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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 energy generation and storage 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. Initial comparisons of measured and calculated interdiffusion coefficients for some of the binary intermetallic compounds in the system are shown in the figure below.
Hydrogenation/dehyrogenation reaction path for 6LiBH4+CaH2
For the Laves phase based hydrogen storage materials databases for four constituent ternary systems (Cr-Ni-Ti, Cr-Ni-Zr, Cr-Ti-Zr and Ni-Ti-Zr) were assembled from descriptions in the literature. Effective ternary compositions were chosen for comparison of the calculated liquid composition paths and their associated solid phase formation sequences with the experimentally observed solidification path of the nine component alloy (Ni, Zr, Ti, V, Co, Mn, Cr, Al, Sn). The features of the ternary systems, Cr-Ni- Zr and Cr-Ti-Zr, are consistent with the observed microstructure, primary solidification of a C14 phase followed by a C15 phase. The intermetallic phases completing solidification are also predicted to occur in some of the four ternaries. The figure shows the liquidus surface of the Cr-Ni-Zr system and the solidification path for the effective composition. The results suggest that development of a full multicomponent Laves phase thermodynamic database will be useful for efficient design of alloys for use as metal hydrides in battery technology.
While extending the CALPHAD methodology to specific materials systems of interest, NIST is also working to establish a set of reference self-diffusion mobilities to enable more efficient construction of multicomponent diffusion mobility databases in the future.Just as the lattice stabilities for the free energy parameters describing the thermodynamics must be self-consistent within a multicompoent system, so must the pure element self-diffusion mobility coefficients. That is, the values defining the self-diffusion of fcc Ni in assessed Ni-Al system should be same as those used in assessed Ni-Cu system.This effort was initiated in the spring of 2009 with a meeting of selected international experts in diffusion mobility modeling, first principle calculation methods as applied to diffusion, and experimental experts. The group is currently preparing a set of recommendations for Ni, Al, Cu, and Fe.
Customers: GE, NASA, Questek, CompuTherm, Northwestern University, Pennsylvania State University, HelioVolt
Contributors: University of Central Florida, George Mason University, University of Michigan
Collaborators: DOE Metal Hydride Center of Excellence (MHCoE), Thermo-Calc AB, University of Florida
Project Summary (PDF)
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