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Designing A Flexible and Transparent Ultrarapid Electrothermogenic Film Based on Thermal Loss Suppression Effect: A Self-Fused Cu/ Ni Composite Junctionless Nanonetwork for Effective De-Icing Heater Ryohei Yoshikawa, Mizuki Tenjimbayashi, Takeshi Matsubayashi, Kengo Manabe, Luca Magagnin, Yasuaki Monnai, and Seimei Shiratori ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00268 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Applied Nano Materials

Designing A Flexible and Transparent Ultrarapid Electrothermogenic Film Based on Thermal Loss Suppression Effect: A Self-Fused Cu/Ni Composite Junctionless Nanonetwork for Effective De-Icing Heater Ryohei Yoshikawa,† Mizuki Tenjimbayashi,† Takeshi Matsubayashi,† Kengo Manabe,† Luca Magagnin,‡ Yasuaki Monnai,§ and Seimei Shiratori*,†

†

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ‡

Dipartimento di Chimica, Materiali e Ingegneria Chimica Giulio Natta, Politecnico di Milano,

Via Mancinelli 7, 20133, Milano, Italy §

Center for Applied Physics and Physico-Informatics, School of Fundamental Science and

Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan *[email protected]

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ABSTRACT Transparent heaters (THs) are one-size-fits-all materials used in electronics such as smart windows, wearable applications, and de-icing devices. Copper-based THs, which are the most promising materials, still face some problems such as increasing electrical resistance at the intersections of each nanowire and easy degradation owing to oxidation. To overcome these problems, the formation of a junctionless network whose nanowire intersections are fused by using a composite of copper and the oxidation-resistant material is considered as one of the best strategies. Herein, we report a junctionless copper/nickel-nanonetwork-based THs formed on a polymer nanofiber by combining electrospinning and electroless deposition. This two-step wet process enables to form a junctionless network composed of a copper/nickel alloy. The THs showed outstanding heating characteristics (the power efficiency reached 421.7 °C cm2/W) which are suitable for the de-icing application. Furthermore, we revealed that prominent heating characteristics are realized owing to decreasing thermal loss at intersections during applying current, which we term “thermal loss suppression effect”. Simulating thermal losses at intersection models of a junctionless network and a junction network based on finite element method, we estimated the thermal loss originated from the network geometry. This insight may contribute to the design of high-performance electrothermal materials.

KEYWORDS transparent conductive film, de-icing, thermal loss, junctionless, electrospinning, electroless deposition


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INTRODUCTION Transparent conductive films (TCFs) are necessary materials for recent electronic devices such as flexible solar cells, wearable touch displays, electronic skin, nanogenerators, smart windows, and de-icing heaters.1–5 Indium tin oxide (ITO), which possesses high transparency and low sheet resistance, is the most famous TCF material. However, ITO faces several problems such as mechanical brittleness and cost volatility caused by the depletion of Indium. In addition, it has a high specific heat, which results in a poor thermal response.1,6 Other metal oxide films such as fluorine-doped tin oxide and zinc oxide also suffer from same problems. Because of this, carbon-based materials (carbon nanofiber, graphene), conductive polymers, metal nanonetworks, and a hybrid of organic materials have been studied over the last few decades.1,2,7–10 In particular, metal nanonetworks potentially have a large number of free electrons and can exhibit comparable conductivity and transparency to ITO; therefore, high-performance TCFs based on noble metals (Au, Ag) have been reported.11–13 Recently, Copper (Cu), which has the second highest conductivity after Silver (Ag), has also been utilized for TCFs to reduce material cost.14–17 To compensate for the lower performance compared with that of Ag-based TCFs, many Cu-based TCFs that form a junctionless network have been reported.18–20 A significant reduction in junction resistance can be achieved by fusing intersections between each wire and mesh.21 Moreover, the formation of a junctionless network contributes to not only improving the conductivity but also improving the mechanical durability.22 Generally, a junctionless network is formed by post-processing such as thermal pressing, chemical etching, laser welding, and plasmonic welding techniques18–20,22,23 or using a randomly cracked template, which is removed after fabrication.24 However, such additional steps require precise control to avoid damage to the film and lead to complication of the process and 3 ACS Paragon Plus Environment


