Silver-based Inks Research Articles

Assessment and improvement of adhesion of printed silver-based inks on alumina produced by fused filament fabrication

More efficient electronic systems need more temperature-stable substrate materials and more flexible production possibilities. Ceramics such as alumina can be used for 2D and 3D base bodies, since they provide better thermal properties than their polymeric counterparts and can be additively manufactured utilizing e.g. Fused Filament Fabrication. Proper functionalization can be achieved by applying methods of printed electronics. Here, adhesion of the functional ink to a substrate is a crucial issue and especially challenging on ceramic substrates. This contribution reached a noteworthy increase in this matter, using a modified silver-based ink with piezojet-printing, while preserving very low electrical resistance after convection sintering. Adhesion is determined by pull-off testing, based on ISO 4624. By the approach shown in this work, printed ceramics could become a more viable choice for use in functional electronic systems.

On the Development of Inkjet-Printed Band Pass Filters Based on the Microstrip Hairpin Structure

In recent years, inkjet printing has emerged as a promising advanced fabrication technology in the field of electronics, offering remarkable advantages in terms of cost-effectiveness, design flexibility, and rapid prototyping. For these reasons, inkjet printing technology has been widely adopted in various applications, including printed circuit board fabrication, sensor development (e.g., temperature, humidity, and pressure sensing), and antenna and filter production, up to the microwave frequency range. The present paper is focused on the investigation of a methodology based on Monte Carlo simulations for quantitatively assessing the influence of fabrication tolerances on the performance of inkjet-printed microwave devices. In particular, the proposed methodology is applied to an inkjet-printed hairpin band pass filter specifically tailored for operation in the L band (i.e., from 1 GHz to 2 GHz). The initial design phase involved the use of computer aided design (CAD) software to optimize the geometric dimensions of the designed filter to closely match the desired performance specifications in terms of bandwidth, insertion loss, and return loss. Later, a Monte Carlo analysis was conducted to evaluate the propagation of tolerances in the fabrication process throughout the design and to estimate their effects on device performance. The fabrication process exploited the advanced capabilities of the Voltera inkjet printer, which was used to deposit a silver-based conductive ink on a commercial Rogers substrate. The device’s performance was evaluated by comparing the simulated scattering parameters with those measured on the developed filter using a vector network analyzer (VNA), thus ensuring accurate validation of real-world performance.

Additive manufacturing of sensor prototype based on 3D-extrusion-printed zirconia ceramics

Additive manufacturing of ceramics has attracted large interest due to its unique ability to rapidly prototype, low cost, and increased geometric complexity. Material extrusion technique is often considered for processing and shaping fine and dense ceramic structures because of the high solid loading in ink. In this work, 8 mol.% yttria-stabilized zirconia ink with 70 wt% ceramic loadings was prepared to print zirconia samples in horizontal and vertical directions, following two different filament orientations: 0/90° and ±45°. The study's main objective was to evaluate printing design influences on mechanical properties. This was assessed through flexural testing and digital image correlation analysis. Experimental findings showed that samples printed horizontally with ±45° filament orientation displayed the highest strength. A detailed inspection of fracture surfaces revealed that printing defects were the critical failure location sites formed after sintering and intimately related to printing design issues. After that, a conductive sensor was integrated into the surfaces of 3D-printed specimens by screen-printing silver-based conductive ink to analyze the structural health of zirconia samples. The sensing capabilities of printed conductive patterns were investigated using the four-point bending tests. The results demonstrated that the electrical resistance of the printed silver patterns increased as the applied load on zirconia substrates rose. It indicates that hybrid material extrusion and screen printing techniques are favored ways to manufacture sensors for structural detection of advanced 3D-printed ceramics.

