Chang Boyce S, Li Chen, Dai Jinghang, Evans Katherine, Huang Jingyu, He Mengdi, Hu Weili, Tian Zhiting, Xu Ting
Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853, United States.
ACS Appl Mater Interfaces. 2022 Mar 30;14(12):14579-14587. doi: 10.1021/acsami.2c00296. Epub 2022 Mar 21.
Thermal percolation in polymer nanocomposites─the rapid increase in thermal transport due to the formation of networks among fillers─is the subject of great interest in thermal management ranging from general utility in multifunctional nanocomposites to high-conductivity applications such as thermal interface materials. However, It remains a challenging subject encompassing both experimental and modeling hurdles. Successful reports of thermal percolation are exclusively found in high-aspect-ratio, conductive fillers such as graphene, albeit at filler loadings significantly higher than the electrical percolation threshold. This anomaly was attributed to the lower filler-matrix thermal conductivity contrast ratio / ∼10 compared to electrical conductivity ∼10-10. In a randomly dispersed composite, the effect of a low contrast ratio is further accentuated by uncertainties in the morphology of the percolating network and presence of other phases such as disconnected aggregates and colloidal dispersions. Thus, the general properties of percolating networks are convoluted as they lack a defined structure. In contrast, a prototypical system with controllable nanofiller placement enables the elucidation of structure-property relations such as filler size, loading, and assembly. Using self-assembled nanocomposites with a controlled 1,2,3-dimension nanoparticle (NP) arrangement, we demonstrate that thermal percolation can be achieved in spite of using spherical, nonconductive fillers ( ∼60) at a low volume fraction (9 vol %). We observe that the effects of volume fraction, interfacial thermal resistance, and filler conductivity on thermal conductivity depart from effective medium approximations. Most notably, contrast ratio plays a minor role in thermal percolation above ∼60─a common range for semiconducting nanoparticles/polymer ratios. Our findings bring new perspectives and insights to thermal percolation in nanocomposites, where the limits in contrast ratio, interfacial thermal conductance, and filler size are established.
聚合物纳米复合材料中的热渗滤——由于填料之间形成网络而导致热传输迅速增加——是热管理领域备受关注的课题，其应用范围涵盖多功能纳米复合材料的一般用途到热界面材料等高导电性应用。然而，它仍然是一个具有挑战性的课题，涉及实验和建模方面的障碍。热渗滤的成功报道仅见于高纵横比的导电填料，如石墨烯，尽管其填料负载量明显高于电渗滤阈值。这种异常现象归因于与电导率（约10⁵-10⁶）相比，较低的填料-基体热导率对比度（约10）。在随机分散的复合材料中，低对比度的影响因渗滤网络形态的不确定性以及其他相（如断开的聚集体和胶体分散体）的存在而进一步加剧。因此，渗滤网络的一般性质因缺乏明确的结构而变得复杂。相比之下，具有可控纳米填料排列的典型系统能够阐明诸如填料尺寸、负载量和组装等结构-性能关系。使用具有可控的一维、二维、三维纳米粒子（NP）排列的自组装纳米复合材料，我们证明尽管使用球形、非导电填料（约60）且体积分数较低（9 vol%），仍可实现热渗滤。我们观察到体积分数、界面热阻和填料电导率对热导率的影响偏离了有效介质近似。最值得注意的是，对比度在高于约60（半导体纳米粒子/聚合物比例的常见范围）时对热渗滤起次要作用。我们的研究结果为纳米复合材料中的热渗滤带来了新的视角和见解，其中确定了对比度、界面热导率和填料尺寸的限制。
