【作者】 陈馨；

【导师】 傅强； 许伟；

【作者基本信息】 四川大学 ， 材料工程（专业学位）， 2023， 硕士

【摘要】 随着科学技术的进步,电子元件逐渐向集成化、小型化和高精度化发展。由于电子设备需要在更小的使用面积上保持更高的功率密度,因此设备产生的热流密度增加,如果这些热量不能得到及时的释放,那将会严重地影响电子设备在运行期间的可靠性和稳定性,甚至使用寿命。研究表明,电子设备中55%的性能问题都源自于温度,因此,为了保证电子器件长期、安全、可靠的运行,研发高性能的导热材料已成为下一代电子器件的迫切需要。聚合物导热材料由于具有低密度、易加工、耐腐蚀、柔韧性和触变性等多种优点,已成为导热材料中不可或缺的一员。其中,向基体中添加高导热填料的方式能够有效地提升基体的导热性能。环氧树脂热固性聚合物,因具有优异的机械性能、高粘度、良好的耐热性和高电阻等特性,常被用作电子设备的基体材料,但由于其本征导热系数较低,常需要通过添加高导热的填料提升其导热性能。但是,尽管导热填料本身的导热系数极高,但在较大的添加量下,复合材料导热性能的提升却极为有限。这是因为当填料分散在聚合物基体中时,填料-聚合物和填料-填料界面上出现大量的声子散射,由此带来的界面热阻会严重阻碍复合材料导热性能的提升。因此,寻找一种宏观上高度连续,沿传热方向的声子散射较小或接近于零的增强相来解决导热复合材料中的界面热阻问题,显得尤为重要。本论文选用了高度连续的具有本征高导热性的聚合物纤维作为基体的增强材料,通过分析纤维的热传输特性,利用加捻、编织等手段设计纤维导热路径,对导热聚合物复合材料进行结构调控及导热性能研究。详细内容与主要结论如下:（1）环氧树脂基导热复合材料在实际生活中的应用场景非常广泛,但常见的添加传统导热填料构建三维导热通路的方法由于难以避免的界面热阻问题导致复合材料的导热性能提升有限,且难以同时实现水平和垂直方向上的高导热系数。针对上述问题,本论文选择了高性能的聚对苯撑苯并二恶唑连续纤维（PBO）作为环氧树脂的导热增强材料,连续的聚合物导热纤维在热传输方向上不存在导热相与基体的界面热阻,解决了传统导热填料导热路径上的界面热阻问题。本论文结合加捻和编织的手段调控传热方向,构建三维导热结构,研究了不同PBO含量与加捻度下环氧/PBO纤维复合材料的导热性能。实验结果表明:具有高本征导热性能的连续PBO纤维的引入有效增强了环氧/PBO纤维复合材料的导热性能,通过对纤维结构的调控实现了多维方向上的高导热,当PBO含量为33 vol%时,复合材料垂直与水平方向的的导热系数分别为10.85 W/m K和7.15W/m K,同时PBO纤维的添加对于复合材料力学性能与透波性能也起到了增强作用。这种利用本征导热性优异的连续纤维有效实现多维方向高导热的方法为导热复合材料的制备提供了一种新的思路。（2）通过添加高本征导热性的聚合物连续纤维作为导热填料的方法在较低添加量下有效提升了导热性能,得到了多维方向的高导热系数,但连续纤维作为导热相时复合材料内部的界面热阻是否降低尚未得到理论数据证明。因此,本论文结合分子动力学、声子模拟和经典的非线性导热模型,分别从分子链结构、聚集态结构、以及微观复合结构等多个方面对PBO连续纤维的导热机理进行了分析。研究结果表明,PBO连续纤维内部的平均界面热阻仅为10^-10m²W/K,几个数量级地小于聚合物-填料的界面热阻10^-8m²W/K,说明PBO纤维在添加进基体中时有效地克服了传统的填料聚合物复合材料中界面热阻大的问题。同时,模拟分析结果证明,PBO晶区和无定形区域大分子链在低频区域的声子态密度高度重叠,两者的声子振动模式接近,无定形区域大分子链的连续构象就像热桥一样将相邻的晶区串联起来,在其界面处的声子散射几乎可以忽略,因此内部的界面热阻极低。该工作证明了利用连续纤维能有效降低复合材料的界面热阻,且支持了研究人员认为无定向区域取向更为关键的观点,对于纤维导热机理的分析可能可以提供一些可参考的空间。（3）通过引入连续的聚合物纤维替代纳米、微米级导热填料能够有效降低复合材料的界面热阻,提升多维方向的导热性能,这一思路具有普遍适用性。本论文通过石墨化的方法,调控纤维中石墨晶体结构,提升纤维的本征导热性能,进而提升复合材料的导热性能。通过对PBO纤维及纤维织物进行高温石墨化处理,得到了PBO基的碳纤维及碳纤维布材料。研究结果表明,通过高温石墨化处理,在纤维石墨化程度为0.34时,PBO基碳纤维的导热系数可达到805.22W/m K,比PBO纤维的本征导热系数（47.56 W/m K）高出17倍。对纤维织物直接高温石墨化的处理保证了纤维布三维结构的完整性,得到的环氧树脂/PBO基碳纤维块体复合材料实现了水平方向和垂直方向的高导热,导热系数分别为9.718 W/m K和5.784 W/m K,但复合材料的导热性能与预期的高效提升并不相符,这可能源于石墨化后纤维变脆,导致后处理过程中纤维部分连续结构被破坏,但这一设计思路仍可能对于碳纤维复合材料在高性能热管理应用方面提供全新的思路与机会。更多还原

【Abstract】 As the 5G era emerges,electronic components are advancing towards integration,miniaturization,and high precision.To maintain high power density and reduce the size of electronic products,heat dissipation issues have become increasingly prominent due to the higher heat flux density generated by electronic devices.Inadequate dissipation of heat can have serious implications on the operational reliability,stability,and service life of electronic equipment.Thus,the development of high-performance thermal conductive materials is an immediate requirement for ensuring the safe,reliable,and long-term operation of the next generation of electronic devices.Due to their advantageous properties such as low density,easy processing,corrosion resistance,flexibility,and thixotropy,polymer thermal conductive materials have become an essential component of thermal conductive materials.Among them,filled polymer thermal conductive materials are obtained by adding high thermal conductive fillers to the matrix,which can achieve high thermal conductivity.Epoxy resin,a thermosetting polymer,is commonly used as a matrix material for electronic equipment due to its superior mechanical properties,high adhesive strength,excellent heat resistance,and electrical insulation.However,its intrinsic thermal conductivity is low,and thus high thermal conductivity fillers are often added to enhance its thermal