Large Positive Heat Of Formation Research Articles

Computational analysis of polyazoles and their N-oxides

The polyazoles have five-membered heterocyclic rings with N/C ratios of at least 2/3. Their high nitrogen contents result in relatively high crystal densities and large positive heats of formation, features that make them attractive as frameworks for energetic materials. However the presence of linked nitrogens (catenation) is accompanied by reduced stability (reflected in the large heats of formation). Several related factors may be involved in this, including the weakness of N-N bonds, the possibility of decomposing through release of the very stable N2(g) and the repulsion between lone pairs of neighboring nitrogens. We show that in the polyazoles a particularly important source of reduced stability is the presence of adjacent doubly-coordinated nitrogens, especially if connected by a formal double bond. Our computed polyazole electrostatic potentials are consistent with significant repulsion between the lone pairs of these nitrogens. Introducing an N-oxide linkage on one of them leads to some stabilization; the oxygen withdraws electronic charge from the heterocyclic ring, reducing the electronic repulsion within it. However the N-oxide derivatives having more nitrogen catenation are still the less stable ones. Comparing the polyazoles to the polyazines (six-membered N/C heterocyclic rings), the π electrons in the former are less delocalized and the computed bond lengths in the polyazole rings are consistent with the formal single and double bonds in their Lewis structures. An interesting consequence is that if a second N-oxide is introduced next to the original one, the bond between the two nitrogens remains intact in polyazoles if it is formally double but it is considerably weakened or broken in polyazines, and in polyazoles if formally single.

Oxygen‐Balanced Energetic Ionic Liquid

Energetic ionic liquids (EILs) are of great interest. They offer enhanced stability, higher densities, no vapor pressure, and, hence, no vapor toxicity. As a general principle, the stability of energetic ionic compounds can be greatly enhanced by making the cation the fuel and the anion the oxidizer. The formal positive charge increases the ionization potential of the fuel cation, and the formal negative charge decreases the electron affinity of the anion. In this manner, the fuel cation becomes more oxidizer-resistant, and the oxidizer anion is protected against premature reduction by the cation. For environmental reasons, it is also desirable to avoid halogen-containing ingredients, such as perchlorates. The previously known EILs consist of small oxidizing anions, such as ClO4 , NO3 , or N(NO2)2 , and large fuel cations containing quaternary nitrogen heterocycles with long, asymmetric, poorly packing side chains. The most serious drawback of these EILs is that they are underoxidized. The small anions do not carry sufficient oxygen for complete oxidation of the large fuel cations to carbon monoxide, resulting in poor performance. In rocket propulsion, a low molecular weight of the exhaust products is very important. 5] Furthermore, at high flame temperatures CO2 is dissociated almost completely to CO and O2 (Boudouard equilibrium). Therefore, it is often sufficient to oxidize the carbon content only to CO and not to CO2 to achieve nearmaximum performance. The aim of this study was the preparation of halogen-free, CO-balanced, EILs. In 1998, the concept of oxidizer-balanced EILs was proposed, and in 2002, its practicability was shown by the preparation of 1-ethyl-3-methylimidazolium tetranitratoborate, a compound that turned out to be indeed an ionic liquid with a freezing point of 25 8C. However, its energy content and thermal stability were marginal. Herein, we report on a significantly improved compound using the tetranitratoaluminate anion as a thermally more stable highoxygen carrier and the 1-ethyl-4,5-dimethyltetrazolium cation as a more energetic counterion (imidazole, DH o f = + 49.8 kJmol ; tetrazole, DH o f =+ 237.1 kJmol 1 ). These are the first CO-balanced EILs. Although an oxygenbalanced tetrazolium salt, 5-aminotetrazolium nitrate, was recently reported, its melting point of 173 8C does not classify it as an ionic liquid. Polynitratoaluminates were first studied in the 1960s in the USA and, subsequently, during the 1970s in the USSR. Several examples of alkali metal, NO2 , and ethylammonium salts of tetra-, penta-, and hexanitratoaluminate anions are known. The tetranitratoaluminate anion contains 12 oxygen atoms; of these, 10.5 are available to oxidize a fuel cation. Alkylated tetrazolium cations were used in this work because of their large positive heats of formation and their potential to form ionic liquids. Ionic salts of the tetranitratoaluminate anion can be prepared in essentially quantitative yields in one-pot reactions in nitromethane solution. The starting materials are the chloride salt of the cation, aluminum trichloride, and dinitrogen tetroxide. The synthesis of 1-ethyl4,5-dimethyltetrazolium tetranitratoaluminate (3) is shown in Scheme 1. The starting material 1-ethyl-4,5-dimethyltetrazo-

