Physicochemical mechanisms of the growth of oxide crystals on the surface of tungsten conductors heated by electric current

Physical and mathematical modeling of stationary thermal modes of heating and oxidation of tungsten conductors heated by electric current in air has been carried out. The dependences of the stationary temperature of the conductor on the strength of the heating current are obtained. The critical values of the current strength are found, which determine the transitions to the unsteady oxidation regime. The results of calculating the temperature regimes describe well the experimental data obtained by us using the electrothermographic method. As a result of experimental studies, the features of the appearance and growth of crystalline oxide structures on the surface of an oxidizing tungsten conductor have been studied. The temperatures at which filamentous crystals appear on the tungsten surface are determined, and the regularities of their growth are investigated. A physicochemical mechanism of the formation and growth of crystalline oxide structures on the surface of a tungsten conductor is proposed. It was found that carbon particles, which are part of the impurity, are the reason for the formation of nitrate crystals of tungsten trioxide on the basic oxide. With an increase in the temperature of the sample, the filaments grow, branch out and transform into dendritic structures of a complex bush-like shape. It has been proven that the rapid growth of crystal structures occurs due to the deposition of clusters and microgranules of WO3 oxide from the gas phase on the crystallization centers on the surface of the conductor. At the initial stage, these are impurity particles or mechanical inhomogeneities of the basic oxide, then a growing crystal. Clusters arise due to large temperature gradients at the surface of the conductor. It has been established that carbon atoms can migrate along the branches of oxide crystal structures. It was found that at the initial stage the crystals grow more intensively in the longitudinal direction. However, upon reaching a certain height, they begin to expand intensively in the transverse direction. The growth rates of crystal structures in the longitudinal and transverse directions are found.

Application of ionic theory to calculate sulfide capacity of slags

The article considers the issues of sulfur removal in the ladle-furnace unit. The sulfur distribution coefficient depends on sulfide capacity of the slag, sulfur activity coefficient, oxidizing potential of the medium and equilibrium constant. The sulfide capacity CS of slags is one of the most important characteristics of refining capacity of the slags used in extra-furnace steel processing. One of the factors affecting the sulfide capacity is temperature. The formula was proposed showing the dependence of sulfide capacity on the optical basicity and temperature, in the temperature range of 1650 – 1400 °C and when the optical basicity Λ is not more than 0.75; the error of the presented formula does not exceed 6 %. The formula for calculating the optical basicity is proposed, which takes into account the influence of basic, acidic oxides and amphoteric oxide Al2O3. It is shown that slags, completely consisting of a homogeneous phase, have an increased optical basicity of aluminum oxide. Heterogeneous slags have a reduced optical basicity of Al2O3 in comparison with homogeneous slags. Perhaps, this fact can be explained by the fact that in homogeneous slags there is a deficiency of the basic oxide CaO and in the conditions under consideration Al2O3 compound begins to exhibit more basic properties than acidic ones, thus, in homogeneous slags, the optical basicity of aluminum oxide is increased and approaches optical basicity of CaO oxide. Calculations carried out on the basis of real heats have shown that with an increase in the content of Al2O3 oxide in the slag, its optical basicity decreases. Known value of the optical basicity makes it possible to determine sulfide capacity of the slag, sulfur distribution coefficient between metal and slag, and, accordingly, final sulfur content in the metal. The research results have shown that it is advisable to apply the ionic theory of slags for the sulfide capacity determination.

High-Temperature Corrosion Behavior of Al-Coated Ni-Base Alloys in Lithium Molten Salt for Electroreduction

The electrolytic reduction of a spent oxide fuel involves the liberation of oxygen in a molten salt LiCl–Li2O electrolyte, which creates a corrosive environment for typical structural materials. In this study, the corrosion behaviors of Al–Y-coated specimens in a Li molten salt kept under an oxidizing atmosphere at 650 °C for 72 and 168 h were investigated. The weight loss fraction of the coated specimen to bare specimen was approximately 60% for 3% Li2O and 54% for 8% Li2O at 72 h, and approximately 38% for 3% Li2O and 30% for 8% Li2O at 168 h. Corrosion was induced in the LiCl–Li2O molten salt by the basic oxide ion O2− via the basic flux mechanism, and the corrosion product was found to be dependent on the activity of the O2− ion. The increase in weight loss may have been caused by the increase in the O2− concentration due to the increase in the Li2O concentration rather than being because of the increased reaction time. The Al–Y coating was found to be beneficial for hot corrosion resistance, which can be useful for handling high-temperature lithium molten salt under an oxidizing atmosphere.

A combined periodic DFT and QM/MM approach to understand the radical mechanism of the catalytic production of methanol from glycerol.

The production of methanol from glycerol over a basic oxide, such as MgO, using high reaction temperatures (320 °C) is a promising new approach to improving atom efficiency in the...

Viscosity and Structure of a CaO-SiO2-FeO-MgO System during a Modified Process from Nickel Slag by CaO

There is a high iron content in nickel slag that mainly exists in the fayalite phase. Basic oxide can destroy the stable structure of fayalite which is beneficial to the treatment and comprehensive utilization of nickel slag. The research was based on the composition of the raw nickel slag, taking the CaO-SiO2-FeO-MgO system as the object and CaO as a modifier. The effect of basicity on the melting characteristics, viscosity and structure of the CaO-SiO2-FeO-MgO system was studied. The relationship between the viscosity and structure of the CaO-SiO2-FeO-MgO system was also explored. The results show as follows: (1) When the basicity is lower than 0.90, the primary phase of the slag system is olivine phase. When the basicity is greater than 0.90, the primary phase of the slag system transforms into monoxide. When the basicity is 0.90, olivine and monoxide precipitate together as the temperature continues to decrease. At the same time, the liquidus temperature, softening temperature, hemispherical temperature, and flow temperature all reach the lowest value. (2) With the increase of basicity, the critical viscosity temperature of the CaO-SiO2-FeO-MgO system decreases first and then increases. Critical viscosity temperature is the lowest at the basicity of 0.90, which is 1295 °C. (3) When the slag system is heterogeneous, the viscosity of the molten slag increases rapidly because of the quantity of solid phase precipitated from the CaO-SiO2-FeO-MgO system. (4) When the slag system is in a homogeneous liquid phase, the molar fraction of O0 decreases with the increase of basicity and the mole fraction of O−, and O2− increases continuously at the basicity of 0.38~1.50. The silicate network structure is gradually depolymerized into simple monomers, resulting in the degree of polymerization, and the viscosity, being reduced. The mole fraction of different kinds of oxygen atoms is converged to a constant value when the basicity is above 1.20.

Microstructural and Mechanical Properties of Alkali Activated Materials from Two Types of Blast Furnace Slags

This paper investigated the effect of blast furnace slags (BFS) characteristics on the properties achievement after being alkali activated. The physical and chemical characteristics of BFS were determined by X-ray fluorescence (XRF), X-ray Diffraction (XRD) and laser granulometry. Multi-technical characterizations using calorimetry, XRD, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetry (TG-DTG), scanning electron microscope (SEM), nitrogen sorption and uniaxial compressive strength (UCS) were applied to give an in-depth understanding of the relationship between the reaction products, microstructure and BFS characteristics. The test results show that the microstructure and mechanical properties of alkali activated blast furnace slags (BFS) highly depend on the characteristics of BFS. Although the higher content of basic oxide could accelerate the hydration process and result in higher mechanical properties, a poor thermal stabilization was observed. On the other hand, with a higher content of Fe, the hydration process in alkali activated BFS2 lasts for a longer time, contributing to a delayed compressive strength achievement.