Low-temperature silver-based ink for highly conductive paths through industrial printing processes suitable for thermally sensitive substrates and beyond

Printed electronics is experiencing tremendous growth in applications and industry interest worldwide. One of the most frequently raised problems is the high curing temperature of commercially available conductive composites. This study describes the process of developing a customized low-temperature silver-based, conductive ink dedicated to the flexographic-printing technique. The formulation was optimized through a series of tests including printing trials, resistance measurements, evaluation of printed samples surface features, and mechanical properties. The appropriate ink viscosity, substrate compatibility, and sinterability at low temperatures were achieved. The dependence of the achieved conductivity on the type of raw materials used was also witnessed and described. The conductivity of the printed inks was evaluated for various polymer vehicles. Samples printed on flexible PET and paper substrates were thoroughly investigated; ink adhesion, scratch resistance, and performance after cyclic bending have been assessed.Graphical abstract

Mechanical Analysis and Constitutive Modeling of Nonlinear Behavior of Silver-based Conductive Ink

Stretchable electronic devices are progressively deployed in many applications of mechanical, electrical, and bio-medical engineering. These circuits are made of stretchable and flexible substrate, as well as conductive ink, and various electronic components. The choice of components and layouts for the substrate and conductive ink can regulate the stretchability of stretchable circuits. On top of that, the substance utilized to create the conductive ink must have high electrical conductivity and strong adherence to the substrate in order to produce a high-quality stretchable printed circuit. Thus, this study focused on the development of stretchable conductive ink using silver powder as a conductive filler and PDMS-OH as a binder. The mechanical properties of the synthesized ink were investigated via simple uniaxial tensile testing method and nanoindentation technique, respectively. Accordingly, the modulus of elasticity, tensile stress and yield stress of the ink were obtained as 5.72 MPa, 1.195 MPa, and 0.86 MPa, congruently at 137% strain before undergoing failure. The experimental stress-strain data was then employed on the elastic-plastic constitutive model to investigate the elastomeric properties of the ink as it is an alternative method of lengthy and expensive procedures of validating different polymers. Moreover, the hardness and reduced modulus of the ink were evaluated by nanoindentation method using 5 mN maximum load with 0.5 mN/s loading/unloading rate and 2 secs holding time. Consequently, the hardness and reduced modulus values were obtained as 1.45 MPa and 34.53 MPa, respectively. These values were further validated by Oliver-Pharr method, and were in a good agreement.

Materials screening and characterization for functional printed automotive interiors parts

Flexible printed electronics (PE) has attracted strong interest during the last two decades and is one of the successful trends in material science, representing the future of PEs. This research work evaluates the use of screen-printing technology and materials for producing functional circuits for automotive interior parts, which can be subsequently processed through in-mold electronics (IME). Since the selection of the materials to build the printed system is of utmost importance, this study evaluates combinations of commercial polycarbonate substrates (LEXAN 8A13E, DE 1-4 060007 and LM 905 2-4 160009) and silver-based inks (ME603, ME604 and CP 6680), all suitable for IME. Different electrically conductive tracks varying in width and spacing (0.5, 0.3 and 0.2 mm) and two capacitive sensors were printed. Tensile tests and surface energy characterizations of the different polycarbonate substrates were carried out, then morphological, electrical, and thermoforming studies were performed on the printed substrates. Morphological characterization showed successful printing for wider lines (0.5 and 0.3 mm), but problems with screen clogging occurred for smaller line widths (0.2 mm). The electrical conductivity of printed tracks was in accordance to the printed layer thickness and ink solids percentage. The proof-of-concept of the electrical functionality was successful, when integrating the sensors into the PCB with SMD LEDs. Thermoforming showed limited functionality, with the best overall performance observed for specific combinations of substrate and ink. In essence, the results indicate that although all the selected substrates and silver-based inks have great compatibility among themselves and can be considered as materials for the production of functional automotive interior parts, there is no ideal pairing of inks and substrates. Therefore, this study emphasizes the importance of defining product specifications for a more suitable material selection.

