Advanced covalent ceramics from organosilicon polymers for sustainable energy and environment

Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringGurpreet SinghControlled thermal degradation of the liquid-phase polymers of molecular precursor-derived ceramics offer them excellent engineering properties. Amorphous ceramic phase and resistance to crystallization up to 1400 °C, high-temperature stability, and intense photoluminescence are some of the remarkable functional properties of these polymer-derived ceramics (PDCs). The ever-increasing demand of high-temperature components to increase the performance efficiency in aerospace applications pushes the industry to look for a new class of materials. Simultaneously, investigations of renewable energy sources lead to the development of efficient energy storage materials. PDC route offers a promising solution to both of these applications owing to PDC’s distinct production route and functional properties. This dissertation focuses on two aspects. Firstly, the use of silicon-based PDCs to fabricate lightweight and strong ceramic matrix composites for high-temperature applications. The efficient infiltration of carbon fibers cloths (disks) and mini-bundles with boron-modified polysilazane and hafnium-modified polysilazane preceramic polymer solutions were investigated using a lab-scale, cost-effective drop coating technique. After the successful infiltration of the fibers was confirmed, the infiltrated fibers were heat-treated at different temperatures to complete the polymer-to-ceramic conversion of the preceramic polymer matrix. The boron-modified polysilazane and hafnium-modified polysilazane coated carbon fibers were crosslinked at 180 °C, followed by pyrolysis at 800 °C in inert environments to achieve Si(B)CN/CF and Si(Hf)CN/CF CMC mini-composites, respectively. Crack and defect free ceramic matric composites were achieved. The as-fabricated mini-composites were then investigated by several techniques to determine the composites’ micro-structures and properties. The effect of the boron and hafnium in the polymer-derived ceramic matrices and micro-structural development of the final ceramic composites were characterized using scanning electron microscopy (SEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The Raman and FTIR spectroscopies showed the complete conversion of the polymer to ceramic phase. The elemental composition and distribution of chemical bonds in the final mini-composites were determined by XPS. The mechanical properties of the mini-composites were investigated by tensile tests. Room-temperature tensile tests showed that the Si(Hf)CN/CF sample could reach a tensile strength of 790 MPa and elastic modulus of 66.88 GPa among the composites. To determine the high-temperature stability the oxidation behavior at various temperatures were studied. The oxidation study of the mini-composites showed stability of the samples up to 1500 °C. Structural and compositional changes of the oxidized samples were also elaborately investigated by XPS and SEM analyses to understand the phase change after oxidation. Secondly, the application of PDCs as free-standing, high capacity electrode materials for energy storage systems. Various preceramic polymer solutions were investigated to fabricate anode materials for lithium-ion batteries. Electrospinning technique was utilized to fabricate free-standing fiber mats from three different siloxanes oligomers. To achieve electrospun fiber mats, the short-chain siloxane oligomers were needed to be mixed with and additional spinning agent such as polyvinylpyrrolidone (PVP). The electrospun fiber mats were then crosslinked at 180 °C and pyrolyzed at 800 °C in Ar environment to obtain three types of SiOC ceramic fiber mats. The electron microscopy of the PDC fiber samples showed rigid surface structures with small diameters in the range of 0.2-3 µm. Raman, FTIR, XPS, and NMR spectroscopies were utilized to outline the ceramization stages of the SiOC fibers. 29Si MAS NMR spectra of the SiOC fibers revealed that mostly SiO4 bonds were formed in the amorphous ceramic phase, which indicated the formation on free carbon phase with limited amount of Si-C bonds after pyrolysis. The higher amount of free carbon along with the SiO-C mixed bonds in the amorphous SiOC samples enabled high lithium reversibility. As a result, when utilized as LIB electrodes, the self-supporting SiOC fiber mats showed excellent specific capacity of 866 mAh gelectrode -1 with a high coulombic efficiency of 72%. Even as supercapacitor electrode, the SiOC fibers maintained 100% capacitance retention over 5000 cycles at a high current density of 3 A g-1. These two approaches for the synthesis of CMC mini-composites and electrode components using PDC materials offer promising potential for the various PDC chemistries to be utilized for both high-temperature and energy storage applications

Enhanced Li-Ion Rate Capability and Stable Efficiency Enabled by MoSe2 Nanosheets in Polymer-Derived Silicon Oxycarbide Fiber Electrodes

