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A Review on Sustainable Inks for Printed Electronics: Materials for Conductive, Dielectric and Piezoelectric Sustainable Inks
In the last decades, the demand for electronics and, therefore, electronic waste, has increased. To reduce this electronic waste and the impact of this sector on the environment, it is necessary to develop biodegradable systems using naturally produced materials with low impact on the environment or systems that can degrade in a certain period. One way to manufacture these types of systems is by using printed electronics because the inks and the substrates used are sustainable. Printed electronics involve different methods of deposition, such as screen printing or inkjet printing. Depending on the method of deposition selected, the developed inks should have different properties, such as viscosity or solid content. To produce sustainable inks, it is necessary to ensure that most of the materials used in the formulation are biobased, biodegradable, or not considered critical raw materials. In this review, different inks for inkjet printing or screen printing that are considered sustainable, and the materials that can be used to formulate them, are collected. Printed electronics need inks with different functionalities, which can be mainly classified into three groups: conductive, dielectric, or piezoelectric inks. Materials need to be selected depending on the ink’s final purpose. For example, functional materials such as carbon or biobased silver should be used to secure the conductivity of an ink, a material with dielectric properties could be used to develop a dielectric ink, or materials that present piezoelectric properties could be mixed with different binders to develop a piezoelectric ink. A good combination of all the components selected must be achieved to ensure the proper features of each ink.
Journal Article
The effect of graphite and carbon black ratios on conductive ink performance
Conductive inks based on graphite and carbon black are used in a host of applications including energy storage, energy harvesting, electrochemical sensors and printed heaters. This requires accurate control of electrical properties tailored to the application; ink formulation is a fundamental element of this. Data on how formulation relates to properties have tended to apply to only single types of conductor at any time, with data on mixed types of carbon only empirical thus far. Therefore, screen printable carbon inks with differing graphite, carbon black and vinyl polymer content were formulated and printed to establish the effect on rheology, deposition and conductivity. The study found that at a higher total carbon loading ink of 29.4% by mass, optimal conductivity (0.029 Ω cm) was achieved at a graphite to carbon black ratio of 2.6 to 1. For a lower total carbon loading (21.7 mass %), this ratio was reduced to 1.8 to 1. Formulation affected viscosity and hence ink transfer and also surface roughness due to retention of features from the screen printing mesh and the inherent roughness of the carbon components, as well as the ability of features to be reproduced consistently.
Journal Article
Artist Intervention
Pablo Bronstein (b. 1977, Buenos Aires) lives and works in London. His upcoming shows include Nottingham Contemporary, Nottingham, UK, and Chatsworth House, Derbyshire (2015). His solo shows include REDCAT, Los Angeles; Centre d'Art Contemporain, Geneva (2013); Institute of Contemporary Art, London (2011); Kunsthal Charlottenborg, Copenhagen (2011); Sculpture Court, Tate Britain, London (2010); and The Metropolitan Museum of Art, New York (2009). In 2013 Konig Books published a major monograph, A Is Building B Is Architecture. Other solo publications include Gilded Keyholes (2013); Postmodern Architecture in London (second edition, 2011); and Ornamental Designs (2008).
Journal Article
Recent Development of Graphene-Based Ink and Other Conductive Material-Based Inks for Flexible Electronics
The promising and extraordinary properties of graphene have attracted significant interest, making graphene an alternative to replace many traditional materials for many applications, particularly in conductive ink for the fabrication of flexible electronics. For the past 10 years, numerous studies have been reported on the synthesis of graphene conductive ink for printing on flexible substrates for various electronic applications. The development of graphene-based ink is reviewed, with the main focus on the types of graphene-like materials in conductive inks, and the compositions and important properties of those inks. Another intention behind this review is to compare the pros and cons of graphene-based ink with those using other common conductive materials, such as gold nanoparticles, silver nanoparticles, copper nanoparticles, conductive polymers and carbon nanotubes. Recent works on graphene hybrid-based ink containing other metallic nanoparticles as an alternative way to improve the electrical properties of the conductive inks are also reported. Brief comparisons between inkjet printing and other printing techniques for the fabrication of flexible electronics are discussed.
