Viscoelasticity in carbon nanotube composites

Polymer composites reinforced by carbon nanotubes have been extensively researched for their strength and stiffness properties. Unless the interface is carefully engineered, poor load transfer between nanotubes (in bundles) and between nanotubes and surrounding polymer chains may result in interfacial slippage and reduced performance. Interfacial shear, although detrimental to high stiffness and strength, could result in very high mechanical damping, which is an important attribute in many commercial applications. We previously reported evidence of damping in nanocomposites by measuring the modal response (at resonance) of cantilevered beams with embedded nanocomposite films. Here we carry out direct shear testing of epoxy thin films containing dense packing of multiwalled carbon nanotube fillers and report strong viscoelastic behaviour with up to 1,400% increase in loss factor (damping ratio) of the baseline epoxy. The great improvement in damping was achieved without sacrificing the mechanical strength and stiffness of the polymer, and with minimal weight penalty. Based on the interfacial shear stress (approximately 0.5 MPa) at which the loss modulus increases sharply for our system, we conclude that the damping is related to frictional energy dissipation during interfacial sliding at the large, spatially distributed, nanotube-nanotube interfaces.     

Oxidation behaviour and protection of carbon/carbon composites

The oxidation behaviour of carbon/carbon composite materials and graphite (in cube form), in flowing air, has been studied in the temperature range 500 to 1100 °C. Gasification for unprotected samples occurred at temperatures around 500 °C. SiC coatings offered only limited protection below their intrinsic protection range due to the diffusion of oxygen along microcracks. Diffusional control was more significant for thicker coatings. However, the use of boron oxide applied on an underlayer of SiC, gave good protection for extended periods at temperatures up to 1000 °C, due to microcrack sealing. The use of borate coatings, both with and without an SiC underlayer, was limited by the volatility of the borate.     

Preparation and microstructural evolution of carbon/carbon composites

Carbon/carbon (C/C) composites with characteristic matrix-crack pattern are key intermediate materials for preparation of carbon/silicon carbide (C/C–SiC) composites. The C/C composites were prepared by pyrolyzing carbon fiber/phenolic resin preform. The change of density, open porosity, mass loss and specially the microstructural evolution of the composites during pyrolysis at 200–900 °C was analyzed, which provided important information for preparation of C/SiC composites by infiltration of silicon. An increasing number of regular spacing cracks were formed above 400 °C. After pyrolysis at 900 °C, the pore volume was 0.17 cm³/g, and the pores in the radius range of 2.44–122.19 μm occupied 81% of the pore volume.     

Graphitization behaviour of carbon fibre-glassy carbon composites

Carbon fibre-carbon composites were fabricated by aligning PAN-based carbon fibre unidirectionally in furfuryl alcohol resin char. The graphitization behaviour was investigated by an X-ray diffraction technique and by the measurement of magnetoresistance. The time-temperature superimposition study for interlayer spacing resulted in an activation energy of 242±35 kcal mol⁻¹. The kinetic study on magnetoresistance agreed with the result of X-ray measurement. The activation energy is that for the graphitization of the layer structure formed in the glassy carbon matrix of the composites. The graphitization mechanism of the layer structure is the same as that of soft carbons.     

碳/碳复合材料的应用研究进展

综述了碳/碳(C/C)复合材料的性能及其在各个领域的应用现状.     

The thermal expansion of carbon/carbon composites

The thermal expansion from 20 to 180°C of three samples of woven cloth based carbon/carbon composites and one sample of unidirectional carbon/epoxy (cfrp) composite (with high modulus fibre) has been measured. In the direction perpendicular to the fibres the cfrp has much the highest expansion, but parallel to the fibres its expansion is considerably less than that parallel to one set of fibres in the C/C samples. One C/C sample contained rayon-based fibre and the other two pan-based fibre. The differences in their behaviour can be explained by assuming that the rayon-based material is not so well aligned crystallographically as the pan material.     

Vitreous joining of SiC-coated carbon/carbon composites

This paper investigated the feasibility of joining of carbon/carbon (C/C) composites by using a silicate as interlayer. A SiC coating was pre-prepared on C/C substrate by pack-cementation technique to improve the wettability of glass on C/C composites, then the joints were prepared by a one-shot and low cost way. Microstructure and morphologies of the as-received joints were characterized by XRD, SEM and EDS. The results indicated that the SiC coating not only had a strong bonding with C/C composites, but also had a good physical and chemical compatibility with silicate glass. The room-temperature shear strength of the joints gives encouraging results, which can be up to 26 MPa. The fracture mode and the fracture behavior were discussed also.     

