Papers by Keyword: Thermodynamic Modeling

Abstract: Experimental techniques like X-ray diffraction (XRD) and pore size distribution determination (MIP), in conjunction with thermodynamic simulation (GEMS) and kinetic modeling of cement hydration (PK), were used to analyze the microstructures and phase compositions of cementitious materials under the coupling effect. This allowed researchers to study the evolution of formation products and pore structure of cementitious materials under the coupling of long-term wet and dry cycling and sulphate erosion. The findings demonstrate that the cement pastes with varying w/b ratios formed the expansion products calcium alumina and gypsum after 180 days of dry and wet cycling by 5% sodium sulphate solution; however, the specimens with lower water-ash ratios had denser structures, fewer erosion products, and better resistance to sulphate erosion. The following illustrates how sulphate erosion products affect the pore structure: during the early stages of erosion, the pore structure becomes more refined and has an increase in the number of transition pores between 10-100 nm due to the erosion products such calcium alumina and sodium sulphate crystals filling the pores. The degree of sulphate erosion is exacerbated by the formation of new cracks and larger macropores, which result in an increase in the number of erosion products, an increase in the average pore size, and a looser pore structure. However, the calcite growing in the transition pores is more destructive, and the crystallisation pressure keeps building up, leading to the destruction of some of the transition pores.

Abstract: The study of the silicon crystallization mechanism by standard analytical methods is difficult since the temperature of liquid silicon in the ladle during oxidative refining is quite high rising to 1500-1600 °C. Therefore, the method of mathematical modelling was used. To understand the mechanism of inclusions formation in silicon the authors applied a method of computer-generated state diagrams of three-component systems using the soft package Diatris. The package allows one to study the behavior of components involved in various physical and chemical transformations, and the interaction of elements with each other during the crystallization of the silicon melt. This work studies the behavior of such impurities as Fe, Ti, B, and C. To achieve the set goals, the ternary diagrams were constructed and analyzed for the Si-Ti-Fe, Si-Ti-B, Si-Ti-C, Si-Fe-B, Si-Fe-C, and Si-B-C systems. To characterize the increase in the number of crystals when a multicomponent alloy passes the crystallization interval, the crystallization rate was calculated. It is established that the most probable impurity compounds in crystalline silicon are SiFe_0.4B_0.06, FeTiSi₅, SiB_0.06Ti_0.33, and SiTi_0.4C_0.1.

Abstract: Changes in thermal expansion during heating of hot-extruded tube made of VT23 titanium alloy (Ti-5.45 Al-4.7 V-2.45 Mo-1.1 Cr-0.7 Fe, wt. %) were analyzed. The volume effect of α→β-transformation, β-transus temperature as well as the volume fraction of α-and β-phase formed during heating were calculated based on the received data of dilatometric analysis. Results obtained were compared to thermodynamic calculations and published data. The change in the CTE value of VT23 alloy upon heating to 975 °C was estimated, and the explanation of different CTE values obtained in the present study and published data was proposed.

Abstract: Portland-limestone cement materials are susceptible to sulfate attack at low temperature and high humidity, because such conditions facilitate the formation of thaumasite, detriment to the structural integrity of calcium silicate hydrates (C─S─H). In this work, the effect of the cation associated with sulfates, concentration of sulfate solution, and limestone content in cement, were thermodynamically simulated. MgSO₄ solution is of higher risk, degrading extensively the structural integrity of C─S─H. Although this phase is partially preserved under the effect of Na₂SO₄ and K₂SO₄ solutions, extensive expansion and thaumasite formation occur. The sulfate content of the corrosive solution and the limestone content in cement are the factors mostly intensifying the attack caused by MgSO₄ and Na₂SO₄/K₂SO₄ solutions, respectively.

Abstract: The use of fluorspar in modern metallurgical slags, incl. slags of the argon-oxygen decarburization (AOD) process, as a fluxing agent, is associated with many disadvantages. Those disadvantages can be solved by using boron oxide as an alternative, which also provides conditions for direct microalloying of steel with boron. The paper presents the results of thermodynamic modeling of the effect of basicity and boron oxide content in slags of the CaO–SiO₂–B₂O₃–Cr₂O₃–Al₂O₃–MgO system on the equilibrium interphase distribution of sulfur and boron, and their equilibrium content in the metal. Modeling was carried out using the HSC 8.03 Chemistry software package (Outokumpu). Slag from the desulfurization period of the AOD-process was used as the oxide phase. As a result, it was shown that, in the range of basicities 2.0-2.5 and a content of 2-4% B₂O₃, it is possible to carry out desulfurization of the metal, providing a sulfur content of 0.001-0.007%, and simultaneous microalloying of steel with boron in an amount of up to 0.0103%.

