Flexible Heater Research Articles

Role of graphene towards long-term stability of flexible heaters made of graphene-coated silver nanowire networks under repeated deformation

Role of graphene towards long-term stability of flexible heaters made of graphene-coated silver nanowire networks under repeated deformation

More comfort in healthcare

Silga (Castelfidardo, Italy) offers flexible heaters that are based on printed electronics. This technology is a possible game changer in the healthcare industry.

A Novel Variable Stiffness and Tunable Bending Shape Soft Robotic Finger Based on Thermoresponsive Polymers

In this study, a soft robotic finger with variable stiffness and tunable bending shape capability is proposed. The finger employs shape memory polymers (SMP) as the variable stiffness element and polyimide (PI) electrothermal films as the flexible heaters for segmental heating. SMP and PI electrothermal films constitute the variable stiffness structure. The SMP part exhibits a reversible transition from glass state to rubber state during the heating/cooling process. The whole transition process results in an elastic modulus range of three orders of magnitude. The finger can obtain multiple bending shapes and adapt to the outer contours of diverse objects because of the segmental stiffness modulation capability of the variable stiffness structure. Supercoiled polymer artificial muscles (SCPAMs) are used as actuators for the finger. SMPs and SCPAMs are thermoresponsive and can be activated by thermal heating with electricity, making the whole finger structure compact. Experimental results show that the stiffness of the finger at room temperature (25 °C) is 10.54 times that at 70 °C when the actuators are on. The performance of the finger is further validated via the sequential motion demonstration and grasping tests with variable stiffness and bending shape control capabilities, showing the proposed design’s potential in soft grippers and robotic design.

Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO2 for Flexible Gas Sensor Application.

Rapid and on-site detection of nitrogen dioxide (NO2) is important for environmental monitoring as NO2 is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO2 sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO2 sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(Rgas - Rair)/Rair × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO2 at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(Rair - Rgas)/Rair × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO2. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O2- and O-) and nitrate species (NO3- and NO32-).

Continuous Patterning of Silver Nanowire-Polyvinylpyrrolidone Composite Transparent Conductive Film by a Roll-to-Roll Selective Calendering Process

The roll-to-roll (R2R) continuous patterning of silver nanowire-polyvinylpyrrolidone (Ag NW-PVP) composite transparent conductive film (cTCF) is demonstrated in this work by means of slot-die coating followed by selective calendering. The Ag NWs were synthesized by the polyol method, and adequately washed to leave an appropriate amount of PVP to act as a capping agent and dispersant. The as-coated Ag NW-PVP composite film had low electronic conductivity due to the lack of percolation path, which was greatly improved by the calendering process. Moreover, the dispersion of Ag NWs was analyzed with addition of PVP in terms of density and molecular weight. The excellent dispersion led to uniform distribution of Ag NWs in a cTCF. The continuous patterning was conducted using an embossed pattern roll to perform selective calendering. To evaluate the capability of the calendering process, various line widths and spacing patterns were investigated. The minimum pattern dimensions achievable were determined to be a line width of 0.1 mm and a line spacing of 1 mm. Finally, continuous patterning using selective calendering was applied to the fabrication of a flexible heater and a resistive touch sensing panel as flexible electronic devices to demonstrate its versatility.

Flexible Heater Fabrication Using Amino Acid-Based Ink and Laser-Direct Writing.

Nature's systems have evolved over a long period to operate efficiently, and this provides hints for metal nanoparticle synthesis, including the enhancement, efficient generation, and transport of electrons toward metal ions for nanoparticle synthesis. The organic material-based ink composed of the natural materials used in this study requires low laser power for sintering compared to conventional nanoparticle ink sintering. This suggests applicability in various and sophisticated pattern fabrication applications without incurring substrate damage. An efficient electron transfer mechanism between amino acids (e.g., tryptophan) enables silver patterning on flexible polymer substrates (e.g., PET) by laser-direct writing. The reduction of silver ions to nanoparticles was induced and sintered by simultaneous photo/thermalchemical reactions on substrates. Furthermore, it was possible to fabricate a stable, transparent, and flexible heater that operates under mechanical deformation.

