ABSTRACT
Molecular dynamics simulations of carbon nanotube (CNT) composites, in which the CNTs are continuous across the periodic boundary, overestimate the experimentally measured mechanical properties of CNT composites along the fiber direction. Since the CNTs in these composites are much shorter than the composite dimensions, load must be transferred either directly between CNTs or through the matrix, a mechanism that is absent in simulations of effectively continuous CNTs. In this study, the elastic and fracture properties of high volume fraction discontinuous carbon nanotube/amorphous carbon composite systems were compared to those of otherwise equivalent continuous CNT composites using ReaxFF reactive molecular dynamics simulations. These simulations were used to show how the number of nanotube-matrix interfacial covalent bonds affect composite mechanical properties. Furthermore, the mechanical impact of interfacial bonding was decomposed to reveal its effect on the properties of the CNTs, the interfacial layer of matrix, and the bulk matrix. For the composites with continuous reinforcement, it was found that any degree of interfacial bonding has a negative impact on axial tensile strength and stiffness. This is due to disruption of the structure of the CNTs and interfacial matrix layer by the interfacial bonds. For the discontinuous composites, the modulus was maximized between 4%-7% interfacial bonding and the strength continues to increase up to the highest levels of interfacial bonding studied. Areas of low stress and voids were observed in the simulated discontinuous composites at the ends of the tubes, from which fracture was observed to initiate. Experimental carbon nanotube yarn composites were fabricated and tested. The results were used to illustrate knockdown factors relative to the mechanical performance of the tubes themselves.
ABSTRACT
Recent availability of carbon nanotubes (CNTs) in quantities and formats amenable to producing macroscale components invites consideration of these materials in space applications where their attractive properties can enable the realization of bold concepts for affordable space exploration. The challenge is to identify relevant systems and quantify the benefits at the systems level. Before significant investment or adoption of CNTs for large aerospace systems can be justified, there must be a plausible path to attain the perceived systems level benefits. This challenging step requires a close collaboration among experts on CNTs and aerospace system communities. This paper provides an overview of a few relevant potential CNTs applications for space systems and the gap that must be overcome for deployment of CNTs. It also provides a simple engineering-level systems analysis approach to quantify the benefits of using CNTs over state of the art material solutions.
ABSTRACT
Thermoset/carbon nanotube (CNT) sheet nanocomposites were successfully fabricated by resistive heating assisted infiltration and cure (RHAIC) of the polymer matrix resin. Resistive heating takes advantage of the electrical and thermal conductivity of CNTs to rapidly and uniformly introduce heat into the CNT sheet. Heating the CNT sheet reduces the viscosity of the polymer resin due to localized temperature rise in close proximity to the resin, which enhances resin flow, penetration, and wetting of the CNT reinforcement. Once the resin infusion process is complete, the applied power is increased to raise the temperature of the CNT sheet, which rapidly cures the polymer matrix. Tensile tests were used to evaluate the mechanical properties of the processed thermoset/CNT sheet nanocomposites. The improved wetting and adhesion of the polymer resin to the CNT reinforcement yield significant improvement of thermoset/CNT nanocomposite mechanical properties. The highest specific tensile strength of bismaleimide(BMI)/CNT sheet nanocomposites was obtained to date was 684 MPa/(g/cm(3)), using 4 V (2 A) for resin infiltration, followed by precure at 10 V (6 A) for 10 min and post curing at 240 °C for 6 h in an oven. The highest specific Young's modulus of BMI/CNT sheet nanocomposite was 71 GPa/(g/cm(3)) using resistive heating infiltration at 8.3 V (4.7 A) for 3 min followed by resistive heating cure at 12.5 V (7 A) for 30 min. In both cases, the CNT sheets were stretched and held in tension to prevent relaxation of the aligned CNTs during the course of RHAIC.
ABSTRACT
Practical approaches are needed to take advantage of the nanometer-scale mechanical properties of carbon nanotubes (CNTs) at the macroscopic scale. This study was conducted to elucidate the salient factors that can maximize the mechanical properties of nanocomposites fabricated from commercially available CNT sheets. The CNT sheets were modified by stretching to improve CNT alignment and in situ polymerization using polyaniline (PANI), a π-conjugated conductive polymer, as a binder. The resulting CNT nanocomposites were subsequently postprocessed by hot pressing and/or high temperature treatment to carbonize the PANI as a means to improve mechanical properties. The PANI/CNT nanocomposites demonstrated significant improvement in mechanical properties compared to pristine CNT sheets. The highest specific tensile strength of PANI/stretched CNT nanocomposite was 484 MPa/(g/cm3), which was achieved in a sample with â¼42 wt % of PANI. This specimen was fabricated by in situ polymerization followed by hot pressing. The highest specific Young's modulus of 17.1 GPa/(g/cm3) was measured on a sample that was hot-pressed and carbonized. In addition, the highest DC-electrical conductivity of 621 S/cm was obtained on a sample prepared by in situ polymerization of PANI on a stretched CNT sheet.
ABSTRACT
To understand the mechanical properties of amorphous carbon (a-C)/boron nitride nanotube (BNNT) nanostructures, in situ mechanical tests are conducted inside a transmission electron microscope equipped with an integrated atomic force microscope system. The nanotube structure is modified with amorphous carbon deposited by controlled electron beam irradiation. We demonstrate multiple in situ tensile, compressive, and lap shear tests with a-C/BNNT hybrid nanostructures. The tensile strength of the a-C/BNNT hybrid nanostructure is 5.29 GPa with about 90 vol% of a-C. The tensile strength and strain of the end-to-end joint structure with a-C welding is 0.8 GPa and 5.2% whereas the lap shear strength of the side-by-side joint structure with a-C is 0.25 GPa.
ABSTRACT
Microwave irradiation was shown to be an effective energy source for the rapid decomposition of organic metal salts (such as silver acetate) in a solid mixture with various carbon and noncarbon substrates under completely solvent-free conditions. The rapid and local Joule heating of microwave absorbing substrates (i.e., carbon-based) resulted in the instantaneous formation of metal and metal oxide nanoparticles on the substrate surfaces within seconds of microwave exposure. Other less absorbing substrates (such as hexagonal boron nitride) required longer exposure times for the salt decomposition to occur. Details of the effects of microwave reaction time, temperature, power, and other experimental parameters were investigated and discussed. The solvent-free microwave method was shown to be widely applicable to various organic metal salts with different substrates including single- and multiwalled carbon nanotubes, graphene, expanded graphite, hexagonal boron nitride and silica-alumina particles, forming substrate-supported metal (e.g., Ag, Au, Co, Ni, Pd, Pt) or metal oxide (e.g., Fe3O4, MnO, TiO2) nanoparticles in high yields within short duration of microwave irradiation. The method was also successfully applied to large structural substrates such as nanotube yarns, further suggesting its application potential and versatility. To demonstrate one potential application, we successfully used both carbon nanotube powder and yarn samples decorated with Ag nanoparticles prepared via the above method to improve data acquisition in surface enhanced Raman spectroscopy.
