title: The Microstructure and Phase Evolution of Pseudomorphic Polymer-derived Ceramic Papers
Creators: name: Peter, Johannes Mauricio
Publisher: name: UNSPECIFIED
Year: 2024

description: Polymer-derived ceramics (PDCs) and their nanocomposites (NCs) are a class of materials accessed through the pyrolytic decomposition of suitable molecular precursors. In contrast to traditional powder sintering techniques, the composition, microstructural characteristics and, consecutively, the performance of PDCs can be conveniently adjusted through the design of the preceramic polymer, which has attracted sustained interest of researchers in the last decades. Most molecular precursors possess exceptional processability in their polymeric state, allowing the employment of advanced fabrication techniques, such as extrusion, coating, infiltration, and even additive manufacturing. Quite recently, heat-treating regular cellulose-based paper templates infiltrated with preceramic polymers have been successfully deployed to conveniently generate polymer-derived ceramic papers (PDCPs) exhibiting structural characteristics closely resembling that of the template. Ceramic papers combine the unique structure of organic papers with the composition and properties of various ceramic materials making them predestined for a wide range of applications, such as filtration, refractories, or as catalyst support. A key concept of the PDCP approach is to unite the processability of cellulose-based papers with the exceptional property design of PDC materials, paving the way for component preform of tailorable ceramics with paper-like structures and complex morphologies. The present dissertation deals with the microstructure and phase characterization of various PDCPs obtained from cellulose paper templates infiltrated with transition metal-modified polysilazane precursors upon heat treatment. The focus lies on assessing the impact of individual synthesis parameters on the structure and phase evolution of the ceramic papers to attain a better understanding of the means applicable to tailor their properties and ultimately, enable future application of this exciting novel material class.
For the present study, various single-source precursors (SSPs) were prepared from modification of either perhydropolysilazane (Durazane 2250) or poly(vinyl)silazane (Durazane 1800) with one of three transition metals. Several different cellulose-based paper templates were dip coated with the SiMN(O,C)-based SSPs (M = Fe, Ni, Pd) prepared from reacting polysilazanes with metalorganic acetylacetonate compounds (Fe(acac)₃, Ni(acac)₂, Pd(acac)₂) and subjected to pyrolysis at up to 1000 °C in Ar or reactive NH₃ atmosphere. In a second step, the as-pyrolyzed ceramic papers were annealed at 1300 °C in flowing Ar or N₂ to investigate their high-temperature evolution and the impact of the atmosphere employed on the phase assemblage generated. Detailed investigation of the interactions between template and precursor, phase evolution, and microstructural characteristics upon heat-treatment of the preceramic composites was conducted employing scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) analyses in combination with X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) spectroscopy. In addition, ultra-thin fiber cross-sections were prepared from each sample and investigated via transmission electron microscopy (TEM) imaging and electron diffraction, allowing the in-situ generated micro- and nanostructure to be elucidated in great detail. 
In all cases, upon pyrolysis, ceramic composites with a paper-like structure were obtained, consisting of cellulose-derived carbon fibers coated with a SiMOC(N)-based PDC layer. The transition from a SiN(O,C)- towards a SiOC(N)-based ceramic is attributed to the interaction between the pyrolysis products of the H₂O-rich cellulose with the polymer during heat-treatment. As revealed by electron microscopic imaging, the coating generated exhibits excellent bonding to the carbon fibers, which was further investigated via FT-IR analyses, indicating that this likely stems from chemical reactions of the polysilazanes with the hydroxyl groups of cellulose during surface modification of the templates. The template-assisted synthesis leads to pseudomorphic PDCPs faithfully reproducing the morphological characteristics of the respective paper template used, whereas other synthesis parameters were found to have a negligible effect. 
