K.T. Jacob

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

Abstract The coexisting phases in the pseudobinary system BaO-Y 2 O 3 have been identified by equilibrating samples containing different amounts of component oxides at 1173, 1273 and 1373 K. Only two ternary oxides, BaY 2 O 4 and Ba 3 Y 4 O 9 , have been found to be stable in the temperature range of investigation. Solid state galvanic cells: Pt, O 2 + BaO + BaF 2 ‖ BaF 2 +2 mol % Al 2 O 3 ‖ BaF 2 + BaY 2 O 4 + Y 2 O 3 + O 2 , Pt and Pt, O 2 + BaO + BaF 2 ‖ BaF 2 +2 mol % Al 2 O 3 ‖ BaF 2 + BaY 2 O 4 + Ba 3 Y 4 O 9 + O 2 , Pt have been employed for determining the Gibbs energies of formation of BaY 2 O 4 and Ba 3 Y 4 O 9 from the component oxides in the range 850 to 1250 K. A composite solid electrolyte incorporating Al 2 O 3 -dispersed BaF 2 was used in the cells. To prevent interaction between the Al 2 O 3 powder and electrode materials, the solid electrolyte was coated with pure BaF 2 . The Gibbs energies of formation of BaY 2 O 4 and Ba 3 Y 4 O 9 from component oxides are given by: Δ f 0 ( BaY 2 O 4 , s )=−128,310+5.211 T (±580) J mol −1 , (850⩽ T ⩽1250 K) and ΔG f o ( Ba 3 Y 4 O 9 , s )= −317,490 −24.704 T (±1100) J mol −1 , (850⩽ T ⩽1250 K).

A galvanic sensor for SO3/SO2 based on the CaF2/CaSO4 couple

Abstract Measurements on the solid state cell, Pt O′ 2 + SO′ 2 + SO′ 3 CaF 2 + CaSO 4 ∥CaF 2 ∥CaF 2 + CaSO 4 O″ 2 + SO″ 2 + SO″ 3 Pt using single crystal CaF 2 as the solid electrolyte and CaSO 4 as an auxiliary electrode, indicate that the EMF is in agreement with that predicted by the Nernst equation when equilibrium is assumed in the gas phase near the electrodes. The cell can be used to measure the SO 2 SO 3 content of gases at temperatures near 1200 K, where approximately 2 h ate required to obtain a steady EMF, without the use of catalysts to improve the kinetics of exchange reaction in the auxiliary electrode. For most applications, the cell EMF will be affected by the presence of water vapour in the gas phase. The cell is well suited for thermodynamic measurements on sulfates, pyrosulfates and their solid and liquid solutions.

Potentiometric determination of activities in the two-phase fields of the system Na2O(α)Al2O3

Three compounds have been found to be stable in the pseudobinary system Na2O---(α)Al2O3 between 825 and 1400 K; two nonstoichiometric phases, β-alumina and β″-alumina, and NaAlO2. The homogeneity of β-alumina ranges from 9.5 to 11 mol% Na2O, while that of β″-alumina from 13.3 to 15.9 mol% Na2O at 1173 K. The activity of Na2O in the two-phase fields has been determined by a solid-state potentiometric technique. Since both β- and β″-alumina are fast sodium ion conductors, biphasic solid electrolyte tubes were used in these electrochemical measurements. The open circuit emf of the following cells were measured from 790 to 980 K: n[GRAPHICS] The partial molar Gibbs energy of Na2O relative to gamma-Na2O in the two-phase regions can be represented as: DELTA-GBAR(Na2O)(alpha- + beta-alumina) = -270,900 + 24.03 T, DELTA-GBAR(Na2O)(beta- + beta-alumina) = -232,700 + 56.19 T, and DELTA-GBAR(Na2O)(beta-alumina + NaAlO2) = -13,100 - 4.51 T J mol-1. Similar galvanic cells using a Au-Na alloy and a mixture of Co + CoAl(2+2x)O4+3x + (alpha)Al2O3 as electrodes were used at 1400 K. Thermodynamic data obtained in these studies are used to evaluate phase relations and partial pressure of sodium in the Na2O-(alpha) Al2O3 system as a function of oxygen partial pressure, composition and temperature.

Thermodynamic Data for Mn3O4, Mn2O3 and MnO2

Thermodynamic properties of Mn3O4, Mn2O3 and MnO2 are reassessed based on new measurements and selected data from the literature. Data for these oxides are available in most thermodynamics compilations based on older calorimetric measurements on heat capacity and enthalpy of formation, and high-temperature decomposition studies. The older heat capacity measurements did not extend below 50 K. Recent measurements have extended the low temperature limit to 5 K. A reassessment of thermodynamic data was therefore undertaken, supplemented by new measurements on high temperature heat capacity of Mn3O4 and oxygen chemical potential for the oxidation of MnO1-x, Mn3O4, and Mn2O3 to their respective higher oxides using an advanced version of solid-state electrochemical cell incorporating a buffer electrode. Because of the high accuracy now achievable with solid-state electrochemical cells, phase-equilibrium calorimetry involving the ``third-law analysis has emerged as a competing tool to solution and combustion calorimetry for determining the standard enthalpy of formation at 298.15 K. The refined thermodynamic data for the oxides are presented in tabular form at regular intervals of temperature.

