Nanocomposites with p-and n-type conductivity controlled by type and content of nanotubes in thermosets for thermoelectric applications

In this work, composites based on epoxy resin and various carbon nanotubes (CNTs) were studied regarding their thermoelectric properties. The epoxy composites were prepared by infiltration of preformed CNT buckypapers. The influence of different types of CNTs on the Seebeck coefficient was investigated, namely lab-made and commercially available multi walled carbon nanotubes (MWCNTs), lab-made nitrogen doped MWCNTs (N-MWCNT) and commercially available single walled carbon nanotubes (SWCNTs). It was found that only by varying the lab-made MWCNT content could both n-and p-type composites be produced with Seebeck coefficients between -9.5 and 3.1 µV/K. The incorporation of N-MWCNTs resulted in negative Seebeck coefficients of -11.4 to -17.4 µV/K. Thus, the Seebeck coefficient of pure SWCNT changed from 37.4 to -25.5 µV/K in the epoxy/1 wt. % SWCNT composite. A possible explanation for the shift in the Seebeck coefficient is the change of the CNTs Fermi level depending on the number of epoxy molecules on the CNT surface. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Molecular simulation of thermosetting polymer hardening: reactive events enabled by controlled topology transfer

We present a quantum mechanical / molecular mechanics (QM/MM) to tackle chemical reactions with substantial molecular reorganization. For this, molecular dynamics simulations with smoothly switched interaction models are used to suggest suitable product states, whilst a Monte Carlo algorithm is employed to assess the reaction likeliness subject to energetic feasibility. As a demonstrator, we study the cross-linking of bisphenol F diglycidyl ether (BFDGE) and 4,6-diethyl-2-methylbenzene-1,3-diamine (DETDA). The modeling of epoxy curing was supplemented by Differential Scanning Calorimetry (DSC) measurements, which confirms the degrees of cross-linking as a function of curing temperature. Likewise, the heat of formation and the mechanical properties of the resulting thermosetting polymer are found to be in good agreement with previous experiments.Comment: To be published in ACS Macromolecule

Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply composites observed by computed tomography

This study investigates the influence of load ratio and impact damage on the fatigue behaviour of high-performance carbon fibre reinforced polymers (CFRP) with areal fibre weights between 30 gsm and 360 gsm. For undamaged samples, the ultimate tensile and compressive strength, as well as the fatigue properties, are evaluated with regard to their layer thicknesses. The fatigue tests were performed under tension-tension (R=0.1), tension-compression (R=-0.5) and compression-compression (R=10) regime. The results are illustrated as a constant-life diagram, and a piecewise linear interpolation examines a first prediction. The results show that static and fatigue performance improves with decreasing layer thickness. Particularly under tension-compression loading, significant improvements are observed, due to the suppression of matrix cracks and delaminations with thinner layers. In addition, the effect of low-energy impact on the fatigue behaviour of Thin- and Thick-Ply laminates is investigated. The tests demonstrate that although the delamination area is larger, Thin-Ply laminates can sustain higher stresses and still reach the same number of load cycles in contrast to Thick-Ply laminates. Computed tomography measurements visualize 3-dimensional the damage progression after various cycles and prove that the Thin-Ply composites show no increase in the damaged area during fatigue. The interlaminar stress at the delamination is not sufficient for expansion. In contrast, in the case of thicker layers, the damage growths progressively throughout the whole sample with increasing number of cycles. © 2021 The Author(s

Influence of carbon nanoparticle modification on the mechanical and electrical properties of epoxy in small volumes

The influence of nanoparticle morphology and filler content on the mechanical and electrical properties of carbon nanoparticle modified epoxy is investigated regarding small volumes. Three types of particles, representing spherical, tubular and layered morphologies are used. A clear size effect of increasing true failure strength with decreasing volume is found for neat and carbon black modified epoxy. Carbon nanotube (CNT) modified epoxy exhibits high potential for strength increase, but dispersion and purity are critical. In few layer graphene modified epoxy, particles are larger than statistically distributed defects and initiate cracks, counteracting any size effect. Different toughness increasing mechanisms on the nano- and micro-scale depending on particle morphology are discussed based on scanning electron microscopy images. Electrical percolation thresholds in the small volume fibres are significantly higher compared to bulk volume, with CNT being found to be the most suitable morphology to form electrical conductive paths. Good correlation between electrical resistance change and stress strain behaviour under tensile loads is observed. The results show the possibility to detect internal damage in small volumes by measuring electrical resistance and therefore indicate to the high potential for using CNT modified polymers in fibre reinforced plastics as a multifunctional, self-monitoring material with improved mechanical properties

