Polymer Derived Ceramics are a promising class of Materials that allow for higher levels of tunability and shaping that traditional sintering methods do not allow for. Polysiloxanes are commonly used as a precursor for these types of material because of their highly tunable microstructures by adjusting the side groups on the initial polymer. These Polymers are generally cross linked and pyrolyzed in inert atmospheres to form the final polymer. The microstructures of Polymer Derived Ceramics is complex and hard to observe due to the size of each microstructure region and the proximity in the periodic table that the elements present have. The process of forming phases such as Graphitic Carbon, Amorphous Carbon, Silicon Carbide. Silicon Oxide, and SiliconOxycarbide are not well understood. Simulation provides a route to understanding the phenomenon behind these phase formations. Specifically, Molecular dynamics simulation paired with the Reaxff forcefield provides a framework to simulate the complex processes involved in pyrolysis such as chemical reactions and a combination of thermodynamic and kinetic interactions. This Thesis examines firstly the size effect that a system can have on phase separation and the change in composition. Showing that size plays a major role in how the system develops and limits the occurrence of specific reactions. Secondly, this thesis shows that using polymer precursors with different initial polymer components leads to vastly different microstructures and yield. This provides insights into how the transition from polymer to ceramic takes place on a molecular level. / Master of Science / Ceramics and Polymers are seen all around the world. Polymers are used in many things from grocery bags to high performance panels on airplanes. Polymers are generally cheap to produce and can be molded into a variety of shapes. Ceramics are generally hard materials and are also used in a wide variety of situations from the concrete in buildings to coatings that protect turbine blades. Ceramics tend to be harder to form specific shapes and more costly to machine. Polymer derived polysiloxanes address this problem by being formed in the polymer state and then transformed into a ceramic by being heated in inert atmospheres. The process of the heating is very complex and the effect that different polymers have on the atomic level is not well understood. This thesis works to address this by using simulation to see what cannot be seen through experimentation alone.

Master of Science / Department of Mechanical and Nuclear Engineering / Gurpreet Singh / Polymer-derived ceramics (PDCs) such as silicon oxycarbide (SiOC) have shown promise as an electrode material for rechargeable Li-ion batteries (LIBs) owing to the synergy between its disordered carbon phase and hybrid bonds of silicon with oxygen and carbon. In addition to their unique structure, PDCs are known for their high surface area (~822.7 m² g⁻¹), which makes them potential candidates for supercapacitor applications. However, low electrical conductivity, voltage hysteresis, and first cycle lithium irreversibility have hindered their introduction into commercial devices. One approach to improving charge storage capacity is by interfacing the preceramic polymer with boron or aluminium prior pyrolysis. Recent research has shown that chemical interfacing with elemental boron, bulk boron powders and even exfoliated sheets of boron nitride leads to enhancements in thermal and electronic properties of the ceramic. This thesis reports the synthesis of a new type of PDC composite comprising of SiOC embedded with boron nitride nanotubes (BNNTs). This was achieved through the introduction of BNNT in SiOC pre-ceramic polymer at varying wt.% loading (0.25, 0.5 and 2.0 wt.%) followed by thermolysis at high temperature. Electron microscopy and a range of spectroscopy techniques were employed to confirm the polymer-to-ceramic transformation and presence of disordered carbon phase. Transmission electron microscopy confirmed the tubular morphology of BNNT in the composite. To test the material for electrochemical applications, the powders were then made into free-standing paper-like electrodes with reduced graphene oxide (rGO) acting as support material. The synthesized free-standing electrodes were characterized and tested as electrochemical energy storage materials for LIBs and symmetric supercapacitor applications. Among the SiOC-BNNT composite paper tested as anode materials for LIBs, the 0.25 wt.% BNNT composite paper demonstrated the highest first cycle lithiation capacity corresponding to 812 mAh g⁻¹ (at a current density of 100 mA g⁻¹) with a stable charge capacity of 238 mAh g⁻¹ when asymmetrically cycled after 25 cycles. On the contrary, the 0.5 wt.% BNNT composite paper demonstrated the highest specific capacitance corresponding to 78.93 F g⁻¹ at a current density of 1 A g⁻¹ and a cyclic retention of 86% after 185 cycles. This study shows that the free carbon content of SiOC-BNNT ceramic composite can be rationally modified by varying the wt.% of BNNT. As such, the paper composite can be used as an electrode material for electrochemical energy storage.

