Additive manufacturing of graded and anisotropic materials for applications at microwave frequencies
Keywords: multi-materials printing
microwave applications
dielectric composite
additive manufacturing

Transformation optics (TO) allows the design of new optical devices for the novel manipulation of electromagnetic (EM) waves based on the spatial variation of relative permittivity and permeability, including at microwave frequencies. However, the practical manufacturing methods to realize these designs are limited. This thesis focuses on the investigation of additive manufacturing (AM) to the manufacture of devices that operate at microwave frequencies and involve spatial variations of dielectric permittivity within the device.
Devices were manufactured using fused filament fabrication (FFF) and required the development of a bespoke relatively high dielectric permittivity filament that comprised barium titanate (BaTiO3) particulates dispersed in acrylonitrile butadiene styrene (ABS). The filament was optimized for printing by additions of a surfactant and a plasticiser to maximise the BaTiO3 fraction while maintaining flexibility and consistency to ensure long hours of uninterrupted printing. The optimized filament contained 32 vol% BaTiO3, 1 wt% surfactant and 5 wt% plasticiser, and had a relative dielectric permittivity of ∼ 11.
Demonstrator microwave devices were printed using the optimized filament and other commercial polymeric filaments typically with relative dielectric permittivity ∼3. The device designs involved spatial variations in the local, place-to-place dielectric permittivity that was achieved by mixing filament types during printing on a sub-wavelength scale. A quarter wave plate (QWP) was printed containing alternating stripes of BaTiO3/ABS and ABS, which successfully converted a linearly polarized wave into a circularly polarized wave at frequencies of 12 to 18 GHz. Three different spiral phase plates (SPPs) were also fabricated that were designed using numerical simulations. Each SPP was designed to provide the same transformation of an incident microwave at 15 GHz, but achieved through different spatial variations of permittivity. The printed SPPs successfully converted a plane wave into a helical wave carrying orbital angular momentum, and the advantages of certain design features were revealed. Multiplexing and demultiplexing were also successfully demonstrated experimentally using both two opposite mode and two same mode SPPs.
Lastly, to further explore the flexibility and degrees of freedom provided by AM of devices working at microwave frequencies, active device tuning was realized by printing of dielectric origami structures. Tuning was achieved through mechanical actuation and transformation of the dielectric element configurations. A suspended patch antenna with a resonant frequency of 1 GHz showed a maximum frequency shift of 0.45 GHz on actuation. Numerical simulations were performed consistent with experiments and suggested further potential for antenna tuning. The device demonstrations showed the potential of AM for realising spatially varying dielectric properties and the prospect for compact and broadband microwave devices.