Wax Conductive Composite

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Healable and Foldable CNT/Wax Conductive Composite Tso-Hsuan Chen, Yu-Chi Yeh, and Ying-Chih Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08310 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Materials & Interfaces

Healable and Foldable CNT/Wax Conductive Composite Tso-Hsuan Chen,1 Yu-Chi Yeh,1 Ying-Chih Liao1* 1

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

E-mail: [email protected]; [email protected]; [email protected]. Abstract: In this study, a composite material with healable and foldable features is formulated to print conductive patterns on rough surfaces, such as paper, cloth, and 3Dprinted objects. Carbon nanotubes (CNT) are mixed with wax to formulate a solid composite for pen writing. The composite has a low percolation threshold of 2.5 wt% CNT and can be written on various rough substrates, such as paper and clothes, to create conductive patterns for electronic conductors. Because of the strong infrared absorption of CNT, the printed patterns can be selectively sintered by a non-contact infrared radiation efficiently to show great electrical conductivity. The electrical resistance of written patterns on paper also show insignificant increase after bending, folding, and crumpling. Furthermore, the conductive composite exhibits great healability after destructive damages. The conductivity of the damaged patterns after severe folding or knife cutting recovers to its original value with thermal or IR heating. Several examples, such as conductive tracks on paper, cloth, or 3D printed objects, are also demonstrated to show the potential of this healable conductive composite for electronic applications.

* Author to whom the correspondence should be addressed. Telephone: 886-23366-9688, e-mail: [email protected]. 0000-0001-9496-4190 ACS Paragon PlusORCID: Environment

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Keywords: foldable conductive patterns, healable material, carbon nanotubes, wax, direct writing

Supporting information SEM images of MWCNT/wax composite on paper substrate, examples for foldable and twistable electronics, and water contact angles of PLA substrate and CNT/Wax composite. This information is available free of charge via the Internet at http://pubs.acs.org/.

ACS Paragon Plus Environment

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Introduction Pen writing is a simple direct printing approach for pattern formation, and has the characteristics of compact, fast period coating and easy accessibility.1 With these adantages, pen writing techniques have been untlized to fabricate various functional electronic devices.2-4 Typical pen writing techniques include brush pen,3 fountain pen,5 ball pen6 and pencil.7 These pens use mostly solution-based inks, and thus needs gravity or capillary forces for writing.1 Take fountain pen as an example; when the nib is upward, the ink cannot be transferred to substrate surfaces due to the reverse ink flow against gravity. Unlike ink-based writing methods, pencil writing only depends on friction forces between the solid pen composites and substrates, and provides an alternative option to avoid gravity effects.1 The friction forces rub off the pencil lead composite on the substrate to form patterns.8-9 Thus, pencils can write patterns in any direction even when the nib is upwards against gravity. Pen writing techniques have also been widely applied in printing electronic devices to provide bendable, elastic, lightweight, and non-breakable features.5, 10-11 For example, Tai’s group fabricated paper based flexible electronics by writing conductive nanosilver ink with a fountain pen.11 However, regular printed flexible electronic devices could only afford low mechanical bending stress. Under folding conditions, permanent creases12 caused by serious deformation are observed in the printed conductive pattterns. These folding creases remain after unfolding. Therefore, common way to manufacture foldable electronics uses both foldable substrates and foldable conductors13 that could withstand deformation,12 maintain stable contact with each other,13 and preserve their operating characteristics after folding. The foldable conductors are usually synthesized by


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mixing conductive fillers with elastic polymers. Because silver nanomaterials provide both great mechanical flexibility and electrical conductivity, silver nanoparticles, nanowires or nanosheets have been widely used in foldable conductors.12, 14-16 However, silver fillers are mostly applied in solution-based pen writing materials. Because conductive solid composites with high silver concentrations are brittle,17 pencils with silver fillers have little ductility for writing patterns. On the other hand, carbon nanotubes (CNT) provide a better choice for conductive filler in pencils. CNT has high electrical conductivity and excellent machanical properties to recover from severe bending and compressing.18 Moreover, CNT

