I. Nerád

Thermodynamic properties of liquid phase in the CaO·SiO2–CaO·Al2O3·2SiO2–2CaO·Al2O3·SiO2 system☆

Abstract Thermodynamic properties consistent with phase equilibria and calorimetric measurements were established for liquid phase in the system CaO·SiO2–CaO·Al2O3·2SiO2–2CaO·Al2O3·SiO2 using molecular regular solution models. The assessed thermodynamic data set involves enthalpy and entropy of formation of crystalline phases, heat capacity (Cp) data of solid and liquid pure components, enthalpy of mixing of liquid pure components, enthalpy and entropy of fusion of solid phases and phase equilibria experiments. Considerable inconsistency was found between experimental phase equilibria and calorimetrically measured enthalpy of the liquid solution. To fix thermodynamic properties of melt in studied system, which reproduce phase diagrams satisfactorily, considerable changes in enthalpy of mixing with respect to the original calorimetric data must be accepted.

Experimental determination of the heat of fusion and calculation of the dissociation degree and dissociation enthalpy of K3FMoO4

Abstract The molar enthalpy of fusion Δ fus H m of KF · K 2 MoO 4 at the temperature of fusion, 1025 K, was measured in the high-temperature Setaram HTC 1800K calorimeter as 58 ± 2 kJ mol −1 . This quantity, together with the known phase diagram of the system KFK 2 MoO 4 and the composition dependence of the enthalpy of mixing at 1273 K, enabled both the dissociation enthalpy Δ dis H and the dissociation degree α of the compound KF · K 2 MoO 4 to be calculated at the temperature of its fusion. The Le Chatelier—Shreder equation, derived without taking hypothetical equilibrium states into account, was also used. The most reliable values of the quantities are those obtained by an iterative procedure using a “quasi-athermal” solution model. Their values are Δ dis,+ H (1, 1025 K) = −1.2 kJ mol −1 and α + (KF · K 2 MoO 4 , 1025 K) = 0.51.

Calorimetric study of melts in the system KF - K2TaF7

Enthalpy increment measurements on melts in the system KF-K2TaF7 were carried out by drop calorimetry at temperatures between 298 K and 1063, 1103 and 1143 K for selected compositions. Heat capacities of the melted mixtures and enthalpies of mixing have been determined. Careful calorimetric experiments showed small but distinct non-ideality of the melt. The molar heat capacity of melt exhibits small positive divergence from additivity. The molar enthalpy of mixing shows negative deviation from ideality which decreases with increasing temperature. The thermal effect at mixing was assigned predominantly to association reactions producing more complex fluorotantalate anions. The formation of complex anions with lower coordination number of Ta may not be excluded.

Enthalpic analysis of melts in the Ca2MgSi2O7CaSiO3 system

Abstract Relative enthalpies of the Ca 2 MgSi 2 O 7 CaSiO 3 system melts have been measured by combination of high-temperature drop calorimetry and solution calorimetry of the same sample in the temperature interval 1760–1930 K. From these results, the relative enthalpy-temperature-composition relation has been found by regression analysis. The enthalpy of mixing of melts is zero at any composition within the experimental errors of measurement. The heat capacities of melts do not depend on temperature and are a linear function of composition. They are discussed with values calculated employing Stebbins et al.s, Richet and Bottingas, and Lange and Navrotskys models.

Determination of enthalpy of fusion of K3FSO4

Abstract The enthalpy of melting of K3FSO4 at 1148 K has been determined using a hightemperature calorimeter, the Setaram HTC 1800 K. ΔfusHm(K3FSO4 is found to equal 86 ± 3 kJ mol−1. The given error is calculated at the level of reliability (1 - α) = 0.95.

Enthalpic analysis of the CaTiSiO5 system

The relative enthalpy of titanite and enthalpy of CaTiSiO5 melt have been measured using drop calorimetry between 823 K and 1843 K. Enthalpies of solution of titanite and CaTiSiO5 glass have been measured by the use of hydrofluoric acid solution calorimetry at 298 K. Enthalpy of vitrification at 298 K, δvitrH(298 K) = (80.7 ± 3.4) kJ mol−1, and enthalpy of fusion at the temperature of fusion 1656 K, δfusH(1656 K) = (139 ± 3) kJ mol−1, of titanite have been determined from experimental data. The obtained enthalpy of fusion is considerably higher than up to the present published values of this quantity.

Estimation of the heat of fusion of binary compounds and of eutectic using thermodynamic balances

Abstract Relations to calculate estimated enthalpies of fusion of binary compounds and eutectics at their equilibrium temperature in binary systems of inorganic substances, based on entropy or enthalpy balances, are presented. Their simplified forms were applied to estimate the enthalpy of fusion of equimolar compounds and binary eutectics in the Na 2 SO 4 -NaF and K 2 SO 4 -KF systems. Estimated enthalpies of fusion of NaF · Na 2 SO 4 and KF · K 2 SO 4 depend on simplified assumptions used for the calculation and are 7–13% lower than measured values. Relative errors of estimated enthalpies of fusion for eutectics, related to the values of these quantities calculated using experimental heats of fusion of binary compounds and measured enthalpies of mixing at corresponding eutectic composition, are within the limits 0–12% depending on input quantities.

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na3FeF6 and for the Na3FeF6 melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δtransH(Na3FeF6, 920 K) = (19 ± 3) kJ mol−1 and enthalpy of fusion at the temperature of fusion 1255 K, δfusH(Na3FeF6, 1255 K) = (89 ± 3) kJ mol−1 have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: Cp(Na3FeF6, cr, α) = (294 ± 14) J (mol K)−1, for 723 = T/K ≤ 920, Cp(Na3FeF6, cr, β) = (300 ± 11) J (mol K)−1 for 920 ≤ T/K = 1233 and Cp(Na3FeF6, melt) = (275 ± 22) J (mol K)−1 for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.

Differential and integral enthalpies of solution of akermanite and pseudo-wollastonite in the melts of the system Ca2MgSi2O7CaSiO3. Isoplethal enthalpies in this system

Abstract It was found that the enthalpy of mixing at formation of the melts in the Ca 2 MgSi 2 O 7 CaSiO 3 system from melts of Ca 2 MgSi 2 O 7 and CaSiO 3 is zero within the temperature limits from 1673 to 1930 K. Consequently, the differential and integral heats of solution of akermanite and pseudo-wollastonite in the melts of this system are equal to their heats of fusion at the corresponding temperature. Using the non-isothermal Hess law and the phase diagram, isoplethal enthalpies were calculated in this system within the temperature range from 298 to 1920 K. The enthalpy and entropy of crystallization for the eutectic melt at the eutectic temperature T e = 1673 K was determined to be Δ cryst H (eut. melt, T e ) = (− 78.3 +- 2.4) J mol −1 and Δ cryst S (eut. melt, T e ) = (−46.8 +- 1.4) J mol −1 K −1 , respectively.

Determination of the enthalpy of fusion of NaMgF3 and KMgF3

Abstract The enthalpy of melting of NaMgF3 and KMgF3 at the dystectic temperature of fusion of these compounds was determined using a high-temperature calorimeter, the Setaram HTC 1800 K. It was found that ΔfusHm(NaMgF3; 1303 K) = (66 ± 4) kJ mol−1 and ΔfusHm(KMgF3; 1343 K) = (96 ± 5) kJ mol−1. The given error is calculated at the level of reliability (1 − α) = 0.95.