Flexible Graphite Film Research Articles

Molecular simulation and experimental investigation of thermal conductivity of flexible graphite film

Flexible graphite film (FGF), as a traditional interface heat dissipation material, has high anisotropy. It is a challenge to enhance both in-plane and through-plane thermal conductivity of FGF. For this reason, the effects of oxygen content, layer spacing, density and particle size on the in-plane and through-plane thermal conductivity of FGF were studied by both molecular simulation and experimental investigation. The simulation results indicate that the ways to improve the thermal conductivity of FGF include reducing oxygen content and layer spacing, increasing the density and matching the size of graphite sheets. The FGF prepared from room temperature exfoliated graphite (RTFGF) has a wide range of adjustable density (1.3–2.0 g/cm3) and thickness (50–400 μm). The thermal conductivity of the RTFGF is significantly improved after heat treatment owing to reduced oxygen content and layer spacing, which is consistent with the simulation results. Moreover, RTFGF with both high in-plane (518 W⋅m−1⋅K−1) and through-plane (7.2 W⋅m−1⋅K−1) thermal conductivity can be obtained by particle size matching of graphite.

Flexible graphite films with high cross-plane thermal conductivity prepared by graphitization of polyimide catalyzed by Ni-coated-CNTs

Graphite films are widely used in microelectronic devices as heat-dissipation materials due to their high thermal conductivity, low thermal expansion coefficient and lightweight.

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Two-dimensional vanadium sulfide flexible graphite/polymer films for near-infrared photoelectrocatalysis and electrochemical energy storage

Modern wearable electronics require scalable, flexible, and conductive electrodes with tunable properties. Abundant materials such as graphite as a conductive component and polymer as a flexible component forming a composite film (electrode) via simple synthesis technique are particularly captivating. This approach conveniently satisfies the fundamental needs of an ideal electrode yet provides a conductive platform to accommodate a secondary material for various purposes in electrochemical energy conversion and storage. Accordingly, we optimize a graphite-polymer composite film with good conductivity and flexibility to incorporate two-dimensional (2D) VSx (mixed phase predominated by V5S8) as an active material within the film. We exemplify the dual functionalities of the VSx/graphite flexible electrode as i) a photo-electrocatalyst for enhanced hydrogen evolution reaction by visible and near-infrared light irradiation (overpotential ≈500 mV at the current density of −10 mA cm−2), and ii) a conductive electrode for symmetrical solid-state supercapacitor with pseudocapacitive charge storage mechanism (areal capacitance of 123 mF cm−2 and areal capacity of 34 µAh cm−2 at the current density of 0.5 mA cm−2). Our work demonstrates the versatility of graphite films in terms of size, shape, flexibility, and scalability, with tunable physical, optical, and electrical properties by integrating other secondary materials. We combine flexible graphite film and 2D vanadium sulfide with near-infrared photoresponse and pseudocapacitive properties, as an economically feasible avenue for energy harvesting, outer space application, and wearable devices.

A Low-Profile Ultrawideband Antenna Based on Flexible Graphite Films for On-Body Wearable Applications

This paper presents a low-profile ultrawideband antenna for on-body wearable applications. The proposed antenna is based on highly conductive flexible graphite films (FGF) and polyimide (PI) substrate, which exhibits good benefits such as flexibility, light weight and corrosion resistance compared with traditional materials. By introducing flaring ground and an arrow-shaped slot, better impedance matching is achieved. The wearable antenna achieves a bandwidth of 122% from 0.34 GHz to 1.4 GHz, with a reflection coefficient of less than −10 dB, while exhibiting an omnidirectional pattern in the horizontal plane. To validate the proposed design, the wearable antenna with a profile of ~0.1 mm was fabricated and measured. The measured results are in good agreement with simulated ones, which indicates a suitable candidate for on-body wearable devices.

Polyacrylonitrile-derived thermally conductive graphite film via graphene template effect

Stable operation of electronic devices requires flexible and thermally conductive graphite film (GF) to spread the locally generated heat in devices. However, the tedious and harsh production procedure, high cost, and brittle nature of commercially available GF derived from polyimide (PI) limited its further applications. Here, we used graphene sheets as a template to induce graphitization of polyacrylonitrile (PAN) in confined 2D space, and thereby generated high crystalline GF. The obtained flexible GF showed a high in-plane thermal conductivity (K) of 1282 W m−1 K−1 and conductivity (σ) of 9.94 × 105 S m−1. The K is 7.3 times than that of pure PAN-derived film, and even higher than pure graphene film (1201 W m−1 K−1). Besides, it is easy to produce thick GF (80 μm), which is difficult to achieve in conventional artificial GF. Our work promises low-cost and easy preparation of high-quality GFs from non-graphitizable synthetic and natural macromolecules with the aid of graphene template structure-directing effect.

