Morphological Instabilities of Nanorods at Elevated Temperature.

<br/>Recent advances in high-precision manufacturing and self-assembly techniques have enabled the fabrication of materials architectures with intricate nanoscale features. Notable examples include nanolattices, nanoscale rods, nanoporous materials, and stochastic nanostructures. Integrating complex designs with nanoscale induced materials properties enables a myriad of applications, including electronics, energy and microfluidic devices, structural, and photonics. While such materials enable enticing combinations of properties, the microstructural stability of nanoscale morphologies under high-temperature environments remains poorly understood. Owing to the increased interfacial contribution to the total free energy in nanoscale geometries, these structures are at highly non-equilibrium states and evolve by a wide range of mechanisms. Recent experiments demonstrate a morphological instability in which a polycrystalline micro- or nanoscale rod breaks up into single-crystal domains, a phenomenon reminiscent of the Plateau-Rayleigh instability in liquid jets. Our high temperature in situ annealing studies of a model alumina rod with a bamboo grain structure demonstrated the pinch-off instability initiated at a GB. Here, we develop a theoretical model to investigate the impact of grain boundaries (GBs) on the morphological instabilities of polycrystalline nanoscale rods. A neutral stability surface is obtained, which demarcates stable and unstable perturbations with respect to the breakup. Our analysis shows that GBs play a destabilizing role in which the critical wavelength for the instability decreases with increasing the GB energy. Further, it is shown that perturbation amplitudes introduced through grain size differences influence the magnitude of the critical wavelength for the instability. We complement our thermodynamic treatment with phase-field simulations that capture the kinetics of morphological instability. The study reveals that increasing the GB energy leads to accelerated pinch-off kinetics. Moreover, it is shown that variations in grain size could provide a pathway for the morphological instability, in which GB migration and shrinkage of small grains result in the development of free surface perturbations whose wavelengths exceed the critical value for the instability. In broad terms, our approach provides avenues to investigate the microstructural stability of nanolattices and nanoporous materials.