Following on from low-temperature chemistry, this lecture looks at hydro(solvo)thermal synthesis. We begin with a brief historical review, highlighting the fact that hydrothermal synthesis has its roots in the field of geology/geochemistry. Nature is rich in hydrothermal systems based on the heating of groundwater by magmatic rocks, enabling the water to reach high temperatures and pressures, thus acting as a supercritical fluid. Geysers, hot springs that intermittently gush water and steam at high temperature and pressure, are just one of the everyday examples. These natural hydrothermal reactions are responsible for the weathering of rocks, resulting in magnificent landscapes. They are also responsible for the formation of metalliferous deposits, the transport and deposition of oxides, and, of course, most natural crystals of varying colors, easily reaching kilograms and beyond. We'll be looking to see how the chemist has been able to mimic this at the laboratory level via i) the use of autoclaves and mineralizers to adjust the solubility of precursor products and ii) the choice of various heat transfer fluids (non-aqueous, alcohol, glycol, ammonia) giving rise to solvo(alco)(glycol)(ammono)-thermal syntheses. This process will be applied to the synthesis of numerous electrode materials, nanometric particles, ferroelectric compounds and others.
In addition to the use of autoclaves, a notable advance in the realization of hydrothermal syntheses is the recent use of microwave devices. These devices, whose operation is described here, enable certain fluids to be placed in their supercritical regimes, and thus advanced electrode materials to be obtained in record synthesis time (a few minutes). Hydrothermal synthesis is certainly an integral part of many technological sectors, even if the examples given here come mainly from environmental technologies (including recycling, and others).
