to the force field.For the missing parameters,by adding the bond length to the bond radius, the bond angle is obtained from hydride orbital hybridization, the geometric or arithmetic mean is adopted as the default parameter, and the electrostatic term is generally calculated by the method. TKX-50 and AP were calculated by DREDING force field. It is found that the errors of crystal density and lattice size are within acceptable range.Therefore,the whole molecular dynamics work was carried out under DREDING force field.

2.2.2. Construction of TKX-50-AP mixed crystal structure

The structures of TKX-50 [26] and AP [27,28] crystals are obtained from X-ray diffraction results. TKX-50 crystal cell is composed of four (NH3OH)+cations and two oxygen-coupled tetrazole rings(C2N8O2)-anions(Fig.1).AP is an ionic crystal,the space group is Pna21, and each cell contains four molecules. Each perchlorate ion is surrounded by seven ammonium ions. The cell length are a=0.9220 nm, b=0.7458 nm, c=0.5814 nm, and the cell angle areα=β=γ=90°.

TKX-50 (3×3×3) supercell and AP (4×4×4) supercell were established. The initial structures of TKX-50 (020), TKX-50 (020),TKX-50 (011), TKX-50 (100) and TKX-50 (11-1) were obtained by cutting TKX-50 (3×3×3) supercell along (020), (011), (100) and(11-1) faces. The initial structure of TKX-50 and AP was simulated by using NVT MD for 20 ps at 298 K. Then the equilibrium structures were obtained. The layered structures of TKX-50 (020), TKX-50 (011), TKX-50 (100) and TKX-50 (11-1) and AP were established, and the NVT MD simulation at room temperature was carried out under DREDING force field to achieve equilibrium. The simulation time was set as 20 ps Finally,the volume of the periodic chamber was further reduced, and MD simulation was carried out to achieve a new equilibrium. The process was repeated until the density of the system approaches its theoretical density.

2.2.3. Molecular dynamics simulation

Under ‘Forcite’ module, the structure of TKX-50 was optimized under DREDING force field. Then, the optimized cell structure was simulated by ‘Growth Morphology’ method in ‘Morphology module’.The morphological important crystal faces((020),(011),(100)and (11-1))of TKX-50 were obtained.

NVT MD simulations were initially carried out to equilibrate these systems. After that, the MD simulation was carried out for another 100 ps to ensure that the systems were truly in the thermal equilibrium condition. Further NPT simulations of 200 ps were performed,in which trajectories were collected at an interval of 50 fs for the binding energy analysis. All the simulations were implemented on computers with Materials Studio from Accelrys Inc.The equilibrium of the system can be determined by the simultaneous equilibrium of temperature and energy.When the temperature and energy fluctuate are in the range of 5%-10%, the system can be considered to have reached equilibrium.

Fig.1. Configurations of TKX-50-AP compounds.

2.3. Experimental methods

TKX-50-AP compounds were prepared via solvent/non-solvent method. TKX-50 and AP were dissolved in dimethyl sulfoxide with mass ratio of 7:3,and ethanol was added into the solution to obtain the mixed crystals of TKX-50-AP compounds. Then, filtered and dried to obtain colorless transparent crystals.Place the sample in a container for later use.

The micro-morphologies of TKX-50-AP compounds were measured by SU8010 field emission scanning electron microscopy,and the high-resolution infrared spectra of TKX-50 and TKX-50-AP compounds were measured by potassium bromide tablet method.The infrared spectra of each sample were measured from 400 cm-1to 4000 cm-1. The test instrument was 60 SXR Spectrometer FT-IR instruments produced by Nicolet Company of the United States,whose resolution is ±0.5 cm-1.

3. Results and discussions

3.1. Geometric structure, electrostatic potential and binding energy of TKX-50-AP compounds

In order to study the feasibility and the mechanism of the combination of TKX-50 and AP,and provide the basis for the design and preparation of TKX-50-AP mixed crystals. Considering the irregular character of the interaction of two molecules in TKX-50-AP mixed crystals, three configurations (TKX-50-AP 1, TKX-50-AP 2, TKX-50-AP 3) of TKX-50-AP compounds were randomly constructed and optimized, as shown in Fig. 1. Then the geometric structure, electrostatic potential and binding energy of TKX-50-AP compounds were calculated.

