Flexible Transparent Film Heaters Research Articles

Large-Scale Preparation of Silver Nanowire-Based Flexible Transparent Film Heaters by Slot-Die Coating.

Highly flexible silver nanowire-based transparent conductive films (AgNWs TCFs) were large-scale fabricated by slot-die coating AgNWs inks on a flexible polyethylene terephthalate (PET) substrate, and further fabricated into a transparent film heater. Appropriate flow rate, coating speed, and AgNWs concentration allow the construction of the 15 cm × 15 cm AgNW TCFs with a sheet resistance (Rs) of less than 20 Ω/sq, a transmittance (T) at 550 nm higher than 95%, and a haze less than 3.5%. The resultant AgNW TCFs heater possesses high uniformity and superior mechanical stability and can reach a Joule heating temperature of 104 °C with a voltage of 12 V. The slot-die coating method has great potential for large-scale production of AgNW based film heaters promisingly used in window defrost and deicer systems.

Highly flexible and transparent film heater with electrospun copper conductive network via junction-free structure

Flexible transparent film heaters (FTHs) are used in a variety of applications, including smart windows and wearable devices, because they are transparent to visible light while generating Joule heat. FTHs are designed for high transparency and conductivity, with thermal stability and mechanical durability as secondary requirements. In general, there is a trade-off between transparency and conductivity, such that both factors must be optimized simultaneously to maximize performance. In this study, a conductive nanofiber network was fabricated on a flexible substrate via electrospinning. Copper (Cu) heaters were materialized on the nanofiber network via electroless deposition. Overlap between nanofibers was removed by heat treatment, resulting in a junction-free Cu nanofiber network without contact resistance. The properties of these FTHs could be actively controlled by optimizing the electrospinning and electroless deposition times, yielding a transparency of more than 90% and sheet resistance (RS) of several Ω/sq. As heaters, the optimized FTHs exhibited a uniform temperature distribution over their entire area, even when an external force was applied. The FTH fabrication method developed herein was based on direct electrospinning patterning and did not require any lift-off or transfer processes, which can result in defects. In addition, since electroless deposition has a low process complexity, it can be used in large-area processes and easily extended to fundamental technologies in various fields.

Silver‐Nanowire‐Embedded Photopolymer Films for Transparent Film Heaters with Ultra‐Flexibility, Quick Thermal Response, and Mechanical Reliability

AbstractA novel approach to fabricate mechanically robust and flexible transparent film heaters (TFHs) in which a highly conductive silver nanowire (Ag NWs) network is sandwiched by photo‐curable polymer layers is demonstrated. The photopolymer provides an excellent platform as a substrate and a passivation layer of the solution‐processed Ag NWs network due to its favorable optical and mechanical properties. The flexible TFHs exhibit excellent heat‐generating properties, reaching to >130 °C within 15 s at a driving voltage of 4 V. The unique bilayer structure also provides an important role for protecting the Ag NWs network from external stimuli, such as waterproof properties, physical damages, and ambient moisture, which promises practical wearable, medical, or industrial heating applications.

Ultrathin, flexible transparent Joule heater with fast response time based on single-walled carbon nanotubes/poly(vinyl alcohol) film

Ultrathin flexible transparent film heaters (TFHs) with fast response time are fabricated by embedding single-wall carbon nanotubes (CNTs) into transparent poly(vinyl alcohol) (PVA) film using a green all-water based solution process. Typically, CNTs network was firstly constructed on the surface of commercial polycarbonate film by a continuous hot roll pressing and spraying technique. The obtained CNTs network was then transferred to the surface of ultrathin PVA film using a transfer spinning technique coupled with hot pressing approach. The resulting film not only shows well optical and electrical properties of 475 Ω/aq with a transmittance of 77.3%, but also exhibits strong interfacial adhesion (standing 100 repeated scratches with 3 M sticky tape) and good flexibility (allowing more than 1000 bending cycles). Very interestingly, the fabricated PVA/CNT TFHs presents a quick Joule heating effect with a very fast heating time and rate of 8 s and 11.4 °C/s, respectively, under low impressed voltage (<15 V). These results demonstrate that our TFH is therefore considered suitable for providing anti-fogging or de-icing in optics and optoelectronic devices, as well as for wearable heating systems.

Ultrasonic Modification of Ag Nanowires and Their Applications in Flexible Transparent Film Heaters and SERS Detectors.

Ultrasonic morphology modification of silver (Ag) nanowires and their applications in transparent film heaters for defogging in electric vehicles and surface-enhanced Raman scattering (SERS) detectors have been studied. With 10 min ultrasonic treatment of Ag nanowires, the electro-thermal conversion capability of Ag nanowire based transparent film heaters is efficiently improved (about 50% increase in temperature rise), which can be mainly attributed to the cross-section area reduction and the serious agglomerations of the ultrasonic modified Ag nanowire films. Furthermore, the bending or fracture caused by deformation of Ag nanowires after ultrasonic treatment provides more hot spots for SERS, and therefore lead to a significant SERS signal enhancement. This work not only greatly improves the performance of Ag nanowire based transparent film heaters and SERS detectors, but provides a new way for the functional modification of Ag nanowires.

