SiOC Ceramic Foams through Melt Foaming of a Methylsilicone Preceramic Polymer

A process for the production of SiOC ceramic foams has been for the first time developed through melt foaming of a siloxane preceramic polymer with the help of a blowing agent, followed by pyrolysis under an inert atmosphere. The raw material consisted of a methylsilicone resin, a catalyst (which accelerated the cross-linking reaction of the silicone resin) and a blowing agent (which generated gas above 210°C). Methylsilicone resin foams were obtained through controlling the melt viscosity around 210°C, temperature where the blowing agent started to decompose, by varying the initial molecular weight of the preceramic polymer and the amount of the catalyst. The obtained SiOC ceramic foams exhibited excellent oxidation stability up to 1000°C, as shown by thermal gravimetric analysis (TGA). As expected, the mechanical properties of the SiOC ceramic foams varied as a function of their bulk density, possessing a flexural strength up to 5.5 MPa and a compression strength up to 4.5 MPa. The main steps in the process, namely foaming and pyrolysis, were analyzed in detail. The viscosity change was analyzed as a function of temperature by the dynamic shear measurement method. The pyrolysis process of foams was analyzed by TGA coupled with infrared spectroscopy (IR).     

Silicon Oxycarbide Ceramic Foams from a Preceramic Polymer

Open-cell ceramic foams were obtained from the pyrolysis, at 1000° to 1200°C under nitrogen, of a preceramic polymer (a silicone resin) and blown polyurethanes. The morphology of the expanded polyurethane was reproduced in the final architecture of the ceramic foam. The foams produced in this way consisted of an amorphous silicon oxycarbide ceramic (SiOC), having a bulk density ranging from 0.1 to 0.4 g/cm³ and variable cell size (300 to 600 µm). Young's modulus ranged from 20 to 170 MPa, and the compression strength from 1 to 5 MPa. The foams displayed excellent dimensional stability up to their pyrolysis temperature.     

An optimized process for in situ formation of multi-walled carbon nanotubes in templated pores of polymer-derived silicon oxycarbide

A reliable and optimized process to grow carbon nanotubes (CNTs) in templated pores of polymer derived ceramic (PDC) matrix was developed. It is realized through the pyrolysis of a preceramic polymer, i.e., poly (methyl-phenyl-silsesquioxane) (denoted as PMPS), in argon atmosphere at 1000 °C together with nickel-catalyst-coated poly-methyl-methacrylate (PMMA) microbeads (denoted as PMMA-Ni). PMPS served as both a precursor for the ceramic matrix and a carbon source for the CNT growth. PMMA microbeads were used as sacrificial pore formers and coated with nickel via an electroless plating method, which provides an improved control of particle size of the catalyst and its distribution in the material. The influence of PMMA-Ni loading on the in situ growth of CNTs and the properties of CNTs/SiOC nanocomposites were studied through thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and density/porosity measurements. Under optimized conditions, uniform distribution of in situ grown CNTs was observed within the templated pores of the SiOC matrix. The optimized process leads to reproducible high yield of CNTs in the pores. The development of such novel CNT/cellular ceramic nanocomposite materials is of significant interest for a variety of sensor applications.     

Microstructure dependent ablation behaviour of precursor derived SiOC ceramic foam for high temperature applications

Ablation behaviour of poly(hydridomethylsiloxane) derived open and closed porous structured SiOC ceramic foams was evaluated using oxy-acetylene flame at 1500 °C for various time durations. X-ray diffraction and scanning electron microscopy analyses of ablated SiOC ceramic foams revealed the formation of a thin protective SiO₂ layer inhibiting further oxidation. The closed porous structured SiOC ceramic foams exhibited very low mass ablation rate in contrast to open porous structured SiOC ceramic foams owing to the differences in thermal energy dissipation mechanism. The feasibility of the plausible foam reduction reactions pertaining to the ablation mechanism was further investigated by computing the Gibbs energy and HR-TEM analysis. The study corroborated the significance of tailoring the microporous structured SiOC ceramic foams as potential thermal protection material for high temperature applications.     

