Yu. P. Dikov

Corrosion testing of platinum metals CVD coated titanium anodes in seawater-simulated solutions ☆

Abstract A chemical vapour deposition (CVD) technique is widely used for the preparation of protection coatings. In the present work titanium anodes with CVD coatings of iridium, platinium and Ir—Pt composite were made The coatings were obtained by thermolysis of platinum(II) acetylacetonate and iridium(III) acetylacetonate in the oxygen or hydrogen contained atmosphere. The investigation of the samples by means of XPS showed the presence of oxide phases in the coatings. Tests of the resulting samples in the electrolysis of seawater-simulated solutions under active evolution of oxygen were conducted. Platinum coatings were found to be more stable than iridium ones, which is, to a substantial extent, due to mechanical strain at the titanium-to-coating boundary. Iridium-coated titanium foil anodes were tested in an experimental electrodialysis unit. Their service life exceeded 1 year.

Valence state of iron in a condensate from the Luna 16 regolith

This paper reports the results of an X-ray photoelectron spectroscopic study of the condensate phase of regolith sample L1639 returned by the Luna 16 mission. The reduced Si0, Si2+, Al0, Ti2+, and Ti3+ forms were detected in the sample. Iron occurs in all valence states, and Fe3+ species were detected for the first time in the condensate. Minor Fe3+ concentrations were observed in the upper layers of the sample containing the maximum amounts of condensate products. The fraction of ferric Fe is 22%, and the Fe0: Fe2+: Fe3+ proportion is 33: 45: 22. The appearance of ferric Fe in the lunar condensate is explained by the reaction of FeO disproportionation occurring either at the stage of the expansion and cooling of impact-related vapor or directly in the condensed phase on the surface of regolith particles. This interpretation is supported by the results of a model experiment on augite vaporization and condensation. The experiment simulating impact vaporization was carried out on a laser set-up at a temperature of ∼3000–4000 K and a pulse duration of ∼10−3 s in a He atmosphere (P = 1 atm). The results of analyses provided compelling evidence that the condensate produced after augite vaporization contains Fe in all oxidation states, and the proportions of different valence forms approach the stoichiometry of the disproportionation reaction.

Structure of colloid particles in groundwaters on the territory of the Mayak production association and its impact on the colloid transport of radionuclides in subsoil environments

Colloid particles were examined in groundwaters sampled at the large contamination area of the subsoil environment at the Mayak production association in the Southern Urals. Colloid particles were separated from groundwater samples by ultrafiltration, and their composition and structure were analyzed by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy (AS). Our results indicate that the chemical composition of the particles significantly varies at their surface and along their radii, from the peripheries to the cores of the particles. The data are used to estimate the possibility of utilizing properties of colloid particles contained in groundwaters (the composition, structure, and zeta potential of such particles) to numerically simulate the spreading of radioactive contamination with groundwaters with regard to the colloid mode of radionuclide transport. It is demonstrated that, along with transmitting electron spectroscopy (TEM) coupled with X-ray microprobe analysis and electron diffraction, the XPS and AS techniques make it possible to obtain information on heterogeneity in the distribution and modes of occurrence of various elements in colloid particles.

Experimental data on the thermal reduction of phosphorus and iron and their significance for the interpretation of the impact reworking of lunar materials

Experiments simulating impact melting and vaporization were performed to study variations in the valence state of P on the Moon. It was shown that P changes its valence and is partially transformed from oxidized (phosphate) to reduced (P0) species at high temperatures typical of the impact process. The reduction of P occurs concurrently with Fe reduction from silicates. The metallization of Fe and P is due to the effect of thermal reduction only. This supports the hypothesis of R. Hunter and L. Taylor that P0 produced from lunar phosphates (apatite and whitlockite) interacted with reduced Fe to form typical P-bearing minerals of lunar highland rocks, schreibersite and Fe-P alloys.

“Invisible” gold in covellite (CuS): Synthesis and studies by EPMA, LA-ICP-MS, and XPS techniques

Samples of covellite CuS(cr) containing up to 0.3 wt % of gold in “invisible” form were obtained by means of hydrothermal synthesis (450–475°C, 1 kbar) and synthesis in eutectic chloride melts (495°C). The studies using EPMA and LA-ICP-MS techniques showed that gold was uniformly dispersed within the volume of covellite grains and Au content increased with increasing of sulfur fugacity. The XPS-analysis of the samples obtained showed no chemical shift of the Au-4f7/2 line compared to Au and Au2S, whereas a positive chemical shift of this line was revealed for mixed Au-Ag sulfides and tellurides. The obtained data allow one to assume that gold does not occur in covellite as an isomorphous admixture (solid solution) but forms nanoparticles of Au or gold sulfide.

Influence of host rocks on composition of colloid particles in groundwater at the Karachai Lake site

Abstract The radioactive contaminant plume in groundwater at the Karachai Lake site has been examined further. The source of pollution is the Karachai Lake which was used as a reservoir for liquid radioactive waste (LRW) from the Radiochemical Production Association ‘‘Mayak’’ (South Urals, Russia). Taking into account that colloid-facilitated transport of radionuclides can substantially increase the contaminant migration velocity, the composition and structure of colloid particles from the groundwater within the contaminant plume were studied. Sampling of groundwater was carried out in the observation well within the contaminant plume. Different fractions of colloid particles were extracted from the groundwater samples by sequential filtration through membranes with decreasing pore size. Chemical and mineralogical compositions of the colloid particles were determined by X-ray photoelectron spectroscopy in combination with ion etching of the particle surfaces. The host rocks were studied using characteristic samples from the surface outcrop nearest to the observation well. The analysis showed that the colloid particles consisted of an inorganic core with a discontinuous organic cover. The inorganic core was close in composition to the host rocks along the flow path with a predominance of layered hydrosilicates.

