Robert Castanet

Structural and thermodynamic data on the pseudobinary phases R(Co1−xCux)5 with R≡ Sm, Y, Ce

For the phases R(Co1−itxCuitx)5 with R ≡ Sm, Y, Ce, homogenized at 950 °C, the enthalpies of formation were determined by a calorimetry technique involving dissolution in molten tin and the lattice parameters were measured using X-ray diffraction in a focusing camera. In order to study the effect of substitution of cobalt for copper, the lattice parameters were corrected for the contribution of the departure of R from stoichiometry. For the R(Co1−itxCuitx)5 phases with R ≡ Sm, Y, the lattice parameters were also measured after heat treatment at 400 °C. The x dependences of the enthalpies of formation and lattice parameters indicate an ideal behaviour for the mixing of SmCo5 and SmCu5 on one hand, and for YCo5 and YCu5 on the other hand. This result is consistent with the homogeneous state found for Sm(Co1−itxCuitx)5 and Y(Co1−itxCuitxon the whole x range at 400 °C. For the mixture of CeCo5 and CeCu5, the enthalpies of formation exhibit a slightly positive deviation which predicts the solid state miscibility gap previously evidenced in the CeCo5-CeCu5 section below 800 °C.

Configurational entropy and the regular associated model for compound-forming binary systems in the liquid state

Abstract A statistical model is developed which permits the configurational entropy of compound-forming binary liquid alloys to be described. The different entropies defined on the basis of ideal mixing and of Florys model are compared. The usefulness of a representation of the configurational entropy that is compatible with the fundamental description of the model is emphasized. The results for a regular associated model in which it is assumed that the binary solution consists of A and B atoms and their chemical associates A a B b are presented. Finally the general treatment is applied to Tl-Te and Cu-Si alloys and the results are compared with experimental data.

Enthalpies of formation of SmCo alloys in the composition range 10–22 at. % Sm

Abstract Several intermetallic compounds exist in the composition range 10–22 at.% Sm(Sm 2 Co 17 , SmCo 5 , Sm 2 Co 7 ) but their preparation as single-phase specimens is very difficult. In order to determine the enthalpies of formation of these compounds, measurements were carried out on four alloys containing respectively 12.9 at.% Sm, 16.4 at.% Sm, 17 at.% Sm and 19.8 at.% Sm, annealed in the temperature range 950–1100 °C. The compositions of the phases present in each specimen were deduced from the characterization of the measured alloys by scanning electron microscopy, electron microanalysis and X-ray diffraction. The heats of formation were deduced from solution calorimetry in molten tin. The variation of the experimental results as a function of the samarium content enabled the enthalpy of formation of SmCo 5 ( − 40.8 kJ mol −1 ) to be determined. The same ΔH f value as determined for the phase quenched from 950 °C was measured for SmCo 5 kept at room temperature after very slow cooling. This result did not confirm the eutectoid decomposition previously reported for SmCo 5 . The extrapolation of the measured values for the higher and lower samarium contents leads to the evaluation of the enthalpies of formation of Sm 2 Co 17 (−152 kJ mol −1 ) and Sm 2 Co 7 (−99kJ mol −1 ).

Experimental Evidence From Heat Capacity Measurements of a Transition in the Vicinity of the Melting Temperature in a Liquid Eutectic GeTe Alloy

Abstract The temperature dependence of the enthalpy of the Te-rich eutectic Ge-Te alloy was measured by drop calorimetry in the crystalline, amorphous and liquid states using a high temperature Calvet calorimeter. This dependence leads to a second order transition which takes place around the eutectic temperature. Such a behaviour is explained by the destruction of the short range order in the liquid phase by heating.

Thermodynamic investigation on (a transition metal + germanium) I. (Platinum + germanium) and (palladium + germanium) alloys

Abstract The integral molar enthalpy of formation of the alloys { x Pt + (1 − x )Ge} at 1352 and 1275 K and of the alloys { x Pd + (1 − x )Ge} at 1270 K were measured by direct reaction calorimetry (drop method) using a Calvet calorimeter, and the molar enthalpies of fusion of PtGe and Pd 2 Ge were deduced. The strong negative departures from ideality of the two alloys suggest chemical bonding in the liquid state. However, the nature of the chemical short-range order seems to differ and to be closely related to the stability of the congruently melting compounds.

Thermodynamic functions and structure of gallium + tellurium liquid alloys

In the frame of our systematic investigation on strongly interacting alloy systems, we have measured the molar enthalpy of formation, ΔHf, of liquid Ga + Te alloy at 1200 and 1238 K by direct reaction calorimetry, using a Calvet microcalorimeter. The enthalpy of formation, with reference to the pure liquid components, is negative over the whole range of mole fractions x and has a minimum at xTe ≈ 0.6. ΔHf(l, xTe = 0.61, 1200 K) = −(38.8 ± 0.8) kJ mol−1. This is evidence for strong chemical interactions in the liquid phase with formation of Ga2Te3 clusters. No significant difference was noted between the enthalpies at 1200 and 1238 K. Comparison of the molar integral enthalpies and entropies of formation of liquid Me0.4IIITe0.6 alloys (MeIII  Al, Ga, In, and Tl) shows that the stability of the Me2Te3 clusters decreases in the series Al > Ga > In > Tl.

