Radio Frequency Induction Welding of Silver Nanowire Networks for Transparent Heat Films

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Article Radio Frequency Induction Welding of Silver Nanowire Networks for Transparent Heat Films

Jisoo Oh 1,†, Long Wen 1,†, Hyunwoo Tak 1, Heeju Kim 1 , Gyowun Kim 1, Jongwoo Hong 1 , Wonjun Chang 1 , Dongwoo Kim 1 and Geunyoung Yeom 1,2,*

1 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] (J.O.); [email protected] (L.W.); [email protected] (H.T.); [email protected] (H.K.); [email protected] (G.K.); [email protected] (J.H.); [email protected] (W.C.); [email protected] (D.K.) 2 SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 16419, Korea * Correspondence: [email protected] † Both authors contributed equally to this work.

Abstract: Transparent heat films (THFs) are attracting increasing attention for their usefulness in various applications, such as vehicle windows, outdoor displays, and biosensors. In this study, the effects of induction power and radio frequency on the welding characteristics of silver nanowires (Ag NWs) and Ag NW-based THFs were investigated. The results showed that higher induction frequency and higher power increased the welding of the Ag NWs through the nano-welding at the junctions of the Ag NWs, which produced lower sheet resistance, and improved the adhesion of the Ag NWs. Using the inductive welding condition of 800 kHz and 6 kW for 60 s, 100 ohm/sq of Ag NW thin film with 95% transmittance at 550 nm after induction heating could be decreased to 56.13 ohm/sq, without decreasing the optical transmittance. In addition, induction welding of the Ag NW-based THFs improved haziness, increased bending resistance, enabled higher operating Citation: Oh, J.; Wen, L.; Tak, H.; Kim, H.; Kim, G.; Hong, J.; Chang, W.; temperature at a given voltage, and improved stability. Kim, D.; Yeom, G. Radio Frequency Induction Welding of Silver Keywords: silver nanowires; nano welding; induction heating; rf frequency; transparent heat film Nanowire Networks for Transparent Heat Films. Materials 2021, 14, 4448. https://doi.org/10.3390/ma14164448 1. Introduction Academic Editor: Alina Maria Holban Transparent and flexible heaters are visually transparent devices that contain an electrically conductive layer. When a current across the transparent heater generates Received: 5 July 2021 heat by the Joule effect, this heat can be efficiently used in many devices. Consequently, Accepted: 5 August 2021 numerous applications require transparent and flexible heaters, and the associated market Published: 8 August 2021 comprising many types of printed electronic devices such as smart windows, defoggers, displays, sensors, etc., is growing fast [1–5]. Great attention has been devoted to various Publisher’s Note: MDPI stays neutral flexible and transparent heaters (or electrodes) based on printed electronics due to different with regard to jurisdictional claims in reasons such as devices having nonplanar and flexible substrates, the potential scarcity of published maps and institutional affil- indium, and the lack of transparency in the near-infrared spectrum [6–8]. In these printed iations. electronic products, conductive materials with low resistance, high transmittance, high flexibility, etc., are essential in the performance of the devices, and various conductive materials such as transparent conductive oxide, metallic nanowires, conductive polymers, and carbon-based electrodes have been investigated [9–14]. Copyright: © 2021 by the authors. Among the various conductive nanomaterials investigated for printed electronic prod- Licensee MDPI, Basel, Switzerland. ucts, silver nanowires (Ag NWs) are considered to be one of the ideal alternative candidates This article is an open access article for future applications in electronics, owing to their advantageous characteristics, which distributed under the terms and include their ductility, low sheet resistance, and low cost. Moreover, they can be fabricated conditions of the Creative Commons by facile low-cost solution-based processes, such as spin coating, spray coating, soft im- Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ printing, and vacuum filtration, and electrospinning process coating [15–22]. However, 4.0/). there are several issues to be solved for the practical application of Ag NWs in the area of

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electronics, such as the significant contact resistance of the nanowire junctions due to the weak bonding between nanowires, high surface roughness, and instability due to the high specific surface area. All the factors mentioned above can degrade the performance of Ag NW networks and impede the application of Ag NW networks in electronics [23–26]. To improve the electrical and mechanical characteristics of the transparent electrode using Ag NWs, the welding process is necessary to enhance their performance. Ag NW welding using various methods has been investigated, such as heating, plasmonic effect, capillary force, chemical treatment, laser irradiation, and radiofrequency [27–36]. The welding treatment can significantly reduce the contact resistance of the Ag NWs. Among these methods, induction welding using radiofrequency has additional advantages, because it can heat and weld only the junction area through the eddy current induced on the Ag NWs by the electromagnetic field generated by rf power, and it is a self-terminating process, where when the welding process is complete, the welding stops itself [37–41]. In this paper, we use induction heating systems operated at various radio frequencies to investigate the nanowire network’s welding characteristics according to the radio frequency and power for the Ag NWs with different resistivities, and explore the effects of Ag NW induction welding on the characteristics of Ag NW-based transparent heat films (THFs).

