Positive Enthalpy Of Formation Research Articles

Enthalpies of formation of RCoO3-δ (R = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) perovskite cobaltites

Standard enthalpies of formation at 298.15 K, ΔfH298.15∘, for RCoO3-δ (R = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) perovskite-type rare-earth cobaltites were obtained from the combined results of drop and reduction calorimetry. The relationship between the oxygen nonstoichiometry, δ, and thermodynamics of RCoO3-δ was discussed. The fact that RCoO3-δ with smaller ionic radius of R3+ tend to be less stable, having more positive enthalpy of formation from binary oxides (CoO and R2O3), Δf,oxH298.15∘, was confirmed for all the studied RCoO3-δ. Drop calorimetry results were also used in discussing, comparing and assessing the literature heat capacity, CpT, data for RCoO3-δ (R ≠ Y). For YCoO3-δ, both ΔfH298.15∘ and CpT were obtained for the first time solely from the drop calorimetric results; ΔfH298.15∘ values determined by two different methods perfectly agree with each other. The reference data and CpT of RCoO3-δ were used to calculate ΔfH298.15∘ from the published EMF measurement results so as to compare the available Δf,oxH298.15∘(RCoO3-δ) values. An unusual dependence of ΔfH298.15∘(LaCoO3-δ) on its heat treatment history was identified and supported by the additional isoperibolic solution calorimetry measurements.

Challenges in the Synthesis and Processing of Hydrosilanes as Precursors for Silicon Deposition.

Hydrosilanes are highly attractive compounds, which can be processed as liquids with printing technology to amorphous silicon films on nearly any solid substrate. The silicon layers can be processed for electronic devices like transistors or thin-film solar cells. The endothermic character of hydrosilanes with their positive enthalpies of formation results in favorable properties for processing. The larger the molecules, the lower their decomposition temperature and the higher their photoactivity. Cyclic hydrosilanes such as cyclopentasilane and cyclohexasilane can be easily deposited. The branched neopentasilane is more difficult to deposit but yields better-quality films after processing. The key challenge is the complex synthesis of the precursors and the hydrosilanes. The available preparative methods are presented in this review and their advantages and disadvantages are evaluated. The following synthesis methods are presented and discussed in this article: Wurtz coupling and other reductive coupling processes, dehydrogenative coupling of silanes, plasma synthesis of chlorinated polysilanes, amine- or chloride-induced disproportionations, and transformation of monosilane to higher silanes. Plasma synthesis is already carried out today as a continuous industrial process. The most effective synthesis methods in the laboratory are currently amine- and chloride-induced disproportionations. There is a great need to further optimize the syntheses of hydrosilanes and to develop new simple synthesis variants.

Design and Synthesis of High-Performance Planar Explosives and Solid Propellants with Tetrazole Moieties.

A straightforward synthetic strategy for newly designed nitrogen-rich planar explosives and solid propellants is reported. These materials exhibit high densities (1.69-1.95 g cm-3), high positive enthalpies of formation (approaching 1149.21 kJ mol-1), promising energetic properties (P = 26.36-33.78 GPa, D = 8258-9518 m s-1), acceptable thermal stabilities (Td = 132-277 °C), good sensitivities (IS = 4-40 J, FS = 60-360 N) and excellent propulsive performance (Isp = 176.80-253.06 s).

Polymorphism of group-IV carbides: Structures, (meta)stability, electronic, and transport properties

Herein we investigate the spectra of crystal structures and their metastability for elemental carbon and the entire set of group-IV monocarbides (SiC, GeC, and SnC) using the ab initio random structure sampling. All of the known structures of elemental C and SiC are correctly identified as high probability in the random sampling and the predicted metastabilities of polymorphs are consistent with the existing knowledge. Furthermore, we find the same four structure types, tetrahedrally bonded zincblende, wurtzite, and rhombohedral-$R\overline{3}m$, as well as octahedrally coordinated rocksalt at high pressure, as the only relevant polymorphs for all studied carbides. However, the rocksalt structure in GeC is shown to be dynamically unstable, contrary to SiC and SnC. Additionally, all GeC and SnC structures are found to have positive enthalpies of formation and are therefore unstable relative to decomposition. The tetrahedrally bonded polymorphs, especially the SnC ones, are predicted to have very high intrinsic (phonon-limited) electron mobilities comparable with the best known semiconductors. Additionally, wurtzite SnC is lattice matched to InN suggesting epitaxial growth as a possible avenue to realize wurtzite SnC.

