A study of the surface of carbon fiber by means of X-ray photoelectron spectroscopy—III

This paper reports a study of the surface composition of carbon fibers treated by various methods by means of X-ray photoelectron spectroscopy (SPS). C_1S X-ray photoelectron spectra showed that after the surface treatment of carbon fibers, the carbon atoms in the hydrocarbon were changed into

etc. oxygen-containing groups, that is, the results of surface oxidation and concentration increased with time but finally reached a constant level. Comparing experimental results for the treatments used, we found that all of these methods resulted in concentrations of oxygen groups on the surface in the order:

.Evidence was found for the formation of lactone groups

during treatment in an oxygen or nitrogen plasma, but not during treatment by nitric acid or anodic oxidation.     

Computational analysis of the thermal conductivity of the carbon–carbon composite materials

Experimental data for carbon–carbon constituent materials are combined with a three-dimensional stationary heat-transfer finite element analysis to compute the average transverse and longitudinal thermal conductivities in carbon–carbon composites. Particular attention is given in elucidating the roles of various micro-structural defects such as de-bonded fiber/matrix interfaces, cracks and voids on thermal conductivity in these materials. In addition, the effect of the fiber precursor material is explored by analyzing PAN-based and pitch-based carbon fibers, both in the same type pitch-based carbon matrix. The finite element analysis is carried out at two distinct length scales: (a) a micro scale comparable with the diameter of carbon fibers and (b) a macro scale comparable with the thickness of carbon–carbon composite structures used in the thermal protection systems for space vehicles. The results obtain at room temperature are quite consistent with their experimental counterparts. At high temperatures, the model predicts that the contributions of gas-phase conduction and radiation within the micro-structural defects can significantly increase the transverse thermal conductivity of the carbon–carbon composites.     

Effect of γ-ray irradiation on the microstructure and self-heating property of carbon fiber/polyethylene composite films

High-density polyethylene composite films filled with various contents of carbon fiber (CF) were manufactured by melt mixing. The electrical and self-heating properties of the composite films were investigated. The composite films containing 10 wt% CF were exposed to γ-ray irradiation. The structural, morphological, and self-heating properties of the irradiated composite films were examined. The results indicated that the surface temperature (T_s) of the composite films was strongly dependent on the applied voltage and filler content. The T_s of the irradiated composite films was higher than that of the non-irradiated films, which contributed to the lower thermal expansion and the higher degree of crystallization of the irradiated composite films. In addition, the mechanical properties of the irradiated composite films were significantly improved. Using a rechargeable battery as the applied voltage source to evaluate the self-heating property of the irradiated composite films, a heating temperature of 54.2 °C was achieved, which lasted for 6 h.     

Effects of Υ-ray irradiation on carbon fiber surface graft modification

为提高碳纤维／环氧树脂复合材料的界面粘结性能,采用7射线共辐照接枝方法对碳纤维表面改性,利用X光电子能谱仪（XPS）、扫描电子显微镜（SEM）、电子万能材料试验机,研究了在缩乙二醇丙酮溶液和环氧氯丙烷丙酮溶液中经200kGy剂量的Υ射线辐照接枝后,碳纤维的表面化学元素及官能团组成、表面形貌、复合材料剪切断面形貌及其层间剪切强度（ILSS）的变化。研究表明,缩乙二醇类接枝液的接枝效果较理想,碳纤维接枝率达7％;辐照处理碳纤维表面O／C比值和含氧官能团含量增加,以此制备的碳纤维／环氧复合材料的ILSS提高,最大提高率达31．2％;同时还发现辐照接枝后的碳纤维表面粗糙度增大。     