In TKX-50-AP 1,there is interaction between O30 atom of C-O in AP and H3 atom of N-H in TKX-50, and the bond length is 0.2069 nm. For TKX-50-AP 2, there is interaction between O30 atom of C-O in AP and H3 atom of N-H in TKX-50, and the bond length is 0.2102 nm. In addition, there is interaction between N10 atom of C-N in AP and N 26 of N-H in TKX-50,and the bond length is 0.1921 nm. For TKX-50-AP 3, there is interaction between O30 atom of C-O in AP and H23 atom of H-O in TKX-50,and the bond length is 0.2082 nm. From the analysis, it can be seen that the intermolecular bond length of H…O and H…N in three configurations ranges from 0.1921 nm to 0.2102 nm. In general, the intermolecular forces include hydrogen bonds and van der Waals forces.The bond length of hydrogen bond is 0.11-0.31 nm,the bond length of the van der Waals force interaction is 0.31-0.50 nm, and the bond length of the weak van der Waals force interaction is more than 0.50 nm.Based on the above analysis,hydrogen bonds exist in the three configurations of TKX-50-AP compounds.

Electrostatic potential(ESP)research has been widely applied to molecular interaction recognition, such as hydrogen bond and halogen bond, and its physical visualization provides convenience for analysis.Herein,the wave functions of AP,TKX-50 and TKX-50-AP compounds were analyzed by ‘Multiwfn’ program. Then the molecular electrostatic potential distributions of AP, TKX-50 and three TKX-50-AP compounds were obtained, as shown in Fig. 2.

Fig.2 displays the molecular electrostatic potentials of AP,TKX-50,TKX-50-AP compounds at B3LYP/6-31++G(d,p)level.Different colors represent different electrostatic potentials. Red represents negative potentials, blue represents positive potentials, and other colors (such as yellow) represent the transition from negative to positive. As can be seen from Fig. 2, the positive electrostatic distribution of AP distributes near the (NH4)+functional group, and the negative charge distributes near the (ClO4)-functional group.The negative electrostatic of TKX-50 distributes near two fivemembered rings. Meanwhile, the positive electrostatic of(NH3OH)+functional group is dense on the side of -OH, and the negative electrostatic of(NH3OH)+functional group is dense on the side of -NH3. Taking the configuration of TKX-50-AP 1 as an example, the color of electrostatic potential distribution on the surface of TKX-50 molecule changed from blue to white, and the color of electrostatic potential distribution on the surface of AP molecule changed from red to white, which indicates that the charge in the contact area of TKX-50-AP 1 changed from negative charge to electric neutrality. The main reason is that the positive electrostatic potential of (NH3OH)+in TKX-50 molecule coincides with the negative electrostatic potential of (ClO4)-in AP molecule,and the positive and negative charge superposition makes the electrostatic potential distributions of TKX-50-AP 1 different from that of TKX-50 and AP. This also shows that there is interaction between AP and TKX-50 molecule in the configuration, which is consistent with the result of configuration analysis. The distributions of electrostatic potential in TKX-50-AP 2 and TKX-50-AP 3 are similar to that in TKX-50-AP 1.

Fig. 2. Electrostatic potential distribution on the molecular surface of AP, TKX-50 and TKX-50-AP compounds.

Table 1 lists the interaction energies between dimers in the fully optimized configuration of DFT-B3LYP/6-311++ G (d, p). The specific calculation methods of intermolecular interaction energy are as follows:

In the formula, Einteris the interaction energy, EABis the total energy of the compound configuration, EAis the energy of removing B to get A in the compound configuration, and EBis the energy of removing A to get B in the compound configuration.

For TKX-50-AP 1,TKX-50-AP 2 and TKX-50-AP 3,the interaction energies (△E) are 39.743 kJ/mol, 61.206 kJ/mol and 27.702 kJ/mol,respectively. Therefore, the stability order of the configurations of three compounds is: TKX-50-AP 2 ＞TKX-50-AP 1 ＞TKX-50-AP 3.There are two hydrogen bonds in TKX-50-AP 2,and the bond length is 0.1921 nm, which is the shortest among all hydrogen bonds.There is only one hydrogen bond in TKX-50-AP 3 with a long bond length of 0.2082 nm, and the stability is the worst. TKX-50-AP 1 is in the middle.It can be found that the stability of the configurations of TKX-50-AP compounds depends mainly on hydrogen bonds,including the number of hydrogen bonds and the length of hydrogen bonds.

3.2. Topological analysis of electron density

As an effective method for studying weak interaction, the atomic theory in molecules(AIM)has been successfully applied to the study of various types of hydrogen-bonded compounds with different strength.In order to further study the interaction mode of TKX-50-AP compounds, the topological analysis of electronic density for three configurations of TKX-50-AP compounds was carried out by means of ‘Multiwfn’ program. The electronic density topological bond saddle points are shown in Fig.3, and the topological properties of electron density are shown in Table 2.