Flexible transparent film heaters using a ternary composite of silver nanowire, conducting polymer, and conductive oxide.

Scientific and technological advances in transparent conductive electrodes improve the heating performance of flexible transparent film heaters (TFHs), which can be utilized for various applications as defrosters and heaters. To achieve high performance as well as practical TFHs, several conditions, such as high optical transmittance, low electrical resistance, heating uniformity, and operational stability in various environmental conditions should be satisfied. However, due to the trade-offs between optical transmittance and electrical resistance, it is not easy to fulfill all the requirements concurrently. Here we report flexible TFHs using a ternary composite of silver nanowire (AgNW), conducting polymer (i.e., poly[3,4-ethylenedioxythiophene]:polystyrene sulfonate [PEDOT:PSS]), and a thin conductive oxide (i.e., indium tin oxide [ITO]) layer, exhibiting higher performance in terms of the maximum heating temperature (>110 °C), operational stability, mechanical flexibility, and optical transmittance (95% at 550 nm), compared to pristine AgNW-based TFHs. We also demonstrated the stable operation of the AgNW–PEDOT:PSS/ITO TFHs soaked in water, showing excellent environmental stability. To analyse the fundamental mechanisms for the improved performance of the AgNW–PEDOT:PSS/ITO TFHs, we investigated the progress of joule heating using a device simulator, and found that the improvement originated not only from reduced electrical resistance but also from enhanced heat dissipation with PEDOT:PSS and ITO. We anticipate that our analysis and results will be helpful for further development of practical flexible TFHs.

Coupling layer-by-layer assembly and multilayer transfer to fabricate flexible transparent film heater

Flexible transparent film heaters (f-TFH) were prepared on a polysulfone (PSU) film with a multilayer transfer of silver nanowires (AgNWs) coated on glass substrates via layer-by-layer (LBL) deposition. First, as-received AgNWs were functionalized with carboxylic acid (AgNW-COOH) and amine (AgNW-NH2) moieties to obtain negatively and positively charged nanowires, respectively. Second, functionalized AgNWs were sequentially coated on a glass substrate via the LBL deposition, which was followed by subjecting the multilayer film to annealing at 125 °C for 30 min to improve the electrical conductivity. Third, the multilayer was transferred from the glass substrate to the polymer film by coating and detaching the PSU. The multilayer film provided optical transmission of 84% and sheet resistance of 12 Ω/□ with 5 bilayer sample, which is comparable to indium tin oxide (ITO) film. The f-TFH reached maximum temperature of 128 °C at 7 V with a response time of 45 s. Moreover, it exhibited good defrosting capability by applying 7 V for 20 s. Cyclic bending test results indicated that the sheet resistance of the flexible multilayer film does not demonstrate any change until 300 cycles, while adhesion test 3-M tapes exhibited no sheet resistance change even after 20 peel cycles showing the superior performance of the multilayer film. In addition, the film showed stable heating performance at 128 °C for 5 h.

Characteristics of indium zinc oxide/silver/indium zinc oxide multilayer thin films prepared by magnetron sputtering as flexible transparent film heaters

Indium zinc oxide/silver/indium zinc oxide multilayer structures with a very low resistivity and high transmittance were deposited on a polyethylene terephthalate substrate by DC magnetron sputtering. The electrical, optical, mechanical, and thermal properties of the multilayer films were investigated at different Ag layer thicknesses (10–20 nm). The multilayers demonstrated relatively constant resistance change (∆R/ R0) over repeated bending tests with a radius of 8 mm. Efficient Joule heating could increase the operating temperature of the film heaters (7.3 Ω/□) to 73.7 °C in ~20 s at 3 V; moreover, the heat distribution remained uniform after bending tests. A high transmittance (84.7%) at a wavelength of 550 nm and low sheet resistance (7.3 Ω/□) of the optimized multilayer was obtained at an Ag layer thickness of 15 nm. These results indicate that the multilayers are promising as transparent film heaters for next-generation flexible applications.

Hybrid Ag nanowire transparent conductive electrodes with randomly oriented and grid-patterned Ag nanowire networks

To improve the electrical properties of silver nanowire (Ag NW) transparent conductive electrodes (TCEs), the density of Ag NW networks should be increased, to increase the number of percolation paths. However, because of the inverse relationship between optical transmittance and electrical resistivity, the optical properties of Ag NW TCEs deteriorate with increasing density of the Ag NW network. In this study, a hybrid Ag NW electrode composed of randomly oriented and grid-patterned Ag NW networks is demonstrated. The hybrid Ag NW electrodes exhibit significantly improved sheet resistances and slightly decreased transmittances compared to randomly oriented Ag NW networks. Hybrid Ag NW TCEs show excellent mechanical flexibilities and durabilities in bending tests with a 5 mm radius of curvature. Moreover, flexible transparent film heaters (TFHs) based on the hybrid Ag NW electrodes show elevated maximum temperatures relative to TFHs based on randomly oriented Ag NW electrodes, when operated at the same input voltages.

