Browsing by Subject "Composites"

Now showing 1 - 9 of 9

A regioselective route to negative coefficient of thermal expansion materials for tuning epoxy thermomechanics
(2021-12-03) Kiker, Meghan; Page, Zachariah
Thermoset polymers are widely used due to their low cost and versatile high-performance material properties. These materials have found primary applications as heterogenous mixtures (composites) in adhesives, encapsulants, and more. However, thermosets and composites are known to have large coefficients of thermal expansion (CTE), which differ from the CTE of the substrates they are often applied to. A large CTE mismatch results in localized stress and sometimes cracking of the substrate, precluding utility of such thermosets/composites in certain applications, such as optoelectronics where adhesives and encapsulants would be highly beneficial. Remedies for this problem include using fillers such as silica or inorganic metals to lower the CTE of the composites, but these solutions can increase the mass and materials cost and shorten device lifetime. The incorporation of negative coefficient of thermal expansion (NTE) materials into the polymer backbone as a tunable handle has emerged as a promising alternative. The following thesis will describe a six-step regiospecific synthesis to obtain a diepoxy-substituted dibenzocyclooctene (DBCO) derivative that is anticipated to exhibit improved NTE behavior relative to state-of-the-art materials
Advanced manufacturing of carbon fiber reinforced PAEK composites : bonding and fracture mechanisms
(2023-08) Heathman, Nathaniel Thomas; Tehrani, Mehran; Seepersad, Carolyn; Kim, Hyonny; Kovar, Desiderio
Fiber reinforced composites are used in applications where lightweight, strength, stiffness, fatigue life, and corrosion resistance are critical. Thermoplastic composites (TPC) offer several advantages over thermosetting ones, including higher toughness, recyclability, weldability, and ease of repair. Over the last decade, the interest in in situ consolidation additive manufacturing (AM) of TPCs has increased exponentially due to their potential for rapid cycle out-of-autoclave processes. To this end, several limitations need to be resolved. Specifically in situ consolidation of TPCs usually result in low interlaminar bonding, weak matrix/fiber adhesion, high void content, and low crystallinity. This dissertation aims to provide a new understanding and solutions to these issues. The material system used in this work is carbon fiber reinforced low-melt polyarlyletherketone (LM-PAEK), processed via two AM methods: fused filament fabrication (FFF) and directed energy deposition (DED) in the form of laser-assisted automated fiber placement (AFP). By combining experiments and modeling, this work aims to develop an understanding of how processing parameters affect interlaminar bonding, void development, and crystallinity for both processes. This dissertation also investigates in situ consolidation of TPCs by examining the underlying physics that control bond strength and fracture toughness at inter-layer interfaces. Overall, this dissertation contributes to the knowledge base in the field, toward the adoption of low-energy and high-rate processes for manufacturing high-performance TPC structures.
Energy dissipation and stiffness of polymeric matrix composites with negative stiffness inclusions
(2016-08) Cortes, Sergio Andres; Kovar, Desiderio; Seepersad, Carolyn C.; Haberman, Michael R.; Bourell, David L.
Typical structural materials have high stiffness to support a static load but offer low damping capacity. These materials easily transmit vibrations that can propagate through the structure, inducing fatigue and premature failure. Thus, structural materials with enhanced damping would increase the operating life of the structure and improve its performance. Here, we study a new class of metamaterials that exhibits simultaneously high damping and stiffness through the use of negative stiffness structures (NSS) embedded into a polymer matrix. Traditional materials have positive stiffness behavior, meaning that the stress increases monotonically with the strain. Similarly, structures made from traditional materials exhibit a positive stiffness, so that the load increases monotonically with displacement applied. NSS structures, however, exhibit a region of negative slope in the force versus displacement response. It has been predicted that the incorporation of these mechanically activated NSS into a polymer matrix would improve the damping behavior, but this has not previously been demonstrated experimentally. A significant part of this work was aimed at determining the geometry of the NSS and the material properties of the NSS and matrix required to achieve high damping. Thus several combinations of NSS geometries, matrix stiffnesses and NSS properties were considered. Analytical and numerical models were developed to guide the design of specimens. Experiments were aimed at producing specimens where damping performance was measured for NSS embedded in a polymer matrix. To conduct these experiments, macro-scale NSS were produced from stainless steel 17-4PH and the properties of the NSS and the NSS embedded in matrices were measured. Results showed that both the design of the NSS and the ratio of the stiffness of the NSS to that of the matrix are important for producing composites that offer simultaneously high damping capacity and high stiffness. Another key challenge is producing NSS at a fine enough scale so that they can be incorporated into a polymer matrix to produce a composite damping material. Amongst potential manufacturing techniques, the multi-filament co-extrusion (MFCX) was selected because it has the potential to produce ceramic, metal or polymer micro-configured geometries in large quantities, quickly and at low cost. This process uses combinations of ceramic-polymer or metal-polymer compounds to reduce an initially macroscopic structure to the microscale while preserving the geometry of the cross-section. When the viscosities of the compounds are ideally matched, co-extrusion is capable of reducing the cross-section by a factor of up to 1000 times (e.g. well into the microscale). However, extensive characterization of the rheology of the compounds is required to achieve very large reductions for complex cross-section such as these. Preliminary results with co-extruded materials were presented to demonstrate the feasibility of this approach.
