Condensed Phase Behavior in the Combustion of Ammonium Dinitramide

The methods for the study of flame structure and kinetics of the thermal decomposition of solid propellant by probing mass spectrometry are described. The developed methods were applied to the study of ammonium dinitramide (ADN) combustion chemistry. The study has shown that along with ADN decomposition, sublimation takes place to give gaseous ADN followed by dissociation to yielding ammonia and dinitraminic acid (HD). Gaseous ADN has been observed in ADN decomposition products. The structure of ADN combustion zones at 1-6 atm was studied using a molecular-beam mass-spectrometry as well as a microthermocouple technique. Three combustion zones have been observed. Gaseous ADN has been discovered in the first cool flame zone at 3 atm. Gaseous ADN dissociation on NH3 and HD followed by HD decomposition in the near-surface zone are key reactions resulting in a temperature rise of about 150 K. The second high-temperature zone is found within 6-8 mm from the ADN burning surface at 6 atm. The main reaction in this zone is ammonia oxidation by nitric acid and the combustion temperature is 1400 K. The third zone was observed at 40 atm, the measured final temperature was —2000 K. The obtained data form the basis for the development of a chemical mechanism of reactions in both the ADN flame and combustion model.

The thermal decomposition behavior and combustion characteristics of mixtures of ammonium dinitramide (ADN) with additives were studied. Micrometer‐sized particles of Al, Fe2O3, TiO2, NiO, Cu(OH)NO3, copper, CuO, and nanometer‐sized particles of aluminum (Alex) and CuO (nano‐CuO) were employed. The thermal decomposition was measured by TG‐DTA and DSC. The copper compounds and NiO lowered the onset temperature of ADN decomposition. The heat value of ADN with Alex was larger than that of pure ADN in closed conditions. The burning rates and temperature of the pure ADN and ADN/additives mixtures were measured. CuO and NiO enhance the burning rate, particularly at pressures lower than 1 MPa, because of the catalyzed decomposition in the condensed phase; the other additives lower the burning rate. This negative effect on the burning rate is explained based on the surface temperature measurements by a physicochemical mechanism, which involves a chemical reaction, a phase change of the ammonium nitrate, and the blown‐off droplets of the condensed phase.

*† ‡ § A multi-phase model for the combustion of ammonium dinitramide (ADN) monopropellant is presented. As part of the work, an in depth study concerning ADN combustion has been undertaken. An extensive literature review has been conducted to extract both experimental data and qualitative theories concerning ADN combustion. Analyses and reviews are presented pertaining to some of the major theories. Based on the literature review a numerical model has also been developed for ADN using the BYU PHASE3 combustion code. The model accurately predicts burning rates, temperature and species profiles, and other combustion characteristics of ADN at pressures preceding the unstable region (20 to 100 atm) of combustion. Burning rates are also predicted in the unstable zone and at higher pressures. Results are discussed and proposed modifications to the present condensed phase model are presented. Nomenclature b = constant in burning rate correlation P = pressure n = pressure exponent

Temperature profiles in the ammonium dinitramide (ADN) combustion wave were measured in the 0.04-10 MPa pressure interval using thin tungsten-rhenium thermocouples. The data obtained suggest that the ADN decomposition reaction in the condensed zone plays a dominant role in burning at low pressures. The heat feedback from the gas to the surface appeared to be negligibly small. It has been concluded that the reaction of AN dissociation to form NH 3 and HNO 3 controls the ADN burning surface temperature. The three-zone flame structure of the ADN combustion wave has been found, and the main chemical reactions occurring in the zones have been proposed. At low pressures (below 1 MPa), the burning of ADN can be satisfactorily described by a condensed-phase combustion model with the rate-controlling reaction being the ADN decomposition in the melt. A gas-phase model with the rate-controlling reaction being HNO 3 decomposition in the first flame can be used at high pressures (above 10 MPa). In the middle pressure interval, ADN shows combustion instability caused by deficiency of heat produced in the condensed material. In this area the fast but energy-limited heat-release process in the condensed phase has to adapt itself to the slow but energy-rich heat-release process in the gas phase.

