Conducting metal oxide materials for printed electronics

Printed electronics as a manufacturing process has many advantages, mainly, it allows for the high throughput rapid fabrication of thin, flexible electronic components with minimal waste. There are many printing processes that can be utilised for printing electronics and although each process can differ vastly, the materials currently used in these processes are generally the same, silver and carbon. However, to develop printing as a more mainstream manufacturing method for electronics, a wider variety of materials are required which can provide better stability and longevity of components, new functionality for printed applications and allow for in-situ processing and tuning of components. Conducting metal oxides are a good candidate for integrating into printed electronics processes, these materials are typically semiconductors, they have bandgaps, and properties can be altered via altering the band gap. They are also oxides, so they cannot oxidise further and therefore atmospheric damage is reduced compared to pure metals. They can also be fabricated into a wide range of particle morphologies, all with advantages in different fields and electronic applications. Therefore, the ability to print these materials is valuable to the field. In this thesis, the integration of conducting metal oxide electro-ceramic materials into the field of printed electronics has been explored. This was performed through the completion of five research objectives including, the selection of appropriate materials for the research, the formulation of conductive inks with the materials, the investigation of post-processing techniques for printed films and further research into passive component fabrication and sensor applications. Firstly, following an extensive literature review, four materials were selected including three doped zinc oxide materials synthesised via different methods. The fourth material is commercially sourced indium tin oxide (ITO). A nitrocellulose vehicle was determined to be the most compatible with the oxides and selected for ink formulation. Inks were then formulated with all four materials, with optical and electrical properties analysed. Gallium doped Zinc Oxide (GZO) and ITO were selected for further investigation based on the excellent conductivity of the indium tin oxide (57.77Ω□-1) and the highly transparent optical properties of the gallium doped zinc oxide (>84% transmittance). Laser processing was selected as a post processing method. It was found that the laser processing dramatically increased conductivity. The GZO improving from a non-conductive film to 10.21% of bulk conductivity. The ITO improved from 3.46% to 40.47% of the bulk conductivity. It was also found that the laser processing invoked a carbothermal reduction process allowing for a rapid manufacturing process for converting spherical particles into useful nanoparticle morphologies (nanorods, nanowires etc). Following this, resistive and capacitive applications involving laser processing and conventionally heat-treated conductive oxide inks were developed. Combining the new materials and manufacturing processes, tuneable printed resistors with a tuning range of 50 to 20M could be fabricated. All metal oxide, ITO based capacitors were also fabricated and characterised. These were then developed into humidity sensors which provided excellent humidity sensing properties, showing linearity between 5 and 95% relative humidity (RH) and sensitivities of up to 7.76pF/RH%, demonstrating higher performance than commercial equivalents (0.2 – 0.5pF/RH%). In conclusion, this work provides a breakthrough for conductive metal oxide materials research and its place in Printed Electronics research by providing insight into the processes required to make these materials conduct and by developing useful manufacturing methods, post processing techniques and applications.</div

Evaluation of microwave characterization methods for additively manufactured materials

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, TE01δ 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

Printed resistors for flexible electronics – thermal variance mitigation and tolerance improvement via oxide-metal coatings

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 [1]. 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 screenprinted carbon resistors were designed and produced. The effects of utilising additional screenprinted 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 500mW 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

Printed, fully metal oxide, capacitive humidity sensors using conductive indium tin oxide inks

© 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

Ferrite-based room temperature negative temperature coefficient printed thermistors

Two screen printing inks were developed for the low-temperature fabrication of printed and flexible thick film negative temperature coefficient (NTC) thermistors able to operate at room temperature. The first of the two screen printing inks developed utilised cobalt ferrite (CoFe2O4) as the temperature sensing material with the second ink incorporating manganese ferrite (MnFe2O4). These were then screen printed onto lithographically printed silver interdigitated electrodes with a 200-micron track and gap using a synthetic paper (Teslin) as the substrate. The inks required a 10-minute curing step at 80oC. Preannealing 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 50oC. Both materials demonstrated typical natural logarithmic responses with high linearity and sensitivity.<br

Fused filament fabrication of functionally graded polymer composites with variable relative permittivity for microwave devices