Near-Zero Transmittance Loss, Highly Durable, Flexible, Transparent Electrode with an Ultrathin Ag Nanoporous Structure

Recent advances in state-of-the-art optoelectrical and photovoltaic devices have necessitated further improvements in the efficiency of transparent electrodes by simultaneously minimizing electrical and optical losses, which remain technically challenging to achieve. In this study, we developed a strategy for constructing a highly efficient transparent electrode that retains near-perfect optical transparency at tunable wavelengths in the visible spectral range and has excellent electrical reliability against heavy processing loading. The optoelectrical features were achieved by integrating an ultrathin Ag nanoporous layer and a substoichiometric electron-transferring SiOx film in a multilayered oxide/metal/oxide configuration. Complete sealing of the Ag nanoporous geometry by a subsequently coated SiOx film facilitates near-zero optical transmittance loss (<1%, averaged at 0.7%) over a wide spectral range of 500–700 nm at a near-bulk resistivity of 7.2 × 10–8 Ω m while ensuring extreme durability against chemical corrosion, electrical/thermal degradation, and mechanical deformation. The unexpected performance of such a defective Ag-layered geometry, which has rarely been considered in the design of transparent electrodes, refutes conventional approaches that emphasize the implementation of a completely continuous layered geometry of Ag to optimize the optical and electrical performances. These results provide an alternative solution for maximizing the performance and durability of Ag-based transparent electrodes that can be synthesized on either glass or polymer substrates for various optoelectrical applications, including flexible transparent Joule heaters.

Reliability and Potential of Inkjet‐Printed Flexible Heaters with Adaptive Temperature Zones for High‐Temperature and Long‐Time Applications

Herein, the reliability and potential of inkjet‐printed flexible heaters are demonstrated by a set of resilience tests, showing never before performed high‐temperature application. The flexible heaters can operate up to 400 °C and failures are not caused by the printed conductor, but due to the temperature sensitivity of the base substrate only. Furthermore, these heaters show an accurate repetition rate with fluctuations &lt;3% (2 °C within ten up–down cycles between 20 and 150 °C) and great long‐lasting capability (200 h on‐time operation at 150–180 °C). The bending strength has been proven by 10 000 cycles with resistance change less than 5%. Additionally, the adaptability and adjustability of defined temperature zones with the help of design rules for the layout definition are illustrated. These inkjet‐printed heaters demonstrate a high potential of usage in high‐temperature and flexible applications with the prerequisite of high stability and reliability.

Multifunctional flexible conductive filament for human motion detection and electrothermal

Highly sensitive wearable e-textiles play an important role in flexible sensors and next-generation personalized electronics with application potential in human physiological monitoring, environmental monitoring, and electronic skin. Here, a scalable and simple strategy for preparing liquid metal (LM)/carbon nanotubes (CNTs)@Ecoflex (LCEF) coaxial conductive filaments were fabricated by wet spinning. The electro-mechanical response behaviors of LCEF sensors were explored to evaluate their sensitivity and stability. The results demonstrated that the filament strain sensors could be used for monitoring both large and subtle human motions. Besides, the LCEF fabrics as a large-area flexible heater exhibited efficient electrothermal response, evidenced by achieving a temperature of over 112.1 °C upon applying a voltage of 12 V with 52.6 °C obtained on the fabrics within 42 s. This work may shed lights on scalable fabrication of flexible strain sensors and electrothermal devices which pave the way for the development of multifunctional wearable electronic devices.

Modeling of Thermal Preparation of Shock Absorbers of Trucks

The use of modern materials and technical fluids allows you to operate cars at negative temperatures, but in some regions of the world (for example, in Siberia), the ambient air temperature can fall below 243 K for several weeks or even months. The operation of trucks at such a temperature refers to extreme conditions that force the special preparation of equipment. This preparation consists not only of special maintenance, but also of carrying out some activities that are carried out immediately before starting the engine and driving. The essence of these measures is, among other things, the thermal preparation of the components and assemblies of the vehicle before departure.This work is devoted to the thermal preparation of truck shock absorbers. It is revealed that the use of oils, the kinematic viscosity of which significantly depends on the ambient temperature, is a limiting factor in the winter operation of shock absorbers. The simulation of the operation of electric flexible heaters for shock absorbers in the SolidWorks Flow Simulation environment was carried out and the preheating efficiency was evaluated. It is established that the temperature distribution of the shock absorber fluid during heating of two-pipe shock absorbers occurs unevenly, but despite this, preheating significantly improves the characteristics of shock absorbers and contributes to the safe and long-lasting operation of trucks.

