Edith Mäder

Single MWNT‐Glass Fiber as Strain Sensor and Switch

It is well known that most materials are susceptible to damage in the form of microcracks, [ 1–3 ] which are usually considered negative and detrimental to the overall mechanical properties of the material. The initiation and growth of microcracks induced by mechanical stresses as well as thermal fatigues are diffi cult to detect at an early stage. In absence of an advance warning, the prevention of catastrophic failure of materials is thus almost impossible. Early detection and eventually even the utilization of microcracks remain two of the most challenging tasks in materials science. In order to early monitor internal microcracks of a material immense efforts have been made over the last decades. Experiments have been reported on embedding optical fi bers, piezoelectric sensors/actuators as well as conductive fi llers in polymer to checks the variations of time-frequency, acoustic emission, Raman band shift signals and so on. [ 4–9 ] However, an embedded sensor has detrimental effects on the integrity of the structure and the implementation of complex equipment remains a technical challenge for fi eld application. [ 10 , 11 ] Since carbon nanotubes (CNTs) with small diameters could act as metals or semiconductors, they could open the door to future low-power fabrication of microdevices. [ 12–14 ] Through embedding CNTs in a sizing or polymer matrix to form electric pathways, the achieved non-uniformly and uniformly dispersed CNTs in composites have successfully been used as health sensors to moni tor damage. [ 15–18 ] Recently, a CNT yarn strain sensor with excellent repeatability and stability was developed by directly embedding the yarn in an epoxy resin. [ 19 ] As a long-standing problem in fi ber-reinforced polymer (FRP) composites, internal microcracks that normally occur around the fi ber/matrix interphase region strongly affect fatigue life and damage tolerance. [ 20 , 21 ]

Multifunctional films composed of carbon nanotubes and cellulose regenerated from alkaline–urea solution

Multifunctional carbon nanotube (CNT)–cellulose films were fabricated by dissolving cellulose and dispersing CNTs homogeneously in aqueous alkaline–urea solution. We found that the resulting composite films with 2–10 wt% CNTs have both normal flexible paper and conducting CNT characteristics, which show a volume resistivity that can be controlled over a wide range of 1.35–540 Ohm cm. The composite films achieved multifunctional sensing ability for monitoring stress–strain, temperature and humidity. Both the mechanical properties and thermal stabilities of the composite films with CNTs were enhanced. This work provided a novel and simple pathway to regenerate cellulose films with carbon nanotubes as multifunctional biomaterials for electronic, magnetic, semiconducting and biotechnological applications.

Novel Carbon Nanotube/Cellulose Composite Fibers As Multifunctional Materials

Electroconductive fibers composed of cellulose and carbon nanotubes (CNTs) were spun using aqueous alkaline/urea solution. The microstructure and physical properties of the resulting fibers were investigated by scanning electron microscopy, Raman microscopy, wide-angle X-ray diffraction, tensile tests, and electrical resistance measurements. We found that these flexible composite fibers have sufficient mechanical properties and good electrical conductivity, with volume resistivities in the range of about 230-1 Ohm cm for 2-8 wt % CNT loading. The multifunctional sensing behavior of these fibers to tensile strain, temperature, environmental humidity, and liquid water was investigated comprehensively. The results show that these novel CNT/cellulose composite fibers have impressive multifunctional sensing abilities and are promising to be used as wearable electronics and for the design of various smart materials.

Electrically conductive aerogels composed of cellulose and carbon nanotubes

Aerogels composed of carbon nanotubes (CNTs) and cellulose were fabricated by the flash freezing/lyophilization process using wet-gel precursors, which were regenerated from a homogeneous dispersion of CNTs and cellulose in alkaline–urea aqueous solution. The morphology and textural properties of the resultant composite aerogels were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and nitrogen adsorption–desorption tests. All the prepared materials exhibited both a nanostructured solid network (specific surface areas between 140 and 160 m2 g−1) and a nanoporous network (including macropores, mesopores and micropores). The results from thermogravimetric analysis and tensile tests revealed that the composite aerogels have good thermal stability and excellent mechanical properties, respectively. The Youngs modulus of the composite aerogels could be tuned to reach about 90 MPa. And composite aerogels with 3–10 wt% CNTs have a conductivity of about 2.3 × 10−4 to 2.2 × 10−2 S cm−1, with the conductivity threshold at MWCNT volume fractions being as low as 3 × 10−3. Furthermore, these cellulose–CNT composite aerogels showed good sensitivity to ambient pressure.

Cellulose fibres with carbon nanotube networks for water sensing

Electroconductive cellulose-based fibres were fabricated by depositing multi-walled carbon nanotubes (MWNTs) on the surface using a simple and scalable dip coating. The morphology, mechanical properties and conductive properties of the resultant MWNT–cellulose fibres were investigated by scanning electron microscopy, tensile testing and electrical resistance measurement, respectively. The resistance (RL) of the single MWNT–cellulose fibre can be controlled in a wide range of 50–200u2006000 kΩ cm−1 by varying the conditions of dip coating. The sensing behaviour of these fibres to liquid water was investigated in detail. The results showed that they exhibit rapid response, high sensitivity and good reproducibility to water, with a relative electrical resistance change of about 100–8000% depending on the initial resistance. It was proposed that the disconnection of MWNT networks caused by swelling effects of the cellulose fibres is the dominant mechanism. Moreover, the sensitivity of the MWNT–cellulose fibres to an electrolyte solution was also investigated.

