High-Performance and Rapid-Response Electrical Heaters Based on

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High-Performance and Rapid-Response Electrical Heaters Based on Ultraflexible, HeatResistant, and Mechanically Strong Aramid Nanofiber/Ag Nanowire Nanocomposite Papers Zhonglei Ma,*,† Songlei Kang,† Jianzhong Ma,*,‡ Liang Shao,† Ajing Wei,† Chaobo Liang,§ Junwei Gu,*,§ Bin Yang,‡ Diandian Dong,‡ Linfeng Wei,‡ and Zhanyou Ji‡ †

College of Chemistry and Chemical Engineering, Key Laboratory of Auxiliary Chemistry and Technology for Chemical Industry, Ministry of Education, Shaanxi Key Laboratory of Chemical Additives for Industry, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, People’s Republic of China ‡ College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, People’s Republic of China § MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China S Supporting Information *

ABSTRACT: High-performance and rapid response electrical heaters with ultraflexibility, superior heat resistance, and mechanical properties are highly desirable for the development of wearable devices, artificial intelligence, and highperformance heating systems in areas such as aerospace and the military. Herein, a facile and efficient two-step vacuumassisted filtration followed by hot-pressing approach is presented to fabricate versatile electrical heaters based on the high-performance aramid nanofibers (ANFs) and highly conductive Ag nanowires (AgNWs). The resultant ANF/AgNW nanocomposite papers present ultraflexibility, extremely low sheet resistance (minimum Rs of 0.12 Ω/sq), and outstanding heat resistance (thermal degradation temperature above 500 °C) and mechanical properties (tensile strength of 285.7 MPa, tensile modulus of 6.51 GPa with a AgNW area fraction of 0.4 g/m2), benefiting from the partial embedding of AgNWs into the ANF substrate and the extensive hydrogen-bonding interactions. Moreover, the ANF/AgNW nanocomposite paper-based electrical heaters exhibit satisfyingly high heating temperatures (up to ∼200 °C) with rapid response time (10−30 s) at low AgNW area fractions and supplied voltages (0.5−5 V) and possess sufficient heating reliability, stability, and repeatability during the long-term and repeated heating and cooling cycles. Fully functional applications of the ANF/AgNW nanocomposite paper-based electrical heaters are demonstrated, indicating their excellent potential for emerging electronic applications such as wearable devices, artificial intelligence, and high-performance heating systems. KEYWORDS: electrical heaters, ANF/AgNW nanocomposite papers, ultraflexible, high-performance, rapid response

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actuation voltage, high heating temperature, and rapid response, as well as good thermal stability and mechanical properties, are urgently demanded to satisfy the highperformance electrical heaters.6−8 In addition, a low resistance

lectrical heaters based on the Joule effect have attracted significant attention owing to their widespread applications including wearable devices, defogging and defrosting, personal thermal management, and heating systems in industry.1−4 Joule heat is generated upon the external supplied voltage as current passes through the conductive materials due to the inelastic collisions between accelerated electrons and phonons.5 Flexibility and transparency, low © 2019 American Chemical Society

Received: January 16, 2019 Accepted: June 18, 2019 Published: June 18, 2019 7578

DOI: 10.1021/acsnano.9b00434 ACS Nano 2019, 13, 7578−7590

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can reach.30,35 High actuation voltages are usually required due to their relatively high electrical resistance. Moreover, these polymer substrates often possess low mechanical properties and weak interface adhesion with the AgNWs. The SBS/ AgNW nanocomposite-based electrical heaters, for example, exhibit a tensile strength of only 1.2 MPa.38 The low thermal stability and mechanical properties of the electrical heaters greatly limit their applications in high-performance heating systems in areas such as aerospace and the military. For instance, the lunar rover vehicle on the moon is located in a harsh environment with a high temperature of 160 °C in daylight and a low temperature of −180 °C at night, which will cause the failure of the precision electronic components and reduced service life. Therefore, flexible, heat-resistant, and mechanically strong electrical heaters are urgently needed to overcome the above challenges. Recently, Kim et al.30 developed electrical heaters based on AgNWs, conductive polymer, and a polyimide (PI) substrate via a layer-by-layer processing method. The electrical heaters show a sheet resistance of ∼30 Ω/sq, which can be heated to a high steady-state temperature exceeding 200 °C at a supplied voltage of 20 V. Aramid nanofibers (ANFs), also called nanoscale Kevlar fibers, consisting of aligned poly(paraphenylene terephthalamide) (PPTA) chains, have been demonstrated to be promising reinforcing phases and high-performance polymer substrates for various applications.40−43 They exhibit superior thermal resistance, mechanical properties, and optical performances due to the high anisotropy ratio and strong interactions between PPTA chains such as hydrogen bonding, π−π stacking, and van der Waals forces.44−46 Since Kotov and his colleagues47,48 discovered that ANFs can be prepared by feasible deprotonation of macroscopic Kevlar fabrics in dimethyl sulfoxide (DMSO) with the presence of potassium hydroxide, the ANF-based high-performance and multifunctional nanocomposites have attracted great attention in many areas.49−51 For instance, multifunctional nanocomposite papers with outstanding thermal stability and mechanical properties have be fabricated based on the high-performance ANFs and highly conductive materials such as graphene,45 multiwalled carbon nanotubes (MWCNTs),52,53 plasmonic nanoparticles,50 poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and polyaniline (PANI)54,55 and exhibit great potential for applications in flexible conductors, supercapacitors, electromagnetic interference shielding, etc. Furthermore, compared with the traditional layer-by-layer processes, the vacuum-assisted filtration (VAF) technique greatly improves the speed of deposition with smaller losses of nanomaterials accompanying the fabrication process and represents a very facile and convenient strategy in the fabrication of the multifunctional nanocomposite papers.56−58 In this work, we report high-performance and rapid-response electrical heaters for wearable electronic applications based on the ultraflexible, heat-resistant, and mechanically strong ANF/ AgNW nanocomposite papers developed by the two-step vacuum-assisted filtration (TVAF) followed by hot-pressing approach. AgNWs are partially embedded into the ANF substrates with extensive hydrogen-bonding interactions, forming highly effective interface adhesion and highly conductive AgNW networks, and thus low sheet resistance. The resultant ANF/AgNW nanocomposite paper-based electrical heaters show ultraflexibility and excellent heating

