石墨烯/碳纳米管复合材料的微波吸收性能研究

石墨烯和碳纳米管都是新型纳米尺寸碳材料,具有极大的比表面积、良好的导电性以及优秀的机械性能等特性。通过微波膨化法在石墨烯(寡层石墨)表面空隙结构内生长了碳纳米管,制备出石墨烯/碳纳米管复合材料,碳纳米管不仅可以发挥连接石墨烯层片结构的作用,还可以与石墨烯共同发挥协同吸波效应;同时生长碳纳米管所添加的催化剂在微波状态下分解为纳米磁性颗粒,提高整体复合材料的吸波性能。通过采用SEM、EDX、XRD等对材料的形貌、化学成分进行表征,并用矢量网络分析仪测试了材料在2~18GHz频带内的复介电常数和复磁导率,利用计算机模拟出不同厚度的微波衰减性能。结果表明,材料的电磁损耗机制由电介质损耗、磁损耗共同构成,微波吸收峰随着材料厚度的增加向低频移动,当厚度为2.5mm时,在14.4GHz时最大损耗值为-28dB,并且在频带12.4~17.7GHz的范围内达到-10dB的吸收。     

掺天然磁铁矿水泥基复合材料电磁波吸收性能研究

以天然磁铁矿粉为低成本吸收剂制备了水泥基吸波材料,采用矩形波导法测试了不同磁铁矿掺量试样的复介电常数与复磁导率,并基于传输线与阻抗理论计算了试样的反射率。结果表明,掺天然磁铁矿水泥复合材料在8.2~12.4GHz频率范围内具有较高的电磁参数,表现出明显的电损耗与磁损耗。磁铁矿水泥复合材料存在2.5和8.5 mm两个匹配厚度。随磁铁矿掺量的增加,吸收峰向低频方向移动。当试样厚度为2.5mm时,磁铁矿掺量15%时,复合材料吸波性能最好,吸收峰值达到-14.8dB,反射率低于-10dB的频带达到2.1 GHz,而试样厚度为8.5mm时吸收峰值较低,但是吸收频带较窄。     

不同厚度的双层碳团簇型微波隐身材料性能研究

通过微波隐身材料结构设计,对不同厚度的双层碳团簇型微波隐身材料在8.2～12.4GHz频率范围内的吸波性能进行了研究.发现对于不同的厚度要求,要使材料具有良好的吸波性能,应选用具有不同电导率σ的碳团簇型材料.对于总厚度为4mm的材料,反射峰值-31.0dB,有效带宽为3.74GHz;对于变换层为1mm、吸收层为2mm、总厚度为3mm的材料,最小反射率为-40.0dB,有效带宽3.8GHz;对于变换层和吸收层分别为1mm的材料,最小反射率达-33dB,有效带宽为3.3GHz.同时发现,当变换层和吸收层厚度相等时,材料的排列顺序不同,所得的吸收效果会存在很大差异,即材料的方向性显著,而当两者厚度不等时,材料的吸波性能基本与其排列顺序无关,方向性不显著.     

多元助剂改性羰基铁粉雷达波低频吸波性能研究

采用三种处理剂对羰基铁粉样品进行表面复合改性,研究了多元助剂对羰基铁粉样品表面改性后的微观结构及电磁参量的影响。结果表明,多元助剂的使用使羰基铁粉表面形成了一层致密的有机绝缘薄膜,能有效降低羰基铁粉的复介电常数,增加复磁导率虚部,提高吸波材料的电磁匹配性能,改善吸收剂的低频吸收效果。根据传输线理论计算吸波材料的反射损耗(Reflection loss,RL),在厚度为2mm时,三元助剂改性羰基铁粉的反射损耗峰值在2GHz附近达到-15dB,在RL-10dB的有效吸收频宽为1GHz(1.6~2.6GHz),具有较好的雷达波低频吸波性能。     

Ni/RGO复合材料的制备及其吸波性能的研究

利用水热还原法在石墨烯(RGO)表面负载磁性金属Ni粒子,通过XRD、FT-IR、拉曼、SEM对其物相、形貌等进行表征分析。结果表明,通过水热原位还原法,直径约100nm的Ni粒子被均匀负载到RGO表面,成功制备了镍负载石墨烯(Ni/RGO)复合材料。利用矢量网络分析仪对其吸波性能进行检测,结果表明,样品1(0.25mmolNi负载RGO)表现出了优异的吸波性能,在吸波涂层厚度为4.5mm,吸波频率为7.2GHz时,最佳反射损耗为-55.9dB,其有效吸波宽度为5.8~9.2GHz。Ni/RGO复合材料是一种很有前途的具有低密度、强吸收、宽频带、厚度薄的微波吸收材料。     