Formation and Structural Anomaly of the Metastable Phases in an Immiscible Ag−Mo System Studied by Ion Beam Mixing and Molecular Dynamics Simulation

For the equilibrium immiscible Ag-Mo system characterized by a large positive heat of formation, the nanosized Ag-Mo multilayered samples are designed and prepared to include sufficient interfacial free energy to elevate their initial energetic states to be higher than that of either the amorphous phase or solid solution and then subject to 200 keV xenon ion irradiation. The results show that a uniform amorphous alloy can be obtained within a composition range, at least, from 25 to 88 atom % of Mo. Interestingly, in the intermediate stage of ion irradiation, a bcc phase, an amorphous phase, and an order (bcc)-disorder coexisting state appear simultaneously in the Ag12Mo88 multilayered sample and extend over the entire bright field image with unanimously homogeneous composition. In thermodynamic modeling, a Gibbs free energy diagram of the Ag-Mo system is constructed, based on Miedema's model, and suggests that within a narrow composition regime of 85-90 atom % of Mo, the energy difference between the bcc and the amorphous phases is extremely small, which is probably the very reason for the order-disorder coexisting state to appear. In atomistic modeling, an ab initio derived Ag-Mo potential is applied to perform molecular dynamics simulations. The simulations not only determine an intrinsic glass-forming ability/range (GFA/GFR) of the Ag-Mo system to be from 10 to 88 atom % of Mo but also reveal the possibility of the formation/appearance of a crystalline and amorphous mixture in a narrow composition regime of 88-92 atom % of Mo. Apparently, the theoretical results are in excellent agreement and/or compatible with the experimental observations in ion beam mixing.

Structural phase transition induced by ion irradiation in the equilibrium immiscible Ag–Co system

In the equilibrium immiscible Ag–Co system characterized by a large positive heat of formation (28 kJ/mol), interesting structural phase transitions were observed in Ag–Co multilayered films upon 200-keV xenon-ion irradiation at 300 K within a dose range of 5×1014 to 9×1015 Xe+/cm2. The formation of a new metastable HCP phase and observation of other structural phase transitions upon ion irradiation were quite in accordance with first principles and thermodynamic calculations. It was observed that in the as-deposited Ag–Co multilayered films, the stress from the Co layers resulted in a condensation effect on the Ag lattice, and that after irradiation to a dose of 5×1014 Xe+/cm2, the Ag lattice recovered to its original size along with the hcp Co transforming into its high-temperature-stabilized fcc structure. Besides, the mechanism of the metastable phase formation as well as the observed structural phase transitions induced by ion irradiation is discussed in terms of thermodynamics and growth kinetics.

Atomistic modeling and thermodynamic interpretation of the bridging phenomenon observed in the Co-Au system

For the Co-Au system, a $n$-body potential is derived by fitting to the properties acquired by ab initio calculations for a few possible metastable Co-Au compounds and proven to be realistic. Based on the derived potential, molecular dynamics simulations are performed, using the $\mathrm{Au}∕\mathrm{Co}∕\mathrm{Au}$ sandwich models, to investigate the bridging phenomenon ever observed in experiments. The simulations reveal that bridging is dependent on the temperature as well as on the thickness of the Co interlayer. A thermodynamic model is proposed and accordingly, it is the energy difference between the interface of $\mathrm{Au}∕\mathrm{Co}$ and the interface of $\mathrm{Au}∕\mathrm{Co}\text{\ensuremath{-}}\mathrm{Au}$ mixture that overcomes an intrinsic repulsion between Co and Au originated from a large positive heat of formation of the Co-Au system and drives the atomic movement as well as accounts for the thickness and temperature dependences in the bridging phenomenon.