R ensures that the electrical heaters can generate enough Joule heat to make the heating temperature T increase to the saturated value at a low supplied voltage.9,10 Although indium tin oxide (ITO) has been widely used as transparent electrical heaters in industry owing to its satisfactory transparency (Tr > 90% at 550 nm), the brittleness, slow thermal response, and complicated fabrication processes limit its applications in wearable and stretchable optoelectronic devices.11,12 Moreover, the low thermal conductivity of ITO results in low heating rates. As a consequence, several conductive alternatives such as graphene,13−17 carbon nanotubes,18,19 conductive polymers,20,21 and their hybrids together22,23 have been investigated extensively for conductors and flexible heaters. Nevertheless, high actuation voltages are usually required for carbon and conductive polymer-based heaters to achieve satisfying heating temperatures due to their relatively high electrical resistance, limiting their specific applications in many areas. Numerous strategies have been developed to realize the fabrication of ultraflexible, transparent, and low-voltage heaters with controllable heating temperatures and rapid response.24,25 Recently, metal nanowires (NWs) or nanofibers (NFs) have shown very promising performances in flexible and transparent conducting electrodes and heaters.26,27 Due to their mesh-like percolating structures, sparse metal nanowire films can exhibit very low resistance (lower than 10 Ω/sq) and high transparency (higher than 90%) simultaneously.10,28 Hsu et al.29 obtained the transparent conducting electrodes with a sheet resistance of 0.36 Ω/sq and a transmittance of 92% by combining the gold nanowires and copper mesoscale wires together. Li et al.9 demonstrated that the incorporation of selffused copper wires endows the electrospun polymer nanofiber based electrodes with a sufficiently low sheet resistance of 8 Ω/ sq and a high transparency of up to 91.4% via thermal evaporation. Park et al.11 found that the electrical heaters based on the electrospun Ag nanofibers (AgNFs) exhibit a low sheet resistance of ∼1.3 Ω/sq and thus a high heating temperature, reaching 250 °C at a supplied voltage of 4.5 V. For flexible heaters composed of polymer substrates and metal NWs, however, low sheet resistance and high transmittance are not the only factors that should to be considered. The thermal stability and mechanical properties of the substrates, as well as the structure design of the electrical heaters, are also of great importance in some high-performance and complex electronic applications.30 Numerous polymer-based elastomers,31−33 fabrics,34,35 and films23,36,37 have been widely used in wearable and stretchable electrical heaters as flexible substrates. For instance, Choi et al.38 developed a stretchable electrical heater for articular thermotherapy based on the styrene−butadiene−styrene (SBS) thermoplastic elastomer and highly conductive silver nanowires (AgNWs). The obtained electrical heaters exhibit a resistance of 0.8 Ω, and the surface temperature increased from room temperature to ∼40 °C at the supplied voltage of 1.0 V. Cheng et al.39 fabricated the stretchable and wearable heating fibers with a hierarchical configuration by coating CuNWs onto the helical polyester (PE) yarn. Lee et al.10 demonstrated the fabrication of roll-to-roll heating films by depositing AgNWs on a rollable polyethylene terephthalate (PET) substrate. The obtained films exhibit a surface temperature of 62 °C with a supplied voltage of 15 V. However, the relatively low thermal stability of these polymer substrates greatly limits the maximum heating temperature (usually lower than 100 °C), which is much lower than the AgNW-based film heaters 7579

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Figure 1. (a) ANF dispersion prepared by the deprotonation of macroscopic aramid fabrics. (b) AgNW dispersion synthesized by the polyol method. (c and d) SEM and TEM images of ANFs, respectively. (e and f) SEM and TEM images of AgNWs, respectively.

