Crystallization of mould powders used in the continuous casting of steel

Mould powders used in the continuous casting of steel play an important role in the heat transfer and lubrication between the liquid metal strand and mould. A range of industrial fluxes was investigated, each sample being decarburized and milled prior to DTA. On heating, the powders undergo silicate crystallization producing exotherms in the range 600°–1000°C, before melting. The activation energy of crystallization (E _a) was determined from the peak shifts of each flux heated at different rates, and ranged from 100–450 kJ/mol.E _a values increased with flux viscosity and decreased with basicity, suggesting that DTA can be applied to mould powder evaluation for use in continous steel casting.     

Mathematical analysis of the heat measured by a power-compensated differential scanning calorimeter during the solidification of a multiphase alloy

The determination of the solidification characteristics of alloys using differential scanning calorimetry (DSC) is difficult because of the unknowns associated with the kinetic of phase transformations and the thermal resistance between the sample and the temperature measuring device. This paper shows how appropriate assumptions coupled with a thermodynamic software package and an accurate mathematical analysis of a power-compensated DSC, can enable a direct comparison between the experimental and the theoretical heat evolutions obtained during the solidification of a multiphase alloy. This comparison is helpful in order to assess the thermodynamic database and to validate the different assumptions made in the solidification model.

     

Bentonite stability,speciation and migration behaviour of some critical radionuclides

The contribution is focused on chemical, geochemical and mineralogical research of bentonite stability with the aim to determine the effect of saturation medium composition and loading by heat on bentonite stability. The main part of the research is directed to the experimental results of bentonite and bentonite leachate samples obtained for the bentonite interaction under laboratory experiments. Computer-modeling methods were used to calculate equilibrium thermodynamic principles, the distributions of predominant aqueous species, and potential solubility controls for the environmentally important oxidation states of each investigated radioactive contaminants. The Eh–pH diagrams of individual chemical species of the tested radionuclides were calculated by the geochemical software tool Geochemist’s Workbench that included the actual chemical compositions of the solid–liquid systems under the given experimental conditions. It was confirmed that smectites are transformed to more stable silicate phases, such as illite/smectite mixed layers, illite. The data obtained from the model calculations conform with experimental results. The effect of the variable aqueous phase composition on bentonite stability using Ca–Mg and Na–Ca bentonites for the experiments was studied. The synthetic granitic waters with the higher concentration of the K⁺ and Mg²⁺ cations were applied for the study of bentonite stability.     

An application of infrared analysis to determine the mineralogical phases formation in fluxes for thin slab casting of steel

The Fourier Transformed Infrared (FTIR) spectra analysis of two fluxes used in the thin slab casting process of steel were carried out in order to identify the mineralogical species present in fluxes as received and after a heat treatment to 1573 K and further solidification at two different cooling velocities. Fluxes as received show the presence of wollastonite (CaO·SiO₂) and a sodium carbonate (Na₂CO₃) as the main components; after the heat treatment, there was almost a whole transformation from the original compounds to cuspidine (3CaO·2SiO₂·CaF₂) and nepheline (Na₂O·Al₂O₃·2SiO₂) phases. These results were confirmed by X-ray powder diffraction (XRD) to the slowly cooling velocity. The FTIR technique is proposed as a useful and complementary technique to X-ray diffraction to study the structure of commercial fluxes for thin slab casting.     

Thermodynamic modelling and phase stability assessment of MO2−X oxides with a fluorite structure

Thermodynamic modelling was performed for O–Th and O–Np systems to assess the phase stability of the MO_2−X phase together with the data in the literature for O–Pu, O–U, and O–Zr systems. Firstly, thermodynamic modelling was demonstrated for O–Th and O–Np systems with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to the phase diagrams of the both systems and the oxygen potential in the ThO_2−X and NpO_2−X phases. Secondly the phase stability of the MO_2−X phase was assessed using the obtained sets of data together with those for O–Pu, O–U, and O–Zr. The phase stability of the MO_2−X phase was discussed with respect to the deviation from the ideal solution in oxygen potential and Gibbs free energy.     

