Is filler orientation always good for thermal management performance: A visualized study from experimental results to simulative analysis

https://doi.org/10.1016/j.cej.2020.124929Get rights and content

Highlights

  • A thinning-then-reassemble strategy was reported to make anisotropic composites.

  • Their thermal management performance was evaluated through a visualized study.

  • High anisotropic thermal conductivity doesn’t equal to good thermal management effect.

Abstract

Alignment of anisotropic thermo-conductive filler, involving in-plane or out-of-plane orientation, is a significant strategy to extend thermal management applications of polymer materials, while the impact of the anisotropic thermal conductivity on thermal management performance is still lack of thoroughgoing study. Herein, a gradually thinning and re-assembling method is reported to fabricate in-plane and out-of-plane orientated polypropylene (PP)/graphite and PP/graphene composites, whose anisotropy could reach 8.37 and directional thermal conductivity achieves more than 4 W/m K. Based on this, a visualized study, supported by infrared imaging and finite simulation analysis, was carried out to investigate their thermal management performance for different applications. Results demonstrate that merely pursuing high directional thermal conductivity does not necessarily achieve optimal thermal management effect, it is equally important to customize oriented structure according to application scenarios. Our out-of-plane orientated polymer composites are proved beneficial to quick heat dispassion along vertical direction, making it perfect candidates as parallel thermal interface materials. However, when applied to scenarios where heat is generated from localized regions, this structure inevitably causes overheat due to the restricted heat flux, while the in-plane orientated composites show much better performance.

Introduction

As the high-power and miniaturization of electronic equipment or devices such as portable computer, light-emitting diodes and lithium ion battery, the study and development of highly efficient thermal management material become indispensable and vital for their dependability, high-performance and long service life [1], [2], [3], [4]. Polymer material is a fascinating choice for thermal management material due to its easy processing, low density and good flexibility [5], [6], [7]. However, its low thermal conductivity largely limits the widespread applications. Novel strategies, such as constructing thermo-conductive three-dimensional filler network [8], [9], [10], utilizing hybrid fillers with different dimensions [11], [12], [13], [14], inducing the orientation of anisotropic fillers [15], [16], [17], [18], [19], surface-modification of fillers [20], [21], [22], and regulating the microcosmic structure of polymer matrix [17], [23], [24], [25], were proposed for improvement of the thermally conductive property of polymer-based materials. Among these methods, inducing the orientation of anisotropic fillers has recently received tremendous attention for achieving noteworthy thermal enhancement at a particular direction [26]. Because thermal conductivity of anisotropic fillers, especially two-dimensional (2D) fillers such as graphite, graphene or hexagonal boron nitride, possess ultra-high thermal conductivity up to several hundreds of W/m·K along the in-plane direction, while exhibiting only several W/m·K at the direction of out-of-plane [27]. Thus, aligning filler along one direction is effective to take advantage of fillers’ anisotropy, facilitating phonon vibration and collision at this direction [28]. However, quantity of time and energy has been consumed to pursue the enhancement of either in-plane or out-of-plane thermal conductivity, while the huge enhancement of thermal conductivity along one direction is at the cost of compromised thermal conductivity along direction perpendicular to fillers’ orientation [29], [30]. It is still unclear whether the pursuit of high thermal conductivity at one direction but sacrificing thermal conductivity at another direction would deteriorate the comprehensive thermal management capability of polymer composites. Meanwhile, there hasn’t yet systematic research that revealed which orientation is more favorable in practical circumstances of thermal management. Therefore, general guidance is urgently needed and significant for the intelligent design of an ideal thermal management material for complicated and multifarious application prospects.

Although a lot of heat conduction model formulas, such as Maxwell model [31], Agari model [32], [33], and effective medium theory model [34], [35], [36], [37] have been formerly proposed by researchers for predicting the thermal conductivity and analyzing the microcosmic mechanism of polymer composites. For example, a layers-in-parallel model has been put forward, which could apply well to the polymer composites at high content of filler with different degrees of in-plane orientation [38]. Effects of fillers aspect ratio, orientation angle and interaction on composites thermal conductivity were also investigated through numerical simulation [39]. However, above models could not explain the differences of thermal conductivity and heat diffusion between in-plane orientation and out-of-plane orientation. Moreover, the widely used heat conduction model formulas without any visual presentation also display an enormous restriction in the analysis of different thermal management scenarios. To sum up, a visual digital simulation combined with experimental validation for illumination between filler orientation and thermal management capability is rare but of great importance.

Herein, polypropylene (PP) composites with respectively in-plane and out-of-plane orientated 2D filler, taking graphite or graphene as examples, were fabricated by a gradually thinning followed with a re-assembling strategy. Based on the achievements of high anisotropy of thermal conductivities and extraordinary directional thermal conductivity of PP composites, a visualized study that combines advantages of infrared imaging and finite element analysis was carried out, which enables us to comprehensively dissect the relationships among filler orientation, thermal conductivities and anisotropic heat transfer process as the change of thermal management scenes, including for interfacial heat conduction or localized heat dispassion. This research is expected to jump out of the stereotype thinking in which thermal conductivity is considered to be equivalent to the thermal management performance, and for the first time point out that which orientated structure is most beneficial for different requirements of thermal managements, providing a significant guidance for fabrication of thermal management materials with optimized heat-conduction performance.

Section snippets

Results and discussion

PP/Graphite composites with loading content of 20 vol% is prepared by mixing PP granules with graphite in internal mixer with the rotation speed of 50 rpm. Then, PP/Graphite with oriented structure were prepared through a gradually thinning and re-assembling strategy. In this article, the orientation along the x-y plane is defined as the in-plane orientation, and the orientation perpendicular to x-y plane is defined as out-of-plane orientation (z direction). As shown in Fig. 1, firstly, PP

Conclusion

In summary, we have demonstrated a strategy for fabricating the in-plane and out-of-plane oriented composites by gradually thinning followed with re-assembling. The obtained composite material exhibits high directional thermal conductivity higher than 4 W/m·K, while anisotropy is as high as 8.37. Moreover, their thermal management performance for different applications is systematically investigated. In scenarios where heat is generated on the entire surface, out of -plane oriented composites

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the Research Start-Up Fund of Nanjing University of Science and Technology, China (Grant No. AE89991/222), the Fundamental Research Funds for the Central Universities, China (Grant No. 30920021121), and the National Natural Science Foundation of China, China (Grant No. 51721091). The authors also acknowledge Analytical & Testing Centre of School of Chemical Engineering in Nanjing University of Science and Technology.

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