has also been widely applied in the fabrication of

foldable electronics. For example, Honda’s et al.19 used CNT/polymer inks as raw materials to fabricate foldable electronics by screen printing. M. A. Darabi’s20 group developed a stretchable and foldable sensor based on CNT and chewing gum membrane. Sekitani et al. also demonstrated the feasibility of applying CNT in the formulation of stretchable and foldable conductors.21 Based on the aforementioned studies, CNT with high conductivity has shown great potentials as candidate conductive fillers in foladable or stretchable condcutive composite. However, these examples used solution-based liquid inks for pen writing, and thus one would encounter aforementioned flow problems when printed under gravity. To address this issue, a new formulation of solid CNT composite pencil is needed. For pen writing with solid composites, wax has been widely used in the formulation due to its great flowability under strong friction forces. For example, crayons are made with wax and coloarants and can be drawn on paper, clothes, or walls. Using similar formulation as crayon, a solid wax/CNT composite material can be easily made for pencil material to draw conductive tracks with precise patterns. As a commonly-used


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additive of hot-melt adhesive, wax has a low boiling point (~ 65 oC)22 and low viscosity.23 Thus, wax composites can be easily processed through simple heating to form compact solids with good filler connectivity or electrical conductivity. Beasides, wax also shows a thermally healable property24 due to its low surface tension25 and great spreading ability to move toward uncovered area.26 In this study, we developed a crayon formulation by mixing CNT and beeswax. This composite can be used to print conductive pattern by direct pen writing on foldable substrates or rough surfaces such as 3D printed objects. The printed conductive pattern can sustain strong strain like bending, folding, and even twisting. The printed patterns also exhibit a healable property and one could repair the damaged conductive tracks by simply heating it with IR radiation or a hair dryer. Several examples will also be demonstrated to show the potential of this composite material in electronic applications.

Experimental section Bee wax pellet with a melting point of 65 °C was obtained from First Chemical Co. Ltd. Two types of multi-walled carbon nanotube (MWCNT) were purchased from Golden Innovation Business Co. Ltd.: highly-directional MWCNT (s-MWCNT, length between 10 and 50 µm) and curly MWCNT (c-MWCNT, length between 10 and 30 µm). Toluene (>99.8%, Alfa Aesar Chemical) and ethanol (>99.8%, Sigma-Aldrich) were used as purchased without further purification. A solvent mixture containing 30 g toluene and 10 g ethanol was first prepared in a glass bottle. 3 g wax with proper amount of CNT powders were then added into the bottle. The bottle containing the CNT/wax mixture was capped and placed in a hot water bath at 90 °C under strong stirring for 2 hours. After


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that, the bottle was opened and placed in a hot bath at 80 °C under slow stirring for 5 hours to evaporate the solvent. After drying at atmospheric pressure, the remnant was further put into a vacuum oven at 85 °C for 5 hours to remove the rest of the solvent residues. The dried CNT/wax composite material was then place in a mold, heated under 80 °C, and compressed to form a pen. The pen in the mold was cooled under room temperature for 15 minutes, and was used to write conductive patterns on substrates. An infrared light (Philips IR-175R-PAR, 175W) was used as a radiative heating source for the printed or written CNT/wax patterns. Morphology of the CNT was examined by using transmission electron microscope (TEM, Tecnai F30, Philips). Four-point probe (GW Instek GDM-8261A) was used to measure the sheet resistance of the written patterns. The line resistance was measured by a multimeter (HOLA DM-2690TU). 3D objects were printed with a 3D printer (D-force Co., Taiwan) using a polylactic acid (PLA) material.

Results and Discussion Percolation and Conductivity of MWCNTs The shapes of MWCNTs strongly affect the conductive network or percolation threshold in the MWCNT/wax composite.27 Thus, selection of MWCNTs with good percolative microstructures is crucial to the composite conductivity. Figure 1 shows the TEM images of two types of MWCNTs. The c-MWCNT has a curly appearance with a cylindrical diameter of 30 nm. On the other hand, the s-MWCNT is straight with a larger diameter of 50 nm. Appaearently, although the c-MWCNT has more branching points, the s-MWCNT has a longer length and theoretically can form more connective points for