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increase in manufacturing cost. To overcome these problems, self-fused Cu networks via facile wet process are reported.25,26 Hsu et al. introduced junctionless metal-nanowire (NW)-based TCFs, which were prepared through electroless deposition on the surface of a polymer nanofiber template.25 Our group also reported a junctionless CuNW using selective electroless metallization, which is a combination of electrospinning and electroless deposition.27 Moreover, Jo et al. characterized the heating characteristics and demonstrated the rapid de-icing properties of such junctionless Cu-based transparent heaters (THs).28 These bottom-up wet process fabrication techniques can be used to easily obtain heaters with excellent performance. Although there has been a significant improvement in the heating performance, and the development of the fabrication process has progressed, applying Cu-based TCFs to heater materials is still a challenge owing to the fast degradation caused by their low oxidation resistance.17 To use the Cu-based THs as an alternative to commercialized ITO- or Ag-based THs, long-term stability is an important factor. Kim et al. reported a cupronickel micromesh, composed of Cu and a small amount of Nickel (Ni) via electron-beam evaporation, which exhibited high heating performance without degradation.29 Such a metal composite networks overcoming the drawbacks of pure Cu attracts attention in scientific and industrial fields. The TH based on a junctionless network composed of a Cu/Ni alloy via wet process can meet requirements for practical use (i.e. high heating performance, facile production, and long-term stability), however, there has not been reported to the best of our knowledge. Herein, we designed a junctionless Cu/Ni nanonetwork as a candidate for high-performance THs through a facile and reproducible method. By utilizing the co-deposition of Cu/Ni alloys in a plating bath, in which the Cu and Ni ion sources are incorporated with sodium phosphinate as a reducing agent, a Cu/Ni alloy nanonetwork can be realized.30 Interestingly, the film doesn’t 4 ACS Paragon Plus Environment

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possess notable conductivity due to containing a low conductive metal (Ni), however, the TH showed a rapid heating and prominent power efficiency. To clarify the advantages of a junctionless network compared with a junction network, we analyzed the heating characteristics of each network model based on the finite element method. The results revealed that the efficient heating of a junctionless network is realized by “thermal loss suppression effect”, which is namely, the thermal loss at the intersections was suppressed. The obtained high-performance Cu/Ni-nanonetwork-based THs based on “thermal loss suppression effect” of a junctionless network should promote the development of designing functional films combined with the physical insight. In terms of de-icing film, the rapid de-frosting performance as well as the practical durability of the materials will contribute to advances in industrial use.

MATERIALS AND METHODS Poly(vinyl butyral) (PVB, Mw=40,000-70,000, Sigma-Aldrich, St. Louis, MO), tin chloride (SnCl2, Sigma-Aldrich, St. Louis, MO), 1-buthanol (Wako Pure Chemical Industries Ltd., Osaka, Japan), and poly(ethylene terephthalate) (PET) substrates with a thicknesses of 188 µm (Toyobo Co., Ltd., Osaka, Japan) were used to fabricate the nanofiber template. Palladium chloride (PdCl2, Sigma-Aldrich, St. Louis, MO) and hydrochloric acid (HCl, conc.=37%, Wako Pure Chemical Industries Ltd., Osaka, Japan) were used in the activation process. Deionized water (DIW) purified by an automatic water distillation apparatus (Advantec, Tokyo, Japan) was utilized in all experiments.

