Compounding a High-Permittivity Thermoplastic Material and Its Applicability in Manufacturing of Microwave Photonic Crystals

Abstract

Additive Manufacturing (AM) techniques allow the production of complex geometries unattainable through other traditional technologies. This advantage lends itself well to rapidly iterating and improving upon the design of microwave photonic crystals, which are structures with intricate, repeating features. The issue tackled by this work involves compounding a high-permittivity material that can be used to produce 3D microwave photonic structures using polymer extrusion-based AM techniques. This material was acrylonitrile butadiene styrene (ABS)-based and used barium titanate (BaTiO₃) ceramic as the high-permittivity component of the composite and involved the use of a surfactant and a plasticizer to facilitate processing. Initial small amounts of the material were compounded using an internal batch mixer and studied using polymer thermal analysis techniques, such as thermogravimetric analysis, rheometry, and differential scanning calorimetry to determine the proper processing conditions. The production of the material was then scaled up using a twin-screw extruder system, producing homogeneous pellets. Finally, the thermoplastic composite was used with a screw-based, material extrusion additive manufacturing technique to produce a slab for measuring the relative permittivity of the material, as well as a preliminary 3D photonic crystal. The real part of the permittivity was measured to be 12.85 (loss tangent = 0.046) in the range of 10 to 12 GHz, representing the highest permittivity ever demonstrated for a thermoplastic AM composite at microwave frequencies.

Keywords: additive manufacturing; compounding; material extrusion; topological structures; twin-screw extrusion.

Figure 1

Schematic of single-screw extruder 3D printer.

Figure 2

Rectangular slab produced to characterize the permittivity of the material.

Figure 3

Internal mixing results comparing torque requirements of composites with and without plasticizer where the solid line is torque and dashed line is temperature (50 rpm, 190 °C).

Figure 4

Complex viscosity (|η*|) of ABS and ABSc without and with a plasticizer (0.1% strain, 160 °C).

Figure 5

TGA measurements of ABSc with and without plasticizer (10 °C/min, Oxygen): (a) Mass loss [%] vs. temperatures [°C]; (b) mass loss rate [%/°C] vs. temperature [°C].

Figure 6

DSC heating curves of neat ABS and ABSc without and with plasticizer (10 °C/min, nitrogen).

Figure 7

The experimental transmission and theoretical fitting transmission spectrum.

Figure 8

Backscattered electron micrographs of the surface of a 3D-printed layer of ABSc at 80× (a) and 1000× (b) magnification showing large areas of isotropically distributed barium titanate particles (white dots). Small (<50 µm) voids are visible on the surface. At 1000× magnification, individual BaTiO₃ particles are visible and are less densely distributed inside the void.

Figure 9

(a) Simulated band structure for a square lattice of dielectric rods. (b) Parallel-plate waveguide apparatus containing a 2D photonic crystal. (c) The measured and simulated (finite-difference time-domain) transmission through the crystal show excellent agreement with a ~45 dB decrease in transmission inside the photonic band gap (highlighted in grey). The direction along which the transmission was measured corresponds to the dashed line in (a).

Figure 10

3D photonic crystal.

Figure 11

Partial print with PLA support structures (left) and PVA (right).

Figure 12

(a) Nozzle cleaning system on the printing bed; (b) 3D photonic crystal with surface defects removed by immersing in acetone for 30 min.