The main objective of the thesis is to study the thermal properties of Si-based polymer derived ceramics (PDCs) at elevated temperatures and to classify the main factors affecting the thermal transport through these ceramics. The polymer derived ceramics with the chemistry Si- O-C-N were prepared starting from commercial polycarbosilane, polysiloxane, and polysilazane precursors. These precursors are cross-linked at room temperature to obtain the preceramic, followed by controlled pyrolysis (at different temperatures ranging from 1200 oC to 1800 oC in argon, nitrogen or carbon-di-oxide atmospheres), to get the final ceramic. The first part of the thesis discusses on development and studies of dense polymer derived thin disks having a basic chemistry, Si-C, Si-O- C, and, Si-C-N-O, developed via a casting technique followed by specific pyrolysis cycles. Having a thickness in the range of 100 μm- 300 μm, these ceramic disks were studied to be nanocrystalline/amorphous at least up to a temperature of 1400 oC and were found to have a significant amount of Cfree phase existing in them along with the intended chemistry. The high-temperature thermal properties were primarily investigated on ceramics prepared at a pyrolysis temperature of 1200 oC (ceramic still in nanocrystalline/amorphous glassy phase). The disks were found to have very low expansion coefficients (CTE) measured up to ~900 oC and the thermal diffusivity (k) and thermal conductivity (l) of these disks were also measured. An attempt to understand the influence of the different phases in a SiOC ceramic (mainly the Cfree phase, studied by enriching the carbon percentages using DVB) in determining the final thermal properties was also conducted. The influence of carbon enrichment on the mechanical properties of these disks is also studied as a sub-part of this work. The second part of the work deals with testing the possibility to use these ceramics for high-temperature insulation applications. ‘Reticulated’ ceramic foams of relatively same chemistries as that of the disks were prepared by a template replica approach, using polyurethane (PU) foams (more open-celled to more closed-celled types of PU foams were used in the study) as the template. Porous structures having densities ranging from as low as 0.02 g.cm-3 to 0.56 g.cm-3 and with a porosity ~ 80 % to ~99% were prepared and tested. The developed foams showed excellent thermal stability up to a temperature of 1400 oC and possessed very low thermal expansion. The thermal conductivity measured on them at RT gave values in the range 0.03 W.m-1.K-1- 0.25 W.m-1.K-1. A Gibson-Ashby modeling approach to explain the thermal conductivity of the porous ceramics was also attempted. The developed foams were also found to be mechanically rigid. In a nutshell, the thesis work studies the thermal properties of Si-O-C- N ceramics in detail and probes into the possibility to develop these class of Si-O-C-N ceramics into promising high-temperature insulation material.

L’objectif de cette thèse est de réaliser des revêtements de nitrure de bore sur différents types de substrats comme le graphite, le quartz, le pyrex, en allant jusqu’aux métaux et en particulier le titane. Le choix de la voie PDCs s’avère intéressant grâce à la maîtrise du précurseur de départ au niveau atomique d’une part et à la facilité du procédé de dépôt, d’autre part. Nos objectifs étaient d’étudier la faisabilité de réaliser des revêtements BN sur différents types de substrat en utilisant un traitement thermique résistif et de mettre en place un dispositif qui nous permette de pyrolyser les films polymériques sur métaux sans dommage pour le substrat, en vue de leur protection contre l’oxydation ou d’autres applications mécaniques. Dans ce sens, nous avons démontré la possibilité d’utiliser un traitement thermique alternatif par lampe halogène émettant dans l’infra-rouge pour densifier les revêtement BN déposer sur substrats métalliques. / The aim of this work was to prepare boron nitride coatings onto different substrates using the Polymers Derived Ceramics (PDCs) approach. In that way, BN coatings were obtained onto graphite, pure silica and metal especially titanium. The first part of this thesis was to study parameters (of the solution used and of the dip-coating process), to obtain the best coatings in terms of morphology, cristallinity and chemical composition. The second part was dedicated to BN coatings obtained onto metal substrates using an alternative thermal treatment allowing the polymer-to-ceramic conversion without any damage for the metal. Actually, annealing by infrared irradiation allows heating only the coating, energy being reflected by the metal.

L'objectif de cette thèse est de réaliser des revêtements de nitrure de bore sur différents types de substrats comme le graphite, le quartz, le pyrex, en allant jusqu'aux métaux et en particulier le titane. Le choix de la voie PDCs s'avère intéressant grâce à la maîtrise du précurseur de départ au niveau atomique d'une part et à la facilité du procédé de dépôt, d'autre part. Nos objectifs étaient d'étudier la faisabilité de réaliser des revêtements BN sur différents types de substrat en utilisant un traitement thermique résistif et de mettre en place un dispositif qui nous permette de pyrolyser les films polymériques sur métaux sans dommage pour le substrat, en vue de leur protection contre l'oxydation ou d'autres applications mécaniques. Dans ce sens, nous avons démontré la possibilité d'utiliser un traitement thermique alternatif par lampe halogène émettant dans l'infra-rouge pour densifier les revêtement BN déposer sur substrats métalliques.

