A study of the electrical properties of carbon nanofiber polymer composites

Tese de doutoramento em FísicaThe interest of industry on using carbon nanofibers (CNF) as a possible alternative to carbon nanotubes (CNT) to produce polymer based composites is due to their lower price, the ability to be produced in large amounts and the their usefulness as a reinforcement filler in order to improve the matrix properties such as mechanical, thermal and electrical. Polymers like epoxy resins already have good-to-excellent properties and an extensive range of applications, but the reinforcement with fillers like CNF, which has high aspect ratio (AR) and surface energy, has the potential to extend the range of applications. The Van der Waals interactions between nanofillers, such as CNF, promote the clustering effect which affects their dispersion in the polymer and may interfere with some properties of the nanocomposites. In this sense, it is very important to use appropriate dispersion methods which are able to disentangle the nanofillers to a certain degree, but avoiding the reduction of the nanofibers AR as much as possible. In fact, the methods and conditions of nanocomposites processing have also influence on the filler orientation, dispersion, distribution and aspect ratio. To the present day, there is a lack of complete information in the literature about the relation between structure and properties, in particular electrical properties, for polymer nanocomposites. The main objective of this work is to study the electrical properties of composites based on CNF and epoxy resin using production methods which can be easily implemented in industrial environments and that provide different dispersion levels, investigating therefore the relationship between dispersion level and electrical response. Some of the requirements for such methods are the adaptability to the industrial processes and facilities which allow large scale productions and provide a good relation between quality and cost of the composite materials. In this work, morphological, electrical and electromechanical studies were performed in epoxy resin composites with vapor-grown carbon nanofibers (VGCNF). First, the electrical properties of VGCNF/epoxy resin composites produced with a simple method were studied. Then, it was investigated the relation between the electrical properties and the dispersion level of VGCNF/epoxy composites produced with different methods, which were selected to provide different levels of dispersion.The level of nanofiber dispersion of the composites produced with the different methods and filler contents was analyzed by transmission optical microscopy (TOM) and greyscale analysis (GSA) and then compared to the electrical conductivity measurements. After this study, the influence of different methods of VGCNF dispersion on the electrical conduction mechanism of the composites was investigated. Then, these composites were submitted to electromechanical tests in order to apply them as piezoresistive sensors. The last study of this work was dedicated to an initial comparison between the epoxy composites with VGCNG and multi-walled carbon nanotubes (MWCNT), in terms of electrical and morphological properties. As the main outcomes of the present work, it can be concluded that a better cluster dispersion seems to be more suitable than good filler dispersion for achieving larger electrical conductivities and lower percolation thresholds. It is also concluded that hopping conductivity is a relevant mechanism for determining the overall conductivity of the composites and that the CNF/epoxy composites are appropriate materials for piezoresistive sensors in particular at concentrations close to the percolation threshold.O interesse da indústria em usar as nanofibras de carbono (CNF) como uma possível alternativa aos nanotubos de carbono (CNT) para produzir compósitos em base polimérica deve-se ao seu baixo preço, facilidade de serem produzidos em grandes quantidades e a sua utilidade como cargas de reforço para aperfeiçoaras propriedades mecânicas, térmicas e elétricas da matriz. Polímeros tais como as resinas epóxidas, já possuem propriedades boas ou até mesmo excelentes e têm uma gama elevada de aplicações, mas o seu reforço com cargas como as CNF, que têm valores elevados de razão entre comprimento e diâmetro (AR) e também de energia de superfície, tem o potencial de estender a gama de aplicações. As interacções de Van der Waals entre cargas nanométricas (nanocargas), tais como as CNF, promovem o efeito de aglomeração que afeta a sua dispersão no polímero e poderá interferir com algumas propriedades dos nanocompósitos. Neste sentido, é muito importante usarem-se métodos de dispersão apropriados que sejam capazes de libertar (desemaranhar) as nanocargas até um determinado grau, de forma a evitar a redução do AR tanto quanto possível. De facto, os métodos e condições de processamento dos nanocompósitos também têm influência nas cargas em termos de orientação, dispersão, distribuição e AR. Hoje em dia existe uma falta de informação generalizada na literatura acerca da relação entre a estrutura e as propriedades dos nanocompósitos poliméricos, em particular nas propriedades eléctricas. O objectivo principal deste trabalho é o estudo das propriedades eléctricas dos compósitos baseados em CNF e resina epóxida usando métodos de produção que possam ser facilmente implementados num ambiente industrial e que permitam vários níveis de dispersão, investigando desta forma a relação entre o nível de dispersão e a resposta eléctrica. Alguns dos pressupostos para esses métodos, são a sua adaptabilidade aos processos e instalações industriais que permitam produções em larga escala e proporcionem uma boa relação entre a qualidade e o custo dos materiais compósitos. Neste trabalho, foram desenvolvidos estudos morfológicos, elétricos e eletromecânicos em compósitos de resina epóxida com nanofibras de carbono de crescimento por vaporização (VGCNF). Primeiramente foram estudadas as propriedades elétricas de compósitos de resina epóxida com VGCNF produzidos a partir de um método simples. De seguida, foi investigada a relação entre as propriedades elétricas e o nível de dispersão de VGCNF nos compósitos de resina epóxida, produzidos com diferentes métodos, os quais foram seleccionados de forma a proporcionarem diferentes níveis de dispersão. O nível de dispersão das nanofibras em compósitos produzidos com diferentes métodos e concentrações de cargas foi analisado através da microscopia ótica de transmissão (TOM) e da análise da escala de cinzentos (GSA), sendo posteriormente comparados os resultados com as medições de condutividade elétrica. Depois deste estudo, foi investigada a influência dos diferentes métodos de dispersão nos mecanismos de condução eléctrica dos compósitos. Seguidamente, estes compósitos foram submetidos a testes eletromecânicos de forma a poderem ser aplicados como sensores piezoresistivos. O último estudo deste trabalho foi dedicado a uma comparação inicial entre os compósitos de resina epóxida com VGCNF e os com nanotubos de carbono multi-parede (MWCNT), em termos de propriedades elétricas e morfológicas. Dos principais resultados deste trabalho pode-se concluir que uma melhor dispersão dos aglomerados parece ser mais adequada do que uma boa dispersão das nanocargas para alcançar condutividades eléctricas elevadas e limiares de percolação reduzidos. Também é possível concluir que a condução por efeito de “hopping” é um mecanismo relevante para determinar a condutividade global dos compósitos e que os compósitos de resina epóxida e CNF são materiais apropriados para serem aplicados como sensores piezoresistivos, particularmente para concentrações próximas do limiar de percolaçãoFundação para a Ciência e Tecnologia (FCT) - SFRH/BD/41191/200