A comprehensive review of thermal decomposition and combustion of ammonium dinitramide (ADN) has been conducted. The basic thermal properties, chemical pathways, and reaction products in both the condensed and gas phases are analyzed over a broad range of ambient conditions. Detailed combustion-wave structures and burning-rate characteristics are discussed. Prominent features of ADN combustion are identified and compared with other types of energetic materials. In particular, the influence of various condensed- and gas-phase processes in dictating the pressure and temperature sensitivities of the burning rate is examined. In the condensed phase, decomposition proceeds through the mechanisms ADN → NH4NO3 + N2O and ADN → NH3 + HNO3 + N2O, the former mechanism being the basic one. In the gas phase, the mechanisms ADN → NH3 + HDN and ADN → NH3 + HNO3 + N2O are prevalent. The gas-phase combustion-wave structure in the range of 5–20 atm consists of a near-surface primary flame followed by a dark-zone temperature plateau at 600–1000°C and a secondary flame followed by another dark-zone temperature plateau at 1000–1400°C. At higher pressures (60 atm and above), a final flame is observed at about 1800°C without the existence of any dark-zone temperature plateau. ADN combustion is stable in the range of 5–20 atm and the pressure sensitivity of the burning rate has the form r b = 20.72p 0.604 [mm/sec] (p = 0.5–2.0 MPa). The burning characteristics are controlled by exothermic decomposition in the condensed phase. Above 100 atm, the burning rate is well correlated with pressure as r b = 8.50p 0.608 [mm/sec] (p = 10–36 MPa). Combustion is stable, and intensive heat feedback from the gas phase dictates the burning rate. The pressure dependence of the burning rate, however, becomes irregular in the range of 20–100 atm. This phenomenon may be attributed to the competing influence of the condensed-phase and gas-phase exothermic reactions in determining the propellant surface conditions and the associated burning rate.

Despite significant progress in studying thermal decomposition of ammonium dinitramide (ADN), the kinetics of the process at the level of elementary stages has not been adequately understood. The aim of this review is to summarize various published data, which are of interest for studying and simulating the processes of thermal decomposition and combustion of ADN. Considerable attention is paid to physical and chemical properties of ADN, dinitramide and its anion N(NO2) 2 - , which play a key role in ADN decomposition. Various paths of decomposition of ADN, dinitramide, and N(NO2) 2 - are discussed. Results illustrating alternative points of view on the decomposition process are presented.

Modeling of ammonium dinitramide (ADN) monopropellant combustion with coupled condensed and gas phase kinetics

Thermal decomposition of eco-friendly propellants such us ammonium dinitramide (ADN) aims to replace hydrazine in satellite systems. ADN, with formula [NH4]+[N(NO2)2]−, is a promising high-performance rocket propellant. It decomposes cleanly, producing gases such as NH3, H2O, NO, N2O, NO2, HONO, and HNO3, making it an attractive alternative to ammonium perchlorate (AP) and hydrazine. This chapter reviews catalyst systems for ADN decomposition, focusing on efficiency and thermal stability. Various catalysts, including metal oxides, transition metal complexes, and nanomaterials, enhance ADN decomposition. Iron and copper oxides lower decomposition temperatures, crucial for energy-efficient propellant compositions. Ruthenium and palladium complexes support homogeneous catalysis. Nanomaterials with high specific surface areas and distinct electronic activity improve ADN decomposition. Alloying carbon nanotubes with metals or using noble metal nanoparticles enhances decomposition rates at lower temperatures while maintaining thermal stability.

