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Due to their small size and large surface area-to-volume ratios, nanoparticles (particles with limiting dimensions smaller than 100 nm) have been widely applied as catalysts. Metal nanoparticles are typically produced in suspensions from metal ion precursors, reducing agents, and organic ligands called capping agents. Capping agents help prevent particle agglomeration, fix nanoparticle size, and promote monodispersity. However, capping agents also affect the morphology and the physico-chemical surface properties of nanoparticles, which can influence catalytic properties in unpredictable ways. While there have been extensive studies focused on examining the relationship between nanoparticle size, shape, composition and catalytic activity, relatively few have investigated the effects of capping agent properties on catalysis, and most studies involving nanoparticle catalysts have been conducted on collections, ensembles, or arrays of particles rather than single nanoparticles. Results obtained for systems composed of multiple nanoparticles dispersed on solid surfaces can be difficult to interpret due to variations in particle loading and interparticle distance, which are often challenging or impossible to control and characterize. The complexity of these unavoidable experimental variables may explain some of the seemingly inconsistent conclusions that have been drawn between nanoparticle properties and catalytic activity in recent reports. Single nanoparticle studies should help overcome limitations associated with investigations based on collections of nanoparticles by helping uncover direct relationships between nanoparticle size, surface properties, and catalytic activity that are unobscured by complex factors such as interparticle distance and particle loading. In this work, we aim to use nitrided carbon fiber ultramicroelectrodes to examine electrocatalytic properties of bare (uncapped) and capped gold nanoparticles at the single nanoparticle level.

Ultramicroelectrodes (UMEs) are useful as probes for evaluating electroactive species in confined spaces (e.g., inside living cells) and for measuring fast electrochemical reactions. However, UME applications often require modification of the electrode surface to improve selectivity and sensitivity towards target analytes. Previous research in our group demonstrated that a simple soft nitriding method introduces surface nitrogen (N)-containing groups on carbon fiber (CF), leading to improved electroreduction of hydrogen peroxide (H2O2) on CF-UMEs. However, sensitivity for H2O2 detection using N-CF-UMEs was low compared to that for other modified UMEs. As an alternative to N-CF-UMEs, a simple strategy for preparing phosphorus (P)-doped CF-UMEs was first investigated. Since P-CF-UMEs performed similarly to N-CF-UMEs, an alternative epoxy sealing strategy for preparing CF-UMEs and doped-CF-UMEs was also developed. Compared to P-CF-UMEs and N-CF-UMEs prepared by traditional laser-assisted pipette pulling, the epoxy-sealed electrodes exhibited 20-50 times higher sensitivities and 2-3 times lower detection limits for H2O2.