High-performance electrically conductive adhesives with aluminum-doped zinc oxide (AZO) for various flexible electronic devices

Electrically conductive adhesives (ECAs) are a kind of attractive alternative to tin solders in microelectronics. However, fabricating high-performance, low-cost ECAs remains challenging due to the need for high-content metal fillers. This work reported a facile preparation of cost-effective ECAs composed of aluminum-doped zinc oxide (AZO) nanoparticles and silver (Ag) micro-flakes. The AZO nanoparticles could serve as a “bridge” to connect neighboring Ag micro-flakes to significantly improve the ECAs′ conductivity (from 1638.8 × 10-5 Ω⋅cm to 8.5 × 10-5 Ω⋅cm). After the aging and bonding performance tests, prototyped applications of the as-prepared ECAs for three flexible electronic devices were demonstrated sequentially: (Ⅰ) The resistance of flexible conductive film showed excellent dynamic stability after being folded for 1000 cycles, bent/released within 6 mm radius for more than 2000 cycles, or applied with 1000 kPa pressure; (Ⅱ) The coil antenna achieved fast transmission of radio frequency signal for near field communication (NFC) system; (III) The flexible heater performed efficient thermal management at 0.4–1.4 V input voltages. Therefore, the fabricated ECAs are expected to provide new opportunities for the development of novel flexible electronic devices for future applications.

High-performance flexible electrothermal Joule heaters from laser reduced F-N Co-doped graphene oxide with extended Sp2 networks

Phonons play an important role in heat generation and propagation across graphene lattice; however, this is hindered by graphene heteroatom doping which generates highly defected structures. Consequently, heteroatom doping has not been primarily explored in electrothermal materials despite presenting a cost-effective and metal-free method of improving electronic activity in flexible 2D materials. Herein, we present a facile and scalable technique for fabrication of fluorine-nitrogen co-doped laser reduced graphene oxide (FN-G) Joule heaters by ultrasonic wetchemical doping and maskless laser writing method. Films containing 6.14 at. % F and 3.22 at. % N, low sheet resistance of 24.49 O/sq. and uniquely high degree of atomic ordering indicated by an ID/IG of 0.16 were successfully fabricated. The heaters showed highly competitive transient electrothermal behaviour with a saturation temperature of 365 °C for only 9 V applied voltage, high heating rate of 385.33 °C/s, and average response time of 0.68 s. An excellent temperature distribution, low power requirement, and a net-zero degradation in the electrical and electrothermal properties even after 500 bending cycles was demonstrated. These high-performance features render the FN-G heater suitable for applications in wearable electronics powered by low voltage portable electrochemical cells.

Silver Nanoparticle-Decorated Multiwalled Carbon Nanotube Ink for Advanced Wearable Devices.

Silver nanoparticles of average size 12-13 nm were successfully decorated on the surface of multiwalled carbon nanotubes (MWCNTs) through a scalable wet chemical method without altering the structure of the MWCNTs. Employing this Ag@MWCNT, a multifunctional room-temperature curable conductive ink was developed, with PEDOT:PSS as the conductive binder. Screen printing of the ink could yield conductive planar traces with a 9.5 μm thickness and a conductivity of 28.99 S/cm, minimal surface roughness, and good adhesion on Mylar and Kapton. The versatility of the ink for developing functional elements for printed electronics was demonstrated by fabricating prototypes of a wearable strain sensor, a smart glove, a wearable heater, and a wearable breath sensor. The printed strain sensor exhibited a massive sensing range for wearable applications, including an impressive 1332% normalized resistance change under a maximum stretchability of 23% with superior cyclic stability up to 10 000 cycles. The sensor also exhibited an impeccable gauge factor of 142 for a 5% strain (59 for 23%). Furthermore, the sensor was integrated into a smart glove that could flawlessly replicate a human finger's gestures with a minimal response time of 225-370 ms. Piezoresistive vibration sensors were also fabricated by printing the ink on Mylar, which was employed to fabricate a smart mask and a smart wearable patch to monitor variations in human respiratory and pulmonary cycles. Finally, an energy-efficient flexible heater was fabricated using the developed ink. The heater could generate a uniform temperature distribution of 130 °C at the expense of only 393 mW/cm2 and require a minimum response time of 20 s. Thus, the unique formulation of Ag@MWCNT ink proved suitable for versatile devices for future wearable applications.

An Analytic Model of Transient Heat Conduction for Bi-Layered Flexible Electronic Heaters by Symplectic Superposition.

