CONTEXT

Metal oxide nanoparticles are usually used in a variety of applications including sensors, varistors, pigments, fillers, electrography and medical materials. Nanoparticles can be produced by several techniques such as precipitation, spray pyrolysis, thermal decomposition and hydrothermal synthesis. In particular, SC Water Hydrothermal Synthesis (scWHS) is an environmentally process for the production of potentially valuable metal oxide nanoparticles (ZnO, TiO2, ZrO2, LiFePO4, BaZrO3, Fe2O3,…) usually applied in the Nanotechnology.

Currently, most scWH syntheses are performed in batch reactor leading to thermal inertia which limits can be removed in a continuous synthesis process (thermal inertia, limited production…).

Continuous hydrothermal production process of oxides and hydroxides nanoparticles synthesized in sub and supercritical conditions. State diagram of water viewing the accessible domain of synthesis using this device.

TECHNICAL DESCRIPTION

This inventions deals with a continuous hydrothermal production process of oxides and hydroxides nanoparticles synthesized in sub and supercritical conditions. In  this apparatus, reactive solutions and deionized water are fed using high-pressure pumps. Pressure is regulated thanks to the back pressure regulator, located at the outlet; and  deionized water is preheated above the desired temperature in the reactor. The three streams are then combined in a tubular reactor.

The residence time (RT) in the reactor depends on pump flows (~ a few seconds). Once leaving the reactor, the solution is rapidly quenched in a cold bath. Two filters made of porous stainless steel remove agglomerated particles. Then, the suspension is centrifuged and washed with deionized water under ultrasonication. The sol formed is freeze-dried to conserve a well-dispersed nanopowder. This apparatus allows performing very fast reactions (RT were only a few seconds in the reactor). Supercritical conditions leading to instantaneous formation of a large number of hydroxide nuclei, short RT allow to reduce particle growth. Therefore, nanopowders were synthesized with a production of around 10 to 15 g/h.

Schematic diagram of the prototype developed in the MaNaPI team

Note that, a new filtration system allows to produce 60 g/h of dry powder. A large set of experimental conditions could be used from soft chemistry to supercritical conditions.

All operating parameters (T, p, concentration of reactive solutions, RT, pH…) could be adjusted independently showing that this process is very flexible. Indeed, thanks to the preheated water flow the temperature of the solution after the mixing zone can be set to the desired temperature, depending on the pumps flows. Moreover close to the H₂O critical point, minor variations of temperature and pressure can generate large changes of solvent properties allowing an adjustment of reaction medium.

BENEFITS

– Continuous process (yield = 95%, high productivity)

– Green chemistry and flexible process: all operating parameters (T, p, concentration of reactive solutions, RT, pH…) can be independently adjusted

– Controlled size ( < 500 nm and especially < 100 nm), composition and morphology of synthesized material

INDUSTRIAL APPLICATIONS

Electronic component, Catalysis, Energy, Optics, Pharmaceutical and cosmetic applications.

DEVELOPMENT STAGE

From this experimental set-up, various nanopowders have been produced in supercritical conditions. The following examples show clearly the ability of H₂O-SC process to obtain performant nanopowders with very interesting nanostructure in terms of cristallite size, size distribution and purity.