Study of metastable alloy formation by thermodynamic calculation and ion beam manipulation in an equilibrium immiscible Au–W system

Based on thermodynamic calculations, a Gibbs free energy diagram was constructed to predict the possibility of metastable alloy formation in the equilibrium immiscible Au–W system characterized by a large positive heat of formation of +17kJ/mol. Accordingly, in the properly designed Au–W multilayered films, a unique super-saturated fcc solid solution was obtained by ion beam irradiation on the Au-rich side. More interestingly, on the W-rich side, an amorphous alloy was formed through a two-step structural transition. First, a low dose (2×1015 Xe+/cm2) irradiation resulted in the formation of a super-saturated bcc solid solution. Second, the bcc solid solution transformed into an amorphous state upon further irradiation up to a dose of 8×1015 Xe+/cm2. We report, in this paper, results of thermodynamic calculations and the experimental observations concerning the metastable alloy formation in the Au–W system together with a brief discussion of the structural transition mechanism.

Interface assisted formation of a metastable hcp phase by ion mixing inan immiscible Ag-Ni system

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in theAg80Ni20 multilayered films, in which the excess interfacial freeenergy provided partial driving force for alloying in an equilibrium immiscibleAg-Ni system characterized by a large positive heat of formation. Themetastable phase was identified to be of D019 hcp structure with astoichiometry of Ag3Ni and can therefore be considered as a Hume-Rothery 7/4electron compound. A thermodynamic calculation showed that when the multilayersconsisted of 8 or more bilayers, the interfacial free energy could elevate theirinitial energy levels up to a state higher than that of the hcp phase, and thatwhile the multilayers composed of 6 or less bilayers, it was thermodynamicallynot favoured to form the hcp phase. Kinetically, the metastable hcp phase wasgrown from the fcc Ag lattice through a fast sliding mechanism. Furthermore, anab initio calculation showed that a minimum total energy of the Ag3Niphase did correspond to the above observed metastable state, indicating that thestability of the metastable Ag-Ni hcp phase was correlated with its electronicstructure.

Burn rate measurements of HMX, TATB, DHT, DAAF, and BTATz

Abstract : In this paper we present new burn rate results for several energetic materials. The burn rates of octahydro- 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), HMX and estane-based binder (PBX 9501), 1,3,5-triamino- 2,4,6-trinitrobenzene (TATB), and TATB and KelF binder (PBX 9502), are reported and compared with existing data. Burn rate data of these common explosives complement and extend existing data sets. Burn rate data of three novel high-nitrogen materials are presented in this work. Specifically, the high-nitrogen monopropellants considered are 3,6-dihydrazino-s-tetrazine (DHT), 4,4'-diamino-3,3' -azoxyfurazan (DAAF), and 3,6-bis(1H-1,2,3,4-tetrazol-5-amino)-s-tetrazine (BTATz). High-nitrogen compounds may be key to meeting the advanced performance objectives of next-generation solid propellants. High-nitrogen solids offer the possibility of high performance, reduced emissions, and lower plume signature (low temperature and no HCl) than current propellant systems. The theoretical specific impulse is comparable to HMX. In contrast to HMX, however, high-nitrogen materials tend to be insensitive to impact. Because high-nitrogen energetic materials have intrinsically large positive heats of formation and produce low-molecular-weight reaction products, they may be suitable for consideration in high-performance propellant applications. BTATz appears particularly interesting because of its rapid burn rate, relatively low-pressure exponent, and high heat of formation. The effect of a small amount of binder is investigated for all but one of these materials.

Unusual alloying behavior observed in the immiscible AgMo and AgNb systems under ion irradiation

Free energy diagrams for the AgMo and AgNb systems which have very large positive heats of formation at equi-atomic stoichiometry, + 56 and + 25 kJ/mol, respectively, were constructed which included consideration of the interfacial free energy present in multilayered films. The calculated diagrams suggested that amorphization was thermodynamically achievable in both systems despite their large positive heats of formation. In the case of AgMo multilayers containing 12 layers, the calculated predictions of amorphization favored Ag- and Mo-rich alloys, in agreement with experimental results of ion mixing conducted at liquid nitrogen temperature. Additionally, new metastable FCC Mo- and Nb-enriched phases were obtained at different irradiation doses. More interestingly, these FCC phases transformed into amorphous states after room temperature aging. This FCC to amorphous transition was also discussed by comparing the free energy of these states.

