Effect of carbonization time on the structure and electromagnetic parameters of porous-hollow carbon fibres

A series of polyacrylonitrile-based porous-hollow carbon fibres (PAN-PHCFs) were prepared by carbonizing PAN porous-hollow cured fibres at 1073 K for different times in nitrogen. The effects of carbonization time on the structure, electrical volume conductivity and electromagnetic parameters were investigated. Results indicate that the degree of graphitization increases as carbonization time increases. The electrical volume conductivity increases as the degree of graphitization and carbonization time increase. The real and imaginary parts of the complex permittivity (′ and ″) increase with carbonization time increasing. The values of ′ and ″ of composites of PAN-PHCFs and paraffin are 13.76 and 10.09 when the carbonization time is 240 min, and the electrical volume conductivity of PAN-PHCFs is 190.47 Ω⁻¹ m⁻¹.     

Recent advances in carbon nanotubes-based microwave absorbing composites

With the blossom of information industry, electromagnetic wave technology shows increasingly potential in many fields. Nevertheless, the trouble caused by electromagnetic waves has also drawn extensive attention. For instance, electromagnetic pollution can threaten information safety in vital fields and the normal function of delicate electronic devices. Consequently, electromagnetic pollution and interference become an urgent issue that needs to be addressed. Carbon nanotubes (CNTs) have become a potential candidate to deal with these problems due to many advantages, such as high dielectric loss, remarkable thermodynamic stability, and low density. With the appearance of climbing demands, however, the carbon nanotubes combining various composites have shown greater prospects than the single CNTs in microwave absorbing materials. In this short review, recent advances in CNTs-based microwave absorbing materials were comprehensively discussed. Typically, we introduced the electromagnetic wave absorption mechanism of CNTs-based microwave absorbing materials and generalized the development of CNTs-based microwave absorbers, including CNTs-based magnetic metal composites, CNTs-based ferrite composites, and CNTs-based polymer composites. Ultimately, the growing trend and bottleneck of CNTs-based composites for microwave absorption were analyzed to provide some available ideas to more scientific workers.     

Effect of carbonization atmosphere on the structure changes of PAN carbon membranes

PAN carbon membranes were prepared by carbonizing the initial PAN membranes in vacuum and Ar at different temperatures. FTIR, Raman and XRD were applied to study the influence of carbonization atmosphere on the structure changes of PAN carbon membranes. The variations in adsorption peaks of FTIR, the intensity, position and FWHM of the Raman peaks, and microcrystallite parameters from XRD (e.g., d₀₀₂, Lc and La) are correlated with the structure change of PAN carbon membranes. Analyses results reveal that vacuum atmosphere can produce PAN carbon membranes with higher order degree than those in Ar atmosphere, although the structures of PAN carbon membranes prepared in the two atmospheres are both amorphous. In addition, vacuum atmosphere can significantly accelerate the degradation reaction of PAN membranes and favors the preparation of carbon membranes with smaller pore size.     

Coefficient variation of the mechanical properties of carbon fiber during carbonization

The approaches of the development for carbon fibers are how to improve their mechanical properties and to reduce the cost of the production as well as to maintain or to control the stability of their properties, i.e., the reproducibility. The coefficient variation of the mechanical properties immediately influenced the reproduction. In this paper, the effect of the operating conditions during carbonization on the coefficient variation of the properties for carbon fiber is discussed.     

Effect of carbonization rate on the properties of a PAN/phenolic-based carbon/carbon composite

The effect of carbonization rate in a wide range (1, 100 and 1000 °C/min) on the properties of a PAN/phenolic-based carbon/carbon (C/C) composite was studied. The results indicated that the composite processed at a higher carbonization rate had a higher porosity level, more large pores and a more graphitic structure than that processed at a lower carbonization rate. After second graphitization the bending properties of composites carbonized at 1 °C/min and 1000 °C/min were comparable. The composite carbonized at 1000 °C/min had the highest fracture energy. The composite carbonized at 100 °C/min showed the worst mechanical performance among three. The large increase in carbonization rate can be beneficial to the industry from an economic point of view.     