<p> An electrically conductive thermoplastic composite (ECTPC) consists of electrically conductive filler(s) at a concentration above percolation threshold distributed in an insulating polymer matrix. The high concentration of the filler required to achieve high electrical conductivity for ECTPC is usually accompanied with the deterioration of mechanical properties and a large increase in the viscosity which prevents feasible processing of these materials in common polymer processing equipments such as injection molding machinery. The initial focus of this work was to control these drawbacks by using combinations of conductive fillers namely carbon fiber (CF) and carbon black (CB) to create a hybrid-filler composite. Cyclic olefin copolymer (COC), an amorphous polyolefin, was used as the matrix material. It was found that carbon black and carbon fiber synergistically contribute to the transport of electrons through the matrix. The synergism exists at various filler concentrations including when one of the fillers was present below its percolation threshold, but not at high carbon fiber content. Results showed that where the concentration of CF was several fold higher than carbon black a good trade-off between viscosity and conductivity can be achieved so that the obtained composites can be reasonably processed tn common processing equipment e.g. in an injection molding machine </p> <p> Carbon fiber is preferred to carbon black as it leads to ECTPC with higher electrical conductivity and lower viscosity. However, the high aspect ratio fibers preferentially align in the flow direction leading to ECTPCs which have electrical conductivity several orders of magnitude greater in the in-plane rather than through-plane. We focused on foaming as a strategy to reorient the fibers toward the through-plane direction in foam injection molding. Through a fractional factorial experimental design, the effect of injection rate, melt temperature and mold temperature on electrical conductivity was screened at two levels for foam and nonfoam COC/CF(lO vol%)-CB(2 vol%) injection molded composites. It was found that foaming significantly enhanced the through-plane fiber orientation and through-plane conductivity of the hybrid composite at low injection rate and high melt temperature. The concurrence of the melt flow and bubble growth was considered to be the key mechanism for fiber reorientation while the cell size and shape should not disrupt the conductive path spanning the bulk of the material. </p> <p> The importance of the relative length scale of the fillers on cell size and subsequently, electrical conductivity was investigated by injection molding. Results showed that where the length scale of the filler was comparable to the cell size, as for foamed COC/CF composites, the conductivity considerably decreases with foaming. The drop was greater in the through plane direction and smaller in the in-plane direction for the composites with larger average fiber length. Also smaller cells led to a larger drop in the composite conductivity. It was observed that where the length scale of the filler was much smaller than the cell size as such for COC/CB composites, foaming enhanced the electrical conductivity particularly in the through-plane directions and its effects became more pronounced at lower carbon black concentrations. It was proposed that induced carbon black coagulation by foaming was the main reason for the observed improvement in conductivity. For COC/CF-CB hybrid composites, enhancement in through-plane conductivity, particularly at CB concentration below percolation, via foaming inferred that CB aggregates significantly contributed in improving fiber-fiber contacts. </p> <p> Reorientation of the fibers by foaming was found to be very dependent on processing conditions. High viscosity and fiber- fiber interactions can hinder fiber rotation. The general understanding of the investigation was that fiber reorientation may occur where the cells are much larger than the fibers. In comparison, a series of nonfoam injection molded composites containing CF, CB and CF-CB were foamed in a batch process to avoid flow effects. The insignificant change in fiber orientation with foaming proved that fibers can not rotate by the growth of an adjacent cell in the absence of shear. Also, a large drop in electrical conductivity with foaming as compared to the foam injection molded composites suggested that particle relocalization can not occur in batch foaming. </p> / Thesis / Doctor of Philosophy (PhD)

Additively manufactured carbon fiber reinforced polymers (CFRP) are vastly studied for their remarkable mechanical properties compared to most other 3D printed materials. Different methods were employed to further increase mechanical performance of CFRP 3D printed parts. The objective of the study is to investigate the effect of autoclave postprocessing on the interlaminar shear behavior between 3D printed CFRP layers. 3D printed CFRP samples were processed with nine combinations of temperature and vacuum in an autoclave. Short beam shear (SBS) tests were performed to characterize the interlaminar shear strength (ILSS) of the samples after autoclave processing. Digital image correlation (DIC) was utilized to quantify the strain and failure mode of the samples during SBS tests. From SBS mechanical tests, the curing temperature and vacuum of 170 C and -90 kPa produced samples with the highest ILSS, 39 MPa, a 46% improvement compared to uncured samples. The observed failure modes were fracture and delamination. Little work in additive manufacturing has applied autoclave as a post-process procedure. This study aims to explore this technique and establish its viability in improving mechanical performance of 3D printed fiber-reinforced parts.

