Molecular-Level Processing of Si-(B)-C Materials with Tailored Nano/Microstructures

Design and Manufacturing of Si-Based Non-Oxide Cellular Ceramic Structures through Indirect 3D Printing

Additive manufacturing of Polymer-Derived Ceramics (PDCs) is regarded as a disruptive fabrication process that includes several technologies such as light curing and ink writing. However, 3D printing based on material extrusion is still not fully explored. Here, an indirect 3D printing approach combining Fused Deposition Modeling (FDM) and replica process is demonstrated as a simple and low-cost approach to deliver complex near-net-shaped cellular Si-based non-oxide ceramic architectures while preserving the structure. 3D-Printed honeycomb polylactic acid (PLA) lattices were dip-coated with two preceramic polymers (polyvinylsilazane and allylhydridopolycarbosilane) and then converted by pyrolysis respectively into SiCN and SiC ceramics. All the steps of the process (printing resolution and surface finishing, cross-linking, dip-coating, drying and pyrolysis) were optimized and controlled. Despite some internal and surface defects observed by topography, 3D-printed materials exhibited a retention of the highly porous honeycomb shape after pyrolysis. Weight loss, volume shrinkage, roughness and microstructural evolution with high annealing temperatures are discussed. Our results show that the sacrificial mold-assisted 3D printing is a suitable rapid approach for producing customizable lightweight highly stable Si-based 3D non-oxide ceramics.

Investigation of polymer-derived Si-(B)-C-N ceramic/reduced graphene oxide composite systems as active catalysts towards the hydrogen evolution reaction

Hydrogen Evolution Reaction (HER) is an attractive technology for chemical conversion of energy. Replacement of platinum with inexpensive and stable electrocatalysts remains a major bottleneck hampering large-scale hydrogen production by using clean and renewable energy sources. Here, we report electrocatalytically active and ultra-stable Polymer-Derived Ceramics towards HER. We successfully prepared ultrathin silicon and carbon (Si–C) based ceramic systems supported on electrically conducting 2D reduced graphene oxide (rGO) nanosheets with promising HER activity by varying the nature and the composition of the ceramic with the inclusion of nitrogen, boron and oxygen. Our results suggest that oxygen-enriched Si-B-C-N/rGO composites (O-SiBCN/rGO) display the strongest catalytic activity leading to an onset potential and a Tafel slope of − 340 mV and ~ 120 mV dec⁻¹ respectively. O-SiBCN/rGO electrodes display stability over 170 h with minimal increase of 14% of the overpotential compared to ~ 1700% for commercial platinum nanoparticles. Our study provides new insights on the performance of ceramics as affordable and robust HER catalysts calling for further exploration of the electrocatalytic activity of such unconventional materials.

In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000–1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiN_xC_y nanocrystals (x + y = 1) are homogeneously formed “in situ” in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si₃N₄, TiN_xC_y (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.

Hydrogen Selective SiCH Inorganic-Organic Hybrid/γ-Al2O3 Composite Membranes

Solar hydrogen production via the photoelectrochemical water-splitting reaction is attractive as one of the environmental-friendly approaches for producing H₂. Since the reaction simultaneously generates H₂ and O₂, this method requires immediate H₂ recovery from the syngas including O₂ under high-humidity conditions around 50 °C. In this study, a supported mesoporous γ-Al₂O₃ membrane was modified with allyl-hydrido-polycarbosilane as a preceramic polymer and subsequently heat-treated in Ar to deliver a ternary SiCH organic–inorganic hybrid/γ-Al₂O₃ composite membrane. Relations between the polymer/hybrid conversion temperature, hydrophobicity, and H₂ affinity of the polymer-derived SiCH hybrids were studied to functionalize the composite membranes as H₂-selective under saturated water vapor partial pressure at 50 °C. As a result, the composite membranes synthesized at temperatures as low as 300–500 °C showed a H₂ permeance of 1.0–4.3 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ with a H₂/N₂ selectivity of 6.0–11.3 under a mixed H₂-N₂ (2:1) feed gas flow. Further modification by the 120 °C-melt impregnation of low molecular weight polycarbosilane successfully improved the H₂-permselectivity of the 500 °C-synthesized composite membrane by maintaining the H₂ permeance combined with improved H₂/N₂ selectivity as 3.5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ with 36. These results revealed a great potential of the polymer-derived SiCH hybrids as novel hydrophobic membranes for purification of solar hydrogen.

Crosslinking chemistry of poly(vinylmethyl-co-methyl)silazanes toward low-temperature formable preceramic polymers as precursors of functional aluminium-modified Si–C–N ceramics

Study of the crosslinking chemistry of liquid polysilazanes with alane hydride-based complex.