CaF2 Solid Electrolyte Research Articles

Experimental Thermodynamics of High Temperature Transformations in Single-Walled Carbon Nanotube Bundles

The thermodynamic quantities associated to the transformation of carbon in single-walled carbon nanotube (SWCNT) bundles to carbon in graphite were determined from 750 to 1015 K by a CaF2 solid electrolyte galvanic cell: (-) Mo | Cr3C2,CrF2,C'' | CaF2s.c. | Cr3C2,CrF2,C' | Mo (+). The trend with temperature of the electromotive force of the cell was found to be greatly dependent on temperature and fully reversible with it. The standard enthalpy DeltaH(T)(o) and entropy DeltaS(T)(o) changes are 7.1, 6.0, and 60.2 kJ mol(-1) and 8.6, 14.7, and 72.8 J K(-1) mol(-1) at 778, 883, and 975 K, respectively. This most likely correlates with the different arrangements and shapes of tubes that deviate from the ideal triangular closed packed structure of the SWCNT bundles. The constraints to the thermal expansion of bundles in the electrode containing them generated high internal pressures that were responsible for deformations of tube shape and lattice. Stable bundle states were formed that interconvert as a function of temperature. Comparative analyses by low angle XRD, microRaman, and HR-TEM of SWCNT bundles before and after experiments support this scenario. The cohesion energy and associated entropy changes are also reported for such states. The formation enthalpy of unbundled SWCNTs was calculated equal to 9.5 +/- 0.4 kJ mol(-1).

Discussion of “thermodynamics of liquid Al-Na alloys determined by using CaF2 solid electrolyte”

Discussion of “thermodynamics of liquid Al-Na alloys determined by using CaF2 solid electrolyte”

Thermodynamics of liquid Al-Na alloys determined by using CaF2 solid electrolyte

The purpose of this work has been to establish activity data on sodium in liquid aluminum-sodium alloys at temperatures applied by the industry in liquid metal refining processes. A coulometric titration technique using a galvanic cell employing CaF2 as a solid electrolyte has enabled measurements to be done under very clean and well-defined conditions over the entire range of compositions from highly diluted up to nearly sodium-saturated solutions. Sodium in liquid aluminum of 99.9999 pct purity is found to exhibit strong negative deviation from Henry’s law, corresponding to a large negative self-interaction coefficient ɛ Na Na as expressed by the equation ɛ Na Na =16,318−(191.1·105 K)·T −1. This behavior is normal for elements, which exhibit strong positive deviation from Raoult’s law and is explained by formation of Na clusters. The activity coefficient at infinite dilution, γ Na o , is expressed by the equation: RT ln γ Na o =86,729−26.237T. The magnitude of γ Na o from this equation agrees with the value predicted from the Miedema’s semiempirical model. Sodium in liquid Al-Si5 pct alloy of 99.9999 pct purity exhibits strong positive deviation from Henry’s law, which is in agreement with earlier investigations of the activity of sodium in liquid Al-Si alloys. The activity coefficient of sodium in pure liquid aluminum at saturation, γ Na sat , is expressed by RT ln γ Na sat =−67,476+102.33T, which gives for the sodium concentration at saturation x Na sat =exp(8115.5/T−12.307). This implies that the solubility of sodium in liquid aluminum at temperatures around the melting point of aluminum is about 10 times higher than previously reported and decreases rapidly with increasing temperature, possibly due to a decreasing stability of Na clusters. Analysis of the experimental conditions used by previous investigators supports these findings.

Discussion of “use of solid electrolyte galvanic cells to determine the activity of CaO in the CaO-ZrO2 system and standard gibbs free energies of formation of CaZrO3 from CaO and ZrO2”

In a recent article, Tanabe and Nagata[1] have used Ca4P2O9 (4CaOzP2O5) as a solid electrolyte in a cell designed to measure the activity of CaO in the system CaOZrO 2 between 1572 and 1877 K. The article is interesting, not only because of new data obtained on an important system, but also because the authors have tried to extend the current experimental capabilities for measuring the activity of CaO to higher temperatures than is possible with the well-established CaF2 solid electrolyte.[2,3,4] At temperatures above 1300 K, CaF2 begins to soften and it is difficult to use the material in solid-state cells. The only other solid electrolyte suitable for measuring the activity of CaO at higher temperatures is Ca b-alumina. The b-alumina phase is metastable in the CaO-Al2O3 system. It is stabilized by the addition of MgO, with the MgO/CaO molar ratio varying from 0.7 to 0.8. However, calcium b-alumina is not in equilibrium with CaO. Hence, pure CaO cannot be used as a reference electrode, despite claims to the contrary by Kumar and Kay[5] and Rog and Kozlowska-Rog.[6] Several intermediate aluminates form at the interface between the electrolyte and the CaO reference electrode.[7] Thus, the development of new solid electrolytes for measuring the activity of CaO at T . 1300 K is important for the development of the theoretical basis of extractive metallurgy and high-temperature ceramic processing. Earlier work by Tanabe et al.[8] had established that Ca4P2O9 is an ionic conductor with transference number of Ca2+ ion close to unity in the temperature range 1373 to 1873 K and oxygen partial pressure range from 105 to 1025 Pa.

