Abstract
Copper (Cu) nanoparticles are considered a promising alternative to silver (Ag) and gold (Au) for printed electronics applications. Because Cu has higher electrical conductivity, it is significantly cheaper than Ag and Au. To study the applicability of electronic printing, we prepared Ag@Cu conductive ink by using a stepwise feeding method to disperse nano Ag and nano Cu in ethanol and water. The ink has the advantages of nontoxic, low content, and low cost. A three-dimensional (3D) model was designed, and a conductive pattern was printed on the photo paper substrate using extrusion 3D printing technology. The influence of waterborne resin on the adhesion of conductive patterns is discussed. The printed conductive pattern can maintain the stability of conductivity after 100 bending cycles. The conductive pattern has good thermal stability. It can be tested 10 times under 2 conditions of 85°C and room temperature to maintain good conductivity. This shows that Ag@Cu conductive ink printed flexible electronic products are competitive.
Keywords: flexible electronics, conductive patterns, nanomaterials, water-based conductive inks, nanosilver, nanocopper
Introduction
Metal nanomaterials have become popular because of their wide application in the field of flexible printed electronics, such as organic light-emitting diodes, radio frequency identification, ultrahigh frequency antennas, thin film transistors, printed circuit boards, electronic packaging, and so on.1–6 Printing technology is an ideal method for low-cost preparation such as conductive inks with good dispersibility, good fluidity, good viscosity, and high solid content by dispersing metal nanomaterials in oil/water solvents.7–9 The additive manufacturing method of printing technology can directly create patterns on many substrates without any complicated photolithography process, including physical or chemical deposition and etching under vacuum conditions. More importantly, owing to the low melting point of metal nanomaterials, the processing temperature of the substrate can be greatly reduced.
Various metal nanomaterials have been developed, such as gold (Au), copper (Cu), and silver (Ag). Cu materials are particularly important for conductive inks because of their high conductivity and low cost. But easy oxidation is the main problem hindering the successful application of Cu nanomaterials. Zhu and colleagues10 synthesized high-purity and monodisperse Cu nanoparticles (NPs) with a diameter between 50 and 500 nm using a modified polyol process. The prepared Cu nanomaterial has strong oxidation resistance even after being stored for 30 days in an ordinary atmosphere. The size of the prepared tannic acid (TA)-covered Cu NPs (TA-CuNPs) is between 20 and 40 nm, and the size distribution is narrow. TA-CuNPs have good oxidation resistance owing to the protection of the TA layer. There is no oxidation even if stored for 90 days under atmospheric conditions.
Luo and colleagues11 reported a method of using TA as a stabilizer and protective agent to synthesize Cu NPs in an aqueous solution without inert gas protection, with a particle size distribution of 20–40 nm. A uniform CuNPs ink is synthesized by simple dispersion of TA-CuNPs in water. An ordinary color printer is used to ink-jet print the ink on the flexible substrate to form a conductive pattern. Fu et al.12 synthesized Cu NPs in the gas phase by transfer arc discharge. Cu NPs can be synthesized continuously at a rate of 1.2–5.5 g/h, and the Brunauer–Emmett–Teller size remains <100 nm.
Using ball milling technology to produce Cu conductive ink, the resistivity of the Cu film sintered at 300°C is 5.4 ± 0.6 μΩ·cm, which is about three times that of bulk Cu (1.7 μΩ·cm). The antioxidation treatments of Cu NPs reported in the above-mentioned research work are all very good. However, the processing temperature of Cu ink is as high as 300°C, which affects the waste of energy consumption and printing applications of more substrates. In the previous work, we explored the application of water-based Ag nanoconductive ink with three-dimensional (3D) printing.13 This work prints a flexible electronic pattern with good conductivity, good bending performance, and high stability on the photo paper substrate. Now it is possible to mix Ag and Cu nanomaterials to prepare green and environmental-friendly conductive inks, which can reduce material costs and broaden application fields.
In this article, a non-toxic, easy-to-operate, low-cost, low-content preparation process of Ag@Cu conductive ink has been developed (Fig. 1). A stepwise addition method is used to disperse the Ag nanomaterials and Cu nanomaterials in an ink mixture with appropriate viscosity, fluidity, and printability. In addition, Ag@Cu ink was printed on photo paper by 3D printing and sintered to obtain different conductive patterns. The ink mixture, printing conditions, sintering temperature, and time are optimized to obtain high-performance and high-resolution conductive pattern. The entire printing process is environmental-friendly, low-cost, easy to operate, and allows practical industrial applications.
