Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity Ag Patterns on Paper Sunshin Jung, Su Jin Chun, and Chae-Hwa Shon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06535 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 23, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity Ag Patterns on Paper Sunshin Jung,†, § Su Jin Chun,† and Chae-Hwa Shon‡

†Nano

Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI),

12, Bulmosan-ro 10Beon-gil, Changwon 51543, Republic of Korea ‡ Power

Apparatus Research Center, Korea Electrotechnology Research Institute (KERI), 12,

Bulmosan-ro 10Beon-gil, Changwon 51543, Republic of Korea. §Department

of Energy and Power Conversion Engineering, University of Science and

Technology (UST), Daejeon, 34113, Republic of Korea KEYWORDS: microwave sintering, microwave heating, cellulose paper, Ag nanoparticles, Ag patterns, paper electronics, flexible printed electronics.

ABSTRACT: Cellulose-based paper is essential in everyday life, but it also has further potentials for use in low-cost, printable, disposable and eco-friendly electronics. Here, a method is developed for the cellulose-mediated microwave sintering of Ag patterns on conventional paper, in which the paper plays a significant role both as a flexible insulating substrate for the conductive Ag pattern and as a lossy dielectric media for rapid microwave heating. The anisotropic dielectric properties of the cellulose fibers mean that a microwave electric field applied parallel to the paper substrate provides sufficient heating to produce Ag patterns with a conductivity 29–38% that of bulk Ag in a short period of time (~1 s) at 250–300 °C.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 35

Significantly, there is little thermal degradation of the substrate during this process. The microwave-sintered Ag patterns exhibit good mechanical stability against 10000 bending cycles and can be easily soldered with lead-free solder. Therefore, cellulose-mediated microwave sintering presents a promising means of achieving short processing times and high electrical performance in flexible paper electronics.

INTRODUCTION Cellulose-based paper consisting of a percolating network of cellulose fibers has become one of the most important and successful materials in human history. As a result, coating and printing methods ranging from simple handwriting to plotting, inkjet printing, screen printing and roll-toroll (R2R) printing have matured alongside paper technology. Indeed, the low cost, ready availability, and recyclability of paper makes everyday life without it barely conceivable. Since paper is also flexible, lightweight, biodegradable, and can be folded into 3D structures, it offers a potential substrate for low-cost, printable, disposable, and eco-friendly electronics.1,2 Interest in paper substrates for electronics and optoelectronics has been recently revived by transparent nanopaper based on nanocellulose.3–7 Paper substrates have been extensively studied for their potential application in thin-film transistors,7,8 energy storage devices,9–11 photovoltaic cells,12 random access memories,13 capacitive touch pads,14,15 sound sources,16 triboelectric nanogenerators,17 radiofrequency identification tags,18 sensors,6,19,20 and displays.21,22 Key to such devices is the need to create highly conductive patterns to function as various electrodes; e.g. contact electrodes, wiring electrodes, and interconnects. These conductive patterns can be deposited on paper either by printing, or by direct-writing or coating with inks or pastes containing conductive nanomaterials


2

Page 3 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


such as metal nanoparticles (NPs) or nanowires (NWs), carbon nanotubes (CNTs), or graphene.11,15,18,23–25 The choice of conductive nanomaterial is primarily based on its compatibility with the printing device and the required properties of the printed pattern, such as its electrical conductivity, optical transparency, adhesion to substrates, and stability during bending and heating. Among the conductive nanomaterials available, Ag NPs such as Ag nanospheres and Ag nanoplates are widely used in conductive inks or pastes since Ag is stable in air and has the lowest resistivity of all metals (1.59 × 10−6 Ω cm). In contrast, Ag NWs can only be used very carefully, as these suffer from the fragmentation that results from their morphology instability at elevated temperatures.26 The use of metal NP-based inks or pastes on heat-sensitive substrates like paper or plastic introduces certain critical problems in post-printing sintering of the printed metal pattern in a hot-air convection oven. Generally speaking, the higher the sintering temperature the shorter the sintering time, but preventing thermal degradation of heat-sensitive substrates requires long periods (tens of minutes) at temperatures lower than the heat-deflection temperature of the substrate. Furthermore, although the ink or paste dries and metal particles draw close to each other during sintering, non-conductive organic stabilizers and other non-volatile components of the ink or paste still remain between the particles.27 These non-conductive components act as barriers that prevent direct contact between particles and limit the percolation network of particles, resulting in low electrical conductivity. For this reason, conventional thermal sintering usually produces low-conductivity metal patterns on heat-sensitive substrates, even with long sintering times, which makes the entire process slow, inefficient, and expensive. It is therefore of great importance to develop a post-printing process to remove non-conductive barriers and to increase the conductivity of printed metal patterns in a short time without damaging heat-


