Ceramic aerogels of the Si-C-N-O system from pre-ceramic polymer

Research in materials science field is often driven by the necessity to overcome problems in fast, reliable and possibly cost effective ways. Many times the starting point to find a solution is a literature survey, to understand if someone already encountered a similar problem and proposed an answer, or if some authors developed a material that can be used to solve the issue. The work performed within this thesis however fits in another type of approach, i.e. research for the research's sake. In this kind of approach the efforts are not dedicated directly to solve a given, precise and detailed problem, but to invent and develop new types of materials, characterizing them so that other researchers can take benefits from both the synthetic way and the measured properties of the new material produced. In particular, this PhD thesis deals with the combination of two "exotic" class of materials, which are aerogels and polymer derived ceramics. Aerogel is actually a shape, more than a material, from the proper chemical point of view. This kind of shape, anyway, is so peculiar that many of the properties are common to all the aerogels' products, similarly to what happen for other class of materials like conductivity for metals, hardness for ceramics and high specific strength for polymers. These common properties are: low density, high specific surface and predominantly mesoporous microstructure. Polymer derived ceramic (PDC) denotes a family of ceramic materials that can be obtained by a controlled thermolysis of a polymeric precursor. These polymers are usually Si based and contain functional groups that allow to control the final chemistry of the ceramic produced, along with the great advantage that the shape can be set already in the polymeric state. Successfully combining the two techniques, i.e. to produce polymer derived ceramic aerogel, is the core of this thesis. Preference was given to the use of commercially available pre-ceramic polymers so ceramic aerogels belonging to the SiOCN system were produced, starting from polycarbosilane (SMP-10), polysilazane (PSZ-20) and polysiloxane (PMHS). A reliable procedure was set up to produce aerogels with different composition and microstructure, leading to a wide range of properties in terms of density, specific surface, high temperature stability, electrochemical functionality etc., as will be better depicted through the thesis. Additionally, some application of the materials produced were tested, in which the aerogel shape, combined with the proper chemistry, was expected to give interesting results

Novel SiC/C Aerogels Through Pyrolysis of Polycarbosilane Precursors

A new approach for forming aerogels with various silicon-based compositions and hybrids between ceramics and carbon has been developed by combining efficient hydrosilylation as the hybridization-crosslinking approach associated with gelation in the presence of solvent and followed by supercritical drying techniques. Highly porous carbon-enriched SiC/C aerogels with adequate mechanical durability have been synthesized, pyrolyzed, and characterized. The “wet” gels were obtained by crosslinking a commercial polycarbosilane with divinylbenzene via Pt-catalyzed hydrosilylation reaction in highly diluted condition (90 vol% of solvent). A supercritical drying was performed after exchanging the solvent (cyclohexane) with liquid CO2 forming undamaged aerogels. A subsequent pyrolysis and heat treatment (up to 1500 °C) in argon flow converted the polymeric aerogel into a SiC/C-based material with bulk density of 166 kg m−3, SSA of 444 m2 g−1, a micro-meso pore volume of 0.79 cm3 g−1, total porosity above 90 vol% and ultimate compressive strength of 1.6 MPa. The final product was compared to its cured gel and intermediates obtained during the pyrolysis process

Isoconversional kinetics of thermal oxidation of mesoporous silicon

Mesoporous silicon (pSi) has several interesting features that makes it suitable for various biomedical applications. In particular, the large surface area make it very sensitive to environment changes. Among other approaches, thermal oxidation is an effective way to passivate its surface. Herein, we present experimental and analytical results concerning kinetics of thermal oxidation reaction of pSi. The experiments were conducted on pSi powders produced from silicon wafer by anodization and converted to particles by sonication. Oxidation experiments were carried out at different heating rates. Structure and morphology of the samples have been investigated by XRD and SEM before and after thermal oxidation. The model-free kinetics proposed by Ozawa–Flynn–Wall (OFW) was used to determine the Arrhenius relationship for the pSi thermal oxidation. The obtained apparent activation energy by OFW was confirmed by Starink method. At low temperature, the oxidation of surface dangling bonds obeys the Avrami–Erofeev mechanism. At high temperature, oxidation is followed by classical bulk oxidation according to diffusion mechanism controlled by the diffusion of oxygen through the silicon dioxide layer on the surface of the pSi. The reaction mechanism was checked by the model fitting kinetics, which confirmed the reaction is a kind of sequential two-stage process, Avrami–Erofeev and 3D diffusion. Finally, differential thermal analysis suggests that the second oxidation step is also possibly affected by phase transformation of the silicon dioxide

Structural Design of Polymer-Derived SiOC Ceramic Aerogels for High-Rate Li Ion Storage Applications

SiOC ceramic aerogels with different porosity, pore size, and specific surface area have been synthesized through the polymer-derived ceramic route by modifying the synthesis parameters and the pyrolysis steps. Preceramic aerogels are prepared by cross-linking a linear polysiloxane with divinylbenzene (DVB) via hydrosilylation reaction in the presence of a Pt catalyst under highly diluted conditions. Acetone and cyclohexane are used as solvent in our study. Wet gels are subsequently supercritically dried with CO2 to get the final preceramic aerogels. The SiOC ceramic aerogels are obtained after a pyrolysis treatment at 900°C in two different atmospheres: pure Ar and H2 (3%)/Ar mixtures. The nature of the solvent has a profound influence of the aerogel microstructure in terms of porosity, pore size, and specific surface area. Synthesized SiOC ceramic aerogels have similar chemical compositions irrespective of processing conditions with ~40 wt% of free carbon distributed within remaining mixed SiOC matrix. The BET surface areas range from 215 m2/g for acetone samples to 80 m2/g for samples derived from cyclohexane solvent. The electrochemical characterization reveals a high specific reversible capacity of more than 900 mAh/g at a charging rate of C (360 mA/g) along with a good cycling stability. Samples pyrolyzed in H2/Ar atmosphere show a high reversible capacity of 200 mAh/g even at a high charging/discharging rate of 20 C. Initial capacities were recovered after whole cycling procedure indicating their structural stabilities resisting any kind of exfoliations