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Grain size and shape and the structure of grain boundaries greatly influence the mechanical, electrical, and optical properties of polycrystalline metals and alloys. In this issue of Chem, Nagaoka et al. take a bottom-up approach to grain boundary engineering by consolidating colloidal nanocrystals into millimeter-scale polycrystalline and metallic glass samples.

A grain-selection template was applied to freeze casting to control nucleation of pore-forming crystals and achieve ceramics with highly aligned pore structures. A polymethylsiloxane preceramic polymer was freeze cast with cyclohexane as a solvent to produce dendritic pores in SiOC. Image analysis and permeability measurements were performed to quantify the influence of various templates on sample properties. Results show that the percentage of porosity aligned along the freezing axis increased from 13.9% without a template to 92.6% with an optimal template. The Darcian permeability constant increased by more than 6-fold, from 3.4 × 10^(−12) to 2.1 × 10^(−11) m^2.