OUT-OF-OVEN CURING OF POLYMERIC COMPOSITES VIA RESISTIVE MICROHEATERS COMPRISED OF ALIGNED CARBON NANOTUBE NETWORKS

The broader adoption of composite materials in next-generation aerospace architectures is currently limited by the geometrical constraints and high energy costs of traditional manufacturing techniques of PMCs such as autoclave and vacuum-bag-only oven curing techniques. Here, an in situ curing technique for PMCs using a resistive heating film comprised of an aligned carbon nanotube (A-CNT) network is presented. A carbon fiber reinforced plastic (CFRP) prepreg system is effectively cured via a single-side CNT network heater incorporated on the outer surface of the laminate without using an autoclave. Evaluation of the curing efficacy shows that composites cured by A-CNT film heaters can achieve degrees of cure that are equivalent or better than composites cured by an autoclave. This manufacturing technique enables highly efficient curing of PMCs while adding multifunctionality to finished composites.United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (contract W91NF-13-D-0001)Kwanjeong Educational Foundation (Korea)National Defense Science and Engineering Graduate (NDSEG) FellowshipConselho Nacional de Pesquisas (Brazil) (Science without Borders Program)United States. Naval Sea Systems Command (contract N00024-12-P-4069 for SBIR topic N121-058

Impact of carbon nanotube length on electron transport in aligned carbon nanotube networks

Here, we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10× with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (contract W911NF-13-D-0001)United States. Air Force Office of Scientific Research (AFRL/RX contract FA8650-11-D-5800, Task Order 0003)National Science Foundation (U.S.) (NSF Award No. ECS-0335765)United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship

Nanomaterial-enabled manufacturing for next-generation multifunctional advanced composite prepreg laminate architectures

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 179-193).Manufacturing of advanced aerospace-grade structural composites has traditionally utilized autoclaves to impart heat and pressure, in addition to vacuum, to create high-quality, void (defect)- free, reproducible structures. Carbon (micro) fiber reinforced polymer (CFRP) composites, which are pre-impregnated with a thermoset or thermoplastic polymer to form prepreg sheets, are in widespread use via autoclave processing due to their ease of use and high fiber volume fraction. However, autoclaves have high capital costs, and incur high operating costs due to the convective heating and applied pressure. Furthermore, the fixed capacity of an autoclave limits the size and design of composite parts, and the production rate is limited by autoclave availability. As a result, there has been an increasing interest in the development of alternatives, for example, out-of-autoclave (OoA) specially-formulated prepregs that only require heat and vacuum (i.e., pressure is not required). OoA prepreg processing also has drawbacks due to their specialized morphological and chemical formulation for vacuum-only conditions, as well as part quality (especially, composite interlaminar properties) that is below autoclave-processed materials. In light of the limitations described above, this dissertation (1) develops a novel prepreg processing technique, termed 'out-of- oven' (OoO) curing, that conductively cures OoA prepregs via nanoengineered resistive heating; (2) expands the applicability of the OoO process to conventional autoclave-formulated prepregs; and (3) introduces multifunctionality in the form of cure status sensing. Characteristics of the OoO process using a CNT film as a heating element are first examined and compared to those of an oven curing process, focusing on an aerospace-grade OoA-formulated unidirectional aerospace-grade CFRP prepreg system. Thermophysical and mechanical property comparisons suggest that there is no difference in laminates cured via OoO and oven curing as evaluated by void content, degree of cure analysis, short beam shear interlaminar testing, dynamic mechanical analysis, and double-edge notch tensile testing. The OoO process reduces electrical energy consumption by two orders of magnitude (from 13.7 to 0.12 MJ) due to conductive vs. convective heating, under a typical industrial curing condition for a small (60 mm x 50 mm) test panel. Modeling shows that for parts beyond a meter-scale, energy savings will also be at least two orders of magnitude. Moreover, comparative finite element modeling of the OoO and oven curing shows excellent agreement with measured values, including the reduction in electrical energy and instantaneous power consumption. Altogether, these findings show that OoO curing works for OoA prepreg systems, with significant energy savings. Given the results of the first study, the next study effectively removes the need for an autoclave by adapting the OoO process to conventional autoclave-formulated prepreg systems that currently require applied pressure of ~700 kPa in addition to vacuum. This technique entails OoO curing plus insertion of a nanoporous network (NPN, e.g., vertically aligned CNT arrays) into the interlaminar regions of autoclave-formulated composite laminates. Capillary pressure due to the NPN is calculated to be of the same order as the pressure applied in conventional autoclave processing. Results show that capillary-enhanced polymer wetting by the NPN enables sufficient reduction of interlaminar voids to levels commensurate with autoclave-processed composites. Thermophysical property comparisons and short beam shear interlaminar strength testing show that OoO-processed composites with NPN are equivalent to those of autoclave-cured composites, with energy and other savings similar to OoO curing with OoA prepreg in the first study. Conformability of the NPN to the micron-scale topology of the prepreg surface, and continuous vacuum channels created by the NPN, are identified as key factors underlying interlaminar void reduction. Finally, this dissertation introduces a multifunctional aspect of the OoO manufacturing: an in situ cure status monitoring technique utilizing the nanostructured CNT-based heating element of the OoO process. The OoO heating elements are nanoporous and CNT-based, but in this study have different morphology (randomly-oriented or in-plane aligned CNTs) than the NPN (vertically aligned CNTs, A-CNTs). As OoO curing proceeds and the heating element is powered, the adjacent polymer flows into the nanoporous heater via capillary action. Based on cure status sensing experiments and theoretical models, it is found that electrical resistance changes of the heating element correspond to several mechanisms associated with different stages in the cure process, including polymer infiltration into the CNT network that causes the average CNT-CNT junction distance to increase, giving a resistance increase. Later in the manufacturing, as the polymer cross-linking occurs after infiltration into the heating element, chemical cure shrinkage decreases the CNT-CNT junction distance, leading to a decrease in resistance. Thus, the heating element is multifunctional as a cure status sensor, and is found to be highly repeatable, demonstrating a new capability to enhance both quality and productivity of composite manufacturing. OoO curing and related processing techniques introduced here are expected to contribute to the design and manufacturing of next-generation multifunctional composite architectures. These processing techniques have several advantages, including: (1) compatibility with a wide range of composite materials, including OoA- and autoclave-formulated prepregs; (2) removal of size and shape constraints on composite components imposed by the use of a heating vessel; (3) manufacturing cost savings by efficient conductive (as opposed to convective) thermal processing; (4) production improvements via the in situ cure status monitoring by multifunctional heating elements as cure sensors; and (5) the potential for spatial heating control to accommodate structural features such as thick and thin transitions. Future work will expand the techniques to thermoplastics and other high-temperature polymers. The OoO techniques are expected to enable several systems-level production and operational savings, such as accelerated cure cycles, that require further study. Other areas of exploration include on-site composite curing and repair, and leveraging the spatial control of heat flux from the OoO technique into other OoA composite processes, such as resin infusion and resin transfer molding.by Jeonyoon Lee.Ph. D