conductivity.However,the current methods for achieving high thermal conductivity in filled polymer thermal conductive materials mainly rely on regulating the three-dimensional thermal conductivity path of the filler,which is a relatively complicated process.Even though these fillers have high thermal conductivity,there is still significant phonon scattering at the interfaces between the polymer matrix and the filler,limiting the improvement in the composite material’s thermal conductivity.Therefore,there is a need to develop new types of fillers or methods to overcome the interface thermal resistance issue caused by severe phonon scattering in thermal conductive composite materials.In this study,the focus is on using polymer fibers with high intrinsic thermal conductivity as the reinforcement material for the matrix.The heat transfer characteristics of the fibers are analyzed,and the thermal conductivity path of the fibers in the composite material is regulated to study the structural regulation and thermal conductivity performance of the resulting thermal conductive polymer composite material.The following sections provide a detailed account of the content and main conclusions of this research.:（1）The conventional approach of using traditional thermal conductive fillers in epoxy resin based thermal conductive composite materials for constructing three-dimensional thermal conductive pathways has limited effectiveness due to interfacial thermal resistance issues,making it difficult to achieve high thermal conductivity in both horizontal and vertical directions.To overcome this limitation,this study utilizes high-performance poly（p-phenylene benzoxazole）continuous fibers（PBO）as the thermal conductivity enhancement material for epoxy resin.By using continuous polymer thermal conductivity fibers for reinforcement,interfacial thermal resistance between the thermal conductivity phase and the matrix in the heat transfer direction can be eliminated,which solves the problem of interfacial thermal resistance in the thermal conductivity path of traditional thermal conductivity fillers.Moreover,the heat can be transmitted completely along the fiber direction,leading to low thermal resistance on the thermal conductivity path.Through twisting and weaving methods,a three-dimensional thermal conductivity structure is constructed,and the thermal conductivity of epoxy/PBO fiber composite materials is studied under different PBO content and twisting degrees.The experimental results demonstrate that incorporating continuous PBO fibers with high intrinsic thermal conductivity enhances the thermal conductivity of epoxy/PBO fiber composites and achieves high thermal conductivity in multi-dimensional directions by adjusting the fiber structure.When the PBO content is 33 vol%,the thermal conductivity of the composite material in the vertical and horizontal directions is 10.85 W/m K and 7.15 W/m K,respectively.The addition of PBO fibers also improves the mechanical and wave transmission properties of composite materials.Therefore,utilizing continuous fibers with excellent intrinsic thermal conductivity provides a new approach for the preparation of thermal conductive composite materials.