Irradiation Induced Amorphization and Free Energy Calculation in Immiscible Fe-Cu Multilayers

Amorphization was achieved by a non-equilibrium process of ion mixing of multilayered films in an immiscible Fe-Cu system with a large positive heat of formation (+19 kJ/mol). The free energy diagram of the Fe-Cu system was constructed. In calculation, the interfacial free energy, ferromagnetic effect and the structure characteristics were considered. The calculated Fe-Cu free energy diagram can explain the amorphization behavior in this immiscible system.

Ion beam mixing of selected binary metal systems with large positive heats of formation

Multilayered samples of the binary systems AuW, AgW and CuW of different overall compositions were irradiated at 77 K with 400 keV Kr + ions at doses up to 5 × 10 16 ions cm −2. In thermal equilibrium these systems have a vanishingly small solid solubility of the components corresponding to a large positive value of the heat of formation. The AgW and CuW systems are known to be immiscible even in the liquid state. Analysis of lattice parameters from our experiments indicates that metastable solid solutions have formed during ion irradiation. This shows that ion beam mixing is a suitable method for producing metastable alloys even in systems with large positive heats of formation.

Different crystalline structures of ruthenium-rich metastable phases formed by ion beam mixing of the binary systems AuRu and AgRu

Multilayered samples of the binary metal systems AuRu, AgRu, AuOs and AgOs were prepared by ultrahigh vacuum deposition of the pure elements on glass and rock salt substrates and were subsequently bombarded at 77 K by 400 keV Kr + ions. Structural analyses were performed by in situ X-ray diffraction using a Seemann-Bohlin arrangement and by transmission electron microscopy. The investigated systems have large positive heats of formation (23–41 kJ mol −1), and therefore in thermal equilibrium the miscibility in the solid state is vanishingly small. For all systems one component has the f.c.c. and the other the h.c.p. structure. From the experiments the conclusion can be drawn that during ion irradiation an extension of the mutual solid solubilities occurred. While with glass as the substrate the crystalline structures of the terminal phases for all the systems did not change, with rock salt as the substrate we observed for the systems AuRu and AgRu after ion irradiation with high doses that two ruthenium-rich phases, the h.c.p. phase already known and a new f.c.c. phase were present.

Amorphization in Fe-Cu system by tracing its microscopic features

Strong evidence that amorphization was achieved by ion-beam mixing in a Fe-Cu system that has a large positive heat of formation (+19kJ/mole) is reported. The composition of the films, Fe70Cu30, was chosen according to those of the localized amorphous phase determined by energy dispersive spectroscopy. The phase identification by transmission electron microscopy, as well as the results from Rutherford backscattering, indicated that the formation of an amorphous phase and the mixing efficiency by ion-beam mixing depended greatly on the alloy composition. Raman spectra were obtained from the metal-metal thin amorphous films in this study and the related results are discussed.

Can Amortization Bs Induced by Ion Mixing in Ag-Cu and Ag-Ni Systems?

ABSTRACTThe extended Structural Difference Rule for amorphous phase formation states that an amorphous phase can be obtained by ion mixing with an alloy with a composition lying in a two-phase region in the equilibrium phase diagram. This criterion has to respond to the challenge that no amorphous alloy has been formed in some early studied systems exhibiting a two-phase region character, e.g. Ag-Cu(typical eutec-tic),Ag-Ni(almost entirely immiscible),etc‥We performed ion mixing experiments for several systems at liquid nitrogen temperature using Xe ions with low current density. Amorphization was indeed observed in both Ag-Cu and Ag-Ni samples, as two halos were seen by TEM SAD immediately after adequate doses ion mixing. These not only support our two-pnase region rule, but also show the possibility of amorphization in a system(Ag-Ni) that has large positive heat of formation.