FIG. 1.
(a) Schematic diagram of the preparation process of Ag@Cu water-based conductive ink. (b) Extrusion type 3D printing conductive pattern. (c) Flat sample of conductive pattern. (d) Bending sample of conductive pattern. (e) Turn on the LED light. 3D, three-dimensional; Ag, silver; Cu, copper.
Materials and Experiment
Materials
The Ag NPs were made in the laboratory. Cu nanopowder was purchased from Yingtai Metal Materials Co., Ltd. (Nangong City, China). Methylcellulose was purchased from Zibo Bailey Chemical Co., Ltd. (China). Waterborne polyurethane resin (WPR) and waterborne acrylic resin (WAR) were purchased from Guangdong Jiajing Printing Materials Co., Ltd. (China). Deionized water and ethanol were purchased from Tianjin Damao Chemical Reagent Company (China).
Ag@Cu water-based nanomaterial conductive ink
The Cu NPs are dispersed in ethanol. Use glass stirring and ultrasonic to disperse ultrasonically. Then Ag NPs were added. The ultrasonic dispersion operation is repeated. Then methyl cellulose was added, and water was used as a dispersion. Finally, WAR was added to the ink. It can improve the adhesion of the ink to the substrate. Each addition required stirring and ultrasonic dispersion. This allows various materials to be dispersed in the mixed ink. The specific material content ratio is given in Table 1.
Table 1.
The Material Content Ratio of Ag@Cu Ink
| Cu | Ag | Ethanol | Deionized water | Methylcellulose | Waterborne acrylic resin |
|---|---|---|---|---|---|
| 0.5 g | 1 g | 2 g | 1 g | 0.1 g | 0.1 g |
Ag, silver; Cu, copper.
3D printing
3D Max software is used to design a 3D model of the conductive pattern. Simplify 3D software was used to slice the 3D model file. Memory card was used to transfer the slice file to the 3D printer. The prepared Ag@Cu ink is filled into the extrusion head of the 3D printer. Set the temperature of the hot bed of the printer, adjust the suitable air pressure, and start the printing operation.
Characterizations
Morphological observations were carried on using a field emission scanning electron microscope (FE-SEM; S4800; Hitachi, Japan). Scanning electron microscope (SEM; Feiner Phenom Pro, Feiner, the Netherlands) was used to examine the microstructure of Cu NPs and Ag@Cu ink printed on photographic paper. X-ray diffraction (XRD) measurements were conducted to confirm the crystal phases of Cu NPs. The X-ray diffractometer (type D8 Advance; Brunker, Germany) was equipped with Cu K alpha x-ray (Cu-Kα rays) radiation (1.5406 Å).
Results and Discussion
Physical properties of Ag@Cu water-based conductive ink
In this study, Ag@Cu conductive ink was prepared by dispersing Ag nanomaterials and Cu nanomaterials in a water solvent through a stepwise addition method. The conductive fillers in the ink are Ag NPs and Cu NPs. Ag NPs are made in the laboratory. Silver nitrate is reduced by sodium borohydride, and polyvinylpyrrolidone is used as a protective agent. As given in Figure 2, the yield of Ag NPs is relatively high, with an average particle size of 20 nm. The surface of Cu NPs is provided with an antioxidation treatment layer. It can be observed by desktop SEM that the average particle size of Cu particles is 150 nm.
FIG. 2.
(a) SEM image of Ag sample. (b) SEM image of Cu powder. (c) Ink penetration deposited on phase paper. (d) SEM diagram of ink deposited on phase paper, with yellow arrows indicating the ink layer thickness. SEM, scanning electron microscope.
The composition of prepared Ag nanomaterials and Cu was measured using XRD. XRD analysis was carried out to verify the component of the prepared Ag NPs. As given in Figure 3, the significant diffraction peaks at the 2-theta angle of 38.12°, 44.3°, 64.5°, and 77.5° were assigned to the (111), (200), (220), and (311) planes of face-centered cubic Ag (JCPDS File No. 04-0783), respectively. A few sharp 2-theta peaks located at 43.38°, 50.48°, and 74.16° are observed in the XRD patterns of the Cu samples, attributed to the (111), (200), (220), and (311) planes of metal Cu with fcc structure, respectively.
FIG. 3.
(a) XRD pattern of sample Ag NPs. (b) XRD pattern of sample CuNPs. NP, nanoparticle; XRD, X-ray diffraction.