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 35

sensitive substrates. This also has major implications for high-throughput R2R manufacturing of printed electronics. Various alternative approaches to thermal sintering have achieved highly conductive metal patterns while overcoming the temperature limit of heat-sensitive substrates, particularly selective or local sintering methods that generate a large temperature difference between the pattern and substrate (e.g., electrical sintering, photonic sintering, and microwave sintering).28 In the past, an Ag NP layer printed on paper was electrically sintered by applying a voltage over the slightly conductive layer using a probe to cause a local sintering.29 This technique allows a highly conductive feature to be fabricated in a short period of time, though requires continuous and reliable physical contact between the voltage probe and printed layer.30 Localized laser sintering of a printed Au NP solution on a polymer substrate is also possible using a focused laser beam,31 but the low writing speed involved (in the order of a few mm s−1) greatly limits its application in R2R processes. Intense pulsed light (IPL) sintering utilizes high-intensity, millisecond light pulses generated by a high-power Xe flash lamp to selectively sinter printed patterns.32 However, as IPL sintering starts at the upper surface of the printed feature, sintered patterns can have problematic inner cavities or detach from the substrate.32,33 The application of antenna-supported microwave sintering to heat-treated Ag lines on plastic substrates at 110 °C has achieved conductivities equivalent to 5–34% that of bulk Ag in about 1 s, with the antenna only required to be in contact with the printed pattern.34 This has led to the development of CNTmediated microwave sintering which can produce Ag patterns with a conductivity ~39% that of bulk Ag within 1 s.35 However, this requires coating or printing CNTs onto the substrate before the pattern can be printed. It has also been reported that polyelectrolyte-Ag NP composite films are spontaneously sintered at room temperature to produce Ag patterns with an electrical


4

Page 5 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductivity ~20% that of bulk Ag, but this approach leaves many voids between the sintered zones.36 This study looks at how great the temperature of paper can be made by cellulose-mediated microwave heating without thermal degradation, and whether this method of sintering can overcome the aforementioned critical issues related to heat-sensitive substrates by reducing the sintering time when printing or plotting metal patterns on cellulose paper. The electrical conductivity and mechanical stability against bending of these Ag lines are also explored with a view to their use as wiring electrodes in an LED display. A flexible touch piano is also demonstrated using microwave-sintered interconnects on paper. RESULTS AND DISCUSSION Paper is a random network of interconnected pulp fibers (or cellulose fibers) comprised of cellulose macrofibrils, which are composed of microfibrils (or nanocellulose) with many polar hydroxyl groups.1,3,4 Nanocellulose has an electric dipole moment along its long axis,37 and can be called cellulose nanofibers or nanocrystals.4 Paper absorbs microwaves through a dielectric loss mechanism associated with its complex dielectric constants, which are dependent on the composition of pulp fibers (e.g. cellulose, lignin and hemicellulose), as well as the presence of any additives (e.g. bulking agents, binders and pigments) used during paper making. Ordinary paper such as copy paper (or office paper) contains more than ~70 wt % cellulose,38 and so this tends to dominate its dielectric properties. Furthermore, paper absorbs solvents and organic components through capillary force when a metal NP ink is printed on its surface. Ordinary paper can match well with such inks to achieve superior electrical conductivity, mechanical strength and reliability when compared to non-porous plastic substrates.38 Thus, a metal-on-


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 35

cellulose configuration offers a unique and effective approach to rapidly sintering Ag nanoparticles by using the microwave absorption of paper itself (Figure 1). The microwave energy absorbed by the paper is promptly converted into heat, which is in turn transferred to the Ag pattern. Moreover, as the microwave-heated paper supplies the pattern with heat, microwave sintering starts at the bottom surface of the pattern. This makes it contrary to photonic sintering, which starts at the top of the pattern.

Figure 1. Schematic illustration of process for creating high-conductivity Ag patterns on paper in a short period of time through rapid and selective microwave heating of cellulose fibers.


6

Page 7 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Schematic illustration and (b) surface temperature of copy paper without Ag patterns under microwave heating with different angles between the microwave electric field and paper sheet in a rectangular waveguide. (c) Surface temperature of copy paper, as measured as a function of time using different levels of microwave power to reach a target temperature for ~1 s. (d) Photographs of paper samples showing their color after microwave (MW) and thermal (TH) heating in relation to the time and temperature. Surface temperature of pure paper as a function of time, as calculated by COMSOL Multiphysics simulations with respect to: (e) the thermal conductivity of paper and (f) the contact area ratio between the substrate and oven wall. Figure 2a shows the microwave irradiation of a paper sheet without Ag patterns using a rectangular waveguide with a fundamental transverse electric mode (TE10; Ez = 0), which


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 35

maximized microwave heating by ensuring that the electric field was parallel to the sheet (Figure 2b). Due to almost the same field strength, the difference in temperature ramping rate was not caused by the variation in the field strength over a specific area (~0.8 cm2) of the sheet, the temperature of which was measured by an IR sensor (see Figure S1 in the Supporting Information). The dependence of microwave heating on the angle between the electric field and paper sheet is presumed to result from the anisotropic dielectric properties of the paper; i.e. the in-plane dielectric constant is likely to be much bigger than the out-of-plane constant. This notion is supported by the fact that a paper sheet exposed to radio and microwave frequencies has a maximum complex dielectric constant when the electric field is parallel to the sheet, while the lowest values occur when the field is perpendicular to it.39,40 The microwave heating was further enhanced by a 3-stub impedance matching technique, resulting in a large and rapid increase in temperature. This allows the formation of high-conductivity Ag patterns in a short sintering time and eliminates the need for any additional antenna structure or material to promote microwave absorption. In this way, paper functions as both a flexible insulating substrate for the Ag pattern and a lossy dielectric media for rapid microwave heating. Figure 2c shows the average temperature over a specific area of common copy paper without Ag patterns as a function of time at different levels of microwave power. The temperature of the paper increased rapidly to the target value in ~1 s, after which the microwave radiation was immediately turned off and the sample allowed to cool down naturally. This means that the time to target temperature can be considered almost the same as the microwave irradiation time. Meanwhile, the moisture content of copy paper was determined to be about 6% using a moisture analyzer (MX-50, ANP). This moisture content turned out to have no effect on the microwave heating of the paper (see Figure S2 in the Supporting Information). The color of the sample after