Mechanical, Thermal, and Electrical Properties of Graphene-Epoxy Nanocomposites—A Review

Monolithic epoxy, because of its brittleness, cannot prevent crack propagation and is vulnerable to fracture. However, it is well established that when reinforced—especially by nano-fillers, such as metallic oxides, clays, carbon nanotubes, and other carbonaceous materials—its ability to withstand crack propagation is propitiously improved. Among various nano-fillers, graphene has recently been employed as reinforcement in epoxy to enhance the fracture related properties of the produced epoxy–graphene nanocomposites. In this review, mechanical, thermal, and electrical properties of graphene reinforced epoxy nanocomposites will be correlated with the topographical features, morphology, weight fraction, dispersion state, and surface functionalization of graphene. The factors in which contrasting results were reported in the literature are highlighted, such as the influence of graphene on the mechanical properties of epoxy nanocomposites. Furthermore, the challenges to achieving the desired performance of polymer nanocomposites are also suggested throughout the article

Polymers and Their Application in 3D Printing

Dear Colleagues, Fused filament fabrication, also known as 3D printing, is extensively used to produce prototypes for applications in, e.g., the aerospace, medical, and automotive industries. In this process, a thermoplastic polymer is fed into a liquefier that extrudes a filament while moving in successive X–Y planes along the Z direction to fabricate a 3D part in a layer-by-layer process. Due to the progressive advances of this process in industry, the application of polymeric (or even composite) materials have received much attention. Researchers and industries now engage in 3D printing by implementing numerous polymeric materials in their domain. In this Special Issue, we will present a collection of recent and novel works regarding the application of polymers in 3D printing

Near infrared photon-assisted polymerization of advanced polymer composites.

