Calphad-database Development: Coupling of Experiment and Thermodynamic Modelling
In various industrial applications thermal storage materials are required, that fulfil many criteria: thermodynamic, kinetic (little or no supercooling), chemical (non-poisonous, non-flammable, non-explosive), economic (available, costs). They should have the required thermal and thermodynamic characteristic (melting point at the desired operation temperature range, high latent heat, high heat capacity, etc.). They should be stable under extreme conditions (high temperature, aggressive environment, mechanical stress) for a long time, also they should be stable in respect to other materials (e.g. containers).
Therefore, the interactions between various materials like molten salts and steel, slag, liquid metal and ceramic etc. need to be known. It is a challenge to find suitable materials according to corresponding requirements, which is nowadays done by time-consuming trial and error.
Thermal energy storage (TES) can be in form of sensible heat, latent heat, or thermochemical heat. All these thermal characteristics can be determined by thermodynamic calculations using a reliable thermodynamic database. Moreover, decomposition and corrosion can be predicted to reduce the time required for experiments. Since the existing/commercial database cannot always give a reasonable solution, the development of a new database using the CALPHAD approach is necessary. Our advantage is that our modelling work is supported by experiments.
Our experimental facilities cover a broad spectrum of thermodynamic properties. Unique combination of thermo-analytical and calorimetrical methods enables measurements in a wide temperature range. KEMS allows to determine thermodynamic characteristics from the equilibria between gas and condensed (solid or liquid) phase. Spectroscopic and structure methods support the study on phase equilibria. All results obtained are integrated into the database along with literature data.
The generation of a thermodynamic database (i.e. Gibbs energy for all phases) implying the Calphad method starts with the reliable experimental data on thermodynamic properties and phase equilibria, which are collected and critically assessed to be used in the optimisation. Next, the initial data are defined for all phases. The data on stoichiometric compounds can be adopted from the thermodynamic handbooks or other literature, or their Gibbs energy can be built using Neumann-Kopp approach. Also, ab-initio calculations can be used. Then, a suitable model is selected for the solution phase, i.e. a mathematical expression for Gibbs energy. The unknown parameters are optimised calculating selected properties and comparing them with experiments using thermodynamic software (FactSage). In case of disagreement, the optimisation is repeated until a good agreement is reached. Finally, the generated database is used for calculations and predictions.
Our database is developed in collaboration with the project partner GTT-Technologies. Including numerous salts, oxides, sulphides, and metals, the database enables calculation of complex equilibria depending on temperature, pressure, and chemical composition for various systems. That helps to calculate and predict the interactions between various materials/container/environment for TES and other applications (e.g. relevant to ceramic materials). One unique feature of our database is the implementation of a single liquid phase described by the non-ideal associate species model. Thus, the equilibrium distribution of elements between several liquids (e.g. metal and oxide) in case of immiscibility is automatically calculated with any preselection of phases. Moreover, a structure based viscosity model has been developed, which utilises the equilibrium composition of the melt calculated with our Calphad database.
Priv.-Doz. Dr. Michael Müller
IMD-1: Head of Division Thermochemistry
- Institute of Energy Materials and Devices (IMD)
- Structure and Function of Materials (IMD-1)
Room 105
- Institute of Energy Materials and Devices (IMD)
- Structure and Function of Materials (IMD-1)
Room 22b
- Institute of Energy Materials and Devices (IMD)
- Structure and Function of Materials (IMD-1)
Room 129