1成果简介
　　石墨烯基气凝胶微球（GAMs）具有出色的阻抗匹配和高损耗能力，是一种有效的电磁波（EMW）吸收体。开发石墨烯气凝胶微球的主要挑战是提高其介电损耗和机械强度，以满足实际应用要求。本文，四川大学王占华 特聘副研究员、夏和生 教授等在《ADVANCED FUNCTIONAL MATERIALS》期刊发表名为“Polyimide Derived Carbon/Graphene Hybrid Aerogel Microspheres for Strong and Wide Bandwidth Microwave Absorption”的论文，研究通过高压喷涂、冷冻成型、冷冻干燥、原位热还原和亚胺化以及高温热解等工艺制备了聚酰亚胺衍生碳/石墨烯混合气凝胶微球（PI∼NC-GAMs）。
　　由于 PI 衍生碳层的增强和支撑作用，制备的低堆积密度超轻混合 PI∼NC-GAMs 具有良好的抗压强度，在抗压应变为 40% 时抗压强度≈10.67 kPa，远优于空白的还原氧化石墨烯气凝胶微球（rGOAMs）。PI~NC-GAMs 可作为超轻、高效电磁波吸收体的有效候选材料，在厚度为 3.43 mm、填充量为 1 wt%、频率为 9.16 GHz 时，其反射损耗（RL）最小，为 -63.6 dB；在厚度为 2.57 mm、频率为 7.45 GHz（10.55-18 GHz）时，其有效吸收带（EAB）最大。PI∼NC-GAMs 的高效电磁波吸收性能源于分层多孔结构引起的多重散射和阻抗匹配、更好的偶极/界面极化以及混合气凝胶微球增强的导电性。
　　2图文导读 

　　图1、PAS@GOAM和PI∼NC-GAM制备过程的示意图，以及扫描电子显微镜 （SEM） 图像。

　　图2、a) Transmission electron microscopy (TEM) images of graphene layers after ultrasonically breaking the rGOAMs into the ethanol. b) PI derived carbon/graphene after ultrasonically breaking PI∼NC-GAMs-2 into the ethanol. c) High-resolution transmission electron microscopy (HRTEM) images of polyimide derived carbon structure after pyrolyzed at 800 °C. d) Energy dispersive spectroscopy mapping (EDS) of PI∼NC-GAMs-2. e) X-ray diffraction (XRD) spectra of PAS@GOAMs, PI@GAMs and PI∼NC-GAMs. f) Raman spectra of PAS@GOAMs, PI@GAMs and PI∼NC-GAMs. g) X-ray photoelectron spectroscopy (XPS) spectra of PI@GAMs and PI∼NC-GAMs.

　　图3、a) Fourier transform infrared spectroscopy (FT-IR) patterns of PAA, PI, PI@GAMs and PI∼NC-GAMs. b) Thermogravimetric analysis (TGA) curves for PAS@GOAMs, PI∼NC-GAMs, PAA and GO. c) Density of the monolith aerogel rGA, PI∼NC-GA-1, PI∼NC-GA-2 and PI∼NC-GA-3, and packing density of the aerogel microspheres rGOAMs, PI∼NC-GAMs-1, PI∼NC-GAMs-2 and PI∼NC-GAMs-3. d) Electrical conductivity of the aerogel microspheres rGOAM, PI∼NC-GAMs-1, PI∼NC-GAMs-2 and PI∼NC-GAMs-3, electrical conductivity of the monolith aerogel rGA, PI∼NC-GA-1, PI∼NC-GA-2 and PI∼NC-GA-3 (inset). e) Compressive strength of the monolith aerogel rGA, PI∼NC-GA-1, PI∼NC-GA-2 and PI∼NC-GA-3. f) Compressive strength and test equipment (inset) of the aerogel microspheres rGOAMs, PI∼NC-GAMs-1, PI∼NC-GAMs-2 and PI∼NC-GAMs-3.

　　图4、Measured frequency dependence of a) ε′, b) ε″, c) tan δε of PI∼NC-GAMs-1, PI∼NC-GAMs-2 and PI∼NC-GAMs-3. d) Cole–Cole curves of PI∼NC-GAMs-1, PI∼NC-GAMs-2 and PI∼NC-GAMs-3. e) Conduction loss and f) polarization loss of 1 wt% PI∼NC-GAMs-1, 1 wt% PI∼NC-GAMs-2 and 1 wt% PI∼NC-GAMs-3. 3D reflection loss and projection plots of g) 1 wt% PI∼NC-GAMs-1, h) 1 wt% PI∼NC-GAMs-2, and i) 1 wt% PI∼NC-GAMs-3.

　　图5、a) Schematic diagram of balancing impedance matching and attenuation loss. b) Attenuation constant and c) wave impedance of 1 wt% PI∼NC-GAMs-1, 1 wt% PI∼NC-GAMs-2 and 1 wt% PI∼NC-GAMs-3. Reflection loss curves in different thickness, matched quarter wavelengths and the impedance matching property of d) 1 wt% PI∼NC-GAMs-1, e) 1 wt% PI∼NC-GAMs-2 and f) 1 wt% PI∼NC-GAMs-3.

　　图6、The radar cross-section (RCS) simulation of all samples: a–c) PI∼NC-GAMs-1, GNCAM-2 and GNCAM-3 samples are covered by the perfect conductive layer (PEC). d) The radar cross-section simulation result diagram of PEC. e) RCS simulated curves of PEC and samples under different scanning angles. f) Simulation model of GAM. g) The electric field distribution with h) cross-sectional diagram of GAM at 10 GHz. i) vectored modal electric field distribution of GAM at 10 GHz. j) RCS simulation model. k) Schematic illustration of microwave absorption mechanism of PI∼NC-GAMs. l) Comparison in the filler loading, EABmax and density of the obtained PI∼NC-GAMs with the reported N-doped graphene-base and derivative materials; The values are listed in Table S2 (Supporting Information).
　　3小结
　　以氧化石墨烯和PI前驱体聚酰胺酸的混合物为喷涂原料，通过高压喷涂、冷冻成型、冷冻干燥和原位热处理制备了具有球形和三维微通道多孔网络结构的PI∼NC-GAMs。PI热解不仅能为 GAMs 的 N 掺杂提供氮源，还能产生残余碳结构，支撑气凝胶微球中的石墨烯网络骨架，改善力学性能。所开发的 PI∼NC-GAMs 的抗压强度高达10.67kPa，是空白 GAMs的533倍。它还具有轻质特性，堆积密度为6-20mg cm-3，具有低填料含量的优势。N 杂原子的存在可以形成更多的缺陷和极化中心，调整电性能，从而具有较强的电磁波吸收能力和较宽的吸收带宽。当填充量为1wt%、样品厚度为3.43mm 时，PI∼NC-GAMs的电磁波吸收最小反射损耗在9.16GHz 时达到 -63.6 dB；当样品厚度为2.57mm 时，EABmax 可达到7.45GHz（10.55-18 GHz）。高效的电磁波吸收性能归功于三维互连石墨烯网络引起的多重反射、分层多孔微球结构增强的阻抗匹配，以及包括偶极损耗、界面损耗和传导损耗在内的介质损耗的增强。
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