Fused filament fabrication (FFF) is a continuously growing additive manufacturing technology that aside from physical prototypes can also deliver functional prototypes and devices for radiofrequency (RF) and microwave applications. The very recent introduction of high-permittivity filaments for FFF has been one of the main facilitators for this major advancement, aiding microwave engineers to realise academics concepts that have thus far been impossible to fabricate and therefore invent new designs. However, the value to the RF community of these devices depends on accurate knowledge and repeatability of the electromagnetic properties of the materials being used which strongly relies on the processing strategy used during printing. This paper investigates the use of a high-permittivity filament and studies the impact of layer height and infill density on the relative permittivity (εr) and loss tangent (tanδ). A maximum relative permittivity of εr = 9.63 ± 0.16 and tanδ = 0.003 ± 0.0003 was achieved with a 200 μm layer thickness and 100% infill density. Finally, the results of this study are used in designing, simulating, 3D printing and measuring the performance of a novel graded-index dielectric lens operating at 10 GHz

Dual circularly polarized 3-D printed broadband dielectric reflectarray with a linearly polarized feed

A broadband dual circularly polarized (dual-CP) reflectarray based on three-dimensional (3D) printed dielectric materials is proposed in this paper. A novel 3D dielectric array element that enables the broadband linearly polarization (LP) to CP transformation is proposed. The unit cell consists of two orthogonal dielectric cuboids, which 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 3D 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 dBi 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

Fabrication of artificial dielectrics via stereolithography based 3D-printing

In this research, stereolithography (SLA) based additive manufacturing (AM) has been investigated as a fabrication method for producing artificial dielectrics. Initially, the effect of the curing time on the microwave electromagnetic properties (X-band) on the photoinitiated resin used was measured and found to be negligible after 15 minutes of UV curing. Artificial dielectric isotropic and anisotropic lattice structures were then designed and fabricated, allowing for varying permittivity between 1.23 and 2.80 through the control of the structure’s density. As a demonstration of the ability to grade permittivity through a high-resolution printing process, lattice structures were embedded into solid substrates. The ability to do this allowed for the printing of a graded permittivity substrate which is showcased in a design for a circularly polarized patch antenna

Synthesis and dielectric characterisation of a low loss BaSrTiO₃/ABS ceramic/polymer composite for fused filament fabrication additive manufacturing

Composite polymer/ceramic filaments for material extrusion-based fused filament fabrication additive manufacturing, using barium strontium titanium oxide (BST) ceramics and acrylonitrile butadiene styrene (ABS) thermoplastics were produced; their dielectric and physical properties have been characterised for the first time. The dielectric properties, relative permittivity (εr), quality factor (Q×f) and dielectric loss (tanδ) were measured as a function of ceramic solid loading (%) at 5 GHz for 3D printed samples. A relative permittivity εr = 6.05, Q×f = 10,433 GHz and dielectric loss tanδ = 0.007 were obtained for a BST/ABS ceramic polymer composite, with 50 wt% (15 vol%) solid loading. The composite materials exhibit reduced dielectric losses compared to standard laminates currently used in the radiofrequency (RF) and telecommunications industry. Based on polymer/ceramic composite filament, a prototype microstrip patch 5 G antenna and a hemispherical dielectric lens were designed and manufactured. Through testing, it shows good antenna performance with a centre frequency of f0 = 3.78 GHz and a (−10 dB) bandwidth of 90.6 MHz. The dielectric lens increased the antenna gain by 3.86 dBi.</p

Indium tin oxide nanowires manufactured via printing and laser irradiation

Metallic and semiconductor nanowires can provide dramatically increased electrical and optical properties in a wide range of fields, ranging from photovoltaics to sensors and catalysts. In this research, a rapid manufacturing process has been developed for printing indium tin oxide microparticles and converting them into nanowires. Microparticle indium tin oxide (ITO) inks were formulated and printed. These were then converted into hierarchical nanowire films via laser irradiation (980nm, NIR) with raster speeds of 40 mm s-1 in air, much faster compared to traditional manufacturing processes. For a 4 cm2 film, only 40 seconds of processing were required. A full materials characterization was performed on the materials pre and post laser processing with the most probable conversion mechanism found to be a laser induced carbothermal reduction process. Microstructural, chemical, and crystallographic evidence of the laser induced carbothermal reduction process were derived from SEM, XRD, XPS and TEM analysis. Compared to conventionally heat-treated printed samples, laser processing was found to increase the conductivity of the printed ITO from 0.88% to 40.47% bulk conductivity. This research demonstrates the ability of printing and laser processing to form nanowires in a high-speed manufacturing context, thereby enabling the development of printed non-transparent ITO nanowire electronics and devices