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network percolation in the MWCNT/wax micture. To evaluate the effects of percolation on conductivity, the composites are rubbed onto a glass slide to create a square solid film of 15.0 × 15.0 × 0.09 mm (length × width × thickness) to measure the corresponding electrical conductivity. The variations in conductivity of CNT/wax films at different MWCNT weight fractions are summarized in Figure 2. With the same weight percentage in wax, the s-MWCNT films always show a much lower resistance than those of cMWCNT films. This difference in film electrical conductivity is attributed to the MWCNT percolation. As shown in Figure 1, the s-MWCNT is much longer than cMWCNT, and is easier to provide percolative contacts.28-29 As a result, the s-MWCNTs can form a dense percolating network with more connections, and exhibits a lower percolation threshold of ~ 5 wt%. For the sake of electrical conductivity, s-MWCNT is used as the conductive filler in the following sections. A classical percolation scaling law30 can be used to explain the conductivity σ in the percolating s-MWCNT network in wax: t

σ = σ ( p - pc )

(1)

where σ is a constant related to the intrisic conductivity of the filler, pc is the critical concentration of filler, p is the MWCNT weight fraction, and t is the conductivity exponent. The best-fitted values after data regression (Figure 2) using Equation (1) shows a critical concentration pc of 4.850% with an exponent t = 1.902, which falls in the regular range between 1.3 and 2.0 for CNT percolation.31

The conductivity of s-

MWCNT/wax composite continues to rise as the concentration of the s-MWCNT increases. However, the lower MWCNT weight fraction can yield a softer pencil for writing at a lower pressure. In addition, lower wax ratio in the composite will lead to a


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weaker healability. Therefore, 15 wt% MWCNT/wax composite is used in the following sections.

Friction Force and Printed Amount The amount of MWCNT/Wax composite printed on paper depends strongly on the applied friction forces. To quantitatively described the relationship, a slender cylinder (5 mm in diameter and 3 cm long) made of MWCNT/Wax composite is placed vertically perpendicular to a piece of paper. After applying a fixed vertical force at by force gauge, the paper substrate is pulled in horizontal direction for 6.5 cm at a fixed speed of 3 cm/s. The weight of written MWCNT/Wax composite on the paper is measured as shown in Figure S1. The result shows that the conductive composite can be printed on paper substrate easily at low normal pressure. Since 3D printed objects have a much rougher surface than paper, this composite can be printed more easily applied on the surface for conductive pattern formation. Moreover, the linear relationship between the applied pressure and coating density allows one to control printed composite amount on the substrates by simply adjusting applied forces.

Post-treatment The resistance of written MWCNT/wax patterns can be decreased after thermal treatment. Because the MWCNT/wax patterns are transferred onto substrates by friction forces, the as-prepared patterns are not compact and the porous MWCNT/wax microstructure reduces the effective pathways for electron transfer. The MWCNT/wax pattern can be solidified by melting the wax. After heating at 80 oC for 10 minutes, a


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compact solid film with reduced resistance is obtained (Figure 3). From the cross sectional examination, the thickness of the written film decreases from 90 µm to 65 µm, representing a 27% decrease in volume. The removal of voids results in more connection between conductive CNT fillers, and therefore the sheet resistance drops to ~200 Ω, which is only 1/3 of its origin value. Although thermal post treatment can be easily applied to papers using a hot plate, it is difficult to apply on thick bulk material, such as 3D printed objects. Alternatively, a non-contact infrared (IR) radiative heating is adopted here to provide surface heating instead of using hot plates. Moreover, both wax and CNT can absorb infrared (IR) radiation effectively,32-33 the MWCNT/wax patterns can be selectively heated. As shown in Figure 4, the temperature of the written MWCNT/wax pattern reaches ~70 o C (above the melting point of the wax) after radiative heating for 2 minutes, while the surface temperature of the 3D printed substrate maintains at less than 50 oC. This IR radiative process can therefore provide an effective local heating on the MWCNT/wax pattern, decreases the voids in the written conductive tracks, and decrease the electrical resistance without causing thermal deformation on the substrate. The resistance reduction ratio, R/R0, of the conductive MWCNT/wax composite shows that a 5-minute exposure under IR lamp can provide satisfactory heating (Figure 5). In particular, the resistance of the composite on paper decreases over 90%, while ~70% on other solid surfaces. This higher resistance reduction on paper is possibly due to the wax absorption in porous paper structures, as the SEM image shown in Figure S2, leading to higher MWCNT lateral density on the paper surface. Nevertheless, the IR post treatment can provide a much better local heating effects even on 3D surfaces, and thus in the following sections, IR


