Positive Enthalpy Of Formation Research Articles

Van der Waals density functional study of the energetics of alkali metal intercalation in graphite

We report on the energetics of intercalation of lithium, sodium and potassium in graphite by density functional theory using recently developed van der Waals (vdW) density functionals. First stage intercalation compounds are well described by conventional functionals like GGA, but van der Waals functionals are crucial for higher stage intercalation compounds and graphite, where van der Waals interactions are important. The vdW-optPBE functional gave the best agreement with reported structure and energetics for graphite and LiC6 and was further applied for intercalation of Na and K. The enthalpy of formation of LiC6 and KC8 were found to be −16.4 and −27.5 kJ mol−1 respectively. NaC6 and NaC8 were unstable with positive enthalpies of formation (+20.8 and +19.9 kJ mol−1). The energetics of stacking of graphene and intercalant layers was investigated from first to fifth stage intercalation compounds. Higher stage compounds of Li and K were stable, but with less negative enthalpy of formation with increasing stage order. The higher stage Na compounds possessed positive enthalpy of formation, but lower in magnitude than the energy difference of 0.6 kJ mol−1 between graphite with AB and AA stacking. The abnormal behaviour of the lower stage Na intercalation compounds was rationalized by the lower energy involved in the formation of the chemical bond between carbon Na relative to the corresponding bond with Li or K. The chemical bond between alkali metal and carbon is characterized by charge transfer from the alkali-metal to carbon resulting in ionized alkali-metals. The intercalation induces only a subtle increase in the in-plane C–C bond lengths, with longer C–C bonds in the vicinity of the alkali metals but without breaking the hexagonal symmetry.

Synthesis, characterization and theoretical study of 2-azido-4-nitroimidazole-based energetic salts

Abstract The 2-azido-4-nitroimidazole-based energetic salts were synthesized in a simple and straightforward manner. The structures of these new salts were determined by IR, 1 H NMR and 13 C NMR spectroscopy and elemental analysis. These salts also exhibited high positive enthalpies of formation, high nitrogen content, and moderate detonation properties.

New Approaches to Functionalizing Metal‐Coordinated N2

N2, can do: Converting the refractory molecule N2 into synthetic chemicals is thermodynamically preferred when the derivatizing reactant is a “high-energy molecule”, one with a positive enthalpy of formation. Alkynes and allenes are thus targeted for this purpose (see scheme), and a range of likely products are identified, especially those with NN bonds, including bipyrroles. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Tetraanionic Nitrogen-Rich Tetrazole-Based Energetic Salts

1,1,3,3-Tetra(1H-tetrazol-5-yl)propane-based energetic salts were synthesized in a simple and straightforward manner. The structures of these new salts were determined by (1)H and (13)C NMR spectroscopy, IR spectroscopy, MS, and elemental analysis. All of these compounds showed good thermal stabilities above 180 °C, as confirmed by thermogravimetric-differential thermal analysis (TG-DTA) measurements. Moreover, these salts also exhibited high positive enthalpies of formation, high nitrogen content, good thermal stabilities, and moderate detonation properties.

The 1,3‐Diamino‐1,2,3‐triazolium Cation: A Highly Energetic Moiety

AbstractThe 1,3‐diamino‐1,2,3‐triazolium cation has been prepared by the amination of 1‐amino‐1,2,3‐triazole with O‐tosylhydroxylamine. The cation shows a very positive enthalpy of formation and is therefore highly suitable as the fuel component in new energetic materials. The cation was then paired with the energetic anions nitrate, nitrotetrazolate‐2‐oxide, 5,5′‐bis(1‐oxidotetrazolate), 5,5′‐azobis(1‐oxidotetrazolate), and nitriminotetrazolate and characterized. All the prepared compounds are high‐performance energetic materials, and when performances were compared with the corresponding triaminoguanidinium salt, those of 1,3‐diamino‐1,2,3‐triazolium were generally superior, as calculated by the EXPLO5 computer code. The heats of formation were calculated at the CBS‐4M level of theory. The impact, friction, and electrical spark sensitivities of the 1,3‐diamino‐1,2,3‐triazolium salts were measured and found to be easily initiated energetic materials.

