Search Results (9)

Precursor-derived silicon carbide (SiC) ceramics have been widely used as absorbing materials, but the residual carbon sink produced by ceramicization limits their application under high-temperature and oxygen-containing conditions, such as the nozzle or jet vane of high-speed aircraft. In this paper, a novel molybdenum carbide/silicon carbide (Mo₂C/SiC) microwave-absorbing ceramic with a two-dimensional sheet structure was obtained through the pyrolysis of polycarbosilane-coated molybdenum sulfide (PCS@MoS₂). The results indicate that addition of an appropriate amount of MoS₂ can react with the free carbon generated during the pyrolysis of PCS, thereby reducing the material’s carbon content and forming Mo₂C. Concurrently, the layered structural characteristics of MoS₂ are utilized to create a two-dimensional composite structure within the material, which enhances the material’s absorption vastly. The as-prepared Mo₂C/SiC ceramics sintered at 1300 °C exhibit a minimum reflection loss (RL_min) of −46.49 dB at 8.96 GHz with a thickness of 2.6 mm. Additionally, the effective absorption bandwidth (EAB) of Mo₂C/SiC spans the entire X-band (8–12 GHz) due to the combined effect of multiple loss mechanisms. Full article

The scale of research for developing and applying silicon carbide (SiC) membranes for gas separation has rapidly expanded over the last few decades. Given its importance, this review summarizes the progress on SiC membranes for gas separation by focusing on SiC membrane preparation approaches and their application. The precursor-derived ceramic approaches for preparing SiC membranes include chemical vapor deposition (CVD)/chemical vapor infiltration (CVI) deposition and pyrolysis of polymeric precursor. Generally, SiC membranes formed using the CVD/CVI deposition route have dense structures, making such membranes suitable for small-molecule gas separation. On the contrary, pyrolysis of a polymeric precursor is the most common and promising route for preparing SiC membranes, which includes the steps of precursor selection, coating/shaping, curing for cross-linking, and pyrolysis. Among these steps, the precursor, curing method, and pyrolysis temperature significantly impact the final microstructures and separation performance of membranes. Based on our discussion of these influencing factors, there is now a good understanding of the evolution of membrane microstructures and how to control membrane microstructures according to the application purpose. In addition, the thermal stability, oxidation resistance, hydrothermal stability, and chemical resistance of the SiC membranes are described. Due to their robust advantages and high separation performance, SiC membranes are the most promising candidates for high-temperature gas separation. Overall, this review will provide meaningful insight and guidance for developing SiC membranes and achieving excellent gas separation performance. Full article

Polymer-derived ceramics (PDCs) based on silicon precursor represent an outstanding material for ceramic coatings thanks to their extraordinary versatile processibility. A promising example of a silicone precursor, polyorganosilazane (Durazane 1800), was studied concerning its crosslinking behavior by mixing it with three different photoinitiators, and curing it by two different UV-LED sources under both nitrogen and ambient atmosphere. The chemical conversion during polymerization and pyrolysis was monitored by FTIR spectroscopy. Pyrolysis was performed in a nitrogen atmosphere at 950 °C. The results demonstrate that polyorganosilazane can be cured by the energy-efficient UV-LED source at room temperature in nitrogen and ambient atmosphere. In nitrogen atmosphere, already common reactions for polysilazanes, including polyaddition of the vinyl group, dehydrogenation reactions, hydrosilylation, and transamination reaction, are responsible for crosslinking. Meanwhile, in ambient atmosphere, hydrolysis and polycondensation reactions occur next to the aforementioned reactions. In addition, the type of photoinitiator has an influence on the conversion of the reactive bonds and the chemical composition of the resulting ceramic. Furthermore, thermogravimetric analysis (TGA) was conducted in order to measure the ceramic yield of the cured samples as well as to study their decomposition. The ceramic yield was observed in the range of 72 to 78% depending on the composition and the curing atmosphere. The curing atmosphere significantly impacts the chemical composition of the resulting ceramics. Depending on the chosen atmosphere, either silicon carbonitride (SiCN) or a partially oxidized SiCN(O) can be produced. Full article

Oxidation barrier coatings on the base of a perhydropolysilazane precursor with different amounts of Si, B, SiB₆ and Mo₅SiB₂ (T2) fillers for the oxidation protection of Mo-Si-B alloys were developed. The influence of different filler compositions as well as the influence of pyrolysis conditions (temperature and atmosphere) was studied. The coating slurries were examined with respect to their rheological behavior, which allows optimization of the coating slurry. Dilatometry studies show that the coefficient of thermal expansion of the composites can be matched to refractory, especially molybdenum alloy based, substrates by varying the content and the composition of the filler. The pyrolyzed coating systems offer a low porosity, which is one of the key parameters to a high oxidation protection capability. Full article

