金属镍粒子的价格低廉，具有高磁导率、高饱和磁化强度及较高的温度稳定性，可用于制造吸波材料。但是，单一的磁损耗机制不能使其具有极高的吸波性能[9]。鉴于此，本文以氧化石墨烯和乙酰丙酮镍为原料，用一步溶剂热法制备具有协同作用的磁性多孔RGO@Ni复合材料，并深入研究其吸波性能。
1 实验方法
1.1 实验用原材料
石墨(粒径8 μm)，高锰酸钾(KMnO4)，浓硫酸(H2SO4)，乙酰丙酮镍(AANi)，聚乙烯醇(PVA)，十六烷基三甲基溴化铵(CTAB)，乙二醇(EG)。
1.2 测试用仪器
用Max-2400型X射线衍射仪分析晶体结构，X射线源为Cu靶(λ=0.15406 nm)，扫描范围在10º~80º内，扫描速度10°/min。使用SENTERRA R200型拉曼光谱仪测量样品的拉曼光谱，激光的波长为633 nm，扫描范围为400~4000 cm-1。使用X射线光电子衍射仪(ESCALAB 250Xi型)分析样品的表面化学元素组成和化学状态。用XFlash 5030型扫描电镜(SEM)和Tecnai F30透射电子显微镜(TEM)观察材料的形貌及微观结构。
使用Aglilent 8720ET型矢量网络分析仪测试样品的电磁参数(ε, μ)，频率范围为1~18 GHz。测试时将吸波粒子与石蜡基体混合均匀，使用同心轴模具制成内径为3.04 mm、外径为7.0 mm、厚度约为3.00 mm的空心环状样品。
1.3 氧化石墨烯的制备
采用改进的Hummers法制备氧化石墨烯(RGO)：将50 mL浓H2SO4倒入冰水浴内的500 mL三口烧瓶中，缓慢加入2.0 g鳞片石墨后搅拌30 min。然后将6.0 g的KMnO4分批缓慢加入，使整体温度低于10℃在冰水浴条件下反应2 h；随后移入35℃水浴锅中继续搅拌3 h，缓慢滴加100 mL去离子水后在95℃反应30 min。最后滴加100 mL去离子水和15 mL双氧水终止反应。将产物自然冷却到室温，用5%的稀盐酸洗涤下层溶液后再用去离子水洗至中性。将此溶液超声处理2h后进行冷冻干燥，得到黄色氧化石墨烯粉末。
1.4 RGO@Ni复合材料的制备
先在冰水浴状态下超声处理将GO粉末分散在乙二醇(EG)中，制备出GO悬浮液(5 mg/mL)。然后在超声处理下将1.5 g AANi溶解在40 mL EG中，并充分搅拌以形成均匀的绿色溶液。然后在机械搅拌条件下将0.5 g CTAB，1 g PVA和自制的40 mL GO/EG溶液添加到上述溶液中，使其均匀混合。将所得溶液转移到不锈钢反应釜(150 mL)中，使其在200℃反应14 h。最后，将产物自然冷却到室温并用去离子水充分洗涤，最后将其冷冻干燥得到多孔磁性RGO@Ni复合材料。
为了进行对比实验，在相同的条件下不添加GO和AANi制备纯Ni和RGO。合成工艺流程，如图1所示。
图1
图1
合成多孔磁性RGO@Ni复合材料的示意图
Fig.1
Schematic illustration for the formation of the porous magnetic RGO@Ni nanocomposites
2 结果和讨论
2.1 结构和物相组成
用XRD、Raman光谱和XPS等手段表征材料的结构特征。用XRD测量鉴定样品的相结构和结晶度。
图2给出了纯RGO、纯Ni和RGO@Ni复合材料的XRD谱图。可以看出，RGO谱中在2θ=24°附近出现较宽的特征衍射峰，对应于RGO的(002)平面，证明GO被成功还原成RGO。RGO@Ni的XRD谱中在44.7°，51.9°，76.5°处出现特征衍射峰，分别对应于金属镍的(111)、(200)、(220)晶面(JCPDS No.04-0850)。谱图中窄而尖的峰表明Ni是高度结晶的，说明合成出的Ni纯度很高。与纯Ni相比，在RGO@Ni样品的衍射谱中可观察到19.7°(d=0.455 nm)处的衍射峰。以上数据表明，在溶剂热反应的条件下GO被还原成RGO，AANi被还原成Ni纳米粒子，说明已经合成出RGO@Ni复合材料。
图2
图2
GO、RGO、Ni以及RGO@Ni复合材料的XRD谱图
Fig.2
XRD patterns of RGO, Ni microspheres, RGO@Ni nanocomposites