2. Experimental In this experiment, commercially available Ag NWs (NANOPYXIS, of 22 ± 5 µm length, and 25 ± 3 nm diameter) were used. These Ag NWs were diluted with isopropyl alcohol (IPA) to form a 0.05 wt.% Ag NW solution. A polyethylene naphthalate (PEN) substrate was coated with the Ag NW solution. Before coating with the Ag NW solution, the substrates were cleaned in sequence using alcohol, and deionized water (DIW) by sonication. The Ag NW film was coated by spin coating of the Ag NW solution at 500 rpm for 5 s, and 4000 rpm for 60 s. Uniform nanowire films with the initial sheet resistances of 30, 50, 100, and 300 ohm/sq could be obtained on the PEN through the multiple coating. To demonstrate the applications of these Ag NW coated transparent flexible films, 25 mm × 25 mm THFs were fabricated using three Ag NWs films (a non-welded Ag NWs film and Ag NW films welded by 100 and 800 kHz inductive heating) for 60 s at 6 kW. The THFs films were fabricated by attaching electrodes on the sides of the films with silver paste and copper tape. The silver paste was painted at the edges of the heat film, and the copper tape was used to connect the heat film with outside electrical wires. Figure1a shows the process sequence of the THF fabrication by Ag NW coating on the PEN. The Ag NWs coated substrates were welded by inductive heating. Figure1b,c show the schematic diagram of the induction welding principle and basic induction heating system, respectively. The induction heating system used in this study is commercially available equipment (Lihua Technology Industrial Co. Ltd., Guangdong, China). The applied input power of the induction heater can be controlled and set to a maximum of 6 kW, and the frequency was fixed at 100, 450, and 800 kHz for each induction heating system. For all systems, the distance between the inductive coil and the substrates was maintained at 5 mm. For the welding, the inductive power of 1–6 kW and the welding time of 30–600 s were varied to observe the resistance change of Ag NWs film. To improve the stability of the Ag NW-based THF during the heating experiment, the Ag NW networks on the PEN substrates were plasma-treated with C4F8 plasma to cover the Ag NW network with a polymer layer. To deposit the fluorocarbon polymer layer on the Ag NW networks, the Ag NW-coated THFs were exposed to c-C4F8 (octa- fluorocyclobutane-fluorocarbon molecules with a circular structure) plasma for 30 s at the substrate temperature of 20 ◦C. The plasma was operated with a 60 MHz capacitively coupled plasma system, at a constant power of 150 W, and at the operating pressure of 30 mTorr C4F8 [42]. To order to observe the changes of the Ag NWs morphologies, samples were pre- pared by coating Ag NW on 25 mm × 25 mm PEN using the spin coating method as Materials 2021, 14, 4448 3 of 12

mentioned above. The welding characteristics of the Ag NWs were observed using ﬁeld emission scanning electron microscopy (FE-SEM; S-4700 Hitachi). The changes of the Ag NWs morphology were observed using atomic force microscopy (AFM; INOVA, MA, USA). Optical transmittance was observed using optical microscopy and measured using ultraviolet–visible (UV–Vis) spectroscopy (UV-3600, Shimadzu, Kyoto, Japan). The haze values were measured by haze meter (BYK-Gardner haze-gard i). The sheet resistance was measured using a four-point probe (CMT-SR2000N, AiT). The mechanical integrity of the Ag NWs ﬁlm (50 mm × 20 mm PEN) was measured using a lab-made bending test system bent to a radius of curvature of 5 mm. The temperature of the THFs was measured as a function of input voltage and heating/cooling cycle times using a thermocouple that Materials 2021, 14, x FOR PEER REVIEW directly touches the surface of THFs, and a thermographic camera (FLIR). 3 of 12

FigureFigure 1. Schematics 1. Schematics of (a of) the (a) theprocesses processes forming forming Ag Ag NW NWss thin ﬁlmfilm heaters, heaters, and and (b )(b the) the principle principle of induction of induction heating heating on on the surfacethe surface of Ag of NW Ag NW networks. networks. (c) (Schematicc) Schematic of of the the basic basic induction heating heating system. system.

