A comprehensive and consistent thermodynamic database with dual solvers.
Thermodynamic database MALT Omega
The major objectives of MALT thermodynamic database are
1. to provide thermodynamic data of a good quality to those who need to make thermodynamic analyses;
2. to facilitate the utilization of thermodynamics in the research and development associated with materials science, new process development, etc.
For those purposes, the followings can be made:
1. Preparation of thermodynamic data for the compound system
2. Analysis based on one chemical reaction
3. Analysis based on the Gibbs energy minimization method
4. Analysis based on the Chemical potential diagram
5. MALTDirect-User's Program
(1) Preparation of thermodynamic data for the compound system
The MALT database contains many compounds or species. Even so, it is not sufficient for the detailed thermodynamic analyses.
This makes it necessary for users to check the validity and coverage of the data stored in the MALT database, to compare with other available data, and finally to obtain the best combination of compound data. The MALT users can register their own data.
For such a purpose, it is also required to have a good facility for checking the consistency among the thermodynamic data of several different compounds.
Given this, it is highly recommended to make use of advanced software to examine the equilibria associated with targeted compounds.
(2) Analysis based on one chemical reaction
This is one of the fundamental thermodynamic analyses. This is quite simple, but it is most important in many fields such as materials science or chemical process analysis. The thermodynamic table provides the normal high-temperature properties, such as heat capacity, entropy, relative enthalpy, Gibbs energy function, the enthalpy change for formation, and the Gibbs energy change for formation at selected temperatures.
The thermodynamic properties change for a chemical reaction can also be tabulated at selected temperatures.
When the results of gem or CHD are reconsidered by analyzing the extracted chemical reaction in more detail, it becomes clearer from a physicochemical point of view. In particular, the redox nature or the acid-base relation can be seen behind the chemical reaction, leading to a correlation between what happens and is observed and what is known from the physicochemical nature.
(3) Analysis based on the Gibbs energy minimization method
The MALT-associated software, the multi-element chemical equilibrium calculation program gem, is based on the Gibbs energy minimization technique. See the example of calculation.
When temperature, pressure, and the initial amount of chemical reactions are given, the corresponding equilibrium amount and associated chemical potential are provided for the respective species involved in equilibrium. This is particularly convenient for the multi-component system in which several chemical reactions can proceed simultaneously.
The Gibbs energy minimization under constant pressure or the Helmholtz energy minimization under fixed volume can be made in a series. The latter makes it possible to make calculations for the system under the selected conditions characterized by the constant chemical potentials. For example, equilibria in air are pretty important in many industrial processes. The equilibria as a function of oxygen potential are also crucial in high-temperature fuel cells.
In a series of calculations, the amounts of reactants can be determined from the reaction products in the previous calculation. This makes it possible to calculate the time-dependent change in composition of those materials that are exposed under a flow of gases containing some reactive impurities.
In a long reaction tube, the changes in substances in the tube can be evaluated under the condition that the gases are in equilibrium with the substance at a point, and then equilibrated gases flow to the next point to react with the substance at that point.
This is a powerful tool in thermodynamic analyses for practical situations in materials science or chemical processes. Since the unique solution can be given for the specified conditions, the results are quite understandable and are easily applied to other further considerations. It is therefore crucial to be familiar with handling gem software in the thermodynamic considerations. As described above, it is also essential to extract the most important chemical reaction out of a vast amount of results produced by this software. This makes it useful for users to understand the phenomena occurring in materials or chemical processes from a physicochemical perspective.
Also see Advanced Usage of gem.
Tips
gem in Omega: The main points of treating aqueous species in gem are as follows:
1. Default setting:
- The default setting will be made for treating the aqueous ideal solution, which consists of the water solvent and the aqueous solvent species.
2. Specification of activity:
- Commands are available to specify the (logarithmic) activity of aqueous species in an analogous way to the gaseous species for which the partial pressure can be specified. In addition, pH can also be specified as -log a(H+).