The viscosity of Ag/Cu conductive ink was measured at different rotational speeds using a cone-plate viscometer, as given in Table 2. The results show that the ink has shear thinning behavior in a certain shear rate range. This shear thinning characteristics is in line with the requirements of micro-pen writing molding. Conductive ink viscosity requirements should be matched to the printer used. The equipment used in this experiment is a micro-pen printer, and the viscosity should be controlled at 200–1000 mPa·s.
Table 2.
Different Speed of Silver/Copper Conductive Ink Viscosity Change
| Speed (rpm) | 200 | 500 | 800 |
| Viscosity (mPa·s) | 919 | 161 | 130.3 |
The pH value of Ag/Cu conductive ink was 8.0 ± 0.3 by ink pH meter. General conductive ink pH quantitative range of neutral alkali value is in the vicinity of 8.0. Ink is weak alkaline, conducive to ink adhesion and drying. Oxidation drying speed of the ink is accerlated, so that the resin is dried and attached to the substrate surface.14
Research on the water penetration of paper substrates
Aqueous penetration test of the substrate
As given in Figure 4, t[s] represents time and I[%] represents ultrasonic intensity. The principle of dynamic osmometer is the ultrasonic strength of paper wetted by solution. The seepage velocity of liquid can be reflected by characteristic curve. The greater the absorption performance of the paper, the faster the speed of ink absorption, that is, the speed of ink penetration into paper. Therefore, the stronger the water absorption of paper, the easier it is to dry. In a certain degree of water-based ink on paper permeability is stronger. This result indicates that the paper has a lower field density and a higher pattern resolution after printing. It can be seen from Figure 4 that the ultrasonic intensity of the photographic paper reached 35% within 10 s in the penetration test, and has been maintained at this value since then. This shows that the permeability of the photographic paper is good and the stability is high.
FIG. 4.
Photo paper penetration in water.
3D printing of conductive patterns
As given in Figure 5, the conductive ink is printed on photo paper by extrusion to form a pattern. The 3D Max software was used to conduct 3D modeling of the circuit diagram and set parameters such as line width, line length, and line thickness, as given in Table 3. The pattern's model file needs to be sliced and layered. The best printing parameters are needle inner diameter of 0.25 mm, bed temperature of 25°C, printing speed of 8 mm/s, and printing pressure of 0.25 MPa·s. The purpose of setting the hot bed temperature to room temperature is to prevent the ink from drying out when it is squeezed out. High temperature will cause ink blockage nozzle, affecting the printing effect. The ink layer of 3D printing is thicker than that of inkjet printing. The load electrons in the ink layer will pass more per unit time. The work response time will also be less.
FIG. 5.
(a) The 3D model of the pattern. (b) The printed conductive pattern.
Table 3.
Comparison of Printed Samples and Models
| Pattern model | Print sample | |
|---|---|---|
| Line width | 0.25 mm | 0.25 ± 0.25 mm |
| Line length | 30 mm | 30 ± 1 mm |
| Line thickness | 0.3 mm | 100 ± 5 μm (SEM) |
SEM, scanning electron microscopy.
Adjusting the best printing conditions can control the thickness of the ink layer. The solute in the ink is sufficiently ground before printing to contribute to the results. This will allow the ink to deposit more fully on the photo paper. The conductive filler in the ink is more evenly dispersed. At fixed moving speed, the amount of material extruded under different air pressure of ink is different. Obviously, as the pressure increases, the material extrudes more under high pressure, and the line width and thickness increase accordingly. When the pressure decreases, the amount of extrusion will also decrease, and the line width and thickness will also decrease. But compared with the line width, the thickness change is not obvious. The results show that the suitable pressure range of ink is 0.20–0.30 MPa. In this range of extruded line width fluctuation is not obvious, thickness deposition changes are normal.
As given in Table 2, line width, line length, and line thickness of the conductive pattern were tested. It can be seen from Figure 2 that the ink is well deposited on the photographic paper. Various phases of the ink are dispersed in the pore structure. The ink layer is ∼100 μm. The printed conductive pattern is dried and sintered in an oven at 70°C. Because the substrate is photo paper, the ignition point is low. Therefore, the substrate cannot be processed by high-temperature annealing. The surface of Cu NPs is treated with antioxidation. The surface is a layer of surfactant ingredients. It will be destroyed at 70°C in the oven, exposing the Cu content inside. The Cu nanomaterial is freely dispersed in the ink. At higher temperatures, they will shrink and join together, and the particles will contact each other to form conductive bridges.
