Carbon based electronic materials: applications in electron field emission

The influence of the different types of bonding present in a range of carbon based materials is discussed as a precursor to describing the field emission characteristics of carbon cold cathode materials. Some of the controlling factors which govern electron emission from carbon based cathodes are discussed. It is shown that from disordered carbon films the interplay between the clustered sp² phase and the insulating sp³ matrix is important. The transition from a ‘back contact’ to ‘front surface’ controlled emission mechanism is described in terms of the sp² content and field penetration. A possible reason for high field enhancement factors found in disordered films also is provided. It is further shown that changes to the sp² phase by current stressing can improve the field emission characteristics. Emission from carbon nanotubes is also discussed and the prospects for new types of nanotube – polymer composite based cathodes are also considered.     

Study of Composite Interface Fracture and Crack Growth Monitoring Using Carbon Nanotubes

Interface fracture of woven fabric composite layers was studied using Mode II fracture testing. Both carbon fiber and E-glass fiber composites were used with a vinyl ester resin. First, the single-step cured (i.e., co-cured) composite interface strength was compared to that of the two-step cured interface as used in the scarf joint technique. The results showed that the two-step cured interface was as strong as the co-cured interface. Carbon nanotubes were then applied to the composite interface using two-step curing, and then followed by Mode II fracture testing. The results indicated a significant improvement of the interface fracture toughness due to the dispersed carbon nanotube layer for both carbon fiber and E-glass fiber composites. The carbon nanotube layer was then evaluated as a means to monitor crack growth along the interface. Because carbon nanotubes have very high electrical conductivity, the electrical resistance was measured through the interface as a crack grew, thus disrupting the carbon nanotube network and increasing the resistance. The results showed a linear relationship between crack length and interface resistance for the carbon fiber composites, and allowed initial detection of failure in the E-glass fiber composites. This study demonstrated that the application of carbon nanotubes along a critical composite interface not only improves fracture properties but can also be used to detect and monitor interfacial damage.     

Young’s moduli of functionalized single-wall carbon nanotubes under tensile loading

This paper quantitatively investigates the effect of chemical functionalization on the axial Young’s moduli of single-walled carbon nanotubes (SWCNTs) based on molecular mechanics (MM) simulation, in which the COMPASS force field is used to model the interatomic interactions in a nonfunctionalized nanotube or a functionalized nanotube grafted with vinyl groups. We obtain the axial Young’s moduli of both functionalized and nonfunctionalized SWCNTs. The influences of the number and distribution density of the sp³-hybridized carbon atoms and the radius and chirality of the SWCNTs on Young’s moduli are studied. The results indicate that Young’s moduli depend strongly on the chirality of the SWCNTs and the distribution density of the sp³-hybridized carbon atoms. A 37.50% content of sp³-hybridized carbon atoms may degrade Young’s modulus by up to 33.36%. In addition, MM simulations show that the functionalization of SWCNTs results in a decrease of Young’s moduli of the corresponding SWCNT/polyethylene composites.     

锌基碳纳米管复合材料的界面特性及增强机理

通过复合电沉积技术制备了锌基碳纳米管复合薄膜。其中, 碳纳米管表面的锌沉积层平滑连续, 无明显界面缺陷, 与其它金属基碳纳米管复合材料相比, 这种独特的界面形貌是值得注意的。在对材料变形区的观察中发现, 在薄膜变形的过程中, 适中的结合强度将允许碳纳米管与基体发生界面脱黏, 碳纳米管被拔出基体后桥联在裂纹中。虽然界面结合受到损伤, 但是仍然可以有效地传递应力。当裂纹继续扩展, 碳纳米管石墨片层开裂, 直至完全断裂。同时, 这些桥连在裂纹中的碳纳米管趋向于向垂直于裂纹的方向滑移, 它们在基体中移动时会对基体造成一定程度的损伤。这些过程都将消耗大量的破断能, 从而起到对基体的增强效果。锌基碳纳米管复合薄膜的平均硬度由HV178.3上升至HV493.5。      

Fracture of a brittle composite: influence of elastic mismatch and interfacial bonding

The influence of the presence or absence of elastic mismatch between the dispersed and matrix phases on local crack particle interactions in brittle composites is reported. The role of interfacial bond strength is also investigated in the presence of elastic stress concentrations. The optimum toughness possible for a brittle composite results when G _iG _m, (G _i, G _m — elastic rigidity moduli for inclusion and matrix), and the interfacial bond strength is sufficient to allow plastic deformation of the dispersed phase. The dispersion of a ductile second phase with high interfacial bond strength but G _i>G _m, reduces effective crack/particle interactions. To increase the toughness in this case, weak interfacial bonding is necessary. Ultrasonic fractography was used to verify the local crack/particle interactions in detail.     