（2）The method of adding high intrinsic thermal conductivity polymer continuous fibers as thermal conductive fillers effectively improves thermal conductivity at a lower dosage,resulting in multi-dimensional high thermal conductivity.However,the true influencing factors of low interfacial thermal resistance in composite materials when continuous fibers are used as thermal conductive phases are not yet clear.Therefore,this paper combines molecular dynamics,phonon simulation,and classical nonlinear thermal conductivity models to analyze the thermal conductivity mechanism of PBO continuous fibers from multiple aspects such as molecular chain structure,aggregated state structure,and micro composite structure.The research results indicate that the average interfacial thermal resistance inside PBO continuous fibers is only 10^-10m²W/K,which is several orders of magnitude smaller than the interfacial thermal resistance of polymer fillers of 10^-6m²W/K.This indicates that PBO fibers effectively overcome the problem of high interfacial thermal resistance in traditional filler polymer composites when added to the matrix.At the same time,the simulation analysis results show that the phonon state density altitude of the macromolecular chains in the PBO crystal region and amorphous region in the low-frequency region is highly overlapped,and their phonon vibration modes are close.The continuous conformation of the macromolecular chains in the amorphous region is like a thermal bridge that connects the adjacent crystal regions in series,and the phonon scattering at its interface can be almost ignored,so the internal interface thermal resistance is extremely low.This work demonstrates that the use of continuous fibers can effectively reduce the interfacial thermal resistance of composite materials,and supports the viewpoint that the orientation of the non-oriented region is more crucial,which may provide some reference space for the analysis of fiber thermal conductivity mechanism.（3）Although the introduction of continuous polymer fibers in the previous section reduced the interfacial thermal resistance of epoxy composite materials and improved multi-dimensional thermal conductivity,the limited inherent thermal conductivity of the fibers due to structural defects of the polymer,such as phonon conduction restrictions,phonon group velocity limitations,and the presence of amorphous regions and molecular chain ends,requires further exploration to reduce internal structural defects and enhance fiber thermal conductivity.This study uses graphitization to regulate the crystal structure of graphite in fibers to improve intrinsic thermal conductivity.PBO-based carbon fibers and carbon fiber cloth materials are obtained through high-temperature graphitization treatment of PBO fibers and fiber fabrics.High-temperature graphitization treatment increases the thermal conductivity of PBO-based carbon fibers to 805.22 W/m K at a fiber graphitization degree of 0.34,which is 17 times higher than the intrinsic thermal conductivity of PBO fibers（47.56W/m K）.Direct high-temperature graphitization treatment of fiber fabrics ensures the integrity of the three-dimensional structure of the fiber cloth.The resulting epoxy resin/PBO-based carbon fiber block composite material achieves high thermal conductivity in the horizontal and vertical directions,with thermal conductivity coefficients of 9.718 W/m K and 5.784 W/m K,respectively.However,fiber embrittlement after graphitization limits the expected improvement in thermal conductivity of the composite material.Nonetheless,this design approach may still provide new ideas and opportunities for high-performance thermal management applications of carbon fiber composite materials,even though the continuous structure of the fiber portion is damaged during the post-treatment process.更多还原