Copper Ink Research Articles

Hybrid Printing Metal-mesh Transparent Conductive Films with Lower Energy Photonically Sintered Copper/tin Ink

With the help of photonic sintering using intensive pulse light (IPL), copper has started to replace silver as a printable conductive material for printing electrodes in electronic circuits. However, to sinter copper ink, high energy IPL has to be used, which often causes electrode destruction, due to unreleased stress concentration and massive heat generated. In this study, a Cu/Sn hybrid ink has been developed by mixing Cu and Sn particles. The hybrid ink requires lower sintering energy than normal copper ink and has been successfully employed in a hybrid printing process to make metal-mesh transparent conductive films (TCFs). The sintering energy of Cu/Sn hybrid films with the mass ratio of 2:1 and 1:1 (Cu:Sn) were decreased by 21% compared to sintering pure Cu film, which is attributed to the lower melting point of Sn for hybrid ink. Detailed study showed that the Sn particles were effectively fused among Cu particles and formed conducting path between them. The hybrid printed Cu/Sn metal-mesh TCF with line width of 3.5 μm, high transmittance of 84% and low sheet resistance of 14 Ω/□ have been achieved with less defects and better quality than printed pure copper metal-mesh TCFs.

Direct Aerosol Printing of Lithium-ion Batteries

Abstract Recently, additive manufacturing (AM) has brought new opportunities to the manufacturing of lithium-ion batteries (LIBs). In this study, aerosol jet printing, as a branch of AM technologies was demonstrated to fabricate lithium-ion batteries for the first time. Printable inks of two pairs of active materials for cathode and anode were developed. The effect of ink composition on the printing characteristics was studied. The developed inks were printed into Li-ion battery electrodes with specific capacities comparable to conventional slurry cast electrodes. Next, to demonstrate fully-printed electrodes, gold and copper inks were printed on top of polymer substrates and thermal/flash sintered as the current collectors for cathode and anode.

A novel method of synthesizing antioxidative copper nanoparticles for high performance conductive ink

Due to low melting temperature, low price and high conductivity, copper nanoparticles have great potential to substitute conductive polymers, silver and gold nanoparticles and others in conductive inks. Here we developed a new, simple and green method to synthesize copper nanoparticles, which have the average size of around 140 nm and show remarkable ability to prevent oxidation. Using the copper nanoparticles for conductive ink, the electrical performance of copper film by depositing copper ink on polyimide was also investigated with the sintering temperature from 150 to 400 °C. The electrical resistivity is around 17.0 μΩ cm under sintering temperature of 250 °C, and can reach as low as 5.7 μΩ cm under the temperature of 400 °C. The proposed approach to synthesize copper nanoparticles for high performance conductive ink presents potential applications in flexible electronics.

Contribution of Ligand Oxidation Products to High Durability of Copper Films Prepared from Low‐Sintering‐Temperature Copper Ink on Polymer Substrates

In electronic printing, ensuring high durability of sintered copper films on flexible substrates to obtain environmental stability and mechanical flexibility has become the most important task from a practical standpoint. In the study reported here, the authors develop solution synthesis of 2‐amino‐2‐ethyl‐1,3‐propanediol (AEP)‐protected copper nanoparticles (AEP–Cu NPs) with sizes of 3–8 nm in ethylene glycol, where the Cu NPs are stabilized via the metallacyclic coordination stability of the AEP ligands. The sintered Cu film exhibits a resistivity of 50 μΩ cm−1 after heating at 150 °C under a nitrogen atmosphere. Most importantly, the resulting Cu films on the polyethylene terephthalate (PET) substrates show excellent durability in terms of bending and adhesion without requiring any additives like nanotubes and nanowires. Furthermore, the authors successfully demonstrated the high environmental stability of the resulting Cu film even after it is exposed to harsh environmental conditions (RH 80%, 60 °C) for 1 month. The environmental durability is further improved by utilizing a composite ink of AEP–Cu NPs with copper microflakes. It is experimentally proven that oxidation products from AEP ligands originating in the sintering process contributed to the high durability of sintered copper films on flexible substrates.

All-copper contacting technology for film-vs-film electric connection using cool plasma sintering

Tiling expansion enables the fabrication of large-area electronics at a much lower equipment cost than manufacturing a large-area device in one piece. Thin connection technology is expected to play a key role there. Contact resistances of the existing film-type flexible connectors are, however, rather high, compared with the conventional connectors or soldering. To realize a lower contact resistance, we inserted copper ink between two mating electrodes on a pair of films and sintered it by cool plasma sintering, a technology recently developed by our group. We achieved a contact resistance of about 20 mΩ, which is sufficiently low and compatible with the most demanding applications.

