Combustion behavior and physico-chemical properties of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50)
Introduction
Concerns about the environmental impact of energetic materials have grown in recent years, demanding new green energetic materials, first of all polynitrogen compounds, which decomposition releases mainly environmentally friendly N2. Tetrazole derivatives are among these compounds. In order to improve energetic properties of the tetrazoles, N-oxides were proposed in several studies as providing even higher densities and stabilities, lower sensitivities and better oxygen balances [1], [2], [3]. A new explosive dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50), which belongs to this chemical class and outperforms all other commonly used explosives, has recently been synthesized [4]. According to published data, TKX-50 can be easily prepared; its explosive performance exceeds the performance of RDX and is comparable to that of CL-20. At the same time, it possesses good thermal stability, low toxicity, and safety of handling is comparable to that of RDX.
The detonation velocity of TKX-50 (D = 9698 ms−1), calculated at maximal density of 1.918 g cm−3 (100 K), is higher than that of β-HMX [5] (D = 9221 ms−1) and also of ϵ-CL-20 (D = 9455 ms−1) [6]. At room temperature (298 K), the density of TKX-50 is 1.877 g cm−3. Next to its impressive performance, the impact sensitivity of TKX-50 is 20 J which is much lower than sensitivities of RDX, HMX, and CL-20, ranging from 4 to 7.5 J [4]. Friction sensitivity of TKX-50 with 120 N is comparable to or lower than sensitivity of RDX, HMX or CL-20 [4]. TKX-50 has an electrostatic sensitivity of 0.1 J [4], which is far higher than the human body can generate (0.025 J). Thermal stability of TKX-50 with a decomposition onset of 221 °C is comparable to that of RDX.
High energetic performance along with high safety allows considering TKX-50 as not only powerful explosive [4], [7], but a promising propellant ingredient also, which could result in superior compositions [8], [9]. Calculations have shown that TKX-50 is a better replacement of RDX in the composite modified double base (CMDB) propellants [8]. Substitution of HMX by TKX-50 in the nitrate ester plasticized polyether (NEPE) propellants results in a two- to five-second increase in the specific impulse. At the same time, neither combustion behavior nor combustion mechanism of TKX-50 has been studied up to date, that are required for purposeful design and adjustment of TKX-50-based rocket propellants.
It is obvious that high energetic characteristics of TKX-50 are primarily due to its high enthalpy of formation. Gas phase enthalpy of formation (131.2 kJ/mol for neutral form of charged components and 1962.3 kJ/mol for noninteracting ions [10]) was computed by combining the atomization method and quantum mechanics [11] (CBS-4 M method [12]). The last value was converted into the solid state enthalpy of formation (446.6 kJ/mol) [4] by subtraction of lattice enthalpy calculated according to Jenkins et al. [13]. The usability of the above method to calculate lattice enthalpy of salts is discussed in [14], indicating that the experimental enthalpy of formation of TKX-50 (439 kJ/mol) proved to be in a good agreement with the calculated one (446.6 kJ/mol).
Without going into discussion on the calculation methods, we note that the present value of the enthalpy of formation of TKX-50 is highly questionable. Indeed, a simple sum of enthalpies of formation of TKX-50 constituents—hydroxylamine (−114.18 kJ/mol [15]) and 5,5′-bis (2-hydroxytetrazole) (∼481 kJ/mol, estimation based on enthalpy of formation of 5,5′-bistetrazole [16]) gives a much lower value of 253 kJ/mol. Heat of reaction between an acid and a base, so-called heat of salt formation, is a difference between enthalpy of formation of salt and sum of enthalpies of formation of salt constituents. Based on comparison of enthalpies of formation of hydroxylammonium perchlorate [17], hydroxylammonium nitrate [15], liquid perchloric acid [15], nitric acid and hydroxylamine [15], heat of salt formation for hydroxylamine salts averages 67 kJ per one mole of hydroxylamine. Taking into account heat of salt formation for two hydroxylamine molecules (∼134 kJ, estimation), the enthalpy of formation of TKX-50 can be calculated as ∼119 kJ/mol, that is more than three times less than the value reported in [4].
The thermal behavior of TKX-50 and the kinetics of its thermal decomposition were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) in nonisothermal condition only [18], the decomposition mechanism of TKX-50 remains unknown. By applying multiple heating rate DSC measurements and Ozawás isoconversional model-free method the activation energy of 143.2 kJ/mol, and pre-exponential factor of 1.99 × 1012 s−1 were calculated from DSC peak maximum temperature vs. heating rate relationship [18].
Initial decomposition steps of TKX-50 were predicted with the help of molecular dynamics simulations on the periodic TKX-50 crystal [19]. The authors showed that continuous heating of this periodic system eventually led to decomposition of the protonated or diprotonated bistetrazole to release N2 and N2O. The reaction barrier for release of N2 and N2O was calculated with help of finite cluster quantum mechanics studies (B3LYP). The author found that transferring one proton to the bistetrazole dianion decreased the reaction barriers. It is a striking result of work [19] that very unstable hydroxylamine, decomposing as it melts near 32 °C, did not undergo any changes upon virtual heating.
The purpose of the present work is to study burning regularities and combustion mechanism of TKX-50 as well as to obtain its thermochemical properties such as thermal stability and enthalpy of formation.
Section snippets
Preparation
TKX-50 was prepared by a modified method published elsewhere [4] via the reaction of dichloroglyoxime with sodium azide in N-methylpyrrolidone. TKX-50 was crystallized from mother solution, isolated by suction filtration, and air dried. The obtained TKX-50 has been characterized by 1H and 13C NMR spectroscopy.
Decomposition study
Experiments on TKX-50 decomposition were carried out in thin-walled glass manometers of the compensation type (the glass Bourdon gauge) at 170–200 °C. A 10 to 15 mg sample was loaded into a
Bomb calorimetric measurements
When ignited in the calorimetric bomb at oxygen pressure of 2 MPa, pressed samples of pure TKX-50 were burned completely without any residues of condensed products. The measured energies of combustion showed satisfactory reproducibility in parallel runs (Table 1). The energy values were calculated as ΔcU (TKX-50) = 8704 ± 53 J g−1 or 2054 ± 16 kJ mol−1 (molecular mass of TKX-50 = 236.15 g mol−1).
The results of combustion experiments correspond to the overall combustion reactions represented by Eq. (1):C2H8N10O
Conclusion
Combustion studies have shown that TKX-50 burns slightly faster than HMX and approaches the burning rate of CL-20. The dependence of the burning rate on pressure reveals a break at a pressure of 5 MPa.
Thermocouple-aided measurements in the combustion wave of TKX-50 have shown that the maximum combustion temperature proved to be much lower than the adiabatic temperature calculated by using the published enthalpy of formation of TKX-50. Analysis of the thermocouple data allowed establishing TKX-50
Acknowledgments
The authors are grateful to Dr. V.Yu. Egorshev and Prof. V.V. Serushkin (MUCT) for fruitful discussions, Dr. P.B. Dzhevakov and Dr. M.A. Topchiy (TIPS) for the assistance in TKX-50 synthesis, graduate student A.O. Suprun (MUST) for the assistance in physico-chemical study. The work was partially supported (to F.S.A.) by the Ministry of Education and Science of Russian Federation (the basic part of government task).
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