Thermal expansion is a fundamental property of materials. It relates a second rank tensor (strain) to a scalar (temperature change). It is a symmetric second rank tensor because the strain tensor is symmetric. The most common method for reporting the thermal expansion of a material is to give the bulk value of thermal expansion. Although this method is elegant in its simplicity, in crystals with less than cubic symmetry, it does little to depict the types of changes that the crystal incurs as the temperature changes. The anisotropy in thermal expansion can lead to development of excessive stresses particularly in low-dimension material systems (e.g. coatings such as EBCs, TBCs) leading to premature failure. Even in polycrystalline materials, microstructure-level stresses can undermine performance and reliability of ceramics and composites.

 

This research focuses on determining the entire coefficient of thermal expansion (CTE) tensor as a function of temperature. The CTE tensor for the general case (a triclinic crystal), has six independent elements, four for monoclinic systems, three for orthorhombic, two for trigonal, tetragonal, and hexagonal crystals, and only one for cubic crystals. Our research group has developed the second generation CTEAS software for this purpose. Together with high temperature X-ray diffraction datasets acquired using our group’s lamp furnace, we can determine the CTE tensor of any crystalline material in the temperature range extending from room temperature to 2000°C in air, and up to 1500°C in controlled atmosphere. CTEAS software allows prediction of thermal expansion along any uvw and normal to any hkl place in the measured temperature range. 3D representation of the CTE as a thermal expansion ellipsoid allows visualization of this key material property.

This research is aimed at developing a comprehensive methodology for quantitative understanding of porosity in natural and man made materials, extending from atomic to macroscale pores, leading to modeling and prediction of their permeability. While the natural materials of interest include shale rocks, cuttlebone and corals, man-made structures will include membranes, concrete, dry wall, etc. Our focus is on seamless integration of experimentally determined materials microstructure properties such as porosity, pore size distribution, surface area, and crystal lattice free volumes, to model and predict permeability in these materials. This work will have direct impact on a wide range of technologies, such as: development of novel methods to enhance oil and gas recovery from unconventional shale; design of robust, fouling-proof membranes for desalination and waste water treatment; functional catalytic meta-materials for high efficiency synthesis of chemicals (e.g. NH3), and biocompatible materials for targeted drug delivery.