1 实验方法
1.1 rGO/Fe3O4 的制备
用水热法制备Fe3O4碳基复合材料。将0.27 g六水合氯化铁、0.26 g聚乙二醇和0.9 g乙酸钠置于30 mL的乙二醇中，用磁力搅拌器充分搅拌后加入10 mg氧化石墨烯，超声搅拌0.5 h使其充分分散。将得到的溶液倒入100 mL水热反应釜中，在200℃反应８h。将得到的试样标记为S10。在其它条件不变的情况下分别将氧化石墨烯添加量改变为30、50、70 mg，将得到的试样分别标记为S30、S50、S70；为了对比，制备不添加氧化石墨烯的试样并标记为S0。
1.2 rGO/Fe3O4/环氧树脂复合材料的制备
将环氧树脂加热到110℃使其全部熔融，然后加入rGO/Fe3O4复合物，搅拌均匀后加入酸酐固化剂(添加量是环氧树脂质量的40%)继续搅拌均匀。将得到的共混物浇注到(已在110℃预热)模具中，抽真空脱泡1 h，升温到150℃固化8 h后关闭烘箱，使其自然冷却至室温。得到的试样用于测试吸波性能(180 mm×180 mm×3 mm)和力学性能的(ASTM D 5942-96)。
1.3 性能测试
用MiniFlex600 X射线衍射仪分析测试氧化石墨烯、Fe3O4的晶体结构，用Tecnai F30 透射电镜和Su8010扫描电镜测试试样的晶面间距、微观结构和形貌以及纳米粒子分布。用矢量网络分析仪(Agilent 8720ET)的弓形法测试方法测试氧化石墨烯/Fe3O4/环氧树脂复合材料的吸波性能，用XJJ-50摆锤式简支梁冲击机测试氧化石墨烯/Fe3O4/环氧树脂复合材料的冲击强度。
2 结果和讨论
图1给出了氧化石墨烯和S30试样的晶面结构。可以看出，氧化石墨烯的XRD图谱中10.8°处出现一个尖峰，属于氧化石墨烯的(001)峰。使用Bragg方程nλ=2dsinθ计算出氧化石墨烯的层间间距为0.92 nm。在S30的XRD谱图中25.4°处出现一个宽峰，是一个非晶态的峰，而在10.8°处没有出峰。其原因是，在高温下氧化石墨烯的部分含氧官能团被脱除，氧化石墨烯被还原。同时，在S30的XRD谱中的18.3°、30.1°、35.4°、43.1°、53.4°、56.9°、62.5°和74.0°处出现了衍射峰，分别对应Fe3O4的(111)、(220)、(311)、(222)、(400)、(422)、(511)、(440)和(533)晶面。这些结果，与Fe3O4的标准XRD卡(JCPDSNO.65-3107)完全相符。X射线衍射分析结果表明，氯化铁在高温水热反应后得到了Fe3O4颗粒，而氧化石墨烯在高温下部分还原变成了还原氧化石墨烯。
图1
图2
图3给出了S30试样的扫描电镜照片。可以看出，Fe3O4颗粒呈正八面体的形貌，是反尖晶石结构Fe3O4的一种典型形貌，其表面覆盖有还原氧化石墨烯，Fe3O4颗粒的尺寸为2~4 μm。
图3
可用冲击强度表征材料的韧性。测试了不同环氧树脂含量的S30试样的冲击强度，以研究rGO/Fe3O4复合物对环氧树脂韧性的影响。参照ASTM D 5942-96标准，测试在 XCJ-4 型简支梁冲击试验机上进行，无缺口样条的尺寸为60 mm×6 mm×4 mm，测试结果如图4所示。可以看出，添加了rGO/Fe3O4复合物的环氧树脂的冲击强度都比纯环氧树脂高。S30添加量为30%的rGO/Fe3O4/环氧树脂复合材料，其冲击强度达到27 kJ/m2，比纯环氧树脂冲击强度提高了58.8%。上述结果表明，rGO/Fe3O4使环氧树脂增韧。其原因是，一方面，Fe3O4作为无机刚性粒子能阻止树脂中微裂纹的扩展；另一方面，还原氧化石墨烯表面大量的羟基、环氧基等活性官能团与环氧树脂和固化剂形成化学键，使rGO和环氧树脂界面间的粘结强度提高，进而提高了环氧树脂的冲击强度。但是，添加过多的rGO/Fe3O4复合物使环氧树脂复合材料的冲击强度下降。其原因是，过多的rGO/Fe3O4不容易均匀分散在环氧树脂中，出现的团聚在环氧树脂基体中产生缺陷而使材料的冲击强度降低。
图4
图4
rGO/Fe3O4/环氧树脂复合材料的冲击强度
Fig.4
Impact strength of rGO/Fe3O4/ epoxy resin composites
石墨烯、Fe3O4以及它们的复合物都是常用的吸波材料，而纯环氧是一种透波材料。为了研究rGO/Fe3O4复合物对环氧树脂吸波性能的影响，用弓形法测试了rGO/Fe3O4/环氧树脂复合材料的反射率值。图5给出了在环氧树脂中添加不同含量S30试样的反射率。可以看出，未添加rGO/Fe3O4复合物的纯环氧树脂没有吸波性能，添加rGO/Fe3O4复合物使吸波性能显著提高。rGO/Fe3O4复合物添加量为20%时，反射率小于-10 dB的频率范围为7.7~12.3 GHz，有效吸收频宽达4.6 GHz，覆盖了整个X波段。rGO/Fe3O4复合物添加量为30%时，最小反射率达-31.7 dB。表1列出了环氧树脂中添加不同含量S30的吸波性能，包括最小反射率值、反射率小于-10 dB的频率范围和频宽。
