Inorganic materials produced by self-combustion and mechanical grinding (principle and examples)

While temperature is important for promoting the diffusion of species during a chemical reaction in the solid state, we'll see that pressure also favors contacts between grains, enabling better reactivity and therefore shorter reaction times. In the final analysis, however, thermodynamics dictates the most stable material. The equipment needed to obtain high pressures ranging from 200 bar to 2,000 kbar (autoclave, flash sintering, diamond anvils, etc.) is described, emphasizing that the best natural furnace is none other than the earth, which at its core can reach temperatures of 5,000°C and pressures of 4,000 kbar. This combined temperature/pressure effect makes it possible to obtain new polymorphs whose electrochemical properties in the high-pressure forms of _V2O5 and _FePO4 have been studied. For example, it enables us to understand natural phenomena linked to the evolution of the structure of silicates within the earth's mantle, a problem very dear to the hearts of our geophysical colleagues. The ecological aspect of high-pressure reactions remains to be improved, however, simply because the quantity of material that can be processed is decreasing. Mechanical crushing is one of the approaches currently being pursued, as it is much simpler to implement. We describe the principle, based on the transformation of mechanical energy into chemical energy, and set out the fundamental rules governing these reactions. We apply them (i) to the shaping of electrode materials and catalysts; (ii) to the manufacture of composites, and even to the design of new materials (oxides, alloys and lithium compounds).