Structural evolution and thermal conductivity of flexible graphite films prepared by carboxylic graphene/polyimide

Graphene (GN) is transformed into carboxylic graphene (GO-COOH) with carboxylic groups by acidizing. Carboxylic graphene/polyimide (GO-COOH/PI) composite films are prepared by in-situ polymerization. The composite films show strong intermolecular force compared to the PI films which leads to excellent tensile strength and thermal conductivity. The composite films can be transformed into carbon films and graphite films by carbonization and graphitization. The GO-COOH can induce the graphitization process and compensate the defects. The thermal conductivity of graphite films prepared by composite films graphitized at the same temperature is higher compared to that of PI films. The addition of GO-COOH has great effects on reducing carbonization and graphitization temperature and improving property of films. The graphite films treated with 2500 °C possess smoother surface and the color of graphite films turn into dark grey. The films can keep the integral state and bend to a certain extent which exhibits that the flexibility of the graphite films is improved. The thermal conductivity of graphite films prepared by composite films treated with 2500 °C is 760.386 W/mK and increases by 128% compared to that prepared by the PI films, which is suitable for the application of heat conduction.

Stretchable and Flexible Thin Films Based on Expanded Graphite Particles

Stretchable and flexible graphite films can be effectively applied as functional layers in the progressively increasing field of stretchable and flexible electronics. In this paper, we focus on the feasibility of making stretchable and flexible films based on expanded graphite particles on a polymeric substrate material, polydimethylsiloxane (PDMS). The expanded graphite particles used in this work are prepared by utilizing bath sonication processes at the ultrasonic frequency of either the commercially available graphite flakes or graphite particles obtained through electrolysis under the interstitial substitution of silver and sulfate, respectively. The X-ray diffraction (XRD) patterns confirm that, due to the action of the bath sonication intercalation of graphite taking place, the resistances of the as-fabricated thin films is ultimately lowered. Mechanical characterizations, such as stretchability, flexibility and reliability tests were performed using home-made tools. The films were found to remain stretchable up to 40% tensile strain and 20% bending strain. These films were also found to remain functional when repeatedly flexed up to 1000 times.

Environment-friendly preparation of exfoliated graphite and functional graphite sheets

Exfoliated graphite (EG) is promising oil sorbent as well as an intermediate product for the preparation of flexible graphite films (FGFs). It has been a critical challenge to energy conservation and pollution abatement for the traditional EG production technique. Here, we propose a simple and effective preparation method to acquire EG in which flake graphite is intercalated and exfoliated at room temperature, not involving any pollutant emission. The influence factors in the preparation process were explored, such as the amount of H2SO4 and H2O2, the temperature for the preparation of room temperature exfoliated graphite (RTEG). In contrast to the EG by high temperature exfoliation (HTEG), RTEG exhibits a homogeneous structure and a significantly increased volume and surface area. Moreover, EG blocks with high oil sorption capacity and excellent reuse performance can be obtained by RTEG method. Especially, FGFs fabricated by RTEG has high flexibility, thermal conductivity and electrical conductivity. It suggests that this environment-friendly technology is suitable for large-scale industrial implementation of graphite-based oil sorbents and flexible materials.

Rapid preparation of expanded graphite at low temperature

The traditional methods for preparing expanded graphite (EG) often require harsh conditions and take a long time resulting in a high-cost and severe environmental pollution. A simple, energy-saving and efficient route for preparing EG was developed, in which flake graphite was exfoliated with K2S2O8 under concentrated H2SO4 at 80 oC for a few minutes. The microstructures, morphology and functional groups of the EG were characterized by XRD, SEM, FT-IR and Raman spectroscopy, and the electrical conductivity was measured by a four-point probe method. Results indicated that the flake graphite transformed into a worm-like structure after expansion with slight damage to the graphite sheets. A maximum expanded volume of 150 mL/g was obtained under the optimal conditions of 80 oC for 5 min with mass ratios of graphite to K2S2O8 of 1:7 and graphite to concentrated H2SO4 of 1:20. The electrical conductivity of the flexible graphite film prepared by rolling from the optimized EG reached 5.47×104 S/m. The mild oxidation and oxygen released by the decomposition of K2S2O8 under the acidic conditions are responsible for the efficiency of the method, which is promising for the mass production of EG due to its simplicity, low cost and low environmental impact.

