Microwave performance and applications of additive manufactured components

Additive manufactured (AM) metals are of increasing interest for their performance in passive microwave applications, however several barriers exist to their large scale uptake. This thesis hopes to help address some of these barriers through the contribution of novel microwave techniques for the characterisation of metal AM parts and reporting of results from supporting experimental studies. A novel parallel plate resonator fixture is developed for the accurate measurement of the surface resistance of at metal plates produced by AM. This allows for microwave current flow in two orthogonal directions by simply exciting a different resonant mode. This has significance for the detection of anisotropy in a given plane that might arise through the laser scan path or vertical layer boundaries, for example, and is used here to assess the performance of individual wall surfaces, as they might appear in a manufactured waveguide component. Experimental studies are performed on the use of AM processes parameters to optimise the manufactured surfaces for low microwave loss, as well as quantifying the effects of several commonly used post-processing treatments. Improving microwave performance of unsupported, downward facing, surfaces is of particular interest and is investigated in this thesis, culminating in a ~ 40% reduction in surface resistance. Finally, a focus on practical applications in satellite technology is given through the evaluation of thermal properties of AM parts. A technique is described that uses fractional frequency shifts to evaluate the thermal expansion (CTE) of a cylindrical AM microwave cavity over an extreme temperature range (6{450 K) without the need for strict calibration. To the authors knowledge, this is the first time that CTE has been assessed over such a wide temperature range for AM parts, as is appropriate for space based components, using a passive microwave structure that can be adopted in a satellite communications system

Measurement technique for microwave surface resistance of additive manufactured metals

Additive manufactured (AM) metals are a subject of much interest for their performance in passive microwave applications. However, limitations could arise due to artifacts, such as surface texture and/or roughness resulting from the manufacturing process. We have, therefore, adopted a parallel plate microwave resonator for the accurate measurement of the surface resistance of flat metal plates, allowing for microwave current flow in two orthogonal directions by simply exciting a different resonant mode (at 5.3 and 6.4 GHz), without the need to remove and refix the sample. The systematic and random errors associated with the measurement of surface resistance are very small, less than 1% and 0.1%, respectively. The technique is demonstrated with measurements on a range of samples of the alloys, AlSi10Mg and Ti6Al4V, manufactured by laser powder bed fusion, in addition to traditionally machined samples of bulk metal alloys of aluminum and brass. For AM samples of AlSi10Mg, we have studied the effect on the surface resistance of directional roughness features, generated by the laser raster paths, in directions transverse or parallel to microwave current flow. Importantly for passive microwave device applications, we demonstrate that these samples exhibit no systematic anisotropy of surface resistance associated with such surface features

Liftoff dielectric resonator for the microwave surface resistance measurement of metal plates

A new method for accurate measurement of the microwave surface resistance of flat metal plates is proposed and verified experimentally, based on a sapphire dielectric resonator. System losses are accounted for by having continuous and controlled variation of the distance between the dielectric and the sample under test

Effect of build orientation and laser power on microwave loss in metal additive manufactured components

The adoption of metal additive manufacturing into the production of passive microwave components is still in its relative infancy. However, it is of increasing interest due to the offer of geometrical design freedom and significant weight reduction. The electrical properties of additive manufactured components are still inferior to traditional manufacturing techniques owing to the poor surface finish, especially on overhanging surfaces, which are unavoidable in three-dimensional microwave components. In this paper we present experimental findings on the disparity in microwave surface resistance values between three common build orientations, as well as findings that establish a connection between increasing downskin laser power and a reduction in surface resistance for overhanging surfaces. Finally, additive manufactured rectangular waveguide sections are measured to assess the influence of combined upward and downward facing surfaces on surface resistance

Evaluating the coefficient of thermal expansion of additive manufactured AlSi10Mg using microwave techniques

In this paper we have used laser powder bed fusion (PBF) to manufacture and characterize metal microwave components. Here we focus on a 2.5 GHz microwave cavity resonator, manufactured by PBF from the alloy AlSi10Mg. Of particular interest is its thermal expansion coefficient, especially since many microwave applications for PBF produced components will be in satellite systems where extreme ranges of temperature are experienced. We exploit the inherent resonant frequency dependence on cavity geometry, using a number of TM cavity modes, to determine the thermal expansion coefficient over the temperature range 6–450 K. Our results compare well with literature values and show that the material under test exhibits lower thermal expansion when compared with a bulk aluminium alloy alternative (6063)

Evaluating the coefficient of thermal expansion of additive manufactured AlSi10Mg using microwave techniques

In this paper we have used laser powder bed fusion (PBF) to manufacture and characterize metal microwave components. Here we focus on a 2.5 GHz microwave cavity resonator, manufactured by PBF from the alloy AlSi10Mg. Of particular interest is its thermal expansion coefficient, especially since many microwave applications for PBF produced components will be in satellite systems where extreme ranges of temperature are experienced. We exploit the inherent resonant frequency dependence on cavity geometry, using a number of TM cavity modes, to determine the thermal expansion coefficient over the temperature range 6–450 K. Our results compare well with literature values and show that the material under test exhibits lower thermal expansion when compared with a bulk aluminium alloy alternative (6063)

Cellular delivery of small interfering RNA by a non-covalently attached cell-penetrating peptide: quantitative analysis of uptake and biological effect

Cell-penetrating peptides (CPPs) have evolved as promising new tools to deliver nucleic acids into cells. So far, the majority of these delivery systems require a covalent linkage between carrier and cargo. To exploit the higher flexibility of a non-covalent strategy, we focused on the characterisation of a novel carrier peptide termed MPGα, which spontaneously forms complexes with nucleic acids. Using a luciferase-targeted small interfering RNA (siRNA) as cargo, we optimised the conditions for MPGα-mediated transfection of mammalian cells. In this system, reporter gene activity could be inhibited up to 90% with an IC(50) value in the sub-nanomolar range. As a key issue, we addressed the cellular uptake mechanism of MPGα/siRNA complexes applying various approaches. First, transfection of HeLa cells with MPGα/siRNA complexes in the presence of several inhibitors of endocytosis showed a significant reduction of the RNA interference (RNAi) effect. Second, confocal laser microscopy revealed a punctual intracellular pattern rather than a diffuse distribution of fluorescently labelled RNA-cargo. These data provide strong evidence of an endocytotic pathway contributing significantly to the uptake of MPGα/siRNA complexes. Finally, we quantified the intracellular number of siRNA molecules after MPGα-mediated transfection. The amount of siRNA required to induce half maximal RNAi was 10 000 molecules per cell. Together, the combination of methods provided allows for a detailed side by side quantitative analysis of cargo internalisation and related biological effects. Thus, the overall efficiency of a given delivery technique as well as the mechanism of uptake can be assessed

Effects of post-processing treatments on the microwave performance of additively manufactured samples

Several flat metal samples, produced by selective laser melting (SLM), have under-gone a variety of post-processing treatments. Microwave surface resistance values at a nominal frequency of 7.5 GHz are used to form a comparison between treatments, whilst also performing a comparison with traditionally manufactured copper PCB and aluminium samples. Measured values are then used to create a 3D simulation of a C-Band waveguide filter, utilising measurements from both horizontally and vertically built samples. Overall performance (quantified via low measured surface resistance) was found to be best in the traditionally manufactured copper samples. However silver plating was found to reduce the surface resistance of some SLM samples close to the values of bulk silver and copper