3. Results and Discussion To order to observe the changes of the Ag NWs morphologies, samples were preparedFigure by 1coatingb shows Ag that NW when on an 25 alternating mm × 25 electrical mm PEN current using is appliedthe spin across coating a conduc- method as tive coil, an alternating magnetic field with the same frequency is generated, and according mentioned above. The welding characteristics of the Ag NWs were observed using field to the Faraday effect, if a conductor is located within the alternating magnetic field, an emissionalternating scanning electric electron current (eddymicroscopy current) (FE-SEM; will be induced S-4700 with Hitachi). the same The frequency changes as of the the Ag NWsmagnetic morphology field. The were eddy observed currents heatusing the atomic material force according microscopy to the well-known(AFM; INOVA, Joule MA, USA).effect Optical [40,43]. transmittance In our experiment, was observed when eddy using current optical flows microscopy to the Ag and NW measured network, a using ultraviolet–visiblehot spot is generated (UV–Vis) at the junctionsspectroscopy due to(UV-3600, the high resistance Shimadzu, at theKyoto, junction, Japan). and The the haze valuesjunctions were are measured preferentially by haze melted meter without (BYK-Gar heatingdner the haze-gard remainder ofi). theThe Ag sheet NW resistance area. The was measuredheating rateusing of thea four-point Ag NW network probe is(CMT-SR2000N, dependent on the AiT). operating The mechanical frequency of integrity induction of the Agpower NWs because,film (50 accordingmm × 20 mm to Faraday’s PEN) was law meas of induction,ured using the induceda lab-made current bending is correlated test system with the operating frequency. The ability to handle internally applied and localized heating bent to a radius of curvature of 5 mm. The temperature of the THFs was measured as a with high heat rates prevents significant thermal stress, degradation, or deformation after functionthe welding of input [43– 46voltage]. and heating/cooling cycle times using a thermocouple that directly touches the surface of THFs, and a thermographic camera (FLIR).

3. Results and Discussion Figure 1b shows that when an alternating electrical current is applied across a conductive coil, an alternating magnetic field with the same frequency is generated, and according to the Faraday effect, if a conductor is located within the alternating magnetic field, an alternating electric current (eddy current) will be induced with the same frequency as the magnetic field. The eddy currents heat the material according to the well- known Joule effect [40,43]. In our experiment, when eddy current flows to the Ag NW network, a hot spot is generated at the junctions due to the high resistance at the junction, and the junctions are preferentially melted without heating the remainder of the Ag NW area. The heating rate of the Ag NW network is dependent on the operating frequency of induction power because, according to Faraday’s law of induction, the induced current is correlated with the operating frequency. The ability to handle internally applied and localized heating with high heat rates prevents significant thermal stress, degradation, or deformation after the welding [43–46].

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The operating frequency applied for radio frequency induction heating is usually higherThe operatingthan 20 kHz frequency and in appliedless than for 1 radioMHz, frequency depending induction on the heatingapplication. is usually And at higherfrequencies than 20 below kHz and100 kHz, in less the than skin 1 depth MHz, is depending large, which on makes the application. it inefficient And to apply at fre- to quenciesnanomaterials below 100[37,47]. kHz, Therefore, the skin depth three is freq large,uency which systems makes were it inefficient selected to within apply tothat nanomaterialsfrequency range [37, 47to]. investigate Therefore, the three effect frequency of induced systems power were and selected frequency within on the that change fre- quencyin sheet range resistance to investigate of the Ag the NW effect network. of induced The powerwelding and process frequency was onperformed the change for in60 s sheetwhile resistance changing of the power Ag NW from network. 1 to 6 ThekW. welding process was performed for 60 s while changingFigure the power2 shows from the 1results to 6 kW. for the initial sheet resistance of (a) 30, (b) 50, (c) 100, and (d)Figure 300 ohm/sq.2 shows The the resultsfigure also for the shows initial that sheet the resistance higher the of (a)initial 30, (b)resistance, 50, (c) 100, the and more (d)significant 300 ohm/sq. resistance The figurechange also that shows was observed that the during higher nanowire the initial welding resistance, using the induction more significantheating. resistanceFigure 2a changeshows that wasfor the observed Ag NW during network nanowire with welding30 ohm/sq, using no induction change in heating.resistance Figure was2 aobserved, shows that even for when the Ag 6 kW NW of network power was with applied 30 ohm/sq, at 100 no kHz, change while in the resistanceresistance was changes observed, of ~11.6 even and when ~12.2% 6 kW were of powerobserved was for applied 450 and at 800 100 kHz, kHz, respectively, while the resistanceat 6 kW. changes Figure of2b–d ~11.6 show and ~12.2%that as werethe init observedial resistance for 450 increased and 800 kHz, to 50, respectively, 100, and at300 6 kW.ohm/sq, Figure respectively,2b–d show thatthe sheet as the resistance initial resistance reduction increased percentage to 50, (Δ 100,R/R and0) at 300a given ohm/sq, power respectively,was further theincreased sheet resistanceby showing reduction 8.1% at 450 percentage kHz and 9.6% (∆R/R at 0800) at kHz a given for 1 powerkW of power, was furtherand 40% increased and 43.7% by showing at 450 and 8.1% 800 at 450kHz, kHz respectively, and 9.6% at for 800 6 kHzkW power. for 1 kW In of addition, power, and even 40%though and 43.7% the reduction at 450 and percentage 800 kHz, respectively, is small, the for reduction 6 kW power. of Insheet addition, resistance even thoughwas also theobserved reduction even percentage with 100 is kHz small, for the 100 reduction and 300 ohm/sq of sheet of resistance Ag NW wassheet also resistance. observed even with 100 kHz for 100 and 300 ohm/sq of Ag NW sheet resistance.