Synthesis, Structure and Property of 5‐Aminotetrazolate Room‐Temperature Ionic Liquids

AbstractEight 5‐aminotetrazolate (AT) salts based on the 1,2,3‐trimethylimidazolium (1), 1,3‐dimethylimidazolium (2), 1‐ethyl‐3‐methylimidazolium (3), 1‐butyl‐3‐methylimidazolium (4), 1‐isobutyl‐3‐methylimidazolium (5), 1‐(3′‐methylbutyl)‐3‐methylimidazolium (6), 1‐hexyl‐3‐methylimidazolium (7) and 1‐methyl‐3‐octylimidazolium (8) cations have been synthesized in high yields and fully characterized by IR and NMR spectroscopy and elemental analysis. White plate crystals of 1 were isolated in methanol/diethyl ether and crystallized. Both the AT anion and the 1,2,3‐trimethylimidazolium cation in 1 were delocalized. Differential scanning calorimetry (DSC) combined with thermogravimetric analysis (TGA) tests were used to assess the thermal stabilities of the AT salts. These salts decomposed within the temperature range 230 °C–262 °C. Salts 3–8 are very fluent room‐temperature ionic liquids, whose glass transition temperatures are low. Their viscosities at 30 °C are in the range from 92 cP to 208 cP. A correlation between the viscosity and temperature was found, and the ionic liquids to do not display an Arrhenius temperature behaviour. In addition, the standard enthalpies of formation of salts 1–8 were investigated and analyzed by the theoretical methods. Salt 2 with the 1,3‐dimethylimidazolium cation gave the highest positive enthalpy of formation. For the assessment of the energetic character of the AT salts, impact sensitivities and friction sensitivities were recorded. These AT salts are insensitive to impact (>40 J) and friction (>360 N) confirmed by UN standards. The reactions of these AT salts with 100 % HNO3 were also determined.

Synthesis and characterization of 3,4-dihydroxyphenylacetic acid imprinted polymers

Eight molecularly imprinted polymers (MIP1–MIP8) were synthesized with different functional monomers and porogens using 3,4-dihydroxyphenylacetic acid (DOPAC) as a template. Thermal, radical bulk polymerization was employed in the presence of ethylene glycol dimethacrylate as a cross-linker. A computational analysis indicated that complexes with four molecules of 4-vinylpyridine, 1-vinylimidazole and acrylonitrile had high positive enthalpies of formation. The polymers synthesized with these monomers showed an imprinting factor below 1. Polymer MIP8 synthesized with allylamine as the functional monomer, with the highest energy of interaction with DOPAC, was characterized by the highest imprinting factor equal to 1.91. Examination of the binding ability of DOPAC and a group of structurally related compounds showed that the strong interactions between amine groups in the polymer and carboxylic groups in the analyte governed the recognition mechanism. The Langmuir adsorption model and the pseudo-second-order mechanism properly evaluated the MIP8 and non-imprinted polymer 8 adsorption characteristics. Scatchard analysis revealed that MIP8 had two classes of heterogeneous binding sites with Kd(1) = 0.12 µmol L−1 and Kd(2) = 1.46 µmol L−1. Finally, the potential application of MIP8 for separation of DOPAC was demonstrated. Copyright © 2011 Society of Chemical Industry

Size‐, Shape‐ and Composition‐Dependent Alloying Ability of Bimetallic Nanoparticles

Based on the surface-area-difference model, the formation enthalpies and the formation Gibbs free energies of bimetallic nanoparticles are calculated by considering size and shape effects. Composition-critical size diagrams were graphed for bulk immiscible bimetallic nanoparticles with the developed model. The results reveal that both the formation enthalpy and formation Gibbs free energy decrease with the decrease of particle size. The effect of rising temperature is similar to the diminishing of particle size on reducing the formation Gibbs free energy. Contrary to the positive formation enthalpy of the bulk immiscible system, a negative formation enthalpy is obtained when the particles are smaller than a critical size. With the decrease of size, the alloying process first takes place in the dilute solute regions, then broadens to the dense solute regions and finally, particles with all compositions can be alloyed. The composition-critical size diagram is classified into three regions by the critical size curves with shape factors of 1 and 1.49, that is, the non-alloying region, alloying region and possible alloying region. The model predictions correspond well with experimental evidences and computer simulation results for Cu-Ag, Au-Ni, Ag-Pt and Au-Pt systems.