Theory of phase diagrams described by thermodynamic potentials with T d symmetry

Phase diagrams of crystals induced by irreducible representations with symmetry group \(L = \bar 43m\) (T _d ) are constructed within the phenomenological theory of second-order phase transitions. A model of the Landau thermodynamic potential is studied, state equations of all symmetry-conditioned phases are obtained, and general conditions for their thermodynamic stability are formulated. Equations for the boundaries of phase areas and lines of phase transitions are obtained for the fourth order of expansion of the potential via components of the order parameter. Some types of the collapse of the multicritical point of the phase diagram for the eighth order of potential expansion are studied using computer calculations. The possible existence of phase diagrams that contain one or more triple points and areas of existence of three and four phases is shown for the first time for the potentials with the above symmetry. Examples are given of crystals that undergo phase transitions in the considered symmetry of the order parameter.     

First‐principles modeling of hafnia‐based nanotubes

Hybrid density functional theory calculations were performed for the first time on structure, stability, phonon frequencies, and thermodynamic functions of hafnia‐based single‐wall nanotubes. The nanotubes were rolled up from the thin free layers of cubic and tetragonal phases of HfO₂. It was shown that the most stable HfO₂ single‐wall nanotubes can be obtained from hexagonal (111) layer of the cubic phase. Phonon frequencies have been calculated for different HfO₂ nanolayers and nanotubes to prove the local stability and to find the thermal contributions to their thermodynamic functions. The role of phonons in stability of nanotubes seems to be negligible for the internal energy and noticeable for the Helmholtz free energy. Zone folding approach has been applied to estimate the connection between phonon modes of the layer and nanotubes and to approximate the nanotube thermodynamic properties. It is found that the zone‐folding approximation is sufficiently accurate for heat capacity, but less accurate for entropy. The comparison has been done between the properties of TiO₂, ZrO₂, and HfO₂. © 2017 Wiley Periodicals, Inc.     

Rigorous nonlinear regression analysis of phase solubility diagrams to obtain complex stoichiometry and true thermodynamic drug-cyclodextrin complexation parameters

This work reports rigorous nonlinear regression procedures aimed at analyzing various types of phase solubility diagrams (PSDs) corresponding to the different soluble and insoluble complex stoichiometries, which are generally encountered in drug-cyclodextrin (CD) complexation studies. These are depicted in final equations that can be modeled to fit experimental data of measured drug solubility against CD concentration utilizing simple spreadsheet software available for all PCs (i.e., the Solver Add-in in Microsoft Excel). They cover all types of guest/host phase solubility diagrams (A-, B_S-and B_I-types) allowing accurate estimation of soluble and insoluble complex stoichiometries generally encountered in drug/CD complexes (1:1, 2:1, 1:2, 2:2, 2:3, 3:2), the corresponding thermodynamic complex formation constants (K₁₁, K₂₁, K₁₂, K₂₂, K₂₃, K₃₂) and solubility product constants (K_sp) of saturated complexes.     

Evaluation of specific heat and related thermodynamic properties of Ge1?xSnxSe2.5 (0?≤?x?≤?0.5) glasses

This paper presents the result of thermodynamic studies on Ge_1−x Sn _x Se_2.5 (0 ≤ x ≤ 0.5) glasses using differential scanning calorimetry. The obtained experimental results on phase transformations have been employed to obtain thermodynamic parameters like entropy difference between metastable states in the glassy region, difference of Gibbs free energy, specific heat, entropy between the glassy and the crystalline phase and the enthalpy released during phase transformation (glassy to crystalline). The results yield that, Ge_0.7Sn_0.3Se_2.5 sample is least stable among all the samples. The stability increases on addition of Sn beyond 0.3 at. mass% upto 0.5 at. mass%.     

PTX phase equilibrium study of new solid solution systems,Cd1−xMxS (M = Mg,Ca, Sr)

Pressure, temperature, and composition phase equilibrium diagrams of new solid solution systems of the Cd_1?xM_xS (M = Mg, Ca, Sr) type were investigated using the quenching method. The stable region for the rock-salt-type phase is widely extended toward the high-temperature/low-pressure region by substituting 10–20 mole% of Cd with Ca or Sr. Temperature and composition phase diagrams for each solid solution system were obtained at 2 GPa. The rock-salt-type phase stability is discussed in view of these phase relations.     