0.30 0.30 (a) 450 kHz (b) 450 kHz 0.25 800 kHz 0.25 800 kHz

0.20 R0 = 30 ohm/sq 0.20 50 ohm/sq 0 0.15 0 0.15

R/R R/R Δ 0.10 Δ 0.10

0.05 0.05

0.00 0.00 123456 123456

Power (kW) Power (kW)

0.2 0.2 R/R R/R Δ Δ

0.1 0.1

0.0 0.0 123456 123456 Power (kW) Power (kW)

FigureFigure 2. Change2. Change of of Ag Ag NWs NWs sheet sheet resistance resistance by by the the welding welding with with different different frequencies frequencies of of 100, 100, 450, 450, and and 800 800 kHz kHz and and with with 1–61–6 kW kW power power for 60for s 60 for s the for Ag the NW Ag networks NW networks with the with initial the sheetinitial resistances sheet resistances of (a) 30, of (b )(a 50,) 30, (c) ( 100,b) 50, and (c ()d 100,) 300 and ohm/sq. (d) 300 ohm/sq. Figure2c,d show that the differences in sheet resistance change of Ag NW networks betweenFigure 450 and 2c,d 800 show kHz that are the not differences significant, in even sheet though resistance they change are significantly of Ag NW different networks frombetween those 450 welded and 800 at 100kHz kHz. are not Therefore, significant, to ev confirmen though the differencethey are significantly between the different two frequencies,from those the welded induction at 100 heating kHz. for Therefore, 30 s with theto powerconfirm of the 1–6 difference kW using 450between and 800 the kHz two wasfrequencies, also investigated, the induction and Figure heating3a,b for show 30 thes with respective the power results. of 1–6 As kW shown using in 450Figure and3a 800, evenkHz when was 6also kW wasinvestigated, applied at and the frequencyFigure 3a,b of 450show kHz, the there respective was no changeresults. ofAs resistance shown in inFigure the 30 3 ohm/sq (a), even Ag when NW network.6 kW was However, applied at the the resistance frequency change of 450 could kHz, be there observed was no change of resistance in the 30 ohm/sq Ag NW network. However, the resistance change

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could be observed for the Ag NW network with the initial resistance of 50 ohm/sq or forhigher. the Ag When NW network6 kW power with was the applied initial resistance for 30 s, the of resistance 50 ohm/sq change or higher. was ~5% When for 6 the kW 50 powerohm/sq was Ag applied NW network, for 30 s, ~20.8% the resistance for the 100 change ohm/sq was Ag ~5% NW for network, the 50 ohm/sqand ~21.5% Ag for NW the network,300 ohm/sq ~20.8% Ag NW for the network. 100 ohm/sq In contrast, Ag NW Figure network, 3b shows and that ~21.5% at 800 for kHz the 300frequency ohm/sq and Ag6 NWkW, the network. resistance In contrast, change ofFigure ~4.4%3b wasshows observ thated at for 800 the kHz initial frequency resistance and of 6 30 kW, ohm/sq the resistanceAg NW networks, change of and ~4.4% the was sheet observed resistance for change the initial of ~23.8 resistance and ~24.1% of 30 ohm/sqfor the 100 Ag and NW 300 networks,ohm/sq, andrespectively, the sheet resistancecould be change observed. of ~23.8 Figure and ~24.1%3c compares for the 100the andchanges 300 ohm/sq, in sheet respectively,resistance of could 100 ohm/sq be observed. Ag NW Figure networks3c compares as a function the changes of welding in sheet time resistancefrom 10 to of 60 s 100at ohm/sq6 kW for Agdifferent NW networks frequencies. as a Although function ofafter welding the welding, time from a change 10 to in 60 sheet s at 6 resistance kW for differentis seen frequencies.at all frequencies, Although the after effect the is welding, negligible a change at 100 inkHz, sheet and resistance the rate is of seen resistance at all frequencies,change increased the effect with is negligibleincreasing at frequency. 100 kHz, and the rate of resistance change increased with increasing frequency.

(a) 0.25 (b) 0.30 50 ohm/sq 30 ohm/sq 0.25 0.20 100 ohm/sq 50 ohm/sq 300 ohm/sq 100 ohm/sq 0.20 300 ohm/sq 0.15 0 0 0.15

R/R R/R Δ 0.10 Δ 0.10

0.05 450 kHz 0.05 800 kHz

0.00 0.00 123456 123456 Power (kW) Power (kW)

30.5 30 27.5 25.8 23.8 20.8 20

10.411.5 10 3.2 1.61.7 1.4 Change of sheet resistance (%) 0 10 20 30 40 50 60 Times (sec)

Figure 3. a b Figure Sheet3. Sheet resistance resistance change change according according to the to inductivethe induct heatingive heating of the of Ag the NW Agnetwork NW network for 30 for s at 30 ( ) s 450 at ( anda) 450 ( )and 800 ( kHz.b) 800 (c)kHz. Comparison (c) Comparison of sheet of resistance sheet resistance change change over induction over induction heating heating time with time 6 with kW power6 kW power applied applied to each to frequency each frequency for 100for ohm/sq 100 ohm/sq of initial of initial resistance resistan Agce NW-coated Ag NW-coated thin ﬁlms. thin films.