Upon pyrolysis at 1000 °C, the transition metal-modified precursor has separated into X/SiOC(N) ceramic nanocomposites (X = metal, metal carbide, metal silicide) consisting of nanosized metal-based precipitates dispersed within an amorphous SiOC(N) matrix. The phase assemblages generated, microstructural characteristics, and even macroscopic properties of the PDCPs were found to vary notably with the transition metal introduced. For instance, while both Ni- and Pd-modified papers encompass metallic and metal silicide phases, they exhibit a remarkably distinct micro- and nanostructure. The former features highly graphitized carbon fibers containing most of the metal-based phases, whereas the latter consists of entirely amorphous pyrolytic carbon encased by a PdₓSi/SiOC-based NC layer. By using Fe for precursor modification, metallic Fe and carbides are generated and the microstructure is somewhat intermediate between the other two systems, with graphitic carbon fibers enveloped by a Y/SiOC layer (Y = α-Fe, Fe₃C) coating and some oxide phases present and the fiber-coating interface. Also, both the Fe- and Ni-modified papers exhibited noteworthy ferromagnetic behavior due to the abundance of ferromagnetic precipitates. To investigate the low-temperature evolution of PDCPs and the effect of a reactive atmosphere on the phase assemblage generated, Fe-modified papers were subjected to pyrolysis in NH₃ at 500-1000 °C. While the ceramic papers are devoid of any crystalline phases upon 500 °C treatment, FeₓO nanocrystals have precipitated within the coating upon ammonolysis at 700 °C. Increasing the temperature further led to a reduction of the oxides towards α-Fe throughout the sample without any Fe₃C present upon pyrolysis at 1000 °C. Moreover, FeXN phases are generated exclusively along the fiber-coating interface at 700 °C, which withstand reduction and grow considerably towards higher temperatures. Both the suppression of carbide formation and generation of metal nitrides is traced back to reactions between the ceramic papers and NH₃ atmosphere and shows that the phase evolution of PDCPs can be effectively controlled via the pyrolysis atmosphere. Conversely, neither the type of polysilazane used nor the kind of paper template had a notable effect on the microstructure and phase assemblage observed in any of the ceramic papers. 
Annealing of the as-pyrolyzed samples at 1300 °C led to profound changes across all systems investigated; most notably, the surface and macropores of the papers were found covered with in situ generated nanowires and whisker structures. Tempering in Ar induced crystallization of β-SiC from the amorphous SiOC matrix and growth of α-SiC whisker structures with aspect ratios of up to 50:1. In N₂, ultra-long α-Si₃N₄ nanowires with aspect ratios of around 1000:1 are produced. Quite often, these nanostructures are decorated with metal silicide tips, suggesting that their growth is facilitated via a transition metal-catalyzed vapor-liquid-solid (VLS) mechanism. The number, morphology, and aspect ratios of the wires were found to vary depending on the transition metal introduced, indicating that precursor modification enables control over these aspects. Apart from generating Si-based nanostructures, tempering led to the conversion of any metal-based phase towards the thermodynamically most stable silicide in the respective system, i.e. Fe₃Si, Ni₂Si, and Pd₂Si, and the ferromagnetism observed in the Fe- and Ni-based papers upon pyrolysis, vanishes in the tempered samples. Also, the graphitization of the cellulose-derived carbon fibers increases significantly with even the formerly amorphous fibers in the Pd-modified papers found to be composed of graphitic nanostructures to a large extend, with Pd₂Si now predominately occurring dispersed throughout the fibers. This indicates that the graphitization of the fibers also is catalyzed by the transition metals, presumably via a solution-precipitation mechanism and within a different temperature range for the three metals introduced, opening additional pathways to tailor micro- and nanostructural attributes of PDCPs.
The results show that the PDC route is a feasible approach to accessing ceramic paper composites exhibiting a variety of microstructural characteristics and interesting properties adjustable to some extend through controlling individual synthesis parameters. While using cellulose-based materials as templates potentially restricts the generation of non-oxide ceramics, their morphological characteristics are precisely transferred onto the ceramic composite, facilitating convenient access to ceramic materials with complex morphologies. Moreover, the data show that besides temperature and atmosphere, precursor modification with transition metals is especially promising to tailor certain features and properties of PDCPs to the needs of a variety of potential applications.