Schematics for fabrication of the ANF/AgNW nanocomposite papers and electrical heaters are presented in Figure 2a. ANF/AgNW nanocomposite papers with partially embedded AgNWs into the ANF substrate were fabricated via the TVAF of ANF and AgNW dispersions, respectively, followed by the hot-pressing and drying treatments. Figure 2b shows the optical photographs of ANF/AgNW nanocomposite papers with various AgNW area fractions. The color of the nanocomposite papers turned from luminous yellow to whitegray, and the transparency decreases gradually as the AgNW area fraction increases. At low AgNW area fractions (0−0.3 g/ m2), the nanocomposite papers exhibit good transparency due to the smaller diameters of ANFs and AgNWs than the wavelength of visible light. The logo of Shaanxi University of Science and Technology is very clear through the nanocomposite papers. Figure 2c−f show the SEM images of pure ANF papers and nanocomposite papers with AgNW area fractions of 0.15, 0.3, and 0.5 g/m2, respectively. The nanocomposite papers present a uniform distribution of AgNWs with a conductive AgNW network, while the pure ANF papers present a smooth surface morphology. With larger AgNW area fraction, more junction points are formed to obtain the highly effective conductive AgNW networks. Figure 2g−j show the fracture surfaces of the pure ANF papers and nanocomposite papers with a AgNW area fraction of 0.3 g/m2, respectively. As can be seen, the papers exhibit a dense and uniform microstructure, while the ANF/AgNW nanocomposite papers exhibit a bilayer structure composed of the partially embedded AgNW layer and ANF substrate layer. It is noted that the average diameter of AgNWs increases above ∼100 nm (Figure 2i and Figure S3), which is larger than that of the original AgNWs, and the surface of AgNWs becomes much coarser. As illustrated in Figure S2 (Supporting Information), a piece of wet viscoelastic ANF gel is first obtained after the first vacuum-assisted filtration. During the second vacuum-assisted filtration after adding the AgNW dispersion into the filter, the AgNWs are partially embedded into the wet viscoelastic ANF gel in the cross section due to their own gravity and the strong suction effect of the vacuumassisted filtration. At the same time, the AgNWs are wrapped by the ANFs owing to the extensive hydrogen-bonding interactions between the carbonyl group of polyvinylpyrroli-

performances with rapid response, as well as superior heat resistance and mechanical properties. The microstructures, sheet resistances, transparency, thermal stability, and heating performances as functions of AgNW area fraction and supplied voltage are detailedly investigated. Moreover, they exhibit sufficient heating reliability, stability, and repeatability during the long-term and repeated heating and cooling cycles owing to the protective effect of the densely wrapped ANFs around AgNWs. The application capabilities of electrical heaters for wearable devices, personal thermal management, and water heating also have been demonstrated, indicating that the materials are suitable for emerging electronics applications.

RESULTS AND DISCUSSION Morphologies and Microstructures of AgNWs, ANFs, and ANF/AgNW Nanocomposite Papers. Figure 1a shows the preparation of an ANF dispersion by the deprotonation of macroscopic aramid fabrics. By weakening the hydrogen bonds and strengthening the electrostatic repulsion between the polymer chains, the macroscale Kevlar fibers were chemically transformed into nanoscale ANFs.50 Figure 1c and d show the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of high-aspect-ratio aramid nanofibers with an average diameter of 10 nm and a length of several micrometers. The hydrophobic attraction and π−π stacking in the polymer backbone counteracted the complete disintegration of the fibers into individual polymer chains. The abundant polar functional groups on ANFs provide excellent compatibility and cohesiveness with other matrixes and good dispersion in deionized water. Moreover, the ANFs possess superior flexibility, heat resistance, and mechanical properties compared with cellulose nanofibers.47 Thus, ANFs can be used as flexible and high-performance polymer substrates for high-tech electronic devices. Figure 1b shows the uniform AgNW dispersion synthesized by the polyol method. AgNWs with a small average diameter of ∼40 nm and a high aspect ratio of ∼800 are synthesized, as can be seen in Figure 1e and f. Five characteristic diffraction peaks of 38.1°, 44.2°, 64.3°, 77.4°, and 81.6° corresponding to the (111), (200), (220), (311), and (222) planes of pure face-centered cubic (fcc) silver crystals can be observed from the X-ray diffraction (XRD) pattern (Figure S1). 7580

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Figure 2. (a) Schematic illustration for fabrication of the ANF/AgNW nanocomposite paper-based electrical heaters. (b) Optical photographs of the nanocomposite papers. The background is the logo of Shaanxi University of Science and Technology (from http://www. sust.edu.cn/xxgk/xxbs/xh.htm with permission). (c−f) SEM images of the nanocomposite papers with AgNW area fractions of 0, 0.15, 0.3, and 0.5 g/m2, respectively. (g and h) Fracture surfaces of the pure ANF papers. (i and j) Fracture surfaces of the nanocomposite papers with partially embedded AgNWs.

while that of the ANF papers presents the band at 1645 cm−1, corresponding to the stretching vibration of CO, and the two bands at 3335 and 1540 cm−1, corresponding to the stretching vibration and deformation of N−H, respectively. For the ANF/AgNW nanocomposite papers, a characteristic transmittance shoulder peak is observed at 1608 cm−1 on the lower side of the band at 1641 cm−1 (CO), and a characteristic transmittance peak of N−H is observed at 3329 cm−1, which is lower than that in the ANF papers. The appearance of the shoulder peak and shift of the N−H peak to the lower band demonstrate the formation of hydrogenbonding interactions between PVP on Ag NWs and ANFs. Figure 3b−d show the XPS wide-scan spectra of AgNWs, ANF papers, and AgNW/ANF nanocomposite papers and the highresolution spectra of C 1s and N 1s. All the spectra are referenced to the C 1s peak with a binding energy of 284.6 eV. As can be seen, beside the main peaks at 284.6 eV for C−C and 285.4 eV for C−N, the peak at 287.3 eV, corresponding to the CO, shifts to a higher binding energy of 287.8 eV for the