Structure prediction and targeted synthesis: A new NanN2 diazenide crystalline structure

Significant progress in theoretical and computational techniques for predicting stable crystal structures has recently begun to stimulate targeted synthesis of such predicted structures. Using a global space-group optimization (GSGO) approach that locates ground-state structures and stable stoichiometries from first-principles energy functionals by objectively starting from randomly selected lattice vectors and random atomic positions, we predict the first alkali diazenide compound Na(n)N(2), manifesting homopolar N-N bonds. The previously predicted Na(3)N structure manifests only heteropolar Na-N bonds and has positive formation enthalpy. It was calculated based on local Hartree-Fock relaxation of a fixed-structure type (Li(3)P-type) found by searching an electrostatic point-ion model. Synthesis attempts of this positive ΔH compound using activated nitrogen yielded another structure (anti-ReO(3)-type). The currently predicted (negative formation enthalpy) diazenide Na(2)N(2) completes the series of previously known BaN(2) and SrN(2) diazenides where the metal sublattice transfers charge into the empty N(2) Π(g) orbital. This points to a new class of alkali nitrides with fundamentally different bonding, i.e., homopolar rather than heteropolar bonds and, at the same time, illustrates some of the crucial subtleties and pitfalls involved in structure predictions versus planned synthesis. Attempts at synthesis of the stable Na(2)N(2) predicted here will be interesting.

First-principles computational study of defect clustering in solid solutions ofThO2with trivalent oxides

The energetics of mixing and defect ordering in solid solutions of fluorite-structured ThO$_{2}$ with oxides of trivalent cations (Sc, In, Y, Nd, La) are investigated by electronic density-functional-theory (DFT). Through DFT calculations of structures enumerated by lattice-algebra techniques, we identify the lowest-energy patterns for defect clustering for four separate dopant concentrations. The most stable structures are characterized by a repulsive interaction between nearest-neighbor vacancies on the oxygen sublattice. The enthalpies of formation with respect to constituent oxides are positive for all dopants considered, and show a tendency to decrease in magnitude as the size and electronegativity of the trivalent dopant decrease. Due to the small positive formation enthalpies and low oxygen-vacancy binding energy with La dopants, La$_{2}$O$_{3}$-ThO$_{2}$ solid solutions are predicted to have relatively high ionic conductivities relative to those for the other aliovalent dopants considered. Our results are compared with those for the more widely studied ZrO$_{2}$ and CeO$_{2}$ fluorite-structured solid solutions with trivalent cations.

Long-range order instead of phase separation in large lattice-mismatch isovalentAX−BXsystems

Large atomic size mismatch between compounds discourages their binding into a common lattice because of the ensuing cost in strain energy. This central paradigm in the theory of isovalent alloys long used to disqualify alloys with highly mismatched components from technological use is clearly broken by the occurrence of stable spontaneous long-range order in mixtures of alkali halides with as much as 40% size mismatch (e.g., LiF-CsF). Our theoretical analysis of these failures uncovered a different design principle for stable alloys: very large atomic size mismatch can lead to spontaneous ordering if the large (small) components have the ability to raise (lower) their coordination number (CN) within the mixed phase. This heuristic design principle has led us to explore via first-principles structure search a few very largely mismatched binary systems whose components have a propensity for CN disproportionation. We find ordered structures for BeO-BaO (37% size mismatch) and BeO-SrO (30%), and ordering in LiCl-KCl (20%), whereas BN-InN (33%) is found to lower its positive formation enthalpy by ∼60% when CN disproportionation is allowed. This new design principle could be used to explore phases unsuspected to order by the common paradigm of strain instability. © 2009 The American Physical Society.