In a flexible electronic heater (FEH), periodic metal wires are often encapsulated into the soft elastic substrate as heat sources. It is of great significance to develop analytic models on transient heat conduction of such an FEH in order to provide a rapid analysis and preliminary designs based on a rapid parameter analysis. In this study, an analytic model of transient heat conduction for bi-layered FEHs is proposed, which is solved by a novel symplectic superposition method (SSM). In the Laplace transform domain, the Hamiltonian system-based governing equation for transient heat conduction is introduced, and the mathematical techniques incorporating the separation of variables and symplectic eigen expansion are manipulated to yield the temperature solutions of two subproblems, which is followed by superposition for the temperature solution of the general problem. The Laplace inversion gives the eventual temperature solution in the time domain. Comprehensive time-dependent temperatures by the SSM are presented in tables and figures for benchmark use, which agree well with their counterparts by the finite element method. A parameter analysis on the influence of the thermal conductivity ratio is also studied. The exceptional merit of the SSM is on a direct rigorous derivation without any assumption/predetermination of solution forms, and thus, the method may be extended to more heat conduction problems of FEHs with more complex structures.

A novel breathable flexible metallized fabric for wearable heating device with flame-retardant and antibacterial properties

Currently, there is an imperative demand for developing a novel flexible heater with high adhesion, breathability, and extreme condition resistance, such as ultra-high temperatures and even fire. Herein, a high-adhesion, flame-retardancy, and anti-bacteria copper nanoparticles networks/nylon 6 woven fabric (CNNs/NWF) wearable heater with a “sandwich-like” structure has been designed and fabricated based on phatic acid/aminopropyltriethoxysilane (PA/APTES) hybrid coating. On the one hand, the CNNs/NWF wearable heater exhibited superb electrothermal behavior working at a peak temperature of 208.8 °C powered with 2.0 V, better than that of wearable heaters in the recently reported literature. On the other hand, the vertical burning test showed that the heater possesses splendid flame retardancy. Furthermore, the PA/APTES coating could deposit a nanoscale porous protective film on the surface of CNNs/NWF, which imparts the heater with relatively excellent oxidation resistance and breathability. In addition, the antibacterial efficiency against Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ) still reached 100% after 50 times of standard washing and being electrified at 2.0 V for 5 min, keeping humans away from the threat of bacterial infections. We think that the heater in this research could be extended to wearable heating devices.

Synthesis of oxidation-resistance copper nanowires-formate for high-performance transparent conductive electrodes

In this study, high-aspect-ratio (~1250) curved copper nanowires (CuNWs) composed of formate (FA) as the surface passivation layer were synthesized by the solvothermal method using sodium formate (HCOONa). Experimental data and density functional theory (DFT) simulations suggest that the growth of CuNWs-FA was due to the high chemisorption energy of FA on the [100] plane of Cu. The relative resistance ratio of CuNWs-FA increased by 1.67 and 1.72 after aging at 25 °C for 60 days and 60 °C for 5 days, showing FA passivation greatly improved the oxidation resistance of CuNWs. The transparent conductive electrodes (TCEs) fabricated from CuNWs-FA showed high transmittance (89.19%), low sheet resistance (Rs = 44.54 Ω sq−1), and excellent mechanical stability against 10,000 bending cycles. The TCEs-based transparent flexible heaters (TFHs) exerted a fast heating rate and a uniform temperature distribution within the 20 s response time, which is a testament to the CuNWs-FA in applications such as electrochromic and optoelectronic devices. This passivation approach sheds light on solving the major drawback of CuNWs oxidation and promotes its use in flexible electronic devices.

Utilization of air injection together with spring wire and twisted tape to enhance performance of vertical tube with downward stream

ABSTRACT Present investigation, provides thermal and exergetic evaluations of downward gas–liquid two-phase flow within a vertical tube, which equipped two types of turbulators (spring wire and twisted tape). The vertical tube was put under fixed value of heat flux, which was provided via flexible wire heaters that were wrapped around the tube. The volume fraction varied between the values of 0.11 and 0.77. For each type of turbulators three different twist ratios of 2.2, 4.4, and 6.6 were considered to check the effect of twist ratio. Findings revealed that the maximum value of the Nu number, observed in the tube with Spring wire turbulator was 17% more than that in the tube with a Twisted tape turbulator. Also, it was found that the minimum value of the C.B.R factor was about 0.81 which was associated with Twisted tape that had the twist ratio of 4.4. Furthermore, the ratio between the maximum value of exergy efficiency associated with Spring wire turbulator and Twisted tape turbulator was about 1.11.

