Hydrothermal synthesis at supercritical conditions is a useful method to produce metal oxide nanocrystals from metal salt aqueous solutions.  The high reaction temperature and the properties of supercritical water as a reaction medium make the reaction rate quite fast and the solubility of dehydrated products extremely low. Consequently, a rapid increase in degree of supersaturation, very high nucleation rates and the mass production of nanocrystals can be achieved.  In such a supercritical hydrothermal synthesis process, continuous flow reactors, in which two streams of metal salt aqueous solution and heated water are mixed at supercritical conditions, are commonly used. Rapid and uniform mixing of the streams is indispensable to produce metal oxide nanocrystals, and the size and its distributions of nanocrystals are strongly affected by how the reactants and supercritical water streams are mixed in the reactor. Therefore, it is important to understand the mixing behaviors of the streams, and the distributions of temperature and supersaturation in the reactor under supercritical conditions. However, the direct observation of the mixing behaviors in the reactor is difficult because the hydrothermal synthesis is performed at high pressure and high temperature in the reactor which is made of metal and consequently is opaque to visible light.
   In this work, we have performed neutron radiography on a tubular flow reactor with a diameter of 1/8 inch which is commonly used for supercritical hydrothermal synthesis of metal oxide nanocrystals (J. Lu et al., ACS Appl. Mater. Interface, 4 (2012) 351), and visualized the mixing behaviors of supercritical water and room-temperature water (corresponding to metal salt aqueous solution) and moreover temperature distributions in the reactor. Here, hydrogen and water are opaque against neutrons, but heavier elements including iron, nickel, and chromium, i.e., the metal wall of the reactor, are more transparent.  In addition, since neutron attenuation coefficient of water depends on its density, the difference between the densities of supercritical water and room-temperature water in the reactor can be visualized by neutron radiography. Figure 1 shows the schematic diagram of experimental setup, where a thermal neutron beam emitted from the B4 port of the KUR at the RRI, Kyoto University was used. The results demonstrated that the mixing behaviors, the distributions of water density and temperature in the reactor were clearly visualized by neutron radiography. The effects of the flow rates of supercritical water and room-temperature water on the temperature distributions in the reactor were also clarified. In addition, numerical simulations using a commercial software FLUENT were carried out to investigate the mixing behaviors in the reactor in detail.  Figure 2 shows the comparison between experimental and numerical results of temperature distributions in a tubular flow reactor (imaging area in Figure 1), where the distributions in (a) were obtained using the relationship between the neutron attenuation coefficient of water and temperature measured experimentally.

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