Laser assisted material extrusion for multi-material metamaterials with radio frequency and microwave applications

In the last two decadesinvestment in additive manufacturing (AM) has been growing rapidly since it has been found as a highly influential technology. Although various AM processes have been investigated and developed so far, many of these are specific to one material system such as polymers, metals or ceramics. On the other hand, amongst these materials AM of ceramics is in early stages and require further research work to catch up with other AM processes used for polymers and metals. Robocasting is one potential technique, which has been used to produce ceramic parts. Low temperature cofired ceramics (LTCCs) that exhibit suitable electromagnetic (EM) properties (e.g. high permittivity, low loss) could be 3D printed with conductive materials concurrently using this technique and can effectively reduce the size and improve the EM performance of RF/microwave devices. Although laser AM of ceramics has been investigated for years, yet fabrication of functional parts using this technique has been rarely reported.

The research provided in this thesis combines laser and robocasting together to generate 3D multimaterial structures made of electroceramics and conductive materials such as silver that corresponds to the design requirements in terms of part resolution and EM properties. This will be realized using multi-stage fabrication processes combining laser assisted robocasting and conventional techniques. The process steps include (a) robocasting, (b) laser burnout/sintering, (c) furnace sintering to fabricate ceramic/Ag part as well as introducing a single-stage process where the effort is put into eliminating the need for conventional techniques. In other words, one stage fabrication process means the material is 3D printed and densified using in-situ laser sintering in a layered manner in order to skip the post-treatment step in conventional furnaces.

The study successfully demonstrated the fabrication of functional electronic devices using laser assisted robocasting of mono-materials and multi-materials. This technique enables us to produce low cost multi-material structures with reduced fabrication time and material waste compared to conventional manufacturing techniques such as screen printing. In screen printing LTCC modules are usually made by stacking green dielectric layers followed by co-firing to make a dense 3D circuit and connection between layers are formed by vias of solder which is a time-consuming process with increased material waste and cost. Therefore, using laser processing (burnout/sintering) opens up new areas for research that leads to reduced process time, less material waste, lower cost fabrication, and potentially greater material system compatibility for integrated metal/ceramic architectures.

A stable and printable slurry was obtained consisting of 82 wt% ceramic powder which was then used to print small and large parts using selective laser burnout (SLB) technique for the subsequent analysis and characterizations. It was indicated how the proposed fabrication technique can positively affect the density, porosity and EM properties of the devices. Sintering BMO for 4 hours at 645°C resulted in parts with ~95% theoretical density whilst maintain phase assemblage and good mechanical (compressive strength of 4097 MPa) and dielectric (Ɛr = 33.8 and tan δ = 0.0004)

properties.

The results also showed how the combination of laser and robocasting allow us to consider 3D printing of dissimilar materials that might be chemically incompatible. The measured EM properties of the fabricated multi-material BMO/Ag devices were improved and showed stability over the investigated frequency range (8-12 GHz). The results represented up to 100% increase in permittivity of BMO/Ag artificial dielectrics (εr 35.005) compared to the BMO only parts (εr 17.47) and the trend observed for the antenna devices agreed with the simulations.

However, such progress was not observed for the selective laser sintering (SLS) of the materials which have been attempted. Single layers of bismuth molybdate (BMO) were successfully laser sintered. BMO layers produced by SLS all suffered shortcomings and exhibit failings at producing defect free layers specially when the energy density was ≥ 0.7 J/mm2. Homogenously sintered layers were achieved at energy densities between 0.5 J/mm2 and 0.6 J/mm2. Unfortunately, it was not possible to achieve SLS of multi-layers due to the restrictions caused by Covid-19 pandemic. Therefore, the remained part of the research could potentially be the future work.