8

Page 9 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microwave heating was used to determine the extent of any thermal degradation, revealing that the paper was stable at temperatures equal to or less than 300 °C under ambient conditions due to the short exposure time. This thermal stability was further confirmed by microwave heating cotton paper (cotton paper filter, KINTO) and cellulose-fiber films on plastic substrates, which showed only a very slight bright yellow color at 300 °C (see Figure S3 in the Supporting Information). In contrast, paper placed in a convection oven at 200 °C (10 s) and 250 °C (5 s) was noticeably changed in color (Figure 2d). The probable reason for this is that although the microwave-heated sample experienced temperatures above the paper’s ignition temperature of approximately 200–250 °C, it was only for a very short period of time.41–43 However, the paper did become brown at 350 °C despite a short heating time of ~1.2 s. It should be noted here that the temperature in this case is the maximum temperature of the sample during microwave heating; therefore, it does not reflect its ambient conditions. Compared to microwave heating, paper samples in a convection oven are generally exposed to high temperatures for longer periods because they have highly rough surfaces and are mainly heated through thermal conduction from the bottom wall and hot air in the oven (see Figure S4 in the Supporting Information). This is supported by the long time that is required to reach the processing temperature according to calculations by COMSOL Mutiphysics simulations, which was little dependent on the thermal contact conductance (or contact area) between the substrate and wall, or the thermal conductivity of the substrate, within this range of values (Figure 2e and 2f).44,45 In these simulations, the wall temperature was kept at 200 °C, as was the initial air temperature. The contact area ratios were chosen to be below 0.2 because the only pressure that comes from the sample’s weight is very small when compared to the Young’s modulus of


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 35

paper.46–48 All this means that both the temperature and time of thermal heating is severely restricted if the thermal degradation of paper is to be avoided.

Figure 3. (a) Electrical resistivity of Ag line patterns processed by microwave (MW) and thermal (TH) sintering as functions of time. Insets show cross-sectional FE-SEM images of an Ag pattern microwave-sintered at 250 °C for ~1 s. (b) Microwave electric field intensity around the Ag patterns on paper, as calculated by COMSOL Multiphysics simulations. (c) Paper samples showing their color around the Ag patterns after microwave sintering at 250 and 300 °C for ~1 s. Figure 3a shows the electrical resistivity of Ag line patterns screen-printed on copy paper as functions of sintering temperature and time during microwave and thermal sintering. The electrical resistivity was calculated using:


10

Page 11 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ρ = R⋅ A/ l

(1)

where R is the resistance, l is the length, and A is the cross-sectional area of the line. After thermal sintering at 150–300 °C for 10 s, the electrical conductivity of the Ag lines was 13.7(±0.8)–32.9(±0.4)% that of bulk Ag. Extending this time to 180 s increased the conductivity up to 32.9(±0.9)–42.6(±0.3)% that of bulk Ag, but also caused a noticeable change in the color of the paper substrate except for samples at 150 °C, which is indicative of thermal degradation due to increased exposure to high temperature. This demonstrates that samples in a convection oven cannot be exposed to high temperatures for the period of time needed to achieve lowresistivity. In contrast, the microwave-sintered Ag lines achieved an electrical conductivity equal to 17.5(±1.1)–41.5(±3.0)% that of bulk Ag at temperatures of 200–350 °C, despite a shorter sintering time of ~1 s. Microwave sintering at higher temperatures significantly increased the electrical conductivity through a progressive decrease in the organic content of the Ag lines, with those sintered at ≥250 °C for 1–2 s exhibiting an electrical conductivity comparable to Ag lines thermally sintered at 200–300 °C for 10–180 s. This electrical performance can be attributed to both the removal of stabilizing and binding materials that make up ~3 wt% of the Ag ink,35 as well as the fact that local temperatures were much higher than those measured by the IR sensor due to the electric field enhancement around the Ag lines during microwave sintering (Figure 3b). This is confirmed by the slight change in the color of the paper around the lines after microwave sintering at 300 °C (Figure 3c), which is in stark contrast to the paper sample that was microwave-heated at the same temperature without Ag lines. Furthermore, because the Ag lines were supplied with heat from the paper, microwave sintering started at bottom surfaces of the lines and there were no inner cavities in the microwave-sintered lines (inset of Figure 3a). Thus, cellulose-mediated microwave sintering was clearly superior to conventional thermal sintering in terms of preventing thermal degradation of the substrate and reducing the processing time


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 35

needed to achieve high-conductivity lines. Microwave sintering of Ag lines for ~1 s produced little change in the color of the paper substrate, even though the processing temperature of 250– 300 °C was higher than its ignition temperature. It should also be noted that the actual temperature in some locations may in fact be much higher than this, for reasons discussed earlier. Selective microwave heating via cellulose paper therefore clearly allows for rapid sintering of Ag lines in direct contact with paper with little thermal degradation, which would account for the high conductivity of the microwave-sintered Ag lines obtained in such a short time.