Cu(II)

sulfate

pentahydrate

(CuSO4·5H2O),

nickel

(II)

sulfate

hexahydrate(NiSO4·6H2O), sodium phosphinate monohydrate (NaPH2O2·H2O), sodium citrate 5 ACS Paragon Plus Environment


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(Na3C6H5O7), and sodium acetate (CH3COONa) were purchased from Wako Pure Chemical Industries Ltd., Osaka, Japan and used for the electroless deposition. Sodium hydroxide (NaOH, 5N, Junsei Chemical Co., Ltd., Tokyo, Japan) was used as pH adjuster. ITO/polyethylene naphthalate (PEN) with a thickness of 125 mm (12±2 Ω/sq, Peccell Technologies, Inc., Kanagawa, Japan) was used in bending tests for comparison. Fabrication of the Cu/Ni nanofiber network film Fabrication of the nanofiber template. The nanofiber template was prepared by an electrospinning method. 8 wt.% PVB and 9 wt.% SnCl2 were dissolved in 1-buthanol by stirring for 12 h. The solution was loaded into a syringe (Volume: ~1 mL; Terumo Co., Tokyo, Japan) with a needle (φ=0.80 mm, Terumo Co., Tokyo, Japan) and then deposited on the PET film at a flow rate at 0.1 mL/h using a syringe pump. To deposit the nanofibers on the PET film directly, the PET film was attached to a Cu plate, which worked as a collector. The applied voltage and distance between the needle tip and the collector were set at 8 kV and 15 cm, respectively. Activation process. After forming the nanofiber template, the PET film with nanofibers was immediately immersed into an activation solution for 10 min, which was prepared by mixing PdCl2 (1.0 g/L), 2.0 mM HCl, and DIW. Then, the film was rinsed in DIW 5 times to remove any residue on the PET substrate. Electroless deposition. A plating bath was prepared by mixing the ion sources, a reducing agent, and the stabilizing materials. CuSO4·5H2O (0.016 M) and NiSO4·6H2O (0.038 M) were the ion sources for electroless deposition. NaPH2O2·H2O (0.094 M) had the role of reducing Cu2+ and Ni2+ on the surface of nanofibers. Na3C6H5O7 (0.090 M) and CH3COONa (0.12 M) had the function as a complexing agent and buffer agent, respectively. All of them were dissolved in 6 ACS Paragon Plus Environment

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DIW by stirring for 12 h. Then, the pH was adjusted to 10 by adding NaOH. The rinsed templates were immersed in the plating bath at 70°C with stirring at 150 rpm for different plating times (15 s, 30 s, 45 s, and 60 s). Characterization The geometry of the film was characterized by field emission scanning electron microscopy (FE-SEM, Hitachi, Tokyo, Japan) with an accelerating voltage of 10 kV. The fiber diameter distributions were measured from five SEM images by using the Image J software (U. S. National Institute of Health, Maryland, USA) for each sample with different plating times. The elemental mapping of the film was obtained by using energy-dispersive X-ray spectrometry (EDX, QUANTAX 70, Bruker Nano GmbH, Berlin, Germany). The sheet resistance was measured by a standard four-probe method using a Loresta GP resistivity meter (MCP-T610, Mitsubishi Chemical Analytech, Co., Ltd., Kanagawa, Japan). For the optical property measurements, a UV-vis spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto, Japan) and a haze meter (NDH-5000, Nippon Denshoku Industries, Tokyo, Japan) with a white light-emitting diode as a light source were used. The PET film was used as a reference in these optical property measurements. A commercialized adhesive tape (Scotch tape, 3M, Tokyo, Japan) was used for the tape peeling test. The durability of the film was determined by an abrasion test with an abrasion device (Tribogear Type 18 L, Shinto Scientific Co., Ltd., Tokyo, Japan), on which a gauze (NS gauze, NISHIO EIZAI Co., Ltd., Aichi, Japan) was attached as an abrasive. The heating characteristics of the Cu/Ni nanonetwork film (dimensions: 12×25 mm), whose ends on the long sides were coated with an Ag paste (DOTITE D-500, Fujikura kasei Co., Ltd., Tokyo, Japan) for electrical contact, were investigated by applying a direct current (DC) voltage 7 ACS Paragon Plus Environment