First-Principles Calculations of Phase Transition and Stability of Si2CN4 under High Pressure

A pressure-induced phase transition and stability in Si2CN4 polymorphs under high pressure are studied by first-principles calculations. The result shows that the phase transition pressure of α- and β-Si2CN4 to the cubic spinal phase is 29.9 GPa and 27.5 GPa predicted by thermodynamic method respectively. Under ambient condition, all of the three Si2CN4 polymorphs are metastable with positive formation enthalpy. Unlike the stability of Si3N4 polymorphs, α-Si2CN4 is more stable than the β phase.

Ab initiostudy of Pb antisite defects inPbZrO3andPb(Zr,Ti)O3

We report an ab initio study of Pb antisite defects in ${\text{PbZrO}}_{3}$ (PZ) and $\text{Pb}(\text{Zr},\text{Ti}){\text{O}}_{3}$ (PZT) perovskites. Also, we calculated the enthalpy of formation of PZ. Our results show that, under strong oxidizing conditions, Pb on the Zr-site antisite defects are unavoidable in PZ. Moreover, a positive enthalpy of formation (0.15 eV) of PZ is found. This indicates that PZ is metastable for low temperature and may help explain the difficulty in synthesizing high-quality Zr-rich PZT crystals. The Pb antisite defects in PZT alloys have low formation energies. This result is in agreement with experiments, which report the predominance of this defect in PZT films. We find that the Pb antisite defect produces electron traps 0.2--0.8 eV below the conduction-band edge.

Observation of largely enhanced hardness in nanomultilayers of the Ag–Nb system with positive enthalpy of formation

Ag ∕ Nb nanomultilayers with different modulation wavelengths Λ were prepared on silicon wafers by electron beam evaporation. Nanoindenter measurements show that with decreasing Λ of the multilayers, the nanohardness increases up to ∼80% for Λ=4nm, whereas the modulus is almost unchanged. This unusual behavior originates from a unique microstructure where amorphous Ag–Nb alloys form at the interfaces and grain boundaries of silver nanoparticles, as observed by cross-section high resolution transmission electron microscopy. The amorphous phases favor hardness enhancement by preventing dislocation emission and movement, whereas they have a negative contribution to the modulus because of their free volume.

Spinel-type gallium oxynitrides attainable at high pressure and high temperature

We study the enthalpy of formation $\ensuremath{\Delta}H$ of spinel-type gallium oxynitrides ($\ensuremath{\gamma}$ galons) as a function of pressure. Formation of the ternary oxynitride from gallia $({\mathrm{Ga}}_{2}{\mathrm{O}}_{3})$ and gallium nitride (GaN) is endothermic at any pressure. $\ensuremath{\Delta}H$ is minimal, however, at the $\ensuremath{\beta}\ensuremath{\rightarrow}\ensuremath{\alpha}$ phase transformation of gallia, which we calculate to occur at 2.6 or $6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, depending on whether we treat the $d$ elctrons of Ga as core or as valence states. Hence, at this pressure the driving force to form $\ensuremath{\gamma}$-galons is maximal. The positive enthalpy of formation may be compensated by entropy contributions arising from a mixing of O and N within the anion sublattice and of Ga atoms and vacancies on octahedral sites of the cation sublattice. Since with increasing pressure the decomposition of gallium nitride is suppressed, we propose the synthesis of ternary spinel-type gallium oxynitrides at high temperature and/or high pressure conditions.

Enthalpy of Formation of Yttria-Doped Ceria

Solid solutions (1 − x)CeO2 − xYO1.5 (0 ≤ x ≤ 0.36) were prepared by coprecipitation and sol-gel methods. Their enthalpy of formation relative to the end-members, fluorite-type cubic CeO2 and C-type YO1.5 was determined by oxide melt solution calorimetry. The enthalpy of drop solution shows a roughly linear trend with composition. Extrapolation to x = 1 gives the transition enthalpy of C-type to cubic fluorite YO1.5 as 22.2 ± 6.7 kJ/mol. This linear behavior is in contrast to the strong curvature seen in the ZrO2 − YO1.5 and HfO2 − YO1.5 systems. The slightly positive enthalpy of formation of CeO2 − YO1.5 is strikingly different from the strongly negative enthalpies of formation of ZrO2 − YO1.5 and HfO2 − YO1.5. The thermodynamics of CeO2 − YO1.5 is analyzed in terms of defect association and oxygen vacancy distribution. Specifically, the association of oxygen vacancies with the tetravalent cations in the zirconia and hafnia systems, in contrast to the preference of vacancies for nearest neighbor yttrium sites in the ceria systems, may explain the different energetics.

