Thermodynamic assessment of the KCl–K2CO3–NaCl–Na2CO3 system

Phase equilibria and thermodynamic properties of the KCl–K₂CO₃–NaCl–Na₂CO₃ system were analyzed on the basis of the thermodynamic evaluation of the KCl–NaCl,KCl–K₂CO₃,NaCl–Na₂CO₃,K₂CO₃–Na₂CO₃ and KCl–K₂CO₃–NaCl–Na₂CO₃ systems. The Gibbs energies of individual phases was approximated by two-sublattice models for ionic liquids and crystals. Most of the experimental information was well described by the present set of thermodynamic parameters. The lowest monovariant eutectic temperature in the KCl–NaCl–Na₂CO₃ system is located at 573 ^°C, with a composition of X_Na2CO3=0.31,X_KCl=0.35 and X_NaCl=0.34.     

A brief history of CALPHAD

Some 35 years ago, Larry Kaufman and Himo Ansara provided the stimulus to bring together a small number of scientists who were working on the calculation of alloy phase diagrams using as basis the required consistency of experimental thermodynamic and phase boundary data. This group represented the origins of CALPHAD and of subsequent developments concerned with computer coupling of phase diagrams and thermochemistry. From those origins, the “CALPHAD Method” has become a successful and widely applied tool in all areas of materials development.     

Summary of the CALPHAD XXXVI 2007 conference

From 6–11 May 2007, 147 registered participants met at State College, Pennsylvania, for the thirty-sixth CALPHAD meeting organized by Zi-Kui Liu, Steve Hansen, Joanne Murray and Phillip Spencer. Activities were held at Toftrees Golf Resort and Conference Center with logistics run by graduate students and postdoctoral fellows from The Pennsylvania State University. Over the course of six days, 57 oral presentations and 84 posters were offered, the abstracts and titles of which are summarized in this report. Posters were on display from Sunday evening until Friday morning. In addition to the continuous display of the posters, two dedicated poster sessions were held in the Monday and Tuesday evenings, which were overwhelmingly enjoyed by the participants.

The CALPHAD Gibbs Triangle Award, an honor given only every three to five years, was bestowed upon Gunnar Eriksson. Fourteen CALPHAD scholarships were awarded to promising students in the field, the Best Paper Award for papers published in 2006 in the CALPHAD journal was conferred, and a Best Poster Award was chosen for one of the conference’s posters.     

Phase equilibria and defect chemistry of the La-Sr-Co-O system

Strontium doped lanthanum cobaltite (LSC, La_1−xSr_xCoO_3−δ) is a promising oxygen electrode material for intermediate temperature solid oxide cells (SOCs) and oxygen membranes. Its application in service is however impeded by its thermodynamic instability. The present work is aiming to identify the phase stability of LSC by thermodynamic modeling of the La-Sr-Co-O system using the CALculation of PHAse Diagrams (CALPHAD) method coupled with experiments. The calculated results based on the developed database are validated using experimental data both from literature and the present work. General agreement is achieved between model predictions and experimental results. Other properties of the LSC perovskite, such as oxygen non-stoichiometry and cation distribution, are also calculated and predicted based on the developed La-Sr-Co-O database. These calculations can provide important guidance in designing LSC-based materials for various applications and the established thermodynamic functions can serve as input for further thermodynamic calculations on altered and even more complex perovskites.     

Thermodynamic Evaluation of Liquid Cd Cathode Containing U and Pu

In our previous study, a mixture of U and Pu was recovered in liquid Cd cathode from molten salt under various conditions of the U:Pu ratio. Two important things were observed. The first was that three kinds of precipitated phase had been detected in the saturated liquid Cd cathode, such as a U metal and two kinds of U-Pu-Cd compound. The compositions of the compounds were roughly (U+Pu):Cd = 1:11 and (U+Pu):Cd = 1:6. The second was that the U metal had selectively precipitated in the saturated liquid Cd cathode under the condition that the U:Pu ratio is higher than about 0.8 in the liquid Cd phase. In the present study, phase diagrams were evaluated by the CALPHAD method on the liquid Cd cathode containing U and Pu. The U-Pu-Cd compounds were modeled as MCd11-type and MCd6-type, respectively, based on the reported binary phase diagrams of U-Cd and Pu-Cd. The calculated result reasonably agreed with the experimental observations. The variations in the U and Pu activities were estimated along with the U:Pu ratio, which is related to the precipitation of various phases in the liquid Cd cathode.     

Thermodynamic Modeling of the Succinonitrile-Water System

Succinonitrile-water (SCN-H₂O) system is a widely used transparent metallic alloy system due to its analog solidification behavior to metals. In the present work, Gibbs energy of pure succinonitrile was derived utilizing temperature as well as enthalpy of transformations, and temperature dependencies of heat capacity available in the literature. The phase diagram for the binary SCN-H₂O system was assessed via the CALPHAD approach using phase equilibrium data available in the literature. Self-consistent thermodynamic parameters were obtained. A good agreement between the experimental and calculated data for the phase diagram has been achieved. The present work contributes to the development of the thermodynamic database of the SCN-H₂O system that can be incorporated into thermodynamic and kinetic codes for computational simulations.     

Hydrogen storage in TiZrNbFeNi high entropy alloys,designed by thermodynamic calculations

The hydrogen storage properties of the novel equiatomic TiZrNbFeNi and non-equiatomic Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅ high entropy alloys (HEAs) were studied. These alloys were designed with the aid of thermodynamic calculations using the CALPHAD method due to their tendency to form single C14 Laves phase, a phase desirable for room-temperature hydrogen storage. The alloys, which were synthesized by arc melting, showed a dominant presence of C14 Laves phases with the (Zr, Ti)₁(Fe, Ni, Nb, Ti)₂ constitution and small amounts of cubic phases (<1.4 wt%), in good agreement with the thermodynamic predictions. Hydrogen storage properties, examined at room temperature without any activation procedure, revealed that a maximum hydrogen storage capacity was reached for the equiatomic alloy in comparison to the non-equiatomic alloy (1.64 wt% vs 1.38 wt%) in the first cycle; however, the non-equiatomic alloy presented superior reversibility of 1.14 wt% of hydrogen. Such differences on reversibility and capacity among the two alloys were discussed based on the chemical fluctuations of hydride-forming and non-hydride-forming elements, the volume per unit cell of the C14 Laves phases and the distribution of valence electrons.