The need for high performance vehicles in the aerospace industry requires materials which can withstand high loads and high temperatures. New developments in launch pads and infrastructure must also be made to handle this intense environment with lightweight, reusable, structural materials. By using more functional materials, better performance can be seen in the launch environment, and launch vehicle designs which have not been previously used can be considered. The development of high temperature structural composite materials has been very limited due to the high cost of the materials and the processing needed. Polymer matrix composites can be used for temperatures up to 260°C. Ceramics can take much higher temperatures, but they are difficult to produce and form in bulk volumes. Polymer Derived Ceramics (PDCs) begin as a polymer matrix, allowing a shape to be formed and cured and then to be pyrolized in order to obtain a ceramic with the associated thermal and mechanical properties. The use of basalt in structural and high temperature applications has been under development for over 50 years, yet there has been little published research on the incorporation of basalt fibers as a reinforcement in the composites. In this study, continuous basalt fiber reinforced PDCs have been fabricated and tested for the applicability of this composite system as a high temperature structural composite material. The oxyacetylene torch testing and three point bend testing have been performed on test panels and the test results are presented.

Polymer-derived ceramics are a new class of materials synthesized by thermal decomposition of polymer precursors. Previous studies have shown that the materials exhibit excellent thermo-mechanical properties and can be stable at temperatures up to 2000oC. Furthermore, the novel polymer-to-ceramics process enables the manipulation of the ceramic structures at the atomic/nano level by designing the chemistry of polymer precursors and controlling the pyrolysis conditions, thereby, the properties of ceramics. In this dissertation, oxidation/hot-corrosion behavior and the structural evolution of Si-Al-C-N ceramics have been studied. The structural evolution and crystallization behavior of the SiCN and SiAlCN ceramics are investigated using FT-IR, XRD, and NMR. The results revealed that aluminum could greatly affect the structural evolution and crystallization behavior of polymer-derived ceramics, resulting to better stability. The oxidation kinetics of the SiCN and SiAlCN ceramics in air is determined by directly measuring the thickness of the oxide scale with SEM as a function of oxidation time. The results revealed that while the oxidation of the SiCN ceramics follows parabolic kinetics in all of the ranges of testing temperatures, oxidation of the SiAlCN ceramics is complicated: their oxidation rates are similar to that of SiCN ceramics at the earlier stage, but they decrease to very low levels after a certain time. The oxidation rate of the SiAlCN ceramics is more than an order of magnitude lower than any other silicon based ceramics previously reported. The transportation behavior of oxygen through the oxide scales is studied by 18O diffusion. The results indicate that oxidation is controlled by molecular oxygen diffusing through the oxides for both SiCN and SiAlCN ceramics; however, the oxygen diffusion rate in the oxides on SiAlCN ceramics is remarkably retarded. The structures of the oxides are characterized by XRD and NMR. A structural model is advanced to account for the aluminum effect on the oxygen diffusion in the oxide. The oxidation and hot-corrosion kinetics of the SiCN and SiAlCN ceramics in water vapor are determined by measuring their weight changes as a function of annealing time. The kinetic constants, kp and kl, are obtained by fitting the weight-change data with a paralinear model. The results reveal that the SiAlCN ceramics have a much better corrosion resistance than the SiCN and CVD SiC/Si3N4. After annealing at 1400oC for 300 hours, the SiAlCN-20 still retains more than 70% of its original strength, while the SiCN only retains about 20% of its original strength. The improvement in oxidation/hot-corrosion resistance of the SiAlCN ceramics is attributed to the low activity of the SiO2 in the Al2O3-containing silica. In summary, I have developed a new class of high-temperature materials, Si-Al-C-N ceramics. It is demonstrated that these new materials have excellent oxidation and corrosion resistance and thermal stability. Together with their easy processability, the materials will find many high temperature applications such as environmental barrier coatings, ceramic matrix composites, and MEMS for harsh environments.