Lightweight Flexible Polyimide-Derived Laser-Induced Graphenes for High-Performance Thermal Management Applications

Laser pyrolysis of polyimide (PI) is a promising approach for preparing porous graphenes with hierarchical structures. In this study, we used a dual-focusing lens with a collimator to generate a highly concentrated CO2 laser beam with a high focal energy density. The resulting PI-derived graphene (PIDG) obtained under high-intensity laser irradiation possessed a high degree of graphitization, a low sheet resistance (27.4 Ω/sq.), and a large surface area (240.4 m2/g). Through optimization of the high-intensity laser parameters, we obtained a hierarchical PIDG possessing a fiber-like structure. We used this PIDG as a flexible heater and efficient heat sink for Si photovoltaic (PV) and light-emitting diode (LED) modules. The developed heater could reach a temperature of 202.6 °C under an applied voltage of 16 V, with rapid responses and remarkable flexibility. As a result of improved convective and radiative heat dissipation, the PIDG-based heat sink lowered the operating temperatures of the Si PV and LED modules efficiently. Accordingly, the power conversion efficiency of the Si PV and the lifetime of the LED module both increased dramatically.

Fabrication of flexible carbon heaters and silver interdigitated electrodes using ink-jet printing technology for electro-chemical gas sensor applications

PurposeThe purpose of this paper is to deal with an identification of a novel ink-jet printing sensor fabrication technology for fabricating flexible carbon heaters of macro and micro sizes, carbon interdigitated (IDT) electrodes and silver IDT electrodes. The technology involved in the proposed ink-jet printing method and materials used for the formulation of homemade nano-conductive inks (digital inks) are discussed in detail. The ink-jet printed flexible carbon heaters of different sizes (macro and micro) and carbon IDT electrodes and flexible silver IDT electrodes can be used as the flexible sensing layers in electrochemical gas sensors for sensitive and selective gas sensing applications. The characterization of ink-jet printed carbon heaters on Kapton substrate and its results are discussed. Similarly, the results of formulation of silver nano-conductive ink and printing of silver IDT electrodes on Kapton and its characterization are reported here for the first time.Design/methodology/approachFlexible carbon heaters of different sizes (macro and micro), carbon micro-IDT electrodes and silver IDT electrodes patterns were developed using AutoCAD 2D and printed on the Kapton (polyimide sheet) flexible substrate using the home-made nano-conductive inks with the help of EpsonT60 commercial piezo-head-based drop-on demand technology printer with standard printing options.FindingsThe proposed novel method is able to print heater patterns and IDT electrode patterns of approximately 12 µm and approximately 1 µm thickness, respectively, on flexible substrate using the home-made nano-conductive inks of carbon and silver by using a commercial low-cost printer. The home-made nano-conductive inks can be re-used for multiple prints up to six months shelf life. The resistance of the carbon heater was measured as 88 O under normal atmospheric condition. The novel flexible carbon heater was tested for its functionality and found to be satisfactory. The resistance of the silver IDT flexible electrodes was measured as 9.5 O which is better than the earlier works carried out in this paper.Research limitations/implicationsThe main challenge is associated with cleaning of printing ink ejection system in the existing commercial printers. The customization of the existing printer in the near future can minimize the printing challenges.Practical implicationsThe novel ink-jet printing technology proposed in this work is cost-effective, capable of achieving bulk production of flexible sensor elements, and consumes the least device fabrication time and high material yielding. The printing can be done with commercial piezo-head-based ink-jet printers with custom-prepared nano-conductive inks. There is a huge market potential for this paper.Originality/valueBoth the carbon heaters and silver IDT electrodes were printed on Kapton flexible substrate by using the commercial printer for the first time. The paper is promising the revolution in flexible low-cost sensor fabrication for mass production, and it is an alternate for thin film and thick sensor fabrication methods. The future of sensor fabrication technology will be the ink-jet printing method. In this paper, the research developments of flexible carbon heaters and flexible silver IDT electrodes for the time are reported. The characterization of carbon heaters and silver IDT electrodes were carried out and confirmed that the results are favourable for gas sensor applications.

Stable Flexible Transparent Electrodes for Localized Heating of Lab‐on‐a‐Chip Devices

AbstractIn situ biological observations require stable, accurate, and local temperature control of specimen. Several heating elements are coupled with microfluidic systems, but few of them are transparent to visible light and therefore compatible with microscopic observation. Traditional transparent electrodes, such as indium tin oxide, still suffer from high fabrication cost and brittleness, which is not fully compatible to emerging microfluidic devices. Here, a lightweight, low‐cost, flexible transparent heater based on percolating silver nanowire networks, protected with a transparent zinc oxide film, for the in situ monitoring of biological experiments is proposed. Using the fluorescence of dyes bound to double‐stranded DNA to monitor its temperature in situ, it is demonstrated that such nanocomposites allow rapid and reproducible heating under low applied voltage. Furthermore, selective heating is achieved in different zones of the same microchannel or for adjacent microchannels of the chip heating at different temperatures, with a single transparent heater and bias.