Design and Synthesis of Nitro Cage Heterocycles as Energetic Materials Derived from Pentacycloundecane (PCUD) Systems

AbstractA variety of pentacycloundecane (PCUD) based cage compounds containing nitro groups were synthesized via a simple synthetic method starting with cage diones. Eight PCUDs were used for nitro functionalization. These cage diones were prepared starting with easily accessible starting materials such as 2,3‐dimethylhydroquinone, 1,4‐dihydroxybenzene, and cyclopentadiene. Here, we have used Diels‐Alder (DA) reaction, [2+2] photocycloaddition, and Henry reaction as key steps. Transannular cyclization plays an important role to generate a nitro substituted cage heterocycles via base promoted reaction using nitromethane as a reagent. Some of these oxa‐cage structures have been further supported by single‐crystal X‐ray diffraction studies. Energetic performance of the title compounds was evaluated using DFT based calculations. These bis(nitromethyl)octahydro‐1H‐1,3,4,6(epiethane‐[1,1,2,2]tetrayl)‐pentaleno[1,6‐bc]furan analogues (11 b–18 b) owned the densities in the range of 1.37–1.60 g/cm3. All the studied compounds possess high positive enthalpy of formation (33.68–230.48 kJ/mol, except compound with dimethyl groups (16 b, −11.38 kJ/mol) and very good thermal (Tm=99–147 °C) and kinetic stabilities. Ammonium perchlorate based various propulsion formulations suggests that these compounds could be excellent candidates in the solid rocket propulsion.

Energetic performance of trinitromethyl nitrotriazole (TNMNT) and its energetic salts.

Little is known about trinitromethyl nitrotriazole (TNMNT) since the crystal structure, density, energetic performance, and thermal properties have not been determined. A detailed characterization of TNMNT and its hydrazinium and potassium salts and their potential as solid propellants and oxidizers has been established. TNMNT exhibits a high density (1.96 g cm-3) and positive enthalpy of formation (ΔHf = +84.79 kJ mol-1). TNMNT and its hydrazinium and potassium salts illustrate excellent detonation properties (P = 34.24 to 36.22 GPa, D = 8899 to 9031 ms-1). TNMNT and its hydrazinium salt exhibit outstanding propulsive properties (Isp = 247.28 to 271.19 s), and these are superior to AP (Isp = 156.63 s) and ADN (Isp = 202.14 s). The results suggest opening the door to utilizing TNMNT and its energetic salts in solid rocket propulsion.

Designing a thermodynamically stable and intrinsically ductile refractory alloy

Developing ductile refractory alloys have remained a challenge. Decreasing the valence electron concentration of refractory alloys has been widely suggested for improving their ductility. However, Re has been used to ductilize W, which goes against the low valency suggestion. The thermodynamic stability of refractory alloys has never been considered while suggesting alloying elements to improve ductility. Here we use first-principles density functional theory simulations to unravel the role of enthalpy of formation in improving the intrinsic ductility of refractory alloys. The intrinsic ductility is assessed using the D-parameter, which is the ratio of surface energy and unstable stacking fault energy. We studied 25 equiatomic binary refractory alloys and found that positive enthalpy of formation improves ductility. The small positive enthalpy of formation could be compensated by sufficiently large entropy; hence the alloy is expected to be a single phase. Our present work explains the role of high-valency and low-valency alloying elements in improving the ductility of refractory alloys. These findings provide a path to design thermodynamically stable and intrinsically ductile high-temperature alloys.

Novel family of nitrogen-rich energetic (1,2,4-triazolyl) furoxan salts with balanced performance.