Atomistic Modeling of Metastable Phase Selection of a Highly Immiscible Ag–W System

Ab initio calculation is performed for predicting the cohesive energies and lattice constants of some possible metastable compounds in the Ag-W system with the largest positive formation enthalpy among the binary transition metal alloys. The calculated results together with the experimental data are then used in deriving an n-body Ag-W potential under the framework of the embedded atom method. Based on the proven realistic Ag-W potential, molecular dynamics simulations are performed to reveal the metastable phase selection over the entire composition of the system and the results predict that the metastable Ag 100-x W x alloy in an fcc structure is more stable than in the bcc structure when 0 ≤ x < 57, whereas the bcc structure becomes energetically favored when 57 < x≤100. Interestingly, the prediction is in good agreement with the experimental results.

Ab initioanalysis of energetics of σ-phase formation in Cr-based systems

A theoretical approach to the explanation of experimentally determined positive enthalpy of formation in Cr-based \ensuremath{\sigma} phases is presented. It turns out that the energy of formation of \ensuremath{\sigma} phases in Cr-Co and Cr-Fe systems is found to be negative taking the pure components in the \ensuremath{\sigma}-phase structure as the standard states. Our treatment represents a first step to a model of an explanation of the stability of the \ensuremath{\sigma} phase.

Donor−Acceptor Complexes of Alkylcarbazole and Dicarbazolylalkane Donors with the Acceptors Tetracyanoethylene and Tetranitromethane

Electron donor−acceptor complexes of conformationally flexible 1,n-dicarbazolylalkanes (C12H8N−(CH2)n−NC12H8), where n = 1−5, were examined. Carbazole, methylcarbazole, ethylcarbazole, and cyanoethylcarbazole also were studied as monochromophoric analogues for comparison. In dichloromethane solution, the dicarbazolylalkanes form 1:1 complexes with the terminal carbazolyl chromophores acting as independent donors when n ≥ 2. With the acceptor tetranitromethane (TNM), the carbazoles form contact complexes displaying small positive enthalpies of formation. In contrast, stable complexes form with the acceptor tetracyanoethylene (TCNE). Crystalline TCNE complexes were isolated for the bichromophoric donors with n = 2−4. The solid complexes and their uncomplexed donor components were analyzed by single-crystal X-ray diffraction. The solid-state stoichiometries of (carbazolyl donor):(TCNE acceptor) were found to depend on the donor conformation. Dicarbazolylalkane donors separated by two or four methylene units ...

The influence of stress during ion beam mixing

Abstract It is well established that thermodynamic forces influence ion mixing. Systems with a negative enthalpy of formation experience enhanced ion mixing while systems with a positive enthalpy of formation experience no mixing or even “de-mixing”. Recent experimental results have suggested that intrinsic stresses should be considered when evaluating the thermodynamic driving forces that influence ion mixing. We have studied the effects of thermal annealing and Ar ion irradiation on the structure and stability of multilayered DC sputtered thin films of TiN/B–C–N. These samples are in a state of bi-axial compression in their as-deposited form. Following thermal annealing, to temperatures as high as 1000°C for 1 h, it is observed that the intrinsic stresses are rapidly relieved and no interdiffusion or break down of the layered structure takes place. However, when samples are ion irradiated, we do observe intermixing and at the highest dose (5×1016 ions cm−2) the multilayered structure partly disappears. The rate of intermixing is well correlated with the magnitude of compressive stress as a function of irradiation dose. We believe that the observed ion mixing, in this otherwise immiscible system, is being controlled by stress derived thermodynamic forces.