Influence of surface properties of carbon fibres on the adsorption of catalase

PAN-based carbon fibres obtained by carbonisation at relatively low temperature and submitted to high-temperature treatment and oxidation in wet conditions provided a range of materials differing according to surface composition and hydrophobicity. The adsorption of catalase by these fibres was studied by XPS and by activity measurements in the liquid and in the adsorbed state. Three typical situations are featured: limited adsorbed amount in the case of low hydrophobicity; significant adsorption and preservation of an appreciable enzyme activity for samples of moderate hydrophobicity; strong adsorption with deactivation of the enzyme in the case of high hydrophobicity.     

Anchorage of carbon nanotubes grown on carbon fibres

A novel reinforcing material based on the concept of an uniform 3-dimensional distribution of carbon nanotubes directly grown on yarns of carbon fibres has been developed. This material shows a potential for applications in polymeric matrix composites, combining the properties of carbon nanotubes with those of a traditional reinforcement.In view of the dipping process of the CNT coated fibres into a polymeric matrix, a good anchorage of CNT to the fibre surface is mandatory. Carbon fibres coated with metallic clusters and CNT were immersed into different liquids (deionised water, ethanol, n-butanol, acetone) and processed with different treatments (immersion, magnetic stirring, centrifugation and ultrasonic bath) in order to test their behaviour in different stressing environments. The morphological features of the samples were characterised by SEM both before and after the tests, demonstrating a good adhesion of the three-component material, which was not destroyed even after the most aggressive test.     

CMK-3 nanostructured carbon: Effect of temperature and time carbonization on textural properties and H2 storage

Abstract

CMK-3 carbons were synthesized varying the carbonization conditions and studying the effect of the templates calcined at different temperatures. The textural characterization of different SBA-15 templates calcined at 350, 450, and 550?°C shows a variation of the specific surface area below 10%. Based on the results, the SBA-15 obtained at 350?°C (the more affordable condition) was used as the final template for the CMK-3 synthesis. The results show that varying the time (from 2 to 6?h) and the temperature (from 600 to 900?°C) on the carbonization step, the textural, structural, and morphological properties of the carbons do not vary in a meaningful way. Thus, a CMK-3 carbon synthesized (using as template an SBA-15 calcined at 350?°C) obtained at 600?°C during 2?h was chosen to be used as adsorbent in hydrogen storage in order to stablish the relationship between the textural properties and its performance. Regarding the hydrogen storage, capacities of 15?mg H₂ g^?1 (1.5% w/w), and up to 28?mg H₂ g^?1 (2.8% w/w) were obtained at 1 and 10?bar, respectively. At high pressure, an important influence of the large micropores and narrow mesopores on the hydrogen adsorption was found.     

Effect of PAN-based and pitch-based carbon fibres on microstructure and properties of continuous Cf/ZrB2-SiC UHTCMCs

In this paper the microstructure and mechanical properties of two different C_f/ZrB₂-SiC composites reinforced with continuous PyC coated PAN-derived fibres or uncoated pitch-derived fibres were compared.Pitch-derived carbon fibres showed a lower degree of reaction with the matrix phase during sintering compared to PyC/PAN-derived fibres. The reason lies in the different microstructure of the carbon. The presence of a coating for PAN-derived fibres was found to be essential to limit the reaction at the fibre/matrix interface during SPS. However, coated bundles were more difficult to infiltrate, resulting in a less homogeneous microstructure.As far as the mechanical properties are concerned, specimens reinforced with coated PAN-derived fibres provided higher strengths and damage tolerance than uncoated pitch-derived fibres, due to the higher degree of fibre pull-out. On the other hand, the weaker fibre/matrix interface resulted in lower interlaminar shear, off-axis strength and ablation resistance.     

陶瓷纤维吸波材料的吸波机理及其结构设计

陶瓷纤维复合材料在结构及隐身技术中得到了广泛的应用。通过分析陶瓷纤维材料的吸波机理以及结构特性对纤维材料的电磁参数和吸波性能的影响,提出了综合不同方法来对吸波纤维进行结构设计的构思。     

Effects of carbonization conditions on the properties of coal-based microfiltration carbon membranes

