Gibbs energies of formation of intermetallic phases in the systems Pt Mg, Pt Ca and Pt Ba and some applications

The standard Gibbs energies of formation of platinum-rich intermetallic compounds in the systems Pt-Mg, Pt-Ca, and Pt-Ba have been measured in the temperature range of 950 to 1200 K using solid-state galvanic cells based on MgF2, CaF2, and BaF2 as solid electrolytes. The results are summarized by the following equations: ΔG° (MgPt7) = −256,100 + 16.5T (±2000) J/mol ΔG° (MgPt3) = −217,400 + 10.7T (±2000) J/mol ΔG° (CaPt5) = −297,500 + 13.0T (±5000) J/mol ΔG° (Ca2Pt7) = −551,800 + 22.3T (±5000) J/mol ΔG° (CaPt2) = −245,400 + 9.3T (±5000) J/mol ΔG° (BaPt5) = −238,700 + 8.1T (±4000) J/mol ΔG° (BaPt2) = −197,300 + 4.0T (±4000) J/mol where solid platinum and liquid alkaline earth metals are selected as the standard states. The relatively large error estimates reflect the uncertainties in the auxiliary thermodynamic data used in the calculation. Because of the strong interaction between platinum and alkaline earth metals, it is possible to reduce oxides of Group ILA metals by hydrogen at high temperature in the presence of platinum. The alkaline earth metals can be recovered from the resulting intermetallic compounds by distillation, regenerating platinum for recycling. The platinum-slag-gas equilibration technique for the study of the activities of FeO, MnO, or Cr2O3 in slags containing MgO, CaO, or BaO is feasible provided oxygen partial pressure in the gas is maintained above that corresponding to the coexistence of Fe and “FeO.”

Metal mould reactions during casting of titanium in zircon sand moulds

Metal-mold reaction during Ti casting in zircon sand molds has been studied using scanning electron microscope, energy and wave length dispersive analysis of X-rays, X-ray diffraction, microhardness measurements, and chemical analysis. Experimental results suggest that oxides from the mold are not fully leached out by liquid Ti, but oxygen is preferentially transferred to liquid Ti, leaving behind metallic constituents in the mold as lower oxides or intermetallics of Ti. The electron microprobe analysis has revealed the depth profile of contaminants from the mold into the cast Ti metal. The elements Si, Zr and O were found to have diffused to a considerable distance within the Ti metals. A possible mechanism has now been evolved in regard to the reactions that occur during casting of Ti in zircon sand molds.

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.