Multifunctional Structural Composites Research Articles

Development of Polyamide 6 (PA6)/Polycaprolactone (PCL) Thermoplastic Self-Healing Polymer Blends for Multifunctional Structural Composites

High-performance composites suffer from fatigue crack propagation during service. Traditional repair methods can be expensive and time-consuming. Therefore, research on composites with self-healing capabilities has considerably increased in the past decade. The aim of this work is to develop a polyamide 6 (PA6) matrix with self-healing properties. Polycaprolactone (PCL) was used as healing agent and melt compounded with PA6. PCL caused a decrease of the mechanical properties of PA6, due to its immiscibility and low mechanical properties. Nevertheless, acceptable fracture toughness values in quasi-static mode were obtained. Samples were thermally mended at 80 and 100 °C, and the healing efficiency was assessed by comparing the fracture toughness of virgin and repaired samples both in quasi-static and in impact mode. The blend with a PCL content of 30 wt% showed limited healing efficiency values (up to 6%) in quasi-static mode, while an interesting repair capability (53%) was detected under impact conditions. This discrepancy was explained through microstructural analysis and correlated to a different fracture morphology. In fact, under quasi-static mode, the PA6 matrix was severely plasticized, while under impact a brittle fracture surface was obtained. This morphology favored the flow of PCL during the thermal healing process.

Tailoring the Mechanical Performance of Carbon Nanotubes Buckypaper by Aramid Nanofibers towards Robust and Compact Supercapacitor Electrode

AbstractCarbon nanotubes (CNTs) buckypaper has shown extraordinary promise for freestanding supercapacitor electrode, but is usually limited by its extremely poor mechanical performance, which is due to the discrete nature of CNTs and rather low packing density. Meanwhile, manipulating the trade‐off between mechanical and electrochemical properties has not yet been realized for buckypaper electrode. Herein, a major breakthrough in optimizing the trade‐off between mechanical performance and compact capacitive delivery for porous buckypaper electrode is demonstrated by using aramid nanofibers (ANFs). Tailoring the microstructure of buckypaper framework by ANFs achieves 6.5‐ and 24.4‐times improvements in strength and toughness, respectively, without significantly sacrificing the volumetric capacitance (only 15.1% decline). Realizing such optimal integration of mechanical and capacitive properties represents a substantial step towards the practically feasible CNTs‐based structural electrodes. Further loading MnO2 conformal coating enables the freestanding electrode to deliver a superior volumetric capacitance of 155.5 F cm−3 while retaining the robust mechanical performance, which is impossible for conventional buckypaper or CNTs‐based bulk electrodes. The maximized structure‐function relationship achieved in this work validates the feasibility of reinforcing buckypaper electrode by ANFs, which may revolutionize the fabrication of advanced CNTs‐based supercapacitor electrodes, and offer a bright avenue for designing multifunctional structural composites.

The role of nitrogen plasma surface treatment and TiO2 seeding onto carbon fibers on the microwave-assisted growth of radially aligned TiO2 nanowires

Radially aligned titanium dioxide (TiO2) nanowires were synthetized on nitrogen plasma-treated carbon fibers (CF) using a two-step microwave-assisted solvothermal method. The TiO2 synthesis consisted in seeding TiO2 nuclei on CF followed by TiO2 nanowire growth. To assess the role of the nitrogen plasma surface treatment and TiO2 seeding, CF and CF-TiO2 hybrids were analyzed by Raman spectroscopy, X-ray diffraction, scanning electron and atomic force microscopies, after each step of the TiO2 synthesis. In addition, X-ray photoelectron spectroscopy was used to analyze the CF chemical composition upon the plasma treatment and TiO2 seeding. Single fiber tensile tests were conducted on CF and CF-TiO2 hybrids to track the mechanical changes induced by the surface treatment and TiO2 synthesis. It was found that both, the nitrogen plasma surface treatment and TiO2 nuclei seeding were essential to homogeneously synthesize radially aligned TiO2 nanowires onto CF. No significant changes in the CF surface roughness after plasma treatment and TiO2 seeding were found; however, upon TiO2 nanowire growth, the surface roughness increased by about twelve times with respect to the pristine ones. The plasma surface treatment generates oxygen- and nitrogen-containing functional groups that promote TiO2 nucleation onto CF. In addition, the CF internal heating due to microwave irradiation triggers a preferential nucleation of TiO2 on the fibers rather than in the reaction medium. The tensile strength of CF remained without significative changes after the surface treatment and TiO2 nanowire growth. The CF-TiO2 hybrid materials developed in this work could be exploited in the development of multifunctional structural composites.

