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Recently, the Dalian Institute of Chemical Physics, Chinese Academy of Sciences has made new progress in phase change energy storage materials. They synthesized a composite graphene film with excellent flexibility using polymer graphene as a raw material, and compounded it with a phase change material to obtain a flexible composite phase change material film. The material has high phase change material loading, exhibits excellent heat storage capacity, and also has excellent light-to-heat conversion ability, showing the potential to be used in the field of human wearable light-thermal management, and for the development of wearable smart fabrics provides a new direction.
Coincidentally, the College of Textiles and Clothing of Xinjiang University prepared a graphene heating membrane, embedded the flexible membrane into the jacket, and controlled the working state and heating rate of a single membrane and multiple membranes through a controller, and obtained a usable electric heating. clothing.
As a new functional material, graphene has good electrical conductivity (about 6000S/cm) and thermal conductivity (the thermal conductivity of single-layer graphene film is (4.84±0.44)×103 ~ (5.30±0.48)×103 W/( m·K)), high specific surface area and excellent mechanical properties and flexibility. Graphene and its nanostructures are mainly used in: (1) conductive films for display electrodes; (2) electronic circuits and sensors, including micro-supercapacitors, ultrasonic sensors, etc.; (3) seawater desalination, separation membranes; (4) genetic screening ; (5) Energy storage, etc.
(Image source: Internet)
Due to the existence of π-π bonds and van der Waals forces, the internal structure of simple graphene films is prone to irreversible agglomeration and stacking, which needs to be modified and doped to increase the available surface area. Therefore, the introduction of metal and polymer modified graphene-based films enables them to be applied in different fields such as sensors, capacitors, and shielding films, and endow them with new functions.
1. Graphene undoped film
Typically, undoped graphene films are prepared by redox methods. Huang et al. used roll coating method and ultra-high temperature heat treatment to obtain tough rGO thin films. The film formation and heat treatment methods are shown in the figure.
(Image source: Wu Sirui et al.: Research progress on graphene-based flexible thin film composites and their functionalization)
In addition to being used as a thermally conductive film, graphene can also be used to fabricate sound pressure sensor films using optical principles. The optical fiber sensor is resistant to electromagnetic interference, corrosion resistance, high temperature resistance, and can also be used in harsh environments. However, the simple graphene film is easy to be broken, and when the detected pressure is large, brittle fracture will occur.
In practical applications, graphene films still have many structural defects, the mobility of carriers is generally not high, and the surface resistance is greater than the theoretical value. Even the graphene films prepared by CVD are not satisfactory. However, the graphene films prepared by the redox method have higher resistance.
2. Metal or oxide modification of graphene films
By adding metal or its oxide to the graphene film, its surface is functionalized and modified, which can be used for gas sensor, electromagnetic interference shielding film, conductive film, thermal conductive film, negative electrode material of lithium ion battery, etc. First of all, due to the high specific surface area of graphene film, it is easy to adsorb gas molecules and cause changes in resistance value, so it is often used in gas sensors. Second, since graphene is nano-scale, it can absorb 2.3% of white light according to theoretical deduction, which endows it with great application potential in wave absorbing materials.
Excellent thermal and electrical conductivity are important properties of graphene-based films. In terms of electrical conductivity, graphene-based films have become an alternative to conductive films. Among them, since the flexible transparent conductive films represented by indium tin oxide (ITO) (mainly used in solar cells, displays, touch panels, etc.) are fragile, the combination of metal grids and graphene films is not only flexible, but also It can show good optoelectronic properties, so the graphene film can fill the large empty area in the metal grid, so that the surface resistance is greatly reduced.
In addition, the three-dimensional connected network structure of the graphene film replaces the traditional metal current collector and provides a fast diffusion channel for the electrons of the electrode.
3. Polymer compounding of graphene
Composite films of graphene and different polymers can be prepared for different purposes. Polymers can be divided into carbon chain polymers, heterochain polymers and elemental organic polymers according to the element structure of the molecular backbone.
3.1 Polymerization of graphene with carbon chain polymers and heterochain polymers
The three-dimensional graphene porous material (3D-rGO) is broken and recombined with the carbon chain polymer polyacrylic elastic substrate (PAA), so that the composite material has the dual piezoresistive effect of tension and compression, which can be stretched 100%, and can be stretched by 100%. From three-dimensional to two-dimensional, it has more excellent elasticity and fit. But in fact, the introduction of PAA did not change its three-dimensional structure, but effectively avoided the stacking and agglomeration of graphene sheets. Since the fragmentation and reorganization of graphene sheets is uncertain, the preparation process is difficult to operate, and secondary stacking may occur inside, so it is not suitable for large-scale production.
The heterochain polymer PI was compounded with graphene, and molybdenum disulfide (MoS2) with a graphene-like structure was added to make up for the lack of zero band gap of graphene. Then, gold nanoparticles (AuNPs) were reduced on the surface of MoS2 by photoreduction. Finally, The immobilization of acetylcholinease (AchE) on the composite material can increase the film stability, compatibility and molecular activity, and the prepared flexible film composite material has a uniform internal structure. Graphene can also be polymerized with carbon chain polymers and heterochain polymers at the same time. For example, the carbon chain polymer sodium polystyrene sulfonate and the heterochain polymer 3,4-ethylenedioxythiophene were mixed with rGO, and the conductivity of the prepared composite film increased significantly, and due to the heavy structure inside the polymer. A strong non-covalent synergy is formed between the sodium polystyrene sulfonates, which enhances the electrical properties and improves the stability. The composite film has low cost, good flexibility and non-toxicity, and is suitable for cancer cell biosensors.
3.2 Polymerization of graphene with elemental organic polymers
The composite films prepared by elemental organic polymer polydimethylsiloxane (PDMS) encapsulated 3D graphene foam (GF) have good toughness, can be repeatedly bent or stretched continuously, and can effectively recover from mechanical deformation and vibration coupled dynamic A static signal is detected in the signal, which can be used for a flexible sound pressure sensor. In addition, the GF/PDMS composite films can also be used as strain sensors.
3.3 Polymerization of graphene and other types of organic polymers
The rGO/CNF ultrathin flexible composite membrane was prepared by blending GO with nanofibers (CNF) and reducing them with hydriodic acid. Nanofibers with biocompatibility, hydrophilicity, elasticity, and uniform distribution in GO were introduced, so that the prepared composites had better electromagnetic interference shielding and thermal conductivity anisotropy. Commonly used in lightweight shielding materials (shielding electromagnetic microwaves, heat), portable electronic devices and wearable devices.
Graphene can also be compounded with conductive polymers. Polypyrrole (PPy) can be combined with rGO by in situ polymerization, and then PET is used as a substrate to construct a thin film gas sensor.
Graphene-based composite films have excellent properties and can be used not only in conductive films, electromagnetic shielding films, electrode materials, but also in biomedicine.