Towards sustainable micro and nano composites from fly ash and natural fibers for multifunctional applications

Manufacturing of petroleum based synthetic materials, exploitation of timber products from forest reserves, improper management of industrial wastes and natural resources greatly persuade the environmental contaminations and global warming. To find viable solutions and reduce such alarming issues, innovative research work on recycling of unutilized materials such as fly ash and natural cellulosic polymers has been reported in this work to develop advanced sustainable hybrid micro/nano composites. In this study, the use of natural cellulosic sisal fibers with fly ash has enhanced the tensile properties and surface finish of composites. Fly ash particulates acted as fillers, additives, as well as surface-finishing medium and sisal fibers as reinforcing elements in achieving glossy finish sustainable composites. The developed composites have been found to be stronger than wood, plastics and have many opportunities for multifunctional applications

Physico‐mechanical properties of nano‐polystyrene‐decorated graphene oxide–epoxy composites.

In this work, nano polystyrene (nPS) decorated graphene oxide (GO) hybrid nanostructures were successfully synthesized using stepwise micro emulsion polymerization, and characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). XRD and FTIR spectra revealed the existence of a strong interaction between the nPS and GO, which implies that the polymer chains were successfully grafted onto the surface of the GO. The nPS decorated GO hybrid nanostructures were compounded with epoxy by hand lay-up technique, and the effect of the nPS-GO on the mechanical, thermal and surface morphological properties of the epoxy matrix was investigated by Universal tensile machine (UTM), Izod impact tester, thermogravimetric analysis (TGA) and contact angle measurement by a goniometer. It was observed that in the epoxy matrix, GO improved the compatibility

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

Electrical and thermal conductivity of aerogel/epoxy composites

This study investigates the electrical and thermal characteristics of two novel carbon aerogel composites containing Aerographite (AG) and a CNT foam. Aerographite of densities between 3 to 16 mg/cm3, and the CNT foam with densities of 17 and 31 mg/cm3 were prepared in the CVD process. Both aerogels were infiltrated with epoxy resin using a vacuum assisted infiltration technique that preserves the interconnected structure. The neat Aerographite showed a maximum electrical conductivity of 10.3 S/m while the CNT foam reached 1.7 S/m. In the epoxy composites the electrical conductivity of the neat materials is adopted, thus resulting in an enhancement of orders of magnitude when compared to neat epoxy. Thermal conductivity was studied using a Xenon flash method. First results show an improvement of thermal conductivity of the composite by 33 % at the low filler content of only 0.26 wt.-% for the Aerographite and of 91 % for the CNT foam at 2.7 wt.-% filler content

3D carbon networks and their polymer composites: fabrication and electromechanical investigations of neat aerographite and aerographite-based PNCs under compressive load

Aerographite is a lightweight 3D nanocarbon network which offers covalent interconnections for polymer nanocomposites (PNCs). Here, the electrical and mechanical properties of neat Aerographite and Aerographite-based PNCs are investigated in detail. The Aerographite filler networks consist of hollow, graphitic tubes of μm-sized diameters and nm-sized wall thicknesses. Different densities of Aerographite in the range of 0.6–13.9 mg/cm3 have been investigated towards their mechanical deformation behavior, electrical conductivities and piezoresistive response under compression. This basic characterization of filler networks is compared to resulting PNCs if the Aerographite is fully embedded in epoxy matrix. It can be shown that the use of 3D interconnected Aerographite results in high electrical conductivities at low filler contents, e.g., 2–8.7 S/m for weight fractions of 0.1–1.2 wt.-%. The neat Aerographite has been characterized in detail by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy techniques. To explain the observed piezoresistive behavior of these 3D nanocarbon-based PNCs, a qualitative micromechanical model is introduced. The model describes the internal graphitic wall slippage and loss of interconnections of the inner electrically conductive networks under load. The piezoresistive response of Aerographite-based PNCs can be directly correlated to the applied outer mechanical loads

Electro-mechanical piezoresistive properties of three dimensionally interconnected carbon aerogel (Aerographite)-epoxy composites

Aerographite (AG) is a carbon aerogel consisting of three-dimensionally (3D) interconnected graphitic microtubes. This study characterizes the electrical and mechanical properties of Aerographite/epoxy composites under tensile load. Aerographite can be used as a highly tailorable filler in polymer nanocomposites (PNCs) where the carbon filler and the matrix form an interpenetrating structure, contrary to particle filled systems. Aerographite networks with densities ranging from 3.0 to 13.9 mg/cm3 were produced in a chemical vapour deposition (CVD) process. An infiltration with epoxy leads to Aerographite/epoxy composites with filler contents in the range of 0.26–1.24 wt%. Their electrical conductivity is in the range of 2–13.6 S/m, thus, orders of magnitude higher compared to CNT-based PNCs at comparable filler contents. Although a large amount of direct interconnections of the graphitic tubes is given, interestingly the Aerographite/epoxy composites show a piezoresistive behaviour comparable to PNCs filled with carbon nanotubes (CNT) or graphene. Unexpected shifts between external mechanical strain and electrical signal have been observed in incremental piezoresistive experiments. Young's moduli and tensile strengths of the PNCs are not affected by embedding Aerographite networks. Fractographic observations identify graphitic wall slippage as the dominating failure mechanism. Both, piezoresistive characterization and fractography studies have been correlated and a model for the observed piezoresistive response is derived