Structural response of oxidation-resistant carbon/carbon composites

Computational simulation of time, temperature, and oxidation-dependent response of carbon/carbon composites is illustrated and compared to experimental data. The material system examined is a SiC-coated, balanced eight-layer 8-harness satin weave carbon fibre cloth with boron carbide inhibitors in the matrix. Two representative cells, in-plane and through-the-thickness of the textile architecture, are developed to describe time(oxidation)-dependent material properties and thermo-mechanical response under static and cyclic loads with two-dimensional finite elements. These intelligible models enable a realistic interpretation of the overall response of the laminate subjected to the complex thermal-oxidative-mechanical loading conditions, and closely agree with the experimental data.     

Acoustic emission in carbon fibre composites

Acoustic emission analyses have been performed on carbon fibre-epoxy composites in order to correlate acoustic activity with well defined fracture processes. Different types of laminates were studied to discriminate the basic processes of fracture within the laminates. Test specimens were subjected to three-point flexural tests so as to separate fracture initiation and propagation. Acoustic emission signals were recorded by digital techniques for off-line analysis.

Results show that during fracture initiation, high amplitude signals are correlated with fracture processes controlled by fibre rupture, and low amplitude signals with matrix fracture. For laminates characterized by major fracture propagation, acoustic emission is generated by the processes induced by the friction of the fracture surfaces created. This acoustic activity, associated with abnormally long duration and high amplitude signals, exceeds the emission generated by the fracture processes themselves.     

Static friction properties of carbon/carbon composites

Preforms were made from 1K PAN plain carbon cloth and densified using the rapid directional diffusion chemical vapor infiltration (RDD CVI) processes. Four carbon/carbon (C/C) composite specimens treated at 1800 (specimen A), 1800+2000 (B), 2000 (C), and 2300 °C (D), respectively, were prepared, then machined into ring-on-ring specimen configurations. The influence of high-temperature heat treatment and the test temperatures on the static friction properties of the C/C composites has been researched. The results show that the high-temperature heat treatment processes has an important impact on the static friction behaviors of RDD CVI C/C composites. With raising the treatment temperature, the interlayer spacing of the matrix carbon in them becomes small, and the crystallite width as well as height increase. Under the test at room temperature, the static friction coefficients (FC) of the specimen treated at 1800 °C (A) are the lowest, but become very big and the highest under the test temperature of 260 °C due to desorption of the water absorbed on the friction surface. The composites treated at 2000 °C (C) exhibit enough high static friction coefficients under room temperature owing to their absorption of less water and the difficult delamination of the matrix carbon. However, in the test temperature of 260 °C or after the dynamic friction tests, their static FC is low. The specimen treated at 2300 °C (D) has a low FC at 260 °C heat condition, but its FC is higher than that of A and C under room temperature and largest after dynamic friction tests. No matter how high the heated temperatures are, the static FC of C/C composites decreases with an increase of the brake-specific pressure. When the specific pressure is very high and exceeds a certain value, the static FC is almost the same for specimens B, C, and D.     

Thermoelastic properties and conductivity of carbon/carbon fiber composites

Unidirectional carbon/carbon composites are modeled as fiber composites with cylindrically orthotropic fibers and matrix. All of the thermoelastic properties and the conductivities are evaluated on the basis of the composite cylinder assemblage (CCA) and the generalized self consistent scheme (GSCS) models. The former for axisymmetric elastic properties, axial shear modulus, thermal expansion coefficients and conductivities; the latter for transverse shear and Young's moduli and Poisson's ratio. Numerical results are given for onion skin, radial and transversely isotropic phase graphitic structures.     

Soluble carbon nanotubes and nanotube-polymer composites

For these two decade, tremendous amount of researches and developments dealing with carbon nanotubes (CNTs) have been carried out. Most of them are focusing on finding the unique and outstanding properties of CNTs and trying to utilizing them as the advanced materials. Whenever we start the research and the development of CNTs, the first difficulty is the dispersion of CNTs into the solvents since the CNTs form strong aggregation. Up to date, large efforts have been carried out for the preparation of CNT dispersion and the typical strategies are summarized. Such a dispersion technique allows us to use CNT as a material. Several applications of the CNT dispersion is also introduced.     

Effective reinforcement in carbon nanotube-polymer composites

Carbon nanotubes have mechanical properties that are far in excess of conventional fibrous materials used in engineering polymer composites. Effective reinforcement of polymers using carbon nanotubes is difficult due to poor dispersion and alignment of the nanotubes along the same axis as the applied force during composite loading. This paper reviews the mechanical properties of carbon nanotubes and their polymer composites to highlight how many previously prepared composites do not effectively use the excellent mechanical behaviour of the reinforcement. Nanomechanical tests using atomic force microscopy are carried out on simple uniaxially aligned carbon nanotube-reinforced polyvinyl alcohol (PVA) fibres prepared using electrospinning processes. Dispersion of the carbon nanotubes within the polymer is achieved using a surfactant. Young's modulus of these simple composites is shown to approach theoretically predicted values, indicating that the carbon nanotubes are effective reinforcements. However, the use of dispersant is also shown to lower Young's modulus of the electrospun PVA fibres.     

