Microstructural Effects on Diffusion and Mechanical Properties in Different Material Systems

Citation

Luo, Shi (2018) Microstructural Effects on Diffusion and Mechanical Properties in Different Material Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z90G3HBV. https://resolver.caltech.edu/CaltechTHESIS:01082018-142110350

Abstract

Material microstructures is a very broad subject that encompasses most of the field of materials science. Advances in materials characterization and small scale mechanical experiments have brought about progress in the understanding of microstructural features and mechanisms down to the nanometer scale. In contrast to bulk features and properties, the small length scale of these microstructures lead to many interesting properties, and often requires a material-by-material, and even localized region-by-region study. While a thorough understanding of microstructural effects even in one material system is way beyond the scope of this thesis, there are nonetheless many common themes and properties that link together microstructures and their effects on different materials, especially in terms of mechanical properties.

In this thesis, the effects of microstructural features such as grain boundaries, surface modification and structural hierarchy are investigated using two sample material systems: Cu-In-Ga-Se (CIGS) thin films and marine diatom frustules. We find that grain structures (or a lack there of) play a major role in both systems, and lead to differences in material stiffness, strength, and diffusion of species. The latter is also significantly affected by material defects across length scales, exemplified in CIGS by both microscopic voids and pores, and atomic scale like substitutional point defects. On the other hand, in diatoms, a low flaw density combined with an effective hierarchical design can propel the mechanical property of relatively simple ingredients like amorphous silica, to achieve extraordinary mechanical strength. We will conclude by showcasing that we can generalize some of these knowledge on microstructural effects across material systems, to help designing manmade structures that fully capture the material-level and structural-level properties of natural marine diatoms.

Thesis Files