Nitrogen-rich energetic materials comprised of a combination of several heterocyclic subunits retain their leading position in the field of materials science. In this regard, a preparation of novel high-energy materials with balanced set of physicochemical properties is highly desired. Herein, we report the synthesis of a new series of energetic salts incorporating a (1,2,4-triazolyl) furoxan core and complete evaluation of their energetic properties. All target energetic materials were well characterized with IR and multinuclear NMR spectroscopy and elemental analysis, while compound 6 was further characterized by single-crystal X-ray diffraction study. Prepared nitrogen-rich salts have high thermal stability (up to 232°C), good experimental densities (up to 1.80 g cm−3) and high positive enthalpies of formation (344–1,095 kJ mol−1). As a result, synthesized energetic salts have good detonation performance (D = 7.0–8.4 km s−1; p = 22–32 GPa), while their sensitivities to impact and friction are quite low.

Improving Shear Strength of Ti/Mg Bimetal Composites Prepared by Hot‐Dip Aluminizing and Solid–Liquid Compound Casting

It is a great challenge to achieve the joining between Ti and Mg for their low solid solubility and positive formation enthalpy value. To fabricate high strength TC4/AZ91D bimetallic composite with metallurgical bonding interface, Al coating is prepared on the surface of TC4 alloy by hot‐dip technology, and Al‐coated TC4/AZ91D bimetal is prepared by solid–liquid compound casting herein. The effect of the hot‐dip process on the thickness and composition of the Al coating on the TC4 sample is discussed. Process parameters including casting temperature and liquid‐to‐solid volume ratio are investigated. The results reveal that the shear strength of Al‐coated TC4/AZ91D bimetal reaches 48.5 MPa when hot dipping at 700 °C for 15 min and casting at 720 °C with the volume ratio of 24, while that of the sample without Al interlayer is only 19.1 MPa. The reason is due to the formation of Ti(Al, Mg)3 intermetallic compound and Al12Mg17 + δ‐Mg eutectic structure at the interface. The results are helpful for the further development of high strength Ti/Mg bimetal by combining hot‐dip technology and solid–liquid composite casting technology.

Energetics and structure of the B‐type solid solution in the Nd2O3–Y2O3 system

AbstractThe system Nd2O3–Y2O3 contains solid‐solution phases with several different structures. Single‐phase B‐type (Nd1−xYx)2O3 solid solutions in the range of were formed at 1873 K and retained on cooling to ambient temperature. They showed a linear composition dependence of lattice parameters, enabling extrapolation to x = 0 and 1 for comparison with the structures of the stable endmembers. A positive enthalpy of formation from A‐type Nd2O3 and C‐type Y2O3 determined using oxide melt solution calorimetry indicated entropic stabilization of the B‐type phase. A positive interaction parameter for mixing in the B‐type solid solution, Ω 47.46 ± 4.04 kJ/mol, was obtained by fitting the data using a regular solution model. This value is significantly more positive than that obtained by previous phase diagram analysis using the CalPhaD approach. Exsolution into two B‐type phases is predicted to occur below 1427 K, making it unimportant for the equilibrium phase relations that will be dominated by other structures at low temperature.

Structural characters and band offset of Ga2O3–Sc2O3 alloys

Alloy engineering is a promising approach to optimize the electronic properties and application of the ultrawide bandgap semiconductor Ga2O3. Here, the structural and electronic properties of (ScxGa1−x)2O3 alloys are studied using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional. Hexagonal (ScxGa1−x)2O3 alloys show more negative formation enthalpies than (AlxGa1−x)2O3 alloys, and the increments in the positive formation enthalpies for monoclinic (ScxGa1−x)2O3 alloys are different from the (AlxGa1−x)2O3 alloys. (ScxGa1−x)2O3 alloys will undergo the compressive strain if grown on the Ga2O3 substrate. The bandgaps range from 4.78 to 5.44 eV for monoclinic (ScxGa1−x)2O3 and from 5.17 to 6.10 eV for hexagonal (ScxGa1−x)2O3. It is noted that Ga2O3/(ScxGa1−x)2O3 heterojunctions keep the type-II band alignments and whose conduction and valence band offsets can be significantly and negligibly enlarged by increasing Sc concentration, respectively. The large conduction band offsets for Ga2O3/(ScxGa1−x)2O3 heterojunctions allow (ScxGa1−x)2O3 alloys to be an electron blocking layer for the Ga2O3 device, and ease the problems of parasitic conduction in the field effect transistor.