Combined homo-hetero doping for enhancement of ionic conductivity

Abstract Electrical conductivity of Na 0.005 Ca 0.995 F 1.995 and Y 0.008 Ca 0.992 F 2.008 samples containing 2 mol% CeO 2− x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaF 2 homogeneously doped with NaF. However, heterogeneous doping with CeO 2− x decreased the conductivity of YF 3 -doped CaF 2 . This suggests that preferential adsorption of F − ions to the CeO 2− x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaF 2 heterogeneously doped with CeO 2− x . By the synergetic effect of homogeneous and heterogeneous doping, the conductivity of polycrystalline CaF 2 can be enhanced by 4.5 orders of magnitude at 630 K.

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.

EMF of bielectrolyte solid state cells with different mobile ions in each phase

Abstract An expression for the emf of an isothermal solid state bielectrolyte cell is developed starting from the basic equations of transport for each solid electrolyte. If the two different migrating species react to form a compound at well defined activity at the interface between the two solid electrolytes, interfacial chemical potentials and the emf are unambiguously defined. The interfacial chemical potentials are determined by the transport properties of the two solid electrolytes. The emf of the bielectrolyte cell will be less than that calculated from the Gibbs energy change for the virtual cell reaction when the interfacial or electrode chemical potentials lie outside the electrolytic conduction domain ( t ion >0.99) of the solid electrolytes. Using estimated values for electonic and hole conductivities in Na β-alumina, the emf of a bielectrolyte combination consisting of β-alumina and (CaO)ZrO 2 is computed for different chemical potentials of sodium and oxygen at the electrodes. The chemical potential gradient for sodium across β-alumina is found to be very small because the conductivity of β-alumina is much larger than that of (CaO)ZrO 2 . When two cationic or anionic conductors with different mobile species are coupled together, the chemical potentials at junction of the solid electrolytes and hence the emf are usually ill defined. A definite relationship between ionic fluxes in each electrolyte is necessary to mathematically define the interfacial chemical potentials.

Theoretical estimation of the effect of minor elements on the solubility of oxygen in silicon melt

The effect of fourteen minor elements (Al, As, B, Bi, C, Ga, Ge, In, N, P, Pb, S, Sb and Sn) on the solubility of oxygen in silicon melt has been estimated using a recently developed theoretical equation, with only fundamental physical parameters such as hard sphere diameter, atomic volume and molar heat of solution at infinite dilution as inputs. The results are expressed in the form of interaction parameters. Although only limited experimental data are available for comparison, the theoretical approach appears to predict the correct sign, but underestimates the magnitude of the interaction between oxygen and alloying elements. The present theoretical approach is useful in making qualitative predications on the effect of minor elements on the solubility of oxygen in silicon melt, when direct measurements are not available.

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.

The standard molar Gibbs energy of formation of YbPt3 and LuPt3 intermetallics

Abstract The standard molar Gibbs energies of formation of YbPt3 and LuPt3 intermetallic compounds have been measured in the temperature range 880 K to 1100 K using the solid-state cells: Ta|Yb+YbF 2 |CaF 2 |YbPt 3 +Pt+YbF 2 |Ta, and Ta|Lu+LuF 3 |CaF 2 |LuPt 3 +Pt+LuF 3 |Ta, The trifluoride of Yb is not stable in equilibrium with Yb or YbPt3. The results can be expressed by the equations: Δ f G m ° ( YbPt 3 ) (J· mol −1 ) =−322100+0.39( T K )±400, Δ f G m ° ( LuPt 3 ) (J· mol −1 ) =−366800+3.82( T K )±400. The standard molar Gibbs energy of formation of LuPt3 is −41.1 kJ · mol−1 more negative than that for YbPt3 at 1000 K. Ytterbium is divalent in the pure metal and trivalent in the intermetallic YbPt3. The energy required for the promotion of divalent Yb to the trivalent state is responsible for the less negative ΔfGmo of YbPt3. The enthalpies of formation of the two intermetallics are in reasonable agreement with Miedemas model. Because of the extraordinary stability of these compounds it is possible to reduce oxides of Yb and Lu with hydrogen in the presence of platinum at T K > 1473 . The equilibrium chemical potential of oxygen corresponding to the reduction of Yb2O3 and Lu2O3 by hydrogen in the presence of platinum is presented in the form of an Ellingham diagram.