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post process with exposure time of 5 minute are used to improve conductivity of the MWCNT/wax composite. Scratch Resistance and Heat Stability The scratch resistance of the printed composite thin films on paper after post treatment is tested by using ASTM-D-3363 standard. The scratch resistance or hardness of the composite thin films is determined by comparing with the hardness of pencil leads. The experiment results show that pure wax has a rally low scratch resistance and is a soft material with pencil hardness of 9B. The hardness increases with increasing MWCNT percentage and is up to 2B when 20% MWCNT is added (Figure S3 (a)), indicating that higher ratio of MWCNT could enhance the mechanical strength of the composite coatings and improve the scratch resistance. The MWCNT/wax composite has great thermal stability and the printed patterns exhibit stable electrical resistance for a long time. To evaluate the heat stability of the MWCNT/wax conductive composite, a rectangular pattern of 1 cm × 4 cm is first printed on a 3D printed PLA plate. The samples are then kept in an oven at 60 °C, and the resistance of the patterns is recorded each day. As shown in Figure S3 (b), there is insignificant change in electrical resistance for 20 days, indicating that the conductive pattern made of MWCNT/Wax composite is thermally stable.

Foldable and twistable characteristics After the IR post-treatment, the MWCNT/wax conductive tracks on paper has a strong mechanical strength under folding or twisting conditions. As shown in the inset figure of Figure 6., the resistance increases slightly (< 6%) under various folding


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conditions. Moreover, the change in resistance, R/R0, is fairly small (< 1.2) in the first 1000 folding cycles (Figure 6.). After that, the change in resistance depends strongly on the folding angles. Under compressive folding conditions (-90° and -180°), the MWCNT/wax composite receives less damage and thus the resistance remains almost the same as the initial value R0 even after 10000 folding cycles. On the other hand, under tensile folding conditions, the MWCNT/wax receives much structural damage and therefore R/R0 rises after 1000 folding cycles. Under tensile flat folding condition (180°), R/R0 increases drastically and reaches 2.44 after 10000 folding cycles. This resistance increase is related to serious substrate damages after 1000 folding cycles. From the SEM images (Figure S4), the protruding fibers break the upper conductive layer, hence leads to a large resistance increase.

Nevertheless, the result reveals that the MWCNT/wax can

resist strong tensile deformation for more than 1000 folding cycles, and can be used as foldable conductor (Figure S5). In addition, after written on clothes and the IR posttreatment, the MWCNT/wax composite remains conductive after being twisted and/or crumpling (Figure S6). These results show that the MWCNT/wax composite has strong mechanical strength and stability under various deformation conditions.

Healable characteristics The damages on the MWCNT/wax composite after folding or cutting can be healed with re-exposure of IR radiation. Because of the low melting point and good flowability of liquid wax,24 the written MWCNT/wax composite pattern can recover its conductivity by IR heating after suffering deformational or destructive damages. Two examples are given here. As shown in Figure 7, the written conductive pattern after


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multiple folding shows a resistance increase to three times of its original value. After applying IR heating on the folded paper, the resistance nearly reduces to its original value because the melting MWCNT/wax composite refills the folding creases and reinforces the conductive networks. The fluidity of the composite at moderate temperatures also provides healability for cut tracks. As shown in Figure 8 (a), the written conductive pattern on a 3D printed plate is cut with a scalpel to open the circuit. After heating the damaged track area, because of the higher surface free energy of the PLA substrate (contact angle of 52°, Figure S7 (a)) than the melted composite (contact angle of 111°, Figure S7 (b)), the melt composite slowly covers the cut area on the substrate and resulted in a healed pattern. As shown in Figure 8 (c), the cut heals after heating with a hair dryer for 15 minutes, and the resistance restore to near its initial value (Figure 8(b)). The healing time dependence for electrical resistance change is also shown in Figure 8 (d). After 1 minute of IR healing, the resistance of the cut sample quickly restores to twice of its original value R0. After 15 minutes, the resistance reaches a plateau and almost restores to the original value. One can repeat the same cutting-healing tests on the same conductive pattern for more than 20 times (Figure 8 (e)) with resistance increase lower than 2 times its original value.

