Chemical potentials of oxygen for fayalite-quartz-lron and fayalite-quartz-magnetite equilibria

The oxygen potentials corresponding to fayalite-quartz-iron (FQI) and fayalite-quartz-magnetite (FQM) equilibria have been determined using solid-state galvanic cells: Pt,Fe + Fe2SiO4 + SiO2/(Y2O3)ZrO2/Fe + \r"FeO,\l"Pt and Pt, Fe3O4 + Fe2SiO4 + SiO2/(Y2O3)ZrO2/Ni + NiO, Pt in the temperature ranges 900 to 1400 K and 1080 to 1340 K, respectively. The cells are written such that the right-hand electrodes are positive. Silica used in this study had the quartz structure. The emf of both cells was found to be reversible and to vary linearly with temperature. From the emf, Gibbs energy changes were deduced for the reactions: 0.106Fe (s) + 2Fe0.947O (r.s.) + SiO2 (qz) → Fe2SiO4 (ol) δG‡= -39,140+ 15.59T(± 150) J mol-1 and 3Fe2SiO4 (ol) + O2 (g) → 2Fe3O4 (sp) + 3SiO2 (qz) δG‡ = -471,750 + 160.06 T±} 1100) J mol-1 The “third-law≓ analysis of fayalite-quartz-wustite and fayalite-quartz-magnetite equilibria gives value for δH‡298 as -35.22 (±0.1) and -528.10 (±0.1) kJ mol-1, respectively, independent of temperature. The Gibbs energy of formation of the spinel form of Fe2SiO4 is derived by com-bining the present results on FQI equilibrium with the high-pressure data on olivine to spinel transformation of Fe2SiO4.

Thermodynamics of GdMnO3 and GdMn2O5 phases determined by the EMF method

Using solid oxide galvanic cells of the type: MnO + Gd2O3 + GdMnO3/O-2/Ni + NiO and Mn3O4 + GdMnO3 + GdMn2O5/O-2/air the equilibrium oxygen pressure for the following reactions :MnO + 1/2Gd(2)O(3) + 1/4O(2) = GdMnO3 1/3Mn(3)O(4) + GdMnO3 + 1/3O(2) = GdMn2O5 was determined in the temperature range from 1073 to 1450 K. From the determined equilibrium oxygen partial pressure the corresponding G i b b s free energy change for these reactions was derived: Delta G(f,GdMnO3)(0) (+/- 425J) 132721(+/ - 2240) +51.91(+/ - 0.81)T Delta G(f,GdMn2O5)(0)(+/- 670J) 121858(+/ - 6176) + 79.52(+/ - 4.83)T From these data, standard G i b b s energies, enthalpies and entropies of formation of GdMnO3 and GdMn2O5 from component oxides and from the elements are derived. Thermodynamic data tables for the two ternary phases are compiled from 298.15 to 1400 K.

Activities in liquid Ga-Te alloys at 1120 k

The activity of gallium in liquid Ga-Te alloys has been measured at 1120 K using a solid state galvanic cell incorporating yttria-stabilized thoria as the solid electrolyte. The cell can be schematically represented as (−) W,Re,Ga(1)+Ga2O3(s)|(Y2O3) ThO2|Ga-Te(1) + Ga2O3(s), Re, W (+) The activity of tellurium was derived by Gibbs-Duhem integration. The activity of gallium shows negative deviation from Raoult's law for XGa < 0.6 and positive deviation from ideality for XGa > 0.6. The activity of gallium was constant in the composition range 0.73 < XGa < 0.89, indicating liquid state immiscibility in this region. The Gibbs energy of mixing and the concentration-concentration structure factor at long wavelength limit show a minimum at XGa ≈ 0.4, suggesting strong interactions in the liquid phase with formation of ‘Ga2Te3‘-type complexes

Refinement of the thermodynamic properties of ruthenium dioxide and osmium dioxide