Transition metal dichalcogenides (TMDs) such as MoSe2 have continued to generate interest in the engineering community because of their unique layered morphology—the strong in-plane chemical bonding between transition metal atoms sandwiched between two chalcogen atoms and the weak physical attraction between adjacent TMD layers provides them with not only chemical versatility but also a range of electronic, optical, and chemical properties that can be unlocked upon exfoliation into individual TMD layers. Such a layered morphology is particularly suitable for ion intercalation as well as for conversion chemistry with alkali metal ions for electrochemical energy storage applications. Nonetheless, host of issues including fast capacity decay arising due to volume changes and from TMD’s degradation reaction with electrolyte at low discharge potentials have restricted use in commercial batteries. One approach to overcome barriers associated with TMDs’ chemical stability functionalization of TMD surfaces by chemically robust precursor-derived ceramics or PDC materials, such as silicon oxycarbide (SiOC). SiOC-functionalized TMDs have shown to curb capacity degradation in TMD and improve long term cycling as Li-ion battery (LIBs) electrodes. Herein, we report synthesis of such a composite in which MoSe2 nanosheets are in SiOC matrix in a self-standing fiber mat configuration. This was achieved via electrospinning of TMD nanosheets suspended in pre-ceramic polymer followed by high temperature pyrolysis. Morphology and chemical composition of synthesized material was established by use of electron microscopy and spectroscopic technique. When tested as LIB electrode, the SiOC/MoSe2 fiber mats showed improved cycling stability over neat MoSe2 and neat SiOC electrodes. The freestanding composite electrode delivered a high charge capacity of 586 mAh g−1electrode with an initial coulombic efficiency of 58%. The composite electrode also showed good cycling stability over SiOC fiber mat electrode for over 100 cycles

Evaluating Use of Boron- and Hafnium-Modified Polysilazanes for Ceramic Matrix Minicomposites

In this study, the potential of polymer-derived ceramic matrix composites (CMCs) is demonstrated by the addition of thin ceramic coatings on carbon fiber (CF) bundles. Boron- and hafnium-modified polysilazane liquid precursors were synthesized and used to infiltrate the fiber bundles of CF to fabricate lab-scale Si(B)CN/CF and Si(Hf)CN/CF CMC minicomposites, respectively by crosslinking and then pyrolysis at 800 °C. The crosslinked precursor to ceramic yield was observed to be as high as 90% when the procedure was carried out in inert environment. The Si(B)CN/CF contained Si–N and B–N bonds, while Si–N and Hf–O–Si bonds were observed for the Si(Hf)CN/CF sample with uniform and dense surfaces. Room-temperature tensile tests showed that the Si(Hf)CN/CF sample could reach a tensile strength of ∼790 MPa and an elastic modulus of 66.88 GPa among the composites. An oxidation study of the Si(Hf)CN/CF minicomposites showed higher stability compared to SiCN/CF and Si(B)CN/CF minicomposites up to 1500 °C

Hybrid HfC‐SiCN matrix for improved oxidation resistance of carbon fiber–reinforced mini‐composites

Abstract Hafnium carbide (HfC) is an ultrahigh‐temperature ceramic with high melting point, chemical stability, hardness, and wear resistance. However, its low fracture toughness and poor thermal shock resistance limit its structural applications in extreme environments. In this study, co‐curing of liquid precursors was carried out prior to complete pyrolysis of individual polymeric precursors. First, HfC preceramic polymer precursor was cured, followed by silicon carbonitride (SiCN) precursor curing on a 2D carbon fiber (CF) cloth using the drop‐coating process. The infiltrated CFs were pyrolyzed at 800°C to achieve CF/HfC‐SiCN ceramic mini‐composites. The cross‐linked precursor‐to‐ceramic yield was observed to be as high as 65% when the procedure was carried out in an inert environment. Although stable up to 1200°C, CF/HfC‐SiCN samples demonstrated susceptibility to oxidation at 1500°C in ambient air. The oxidation of HfC in the presence of SiC leads to the formation of a hafnium‐containing silicate (HfxSiyOz) along with hafnia (HfO2). This compound of silicate and hafnia limits oxygen diffusion better than SiO2 and HfO2 individually. The incorporation of SiCN in HfC ceramic led to improved phase stability compared to a neat HfC system. The results of this study also show that the use of liquid‐phase precursors for HfC and SiCN in the polymer‐infiltrated pyrolysis method is a promising approach to fabricating high‐temperature structural ceramic matrix composites with good oxidation resistance

A perspective on silicon-based polymer-derived ceramics materials for beyond lithium-ion batteries

Energy storage devices beyond lithium-ion batteries (LIBs), such as sodium-ion, potassium-ion, lithium-sulfur batteries, and supercapacitors are being considered as alternative systems to meet the fast-growing demand for grid-scale storage and large electric vehicles. This perspective highlights the opportunities that Si-based polymer-derived ceramics (PDCs) present for energy storage devices beyond LIBs, the complexities that exist in determining the structure-performance relationships, and the need for in situ and operando characterizations, which can be employed to overcome the complexities, allowing successful integration of PDC-based electrodes in systems beyond LIBs