FigureFigure4 shows4 shows the the results results of theof the morphologies morphologies of theof the Ag Ag NW NW networks networks on on PEN PEN substratessubstrates before before and and after after the the welding welding for for 60 60 s ats at 6 kW6 kW at at different different frequencies frequencies that that were were observedobserved by by SEM SEM for for the the Ag Ag NW NW network network of of 100 100 ohm/sq. ohm/sq. Figure Figure4a 4a shows shows the the Ag Ag NW NW networksnetworks after after the the coating coating on on the the PEN PEN substrate substrate without without welding, welding, (b) (b) shows shows the the Ag Ag NW NW networks after welding at 100 kHz, (c) shows after welding at 450 kHz, and (d) shows after networks after welding at 100 kHz, (c) shows after welding at 450 kHz, and (d) shows welding at 800 kHz. Figure4a shows that after the coating without welding, the Ag NW after welding at 800 kHz. Figure 4a shows that after the coating without welding, the Ag junctions were not welded, and were separated from each other. NW junctions were not welded, and were separated from each other. However, Figure4b–d show that with increasing the operating induction frequency, However, Figure 4b–d show that with increasing the operating induction frequency, increased welding of the Ag NW junction parts could be observed, which after the welding increased welding of the Ag NW junction parts could be observed, which after the process resulted in the decrease of sheet resistance. welding process resulted in the decrease of sheet resistance.

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In particular, Figure 4d shows that for the welding at 800 kHz, the Ag NW junction showed complete welding, without changing the morphology of the nanowire. However, Figure 4d inset image shows that even though most of the Ag NW junctions were welded completely at 800 kHz, some of the Ag NW junctions having stacked multiple junctions appeared to not be completely welded. This indicates possible difficulty in welding the Materials 2021, 14, 4448 stacked Ag NW junctions; therefore, difficulty in reducing the sheet resistance6 ofby 12 induction welding for low sheet resistance Ag NW networks containing more stacked Ag NW junctions, such as 30 ohm/sq, was observed in Figures 2 and 3.

Figure 4. SEM image of the 100 ohm/sq Ag NW networks after inductive welding. (a) After the coating on the PEN Figure 4. SEM image of the 100 ohm/sq Ag NW networks after inductive welding. (a) After the coating on the PEN substrate without welding; (b) after welding at 100 kHz; (c) after welding at 450 kHz; and (d) after welding at 800 kHz at substrate without welding; (b) after welding at 100 kHz; (c) after welding at 450 kHz; and (d) after welding at 800 kHz at 6 kW for 60 s. All of inset images are the morphology of nanowire junctions (scale bar: 30 nm). 6 kW for 60 s. All of inset images are the morphology of nanowire junctions (scale bar: 30 nm).

TheIn particular, surface roughness Figure4d of shows a single that Ag for NW the junction welding before at 800 and kHz, after the the Ag welding NW junction at the 800showed kHz completeinductive welding, heating withoutcondition changing shown in the Figure morphology 4d was of investigated the nanowire. using However, AFM. FigureFigure 45a,bd inset show image the showsresults thatfor the even Ag though NWs mostbefore of and the after Ag NW the junctionswelding, wererespectively, welded andcompletely in Figure at S1 800 for kHz, line some scan ofdata the of Ag the NW single junctions Ag NW having and the stacked Ag NW multiple junction junctions before andappeared after the to notwelding be completely (detailed height welded. changes This indicates for single possible Ag NW difficultyand before/after in welding welding the Agstacked NW Agcan NW be junctions;found in therefore,Figure S1, difficulty Supporting in reducing Information). the sheet Before resistance the welding, by induction the heightwelding of forthe Ag low NW sheet junction resistance was ~47.9 Ag NW nm, networkswhile the height containing of the more single stacked Ag NW Ag was NW 25 ±junctions, 3 nm. such as 30 ohm/sq, was observed in Figures2 and3. Therefore,The surface before roughness the welding, of a single a single Ag NW Ag junction NW was before physically and after resting the weldingon top of at anotherthe 800 kHzsingle inductive Ag NW heatingat the junction. condition After shown the welding, in Figure the4d washeight investigated of the Ag NW using junction AFM. wasFigure decreased5a,b show to the ~26.7 results nm, forindicating the Ag NWs the almo beforest complete and after thefusion welding, of the respectively, two Ag NWs and at thein Figure junction, S1 foras shown line scan in Figure data of 4d. the In single addition, Ag NW after and the thewelding, Ag NW the junction surface beforeroughness and ofafter the the Ag weldingNWs network (detailed was height significantly changes decre for singleased, Agdue NW to the and fusion before/after of the two welding Ag NWs Ag atNW the can junction. be found in Figure S1, Supporting Information). Before the welding, the height of the AgVarious NW junction physical was properties ~47.9 nm, of the while Ag theNW height networks of the coated single on Ag PEN NW after was the 25 welding± 3 nm. were Therefore,investigated. before Figure the welding,6a shows a the single optical Ag NWtransmittance was physically before resting and after on top the of welding another ofsingle Ag NW Ag NWnetworks at the on junction. PEN using After different the welding, induction the height frequencies of the Agat 6 NW kW junctionfor 60 s. wasThe Agdecreased NW network to ~26.7 with nm, the indicating sheet resistance the almost of 100 complete ohm/sq fusionbefore ofwelding the two was Ag used. NWs Figure at the junction, as shown in Figure4d. In addition, after the welding, the surface roughness of the Ag NWs network was significantly decreased, due to the fusion of the two Ag NWs at the junction. Various physical properties of the Ag NW networks coated on PEN after the welding were investigated. Figure6a shows the optical transmittance before and after the welding of Ag NW networks on PEN using different induction frequencies at 6 kW for 60 s. The Ag NW network with the sheet resistance of 100 ohm/sq before welding was used. Figure6a shows Materials 2021, 14, x FOR PEER REVIEW 7 of 12 Materials 2021, 14, 4448 7 of 12 Materials 2021, 14, x FOR PEER REVIEW 7 of 12 6a shows that before welding, the optical transmittance of the Ag NW network with 100 6a ohm/sqshows that was before 96.1% welding,at 550 nm the wavelength. optical transmittance of the Ag NW network with 100 ohm/sqthat beforewas 96.1% welding, at 550 the nm optical wavelength. transmittance of the Ag NW network with 100 ohm/sq was 96.1% at 550 nm wavelength.