Phenomenological theory of phase diagrams with multicritical points

The breakup of multicritical points is studied in detail within the phenomenological theory of second-order phase transitions for a thermodynamic potential invariant with respect to the C _3v (3m) group of transformations. The general conditions of this breakup are obtained and possible types of derived diagrams are plotted from a parent phase diagram containing a multicritical point. Examples of experimental phase diagrams are given that qualitatively confirm the results from theoretical and computer simulations of phase equilibria.     

Calorimetric determination of equilibrium phase diagrams of inorganic systems

Calorimetric isobaric determination of the heat involved during mixing processes gives the concentration dependence of the enthalpy of mixing ΔH_M = F(x) and also, under particular conditions, some liquids concentrations of the phase diagram.For some simple typical diagrams (with a eutectic point or miscibility gaps, or a definite compound etc.) direct calorimetric experiments at many temperatures give the liquidus (e.g. NaBrNaNO₃, KBrKNO₃, NaBKNO₃, KBrNaNO₃, GaHg, GaSb).For more complicated or multicomponent systems, the setting up of the equilibrium phase diagram needs both experimental measurement and thermodynamic calculations (e.g. GaInSb).     

Thermodynamic insights into n-alkanes phase change materials for thermal energy storage

n-Alkanes have been widely used as phase change materials (PCMs) for thermal energy storage applications because of their exceptional phase transition performance, high chemical stability, long term cyclic stability and non-toxicity. However, the thermodynamic properties, especially heat capacity, of n-alkanes have rarely been comprehensively investigated in a wide temperature range, which would be insufficient for design and utilization of n-alkanes-based thermal energy storage techniques. In this study, the thermal properties of n-alkanes (C₁₈H₃₈-C₂₂H₄₆), such as thermal stability, thermal conductivity, phase transition temperature and enthalpy were systematically studied by different thermal analysis and calorimetry methods, and compared with previous results. Thermodynamic property of these n-alkanes was studied in a wide temperature range from 1.9 K to 370 K using a combined relaxation (Physical Property Measurement System, PPMS), differential scanning and adiabatic calorimetry method, and the corresponding thermodynamic functions, such as entropy and enthalpy, were calculated based on the heat capacity curve fitting. Most importantly, the heat capacities and related thermodynamic functions of n-heneicosane and n-docosane were reported for the first time in this work, as far as we know. This research work would provide accurate and reliable thermodynamic properties for further study of n-alkanes-based PCMs for thermal energy storage applications.     

A new method for the simultaneous determination of the size and shape of pores: the thermoporometry

A thermodynamic study of the liquid—solid phase transformations in porous materials provides the relationships between the size of the pores in which solidification takes place and the temperature of the triple point of the divided liquid, on the one hand, and between this temperature and the apparent solidification energy on the other hand.The experimental study of the phase transformations, carried out by means of a microcalorimeter, gives the values of the parameters necessary to calculate the free solid = liquid interphase extension energy γ_ls at different temperatures. A formula γ_ls  f(T) is given for water and benzene. Once this factor is known, it is possible to study the numerical relationship between pore-radius and freezing energy at the equilibrium temperature.By using these relations together with the solidification thermogram (the recording of the power evolved by the solidification of a capillary condensate during a linear decrease of temperature) the authors have been able to determine pore distribution curves. An emphasis is put on the comparison between this method, thermoporometry, and the B.J.H. method.Last of all the comparison of the experimental data for solidification and melting provide information concerning pore shape by means of the evaluation of a thermodynamic shape factor or by a method of simulation of porous material.     

Gas Evolution from a Sol-Gel MgO-Al2O3-SiO2-Li2O Powder during Heat Treatment Analysed by Mass Spectrometry

Desorption and vaporisation from MAS-L (MgO-Al₂O₃-SiO₂-Li₂O) sol-gel powders are observed during heat treatment from room temperature to 1000°C using thermogravimetry and Knudsen-cell mass spectrometry. The vaporisation gaseous products are H₂O, CO₂, NO, NO₂ and LiNO₂. Volatile hydroxides are not observed, but the evolution of the partial pressure of H₂O(g) with temperature, when compared with equilibrium thermodynamic calculations, reveals that LiOH (solid or liquid) is present as an independent bulk phase and that Li₂O is likely to be combined with another compound. The hydroxide Mg(OH)₂ is formed from the physically adsorbed and/or condensed water inside the porosity. From the evolution of the CO₂(g) partial pressure, independent carbonated bulk phases could not be detected, although the measured partial pressures crossed the pressure range of thermodynamic stability of known carbonate phases. Besides, both decompositions of LiOH and of nitrates were seen to contribute to the loss of lithium during heat treatment.     