Effect of carboxylic acids on conductivity of metallic films formed by inks based on copper@silver core-shell particles

Conductive copper inks have attracted much attention as low-cost replacement for the currently used silver inks for printed electronics. The copper inks should be stable to oxidation at all stages of fabrication of conductive patterns: ink formulation and storage, printing, and post-printing treatment. In the present study, air-stable copper-silver core-shell (Cu@Ag) submicron particles were utilized in conductive ink formulations. To improve the conductivity of the resulting Cu@Ag coatings, the effect of various carboxylic acids was evaluated. It was found that all acids led to decreased resistivity after sintering at elevated temperatures, while the lowest value was only 4 times higher than the bulk resistivity at 3wt% of oleic. The mechanism governing the effect of carboxylic acids is discussed, in view of possible stabilizer exchange and oxide dissolution.

Relief printing of micron-sized electrical conductive structures on silicon

Copper is the metal of choice for electrical circuits in electronics. It is lower in cost than silver and offers excellent electrical properties like low electrical resistivity and electromigration resistance. Physical vapor deposition and wet-chemical etching or electroforming are the standard processes used to deposit and pattern copper on silicon and in printed-circuit-board technology. Recently, copper inks and pastes have become available for the printing of copper films, with the potential outcome of lowering the production cost beyond that of the established processes. Furthermore, the printing processes are compatible with role-to-role fabrication, and are hence attractive for flexible electronics. In this study, a bi-modal copper paste containing nano- and micro-particles is transferred in a relief printing process by using silicon stamps. It was possible to demonstrate electrical conductive tracks with a linewidth down to 8 μm. The spacing between neighboring tracks is related to their width and can be as low as 10 μm. It was possible to achieve a sheet resistance of less than 8 mΩ sq–1 after formic-acid-assisted sintering at 180 °C. The low sintering temperature enables the direct placement of electronic components without additional soldering and thus saves time and production costs.

Self-Reducing Copper Precursor Inks and Photonic Additive Yield Conductive Patterns under Intense Pulsed Light

Printing conducting copper interconnections on plastic substrates is of growing interest in the field of printed electronics. Photonic curing of copper inks with intense pulsed light (IPL) is a promising process as it is very fast and thus can be incorporated in roll-to-roll production. We report on using IPL for obtaining conductive patterns from inks composed of submicron particles of copper formate, a copper precursor that has a self-reduction property. Decomposition of copper formate can be performed by IPL and is affected both by the mode of energy application and the properties of the printed precursor layer. The energy application mode was controlled by altering three pulse parameters: duration, intensity, and repetitions at 1 Hz. As the decomposition results from energy transfer via light absorption, carbon nanotubes (CNTs) were added to the ink to increase the absorbance. We show that there is a strict set of IPL parameters necessary to obtain conductive copper patterns. Finally, we show that by adding as little as 0.5 wt % single-wall CNTs to the ink the absorptance was enhanced by about 50% and the threshold energy required to obtain a conductive pattern decreased by ∼25%. These results have major implications for tailoring inks intended for IPL processing.

Inks of Organic Cu-Precursors with Short Carbon Chain

Based-Cu printing inks (20wt% Cu) and (9.6wt% Cu) were prepared using the short carbon chain organic Cuprecursors formed during the preparation of the inks, which can easy form a favourable conductive film onto glass slides at 290L and 220L (Fig.1). The resistance of the film induced by the oxidation of Cu and remainder of long carbon chain Cuprecursors markedly decreased, than usual. The influence factors on the formation and conductive mechanism of the copper ink and film were discussed.

Air stable copper-silver core-shell submicron particles: Synthesis and conductive ink formulation

We report on the synthesis of copper-silver core-shell (Cu@Ag) particles with about 1μm-diameter Cu core coated with a thin (∼20nm) silver shell, for application in printed electronics as low cost conductive ink. The process is based on using the environmentally friendly sodium formaldehyde sulfoxylate dehydrate as a reducing agent for copper ions and two types of polymeric stabilizers (nonionic PVP and anionic PAA). The formation of core-shell particles is followed by transmetallation reaction on the surface of the Cu particles, where copper atoms function as the reducer for silver ions. Characterization of the submicron particles by SEM, EDS and XRD confirm the core-shell structure. The resulting Cu@Ag particles enable overcoming a major challenge in copper ink, their rapid oxidation in air. It was found that ink formulations based on propylene glycol as the liquid vehicle and containing a silicone based wetting agent possesses the optimal characteristics (wetting, sintering) for printing on a glass substrate. To obtain conductive metallic structures, thermal sintering of metallic patterns was used. The Cu@Ag coating are stable to oxidation for at least 6 months at room temperature, and also during sintering process which is carried out at temperatures up to 250°C. The conductivity of Cu@Ag coatings after sintering at 250°C was high, 16% of that for bulk copper.

