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Pseudomorphe Umwandlung von Cellulose-basierten Papieren zu funktionalen keramischen Papieren

Fachliche Zuordnung Herstellung und Eigenschaften von Funktionsmaterialien
Förderung Förderung von 2018 bis 2023
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 411442613
 
Erstellungsjahr 2024

Zusammenfassung der Projektergebnisse

For this study, regular cellulose-based papers were infiltrated with transition metal-modified polysilazane polymers and heat-treated in different atmospheres to produce pseudomorphic polymer-derived ceramic papers with a paper-like structure. A variety of synthesis parameters, such as the type of the paper template employed, the carbon content of the precursor, the modification of the precursors with different transition metals, and the influence of consecutive high-temperature treatment in either nitrogen or argon atmosphere, were tested for their impact on the micro-/nanostructure and phase assemblage generated within the ceramic composites. The morphology of the ceramic composites was shown to be directly dependent on the paper template employed, with even fine structural details inherited upon heat treatment. Other parameters, such as the precursor used, the processing temperature and atmosphere, as well as the modification with transition metals had no notable impact on the morphological feature transfer. Meanwhile, upon pyrolysis at 1000 °C, the majority of nitrogen is removed from the system and C/SiMOC-based (M = Fe, Ni, Pd) ceramics are generated, which is attributed to reactions between the pyrolysis products of the H2O-rich cellulose and the oxidation sensitive precursor. Hence, employing cellulose as template material is a useful approach to conveniently generating pseudomorphic ceramics with complex structures, but the effects of the organic papers on the composition of the resulting material have to be considered. TEM analyses of cross-sectional samples revealed that the papers upon pyrolysis have a complex composite structure with cellulose-derived porous carbon fibers encased by a ceramic nanocomposite layer. The PDC coating exhibits excellent adhesion to the fibers, which is attributed to chemical bonding upon reaction of the polysilazanes with the hydroxy surface groups of the cellulose substrate, as indicated by FT-IR measurements. Dispersed within the SiOC-based ceramic layer, depending on the metal modification, a distinct phase assemblage of nanosized metal-based precipitates is generated, ranging from a variety of silicides in the Ni- and Pd-based samples to metals, carbides, and oxides upon Fe-modification. Reactive ammonolysis at 500-1000 °C of the latter revealed the low-temperature phase evolution, beginning with the precipitation of Fe(1-X)O at 700 °C and simultaneous generation of FeXN at the fiber coating interface. Upon 1000 °C treatment, the oxides get reduced resulting in metallic α-Fe, whereas the nitrides withstand reduction and have grown, revealing a distinct nitriding potential within the fibers. This is attributed to the retention of N2 produced from the thermal decomposition of NH3 diffusing through the porous carbon, demonstrating the effect of the pyrolysis atmosphere on the phase assemblage generated. Moreover, the respective transition metal affects the micro- and nanostructural characteristics of the ceramic papers, with varying degrees of fiber graphitization and distinct distribution of the metal-based precipitates within the composites observable. The different papers and polysilazane base-precursors on the other hand, had no noteworthy influence on the microstructural characteristics of the PDCPs. Upon tempering at 1300 °C in either Ar or N2 atmosphere, most of the oxygen present is removed from the papers via carbothermal reduction and associated COX formation. The PDC coatings crystallize forming either SiC or Si3N4, depending on the atmosphere employed, and the phase equilibrium is shifted towards the thermodynamically favorable silicides, regardless of the metal modification. In addition, the cellulose-derived carbon fibers have now undergone extensive graphitization in all papers, revealing different temperature ranges for catalytic carbon graphitization for the different metals and resulting in the generation of C/M/SiC (Ar) and C/M/Si3N4 (N2) microcomposites (M = Fe3Si, Ni2Si, Pd2Si) with overall very similar microstructures. On the surface of the ceramic papers, a multitude of crystalline whisker- and wire-like Si-based nanostructures with high-aspect ratios is generated, which is attributed to a VLS process reacting gaseous SiO and CO or N2 at catalytic transition metal sites forming the one-dimensional nanostructures. Hence, upon high-temperature annealing, some macroscopic characteristics of the papers, such as their color, mechanical properties, and ferromagnetism, change notably. In summary, within the pseudomorphic ceramic papers inheriting the original, intrinsic morphology of the paper template used, a variety of phase assemblages, microstructural characteristics, and functional properties can be generated by controlling certain synthesis parameters. In addition, the concept could enable the convenient production of ceramics with well-defined and specialized morphologies through preforming of paper templates and subsequent conversion into ceramic materials. The results highlight that, besides using different transition metals for precursor modification, the attributes of PDCPs can be conveniently adjusted by controlling temperature and atmosphere during processing.

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