Low-Cost Direct-Writing of Silver-Based Ink for Planar Microwave Circuits up to 10 GHz

Direct ink writing (DIW) of conductive ink is a printed electronics technology that allows a variety of electronic circuits to be produced in a simple way and with minimal waste of materials. In recent years it has been used for rapid prototyping of RF circuits typically working at S-band frequencies (2–4 GHz). In an attempt to extend this frequency range while maintaining cost-effective prototyping, this work has focused on proving the feasibility of DIW of silver-conductive (SC) ink for the fabrication of planar microwave circuits beyond 10 GHz, more specifically, ultra-wideband (UWB) antennas for medical applications. For this purpose, the DC and RF performance of the SC ink, as well as the FR4 substrate used, were first evaluated. Based on the comparison between experimental and simulated results, we have found that the effective RF conductivity of the SC ink is approximately 27.6% of its DC value and 3.4% of the copper conductivity. A few test microstrip circuits were fabricated by DIW, namely two S-band filters and one UWB antenna. The overall measured performance of all of them agreed well with simulations. In particular, the DIW antenna exhibited a bandwidth of 8.2 GHz (between 2.4 and 10.6 GHz), and was compared with an identical copper antenna showing that both have very similar characteristics. It was also found that the lower conductivity of SC ink as compared to copper led to a gain reduction of only 0.3 dB.

Integration of conductive silver sensors on zirconia ceramics by screen-printing for monitoring strain under applied load

There is a growing interest in zirconia ceramics due to their high flexural strength, excellent corrosion resistance and good biocompatibility. The assembly of advanced zirconia material with functions of sensing, actuation and controlling to solve the problems that may arise during its use is critical to ensure long-term service and performance. Within this context, the objective of this work is to analyse the structural health of zirconia samples by screen-printing silver-based conductive ink on the surface subjected to maximum load under flexural testing. In doing so, silver conductive ink was formulated using silver nanoparticles where polyvinyl pyrrolidone acted as the capping agent. Silver ink was then screen-printed onto zirconia samples and sintered at 200 °C. The resistance of the sensors was measured and the sensing capabilities of printed conductive patterns were investigated using the four-point probe method. Experimental findings displayed a reproducible direct correlation between electrical resistance change and strain resulting from surface displacement under applied load in four-point bending.

The need for design-for-recycling of paper-based printed electronics – a prospective comparison with printed circuit boards

The present study compares conventional printed circuit boards (having glass-fibre and epoxy substrates and etched copper circuits) with paper-based printed electronics (offering flexible, bio-based, and biodegradable substrates with circuit design printed using silver-based inks) and assesses the relevance of e-waste recycling to the latter's sustainability. Therefore, a comparative life cycle assessment between these two options has been undertaken and the global warming impacts were calculated.The impact assessment results underscore that printed electronics offer a consistent sustainability advantage over printed circuit boards only through recycling of silver in the former at the end-of-life. Hence, design-for-recycling and recycling as e-waste are crucial to the sustainability of the current generation of printed electronics. Other foreseen waste treatment options for paper-based printed electronics, such as composting, and paper recycling, are likely to limit the sustainability advantage of printed electronics to circuits with small conductive areas.

Cost-Driven Design of Printed Wideband Antennas with Reduced Silver Ink Consumption for the Internet of Things.

The Internet of Things (IoT) accelerates the need for compact, lightweight and low-cost antennas combining wideband operation with a high integration potential. Although screen printing is excellently suited for manufacturing conformal antennas on a flexible substrate, its application is typically limited due to the expensive nature of conductive inks. This paper investigates how the production cost of a flexible coplanar waveguide (CPW)-fed planar monopole antenna can be reduced by exploiting a mesh-based method for limiting ink consumption. Prototypes with mesh grids of different line widths and densities were screen-printed on a polyethylene terephthalate (PET) foil using silver-based nanoparticle ink. Smaller line widths decrease antenna gain and efficiency, while denser mesh grids better approximate unmeshed antenna behavior, albeit at the expense of greater ink consumption. A meshed prototype of with almost 80% ink reduction compared to an unmeshed counterpart is presented. It is capable of providing wideband coverage in the IMT/LTE-1/n1 (1.92–2.17 GHz), LTE-40/n40 (2.3–2.4 GHz), 2.45 GHz ISM (2.4–2.4835 GHz), IMT-E/LTE-7/n7 (2.5–2.69 GHz), and n78 5G (3.3–3.8 GHz) frequency bands. It exhibits a peak radiation efficiency above 90% and a metallized surface area of cm2 (yielding an ink-to-total-surface ratio of 12.2%).