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from a power supply. The surface temperature was measured by using an infrared camera (PI1400, Optris GmbH, Berlin, Germany). The thermal loss was simulated using a finite elemental method provided by commercial software (CST STUDIO SUITE, Dassault Systemes France). All objects were composed of Cu cylinders (outer diameter: 400 nm, inner diameter: 100 nm, length: 10 μm, conductivity:

5.8 × 10 S/m, heat capacity: 0.385 kJ/K/Kg, thermal conductivity: 401 W/m/K) and PVB

cylinders (diameter: 100 nm, length: 10 μm, conductivity: N/A, heat capacity: 1.5 kJ/K/Kg,

thermal conductivity: 0.28 W/m/K). These objects were surrounded with ambient air. The thermal loss was simulated by sequentially running an electromagnetic solver and a thermal solver. The heat source was modeled by the steady state power loss calculated for the current distribution obtained with a stationary input current (6 mA) passing through the terminal’s edges. To investigate the de-icing performance, the heater was attached on a peltier cooler and it was cooled at −20°C for 30 min to accumulate ice on the heater. Then, the DC voltage was applied to the heater. The de-frosting ratio was calculated from the images binalized by Image J.

RESULTS AND DISCUSSION The fabrication procedure of the Cu/Ni nanonetwork is described in Scheme 1. First, PVB/SnCl2 nanofiber was deposited on the PET film by an electrospinning method. Then, the PET film with the electrospun nanofiber was immediately immersed in an activation solution containing PdCl2 to form a catalytic surface on the nanofiber network. Because Sn (II) works as a sensitizer, the Pd (II) ions were reduced to Pd and deposited on the nanofiber surface. After rinsing well with DIW, 8 ACS Paragon Plus Environment

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the resulting film was immersed in the plating solution. The geometry of the Cu/Ni nanonetwork films were investigated by FE-SEM. The polymer nanofiber template is shown in Figure 1a and the electrolessly plated nanofiber networks are shown in Figure 1b-e. Figure 1f shows the average diameters of the electrolessly plated nanofibers. The fiber diameters clearly increased with increasing plating time. The diameter distributions of each film are shown in Figure S1. By using nanofibers with the smallest possible diameter as a template for the Cu/Ni nanonetwork films, the number of nanofibers could be increased while maintaining the same transmittance. Increasing the number of nanofibers provides a lot of pathways to transport electrons. It reduces the risk of short circuit due to the electromigration and forming a hot spot, which is a locally overheated area by preventing the current crowding.31,32 The elemental mapping of the plated nanofibers by EDX indicated that Cu and Ni were deposited on the fiber network (Figure 1g). This indicated that Cu and Ni selectively deposited on the nanofiber surface. Because the reducing agent contains phosphate (P), a small amount of P was also detected. Moreover, we investigated the ratio of each element depending on the plating time (Table S1). The result indicated that the plating materials were uniformly deposited on the fiber, regardless of the plating time.



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Scheme 1. Schematic of the fabrication procedure of the Cu/Ni nanonetwork. (i) The polymer nanofiber template is formed on a PET film by an electrospinning method. (ii) The catalyst is deposited on the surface of polymer nanofiber template. (iii) The Cu/Ni nanonetwork is plated on the polymer nanofiber template by electroless deposition.