Ni0.4Co0.2Zn0.4Fe2O4/BaTiO3纳米纤维双层吸波涂层的微波吸收特性研究

采用静电纺丝法制备了平均直径分别为180 nm和220 nm的BaTiO₃(BTO)和Ni_0.4Co_0.2Zn_0.4Fe₂O₄(NCZFO)纳米纤维, 使用X射线衍射(XRD)、场发射扫描电镜(FESEM)和矢量网络分析仪(VNA)对纤维的物相结构、表面形貌和微波电磁参数进行了表征, 并根据传输线理论分析评估了以BTO和NCZFO纳米纤维为吸收剂的硅橡胶基单层和双层结构吸波涂层在2~18 GHz范围内的微波吸收性能。结果显示, 由于BTO纳米纤维的介电损耗与NCZFO纳米纤维的磁损耗的有机结合和阻抗匹配特性的改善, 以NCZFO纳米纤维/硅橡胶复合体(S1)为匹配层、BTO纳米纤维/硅橡胶复合体(S2)为吸收层的双层吸波涂层比相应单层吸波涂层表现出更为优异的吸收性能。通过调节匹配层与吸收层的厚度, 在4.9~18 GHz范围内反射损耗可达–20 dB以下; 当吸收层和匹配层的厚度分别为2.3 mm和0.5 mm时, 最小反射损耗位于9.5 GHz达–87.8 dB, 低于–20 dB的吸收带宽为5 GHz。优化设计的NCZFO/BTO纳米纤维双层吸波涂层有望发展成为一种新型的宽频带强吸收吸波材料。     

化学镀镍碳纳米管的微波吸收性能研究

采用化学镀的方法对碳纳米管进行表面镀镍,TEM观察证实了碳纳米管上已镀覆了镍层,镀层厚度约8～15nm.采用HP8722ES矢量网络分析仪测量了样品在2～18GHz频率范围内的复介电常数(ε=ε′-jε″)和复磁导率(μ=μ′-jμ″).用吸收屏理论公式计算其反射损耗(R.L.)、匹配厚度(dm)及匹配频率(fm).结果表明,随着匹配厚度的增大,化学镀镍碳纳米管的吸收峰没有发生移动,当匹配厚度dm=0.2mm时,样品最低反射损耗达-11.40dB,对应的匹配频率fm=15.6GHz,而且在整个电磁波频率测试范围内,反射损耗值均＜-10.5dB,能够作为一种理想的电、磁损耗型吸波材料.     

膨胀石墨基纳米镍、铁、钴化学镀制备复合吸波材料

为了获得薄、轻、宽、强等性能理想的吸波材料,采用化学镀的方法在膨胀石墨表面镀覆纳米镍、镍钴、镍铁钴,制备了复合吸波材料.SEM和EDs分析证实,膨胀石墨表面镍层、镍钴层、镍铁钴层的镀覆厚度约为70～150 nm.采用HP8722ES矢量网络分析仪测量了复合吸波材料在2～18 GHz内的复介电常数(ε=ε'-jε")和复磁导率(μ=μ'-jμ").用吸收屏理论公式计算了反射率损耗(R.L)、匹配频段(fm)及匹配厚度(dm).结果表明,当dm=0.3 mm时,镀覆镍铁钴层的复合吸波材料最低的反射损耗达-28 dB,对应的fm=13.5 GHz,R>L<-10 dB时频宽达7.5 GHz.本法制备的复合吸波材料符合"轻、薄、宽、强"的现代要求.     

化学共沉淀法制备的纳米Ni0.5Zn0.5CexFe2-xO4铁氧体微波吸收特性研究

采用化学共沉淀法制备了纳米Ni0.5Zn0.5CexFe2-xO4(x=0,0.005,0.01,0.015)铁氧体吸波材料,用AV3618型微波矢量网络分析仪测试了样品在8.2～12.5GHz范围内的微波吸收特性,实验结果表明:稀土元素铈的含量影响材料的吸波性能,当x=0.01时, 纳米Ni0.5Zn0.5CexFe2-xO4铁氧体的吸波性能最佳.对于Ni0.5Zn0.5Ce0.01Fe1.99O4铁氧体吸波材料,当涂层厚度为1mm时,在测试频段内有三个吸收峰,在8.8GHz处,其最大吸收衰减量为15.4dB,10 dB以上带宽达3.8GHz,适量掺杂稀土元素铈是提高镍锌铁氧体吸波材料性能的一种有效途径.     

