Thermal conductivity of polyimide composite films


In a recent study published in the journal Nano-micro letters, Chinese researchers have developed a hierarchically multifunctional polymer-based polyimide composite film comprising layers of 61.0 wt% graphene oxide / expanded graphite, 23.8 wt% ferric oxide / polyimide and fibers of polyamide for use as flexible material, heat dissipating and electromagnetic interference (EMI) protective film for electronic devices.

To study: Hierarchically multifunctional polyimide composite films with greatly improved thermal conductivity. Image Credit: tj-rabbit / Shutterstock / com

The film demonstrated a coefficient of thermal conductivity in the high plane of 95.40 W (m K)-1, suitable tensile strength of 93.6 MPa, excellent EMI shielding efficiency of 34.0 dB, and fast electric heating response of 5 seconds.

Polymer-based composites for heat dissipation and EMI shielding

As electronic devices become more compact and lightweight, the amount of heat generated and EMI caused by a large number of intertwined circuits becomes a serious problem. Many researchers have tried to develop ultra-thin flexible films that can simultaneously dissipate heat and provide EMI shielding that incorporates water-based carbon / polyurethane nanotubes and graphene oxide (GO) / cellulose (CNF) nanofibers ).

However, the direct mixing of charges in the polymer matrix often leads to thermal conduction pathways of discontinuous charges due to uncontrolled segregation and agglomeration of charges. The solution to this problem is to prepare consolidated filler films first before adding them to the polymer matrix such as graphite nanosheets (GNP) / nylon gauze or GNP / CNF, but they have poor durability due to poor adhesion between the filler film and the matrix polymer.

Schematic diagram of the preparation process of PI composite films.

Schematic diagram of the preparation process of PI composite films. Image credit: Guo, Y et al., Nano-Micro Letters

Electrospun Polyimide (PI) fibers are known for their excellent tensile strength, flexibility and mechanical properties, while expanded graphite (EG) has high thermal conductivity, excellent EMI shielding performance, and low cost. ; thus, EG filler films in the PI electrospun fiber matrix are a promising candidate for this application.

However, EG is brittle in nature and has poor mechanical properties, which makes it unsuitable for processing into a durable film. On the other hand, GO film is flexible, has excellent mechanical properties, high specific surface area and could form π-π interaction with EG magnetic charges.

About the study

In this study, the researchers fabricated a three-layer polymer composite film, which consists of a GO / EG top layer for high thermal conductivity, improved EMI shielding and adequate mechanical properties, a layer of lower substrate in electrospun PI fibers for improved mechanical properties, and ferric oxide intermediate (Fe3oh4) / PI layer for better grip, and EMI shielding.

GO was prepared by the Hummers method and EG was prepared by mixing graphite and (NH4)2S2oh8 in H2SO4 solution followed by stirring, filtration, washing and drying. Subsequently, EG was dispersed in an aqueous GO solution, followed by ultrasonication, filtration, drying and molding in a press to obtain GO / EG films.

?  of PI composite films with fixed addition of Fe3O4 / PI and PI (a) fibers,?  PI composite films contain 61.0 wt% GO / EG change with temperature (b)

λ of PI composite films with fixed addition of Fe3oh4/ PI and PI fibers (a), λ PI composite films contain 61.0 wt% GO / EG change with temperature (b). Image credit: Guo, Y et al., Nano-Micro Letters

In addition, the PI precursor, 4, 4′-oxidianiline (ODA) and pyromellitic dianhydride (PMDA) were dissolved in a dimethylacetamide solvent to obtain a solution of poly (amic acid) (PAA), which was then thermally imidated to form PI fibers. Finally, to synthesize Fe3oh4/ PI middle layer, ODA and Fe3oh4 were dispersed in a solvent and with further addition of PMDA and equimolar ODA in the solution to obtain Fe3oh4Mixture / PAA followed by thermal imidation.

Observations

Scanning electron microscopy (SEM) showed that EG is thicker than GO in the GO / EG top layer, which is desirable for high thermal conductivity, adequate EMI shielding, and mechanical stability. In addition, the entanglement of PI molecular chains between Fe3oh4/ PI layer and PI fiber layer and hydrogen bond and π-π interaction between the top layer Go / EG and Fe3oh4The / PI layer provided a robust hierarchical structure. The GO / EG, Fe layer3oh4The / PI layer and the PI fiber mat had thicknesses of 70 µm, 16 µm and 160 µm, respectively.

The transient plane heat source method revealed that the PI composite had a coefficient of thermal conductivity of 95.40 W (m K)-1 and surface electrical conductivity of 230.0 S cm-1, which only depended on the amount of GO / EG, i.e. 61% by weight, and the amount of Fe3oh4The / PI layer had no effect.

From the vector network analysis (VNA) test, it is evident that the total EMI shielding effect and absorption efficiency of PI composite films increased with the percentage of Fe3oh4/ PI in the middle layer. Moreover, when the Fe3oh4The amount / PI was 23.8% by weight, this EMI shielding efficiency was maximum at 34.0 dB. Likewise, a rapid heating response of 5 seconds was recorded.

Electric heating property of PI composite films.  The time-temperature relationship of PI composite films with different voltages (a, b), the linear adjustment of the experimental temperature with respect to U2 (c), the surface temperatures of PI composite films at gradient modified voltages (d ), the electric heating stability of PI Composite Films over repeated supply voltages (e)

Electric heating property of PI composite films. The time-temperature relationship of PI composite films with different voltages (a, b), linear adjustment of the experimental temperature as a function of U2 (vs), the surface temperatures of PI composite films at gradient modified voltages (D), electrically heated stability of PI composite films under repeated supply voltages (e). Image credit: Guo, Y et al., Nano-Micro Letters

Conclusion

The researcher prepared a three-layer PI-based polymer composite with GO / EG as the top layer, Fe3oh4/ PI as the middle layer, and electrospun PI fibers as the bottom substrate layer. The GO / EG layer provided high thermal conductivity due to the presence of a greater amount of EG, while a small amount of GO provided adequate flexibility.

Likewise, Fe3oh4 in the middle layer improved the EMI shielding effect, and the lower electrospun PI fibers improved the mechanical stability. In addition, all three layers demonstrated strong adhesion to each other. Therefore, the new PI composite is an excellent choice for heat dissipation and EMI shielding in heavily loaded compact electronic devices.

Reference

Guo, Y., Qiu, H., Ruan, K., Zhang, Y., Gu, J., Hierarchically multifunctional polyimide composite films with greatly improved thermal conductivity. Nano-Micro Let. 2021, 14, 26.https: //link.springer.com/article/10.1007/s40820-021-00767-4#citeas

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