Figure 4. FE-SEM images showing change in morphology and particle packing on the top surface of Ag line patterns screen-printed on paper (a) before and after microwave sintering at (b) 200, (c) 250, (d) 300, and (e) 350 °C for ~1 s. The FE-SEM images in Figure 4 show the morphology and particle packing of the Ag lines on paper before and after microwave sintering for ~1 s. The dried printed line shown in Figure 4a contains Ag NPs with a typical diameter of 40–80 nm that are clearly disentangled. This indicates that the particles probably lost a part of their organic protective layer during drying,


12

Page 13 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


causing them to form pathways of electrical conduction, giving rise to the initial conductivity of the as-dried Ag lines. Under microwave irradiation, the organic material is removed and the particles merge into larger clusters with noticeable necking, leading to a more densely packed Ag layer. This process increases the number and conductance of percolating pathways for electrons throughout the line. Higher sintering temperatures create a higher density structure for the same amount of time, which is in good agreement with the aforementioned decrease in electrical resistivity with temperature. A critical change in morphology and particle packing can be clearly observed in the images obtained at ≥250 °C, with the formation of a tight interconnection between Ag clusters or flakes (Figure 4c and also see Figure S5 in the Supporting Information). The structure sintered at these temperatures was much denser and its constitutive clusters even larger than those samples sintered at lower temperatures. As mentioned before, this dense and interconnected structure is attributed to the removal of organic and other non-volatile components at temperatures above 250 °C. Figure 5 shows the mechanical stability of Ag lines microwave sintered at 250 °C for ~1 s and thermally sintered at 150 °C for 180 s on paper, which was characterized by bending the substrate to different radii of curvature (r) between 1.5–15.0 mm, corresponding to tensile strains (ε) of about 3.3–0.3% based on the relation: ε = d/2r, where d is the substrate thickness (~100 µm).49,50 Samples subjected to bending can often experience localized folding of the paper after tens or hundreds of bends, resulting in a substantial reduction in the radius of curvature needed to cause a large increase in resistance. A polycarbonate film supporter was therefore attached to the samples with adhesive tape to keep them from folding during testing. Different from the thermally sintered lines, the relative resistance (R/R0) of the microwave-sintered lines was kept within 140% that of R0 for tensile strains below 0.5% after 10000 cycles, where R0 is the initial


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 35

resistance of the lines prior to testing. After 10000 cycles, fine cracks were observed on the top surface of the Ag lines when the bending radius was above 2.0 mm. In contrast, when the bending radius was 1.5 mm, the cracks were wider and deeper to the extent that some parts of the lines were broken. This would account for the continual and steep increase in resistance without saturation during the bending tests.

Figure 5. Photographs of microwave-sintered Ag line patterns on paper after bending to a radius of: (a) 4.0 mm (top), (b) 2.0 mm (middle), and (c) 1.5 mm (bottom). FE-SEM images show the top surface of the Ag patterns in a flat state after 10000 bending cycles. (d) Relative resistance of Ag lines sintered by microwave (MW) and thermal (TH) heating as a function of bending cycles and corresponding tensile strain. When connected to a red LED and 9 V battery via a switch, 10.6-cm-long Ag lines plotted on paper with a conductive pen and dried on a hot plate at 80 °C for 60 min served well as flexible electrodes regardless of their degree of bending (see Figure S6 and Movie S1 in the Supporting Information). However, the LED greatly diminished in brightness and even went out when the


14

Page 15 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) LED experiments in which Ag lines pen-drawn on paper were wound several times over an electric cable with a radius of 1.6 mm or folded in two places before and after microwave sintering at 250 °C for ~1 s. (b) Soldering quality of screen-printed Ag lines before and after sintering. (c) Bright red LED with leads soldered on microwave-sintered Ag lines connected in series to two 1.5 V batteries (not shown). lines were wound several times over a common electric cable with a radius of 1.6 mm, or folded in two places (Figure 6a). The transfer of Ag particles onto the paper bed and gloves during winding and folding suggests that the as-dried lines have low mechanical stability, whereas Ag lines microwave-sintered at 250 °C for ~1 s continued to work well when wound or folded and


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

left no trace of Ag particles. Even after folding and unfolding several times, the resistance of the folded line increased by ~10% compared to that of the unfolded (see Figure S7 in the Supporting Information). Moreover, the as-dried line did not solder well with lead-free solder at 250 °C, and was mostly removed from the substrate with only a small amount of solder left (Figure 6b). It would therefore seem that the as-dried line is comprised of non-interconnected individual particles that adhere easily to the solder. In contrast, a good connection was made with the microwave-sintered line at the same temperature, with no obvious damage and a significant amount of solder deposited thanks to the interconnected particles (or clusters) of the Ag line. Two leads of a red LED were therefore soldered onto the microwave-sintered Ag lines and connected in series to two 1.5 V batteries. This soldered LED did not detach from the lines and produced a bright light even when overturned (Figure 6c). A simple display with fifteen surface-mount green LEDs was fabricated to demonstrate the feasibility of creating a flexible device using microwave-sintered Ag lines as the wiring electrodes (Figure 7a). This had a multi-layer structure with vertical interconnect holes, which is the advantage of using paper as a substrate.51 The display was capable of turning all the LEDs on and off at the same time, as well as controlling each individual LED. In this way, the four letters representing the abbreviated name of our affiliated organization (i.e. ‘K’, ‘E’, ‘R’ and ‘I’) were displayed in sequence (see Movie S2 in the Supporting Information). Electrical interconnects and a carbon-printed keyboard were also prepared to produce a paper-based flexible touch piano as a further practical example of the metal-on-cellulose approach to microwave sintering (Figure 7b). This keyboard was inserted into the mating side of a commercial connector and electrically connected to the control board via the Ag lines, which allowed it to create eight musical notes


16

Page 17 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(an octave). Using this, we were able to play the tune ‘Twinkle Twinkle Little Star’, thereby confirming the effectiveness of this method (see Movie S3 in the Supporting Information).