Development of New Simple Compositions of Silver Inks for the Preparation of Pseudo-Reference Electrodes.

Silver materials are known to present excellent properties, such as high electrical and thermal conductivity as well as chemical stability. Silver-based inks have drawn a lot of attention for being compatible with various substrates, which can be used in the production uniform and stable pseudo-reference electrodes with low curing temperatures. Furthermore, the interest in the use of disposable electrodes has been increasing due to the low cost and the possibility of their use in point-of-care and point-of-need situations. Thus, in this work, two new inks were developed using Ag as conductive material and colorless polymers (nail polish (NP) and shellac (SL)), and applied to different substrates (screen-printed electrodes, acetate sheets, and 3D-printed electrodes) to verify the performance of the proposed inks. Measurements attained with open circuit potential (OCP) attested to the stability of the potential of the pseudo-reference proposed for 1 h. Analytical curves for β-estradiol were also obtained using the devices prepared with the proposed inks as pseudo-references electrodes, which presented satisfactory results concerning the potential stability (RSD < 2.6%). These inks are simple to prepare and present great alternatives for the development of pseudo-reference electrodes useful in the construction of disposable electrochemical systems.

Silver-Copper Alloy Nanoinks for Ambient Temperature Sintering.

There is an increasing need to reduce the silver content in silver-based inks or pastes and achieve low-temperature sintering for scalable and low-cost production of printed wearable electronics. This need depends on the ability to control the metal composition and the surface properties of the nanoinks. Alloying silver with copper provides a pathway for meeting the need in terms of cost reduction, but little is known about the composition controllability and the low-temperature sintering capability. We report herein a scalable wet chemical synthesis of bimetallic silver-copper alloy nanoinks with room temperature sintering properties. The bimetallic alloy nanoparticles with a controllable composition can be formulated as stable nanoinks. The nanoinks printed on paper substrates are shown to sinter under room temperature. In addition to composition dependence, the results reveal an intriguing dependence of sintering on humidity above the printed nanoink films. These findings are assessed based on theoretical simulation of the sintering processes via surface-mediated sintering and interparticle necking mechanisms in terms of nanoscale adsorption, adhesion and diffusion, and surface free energies. Implications of the findings for room temperature fabrication of wearable sensors are also discussed.

Recent development in silver-based ink for flexible electronics

Silver based flexible printed electronics offer the possibility to design low-cost environmentally friendly products through versatile manufacturing procedures. In this regard, conductive silver inks are commonly printed on flexible substrates because in contrast to their macroscale analogues, the silver nanoparticles (AgNPs) present different salient physical and chemical properties. However, an extensive application of silver inks is often limited by factors such as a complicated ink preparation process, conflict between ink stability and nanoparticle content, overall ink formulation and selection of suitable sintering techniques. Nevertheless, advances in nanotechnology and printing techniques have opened new rooms to improve the synthesis, stability, processing, and application of silver inks. This review article presents the progress in AgNPs processing, Ag-conductive ink formulation and important printing techniques, especially as it relates to the manufacture of flexible electronics. Furthermore, the significance of the sintering process and the importance of various kinds of sintering methods for improving ink quality and adhesion to substrates are discussed. In addition, some of the notable applications of printed flexible electronics are highlighted.

Green Synthesis of Nanoparticles Using Bio-Inspired Systems and Electrically Conductive Pattern Fabrication through Laser-Direct Writing.