Polymer-derived ceramics (PDCs) are exceptional ultra-high temperature and stable multifunctional class of materials that can be synthesized from a polymer precursor through thermal decomposition. The presented research focuses on 1-D nanofibers, 2-D films and 3-D bulk, carbon-rich silicon carbon nitride (SiCN) ceramics. 1-D nanofibers were prepared via electrospinning for light weight, flame retardant and conductive applications. The commercially available CerasetTM VL20, a liquid cyclosilazane pre-ceramic precursor, was mixed with polyacrylonitrile (PAN) in order to make the cyclosilazane electrospinnable. Carbon-rich PDC nanofibers were fabricated by electrospinning various ratios of PAN/cyclosilazane solutions followed by pyrolysis. Surface morphology of the electro spun nanofibers characterized by SEM show PDC nano?bers with diameters ranging from 100-300 nm. Also, thermal stability towards oxidation showed a 10% mass loss at 623°C. 2-D carbon/SiCN films were produced by drop-casting a mixture of PAN/cyclosilazane onto a glass slide followed by pyrolysis of the film. Samples ranging from 10:1 to 1:10 PAN:cyclosilazane were made by dissolving the solutes into DMF to produce solutions ranging from 1% to 12% by weight. Green, heat-stabilized, and pyrolyzed 8% films were examined with FTIR to monitor the change in chemical structure at each step of the ceramization. SEM shows that high PAN samples produced films with ceramic embedded spheroid components in a carbon matrix, while high cyclosilazane samples produced carbon embedded spheroid. Finally, this research focuses on the challenge of making fully dense, 3-D bulk PDCs materials. Here we present a composite of SiCN with reduced graphene oxide (rGO) aerogels as a route for fully dense bulk PDCs. Incorporation of the rGO aerogel matrix into the SiCN has its pros and cons. While it lowers the strength of the composite, it allows for fabrication of large bulk samples and an increase in the electrical conductivity of the PDC. The morphology, mechanical, electrical properties and thermal conductivity of graphene-SiCN composite with varying rGO aerogel loading (0.3-2.4%) is presented. The high temperature stability, high electrical conductivity and low thermal conductivity of these composites make them excellent candidates for thermoelectric applications. Generally, carbon-rich SiCN composites with improved thermal and electrical properties are of great importance to the aerospace and electronics industries due to their expected harsh operating environments.

Polymer-derived ceramics (PDCs) are a new class of high-temperature materials synthesized by thermal decomposition of polymeric precursors. These materials possess many unique features as compared with conventional ceramics synthesized by powder metallurgy based processing. For example, PDCs are neither amorphous nor crystalline. Instead, they possess nano-domain structures. Due to the direct chemical-to-ceramic processing, PDCs can be used for making components and devices with complex shapes. Thus, understanding the properties and structures of these materials are of both fundamental and practical interest. In this work, the structures and electronic behavior of polymer-derived amorphous silicon carbonitrides (SiCNs) were investigated. The materials were synthesized by pyrolysis of a commercially available liquid precursor. Ceramic materials with varied structures/properties were successfully synthesized by modifying the precursor and using different pyrolysis temperatures. The structures of the obtained materials were studied using XRD, solid state NMR, EPR, FTIR and Raman Spectroscope. The electronic behavior of the materials was investigated by measuring I-V curves, Hall effects, temperature dependent conductivity. The experiments were also performed to measure UV-Visible absorption and dielectric properties of the materials. This work leads to the following significant progresses: (i) developed quantitative technique for measuring free carbon concentration; (ii) achieved better understanding of the electronic conduction mechanisms and measured electronic structures of the materials for the first time; and (iii) demonstrated that these materials possess unusual dielectric behavior and provide qualitative explanations.

Ceramic additive manufacturing is gaining popularity with methods like selective laser sintering (SLS), binder jetting, direct ink writing and stereolithography, despite their disadvantages. Laser sintering and binder jetting are too expensive, while direct ink writing lacks resolution and stereolithography lacks scalability. The project aims to combine one of the most versatile, affordable, and readily available 3D printing methods: fused filament fabrication (FFF) with polymer derived ceramics to produce cellular ceramics to overcome the disadvantages posed by the other methods. The process uses a two-step approach. The first step is to 3D print the part using a polymer FFF 3D printer with a thermoplastic polyurethane filament and the second step is to impregnate the part in a polysilazane preceramic polymer and then pyrolyze it in an inert environment up to 1200C. The resulting product is a high-resolution cellular ceramic of the composition SiOC(N). This type of cellular ceramic can find an application in several fields such as scaffolds for bone tissue regeneration, liquid metal filtering, chemical and gas filtering, catalytic converters and electric applications. The process can provide an affordable alternative to the products used in these fields currently.