The size deformation of line pattern is small. The average line width is ∼500 μm. The model line length is set to 30 mm with almost no deformation. The ink layer for the printed line pattern is ∼100 μm thick, whereas the 3D model design is 150 μm thick. The results show that the thickness of the pattern is smaller than that of the model. This is because photo paper has a capillary structure. Ink penetrates into the capillary structure (Fig. 2c), resulting in a reduction in the thickness of the pattern.15 Figure 2d shows the cross-section of ink on paper, which proves that some ink penetrates the capillary structure of the paper.
Mechanical properties and thermal stability of conductive patterns
Bending performance test
A section of the printed line is selected on the conductive pattern. The initial resistance (R0) is 235 Ω. Resistance change of 100 cycles before and after the conductive pattern is bent into a semicircular state is tested. It can be seen from Figure 6 that there are some slight fluctuations before and after the conductive pattern is bent, but the fluctuations do not change much. As the number of bending increases, the resistance change of the pattern also increases. This is because the bending action will cause cracks or voids in the electronic filler passage inside the pattern printing line.
FIG. 6.
(a) The resistance change of the conductive pattern with different bending times. (b) The resistance change of the conductive pattern with different bending angles.
In addition, the pattern has also been discussed in terms of bending angle. The bending range from 30° to 360° summarizes the changes in the resistance of the pattern before and after bending. It can be seen from the data in the figure that as the bending angle increases, the resistance of the pattern changes more and more. It also shows that the bending angle of the pattern becomes so large that the movement of the load electrons in the ink layer is hindered. On the macrolevel, the resistance will have an increasing trend. However, owing to the sintering of the pattern, the resistance fluctuates little, still within the acceptable range.
Thermal stability test
The conductive pattern is placed in a high temperature environment (oven) of 85°C for a period of time. Resistance change after cooling to room temperature is tested. The R0 at room temperature is 312 Ω. Cycle test 10 times under high temperature/cooling. It can be seen from Figure 7 that the conductive pattern is tested in the high temperature/cooling cycle. As the number of cycles increases, resistance gradually increases. The resistance of the conductive pattern gets smaller in the first few cycles. This is because the particles in the conductive filler agglomerate at high temperatures. The conductive path formed is dense.
FIG. 7.
Resistance change of conductive pattern in thermal/cooling cycle.
Resistance increases again after the next few cycles of testing. It shows that the excessive high temperature/cooling cycle makes the conductive material of the pattern exceed the fatigue threshold. The agglomeration of particles in the filler no longer continues. Some particles have splits and pores, which will lead to an increase in electrical resistance. The test of the thermal stability of the conductive pattern has not been seen in other similar studies. The necessity of the thermal stability test is that the conductive pattern needs to continuously deliver current when it is working. Maintaining high-intensity work will generate a lot of heat. It is in a cooling state when not working. The thermal stability test simulates the operation of this situation.
Hundred grid test
Tape is used for adhesion performance test. After 50 times of tearing, it was found that the ink layer was broken off. In addition, the influence of waterborne resin on the adhesion of conductive patterns was studied. There are also changes in pattern conductivity. Water-based resin is used as an additive in the ink. It can significantly improve the adhesion of the ink on the substrate. The result we need is not only to improve the adhesion, but also to ensure the stability of the pattern's conductivity. We conducted experiments to explore the different contents of WPR and WAR. As given in Figure 8 an increase in the content of the WPR leads to an increase in the resistance of the pattern. When the content of the WAR increases, the resistance of the pattern does not change significantly, although the addition of resin will improve the adhesion of the pattern on the photo paper. But it will increase the resistance of the pattern. So the final resin chosen was WAR. Finally, the conductive patterns were used to demonstrate the lighting of an LED bulb using a 1.5 V battery, as given in Figure 9. This result provided some ideas for paper-based circuits in the future.
FIG. 8.
(a) The effect of different content of WPR on the pattern. (b) The effect of different content of WAR on the pattern. WAR, waterborne acrylic resin; WPR, waterborne polyurethane resin.
FIG. 9.
Light an LED bulb on the conductive pattern. (a) Vertical line conductive pattern; (b) Horizontal line conductive pattern.
