The Self-assembly of Organosilane Films on Aluminium Oxide and their Application to Si-Al Composite Materials for 3D Printing.

Author: Ruby Sims

Sims, Ruby, 2019 The Self-assembly of Organosilane Films on Aluminium Oxide and their Application to Si-Al Composite Materials for 3D Printing., Flinders University, College of Science and Engineering

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Organosilanes provide a unique opportunity to modify the physical and chemical properties of a hydroxylated surface in an environmentally friendly and cost-effective manner. The water activated self-assembly mechanism of Organosilanes in solution has been extensively studied however, the mechanism by which they interact with a surface and condense to form thin films is not well known. The original contribution to knowledge presented in this thesis is a comprehensive picture of silane-substrate interactions through both the ex-situ and in-situ monitoring organosilane films on a molecular scale. A link has been successfully made between the time dependant oscillatory surface coverage of silane films measured using X-ray Photoelectron Spectroscopy and the presence of more than one hydrolysable group on the central silicon atom. This along with the absence of such behaviour for silanes which contain just one hydrolysable group and follow a time dependant Langmuir-type growth profile indicates that the origin of this behaviour lies with the ability of silanes to oligomerise both in solution and on the surface.

Conformation of oligomerised silane islands on the surface of oscillating Propyltrimethoxysilane and Propylmethyldimethoxysilane films and the absence of these islands in Propyldimethylmethoxysilane films imaged using Auger Electron Spectroscopy, led to the proposal of a surface condensation mechanism involving the growth and desorption of oligomerised islands, the collective behaviour of which creates a measurable oscillation in surface coverage. In order to further probe the interactions of silane molecules and the substrate, the role of covalently and hydrogen bound silane species was explored. The removal of hydrogen bound species while reducing the amount of silane on the surface did not remove the oscillatory behaviour, confirming that the oscillation involves the removal of covalently bound species from the surface. This mechanism was related directly to a reversal of the equilibrium condensation reaction as a result of an increase in localised water concentrations, a product of the condensation reaction itself. The kinetics of these interactions were proposed through the creation of a theoretical 10-component mathematical model requiring competing silane-substrate and water-substrate interactions. For each oscillation to occur there must be two components present in solution, a silicon containing species and a non-silicon containing species, thought to be water.

Probing the silane-substrate interface using Sum Frequency Generation spectroscopy not only confirmed the presence of a monolayer-like coverage amongst the oligomerised islands of Propyltrimethoxysilane films, but that the order and number of defects within this film is directly related to molecular packing on the surface. The presence of an ordered, physisorbed bilayer on top of covalently bound Propyltrimethoxysilane films was also identified as a key process of the self-assembly mechanism.

This extensive investigation into the mechanism of silane film self-assembly and the knowledge accrued from it was then used to develop new application for silane films via the creation of a novel, 3D printed Al-Si composite material. The silane functionalisation of aluminium powders used as starting material for 3D-printing not only presented a new method of incorporating silicon into a 3D-printed aluminium matrix but also improved the flowability of these powders, a key property required for the Laser Metal Deposition 3D-printing process; an improvement so significant that it has allowed for the successful printing of aluminium powders previously deemed unprintable.

Keywords: Organosilane, alkoxysliane, self-assembly, 3D Printing, oscillatory adsorption, Auger Electron Spectromicroscopy, X-ray Photoelectron Electron Spectroscopy, film Morphology, thin films, aluminium oxide.

Subject: Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Professor Jamie S Quinton