A screen-printing method for manufacturing of current collectors for structural batteries

Structural carbon fibre composite batteries are a type of multifunctional batteries that combine the energy storage capability of a battery with the load-carrying ability of a structural material. To extract the current from the structural battery cell, current collectors are needed. However, current collectors are expensive, hard to connect to the electrode material and add mass to the system. Further, attaching the current collector to the carbon fibre electrode must not affect the electrochemical properties negatively or requires time-consuming, manual steps. This paper presents a proof-of-concept method for screen-printing of current collectors for structural carbon fibre composite batteries using silver conductive paste. Current collectors are screen-printed directly on spread carbon fibre tows and a polycarbonate carrier film. Experimental results show that the electrochemical performance of carbon fibre vs lithium metal half-cells with the screen-printed collectors is similar to reference half-cells using metal foil and silver adhered metal-foil collectors. The screen-printed current collectors fulfil the requirements for electrical conductivity, adhesion to the fibres and flexible handling of the fibre electrode. The screen-printing process is highly automatable and allows for cost-efficient upscaling to large scale manufacturing of arbitrary and complex current collector shapes. Hence, the screen-printing process shows a promising route to realization of high performing current collectors in structural batteries and potentially in other types of energy storage solutions

Compression fracture of CFRP laminates containing stress intensifications

For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers

Experimentelle und modellhafte Untersuchung von FKV-Verbindungselementen bei statischer und zyklischer Belastung

Diese Arbeit beschreibt anhand einer detaillierten experimentellen Studie das System- und Versagensverhalten neuartiger Verbindungselemente aus Faser-Kunststoff-Verbund (FKV) bestehend aus Kohlenstofffasern (engl. carbon fibre = CF) und einer thermoplastischen Matrix Polyether-Etherketon (PEEK), die für konventionelle FKV-Verbindungen entwickelt wurden. Das System- und Versagensverhalten wird anhand einer einfach überlappenden Verbindung mit zwei CF-PEEK Senkkopfbolzen bei quasi-statischer Zug- und zyklischer Zug-Zug-Belastung mittels akustischer Emission untersucht. Die Ergebnisse stimmen gut mit dem entwickelten mathematischen Modell, einem auf Feder-Masse-Modell basierenden Ansatz, überein.This Work presents a detailed experimental study on carbon fibre (CF) polyether etherketone (PEEK) composites fasteners designed to join conventional high performance composites (CFRP). The failure mechanisms of two CF-PEEK fasteners with countersunk heads joining two laminate plates in a single-lap configuration were investigated under static (tensile) and cyclic loading (tension-tension). The failure process of the bolted joints is described in detail using acoustic emission and microscopic cut views. The experimental results are in good agreement with the newly developed “closed-form” model. This enhanced analytical approach is a closed-form extension of the spring based method, where bolts and laminates are represented by an arrangement of springs and masses.Bundesministerium für Bildung und Forschung (BMBF

Elektrische, piezoresistive und thermische Charakterisierung der Kohlenstoffstruktur "Aerographit" und deren Epoxidkomposite