Carbon membranes, a novel porous inorganic membrane, have considerable potential applications in many industrial fields owing to their better stability in aggressive and adverse environments. However, the high cost of precursor materials has hampered their wide applications on commercial scale. In this study, coal, a cheap material, is used to prepare the tubular microfiltration carbon membranes. The effects of carbonization conditions on the properties of coal-based carbon membrane were investigated by the variation of the weight loss, shrinkage ratio of tube size and pore structure characteristics during carbonization. The results show that carbonization conditions greatly affect the properties of coal-based carbon membranes. The carbon membranes carbonized in the inert gases have more “open” porous structure and high gas flux compared to those carbonized in vacuum which makes the carbon membrane possess smaller pores and low gas flux. The carbonization temperature plays an important role in the determination of the pore structure and densification of carbon matrix. At the temperature below 600°C, the pore structure and carbon matrix of carbon membrane are formed with more than 95% of the total weight loss and only 48% of the total size shrinkage ratio. The matrix of carbon membrane gets more compact with the temperature increasing from 600°C to 900°C, in which the size shrinkage ratio is up to 52% with only 5% of the total weight loss. The low heating rate should favor the preparation of the carbon membranes with small average pore size and narrow pore size distribution, and the high gas flow rate can produce the carbon membranes with large average pore size and high porosity.     

Effect of carbon fibres on the mechanical properties and corrosion levels of reinforced portland cement mortars

The changes in mechanical properties of portland cement mortars due to the addition of carbon fibres (CF) to the mix have been studied. Compression and flexural strengths have been determined in relation to the amount of fibres added to the mix, water/binder ratio, curing time and porosity. Additionally, the corrosion level of reinforcing steel bars embedded in portland cement mortars containing CF and silica fume (SF) have also been investigated and reinforcing steel corrosion rates have been determined. As a consequence of the large concentration of oxygen groups in CF surface, a good interaction between the CF and the water of the mortar paste is to be expected. A CF content of 0.5% of cement weight implies an optimum increase in flexural strength and an increase in embedded steel corrosion.     

The effects of carbonization temperature on the properties and structure of PAN‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate and then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of carbonization temperature of PAN hollow fiber precursor on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based carbon hollow fiber (PAN‐CHF) and PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the surface area of the PAN‐ACHF increased very remarkably, reaching 900 m² g^?1 when carbonization is 1000°C, and the adsorption ratios to creatinine and VB₁₂ of ACHF were much higher than those of CHF, especially to VB₁₂. The different adsorption ratios to two adsorbates including creatinine and VB₁₂ reflect the number of micropores and mesopores in PAN‐ACHF. The dominant pore sizes of mesopores in PAN‐ACHF are from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2155–2160, 2005     

Targeted design and analysis of microwave absorbing properties in iron-doped SiCN/Si3N4 composite ceramics

The SiCN(Fe)/Si₃N₄ composite ceramics containing C, β-SiC and α-Fe were designed by impregnating polysilazane followed by pyrolysis at 1–5 times with porous silicon nitride matrix. After the heat treatment, these new phases gradually formed and filled in the pores of the matrix to form a large whole. They bond together to facilitate the formation of conductive networks, which can improve the storage and transport capacity of the charge, thus improving the dielectric properties of the material. In this way, the highest dielectric and magnetic loss tangent values were achieved as 0.49 and 0.29 after third and fourth cycles, respectively. The electromagnetic wave can be absorbed ~80–90% in the range of 8–18 GHz with the thickness of 2 mm. The minimum reflectivity was ?12.5 dB at 16 GHz with the thickness of 5 mm, showing >90% electromagnetic wave absorption at this frequency.     

Changes in pore properties of phenol formaldehyde-based carbon with carbonization and oxidation conditions

Porous carbon beads were prepared by carbonizing at 700 and 1000 °C under N₂ (NN-series) or CO₂ atmospheres (CO-series) and a subsequent oxidization with boiling nitric acid solution (13%, v/v) for 3 h (ANN- and ACO-series). BET surface area of both CO- and NN-series samples tends to increase with increasing carbonization temperature, but CO-series samples show higher BET surface areas (700 °C: 610 m²/g, 1000 °C: 780 m²/g) compared with those of NN-series samples (700 °C: 380 m²/g, 1000 °C: 580 m²/g). After acid oxidation, BET surface areas of NN-series samples increased from 580 to 650 m²/g, whereas those of CO-series samples decreased from 780 to 600 m²/g. On subsequent acid oxidation, acidic surface functional groups increased in both CO- and NN-series samples, but the CO-series samples tend to have much more acidic surface functional groups. Adsorption of halogenated methanes, such as tetra-, tri- and dichloromethanes, onto the samples of CO-, NN-, ACO- and ANN-series was thought to be driven initially by the basicity, but the acidic functional groups that could attract adsorbates via dipole-dipole interaction might hinder the adsorption of subsequently incoming adsorbates.     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