Figure 7. Photographs of paper-based flexible devices: (a) display with fifteen surface-mount green LEDs using microwave-sintered Ag lines as wiring electrodes, and (b) touch piano consisting of a carbon keyboard, microwave-sintered Ag interconnects, and an Arduino-based control board connected to a speaker.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

CONCLUSIONS This study has demonstrated that microwave irradiation of metal-on-cellulose is an effective approach to creating high-conductivity Ag patterns on heat-sensitive paper by sintering Ag particles. In this process, cellulose paper provides both a flexible insulating substrate for Ag patterns and a lossy dielectric media for rapid microwave heating. The anisotropic dielectric properties of cellulose fibers in particular mean that microwave heating is maximized when the electric field is parallel to the paper substrate. The printed or plotted Ag nano-/micro-particles can therefore be rapidly sintered by heat transferred from the paper substrate, which is directly heated by microwaves with little thermal degradation. This allows conductivities as high as ~29– 38% that of bulk Ag to be achieved through microwave sintering of Ag patterns on paper at 250– 300 °C for ~1 s. Furthermore, these microwave-sintered Ag lines retain good mechanical stability after 10000 bending cycles, and can be readily soldered with lead-free solder. The practicality of this approach was confirmed by the successful fabrication of an LED display and touch piano in which microwave-sintered Ag lines acted as flexible electrodes and interconnects. Cellulosemediated microwave sintering of metal-on-cellulose is therefore considered a promising means of reducing the processing time and electrical resistance in flexible paper electronics. EXPERIMENTAL SECTION Preparation of materials and samples. Samples were prepared on common copy paper (Double A®, KHAN-NA) by screen-printing Ag lines with a commercial Ag ink (TEC-PA-010, InkTec) or direct-writing with a commercial Ag pen (Conductive Ink Pen, Circuit Scribe) installed in a plotter (CAMEO, Silhouette). The screen-printed Ag lines were 235–260 µm wide and 4–6 µm thick. All samples were dried at 80 °C for 1 h before microwave sintering. An


18

Page 19 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aqueous dispersion of 25.0 wt% cellulose fibers (Sigmacell® Cellulose S6790, Sigma Aldrich) was then bar-coated on polycarbonate substrates (thickness: 1.0 mm), and the resulting cellulose film samples dried overnight under ambient conditions. Microwave heating of samples. The 2450 MHz microwave applicator used for heating the samples consisted of a moving metal short and rectangular parallelepiped waveguide (WR340) with a long narrow slit on its broad wall. The microwave electric field in the waveguide is defined as the fundamental transverse electric mode (TE10; Ez = 0), such that the microwave field is constant along the y-axis. Samples were inserted through the slit, and then positioned in the yzplane parallel to the electric field of the microwave so that the field intensity was practically constant across the sample. The moving short reflected the incident microwaves and produced a standing wave in the waveguide. The surface temperature of the samples was measured using an infrared (IR) optical sensor (MI3-LT, Raytek), which was installed on the exterior of the waveguide sidewalls and therefore unaffected by microwave radiation. A computer was used to control the microwave sintering system based on the measured temperature. Characterization of samples. A field-emission scanning electron microscope (FE-SEM; S4800, Hitachi) was used to investigate the surface and cross-sectional morphology of the Ag line patterns before and after microwave or thermal sintering under different conditions. The electrical resistance of the Ag lines was measured at room temperature by the four-wire Kelvin method (PM6340 LCR meter, Fluke). The cross-sectional area was determined by numerical integration of a measured surface profile (Alpha-Step 500, KLA-Tencor), as well as from crosssectional FE-SEM images.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

Fabrication of a paper-based flexible display. A flexible display was fabricated using fifteen surface-mount green LEDs (2.0 mm × 1.25 mm × 0.8 mm, Lumex Inc.) and two sheets of paper with microwave-sintered Ag lines. One of these sheets had fifteen holes beside the lines and covered the other so that only parts of the horizontal Ag lines were exposed through the holes. Each LED was placed and bonded on the vertical and horizontal line with conductive paint (Electric Paint, Bare Conductive). This LED array was then connected to an Arduino-based control board (Orange Board, kocoafab) via cables. Fabrication of a paper-based touch piano. A flexible touch piano was fabricated on paper using electrical interconnects and a touch keyboard. The interconnects were fabricated by plotting eight Ag lines with the Ag pen, which were then microwave-sintered. The keyboard was produced by stenciling carbon paste (CSP-3225, Chang Sung Co.) onto paper, to which an eightkey mask was attached. Eight terminal pins of a commercial connector were then bonded on their corresponding Ag lines by ACF (Electrically Conductive Adhesive Transfer Tape 9703, 3M), and then pressed hard to ensure electrical contact with each line. Finally, the interconnects were attached to the bottom of the terminal electrodes of an Arduino-based control board (Touch Board, Bare Conductive). ASSOCIATED CONTENT Supporting Information Electric field strength of a TE10-mode in a rectangular waveguide; moisture content of copy paper and its effect on the microwave heating of the paper; surface temperature and color of cotton paper and cellulose films on polycarbonate after microwave heating; surface roughness of copy paper; FE-SEM images of pen-drawn Ag lines after microwave sintering; pen-drawn Ag


20

Page 21 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lines as flexible electrodes during bending; change in resistance of a microwave-sintered Ag line pen-drawn on paper after folding, and the curvature of folding; movie showing the creation of patterns on paper with an Ag pen used in a plotter; movie of a paper-based LED display; movie of a paper-based touch piano. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author Dr. Sunshin Jung, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the Korea Electrotechnology Research Institute (KERI) Primary Research Program through the National Research Council of Science & Technology (NST), as funded by the Ministry of Science, ICT and Future Planning (MSIP) (no. 16-12-N0101-18). ACKNOWLEDGMENT The authors would like to thank Jin Man Bae at Gyeongsang National University for his help with Arduino coding. REFERENCES 1. Tobjörk, D.; Österbacka, R. Paper Electronics. Adv. Mater. 2011, 23, 1935–1961.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