done (PVP) on AgNWs and the amino group of ANFs. Subsequently, the hot-pressing process will promote the further embedding of AgNWs into the ANF substrate and causes the desiccation and densification of the ANF/AgNW wet papers, obtaining the ultrathin ANF/AgNW nanocomposite papers with a bilayer structure. The bilayer structure will endow the papers with high conductivity on the AgNWrich side and high insulation on the AgNW-free side. Importantly, the densely wrapped ANFs can prevent the permeation of oxygen gas and retard the oxidation of Ag in air, which have a protective effect on the AgNWs especially in air and humid conditions. The hydrogen-bonding interactions between the carbonyl group (CO) of PVP on AgNWs and the amino group (N− H) of ANFs have been proved by the Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) analysis. As shown in Figure 3a, the FTIR spectrum of the AgNWs presents the band at 1629 cm−1 corresponding to the stretching vibration of the carbonyl group (CO) of PVP, 7581

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Figure 3. FTIR spectra (a) and XPS wide-scan spectra (b) of AgNWs, ANF papers, and AgNW/ANF nanocomposite papers. High-resolution XPS spectra of C 1s (c) for AgNWs and AgNW/ANF nanocomposite papers and N 1s (d) for ANF papers and AgNW/ANF nanocomposite papers.

Figure 4. (a−c) XRD patterns, transparency, and sheet resistance of ANF/AgNW nanocomposite papers. (d) Real-time relative resistance upon repeated bending and stretching. (e) Lighting of the “SUST” LED lamps via the nanocomposite paper. (f) Relative resistance of the nanocomposite papers upon washing followed by drying cycles. (g) TGA curves of nanocomposite papers. (h) Tensile stress−strain curves of the nanocomposite papers. (i) Tensile strength and tensile modulus of the nanocomposite papers.

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Figure 5. (a and b) Time-dependent surface temperatures of the ultraflexible electrical heaters with different supplied voltages for AgNW area fractions of 0.3 and 0.5 g/m2, respectively. (c) Experimental data and linear fitting of saturation temperature versus U2. (d) Tailored surface temperatures of the electrical heaters upon gradiently changed voltages. (e) Long-term time−temperature curve at a constant voltage of 2 V for the electrical heaters. (f) Heating stability and repeatability of the electrical heaters upon repeated supplied voltages. (g) Illustration for the heating and cooling cycles of the electrical heaters.

points. When the area fraction increases to 0.3 g/m2, the Rs rapidly decreases to 3.2 Ω/sq by 2 orders of magnitude owing to the formation of numerous junction points and highly conductive AgNW networks. With a higher area fraction of 1.0 g/m2, the nanocomposite papers exhibit an extremely low Rs of 0.12 Ω/sq, which is much lower than those of ITO (higher than 50 Ω/sq) and CVD-synthesized graphene (higher than 250 Ω/sq). The significantly enhanced brightness of LED lamps (insets) with increasing AgNW area fraction also demonstrates the formation of the highly conductive AgNW networks. The relationship between sheet resistance and AgNW area fraction can be used to fabricate conductive papers with desired sheet resistance to meet the requirements for different applications. Figure 4d shows that the nanocomposite papers exhibit a very stable and constant real-time relative resistance (R/R0, between 0.99 and 1.02) upon repeated bending (radius of curvature: about 6 mm) and stretching with a frequency of 1 cycle per 1.5 s, which is comparable to those reported in previous studies.9,24 The results demonstrate the superior flexibility and electrical stability of the flexible electrical heaters under external applied stress/strain fields. The “SUST” LED lamps can also be lighted via the flexible electrical heaters and maintain a steady brightness upon repeated bending and stretching, as shown in Figure 4e and Movie S1. Figure 4f shows that the relative resistance of the nanocomposite papers stays constant upon repeated water washing followed by drying cycles for 20 times (Movie S2). It is believed that the outstanding electrical stability mainly results from the partially embedded AgNWs in

AgNW/ANF nanocomposite papers. Moreover, the XPS of N 1s for N−H of ANF papers shifts from 398.5 eV to 399.6 eV. The results indicate that the chemical environment of CO and N−H has been changed, further demonstrating the formation of hydrogen-bonding interactions between PVP on Ag NWs and ANFs. XRD Analysis and Optical, Electrical, Thermal, and Mechanical Properties of ANF/AgNW Nanocomposite Papers. Figure 4a shows the XRD patterns of the pure ANF papers and nanocomposite papers. The nanocomposite papers present similar diffraction peaks and crystal planes to the facecentered cubic (fcc) crystalline silver, while the ANF papers present no distinct diffraction peak. The intensity of the diffraction peaks increases gradually as the AgNW area fraction increases. Figure 4b shows that the pure ANF papers and nanocomposite papers with low AgNW area fractions have good transparency in the visible and near-infrared region. With increasing AgNW area fraction, the optical transmittance gradually decreases due to the stronger scattering and reflection of photons. In the ultraviolet region, the ANF/ AgNW nanocomposite papers show extremely low transmittance due to the absorption of light ranging from 200 to 450 nm, indicating that the nanocomposite papers have a UVshielding effect. Figure 4c shows the sheet resistance of ANF/AgNW nanocomposite papers as a function of AgNW area fraction. At the low area fraction of 0.1 g/m2, the nanocomposite papers exhibit a relatively high sheet resistance (Rs) of 390 Ω/sq due to the sparsely distributed AgNWs and very few junction 7583