Figure 1. The SEM images of the polymer nanofibers prepared at different plating times: (a) 0 s, (b) 15 s, (c) 30 s, (d) 45 s, and (e) 60 s. (f) Diameter of the plated fibers as a function of plating time. (g) Elemental mapping of the Cu/Ni nanonetwork. 10 ACS Paragon Plus Environment

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Analysis of the Optical and Electrical Properties The transmittance and sheet resistance of the Cu/Ni nanonetwork films prepared with different plating times are shown in Figure 2a. The transmittance decreased and the conductivity increased with increasing plating time. Generally, these properties of the metal nanonetwork fabricated by a solution process are determined by the length and diameter of the wire (or mesh).33 Therefore, wires (or meshes) with different diameters need to be prepared to control the morphology. Conversely, in the case of metal nanonetworks fabricated by plating, the amount of metal deposited is controlled by tuning the plating times; hence, the transparency and conductivity can be easily controlled. The transparency of metal-based TCFs is dominated by the shape and area fraction of free spaces.25 The Cu/Ni nanonetwork film has several micrometer-sized free spaces that are large enough for visible light to penetrate. This resulted in flat transmittance spectra in the visible region (Figure S2). We can confirm the transparency of the film by comparing the photograph taken by the pristine camera lens (Figure S3a) and by the one which the film attached on (Figure S3b). Moreover, the total transmittance including the diffusive transmittance was above 80% even after plating for 60 s (Figure S4). These features, which contribute to an increase in the total amount of incident light that goes through, are desired for photoelectronic devices such as solar cells. Figure 2b shows the performance of our material compared with that of other TCFs, as determined by the transmittance and sheet resistance. The film prepared in this work exhibited comparable performance to that of other materials, even though it contained approximately 20 wt.% of low conductive materials (Ni and P). This was attributed to a decreased junction resistance through the formation of a junctionless network. Figure 2c shows the tilted SEM images of the polymer nanofiber network, which shows that the fibers make physical contact with each other at the intersections. The intersections are covered 11 ACS Paragon Plus Environment


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by the deposited metal during the plating step. Therefore, a self-fused junctionless network was formed, as shown in Figure 2d. This can suppress the increasing resistance of electron transport at the intersections.

Figure 2. (a) The dependence of the transmittance and sheet resistance on the plating time. (b) The optoelectronic performance plots of the Cu/Ni nanonetwork film and previously reported materials (AgNWs,34-36 CuNWs,37,38 and carbon nanotubes39,40). (c) Tilted SEM image of the electrospun nanofiber template. (d) Tilted SEM image of Cu/Ni nanonetwork film (plating time: 45 s). Assessment of damage tolerance and long-term stability A schematic representation of the bending test is shown in Figure 3a. Both sides of the film were fixed by clamps and then one side was pushed until the target bending radius (1.5–24 mm) was 12 ACS Paragon Plus Environment

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reached. The bending radius was defined by following equation: 41

=

#=

"

"

,

(1)

in which r is the bending radius, d is the distance between two electric contact points, and L is the length of the sample. Figure 3b shows the resistance change depending on the bending radius. The resistance of the ITO/PEN drastically increased, which is consistent with the previous report.41 In addition, the Cu/Ni thin film fabricated on PET without a polymer template also showed large resistance change (>600%) against the original resistance with decreasing bending radius. Conversely, the Cu/Ni nanonetwork suppressed the resistance change to ~10% compared with the original resistance in the bending radius range of 1.5–24 mm. These results indicated that the existing polymer template improves the flexibility, which results in superior bending durability compared with that of the Cu/Ni film. Figure 3c shows the resistance change during the bending cycle test with a fixed bending radius of 15 mm. The change in resistance of the Cu/Ni nanonetwork film was 0.5% after 100 cycles. For the practical application of THs, the durability of not only the bending but many other types of external forces such as peeling and abrasion is required. Figure 3d shows the resistance change during the tape peeling test. The tape was attached to a film followed by peeling off of the tape. The tape was changed every five times and the peeling speed was kept at approximately 2.4 mm/s by hand. Although the resistance gradually increased, the Cu/Ni nanonetwork film showed no significant resistance change (lower than 10%) even after peeling 200 times. Figure 3e shows the resistance change during a 1000 cycle abrasion test. No apparent change in the resistance was observed at 5 kPa, and only ~5% change in resistance was observed at 50 kPa after 1000 cycles. According to previous reports,



