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Since its discovery, graphene has been in the spotlight for its excellent properties, especially the high electrical conductivity of monolayer graphene of up to 5 × 106 s-m-1. Nowadays, graphene powders have been used in a variety of applications, including electrodes for energy storage devices and conductive fillers for polymer nanocomposites. However, in practical applications, it is usually difficult to utilize the excellent conductive properties of graphene-polymer composites. The main reason for this is that the interfacial contact between graphene sheets hinders the transmission of electrical energy, and thus the conductivity of graphene materials is usually much lower than the inherent conductivity of individual graphene sheets. To face this problem, some researchers have attempted to introduce similar sp2-hybridized covalently bonded carbon atom structures to connect graphene flakes while retaining the original excellent in-plane conductivity of graphene. For example, M. M. Slepchenkov and colleagues used single-walled carbon nanotubes to form single-walled carbon nanotube-graphene junctions of varying diameters in graphene nanopores to control the material's conductivity. Another group of researchers has constructed conductive networks using polymer materials as substrates. For example, J. Y. Suh and colleagues constructed conductive networks using a hot-pressing method in which 30 percent graphene was encapsulated in a polytetrafluoroethylene powder.
Among these attempted polymers, polyimide has been used by many researchers as a polymer matrix for graphene composites due to its promising applications in areas such as thin-film electrodes, integrated-circuit heat sinks, high-temperature structural materials, and lithium-ion batteries. In addition, it is widely used as one of the best pyrolytic precursors for the preparation of carbon thin films by carbonization and graphitization.
The main point of this paper
1.Background of the study:
R. K. Biswas et al. converted polyimide to graphene on copper substrate by pulsed CO2 laser.
The dispersion of graphene and polyimide composites is not ideal and requires modification or the use of dispersants.
2.C. Lim's team's study:
Preparation of graphene/polyimide films by electrostatic discharge method. Increased graphene content led to an exponential increase in thermal conductivity, but above 40 wt% the films could not retain their shape.
3.Z. Xu's team:
Flexible electrically heated films with 8 wt% graphene content were prepared, capable of rapid temperature increase to 390°C at 24 V. Joule heating method was used.
4.P. Zhang's team:
Preparation of polyimide/evaporation-grown carbon fiber composites. Investigate the frequency-temperature relationship between Joule thermal modulus and electron transfer mechanism.
5.Methods of this research:
Preparation of graphene/polyimide thin films by in-situ synthesis method to ensure the dispersion of graphene. Conductive networks were constructed based on polyimide substrate and 50 wt% graphene.
6.Joule heating treatment:
The films were subjected to a simple and fast Joule heating treatment.
After the treatment, the polyimide generates a graphene-like structure of sp2-hybridized covalently bonded carbon atoms.
7.Performance Enhancement:
The conductivity of the graphene/polyimide film was improved by about 76.85%.
Study of the effect of Joule heating treatment on the electrical conductivity and interfacial structure of graphene/polyimide thin films
Graphene electrical properties:
Graphene has received widespread attention for its excellent electrical properties, but the difficulty of interlayer electron transfer limits its application in composite materials.
Effect of Joule heating treatment:
Joule heating treatment of graphene/polyimide films significantly increased the electrical conductivity of the films by about 76.85%. After several Joule heating cycle treatments, the conductivity continued to increase, but the increase slowed down, and finally increased by about 93.94%.
Interfacial atomic rearrangement:
Joule heating treatment leads to atomic rearrangement of polyimide near the interface with graphene bonding, forming a new crystalline phase favorable for electron transport.
Double-layer capacitance microstructure model:
A graphene/polyimide double-layer capacitance microstructure model is proposed, and the Joule heating treatment reduces the distance between graphene electrode plates and increases the number of charge carriers.
Experimental characterization results:
TEM and WAXS characterization results show that the atomic structure of the graphene/polyimide bonding interface has changed, confirming the rearrangement of interface atoms.
Performance enhancement mechanism:
Experiments showed that Joule heating treatment improved the electrical conductivity of graphene/polyimide composites through atomic rearrangement and recrystallization, which provides a new method to utilize the excellent electrical conductivity of graphene.
This work demonstrates a new method to improve electron transfer between graphene sheets while developing the electrical properties of graphene. The results show that the electrical conductivity of graphene/polyimide composite films was significantly improved by about 76.85% under Joule heating treatment. A series of characterization results suggest that this change in performance may be due to rearrangement and recrystallization of polyimide atoms at the graphene/polyimide interface. During the Joule heating treatment, the polyimide molecules with graphene as the template underwent atomic rearrangement under the action of an electric current, forming a regular phase more favorable for electron transport. Eight experimental cycles showed that this structural change was irreversible and could be further accomplished by multiple Joule heating treatments, culminating in a conductivity improvement of about 93.94%. Accordingly, the researchers proposed a microstructural model of a double-layer capacitor with graphene electrode plates and polyimide insulating dielectric. The model shows that Joule heat treatment shortens the distance between the graphene sheets and increases the number of off-domain electrons, thus enlarging the diffusion region for electron reactions. This work provides a viable way to better utilize graphene's excellent electrical conductivity in practice, which is key to many graphene composite applications. This study helps researchers achieve better graphene properties in areas such as electrically and thermally conductive materials. In addition, it provides a viable idea for the induced carbonization of polyimide.