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the flexible electrical heaters to achieve more excellent heating performances. In order to fully understand the mechanism underlying the heating behavior, thermodynamic analysis of heating performance for the ANF/AgNW nanocomposite paper-based electrical heaters was conducted. Based on the energy balance principle, the surface temperature of the electrical heaters would reach an equilibrium when the dissipated power by Joule heating becomes equal with the power losses through conduction, convection, and radiation.4,39 Accordingly, the surface temperature of the electrical heaters could be calculated by establishing the following equation:

the ANF substrate and strengthened interface adhesion between AgNWs and ANFs. The heat resistance and mechanical properties of the ANF/ AgNW nanocomposite papers are critical for the design of high-performance electrical heaters. Figure 4g shows that the ANF/AgNW nanocomposite papers exhibit high thermal degradation temperatures above 500 °C mainly benefiting from the heat-resistant ANF substrate, which can meet the maximum heating temperature of AgNW-based electrical heaters.11 Figure 4h and i show that the ANF/AgNW nanocomposite papers with various AgNW area fractions all exhibit outstanding mechanical properties. A strong tensile strength of 159.6 MPa and a high tensile modulus of 2.05 GPa are obtained due to the extensive hydrogen-bonding interactions between ANFs. The incorporation of AgNWs significantly improves the tensile strength and modulus of the nanocomposite papers. At a AgNW area fraction of 0.4 g/m2, for example, the nanocomposite papers present a stronger tensile strength of 285.7 MPa (improved by 79.0%) and a higher tensile modulus of 6.51 GPa (improved by 217.6%) as a result of the enhancement effect of partially embedded AgNWs in the ANF substrate and extensive hydrogen-bonding interactions between AgNWs and ANFs. The stress could be effectively transferred to the strong AgNW frameworks under tensile strain, thereby obtaining high mechanical properties. As the AgNW area fraction increases, the tensile modulus constantly increases (7.76 GPa at 1.0 g/m2), while the tensile strength increases first before 0.4 g/m2 and then decreases (242.3 MPa at 1.0 g/m2). The decrease of tensile strength at higher area fraction may be ascribed to the stress concentration induced by the aggregation of AgNWs. Nevertheless, the tensile strength of the nanocomposite papers is much higher than that of the pure ANF papers. The excellent heat resistance and mechanical properties of the ANF/AgNW nanocomposite papers will endow the flexible electrical heaters with longer service life especially in harsh environments in areas such as aerospace and the military. Heating Performances of ANF/AgNW Nanocomposite Papers. The ultraflexible, heat-resistant, and mechanically strong ANF/AgNW nanocomposite papers with low sheet resistance can work as high-performance electrical heaters based on the Joule heating effect. Figure 5a,b and Figure S5 (Supporting Information) show the time-dependent surface temperatures of the ANF/AgNW nanocomposite paper-based electrical heaters with different AgNW area fractions and supplied voltages. The steady-state saturation temperature (Ts) in all cases increases with the increasing supplied voltage as more Joule heat power is generated from the electrical heaters. At higher area fraction, which means a larger AgNW density and a more effective conductive network, a lower voltage U is required to obtain similar surface temperatures. For instance, a Ts of ∼103.5 °C is obtained at 5 V at a AgNW area fraction of 0.15 g/m2, whereas a similar Ts of ∼100.2 °C is obtained at only 2 V at a AgNW area fraction of 0.3 g/m2. Furthermore, a Ts of ∼104.3 °C is obtained at an even lower voltage of 1.5 V when the area fraction is increased to 0.5 g/m2. At 2.5 V, the surface temperature of the electrical heaters with an area fraction of 0.5 g/m2 quickly reaches 200 °C within 20 s (Movie S3). The electrical heaters with an area fraction of 0.1 g/m2 (Rs = 390 Ω/sq) present a relatively low surface temperature of ∼42.5 °C at 5 V, while those with an area fraction of 1.0 g/m2 (Rs = 0.12 Ω/sq) present a high surface temperature of ∼260 °C at only 2.5 V. Therefore, a lower sheet resistance will allow

U2 dT = mc + hA(T − T0) R dt

(1)

where U is the supplied voltage, R is the resistance of the electrical heaters, m is the mass of the heater, c is specific heat capacity, h is the convective heat-transfer coefficient, A is the area of the electrical heater, T is the surface temperature of the electrical heater, and T0 is the initial ambient temperature, respectively. By integrating eq 1 and considering that T(t = 0) = T0, the time-dependent surface temperature and saturation temperature (Ts) of the flexible electrical heaters could be acquired, respectively: T = T0 +

U2 (1 − e−(hA / mc)t ) RhA

(2)

Ts = T0 +

U2 RhA

(3)