3. Restriction:
- A new function is introduced to indicate whether the temperature region of calculation is beyond the valid temperature region of the respective species. Particularly, this is important for the aqueous species that have no high-temperature heat capacity.
Batch process is now possible by gem (common in both Omega and Basic: See gem batch process).
(4) Analysis based on the Chemical potential diagram
See What is Chemical Potential Diagram? for further details.
The chemical potential diagram is constructed on the basis of the Gibbs phase rule. In the ternary system at fixed temperature and pressure, three-phase coexistence gives no freedom, so that the chemical potentials of all elements are uniquely fixed. As a result, the chemical potential of other species/compounds can be uniquely determined.
This state corresponds to the point in the chemical potential space. From this equilibrium point, three lines are extended; those are the two-phase coexistence equilibrium. Those geometric features can be plotted to construct the chemical potential diagram with two axis values. Usually, those axis values are selected among the environmentally controllable properties such as temperature, log p(O2), log p(CO2), etc. Even so, more general variables can be adopted as the variables. This is a so-called generalized chemical potential diagram.
CHD is a strong tool for constructing the generalized chemical potential diagrams using the powerful polyhedron algorithm.
This makes it possible to construct the high-temperature stability diagrams as well as the Pourbaix diagrams using the same software. In addition, the profile diagram is also constructed to show the variation of the partial pressure of the gaseous species or the logarithmic activities of the aqueous species along the fixed line in the diagram.
The Pourbaix diagram is newly prepared in the recent version update procedure. Since the special treatments are needed to construct the Pourbaix diagram, the default settings are widely adopted when CHD is run for the chemical system that contains aqueous species. For example, the temperature is fixed at 298.15 K, as the availability of aqueous data is optimal at this temperature. The fixation of H2O(l) at activity of unity is adopted together with the adoption of pH and E/V as axes. In the multi-component O-H-X-M system, the element of M is selected as the target element according to the order of the NBS table, when the NBS order of M is higher than that of X. In the <O,H,S,Fe> or <O,H,P,Fe> system, the element Fe is selected as target, while in the <O,H,Fe,Ti> or <O,H,Fe,Na> system, the element Fe will not be selected as target.
Tips
CHD in Omega: The main features of treating the aqueous species in CHD are given below.
Also see How is Pourbaix Diagram constructed?
1. Default setting for Pourbaix diagram:
- The chemical potential diagrams for aqueous systems are well recognized as Pourbaix diagrams and have been widely utilized in various fields. Therefore, the present setting adopts the Pourbaix diagrams as the default setting.
- Even so, where the aqueous solutions are not identified from the selected species to be used in the construction of the diagrams, such default settings will not be adopted, and the standard settings will be validated as normal functions in CHD.
2. Normal diagram, Predominance area diagram, Profile diagram:
- There are several variations of the well-known Pourbaix diagrams. The most popular diagrams are those given in the pH-pE (or E/V) plot, which indicate the type of chemical state that is stable for the selected element, such as Fe or Mn.
- Those diagram that excludes the solid or other condensed phase and contain only aqueous species are called the Predominance area diagram.
- Furthermore, this analysis deals with a mixture consisting of many species, so that the profile diagram is also widely utilized to show the concentration or activity of such species as a function of a selected variable, such as pH.
3. Pourbaix diagram for Multi-component systems:
- The Pourbaix diagrams for the multi-component systems can also be constructed by following the strategy of generalized chemical potential diagrams and a well-developed way of representing the stability diagram for the systems containing two target elements, such as Fe and S.
- Usually, the stability regions of the Fe-containing compound/species are preferentially selected to be visualized. This can be done in the generalized chemical potential diagrams by adopting the method of unvisualizing the polygons that do not contain the Fe component. This can be manually operated by changing the transparency of polygons for such items to be shown.