Functional polymer/graphene oxide composites synthesis, characterization and applications
(2016-08) Ha, Heonjoo; Ellison, Christopher J.; Willson, Carlton G; Freeman, Benny D; Akinwande, Deji
Polymer nanocomposites have been identified as a growth area for the last several decades. The synergy between inorganic and organic compounds has played a major role in developing advanced functional materials for many emerging applications, including enhancing physical/chemical properties of base polymers, replacing metal counterparts, and introducing new energy storage materials, among others. While a number of carbon based nanoparticles have been considered as nanofiller material, significant research effort has been devoted to studies on graphene and its graphene oxide (GO) or reduced graphene oxide (rGO) derivatives. Graphenes are 2D sheets of carbonaceous material that possess extraordinary mechanical properties, thermal and electrical conductivities, and high surface area. However, most of the methods that have been developed to mass produce graphenes often require costly and tedious purification, along with associated high energy consumption, to achieve the most attractive forms of the material. One of the solutions to reduce the cost of synthesizing graphene while preserving its excellent properties is to use a precursor such as GO. While preparing GO, GO sheets can be functionalized with numerous reactive groups including carboxylic acids, hydroxyls, and epoxides that can be exploited for materials design. The work in this thesis outlines the synthesis of functional polymer/GO composites by utilizing secondary interactions, such as hydrogen bonding and π-π interactions, and covalent bonds between functional polymer and GO sheets. To understand the impact of these approaches, a fundamental investigation directed towards characterizing various chemical and physical properties for a range of GO-containing materials is discussed in full detail. In addition, different functional polymer/GO composites proposed in this work are evaluated for their utility in a number of different applications. Finally, it is expected that these composite materials will be cost-effective, commercially relevant and reasonable to scale-up for mass production. Therefore, this research will not only contribute to enriching fundamental knowledge but it also has potential to impact society and the economy.
Graphite oxide and its applications in the preparation of small molecules, polymers, and high performance polymer composites
(2012-05) Dreyer, Daniel Robert; Bielawski, Christopher W.; Ruoff, Rodney S.; Willets, Katherine A.; Anslyn, Eric V.; Siegel, Dionicio R.
Graphite oxide (GO), a carbon material prepared in one step from low cost commercial materials, and graphene oxide have been found to catalyze a wide range of reactions including oxidations, hydrations, and dehydrations, as well as cationic or oxidative polymerizations. Applicable in both small molecule and polymer chemistry, this single, metal-free catalyst shows remarkable breadth, including the combination of the aforementioned reactions in an auto-tandem fashion to form advanced substrates, such as chalcones, from simple starting materials. Some of these reactions, such as the selective oxidation of alcohols to aldehydes, have been shown to be dependent on the presence of molecular oxygen, suggesting that this may be the terminal oxidant. Aside from its eminently valuable reactivity, the use of GO as a catalyst also presents practical advantages, such as its heterogeneous nature, which facilitates separation of the catalyst from the desired product. The use of this simple material in synthetic chemistry, as well as others like it, is distinct from other forms of catalysis in that the active species is carbon-based, heterogeneous and metal-free (as confirmed by ICP-MS and other spectroscopic techniques). This has led us to propose the term “carbocatalyst” to describe such materials. With dwindling supplies of precious metals used in many common organic reactions, the use of inexpensive and widely available carbocatalysts in their place will ensure that commercial processes of fundamental importance can continue unabated. Moreover, as we have shown with just one material, carbons are capable of facilitating a broad range of reactions.