As can be seen, the saturation temperature of the electrical heater is determined by the square of the supplied voltage, resistance, and convective heat-transfer coefficient of the electrical heaters. The experimental data (extracted from Figure 5a,b and Figure S5a−e in the Supporting Information) and the linear fitting of saturation temperature versus U2 are presented in Figure 5c. Note that the saturation temperature of the electrical heaters with various AgNW area fractions exhibits an excellent linear relationship with the square of the supplied voltage, demonstrating the accuracy of the theoretical prediction of saturation temperature at various supplied voltages, which is in agreement with that in previous studies.11,24 At a given supplied voltage, the electrical heaters with higher area fractions exhibit increased saturation temperatures owing to the decreased sheet resistance. According to eq 2, the time-dependent surface temperature of the electrical heaters increases as the specific heat capacity c decreases for a given input power. Nevertheless, the saturation temperature shows no dependence on the specific heat capacity, as can be derived from eq 3.25 For the electrical heaters with a constant thickness and aspect ratio, the higher convective heat-transfer coefficient h and larger area A result in a lower heating rate and saturation temperature.20,59 Furthermore, the electrical heaters can be rapidly heated from room temperature to the saturation temperature within 10−30 s, demonstrating the rapid response of the flexible heaters. The above results indicate that the heating performance of the ANF/AgNW nanocomposite paper-based electrical heaters could be tuned by simply controlling the AgNW area fraction or supplied voltage. Figure 5d shows the tailored surface temperatures of the electrical heaters upon the gradiently changed voltages for AgNW area fractions of 0.15 and 0.3 g/m2, respectively. When the supplied 7584

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Figure 6. IR camera images of the electrical heaters upon repeated bending and stretching (a), with the irregular shape of the letter “C” (b), and upon vapor (c), solvent (d), and water (e) treatments.

Figure 7. (a) Digital and IR camera images of the electrical heaters in wearable thermotherapy with supplied voltages of 0.5, 1, and 1.5 V. (b) Digital and IR camera images of the electrical heaters in personal thermal management. (c) IR camera images of the electrical heaters affixed to human hands. (d) Digital images of the electrical heaters in water heating with a voltage of 1.5 V. 7585

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ACS Nano voltage is gradiently increased or decreased, the flexible heater immediately responded with higher or lower surface temperature due to the highly responsive power generated by the passing current, indicating that the flexible electrical heaters have great potential for electronic applications in heating devices with intelligently controlled temperatures.60−62 The long-term time-dependent heating temperatures upon the constant and repeated supplied voltages were performed respectively to evaluate the heating reliability, stability, and repeatability of the ANF/AgNW nanocomposite paper-based electrical heaters. Figure 5e and Figure S6 (Supporting Information) show the long-term heating performances of the electrical heaters at a constant voltage of 2 V. The electrical heaters show a very stable surface temperature of around 104 °C due to the constant passing current within the duration longer than 2 h, confirming the good long-term heating reliability of the electrical heaters owing to the protective effect of the densely wrapped ANFs around AgNWs. Figure 5f and Figure S7 (Supporting Information) show the steadily ascending and descending surface temperatures upon the repeated supplied voltage and current for 120 cycles and 2 h, demonstrating the excellent heating stability and sufficient repeatability of the electrical heaters. As illustrated in Figure 5g, the surface temperature of the electrical heaters increases immediately due to the generated Joule heat upon the supplied voltage through inelastic collisions between accelerated electrons and phonons and decreases immediately due to the absent Joule heat upon the released voltage. Figure 6a shows the IR camera images of the flexible electrical heaters upon repeated bending (radius of curvature: about 3 mm) and stretching, indicating a uniform temperature distribution even during the mechanical bending. Figure 6b shows the heating performance of the electrical heaters with an irregular shape of the letter “C”, demonstrating that the electrical heaters can be used in certain applications requiring patterned shapes. Moreover, the electrical heaters located above a container with boiling water exhibit a stable surface temperature, which is unaffected by the released vapor (Figure 6c). The heating performances of the electrical heaters upon the solvent (ethyl alcohol) and water treatments have also been evaluated, as shown in Figure 6d and e, respectively. When the cool liquid drips onto the electrical heaters, the surface temperature immediately decreases from about 110 °C to a low temperature. Due to the Joule heat generated from the electrical heaters, the liquid drops evaporate quickly and the surface temperature rises to the original values. The outstanding heating performances of the electrical heaters in a harsh environment result from the AgNWs being densely wrapped by the ANFs, which can prevent the permeation of oxygen gas and retard the oxidation of Ag in air, thus leading to the long service time of the electrical heaters in air and humid conditions. The comparison of electrical properties, heating performance, and mechanical properties between the ANF/ AgNW nanocomposite paper-based electrical heaters and the previously reported flexible electrical heaters is provided in Table S1 (Supporting Information). The ultraflexible electrical heaters in this work exhibit preponderant performances in terms of low resistance, low drive voltage, high saturation temperature, high tensile strength, and high tensile modulus. Compared with the previously reported graphene papers (tensile strength of 131 MPa)16 and waterborne polyurethane (WPU)-based nanocomposites (tensile strength of 12.5 MPa and tensile modulus of 0.25 MPa),22 the ANF/AgNW

