This award supports theoretical and computational research and education aimed at achieving a fundamental understanding of how thermal energy is transported in crystals with large unit cells. In a crystalline material, the arrangement of atoms can be described in terms of a unit cell which periodically repeats in all directions to make up the bulk crystal. In some crystals such as gold and silicon, the unit cell contains only a few atoms, while in others, the unit cell may contain upwards of hundreds of atoms. Examples of large unit cell crystals include "zeolites", which have application in catalysis, molecular separation, and gas storage, and "fullerenes", which have applications in molecular electronics and solar energy conversion. Understanding thermal transport in these materials is critical for predicting how they will respond to temperature fluctuations in their surroundings and how they can dissipate excess heat generated during device operation.
The way that heat flows through a crystalline material depends on its unit cell. Conventional understanding of thermal transport is based on theory developed for small unit cell crystals. Experimental evidence, however, suggests that these theories are not suitable for modeling thermal transport in large unit cell crystals. The objective of this project is to use atomic-level computational tools to develop a framework for predicting the thermal conductivity of large unit crystals and apply it to zeolites and fullerenes. The results will be of direct importance to scientists and engineers using these materials in everyday applications. The computational tools developed will be suitable for modeling other large unit cell crystals.
Through this project, the research team will integrate nanoscience into the undergraduate engineering curriculum at Carnegie Mellon University through a lecture series and distribute the materials through the National Science Foundation-supported nanoHUB. Outreach activities related to large unit cell crystals will be developed and presented to middle school and high school students in Pittsburgh.
This award supports theoretical and computational research and education aimed at achieving a fundamental understanding of how thermal energy is transported in crystals with large unit cells. Crystalline materials with large unit cells are relevant in a wide range of energy-related challenges and opportunities, such as in catalysis, molecular separation, gas storage, thermoelectric energy conversion, and solar energy conversion. In many of these areas, thermal transport plays a critical role, but has received minimal attention. The central hypothesis of this project is that not all vibrational modes in a large unit cell crystal propagate and that energy transport mechanisms at length scales smaller than the lattice constant are important. The overarching objective is to uncover the underlying physical mechanisms and to suggest strategies for the design of materials with tailored thermal properties.
Two distinct materials systems will be considered using a suite of atomic-level computational tools including molecular dynamics simulations and lattice dynamics calculations. First, zeolites, where all atoms are strongly bonded due to covalent and electrostatic interactions will be investigated. Anharmonic and harmonic effects on thermal transport will be rigorously modeled to quantify the contributions of different types of vibrational modes to thermal conductivity. The effects of framework aluminum, non-framework cations, and adsorbed species on thermal conductivity will be quantified. Second, fullerene-based molecular crystals, where the intramolecular interactions are strong but the intermolecular interactions are weak due to van der Waals forces, will be investigated. At low temperatures, a reduced-order model will be developed, while at higher temperatures a network model based on the thermal conductance between molecules will be explored. The modeling predictions will be validated through collaboration with experimental research groups. The methods and tools developed will translate to studies of thermal transport in other large unit cell crystals, such as clathrates, skutterudites, Zintl compounds, gas hydrates, and metal-organic frameworks.
Through this project, the research team will integrate nanoscience into the undergraduate engineering curriculum at Carnegie Mellon University through a lecture series and distribute the materials through the National Science Foundation-supported nanoHUB. Outreach activities related to large unit cell crystals will be developed and presented to middle school and high school students in Pittsburgh.