Advanced composites play important roles in the materials sciences, military, space and commercial applications. The desirable load transfer and mechanical strength of reinforced polymers are crucial for developing advanced composites. Owing to their excellent mechanical properties derived from the sp2 bonding structure and the nanoscale size, nano-carbons are attractive materials used for nanoscale reinforcement of polymer composites. This dissertation describes a novel method to develop polymer composites using near infrared (NIR) photon-assisted polymerization and nanoscale reinforcement. We used multi-walled carbon nanotubes (MWNTs), reduced graphene oxide (RGO), and graphene nanoplatelets (GNPs) to make polymer composites, and explored in-situ NIR photon assisted heating of these nano carbons to enhance polymerization of the nano-carbon/polymer interface, thus achieving significant load transfer and improved mechanical properties. To specify, nano-carbon was dispersed into the polymer matrix by shear or evaporation mixing method to attain a uniform distribution in the prepared thin film composite. The thin film was exposed to NIR light during polymerization instead of conventional oven based heating. NIR was effectively absorbed by nano-carbons and also atoms from polymer molecule; the induced photo-thermal heat was transferred into the polymer matrix to induce polymerization of the composite and the covalent bonding between nano-carbons and polymer matrix at the interface. Scanning electron microscope (SEM), Raman spectroscopy, and RSA were used to evaluate the load transfer and mechanical strength of the polymerized composite samples. Investigating first the nanotube/polymer composites synergized by NIR photon-assisted polymerization, large Raman shifts (20 cm-1 wavenumber for up to 80% strains) of the 2D band were recorded for the NIR light polymerized samples and an increase in Young‘s modulus by ~130% was measured for the 1 wt. MWNT/poly(dimethylsiloxane) (PDMS) composites. While at first it was thought that NIR radiation during polymerization heated the nano-carbons inside resulting in strengthening of the nano-carbon/polymer interface, it was seen after further experimentation with graphene reinforcements that other light induced bonding effects apart from heat were also responsible. Raman spectroscopy revealed that mixing graphene in polymer has a profound effect on the G, D and 2D bands. Investigating G bands for pure RGO and GNPs and comparing them with their polymer counterparts showed large shifts in the G band indicating lattice compression. The comparison of the NIR polymerization with the conventional oven based polymerization for both RGO and GNPs revealed large changes in wavenumbers and indicated increased load transfers for the NIR photon-assisted polymerization method. The Full Width Half Maximum (FWHM) data of the NIR treated samples exhibited smaller change at large strains compared to conventionally polymerized samples indicating the minimum slippage in the former. Finally, the stress-strain curves showed more than three times improvement in the Young‘s modulus of the composites fabricated using the NIR treatment in comparison to the conventional baking for both types of graphene. These results are compared to the carbon nanotube (CNT) counterparts in PDMS. The study provided insights on how to use stress-sensitive shifts in Raman spectroscopy for the development of advanced polymer composites. While NIR light induced polymerization showed increased load transfer and mechanical strength of nanotube and graphene polymer composites, investigation into two types of nano-carbon of different dimensionalities yielded extraordinary synergy between nano-carbons. Synergistic effects in binary mixtures of nano-carbon/polymer composites polymerized by NIR photon-assisted polymerization are observed. Small amounts of MWNT0.1 dispersed in RGO0.9/PDMS samples (subscripts represent weight percentage) reversed the sign of the Raman stress-sensitive wavenumbers from positive to negative values demonstrating the reversal of the lattice stress itself on applied uniaxial strain. A wavenumber change from 10 cm-1 in compression to 10 cm-1 in tension, and an increase in the Young‘s modulus of ~103% was observed for the MWNT0.1RGO0:9/PDMS with applied uniaxial tension. Extensive scanning electron microscopy measurement revealed the bridging of MWNT between two graphene plates in polymer composites. Mixing small amounts of MWNTs in RGO/PDMS eliminated the previously reported compressive deformation of RGO and significantly enhanced the load transfer and the mechanical strength of composites in tension. This is a direct indication of the cooperative effects of binary nano-carbons that produces an overall dramatic increase in load transfer (100% change). The orientation order of MWNTs with the application of uniaxial tensile strain directly affected the shift in the Raman wavenumbers (2D-band and G-band) and the load transfer. It is observed that the cooperative behavior of binary nano-carbons in polymer composites resulted in enhanced load transfer and mechanical strength. Such binary compositions could be fundamentally interesting for developing advanced composites such as nano-carbon based mixed dimensional systems. The NIR polymerization could be used to control aspects such as polymer chain entanglement between nano-carbons of different dimensional states, polymer chain lengths, mobility and eventual mechanical and electrical properties. At first it was thought that NIR light based polymerization only heated the nano-carbons and strengthened the interface, further studies using X-ray photoelectron spectroscopy (XPS) suggested other light induced bond formation was also responsible mechanism for improved interfacial strength, load transfer and mechanical properties. XPS data on RGO/polymer composites suggested activation of hydroxyl and carbonyl groups on the RGO that opens the carbon-carbon double bond of the PDMS oligomer thereby assisting in the formation of the C-O bonds between the PDMS matrix and the graphene filler. High absorption of NIR photons causes the free radical reaction between SiH group on PDMS crosslinker and hydroxyl/carbonyl groups on the RGO. The increase in the number of C-O and Si-O bonds at the graphene/polymer interface assists in the improved load transfer and eventual mechanical properties of the composites. This is the first such study which shows direct correlation between bond formation, load transfer and mechanical properties without degrading the interface. While surface chemical functionalization is attractive, past reports have shown that improvement in interfacial adhesion due to surface functionalization of nanotubes does not always promote improvement in mechanical properties. This is due to the surface degradation of nanotubes/graphene during functionalization process. Compared to these techniques, the NIR light induced technique is benign, environmentally friendly and also results in high interfacial shear strength, load transfer and excellent mechanical properties. As a demonstration of applications, PDMS/RGO/PDMS sandwiched structure strain sensor, a demo application of the NIR photon-assisted polymerization was investigated. High sensitivity and high Gauge Factor (GF) are addressed. These results shown in this dissertation suggest that the NIR photon-assisted polymerization can be practically developed as a scalable nanomanufacturing technique to create large panels of advanced composites with strong interface and better mechanical properties compared to conventional oven based heating methods. It also suggests that it is possible to fabricate large-scale flexible smart device like high sensitivity strain sensors