2. Siegel, A. C.; Phillips, S. T.; Dickey, M. D.; Lu, N.; Suo, Z.; Whitesides, G. M. Foldable Printed Circuit Boards on Paper Substrates. Adv. Funct. Mater. 2010, 20, 28–35. 3. Zhu, H.; Fang, Z.; Preston, C.; Li, Y.; Hu, L. Transparent Paper: Fabrications, Properties, and Device Applications. Energy Environ. Sci. 2014, 7, 269–287. 4. Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: a New Family of Nature-Based Materials. Angew. Chem. Int. Ed. 2011, 50, 5438–5466. 5. Hsieh, M.-C.; Kim, C.; Nogi, M.; Suganuma, K. Transparent Nanopaper with Tailored Optical Properties. Nanoscale 2013, 5, 9289–9295. 6. Mahadeva, S. K., Walus, K., Stoeber, B. Paper as a Platform for Sensing Applications and Other Devices: a Review. ACS Appl. Mater. Interfaces 2015, 7, 8345–8362. 7. Jung, Y. H.; Chang, T.-H.; Zhang, H.; Yao, C.; Zheng, Q.; Yang, V. W.; Mi, H.; Kim, M.; Cho, S. J.; Park, D.-W.; Jiang, H.; Lee, J.; Qiu, Y.; Zhou, W.; Cai, Z.; Gong, S.; Ma, Z. HighPerformance Green Flexible Electronics Based on Biodegradable Cellulose Nanofibril Paper. Nat. Commun. 2015, 6, 7170. 8. Lim, W.; Douglas, E. A.; Kim, S.-H.; Norton, D. P.; Pearton, S. J.; Ren, F.; Shen, H.; Chang, W. H. High Mobility InGaZnO4 Thin-Film Transistors on Paper. Appl. Phys. Lett. 2009, 94, 072103. 9. Hu, L.; Choi, J. W.; Yang, Y.; Jeong, S.; Mantia, F. L.; Cui, L.-F.; Cui, Y. Highly Conductive Paper for Energy-Storage Devices. Proc. Natl. Aca. Sci. USA 2009, 106, 21490–21494. 10. Nyström, G.; Razaq, A.; Strømme, M.; Nyholm, L.; Mihranyan, A. Ultrafast All-Polymer Paper-Based Batteries. Nano Lett. 2009, 9, 3635–3639.


22

Page 23 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11. Zhang, L.; Zhu, P.; Zhou, F.; Zeng, W.; Su, W.; Li, G.; Gao, J.; Sun, R.; Wong, C.-P. Flexible Asymmetrical Solid-State Supercapacitors Based on Laboratory Filter Paper. ACS Nano 2016, 10, 1273–1282. 12. Águas, H.; Mateus, T.; Vicente, A.; Gaspar, D.; Mendes, M. J.; Schmidt, W. A.; Pereira, L.; Fortunato, E.; Martins, R. Thin Film Silicon Photovoltaic Cells on Paper for Flexible Indoor Applications. Adv. Funct. Mater. 2015, 25, 3592–3598. 13. Lien, D.-H.; Kao, Z.-K.; Huang, T.-H.; Liao, Y.-C.; Lee, S.-C.; He, J.-H. All-Printed Paper Memory. ACS Nano 2014, 8, 7613–7619. 14. Mazzeo, A. D.; Kalb, W. B.; Chan, L.; Killian, M. G.; Bloch, J.-F.; Mazzeo, B. A.; Whitesides, G. M. Paper-Based, Capacitive Touch Pads. Adv. Mater. 2012, 24, 2850–2856. 15. Li, R.-Z.; Hu, A.; Zhang, T.; Oakes, K. D. Direct Writing on Paper of Foldable Capacitive Touch Pads with Silver Nanowire Inks. ACS Appl. Mater. Interfaces 2014, 6, 21721–21729. 16. Tian, H.; Ren, T.-L.; Xie, D.; Wang, Y.-F.; Zhou, C.-J.; Feng, T.-T; Fu, D.; Yang, Y.; Peng, P.-G.; Wang, L.-G.; Liu, L.-T. Graphene-on-Paper Sound Source Devices. ACS Nano 2011, 8, 4878–4885. 17. Fan, X.; Chen, J.; Yang, J.; Bai, P.; Li, Z.; Wang, Z. L. Ultrathin, Rollable, Paper-Based Triboelectric Nanogenerator for Acoustic Energy Harvesting and Self-Powered Sound Recording. ACS Nano 2015, 9, 4236–4243. 18. Yang, L.; Rida, A.; Vyas, R.; Tentzeris, M. M. RFID tag and RF Structures on a Paper Substrate Using Inkjet-Printing Technology. IEEE Trans. Microwave Theory Tech. 2007, 55, 2894–2901.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

