Thermodynamic studies on the Mg-Ga system

By means of concentration cells of the following type: Mg(s) |MgCl₂ in (LiCl-KCl)_eut (1) |Mg-Ga (1) the partial thermodynamic data of Mg in Mg-Ga liquid solutions have been obtained in the composition range 0. l≤x_Mg≤0.7 and at temperatures from 760 to 900 K. In addition, the enthalpies of mixing of liquid Gallium with liquid Magnesium have been measured calorimetrically at 973 K. The experimental values for the enthalpies of mixing from emf and calorimetric measurements are in good agreement. These values are compared with thermodynamic data reported in the literature and used for the calculation of thermodynamic data of intermetallic compounds existing in the Mg-Ga-system. Additionally, the heat of formation of Mg₅Ga₂ has been determined by solution calorimetry. The thermodynamic data of the liquid alloys can be described by an association model, and these results explain the possibility to obtain glassy metals by rapid cooling of these melts.     

Crystallization studies in the lead-rich corner of the system Pb−Mg−Bi

Crystallizing experiments using slow cooling rates and uninterrupted stirring while periodically sampling the liquid phase are used to determine the areas of primary crystallization of Mg_0.6Bi_0.4 and lead in Pb−Mg−Bi alloys and the location of the line of the eutectic trough which separates these areas. The procedure involves the use of highly sheared and relatively large baths of lead alloys under a protective atmosphere of argon over a layer of silicone fluid, and a close approach to equilibrium conditions is demonstrated by approaching the eutectic trough from both sides. A maximum temperature in the eutectic trough occurs at 589.4 K, 0.18 pct Mg, and 1.84 pct Bi, and the ternary point occurs at 525.2 K, 2.334 pct Mg, and 0.0247 pct Bi. The solubility data are correlated using a thermodynamic procedure, and the equations obtained are used to calculate lines representing the Mg_0.6Bi_0.4 isotherms and the eutectic trough in satisfactory agreement with the experimental data. Calculated tie lines representing equilibrium between lead crystals and liquid along the eutectic solidus and trough at appropriate temperatures are also given, including that which is collinear with Mg_0.6Bi_0.4 at the point of maximum temperature of the eutectic trough. J. D. ESDAILE, formerly Senior Principal Research Scientist, now deceased     

Thermodynamic properties of magnesium-lead alloys

Magnesium activities in liquid magnesium-lead alloys have been measured with electromotive force cells over the temperature range 745 to 868 K and over the mole fraction range 0.03 <X _Mg<0.50. The exper imental data were used to evaluate the parameters in the empirical temperature-composition relationship proposed by Krupkowski for describing the activity coefficients. Values generated from the Krupkowski formulae were utilized with data for liquidus compositions to evaluate the free energies of formation of the solid intermediate phases and compared with available experimental data for Mg₂Pb.     

Luminescent ionic lattice occupation and wide tunable emission spectra of La2MgZrO6:Bi3+,Eu3+ double perovskite phosphors for white light LED

La₂Mg_1-x/2Zr_1-x/2O₆:xBi³⁺(x=0.01-0.035,abbreviated as LMZ:Bi³⁺) and La_2-yMg_0.99Zr_0.99O₆:0.02Bi³⁺,yEu³⁺(y=0.1-0.11,abbreviated as LMZ:Bi³⁺,Eu³⁺) double-perovskite phosphors were prepared through high-temperature solid-phase method.The emission spectrum of LMZ:xBi³⁺(x=0.01-0.035)phosphors excited at 353 nm is asymmetric in the range be...     

Thermodynamic analysis of ordered liquid solutions by a modified quasichemical approach—Application to silicate slags

A system of semi-empirical equations has been developed for the analysis of the thermodynamic properties of ordered liquid solutions such as slags. The equations, which are based upon modifications of the quasichemical theory, take into account the concentration and temperature dependence of the solution properties of ordered systems and thus enhance the reliability of interpolations and extrapolations of data. For binary systems, these equations have been coupled with an optimization computer program to analyze simultaneously all available thermodynamic data including phase diagrams, Gibbs energies and enthalpies of formation of compounds, activities, enthalpies of mixing, entropies of fusion, miscibility gaps,etc. In this manner, data for several binary slag systems have been analyzed. In the present article, analyses for the CaO-SiO₂, FeO-SiO₂, and CaO-FeO systems are presented. The resulting equations represent all the binary data, including the phase diagrams, essentially within experimental error limits. The calculations have been extended to ternary systems, thereby permitting ternary thermodynamic properties to be approximated solely from data from the subsidiary binary systems. Results for the SiO₂-CaO-FeO system are in excellent agreement with measured ternary data.     