Thermal decomposition of a famous high oxidizer ammonium dinitramide (ADN) under high temperatures (2000 and 3000 K) was studied by using the ab initio molecular dynamics method. Two different temperature-dependent initial decomposition mechanisms were observed in the unimolecular decomposition of ADN, which were the intramolecular hydrogen transfer and N—NO2 cleavage in N(NO2)−. They were competitive at 2000 K, whereas the former one was predominant at 3000 K. As for the multimolecular decomposition of ADN, four different initial decomposition reactions that were also temperature-dependent were observed. Apart from the aforementioned mechanisms, another two new reactions were the intermolecular hydrogen transfer and direct N—H cleavage in NH4+. At the temperature of 2000 K, the N—NO2 cleavage competed with the rest three hydrogen-related decomposition reactions, while the direct N—H cleavage in NH4+ was predominant at 3000 K. After the initial decomposition, it was found that the temperature increase could facilitate the decomposition of ADN, and would not change the key decomposition events. ADN decomposed into small molecules by hydrogen-promoted simple, fast and direct chemical bonds cleavage without forming any large intermediates that may impede the decomposition. The main decomposition products at 2000 and 3000 K were the same, which were NH3, NO2, NO, N2O, N2, H2O, and HNO2.

In this work, the evaporation and combustion processes of liquid propellant ammonium dinitramide–methanol aqueous solution were investigated by a thermogravimetric analyzer and a Curie-point pyrolysis unit, respectively, and the gas-phase products were measured by a Fourier transform infrared spectroscopy. The results showed that methanol and water evaporated first, followed by the decomposition of ammonium dinitramide at heating rates of 5–. When a catalyst was added, ammonium dinitramide decomposed much faster and released heat that let methanol evaporate immediately as the propellant solution was in contact with the catalyst particles. To explore how the decomposition of ammonium dinitramide is coupled with the oxidation of methanol, the propellant solution was heated by a Curie-point pyrolysis unit at a rate of to temperatures ranging from 160 to 670°C. Large amounts of nitrous oxide and ammonia were detected at all temperatures, suggesting that most of the decomposition of ammonium dinitramide occurred at the condensed phase. Finally, a detailed gas-phase ammonium dinitramide–methanol reaction model was built to analyze the coupling between the decomposition of ammonium dinitramide and the oxidation of methanol.

The paper presents a detailed analysis of the ammonium dinitramide (ADN) combustion behavior depending on organic and inorganic additives, material of the surrounding shell, and the sample cross-section size. In spite of the fact that ADN is an oxidizer, combustible surroundings have been found to exert practically no effect on the burning rate of ADN pressed strands. In contrast, minor amounts of organic and inorganic admixtures were shown to have a strong effect on ADN burning behavior, extending considerably the pressure limit of sustained combustion to the vacuum area. Within the pressure range of 1-10 MPa, a decrease in the strand cross-section size leads to a decrease in the ADN burning rate, accompanied by a notable burning-rate data scatter. The main reason for the observed combustion behavior is assumed to be a dominant role of exothermic condensed-phase decomposition reactions in burning of ADN. A descriptive mechanism has been proposed to explain the influence of small amounts of different substances added to ADN on its combustion behavior and the low-pressure limit of self-sustained burning.

Ammonium dinitramide (ADN) is a promising new oxidizer for solid propellants because it possesses both high oxygen balance and high energy content, and does not contain halogen atoms. A necessary characteristic of solid propellants is chemical stability under various conditions. This study focused on the thermal decomposition mechanism of ADN under pressurized conditions. The pressure was adjusted from 0.1 to 6 MPa, while ADN was heated at a constant rate. The exothermal behavior and the decomposition products in the condensed phase during heating were measured simultaneously using pressure differential scanning calorimetry (PDSC) and Raman spectrometry. PDSC analyses showed the multiple stages of exotherms after melting. The exothermal behavior at low temperatures varied with pressure. Analysis of the decomposition products indicated that ammonium nitrate (AN) was generated during decomposition of ADN at all pressures. At normal pressure, AN was produced at the same time as start of exotherm. However, the temperature at which the ratio of ADN in chemical species in the condensed phase began to decrease under high pressure was higher than that at atmospheric pressure despite the existence of significant exotherm. At initial stage, thermal decomposition of ADN that does not generate AN was thought to be promoted by increased pressure.