Impact of the Zr-substitution on phase composition,structure, magnetic,and microwave properties of the BaM hexaferrite

Rapid developments in information technologies and a large rise in electrical and electronic equipment have generated different forms of electronic environmental contamination. Microwave absorption materials are important to avoid the damage that can be caused by electromagnetic radiation. A necessary condition for the absorption of the most incoming radiation is balanced wave impedance at the air/shield interface, which depends on the studied materials' magnetic and electrical properties. The paper introduces the preparation of BaFe_12-xZr_xO₁₉ (x = 0.1, 0.3, 0.6, 0.9, and 1.2) using a solid-state reaction technique. The studied samples were examined by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer, and a vector network analyzer. The studied samples showed that controlling the grain size and the prepared samples' magnetic properties play an important role in enhancing the microwave radiation absorption. The examined samples can be a promising absorption material in electromagnetic shielding applications.     

Silicon carbide nano-fibers in-situ grown on carbon fibers for enhanced microwave absorption properties

Silicon carbide nano-fibers (SiCNFs) were in-situ grown on the surface of carbon fibers by catalysis chemical vapor deposition (CCVD) with Ni nano-particles as catalyst at 1000 °C. The phase composition, microstructures, oxidation resistance and microwave absorption properties of the SiCNFs coated carbon fibers were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermal gravity analysis (TGA) and Vector network analyzer, respectively. The results show that the as-grown nano-fibers which are mainly composed of β-SiC, present a withe-like morphology with diameter of 20–50 nm and aspect ratio of 100–150. Additionally, the TGA curves indicate that the oxidation resistance of the SiCNFs coated carbon fibers is significantly improved in comparison to the pure carbon fibers. Moreover, the investigation reveals that the microwave absorption properties of the SiCNFs coated carbon fibers are effectively enhanced. The reflectivity of the SiCNFs coated carbon fibers is less than −10 dB within the frequency ranging from 9.2 to 11.7 GHz and the lowest value of reflectivity can approach −19.9 dB when the thickness of specimen is 2 mm. While the reflection loss of the pure carbon fibers is higher than −2.1 dB within the whole band ranging from 2 and 18 GHz. The superior microwave absorbing performance of the SiCNFs coated carbon fibers is mainly attributed to the improved impedance matching as well as dissipation resulted from hopping migration. In conclusion, this study provides an effective modification approach to improve the microwave absorption properties of carbon fibers. Finally, the SiCNFs coated carbon fibers could be considered as a promising candidate in light-weight microwave absorbing materials.     

具备雷达吸波功能的碳化硅纤维的研究进展

概述了雷达吸波材料的特点、应用领域,以及碳化硅纤维作为雷达结构吸波材料的吸收剂和增强剂的要求和应用。综述了赋予碳化硅纤维雷达波吸收功能的4种方法(即高温处理法、表面处理法、掺杂异元素法和改变纤维截面形状法)的研究进展,比较了这4种方法的优缺点,并指出了今后的研究方向。     

Fabrication and microwave absorbing property of WO3@WC with a core-shell porous structure

Special microstructure can significantly improve the microwave absorption property of rare materials. In this paper, porous WC powders were successfully synthesized by spray granulation method. Then, WO₃@WC materials with core-shell porous structure can be prepared after 410 °C heating treatment at different time to form the outer WO₃ oxidation layer. In addition, the microstructure, morphology, phase analysis and electromagnetic property were fully studied by investigating the WC-based materials in different structures. For WO₃@WC core-shell porous materials, when the coating thickness was 2.1 mm, the maximum reflection loss can reach ?19.4 dB at 12.6 GHz, which shows quite good microwave absorbing effects. The core-shell porous structure enhances the original microwave absorption performance due to the multiple reflection reflections and polarizations.