Systems existing in nature have evolved to operate efficiently over a long period of time, enabling efficient material transformation and processing. These natural systems provide hints for the synthesis of metal nanoparticles through efficient electron generation and transport towards metal ions for the reduction process. In this study, based on the efficient electron transfer mechanism between tryptophan (Trp) in the living body, the possibility of advanced silver patterning on flexible substrates has been presented through laser-direct writing. Irradiation of a low-power laser on the precursor induces the reduction of silver ions to nanoparticles. The sintering of these generated nanoparticles induces a silver conductive pattern by a photothermal/chemical reaction. The method of this study has strength as it supports the possibility of conductive pattern fabrication on various substrates (e.g., glass and PDMS) using a silver-based organic ink with low laser power compared to the conventional nanoparticle-based sintering method. It also suggests its suitability to various applications in terms of sophisticated pattern fabrication with minimized substrate denaturation.

A silver-based ink for assessing low activity photocatalytic films

A new type of photocatalytic activity indicator ink, paii, is described based on the photocatalysed reduction of Ag ions to the metal, rather than that of a dye, such as resazurin, Rz. UVA irradiation of an Ag ink film coating on a photocatalytic surface produces a brown colouration due to Ag nanoparticle formation. The Ag ink is easy to make and provides a simple method for the rapid assessment of photocatalytic films, with the added advantage of being particularly sensitive to low activity photocatalytic materials, such as commercial self-cleaning tiles for which no International Standards Organisation, ISO, test exists, including a Rz paii ISO. A felt-tipped pen applied Ag ink film is used to identify the presence of activity in a wide range of different materials, including examples of commercial self-cleaning glass, tile, paint and plastic awning materials. Unlike its well-established Rz paii counterpart, the Ag ink is effective on very hydrophobic and delicate, i.e. easily removed, photocatalytic films. A protocol is employed for the Ag ink to provide a quantitative measure of photocatalytic activity on both a low activity photocatalytic material, i.e. a Hytect™ tile, and a moderately active, reference photocatalytic material, Activ™ glass. The potential of the Ag ink to aid research and development into, and quality assurance of, low activity photocatalytic films, and also help in identifying activity in films previously considered inert, is discussed briefly.

Origins of strain localization in a silver-based flexible ink under tensile load

Flexible electronics often employ composite inks consisting of conductive flakes embedded in a polymer matrix to transmit electrical signal. Recently, localized necking was identified as a cause of a substantial increase in normalized resistance with applied strain thereby adversely impacting electrical performance. The current study explores two possible contributing factors for the formation of such localization—ink surface roughness and local variations in silver flake volume fraction. Uniaxial tension experiments of a DuPont 5025 type ink are used to inform a constitutive model implemented using finite element method on different substrates. Surface roughness was modeled by sinusoidal variation in ink height, whose amplitude and wavelength are informed by experimental laser profilometry scan data. Local flake fraction variations obtained from experimental measurements before applying any strain, were modeled as local variations in the elastic modulus according to an inverse rule of mixtures between the silver flake and acrylic binder material properties. The study identified that the ink height roughness is the most impactful contributor to the subsequent strain localization. The substrate elastic properties impact the number and magnitude of localization bands, with the stiffer substrate delocalizing strain and averting catastrophic crack formation seen with a more compliant substrate. The model incorporating surface roughness closely matches experimental measurements of local strain across different substrates. The study can inform designers of the adverse impact of ink surface roughness on localization and subsequent detrimental increase of the resistance.

Pad-Printing as a Fabrication Process for Flexible and Compact Multilayer Circuits.