nanocomposite papers in this work exhibit a much higher tensile strength of 285.7 MPa and tensile modulus of 6.51 GPa as a result of the enhancement effect of AgNWs partially embedded in the ANF substrate and the extensive hydrogenbonding interactions between AgNWs and ANFs. Applications of ANF/AgNW Nanocomposite PaperBased Electrical Heaters. Applications of the ultraflexible, heat-resistant, and mechanically strong ANF/AgNW nanocomposite paper-based electrical heaters in wearable thermotherapy, personal thermal management, and water heating have been demonstrated. In order to evaluate the heating performances, the ultraflexible heaters were attached to different object surfaces and supplied with prescribed voltages. Figure 7a shows the digital and IR camera images of the flexible electrical heaters in wearable thermotherapy with supplied voltages of 0.5, 1, and 1.5 V, respectively. Obviously, the thermal images present a uniform temperature distribution, which is regarded as an important criterion in electrical heaters.63 With a low supplied voltage of 0.5 V, the electrical heaters affixed to the chest and knee are heated from a room temperature of about 20.5 °C to 33.5 and 35 °C, respectively, which lie in the comfortable temperature range for the human body (between 33 and 38 °C).64 Furthermore, the temperatures of the electrical heaters are both elevated to higher than 42 and 60 °C with increased voltages of 1.0 and 1.5 V, respectively. The temperatures are suitable for uses in wearable devices, which usually need Joule-heated temperatures of approximately 41 to 77 °C to relieve pain and stiffness.22 Figure 7b shows digital and IR camera images of the electrical heaters in personal thermal management under cold conditions. The electrical heaters can elevate the temperature from 20.5 °C to 41.2 °C with a supplied voltage 1 V and are suitable for those who work in cold and remote areas. Figure 7c presents IR images of the skin-attached thermal patches for hand warming with comfortable contact. Informed consent was obtained from the human subjects. The wearer could feel the heat transferred from the flexible heater to the back of his hand a few seconds after the voltage was applied. Figure 7d shows digital images of the electrical heater with a surface temperature of 100 °C in water heating with a AgNW area fraction of 0.5 g/m2 at 1.5 V. The electrical heater with a dimension of 3 cm × 2 cm is attached to the bottom of the glass bottle filled with 5 mL of DI water. The real-time temperature of the DI water is measured by a mercury thermometer. Upon the supply of voltage, the heat generated from the electrical heater is efficiently transferred to the water inside. The temperature increases from the initial 28 °C to 58 °C after heating for 10 min and then reaches 84 °C after heating for 30 min. Fog is observed on the upper and inner wall of the glass bottle due to steam evaporation and condensation. The electrical heaters can also be wrapped around the side wall of the glass bottle due to its ultraflexibility. The results indicate that the ultraflexible, heat-resistant, and mechanically strong ANF/AgNW nanocomposite papers in this study have excellent potential for emerging electronic applications such as wearable devices, artificial intelligence, and high-performance heating systems.

CONCLUSION In conclusion, this work demonstrates the fabrication of highperformance and rapid-response electrical heaters based on ultraflexible, heat-resistant, and mechanically strong ANF/ AgNW nanocomposite papers by the facile and efficient two7586

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ACS Nano step vacuum assisted filtration followed by hot-pressing approach. The microstructures, transparency, thermal stability and mechanical properties, electrical and heating properties, and application performances of the ANF/AgNW nanocomposite papers were detailedly investigated. The highly effective conductive AgNW networks partially embedded in the ANF substrate result in a significantly reduced sheet resistance and excellent electrical stability upon repeated bending and washing. The resultant electrical heaters exhibit satisfyingly high heating temperatures with a rapid response at low AgNW area fractions and supplied voltages and possess sufficient heating reliability, stability, and repeatability during long-term and repeated heating and cooling cycles. For example, the electrical heaters with a AgNW area fraction of 0.5 g/m2 show a stable heating temperature higher than 200 °C at 2.5 V. With a higher AgNW area fraction and supplied voltage, the heating temperature increases and the response time decreases owing to more generated Joule heat. In addition, the ultraflexible electrical heaters exhibit outstanding heat resistance and mechanical properties due to the highperformance ANF substrate, partially embedded AgNWs, and extensive hydrogen-bonding interactions. The protective effect of the densely wrapped ANFs around AgNWs endows the electrical heaters with sufficient heating reliability, stability, and repeatability during the long-term and repeated heating and cooling cycles. They also present favorable optical transmittance in the visible and near-infrared region and have a UVshielding effect in the ultraviolet region. Applications of the ANF/AgNW nanocomposite paper-based electrical heaters in wearable thermotherapy, personal thermal management, and water heating have been demonstrated. We believe that this facile and efficient strategy allows the practical fabrication of electrical heaters for emerging electronic applications such as wearable devices, artificial intelligence, and high-performance heating devices.

