Instant formation of AlN whiskers and monocrystalline silicon with the separation of silicon and aluminum elements from 7 Å halloysite during thermite reduction

Abstract

Natural halloysite have attracted great attention owing to their unique hollow tube and strong adsorption capacity. Nevertheless, the straightforward utilization of halloysite without further processing cannot fully discover its potential will lead to the waste of resources. In this study, an environmentally friendly method for the simultaneous separation of silica and aluminum from natural 7 Å halloysite via the aluminum thermal reduction method upon 700 °C under the system of aluminum powder and NaCl in N₂ atmosphere was developed, concurrently occurs with the structural transformation from the raw hollow tubular halloysite to AIN whiskers and monocrystalline silicon droplet tips was observed. The formation of AlN whiskers can be concluded as three steps: (1) the reduction of Si⁴⁺ in SiO tetrahedron into Si⁰ atoms; (2) the separation of silicon and aluminum in the 1:1 meta-halloysite layer and the Si⁰ atoms transportation to the end of whisker rod to form the monocrystalline silicon microsphere; (3) the formation of AlN whiskers by the replacement of the remaining oxygen atoms in the aluminum‑oxygen octahedra. This in-situ replacement of oxygen atoms by nitrogen atoms in the AlO octahedron of tubular meta-halloysite plays a pivotal role in decreasing the reaction temperature. This work provides a novel idea and opens up a new technical route to prepare aluminum nitride and monocrystalline silicon with green efficiency and low cost.