A High-Energy 5 V-Class Flexible Lithium-Ion Battery Endowed by Laser-Drilled Flexible Integrated Graphite Film.

Due to its highly in-plane oriented crystal structure, the flexible graphite film (GF) possesses excellent electrochemical corrosion resistance, high planar electrical conductivity, and considerable mechanical strength. In this work, the laser-drilled integrated graphite film (porous-GF, PGF) is unprecedentedly used as a key to fabricate a high-performance high-energy 5 V-class flexible PGF/PGF-LiNi0.5Mn1.5O4 full cell, where the flexible PGF is a self-standing flexible graphite anode for lithium-ion intercalation/deintercalation and a high-voltage resistance cathode current collector. This unique design based on the flexible PGF will endow the future flexible batteries with excellent characteristics of thin, lightweight, simple fabrication, and high energy. More encouragingly, unlike previously reported flexible electrodes using carbon nanomaterials as the nonmetal current collector, the mass production and processability of the flexible GF and PGF are feasible with the aid of commercially available roll-to-roll laser drilling technology.

Motion Prediction with Artificial Neural Networks Using Wearable Strain Sensors Based on Flexible Thin Graphite Films

A human motion prediction system can be used to estimate human gestures in advance to the actual action for reducing delays in interactive system. We have already reported a method of simple and easy fabrication of strain sensors and wearable devices using pyrolytic graphite sheets (PGSs). The wearable electronics could detect various types of human motion, with high durability and fast response. In this study, we have demonstrated hand motion prediction by neural networks (NNs) using hand motion data obtained from data gloves based on PGSs. In our experiments, we measured hand motions of subjects for learning. We created 4-layered NNs to predict human hand motion in real-time. As a result, the proposed system successfully predicted hand motion in real-time. Therefore, these results suggested that human motion prediction system using NNs is able to forecast various types of human behavior using human motion data obtained from wearable devices based on PGSs.

Fabrication of Flexible Graphite Film Using a Mechanical Method and Its Application in Tio2/Graphite Solar Cell as a Flexible Counter Electrode

We have successfully fabricated solar cell using flexible counter electrode of graphite powder. TiO2 / graphite suspension was simply poured onto heated Fluorine Tin Oxide (FTO) substrate and kept heated at 200°C for 2 hours. PVA.LiOH polymer electrolyte was employed as a positive charge transfer medium. The counter electrode was made of graphite powder which is manually deposited on transparent plastic using tube-shaped metal which its surface covered with tissue of 2 mm thick. Graphite powder was deposited by straight motion of 20 movements for one coating or one layer. The same activity was repeated on the stacked film to increase the conductivity of the film. Resistivity and transmittance measurement of the film was performed by four-point probe method and UV-Vis equipment respectively. The film thickness and 2-D porosity were measured using an electronic microscope and ImageJ software orderly. The film resistivity and transmittance gradually decreased with the increase of coating number and reached the resistivity of 0.35 Ω.cm and transmittance of 5% for 40 times of coating. At the same time, the film thickness was saturated at the 30 times coating and reached the thickness of 9.4 μm. The film 2-D porosity which indicated how well the surface of the film covered the graphite also decreased to reach the film porosity of 18%. The most conductive film (40 times coating) was used as a flexible counter electrode in TiO2/graphite solar cell. The efficiency of this structure was 0.4 %, a reasonable efficiency achievement for a cheap material and easy method used.

Preparation and Properties of Ultrathin Flexible Expanded Graphite Film via Adding Natural Rubber

Expanded graphite (EG) films exhibit potential use in a wide field including thermal management, conductive applications, and electromagnetic interference (EMI) shielding. However, their poor tensile strength and brittleness are crucial deficiencies for commercial applications. To address these defects, in our work, natural rubber (NR) is employed to improve EG films for better mechanical strength and flexibility. The origin of the strengthening effect of EG films by the addition of natural rubber mainly arises from the formation of a simulate shell structure. Compared to the neat EG films, the addition of merely 2 wt% NR can give rise to superior ductility. Further, the loading of 10 wt% NR realizes a significant mechanical enhancement of the EG/NR films, i.e., 2.4 and 11.4 times increase in tensile strength and elongation at break, respectively. Besides, EG/NR films containing 10 wt% NR can still sustain excellent thermal and electric conductivities of 173 W·n−1·K−1 and 75 S·cm−1, respectively. Furthermore, a very high EMI of 41.4 dB is achieved as the film thickness reaches 50 μm. Thus, the lightweight EG/NR films with comprehensive performance as well as their virtue of green and simple large-scale preparation endow them with the possibility of designing next-generation flexible electronics.