Subsequently, 20 mL of AgNO3 (0.35 M) and NaBr (2 mM) solution was added into the mixture and magnetically stirred for 5 min; the solution was allowed to stand for 40 min at 170 °C, followed by cooling to room temperature. Finally, the products were adequately washed with acetone and deionized (DI) water by suction filtration two and three times, respectively. The obtained AgNWs were dispersed again in DI water to get a 2.5 mg·mL−1 dispersion. Fabrication of Ultraflexible Electrical Heaters. ANF/AgNW nanocomposite papers were fabricated via the TVAF followed by hotpressing approach. First, the ANF dispersion ultrasonicated for 30 min was vacuum filtrated (Vacuum filtration I) onto a porous nylon membrane for 10 min to get a piece of wet viscoelastic ANF gel. Subsequently, the AgNW dispersion was added into the filter and further filtrated (Vacuum filtration II) for 1 h. Then, the filter membranes were transformed into the hot press machine for hotpressing at 1 MPa and 80 °C for 12 h. Finally, several drops of alcohol were applied onto the bottom side of the ANF papers, and the ANF/ AgNW nanocomposite papers with partially embedded AgNWs could be easily peeled off from the porous membrane. A series of ultraflexible ANF/AgNW nanocomposite papers with AgNW area fractions of 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, and 1.0 g/m2 were fabricated by adding different volumes of AgNW dispersions, respectively. Silver paste was brushed onto the two ends of the heaters and cured at 70 °C for 30 min to act as electrodes. Pure ANF papers without AgNWs were also prepared via a one-step vacuum-assisted filtration for comparison. Characterizations. Morphologies and microstructures of ANFs, AgNWs, and ANF/AgNW nanocomposite papers were observed with a FEI Verios 460 SEM and a Tecnai G2 F20 S-Twin TEM. FTIR spectra were recorded on a Vertex-70 (Bruker, Germany) spectrometer with attenuated total reflectance in the scanning range of 400−4000 cm−1. XPS was conducted using an Axis Supra (Kratos Analytical, U.K.) with a monochromatic Al Kα source (1486.6 eV).65 XRD measurements were performed on a D8 AdvanceX diffractometer using Cu Kα radiation. The four-point probe method was used for the sheet resistance measurement using a 2258C probe instrument (Suzhou Lattice Electronics Co., Ltd.) similar to those in the previous studies.11,24 Thermogravimetric analysis (TGA) was conducted using a TA Q200 thermogravimetric analyzer under a nitrogen atmosphere from room temperature to 900 °C with a heating rate of 10 °C/min. The tensile properties were measured using a universal testing machine (SANS CMT8502) at a rate of 5 mm/ min.66 For the Joule heating investigation, constant or repeatedly or gradiently changed voltages were applied to the electrical heaters with a dimension of 3.2 cm × 2.5 cm using a Princeton 4000+ electrometer.67−69 The surface temperature of the electrical heaters was recorded using a UT300S infrared thermometer (Uni-Trend Technology Co., Ltd.). An infrared thermal imager (Fluke Ti300) was used to determine the heat distribution over the entire ANF/AgNW nanocomposite paper-based heaters using the method reported in a previous study.70

MATERIALS AND METHODS Chemicals and Materials. Bulk Kevlar 49 thread was purchased from DuPont, USA. Chemicals including silver nitrate (AgNO3), ethylene glycol (EG), ferric chloride (FeCl3), copper chloride dihydrate (CuCl2·2H2O), sodium bromide (NaBr), and PVP of analytical grade for preparation of silver nanowires were used as received without further purification. The nylon membranes with a pore size of 0.22 μm were supplied by Shanghai Xin Ya Purification Equipment Co., Ltd., China. The conductive silver adhesives SS5200 were purchased from Shanghai Sunrise Electronic Materials Co., Ltd., China. Preparation of Aramid Nanofiber Dispersions. The ANF dispersions were prepared by deprotonation of macroscopic aramid fabrics according to the previously reported method.47 Specifically, 1.0 g of bulk Kevlar 49 thread was added into 500 mL of DMSO in the presence of 1.5 g of potassium hydroxide. After magnetically stirring for 1 week at room temperature, a homogeneous, viscous, and crimson ANF dispersion with a concentration of 2 mg·mL−1 was obtained. By weakening the hydrogen bonds and strengthening the electrostatic repulsion between the polymer chains, the macroscale aramid fibers can be chemically transformed into nanofibers. The ANFs can be dispersed well in deionized water by ultrasonication to obtain a homogeneous ANF dispersion. Preparation of Silver Nanowires. The AgNWs were synthesized using the polyol method under ambient conditions. Briefly, 0.80 g of PVP (Mw ≈ 1 300 000) was first added into 50 mL of EG and magnetically stirred at 170 °C to ensure the complete dissolution. A 2 mL amount of CuCl2·2H2O (10 mM) and FeCl3 (4 mM) solution was then added to the PVP solution to achieve AgNWs with an ultrahigh aspect ratio. The Cu2+/Fe3+ molar ratio was 5:2.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b00434. SEM image of AgNWs; X-ray diffractograms of AgNWs and ANF/AgNW nanocomposite papers; schematic illustration and SEM images for fabrication of the nanocomposite papers with partially embedded AgNWs; heating performances of the nanocomposite paper-based electrical heaters; performance comparison of the flexible electrical heaters with those reported in the literature (PDF) Movie showing the steady brightness of LED lamps upon repeated bending and stretching (Movie S1) (AVI) Movie showing the water washing and drying processes (Movie S2) (AVI) 7587

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Movie showing the heating temperature of ANF/AgNW nanocomposite papers (Movie S3) (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail (Z. Ma): [email protected]. *E-mail (J. Ma): [email protected]. *E-mail (J. Gu): [email protected]. ORCID

Zhonglei Ma: 0000-0002-3069-1557 Jianzhong Ma: 0000-0003-0512-702X Bin Yang: 0000-0003-3430-0522 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.9b00434 ACS Nano 2019, 13, 7578−7590

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DOI: 10.1021/acsnano.9b00434 ACS Nano 2019, 13, 7578−7590