Elucidating Thermally Induced Structural and Chemical Transformations in Kaolinite Using Reactive Molecular Dynamics Simulations and X-ray Scattering Measurements

The lack of reliable predictive modeling methods and robust experimental techniques has hindered the rational design of hierarchical materials with desired structure-property-performance attributes suitable for extreme environments. With this context in mind, we explore the utility of ReaxFF reactive molecular dynamics (MD) simulations in combination with in-operando wide-angle X-ray scattering (WAXS) and X-ray pair distribution function (PDF) analyses. To demonstrate the method, we consider kaolinite, a natural hierarchical material, as the candidate and determine thermally induced chemical and structural transformations when heated from 298 to 1673 K. We first compare the key structural features from the PDF data and WAXS peaks obtained experimentally to those calculated from MD simulations. Upon observing excellent agreement, we proceed to elucidate the underlying chemical reaction mechanisms associated with dehydroxylation and sintering, identify intermediate and transition states, and also estimate energy barriers of individual reactions and their effects on the structural organization of kaolinite obtained using MD simulations. On heating from 298 to 873 K, dehydroxylation reactions lead to the transformation of crystalline kaolinite with octahedrally coordinated aluminum atoms to semicrystalline metakaolin with ∼90% tetrahedrally coordinated aluminum atoms. Sintering reactions and the subsequent emergence of mullite (a high-temperature phase of kaolinite) are observed on heating metakaolin from 1055 to 1673 K. We also find that heating rates have a significant effect on the onset temperature of dehydroxylation and sintering reactions. A rapid heating rate leads to early dehydroxylation (425 K) and sintering (1055 K), whereas a 10 times slower heating rate delays dehydroxylation (622 K) and sintering reactions (1100 K). An outcome of our method is a regime map that illustrates the degree of agreement between the experimental data and simulation results in describing the thermally induced onset of atomic-level reorganizations in materials. Herein, quantitative agreement between simulation predictions and experimental data is noted at lower temperatures (T < 1000 K), and minor deviations between these methods is noted for T > 1000 K. The remarkable agreement between the methods observed in our study reiterates the reliability of the combined ReaxFF approach in predicting material properties under chemically reactive and extreme conditions.