Thermodynamic Measurements of Binary and Ternary Intermetallic Compounds

Illinois Institute Of Technology, Chicago IL

Investigators

Abstract

Non-Technical Abstract Compounds based on metals are important functional materials that provide many properties of value to society and are part of a class of "smart materials". Their properties can, for example, provide energy harvesting through conversion of waste heat to electricity, efficient refrigeration through the elimination of moving parts, mechanical positioners operated by heat or magnetic field and medical devices such as stents for circulatory problems, braces for straightening teeth and micro-robots for treatment of eye diseases. The development of these materials requires a knowledge of what scientists call phase equilibria, which means what structure an alloy composition consists of at a particular temperature, so that suitable processing can be developed and so it can provide possible multi-functionality. A knowledge of the structure is also necessary as this determines the properties which may be tuned by modifying the structure through adjustment of composition and process heat treatments. This project is focused on the determination of the phase equilibria, structure and properties of a class of compounds based on metals. The work involves both experimental and computational research into the nature and properties of these compounds. For device manufacturers, the results of this research will enable them to incorporate these materials into existing devices as replacements for current materials or enable them to develop new devices for the benefit of society. Technical Abstract Intermetallic compounds such as Heusler type X2YZ and "half-Heusler" compounds of type XYZ, where X and Y are transition metals and Z is a group III-V element and Laves phases are of great interest for their properties such as shape memory, thermoelectric, ferromagnetic and spin polarization effects which could lead to new mechanical and electronic devices. In this project the thermodynamic properties, crystal structures and phase equilibria of these compounds will be systematically determined. Using drop calorimetry the standard heat of formation and heat capacity as a function of temperature of the compounds will be determined. Phase equilibria and crystal structures will be determined using differential scanning calorimetry, energy dispersive x-ray analysis and x-ray diffraction. The data will be used to benchmark first-principles calculations for these compounds and to assess the general accuracy of DFT predictions. The database will be expanded to include DFT structural and energy results as validations for experimental data as well. Although potentially there are thousands of compounds, rapid screening of composition space will be performed using systematic analysis of phase diagrams and high-throughput first principles calculations. The thermodynamic data obtained will be incorporated into an on-line database for access by the scientific community directly for use in alloy development and computational materials design. The data will be incorporated in thermodynamic Gibbs energy descriptions of the compounds for use in predicting phase equilibria using the Calphad approach. The experimental data will provide a unique opportunity for the DFT community to assess the accuracy of their calculations. DFT defect calculations for non-stoichiometric intermetallics performed in this project will provide the community with a greater understanding of the effect of constitutional defects on structural stability. The project involves a substantial international collaboration with researchers in China, Czech Republic, and Austria. The thermodynamic database is publicly available for all to access via the web.