The standard Gibbs energies of formation of RuO2 and OsO2 at high temperature have been determined with high precision, using a novel apparatus that incorporates a buffer electrode between the reference and working electrodes, The buffer electrode absorbs the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential, The buffer electrode prevents polarization of the measuring electrode and ensures accurate data, The standard Gibbs energies of formation (Delta(f)G degrees) of RuO2, in the temperature range of 900-1500 K, and OsO2, in the range of 900-1200 K, can be represented by the equations Delta(f)G degrees(RuO2)(J/mol) = -324 720 + 354.21T - 23.490T In T Delta(f)G degrees(OsO2)(J/mol) = -304 740 + 318.80T - 18.444T In T where the temperature T is given in Kelvin and the deviation of the measurement is +/- 80 J/mol, The high-temperature heat ;capacities of RuO2 and OsO2 are measured using differential scanning calorimetry. The information for both the low- and high-temperature heat rapacity of RuO2 is coupled with the Delta(f)G degrees data obtained in this study to evaluate the standard enthalpy of formation of RuO2 at 298.15 K (Delta(f)H degrees(298.15K)). The low-temperature heat capacity of OsO2 has not been measured: therefore, the standard enthalpy and entropy of formation of OsO2 at 298.15 K (Delta(f)H degrees(298.15K) and S degrees(298.15K), respectively) are derived simultaneously through an optimization procedure from the high-temperature heat capacity and the Gibbs energy of formation. Both Delta fH degrees(298.15K) and S degrees(298.15K) are treated as variables in the optimization routine, For RuO2, the standard enthalpy of formation at 298.15 K is Delta fH degrees(298.15K) (RuO2) -313.52 +/- 0.08 kJ/mol, and that for OsO2 is Delta(f)H degrees(298.15K) (OSO2) = -295.96 +/- 0.08 kJ/mol. The standard entropy of OsO2 at 298.15 K that has been obtained from the optimization is given as S degrees(298.15K) (OsO2) = 49.8 +/- 0.2 J (mol K)(-1).

Phase relations and thermodynamic properties of compounds in the pseudobinary system BaO-Y2O3

The coexisting phases in the pseudobinary system BaO-Y2O3 have been identified by equilibrating samples containing different amounts of component oxides at 1173, 1273 and 1373 K. Only two ternary oxides, BaY2O4 and Ba3Y4O9, have been found to be stable in the temperature range of investigation. Solid state galvanic cells: Pt, O2+BaO+BaF2double vertical barBaF2+2mol%Al2O3double vertical barBaF2+BaY2O4+Y2O3+O2, Pt and Pt, O2+BaO+BaF2double vertical barBaF2+2mol% Al2O3double vertical barBaF2+BaY2O4+Ba3Y4O9+O2, Pt have been employed for determining the Gibbs' energies of formation of BaY2O4 and Ba3Y4O9 from the component oxides in the range 850 to 1250 K. A composite solid electrolyte incorporating Al2O3-dispersed BaF2 was used in the cells. To prevent interaction between the Al2O3 powder and electrode materials, the solid electrolyte was coated with pure BaF2. The Gibbs' energies of formation of BaY2O4 and Ba3Y4O9 from component oxides are given by: Δf0 (BaY2O4, s)=−128,310+5.211T (±580) J mol−1, (850less-than-or-equals, slantTless-than-or-equals, slant1250 K) and ΔGfo(Ba3Y4O9, s)= −317,490 −24.704T (±1100) J mol−1, (850less-than-or-equals, slantTless-than-or-equals, slant1250 K).

Photoacoustic investigation of the optical absorption and thermal diffusivity in SixTe100−x glassesstar

The photoacoustic technique is used to determine the optical energy gap E0 of bulk SixTe100−x glasses in the glass-forming region 10 ≤ x ≤ 28. The thermal diffusivity α of these samples has also been measured. The variation of E0 and α with x is reported. It is found that E0 increases with x nearly linearly with a sharp decrease in the rate of increase beyond x = 20. The thermal diffusivity also increases with x up to x = 20 but decreases for compositions with higher values of x. The observed behaviour is explained on the basis of a chemical bond approach. It is accounted for in terms of the increase in the number of Te---Te bonds and formation of SiTe4 tetrahedra with an increase in the chalcogen content.