Seismic retrofit of RC columns with FRP composites and anchorage system
(2015-05) Psaros Andriopoulos, Apostolos; Jirsa, J. O. (James Otis); Hrynyk, Trevor
Research on the use of composite materials in structural applications started more than 30 years ago but still remains active. The challenges that accompany those applications are diverse and seem to increase as the variety of applications grows. There are several fiber-reinforced polymer (FRP) systems that have been introduced through the years for strengthening reinforced concrete (RC) structures. Those systems focus on strengthening of slabs, beams and columns. The present study pertains to seismic retrofit of rectangular RC columns. The typical FRP materials used in structural applications are introduced, as well as, how FRP materials become an integral part of the force-resisting system. In addition, analysis work pertaining to a series of strengthened RC columns was conducted and the results were compared to the experimental data. Moreover, deficiencies of typical material models were highlighted. Design guidelines are discussed and recommendations about current design practices are provided. Finally, research gaps and future research recommendations are identified.
Silicon Carbide Preforms for Metal Infiltration by Selective Laser Sintering™ of Polymer Encapsulated Powders
(1993) Vail, N.K.; Barlow, J.W.; Marcus, H.L.
A polymer encapsulated silicon carbide system has been developed for use with Selective Laser Sintering. Extensive studies with this material have provided information pertaining to processing and material parameters which most affect the strengths and densities of resulting green parts. The important parameters considered were particle size distribution of the powders, laser scanning conditions, and laser beam diameter. Simple and complex shapes were easily produced with this material using optimized parameters. Green objects were infused with metal by Lanxide using their pressureless infiltration process to produce both metal matrix and ceramic matrix composites. (Key Words: Silicon Carbide, Encapsulation, Polymer, Selective Laser Sintering, Composites).
Surface chemistry and material integration of metal oxide nanocrystals
(2022-09-12) Lakhanpal, Vikram Shri; Milliron, Delia (Delia Jane); Ganesan, Venkat; Lynd, Nathaniel; Yu, Guihua
Metal oxide nanocrystals have a variety of chemical, electronic, and optical properties unique not only to their material composition but also to their size and shape. Control and tunability over these physical parameters can be achieved through colloidal synthesis. In this process, small nuclei form in a heated mixture and long organic molecules known as ligands, which are typically amines or carboxylates, regulate their growth. These ligands also provide long term stability to the nanocrystals when dispersed in solution, as their bulky chains prevent the particles from aggregating. However, many of the properties that make nanocrystals so intriguing involve interaction with their surfaces, which necessitates the removal of the ligands. A variety of methods for ligand stripping have been explored, all with the goal of obtaining ligand-free nanocrystals that retain their properties in dispersions that are stable long enough to combine with other materials, such as polymers, or to process into a functional device, such as a film or coating. In this dissertation, I relate my work during my PhD in various methods of ligand stripping and nanocrystal processing. In particular, I focus on nanocrystalline cerium oxide, a rare-earth metal oxide with a highly reactive surface, and cerium-doped indium oxide, an optically transparent conductor. Cerium oxide nanocrystals are ligand stripped with an organic salt and transferred into dimethylformamide, a polar organic solvent, after which they are mixed with poly(ethylene oxide) to make composite thin films. Protons are generated from water vapor by the cerium oxide surface, turning the films into a proton conducting electrolyte. Cerium-doped indium oxide nanocrystals are stripped with potassium hydroxide in order to transfer them into water, where it is mixed with the conductive polymer PEDOT:PSS in order to create conductive films with enhanced transparency over more opaque films containing just the polymer. Following the mixed results of these projects, I developed and adapted the potassium hydroxide ligand stripping process to a broader range of metal oxide nanocrystals, providing a general method for ligand stripping and transfer into aqueous media.
Towards an Automated Methodology for Optically Cured, High Thickness Composite Polymer Pastes: An Iso-based Approach for Large Area Additive Manufacturing
(2022) Kerr, C.J.; Munguia Valenzuela, J.; Nixon-Pearson, O.; Forrest, M.; Gosling, P.D.
Optically cured polymer composites can offer numerous advantages over thermally- processed composites. This is especially applicable with large beads generated using Large Area Additive Manufacturing (LAAM) extrusion. This paper proposes a low-cost, semi- automated, off-the-shelf desktop system to quickly and precisely cure 8 mm thick, glass-filled (di-)methacrylate composite specimens for three-point flexural testing. In the absence of a clear preparation method for specimens of this thickness, we integrate aspects of two existing ISO Standards: ISO 178:2019, and ISO 4049:2019, to provide a rapid and reliable basis for specimen preparation and flexure testing. We propose two distinct curing strategies and compare these in terms of the flexural performance of prepared specimens, and the total cure time. Flexural data is critical to understanding the structural performance of light-activated composite pastes; and the curing methodology can be applied to potential future technologies which involve high-speed in-situ curing, from direct part/feature repair to functional production.