Quasi-Chemical Viscosity Model for Fully Liquid Slag in the Al2O3-CaO-MgO-SiO2 System. Part II: Evaluation of Slag Viscosities

A model is presented that enables viscosities to be predicted reliably over the whole range of compositions and temperatures in the Al₂O₃-CaO-MgO-SiO₂ slag system above liquidus in the temperature range from 1543 K to 2643 K (1270 °C to 2370 °C). Experimental procedures and data from the studies reported in the literature have been collected and critically reviewed with particular attention to the viscometry methods and possible contamination of slag samples to select reliable data points for further model development. Relevant revised formalism to describe the complex viscosity trends including charge-compensation effect of the Ca²⁺ and Mg²⁺ cations on the formation of tetrahedrally coordinated Al³⁺ was introduced. Parameters of the quasi-chemical viscosity model have been optimized to reproduce within experimental uncertainties most of the selected experimental data in the Al₂O₃-CaO-MgO-SiO₂ system and all subsystems. This study is part of the overall development of the self-consistent viscosity model of the Al₂O₃-CaO-FeO-Fe₂O₃- MgO-Na₂O-SiO₂ multicomponent slag system.     

Thermodynamic study of the Bi-Ca-O system

The Gibbs energy changes for syntheses of the interoxide compounds in the Bi-Ca-O system have been determined from thermogravimetry and solid-state electrochemical measurements in the temperature range 850 to 1000 K. Thermogravimetry was used in the measurements of CO₂ partial pressures equilibrated with calcium carbonate and a pair of the compounds in the Bi-Ca-O system: Bi₆Ca₇O₁₆ + Bi₁₀Ca₇O₂₂ (Bi₁₀Ca₇O₂₂ + Bi₂CaO₄ or Bi₂CaO₄ + Bi₁₄Ca₅O₂₆). Cells employing yttria-stabilized zirconia as electrolyte were used in the study of Bi₆Ca₇O₁₆ where an intermetallic compound BiPt was employed for the protection of Pt lead wire of the cell and to lower the vaporization of Bi in the cell. Based on the results of thermogravimetric study and electromotive force (emf) measurement, the Gibbs energy changes were calculated for the reactions in which the respective compounds were synthesized from Bi₂O₃ and CaO.     

Thermodynamic Properties of the Quasibinary System Bi2Se3 - Bi2Te3 over a Broad Temperature Range

We have studied the heat capacity and enthalpies of solid solutions in the quasibinary system Bi₂Se₃ - Bi₂Te₃ at low and high temperatures. Based on low-temperature measurements of the heat capacity, we have calculated the values of the enthalpy, entropy, and reduced Gibbs energy of the compounds under standard conditions. We have determined the temperature dependences of the thermodynamic functions for the solid solutions Bi₂Se₃ - Bi₂Te₃ (50, 70, and 80 mole% Bi₂Te₃) in the temperature range 298.15 K - T_mp. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 80–85, July–August, 2005.     

Determination of the standard gibbs energies of formation of Ca3As2, Ca3Sb2, and Ca3Bi2

The standard Gibbs energies of formation of Ca₃As₂, Ca₃Sb₂, and Ca₃Bi₂ were determined by a chemical equilibration technique yielding the following results: 3Ca(1)+2As(1)=Ca₃As₂ (s) ΔG°=−723,800+172.8T (±23,700)(J/mol) 1273 to 1573 K 3Ca(1)+2Sb(1)=Ca₃Sb₂(s) ΔG°=−726,300+159.3T(±24,600) (J/mol) 1273 to 1573K 3Ca(1)+2Bi(1)=Ca₃Bi₂(s) ΔG°=−696,400+195.6T(±23,200) (J/mol) 1148 to 1323 K The thermodynamic data for removal of arsenic, antimony, and bismuth by other experimental investigations were discussed in terms of the activity coefficients of these compounds in slags. The stabilities of these compounds were also discussed by using the critical oxygen partial pressures calculated from the above equations. D.J. MIN, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan     

Thermodynamic properties of Mg−Ge alloys

The thermodynamic properties of liquid alloys and two-phase mixtures in the Mg?Ge system have been investigated in the approximate temperature range 660° to 1130°C by means of a galvanic cell based on MgCl₂ as electrolyte. Derived thermodynamic properties for the germanium-rich liquid alloys have been found to be in good agreement with previous work. The standard free energy of formation of solid Mg₂Ge from liquid magnesium and solid germanium has been derived from this work, and from previous studies as: \(\vartriangle F_{Mg_2 Ge}^o = - 29,900 + 7.98T cal.\) valid from 687° to 1115°C. The phase diagram for the system has been found to differ only slightly from that due to Klemm and Westlinning.     