嵌入分形频率选择表面的低频超薄吸波层的设计

研究了频率选择表面对超薄多层微波吸波体在低频(L和S频段)吸波性能的影响. 分别采用硫化工艺和激光刻蚀方法制备出传统的微波吸收材料(MAM)--橡胶板和FSS层, 然后利用它们合成多层微波吸波体(MMA)样品, 在NRL弓形法测试系统中测量该样品的反射率. 发现随着FSS层在传统吸波材料层中的引入, 确实可以增强整个多层吸波体在低频段的吸波性能. 实验结果显示, 当两个FSS层在多层吸波体中适当排列时, 可以在1 GHz得到一个–3.49 dB的反射率峰值, 最大反射峰值可达–9.35 dB, 这时的样品厚度是1.8 mm. 本研究为吸波材料的吸波性能向低频段的拓展提供了一种有效的方法.     

Highly conducting graphene sheets and Langmuir-Blodgett films

Graphene is an intriguing material with properties that are distinct from those of other graphitic systems. The first samples of pristine graphene were obtained by 'peeling off' and epitaxial growth. Recently, the chemical reduction of graphite oxide was used to produce covalently functionalized single-layer graphene oxide. However, chemical approaches for the large-scale production of highly conducting graphene sheets remain elusive. Here, we report that the exfoliation-reintercalation-expansion of graphite can produce high-quality single-layer graphene sheets stably suspended in organic solvents. The graphene sheets exhibit high electrical conductance at room and cryogenic temperatures. Large amounts of graphene sheets in organic solvents are made into large transparent conducting films by Langmuir-Blodgett assembly in a layer-by-layer manner. The chemically derived, high-quality graphene sheets could lead to future scalable graphene devices.     

Highly efficient electron field emission from graphene oxide sheets supported by nickel nanotip arrays

Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface due to a strong electric field. Graphene and its derivatives are expected to be efficient field emitters due to their unique geometry and electrical properties. So far, electron field emission has only been achieved from the edges of graphene and graphene oxide sheets. We have supported graphene oxide sheets on nickel nanotip arrays to produce a high density of sharp protrusions within the sheets and then applied electric fields perpendicular to the sheets. Highly efficient and stable field emission with low turn-on fields was observed for these graphene oxide sheets, because the protrusions appear to locally enhance the electric field and dramatically increase field emission. Our simple and robust approach provides prospects for the development of practical electron sources and advanced devices based on graphene and graphene oxide field emitters.     

Preparation and characterization of graphene/NiO nanocomposites

Graphene-based nanocomposites are emerging as a new class of materials that hold promise for many applications. In this article, we present a facile approach for the preparation of graphene/NiO nanocomposites using graphite oxide and nickel chloride as starting materials. The as-synthesized composites were characterized using X-ray diffraction, Fourier transform-Infrared spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, thermogravimetry, and differential scanning calorimetry analyses. It was shown that graphene sheets were decorated by the in situ-formed NiO nanoparticles to form a film-like composite structure and as a result, the restacking of the as-reduced graphene sheets was effectively prevented. The NiO-coated graphene nanocomposites can be expected to remarkably improve the electrochemical properties of NiO and would be the promising candidates for a variety of applications in future nanotechnology.     

Nanoscale synthesis and characterization of graphene-based objects

Graphene-based nano-objects such as nanotrenches, nanowires, nanobelts and nanoscale superstructures have been grown by surface segregation and precipitation on carbon-doped mono- and polycrystalline nickel substrates in ultrahigh vacuum. The dominant morphologies of the nano-objects were nanowire and nanosheet. Nucleation of graphene sheets occurred at surface defects such as step edges and resulted in the directional growth of nanowires. Surface analysis by scanning tunneling microscopy (STM) has clarified the structure and functionality of the novel nano-objects at atomic resolution. Nanobelts were detected consisting of bilayer graphene sheets with a nanoscale width and a length of several microns. Moiré patterns and one-dimensional reconstruction were observed on multilayer graphite terraces. As a useful functionality, application to repairable high-resolution STM probes is demonstrated.     