Estimation of temperature conditions for the formation of HASP and GASP glasses from the lunar regolith

Impact cratering on the Moon’s surface was accompanied by the high-temperature melting of rocks, melt evaporation, and silicate vapor condensation. Evidence for the extensive evaporative fractionation of melts was found in HASP (High-Alumina Silica-Poor) glasses from the lunar regolith. Numerous objects of condensation origin were found in the Apollo 14 regolith breccia. They are referred to as GASP (Gas-Associated Spheroidal Precipitates). With respect to chemical characteristics, namely FeO and SiO2 contents, GASP were subdivided into Fe-rich (FeGASP) and Si-rich (SiGASP) condensates. Based on experimental data on the evaporation of aluminous basalt sample 68415.40 from the Apollo 16 collection and the calculated compositions of residual melts and complementary vapors at various temperatures, we compared the obtained compositions with the chemical analyses of the HASP glasses and GASP condensates. The comparison was aimed at estimating the temperature conditions of HASP and GASP formation. The comparison showed that the compositions of the HASP glasses and GASP condensates are consistent with the compositions obtained in the equilibrium experiment. In accordance with the experiment, the temperature range of the evaporation of HASP glasses was estimated as ∼1750–1870°C. The temperature interval of condensation, with allowance for the effect of vapor supercooling, is ∼1700–1500°C for FeGASP and no higher than 1700–1750°C for SiGASP. This paper discusses the problems of establishing interphase thermodynamic equilibrium during the dispersion of a vapor-melt cloud, vapor supercooling during its condensation, and the influence of the curvature of melt and condensate particles on the character of evaporation and condensation.

The Nature of Volatiles in the Lunar Regolith

Layer-by-layer analysis of the fine fraction of Luna-16 sample L1635 by X-ray photoelectron spectroscopy (XPS) revealed a zonal structure of its constituent particles. The outer zone, to a depth of 1200 Å, is enriched with Si and Fe and depleted in refractory elements Mg, Ca, and Al as compared to the bulk composition of the sample. The second zone, in the depth range from 1200 to 4500 Å, is of very unusual composition: it is enriched with carbon (up to 60 at. %) and zinc (up to 4 at. %) and contains some other volatiles. The lower zone, at depths from 4500 to 7000 Å, is composed of a silicate material; however, the concentrations of refractory elements are noticeably higher than those in the near-surface layer. There is a difference in structural and chemical characteristics of most of the elements between different zones. These findings point to the exogenous origin of the enrichment of the studied sample with volatiles and are related to the fall of a volatile-rich body (comet or carbonaceous chondrite) on the Moon. The hypothesis of the cometary (carbonaceous-chondrite) impact explains the main patterns of the volatile distribution in the lunar regolith, including the formation of green and orange glasses from Apollo 15 and 17 collections.

Effect of the disproportionation reaction of ferrous iron in impact-evaporation processes

Evidence for the disproportionation of iron was found in model experiments imitating impact melting, evaporation, and condensation. The experiments were carried out using a laser system at a characteristic temperature of ∼3000–4000 K and a pulse duration of ∼10−3 s in a He atmosphere (P = 1 atm). Augite and mixtures of peridotite with MnO2 and WO3 were used as starting target materials. Experimental products (condensed vapor phase) were analyzed by X-ray photoelectron spectroscopy. The results of condensate analysis provided compelling evidence for the presence of iron in three oxidation states (Fe0, Fe2+, and Fe3+). In an experiment with augite, the proportions of iron species of different valences were similar to the stoichiometry of the disproportionation reaction. Similar evidence for this reaction was first found in a condensate from the samples of the fine fraction of the Luna 16 regolith. In the layers of the lunar condensate, the proportions of the valence states of iron were on average Fe0:Fe2+:Fe3+ = 1.2: 1.9: 0.7.

Cluster type of silicate vaporization: Newly obtained experimental data

Impact cratering is usually associated with the partial or complete vaporization of the high-temperature impact melts. According to its chemical characteristics, the vaporization of major oxides, silicate minerals, and rock melts can be classified into the following four types: (1) congruent vaporization without decomposition of the compound in the vapor phase, (2) congruent vaporization with the decomposition of the compound in the vapor phase, (3) incongruent vaporization, and (4) cluster vaporization. The latter type of vaporization pertains to the transfer of material into vapor phase in the form of complicated atomicmolecular groups (clusters) of certain stoichiometry. Cluster vaporization takes place at superhigh temperatures typical of impact processes. The clusters can comprise compounds of different individual volatility, and this often results in the enrichment of the vapor phase in elements traditionally thought to be refractory. Examples of cluster vaporization are offered by lately obtained experimental results on laser-pulse vaporization of larnite, merwinite, and wollastonite. Condensed vapor generated at the vaporization of orthosilicates (larnite and merwinite) was proved to be dominated by chain bonds of Si-O tetrahedrons and to contain molecular groups of wollastonite and pseudowollastonite stoichiometry.