Membrane catalytic heater

Flexible transparent film heaters (FTHs) are used in a variety of applications, including smart windows and wearable devices, because they are transparent to visible light while generating Joule heat. FTHs are designed for high transparency and conductivity, with thermal stability and mechanical durability as secondary requirements. In general, there is a trade-off between transparency and conductivity, such that both factors must be optimized simultaneously to maximize performance. In this study, a conductive nanofiber network was fabricated on a flexible substrate via electrospinning. Copper (Cu) heaters were materialized on the nanofiber network via electroless deposition. Overlap between nanofibers was removed by heat treatment, resulting in a junction-free Cu nanofiber network without contact resistance. The properties of these FTHs could be actively controlled by optimizing the electrospinning and electroless deposition times, yielding a transparency of more than 90% and sheet resistance (RS) of several Ω/sq. As heaters, the optimized FTHs exhibited a uniform temperature distribution over their entire area, even when an external force was applied. The FTH fabrication method developed herein was based on direct electrospinning patterning and did not require any lift-off or transfer processes, which can result in defects. In addition, since electroless deposition has a low process complexity, it can be used in large-area processes and easily extended to fundamental technologies in various fields.

Highly flexible transparent thin film heaters based on silver nanowires and aluminum zinc oxides

In this work, we developed highly flexible transparent film heaters (f-TFHs) composed of Ag nanowire networks (AgNWs) and aluminum zinc oxide (AZO). Uniform AgNWs were roll-to-roll coated on polyethylene terephthalate (PET) substrates using the Mayer rod method, and AZO was sputter-deposited atop the AgNWs at room temperature. The sheet resistance (Rs) and transparency (Topt) of the AZO-coated AgNWs changed only slightly compared with the uncoated AgNWs. AZO is thermally less conductive than the heat pipes, but increases the thermal efficiency of the heaters blocking the heat convection through the air. Based on Joule heating, a higher average film temperature (Tave) is attained at a fixed electric potential drop between electrodes (ϕ) as the Rs of the film decreases. Our experimental results revealed that Tave of the hybrid f-TFH is higher than AgNWs when the ratio of the area coverage of AgNWs to AZO is over a certain value. When a ϕ as low as 3V/cm was applied to 5cm×5cm f-TFHs, the maximum temperature of the hybrid film was over 100°C, which is greater than that of AgNWs by more than 30°C. Furthermore, uniform heating throughout the surfaces is achieved in the hybrid films while heating begins in small areas where densities of the nanowires (NWs) are the highest in the bare network. The non-uniform heating decreases the lifetime of f-TFHs by forming hot spots. Cyclic bending test results indicated that the hybrid films were as flexible as the AgNWs, and the Rs of the hybrid films changes only slightly until 5000cycles. Combined with the high-throughput coating technology presented here, the hybrid films will provide a robust and scalable strategy for large-area f-TFHs with highly enhanced performance.

Highly thermal conductivity and infrared emissivity of flexible transparent film heaters utilizing silver-decorated carbon nanomaterials as fillers

A flexible transparent film heater using functionalized few-walled carbon nanotubes and graphene nanosheets decorated with silver nanoparticles as fillers and poly(3,4-ethylenedioxythiophene)- poly(4-stryrenesulfonate) (PEDOT:PSS) as a dispersant possesses excellent optoelectronic and electrothermal properties. The film possesses a low sheet resistance of 53.0 ± 4.2 ohm · sq−1, a transmittance of 80.2 ± 0.8% at a wavelength of 550 nm, a high thermal conductivity of 142.0 ± 9.6 W · m−1 · K−1, a quick response time of less than 60 s, stable heating performance, good reliability, low power consumption, flexibility, and uniform heat diffusion. Besides, the film shows an average infrared emissivity of 0.53 in the wavelength range of 4 to 14 μm, which shows an outstanding heat release performance by radiation. The flexible transparent film heaters adopting graphene and carbon nanotubes as fillers boast excellent electrothermal performance through heat conduction and infrared radiation, suggesting that they are good substitutes for traditional metallic and indium tin oxide film heaters.

Highly flexible transparent film heaters based on random networks of silver nanowires

We demonstrate a new concept for the fabrication of flexible transparent thin film heaters based on silver nanowires. Thanks to the intrinsic properties of random networks of metallic nanowires, it is possible to combine bendability, transparency and high heating performances at low voltage, typically below 12 V which is of interest for many applications. This is currently not possible with transparent conductive oxide technologies, and it compares well with similar devices fabricated with carbon nanotubes or graphene. We present experiments on glass and poly(ethylene naphthalate) (PEN) substrates (with thicknesses of 125 μm and extremely thin 1.3 μm) with excellent heating performances. We point out that the amount of silver necessary to realize the transparent heaters is very low and we also present preliminary results showing that this material can be efficiently used to fabricate photochromic displays. To our knowledge, this is the first report of metallic nanowire-based transparent thin film heaters. We think these results could be a useful approach for the engineering of highly flexible and transparent heaters which are not attainable by existing processes. Open image in new window