These results indicate that the healable

characteristic of the written conductive circuit could be applied on the surface of 3D printed objects. The healed conductive tracks exhibit similar conductivity as the pristine ones, but the mechanical strength of the healed tracks can be deteriorated due to damages on substrates. To evaluate the mechanical strength of the healed conductive tracks, similar folding experiment in Figure 6 is applied to cut-and-healed samples. As shown in Figure


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S8, the resistance shows significant change within 100 folding cycles, but starts to increase afterwards. Comparing to the results in Figure 6, in which the uncut samples show insignificant resistance increase before 1000 folding cycles, the healed tracks show higher resistance increase. It is suspected that the paper substrate is damaged in the cutting process and thus is more susceptible in the repetitive folding process. Nevertheless, despite the slightly lower mechanical strength, the healed track can still maintain good electrical property after repetitive folding.

Automated pencil printing on curvilinear surfaces This composite can be printed on 3D surfaces by robots to create foldable and healable conductive circuit. As shown in Figure 9 (a), a pencil made of the MWCNT/wax composite could be installed on a five-axis automated robot to print conductive patterns on the surface of a 3D-printed car. The LED light on the 3D printed car could illuminate after electrified on the conductive patterns (Figure 9 (b)). In Figure 9 (c-d), a line with a width of 0.48 mm is cut on the conductive patterns, and the light turned off due to the open circuit. The damaged conductive patterns are then heated with a hair drier for five minutes (Figure 9 (e)). The healed conductive patterns after the thermal treatment retains it conductivity, and the LED light recovers its illumination (Figure 9 (f)). After careful examination of the cutting area (Figure 9 (g) vs. 9 (d)), the cut line on the pattern was clearly healed after heating with a hair dryer. In addition, if there is a need for higher conductivity of such kind of composite, we can simply add some nanosilver wire into the composite. This result indicates that the MWCNT/wax composite could be applied on the mass production of conductive patterns with mechanized and automatic methods.


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Conclusions In this study, a composite made of MWCNT and wax is synthesized for foldable and healable conductors. After mixing MWCNT and wax in organic solvents, the MWCNT/wax mixture is dried to form a writable pencil for conductive pattern formation. Among the tested MWCNTs with different mophorlogies, s-MWCNT shows a lower percolation threshold due to its long length for better network percolation and is chosen in the composite formulation. Because the pen writting process utilizes frictional forces, voids are formed in the written MWCNT/wax patterns and reduce the conductivity. These voids can be removed by heating the patterns above the melting temperature of the wax (~80o C), and the conductivity can be improved by 5 times on solid substrates and 10 times on porous paper substrates. To avoid substrate deformation, an IR radiation process is developed to provide selective heating on the composite by utilizing the strong IR absorption of MWCNT. After the IR heating, the composite conductor exhibits great mechanical strength and stability under folding, twisting, and crumpling conditions. The resistance of the written conductive patterns increases less than 20% after 1000 repetitive folding and unfolding cycles. After written on paper or cloth substrates, the composite conductors can provide effective conducitve circuits for LED lights even after serious deformations. The conductivity of the composite after deformational or desctructive damages, such as folding or knife cutting, can also be recovered by IR radiation or using a hair dryer. This composite can also be printed with automated robots to create foldable and healable conductive circuit on 3D curvilinear surfaces. These examples show the


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great potential of this healable and foldable composite material and can be used in many electronic applications. Acknowledgement This study is supported by the “Advanced Research Center of Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F002-001 and 106-2628-E-002 -008 -MY3).

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Stauffer, D.; Aharony, A.; Redner, S., Introduction to Percolation Theory. Physics

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Sandler, J.; Kirk, J.; Kinloch, I.; Shaffer, M.; Windle, A., Ultra-low Electrical

Percolation Threshold in Carbon-nanotube-epoxy Composites. Polymer 2003, 44 (19), 5893-5899. 32.

Huang, Z.; Zheng, S.; Fogler, H. S., Wax Deposition: Experimental

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Absorption Properties of Carbon Nanotube/Nanodiamond Based Thin Film Coatings. Journal of Microelectromechanical Systems 2014, 23 (1), 191-197.