Adding fillers to polysilazane (PSZ)-derived ceramic coating is one of the main methods used to reduce PSZ porosity. In this study, we designed a sinter visualization device for understanding the effect of fillers on coating porosity and observed pore evolution within the coating sintering process using different filler ratios. When there was no filler in the coating, gas evolution occurred at the initial sintering stage due to a PSZ pyrolysis reaction. In the final stage, numerous cracks appeared because of volume shrinkage. It was determined that such coatings cannot provide good protection. Although the cracks disappeared after adding glass powder, many bubbles appeared. After adding ZrO₂, the bubbles in the coating significantly reduced. When the volume ratio of PSZ/glass powder/ZrO₂ was 1:2:1, the coating porosity after sintering was the lowest. Based on our visualization experimental results, we concluded that the glass powder’s healing effect and the ZrO₂ skeleton effect were the main reasons for the reduced coating porosity. In addition, the sinter visualization device can be used to observe the surface morphology of other similar coatings during the sintering processes. Full article

Polysilazane (PSZ) have been used for many years as precursors for the development of ceramic materials. Recently, hydrocarbon-substituted polysilazane, which is called organopolysilazane (OPSZ), has been proposed as possible alternative to silanes for the corrosion protection of metals by the sol gel route. In this work, polymethyl(hydro)/polydimethylsilazane-derived coatings were deposited on low-carbon steel for corrosion protection purposes. The effect of the OPSZ precursor concentration (10–40 v/v %) in butyl-acetate on the final properties of the coatings was investigated. Coatings in the thickness range of 1 to 3.5 µm were obtained. The experimental results showed that the concentration of OPSZ in the solvent affects the structural properties as well as the dry film thickness of the hybrid layer. In particular, the network arrangement seems to be influenced by the dilution of the OPSZ precursors solution. The electrochemical characterization revealed that a minimum thickness of about 2 to 3 µm is needed to provide the mild steel substrate with enhanced corrosion protection properties compared to the bare substrate. Comparing the obtained results with literature data, it seems that OPSZs are a potential alternative to coatings derived from organisilicon precursors. Full article

Hexagonal boron nitride (h-BN) is a well-known material whose use is almost restricted to lubricating applications in domains ranging from metallurgy to cosmetics. Howover, h-BN displays many other interesting properties, opening new perspectives for other engineering applications, such as as a solid lubricant in aeronautics, as the perfect substrate to graphene for electronic devices, etc. However, all these promising developments require tailored h-BN shapes displaying a high level of crystallization, ensuring its properties for the long term. Here, we developed three strategies, all associated with the Polymer Derived Ceramics (PDCs) route, to prepare highly crystallized supported thick coatings and self-standing nanosheets. The first strategy concerns the innovative implementation of a Rapid Thermal Annealing to prepare micrometric h-BN coatings on thermal sensitive substrates. Compared to conventional treatment the crystallization of h-BN has successfully lowered to about 300 °C. The second strategy consists of an additivation of the used polymer precursor. Effect of lithium nitride as a crystallization promoter was investigated lowering the onset crystallization temperature from 1400 °C (traditionally) to 1000 °C. This novel synthetic route allows preparing self-standing highly crystallized h-BN nanolayers. Finally, the third strategy is based on a unique combination of the PDCs route with Spark Plasma Sintering to profit of both approaches. This original method leads to large and well-crystallized flakes available for a subsequent exfoliation. Full article

Boron nitride (BN) is a III-V compound which is the focus of important research since its discovery in the early 19th century. BN is electronic to carbon and thus, in the same way that carbon exists as graphite, BN exists in the hexagonal phase. The latter offers an unusual combination of properties that cannot be found in any other ceramics. However, these properties closely depend on the synthesis processes. This review states the recent developments in the preparation of BN through the chemistry, shaping and ceramic conversion of borazine derivatives. This concept denoted as Polymer-Derived Ceramics (PDCs) route allows tailoring the chemistry of precursors to elaborate complex BN shapes which cannot be obtained by conventional process. The effect of the chemistry of the molecular precursors, i.e., borazine and trichloroborazine, and their polymeric derivatives i.e., polyborazylene and poly[tri(methylamino)borazine], in which the specific functional groups and structural motifs determine the shaping potential by conventional liquid-phase process and plastic-forming techniques is discussed. Nanotubes, nano-fibers, coatings, monoliths and fiber-reinforced matrix composites are especially described. This leads to materials which are of significant engineering interest. Full article

Preceramic polymers, i.e., polymers that are converted into ceramics upon heat treatment, have been successfully used for almost 40 years to give advanced ceramics, especially belonging to the ternary SiCO and SiCN systems or to the quaternary SiBCN system. One of their main advantages is the possibility of combining the shaping and synthesis of ceramics: components can be shaped at the precursor stage by conventional plastic-forming techniques, such as spinning, blowing, injection molding, warm pressing and resin transfer molding, and then converted into ceramics by treatments typically above 800 °C. The extension of the approach to a wider range of ceramic compositions and applications, both structural and thermo-structural (refractory components, thermal barrier coatings) or functional (bioactive ceramics, luminescent materials), mainly relies on modifications of the polymers at the nano-scale, i.e., on the introduction of nano-sized fillers and/or chemical additives, leading to nano-structured ceramic components upon thermal conversion. Fillers and additives may react with the main ceramic residue of the polymer, leading to ceramics of significant engineering interest (such as silicates and SiAlONs), or cause the formation of secondary phases, significantly affecting the functionalities of the polymer-derived matrix. Full article