19. Wang, L.; Chen, W.; Xu, D.; Shim, B. S.; Zhu, Y.; Sun, F.; Liu, L.; Peng, C.; Jin, Z.; Xu, C.; Kotov, N. A. Simple, Rapid, Sensitive, and Versatile SWNT−Paper Sensor for Environmental Toxin Detection Competitive with ELISA. Nano Lett. 2009, 9, 4147–4152. 20. Liu, X.; Mwangi, M.; Li, X. J.; O’Brien, M.; Whitesides, G. M. Paper-Based Piezoresistive MEMS Sensors. Lab Chip 2011, 11, 2189–2196. 21. Andersson, P.; Forchheimer, R.; Tehrani, P.; Berggren, M. Printable All-Organic Electrochromic Active-Matrix Displays. Adv. Funct. Mater. 2007, 16, 3074–3082. 22. Siegel, A. C.; Phillips, S. T.; Wileya, B. J.; Whitesides, G. M. Thin, Lightweight, Foldable Thermochromic Displays on Paper. Lab Chip 2009, 9, 2775–2781. 23. Russo, A.; Ahn, B. Y.; Adams, J. J.; Duoss, E. B.; Bernhard, J. T.; Lewis, J. A. Pen-on-Paper Flexible Electronics. Adv. Mater. 2011, 23, 3426–3430. 24. Han, J.-W.; Kim, B.; Li, J.; Meyyappan, M. Carbon Nanotube Ink for Writing on Cellulose Paper. Mater. Res. Bull. 2014, 50, 249–253. 25. Kamyshny, A.; Magdassi, S. Conductive Nanomaterials for Printed Electronics. small 2014, 10, 3515–3535. 26. Langley, D. P.; Lagrange, M.; Giusti, G.; Jiménez, C.; Bréchet, Y.; Nguyen, N. D.; Bellet, D. Metallic Nanowire Networks: Effects of Thermal Annealing on Electrical Resistance. Nanoscale 2014, 6, 13535–13543. 27. Perelaer, J.; Smith, P. J.; Mager, D.; Soltman, D.; Volkman, S. K.; Subramanian, V.; Korvinkdf, J. G.; Schubert, U. S. Printed Electronics: the Challenges Involved in Printing Devices, Interconnects, and Contacts Based on Inorganic Materials. J. Mater. Chem. 2010, 20, 8446–8453.


24

Page 25 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28. Wunscher, S.; Abbel, R.; Perelaer, J.; Schubert, U. S. Progress of Alternative Sintering Approaches of Inkjet-Printed Metal Inks and Their Application for Manufacturing of Flexible Electronic Devices. J. Mater. Chem. C 2014, 2, 10232–10261. 29. Allen, M. L.; Aronniemi, M.; Mattila, T.; Alastalo, A.; Ojanperä, K.; Suhonen, M.; Seppä, H. Electrical Sintering of Nanoparticle Structures. Nanotechnology 2008, 19, 175201. 30. Cummins, G.; Desmulliez, M. P. Y. Inkjet Printing of Conductive Materials: a Review. Circuit World 2012, 38, 193–213. 31. Ko, S. H.; Pan, H.; Grigoropoulos, C. P.; Luscombe, C. K.; Fréchet, J. M. J.; Poulikakos, D. Air Stable High Resolution Organic Transistors by Selective Laser Sintering of Ink-Jet Printed Metal Nanoparticles. Appl. Phys. Lett. 2007, 90, 141103. 32. Lee, D. J.; Park, S. H.; Jang, S.; Kim, H. S.; Oh, J. H.; Song, Y. W. Pulsed Light Sintering Characteristics of Inkjet-Printed Nanosilver Films on a Polymer Substrate. J. Micromech. Microeng. 2011, 21, 125023. 33. Park, S.-H.; Jang, S.; Lee, D.-J.; Oh, J.; Kim, H.-S. Two-Step Flash Light Sintering Process for Crack-Free Inkjet-Printed Ag Films J. Micromech. Microeng. 2013, 23, 015013. 34. Perelaer, J.; Klokkenburg, M.; Hendriks, C. E.; Schubert, U. S. Microwave Flash Sintering of Inkjet-Printed Silver Tracks on Polymer Substrates. Adv. Mater. 2009, 21, 4830–4834. 35. Jung, S.; Chun, S. J.; Han, J. T.; Woo, J. S.; Shon, C.-H.; Lee, G.-W. Sub-second CarbonNanotube-Mediated Microwave Sintering for High-Conductivity Silver Patterns on Plastic Substrates. Nanoscale 2016, 8, 5343–5349. 36. Magdassi, S.; Grouchko, M.; Berezin, O.; Kamyshny, A. Triggering the Sintering of Silver Nanoparticles at Room Temperature. ACS Nano 2010, 4, 1943–1948.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

37. Frka-Petesic, B.; Jean, B.; Heux, L. First Experimental Evidence of a Giant Permanent Electric-Dipole Moment in Cellulose Nanocrystals. EPL 2014, 107, 28006. 38. Wu, H.; Chiang, S. W.; Lin, W.; Yang, C.; Li, Z.; Liu, J.; Cui, X.; Kang, F.; Wong, C. P. Towards Practical Application of Paper based Printed Circuits: Capillarity Effectively Enhances Conductivity of the Thermoplastic Electrically Conductive Adhesives. Sci. Rep. 2014, 4, 6275. 39. Osaki, S. Quick Determination of Dielectric Anisotropy of Paper Sheets by Means of Microwaves. J. Appl. Polym. Sci. 1989, 37, 527–540. 40. Borch, J.; Lyne, M. B.; Mark, R. E.; Habeger, Jr., C. Handbook of Physical Testing of Paper, Vol. 2; CRC Press: New York · Basel, 2001, 351–353. 41. Babrauskas, V. Ignition of Wood: a Review of the State of the Art. Proceedings of the International Conference on Fire Science and Engineering, Edinburgh, Scotland, Sept 17–19; Interscience Communications Ltd.: London, 2001. 42. Tobjörk, D.; Aarnio, H.; Pulkkinen, P.; Bollstöm, R.; Määttänen, A.; Ihalainen, P.; Mäkelä, T.; Peltonen, J.; Toivakka, M.; Tenhu, H.; Österbacka, R. IR-Sintering of Ink-Jet Printed MetalNanoparticles on Paper. Thin Solid Films 2012, 520, 2949–2955. 43. Matsuo, M.; Umemura, K.; Kawai, S. Kinetic Analysis of Color Changes in Cellulose during Heat Treatment. J. Wood Sci. 2012, 58, 113–119. 44. Lavrykov, S. A.; Ramarao, B. V. Thermal Properties of Copy Paper Sheets. Dry. Technol. 2012, 30, 297–311. 45. Cooper, M. G.; Mikic, B. B.; Yovanovich, M. M. Thermal Contact Conductance. Int. J. Heat Mass Transfer 1969, 12, 279–300. 46. Yang, C.; Persson, B. N. J. Contact Mechanics: Contact Area and Interfacial Separation from Small Contact to Full Contact. Int. J. Phys. Condens. Matter 2008, 20, 215214.