Enhancement of pullout energy in a single-walled carbon nanotube-polyethylene composite system via auxetic effect

Inter-tube bridging of carbon nanotubes (CNTs) is a reliable way to improve the inter-tube stress transfer abilities. The work describes the interfacial interactions provided by a wall-to-wall inter-tube bridging between two single-walled carbon nanotubes (SWCNTs) embedded in a polyethylene (PE) matrix. Molecular dynamics (MD) models of tube pullout phenomena represent by the embedding (10, 10)–(10, 10) SWCNT with interconnections into an amorphous PE matrix. The simulations show that the inter-tube bridging enhances the pullout energies significantly due to the three energy dissipative micro-mechanisms: stress-induced tube deformation with localized auxetic effect, “cutting through” (penetration) between linker and matrix, and the accompanying tube pullout. Moreover, the results also predict that linkers with longer aliphatic chains or aromatic rings provide further increase to the levels of the nanotube pullout energies. These are of potential importance in guiding the design of CNT/polymer composites through inter-tube linkage.     

碳纳米管增强金属基复合材料的研究进展

碳纳米管增强金属基复合材料由于高的比强度、比模量以及优异的热、电性能在航空航天领域具有很好的应用潜力,本文在分析大量文献的基础上,评述该类材料的制备技术和界面研究进展,对其典型性能进行归纳,指出碳纳米管的分散技术以及碳管、基体之间的界面特性应该是今后本领域的重点研究方向。     

High-cycle fatigue of hybrid carbon nanotube/glass fiber/polymer composites

Glass fiber polymer composites have high strength, low cost, but suffer from poor performance in fatigue. Mechanisms for high-cycle (>10⁴ cycles) fatigue failure in glass fiber composites consist primarily of matrix-dominated damage accumulation and growth that coalesce and propagate into the fibers resulting in ultimate fatigue failure. This investigation shows that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) in the matrix results in a significant increase in the high-cycle fatigue life. Cyclic hysteresis measured over each cycle in real time during testing is used as a sensitive indicator of fatigue damage. We show that hysteresis growth with cycling is suppressed when CNTs are present with resulting longer cyclic life. Incorporating CNTs into the matrix tends to inhibit the formation of large cracks since a large density of nucleation sites are provided by the CNTs. In addition, the increase in energy absorption from the fracture of nanotubes bridging across nanoscale cracks and nanotube pull-out from the matrix is thought to contribute to the higher fatigue life of glass composites containing CNTs. High-resolution scanning electron microscopy suggests possible mechanisms for energy absorption including nanotube pull-out and fracture. The distributed nanotubes in the matrix appear to inhibit damage propagation resulting in overall improved fatigue strength and durability.     

碳纳米管增强铝基复合材料分散方法研究进展

碳纳米管与铝基体的结合,可以获得导电和导热性良好及综合力学性能优异的复合材料,有望成为新一代轻质高强、结构功能一体化的复合材料.在制备碳纳米管增强铝基复合材料过程中,碳纳米管的团聚将降低界面结合,诱发缺陷产生,导致性能大幅下降,因此,调控优化碳纳米管的分散状态、含量成为获取良好界面结合,获得高性能碳纳米管增强铝基复合材...     

Electronic structures and three-dimensional effects of boron-doped carbon nanotubes

Abstract

We study boron-doped carbon nanotubes by first-principles methods based on the density functional theory. To discuss the possibility of superconductivity, we calculate the electronic band structure and the density of states (DOS) of boron-doped (10,0) nanotubes by changing the boron density. It is found that the Fermi level density of states D(?_F) increases upon lowering the boron density. This can be understood in terms of the rigid band picture where the one-dimensional van Hove singularity lies at the edge of the valence band in the DOS of the pristine nanotube. The effect of three-dimensionality is also considered by performing the calculations for bundled (10,0) nanotubes and boron-doped double-walled carbon nanotubes (10,0)@(19,0). From the calculation of the bundled nanotubes, it is found that interwall dispersion is sufficiently large to broaden the peaks of the van Hove singularity in the DOS. Thus, to achieve the high D(?_F) using the bundle of nanotubes with single chirality, we should take into account the distance from each nanotube. In the case of double-walled carbon nanotubes, we find that the holes introduced to the inner tube by boron doping spread also on the outer tube, while the band structure of each tube remains almost unchanged.     

