Cross-Scale Industrial Manufacturing of Multifunctional Glass Fiber/Epoxy Composite Tubes via a Purposely Modified Filament Winding Production Line.

In the present research work is demonstrated a cross-scale manufacturing approach for the production of multifunctional glass fiber reinforced polymer (GFRP) composite tubes with a purposely redesigned filament winding process. Up until now, limited studies have been reported towards the multiscale reinforcement direction of continuous fibers for the manufacturing of hierarchical composites at the industrial level. This study involved the development of two different multi-walled carbon nanotube (MWCNT) aqueous-based inks, which were employed for the modification of commercial glass fiber (GF) reinforcing tows via a bath coating unit in a pilot production line. The obtained multifunctional GFRP tubes presented a variety of characteristics in relation to their final mechanical, hydrothermal aging, electrical, thermal and thermoelectric properties. Results revealed that the two individual systems exhibited pronounced differences both in crushing behavior and durability performance. Interestingly, for lateral compression the MWCNT coatings comprising a polymeric dispersant minorly affected the mechanical response of the produced tubes. The crashworthiness indicators of the multifunctional tubes displayed a slight 5% variation to the respective reference values, combined with a more ductile behavior. Moreover, regarding the bulk electrical and thermal conductivity values, as well as the Seebeck coefficient factor, the corresponding tubes displayed a variance of 233% and 19% and an opposite semi-conducting sign denoting a p- and n-type character, respectively.

Enhancing Epoxy Composite Performance with Carbon Nanofillers: A Solution for Moisture Resistance and Extended Durability in Wind Turbine Blade Structures.

The increasing prominence of glass-fibre-reinforced plastics (GFRPs) in the wind energy industry, due to their exceptional combination of strength, low weight, and resistance to corrosion, makes them an ideal candidate for enhancing the performance and durability of wind turbine blades. The unique properties of GFRPs not only contribute to reduced energy costs through improved aerodynamic efficiency but also extend the operational lifespan of wind turbines. By modifying the epoxy resin with carbon nanofillers, an even higher degree of performance can be achieved. In this work, graphene nanoplatelet (GNP)-enhanced GFRPs are produced through industrial methods (filament winding) and coupons are extracted and tested for their mechanical performance after harsh environmental aging in high temperature and moisture. GNPs enhance the in-plane shear strength of GFRP by 200%, while reducing their water uptake by as much as 40%.

Capsule-Based Self-Healing and Self-Sensing Composites with Enhanced Mechanical and Electrical Restoration.

In this work, we report for the first time the manufacturing and characterization of smart multifunctional, capsule-based self-healing and self-sensing composites. In detail, neat and nanomodified UF microcapsules were synthesized and incorporated into composites with a nanomodified epoxy matrix for the restoration of the mechanical and electrical properties. The electrical properties were evaluated with the use of the impedance spectroscopy method. The self-healing composites were subjected to mode-II fracture toughness tests. Additionally, the lap strap geometry that can simulate the mechanical behavior of a stiffened panel was used. The introduction of the nanomodified self-healing system improved the initial mechanical properties in the mode-II fracture toughness by +29%, while the values after the healing process exceeded the initial one. At lap strap geometry, the incorporation of the self-healing system did not affect the initial mechanical properties that were fully recovered after the healing process.

The Use of Electrochemical Impedance Spectroscopy as a Tool for the In-Situ Monitoring and Characterization of Carbon Nanotube Aqueous Dispersions.

So far, there is no validated technology for characterizing the dispersion and morphology state of carbon nanotubes (CNTs) aqueous dispersions during sonication. Taking advantage of the conductive nature of CNTs, the main hypothesis of the current study is that Electrochemical Impedance Spectroscopy (EIS) is an appropriate technique for the in-situ monitoring and qualification of the dispersion state of CNTs in aqueous media. To confirm our hypothesis, we monitored the Impedance |Z| during the sonication process as a function of type CNTs/admixtures used for the preparation of the aqueous solutions and of crucial process parameters, such as the applied sonication power and duration (i.e., sonication energy). For dispersions above the percolation threshold, a drop of |Z| by approximately seven orders of magnitude was observed, followed by a linear reduction. The dramatic change in |Z| is regarded as an indication of the formation of a conductive path or destruction of an existing one during sonication and can be used to characterize the dispersion and morphology state of CNTs. The results of the EIS provide, straightforwardly and reliably, the required information to create an optimum dispersion protocol for conductive CNT suspensions. The produced dispersions are part of research focusing on the manufacturing of cement-based composite materials with advanced thermoelectric functionalities for energy harvesting. Such dispersions are not only limited to energy harvesting applications but also to applications where functionalities are introduced through the use of conductive-based suspensions.

