Materials Processing and Recycling with Near- and Supercritical CO2-based Solvents
Résumé
This chapter describes one area of applications where supercritical CO2 (scCO2) is effectively used as a solvent. CO2-based solvents can be used to perform physical or chemical transformations, the two main routes introduced in this chapter to prepare advanced materials. For processes based on physical transformations, CO2 can be used as a solvent, antisolvent or as a solute for the controlled design of organic particles. Switching from physical to chemical transformations gives access to the design of advanced inorganic materials, such as nanopowders or films using supercritical CO2-based solvent systems. The choice of the reactor technology allows working with homogeneous (continuous reactor) or heterogeneous nucleation (cold wall reactor). With a continuous reactor, it is possible to synthesise metal or metal oxide nanoparticles. Changing to a cold wall reactor allows for the deposition of thin films (metals or metal oxides) on different kinds of substrates, i.e. the Supercritical Fluid Chemical Deposition (SFCD) process forming continuous, conformal and dense coatings. The SFCD technology has also been used for the deposition of heterostructures on other supports, such as silica beads, carbon nanotubes, carbon aerogels, etc., in hot-walled batch reactors. A more recent application of CO2-based solvents is in the field of materials recycling. scCO2 is very interesting as it can easily contribute to improving processes, e.g. in terms of metal leaching, by shortening their duration. It can also be included in current recycling processes as a supplementary step, as shown for lithium ion battery electrolyte extraction or printed circuit board delamination.