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NW-based TCFs tend to increase the resistance easily owing to lots of junction points and the weak adhesion force to the substrate.42 However, by applying electroless deposition, which can realize high adhesion to the substrate, the Cu/Ni nanonetwork film showed strong mechanical durability. Furthermore, exposure durability to ambient air of the Cu/Ni nanonetwork was evaluated at room temperature and 30%–40% relative humidity (RH). In the first three days, the resistance slightly decreased and then the degradation gradually progressed, as sown in Figure 3f. In the case of the ordinary CuNW film, the resistance increased by more than five times after ten days;43 however, the Cu/Ni nanonetwork maintained its resistance, which increased as low as ~10% after one month. Such a high durability was attributed to the passivation properties of Ni and P.

Figure 3. (a) The illustration of the bending test (L is the length of the sample and d is the distance between the two electric contact points). The resistance change of the Cu/Ni nanonetwork film depends on (b) bending radius, (c) bending cycles at bending radius of 15 mm, (d) tape peeling cycles, (e) abrasion times, and (f) exposure days under ambient condition. R and R0 are the electrical resistance of the film before and after the test, respectively. 14 ACS Paragon Plus Environment

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Heater Performance To assess the heating performance, the Cu/Ni nanonetwork (Plating time: 60 s)-based THs were prepared as shown in the schematic illustration in Figure 4a. The heater operated by applying a DC voltage through the Ag electrode. The surface temperature of the heater was recorded by an infrared camera. Figure 4b shows the time-dependent temperature profiles of the heater with different applied DC voltage. After the voltage was applied, the surface temperature reached a maximum temperature after approximately 15 s and the saturated temperature was stable until the voltage was switched off. When the applied voltage was above 6 V, reliable temperature measurement could not be conducted owing to distortion of the PET substrate. A summary of the heating characteristics of heaters prepared with different materials is shown in Table 1, which demonstrates that the Cu/Ni nanonetwork exhibited rapid thermal response (saturation time ~15 s). Table 1. The heating characteristics of various types of THs (%: transmittance, &' : sheet resistance, (: applied voltage, %' : saturation surface temperature, )' : saturation time). Material AgNW AgNW/PEDOT:PSS CuNW/PMMA Cu/Zr nanotrough Cu/Ni micromesh Cu/NiNW Cu/Ni nano-network

Substrate PET glass PET PDMS PES glass PET

% (%) &' (Ω/sq) 90 10 70 4 84 17 90 3.8 86 16.2 76 300 71 14

( (() 5 4 5 7 9 6 6

%' (℃) 70 ~70 150 180 225 41 109

)' () ~50 300 ~70 90 60 80 ~15

ref. 35 44 45 46 29 47 This work

From the joule heating theory, the total amount of generated heat (12 ), which is equals to the power consumption of the film, is described as 15 ACS Paragon Plus Environment


12 =

34 5

.

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(2)

Here, 6 is the applied voltage for the film and & is the electrical resistance of the film. 12 is separated into stored heat (1' ) and dissipated heat (1 ).

Furthermore, 1' is estimated from

12 = 1' + 1 .

1' = 89

(3)

:(;) ;

,

(4)

in which 8 is the heat capacity of the film, 9 is the film mass, and %()) is the surface

temperature as a function of the heating time ()). Although 1 is caused by the heat transfer of radiation and the thermal loss (1: ) owing to air convection related with heat transfer coefficient

(ℎ) and surface area (=), the latter is dominant below 150°C;48 hence 1 can be treated as 1 ~1: ,

1: = ℎ=(%()) − %@ ),

(5) (6)

in which %@ is the initial surface temperature. Therefore, the surface temperature was dependent

on the generated heat (12 ) and thermal loss (1: ) coming from air convection. From the above equations, %()) can be written as equation 7. %()) =

3 4A 5 D1 BC

− EFG(− HI ))J + %@ . BC

(7)