New Class of Titanium Niobium Oxide for a Li-Ion Host: TiNbO4 with Purely Single-Phase Lithium Intercalation

Entropy-stabilized titanium niobium oxides (TNOs) with crystallographic shear structures (e.g., TiNb2O7 and Ti2Nb10O29) are generally synthesized by high-temperature calcination in an air or an oxygen atmosphere to compensate for their positive enthalpies of formation. In this work, we demonstrate that changing the reaction atmosphere into a slightly reductive environment using in situ carbonization leads to the creation of a new class of TNO with a formula of TiNbO4. Unlike its predecessors, this new lithium reservoir is a rutile phase, and most strikingly, in situ X-ray diffraction analysis revealed that its lithium intercalation occurs via a purely solid-solution process. Since solid-electrolyte-interface-free, high capacity anode materials with long cyclic life are required to meet the stringent requirements of widespread lithium-ion battery utilization, this finding of a new electrode material with purely single-phase lithium intercalation is of great interest for the development of high-performance anode materials. Distinctive electrochemical behavior that is different from that of crystallographic shear structured TNO is revealed by in-depth electrochemical analyses, which is ascribed to the unique structural and electronic properties of TiNbO4. We believe this work opens a new avenue for the development of feasible oxide-based alternatives to graphite, which can be safer and suitable for high-power performance.

Synthesis and thermodynamics of uranium-incorporated α-Fe2O3 nanoparticles

• Uranium-incorporated hematite is thermodynamically metastable. • Increasing uranium incorporation within hematite increases its formation enthalpy. • Hydrothermal treatment can increase uranium concentrations in hematite. • U-Fe 2 O 3 coordination in radioactive waste environments may be process-dependent. Hematite nanoparticles were synthesized with U(VI) in circumneutral water through a coprecipitation and hydrothermal treatment process. XRD, TEM, and EXAFS analyses reveal that uranium may aggregate along grain boundaries and occupy Fe sites within hematite. The described synthesis method produces crystalline, single-phase iron oxide nanoparticles absent of surface-bound uranyl complexes. EXAFS data were comparable to spectra from existing studies whose syntheses were more representative of naturally occurring, extended aging processes. This work provides and validates an accelerated method of synthesizing uranium-immobilized iron oxide nanoparticles for further mechanistic studies. High temperature oxide melt solution calorimetry measurements were performed to calculate the thermodynamic stability of uranium-incorporated iron oxide nanoparticles. Increasing uranium content within hematite resulted in more positive formation enthalpies. Standard formation enthalpies of U x Fe 2–2x O 3 were as high as 76.88 ± 2.83 kJ/mol relative to their binary oxides, or -764.04 ± 3.74 kJ/mol relative to their constituent elements, at x = 0.037. Data on the thermodynamic stability of uranium retention pathways may assist in predicting waste uranyl remobilization, as well as in developing more effective methods to retain uranium captured from aqueous environments.

Crystal phase induced direct band-gap modifications in bulk GaP and GaAsP

Wurtzite III-phosphide GaP and GaAsP compounds have shown tunable direct band-gap making them promising candidates in a variety of future optoelectronic and photonic devices. We explore here, through DFT their corresponding electronic structure properties of bulk cubic and wurtzite polytypes. It is shown that the calculated weak positive formation enthalpy of GaAsP crystal-phases suggest slight thermodynamical instability. In addition, from accurate quasiparticle (local density approximation-1/2) we predict a direct band-gap for WZ 2H, 4H, and 6H polytypes of GaP and GaAsP. The emitted wavelength can be then tuned across an important range of the visible light yellow-green spectrum for GaP (2.13eV(2H)-2.33eV(4H)) and the visible light red spectrum for GaAsP (1.64eV(6H) to 1.82eV(4H)).