Studies on nonisothermal solid state galvanic cells — effect of gradients on EMF

An expression for the EMF of a nonisothermal galvanic cell, with gradients in both temperature and chemical potential across a solid electrolyte, is derived based on the phenomenological equations of irreversible thermodynamics. The EMF of the nonisothermal cell can be written as a sum of the contributions from the chemical potential gradient and the EMF of a thermocell operating in the same temperature gradient but at unit activity of the neutral form of the migrating species. The validity of the derived equation is confirmed experimentally by imposing nonlinear gradients of temperature and chemical potential across galvanic cells constructed using fully stabilized zirconia as the electrolyte. The nature of the gradient has no effect on the EMF.

A modified least squares algorithm to treat associations in liquid alloys

The associated model for binary systems has been modified to include volume effects and excess entropy arising from preferential interactions between the associate and the free atoms or between the free atoms. Equations for thermodynamic mixing functions have been derived. An optimization procedure using a modified conjugate gradient method has been used to evaluate the enthalpy and entropy interaction energies, the free energy of dissociation of the complex, its temperature dependance and the size of the associate. An expression for the concentration—concentration structure factor [Scc (0)] has been deduced from the modified associated solution model. The analysis has been applied to the thermodynamic mixing functions of liquid Ga-Te alloys at 1120 K, believed to contain Ga2Te3 associates. It is observed that the modified associated solution model incorporating volume effects and terms for the temperature dependance of interaction energies, describes the thermodynamic properties of Ga-Te system satisfactorily.

Design of temperature-compensated reference electrodes for non-isothermal galvanic sensors

The criterion for the design of a temperature-compensated reference electrode for non-isothermal galvanic sensors is deduced from the basic flux equations of irreversible thermodynamics. It is shown that when the Seebeck coefficient of the non-isothermal cell using a solid oxygen ion-conducting electrolyte under pure oxygen is equal to the relative partial molar entropy of oxygen in the reference electrode divided by 4F, then the EMF of the non-isothermal cell is the same as that of an isothermal cell with the same electrodes operating at the higher temperature. By measuring the temperature of the melt alone and the EMF of the non-isothermal galvanic sensor, one can derive the chemical potential or the concentration of oxygen in a corrosive medium. The theory is experimentally checked using sensors for oxygen in liquid copper constructed with various metal+oxide electrodes and fully stabilised (CaO)ZrO2 as the electrolyte. To satisfy the exact condition for temperature compensation it is often necessary to have the metal or oxide as a solid solution in the reference electrode.

Stability constraints in the design of galvanic cells using composite electrolytes and auxiliary electrodes

Recent trends in the use of dispersed solid electrolytes and auxiliary electrodes in galvanic cells have increased the need for assessment of materials compatibility. In the design of dispersed solid electrolytes, the potential reactions between the dispersoid and the matrix must be considered. In galvanic cells, possible interactions between the dispersoid and the electrode materials must also be considered in addition to ion exchange between the matrix and the electrode. When auxiliary electrodes, which convert the chemical potential of a component present at the electrode into an equivalent chemical potential of the neutral form of the migrating species in the solid electrolyte are employed, displacement reactions between phases in contact may limit the range of applicability of the cell. Examples of such constraints in the use of oxide dispersoids in fluoride solid electrolytes and NASICON/Na2S couple for measurement of sulphur potential are illustrated with the aid of Ellingham and stability field diagrams.

