Development of Continuous Apparatus using Microreactor for Production of Nanoparticles in Supercritical Water

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Over the past ten years, a flow-through hydrothermal synthesis method with microreactor for continuous production of metal oxides have been established for forming micro or nano size metal oxide fine particles in subcritical and supercritical water. This method promises to heat starting solutions rapidly to the given temperature and to control reaction time exactly. When water in its near-critical or supercritical state is used, the hydrothermal reaction rate and metal oxide solubility can be controlled by changing temperature and pressure since the reaction solvent properties are strongly dependent on the conditions in these regions. Therefore, this method has great potential for producing nano-size metal oxide fine particles. There are many reports for producing nanoparticle by this method. However, there is no report that the controllability of the particle size is discussed on the basis of metal oxide solubility and supersaturation. In this work, to develop this method as a generic … More technology for producing nanocrystals we focus on the quantitative clarification of experimental conditions for producing single nano-sized metal oxide particles by this method and also its crystallinity on the basis of estimated metal oxide solubilities.We carried out hydrothermal synthesis of metal oxides (AlO(OH), CuO, Fe2O3, NiO, ZrO2) nanoparticles from metal nitrates aqueous solution at 673 K and 30 MPa by this method. Particle size, phase and crystallinity of the obtained particles were characterized by TEM, XRD and TG, respectively. Effect of the difference of the metals in starting materials on particle size, conversion and crystallinity was analyzed on the basis of supersaturation, which was evaluated by estimated metal oxide solubilities. The result suggests that supersaturation should be set to higher than 104 in this method to obtain particles in single nanoscale. Further, crystallinity of the obtained particles was evaluated as weight loss through TG analysis. In addition that longer reaction time increased the crystallinity, it was found that smaller supersaturation also increased the crystallinity. This result can be explained that large supersaturation lead to include water molecule on the way to formation of particles. Less