CNT-grafted glass fibers as a smart tool for epoxy cure monitoring, UV-sensing and thermal energy harvesting in model composites

A ‘hierarchical’ reinforcement of glass fibers (GFs) chemically grafted with multiwall carbon nanotubes (MWCNTs) has been utilized for epoxy cure monitoring, UV-sensing, and thermal energy harvesting in model composites. MWCNTs were covalently attached to the surface of glass fiber yarns (GF-yarns) in a dip-coating deposition process. Hereafter, the hybrid yarns are denoted as GF-CNT. Scanning electron microscopy (SEM) demonstrated a highly uniform CNT-layer covering the fiber surfaces. In turn, GF-CNT reached a maximum conductivity of 2060 S m−1, being of the same order of magnitude as the CNT-only bucky paper film. A GF-CNT in a uni-directional arrangement within a dog-bone shaped mould was employed for epoxy cure monitoring, recording the resistance changes during the curing process. In addition, three yarns connected in parallel highlighted the potential for detecting the resin position upon filling a mould. GF-CNT embedded in epoxy has been proposed also as an integrated non-invasive composite UV-sensor, allowing polymer matrix health monitoring. Besides, the semi-conductive nature of MWCNTs offered the opportunity of thermoelectric energy harvesting by the GF-CNT and its model composite when exposed to a temperature gradient. This work reports some new insights into and potential of fiber/CNT multi-scale reinforcements giving rise to multi-functional structural composites.

Coating of Carbon Nanotube Fibers: Variation of Tensile Properties, Failure Behavior, and Adhesion Strength

An experimental study of the tensile properties of CNT fibers and their interphasial behavior in epoxy matrices is reported. One of the most promising applications of CNT fibers is their use as reinforcement in multifunctional composites. For this purpose, an increase of the tensile strength of the CNT fibers in unidirectional composites as well as strong interfacial adhesion strength is desirable. However, the mechanical performance of the CNT fiber composites manufactured so far is comparable to that of commercial fiber composites. The interfacial properties of CNT fiber/polymer composites have rarely been investigated and provided CNT fiber/epoxy interfacial shear strength of 14.4 MPa studied by the microbond test. In order to improve the mechanical performance of the CNT fibers, an epoxy compatible coating with nano-dispersed aqueous based polymeric film formers and low viscous epoxy resin, respectively, was applied. For impregnation of high homogeneity, low molecular weight epoxy film formers and polyurethane film formers were used. The aqueous based epoxy film formers were not crosslinked and able to interdiffuse with the matrix resin after impregnation. Due to good wetting of the individual CNT fibers by the film formers, the degree of activation of the fibers was improved leading to increased tensile strength and Young’s modulus. Cyclic tensile loading and simultaneous determination of electric resistance enabled to characterize the fiber’s durability in terms of elastic recovery and hysteresis. The pull-out tests and SEM study reveal different interfacial failure mechanisms in CNT fiber/epoxy systems for untreated and film former treated fibers, on the one hand, and epoxy resin treated ones, on the other hand. The epoxy resin penetrated between the CNT bundles in the reference or film former coated fiber, forming a relatively thick CNT/epoxy composite layer and thus shifting the fracture zone within the fiber. In contrast to this, shear sliding along the

Analysis of a pull-out test with real specimen geometry. Part I: matrix droplet in the shape of a spherical segment

We compared two models of the pull-out specimen – the ‘equivalent cylinder’ and the platelet models in which the matrix droplet is represented as a set of thin parallel disks with the diameters varying along the embedded fiber to approximate the real droplet shape. Analytical expressions for the profiles of the fiber tensile stress and the interfacial shear stress have been derived for the matrix droplet in the shape of a spherical segment, including the effects of residual thermal stresses and interfacial friction. Using these expressions, we analyzed the process of crack initiation and propagation in the platelet model and investigated the effect of the specimen shape on the force–displacement curves. The interfacial stress near the loaded fiber end in the platelet model is higher than in the equivalent cylinder model, which gives rise to earlier crack initiation and smoother shape of the force–displacement curve. As a result, the calculated interfacial shear strength values may be underestimated by 10–20%, if the equivalent cylinder is used instead of the real droplet shape. A method of correction to the equivalent cylinder model in order to avoid this underestimation is proposed.

Variable structural colouration of composite interphases

Variable structural colouration results in brilliant colour changes in nature, due to the interaction of light with periodic photonic nanostructures. We report the observations of variable structural colouration from red, orange, yellow to green in a composite interphase region. By overlapping graphene nanoplatelets (GNPs) with ordered and disordered features using a special deposition approach, unique “fish scale” like structures are achieved. Variable structural colouration is observed through the mechanical tuning of fine parallel multilayers. Moreover, the method with incorporated variable structural colouration and electrical sensing functionality brings a first valuable step towards danger rating and the early warning of microcracks prior to a material’s failure, using a few colours for addressing danger, alarm and safety in a “traffic light” system.

Surface, interphase and tensile properties of unsized, sized and heat treated basalt fibres

Recycling of fibre reinforced polymers is in the focus of several investigations. Chemical and thermal treatments of composites are the common ways to separate the reinforcing fibres from the polymer matrices. However, most sizings on glass and basalt fibre are not designed to resist high temperatures. Hence, a heat treatment might also lead to a sizing removal, a decrease of mechanical performance and deterioration in fibre-matrix adhesion. Different basalt fibres were investigated using surface analysis methods as well as single fibre tensile tests and single fibre pull-out tests in order to reveal the possible causes of these issues. Heat treatment in air reduced the fibre tensile strength in the same level like heat treatment in nitrogen atmosphere, but it influenced the wetting capability. Re-sizing by a coupling agent slightly increased the adhesion strength and reflected a decreased post-debonding friction.