Quasicrystal Growth Mechanisms and Generalized Defect Detection in Crystals

Abstract

Quasicrystals are aperiodic crystals known to exhibit properties unexpected for their composition. These materials have potential applications as solar absorbers, Teflon alternatives, and mechanical reinforcement. Unfortunately, the presence of defects, dislocations, and grain boundaries in synthesized quasicrystals impede the study and commercialization of these materials. Conventional methods to detect defects and grains assume periodicity, are limited to simple crystals, or are developed for a specific crystal structure. Although theories of defects and dislocations in quasicrystals exist, they can be difficult to implement on large systems due to their complexity and computational cost. This dissertation aims to bridge this gap by deepening our understanding of quasicrystal growth mechanisms, offering a versatile tool for defect and dislocation detection, and ultimately enhancing the quasicrystal manufacturing processes. Due to the difficulties associated with defect detection in quasicrystals, research on the growth interactions of these materials remains limited. To address this gap in the literature, I use molecular dynamics simulation to model two novel growth behaviors discovered by our experimental collaborators. First, I elucidate how phasons, the configurational degrees of freedom imparted by aperiodicity, enables the formation of single, defect free quasicrystals upon collision of two grains with small misorientation. I show how phasons enable quasicrystals to redistribute direct space strain (i.e. phonon strain) upon collision and rotation of misoriented grains. Second, I detail the role of multiple length scales in phason-mediated coalesence mechanisms upon quasicrystal collison and engulfment of shrinkage pores. This phason-mediated mechanism results in a low-energy region at the site of growth front collision, and is agnostic to pore collision conditions. These works highlight the role phasons play in redistributing strain upon collision of growth fronts. Understanding how the presence of phasons affects quasicrystal growth behavior will give experimentalists the tools they need to develop better manufacturing processes for commercially viable quasicrystal coatings. Although Fourier filtering is traditionally used to detect strain and dislocations in experimental crystals imaged at atomistic resolution, this technique has seen limited usage for the analysis of phason and classical strain in quasicrystals. Additionally, Fourier filtering relies on manual inspection of structural data and often requires specialized knowledge of proprietary software. For systematic studies over large parameter spaces, manual inspection becomes infeasible. Consequently, Fourier filtering for defect detection has seen limited usage in simulated systems. To process the large volumes of data required for our systematic study of obstacles and temperature on quasicrystal synthesis, I develop a structure agnostic algorithm to automate defect and strain detection. The algorithm is robust to noise and artifacts originating from disordered regions or misaligned grains, effective at segmenting misoriented grains in polycrystalline samples, and effective at identifying defects and dislocations. I leverage this algorithm to analyze phason trail relaxation in simulated quasicrystals and demonstrated the algorithm’s generalizability across a diverse array of simulated and experimental crystals, including images of non-spherical particles, three dimensional experimental data, and three dimensional simulation data. This dissertation aims to advance our understanding of quasicrystals by exploring their growth behaviors, offering a robust defect detection tool, and providing valuable insight for material scientists, crystallographers, and other researchers specializing in quasicrystals. Through the integration of molecular dynamics simulations and innovative algorithms, this research promises to facilitate significant advancements in the comprehension and commercialization of these remarkable materials.