拉曼光谱可用于检测碳材料的石墨化程度和内部结构缺陷。石墨、GO以及RGO@Ni复合材料的拉曼光谱图，如图3所示。从图3可见，三条曲线均在1350 cm-1和1590 cm-1附近处出现特征峰，分别对应于D和G波段。D带对应于碳原子无序诱导或晶格结构中的缺陷；而G波段可归因于碳原子sp2杂化的面内拉伸振动，代表材料近似石墨结构的程度[10~13]。值得注意的是，D和G谱带的面积强度比(ID/IG值)与碳原子晶体的缺陷有关。ID/IG值越高，则缺陷程度越高，石墨化程度越低[10~13]。随着反应的进行，与原始石墨(ID/IG=0.78)相比，GO的ID/IG值(1.34)显著增大，表明碳骨架结构中有更多的缺陷。值得注意的是，RGO@Ni复合材料的ID/IG(1.13)值小于GO，表明在还原过程中产生了新的sp2结构域，并且碳原子晶格缺陷得到了还原，有利于改善阻抗匹配，产生更多的极化中心使电磁波衰减。
图3
图3
石墨、GO和RGO@Ni复合材料的拉曼谱图
Fig.3
Raman spectra of graphite, GO, RGO@Ni nanocomposites
用X射线光电子能谱(XPS)表征了GO和RGO@Ni的表面化学组成和元素价。如图4a所示，在RGO@Ni纳米复合材料的XPS低分辨率总谱中出现特征峰，表明样品完全由Ni，O和C元素组成。在全谱中未检测到其他元素，与XRD表征结果一致，表明合成出的材料比较纯。Ni 2p的高分辨率光谱，如图4b所示。图中的两个明显的峰值分别位于855.9 eV和873.6 eV，对应于镍的Ni 2p3/2和Ni 2p1/2(0价)。尤其是在861.5 eV和879.7 eV处也出现了与Ni2+的Ni 2p3/2和Ni 2p1/2相对应的峰，表明生成了Ni-O键，也证明镍在空气中其表面有一层致密的氧化膜[14]。RGO@Ni纳米复合材料的光谱，如图4c所示。分峰拟合结果表明，位于结合能为284.7、286.2和288.5 eV的三个峰分别归因于C-C/C=C，C-O-C和O-C=O基团。GO的C 1s光谱如图4d所示，位于结合能为284.8、286.9和288.7 eV的三个峰分别归因于C-C/C=C，C-O-C和O-C=O基团。显然，与GO C 1S的光谱相比，RGO@Ni的C-O-C和O-C=O基团的峰值明显减少，表明在溶剂热反应中含氧基团部分消失了，进一步表明GO已经还原为RGO。
图4
图4
RGO@Ni的XPS全光谱、RGO@Ni的Ni 2p光谱、RGO@Ni的C 1s频谱以及GO的C 1s光谱
Fig.4
XPS full spectrum of RGO@Ni (a), Ni 2p spectrum of RGO@Ni (b), C 1s spectrum of RGO@Ni (c) and C 1s spectrum of GO (d)
2.2 微观结构
图5
2.3 吸波性能
最小反射损耗(RLmin)和有效吸收带宽(EAB)，是评价吸波性能的重要标准。根据传输线理论，由给定频率范围内各厚度下的相对磁导率和介电常数，可计算材料的反射损耗(RL)：
图6a~c给出了在1~18 GHz范围内，填充剂含量(质量分数)为25%不同厚度的Ni、RGO和RGO@Ni复合材料的三维反射率图像。可以看出，厚度从2 mm增大到5 mm纯Ni粒子和纯RGO的电磁波反射损耗值在1~18 GHz范围内都未达到-10 dB，说明单一损耗机制的吸波粒子性能不高。而纳米复合材料其EMW的吸收性能大大提高，可归因于RGO和Ni之间的协同作用。显然，随着厚度的增加反射损耗的峰值从高频转移到低频。这种现象可以用四分之一波长公式
图6
图6
Pure Ni、RGO以及RGO@Ni.的三维反射率图像和2.2 mm处Ni、RGO和RGO@Ni的RL值
Fig.6
Three-dimensional representation of RL values for pure Ni (a); RGO (b); RGO@Ni (c) and RL values for Ni, RGO, RGO@Ni at 2.2 mm (d)
解释，式中c为光速，
2.4 电磁参数
材料的电磁波(EMW)吸收性能与相对复磁导率(μr=μ′-jμ″)和相对复介电常数(εr=ε′-jε″)有关[16]。图7给出了Ni，纯RGO和RGO@Ni复合材料在1~18 GHz范围内的相对复介电常数和相对复磁导率。图8a~b给出了各个样品的介电常数实部(ε′)和介电常数虚部(ε″)。由于共振和介电弛豫，RGO和RGO@Ni的ε′和ε″值表现出对频率的明显依赖性，随着频率的提高呈现降低的趋势。重要的是，在RGO@Ni的ε″值的图像中出现多种共振峰，可归因于偶极极化和界面极化引起的极化损耗[17]。偶极极化通常是多孔RGO@N复合材料中的残留缺陷和官能团导致偶极子重定向引起的；石墨烯和Ni之间的电荷转移导致在Ni和多孔石墨烯层的界面处形成电偶极子，从而产生界面极化损失。
