Gruar, Robert I. J.; (2013) Synthesis and Characterisation of Nanomaterials Produced Using Laboratory and Pilot Scale Continuous Hydrothermal Flow Reactors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Due to their small particle size, nanoparticles ( < 100 nm in diameter) have an increased surface area to volume ratio compared to larger particles, meaning that surface attributes become increasingly important over bulk properties. Chemically, this means more atoms in the material have unsatisfied coordination environments compared to atoms in the bulk of the particle. In many cases, this leads to materials with significantly different bulk properties compared to much larger particles; some of these unique properties are desirable in high technology applications such as sun screens, catalysts, etc. This thesis explores the use of Continuous Hydrothermal Flow Synthesis (CHFS) reactors as a niche technology to controllably produce nanoparticles at different process scales. In CHFS, a metal ion feed is mixed with superheated water (the latter is typically above the critical temperature and pressure of water, i.e. 374 °C, 22.1 MPa), and nanoparticles are precipitated. This thesis presents data relevant to an evaluation of a laboratory scale CHFS process (able to produce ca. 100 g a day of nanoparticles). This included the development of a new type of mixer for this type of process suitable for the continuous precipitation of nanoparticles. The knowledge gained from in situ measurements and particle property measurements was then applied to the successful scale up of the technology to produce up to ca. 2 kg per hour of nanoparticles. It was demonstrated that the versatility of a flow process and the rapid crystallising environment in a CHFS system could be effectively exploited for the production of target nanoparticles when appropriate synthesis conditions were used. This thesis has also demonstrated the versatility of CHFS in that as formed particles could be surface functionalised in flow by use of an additional feed in process. The outcomes of this thesis have been demonstrated using a variety of material compositions; Hydroxyapatite, ZnO, iron oxides, yttrium oxyhydroxide, yttrium oxide and the binary oxide series Ce-Zn. Where each material composition was used to probe different aspect of the continuous hydrothermal process reported in this work. In summary, the CHFS process has been evaluated and developed to allow for synthesis of a wide range of nanoparticle compositions with different particle properties. Process modifications have been evaluated and shown to be suitable for the synthesis of target materials.

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