Modal separation of work-of-fracture of carbon/carbon composites

Modal separation of work-of-fracture was carried out for unidirectional C/C composites in order to characterize their multi-modal fracture behavior. The C/C composites were prepared by filament winding method with a coal tar pitch as a matrix precursor, and followed by heat treatment at temperatures between 1000°C and 2800°C under argon flow. During single edge-notched beam tests, tensile fracture and interlaminar shear fracture were simultaneously observed in the specimens. In proportion to the fracture surface area, the total work done during fracture was divided into each fracture mode, and the modal work-of-fracture was obtained by dividing each modal dissipation energy by twice the corresponding modal fracture surface area. With increasing the heat treatment temperature, the modal work-of-fracture for the tensile mode gradually increased and reached a maximum of around 6 kJm^–2 at a heat treatment temperature of 2000°C, whereas the modal work-of-fracture for the interlaminar shear mode remained constant around 20 Jm^–2. Determination of these modal work-of-fracture values revealed quantitatively the contribution of each fracture mode to the total dissipation energy of the composites.     

定向碳纳米管/炭纳米复合材料形貌和显微结构

以CVD法定向碳纳米管(ACNTs)阵列为骨架,利用化学气相渗透(CVI)工艺制备了新型定向碳纳米管/炭(ACNT/C)纳米复合材料。通过偏光金相显微镜(PLM)、扫描电子显微镜(SEM)、高分辨透射电子显微镜(HRTEM)、X射线衍射(XRD)和Raman光谱等分析方法对其显微结构和热解炭沉积机理进行了研究。结果表明:所制ACNT/C纳米复合材料的热解炭结构主要为类粗糙层结构,围绕碳纳米管生长的热解炭石墨层片结构清晰,并且碳纳米管和热解炭之间具有良好的界面结合;而在相同工艺条件下围绕炭纤维生长的热解炭为典型的光滑层结构。这可能是由于在热解炭沉积过程中存在碳纳米管"诱导"沉积过程,即沿着碳纳米管径向的离域化共轭π键和具有类似结构的芳香族大分子通过π-π非共价键作用相结合,并在CNTs纳米尺寸的影响下,芳香族大分子按照"软取向"(Softepitaxy)围绕碳纳米管生成环形层片状类石墨结构的热解炭。该研究结果有望为热解炭的可控沉积起到一定的借鉴作用。     

烧蚀过程中C/C复合材料的结构演变

利用SEM和TEM考察了3D C／C复合材料在电弧加热器上烧蚀后的材料表层显微组织结构变化。发现炭纤维和基体炭中的石墨微晶经过烧蚀后都得到了明显的发展。在距烧蚀表面几个纳米的深度范围内形成了高度取向的带状石墨织构，同时，在带状织构中间也形成了许多孔洞和缝隙。在距烧蚀表面几个微米的深度形成了卷曲柱状结构，在这些柱状结构周围有许多缺陷。在纤维和基体炭的界面区域，靠近纤维一侧形成了带状织构，且织构间的缝隙变大。     

Joining of carbon/carbon composites for nuclear applications

Using hot pressing, carbon/carbon composites were joined using a Ti₃SiC₂/SiC interlayer which was in situ synthesized by the reaction of TiC and Si. Phase composition of the interlayer was characterized by X-ray diffraction. Morphologies of the joints before and after shear test were determined by scanning electron microscope and energy dispersive spectroscopy. The mechanical strength of the joints was assessed by shear strength test. Phase analysis reveals that the interlayer was mainly composed of ternary Ti₃SiC₂, SiC, and little TiC. The microstructure observation results show that the dense and uniform interlayer adheres firmly to the C/C composites. A composition gradient reaction layer was formed at the joining interface between C/C substrates and interlayer. The room temperature average shear strength of the joints is about 38.9 ± 3.6 MPa. The joining mechanism and failure behavior of the joints were also discussed.     

Characterization of fibre/matrix interfaces in carbon/carbon composites

The present paper proposes an approach to characterizing fibre/matrix (F/M) interface in carbon/carbon (C/C) composites with respect to both modes of loading that may be expected: opening or shearing. Push-out and tensile tests were used. The former tests involve the shearing mode whereas the latter ones involve the opening one. Push-out tests use a diamond indenter to load the fibres. The interface sliding shear stress was obtained from the load-fibre displacement curve. The tensile tests were conducted on specimens having fibres oriented at 90° with respect to loading direction in order to preferentially open the interfaces. Interface opening strength was extracted from the composite tensile stress–strain behaviour. The specimens were examined under load and after ultimate failure by optical microscopy (OM). The mechanical properties of the F/M interfaces were then discussed.