Gibbs energies of formation of rare earth MPt5 compounds

The Gibbs energies of formation of MPt5 (MNd, Dy, Ho, Er) intermetallic compounds were determined in the temperature range 900–1100 K using the solid state cell Ta,M+MF3¦CaF2¦MPt5+Pt+MF3,Ta For M ≡ Sm, a mixture of Gd + GdF3 was used as the reference electrode. In the case of Eu, a mixture of Eu + EuF2 served as the reference electrode. The trifluorides of Sm and Eu are not stable in equilibrium with the metal. The fluoride phase coexisting with a SmPt5 + Pt mixture is SmF3, whereas EuF2 is the equilibrium phase in contact with EuPt5 + Pt. All the MPt5 compounds studied (except EuPt5) exhibit similar stability. Europium is divalent in the pure metal and trivalent in EuPt5. The energy required for the promotion of divalent Eu to the trivalent state accounts for the less negative Gibbs energy of formation of EuPt5. The enthalpies of formation of all the MPt5 compounds obtained in this study are in good agreement with Miedema's model.

Thermodynamic properties of Pt5La, Pt5Ce, Pt5Pr, Pt5Tb and Pt5Tm intermetallics

The Gibbs’ energies of formation of Pt5La, Pt5Ce, Pt5Pr, Pt5Tb and Pt5 Tm intermetallic compounds have been determined in the temperature range 870–1100 K using the solid state cell:Ta,M + MF3 /CaF2 /Pt5 M + Pt + MF3 ,TaTaM+MF3CaF2Pt5M+Pt+MF3Ta.The reversible emf of the cell is directly related to the Gibbs’ energy of formation of the Pt5M compound. The results can be summarized by the equations:DGf° á Pt5 La ñ = - 373,150 + 6 ·60 T( ±300 )J mol - 1 DGf° á Pt5 Ce ñ = - 367,070 + 5 ·79 T( ±300 )J mol - 1 DGf° á Pt5 Pr ñ = - 370,540 + 4 ·69 T( ±300 )J mol - 1 DGf° á Pt5 Tb ñ = - 372,280 + 4 ·11 T( ±300 )J mol - 1 DGf° á Pt5 Tm ñ = - 368,230 + 4 ·89 T( ±300 )J mol - 1 Unknown control sequence '\hfill'relative to the low temperature allotropic form of the lanthanide element and solid platinum as standard states The enthalpies of formation of all the Pt5M intermetallic compounds obtained in this study are in good agreement with Miedema’s model. The experimental values are more negative than those calculated using the model. The variation of the thermodynamic properties of Pt5M compounds with atomic number of the lanthanide element is discussed in relation to valence state and molar volume.

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

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:View the MathML source and View the MathML source The trifluoride of Yb is not stable in equilibrium with Yb or YbPt3. The results can be expressed by the equations: View the MathML source View the MathML source 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 Miedema's 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 View the MathML source. 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.

Measurement of the Gibbs energy of formation of URh3 using an oxide solid electrolyte

The Gibbs' energy offormation of the intermetallic compound URh3has been measured in the temperature range 980 to 1320 K using an oxide solid state cell incorporating yttria-doped thoria as the solid electrolyte and a mixture of manganese and manganese oxide as the reference electrode. The cell can be represented as Pt, Mn + MnO I (Y203)Th02 I Rh + URh3 + U02 + x' Rh, Pt The reversible emf of the cell was a linear function of temperature E = 15.60 +0.0237 T (±0.8) mY. Using auxiliary thermodynamic data for MnO and U02+ x the Gibbs' energy of formation of URh3 from component metals has been computed. The results can be expressed by the equation L'.G?< URh3 > = -316240 + 13.22 T (± 3000) J mol-1. The "third-law" enthalpy of formation of URh3at 298 K is -293.2 (± 4) kJ mol-1, significantly more negative than the value of -181.5 kJ mol-1 calculated using Miedema's model.

