Thermodynamic database MALT Omega
The major objectives of MALT thermodynamic database are
1. to provide thermodynamic data with 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 a best combination of compound data.
For such a purpose, it is also required to have a good facility of checking the consistency among the thermodynamic data of several different compounds.
In view this, it is highly recommended to mae use of advanced software in order to examine the equilibira associated with targeted compounds.
(2) Analysis based on one chemical reaction
This is one of the fundamental thermodynamic analyses. This is quite simple but most important in many fields such as materials science or chemical process analyses. 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 chemical reaction can be also tabulated at selected temperatures.
When results of gem or CHD are reconsidered by analyzing in more details on the extracted chemical reaction, it becomes more clear from the physicochemical point of view. In particular, the redox nature or the acid-base relation can be seen behind the chemical reaction, leading to correlation between what happened and are observed and what are known from the physicochemical nature.
(3) Analysis based on the Gibbs energy minimization method
The MALT associated software, multi-element chemical equilibrium calculation program gem is based on the Gibbs energy minimization technique.
When temperature, pressure and the initial amount of chemical reactions are given, the corresponding equilibrium amount and associated chemical potential are given for respective species involved in equilibrium. This is particularly convenience for the multi-component system in which several chemical reactions can proceed simultaneously.
The Gibbs energy minimization under the constant pressure or the Helmholtz energy minimization under the 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 quite important in many industrial processes. The equilibria as a function of oxygen potential are also important 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 calculation the time dependent change in composition of those materials which 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 by the condition that the gases are in equilibria with the substance in a point and then equilibrated gases flows to the next point to react with substance at the point.
This is the strong tool in the thermodynamic analyses for the practical situations of materials science or chemical processes. Since the unique solution can be given for the specified conditions, results are quite understandable and are easily applied for 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 huge amount of results produced by this software. This makes it useful for users to understand the phenomena occurring on materials or chemical processes from the physicochemical reasons.
(4) Analysis based on the Chemical potential diagram
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 and as a result, the chemical potential of other species/compound 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 feature can be plotted to construct the chemical potential diagram having 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 so called generalized chemical potential diagram.
CHD is a strong tool of 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 by the same software. In addition, the profile diagram is also constructed to show the variation of the partial pressure of the gaseous species or of the logarithmic activities of the aqueous species along the fixed line in the diagram.
The Pourbaix diagram is newly prepared in the recent version up procedure. Since the special treatments are needed to construct the Pourbaix diagram, the default setting are widely adopted when CHD is run for the chemical system which contains aqueous species. For example, temperature is fixed at 298.15 K since the availability of aqueous data is best at 298.15 K. 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 target element according to the order of 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 selected as target.
(5) MALT Direct - User's Program
MALT suggests users to write their own programs and to make their own analyses on 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.