Additive manufacturing of advanced ceramics for demanding applications
thesisposted on 01.07.2019, 08:31 authored by William Rowlands
Interest and investment in additive manufacturing (AM) has been growing exponentially in recent years. Many AM processes for polymers and metals are nearing maturity, however for ceramics there are fewer processes available and they are in their early stages of development, with a significantly narrower material selection. AM offers benefits of light-weighting and increased efficiency through complex functional designs not possible by conventional manufacturing techniques whilst reducing material waste and tooling costs for different components. Ceramic heaters that exhibit positive temperature coefficient of resistance (PTCR) could be fabricated by AM for improved use in automotive and aerospace applications. This project demonstrates the feasibility of using AM to fabricate PTCR heating elements in various shapes and sizes including honeycomb lattice structures with the same material extrusion equipment – thus signifying the flexibility and suitability of AM to manufacture complex heater geometries. The project work traverse through an entire power-to-product processing excursion to achieve a functional prototype PTCR ceramic heater.
Barium titanate (BT) and lanthanum/manganese doped barium titanate (BT) based PTCR functional heater elements/structures were fabricated with desirable electrical properties for the first time using additive manufacturing by material-extrusion (robocasting). 3D printed components of varying size and shape, and prototype honeycomb lattices with high density were achieved with comparable and in some cases superior electrical properties. Aqueous, less organic containing (2.5 wt% additives versus 10-30 wt% added typically), eco-friendly ink formulations were developed with suitable rheological properties for 3D printing. For BT prints, the sintered densities of the 3D ceramic parts were found to be >99% TD, this is the highest reported value so far. The rheology of pastes required for robocasting was investigated in detail to understand the effect of dispersant, solids loading and viscofier contents. The rheological characteristics studied included yield stress, viscosity, shear stress, storage modulus and loss modulus. Different BT based PTCR mixtures were investigated through 3D printing, calcination, sintering along with conventional pressing & sintering for comparison. Effect of printing parameters on print quality and print microstructure were investigated. Drying of the prints was investigated as a parameter to allow printing of a wider range of pastes with different viscosity and yield stress. Yield stress of paste formulations and controlled drying were identified as key contributors to the successful 3D fabrication via robocasting. The microstructure, electrical properties and heating characteristics of the printed PTCR components were studied in detail and their thermal stability evaluated using infrared imaging and benchmarked against commercial PTCR heating element. The heating behaviour of the solid and porous 3D printed components was demonstrated to be similar, paving the way for light weight ( ̴47% reduction in weight) heaters suitable for automotive, aeronautical and even astronautical applications and the technique can also lead to significant materials savings during device fabrication.
The general applicability of the robocasting-based AM technique augers well for its use to manufacture complex shaped advanced ceramics for demanding applications, for example, in automotive (heaters as described here), healthcare (biomedical implants - personalised dental and hip/knee implants), defence (high frequency, high temperature conformal antenna structures) and energy (battery components) sectors.