Carbon fibres are used in many applications thanks to their exceptional tensile properties in relation to their relatively low weight. These thin strands of at least 90% carbon are mixed with a matrix such as epoxy to form composites used for cars, wind turbines, prosthetics, and many other things. Carbon fibres are often manufactured from polyacrylonitrile (PAN) based on fossil oil, an expensive and non-renewable resource. Developing carbon fibres from renewable resources, like products from the forest industry such as lignin and cellulose could reduce the environmental impact of carbon fibre production. Cellulose precursors have long been used for carbon fibre production, but adding lignin could potentially increase the yield and improve the tensile properties of the carbon fibres. The transition from PAN carbon fibres to lignin-cellulose carbon fibres might also lower the cost. Currently, the properties of cellulose- and lignin-based carbon fibres are poor compared to other fibres on the market. Research like this work focuses on improving the tensile properties of the carbon fibres to make them viable alternatives. The focus of this work was to further develop the three main parts of thermal conversion: stabilisation, low-temperature carbonisation and high-temperature carbonisation, by evaluating the impact of temperature and tension on the tensile properties of the fibre. The fibres were converted using a continuous setup for each process step. The stabilisation profiles tested ranged between 245-260 ⁰C, low-temperature carbonisation between 460-600 ⁰C and high- temperature carbonisation between 1100-1600 ⁰C. Batch conversion was conducted to compare conversion methods. The results showed that the temperature of the low-temperature carbonisation had a large impact on the ability to stretch the carbon fibre and increase the tensile properties with 600 ⁰C being the optimal temperature out of the temperatures tested on the setup used in this thesis. The stretching during the high-temperature carbonisation and the temperature of the stabilisation had less impact on tensile properties. The properties of the precursor fibre are not always indicative of the properties of the carbon fibre, but the methods used for creating the precursor fibres have a great impact on the behaviour during thermal conversion.

Master of Science / Material extrusion is a common form of 3D printing that has historically been limited to producing prototypes, models, and low load-bearing parts. This is primarily because parts are manufactured layer-by-layer, resulting in poor adhesion along the build direction, and machines struggle to print with high-strength polymers, which tend to shrink significantly as they cool. However, one way to address these limitations is to use fiber-reinforced materials in combination with multi-axis deposition strategies. In material extrusion, embedded fibers will align themselves along the deposition path, providing structural, thermal, and chemical improvements. Multi-axis toolpathing can enable the deposition of this fiber-filled material in full 3D along a part's expected stress paths. This is possible using a complex kinematic system like an industrial robot arm that can rotate the angle of the tool relative to the part as it is printing. The objective of this work was to develop and test a tool capable of multi-axis continuous carbon fiber reinforcement, which required a dedicated cutting mechanism to shear the fiber at the end of each deposition path, control over the amount of fiber used, and a slender tool profile to avoid collisions during multi-axis printing. The findings of this work revealed that while the use of continuous carbon fiber further reduced the adhesion between deposition paths, it substantially improved the strength of the part along them. To validate the multi-axis capability of the system, a toolpath was generated for a curved tensile bar. The results showed that the continuous carbon fiber multi-axis toolpath resisted a load 820.57% higher than an XY-planar sliced part printed with traditional filament, confirming the effectiveness of the presented approach. Multi-axis motion can also be used for avoiding support material requirements. In traditional 3-axis material extrusion, steep overhanging features often require additional, sacrificial material to be printed underneath. This leads to longer print times, more material waste, and a poor surface finish left behind on the final part. To minimize the amount of support material required, various techniques have been explored, including changing the toolpath, part geometry, or material processing parameters. However, none of these techniques have been successful in eliminating the need for supports entirely. A promising approach to address this issue is multi-axis material extrusion, where the angle of the printing tool and the direction of the layers can be precisely controlled during the printing process. This technique can be used to ensure that the tool is always extruding material onto a well-supported surface, rather than over thin air. However, research to date has not yet fully explored how the range of achievable overhang features changes as the tool is rotated. To address this knowledge gap, this work used an industrial robot arm equipped for material extrusion to investigate the relationship between tool angle, build direction, and achievable overhang threshold. The results showed that the same overhang limitations that exist in the XY plane will rotate with the tool and are unaffected by gravitational forces. These findings provide valuable insights for advancing the use of multi-axis material extrusion in the production of complex and intricate 3D objects without the need of supports.

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