Figure 5. Surface roughness of a single Ag NW junction measured by 2-dimensional AFM. (a) before and (b) after welding Figureat theFigure 5. 800Surface 5.kHzSurface roughness frequency roughness ofwith a ofsingle the a single condition Ag Ag NW NW junc shown junctiontion in measured measuredFigure 4d. by 2-dimensional AFM. AFM. (a )(a before) before and and (b) after(b) after welding welding at the at800 the kHz 800 frequency kHz frequency with with the thecondition condition shown shown in in Figure Figure 44d.d.

FigureFigure 6. 6.(a)( aOptical) Optical transmittance, transmittance, ((bb)) haze,haze, and and (c ,(dc,)d resistance) resistance change change with with bending bending cycles cy ofcles the of Ag the NW Ag networks NW networks on on PEN that were not, and were welded at 100, 450, and 800 kHz frequency at 6 kW for 60 s. FigurePEN 6. that (a) wereOptical not, transmittance, and were welded (b) haze, at 100 and, 450, (c, dand) resistance 800 kHz changefrequency with at bending6 kW for cy 60cles s. of the Ag NW networks on PEN that were not, and were weldedAfter at 100 welding,, 450, and the 800 optical kHz frequency transmittances at 6 kW of the forAg 60 s. NW networks welded at 100, 450, and 800After kHz welding, were 95.5%, the optical 95.0%, transmittances and 95.1%, respectively, of the Ag indicating NW networks that after welded welding, at 100, 450, andthereAfter 800 were welding, kHz no were noticeable the 95.5%, optical changes 95.0%, transmittances in and optical 95.1%, transmittance of therespectively, Ag NW for networks all indicating frequencies. welded that Figure after at 100,6 bwelding, 450, andthereshows 800 werekHz the haze wereno noticeable percentage 95.5%, 95.0%, ofchanges the and Ag NWin 95.1%, optical networks respectively, transmittance in Figure 6indicatinga. Beforefor all thefrequencies. that welding, after welding, the Figure 6b thereshows were the no haze noticeable percentage changes of the in Agoptical NW transmittancenetworks in Figure for all 6a. frequencies. Before the welding,Figure 6b the showshaze the percentage haze percentage was 10.09%, of the and, Ag NWafter netw the orkswelding in Figure at 100, 6a. 450, Before and the800 welding, kHz, the the haze hazepercentage percentage was was continuously 10.09%, and, decreased after the towelding 9.04%, at7.76%, 100, 450,and and6.81%, 800 respectively, kHz, the haze with percentageincreasing was the continuously induction frequency. decreased This to 9.04%,decrease 7.76%, in haze and is 6.81%, due to respectively, the decreased with light increasingscattering the and induction lower angle frequency. scattering This by decrease the increased in haze fusion is due of Agto the NW decreased junctions light[29]. scattering and lower angle scattering by the increased fusion of Ag NW junctions [29].