Healing Efficiency of CNTs-Modified-UF Microcapsules That Provide Higher Electrical Conductivity and EMI Shielding Properties.

In this study, the effect of the addition of multi-walled carbon nanotubes (MWCNTs), at three percentages, into the urea-formaldehyde (UF) shell-wall of microcapsules on the healing efficiency is reported. The modified shell-wall created a conductive network in semi-conductive epoxies, which led to an improvement of the electromagnetic interference shielding effectiveness (EMI SE); utilizing the excellent electrical properties of the CNTs. The microcapsule's mean diameter and shell wall were examined via scanning electron microscopy (SEM). Thermal stability was evaluated via thermogravimetric analysis (TGA). The healing efficiency was assessed in terms of fracture toughness, while the electrical properties were measured using impedance spectroscopy. The measurements of the EMI SE were carried out in the frequency range of 7-9 GHz. The derived results indicated that the incorporation of the CNTs resulted in a decrease in the mean size of the microcapsules, while the thermal stability remained unchanged. In particular, the introduction of 0.5% w/v CNTs did not affect the healing efficiency, while it increased the initial mechanical properties of the epoxy after the incorporation of the self-healing system by 27%. At the same time, it led to the formation of a conductive network, providing electrical conductivity to the epoxies. The experimental results showed that the SE increased on average 5 dB or more after introducing conductive microcapsules.

Printed Single-Wall Carbon Nanotube-Based Joule Heating Devices Integrated as Functional Laminae in Advanced Composites.

This work reports the design and fabrication of novel printed single-wall carbon nanotube (SWCNT) electrothermal Joule heating devices. The devices are directly deposited on unidirectional (UD) glass fiber (GF) fabrics. The GF-SWCNT Joule heaters were integrated during manufacturing as "system" plies in carbon fiber reinforced polymer (CFRP) composite laminates. Specific secondary functions were imparted on the composite laminate endowing thus a multifunctional character. The efficient out-of-oven curing (OOC) of a CFRP laminate was demonstrated using a sandwich configuration comprising top/bottom GF-SWCNT system plies. A total power consumption of ca. 10.5 kWh for the efficient polymerization of the thermoset matrix was required. Infrared thermography (IR-T) monitoring showed a uniform and stable temperature field before and after impregnation with epoxy resin. Quasi-static three-point bending and dynamic mechanical analysis (DMA) revealed a minor knock-down effect of the OOC-CFRP laminates properties compared to oven cured CFRPs, whereas the glass transition temperature (Tg) was almost identical. The OOC-CFRP laminates were efficient in providing additional functions such as deicing and self-sensing that are highly sought in the energy and transport sectors, i.e., wind turbine blades or aircraft wings. The novel modular design provides unique opportunities for large-area applications via multiple interconnected arrays of printed devices.

Advanced Glass Fiber Polymer Composite Laminate Operating as a Thermoelectric Generator: A Structural Device for Micropower Generation and Potential Large-Scale Thermal Energy Harvesting.

This study demonstrates for the first time a structural glass fiber-reinforced polymer (GFRP) composite laminate with efficient thermal energy harvesting properties as a thermoelectric generator (TEG). This TEG laminate was fabricated by stacking unidirectional glass fiber (GF) laminae coated with p- and n-type single-wall carbon nanotube (SWCNT) inks via a blade coating technique. According to their thermoelectric (TE) response, the p- and n-type GF-SWCNT fabrics exhibited Seebeck coefficients of +23 and -29 µV/K with 60 and 118 µW/m·K2 power factor values, respectively. The in-series p-n interconnection of the TE-enabled GF-SWCNT fabrics and their subsequent impregnation with epoxy resin effectively generated an electrical power output of 2.2 µW directly from a 16-ply GFRP TEG laminate exposed to a temperature difference (ΔT) of 100 K. Both experimental and modeling work validated the TE performance. The structural integrity of the multifunctional GFRP was tested by three-point bending coupled with online monitoring of the steady-state TE current (Isc) at a ΔΤ of 80 K. Isc was found to closely follow all transitions and discontinuities related to structural damage in the stress/strain curve, thus showing its potential to serve the functions of power generation and damage monitoring.

Thermoelectric Energy Harvesting from Single-Walled Carbon Nanotube Alkali-Activated Nanocomposites Produced from Industrial Waste Materials.

