Microstructure Development and Phase Evolution of Alumina- mullite Nanocomposite

In this work, alumina-mullite composites (5-15 vol.%) were prepared using sol-gel derived alumina composite nanopowders. The results revealed the formation of intragranular and intergranular mullites inside and between the alumina grains, respectively. Accordingly, the intragranular mullites (average grain size, 0.3 ?m) were smaller than the intergranular mullites (average grain size, 0.5 ?m). Moreover, the alumina grains (average grain size, 1.0 ?m) are larger than the mullites. Meanwhile, the mullites showed positive results in the prevention of the alumina grains growth and the retardation of densification. The relative density of alumina-15 vol.% mullite that was sintered at 1650?C for 2 h, was obtained as 98.7 %. After sintering at 1750?C for 2 h, the mullite was decomposed

Novel polysiloxane and polycarbosilane aerogels via hydrosilylation of preceramic polymers

We report new polysiloxane and polycarbosilane aerogels, which have been obtained by crosslinking Si–H-containing polymers with a CC-containing crosslinker via hydrosilylation reactions. The crosslinking reaction has been carried out in a highly diluted solution using up to 97 vol% of solvent. The obtained aerogels have a colloidal structure with meso- and macropores. Density as low as 0.17 g cm−3 has been reached, which implies a porosity of ca. 84 vol%

Polymer‐Derived Ceramic Aerogels to Immobilize Sulfur for Li‐S Batteries

Lithium–sulfur batteries are among the promising high‐capacity candidates owing to the superior theoretical capacity of sulfur, when compared with conventional cathodes such as LiCoO₂. However, several issues must be addressed before these batteries can be considered fully operational. Major issues regard the insulating nature of sulfur and the so‐called shuttle effect of soluble polysulfides, which dramatically reduces the cathode capacity upon cycling. Herein, three carbon‐containing polymer‐derived ceramic aerogels are characterized belonging to the Si‐C‐O and Si‐C‐N systems, infiltrated with sulfur to work as cathodes for Li‐S batteries. The electrochemical performances are evaluated in relation to the microstructural and chemical features of such materials. In particular, the effect of the pore size of the ceramic matrices on the shuttling behavior of polysulfides is investigated. Despite the high initial specific capacities exceeding hundreds of mAh g⁻¹, all types of cathodes show stable capacities in the 60–120 mAh g⁻¹ range after 100 cycles

Viscosity, Boson Peak and Elastic Moduli in the Na2O-SiO2 System

The temperature and chemical dependence of the melt viscosity are ubiquitous in the model development of the volcanic dynamics, as well as in the glass production and design. We focussed on the yet-explored relationship between the bulk and shear moduli ratio and boson peak with the melt fragility of their parental glasses. Here, we explored the extension of the observed trend by testing the conventional binary system Na2O-SiO2, thus providing new evidence supporting the link between the flow of melts and supercooled liquids and the vibrational dynamics of their parental glasses. This was accomplished by integrating new low-frequency Raman measurements and integrating data from the literature on Brillouin light scattering and viscometry. This approach allows us to feed the MYEGA equation with reliable input parameters to quantitatively predict the viscosity of the Na2O-SiO2 system from the liquid up to the glass transition

Phase Separation in an SiCO Glass Studied by Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy

International audienc

Polymer-derived SiOC replica of material extrusion-based 3-D printed plastics

A promising method for obtaining ceramic components with additive manufacturing (AM) is to use a two-step process of first printing the artifact in polymer and then converting it to ceramic using pyrolysis to form polymer derived ceramics (PDCs). AM of ceramic components using PDCs has been demonstrated with a number of high-cost techniques, but data is lacking for fused filament fabrication (FFF)-based 3-D printing. This study investigates the potential of lower-cost, more widespread and accessible FFF-based 3-D printing of PDCs. Low-cost FFF machines have a resolution limit set by the nozzle width, which is inferior to the resolutions obtained with expensive stereolithography or selective laser sintering AM systems. However, to match the performance a partial PDC conversion is used here, where only the outer surface of the printed polymer frame is converted to ceramic. Here the FFF-based 3-D printed sample is coated with a preceramic polymer and then it is converted into the corresponding PDC sample with a high temperature pyrolysis process. A screening experiment is performed on commercial filaments to obtain ceramic 3-D prints by surface coating both hard thermoplastics: poly lactic acid (PLA), polycarbonate (PC), nylon alloys, polypropylene (PP), polyethylene terephthalate glycol (PETG), polyethylene terephthalate (PET), and co-polyesters; and flexible materials including: flexible PLA, thermoplastic elastomer and thermoplastic polyurethane filaments. Mass and volume changes were quantified for the soaking and pyrolysis steps to form a hollow ceramic skin. All 3-D printing materials extruded at 250 microns successfully produced hollow ceramics skins of less than 100 microns. Details on the advantages and disadvantages of the different 3-D printing polymer precursors are discussed for this processing regime. The novel results developed here can be used to choose FFF-based polymers to use for PDC processing on a wide range of applications such as heat exchangers, heat sinks, scaffoldings for bone tissue growth, chemical/ gas filters and custom scientific hardware

N-doped polymer-derived Si(N)OC: The role of the N-containing precursor

Polymer precursors for Si(N)OC ceramics have been synthesized by hydrosilylation reaction of polyhydridomethylsiloxane (PHMS) with three different nitrogen containing compounds. The results obtained by combining characterization techniques such as FTIR, 13C- and 29Si-NMR confirm the occurrence of the cross-linking reaction between Si–H and vinyl groups. The structural characterization of the corresponding ceramic phase shows that the type of N-containing compounds strongly influences the pyrolytic transformation as well as the crystallization behavior of the final ceramics. Elemental analysis clearly indicates that N is present in the Si(N)OC matrix and the degree of N retention after pyrolysis is related to the type of N-containing starting compound. XPS data show that N–C bonds are present in the Si(N)OC ceramic samples even if only N–Si bonds are present in the starting N-containing precursors. However, if nitrogen atoms form bonds with sp2 carbon atoms in the preceramic polymer then a larger fraction of C–N bonds is retained in the final Si(N)OC ceramic

On the shrinkage during pyrolysis of thin films and bulk components: The case of a hybrid silica gel precursor for SiOC glasses

International audienceThis paper compares the shrinkage during pyrolysis of a gel precursor as thin film and as bulk sample. The hybrid silica gel, precursor for SiOC glasses, contains Si–CH3 and Si–H moieties. The shrinkage of bulk samples has been measured with conventional dilatometry. Shrinkage of thin films has been studied for the first time with in situ dilatometry allowing to measure the thickness and the refractive index during pyrolysis. Thin films shrink more compared to bulk samples and the pyrolytic transformation occurs at lower temperature (100–150 °C) compared to the bulk samples