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Nanoparticle Dynamics

Inorganic nanoparticles are structures with diameters in the range of 0.1 to 1000 nm. The properties of these particles are, in some cases, enhanced from those of the bulk material they originate from. These could be physical properties such as tensile strength, optical properties like reflectivity or any of a number of other physical attributes. A good example is iron oxide, or more specifically maghemite (y - Fe₂O₃) which, in the bulk phase, is used as a recording medium in cassette tapes and disc drives due to its magnetic properties. In the nanophase however, these properties become superparamagnetic which has led to new applications in MRI and magnetocaloric refrigeration.

Nanoparticles can be synthesized using either gas-phase or liquid-phase processes. In general, the gas-phase processes are prefered as they offer some advantages over the liquid routes, especially as regards the purity of the final product. Three main types of gas-phase reactor are used in the synthesis of nano materials:

• Tube reactors
• Plasma CVD chambers
• Flame reactors

Flame reactors can be further split into two main types, namely diffusion flames and laminar flat flames.

Most of the research in the CoMo group focuses on the morphology of nanoparticles formed from gas-phase reactions. In particular we study low-pressure premixed laminar flames producing SiO₂ and Fe₂O₃ and reactors producing TiO₂. Using simplified mechanisms for the combustion of H₂/O₂/Ar flames doped with various precursors, we are able to determine the rate of particle inception into the system along with the temperature and velocity profiles. This information allows us to run a simulation on the growth and morphology of the particles within the flame. We make use of the Smoluchowski coagulation equation, which has been modified to account for particle sintering, particle inception and surface growth. The mechanisms involved are explained below.

Sintering is the mechanism by which an aggregate particle will reduce its surface area over time whilst retaining all its mass:

Coagulation involves two particles coming together to make one larger particle:

Particle inception is the addition of new mass to the system from the gas phase in the form of a monomer particle:

Finally, surface growth is the addition of new mass to the system from the gas phase directly on to an existing particle:

To solve the overall particle balance model, we make use of stochastic particle algorithms. These methods allow the moments of the distribution to be determined along with the full PSD, and are also computationally very efficient. This fact allows multi-dimensional simulations to be run but without the excessive computational expense that is associated with standard finite element methods.