Influence of carbon-fiber felts on the development of carbon–carbon composites

Oxidized PAN-fiber felt was carbonized to 600, 1000, and 1800 °C, respectively. Different carbon/carbon composites (C/C composites) were prepared from oxidized PAN-fiber felt, the carbonized felts, and resol-type phenol–formaldehyde resin. These composites were then carbonized and graphized at temperatures of between 600 and 2400 °C. The C/C composite made with oxidized PAN-fiber felt showed a strong fiber/matrix bonding, and those developed from the carbonized felt (heat-treatment of 1800 °C) showed a poor fiber/matrix bonding. The graphitized composites reinforced with the oxidized PAN-fiber felt resulted in having a high flexural strength (325 MPa), and the graphitized composites reinforced with the carbonized felt (carbonized at 1800 °C) had a low flexural strength (9 MPa). It was found that the stress-orientation promoted the formation of the anisotropic texture around the fibers as well as between the fibers. This felt may very well be able to provide a low-cost route for producing multidimensional C/C composites.     

Influence of the interface structure on the thermo-mechanical properties of Cu–X (X = Cr or B)/carbon fiber composites

This study focuses on the fabrication, for power electronics applications, of adaptive heat sink material using copper alloys/carbon fibers (CF) composites. In order to obtain composite material with good thermal conductivity and a coefficient of thermal expansion close to the ceramic substrate, it is necessary to have a strong matrix/reinforcement bond. Since there is no reaction between copper and carbon, a carbide element (chromium or boron) is added to the copper matrix to create a strong chemical bond. Composite materials (Cu–B/CF and Cu–Cr/CF) have been produced by a powder metallurgy process followed by an annealing treatment in order to create the carbide at the interphase. Chemical (Electron Probe Micro-Analysis, Auger Electron Spectroscopy) and microstructural (Scanning and Transmission Electron Microscopies) techniques were used to study the location of the alloying element and the carbide formation before and after diffusion. Finally, the thermo-mechanical properties have been measured and a promising composite material with a coefficient of thermal expansion 25% lower than a classic copper/carbon heat sink has been obtained.     

Influence of embedded gap and overlap fiber placement defects on the microstructure and shear and compression properties of carbon–epoxy laminates

This paper presents results from an experimental study of the influence of embedded defects created during automated fiber tape placement, on the mechanical properties of carbon/epoxy composites. Two stacking sequences have been examined, [(−45°/+45°)₃/−45°] and [90°₄/0°₃/90°₄], in which gaps and overlaps have been introduced during fiber placement. These materials have been cured in an autoclave either with or without a caul plate, then analyzed by ultrasonic C-scan. The microstructures were characterized by scanning electron microscopy. In-plane shear tests were performed on the ±45° laminates and showed that the use of a caul plate does not affect mechanical behavior of plies in the embedded defect region. Compression tests were performed on 0°/90° laminates and in this case the presence of a caul plate is critical during polymerization as it prevents thickness variations and allows defects to heal.     

Thermal conductivity improvement of copper–carbon fiber composite by addition of an insulator: calcium hydroxide

The effects of adding calcium hydroxide (Ca(OH)₂) to a copper–CF (30 %) composite (Cu–CF(30 %)) were studied. After sintering at 700 °C, precipitates of calcium oxide (CaO) were included in the copper matrix. When less than 10 % of Ca(OH)₂ was added, the thermal conductivity was similar to or higher than the reference composite Cu–CF(30 %). A thermal conductivity of 322 W m^?1 K^?1 was measured for the Cu–Ca(OH)₂(3 %)–CF(30 %) composite. The effects of heat treatment (400, 600, and 1000 °C during 24 h) on the composite Cu–Ca(OH)₂(3 %)–CF(30 %) were studied. At the lower annealing temperature, CaO inside the matrix migrated to the interface of the copper matrix and the CF. At 1000 °C, the formation of the interphase calcium carbide (CaC₂) at the interface of the copper and CFs was highlighted by TEM observations. Carbide formation at the interface led to a decrease in both thermal conductivity (around 270 W m^?1 K^?1) and the coefficient of thermal expansion (CTE (10.1 × 10^?6 K^?1)).     