The type of chemical bond can be explained by the Laplacian quantity▽2ρ(r) and the energy density value H(r) of electron density at the saddle point.If ▽2ρ(r)at the saddle point is negative,the covalent bond is the main component. However, if ▽2ρ(r) at the saddle point is positive, the weak interaction is closed shell interaction. Laplace quantity and energy density can be used as criteria to measure hydrogen bond strength. When▽2ρ(r)＞0 and H (r)＞0, the hydrogen bond strength is weak. When ▽2ρ(r)＜0 and H(r)＜0,the hydrogen bond strength is strong.When▽2ρ(r)＞0 and H(r)＜0, the hydrogen bond strength is in medium.

Table 1 Energy parameters of TKX-50, AP and TKX-50-AP.

Fig. 3. Electron density topological bond saddle point diagram of TKX-50-AP compounds.

Table 2 Characteristic parameters of electron density topology.

In addition, the kinetic energy density G (r) of electrons is positive, and the potential energy density H (r) is negative. -G(r)/H(r) illustrate the essential characteristics of chemical bonds,whether the region is covalent or non-covalent.When-G(r)/H(r)＞1, the interaction between TKX-50-AP compounds is non-covalent interaction. When -G(r)/H(r)＜0.5, the interaction between TKX-50-AP compounds is covalent interaction. In addition, if -G(r)/H(r)is between 0.5 and 1, the interaction between TKX-50-AP compounds is partial covalent interaction.

From the analysis of Table 2, the values of ▽2ρ(r) of the three configurations of TKX-50-AP compounds are greater than 0, and the values of-G(r)/H(r)are greater than 1,which indicates that the interaction of TKX-50-AP compounds are non-covalent interaction,and it is also closed shell weak interaction.In TKX-50-AP 1,H(r)of O30…H3 is less than 0,H(r)of O30…C20 and O32…N14 is greater than 0,indicating that the intermolecular interaction between TKX-50-AP 2 mainly depends on medium-strength hydrogen bond interaction, and there are some van der Waals forces. H(r) of H3…O30 in TKX-50-AP 2 configuration is less than 0, and H(r) of N16…H26 is greater than 0,indicating that the interaction between TKX-50-AP 2 molecules mainly depends on medium-strength hydrogen bond interaction, and there are part weak hydrogen bond interaction.In TKX-50-AP3,H(r)of H21…O33 and H23…O30 is less than 0, and H(r) of N14…O31 is greater than 0, mainly depending on medium strength hydrogen bond interaction, and there are some weak hydrogen bond interactions. The order of electron density at BCP is TKX-50-AP 3 ＞TKX-50-AP 1 ＞TKX-50-AP 2. Among three configurations, TKX-50-AP1 has the highest intermolecular interaction intensity, and the results of the topology analysis of electron density are consistent with that of energy analysis.

3.3. The intermolecular interaction mechanism of TKX-50-AP mixed crystals

In order to study the mechanism of intermolecular interaction in TKX-50-AP mixed crystals, it is necessary to study the radial distribution function between AP and the atoms on the main growth surface of TKX-50. Herein, the optimized TKX-50 cell is simulated by the ‘Growth Morphology’ method in the ‘Morphology’ module.Crystal growth morphology of TKX-50 in vacuum is obtained.Fig.4 displays that the morphology of TKX-50 is polyhedron.As displayed in Table 3, the main growth surfaces (percentage of total area is larger than 5%)for TKX-50 crystal are(0 2 0),(0 11),(1 0 0)and(11-1) crystalline surfaces, whose growth area account for 10.25%,27.55%, 6.29% and 55.91% of the total area, respectively. The main growth surface for TKX-50 crystal is (1 1 -1) surface.

Fig. 4. The crystal morphology of TKX-50.

Table 3 Main crystal growth surfaces of TKX-50.

The equiIibrium structures of TKX-50(0 2 0)/AP,TKX-50(0 1 1)/AP,TKX-50(1 0 0)/AP and(1 1-1)/AP are gained by MD simulation.It can be seen from Fig.5 that there are tight contacts between TKX-50 and AP in mixed crystals,which shows that TKX-50 and AP have good physical compatibility. Then, the radial distribution function of TKX-50-AP mixed crystals is calculated,and the results are listed in Fig. 6.