Conductive layer-based multifunctional structural composites for electromagnetic interference shielding

The doped polyaniline (PANI) mixed with divinylbenzene (DVB) as a crosslinking thermoset polymer has been presented to exemplify as a structural electromagnetic interference (EMI) shielding material. Both theoretical and experimental analysis was conducted to analyze the shielding mechanisms. The effects of shielding layer thickness were studied to optimize the EMI shield properties. Preliminary studies reveal that the maximum EMI shielding effectiveness (SE) and electrical conductivity values of ~20 dB in the X-band (8.2–12.4 GHz) and 100 S/m, respectively, have been reached in the case of 1 mm thick PANI-based thermoset. It is also found from the experimental results that; absorption mechanism was the primary shielding mechanism for this material due to high conductivity. Further, both dielectric constant and permeability were estimated using Nicolson-Ross-Weir (NRW) algorithm and the S-parameters obtain from the vector network analyzer (VNA) to understand and correlate the shielding mechanisms. Finally, the PANI-based conductive layer successfully installs on structural fiber-reinforced composites to demonstrate its application as a self-EMI shielding structural composite materials. Both glass fiber reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) composites were able to shield the EMI radiation with a maximum SE value of 17 and 45 dB, while keeping the layer thickness of 1 mm.

Preparation of multifunctional structural P-CF@ZnCo2O4 composites used as structural anode materials

Abstract PAN-based carbon fibers have been studied in multifunctional structural composites with broad application prospects. One of the critical problems in their applications is how to improve their energy storage capacity while maintaining their tensile strength. In this paper, we prepared a novel P-CF@ZnCo2O4 composite (PAN-based carbon fiber/ZnCo2O4) by coating ZnCo2O4 thin films tightly onto the surface of carbon fibers. Due to the combined advantages of ZnCo2O4 with high theoretical capacity and P-CF with strong tensile strength, the prepared P-CF@ZnCo2O4 composite is expected to have outstanding comprehensive performance. As a result, a high reversible capacity of 787.2 mAh·g−1 is maintained after 150 cycles at a current density of 100 mA g−1 with a huge increase of almost 5.15 times as high as for the pure carbon fibers of 153 mAh·g−1. Besides, even at a density of 2000 mA g−1, the discharge capacity can still reach 289.5 mAh·g−1, showing excellent rate performance. Such superior comprehensive electrochemical characteristics can be ascribed to the synergistic effects of carbon fibers and ZnCo2O4.

Printing with 3D continuous carbon fiber multifunctional composites via UV-assisted coextrusion deposition

This study proposes to print multifunctional composite materials with 3D continuous carbon fibers. Coextrusion was implemented to deposit continuous carbon fibers simultaneously with doped functional photopolymer resin, subsequently rapidly cured by an ultraviolet laser. The technique allowed for direct printing of free-standing compliant carbon fiber reinforced composite structures and composites with conductive channeling. In particular, a novel functional, full lithium-ion structural battery was successfully printed in one single step with each coated carbon fiber acting as a micro-battery cell. The characterized mechanical and electrochemical performance well demonstrated the potentials of the proposed method in direct printing multifunctional structural composites.