Aufgrund ihres großen Potentials für Anwendungen in der Energiespeicherung, Sensorik oder Optik sind dreidimensional strukturierte Kohlenstoffmaterialien und deren Polymerkomposite immer öfter Gegenstand aktueller Forschung. Eine dieser Kohlenstoffstrukturen – Aerographit – wird in dieser Dissertation behandelt. Ziel dieser Arbeit ist eine umfassende Charakterisierung von Aerographit hinsichtlich mechanischer, elektrischer und thermischer Eigenschaften. Diese werden sowohl für das reine Aerographit als auch für dessen Epoxidkomposite diskutiert. Der Durchdringungsverbund, der durch Infiltration des Aerographits mit Epoxidharz entsteht, stellt eine Besonderheit gegenüber partikelmodifizierten Kohlenstoffnanokompositen dar. Aerographit wird zunächst auf Basis von hochporösen Zinkoxid-Templaten bestehend aus tetrapodenförmigen Partikeln mittels chemischer Gasphasenabscheidung in verschiedenen Dichten hergestellt. Die Dichten liegen dabei im Bereich von 0,6 mg/cm³ bis 13,9 mg/cm³. Die hergestellten Proben werden mittels Rasterelektronenmikroskopie und Transmissionselektronenmikroskopie hinsichtlich ihrer Morphologie charakterisiert. Eine Bewertung der Graphitqualität erfolgt mittels thermogravimetrischer Analyse und Ramanspektroskopie. Ein nanokristalliner Aufbau der graphitischen Wände konnte identifiziert werden. Zu Vergleichszwecken wird ein Teil der Proben einer thermischen Nachbehandlung unterzogen, bei der eine Nachgraphitisierung erfolgt. Vor der Herstellung des Aerographitkomposites werden außerdem mechanische, elektrische sowie piezoresistive Eigenschaften des reinen Aerographits bestimmt. Anschließend erfolgt die Weiterverarbeitung des Aerographits zu einem Komposit, indem es in einem vakuumassistierten Infiltrationsverfahren mit Epoxidharz ausgefüllt wird. Neben der elektrischen Leitfähigkeit im Ausgangszustand werden piezoresistive Eigenschaften unter verschiedenen Lastzuständen ermittelt und in Abhängigkeit des Füllgrads diskutiert. Die elektrische Leitfähigkeit ist um Größenordnungen höher als bei partikelmodifizierten Polymerkompositen und erreicht Werte von bis zu 13,6 S/m. Die elektrische Widerstandsantwort wird unter Druckbelastung sowie unter quasistatischer, zyklischer und inkrementeller Zugbelastung ausgewertet. Die erhaltenen Widerstandsverläufe werden mit Hilfe phänomenologischer Modelle unter Berücksichtigung der besonderen Aerographitmorphologie erklärt. Durch eine Analyse der Bruchflächen nach dem quasistatischen Zugversuch konnte das aneinander Abgleiten von Graphitlagen als dominierender Versagensmechanismus des Komposites identifiziert werden. Als ursächlich für charakteristische Widerstandsantworten unter Belastung wird vor allem das, bedingt durch Reib- und Van-der-Waals-Kräfte, zeitabhängige Verformungsverhalten des Aerographitnetzwerkes sowie das teleskopartige Auseinanderziehen einzelner Tetrapoden gesehen. In einer weiteren Untersuchung wird die thermische Leitfähigkeit der Aerographitkomposite bestimmt. Anders als bei der elektrischen Leitfähigkeit ist die Verbesserung hier gering. Letztlich werden die elektrische und thermische Leitfähigkeit der Komposite mit wärmebehandeltem Aerographit dargestellt. Die Nachgraphitisierung hat einen erheblichen Einfluss und führt zu einer Verbesserung beider Leitfähigkeiten.Due to their great potential for applications in energy storage, sensor technology or optics, three-dimensionally structured carbon materials and their polymer composites are increasingly the subject of current research. One of these carbon structures - Aerographite - is treated in this dissertation. The aim of this thesis is a comprehensive characterization of Aerographite with regard to mechanical, electrical and thermal properties. These are discussed for the pristine Aerographite and for its epoxy composites. The interpenetrating compound, which is formed by infiltration of the Aerographite with epoxy resin, is a special feature compared to particle-modified carbon nanocomposites. Aerographite is first produced in different densities on the basis of highly porous zinc oxide templates consisting of tetrapod-shaped particles by means of chemical vapor deposition. The densities are in the range from 0.6 mg/cm³ to 13.9 mg/cm³. The samples produced are characterized by means of scanning electron microscopy and transmission electron microscopy with respect to their morphology. The graphite quality is evaluated by means of thermogravimetric analysis and Raman spectroscopy. A nanocrystalline structure of the graphitic walls could be identified. For purposes of comparison, some of the samples are subjected to a thermal post-treatment in which graphitization takes place. Prior to the production of the Aerographite composite, mechanical, electrical and piezoresistive properties of the pristine Aerographite are also determined. Subsequently, the further processing of the Aerographite into a composite is performed by filling it with epoxy resin in a vacuum-assisted infiltration process. In addition to the electrical conductivity in the initial state, piezoresistive properties under different load conditions are determined and discussed depending on the filler content of the composite. The electrical conductivity is by orders of magnitude higher than in particle- modified polymer composites and assumes values of up to 13.6 S/m. The electrical response is evaluated under compressive load as well as under quasi-static, cyclic and incremental tensile load. The obtainedresistance curves are explained by means of phenomenological models, taking into account the unique morphology of Aerographite. By analyzing the fracture surfaces after the quasi-static tensile test, the sliding off of graphitic layers could be identified as the dominant failure mechanism of the composite. The reasons for characteristic resistance responses under stress are the time dependent deformation behavior of the Aerographite network due to friction and Van der Waals forces, as well as a possible the telescopic extension of individual tetrapods. The thermal conductivity of the Aerographite composites was determined. Unlike the electrical conductivity, the improvement is small. Finally, the electrical and thermal conductivity of the composites are presented. Post-graphitization has a considerable influence and leads to an improvement in both conductivities