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FIGURE CAPTIONS Figure 1. TEM images of (a) c-MWCNT and (b) s-MWCNT. Figure 2. Conductivity of MWCNT/wax composite at different weight fraction of sMWCNT and c-MWCNT. Figure 3. The cross-section images (a) before and (b) after the thermal post-process. (c) The top view images (c) before and (d) after the thermal post-process. The MWCNT/wax was printed on a glass substrate and heated on a hotplate 80 oC for 10 minutes. Figure 4. The temperature profile of s-MWCNT/wax pattern under IR radiation: (a) top view image, (b) IR radiation setup, (c) heating process, and (d) thermal profile after 5minute IR-radiation. Figure 5. Relative resistance of s-MWCNT/wax composite pattern after IR radiation process on (a) paper (b) 3D-printed PLA plate. The pattern is 20 × 5 (mm). Figure 6. Relative resistance of printed s-MWCNT/wax tracks on paper after repetitive folding process at various folding angles. The inset figure shows the schematic diagram of folding angles and the relative resistance after first folding cycle. Figure 7. The images and resistance (a-b) before and (c-d) after crumpling. (e-f) The image and resistance after IR radiation. Figure 8. The resistance of a printed track (a) after knife cut and (b) after healed with IR radiation. (c) Optical images the knife cut area at various times. (d) Variation of electrical resistance with IR radiation time for a cut sample. (e) The relative resistance of the conductive track after repetitive cutting and healing cycles. Figure 9. (a) A pencil made of MWCNT/wax composite is installed on a five-axis automated robot to print conductive tracks on a 3D printed object. (b) The finished MWCNT/wax conductive track on the 3D object. (c)(d) A line was cut on the conductive tracks. (e) Healing the conductive tracks with a hair dryer. (f)(g) The conductive track on the car after healing process.


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Figure 1. TEM images of (a) c-MWCNT and (b) s-MWCNT.


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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 (S/m)


100

Page 22 of 30

s-MWCNT σ ∝ (p - 4.85)1.95

1 c-MWCNT σ ∝ (p - 8.79)1.66

0.01

1E-4

0.05

0.10

0.15

0.20

CNT (g/g) Figure 2. Conductivity of MWCNT/wax composite at different weight fraction of sMWCNT and c-MWCNT.


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Figure 3. The cross-section images (a) before and (b) after the thermal post-process. (c) The top view images (c) before and (d) after the thermal post-process. The MWCNT/wax was printed on a glass substrate and heated on a hotplate 80 oC for 10 minutes.


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Figure 4. The temperature profile of s-MWCNT/wax pattern under IR radiation: (a) top view image, (b) IR radiation setup, (c) heating process, and (d) thermal profile after 5minute IR-radiation.


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1.2

Paper 3D

1.0

R/R0 (-)

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


0.8 1 cm

1 cm

0.6

3D printed plate

Paper

0.4 0.2 0.0 0

5

10

15

20

25

30

Time (min) Figure 5. Relative resistance of s-MWCNT/wax composite pattern after IR radiation process on (a) paper (b) 3D-printed PLA plate. The pattern is 20 × 5 (mm).


25


3.5 3.0

R/R0 (-)

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 30

2.5

180°°

2.0 1.5

90°° -180°° -90°°

1.0 0.5

1

10

100

1000

10000

Folding Cycles (-) Figure 6. Relative resistance of printed s-MWCNT/wax tracks on paper after repetitive folding process at various folding angles. The inset figure shows the schematic diagram of folding angles and the relative resistance after first folding cycle.


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Figure 7. The images and resistance (a-b) before and (c-d) after crumpling. (e-f) The image and resistance after IR radiation.


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(d) 3.0

∞

2.5 2.0

R/R0 (-)

R/R0 (-)

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 30

1.5 1.0

10

0.5 1

0.0

0

5

10

15

20

0

Healing Time (s)

5

10

15

20

25

Cutting Times (-)

Figure 8. The resistance of a printed track (a) after knife cut and (b) after healed with IR radiation. (c) Optical images the knife cut area at various times. (d) Variation of electrical resistance with IR radiation time for a cut sample. (e) The relative resistance of the conductive track after repetitive cutting and healing cycles.


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Figure 9. (a) A pencil made of MWCNT/wax composite is installed on a five-axis automated robot to print conductive tracks on a 3D printed object. (b) The finished MWCNT/wax conductive track on the 3D object. (c)(d) A line was cut on the conductive tracks. (e) Healing the conductive tracks with a hair dryer. (f)(g) The conductive track on the car after healing process.


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TOC


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Wax Conductive Composite

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