图7
图7
纯N、纯RGO和RGO@Ni复合材料的介电常数
Fig.7
Relative complex permittivity and relative complex permeability (a) real part (ε′)； (b) imaginary part (ε″)； (c) real part (μ′) and (d) imaginary part (μ″)
图8
图8
介电损耗正切、磁损耗正切(tanδμ)、Cole-Cole半圆(ε′对ε″)以及1~18 GHz频率范围的C0值
Fig.8
dielectric loss tangent (tanδε) (a), magnetic loss tangent (tanδμ) (b), Cole-Cole semicircles (ε′ versus ε″) (c~e) and C0 versus f in the range of 1~18 GHz (f)
除介电损耗外，磁损耗也是影响电磁波吸收的关键因素。图7c和d给出了Ni、纯RGO和RGO@Ni复合材料在1~18 GHz范围内的磁导率实部图(μ′)和磁导率虚部(μ″)曲线。可以看出，RGO@Ni复合材料的μ′值随着频率的提高呈现降低的趋势。在1~18 GHz范围内的共振峰，可归因于磁矩运动松弛作用。
2.5 介电损耗和磁损耗
图8a和b给出了Ni、RGO和RGO@Ni复合材料在1~18 GHz频率范围内的介电损耗角正切tanδε和磁损耗角正切tanδμ曲线。可以看出，在整个频率范围内RGO@Ni的tanδε值远高于tanδμ值，表明介电损耗是影响RGO@Ni复合材料微波吸收性能的主要因素。在tanδε和tanδμ的图像中出现多重共振峰，表明RGO@Ni复合材料具有多种磁损耗机制。总之，强介电损耗和弱磁损耗之间的协同效应，对微波吸收有重要的影响。
根据Debye偶极弛豫[17]理论，
其中εs为静态介电常数，τ和ε∞分别为极化弛豫时间和高频极限处的相对介电常数。若介电损耗只与Debye偶极松弛有关，则
除了介电损耗，磁损耗是影响电磁波损耗的另一个关键因素。材料的磁损耗主要与磁滞、自然共振、涡流效应、畴壁位移以及交换共振等有关[19]。在1~18 GHz范围内磁滞损耗、畴壁共振可忽略不计。
2.6 阻抗匹配和衰减常数
电磁波在传播过程中入射到材料内部，在入射面发生电磁波反射和透射。为了使材料更有效地吸收电磁波，入射的电磁波应该尽可能多地进入材料内部而不是发射到表面，这需要较好的阻抗匹配。同时，在透射过程中进入材料内部的电磁波应该完全衰减掉而转换成其他形式的能量，也要求满足衰减匹配。因此，阻抗匹配(Z)和衰减常数(α)是确定材料优异吸收特性的两个重要因素[22]，可分别由
和
图9
图9
Ni、RGO和RGO@Ni复合材料的衰减系数和阻抗匹配系数
Fig.9
Attenuation constant and Impedance matching ratio of Ni、RGO and RGO@Ni composites
由此可见，RGO@Ni复合材料优异的电磁波吸收特性主要由以下因素决定：(1)RGO@Ni泡沫独特的3D多孔结构引起多次反射和散射，导致电磁波在受限的空隙中传播很长的距离，并将其转换为热能或其他类型的能量。(2)在RGO与Ni之间有大量界面，从而引起界面极化。这种界面极化与RGO上的缺陷引起的偶极极化相关，从而使微波能量衰减。(3)在1~18 GHz频率范围内的磁损耗，由自然共振、交换共振和涡流损耗确定。更重要的是，介电损耗和磁损耗之间的协同作用不仅抑制了涡流效应，而且得到合适的复介电常数，使微波更容易进入吸收体而增大电磁波损耗，从而实现更高的电磁波(EMW)吸收性能。
3 结论
以氧化石墨烯和乙酰丙酮镍为原料用一步自组装溶剂热法可制备具有多孔结构的三维(3D)RGO@Ni纳米复合材料。与纯Ni纳米晶体和石墨烯相比，RGO@Ni纳米复合材料在最小反射损耗(RLmin)和最大有效吸收带宽(EAB)方面显示出优异的EMW吸收性能。RGO@Ni纳米复合材料在2.2 mm厚度下的RLmin为-61.2 dB，在2.5 mm的匹配厚度下覆盖的EAB范围最广，为6.6 GHz(10.5~17.1 GHz)。RGO@Ni纳米复合材料优异的微波吸收性能，可归因于多重极化弛豫的协同效应和特殊的多孔结构。
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