Flexible Heater Research Articles

High‐Performance Electrothermal Film Based on Laser‐Induced Graphene

Herein, flexible and robust heaters are proposed based on laser‐induced graphene (LIG) fabricated photothermally on polyimide (PI) substrates. The heaters are precisely defined by a computer numerical control (CNC) platform‐driven laser diode with a one‐step process. The electrothermal performance of the heaters can be adjusted by tuning the laser driving speed in a certain range under a given input power. The fabricated heater shows a high saturation temperature (270 °C), the fast response time (10 s), and satisfactory convective heat‐transfer coefficient (42 W m−2 °C−1). Simultaneously, the developed heaters demonstrate excellent flexibility, robust adhesion, and good long‐time stability. The experimental results pave the way for high‐performance, environmentally friendly, robust, and flexible heaters.

Superior Stable, Highly Efficient, and Anisotropic Electrothermal Composite Heater in Various Directions with Fast Response Based on Aligned Carbon Nanotube Sheets

Flexible electrothermal heaters have a great potential for applications of warm‐keeping and thermotherapy. However, those common electrothermal heaters are so sensitive to strain that they are hard to resistant to deformation while still keeping the stable and outstanding electrothermal performance. Here, a flexible and elastic composite heater with a stable electrothermal property under varied strain with large deformation is developed by mimicking the wrinkle feature of human skin in a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) bilayered structure. The created wrinkle feature of CNT film from the resilience of PDMS makes the electrothermal heater a stable electrothermal performance within 30–40% strain in different directions. The aligned preference of CNTs in the CNT film gives the electrothermal heater an ability that shows different electrothermal performance depending on different angles between the direction of CNT alignment and the long axis of the sample. This composite electrothermal heater also presents a highly efficient electrothermal performance that can reach 200 °C within 30s upon 2 V.

Polyethylene/carbon fiber composites reinforced by a non-covalent compatibilization approach for flexible electric heater and structural self-monitoring

The rational modulation for the interfaces of polymer/filler composites is the key to maximizing its mechanical property and functional performance. In this work, the interface of recycled carbon fiber (CF) enhanced linear low-density polyethylene (LLDPE) composites was modulated via graphene oxide (GO) bridging and alkyl group modification to increase the interface roughness and improve the compatibility. The optimized composite displays improved mechanical, thermal, and electrical properties due to the modulated interface synergistic effect, particularly as a flexible heater, the composites film displayed excellent electrical heating behavior at a fast speed of heating up in 50 s to a steady-state temperature of 40 °C under the low voltage of 5 V. In addition, the structural self-monitoring of this conductive composite could be reflected by the electrical resistance change based on the results of finite element analysis (FEA) and in-situ resistance-tensile measurements. This facile compatibilization approach and in-situ self-monitoring method provide a new route for expanding the application of functional polyethylene materials.

Flexible transparent Ag nanowire/UV-curable resin heaters with ultra-flexibility, high transparency, quick thermal response, and mechanical reliability

The non-fast thermal response and unstable mechanical properties of flexible transparent heaters (FTHs) make it difficult for them to meet the requirements for wearable electronics. In this paper, the influences of heater thickness and resistance of Ag nanowire networks, on the thermal response, optical and mechanical flexible properties are investigated systematically. The relationships between the mechanisms behind the thermal response (bending behaviors) and the thickness, which is important for enhancing the thermal and flexible performances of FTH, are explored. The optimized Ag NW/UV-curable resin -based FTHs, with a thickness of 2 µm and sheet resistance of 9.9 Ω/□, exhibit a quick thermal response time within 12 s, a high optical transmittance of 86.4% and superior mechanical properties. The resistance of the FTH shows almost no change after bending 1000 times at a curvature radius of 0.5 mm. The fast-heating characteristics associated with its transparency and flexibility make the Ag NW/UV-curable resin -based FTHs a potential candidate in wearable electronics.