Vibrational Analysis of Benziodoxoles and Benziodazolotetrazoles

Tetrazoles are well known for their high positive enthalpy of formation which makes them attractive as propellants, explosives, and energetic materials. As a step towards a deeper understanding of the stability of benziodazolotetrazole (BIAT)-based materials compared to their benziodoxole (BIO) counterparts, we investigated in this work electronic structure features and bonding properties of two monovalent iodine precursors: 2-iodobenzoic acid and 5-(2-iodophenyl)tetrazole and eight hypervalent iodine (III) compounds: I-hydroxybenzidoxolone, I-methoxybenziodoxolone, I-ethoxybenziodoxolone, I-iso-propoxybenziodoxolone and the corresponding I-hydroxyben ziodazolotetrazole, I-methoxybenziodazolotetrazole, I-ethoxybenziodazolotetrazole and I-iso- propoxybenziodazolotetrazole. As an efficient tool for the interpretation of the experimental IR spectra and for the quantitative assessment of the I−C, I−N, and I−O bond strengths in these compounds reflecting substituent effects, we used the local vibrational mode analysis, originally introduced by Konkoli and Cremer, complemented by electron density and natural bond orbital analyses. Based on the hypothesis that stronger bonds correlate with increased stability, we predict that, for both series, i.e., substituted benziodoxoles and benziodazolotetrazoles, the stability increases as follows: I-iso-propoxy < I-ethoxy < I-methoxy < I-hydroxy. In particular, the I−N bonds in the benziodazolotetrazoles could be identified as the so-called trigger bonds being responsible for the initiation of explosive decomposition in benziodazolotetrazoles. The new insight gained by this work will allow for the design of new benziodazolotetrazole materials with controlled performance or stability based on the modulation of the iodine bonds with its three ligands. The local mode analysis can serve as an effective tool to monitor the bond strengths, in particular to identify potential trigger bonds. We hope that this article will foster future collaboration between the experimental and computational community being engaged in vibrational spectroscopy.

Novel energetic CNO oxidizer: Pernitro-substituted pyrazolyl-furazan framework

There is a need for dense energetic oxidizers whose composition is restricted to carbon, hydrogen, nitrogen and oxygen atoms as chlorine-free alternative to current oxidizers of energetic materials such as explosives, propellants and pyrotechnics. High nitrogen heterocyclic frameworks containing trinitromethyl units are an attractive and increasingly important class of oxygen-rich compounds. Herein, for the first time, a synthetic method has been developed for the preparation of a new (pyrazole-3-yl)furazan framework bearing a nitro group in the furazan ring. From this framework, 3-nitro-4-(4-nitro-1-(trinitromethyl)-1H-pyrazol-3-yl)furazan (15) and 3-(3,4-dinitro-1-(trinitromethyl)-1H-pyrazol-5-yl)-4-nitrofurazan (16) have been produced. The combination of positive enthalpy of formation, high density, favorable physical and thermal properties, and reasonable sensitivity with the promising theoretical energetic performance of these oxygen-rich compounds offers materials not only of fundamental interest, but also for potential practical applications, for example, as promising candidates to new environmentally benign rocket propellants.

Effects of AgTi3 intermetallic on suppression of Ag agglomeration: a theoretical study

ABSTRACT The mechanical and electornic properties of AgTi, AgTi2 and AgTi3 intermetallic phases at zero pressure have been systemically investigated by first-principles calculations using ultrasoft pseudopotential and generalised gradient approximation. Mechanical properties calculation indicates AgTi3 is mechanically stable and shows a better ductility compared with AgTi and AgTi2. The positive formation enthalpy of AgTi3 confirmed that its formation is hard at 0 K but would be overcome with the aid of interfacial effect and high manufacture temperature. The adsorption behaviours of Ag atoms on AgTi3(111), AgTi3(011) and Ti(0001) surfaces were also investigated theoretically. The results show that Ag atoms prefer to be adsorbed on hollow sites on these surfaces. Diffusion barriers calculation reveals that AgTi3 surface can suppress Ag diffusion compared with Ti(0001) surface, significantly. The good adhesive character of Ag(111)/AgTi3(111) interface provides a powerful guarantee of long-term Ag-Agglomeration suppression.