Impact behavior and fractographic study of carbon nanotubes grafted carbon fiber-reinforced epoxy matrix multi-scale hybrid composites

Carbon nanotubes are the most promising reinforcement for high performance composites. Multiwall carbon nanotubes were directly grown onto the carbon fiber surface by catalytic thermal chemical vapor deposition technique. Multi-scale hybrid composites were fabricated using the carbon nanotubes grown fibers with epoxy matrix. Morphology of the grown carbon nanotubes was investigated using field emission scanning electron microscopy and transmission electron microscopy. The fabricated composites were subjected to impact tests which showed 48.7% and 42.2% higher energy absorption in Charpy and Izod impact tests respectively. Fractographic analysis of the impact tested specimens revealed the presence of carbon nanotubes both at the fiber surface and within the matrix which explained the reason for improved energy absorption capability of these composites. Carbon nanotubes presence at various cracks formed during loading provided a direct evidence of micro crack bridging. Thus the enhanced fracture strength of these composites is attributed to stronger fiber–matrix interfacial bonding and simultaneous matrix strengthening due to the grown carbon nanotubes.     

碳纳米管增强铝基复合材料的界面特性及增强机理研究进展

采用简单的电热板在空气气氛中、430℃加热氧化Fe片以及沉积在硅基片上的Fe膜,在Fe基体表面分别制备出了一维α-Fe2O3纳米线和纳米带,并研究了不同Fe基体热氧化制备的纳米结构的场发射特性。结果表明：Fe片和Fe膜热氧化获得的α-Fe2O3纳米结构的开启电场分别为14．5V／μm和13．3V／μm;α-Fe2O3纳...     

Fabrication and mechanical properties of β-sialon–Fe x Si y –CNTs composites

Silicon nitride composites were fabricated by adding Fe₃Al and carbon nanotubes and hot-pressing at a low sintering temperature of 1600 °C. The resulted composites were characterized by X-ray diffraction, Fourier-transform infrared spectrum, and field emission scanning electron microscopy. It was found that the Fe₃Al could react with Si₃N₄ to form the series of compound of Fe _x Si _y , and CNTs could keep chemical stability in the system. Mechanical properties of the composites were also investigated. For Fe₃Al as the additive, the relative density could reach to 93.6 % with the maximum hardness of 15.7 GPa. When the Fe₃Al and CNTs were added into matrix simultaneously, the relative density reached to 92.6 %, and the maximum fracture toughness was 6.7 MPa m^1/2. Finally, the toughening mechanism of Fe₃Al and CNTs in sialon composites, containing crack deflection and bridging, and nanotubes pullout and bridging, were also discussed.     

Formation of Single Carbon Nanotube T‐Junctions

Abstract

We investigate the formation of single wall carbon nanotube T‐junctions via the fusing of two nanotubes. We propose energetically efficient pathways for formation (9, 0)–(10, 0)–(9, 0) and (5, 5)–(10, 0)–(5, 5) T‐junctions. In the proposed scheme all carbon atoms maintain their sp ² arrangements throughout and transformation is achieved through creation/annihilation of topological defects. We use tight‐binding molecular dynamics simulation to investigate energetic of proposed mechanism.     

碳纳米管纤维/环氧树脂复合材料的界面剪切强度及微观结构

研究了碳纳米管纤维的微观结构和拉伸性能,并进一步分析了其与环氧树脂形成界面剪切强度及微观结构。采用单丝断裂试验测试了碳纳米管纤维/环氧树脂复合材料体系的界面剪切强度,结合单丝断裂过程中的偏光显微镜照片、复合材料的拉曼谱图和断口扫描电镜照片,研究了碳纳米管纤维/环氧树脂复合材料界面的微观结构。结果表明: 碳纳米管纤维/环氧树脂复合材料的界面剪切强度约为14 MPa;在碳纳米管纤维和环氧树脂形成界面的过程中,环氧树脂可以浸渍纤维,形成具有一定厚度的复合相,这种浸渍过程和界面相的形成都有利于碳纳米管纤维与基体之间的连接。     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.     