Brush-painting and photonical sintering of copper and silver inks on cotton fabric to form antennas for wearable ultra-high-frequency radio-frequency identification tags

Additive deposition of inks with metallic inclusions provides compelling means to embed electronics into clothing in a seamless way. We present a simple single-step brush-painting method for depositing copper and silver inks directly on ordinary cotton fabric and discuss the optimization of the photonical sintering of both materials. For the first time, we demonstrate the deposition and sintering of copper oxide ink on fabric achieving high electrical conductivity suitable for radio-frequency electronics. The results from wireless testing confirmed that the radio-frequency identification tags based on the copper and silver ink antennas were readable from 6 and 10 m, respectively.

Product design: Metal nanoparticle‐based conductive inkjet inks

A systematic procedure that provides an efficient workflow for formulating conductive inks is developed. Qualitative product attributes of the inkjet ink are first identified and transformed into quantitative product specifications. Ingredients are then selected based on heuristics, mechanistic, and empirical models to meet the product specifications. Printability checks based on theoretical criteria are used to ensure that stable droplets can in principle be formed and coalesced properly to form a printed line of ink. Then, the conductive ink is prepared and printed to evaluate the performance of the inkjet ink experimentally. An example of preparing a copper ink for inkjet printing is used to illustrate the systematic procedure. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2740–2753, 2016

Self-reducible copper inks composed of copper–amino complexes and preset submicron copper seeds for thick conductive patterns on a flexible substrate

Submicron copper inks can be sintered at a low temperature of 140 °C by the in situ generation of fresh copper nuclei.

Laser sintering of copper nanoparticles on top of silicon substrates

This study examines the sintering of inkjet printed nanoparticle copper ink in a room environment using a laser as a high speed sintering method. Printed patterns were sintered with increasing laser scanning speed up to 400 mm s−1. The resistivities of the sintered structures were measured and plotted against the scanning speeds. Increased resistivity seems to correlate with increased scanning speed. A selections of analytical methods was used to study the differences in microstructure and composition of the sintered structures. Based on the results, no discernable difference in the microstructure was noticed between the structures sintered using 20 mm s−1 to 400 mm s−1 scanning speeds; only the structure scanned using 5 mm s−1 speed showed a vastly different microstructure and no resistivity was measurable on this structure. Compositional studies revealed that, apart from the structure scanned with 5 mm s−1 speed which contained the highest oxygen, the rest of the structures showed a steady oxygen increase with increased scanning speed.

A Study of the Preparation and Properties of Antioxidative Copper Inks with High Electrical Conductivity.

Conductive ink using copper nanoparticles has attracted much attention in the printed electronics industry because of its low cost and high electrical conductivity. However, the problem of easy oxidation under heat and humidity conditions for copper material limits the wide applications. In this study, antioxidative copper inks were prepared by dispersing the nanoparticles in the solution, and then conductive copper films can be obtained after calcining the copper ink at 250 °C in nitrogen atmosphere for 30 min. A low sheet resistance of 47.6 mΩ/□ for the copper film was measured by using the four-point probe method. Importantly, we experimentally demonstrate that the electrical conductivity of copper films can be improved by increasing the calcination temperature. In addition, these highly conductive copper films can be placed in an atmospheric environment for more than 6 months without the oxidation phenomenon, which was verified by energy-dispersive X-ray spectroscopy (EDS). These observations strongly show that our conductive copper ink features high antioxidant properties and long-term stability and has a great potential for many printed electronics applications, such as flexible display systems, sensors, photovoltaic cells, and radio frequency identification.

Microstructure and electrical property of copper films on a flexible substrate formed by an organic ink with 9.6 % of Cu content

An organic copper ink with 9.6 wt% of Cu content derived from a short carbon chain organic copper precursor was successfully applied on a modified PI substrate and easily formed a favorable conductive copper film by self-reduction in the sintering process, which showed excellent conductivity. The effects of sintering temperature and time on the microstructure and conductivity action of the copper films were studied by XRD, EDS and SEM and electrical measurements, respectively. The sheet resistance and resistivity were determined to be as low as 0.11 Ω/□ and 2.2 × 10−5 Ω·cm. The conduction mechanism is discussed in terms of the percolation theory.