Capacitive Micromachined Ultrasonic Transducers (CMUTs) for Humidity Sensing

In the last two decades, capacitive micromachined ultrasonic transducers (CMUTs) have proven themselves to be promising for various ultrasound imaging and chemical sensing applications. Although holding many benefits for ultrasound imaging, CMUTs have certain weaknesses such as the relatively low output pressure at transmission, which hinder their development in the diagnostic imaging application. In the sensing area, CMUTs have shown attractive features such as high mass sensitivity, miniaturized array configuration, and ease of functionalization. However, their potential for humidity sensing is less explored. The objectives of this thesis lie in two aspects. One is to offer a solution to overcome the limitation of low output pressure, and the other is to develop CMUTs as resonant gravimetric humidity sensors. The major efforts are made on the second task. For the first objective, a novel dual-element ultrasonic transducer is proposed. It incorporates two transducer technologies by using a circular piezoelectric element for ultrasound transmission and an annular CMUT element for reception. The hybrid transducer combines the broad bandwidth and high receive sensitivity of the CMUT and the high output power of the piezoelectric transducer to improve the overall sensitivity and axial resolution. The annular CMUT is designed, fabricated, and concentrically aligned with the piezoelectric probe via a custom housing. Immersion measurements show that the hybrid dual-element transducer improves the axial resolution by 25.58% and the signal-to-noise ratio by 8.55 dB over the commercial piezoelectric probe. For the second objective, a CMUT-based resonant humidity sensor is first developed with the direct wafer bonding technique. Graphene oxide (GO) is employed as the sensing material. Due to combination of the mass-sensitive CMUT and the moisture-sensitive GO, the sensor exhibits rapid response/recovery, good repeatability, and higher sensitivity than most of its competitors. The second generation of CMUT-based humidity sensors aims to further improve the relative humidity (RH) sensing performance by adopting the nitride-to-oxide wafer bonding technology for CMUT fabrication. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the new process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. It provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Additional benefits of the nitride-to-oxide wafer bonding technique are the reduced fabrication complexity and more controllable membrane thickness. Finally, a dual-frequency (10/14 MHz) CMUT is developed using this fabrication technique. It generates two RH response curves and can provide more accurate RH sensing. Due to the independence of the two resonance frequencies, the dual-frequency CMUT also shows great potential for identification of different chemicals. This thesis demonstrates that CMUT sensors can be strong candidates for miniaturized, highly sensitive, and reliable humidity sensors

Multi-Layer-Graphene-Nanoclay-Epoxy Nanocomposites – Theory and Experimentation

The influence of Multi-Layer Graphene (MLG) and nanoclay on the performance of epoxy based nanocomposites has been studied. First, the theoretical aspects of nano-fillers and their impact on mechanical, thermal, and electrical properties of nanocomposites have been discussed. Then, nanocomposites were produced with varying weight fraction of nano-fillers (0.05, 0.1, 0.3, 0.5, and 1.0 wt%). It was observed that organic solvent, if not completely removed, causes porosity which acts as stress raiser and deteriorates the mechanical properties. The influence of reinforcement morphology on the mechanical properties of epoxy nanocomposites was studied using two nano-fillers: MLG and nanostructured graphite (NSG). It was observed that mechanical properties of nanocomposites were higher when the filler had corrugated and fluted topography. Modeling and simulation of epoxy nanocomposites were carried out using finite element method. It was observed that graphene based nano-fillers are efficient in scattering and dissipation of heat flux thereby increasing the thermal stability of epoxy nanocomposites. The macro-topography of bulk samples of monolithic epoxy and nanocomposites was modified by treating the samples with the abrasive papers. It was observed that surface notches, when exceed certain depth, cause degradation in mechanical properties. It was further observed that tensile properties are more sensitive to topography than flexural properties