Low oxygen potential boundary for the stability of YBA2Cu3O7−δ

On lowering the oxygen potential, the tetragonal phase of YBa2Cu3O7−δ was found to decompose into a mixture of Y2BaCuO5, BaCuO2 and BaCu2O2 in the temperature range 773–1173 K. The 123 compound was contained in a closed crucible of yttria-stabilized zirconia in the temperature range 773–1073 K. Oxygen was removed in small increments by coulometric titration through the solid electrolyte crucible at constant temperature. The oxygen potential was calculated from the open circuit e.m.f. of the solid state cell after successive titrations. Pure oxygen at a pressure of 1.01 × 105 Pa was used as the reference electrode. The decomposition of the 123 compound manifested as a plateau in oxygen potential. The decomposition products were identified by X-ray diffraction. At temperatures above 1073 K there was some evidence of reaction between the 123 compound, solid electrolyte crucible and platinum. For measurements above 1073 K, the 123 compound was contained in a magnesia crucible placed in a closed outer silica tube. The oxygen potential in the gas phase above the 123 compound was controlled and measured by a solid state cell based on yttria-stabilized zirconia which served both as a pump and sensor. The lower oxygen potential limit for the stability of the 123 compound is given by View the MathML source The oxygen non-stoichiometric parameter δ for the 123 compound has a value of 0.98 (View the MathML source) at dissociation.

Phase equilibria in the Co Ni F system and activities in (CoxNi1 x)F2 solid solutions at 1373 K

The tie-lines delineating equilibria between CoF2-NiF2 and Co-Ni solid solutions in the ternary Co-Ni-F system at 1373 K have been determined by electron microprobe and EDAX point count analysis of the equilibrated phases. Activities in the fluoride solid solution have been derived from the knowledge of activitycomposition relation in the metallic solid solution and tie-line data,using a modified form of the Gibbs-Duhem integration. The fluorine potentials corresponding to the tie-line compositions have been calculated.The excess Gibbs' energy of mixing for the fluoride solid solution derived from the present data can be represented by the expression

Physical chemistry of the reduction of calcium oxide with aluminium in vacuum

The physical chemistry of "aluminothermic" reduction of calcium oxide in vacuum is analyzed. Basic thermodynamic data required for the analysis have been generated by a variety of experiments. These include activity measurements in liquid AI-Ca alloys and determination of the Gibbs energies of formation of calcium aluminates. These data have been correlated with phase relations in the Ca-AI-0 system at 1373 K. The various stages of reduction, the end products and the corresponding equilibrium partial pressures of calcium have been established from thermodynamic considerations. In principle, the recovery of calcium can be improved by reducing the pressure in the reactor. However,, the cost of a high vacuum system and the enhanced time for reduction needed to achieve higher yields makes such a practice uneconomic. Aluminum contamination of calcium also increases at low pressures. The best compromise is to carry the reduction up to the stage where 3CaO-Al,O, is formed as the product. This corresponds to an equilibrium calcium partial pressure of 31.3 Pa at 1373 K and 91.6 Pa at 1460 K. Calcium can be extracted at this pressure using mechanical pumps in approximately 8 to 15 hr, depending on the size and the fill ratio of the retort and porosity of the charge briquettes.

Zircon Sand--a Viable Alternative Moulding System for Titanium Castings

The experimental observations of casting titanium in sodium silicate bonded zircon sand mould are presented in this paper. Metal-mould reactions, in general, involved dissolution of oxides in liquid titanium resulting in contamination of the casting. Minimal metal-mould reactions occurred when titanium was cast in zircon sand mould containing about 7.5 wt% of ZrO2. It has been further shown that the metal-mould reaction is considerably reduced if moulds were fired at high temperatures (> 1273K). This ensured elimination of moisture from the mould and also resulted in some beneficial changes in the mould chemistry. The reduction in metal-mould reaction is reflected in the decrease in oxygen and hydrogen contamination and decrease in hardness. Thus microhardness profile and oxygen analysis seems to provide a good index for evaluation of severity of metal-mould reaction. The method has been demonstrated to be satisfactory for casting titanium components.

Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

The high temperature ceramic oxide superconductor YBa2Cu3O7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF2 as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 105 Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O2, YBa2Cu3O7−x , Y2BaCuO5, CuO, BaF2/CaF2/BaF2, BaZrO3, ZrO2, O2, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO3 and Y2BaCuO5 from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol−1.