A waste-originated one-part alkali-activated nanocomposite is introduced herein as a novel thermoelectric material. For this purpose, single-walled carbon nanotubes (SWCNTs) were utilized as nanoinclusions to create an electrically conductive network within the investigated alkali-activated construction material. Thermoelectric and microstructure characteristics of SWCNT-alkali-activated nanocomposites were assessed after 28 days. Nanocomposites with 1.0 wt.% SWCNTs exhibited a multifunctional behavior, a combination of structural load-bearing, electrical conductivity, and thermoelectric response. These nanocomposites (1.0 wt.%) achieved the highest thermoelectric performance in terms of power factor (PF), compared to the lower SWCNTs' incorporations, namely 0.1 and 0.5 wt.%. The measured electrical conductivity (σ) and Seebeck coefficient (S) were 1660 S·m-1 and 15.8 µV·K-1, respectively, which led to a power factor of 0.414 µW·m-1·K-2. Consequently, they have been utilized as the building block of a thermoelectric generator (TEG) device, which demonstrated a maximum power output (Pout) of 0.695 µW, with a power density (PD) of 372 nW·m-2, upon exposure to a temperature gradient of 60 K. The presented SWCNT-alkali-activated nanocomposites could establish the pathway towards waste thermal energy harvesting and future sustainable civil engineering structures.

An Approach toward the Realization of a Through-Thickness Glass Fiber/Epoxy Thermoelectric Generator.

The present study demonstrates, for the first time, the ability of a 10-ply glass fiber-reinforced polymer composite laminate to operate as a structural through-thickness thermoelectric generator. For this purpose, inorganic tellurium nanowires were mixed with single-wall carbon nanotubes in a wet chemical approach, capable of resulting in a flexible p-type thermoelectric material with a power factor value of 58.88 µW/m·K2. This material was used to prepare an aqueous thermoelectric ink, which was then deposited onto a glass fiber substrate via a simple dip-coating process. The coated glass fiber ply was laminated as top lamina with uncoated glass fiber plies underneath to manufacture a thermoelectric composite capable of generating 54.22 nW power output at a through-thickness temperature difference οf 100 K. The mechanical properties of the proposed through-thickness thermoelectric laminate were tested and compared with those of the plain laminates. A minor reduction of approximately 11.5% was displayed in both the flexural modulus and strength after the integration of the thermoelectric ply. Spectroscopic and morphological analyses were also employed to characterize the obtained thermoelectric nanomaterials and the respective coated glass fiber ply.

High-Power All-Carbon Fully Printed and Wearable SWCNT-Based Organic Thermoelectric Generator.

In this study, we introduce the fabrication process of a highly efficient fully printed all-carbon organic thermoelectric generator (OTEG) free of metallic junctions with outstanding flexibility and exceptional power output, which can be conveniently and rapidly prepared through ink dispensing/printing processes of aqueous and low-cost CNT inks with a mask-assisted specified circuit architecture. The optimal p-type and n-type films produced exhibit ultrahigh power factors (PFs) of 308 and 258 µW/mK2, respectively, at ΔΤ = 150 K (THOT = 175 °C) and outstanding stability in air without encapsulation, providing the OTEG device the ability to operate at high temperatures up to 200 °C at ambient conditions (1 atm, relative humidity: 50 ± 5% RH). We have successfully designed and fabricated the flexible thermoelectric (TE) modules with superior TE properties of p-type and n-type SWCNT films resulting in exceptionally high performance. The novel design OTEG exhibits outstanding flexibility and stability with attained TE values among the highest ever reported in the field of organic thermoelectrics, that is, open-circuit voltage VOC = 1.05 V and short-circuit current ISC = 1.30 mA at ΔT = 150 K (THOT = 175 °C) with an internal resistance of RTEG = 806 Ω, generating a 342 µW power output. It is also worth noting the remarkable PFs of 145 and 127 µW/mK2 for the p-type and n-type films, respectively, at room temperature. The fabricated device is highly scalable, providing opportunities for printable large-scale manufacturing/industrial production of highly efficient flexible OTEGs.

Production of hierarchical all graphitic structures: A systematic study.

We report the production of hierarchical all graphitic structures through a systematic study involving the use of wet chemical treatments for dip coating of carbon fibers (CFs) for surface grafting with multiwalled carbon nanotubes (CNTs). Realization of a thin homogeneous veil of CNTs onto the CF surface was achieved through an extensive parametric survey. Optimization of aqueous dispersions of CNTs eliminated the need for oxidation of the CF surface. The effects of chemical processes onto the surface and structural characteristics of the involved graphitic species were evaluated via thermogravimetric analysis, and X-ray photoelectron, infrared and Raman spectroscopies. The dielectric properties of the produced CNT aqueous dispersions were monitored via electrochemical impedance spectroscopy. A final assessment of the produced hierarchical CFs was performed through scanning electron microscopy.