Experimental and Computational Characterization of Alloy--Spinel--Corundum Equilibrium in the System Fe--Co--Al--O at 1873K

The three-phase equilibrium between alloy, spinel solid solution and alpha -Al sub 2 O sub 3 in the Fe--Co--Al--O system at 1873k was fully characterized as a function of alloy composition using both experimental and computational methods. The equilibrium oxygen content of the liquid alloy was measured by suction sampling and inert gas fusion analysis. The O potential corresponding to the three-phase equilibrium was determined by emf measurements on a solid state galvanic cell incorporating (Y sub 2 O sub 3 )ThO sub 2 as the solid electrolyte and Cr + Cr sub 2 O sub 3 as the reference electrode. The equilibrium composition of the spinel phase formed at the interface between the alloy and alumina crucible was measured by electron probe microanalysis (EPMA). The experimental results were compared with the values computed using a thermodynamic model. The model used values for standard Gibbs energies of formation of pure end-member spinels and Gibbs energies of solution of gaseous O in liquid Fe and cobalt available in the literature. The activity--composition relationship in the spinel solid solution was computed using a cation distribution model. The variation of the activity coefficient of O with alloy composition in the Fe--Co--O system was estimated using both the quasichemical model of Jacob and Alcock and Wagner's model along with the correlations of Chiang and Chang and Kuo and Chang. The computed results of spinel composition and O potential are in excellent agreement with the experimental data. Graphs. 29 ref.--AA

Novel phase-transition behavior in an aqueous electrolyte solution

We have investigated the near-critical behavior of the susceptibility of a ternary liquid mixture of 3-methylpyridine. water, and sodium bromide as a function of the salt concentration. The susceptibility was determined from light-scattering measurements performed at a scattering angle of 90 degrees in the one-phase region near the locus of lower consolute points. A sharp crossover from asymptotic Ising behavior to mean-field behavior has been observed at concentrations ranging from 8 to 16.5 mass% NaBr. The range of asymptotic Ising behavior shrinks with increasing salt concentration and vanishes at a NaBr concentration of about 17 mass%. where complete mean-field-like behavior of the susceptibility is observed. A simultaneous pronounced increase in the background scattering at concentrations above 15 mass%, as well as a dip in the critical locus at 17 mass % NaBr, suggests that this phenomenon can be interpreted as mean-field tricritical behavior associated with the formation of a microheterogeneous phase due to clustering of the molecules and ions. An analogy with tri critical behavior observed in polymer solutions as well as the possibility of a charge-density-wave phase is also discussed. In addition, we, have observed a third soap-like phase an the liquid-liquid interface in several binary and ternary liquid mixtures.

Extension of Darken's Quadratic Formalism to Dilute Multicomponent Solutions

Darken's quadratic formalism is extended to multicomponent solutions. Equations are developed for the representation of the integral and partial excess free energies, entropies and enthalpies in dilute multicomponent solutions. Quadratic formalism applied to multicomponent solutions is thermodynamically consistent. The formalism is compared with the conventional second order Maclaurin series or interaction parameter representation and the relations between them are derived. Advantages of the quadratic formalism are discussed.

properties of Na2O in Nasicon solid solution, Na1+xZr2SixP3-xO12

The thermodynamic activity of sodium oxide (Na2O) in the Nasicon solid solution series, Na1+xZr2SixO12, has been measured in the temperature range 700–1100 K using solid state galvanic cells: Pt|CO2 + O2|Na2CO3?Na1+xZr2SixP3-xO12?(Y2O3)ZrO2?In + In2O3|Ta, Pt for 1 = ? = 2.5, and Pt?CO2 + O2?Na2CO3?ß-alumina?Na1+xZr2SixP3-xO12?Ar + O2?Pt for x = 0, 0.5, 2.5, and 3. The former cell, where the Nasicon solid solution is used as an electrolyte along with yttria-stabilized zirconia, is well suited for Nasicon compositions with high ionic conductivity. In the latter cell, ß-alumina is used as an electrolyte and the Nasicon solid solution forms an electrode. The chemical potential of Na2O is found to increase monotonically with x at constant temperature. The partial entropy of Na2O decreases continuously with x. However, the partial enthalpy exhibits a maximum at x = 2. This suggests that the binding energy is minimum at the composition where ionic conductivity and cell volume have maximum values.