Nickel/carbon hybrid nanostructures as microwave absorbers

Magnetic metal/carbon hybrid nanostructures are novel materials having multifunctional properties. Here we report the microwave absorbing properties of nickel/carbon nanostructures synthesized by a controlled pyrolysis method. Their complex dielectric permittivity and magnetic permeability were measured at different microwave frequencies using the technique of cavity perturbation. Reflection losses were evaluated and found to be less than −10 dB over the entire X-band (8-12 GHz) for a thickness of 2.2-2.8 mm. A minimum reflection loss of −45 dB was attained for an absorber thickness of 6.6 mm at 3.13 GHz.     

Boron-doped helical carbon nanotubes: lightweight and efficient microwave absorbers

Boron-doped helical carbon nanotubes (B-HCNTs) were obtained by annealing HCNTs under the boric oxide presence. The morphology and structure of HCNTs remained unchanged even after annealing and B-doping. HCNTs displayed excellent electromagnetic wave (EMW) absorption, judging by the corresponding optimal reflection loss and the absorption bandwidth values equal to ? 47.86 dB and 3.20 GHz, respectively. Quick and straightforward synthesis process, excellent chemical stability and low density make our B-HCNTs promising as lightweight and efficient microwave absorbers.

     

含铁碳纳米纤维的制备与表征

在聚丙烯腈的N,N-二甲基甲酰胺和四氢呋喃的混合溶剂中添加聚二茂铁硅烷,通过溶液的静电纺丝,制备了含聚二茂铁硅烷微相分离的微纳前驱体纤维,经预氧化和炭化,得到负载Fe的碳纳米纤维膜;利用SEM、SEM-EDS和XPS分别对纳米纤维的形貌、尺寸、结构和组成进行了表征。     

Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures

The radar absorbing structures (RAS) having sandwich structures in the X-band (8.2–12.4 GHz) frequencies were designed and fabricated. We added conductive fillers such as carbon black and multi-walled carbon nanotube (MWNT) to composite prepregs and polyurethane foams so as to efficiently increase the absorbing capacity of RAS. In order to improve the mechanical stiffness of RAS, we adopted the sandwich structures made of composite face sheets and foam cores. Glass fabric/epoxy composites containing conductive carbon black and carbon fabric/epoxy composites were used for the face sheets. Polyurethane foams containing MWNT were used as the core material. Their permittivity in the X-band was measured using the transmission line technique. The reflection loss characteristics for multi-layered sandwich structures were calculated using the theory of transmission and reflection in a multi-layered medium. Three kinds of specimens were fabricated and their reflection losses in the X-band were measured using the free space technique. Experimental results were in good agreement with simulated ones in 10-dB absorbing bandwidth.     

基材含微粒的空腔结构吸声器吸声性能预报的研究

采用一种近似方法计算含球形微粒和空腔结构吸声器的吸声系数,即首先用自洽理论,计算掺入微粒的复合材料的等效力学参数,再用求空腔结构吸声器的等效力学参数方法,计算这种复合材料具有空腔结构的该参数,最后用声波在分层吸收媒质中传播模型来计算能量吸收系数。所得数值模拟结果表明:基材掺入微粒形成的复合材料其吸声性能有不同程度的变化,性能的提高与否与基材和微粒的种类、孔隙率及粒子分布概率密度等因素有关;对于某些含微粒的复合材料若开空腔则可进一步提高吸声器的低频吸声性能。总之,基材含微粒的空腔结构吸声器的吸声系数是以不同程度的灵敏度受控于表征微粒的参数以及空腔结构。     

