Jack R. McGhee

A broadband dual circularly polarized (dual-CP) reflectarray based on 3-D printed dielectric materials is proposed in this article. A novel 3-D dielectric array element that enables the broadband linearly polarization (LP) to CP transformation is proposed. The unit cell consists of two orthogonal dielectric cuboids that adjust the phases of the two orthogonal LP waves independently and then combine them into a CP wave. The innovative unit cell design provides an extra degree of freedom in varying the geometries of the array elements in all three dimensions, which enables us to independently control the phases of the two LP waves. This maintains an equal amplitude and 90° phase difference condition across the entire reflectarray surface, realizing a broadband and high gain LP–CP reflectarray. The placement of the feed is also optimized to achieve the highest aperture efficiency. Finally, an off-set reflectarray was designed and fabricated using lost-cost 3-D printing. The reflectarray is able to provide both left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP), with just an LP feed. The measurements agree well with simulated results where the maximum realized gain and directivity at 34 GHz are measured as 27.9 and 28.1 dBi, respectively. The measured 3 dB gain bandwidth and aperture efficiency are 30% and up to 38%, respectively. More importantly, a broad 3 dB axial ratio (AR) bandwidth greater than 40% has been achieved for both LHCP and RHCP, covering almost the entire frequency band of interest, ranging from 26 to 40 GHz.

Two screen printing inks were developed for the low‐temperature fabrication of printed and flexible thick film negative temperature coefficient thermistors able to operate at room temperature. The first of the two screen printing inks developed utilised cobalt ferrite (CoFe ₂ O ₄ ) as the temperature sensing material with the second ink incorporating manganese ferrite (MnFe ₂ O ₄ ). These were then screen printed onto lithographically printed silver interdigitated electrodes with a 200 µm track and gap using a synthetic paper (Teslin) as the substrate. The inks required a 10 min curing step at 80°C. Pre‐annealing of the ferrite powders before ink formulation enabled the avoidance of high‐temperature processing post‐fabrication typically required in industrial thermistor production. The printed thermistors were tested at a controlled constant humidity between 15 and 50°C. Both materials demonstrated typical natural logarithmic responses with high linearity and sensitivity.

In this research, fully metal oxide, capacitive humidity sensors for printed electronic applications have been designed and fabricated through the development of conductive indium tin oxide and dielectric aluminum oxide inks for the screen-printing process. Sensors were printed in a parallel plate configuration in 4 and 9 cm² conductive plate areas. Typically, commercially available discrete humidity sensors have a sensitivity in the range of 0.2–0.5 pF/RH%, whereas the printed humidity sensors presented in this paper have sensitivities of 0.85–7.76 pF/RH% depending on the sensor size, allowing for customizable properties. Response times were measured using a weighted average and found to be 21.4 s on average and recovery times were 4.8 s on average. The sensing performance was highly linear (R² > 0.97) for sensors of all sizes across the measured humidity range of 5–95%. Impedance spectroscopy was used to determine the sensing mechanism, and the mechanism was simulated and matched with experimental data. The sensing mechanism analysis shows that the sensing is primarily dictated by alumina at a lower relative humidity. The ITO contributes through increasing the ionic conductivity at a higher relative humidity, contributing to the high linearity of the sensor.

Manufactured resistors in conventional electronics are classified into tolerance groups ranging from <1% for high stability film types (E192) to 20% (E6) which are often carbon-based and utilised in less critical resistance value contexts such as current limiting or pull-up/down applications [IEC 60063:2015, Preferred number series for resistors and capacitors. One of the major identified challenges in the printed electronics industry currently is the ability to match this manufacturing capability for printed resistors in terms of initial tolerance, stability over time and power capabilities. In this work, a variety of screen-printed carbon resistors were designed and produced. The effects of utilising additional screen-printed ZnO and Ag layers as thermal variance management for the carbon resistors are investigated with the aim of improving the resistors power rating and stability. The introduction of ZnO or ZnO/Ag layers to carbon resistors saw notable improvements in the peak power capability, stability when sustaining 500 mW power dissipation, and stability in varying environmental conditions. Utilizing ZnO and Ag layers also notably improved the initial tolerance groupings when compared to basic uncoated carbon resistors.