Herein, the radial distribution function (RDF) is the ratio of the regional density to the average density of the system.The density of the region near the molecule is different from the average density of the system. However, when the region is far away from the molecule,the density should be the same as the average density.It means that when the value of R is large,the RDF is close to 1.‘g(r)’is usually understood as the geometric distribution of other particles in space(how far away from a given particle)given the coordinates of one particle. Generally, intermolecular forces include hydrogen bonds and van der Waals forces. The hydrogen bond length is 0.11-0.31 nm, the bond length of strong van der Waals force interaction is 0.31-0.50 nm, and the bond length of weak van der Waals force interaction is longer than 0.5 nm.

In Fig. 6, the radial distribution function of H(TKX-50)-O(AP),H(TKX-50)-N(AP), O(TKX-50)-H(AP) and N(TKX-50)-H(AP) are analyzed.Take TKX-50(0 2 0)-AP mixed crystals as an example,the number of H (TKX-50)-O (AP) with bond lengths of 0.17 nm,0.22 nm and 0.27 nm peaked, which indicates H atoms of TKX-50 interact with O atoms of AP mainly via hydrogen bond interaction. The number of H (TKX-50)-N (AP) with bond lengths of 0.32 nm peaked,it indicates that H atoms of TKX-50 interact with N atoms of AP mainly via van der Waals force interaction.The number of O (TKX-50)-H (AP) with bond lengths of 0.33 nm peaked, it indicates that O atoms of TKX-50 interact with H atoms of AP mainly via van der Waals force interaction. The number of N(TKX-50)-H(AP) with bond lengths of 0.12 nm and 0.22 nm peaked, it indicates that N atoms of TKX-50 interact with H atoms of AP mainly via hydrogen bond interaction. Generally, the interaction between TKX-50(0 2 0)and AP in TKX-50(0 2 0)-AP mixed crystals mainly depends on hydrogen bond and van der Waals force. The number and strength of hydrogen bond are significantly greater than that of van der Waals force. The analysis of Fig.5(b), 5(c)and 5(d)reveals that the mode and mechanism of action for TKX-50 molecules and AP molecules in TKX-50 (11-1)-AP, TKX-50 (001)-AP and TKX-50(100)-AP mixed crystals are in a manner similar to that of TKX-50(020)-AP mixed crystals.

3.4. Chemical structure of TKX-50-AP composites

The infrared spectroscopy of TKX-50 is characterized,as shown in Fig. 7 3427 cm-1in the spectroscopy represents the stretching vibration of hydrogen-oxygen bond of hydroxyl group on(NH3OH)+,and hydrogen in air also causes blue shift to the spectral region (moving to high wavenumber). Characteristic absorption band of 2509-3221 cm-1represents the hydrogen bond between TKX-50 molecules. 3091 cm-1, 1581 cm-1and 1530 cm-1are the characteristic absorption bands of tetrazole ring. Characteristic absorption band of 814 cm-1represents the nitrogen-oxygen bond.The two five-membered aromatic rings are not left-right symmetrical structures, and the characteristic absorbs band appearing in 716 cm-1represents the contractive vibration between C-C.

TKX-50-AP composite particles were prepared via solvent/nonsolvent method.From Fig.8(a),it can be seen that some AP particles adsorbed on the surface of TKX-50 crystals. The close contact between them indicates that the binding energy of the two substances is positive, and the adsorption is stable, which verifies the results of theoretical calculation.Fig.4 is the infrared spectroscopy of TKX-50 and AP composites with mass ratio of 7:3.It is found that the absorption peak of TKX-50 at 3427 cm-1was advanced to 3417 cm-1after combined with AP,which represents the stretching vibration of hydrogen-oxygen bond of hydroxyl group on(NH3OH)+. A certain red shift has also occurred for the absorption peak of TKX-50 at 3091 cm-1,1581 cm-1and 1530 cm-1, and they are the characteristic absorption bands of tetrazole ring of TKX-50.Therefore,the formation of hydrogen bonds can reduce the original chemical bond force involved in the formation of hydrogen bond and make the absorption peak shift to low wavenumber.In Fig.8(b)and 3221 cm-1,1413 cm-1,1010 cm-1, 938 cm-1and 620 cm-1on the curve of the infrared spectroscopy represent the absorption peak of AP.Therefore,both TKX-50 and AP exist in the test samples.

Fig. 5. The equiIibrium structures of TKX-50-AP mixed crystals.

Fig. 6. Radial distribution function of TKX-50-AP mixed crystals.

Fig. 7. Infrared spectroscopy of TKX-50.