The Flexible and Transparent Film Heaters Based on Regenerated Cellulose and Carbon Nanotubes

The flexible and transparent film heaters (FTFHs) with the advantages of mechanical flexibility, portability, and excellent electrothermal performance, are key to the next generation portable, wearable heaters and thermal protection systems. However, the present flexible and transparent substrates of FTFHs are fabricated by typical plastic PET films, which suffer from poor interfacial adhesion with the thermally conductive materials. In this work, the transparent regenerated cellulose fibers made of completely dissolved in NMMO solution followed by regeneration process is presented to disperse and support carbon nanotubes (CNTs) by a vacuum-dewatering process. In the presence of cellulose fibers, these CNTs have strong hydrogen bonding properties in the dehydration-deposition process and thus respond to tight intertwining structures in fibrous composites. The resulting regenerated cellulose fibers exhibit high optical transparency of 88% (@550 nm) and good mechanical properties (30 MPa). Interestingly, the FTFHs show a rapid heating response, high generation temperature, and resistance stability for up to 2 h. The FTFHs made with earth-abundant, cost-effective, and recyclable materials, have excellent potential in the areas of green flexible and transparent film heaters.

Charge-assisted coating of silver nanowire transparent conductive layer and application to flexible heater

Transparent conductive layer (TCL) were fabricated from flexible silver nanowires (AgNWs) using charge-assisted coating (CAC) and were subsequently applied in a transparent flexible heater (TFH). The CAC process is a simple and inexpensive process of immersing a substrate in an aqueous solution of charged species and assembling them on the substrate via electrostatic attraction. The AgNWs were synthesized by the polyol method, functionalized with 3-mercaptopropionic acid, and assembled on a polyethylene terephthalate substrate. The optical transmittance of the TCL was controlled by regulating the AgNW solution concentration. The fabricated films were post-processed by calendering. The calendering post-treatment decreased the sheet resistance and increased the structural stability of the AgNW TCL. When the concentration of the AgNW solution was 50 µg/mL, the fabricated AgNW layer exhibited the highest figure of merit possessing a sheet resistance of 8.2 Ω/sq and a transmittance of 82.7%. With a protective layer of polyurethane, AgNW TFH showed a heating temperature of 110°C at bias voltage of 5 V. The dehumidification performance of the TFH was also successfully demonstrated.

Flexible stretchable electrothermally/photothermally dual-driven heaters from nano-embedded hierarchical CuxS-Coated PET fabrics for all-weather wearable thermal management

The multifunctional photoelectronic devices are recently attracting much more attention due to their potential enlarged applications. The flexible stretchable electrothermally/photothermally dual-driven heaters for all-weather wearable thermal management are presented in this work with nano-embedded hierarchical CuxS-coated PET fabrics. Herein, the hierarchical nano-embedded CuxS film is fabricated via a simple chemical bath method for high electrical conductivity and highly efficient inelastic collision of electro/photo-generated carriers. The hierarchical nano-embedded CuxS morphology produces the low sheet resistance of 1.26 Ω sq-1 and the super low total heat transfer coefficient of 3.256 × 10-5 W/oC·mm2, which lead to the high-efficient electro/photo-dual-driven heating effect in the CuxS@PET fabrics. The saturated temperature on the as-fabricated flexible wearable heaters reaches up to 172 °C. The thermal conversion devices also bear the excellent stability, reproducibility, stretchability, controllability and corrosion-resistant characteristics. Interestingly, their excellent thermal conversion performance could be achieved by freely exchanging the driving power sources, such as electricity-supplying equipment, 635-nm laser, infrared physiotherapy lamp and solar simulator, which provides a necessary precondition for the all-weather applications of flexible wearable heaters. The as-fabricated electro/photo-dual-driven heaters on the CuxS@PET fabrics have the promising applications in wearable electronics, all-weather self-heating facilities, out/in-vivo physiotherapy, and so on.

Highly sandwich-structured silver nanowire hybrid transparent conductive films for flexible transparent heater applications

Hybridization with other materials has been an effective method to improve the photoelectric performance and stability of silver nanowire (AgNW) based transparent conductive films (TCFs). In the work, hybrid TCFs with a sandwiched structure composed of MXene, AgNW, and graphene were proposed. MXene sheets in the bottom acted as the intermediate layer between substrates and AgNWs could significantly improve the adhesion, graphene could be used as protective layer, and both filled the AgNW network to improve conductivity and flatness. As a result, the TCF displayed good photoelectric performance (14.4 Ω/sq with 87.5% transmittance at 550 nm), low surface roughness, enhanced adhesion and stability. Furthermore, transparent heaters (THs) were fabricated with the MXene/AgNW/graphene hybrid TCFs. The THs exhibited uniform heating distribution, fast thermal response, good repeatability and long-term working stability. Therefore, flexible THs proposed are expected to be widely used in defogging systems, smart windows, and other flexible electronic devices.