Corresponding to equation 7, the obtained saturation surface temperature was proportional to the square of the applied voltage (Figure S5). 16 ACS Paragon Plus Environment

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One of the most important parameters for THs is the power efficiency because the energy consumption is regarded as the most crucial issue for de-icing by THs. The power efficiency is normalized factor because it is the slope of the saturation surface temperature as a function of power consumption per unit area. Therefore, the power efficiency is able to be used for performance comparison factor among many types of the TH.44,48 As shown in Figure 4c, the power efficiency of the Cu/Ni nanonetwork was 421.7 °C cm2/W. The obtained value was much higher than that reported in the literature (see Table S2 for a comparison). The performance repeatability of the film was investigated by a heating/cooling test for 20 cycles. Figure 4d shows the time-dependent surface temperature when the applied voltage was repeatedly switched between 0 V and 4 V every 30 s. The surface temperature sharply increased from room temperature to 60°C after a voltage was applied repeatedly, without clear degradation. The infrared images of the heating state and cooling state are shown in Figure 4e. Although a small temperature gradient was confirmed in the heating state, the heat spread throughout the film, which indicated that the material could be applied in de-icing devices.



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Figure 4. Heater performance of the Cu/Ni-nanonetwork-based transparent heater (dimensions: 12 × 25 mm). (a) A schematic of the Cu/Ni-nanonetwork-based transparent heater. (b) Time-dependent heating characteristics at different voltages. (c) An energy efficiency plot. (d) The heating cycle test by switching the voltage (0 V and 4 V) every 30 s. (e) Thermal images of the heater at 0 V (without applying voltage) and at 4 V. Thermal loss simulation Even though the Cu/Ni nanonetwork did not possess notable conductivity, it showed prominent heating characteristics. Such a result was also reported in a previous study using a junctionless cupronickel-micromesh-based TH29 but the reason for the excellent heating characteristics was not determined. To further understand the difference in the heating characteristics between a junction network and junctionless network, the thermal loss originating mainly from air convection (as described by Equation 6) during the application of a current was numerically investigated by a finite element method. By setting the material parameters (size, conductivity, heat capacity and thermal conductivity), the objects composed of Cu and PVB were modeled 18 ACS Paragon Plus Environment

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with two types of intersections: contact junction and fused junction. The thermal losses at each intersection were simulated by combining a stationary current solver and a thermal solver. The simulated result is shown in Figure 5a. The magnified views of the contact junction and the fused junction in the two insets show that the thermal loss was concentrated around the contact junction area. Outputting the thermal loss at the surface (y=0, z=0.2) as a function of the object position along the white dotted line in figure 5a, a clear difference was confirmed between the

two models (Contact junction region: −4.2 ≤ x ≤ −3.8, fused junction region: 3.8 ≤ x ≤ 4.2). As shown in Figure 5b and c, the maximum value of the thermal loss was 3.67 × 10OP W/mQ

in the contact junction region; conversely, in the fused junction region the value was suppressed

to 2.51 × 10OQ W/mQ when the stationary current was set at 6 mA (the spatial profile is shown in Figure S6).

Therefore, a junctionless network has benefits for the design of an efficient

joule heater by suppressing the thermal loss, which means a small 1 in Equation 7 as well as improved conductivity (See Note S1 for a further discussion of the “thermal loss suppression effect” caused by a junctionless network).