Conclusions
In paper, a nontoxic, easy-to-operate, and low-cost preparation process of Ag@Cu conductive ink is developed. A stepwise addition method is used to disperse the Ag nanomaterials and Cu nanomaterials in an ink mixture with appropriate viscosity, fluidity, and printability. In addition, Ag@Cu ink is 3D printed on a flexible substrate, and then sintered to obtain different conductive patterns. The ink mixture, printing conditions, sintering temperature, and time are optimized to obtain high-performance and high-resolution conductive modes. To study the applicability of electronic printing, a conductive pattern was printed on a photo paper substrate using extrusion 3D printing technology. The influence of waterborne resin on the adhesion of conductive patterns is discussed. The printed conductive pattern can maintain the stability of conductivity after 100 bending cycles. At the same time, the conductive pattern has good thermal stability. It can be tested 10 times under two conditions of 85°C and room temperature to maintain good conductivity. The entire printing process is environmental-friendly, low-cost, easy to operate, and allows practical industrial applications.
Authors' Contributions
The article was written through contributions of all authors. All authors have given approval to the final version of the article.
Author Disclosure Statement
No competing financial interests exist.
Funding Information
No funding was received for this article.
References
- 1. Tam SK, Fung KY, Ng KM. Copper pastes using bimodal particles for flexible printed electronics. J Mater Sci 2016;51:1914–1922. [Google Scholar]
- 2. Li JJ, Cheng CL, Shi TL, et al. Surface effect induced Cu–Cu bonding by Cu nanosolder paste. Mater Lett 2016;184:193–196. [Google Scholar]
- 3. Kim D, Jeong S, Moon J. Synthesis of silver nanoparticles using the polyol process and the inflfluence of precursor injection. Nanotechnology 2006;17:4019–4024. [DOI] [PubMed] [Google Scholar]
- 4. Perelaer J, Smith PJ, Mager D, et al. Printed electronics: The challenges involved in printing devices, interconnects, and contacts based on inorganic materials. J Mater Chem 2010;20:8446–8453. [Google Scholar]
- 5. Guo R, Yu Y, Xie Z, et al. Matrix-assisted catalytic printing for the fabrication of multiscale, flflexible, foldable, and stretchable metal conductors. Adv Mater 2013;25:3343–3350. [DOI] [PubMed] [Google Scholar]
- 6. Jiu J, Zhang H, Nagao S, et al. Die-attaching silver paste based on a novel solvent for high-power semiconductor devices. J Mater Sci 2016;51:3422–3430. [Google Scholar]
- 7. Komoda N, Nogi M, Suganuma K, et al. Highly sensitive antenna using inkjet overprinting with particle-free conductive inks. ACS Appl Mater Interfaces 2012;4:5732–5736. [DOI] [PubMed] [Google Scholar]
- 8. Dong Q, Huang C, Duan G, et al. Facile synthesis and electrical performance of silica-coated copper powder for copper electronic pastes on low temperature co-fifired ceramic. Mater Lett 2017;186:263–266. [Google Scholar]
- 9. Deng D, Jin Y, Cheng Y, et al. Copper nanoparticles: Aqueous phase synthesis and conductive fifilms fabrication at low sintering temperature. ACS Appl Mater Interfaces 2013;5:3839–3846. [DOI] [PubMed] [Google Scholar]
- 10. Zhang Y, Cui C, Yang B, et al. Size-controllable copper nanomaterials for flexible printed electronics. J Mater Sci 2018;53:12988–12995. [Google Scholar]
- 11. Hao Y, Zhang N, Luo J, et al. Tannic acid stabilized antioxidation copper nanoparticles in aqueous solution for application in conductive ink. J Mater Sci Mater Electron 2018;29:20603–20606. [Google Scholar]
- 12. Fu Q, Stein M, Li W, et al. Conductive films prepared from inks based on copper nanoparticles synthesized by transferred arc discharge. Nanotechnology 2019;31:025302. [DOI] [PubMed] [Google Scholar]
- 13. Zhao C, Wang J, Lu L. Preparation and application of water-based nano-silver conductive ink in paper-based 3D printing. Rapid Prototyping J 2022;28:747–755. [Google Scholar]
- 14. Wlodarczyk-biegun MK, Paez JI, Villiou M, et al. Printability study of metal ion crosslinked PEG-catechol based inks. bioRxiv 2019;12:035009. [DOI] [PubMed] [Google Scholar]
- 15. Bao L, Wei B, Xiao AY. Conductive coating formulations with low silver content. 2007 Proceedings 57th Electronic Components and Technology Conference 2007; IEEE: pp.. 494–500. [Google Scholar]