The Effectiveness of Al-Si Coatings for Preventing Interfacial Reaction in Al-Mg Dissimilar Metal Welding

The dissimilar welding of aluminum to magnesium is challenging because of the rapid formation of brittle intermetallic compounds (IMC) at the weld interface. An Al-Si coating interlayer was selected to address this problem, based on thermodynamic calculations which predicted that silicon would change the reaction path to avoid formation of the normally observed binary Al-Mg IMC phases (β-Al₃Mg₂ and γ-Al₁₂Mg₁₇). Long-term static heat treatments confirmed that a Si-rich coating will preferentially produce the Mg₂Si phase in competition with the less stable, β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ binary IMC phases, and this reduced the overall reaction layer thickness. However, when an Al-Si clad sheet was tested in a real welding scenario, using the Refill^? friction stir spot welding (FSSW) technique, Mg₂Si was only produced in very small amounts owing to the much shorter reaction time. Surprisingly, the coating still led to a significant reduction in the IMC reaction layer thickness and the welds exhibited enhanced mechanical performance, with improved strength and fracture energy. This beneficial behavior has been attributed to the softer coating material both reducing the welding temperature and giving rise to the incorporation of Si particles into the reaction layer, which toughened the brittle interfacial IMC phases during crack propagation.     

Thermodynamic study and the phase diagram of the Mg-Sn system

By means of concentration cells of the following type, Mg (l)MgCl2 in (LiCl-KCl)_eut (l)Mg-Sn (1), the partial thermodynamic data of Mg in Mg-Sn liquid solutions have been obtained in the composition range of 0.1 ≤X _Mg ≤ 0.75 and at temperatures from 950 to 1100 K. These values are compared with thermodynamic data reported in the literature and used for the evaluation to obtain a complete set of thermodynamic functions for phase diagram calculations and for further interpretation by the associate model. This model, which accepts the existence of ‘Mg₂Sn as-sociates’ in the liquid alloys, enables calculations of viscosity by Kucharski’s method corre-lating properly with experimental data. Mutual correlations between thermodynamic properties, physical properties, structure, and the phase diagram of the Mg-Sn system were shown to in-dicate a maximum chemical short-range order close to the composition Mg2Sn.     

Thermodynamic properties of dilute solutions of magnesium in zinc

Concentration cells of the type: Mg(s) |MgCl₂ in (LiCl-KCl)_eut(l) |Mg-Zn(l) have been employed for a study of the thermodynamic properties of dilute Mg-Zn alloys in the range from 0.01 < X_Mg < 0.17 and at temperatures below the melting point of Mg, 922 K. The data were discussed in terms of the Krupkowski formulae, Wagner's linear relations of In γ_Mg vs X_Mg and Darken's quadratic formalism. This research was done at the Institute of Inorganic Chemistry, The Technical University of Norway, Trondheim, Norway.     

Electrochemical Properties of Intermetallic Compounds Mg2Ni,MgNi2, and the Hydride Mg2NiH4 in 30% KOH Solution

Using the potentiodynamic polarization curve method, x-ray phase analysis, and Auger electron spectroscopy, we have studied the mechanisms for cathodic and anodic processes occurring in a 30% KOH solution on the surface of the intermetallic compounds Mg₂Ni, MgNi₂ and the hydride Mg₂NiH₄. In contrast to MgNi₂, we observed hysteresis of the cathodic polarization curve for Mg₂Ni due to formation of a surface layer (a mixture of the hydride phases β‐Mg₂NiH₄ and Mg₂NiH_3.85) on the sample, as a result of reaction between Mg₂Ni and atomic hydrogen. We have shown that the surface of the samples is considerably enriched in magnesium and depleted of nickel, and their corrosion resistance in the indicated electrolyte is determined by active dissolution of nickel and formation of a surface layer (～25 nm) on the electrodes consisting of MgO and Mg(OH)₂ in the case of Mg₂Ni and Mg₂NiH₄, and also of MgO, Mg(OH)₂, and NiO in the case of MgNi₂. The intermetallic compound MgNi₂ was the most corrosion-resistant, probably because of the presence of a monolayer of the oxides MgO and NiO on the original surface.     