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haze percentage was 10.09%, and, after the welding at 100, 450, and 800 kHz, the haze percentage was continuously decreased to 9.04%, 7.76%, and 6.81%, respectively, with increasing the induction frequency. This decrease in haze is due to the decreased light scattering and lower angle scattering by the increased fusion of Ag NW junctions [29]. Mechanical strength was also measured by cyclic bending test of the Ag NW networks coated on PEN before and after welding at different induction frequencies with the conditions in Figure6a, and Figure6c,d shows the changes in sheet resistance measured as a function of the bending cycle. For the bending test, the Ag NW network with initial resistance of about 100 ohm/sq was spin-coated on a 50 × 20 mm PEN substrate. The bending test was performed by repeating tensile/compressive cycles up to 10,000 cycles with a radius of 5 mm using a bending machine. Figure6c shows that the Ag NW network coated on PEN without welding showed rapid increase of sheet resistance, and before the cyclic bending cycle reached 10,000 cycles, the Ag NW network was broken by showing inﬁnite resistivity. In contrast, after welding, the Ag NW networks appeared to show negligible change in resistance with increasing bending cycles, regardless of the induction frequency. However, Figure6d shows that, when the change of resistance was more accurately observed, the higher induction frequency showed lower change in sheet resistance by showing the resistance change of 1.62%, 0.04%, and 0.02% for 100, 450, and 800 kHz condition, respectively, after 10,000 cycles. The increase in sheet resistance can be attributed to broken junctions in the random Ag NW network. The more changes in sheet resistance under a tensile stress state than a compressive stress state indicates that the random Ag NW network is more vulnerable to the failure under a tensile stress. It is believed that non-linear behavior of resistance change observed at 100 kHz is related to more signiﬁcant resistance changes due to less welded junctions. It is also believed that the smaller change of sheet resistance for the higher induction frequency is related to the increasing number of fully welded Ag NW junctions. Using the Ag NWs coated PEN, THFs were fabricated, and their heater characteristics were investigated for the Ag NW networks before and after the welding at 100 and 800 kHz at 6 kW for 60 s, and Figure7 shows the results. The sheet resistivity of the Ag NW network before the welding was ~100 ohm/sq, while after the welding at 100 and 800 kHz, the resistivities were decreased to ~90.6 and ~56.1 ohm/sq, respectively, without changing the optical transmittance. The temperature of the THFs was measured using a thermocouple attached to the surface of the THFs. Figure7a shows the saturated temperature of the Ag NW-based THFs measured as a function of applied voltage. Figure7a shows that the temperature for the non-welded Ag NW-based THF was ~36 ◦C at 6 V, and increased to 53 ◦C at 13 V. In the case of Ag NW-based THF welded at 100 and 800 kHz, the temperature was increased to ~65 and ~79 ◦C at 13 V, respectively. Therefore, the increase of applied voltage increased the saturated temperature of all THFs; however, at the same voltage, the highest temperature was observed for the Ag NW network welded at 800 kHz, and the lowest temperature for the Ag NW network before welding. Figure7b shows the temperature response over time for the Ag NW-based THFs at 6 V DC bias voltage with the Ag NW networks before and after the welding at 100 and 800 kHz. For non-welded Ag NW-based THF, it reached the saturation temperature of ~36 ◦C at ~200 s, while the THFs welded at 100 and 800 kHz reached the saturation temperature of ~41 and ~48 ◦C, respectively, at ~100 s. In addition, the THFs with the welded Ag NW networks showed more stable temperature increase rate, compared to that with the non-welded Ag NW network. Figure7c shows the result of the cyclic switching test using the Ag NW-based THFs welded at 800 kHz. The test was conducted at the operating voltage of 6 V. Cyclic heating and cooling tests were performed for 10 cycles (1 cycle = 250 s on-time/250 s off-time). If Ag NWs are exposed to atmospheric environment at high temperature for a long time, due to the inherent properties of nanowires, the Ag NWs can be easily damaged by the external environment, such as by oxidation. Therefore, before the cyclic switching test that requires a lengthy exposure to air at high temperature, the Ag NW-based THF surfaces Materials 2021, 14, x FOR PEER REVIEW 8 of 12

Mechanical strength was also measured by cyclic bending test of the Ag NW networks coated on PEN before and after welding at different induction frequencies with the conditions in Figure 6a, and Figure 6c,d shows the changes in sheet resistance measured as a function of the bending cycle. For the bending test, the Ag NW network with initial resistance of about 100 ohm/sq was spin-coated on a 50 × 20 mm PEN substrate. The bending test was performed by repeating tensile/compressive cycles up to 10,000 cycles with a radius of 5 mm using a bending machine. Figure 6c shows that the Ag NW network coated on PEN without welding showed rapid increase of sheet resistance, and before the cyclic bending cycle reached 10,000 cycles, the Ag NW network was broken by showing infinite resistivity. In contrast, after welding, the Ag NW networks appeared to show negligible change in resistance with increasing bending cycles, regardless of the induction frequency. However, Figure 6d shows that, when the change of resistance was Materials 2021, 14, 4448 more accurately observed, the higher induction frequency showed lower change9 of in 12 sheet resistance by showing the resistance change of 1.62%, 0.04%, and 0.02% for 100, 450, and 800 kHz condition, respectively, after 10,000 cycles. The increase in sheet resistance can be

