Impact-induced chemistry and physics through high-energy ball milling*

Mechanochemical activation by high-energy milling has become a widely used method for solid state synthesis, and alternative to high-temperature processes. It has been successfully used for synthesis of crystalline and amorphous alloys, intermetallic compounds, metastable phases, nanomaterials and metal-ceramic composites. Mechanochemical synthesis utilizes high-energy impact phenomena to initiate chemical reactions or structural transformations. The peak impact pressures which the individual sample particles experience vary depending on the type of mill, milling speed, as well as size, shape and density of the attritor components, but can reach 20 GPa, while the temperature typically remains below 100C. Remarkably, this is achieved with significant sample quantities (grams), over a short period of time, and very inexpensively. Whereas mechanisms and kinetics of solid-state reactions induced by temperature or static pressure are fairly well understood, transformations of materials under impact in a milling assembly remain largely unexplored and are based almost exclusively on ex situ studies. During the mechanical activation particles undergo heavy deformation and experience significant strains. This results in formation of dislocations, vacancies, stacking faults and increased number of particle boundaries, making the milled material energetically less stable. This presentation will discuss results of ball milling experiments with simple oxides such as GeO₂ and TiO₂, aimed at inducing structural phase changes, as well as mechanochemical solid state reactions leading to formation of olivine-type compounds. I will also introduce a new project to develop more economical mechanochemical paths towards synthesis of the cubic phase of boron nitride.