Heat Capacity and Enthalpy of Bi2Si3 and Bi2Te3 in the Temperature Range 58-1012 K

We have studied the heat capacity and enthalpy of Bi₂Si₃ and Bi₂Te₃ at low and high temperatures. Based on low-temperature measurements of the heat capacity, we have calculated the enthalpy, entropy, and reduced Gibbs energy of the compounds under standard conditions. We have determined the temperature dependences of the thermodynamic functions of Bi₂Si₃ and Bi₂Te₃ in the range 298.15-968 K and 298.15-874 K, respectively. We have determined the melting points and the enthalpies and entropies of melting for the compounds.     

Diffusion Brazing of Al6061/15 Vol. Pct Al2O3p Using a Cu-Sn Interlayer

Diffusion brazing of Al-6061 alloy containing 15 vol. pct Al₂O₃ particles was attempted using Cu-Sn interlayer. Joint formation was attributed to the solid-state interdiffusion of Cu and Sn followed by eutectic formation and subsequent isothermal solidification. Examination of the joint region using scanning electron microprobe analyzer (EPMA), wavelength dispersive spectroscopy (WDS) and X-ray diffraction (XRD) showed the formation of intermetallic phases such as Al₇Cu₃Mg₃, Mg₂Cu₆Al₅, Cu₃Sn, and Mg₂Sn. The results indicated an increase in joint strength with increasing bonding time giving the highest joint shear strength of 94 MPa at a bonding duration of 3 hours.     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the MnO-TiO2, MgO-TiO2, FeO-TiO2, Ti2O3-TiO2, Na2O-TiO2, and K2O-TiO2 systems

All available thermodynamic and phase diagram data have been critically assessed for all phases in the MnO-TiO₂, MgO-TiO₂, FeO-TiO₂, Ti₂O₃-TiO₂, Na₂O-TiO₂, and K₂O-TiO₂ systems at 1 bar pressure from 298 K to above the liquidus temperatures. All reliable thermodynamic and phase diagram data have been simultaneously optimized to obtain, for each system, one set of model equations for the Gibbs energy of the liquid slag as a function of composition and temperature and equations for the Gibbs energies of all compounds as functions of temperature. The modified quasichemical model was used for the molten slag phases.     

Some Thermodynamic Aspects of the Oxides of Chromium

To understand Cr emissions from slag melts to a vapor phase, an assessment of the stabilities of the chromium oxides at high temperatures has been carried out. The objective of the present study is to present a set of consistent data corresponding to the thermodynamic properties of the oxides of chromium, with special reference to the emission of hexavalent chromium from slags. In the current work, critical analysis of the experimental data available and a third analysis in the case of Cr₂O₃ have been carried out. Commercial databases, Fact Sage and ThermoCalc along with NIST-JANAF Thermochemical Tables, have been used for the analysis and comparisons of the results that are presented. The significant discrepancies in the available data have been pointed out. The data from NIST-JANAF Thermochemical Tables have been found to provide a set of consistent data for the various chromium oxides. An Ellingham diagram and the equations for the ΔG° (standard Gibbs free energy change) of formation of CrO _x have been proposed. The present analysis shows that CrO₃(g) is likely to be emitted from slag melts at high oxygen partial pressures.     

The thermodynamic properties of solid Al−Mg alloys

By measurements of the electromotive force of reversible galvanic cells of the form Mg (solid)/MgCl₂ in fused LiCl/KCl eutectic/Mg _x Al _y (solid) between 370° and 550°C, values have been determined for the free energies, entropies, and heats of formation of all the solid phases of the Al?Mg binary alloy system. The aluminum-rich solid solution appears to be essentially regular with a small positive heat of formation, while the magnesium-rich terminal solution shows negative heats and entropies of formation. The intermediate phases β, γ, and ε all exhibit moderate negative heats and large negative excess entropies of formation; in the case of β and γ these properties pass through pronounced minima, at compositions within their homogeneity ranges, indicative of highly ordered stoichiometric conditions. The results are discussed in relation to known phase structures.