The paper presents a study of the burning behavior and flame structure of compositions consisting of ammonium dinitramide (ADN) and glycidylazidepolymer (GAP). Unlike neat ADN and GAP, the ADN/GAP binary mixtures appeared to burn steadily within the whole pressure interval, showing no breaks on the burning rate vs. pressure dependences. In the lowpressure area, the mixtures burn with rates less than those of pure ADN and exceed it as pressure grows above 10 MPa. In the whole pressure interval, the mixtures burn with the rates higher than those of pure GAP. It was determined on the basis of flame structure investigation that the surface temperature of ADN/GAP mixtures is controlled by the dissociation reaction of ammonium nitrate (AN) formed in the condensed phase during ADN decomposition. Physical and chemical processes responsible for the observed combustion behavior of the mixtures are discussed.

A kind of composite propellant based on a click-chemistry binder, the propargyl-terminated ethylene oxide-tetrahydrofuran copolyether (PTPET), and ammonium perchlorate (AP), aluminum (Al) were studied with ammonium dinitramide (ADN), hexahydro-1,3,5-trinitro-s-triazine (RDX) and hexanitroisowurtzitane (CL20), respectively. Thermogravimetric analysis (TG) and energy calculation of the composite propellants containing ADN and CL20 were carried out. The burning rates of the composite propellants were measured in the pressure range of 3–17 MPa with measurement system of Charge Coupled Device (CCD) line scan camera and high-pressure chamber. Scanning electron microscopy was used to characterize the quenched surfaces of the burning propellants. Microscopic high-speed photography was used to record the flame structure and dynamic burning surface of the propellants in the argon atmosphere. The replacement of RDX with CL20 had less effect on the thermal decomposition of the components in the propellant, but increased the burning rates of the propellant. The replacement of AP with ADN could significantly accelerate the thermal decomposition of the plasticizer A3, AP and PTPET binder, so that the burning rates under high pressure was raised obviously. For the propellants containing ADN and CL20, there were dense and thick molten layer of ADN on the burning surface. The propellants containing ADN and CL20 exhibited characteristic flame different from the propellant containing only AP and RDX. These results were considered to correlate with the early thermal decomposition of ADN and higher burning rate of the propellants.

The initial interaction mechanism is very important for the design and safety of nano-scale composite energetic materials composed of ammonium dinitramide (ADN) and nitrocellulose (NC). The thermal behaviors of ADN, NC and an NC/ADN mixture under different conditions were studied by using differential scanning calorimetry (DSC) with sealed crucibles, an accelerating rate calorimeter (ARC), a self-developed gas pressure measurement instrument and a DSC-thermogravimetry (TG)—quadrupole mass spectroscopy (MS)—Fourier transform infrared spectroscopy (FTIR) combined technique. The results show that the exothermic peak temperature of the NC/ADN mixture shifted forward greatly in both open and closed circumstances compared to those of NC or ADN. After 585.5 min under quasi-adiabatic conditions, the NC/ADN mixture stepped into the self-heating stage at 106.4 °C, which was much less than the initial temperatures of NC or ADN. The significant reduction in net pressure increment of NC, ADN and the NC/ADN mixture under vacuum indicates that ADN initiated the interaction of NC with ADN. Compared to gas products of NC or ADN, two new kinds of oxidative gases O2 and HNO2 appeared for the NC/ADN mixture, while NH3 and aldehyde disappeared. The mixing of NC with ADN did not change the initial decomposition pathway of either, but NC made ADN more inclined to decompose into N2O, which resulted in the formation of oxidative gases O2 and HNO2. The thermal decomposition of ADN dominated the initial thermal decomposition stage of the NC/ADN mixture, followed by the oxidation of NC and the cation of ADN.