(5) MALT Direct - User's Program
MALT suggests that users write their own programs and make their own analyses of their problems. For this purpose, several sample programs written in Delphi are prepared to show how user programs can be written to utilize the MALT-supplied procedures for handling thermodynamic data. In order to transfer the thermodynamic data in the MALT to the User's programs, the special function of "MALT Direct" is prepared. In the MALT-related software, gem and CHD make use of the same MALT Direct function to receive the data from the MALT Data Management System.
Detailed Explanation of Data
Considered matters:
Some difficulties were encountered during the optimization of their thermodynamic properties in view of consistency with phase diagram behavior and other sources derived from the plausible kinetic effects behind the observed thermodynamic behaviors.
- A special remark is needed for a particular compound with controversial discussions.
- Special treatment was adopted for better understanding the behavior of a family of compounds, such as perovskite compounds and related Ruddlesden-Popper phases.
- Special treatment was adopted for high-temperature heat capacities of gaseous or aqueous species to expand the usability of compound data.
(1) Perovskite Related Ruddlesden-Popper Phases
Those oxide compounds in the perovskite-type structure or related Ruddlesden-Popper phases exhibit extensive solid solution behaviors among the family of compounds. See the details.
- This makes it attractive to treat this type of family as the ideal associated solution. For such a purpose, the chemical formula has been changed, like A1.5BO3.5 instead of A3B2O7 and AB0.75O2.5 instead of A4B3O10.
- This is because the mixing entropy in the ideal associated solutions strongly depends on the atom numbers in one molecule (the hypothetical component in the solution).
(2) About La2Zr2O7
No change has been made, in spite of many recent investigations on the thermodynamic data of La2Zr2O7 after the first thermodynamic evaluation was made in the early 1990s. A detailed explanation was reported.
(3) NaSiCon and related Phases in the Na-Zr-Si-P-O system
Old data for NASICON were based on electrochemical investigations (ref 162). New calorimetric investigations, as well as phase relations, have been made extensively. Thus, attempts were made to evaluate the thermodynamic data that are consistent with the experimental phase relations. See the detail.
(4) Li-Mn-O system
This system is renowned in the field of technology associated with Li batteries. The materials' behavior and electrochemical behaviors have been well investigated in the vicinity of room temperature, whereas the high-temperature phase behaviors have also been extensively investigated. There can be some inconsistency in related information. The compiled thermodynamic data have been derived with the understanding that the room temperature electrochemical characteristics are not necessarily consistent with the most stable behaviors among compounds. See the detail.
(5) Li-Fe-P-O system
This system is also important in Li battery technology. In this sense, new features appear in the utilization of thermodynamic data in materials science. This is the deep utilization of the first principle energy calculations. In the Li-Fe-P-O system, a systematic approach has been adopted to make clear the phase stability in the multicomponent systems. In such a multicomponent system, experimental investigations have been focused on clarifying the high-temperature phase relations. On the other hand, the calorimetric investigations are few and therefore not systematic. In view of this, the system provides important examples concerning how to harmonize experimental and theoretical efforts. In the Example diagrams 9a and 9b in CHD manuals, this is partly shown.
(6) High Temperature Heat Capacity of Aqueous Species
Recently, the heat capacities of aqueous species related to the geology have been evaluated using a specialized heat capacity equation, which is not consistent with the MALT adopted equations. A description is given to show how those coefficients of such equations can be transferred to those adopted in the MALT database. See the detail.
(7) High Temperature Heat Capacity of Gaseous Species
High-temperature Mass Spectrometry has determined a large number of thermodynamic data for gaseous species. However, in many cases, only the enthalpy of formation at 298 K or 0K is given without entropy or heat capacity information. Some attempts have been made to provide estimated values by simple considerations. See the detail.
(8) High Temperature Alloys
Only a limited number of alloys are stored in the MALT database. This is because the MALT system has a strategy that the nonideal solutions are not treated, and therefore only the intermetallic compounds are stored. In a particular composition of alloy solution, a phase can be given to provide a rough idea about the stability of the target alloy solutions.