SYNTHESIS OF POROUS FILMS FROM NANOPARTICLE AGGREGATES AND STUDY OF THEIR PROCESSING-STRUCTURE-PROPERTY RELATIONSHIPS

Porous films made from titania nanoparticle aggregates have a variety of uses in high surface area applications such as gas sensors, photocatalysts in treatment of wastewater and air pollutants, optical filters, and photovoltaic electrodes for low cost solar cells. A hybrid process based upon gas-to-particle conversion and particle precipitated chemical vapor deposition was used to synthesize porous films of titania nanoparticle aggregates. The residence time of particles in the reactor was varied and the influence on particle morphology and mechanical properties was studied. An increase in residence time resulted in an increase in primary particle diameter but did not significantly affect aggregate diameter, over the range of residence times considered in this study. The Young's modulus is shown to increase with a decrease in primary particle diameter. A study of the effect of post processing annealing on the particle morphology and mechanical properties was conducted. Increasing the annealing temperature resulted in particle growth at different temperatures and aggregate growth only at the highest temperature studied. The Young's modulus, however, shows only an influence of aggregate diameter, increasing as aggregate diameter increased. It is interesting to note that annealing did not result in a significant increase in Young's modulus or hardness until most of the surface area was lost. This suggests that annealing may not be the most effective process for strengthening films, if preservation of high surface area is desired. To better understand the effect of change in particle and aggregate diameters on Young's modulus, Monte Carlo and continuum methods were employed to explore structure-property relationships. A Monte Carlo method was used to simulate particle deposits and a finite element method was used to calculate the Young's modulus from strain energy of the deposits simulated. The results of this study indicate that a decrease in particle diameter increases the Young's modulus, especially below 15 nm. Aggregate size was not seen to have any effect on the Young's modulus, for the range of aggregate sizes considered. The results of these studies can be used to optimize the mechanical properties of titania films, made up of nanoparticle aggregates, for different desired applications