Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts

Thermodynamic properties and phase equilibria in the Si-B system

The vapor composition and thermodynamic properties of Si-B alloys with boron content from 1.5 up to 100 at.% were investigated in the temperature interval of 1522 to 1880 K by Knudsen effusion mass spectrometry. Thermodynamic functions of the SiB6 and SiBn borides, primary solid solutions, and liquid solution were obtained. The thermodynamic functions of the Si-B melt were approximated by the ideal associated-solutions model under the assumption that only one complex, SiB3, existed. The established thermodynamic functions of the melt and of the SiB6 and SiBn compounds were used for computation of the phase diagram of the Si-B system. Good agreement with the available experimental data was obtained

Periodic pattern formation in metal-ceramic reactions

Formation of diffusion zone morphologies periodic in time and space during metal-ceramic reactions is considered as a manifestation of the Kirkendall effect. In a diffusion-controlled interaction, the Kirkendall marker plane can bifurcate, which is attributed to diverging vacancies fluxes in the reaction zone. When the Kirkendall plane is present in a phase layer, it attracts in situ-produced inclusions of secondary-formed phase , which in turn, can result in a highly patterned microsctucture

Concentration dependence of interdiffusion coefficients in Cu-Fe-Ni system

Conventional and effective interdiffusion coefficients have been determined in a Cu-Fe-Ni system for 1000°C based on our experimental data [13] on the distribution of concentrations of components in six diffusion couples of this system. The possibilities of applying these coefficients to interpret peculiar features of interdiffusion in ternary systems are analyzed. It has been demonstrated that effective interdiffusion coefficients are in a certain correlation with thermodynamic characteristics of the system

On diffusion in the β-NiAl phase

Interdiffusion coefficients in the ß-NiAl phase over the homogeneity range are determined by the diffusion couple technique in the temperature range of 1000–1200 °C. Intrinsic diffusivities of the species at 1000 °C at different compositions are measured by Kirkendall marker experiments. The variations of the molar volume with composition and partial molar volumes of the species, required for the determination of the diffusion parameters, are calculated using lattice parameter and vacancy concentration. Tracer diffusion coefficients of the species were calculated from the knowledge on intrinsic diffusivities and compared with the data available in the literature, which were measured directly by tracer methods. The influence of the vacancy wind effect is determined on the calculated results

Nitridation of Ni-based alloys: thermodynamics, kinetics and deformation phenomena accompanying internal precipitation

When a moderately stable phase is precipitated out during an internal reaction, the behaviour of the penetrating atoms within the diffusion zone can be interpreted based on thermodynamic considerations. Evidence for up-hill diffusion of the penetrating species through the matrix towards the precipitation front during the internal nitridation of Ni-Cr alloys at 1125 °C and 6000 bar of N2-pressure was predicted. Such behaviour of nitrogen is opposite to the boundary conditions in Wagner's description of internal reactions. A volume change associated with the precipitation reaction resulted in a stress gradient between alloys surface and the internal nitridation front. Stress relief occurred mainly by transport of nickel to the gas/metal interface. Pipe diffusion-controlled creep is the dominant stress accommodation mechanism during nitriding of dilute Ni-Cr alloys at 700 °C under a flowing NH3 + H2 gas mixture

Intermetallic growth and Kirkendall effect manifestations in Cu/Sn and Au/Sn diffusion couples

The Kirkendall-effect induced migration of inert markers during a diffusion-controlled growth of intermetallic compounds in the Cu/Sn and Au/Sn couples at 215 and 180 °C were studied. It was shown that the behaviour of markers in multiphase reaction zones can be rationalized in terms of Kirkendall velocity construction. Observations on the microstructural features of the product intermetallic layers and the role of the Kirkendall effect in the morphogenesis of the interdiffusion systems are discussed. It was demonstrated that the velocity of markers in a product layer, the appearance of the Kirkendall plane(s), their location(s) and the morphological evolution of the reaction products can also be explained from a purely chemical point of view considering the diffusion-controlled interactions at the interphase interfases. A representation of the reaction scheme and kinetics of intermetallic growth in each of teh two systems is given