Effect of a Competitive Solvent on Binding Enthalpy and Chain Intermixing in Hydrogen-Bonded Layer-by-Layer Films

This work is focused on the effect of a small-molecule/hydrogen-bonding competitor on the heat of complexation of hydrogen-bonding polymers in aqueous solutions, as well as on deposition of these molecules within layer-by-layer (LbL) films. Specifically, binding between poly(methacrylic acid) (PMAA) and poly(vinylpyrrolidone) (PVP) under acidic conditions in the presence of dimethyl sulfoxide (DMSO) has been explored. Isothermal titration calorimetry showed that with increasing concentration of DMSO in aqueous solution, the positive enthalpy of formation of PMAA/PVP complexes decreased and switched sign at ∼25 vol % DMSO. This change is interpreted as a transition from endothermal association between PMAA and PVP in water, driven by release of solvation water molecules, to exothermic association driven by the enthalpy of interpolymer hydrogen bonding. This switch is interpreted as destruction of polymer hydration shells by DMSO molecules. The content of DMSO also strongly impacted the growth of LbL films. With an increase in DMSO concentration from 0 to 60 vol %, polymer mass deposited within the bilayer increased, while larger DMSO concentrations suppressed film deposition. The effect of DMSO was also revealed in neutron reflectometry measurements, which showed enhanced interpenetration of deuterated PMAA chains within a hydrogenated LbL matrix at increasing concentration of DMSO. The dependence of film intermixing on DMSO content was strongly history dependent. While assembly of LbL films in aqueous solutions followed by exposure to a mixed solvent post-assembly did not significantly influence the internal film structure, the effect of DMSO on film intermixing was strongest when DMSO was present in the film deposition solutions.

Computational Design of Mixed-Valence Tin Sulfides as Solar Absorbers.

Binary tin sulfides, such as SnS and SnS2, are appealing because of their simple stoichiometry and semiconducting properties and are, therefore, being pursued as potentially cost-effective materials for optoelectronic applications. The multivalency of Sn, that is, Sn(+2) and Sn(+4) allows yet more intermediate compositions, SnxSy, whose structures and properties are of interest. Sn2S3 is already under consideration as a mixed-valence semiconductor. Other intermediate compositions, for example, Sn3S4 and Sn4S5 have remained elusive, although their existences have been alluded to in literature. Here we report a comprehensive study of phase stability of the SnxSy series compounds, utilizing swarm-intelligence crystal structure search method combined with first-principles energetic calculations. We find that the stability of mixed-valence SnxSy compounds with respect to decomposition into pure-valence SnS and SnS2 is in general weaker than the SnxOy counterparts, likely due to differences in chemical bonding. Besides identifying the experimentally discovered stable phases of Sn2S3, our calculations indicate that the Sn3S4 phase is another mixed-valence composition which shows marginal stability with respect to decomposition into SnS and SnS2. Other studied compositions may be metastable under ambient conditions, with slightly positive formation enthalpies. We find two structures of Sn3S4 having comparably low energies, both of which feature one-dimensional chain-like fragments obtained by breaking up the edge-connected octahedral layers of SnS2. Both structures indicate lattice phonon stability and one shows quasi-direct band gap with a calculated value of 1.43 eV, ideal for solar absorbers. A further analysis of the composition-structure-property relationship supports the notion that low-dimensional Sn-S motifs and van der Waals interaction may lead to diverse structure types and chemical compositions, having functional properties that are yet to be identified in the SnxSy series with mixed valency.

A novel class of oxynitrides stabilized by nitrogen dimer formation

Despite the wide applicability of oxynitrides from photocatalysis to refractory coatings, our understanding of the materials has been limited in terms of their thermodynamics. The configurational entropy via randomly mixed O/N or via cation vacancies are known to stabilize oxynitrides, despite the positive formation enthalpies. Here, using tin oxynitrides as a model system, we show by ab initio computations that oxynitrides in seemingly charge-unbalanced composition stabilize by forming pernitrides among metal-(O,N)6 octahedra. The nitrogen pernitride dimer, =(N-N)=, results in the effective charge of −4, facilitating the formation of nitrogen-rich oxynitrides. We report that the dimer forms only in structures with corner-sharing octahedra, since the N-N bond formation requires sufficient rotational degrees of freedom among the octahedra. X-ray photoemission spectra of the synthesized tin oxynitride films reveal two distinct nitrogen bonding environments, confirming the computation results. This work opens the search space for a novel kind of oxynitrides stabilized by N dimer formation, with specific structural selection rules.