Main development directions in the application of microwave irradiation to the synthesis of nanopowders

Microwaves, MW, applications in synthesis of nanoparticles is a large and dynamically growing research field. The driving force is search for methods that permit production of nanoparticles with well controlled and tailored properties: degree of crystallinity and/or defect free crystalline lattice, dopped or alloyed with optically or magnetically active ions, crystals size and crystal size distribution, shape, capping with functional layers, etc. Although Europe was pioneering in research on microwave technology applied to chemical synthesis, recently the number of papers coming from Asia, and particularly China, has outnumbered these produced in Europe and in USA. This is mainly due to relatively low cost of experiments where modified commercially available microwave ovens or even kitchens can be used to make MW-assisted chemical synthesis. It has been proved worldwide that MW driven reactions provide fast heating of the reactants, in some conditions accelerated reaction rate, and high purity conditions. Microwave heating is particularly useful for reactions carried out under elevated pressures, because of contactless delivery of energy to the reading fluids, high energy density possible, and short heating times leading to nanoparticles weakly agglomerated, with high crystallinity and narrow grain size distribution. This progress was possible with the use of specially designed reactors, permitting pressure and temperature control and high purity conditions. Prospective research directions are development of continuous MW&HP reactors, increase of MW field strength, in situ preparation of functionalised nanopowders, investigation of the possible level of doping with functional ions, structure and properties of the nanoparticles, as well as extension of the range of fluids used as solvents. It is needed that laboratories working on nanopowders synthesis work closely together with the producers of the final commercial products and tailor the particles to their needs, while in parallel scaling up the production capacities.