Influence of the fiber volume fraction and the fiber Weibull modul on the behavior of 2D woven SiC/SiC — a finite element simulation

Summary The behavior of two-dimensional woven SiC/SiC ceramic matrix composite (CMC) is studied by numerical simulations based on the finite element method (FEM). Starting point of the investigations is a micromechanical model regarding a three-dimensional unit cell, which takes damage and fracture of the single components—fiber bundles and inter yarn matrix—into account. The scattering of the strength values which is characteristic for ceramic material is involved using Weibull distribution. In a first step the unit cell regarded within the simulations is cooled down to consider the residual thermal stresses resulting from the fabrication process. In a second step the unit cell is subjected to tensile loading and its behavior—especially the influence of the scattering of the strength values—is studied. To be able to estimate the influence of important parameters on the behavior of the composite a macrostructure is built up using the results obtained for a large number of unit cell. Thus an averaging effect is reached and the behavior obtained for the macrostructure should be characteristic for the composite. Doing so, the influence of the fiber volume fractionv _f and the fiber Weibull modulM _f on the composite behavior can be studied.Dedicated to Prof. Dr.-Ing. Dr.-Ing. E. h. mult. Oskar Mahrenholtz on the occasion of his 70th birthday     

Experimental research of carbon fiber filament＇s fracture toughness based on focus iron beam

以商用碳纤维T300和T800为研究对象,采用聚焦粒子束（FIB）技术精确刻蚀了碳纤维单丝的缺陷,分析了碳纤维单丝的断裂性能。通过单丝拉伸试验获得碳纤维拉伸强度,并利用扫描电镜观察试件的断裂截面,基于镜像方法和Griffith断裂理论获得拉伸强度与镜像半径之间的关系,进而对碳纤维单丝的断裂韧性KⅠC进行了估算。结果表明：采用FIB刻蚀缺陷的方法估算得到的T300碳纤维单丝的KⅠC为1.32MPa.m1/2,与采用试剂溶解方法得到的数据（1.25MPa.m1/2）相比较,两者相差小于10%。     

Basalt fiber–epoxy laminates with functionalized multi-walled carbon nanotubes

Cross-ply laminates reinforced with basalt fibers and functionalized multi-walled carbon nanotubes (MWCNTs) were fabricated from unidirectional epoxy prepregs. MWCNTs with varied surface conditions were prepared by oxidization or esterification, and then dispersed into a DGEBA epoxy system. The dispersion of the MWCNTs in the epoxy was improved by surface modification, resulting in improved composite mechanical properties as well. Significant increases in elastic modulus and strength were observed for epoxies with functionalized MWCNTs, especially for esterified species. When MWCNT – filled epoxies were used as matrices for basalt fiber/epoxy laminates, however, the reinforcement effects of MWCNTs on the composite elastic modulus exceeded micromechanics based semi-empirical predictions and were independent of surface functionalization. SEM morphological observations and the results of the micromechanical model revealed that nanotube re-distribution and orientation during processing was responsible for the enhancement of fiber-dominated mechanical properties. This work demonstrated the feasibility of in situ alignment and dispersion of functionalized nanotubes in multi-scale composite laminates.     

A new carbon structure in annealed film coatings of the carbon–lead system

Carbon–lead solid solutions coexisting with amorphous carbon have been obtained for the first time in a film coating deposited by ion-plasma sputtering. During subsequent vacuum annealing of carbon–lead films containing more than 68.5 at % Pb, this element almost completely evaporates to leave an amorphous carbon coating on a substrate. During annealing at 1100°C, this amorphous carbon crystallizes into a new hexagonal lattice with unit cell parameters a = 0.7603 nm and c = 0.8168 nm. Characteristic X-ray diffraction data for the identification of this phase are determined.