具有有序宏观结构纳米碳纤维的制备及其性能应用

以CH4为碳源,金属Ni为催化剂,采用化学气相沉积法(CVD)在有序宏观基体材料(SiO2纤维,Al2O3纤维)上制备出纳米碳纤维。利用场发射扫描电镜(FE-SEM)、透射电镜(TEM)、拉曼光谱(Raman)以及热重分析(TG)对产物进行了微观形貌和结构的检测。结果表明,所制备的纳米碳纤维具有取向性,能够在有序宏观基体材料上形成致密有序的纳米碳纤维层;与Al2O3纤维相比,SiO2纤维更易于生成高质量的纳米碳纤维;所制备的纳米碳纤维遵循顶端生长模式。此外,采用纳米碳纤维作模板可以原位合成出具有有序宏观结构的纳米LaMnO3,它能明显降低碳黑颗粒的起燃温度,具有潜在的应用前景。     

Synthesis of zinc oxide particles coated multiwalled carbon nanotubes: Dielectric properties,electromagnetic interference shielding and microwave absorption

Zinc oxide (ZnO) nanoparticles were coated on the surfaces of multiwalled carbon nanotubes (MWCNTs). High resolution transmission electron microscopy images show that the wurtzite ZnO immobilized on the MWCNTs is single-crystalline with a preferential [0 0 0 2] growth direction. A capacitor was generated by the interface of ZnO and MWCNTs, and a resistor–capacitor model could well describe the relationships between the structure and the dielectric properties, electromagnetic interference shielding and microwave-absorption of the composites in the frequency range of 2–18 GHz. The network built by ZnO-immobilized MWCNTs could contribute to the improvement of electrical properties. Resonant peaks associated with the capacitor formed by the interface were observed in the microwave absorption spectra, which suggest that reflection–loss peaks greatly broadens the absorption bandwidth.     

Fabrication of multi-walled carbon nanotube reinforced polyelectrolyte hollow nanofibers by electrospinning

Hybrid hollow multi-walled carbon nanotubes (MWCNTs)/polyelectrolytes (PE) nanofibers were prepared by a combination of the electrospinning method and layer-by-layer (LbL) technique. The mixed polystyrene (PS)/MWCNTs nanofibers were obtained by electrospinning method, which were employed as templates to self-assembly multilayered polyelectrolytes by LbL technique. Hollow MWCNTs/PE nanofibers were obtained by selectively removed part of the template: PS, which is confirmed by Raman spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM).     

平直碳纳米纤维与螺旋碳纳米纤维的共同生长

研究了采用乙醇催化燃烧法制备的碳纳米纤维的形貌和结构，并且讨论了平直碳纳米纤维与螺旋碳纳米纤维分别对应的生长机制。分析结果表明在特定的实验条件下，可以制备出平直碳纳米纤维与螺旋碳纳米纤维的共生材料。螺旋碳纳米纤维的生长机制是基于催化剂颗粒的各向异性。本实验方法具有制备工艺简单，碳源无毒性，制备过程无环境污染等特点，因而有望实现大量生产。     

Electrical conductivity of lanthanum oxide based composites containing carbon nanofibers

We have measured the electrical conductivity of lanthanum oxide based composite materials containing different concentrations of carbon nanofibers as additives. The conductivity has been shown to increase sharply (by two orders of magnitude) at carbon nanofiber contents from 2 to 3 wt % owing to the formation of a three-dimensional network of nanofibers in the bulk of the composite. Particular attention has been paid to the morphology of the particles of the constituent components of the composites and to the chemical and phase compositions of the matrix material.     

Nano-epoxy resins containing electrospun carbon nanofibers and the resulting hybrid multi-scale composites

For the first time, electrospun carbon nanofibers (ECNFs, with diameters and lengths of ∼200 nm and ∼15 μm, respectively) were explored for the preparation of nano-epoxy resins; and the prepared resins were further investigated for the fabrication of hybrid multi-scale composites with woven fabrics of conventional carbon fibers via the technique of vacuum assisted resin transfer molding (VARTM). For comparison, vapor growth carbon nanofibers (VGCNFs) and graphite carbon nanofibers (GCNFs) were also studied for making nano-epoxy resins and hybrid multi-scale composites. Unlike VGCNFs and GCNFs that are prepared by bottom-up methods, ECNFs are produced through a top-down approach; hence, ECNFs are more cost-effective than VGCNFs and GCNFs. The results indicated that the incorporation of a small mass fraction (e.g., 0.1% and 0.3%) of ECNFs into epoxy resin would result in substantial improvements on impact absorption energy, inter-laminar shear strength, and flexural properties for both nano-epoxy resins and hybrid multi-scale composites. In general, the reinforcement effect of ECNFs was similar to that of VGCNFs, while it was higher than that of GCNFs.     