In situ curing of polymeric composites via resistive heaters comprised of aligned carbon nanotube networks

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 83-87).The widespread application of polymer matrix composites (PMCs) has encouraged the use of nanofibers, especially carbon nanotubes (CNTs), to concurrently enhance the physical properties of such composites while adding multi-functionality. However, current-generation manufacturing routes of PMCs have drawbacks including geometrical limitations and high energy utilization. Improvements to manufacturing processes are needed, and in this thesis the in situ curing of a PMC using a resistive heating film comprised of an aligned CNT network is developed, and the underlying physics that govern the electron transport properties of the aligned CNT network as a function of temperature and orientation are explored. Aligned CNT film heaters have anisotropic electrical properties and show negative coefficient of resistance with temperature. A carbon fiber reinforced plastic (CFRP) system is effectively cured by a single (top layer) CNT network heater. Evaluation of the curing efficacy (via degree of cure) shows that the in-plane spatial variation of the degree of cure directly correlates to maximum temperature during cure as evaluated with a thermal camera. Through-thickness spatial variation in degree of cure is found to be < 8% for the one-sided curing heater. Future work will include characterization and modeling of the underlying physics that govern the performance of aligned CNT networks as resistive heaters, exploration of possible methods to scale-up the in situ curing process to laminates with sizes on the order of meters, and evaluation of the structure and properties of composites manufactured using the method reported here to elucidate any difference in composite performance when compared to materials synthesized using current-generation techniques.by Jeonyoon Lee.S.M

Aligned Carbon Nanotube Film Enables Thermally Induced State Transformations in Layered Polymeric Materials

The energy losses and geometric constraints associated with conventional curing techniques of polymeric systems motivate the study of a highly scalable out-of-oven curing method using a nanostructured resistive heater comprised of aligned carbon nanotubes (A-CNT). The experimental results indicate that, when compared to conventional oven based techniques, the use of an “out-of-oven” A-CNT integrated heater leads to orders of magnitude reductions in the energy required to process polymeric layered structures such as composites. Integration of this technology into structural systems enables the in situ curing of large-scale polymeric systems at high efficiencies, while adding sensing and control capabilities.United States. Army Research Office (Contract W911NF-07-D-0004)United States. Army Research Office (Contract W911NF-13-D-0001

NANOENGINEERED GLASS FIBER REINFORCED COMPOSITE LAMINATES WITH INTEGRATED MULTIFUNCTIONALITY

Combining one or more functional capabilities of subsystems within a structure can provide system-level savings, particularly for weight-critical applications such as air and space vehicles. Nanoengineering presents a significant opportunity for additional functionalities on the nanoscale without the necessity to modify shape, design, or load carrying capacity of the structure. Here, an integrated-multifunctional nano-engineered system was preliminarily studied in composite laminate structures. The study would support the exploration of a system designed to serve independent yet synergistic functionalities in life-cycle enhancements, energy savings during manufacturing, in-situ cure (manufacturing) monitoring, and in-service damage sensing. For the preliminary study, an integrated multifunctional composite (IMC) laminate was created via aligned nanofiber introduction into the composite interlaminar region and the laminate surfaces of Hexcel E-glass/913 unidirectional glass fiber prepreg. Various heights ranging from 10 - 40 μm-tall vertically aligned carbon nanotube (VA-CNT) arrays, as well as patterned and buckled VA-CNT architectures, were used to reinforce the weak interlaminar regions within the laminates showing a ~ 4 - 5% increase in short beam strength of VA-CNT reinforced specimens hence demonstrating interlaminar enhancement for life-cycle advancements. The same layers, being electrically conductive, can provide several additional multifunctionalities.</jats:p