Optimized microstructures for multifunctional structural electrolytes

Multifunctional structural materials offer compelling opportunities to realize highly efficient products. However, the need to fulfil disparate functions generates intrinsically conflicting physical property demands. One attractive strategy is to form a bi-continuous architecture of two disparate phases, each addressing a distinct physical property. For example, structural polymer electrolytes combine rigid and ion-conducting phases to deliver the required mechanical and electrochemical performance. Here, we present a general methodology, based on topology optimization, to identify optimal microstructures for particular design considerations. The numerical predictions have been successfully validated by experiments using 3D printed specimens. These architectures are directly relevant to multifunctional structural composites whilst the methodology can easily be extended to identify optimal microstructural designs for other multifunctional material embodiments.

Docosane-Organosilica Microcapsules for Structural Composites with Thermal Energy Storage/Release Capability.

Organic phase change materials (PCMs) represent an effective solution to manage intermittent energy sources as the solar thermal energy. This work aims at encapsulating docosane in organosilica shells and at dispersing the produced capsules in epoxy/carbon laminates to manufacture multifunctional structural composites for thermal energy storage (TES). Microcapsules of different sizes were prepared by hydrolysis-condensation of methyltriethoxysilane (MTES) in an oil-in-water emulsion. X-ray diffraction (XRD) highlighted the difference in the crystalline structure of pristine and microencapsulated docosane, and 13C solid-state nuclear magnetic resonance (NMR) evidenced the influence of microcapsules size on the shifts of the representative docosane signals, as a consequence of confinement effects, i.e., reduced chain mobility and interaction with the inner shell walls. A phase change enthalpy up to 143 J/g was determined via differential scanning calorimetry (DSC) on microcapsules, and tests at low scanning speed emphasized the differences in the crystallization behavior and allowed the calculation of the phase change activation energy of docosane, which increased upon encapsulation. Then, the possibility of embedding the microcapsules in an epoxy resin and in an epoxy/carbon laminate to produce a structural TES composite was investigated. The presence of microcapsules agglomerates and the poor capsule-epoxy adhesion, both evidenced by scanning electron microscopy (SEM), led to a decrease in the mechanical properties, as confirmed by three-point bending tests. Dynamic mechanical analysis (DMA) highlighted that the storage modulus decreased by 15% after docosane melting and that the glass transition temperature of the epoxy resin was not influenced by the PCM. The heat storage/release properties of the obtained laminates were proved through DSC and thermal camera imaging tests.

Effect of fiber lay-up configuration on the electromagnetic interference shielding effectiveness of continuous carbon fiber polymer-matrix composite

Continuous carbon fiber polymer-matrix multifunctional structural composites capable of electromagnetic interference (EMI) shielding are needed for electronics and radiation sources. The laminate's fiber lay-up configuration affects the shielding effectiveness, as shown in this work for unmodified conventional carbon fiber polymer-matrix composite laminates with high-strength PAN-based carbon fiber and a polyamide thermoplastic matrix. The radiation is normal-incidence unpolarized plane wave, as commonly used. The shielding is dominated by absorption rather than reflection – more so for crossply composites than unidirectional composites. Due to the electrical conductivity longitudinal-to-transverse ratio of 930 for a lamina, the absorption-loss/thickness longitudinal-to-transverse ratio is 30 at 1.0 GHz. This factor of 30 means that the contribution of the fibers transverse to the electric field to absorption is negligible compared to that of the fibers parallel to the electric field. The ratio of absorption-loss/thickness for the crossply composite to that for the unidirectional composite with the same number of laminae is ∼4, and the ratio of the reflection loss for the crossply composite to that for the unidirectional composite is ∼2. The values of these ratios are consistent with electromagnetic theory for unpolarized radiation. This work strengthens the science base for the design of continuous fiber composites for shielding.