Fabrication, characterization and mode locking application of single-walled carbon nanotube/polymer composite saturable absorbers

We present the fabrication of a high optical quality single-walled carbon nanotubes (SWNTs) polyvinyl alcohol (PVA) composite film. The composites demonstrate strong saturable absorption at ～1.5 μm, the spectral range for optical communications. By measuring the nonlinear transmission of a sub-picosecond pump pulse through the film, we were able to deduce a saturation fluence of ～13.9 μJ/cm² and a modulation depth ～16.9% (in absorption) at a high pulse fluence ～200 μJ/cm². Transient saturable absorption is investigated by measuring the transmitted autocorrelation traces at various incident power levels. Observed side-peak suppression indicates a fast recovery time on the scale of ～1 ps for our saturable absorber devices. Furthermore, we use these SWNT-PVA composite saturable absorbers as mode-lockers in an Er³⁺ fiber ring laser and achieve ～560 fs pulse generation with good jitter performance and long term stability. The laser performance is also associated with the parameters of our SWNT based saturable absorber.     

Fabrication of bimetallic ZIF/carbon nanofibers composite for electrochemical sensing of adrenaline

Journal of Materials Science - The detection method with potential ability to diagnose drugs in human serum rapidly, highly sensitivity and selectivity is of vital importance for clinical...     

A mechanistic study on electromagnetic shielding effectiveness of polysulfone/carbon nanofibers nanocomposites

The present study aims to produce a light weight electromagnetic interference (EMI) shielding material from carbon nanofibers (CNFs)-based polysulfone (PSU) nanocomposites. EMI shielding effectiveness (EMI SE) was studied by analyzing the electromagnetic wave transmission, reflection, and absorption characteristics of nanocomposites. The electrical conductivity and EMI SE of the nanocomposite with different weight percentage of CNFs (3–10 wt%) were investigated at room temperature and the measurement of EMI SE was carried out in a frequency range of 8.2–12.4 GHz (X-band). The mechanism of EMI shielding of PSU/CNFs nanocomposite has been well explained by comparing the contribution of reflection and absorption to the total EMI SE. The state of dispersion of CNFs and PSU–CNFs interaction was studied by high resolution transmission electron microscopy and scanning electron microscopy. The thermal stability of nanocomposite studied from thermogravimetric analysis was increased after addition of CNFs to PSU matrix. Electrical conductivity of nanocomposite followed power law model of percolation theory having a percolation threshold Φ_c = 0.0079 vol% (0.9 wt%) and exponent t = 1.73. The EMI SE of nanocomposites with thickness of 1 mm was 19–45 decibel (dB) at 3–10 wt% CNFs loading. This high thermal stability and high EMI SE suggest the potential use of PSU/CNFs nanocomposite as effective lightweight EMI shielding material in different electronic applications.     

纳米磁性多层膜的微波电磁特性研究

采用射频磁控溅射法制备了FeCoB／SiO2纳米磁性多层膜,并对其微波特性进行研究。用微波谐振腔法测量了该薄膜的复磁导率．2GHz频率处复磁导率实部及虚部分别可达251和22．7。重点研究了多层膜结构中介质层厚度及薄膜总厚度对磁导率的影响,同时结合直流电阻率ρ、静态磁参数Bs、Hc的测量结果分析．探讨了纳米磁性多层膜的微波高磁导率机理．并提出了微波频率下获得高磁导率及低磁损耗的途径。实验结果也表明这种纳米磁性多层膜材料有望在高频平面变压器、平面电感器、薄膜电磁干扰抑制器等领域获得应用。