Standard Gibbs free energy of formation of the high-Tc superconducting compound YBa2Cu3O6.5+δ

Cells were constructed with a single-crystal CaF2 solid electrolyte and the cell EMF was measured. The standard change in free energy of formation of BaCuO2 (011 phase) and YBa2Cu3O6.5+δ (123 phase) from the constituent oxides was estimated. The oxygen non-stoichiometric fraction, δ, was taken into account in the estimation of ΔG123+δ0.

Measurement of Gibbs energies of formation of CoF2 and MnF2 using a new composite dispersed solid electrolyte

Gibbs energies of formation of CoF2 and MnF2 have been measured in the temperature range from 700 to 1100 K using Al2O3-dispersed CaF2 solid electrolyte and Ni+NiF2 as the reference electrode. The dispersed solid electrolyte has higher conductivity than pure CaF2 thus permitting accurate measurements at lower temperatures. However, to prevent reaction between Al2O3 in the solid electrolyte and NiF2 (or CoF2) at the electrode, the dispersed solid electrolyte was coated with pure CaF2, thus creating a composite structure. The free energies of formation of CoF2 and MnF2 are (± 1700) J mol−1; {fx37-1} The third law analysis gives the enthalpy of formation of solid CoF2 as ΔH° (298·15 K) = −672·69 (± 0·1) kJ mol−1, which compares with a value of −671·5 (± 4) kJ mol−1 given in Janaf tables. For solid MnF2, ΔH°(298·15 K) = − 854·97 (± 0·13) kJ mol−1, which is significantly different from a value of −803·3 kJ mol−1 given in the compilation by Barinet al.

Thermodynamics of formation of Y-Ni alloys

The Gibbs free energies, enthalpies, and entropies of phase formation were determined for nine Y-Ni intermediate phases from electromotive force measurements. With CaF2 solid electrolyte, emf measurements were made over the temperature range 900 to 1225 K. The Gibbs energies of formation per mole of Y reactant were found to be nearly constant for the four Y-poor phases; this is in keeping with the close structural relationship among these four phases. The Gibbs energies of formation of the Y-Ni phases at 973 K have been compared with those of the Th-Fe, Th-Co, Th-Ni, La-Co, La-Ni, Y-Fe, and Y-Co systems, and an increasing bond strength with increasing number of bonding electrons is evident. Comparison of the experimental values for the enthalpies of formation of the Y-Ni phases has been made with values predicted by the Miedema model and by the Watson-Bennett model. Both predictions yield values that are within ±50 pet of the experimental values which is the range of scatter that is normally expected from these simple models.

Thermodynamic Properties of Liquid Non‐Metallic NaNaBr Solutions

AbstractThe thermodynamic properties of molten NaNaBr mixtures were determined at 800°C for concentrations of Na up to 5 atom‐% using an emf technique with CaF2 solid electrolyte. Concentrations were controlled by the method of coulometric titration. The activity coefficient is found to pass through a maximum at 0.5 atom‐% Na. An atomic model is presented to explain the thermodynamic results of these solutions. It predicts the concentration dependence of the number of F‐centers as well as the number of F‐center associates, e. g., dimers. Calculations using the model also explain results from spectroscopic, magnetic, and electrical conductivity measurements quantitatively.

Thermodynamics of Liquid Na — Sn Alloys Using CaF2 Solid Electrolytes

AbstractThe thermodynamic properties of mixing for liquid Na – Sn alloys were determined between 750°C and 850°C by emf measurements using a CaF2 solid electrolyte. Concentrations were controlled by coulometric titration. Results are presented for partial and integral Gibbs energies, entropies, and heats of mixing across the entire composition range.

Thermodynamic Properties of Pd‐X‐Alloys, with X = Gd, Y, Ce

AbstractElectromotive force measurements on cells with a CaF2 solid electrolyte were used to determine the activities of Gd, Y, and Ce in Pd‐rich alloys between 700°C and 800°C. The thermodynamic properties of these alloys are characterized by extreme negative deviations from ideal mixing behavior, up to −400 kJ/mol for the partial molar excess Gibbs energy of Ce and up to −320 kJ/mol for Gd and Y. The high thermodynamic stability of these alloys is attributed to a transfer of valence electrons to the electron gas of the alloy. At infinite dilution the resulting bonding contribution to the partial excess free energy is determined by the difference of the Fermi levels of the components and the valency of the solute. Including the size difference of the components as an additional effect, the mixing behavior of the alloys can be well described. – Within the homogeneity ranges of the intermetallic phases Pd3Y and Pd3Gd the activities of the unnoble components increase by more than ten orders or magnitude.

Über die extrem hohe thermodynamische Stabilität von Pd-Th-Legierungen / On the Extremely High Thermodynamic Stability of Pd-Th Alloys

The thermodynamic properties of Pd-Th solid solutions containing up to 16 at.% Th have been determined by the e.m.f. method between 700 and 800 °C with CaF2 solid electrolyte cells. The mixing behaviour of the alloys is characterized by extreme negative deviations from ideality, up to -410 kJ/mol for the partial molar excess free energy of Thorium. The extraordinary stability is attributed to electronic interactions brought about by the transfer of the Thorium valence electrons to the electron gas of the alloy. At vanishing Thorium concentration the resulting bonding contribution to the thermodynamic excess quantities is determined by the difference of the Fermi energies of the pure components, approximately equal to the difference of the work functions. Including the lattice distortion as a small additional effect, the mixing behaviour of these and other palladium alloys can be well described