Interface characterization and fracture of calcium aluminosilicate glass-ceramic reinforced with nicalon fibres

An investigation of the structure and properties of a calcium aluminosilicate glass-ceramic reinforced with Nicalon fibres is described. Microstructural analysis of the interface showed that during manufacture of the composite a reaction zone rich in carbon formed between the Nicalon fibre and the anorthite matrix. Tensile strengths were approximately 330 MPa for unidirectional material and around 210 MPa for a (0°/90°)_3s. composite, little more than half that predicted by the mixtures rule. Flexural strengths were, however, higher than tensile strengths, by a factor 1.5–2.5 depending on lay-up. Studies carried out on specimens heat treated in air for 24 h at temperatures in the range 600–1200 °C showed a progressive change of interface microstructure in the outermost regions of the specimens due to oxidation of the carbon-rich layer; at 1000 °C and above the carbon had disappeared to leave voids and silica-rich bridges between fibre and matrix. These changes affected the strength of the interfacial bond, as measured by an micro-indentation technique, and also the degree of fibre pull-out produced in mechanical tests. Thus as-received material exhibited appreciable pull-out whilst heattreated samples were characterized by brittle behaviour in the outer (oxidized) regions. Nevertheless, the composites whilst in the unstressed condition appeared to survive these short-term exposures to oxidizing environments. An interfacial shear stress of around 5 MPa was calculated by applying the Aveston, Cooper and Kelly theory to crack spacings measured in our room-temperature deformation experiments, a value which agreed well with the 3–5 MPa obtained by the micro-indentation method.     

Effect of functionalized carbon nanotubes on the mechanical properties of epoxy-based composites

Abstract

In this research, composites reinforced with multi-walled carbon nanotubes (MWCNTs) in terms of their capability of absorbing energy during ballistic impact and tensile test were investigated. In order to investigate the effect of functional groups on mechanical properties of multi-wall carbon nanotubes composites, multi-walled carbon nanotubes were functionalized by COOH and NH₂. In order to investigate the penetration resistance of produced composites, the ballistic impact test with the high velocity was used. Results of the ballistic impact test imply that composites which benefit from carbon nanotubes with the carboxylic functional group have the highest value of the energy absorption. Results of the tensile test indicate, the interaction between carbon nanotubes with the carboxylic agent and the polymer matrix is much better. The ultimate stress for the sample containing nanotubes with hydroxyl functional group is 55% higher compared with the sample which does not contain any nanotubes. Reinforced composite samples which contain carbon nanotubes with the carboxylic functional group absorb 57.63% of the projectile energy and represent a better performance compared to the sample without nanotubes and the sample which consists of nanotubes with the amine functional group. The results of this study show that, uniform dispersion of nanotubes in the polymer matrix can profoundly affect mechanical properties especially the tensile strength. By functionalizing nanotubes and creating suitable functional groups, the cohesion between nanotubes and the matrix can be improved.     

Enhancement of delamination fatigue resistance in carbon nanotube reinforced glass fiber/polymer composites

We show that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) to the matrix of glass–fiber composites reduces cyclic delamination crack propagation rates significantly. In addition, both critical and sub-critical inter-laminar fracture toughness values are increased. These results corroborate recent experimental evidence that the incorporation of CNTs improve fatigue life by a factor of two to three in in-plane cyclic loading. We show that in both the critical and sub-critical cases, the degree of delamination suppression is most pronounced at lower levels of applied cyclic strain energy release rate, ΔG. High-resolution scanning electron microscopy of the fracture surfaces suggests that the presence of the CNTs at the delamination crack front slows the propagation of the crack due to crack bridging, nanotube fracture, and nanotube pull-out. Further examination of the sub-critical fracture surfaces shows that the relative proportion of CNT pull-out to CNT fracture is dependent on the applied cyclic strain energy, with pull-out dominating as ΔG is reduced. The conditions for crack propagation via matrix cracking and nanotube pull-out and fracture are studied analytically using fracture mechanics theory and the results compared with data from the experiments. It is believed that the shift in the fracture behavior of the CNTs is responsible for the associated increase in the inter-laminar fracture resistance that is observed at lower levels of ΔG relative to composites not containing CNTs.