Growth of few layer graphene from commercial C2 hydrocarbons at low temperature and controlled surface functionalization

Graphene is mostly grown from methane on copper foils at a high temperature about 1000°C. In this research, a commercial ethylene-acetylene-ethane mixture was used as a clean precursor for graphene synthesis on nickel foils in a chemical vapor deposition reactor at 750°C. Furthermore, controlled functionalization of graphene sheets was achieved via hydrothermal oxidation at moderate pressure and temperature using nitric acid. Broadened 2D band and G band frequencies in Raman spectra indicated that pristine graphene (PG) was of high quality with low defects. X-ray diffraction results confirmed that PG has five layers. Transmission electron microscopy and N₂ adsorption-desorption analyses affirmed that the graphene is of a good quality, large surface area (562 m²/g) and small pore size. Fourier transform infrared spectroscopy confirmed functionalization process performance. Thermogravimetric analysis affirmed that the thermal stability of PG was drastically decreased after the functionalization process.     

Nanoscale synthesis and characterization of graphene-based objects

Abstract

Graphene-based nano-objects such as nanotrenches, nanowires, nanobelts and nanoscale superstructures have been grown by surface segregation and precipitation on carbon-doped mono- and polycrystalline nickel substrates in ultrahigh vacuum. The dominant morphologies of the nano-objects were nanowire and nanosheet. Nucleation of graphene sheets occurred at surface defects such as step edges and resulted in the directional growth of nanowires. Surface analysis by scanning tunneling microscopy (STM) has clarified the structure and functionality of the novel nano-objects at atomic resolution. Nanobelts were detected consisting of bilayer graphene sheets with a nanoscale width and a length of several microns. Moiré patterns and one-dimensional reconstruction were observed on multilayer graphite terraces. As a useful functionality, application to repairable high-resolution STM probes is demonstrated.     

Cytotoxicity of graphene oxide and graphene in human erythrocytes and skin fibroblasts

Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas aggregated graphene sheets exhibited the lowest hemolytic activity. Coating graphene oxide with chitosan nearly eliminated hemolytic activity. Together, these results demonstrate that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, the cytotoxicity of graphene oxide and graphene sheets was investigated by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. However, appropriate alternate assessments, using the water-soluble tetrazolium salt (WST-8), trypan blue exclusion, and reactive oxygen species assay reveal that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e., whether or not aggregation occurs) and mode of interaction with cells (i.e., suspension versus adherent cell types).     

A study of graphene films synthesized on nickel substrates: existence and origin of small-base-area peaks

Large-area graphene films, synthesized by the chemical vapor deposition (CVD) method, have the potential to be used as electrodes. However, the electrical properties of CVD-synthesized graphene films fall short of the best results obtained for graphene films prepared by other methods. Therefore, it is important to understand the reason why these electrical properties are inferior to improve the applicability of CVD-grown graphene films. Here, we show that CVD-grown graphene films on nickel substrates contain many small-base-area (SBA) peaks that scatter conducting electrons, thereby decreasing the Hall mobility of charges in the films. These SBA peaks were induced by small peaks on the nickel surface and are likely composed of amorphous carbon. The formation of these SBA peaks on graphene films was successfully suppressed by controlling the surface morphology of the nickel substrate. These findings may be useful for the development of a CVD synthesis method that is capable of producing better quality graphene films with large areas.     

Excellent photocatalytic performance under visible-light irradiation of ZnS/rGO nanocomposites synthesized by a green method

ZnS/graphene nanocomposites with different graphene concentrations (5, 10 and 15 wt.%) were synthesized using L-cysteine as surfactant and graphene oxide (GO) powders as graphene source. Excellent performance for nanocomposites to remove methylene blue (MB) dye and hexavalent chromium (Cr(VI)) under visible-light illumination was revealed. TEM images showed that ZnS NPs were decorated on GO sheets and the GO caused a significant decrease in the ZnS diameter size. XRD patterns, XPS and FTIR spectroscopy results indicated that GO sheets changed into reduced graphene oxide (rGO) during the synthesis process. Photocurrent measurements under a visiblelight source indicated a good chemical reaction between ZnS NPs and rGO sheets.     

A facile method to prepare stable noncovalent functionalized graphene solution by using thionine

A novel noncovalent functionalization approach was presented here to exfoliate and stabilize the chemical converted graphene and the low-temperature exfoliated graphene in aqueous solution by using thionine. It was found that the thionine exhibited the π-π stacking force with the graphene sheets, and the attachment of thionine molecules onto the graphenes’ surfaces could obviously improve their solubility in water. The AFM observation further verified that the graphene sheets with single-layer to double-layer were existed in the dispersions. The electronic test indicated that the modified graphene sheets possessed excellent electronic properties.