26

Page 27 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47. Khoury, M.; Tourtollet, G. E.; Schröder, A. Contactless Measurement of the Elastic Young’s Modulus of Paper by an Ultrasonic Technique. Ultrasonics 1999, 37, 133–139. 48. Stenberg, N. On the Out-of-Plane Mechanical Behaviour of Paper Materials. Doctoral Thesis, Royal Institute of Technology, 2002. 49. Hyun, W. J.; Lim, S.; Ahn, B. Y.; Lewis, J. A.; Frisbie, C. D.; Francis, L. F. Screen Printing of Highly Loaded Silver Inks on Plastic Substrates Using Silicon Stencils. ACS Appl. Mater. Interfaces 2015, 7, 12619–12624. 50. Suo, Z.; Ma, E. Y.; Gleskova, H.; Wagner, S. Mechanics of Rollable and Foldable Film-onFoil Electronics. Appl. Phys. Lett. 1999, 74, 1177–1179. 51. Liu, J.; Yang, C.; Wu, H.; Lin, Z.; Zhang, Z.; Wang, R.; Li, B.; Kang, F.; Shi, L.; Wong, C. P. Future Paper Based Printed Circuit Boards for Green Electronics: Fabrication and Life Cycle Assessment. Energy Environ. Sci. 2014, 7, 3674–3682.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

Table of Contents Cellulose-based paper functions well as both a flexible insulating substrate for conductive Ag patterns and a lossy dielectric media for rapid microwave heating. Printed Ag nanoparticles can be rapidly (~1 s) sintered by heat transferred from the microwave-heated paper substrate with little thermal degradation of the substrate, resulting in highly conductive Ag patterns with a conductivity of ~29–38% that of bulk Ag.


28

Page 29 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic illustration of process for creating high-conductivity Ag patterns on paper in a short period of time through rapid and selective microwave heating of cellulose fibers. 721x652mm (72 x 72 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) Schematic illustration and (b) surface temperature of copy paper without Ag patterns under microwave heating with different angles between the microwave electric field and paper sheet in a rectangular waveguide. (c) Surface temperature of copy paper, as measured as a function of time using different levels of microwave power to reach a target temperature for ~1 s. (d) Photographs of paper samples showing their color after microwave (MW) and thermal (TH) heating in relation to the time and temperature. Surface temperature of pure paper as a function of time, as calculated by COMSOL Multiphysics simulations with respect to: (e) the thermal conductivity of paper and (f) the contact area ratio between the substrate and oven wall. 716x1004mm (72 x 72 DPI)


Page 30 of 35

Page 31 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Electrical resistivity of Ag line patterns processed by microwave (MW) and thermal (TH) sintering as functions of time. Insets show cross-sectional FE-SEM images of an Ag pattern microwavesintered at 250 °C for ~1 s. (b) Microwave electric field intensity around the Ag patterns on paper, as calculated by COMSOL Multiphysics simulations. (c) Paper samples showing their color around the Ag patterns after microwave sintering at 250 and 300 °C for ~1 s. 602x754mm (72 x 72 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. FE-SEM images showing change in morphology and particle packing on the top surface of Ag line patterns screen-printed on paper before and after microwave sintering at (b) 200, (c) 250, (d) 300, and (e) 350 °C for ~1 s. 766x335mm (72 x 72 DPI)


Page 32 of 35

Page 33 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Photographs of microwave-sintered Ag line patterns on paper after bending to a radius of: (a) 4.0 mm (top), (b) 2.0 mm (middle), and (c) 1.5 mm (bottom). FE-SEM images show the top surface of the Ag patterns in a flat state after 10000 bending cycles. (d) Relative resistance of Ag lines sintered by microwave (MW) and thermal (TH) heating as a function of bending cycles and corresponding tensile strain. 764x343mm (72 x 72 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) LED experiments in which Ag lines pen-drawn on paper were wound several times over an electric cable with a radius of 1.6 mm or folded in two places before and after microwave sintering at 250 °C for ~1 s. (b) Soldering quality of screen-printed Ag lines before and after sintering. (c) Bright red LED with leads soldered on microwave-sintered Ag lines connected in series to two 1.5 V batteries (not shown). 301x471mm (72 x 72 DPI)


Page 34 of 35

Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Photographs of paper-based flexible devices: (a) display with fifteen surface-mount green LEDs using microwave-sintered Ag lines as wiring electrodes, and (b) touch piano consisting of a carbon keyboard, microwave-sintered Ag interconnects, and an Arduino-based control board connected to a speaker. 282x525mm (72 x 72 DPI)


Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity

Recommend Documents