微电子技术的快速发展对器件的散热提出了更高的要求，进而对高导热绝缘材料提出了更高的要求。高分子材料综合性能优异，受到研究者广泛关注。但高分子热导率较差，块体高分子热导率仅为0.1W/(m·K)量级。高分子材料中存在大量长链共价键，理论上可作为声子传导的通道，但块体的本征热导率很低，主要原因为块体高分子的分子链取向差，声子散射强。电纺高分子纤维，可提高分子链取向，提升热导率。向电纺纤维中添加纳米材料，可以进一步提升分子链取向，进而提升高分子热导率。聚酰亚胺（PI）是一种综合性能非常优异的高分子材料，是柔性电路板的基板材料。提升其热导率，制备高导热PI材料，可以提高其应用潜力。本文电纺PI纤维中引入氮化硼（BN）纳米片，使用原位聚合-静电纺丝法成功制备了BN/PI纤维，并研究了BN加入对纤维热导率和分子链取向度的影响，发现BN的添加可以提升分子链取向度，进而提升热导率。分散良好的BN可以在电纺过程中诱导高分子链的取向，抑制解取向作用。BN添加量为2%时，热导率最高达到10.76 W/(m·K)，较纯PI纤维有260%的提升。BN添加量超过2%时，BN分散性下降，分子链取向度和热导率下降，表明高导热的BN未起到导热填料的作用。芳香族聚酰亚胺还是一种良好的石墨化前驱体，石墨化后的PI基石墨材料具有优异的导热性能。硼元素的加入可诱导碳纤维的石墨化，提升石墨化度。本文将电纺BN/PI纤维碳化、石墨化，成功制备了BN/PI基石墨纤维，并研究了BN加入对石墨纤维热导率、石墨化度的影响，发现氮化硼的添加可以提升石墨化度，增大石墨微晶尺寸，进而石墨纤维热导率。BN添加量为2%时，热导率最高达到244.80 W/(m·K)，较纯PI基石墨纤维有36%的提升。BN添加量与石墨纤维热导率的关系和与高分子纤维热导率的关系一致，表明高分子链取向度与石墨化后的声子输运有相关性。加入BN提升石墨化度的机理一方面是高分子纤维的取向度提升，另一方面可能为氮化硼可以与碳形成固溶体，诱导石墨形核和平面取向，起到诱导石墨化作用。
The rapid development of microelectronics technology has put forward higher requirements for heat dissipation of devices, and further higher requirements for high thermal conductivity insulating materials. Polymer materials with excellent comprehensive performance have attracted wide attention from researchers. However, thermal conductivity of polymer is poor, and thermal conductivity of the bulk polymer is at the level of 0.1W/(m·K). There are a large number of long-chain covalent bonds in polymer materials, which can theoretically be used as phonon conduction channels, but the intrinsic thermal conductivity of the block is very low, mainly due to the poor molecular chain orientation and strong phonon scattering. Electrospun polymer fibers can improve molecular chain orientation and increase thermal conductivity. Adding nanomaterials to electrospun fibers can further improve the molecular chain orientation, thereby increasing the thermal conductivity of the polymer.Polyimide (PI) is a polymer material with excellent overall performance, and it is the substrate material of FPC (Flexible Printed Circuit). Increasing its thermal conductivity and preparing high thermal conductivity PI materials can increase its application potential. In this paper, boron nitride (BN) nanosheets were introduced into electrospun PI fibers. BN/PI fibers were successfully prepared using in-situ polymerization-electrospinning method, and the effects of BN addition on fiber thermal conductivity and molecular chain orientation were studied. It was found that the addition of BN can increase the degree of molecular chain orientation, thereby increasing the thermal conductivity. The well-dispersed BN can induce the orientation of the polymer chain during the electrospinning process and suppress the deorientation effect. When the amount of BN added is 2%, the maximum thermal conductivity reaches 10.76 W/(m·K), which is a 260% improvement over pure PI fiber. When the amount of BN added exceeds 2%, the dispersion of BN decreases, and thermal conductivity decrease, indicating that BN does not function as a thermally conductive filler.Aromatic polyimide is also a good graphitization precursor. PI-based graphite material after graphitization has excellent thermal conductivity. The addition of boron element can induce the graphitization of carbon fiber and improve the graphitization degree. In this paper, electrospun BN/PI fiber was carbonized and graphitized, and BN/PI-based graphite fiber was successfully prepared. The effect of BN addition on the thermal conductivity and graphitization of graphite fiber was studied. It was found that the addition of boron nitride can improve graphitization degree, increase the size of graphite crystallites, and then thermal conductivity of graphite fibers. When the amount of BN added is 2%, the maximum thermal conductivity reaches 244.80 W/(m·K), which is a 36% improvement over pure PI-based graphite fibers. The relationship between the amount of BN added and the thermal conductivity of the graphite fiber is consistent with the thermal conductivity of the polymer fiber, indicating that the orientation degree of the polymer chain is related to the phonon transport after graphitization. The mechanism of adding BN to increase the degree of graphitization is on the one hand to increase the degree of orientation of the polymer fiber, on the other hand, it may be that boron nitride can form a solid solution with carbon, induce graphite nucleation and planar orientation, and play a role in inducing graphitization.
