Polymer derived ceramics with β-eucryptite fillers: A novel processing route to negative and near zero expansion materials

A polymer derived ceramic with near-zero or negative thermal expansion was manufactured from a powder mixture consisting of β-eucryptite fillers having a negative thermal expansion, and a polymethylsilsesquioxane preceramic polymer. The composite starting material was manufactured by ball-milling and warm-pressing and pyrolysis in inert atmosphere. The thermal expansion behavior of the composite material was controlled via the filler volume fraction and via the pyrolysis temperature. An influence of the filler material on the pyrolysis process was found.     

Experimental investigation on the indentation hardness of precursor derived Si–B–C–N ceramics

The elasto-plastic response of the precursor derived Si–B–C–N ceramics upon contact loading was determined by depth sensing nanoindentation technique. The indentation response of as-thermolyzed Si–B–C–N ceramic was compared with the heat-treated counterpart. The as-thermolyzed ceramic was X-ray amorphous and the heat-treated ceramic was phase separated and crystallized. The hardness and reduced elastic modulus values of the as-thermolyzed ceramic were ～16 GPa and ～172 GPa, respectively. The reduction in hardness to ～9 GPa in the heat-treated ceramic was attributed to phase separation and crystallization of SiC and Si₃N₄. Furthermore, high elastic recovery with a plastic work ratio of ～0.3 was observed and ascribed to volume controlled deformation mechanism.     

A semi-empirical power-law model for the dip-coating of a substrate into a particle-containing,non-Newtonian,complex fluid system

The apparent viscosity of a particle suspension of ZrSi₂ particles, polyhydromethylsiloxane (PHMS) preceramic polymer and n-Octane solvent, used to process polymer-derived ceramic composite coatings, is shown by viscometric experiments to be shear-thinning. The suspension is dip-coated onto substrates and the measured entrained coating thickness, h_0, is observed to be a power-law function of U, the substrate extraction speed, as h₀ = 0.5051U^0.5. The experimentally observed semi-empirical model is directly compared to the results of a variety of theoretically derived Landau-Levich scaling laws and other models that have similar liquids and that include other effects. None of these cases predicts the scaling observed in these experiments. A correction factor is introduced to quantify the difference between the semi-empirical model with existing theoretical models. Possible explanations for the observed scaling behavior are presented.     

闭孔泡沫陶瓷的研究现状及应用前景

闭孔泡沫陶瓷由于结合了陶瓷材料自身的优异性能以及其特殊的多孔结构,从而具有良好的保温、绝热、隔音、耐腐蚀、防水等性能,可作为复合功能材料在建筑领域中应用,对环境保护、能源节约具有重要的意义,已得到了行业,科研机构的广泛关注。文章介绍了闭孔泡沫陶瓷的国内外研究现状,对其在制备工艺、成果推广等方面存在的问题进行了分析,对其应用前景进行了展望。     

Pressureless synthesis of fully dense and crack-free SiOC bulk ceramics via photo-crosslinking and pyrolysis of a polysiloxane

This paper presents the pressureless preparation of fully dense and crack-free SiOC ceramics via direct photo-crosslinking and pyrolysis of a polysiloxane. Elemental analysis revealed the presence of high levels of carbon in the SiOC ceramics. Thus, the samples showed the highest content (78-86 mol%) of segregated “free” carbon reported so far. XRD investigations indicated that the materials prepared at 1100 °C were X-ray amorphous, whereas the sample prepared at 1400 °C contained a turbostratic graphite-like phase and silicon carbide as crystalline phases, as additionally confirmed by TEM and Raman spectroscopy. Vickers hardness was measured to be 5.5-8.6 GPa. The dc resistivity of the prepared material at 1100 °C was 0.35 Ω m, whereas the ceramic pyrolyzed at 1400 °C showed a value of 0.14 Ω m; both values are much lower than those of other known SiOC materials. This latter feature was attributed to the presence of a percolating carbon network in the ceramic.     

Polymer-derived mesoporous Ni/SiOC(H) ceramic nanocomposites for efficient removal of acid fuchsin

In this paper, magnetic porous Ni-modified SiOC(H) ceramic nanocomposites (Ni/SiOC(H)) were successfully prepared via a template-free polymer-derived ceramic route, which involves pyrolysis at 600 °C of nickel-modified allylhydridopolycarbosilane (AHPCS-Ni) precursors synthesized by the reaction of allylhydridopolycarbosilane (AHPCS) with nickel(II)acetylacetonate (Ni(acac)₂). The resultant Ni/SiOC(H) nanocomposites are comprised of in-situ formed nanoscaled Ni socialized with small amounts of NiO and nickel silicides embedded in the amorphous SiOC(H) matrix. The materials show ferromagnetic behavior and excellent magnetic properties with the saturation magnetization in the range of 1.71–7.08 emu g⁻¹. Besides, the Ni/SiOC(H) nanocomposites are predominantly mesoporous with a high BET surface area and pore volume in the range of 253–344 and 0.134–0.185 cm³ g⁻¹, respectively. The measured porosity features cause an excellent adsorption capacity towards a template dye acid fuchsin with the adsorption capacity Q_t at 10 min of 80.7–85.8 mg g⁻¹ and the Q_e at equilibrium of 123.8–129.8 mg g⁻¹.     

Oxidation resistant ceramic foam from a silicone preceramic polymer/polyurethane blend

Silicon oxycarbide (SiOC) ceramic foams, produced by the pyrolysis of a foamed blend of a methylsilicone preceramic polymer and polyurethane (PU) in a 1/1 wt.% ratio, exhibit excellent physical and mechanical properties. The proposed process allows to easily modify the density and morphology of the foams, making them suitable for several engineering applications. However, it has been shown that, due to residual carbon present in the oxycarbide phase after pyrolysis, the foams are subjected to an oxidation process that reduces their strength after high temperature exposure to air (12 h 1200°C). A modified process, employing the same silicone resin preceramic polymer but a much lower PU content (silicone resin/PU=5.25/1 wt.% ratio), has been developed and is reported in this paper. Microstructural investigations showed that carbon rich regions deriving from the decomposition of the polyurethane template are still present in the SiOC foam, but have a much smaller dimension than those found in foams with a higher PU content. Thermal gravimetric studies performed in air or oxygen showed that the low-PU containing ceramic foams display an excellent oxidation resistance, because the carbon-rich areas are embedded inside the struts or cell walls and are thus protected by the dense silicon oxycarbide matrix surrounding them. SiOC foams obtained with the novel process are capable to maintain their mechanical strength after oxidation treatments at 800 and 1200°C (12 h), while SiOC foams obtained with a higher amount of PU show about a 30% strength decrease after oxidation at 1200°C (12 h).     

Crack healing of ferrosilicochromium-filled polymer-derived ceramic composites

In this study, SiOC/FeSiCr/SiC-ceramics were derived by using polymethylsilsesquioxane as a polymeric precursor and FeSiCr (iron-chromium-silicide) and SiC powders as ceramic fillers in a polymer-extrusion process followed by pyrolytic conversion in a nitrogen atmosphere. The crack healing properties of the ceramic in an oxidising atmosphere were subsequently investigated. The SiOC/FeSiCr/SiC-ceramics showed crack closure and strength recovery behaviour after oxidation treatments in air from 600 to 1300?°C with various holding times from 2 to 1000?min. The crack healing mechanisms at different oxidation temperatures and with various dwell times are discussed.