Mikrostrukturelle Betrachtung des Einflusses von Poren auf die mechanischen Eigenschaften von faserverstärkten Kunststoffen

Die vorliegende Arbeit befasst sich mit der Untersuchung des Einflusses von Poren auf die mechanischen Eigenschaften von Faser-Kunststoff-Verbunden (FKV) unter Drucklast. Um die Schadensentwicklung durch Poren in einem FKV näher be\-schrei\-ben zu können, wurde der komplexe dreidimensionale strukturelle Aufbau des Verbundes inklusive der Poren analysiert und dieser in einen vereinfachten modellhaften Verbund überführt. Der modellhafte Verbund besteht aus der Matrix und mindestens zwei Fasern, zwischen denen eine einzelne Pore platziert wurde. Neben der experimentellen Untersuchung des modellhaften Probekörpers, welche die Betrachtung des Spannungs- und Dehnungsverhaltens der Matrix mittels der optischen Spannungsanalyse und digitalen Bildkorrelation beinhaltet, wurde zusätzlich der Verbund mit Hilfe der Finite-Elemente-Methode numerisch in einem parametrisierten Modell abgebildet und das Stabilitätsverhalten einer Faser analytisch beschrieben. Insbesondere die experimentelle Untersuchung erlaubte es, bei der Verwendung von zehn Fasern im modellhaften Probekörper die Schadensentwicklung in der unmittelbaren Umgebung einer Pore schrittweise zu beobachten. Gestützt durch Erkenntnisse aus der numerischen und analytischen Betrachtung konnte festgestellt werden, dass sowohl die Art und Weise der Stützung der Faser durch die Matrix, als auch deren Haftung untereinander versagensrelevant sind. Beide Aspekte führen zu einer frühzeitigen longitudinalen Ablösung der Faser von der Matrix mit anschließendem Stabilitätsverlust der Faser durch deren Ausknicken. Dies ist Ursache für weitere Faserbrüche, die aufgrund der Lastumlagerung initiiert werden. Untersuchungen an einem transparenten glasfaserverstärkten Kunststoff mit einer einzeln eingebrachten Pore boten darüber hinaus die Möglichkeit, die Erkenntnisse aus den Versuchen mit dem modellhaften Probekörper auf anwendungsnahe faserverstärkte Kunststoffe zu transferieren.The subject of this work is the investigation of the influence of voids on the mechanical properties of fibre-reinforced polymers (FRP) under compression. To specify the damage accumulation of FRP in the presence of voids, the complex three dimensional structure of the composite including several voids were analysed and a reduced mechanical model composite was derived. The reduced model consists of the matrix system and a unique void, which is squeezed between two fibres by using an injection method. The experimental investigation of the model composite included the description of the stress- and strain behaviour of the matrix using photoelasticity and digital image correlation technology. Additionally, a numerical examination of a parameterised model composite and an analytical study of the stability of a single fibre was conducted. As a result of the experimental investigation of the model composite consisting of ten fibres embedded in a matrix, the failure initiation and propagation could be observed. Supported by the findings from the numerical examination and the analytical study, the most impact on the failure initiation has the foundation of the fibre as well as the bonding between fibre and matrix. Both facts are leading to a premature fibre-matrix debonding with ongoing loss of stability of the fibre finally resulting in fibre kinking. Because of the rearrangement of stresses further overloaded fibres failed. Additional studies on transparent glassfibre reinforced polymers including a unique void showed, that the gained experience made on the examination of the model composite could be transferred to real existed composites