Thermodynamic investigation of PtGe and PdGe binary alloys

Abstract The enthalpy of formation, ΔHf, of Pt-Ge and Pd-Ge alloys was measured in the temperature range 1270–1680 K with respect to composition by direct reaction calorimetry (drop method) with the help of high temperature Calvet-type calorimeters. The results agree well with our previous measurements. The enthalpies of formation and melting of the PtGe and Pd2Ge intermediate phases were also obtained. Furthermore, we determined also some phase boundaries from the breaks of the ΔHf(x) curves in agreement with the published phase diagram. From our enthalpy data and those of the free enthalpy of formation of the melts determined by Knudsen cell effusion mass spectrometry we were able to derive also the thermodynamic properties of PtGe and Pd2Ge at their melting points. These lead to the calculation of solid compound-liquid domains in agreement with the published ones and our experimental determinations. The thermodynamic behaviour of the two melts is characterized by strong negative deviations to ideality, suggesting strong chemical short-range order. The stoichiometries of the associates are strongly connected with the highest melting intermediate compounds of both binaries, i.e. Pd2Ge and PtGe. Their stability seems very high, since the excess heat capacities of the liquid phases are particularly low. Such a conclusion is in agreement with the low values of the entropy of melting of the compounds when compared with those deduced from additivity.

Enthalpy of formation for CoGe, CoSn, Ni3.14Sn4, Ni3.50Sn4, AuCo1.66Sn4, AuNi2Sn4 and Au1.17Pt1.82Sn4

Abstract The enthalpies of dissolution, H diss , of CoGe, CoSn, Ni 3.14 Sn 4 , Ni 3.50 Sn 4 , AuCo 1.66 Sn 4 , AuNi 2 Sn 4 and Au 1.17 Pt 1.82 Sn 4 and their components have been measured in molten tin at 874 K by drop calorimetry. The enthalpy of formation of these phases, Δ H f 298 , has been derived for 298 K. Earlier measurements of the enthalpy of formation for the elements, disregarding cobalt, are in close agreement with the present results. The values for pure Co show significant temperature dependence, and also deviate from earlier findings. For the binary and ternary phases the accordance with earlier results for the Ni 3+ v Sn 4 phase is perhaps acceptable when the differences in specimen compositions are taken into account. The corresponding comparison for CoGe and CoSn has a much poorer outcome. The latter discrepancies are tentatively coupled to distinctions between the species of dissolved cobalt in liquid tin at different temperatures. No enthalpy of formation measurements have been reported for any of the ternary phases. Comparison of theoretically computed data for the enthalpy of formation for the binary and ternary phases shows close agreement with the measured values obtained in this study.

Etude thermique et thermodynamique du cyclohexaphosphate de lithium Li6P6O18.5H2O

We report in this paper the results of our thermal and thermodynamic investigation on lithium cyclohexaphosphate, Li6P6O18·5H2O between 298 and 1007 K. The different transitions with respect to temperature (successive dehydrations, solid-solid transition and melting) were studied with the help of differential thermal analysis and thermogravimetry. The different phases were characterized by X-ray diffraction and by infrared absorption. Finally, the enthalpy of these phasesvs. temperature was measured by isothermal drop calorimetry. Their heat capacities as well as the enthalpies of dehydration, of solid-solid transition and of melting were deduced.We pointed out that the lithium cyclohexaphosphate loses a molecule of water at 333 K (54.3 kJ·mol−1), three molecules of water at 413 K (151 kJ·mol−1) and the last one at 488 K (50.6 kJ·mol−1). The anhydrous lithium cyclohexaphosphate, Li6P6O18, give the polyphosphate, LiPO3, at 708 K (second order transition) and melt at 933 K (24.6 kJ·mol−1).ZusammenfassungVorliegend wird über die Resultate der thermischen und thermodynamischen Untersuchung an Lithiumcyclohexaphosphat, Li6P6O185H2O im Temperaturbereich 298-1007 K berichtet. Mittels DTA und TG wurden in Abhängigkeit von der Temperatur die einzelnen Umwandlungen untersucht (in der Reihenfolge: aufeinanderfolgende Dehydratationen, Feststoff-Feststoff-Umwandlungen, Schmelzen). Die einzelnen Phasen wurden durch Röntgendiffraktion und Infrarotabsorption charakterisiert. Letztlich wurde in Abhängigkeit von der Temperatur die Enthalpie dieser Phasen mittels eines isothermen Drop-Kalorimeters vermessen. Abgeleitet wurden für die Feststoff-Feststoff-Umwandlung und für den Schmelzvorgang weiterhin deren Wärmekapazitäten, Dehydratationsenthalpien.Lithiumcyclohexaphosphat gibt bei 333 K ein Mol Wasser (54.3 kJ/mol), bei 413 K drei Mol Wasser (151 kJ/mol) und bei 488 K das letzte Mol Wasser (50.6 kJ/mol) ab. Das wasserfreie Lithiumcyclohexaphosphat Li6P6O18 ergibt bei 708 K das Polyphosphat, LiPO3 (Umwandlung zweiter Ordnung) und schmilzt bei 933 K (24.6 kJ/mol).

Calorimetric investigation of the Au-Pb system

Abstract The enthalpy of formation hf of Au-Pb alloys was measured at 695, 871 and 1123 K by direct reaction calorimetry (drop method) with the help of a high temperature Calvet calorimeter. The measurements led to the following equations: hf(kJ mol−1)=χAu(1-χAu) (−12.2686 + 8.1423χAu) at 695 K; hf (kJmol−1)=χAu(1 − χAu) (−6.1750 + 3.4653χAu) at 871 K; hf (kJ mol−1)=χAu(1-χAu (−4.4416 + 5.6702χAu) at 1123 K. We also obtained the boundary of the liquid + Au(s) region from the breaks in the hf(χAu) curves. From our enthalpic data and those of the literature for the free enthalpy of formation we calculated at 695, 871 and 1123 K the mole fractions corresponding to the gold-rich liquidus were in good agreement with our experimental values. Our investigation allows us to state definitely that the enthalpy of formation depends strongly on the temperature. The positive excess heat capacity suggests accentuated short-range order in the melts which has to be investigated with reference to associated models.