Thermal analysis for generating phase diagrams of systems alkali metal chloride/divalent metal chloride

After a historical survey of the application of differential thermal analysis (DTA) for the generation of phase diagrams, particularly those of pseudobinary halide systems, the authors' research work on the systems ACl/MCl₂ (A = NaCs) is reported. In addition to DTA measurements the thermodynamic properties of the ternary compounds A_nMCl_n+2 are investigated by solution calorimetry and EMF measurements dependent on temperature with a newly developed galvanic cell for solid electrolytes. This method is also a useful tool for elucidating phase diagrams, especially for reactions occurring in the solid state.     

Liquid-solid equilibrium for the NaCl-NaHCO3-Na2CO3-H2O system at 45°C. Validation of mixed solvent electrolyte model

The main objective of this work was to verify the mixed solvent electrolyte (MSE) thermodynamic model for the prediction of the liquid-solid equilibrium in the soda system. The full quaternary system NaCl-NaHCO₃-Na₂CO₃-H₂O and its three subsystems were considered. Validation of the thermodynamic model was based on experimental data from literature. Phase diagrams for the soda system are plotted for isotherms of 45°C. Good agreement of the MSE results with those from experiments was found at 45°C. Similar phase diagrams for other temperature levels can be constructed based on the validated MSE model.     

Experimental verification of hematite ingot mould heat capacity and its direct utilisation in simulation of casting process

Heat capacity of alloys (metals) is one of the crucial thermophysical parameters used for process behaviour prediction in many applications. Heat capacity is an input variable for many thermodynamical (e.g. Thermocalc, Pandat, MTData, …) and kinetic programs (e.g. IDS-Solidification analysis package, …). The dependences of heat capacity on common variables (temperature, pressure, ...) are also commonly used as the input data in software packages (e.g. ProCast, Magmasoft, ANSYS Fluent, …) that are applicable in the field of applied research for simulations of technological processes. It follows from the above that the heat capacities of materials, alloys in our case, play a very important role in the field of basic and applied research. Generally speaking, experimental data can be found in the literature, but corresponding (needed) data for the given alloy can very seldom be found or can differ from the tabulated ones. The knowledge of proper values of heat capacities of alloys at the corresponding temperature can be substantially used for addition to and thus towards the precision of the existing database and simulation software. This study presents the values of C _p measured for the hematite ingot mould and comparison of the measured data with the C _p values obtained using the software CompuTherm with respect to simulation of technological casting process.     

Volumes,heat capacities and solubilities of amyl compounds in decyltrimethylammonium bromide aqueous solutions

Apparent molar heat capacities and volumes of amylamine (PentNH₂) 0.02m, capronitrile (PentCN) 0.02m and nitropentane (PentNO₂) 0.009m in decyltrimethylammonium bromide (DeTAB) micellar solutions, in water and in octane were measured at 25°C. By assuming that their concentration approaches the standard infinite dilution state, heat capacities and volumes were rationalized by means of previously reported equations following which the distribution constant between the aqueous and the micellar phase and heat capacity and volume of the additives in both phases are simultaneously derived. The present results are compared to those we have previously obtained for pentanol (PentOH). The thermodynamic properties of PentNH₂ in water and in micellar phase are substantially identical to those of PentOH but different from those of PentCN and PentNO₂ whereas the opposite behavior was observed in their pure liquid state and in octane. The nature of the solvent medium seems to affect the thermodynamic behavior of PentNH₂. Also, the study of the apparent molar heat capacities of the amyl compounds investigated here in micellar solutions as a function of surfactant concentration shows evidence of a maximum at about 0.4m DeTAB, which can be attributed to a micellar structural transition. Accordingly, the solubilities of PentCN and PentNO₂ as a function of the DeTAB concentration drop in the neighborhood of the concentration where heat capacities display the maximum.