Thermodynamics of Aluminium -Strontium Alloys

The activity of strontium in liquid Al-Sr alloys (X(Sr) less-than-or-equal-to 0.17) at 1323 K has been determined using the Knudsen effusion-mass loss technique. At higher concentrations (X(Sr) greater-than-or-equal-to 0.28), the activity of strontium has been determined by the pseudoisopiestic technique. Activity of aluminium has been derived by Gibbs-Duhem integration. The concentration - concentration structure factor of Bhatia and Thornton at zero wave vector has been computed from the thermodynamic data. The behaviour of the mean square thermal fluctuation in composition and the thermodynamic mixing functions suggest association tendencies in the liquid state. The associated solution model with Al2Sr as the predominant complex can account for the properties of the liquid alloy. Thermodynamic data for the intermetallic compunds in the Al-Sr system have been derived using the phase diagram and the Gibbs' energy and enthalpy of mixing of liquid alloys. The data indicate the need for redetermination of the phase diagram near the strontium-rich corner.

Alloy oxide equilibria in the system Ca Al O at 1373 K

The tie lines delineating equilibria between different oxides of the Ca-Al-O system and liquid Ca-Al alloy has been determined at 1373 K. Equilibration of the alloy with two adjacent oxide phases in the CaO-Al2O3 pseudo-binary system was established in a closed cell made of iron. Equilibrium oxide phases were confirmed by x-ray analysis and alloy compositions were determined by chemical analysis. The compound 12CaO.7Al2O3 Ca12Al14O33 was found to be a stable phase in equilibrium with calcium alloys. The experimental diagram is consistent with that calculated from the free energies of formation of the oxide phases and activities in liquid Ca-Al alloys at 1373 K reported in the literature.

Use of CaC12 as a chlorinating agent for oxides - A thermodynamic analysis

Presented is a thermodynamic feasibility analysis of extracting base metal chlorides fiom low-grade,multimetallic oxide ores using CaClz as a chlorinating agent in the presence of SOz undoz. The oxides react to form corresponding chlorides, while CaClz is converted to CaS04. The Ellingham diagram is usedfor comparing the standard Gibbs' fiee energy chanlpef or the su(fation-chlorinationr eaction of a large number of oxides. Except for alumina, silica and chromia, most of the other metal oxides will be converted to their respective chlorides. The volatile chlorides can be condensed, and the chlorides present in the condensed state can be leached. A process is proposed that uses a nontoxic chlorinating agent and gives an eficient sepurutiort cftlte metallic vuluesfr.om the garlgue.

Variation of the partial thermodynamic properties of oxygen with composition in YBa2Cu3O7−δ

The variation of equilibrium oxygen potential with oxygen concentration inYBa 2Cu3O7-δhas been measured in the temperature range of 773 to 1223 K. For temperatures up to 1073 K, the oxygen content of theYBa 2Cu3O7-δsample, held in a stabilized-zirconia crucible, was altered by coulometric titration. The compound was in contact with the electrolyte, permitting direct exchange of oxygen ions. For measurements above 1073 K, the oxide was contained in a magnesia crucible placed inside a closed silica tube. The oxygen potential in the gas phase above the 123 compound was controlled and measured by a solid-state cell based on yttria-stabilized zirconia, which served both as a pump and sensor. Pure oxygen at a pressure of 1.01 × 105 Pa was used as the reference electrode. The oxygen pressure over the sample was varied from 10-1 to 105 Pa. The oxygen concentrations of the sample equilibrated with pure oxygen at 1.01 × 105 Pa at different temperatures were determined after quenching in liquid nitrogen by hydrogen reduction at 1223 K. The plot of chemical potential of oxygen as a function of oxygen non-stoichiometry shows an inflexion at δ ∼ 0.375 at 873 K. Data at 773 K indicate tendency for phase separation at lower temperatures. The partial enthalpy and entropy of oxygen derived from the temperature dependence of electromotive force (emf ) exhibit variation with composition. The partial enthalpy for °= 0.3, 0.4, and 0.5 also appears to be temperature dependent. The results are discussed in comparison with the data reported in the literature. An expression for the integral free energy of formation of YBa2Cu3O6.5 is evaluated based on measurements reported in the literature. By integration of the partial Gibbs’ energy of oxygen obtained in this study, the variation of integral property with oxygen concentration is obtained at 873 K.

A Sensor for Sulfur Gas Based on Silver β-Alumina

EMF measurements were made with an electrochemical cell of the type ~t/&(s)/&+-beta alumina/Ag~S(s)S. 2(g). S(s or 1)/R at temperatures between 95 and 241°C. Sflver $- alumina was prepared with the ion exchange technique. The patial pressure of diatomic gas obtained from cell voltages agreed with the literature data.