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Figure 5. Thermal loss simulation. (a) The estimated thermal loss distribution on two different models; contact junction and fused junction. (b) The simulated thermal loss of the contact junction region (-4.2≤x≤-3.8, y=0, z=0.2). (c) The simulated thermal loss of the fused junction region (3.8≤x≤4.2, y=0, z=0.2). De-frosting performance To evaluate the practical application of the TH for de-icing, a de-frosting test was demonstrated. Superhydrophobic, hydrophilic, or slippery surfaces have been investigated for the design of anti-icing coating materials;49–51 however, these materials only delay frost formation on the surface and it is impossible to obtain long-term stable frost resistance. Thus, our proposed cost-effective heater is the most promising approach.48 The film was attached to Peltier cooler, whose temperature was set to −20°C to realize cold district conditions. After the film was covered with the accumulated frost, 10 V was applied. Despite high voltage is needed due to the 20 ACS Paragon Plus Environment

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endothermic property of the Peltier cooler, the film worked without the effect of the electromigration. Even if the driven voltage is much higher than its experiment, the film can be utilized after passivation treatment such as capping.31 Figure 6a shows the optical images during the test taken every 15 s. To quantify the result, the ratio of the de-frosted area (RSTUV';@W2 ) to

the total area (R;V;XY ) was obtained from binalized optical images. Figure 6b shows the

time-dependent de-frosting ratio. Because the film temperature did not increase and the frost melting occurred at the film-ice interfaces, the ratio did not change in the first several seconds. Then, the de-frosting rapidly progressed and all the frost on the film was removed within ~35 s. The progress of the de-frosting is shown in Movie S1.

Figure 6. De-frosting demonstration. (a) Optical images taken every 15 seconds during the de-frosting demonstration. (b) The Sde-frosting/Stotal ratio as a function of duration time. The inset illustrates the experimental setup. The TH was attached on a peltier cooler set at -20°C. After frost was accumulated, the voltage was applied to the TH through a silver electrode.



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CONCLUSION Effective THs that utilize the electrothermogenic properties of a metal composite junctionless nanonetwork were prepared through a facile two-step wet process combining electrospinning and electroless deposition. Mixing two kinds of ion sources (Cu and Ni) in the plating bath, the Cu/Ni alloy was deposited on the nanofiber template at the same time. The polymer played a role as a plating template and improved the flexibility of the film. The self-fused Cu/Ni junctionless nanonetwork effectively generated electric power to heat without degradation. Taking advantage of these materials, a film exhibiting excellent heating characteristics including rapid thermal response (saturation time ~15 s) and high power efficiency (421.7 °C cm2/W) was realized as a TH. Moreover, the thermal loss of the metal nanonetworks during the application of a current were investigated by the finite element method. Comparing the results of two types of intersections (contact junction and fused junction), we showed that the junctionless network enabled more efficient heating owing to the “thermal loss suppression effect”. Therefore, we showed that the network geometry, as well as the conductivity, are important factors for the heating characteristics of THs. The strategy and investigation reported in this work could inspire the preparation of next-generation electronics.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXX. Figure S1-S7 representing diameter distribution of plated nanofiber, transmittance spectra of Cu/Ni nanonetwork film, the photograph taken by the camera which the Cu/Ni nanonetwork film attached on, the optical properties of the Cu/Ni nanonetworks, the saturation surface temperature of the Cu/Ni nanonetwork based THs with different applied voltage, the simulated thermal loss of whole region, and the thermal loss with different intersection models, respectively and Table S1 and S2 summarizing the weight and atomic ratio of each element, and the power efficiency of the THs using different materials, respectively. (PDF) Movie S1 showing the De-frosting progress of the Cu/Ni nanonetwork based THs. (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions R.Y., T.M., K.M. designed the concept of this work. R.Y. conducted and analyzed all experiments. R.Y. and M.T. wrote the manuscript. M.T., T.M., K.M. and L.M. provided



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substantially scientific supports. M.T. and Y.M. supported the analysis by using finite element method. S.S. supervised the project. Funding Sources Part of this work was supported by JSPS KAKENHI (grant number JP 16J06070) awarded to M. T. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful to Mr. Issei Takenaka, Mr. Testa Andrea, and Mr. Pecorelli Pietro whose suggestion and comments were valuable for our work. M. T. thanks predoctoral fellowship (PD) from Japan Society of Promotion of Science (JSPS).


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