From polymers to covalent glasses (PolGla)
Abstract
Polymer-derived ceramics (PDCs) are a wide family of advanced ceramics and glasses obtainedthrough the high-temperature pyrolysis of organosilicon polymers in a controlled atmosphere. Differentcompositions (SiC, SiOC, SiN, SiON, SiCN, SiBCN…, usually including also a disordered free C phase)can be obtained by tailoring the polymer chemistry, the pyrolysis atmosphere, and temperature. Inparticular, amorphous PDCs are of a certain interest because they offer a unique combination ofmechanical, thermal, optical, chemical, and electrochemical properties as well as highresistance to devitrification (the highest ever measured to date) and to high-temperature creep.Moreover, PDCs can be shaped by any polymer-forming technology allowing the development of porousstructures and fibers. All these features make PDCs attractive for high-temperature structuralapplications, as well as high-temperature insulators, thermal barriers, and anti-corrosion-antiwearcoatings.Despite their technical and scientific relevance, still, little is known about (i) how their structureevolves during the ceramization process (when the polymer is converted into an amorphous ceramicnetwork) and (ii) about the medium-range order of the amorphous PDC structure. Indeed, the localorganization of the glass structure is, however, of pivotal relevance for understanding their unique setof properties and tailoring new amorphous structures.PolGla aims at unveiling the medium-range order and structural evolution of amorphous PDCsduring ceramization, with a specific focus on the local arrangement of the glass-forming units, thefree volume, and correlation length (the distance over which short-range order is maintained). Thestudy will be conducted on a model system constituted by microspheres of methyl-silsesquioxane whichwill be pyrolyzed in different atmospheres Ar, CO2, Ar-H2, and air, to obtain different glass compositions(SiOC+Cfree, SiOC, SiO2). The structural evolution study during pyrolysis will be achieved byapplying positron annihilation spectroscopy (PAS) to PDCs pyrolyzed at different temperatures(i.e., with different levels of ceramization) and in different atmospheres. PAS will be carried out boththrough positron lifetime (very sensitive to the free volume size) and Doppler broadening studies (i.e.,the broadening of the gamma-rays emission due to the annihilation between e+ and e-, sensitive to the amountof free volume). In parallel, the materials will be studied by a set of conventional techniques such asXRD, N2 adsorption-desorption, nuclear magnetic resonance (MAS-NMR), Raman, and Fourier-transformedinfrared spectroscopy (FT-IR) and chemical analysis (Si, C, O, H) to get a complete pictureof their composition, structure, microstructure, and organization of the SiX4 units. Furthermore, thecorrelation length will be studied by vibrational spectroscopy, namely low-frequency Ramanspectroscopy (Boson peak position) and Brillouin scattering.The structural features of amorphous PDCs will be correlated to their properties. In particular, weaim at understanding the correlations between the glass structure (type, arrangement, and free volumeof the glass-forming units and microporosity of the glass structure) with mechanical and thermalproperties. The mechanical characterization will mainly involve measurements of the elastic modulus(nano-hardness tests), hardness and fracture toughness. Finally, the thermal conductivity of amorphousPDCs will be investigated by laser flash analysis (LFA) and correlated with their free volume.In summary, PolGla will improve our fundamental understanding of the organic-to-inorganictransformation, will expand our knowledge of covalent glass structures obtained by PDC route,and will improve our knowledge of the correlations between PDCs structure and their thermaland mechanical properties.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 22, 2024
- Source ID
- FA86552317241
Entities
People
- Gian Soraru
Organizations
- Air Force Office of Scientific Research
- United States Air Force
- University of Trento