Ladislav Kosa

Determination of the enthalpy of fusion of Na3FSO4

The areas of the fusion and crystallization peaks of Na3FeF6 and of four calibration substances (KCl, NaCl, Na2SO4, and K2SO4) were measured using the DSC mode of a high-temperature calorimeter. Using the measured quantities and known values of the enthalpy of fusion of the calibration substances, the enthalpy of fusion of Na3FeF6 was determined. Its value at the temperature of fusion 1224 K was 70 ± 4 kJ mol−1.

Phase transitions of K2TaF7 within 680–800°C

Three thermal effects on heating/cooling of K2TaF7 in the temperature interval of 680–800°C were investigated by the DSC method. The values determined for the enthalpy change of the individual processes are: ΔtransIIHm(K2TaF7; 703°C) = 1.7(2) kJ mol−1, ΔtransIHm(K2TaF7; 746°C) = 19(1) kJ mol−1 and ΔtransIIIHm(K2TaF7; 771°C) = 13(1) kJ mol−1. The first thermal effect was attributed to a solid-solid phase transition; the second to the incongruent melting of K2TaF7 and the third to mixing of two liquids. These findings are supported by in situ neutron powder diffraction experiments performed in the temperature interval of 654–794°C.

Enthalpic analysis of melts in the CaO · SiO2 (CS)-CaO · Al2O3 · 2SiO2 (CAS2) -2CaO · Al2O3 · SiO2 (C2AS) system

Abstract Relative enthalpies for melts in the CS-CAS2-C2AS system have been measured by combination of high-temperature drop calorimetry and solution calorimetry of the same sample at 298 K. From the measured values of relative enthalpy and from those determined using the literature data the temperature-composition dependences of relative enthalpy of melts related to 1 mol of mixture of the CS-CAS2-C2AS system and/or C(CaO)-A(Al2O3)-S(SiO2) system were determined. From these dependences the values of enthalpy of mixing in formation of melts of the CS-CAS2-C2AS system either from the melts of CS, CAS2 and C2AS, or from C, A and S, as well as the values of partial molar enthalpies of mixing of C, A and S in this system in the temperature interval 1600–1950 K, were calculated. The analysis has shown that the values of enthalpy of mixing in formation of the equilibrium melts in the CS-CAS2-C2AS system from melts of components CS, CAS2 and C2AS and/or C, A and S are mostly negative. In the region of compositions close to the binary system CAS2-C2AS and in this system itself, the melts in the temperature interval 1600–1950 K within the error range of ΔHmix behave as the athermic solutions. The formation of the athermic solutions in the CAS2-C2AS system shows that new structural particles are not created in mixing of both basic substances. Different structural roles of oxides are also evident.

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.

Determination of dissociation degrees of K3NbF8 and K3TaF8 by thermodynamic analysis of subsystems of the KF-K2NbF7 and KF-K2TaF7 systems

A special form of the LeChatelier-Shreder equation describing the equilibrium between the crystalline phase and the melt in system A-AB in which the substance AB partially dissociates upon melting was applied to systems KF-K3NbF8, K2NbF7-K3NbF8 and to KF-K3TaF8, K2TaF7-K3TaF8 subsystems of the binary systems KF-K2NbF7 and KF-K2TaF7 in which the additive compounds K3NbF8 and K3TaF8 are formed. Using the phase diagram of the system KF-K2NbF7 determined by McCawley and Barclay (1971) and the values of the fusion enthalpy of K3NbF8 taken from literature, the intervals of the dissociation degree values of K3NbF8 for both branches of the liquidus curve of K3NbF8 were calculated. The calculated values of the dissociation degree depend on the coordinates of the liquidus curve of K3NbF8 of the pertinent phase diagram, on its used branch and section, and on the value of the fusion enthalpy of K3NbF8. For the measured fusion enthalpy of K3NbF8 (57 kJ mol−1), a common interval of the dissociation degree values of K3NbF8 for both branches of the liquidus curve of K3NbF8 is 0.71–0.72. Similarly, intervals of the dissociation degree values of K3TaF8 for both branches of the liquidus curve of K3TaF8 were calculated using the phase diagram of the system KF-K2TaF7 determined by Boča et al. (2007) and the measured fusion enthalpy of K3TaF8 ((52 ± 2) kJ mol−1). The error of the determination of the fusion enthalpy of K3TaF8, the common interval of the dissociation degree values of K3TaF8 for both branches of the liquidus curve of K3TaF8 is 0.68–0.69.

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.

Determination of the enthalpy of fusion of K3TaO2F4 and KTaF6

Areas of fusion and crystallization peaks of K3TaO2F4 and KTaF6 were measured using the DSC mode of a high-temperature calorimeter (SETARAM 1800 K). On the basis of these quantities, considering the temperature dependence of the calorimeter sensitivity, values of the fusion enthalpy of K3TaO2F4 at the fusion temperature of 1181 K of (43 ± 4) kJ mol−1 and of KTaF6 at the fusion temperature of 760 K of (8 ± 1) kJ mol−1 were determined.

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.