Transparent and flexible heaters using plasma polymer fluorocarbon/silver bilayer thin films

In this study, a transparent and flexible heater is fabricated using plasma polymer fluorocarbon (PPFC)/silver (Ag) bilayer thin films. Compared with polymer/metal/polymer trilayer thin film structures, the polymer/metal bilayer thin film structure allows a simple process while improving the optical transmittance in the visible light range. The optimized PPFC/Ag bilayer thin film demonstrates an optical transmittance of 0.80 at a wavelength of 440 nm, a sheet resistance of 6.67 Ω·sq–1, and a deionized water contact angle of 103.44°. Results from various mechanical flexibility tests indicate that the heater can be used in flexible and rollable devices without severe performance degradation. This study provides insights into the use of transparent and flexible heaters in the exterior walls of buildings, flexible devices, and automobile glasses for preventing frosting and fogging.

MODERNIZATION OF EQUIPMENT FOR PRODUCTION OF CHEESE PRODUCT

The article substantiates the modernization of the apparatus for melting cheese masses, which differs in the method of heat supply of working tanks by replacing the steam jacket for heating with a flexible film resistive electric heater of the radiating type (GPREnVT). The heat transfer surface of the working tank is increased in the apparatus due to the heating of the mixing device. The advanced installation is characterized by the use of secondary thermal energy of melting of cheese masses (35… 95°C) by converting its Peltier elements into low-voltage supply voltage of autonomous fans (3.5…12 W) to cool the control unit. This will increase the efficiency of the proposed design, due to the reduction of dimensional and weight parameters of the apparatus for melting cheese masses by replacing the steam method of heating with electric. The reduction of the duration of the equipment output to the stationary mode (85 ° С) during melting of cheese masses was experimentally confirmed: for the bowl of the research apparatus - 575 s, in comparison with the analogue – 725 s. This scientific work confirms the reduction of the duration of entering the stationary mode by 2% compared to the basic device B6-OPE-400. It is proved that the main indicator of resource efficiency of specific energy consumption for heating the volume of a unit of product has decreased 1.2 times. In the modernized apparatus for melting cheese masses, it is equal to 3037,2 kJ/kg, and the indicator of the basic version B6-OPE-400 – 3672.5 kJ/kg. According to the obtained results, there is an increase in resource efficiency, which is achieved by: elimination of steam heating networks; increasing the heat transfer surface of the working bowls by heating the stirrer with a flexible film resistive electric heater of the radiating type. Heat dissipation of the heater improves the performance of the temperature stabilization system in the bowl of the cheese melter. The obtained results can be used in the design of thermal equipment with electric heating in the use of secondary thermal energy.

A wearable gamma radiation-responsive granulocyte colony-stimulating factor microneedle system protecting against ionizing radiation-induced injury

Exposure to a nuclear accident or a radiological attack may cause serious death events due to ionizing radiation-induced injury and acute radiation syndrome (ARS). Recombinant human granulocyte colony-stimulating factor (G-CSF) is now used for the treatment of ARS. However, the current injection formulation might not ensure treatment as early as possible after a nuclear accident, resulting in a decrease in therapeutic efficiency. In the present study, we have developed a G-CSF wearable system (GWS) consisting of a commercial microchip, a temperature sensor, a gamma-ray detection sensor, a flexible heater, and a G-CSF temperature-sensitive microneedle (GTSMN) patch. G-CSF-containing hyaluronic acid solutions were cast into the mold to obtain G-CSF microneedles (GMNs), which were coated with a temperature-sensitive layer of dodecanoic acid-cetylamine salt to obtain GTSMNs. The flexible heater was prepared by jet printing Ag nanoparticle inks. The GWS and its components are explored and optimized in the aspects of electronics, mechanics, heat transfer and drug diffusion. The γ radiation signal is sensitively monitored by the GWS. The wearable G-CSF system immediately releases G-CSF into the body in response to signal feedback and provides maximal protection against ionizing radiation-induced injury. Therefore, the GWS is a promising wearable system against emergent ionizing radiation injury. Statement of significanceIonizing radiation-induced injury is always the very important public health problem all the global people care. Some medicines have been applied to protect the body from the injury. Unfortunately, sometimes the injuries accidently happen and the medicines cannot be administered in time, leading to serious acute radiation syndrome. Here, we design a wearable system loading G-CSF that has been approved by FDA to protect the body from ionizing radiation-induced injury. This system consists of a commercial microchip, a temperature sensor, a Gamma-ray detection sensor, a flexible heater, and a G-CSF temperature-sensitive microneedle patch. It can monitor γ radiation and immediately release G-CSF into the body to protect the body to the maximal extent. Therefore, the system is a promising wearable medical device against emergent ionizing radiation injury.