Additive manufacturing (AM) has become more important and common in recent years. Advantages of AM include the ability to rapidly design and fabricate samples much faster than traditional manufacturing processes and to create complex internal geometries. Materials are crucial components of microwave systems and proper and accurate measurement of their dielectric properties is important to aid a high level of accuracy in design. There are numerous measurement techniques and finding the most appropriate method is important and requires consideration of all different factors and limitations. One limitation of sample preparation is that the sample size needs to fit in the measurement method. By utilizing the advantage of additive manufacturing, the material can be characterized using different measurement methods. In this paper, the additive manufacturing process and dielectric measurement methods have been critically reviewed. The test specimens for measuring dielectric properties were fabricated using fused filament fabrication (FFF)-based additive manufacturing and were measured using four different commercial dielectric properties measurement instruments including split post dielectric resonator (SPDR), rectangular waveguide, TE_01δ cavity resonator, and open resonator. The measured results from the four techniques have been compared and have shown reasonable agreement with measurements within a 10 percent range.

In this research the use of micron and nanoparticle metal oxide materials for conductive and dielectric screen printing inks were investigated. Screen printed parallel plate capacitors were fabricated to research the applications of these inks. Conductive, micron-particle indium tin oxide (ITO) screen printing inks were formulated for the conductive plates. A dielectric, nanoparticle aluminium oxide (Al ₂ 03) ink was formulated for the dielectric barrier. Capacitors of varying sizes (1cm ² - 9cm ² ) were printed onto flexible polyethylene terephthalate (PET) sheets. The effect of dielectric layer thickness was investigated to find the optimal fabrication process. Impedance analysis was performed on the capacitors to characterise both the performance of the capacitors and the dielectric ink. Analysis of the capacitors was also performed under flex (up to 30% strain) to investigate the effect of bending on the electronic properties. The capacitors were found to be highly stable under bending enabling their use in flexible printed electronics applications.

The humidity sensing properties of sputter-coated indium-tin oxide (ITO) and printed dielectric structures were tested for samples with sheet resistances ranging from 10 to 50 Q/sq. ITO/Polymer composite sensors were fabricated to form a parallel-plate capacitive-based humidity sensor that could detect relative humidity within a tested range of 5%-95%. The sensors were most stable and gave a linear response between 5% and 75% relative humidity. The capacitive sensors were characterized, using a range of techniques, to establish their capability and performance as humidity monitors. Response time of the humidity sensors was measured to be an average of 31.5 s and the recovery time was measured at an average of 31 s in the capacitive mode. Complex impedance spectroscopy was used to determine the mechanism of action for the sensors, which was found to be both the diffusion of water molecules into the dielectric layer and an increase of ionic conductivity within the dielectric layer. Stability of the humidity sensors was tested at three different humidity levels over seven days and sensors were found to be stable or follow a predictable change for this time span.

Three types of commercially available conductive three‐dimensional (3D) printing filament are electrically characterised for use in 3D‐printed functional devices. The three plastics were carbon dispersed acrylonitrile butadiene styrene, carbon dispersed polylactic acid (PLA) and graphene dispersed PLA. The method of 3D printing used was material extrusion and prints were made in both single and dual extrusion modes. The plastics were found to be piezoresistive, enabling them to be characterised as strain sensors. The electrical characteristics of these materials enabled the measurement of strain using low cost, readily available prototyping equipment and minimising the requirement for dedicated instrumentation components (e.g. Wheatstone bridges). Increasing the thickness of the plastics improved conductivity. However, this also decreased the reliability and reproducibility of strain sensor data due to a complex internal 3D structure. The recommendation for reliable use in prototyping and manufacturing is to print tracking under 0.8 mm thickness, thus producing resistance measurements that are predictable and follow a linear regression up to R ² = 0.9991. A dual extruded 3D print was fabricated as a final demonstration. A force sensing resistor interface was created. The final demonstration uses a PIC18F45K20 microcontroller to process sensor inputs, outputting to an alphanumeric LCD.