Positive Enthalpy Of Formation Research Articles

Liquid-flame combustion II: Some physical and chemical characteristics of the burning process

To gain further insight into the nature of the liquid-flame structure (LFS), which is characteristic of compressed solid mixtures, containing tetrazole (64 wt%) and sodium tetrazolate monohydrate, a number of physical and chemical characteristics of the liquid flame was studied. The temperature distribution on the surface of the LFS and the temperature profile in a combustion wave were investigated using optical pyrometry, as well as thermocouples and electrochemical methods. The LFS is found to be characterized by highly uniform heat radiation; the temperature on the LFS surface varied from 940 to 1000 K. The small LFSs (up to 5–6 mm in diameter) are capable of voluminous radiation, while the LFSs of larger sizes irradiate as hollow envelopes, with a wall thickness of ∼ 1–2 mm. The temperature and electrochemical measurements in the combustion wave indicate that the LFS represents a spheroidal formation, having to a considerable extent a hollow envelope of melt characterized by a relatively even temperature distribution inside the LFS, the temperature being 1050–1250 K, depending on the LFS size. The chemical compositions of the gaseous and condensed products of the thermal decomposition and combustion of pure tetrazole, sodium tetrazolate, and a mixture of them were studied by 1H NMR, IR, mass spectrometry, gas chromatography, elemental, and x-ray analyses. The data obtained allow one to identify the chemical reactions in the liquid flame. The thermal decomposition of sodium tetrazolate results in the formation of chemically stable sodium acid cyanamide; this is the basic reaction proceeding in the liquid flame. The fragmentation of the tetrazole leads to cyanamide polymer; partial decomposition of sodium tetrazolate proceeds in the melt layer below the liquid flame. Both reactions are accompanied by the evolution of considerable heat and gaseous nitrogen. An analysis of all the measurements suggests that the necessary conditions for an LFS and its development on combustion are a large amount of nitrogen in the initial substances, which must have considerable thermal stability, along with high positive enthalpies of formation, together with the presence of definite chemical fragments and ions of alkali metals in molecules of at least one of the components. In accordance with these criteria, mixtures of tetrazolates and azides of alkali metals with tetrazole, 1,2,3-triazole, 1,2,4-triazole, guanidine, and some of their derivatives are shown to be capable of forming an LFS on combustion.

Formation of amorphous alloys through the multilayer method in an immiscible Y - Nb system

Amorphization was achieved by room-temperature 190 keV xenon ion mixing of multilayered films in the Y - Nb system, which has a positive enthalpy of formation exceeding . To give further insight into the singularity of the amorphization behaviour of the system, a Gibbs free-energy diagram of the system was constructed by calculating the free energies of the amorphous phase and those of the as-deposited Y - Nb multilayers, in which the interfacial free energy was estimated and added. The established diagram can explain the irradiation-induced amorphization behaviour in the Y - Nb system well.

Calculation of the physical and electronic properties of rubidium polybenzide Rb2C24.

We have calculated the physical and electronic properties of rubidium polybenzide ${\mathrm{Rb}}_{2}$${\mathrm{C}}_{24}$. It is a metal whose five lowest conduction bands have their minima at the (1/21/21/2) corner of the cubic Brillouin zone. Its bulk modulus is 5.3% greater than that of polybenzene while its positive formation enthalpy (with respect to Rb intercalated graphite) is appreciably smaller than that of polybenzene (with respect to graphite).

Formation of strained superlattices with a macroscopic period via spinodal decomposition of III–V semiconductor alloys

The theory of (in) stability of III–V A 1− x B x C 1− y D y -type quaternary alloys with respect to the coherent phase separation is developed on the basis of the regular solution approximation. It is shown that, in spite of the stabilizing factor of coherency elastic strains, the III–V quaternary alloys which have positive formation enthalpy become at some temperature unstable with respect to coherent phase separation. The equilibrium structure resulting after the phase separation is a one-dimensional modulated two-phase structure with alternating alloy composition ( a system of compositional domains, or, in the other words, a superlattice with a macroscopic periodicity ). Alloy compositions of both phases are determined by the conditions of the minimal homogeneous bulk free energy; the superlattice periodicity is governed by the interplay of the interface free energy and the inhomogeneous part of the elastic free energy. The coherent phase separation diagram and the periodicity of spontaneously formed superlattices are calculated for Ga 1− x In x As 1− y P y .

Endothermic Crystallization of Lysozyme to the Orthorhombic Crystal from the Aqueous Solution at 35°C

Enthalpies of formation of lysozyme crystals were measured by a commercial twin microcalorimeter. The enthalpy of formation of the tetragonal and the orthorhombic crystals were -105±21 kj mol -1 and 54±13 kj mol -1 , respectively. The positive enthalpy of formation of the orthorhombic crystal was attributed to the hydrophobic interaction among lysozyme molecules in the crystal.