The purpose of this paper is to present a newly developed process for the fabrication of multilayer circuits based on the pad-printing technique. Even though the maturity level, in terms of accuracy, substrate type and print size of several printing industrial processes is relatively high, the fabrication complexity of multilayer printed electronics remains relatively high. Due to its versatility, the pad-printing technique allows the superposition of printed conductive and insulating layers. Compared to other printing processes, its main advantage is the ability to print on various substrates even on flexible, curved or irregular surfaces. Silver-based inks were used for the formulation of conductive layers while UV inks were employed to fulfil the functionality of the insulating layers. To demonstrate the functionality of the pad-printing results, a multilayer test pattern has been designed and printed on Kapton®. Furthermore, to demonstrate the efficacy of this approach, a multilayer circuit composed of three stacked layers has been designed and printed on various substrates including Kapton®, paper and wood. This electronic circuit controls an array of LEDs through the manipulation of a two-key capacitive touch sensor. This study, allowed us to define recommendations for the different parameters leading to high printing quality. We expect a long-term beneficial impact of this study towards a low-cost, fast, and environmental-friendly production of printed electronics.

Transfer of printed electronic structures using graphene oxide and gelatin enables reversible and biocompatible interface with living cells

We present a low-cost, easy-to-implement platform for printing materials and interfacing them with eukaryotic cells. We show that thermal or chemical reduction of a graphene oxide thin film allows water-assisted delamination of the film from glass or plastic. The chemical and physical properties and permeability of the resulting film are dependent on the method of reduction and deposition of the graphene oxide, with thermal reduction removing more oxidized carbon functionality than chemical reduction. We also developed a method to attach the films onto cell surfaces using a thin layer of gelatin as an adhesive. In general, the films are highly impermeable to nutrients and we observed a significant amount of cell death when gelatin was not used; gelatin enables diffusion of nutrients for sustained cell viability. The combination of nanoscale membranes with a low melting point biopolymer allows us to reversibly interface cells with cargo transferred by graphene oxide while maintaining cell viability. To demonstrate delivery of electronic structures, we modified a commercial off-the-shelf printer to print a silver-based ink directly onto the reduced graphene oxide films which we then transferred to the surface of the cells.

The role of strain localization on the electrical behavior of flexible and stretchable screen printed silver inks on polymer substrates

This work explores conductivity under applied uniaxial strain of two silver-based inks with similar flake volume fractions of ~50% that are screen-printed with a single pass (thickness: 10 µm) onto three different polymer substrates. The normalized resistance increases more rapidly with applied strain for the “flexible” ink (5025 with an acrylic binder), and has three times greater resistance at 35% strain when compared to the “stretchable” ink (PE874 with polyurethane binder). While resistance increase is qualitatively consistent with percolation theory, the in-situ strain map analysis and post-mortem fractography reveal drastic differences in the root causes of the inks’ electrical behavior. Both inks form strain localization bands with similar spacing. For the flexible ink (5025), strain localization is accompanied by local necking and flake fraction reduction. For the stretchable ink (PE874), strain localization is associated with surface cracking initiated by pre-existing voids, with minimal changes in the flake fraction. A model incorporating strain localization through a Gaussian distribution of flakes evolving with applied strain more closely accounts for the 5025 ink's normalized resistance increase compared to models that assume uniform strain and a uniform flake distribution. Overall, local necking and reduction of the flake fraction appear to be more detrimental to the resistance than the formation of surface cracks.

Facile and High-Yield Replacement Reaction-Assisted Synthesis of Silver Dendrites by Jet for Conductive Ink.

Metallic dendrites with uniform morphology, high purity, and large yield remain challenging to synthesize. In this work, single-crystalline silver (Ag) dendrites with uniform morphology, high purity, and large yield are successfully synthesized by employing single replacement reaction between aqueous silver nitrate (AgNO3) and solid copper (Cu) by jet. The combined effect of diffusion-limited aggregation and the locally oriented attachment of Ag particles is responsible for the formation of silver dendrites under nonequilibrium conditions. Finally, the potential applications of as-synthesized silver dendrites are demonstrated by successfully preparing silver-based conductive ink for flexible electronics and wearable equipment. The conductive pattern exhibits resistivity of 7.2 μΩ·cm, showing good conductivity of the prepared conductive material. This facile and time-efficient synthetic route can be extended to synthesize other noble metal nanostructures with desired morphologies by adopting selective precursor salt concentrations and substrate metals with proper redox potentials.