     

Shot peen forming of fiber metal laminates on the example of GLARE®

GLARE (GLAss-fiber REinforced aluminum) is a sandwich material that combines thin aluminum sheets with intermediate layers of glass fiber that are bonded using epoxy. Due to the resulting low specific weight and high strength as well as superior deterioration resistance the material has found its application in aircraft structures. GLARE parts are typically manufactured using the so-called self-forming technique, which is a very expensive and labor-intensive manufacturing process. If it was feasible to form GLARE from flat stock material using conventional forming processes, substantial savings could be achieved. Several attempts to form GLARE from flat stock reported in the literature are restricted by the limited formability of the glass fibers and/or delamination of the layers. This work analyses the possibilities to form GLARE using shot peen forming (SPF), which is an established forming process, e.g. for the production of fuselage parts. It is shown that GLARE shows a similar deformation behavior as monolithic sheets under quasi-static indentation with single steel balls. The process limits are analyzed using SPF tests and lock-in thermography, which is a non-destructive testing procedure for the detection of delamination. A process window for shot peen forming of GLARE is established, and it is shown that curvature radii of less than 2500 mm can be accomplished with no evidence of failure, which is a typical curvature radius of fuselage components for the Airbus A380.     

Development of titanium-doped carbon–carbon composites

The development of titanium-doped carbon matrix–carbon fibre reinforced composites (CCCs) via liquid impregnation of carbon fibre preforms using mesophase pitch is studied. Two different approaches for introducing the dopant into the carbon material are investigated. One consists of doping the matrix precursor followed by the densification of the preform with the doped precursor. The second approach consists of doping the porous preform prior to densification with the undoped mesophase pitch. Titanium-doped CCCs with a very fine distribution of dopant (in the nanometric scale) are obtained by adding TiC nanoparticles to the matrix precursor. Thermal decomposition of titanium butoxide on the carbon preform prior to densification yields doped CCCs with higher titanium content, although with larger dopant size. The combination of these two methods shows the best results in terms of dopant content.     

Computational study of the effect of core–skin structure on the mechanical properties of carbon nanofibers

Journal of Materials Science - The effect of the core–skin structure on the mechanical properties of carbon nanofibers is investigated in large-scale molecular dynamics simulations of tensile...     

Self-sensing of carbon fiber/carbon nanofiber–epoxy composites with two different nanofiber aspect ratios investigated by electrical resistance and wettability measurements

Electro-micromechanical techniques, wettability test, and acoustic emission (AE) were use to compare self-sensing and stress-transferring effects in single carbon fiber embedded in carbon nanofiber (CNF)–epoxy composites with two different aspect ratios. Electrical resistivity and standard deviation were used as indirect measures of comparative dispersion degree of CNF. The dispersion was observed to decrease with increasing CNF content due to an increase in the electrical contacts. Composites with higher aspect ratio exhibited better self-sensing than lower aspect ratio case. This was attributed to differences in dispersion, orientation, coagulation of CNF with different aspect ratios. The opposite effect was observed for apparent Young’s modulus, which was larger for composites with lower aspect ratio. This is probably related to better stress transfer linked to orientation effects. Work of adhesion consistently followed same trend as apparent Young’s modulus. Single carbon fiber pull-out tests and AE provided additional information on the effects of aspect ratio.     

Effect of glass fiber sizing on the cure kinetics of vinyl–ester resins

The cure characteristics of thermosetting resins are affected by the presence of reinforcements as a result of surface–resin interactions. Surface treatments and sizing can significantly affect such interactions; hence, sizing or surface treatment selection may significantly affect resin cure characteristics. This is of particular concern in the processing of composite materials, since neat resin cure characteristics often will not provide the appropriate basis for predicting the cure behavior of the composite. In this work, the effect of several commercially sized S-2 glass systems on the cure of vinyl–ester resin was investigated. Generally, a significant increase in the cure rate of the glass-modified systems is observed. Furthermore, a relationship between the surface energy characteristics of the fibers and the degree of cure acceleration is established, and possible mechanisms for the effect are discussed. It is apparent that sizing selection can significantly affect cure processes for vinyl–ester systems.     

Modelling of carbon–carbon composite manufacturing processes

This paper is devoted to the modelling of technological processes of manufacturing of siliconized carbon–carbon composites. The developed model describes the changes that occur in the properties of the composites (strength, elastic moduli, shrinkage) during the technological cycle of manufacturing and also the residual stresses generated in composite structures. It is shown that the level of the residual stresses and the character of changes in the properties of carbon–carbon composites essentially differ from those of polymer–matrix composites.     

Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.