图5
图5
S30不同含量复合材料的反射率
Fig.5
Reflectance value with different S30 content in epoxy resin
表1 S30含量不同的复合材料的吸波性能
Table 1
| S30 content in epoxy resin | RLmin/dB | Frequency range/GHz (RL≤-10 dB) | The bandwidth /GHz (RL≤-10 dB) |
|---|---|---|---|
| 0% | -1.4 | - | - |
| 10% | -22.5 | 8.1-12.3 | 4.2 |
| 20% | -28.4 | 7.7-12.3 | 4.6 |
| 30% | -31.7 | 7.1-11.5 | 4.4 |
| 40% | -25.5 | 7.2-10.9 | 3.7 |
为了研究rGO和Fe3O4颗粒的配比对复合材料吸波性能的影响，分别在环氧树脂中添加30%的S0、S10、S30、S50、S70制备出五组对比样，用弓形法测试其吸波性能。图6表明，只添加Fe3O4颗粒虽然对环氧树脂的吸波性能有提高，但反射率值都大于-10 dB，并且其吸收频段在高频区间，S70样品改性后的环氧树脂吸收频段在低频区间，并且反射率值都大于-10 dB。表2列出了添加S0、S10、S30、S50、S70的环氧树脂复合材料的吸波性能，包括最小反射率值及其对应的频率、反射率小于-10 dB的频率范围和频宽。结果表明，磁性Fe3O4颗粒有利于高频电磁波吸收，介电材料rGO有利于低频电磁波吸收，并且磁性材料和介电性材料有合理的配比。对比S0、S10、S30、S50、S70五个试样改性后环氧树脂复合材料最小反射值出现的频率位置发现，随着石墨烯含量的提高试样达到最小反射率的位置向低频位置移动。其原因是，复合物中石墨烯相对含量提高使Fe3O4的相对含量降低，结果是rGO/Fe3O4复合物的介电常数明显上升，磁导率降低；但是，磁导率与介电常数的乘积呈现升高的趋势。根据四分之一波长公式，磁导率和介电常数的乘积升高，材料的谐振频率降低。因此，随着石墨烯含量的提高试样达到最小反射率的位置向低频位置移动。这表明，控制石墨烯和Fe3O4的相对含量可调控环氧树脂复合材料的吸波性能，使其用于吸收不同频率的电磁波。
图6
图6
添加S0、S10、S30、S50、S70的环氧树脂复合材料的反射率
Fig.6
Reflectance values of epoxy resins composite materials which added S0, S10, S30, S50 and S70
表2 添加有S0、S10、S30、S50、S70的环氧树脂复合材料吸波性能
Table 2
| Samples | The ratio of Fe to rGO | RLmin/dB | Frequency/GHz | Frequency range /GHz(RL≤-10 dB) | The bandwidth /GHz(RL≤-10 dB) |
|---|---|---|---|---|---|
| S0 | 56 : 0 | -7.9 | 10.6 | - | - |
| S10 | 56 : 10 | -22.1 | 10.35 | 8.7~12.5 | 3.8 |
| S30 | 56 : 30 | -31.7 | 9.2 | 7.1~11.5 | 4.4 |
| S50 | 56 : 50 | -17.7 | 8.1 | 6.5~10.9 | 4.4 |
| S70 | 56 : 70 | -6.7 | 4.0 | - | - |
3 结论
(1) 以氧化石墨烯、三氯化铁为原料用高温水热法制备的还原氧化石墨烯/Fe3O4复合物，Fe3O4颗粒呈正八面体的形貌，尺寸为2~4 μm，还原氧化石墨烯覆盖在Fe3O4颗粒表面。
(2) 在环氧树脂中添加rGO/Fe3O4复合物制备的复合材料，其冲击强度都比纯环氧树脂高，S30添加量为30%的rGO/Fe3O4/环氧树脂复合材料其冲击强度达到27 kJ/m2，比纯环氧树脂的冲击强度提高了58.8%。
(3) 在环氧树脂中添加rGO/Fe3O4复合物使复合材料吸波性能显著提高，rGO/Fe3O4复合物添加量为20%的复合材料其反射率在小于-10 dB的频率范围为7.7~12.3 GHz，有效吸收频宽达4.6 GHz，覆盖了整个X波段，rGO/Fe3O4复合物添加量为30%时，最小反射率达-31.7 dB。
(4) 随着石墨烯含量的提高，rGO/Fe3O4/环氧树脂复合材料达到最小反射率的位置向低频位置移动。这表明，控制石墨烯和Fe3O4的相对含量可调控环氧树脂复合材料的吸波性能。
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