High-Temperature Flexible Transparent Heater for Rapid Thermal Annealing of Thin Films

Rapid heating is highly sought-after for compositional control in the sintering-annealing process of materials containing volatile elements. Although several innovative approaches have been reported, suitable processes for the rapid annealing of thin films are still very rare. Herein, we develop a high-temperature rapid heating method for thin-film preparation through employing an indium tin oxide (ITO)/mica-based flexible transparent heater (FTH). The ITO/mica FTH shows an extremely fast thermal response (2 s), high saturation temperature (>830 \ifmmode^\circ\else\textdegree\fi{}C), and high heating-cooling rates (>${10}^{4}$ \ifmmode^\circ\else\textdegree\fi{}C ${\mathrm{min}}^{\ensuremath{-}1}$). Thermodynamic analysis reveals that the excellent heating performance can be largely attributed to the high heat-transfer coefficient and the low thickness of the mica substrates. The remarkable potential of our ITO/mica FTH for practical use is unambiguously evident by the demonstration of water heating and rapid thermal annealing of ferroelectric oxide thin films.

Dry-transferred single-walled carbon nanotube thin films for flexible and transparent heaters

Single-walled carbon nanotubes (SWCNTs) are considered to be ideal candidates for flexible and transparent heaters (THs). However, the conventional wet-fabrication of SWCNT thin films makes the performance of SWCNT THs fail to meet the expectation. Herein, we synthesized high-quality SWCNT thin films by floating catalyst chemical vapor deposition (FCCVD), allowing the dry and direct process of transferring films on polyethylene terephthalate (PET) as THs. A pristine SWCNT TH with a transmittance of 73.5% displayed a reasonable heating performance, reaching the saturated temperatures of ∼53 and 85 °C at the driven voltages of 7 V and 12 V, respectively. Nevertheless, after doping the identical SWCNT film by AuCl3, it rises to ∼70 and 139 °C at 7 V and 12 V, respectively. We held the point that the improved heating performance was mainly attributed to the reduced sheet resistance and film densification by AuCl3 doping. We also found our SWCNT TH showed excellent thermal stability and mechanical flexibility. Due to the continuous synthesis and facile transfer of FCCVD, our SWCNT THs demonstrate potential applications in modern technologies, such as smart windows and defoggers.

Thermal Management of Serpentine Flexible Heater Based on the Orthotropic Heat Conduction Model.

Flexible heaters can perfectly fit with undevelopable surfaces for heating in many practical applications such as thermotherapy, defogging/deicing systems and warming garments. Considering the requirement for stretchability in a flexible heater, certain spacing needs to be retained between serpentine heat sources for deformation which will inevitably bring critical challenges to the thermal uniformity. In order to reconcile these two conflicting aspects, a novel method is proposed by embedding the serpentine heat source in orthotropic layers to achieve comprehensive performance in stretchability and uniform heating. Such a scheme takes advantage of the ability of orthotropic material to control the heat flow distribution via orthotropic thermal conductivity. In this paper, an analytical heat conduction model with orthotropic substrate and encapsulation is calculated using Fourier cosine transform, which is validated by finite element analysis (FEA). Meanwhile, the effects of the orthotropic substrate or encapsulation with different ratios of thermal conductivity and the geometric spacing on the thermal properties are investigated, which can help guide the design and fabrication of flexible heaters to achieve the goal of uniform heating.