Multifunctional epoxy/carbon fiber laminates for thermal energy storage and release

This work is focused on the preparation and characterization of novel multifunctional structural composites with thermal energy storage (TES) capability. Structural laminates were obtained by combining an epoxy resin, a paraffinic phase change material (PCM) stabilized with carbon nanotubes (CNTs), and reinforcing carbon fibers. The stabilized paraffin kept its ability to melt and crystallize in the laminates, and the melting enthalpy of the composites was proportional to the paraffin weight fraction with a maximum value of 47.4 J/cm3. This thermal response was preserved even after fifty consecutive heating-cooling cycles. Moreover, the thermal conductivity of the laminates through thickness direction resulted to increase proportionally to the content of CNT-stabilized PCM. The capability of the developed TES laminates to contribute to the thermal energy management was also proven by monitoring their cooling rates through thermal imaging. The flexural modulus was only slightly affected by the presence of the PCM, while a decrease of flexural strength, strain at break and interlaminar shear strength was detected. Optical microscopy highlighted that this could be attributed to the preferential location of the PCM in the interlaminar region. The obtained results demonstrated the feasibility of the concept of multifunctional structural TES composites.

Wax Confinement with Carbon Nanotubes for Phase Changing Epoxy Blends.

A paraffin wax was shape stabilized with 10 wt % of carbon nanotubes (CNTs) and dispersed in various concentrations in an epoxy resin to develop a novel blend with thermal energy storage capabilities. Thermogravimetric analysis showed that CNTs improve the thermal stability of paraffin, while differential scanning calorimetry showed that the paraffin kept its ability to melt and crystallize, with enthalpy values almost proportional to the paraffin fraction. In contrast, a noticeable loss of enthalpy was observed for epoxy/wax blends without CNTs, which was mainly attributed to the partial exudation of paraffin out of the epoxy matrix during the curing phase. Dynamic mechanical thermal analysis contributed to elucidate the effects of the melting of the paraffin phase on the viscoelastic properties of the epoxy blends. Flexural elastic modulus and strength of the blends decreased with the wax/CNT content according to a rule of mixtures, while flexural strain at break values deviate positively from it. These results show the potentialities of the investigated epoxy blends for the development of multifunctional structural composites.

Design and physical properties of multifunctional structural composites reinforced with nanoparticles for aeronautical applications

The multifunctional structural composites (MSCs) are used in engineering applications mainly in the aeronautical area because they meet the necessary requirements in new multifunctional systems. These composites exhibit good overall mechanical and thermal performance and potentially offer a large variety of functional properties. The present paper discusses the design, manufacturing and characterisation of these multifunctional structural composites. Generally, a combination of structural and energetic functions is achieved using different nanoparticle reinforcements in epoxy–rubber composites This type of material design gives an exigent task to the designers looking to integrate more functionality into the base material of their structure to achieve overall improved system performance. This paper is focused on the design of MSCs reinforced with Ni + Al mixture with Fe304 nanopowders in different ratios in a matrix of epoxy – fresh scrap rubber. We expect this material would be attractive for industrial applications because of the readily available recycled materials that are utilised. The mechanical and some physical properties of these composite systems were studied in this research. Microstructural characterisation revealed that micro- and nano-sized reinforcements were dispersed homogeneously in the epoxy–rubber matrix. Magnetic properties and surface wear resistance have been evaluated in detail as a function of the microstructure obtained for different compositions.

Integration of Single-Walled Carbon Nanotubes into a Single Component Epoxy Resin and an Industrial Epoxy Resin System

Single-walled carbon nanotubes (SWCNT) exhibit amongst the best mechanical, thermal and electrical properties of any known material. With their very high aspect ratios, SWCNT are well-suited to making ultra-light multifunctional structural composites. In this work, covalent chemistry is used to integrate SWCNT into a single component epoxy resin (aerospace grade MY0510) as well as an industrialized epoxy resin system for sporting goods. In particular, reduced SWCNT react directly with epoxide groups to create direct connections to the resin backbone. As the reduction process naturally exfoliates the SWCNT bundles, excellent dispersion is readily obtained. Substantial mechanical property improvements of the modified resin and carbon fibre composites have been observed through well-controlled processing. Their mechanical properties, specifically impact resistance, compression after impact strength and fracture toughness of the modified resin and fibre composites are discussed.