Intense Pulsed Light Sintering of Copper Nanoink for Conductive Copper Film

Conducting electrodes and films are becoming increasingly important for the printed electronics which can be applied in various fields such as RFID tags, photovoltaics, display and PCB. And copper nanoparticle inks serve as an attractive potential replacement to silver nanoparticle inks because its low cost and good electrical conductivtiy.In this paper, the commercial copper nanoparticle powder with oxide shells were dispersed in the solvent of ethylene glycol. Poly (N-vinylpyrrolidone) (PVP K30) was added to the solvent as capping materials. The prepared copper ink was spin-coated on glass substrate under ambient conduction. Then, the deposited Cu ink was sintered by an intense pulsed light (IPL) system. The sintering effects were investigated with different sintering conditions including pulse energy and pulse duration. The resulting electrical resistivity was 94.1uΩ·cm which is about fifty times as high as that of bulk copper. The high resistivity may be mainly attributed to the porosity of sintered film. Also, crystal phase analysis was performed using x-ray diffraction (XRD) and was found the XRD peaks corresponding to cuprous oxide disappeared after IPL sintering. It was believed that PVP reduced the copper oxide to copper in the process of IPL sintering.

The application of a piezo-resistive cardiorespiratory sensor system in an automobile safety belt.

Respiratory and heart failure are conditions that can occur with little warning and may also be difficult to predict. Therefore continuous monitoring of these bio-signals is advantageous for ensuring human health. The car safety belt is mainly designed to secure the occupants of the vehicle in the event of an accident. In the current research a prototype safety belt is developed, which is used to acquire respiratory and heart signals, under laboratory conditions. The current safety belt is constructed using a copper ink based nonwoven material, which works based on the piezo-resistive effect due to the pressure exerted on the sensor as a result of expansion of the thorax/abdomen area of the body for respiration and due to the principle of ballistocardiography (BCG) in heart signal sensing. In this research, the development of a theoretical model to qualitatively describe the piezo-resistive material is also presented in order to predict the relative change in the resistance of the piezo-resistive material due to the pressure applied.

Electromechanical properties of printed copper ink film using a white flash light annealing process for flexible electronics

We report on a systematic study of the electromechanical properties of flexible copper (Cu) thin film for flexible electronics. Cu ink is synthesized with chemical reduction process. Cu ink film spin-coated on a polyimide substrate is annealed with white flash light, also known as intense pulsed light (IPL), which guarantees a room temperature and sub-second process in ambient conditions. IPL annealed Cu film shows the electrical resistivity of 4.8μΩcm and thickness of 200nm. The electromechanical properties of IPL annealed Cu film are investigated via outer/inner bending, stretching, and adhesion tests, and it is compared with conventional electron-beam evaporated Cu film. IPL annealed Cu film shows a constant electrical resistance within a bending radius of 6mm. The bending fatigue test shows that the Cu film can withstand 10,000 bending cycles. In the stretching test, the Cu film shows a 50% increase in resistance when a strain of 2.4% was induced. At 4% strain, the resistance increases more than 200%. Meanwhile, the electron-beam evaporated film shows a constant resistance up to a strain of 4%. Lower stretchability of IPL annealed Cu film is attributed to its inherent cracks and porous film morphologies. IPL annealing induces the local melting at the interface between the substrate and Cu film, which increases the adhesion strength of the Cu film. These results provide useful information regarding the mechanical flexibility and durability of the nanoparticle films for the development of flexible electronics.

Flash light sintered copper precursor/nanoparticle pattern with high electrical conductivity and low porosity for printed electronics

In this work, the hybrid copper inks with precursor and nanoparticles were fabricated and sintered via flash light irradiation to achieve highly conductive electrode pattern with low porosity. The hybrid copper ink was made of copper nanoparticles and various copper precursors (e.g., copper(II) chloride, copper(II) nitrate trihydrate, copper(II) sulfate pentahydrate and copper(II) trifluoroacetylacetonate). The printed hybrid copper inks were sintered at room temperature and under ambient conditions using an in-house flash light sintering system. The effects of copper precursor weight fraction and the flash light irradiation conditions (light energy and pulse duration) were investigated. Surfaces of the sintered hybrid copper patterns were analyzed using a scanning electron microscope. Also, spectroscopic characterization techniques such as Fourier transform infrared spectroscopy and X-ray diffraction were used to investigate the crystal phases of the flash light sintered copper precursors. High conductivity hybrid copper patterns (27.3μΩcm), which is comparable to the resistivity of bulk copper (1.68μΩcm) were obtained through flash light sintering at room temperature and under ambient conditions.