Abstract: Thermodynamic modeling of phase equilibria with the subsequent construction of the phase diagram of the SrO–Al₂O₃ system has been carried out. To calculate the activities of the oxide melt in the course of this work, we used the approximation of the theory of subregular ionic solutions, with the most optimal values of the energy parameters Q₁₁₁₂ = –104 349: Q₁₁₂₂ = –217 689; Q₁₂₂₂ = –104 436 J/mole. The results obtained for the liquidus line in this work are in good agreement with the literature experimental data. In the course of the calculation, the values of the equilibrium constants for the formation of strontium aluminates from the components of the oxide melt were estimated.

Abstract: The behavior of С₅₆ fullerene when heated in a nitrogen atmosphere at a pressure of 10⁵ Pa was studied using computer thermodynamic modeling. The modeling consisted in a complete thermodynamic analysis of the system using the TERRA software package, which is one of the most developed and efficient ones that implements such thermodynamic calculations. Experiment temperature ranges are from 273 to 3373 К. Based on the calculated data, a graph of the carbon balance in the С₅₆-N₂ system was constructed, the ongoing physicochemical processes were described, divided into four classes: sublimation, dissociation in the gas phase, chemical reactions occurring in the gas phase, dissociation and chemical reaction in the gas phase. Temperature intervals of reactions are identified. The equilibrium constants of the reactions are calculated and described, as well as the coefficients of these constants are found using the least squares method. The temperature interval of thermal stability of the condensed C₅₆ fullerene and C₅₆ vapors is defined. This work is one of the series of works on the properties of nanoparticles, in the future it is planned to study the thermal properties of higher fullerenes. The data obtained can be used to determine the explosive and fire hazardous properties of fullerenes as a dispersed solid.

Abstract: Austenitic stainless steels are widely used in industry. Increased requirements for the quality of products from these steel grades, the difficulties associated with the implementation of technological processes, as well as the high cost of steel, determine the necessity to assess probable causes of defects. This article presents an analysis of the influence of main process parameters on the quality of products from the grade 08X18H10T steel. Based on the results of statistical analysis and thermodynamic modeling, it was concluded that the increased content of titanium and nitrogen affects the quality of products, which is caused by the formation of titanium carbonitrides in the process of steel solidification.

Abstract: The structural changes of condensed fullerenes C₆₀ and C₂₈ at a temperature increase from 200 K to 2000 K have been studied by computational methods using the TERRA software for carbon-argon systems. The processes of destruction of fullerenes C₆₀ and C₂₈ molecules are presented, and the temperature ranges of their thermal stability are determined: up to 1000 K and up to 400 K, respectively. The following thermophysical parameters of the C₆₀-Ar and C₂₈-Ar systems are considered: specific volume, entropy, total enthalpy, total internal energy, equilibrium specific heat, molar mass of the gas phase, gas constant, and mass fraction of the condensed phase. A comparative analysis of their changes with increasing temperature is carried out. The results obtained in the course of thermodynamic modeling are similar to the results of a full-scale experiment conducted under similar conditions. In the future, the obtained data can be used to determine the explosive and fire-hazardous properties of fullerenes as a dispersed solid.

Abstract: Thermodynamic modeling of coordinates of phase diagrams’ liquidus lines of the FeO–MgO, FeO–Al₂O₃, MgO–Al₂O₃ systems and coordinates of phase diagram’s liquidus surface of the FeO–MgO–Al₂O₃ system has been carried out. In the course of work, a thermodynamic model which describes activity of oxide melt had been selected for each of the systems; energy parameters of the model have been determined. Regions of thermodynamic stability of solid phases which are at equilibrium with the oxide melt have been determined. Results of the modeling have been compared with experimental data existing in the literature. Modeling technique has also allowed evaluating enthalpies and entropies of FeAl₂O₄ and MgAl₂O₄ compounds’ formation out of components of the oxide melt. The obtained results are of interest for steelmaking industry processes when determining the melt temperature of a slag containing oxides of iron, magnesium and aluminum.