Thermodynamic & Kinetic Data for Sustainable Energy

Industry needs thermodynamic and kinetic data for the development of new materials for sustainable energy applications; expanded and new databases are needed for experimental and computational methods in materials development. The goal of this project is to provide thermodynamic and diffusion mobility databases as an efficient method of storing the wealth of these data, focusing on needs in the gas turbine, hydrogen storage and photovoltaics industries.

Knowledge of the thermodynamic, phase equilibria and diffusion properties of potential novel materials can greatly accelerate their development. However, the data needed for new, multi-component materials are often not available.

The CALPHAD (Computer Coupling of Phase Diagrams and thermochemistry) approach allows the development of thermodynamic and diffusion mobility databases enabling the extrapolation of higher systems from binary and ternary systems. Phase equilibria calculations using the thermodynamic databases determine the phases present and compositions, as well as enthalpy contents, temperature and concentration dependence of phase boundaries, and enable the coupling of microscopic and macroscopic models.

Going beyond equilibrium properties, these methods can also be applied to the dynamics of materials. Diffusion rates can be expressed as a product of a thermodynamic factor and diffusion mobility. Diffusion mobility functions are optimized with the CALPHAD method using a variety of diffusion data and a given thermodynamic description. These diffusion mobility databases have become crucial in numerical diffusion process simulations for multi-component alloys where composition and temperature-dependent diffusion-coefficient matrices are needed at each point in the material.

Currently NIST is focused on extending the CALPHAD methodology beyond traditional metallic systems as it works to develop improved thermodynamic and diffusion mobility descriptions for photovoltaic and hydrogen storage materials.

Working in conjunction with researchers at the University of Florida, thermodynamic and diffusion mobility descriptions for the Cu-In-Ga-Se system are being developed.  This system is critical in the processing of the α-Cu(In,Ga)Se₂ (CIGS) photovoltaic absorber material.  Advantages of the CIGS-based photovoltaic cells include high efficiency (19.9%), the possibility of direct band gap engineering, high optical absorption coefficient, high radiation resistance, high reliability, and use on flexible substrates. Production costs, however, prevent CIGS from being widely used.  To reduce the processing cost requires reducing the current processing times from an average of 30 minutes to less than 3 minutes.

NIST is developing a diffusion mobility database for the Cu-In-Ga-Se system to use with thermodynamic descriptions to enable modeling of a variety of different processing routes.  The mobility descriptions are optimized using available measured diffusion coefficients and composition profiles from the literature and growth rate constants measured during in-situ high temperature x-ray diffraction studies by U. Florida and Oak Ridge National Laboratory. The figure shows the calculated ternary Cu-In-Se isothermal section at 500 °C. The complexity of the isothermal section demonstrates the need for diffusion simulations to investigate the numerous possible processing routes to form the α-CIS phase.  

Initial comparisons of measured and calculated interdiffusion coefficients for some of the binary intermetallic compounds in the system are shown in the figure below.

Interdiffusion coefficients for binary intermetallics

To support materials development efforts in hydrogen storage for the Hydrogen Economy, NIST has developed a thermodynamic database for Calphad modeling from the available literature, which includes the elements Li, Mg, Ca, B, Si, and H and their respective binary phases.

The Calphad approach incorporates experimental data and first-principles calculation results into an overall temperature - pressure – composition framework for metal-hydrogen systems with three or more components. Data and functions have been compiled into a consistent database describing the multi-component systems. Missing quantities have been identified, and descriptions are being developed in collaboration with MHCoE partners. The Neumann-Kopp rule for the prediction of heat capacities was modified for more accurate predictions for compounds with anion complexes. This database has been used to calculate the reaction paths in multi-component hydrogen systems. These results are the basis for the evaluation of the suitability of a series of promising light weight hydride mixtures, such as 6LiBH₄ + CaH₂ or xLiBH₄ + MgH₂, that have the potential for a favorable hydrogenation/dehydrogenation temperature and storage capacities suitable for automotive applications.

Hydride reaction path for 6LiBH4+CaH2