attributedwere exposed to broken to a C junctions4F8 plasma in to the form random a fluorocarbon Ag NW passivation network. layerThe more with the changes condition in sheet resistancein the experimental under a tensile section, stress and the state Ag NW-basedthan a compressive THFs with andstress without state the indicates passivation that the randomlayer wereAg NW also compared.network is Figure more7 cvulnerable shows that to when the thefailure Ag NW-based under a tensile THF was stress. not It is passivated, the THFs were gradually degraded by oxidation during the cyclic switching believed that non-linear behavior of resistance change observed at 100 kHz is related◦ to moretest; significant however, when resistance the passivated changes Ag due NW-based to less welded THF was junctions. repeatedly It heatedis also to believed ~53 C, that and cooled to near room temperature, it showed good response time, operational stability, the smaller change of sheet resistance for the higher induction frequency is related to the and repeatability (the passivated Ag NW-based THF showed slightly higher saturation increasingtemperature number at the of same fully operating welded Ag voltage NW thanjunctions. that without the passivation layer, but theUsing optical the transmission Ag NWs coated and thePEN, temperature THFs were behavior fabricated, were and similar their toheater those characteristics without werepassivation). investigated Therefore, for the for Ag the NW passivated networks Ag before NW-based and THF,after stable the welding cyclic temperature at 100 and 800 kHzbehavior at 6 kW was for observed 60 s, and during Figure the 7 switchingshows the test. results. Figure The7d showssheet resistivity the optical of image the Ag of NW networkthe passivated before the Ag welding NW-based was THF ~100 fabricated ohm/sq, on while PEN and after the the images welding of the at heat 100 radiationand 800 kHz, themeasured resistivities with were an infrared decreased (IR) to camera. ~90.6 Theand IR~56.1 camera ohm/sq, was used respectively, to visually without confirm changing that thethe optical welded transmittance. Ag NW-based THFs The not temperat only showedure higherof the temperature THFs was rise comparedmeasured to using the a unwelded Ag NW-based THFs, but also showed more uniform temperature rises over the thermocouple attached to the surface of the THFs. Figure 7a shows the saturated entire area of the heat film, even when bent and twisted (a more detailed figure is shown in temperatureFigure S2 of of the the supporting Ag NW-based information). THFs measured as a function of applied voltage.

Figure 7. Ag NW THFs before and after welding at 100 and 800 kHz at 6 kW for 60 s. (a) Temperature of the THFs as a function of applied voltage. (b) Time-dependent temperature proﬁle of the THFs at 6 V of applied voltage. (c) Temperature response of the THFs (welded at 800 kHz at 6 kW for 60 s) over 10 switching cycles with the applied voltage of 6 V (one

cycle = 250 s on-time/250 s off-time) with, and without, a passivation layer formed by a C4F8 plasma. (d) Thermal imagery of the Ag NW THFs welded at 800 kHz with the condition shown in (a). For (c,d), to protect the Ag NW network from the environment during the long-time exposure to atmosphere, the THFs were coated with a ﬂuorocarbon polymer layer

formed by a C4F8 plasma. Materials 2021, 14, 4448 10 of 12

4. Conclusions In this work, Ag NW networks on PEN with different sheet resistances were welded by induction heating with different induction frequencies and power, and the optical, electrical, and mechanical properties of the Ag NW coated PEN before and after the welding was investigated. The eddy current induced by induction heating welds only the Ag NW junction area and remotely from the junction area; therefore, it was an efficient Ag NW welding method showing no change in optical transmittance after the welding. The results showed that the higher the induction frequency and higher the induction power, the more significant the welding effect that could be observed for the Ag NW network with the same resistivity. For the same induction frequency and power, the Ag NW network with higher sheet resistance showed higher welding effect, by showing more significant change in resistivity after welding. The induction welding of the Ag NW networks at higher frequency not only decreased the sheet resistance, but also decreased the haze by decreasing the total height of the Ag NWs during the joining of the junction area, without changing the optical transmittance. The Ag NW network on PEN welded at higher induction frequency also showed better stability during the cyclic bending test, and THF fabricated with welded Ag NW networks by induction heating at the higher frequency showed higher temperature rise at a given applied voltage. It is believed that the inductive welding of nanowire films would be widely applicable to THF as well as the various flexible electronics such as solar cells, touch panels, and biosensors.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/ma14164448/s1, Figure S1: Line can data of the single Ag NW and the Ag NW junction before and after the welding. Figure S2: IR images of Ag NW THFs while bending and twisting at 6 V. (induction heating welded at 800 kHz condition). To protect Ag NW network from the environment, the THFs were coated with a fluorocarbon polymer layer. Author Contributions: Conceptualization, J.O. and L.W.; methodology, J.O., L.W., H.T. and H.K.; formal analysis, J.O. and L.W.; investigation, J.O., L.W., G.K., J.H. and W.C.; writing—original draft preparation, J.O.; writing—review and editing, L.W., D.K. and G.Y.; supervision, G.Y.; project admin- istration, D.K. and G.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by Ministry of Trade, Industry and Energy (MOTIE, Korea) (20011977). Conflicts of Interest: The authors declare that there is no conflict of interest.

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