Transparent and flexible passivation of MoS2/Ag nanowire with sputtered polytetrafluoroethylene film for high performance flexible heaters

We demonstrated highly transparent and flexible polytetrafluoroethylene (PTFE) passivation for the MoS2/Ag nanowire (Ag NW) electrodes used in thin film heaters (TFHs). The electrical, optical, and mechanical properties of PTFE coated MoS2/Ag NW electrode were compared to the bare MoS2/Ag NW electrode to demonstrate effective passivation of the sputtered PTFE films before and after the 85 °C–85% temperature-relative humidity environment test. In addition, we investigated the performances of TFHs with PTFE/MoS2/Ag NW as a function of PTFE thickness from 50 to 200 nm. The saturation temperature (87.3 °C) of TFHs with PTFE/MoS2/Ag NW electrode is higher than that (61.3 °C) of TFHs with bare MoS2/Ag NW, even after the 85 °C–85% temperature-relative humidity environment test, due to effective passivation of the PTFE layer. This indicates that transparent PTFE film prepared by sputtering process provides effective thin film passivation for the two-dimensional (2D) MoS2 and Ag NW hybrid electrode against harsh environment condition.

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.

Simultaneous Enhancement in Visible Transparency and Electrical Conductivity via the Physicochemical Alterations of Ultrathin-Silver-Film-Based Transparent Electrodes.

A methodology for the simultaneous modulation of the chemical and physical states of an amorphous TiOx layer surface and its impact on the subsequent deposition of a polycrystalline Ag layer are presented. The smoothened TiOx layer surface comprising chemically altered, oxygen-deficient states serves as a nucleating platform for Ag deposition, facilitating a marked increase (∼75%) in the nucleation number density, which strongly enhances the wettability of ultrathin Ag layers. The physically smoothened TiOx/Ag interface further reduces the optical and electrical losses. When the proposed methodology is applied to TiOx/Ag/ZnO transparent conductive electrodes (TCEs), exceptional TCE properties are yielded owing to the simultaneous improvement in visible transparency and electrical conductivity; specifically, a record-high 0.22 Ω-1 Haacke figure of merit is realized. TCEs are prepared on flexible substrates to verify their applicability as stand-alone flexible transparent heaters and as integrated heaters within electrochromic devices to enhance color-switching reactions.

A Healable Resistive Heater as a Stimuli-Providing System in Self-Healing Soft Robots

Self-healing polymers can address the damage susceptibility in soft robotics. However, in most cases, their healing requires a heat stimulus, provided by an external device. This letter presents a self-healing soft actuator with an integrated healable flexible heater, functioning as the stimuli-providing system. The actuator is constructed out of thermoreversible elastomers that are crosslinked by the Diels-Alder (DA) reaction, which provides the healing ability. The heater is manufactured from a DA-based composite network filled with 20 wt% carbon black to provide electrically conductive properties for resistive Joule heating. The flexibility of the heater does not compromise the actuator performance upon integration and the self-healing properties of both heater and actuator allow for damage repair. This includes very large damages, as both heater and actuator can recover (near 100%) from being cut completely in two pieces, using Joule heating at 90°C with a bias voltage of about 30 V. The embedded heater avoids the need for external intervention in the healing process, and provides healing quality assessment and a healing on-demand mechanism, paving the way for an optimum healing solution of damage resilient soft robots that require heat as a healing stimulus.

Flexible, Air-Stable, High-Performance Heaters Based on Nanoscale-Thick Graphite Films.

Graphite sheets are known to exhibit remarkable performance in applications such as heating panels and critical elements of thermal management systems. Industrial-scale production of graphite films relies on high-temperature treatment of polymers or calendering of graphite flakes; however, these methods are limited to obtaining micrometer-scale thicknesses. Herein, we report the fabrication of a flexible and power-efficient cm2-scaled heater based on a polycrystalline nanoscale-thick graphite film (NGF, ∼100 nm thick) grown by chemical vapor deposition. The stability of these NGF heaters (operational in air over the range 30-300 °C) is demonstrated by a 12-day continuous heating test, at 215 °C. The NGF exhibits a fast switching response and attains a steady peak temperature of 300 °C at a driving bias of 7.8 V (power density of 1.1 W/cm2). This excellent heating performance is attributed to the structural characteristics of the NGF, which contains well-distributed wrinkles and micrometer-wide few-layer graphene domains (characterized using conductive imaging and finite element methods, respectively). The efficiency and flexibility of the NGF device are exemplified by externally heating a 2000 μm-thick Pyrex glass vial and bringing 5 mL of water to a temperature of 96 °C (at 2.4 W/cm2). Overall, the NGF could become an excellent active material for ultrathin, flexible, and sustainable heating panels that operate at low power.

Laser-induced graphene via the far-infrared irradiation of polyimide films for flexible electric heater applications

Laser-induced graphene via the far-infrared irradiation of polyimide films for flexible electric heater applications