Symeta has many partners and the Institute of Circuit Technology has supported the project from the start. In the latest edition of their Journal [The Journal of the Institute of Circuit Technology Autumn Issue Vol.10 No.4 Page 3] they publish a review of the project to date written by Project Engineer, Dr Darren Cadman.
In the autumn of 2015, the UK’s Engineering and Physical
Sciences Research Council awarded Loughborough University £4 million to lead the Grand Challenge project “Synthesising 3D metamaterials for RF, microwave and THz Applications: SYMETA”. In collaboration with Exeter, Oxford, Sheffield and Queen Mary Universities, the project aims to deliver a palette of metamaterials that designers can deploy when creating high frequency circuits and components. In doing so it will provide microwave circuit and component engineers greater design freedom. The manufacturing processes to fabricate these circuits and components will be based on additive manufacturing principles. This article gives an overview of additive manufacturing and metamaterials, and how we aim to combine them.
Additive manufacturing (AM) describes a set of processes driven by technological advancements that are underpinned by the ethos of waste reduction to create products. It’s a market that is predicted to see £600 million investment in the UK over the next 5 years, with the UK AM community recently launching a National Strategy. The AM processes available enable the creation of functional structures that it would not have been possible to produce using standard manufacturing processes such as injection moulding or CNC milling. Typically, AM processes create products layer by layer. There are various methods available to do this, often dictated by the material that is being used to create the product, and of course, the product performance requirements. Examples include powder bed melting and sintering for creating products in metals or nylon. Recently, Airbus Space and Defence have produced a metal microwave waveguide that is space qualified for the European Space Agency.
There is also Stereolithographic Apparatus (SLA) that produces structures made from a vat of UV curable polymer. These processes are compatible for single material products, but can be modified or functionalised with post processing. Swissto12 are a company producing SLA manufactured waveguides that are then copper plated for deployment at microwave and terahertz frequencies.
For electronics products, the real interest in AM is those processes that can handle multiple materials, in particular insulators and conductors. Ink jet processes have been the foundation for printed and plastic electronics and can deliver multi-material deposition capability.
Key challenges for deploying AM processes for microwave and high frequency applications reside in the material choice available; UV curable polymers are relatively lossy at GHz frequencies and are therefore not suitable. Additionally, inkjet printing doesn’t lend itself to building large structures such as lenses whereby the structure’s dielectric constant is varied through the designed placement of air cavities.
Extrusion based processes, now popularised by consumer units such as those made by Ultimaker and referred to as 3D printers, offer the advantages of multi-material capability, can create relatively large structures in the X, Y and Z dimensions on a build plate, have modest resolution, and from suppliers such as PREMIX there are now materials emerging that are tailored for microwave applications.
Fused Filament Fabrication (FFF) is a process that involves the deposition of thermoplastic polymer layers through a heated nozzle. Such polymers include those based on polylactic acid (PLA) and
acrylonitrile butadiene styrene (ABS) and have been typically used for rapid prototyping and model creation. More recently there has been an interest in the development of conductive filaments that are polymer based and loaded with copper particles, such as that produced by Electrifi®. FFF lends itself to multi-material processing whereby a second nozzle and filament, or a second extrusion head, can be deployed as demonstrated by products from nScrypt® and Voxel8®.
Each of these have one head for FFF or material extrusion and a second for highly conductive silver ink extrusion.
Metamaterials have distinct properties not found in naturally occurring materials, in domains such as acoustics, optics, mechanics and, as presented here, in the microwave band of the electromagnetic(EM) spectrum. In the EM domain, these new materials can control wave propagation and be tailored to create localized and designed dielectric properties. As such they can be used to fabricate new microwave substrates that have manufacturing, physical and EM advantages over conventional dielectric materials. The metamaterials are designed to have certain EM properties by having dielectric or metallic inclusions within a host material. The spacing, size, material, shape and design of these inclusions determine the EM properties of the bulk material.
The 3D printing of a solid piece of PLA generally produces a material that has r ~ 2.75, μ r = 1 and a loss tangent tan ~ 0.008 (properties can vary between filaments and 3D print quality). The insertion of inclusions of a different dielectric, air for example with a lower relative permittivity as illustrated in Fig. 1, enables a designer to create substrates with a bulk effective permittivity eff < 2.75. As the volume of those air inclusions increases then the permittivity of the composite substrate tends closer to that of air.
Fig. 1: Air inclusions within a PLA 3D printed substrate, prior to encapsulation
with additional layers of PLA.
Instead of dielectric inclusions, if conductive elements are introduced, then rather than decreasing the host PLA dielectric constant, it can be enhanced as a result of introducing additional capacitance. Fig. 2 shows the Voxel8 printer creating silver inclusions within a PLA ‘ice cube tray’ which are then encapsulated with further layers of PLA on top. The structure shown in Fig. 2 displayed a dielectric constant of 4.5 at 10 GHz, in contrast to a solid piece of PLA with a
dielectric constant of 2.75.
Fig. 2: Silver inclusions printed within a 3D printed PLA substrate for assessing at 10 GHz.
Increasing the volume fraction of metal within the PLA does however have a drawback. The relative permeability decreases as shown in Fig. 3.
Fig.3: Trade off of increasing conductor content within a PLA substrate against relative permittivity and permeability.
This effect can be mitigated to some degree through the design of the metallic inclusions. Such design consideration can inhibit or disrupt the surface currents induced upon the metallic inclusions.
Potential for substrates and microwave circuit design: By being able to regionally create distinct substrate EM properties using AM processes, miniaturised filters, antennas and lenses could all be built in a single manufacturing step. To further push these concepts, ceramic-based materials are being developed within the consortium that have high dielectric constants and low loss, and that can be 3D printed to create novel microwave substrates. The challenges that lie ahead reside not so much in the AM processes, but in the materials that can be used with an end application in mind.
With thanks to the Institute of Circuit Technology for allowing us to reprint this article here.