Journal Article
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.
Journal Article
Artistic anti-counterfeiting with a pH-responsive fluorescent ink using DFT and molecular electrostatic potential mapping insights
The observed fluorescence behavior of the sulfur, nitrogen-doped carbon dots (S, N-CDs) ink which derived from onion peel wastes (OW) demonstrates its pH-sensitive nature, making it suitable for applications where visual or fluorescent changes upon pH variation are desired. The initial lack of fluorescence under UV light suggests that the S, N-CDs in the ink are in a non-fluorescent state. However, upon treatment with acid, the ink exhibits a faint yellow color under light and fluoresces under UV light. This indicates a shift in the electronic structure of the S, N-CDs, likely due to protonation. The return to non-fluorescence after re-treatment with alkaline solution suggests that the de-protonation process reverses the effect of acid, restoring the S, N-CDs to their original non-fluorescent state. This reversible pH-sensitivity is a valuable asset for various applications. The synthesized S, N-CDs exhibited a reversible change in fluorescence intensity under acidic and alkaline conditions, transitioning from non-fluorescent to fluorescent under acidic conditions and back to non-fluorescent in alkaline media. Density Functional Theory (DFT) calculations revealed that S, N-doping resulted in a narrower energy gap (0.2779 eV compared to 0.3199 eV for N-CDs) and a higher dipole moment (2.640 Debye), enhancing their reactivity towards protons and leading to more pronounced color and fluorescence changes across different pH conditions. The S, N-CDs displayed dual fluorescence emission peaks at 443.00 nm and 502.00 nm upon excitation at 350 nm, and fluorescence contour maps (FCM) confirmed their multicolor emission capabilities. The calculated quantum yield for the S, N-CDs was notably high at 37.76%. Fourier Transform Infrared (FTIR) spectroscopy confirmed the successful incorporation of sulfur (S–H at 2368 cm⁻
1
, C–S at 750 cm⁻
1
) and nitrogen (N–H at 3552 cm⁻
1
, C–N at 989 cm⁻
1
) functionalities into the carbon dot structure. Furthermore, Molecular Electrostatic Potential (ESPM) mapping indicated regions of high negative potential around S, OH, and C=O groups, particularly pronounced under acidic and basic conditions, supporting the observed pH sensitivity.
Journal Article
Formulation of Nickel Oxide–Graphene Composite Ink and the Fabrication of Thin-Film Electrodes Using Direct Ink Writing
Nickel oxide (NiO) is a p-type material with a wide bandgap of 3.6 to 4 eV, and graphene is a zero-bandgap material. When the two materials are combined to form a composite, their structural, optical, and electrical properties are enhanced. This work presents the formulation of nickel oxide (NiO)–graphene composite ink (with ethylene glycol as a solvent) for the fabrication of thin-film electrodes using a direct ink writing technique. NiO is used in nanopowder form, which is prepared via a wet-chemical approach. NiO ink is prepared in three different concentrations of 40 mg/ml (N1), 60 mg/ml (N2), and 70 mg/ml (N3). Graphene ink (G) is also prepared in concentrations of 0.5 mg/ml (G1), 1 mg/ml (G2), and 2 mg/ml (G3). Among the pure NiO inks, it was observed that low resistance and uniform printing were obtained for N2. Therefore, N2 is mixed with different concentrations of graphene to form the composite ink, which is further printed on the substrates. The rheological properties of all the formulated inks are measured. An extrusion-based direct-write system is employed for printing the inks on a glass substrate. The printed samples are characterized before and after annealing at 250°C for 2 h. The resistance of the composite ink (N2G1) is decreased by 53% in comparison with the pure NiO ink (N2). As the conductivity of the composite ink thin film is in the range of MΩ, they could find application in the field of gas sensing and thermistors.
Journal Article
