This thesis investigates the morphology and polymorphism of TiO$2$ nanoparticles prepared in premixed stagnation flames in order to understand the factors controlling the formation of the particles. Computational models are developed and experimental measurements are performed to characterise the effect of varying process conditions on the particle properties. The model predictions and the experimental results are critically assessed to gain insights into the fundamental processes involved in particle formation.

A univariate particle model is implemented in a flame solver to simulate TiO$2$ particle formation from titanium tetraisopropoxide (TTIP) in a stagnation flame reactor. The particle model is solved using the method of moments with interpolative closure (MoMIC) and is fully coupled with the gas-phase chemistry and flow models. The model predictions are compared against literature data which reveals a good agreement for particle size but some discrepancy in particle geometric standard deviation (GSD). This suggests that the particle morphology is more complex that is assumed by the univariate particle model.

The morphology of TiO$2$ particles is characterised experimentally using transmission electron microscopy (TEM) image analysis and mobility measurements. The analysis allows a quantification of the particle sphericity. The detailed particle morphology is simulated using a post-processing method with a stochastic population balance solver. The model predictions show an excellent agreement with measurements for both particle size and GSD. The model is subsequently used to quantify the uncertainties in the measured particle morphology.

The phase composition of the experimentally produced TiO$2$ particles is analysed. The results demonstrate a high sensitivity of the phase composition to the premixed gas equivalence ratio, especially for near-stoichiometric mixtures. Metastable phases TiO$2$-B and TiO$2$-II are identified which provide new insights into the phase formation mechanism. It is suggested that TiO$2$-II is a pre-rutile phase and it is formed through sub-oxide intermediates, TiO.

The sensitivity of phase composition to the flame dilution is investigated experimentally for stoichiometric flames. The results demonstrate a strong influence of the flame dilution on anatase-rutile stability. Different hypotheses to explain the origin of this sensitivity are evaluated using the stochastic population balance solver with a size-dependent transformation model. The analysis suggests that the nascent particle composition may play a more important role in the anatase-rutile stability than the particle size.