Nanocrystalline ceramics synthesized of highly abundant raw materials under extreme pressure and temperature conditions
The development of hard and tough materials remains a great challenge. The nature demonstrates that the high-pressure polymorphs of highly abundant bulk materials formed deep in the earth’s interior are frequently those with the highest mechanical resistance. The most famous example is diamond. Even harder nano-polycrystalline diamond can be fabricated through the reduction of the grain size. Grain size refining is not only a hardening mechanism; it can also improve the fracture toughness and the optical properties of polycrystalline ceramics consisting of intrinsically transparent crystals. Exploring the mechanisms responsible for these improvements requires the fabrication of polycrystalline materials with certain controls on the grain size down to the nanometer scale. In the current study, nano-polycrystalline ceramics are synthesized from glass starting materials using a state-of-the-art high-pressure high-temperature technique at pressures above several GPa and temperatures above 1200 K. Most of the starting glasses were prepared by an aerodynamic levitation furnace from simple and highly available oxide compounds. The fabricated ceramics reproducibly exhibit average grain sizes from the micro- to few tens of nanometers. The reduction in grain size results in the material reinforcement and greatly improved optical transparency of the synthesized ceramics, which consist of intrinsically birefringent crystals. In particular, nanocrystalline ceramics composed of triclinic kyanite monocrystals are highly transparent. This is noteworthy because most of the transparent polycrystalline ceramics are restricted to cubic crystals and, so far, have not be produced in the same quality when composed of the more common crystals of low symmetry. All of the synthesized ceramics are hardened and toughened in contrast to the transparent brittle single crystals and glasses that are mainly used in photonic applications.