Flexible graphite films with high conductivity for radio-frequency antennas

The low conductivity and fragile mechanical toughness of conventional carbon material based films are major barriers for their applications in modern electronics, especially in flexible radio frequency electronics, such as antennas. In order to optimize their performance, the electrical and mechanical properties of the materials need to be improved. In this work, flexible graphite films, fabricated by high temperature thermal treatment of polymer precursor and subsequence compression rolling, are designed and explored to create dipole antennas for radio frequency applications. It is shown that the flexible graphite films with 808.8 cm2/V·sec carrier concentration have a high conductivity of 1.1 × 106 S/m. We demonstrate that the resulting flexible graphite film dipole antennas can not only produce a relatively high peak gain of 1.45 dB with comparable return loss, bandwidth, and radiation patterns to an identical copper antenna, but also have excellent structure stability and mechanical flexibility. Moreover, the density of the graphite film is 5 times less than the copper film. This indicates that the tailored flexible graphite film could be a new alternative way to produce superb flexible and efficient radio-frequency antennas.

A flexible anti-clogging graphite film for scalable solar desalination by heat localization

A flexible scalable anti-clogging graphite film for efficient solar desalination is demonstrated with long-term performance.

Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultra-thin, flexible and free-standing graphite oxide films

In this work we present graphene-based in-plane flexible interdigitated micro-supercapacitor devices fabricated through direct laser writing onto ultra-thin graphite oxide (GO) films.

Flexible graphite film with laser drilling pores as novel integrated anode free of metal current collector for sodium ion battery

In this work, flexible polyimide graphite films with laser drilling pores were prepared by laser drilling pore technique. Then a full cell was built based on the porous graphite film anode, Na3V2(PO4)3 cathode, and NaPF6 in diglyme electrolyte. This unique battery system makes the best of co-intercalation mechanism of Na+-solvent into porous graphite film host. Moreover, the porous graphite film is directly served as an integrated anode with no electrochemically inactive components, such as binders, conductive agents and metallic current collectors. The above merits allow remarkable progress in electrochemical performance, especially the high energy density and much improved cycle life, which will significantly boost the high energy batteries. Our sodium-ion system is potentially promising power sources for promoting the substantial use of low-cost energy storage systems.

Nanothermal Interface Materials: Technology Review and Recent Results

Thermal interface materials (TIMs) play a critical role in conventionally packaged electronic systems and often represent the highest thermal resistance and/or least reliable element in the heat flow path from the chip to the external ambient. In defense applications, the need to accommodate large differences in the coefficients of thermal expansion (CTE) among the packaging materials, provide for in-field reworkability, and assure physical integrity as well as long-term reliability further exacerbates this situation. Epoxy-based thermoplastic TIMs are compliant and reworkable at low temperature, but their low thermal conductivities pose a significant barrier to the thermal packaging of high-power devices. Alternatively, while solder TIMs offer low thermal interface resistances, their mechanical stiffness and high melting points make them inappropriate for many of these applications. Consequently, Defense Advanced Research Projects Agency (DARPA) initiated a series of studies exploring the potential of nanomaterials and nanostructures to create TIMs with solderlike thermal resistance and thermoplasticlike compliance and reworkability. This paper describes the nano-TIM approaches taken and results obtained by four teams responding to the DARPA challenge of pursuing the development of low thermal resistance of 1 mm2 K/W and high compliance and reliability TIMs. These approaches include the use of metal nanosprings (GE), laminated solder and flexible graphite films (Teledyne), multiwalled carbon nanotubes (CNTs) with layered metallic bonding materials (Raytheon), and open-ended CNTs (Georgia Tech (GT)). Following a detailed description of the specific nano-TIM approaches taken and of the metrology developed and used to measure the very low thermal resistivities, the thermal performance achieved by these nano-TIMs, with constant thermal load, as well as under temperature cycling and in extended life testing (aging), will be presented. It has been found that the nano-TIMs developed by all four teams can provide thermal interface resistivities well below 10 mm2 K/W and that GE's copper nanospring TIMs can consistently achieve thermal interface resistances in the range of 1 mm2 K/W. This paper also introduces efforts undertaken for next generation TIMs to reach thermal interface resistance of just 0.1 mm2 K/W.