The experimental results and density functional theory calculations both show that there exists hydrogen bond interaction between AP and TKX-50’s five-membered rings,AP and(NH3OH)+of TKX-50. The experimental results are in good agreement with the density functional calculations.

3.5. Properties of TKX-50-AP composites

Density and oxygen balance(OB)are two important parameters of energetic composites containing oxidant. According to C. Zhang[29] and Y. Wei’s [30] methods, theoretical mix density (dmix) is calculated by Eq. (2), and oxygen balance (OB) of TKX-50-AP composites is calculated according to literature [31]. It is assumed that the system is composed of pure components.

In the formula, miis the mass fraction of component i. All calculation results are shown in Table 3.

As displayed in Table 4, the density of TKX-50-AP composite increases gradually with the increase of AP content in the composite. The OB of TKX-50 is -27.12%, and the oxidation ability of TKX-50 to metal powder is weak. The oxygen balance of AP is 34.04%,which is a common oxidant.When the mass ratio of TKX-50 to AP is about 7:3,the TKX-50-AP complex approaches zero oxygen equilibrium.

Fig. 8. Micro-morphology and infrared spectroscopy of TKX-50-AP composites.

Table 4 Density and oxygen balance of TKX-50-AP composites.

Fig.9. Infrared absorption intensity of tetrazole ring in TKX-50,TKX-50-AP composite varies with temperature.

Fig.9 shows that the infrared absorption intensity of TKX-50 and TKX-50-AP composite tetrazole ring varies with temperature when the heating rate is 10°C/min. The effect of AP content on the thermal decomposition of TKX-50 has not been studied here,but a qualitative analysis of the effect of AP on the thermal decomposition of TKX-50 has been carried out. It can be seen that the infrared absorption intensity of TKX-50 and AP begins to decrease at 100°C, which can be inferred that the intermolecular hydrogen bond is destroyed by the temperature rise at this stage.Because TKX-50 and AP mixtures have stronger hydrogen bond interaction, the degradation rate of TKX-50 and AP mixtures is slightly faster. When the temperature rises to 170°C, it can be observed that the decomposition temperature of the composite is delayed by about 15°C compared with TKX-50, and the decomposition time of TKX-50 mixtures is prolonged. However, the relative infrared absorption intensities of TKX-50 and TKX-50-AP are both close to zero. In general, AP can increase the thermal decomposition temperature of TKX-50 and prolong the decomposition time of TKX-50, but AP does not affect the final decomposition completeness of TKX-50.

4. Conclusions

(1) Hydrogen bonds exist in the three configurations of TKX-50-AP compounds,and the intermolecular bond length of H…O and H…N in the configurations ranges from 0.1921 nm to 0.2102 nm. The positive electrostatic potential of (NH3OH)+in TKX-50 molecule coincides with the negative electrostatic potential of(ClO4)-in AP molecule,it makes the electrostatic potential distributions of the two molecules in the configuration different from that of one molecule alone.

2) The interaction energies of TKX-50-AP 1, TKX-50-AP 2 and TKX-50-AP 3 are 34.604 kJ/mol,56.872 kJ/mol and 22.031 kJ/mol, respectively. The stability order of three compound configurations is TKX-50-AP 2＞TKX-50-AP 1＞TKX-50-AP 3,and stability depends on the number of hydrogen bonds and the length of hydrogen bonds.

(3) The interaction between TKX-50-AP 1 molecules mainly depends on medium-strength hydrogen bond interaction and part van der Waals forces.The interaction between TKX-50-AP2 molecules mainly depends on medium-strength hydrogen bond interaction and part weak hydrogen bond interaction. The interaction between TKX-50-AP3 molecules mainly depends on medium strength hydrogen bond interaction and part weak hydrogen bond interactions.The order of electron density at BCP is TKX-50-AP 3 ＞TKX-50-AP 1 ＞TKX-50-AP 2.

(4) The interaction between TKX-50 molecules and AP molecules in TKX-50-AP mixed crystals mainly depends on hydrogen bond and van der Waals force. The number and strength of hydrogen bond are significantly greater than that of van der Waals force.

(5) Part AP particles adsorbed on the surface of TKX-50 crystals in TKX-50-AP composite particles prepared via solvent/nonsolvent method,and the contact is close.The composition of AP and TKX-50 makes the absorption peak of the fivemembered rings and NH3OH+of TKX-50 shift to low wavenumber. AP can increase the thermal decomposition temperature of TKX-50 and prolong the decomposition time of TKX-50. The experimental results are in good agreement with the density functional calculations.

Declaration of competing interest

No conflict of interest exits in the submission of this manuscript.