title: Fabrication of dense polymer derived mullite ceramics by fused deposition modeling (FDM) method
Creators: name: Sarraf, Fateme
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology

description: Using preceramic polymers (PCP) for additive manufacturing of oxide and non-oxide ceramics has attracted a lot of attention since a few years ago due to their extraordinary properties. Various kinds of polymer derived ceramics produced by utilizing different PCPs have shown excellent thermal stability, corrosion and oxidation resistance at high temperatures, biocompatibility, dielectric properties and so on. 
Thermoplastic properties of PCPs provide the plasticity and flowability desired for the additive manufacturing (AM) shaping processes. 
Fabrication of PDCs using material extrusion based additive manufacturing (MEX-AM) method, formerly called fused deposition modeling (FDM) or fused filament fabrication (FFF) process, has been addressed in a very limited set of studies with a narrow focus. Further investigations are needed to explore the full potential of this method for the fabrication of dense, crack-free polymer derived ceramics. 
This PhD thesis aimed to develop a fused deposition modeling (FDM) process for fabricating polymer-derived mullite ceramics, with a focus on achieving dense sintered parts with high wall thickness. The research questions addressed in this study are 1) the promotion of densification through the addition of MgO sintering aid, 2) the evaluation of ceramic yield and the effect of heating rate on the polymer derived mullite material, and 3) the prevention of structural defects through the creation of interconnected small pores. 
In this study, it was demonstrated that the addition of 1.0 wt% MgO significantly influenced the sinterability of mullite ceramics, resulting in increased densification and improved flexural strength. Furthermore, the impact of printing orientation on mechanical properties was mitigated by the presence of MgO. However, defects in the cross-section of the sintered samples, attributed to trapped water molecules, were still present. Therefore, the utilization of submicron alumina powder as an active filler, combined with a small amount of MgO, was necessary to obtain a pure mullite phase after sintering. 
Further, the ceramic yield of commercially available methyl-silsesquioxane (SILRES MK) was evaluated under different atmospheres and heating rates. Stable yields were observed under inert atmospheres and at higher heating rates in air, while lower heating rates led to decreased yields in air atmosphere. The heating rate was found to influence the composition of Si-O-Si bonds, with slower rates resulting in the dominance of cages over network structures. This 
finding suggested that volatile polyhedral silsesquioxanes (POSS) molecules were more likely to evaporate during the heat-treatment at lower heating rates. 
Finally, a new approach involving the modification of the thermoplastic binder system was proposed. By adding a water-soluble polyvinyl alcohol (PVA) grade, interconnected porous channels were formed, enabling the escape of released gases during post-processing. The use of a screw-based FDM printer facilitated the printing of feedstocks in the form of pellets, allowing for the fabrication of bulk polymer-derived ceramics without restrictions related to filament production. The choice of suitable ethylene vinyl alcohol (EVA) properties influenced the removal of gaseous products and the printability of the feedstocks. 
In general, the research presented in this thesis contributes to the development of FDM-based fabrication of polymer-derived mullite ceramics, and the methods and findings presented have the potential for application in the manufacturing of ceramic objects with diverse shapes using a range of plastic-forming technologies.