Thermal Shock Behavior of Silicon Oxycarbide Foams

Silicon oxycarbide (SiOC) ceramic foams, obtained from the pyrolysis of a preceramic polymer, were subjected to thermal multiple cycles from 800°–1200°C to room temperature in a water bath. Flexural and compression strengths, as well as elastic modulus, were characterized before and after quenching. Excellent thermal shock and cycling resistance behavior was observed, with only moderate strength and stiffness degradation. The phase assemblage of the foam remained unchanged, and no crack formation in the foams was observed. However, microstructural characterization revealed the development of porosity in the struts and cell walls due to the oxidation of residual carbon in the amorphous SiOC material, thereby contributing to a small decrease in stiffness after quenching.     

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).     

Effect of additive structure and size on SiO2 formation in polymer‐derived SiOC ceramics

Silicon oxycarbide (SiOC) ceramics with highly adjustable properties and microstructures have many promising applications in batteries, catalysis, gas separation, and supercapacitors. In this study, additive structures on the nucleation and growth of SiO₂ within SiOC ceramics are investigated by adding cyclic tetramethyl‐tetravinylcyclotetrasiloxane (TMTVS) or caged octavinyl‐polyhedral oligomeric silsesquioxane (POSS) to a base polysiloxane (PSO) precursor. The effects of the 2 additives on the polymer‐to‐ceramic transformation and the phase formation within the SiOC are discussed. POSS encourages SiO₂ nucleation and leads to more SiO₂ formation with significantly increased ceramic yield, which subsequently leads to higher specific surface of 1557 m²/g with a larger pore size of ～1.8 nm for the porous SiOC. High TMTVS content decreases both the specific surface area and pore volume of the resulting porous SiOCs. This study demonstrates a new approach of using Si‐rich additive POSS to increase the SiOC yield while maintaining or even increasing the specific surface area.     

Synthesis and characterization of Ce/SiOC nanocomposites through the polymer derived ceramic method and evaluation of their catalytic activity

Cerium oxide and silicon oxycarbide (Ce/SiOC) porous nanocomposites have been synthesized through the polymer derived ceramic route. In the synthesis of the preceramic precursors, the addition of urea facilitates the deposition of Cerium atoms on the surface of SiO₂ nanoparticles since it prevents the SiO₂ from agglomeration. Both Ce and urea affects the structural and textural parameters of the obtained ceramics. Less crosslinked structures are formed when the urea concentration increases and it also provokes a reduction of the carbon crystallite size. Cerium, on the other hand, induces an increase of the carbon size as well as the number of SiOC units. Pore anisotropy and smoothness of the surface are also dependent on the composition of the material. As expected, the better thermocatalytic behavior against CO₂ decomposition is found at the largest Ce amounts but also, smooth surfaces and low pore anisotropies favor the accessibility of the gases to the thermocatalytic centers.     

Macroporous polymer-derived SiO2/SiOC monoliths freeze-cast from polysiloxane and amorphous silica derived from rice husk

A freeze-casting route towards macroporous SiOC/SiO₂ ceramic nanocomposites from preceramic polymers was developed. Amorphous SiOC/SiO₂ monolith with pore channels aligned along the freezing direction were obtained from commercially available methyl-phenyl-vinyl-hydrogen polysiloxane (Silres® H62C) and amorphous silica derived from rice husk ash freeze-cast with water or tert-butyl alcohol, crosslinked and pyrolyzed at 1100 °C in nitrogen. The influence of processing parameters such as solvent (tert-butyl alcohol or water), polymer to silica ratio (2:1, 1:1, 1:2), cooling rate (2, 4, 6 °C/min) and pre-crosslinking of polysiloxane on the porosity and structure of the obtained ceramic nanocomposites were assessed by X-ray tomography, XRD, solid state NMR, scanning electron microscopy and mercury porosimetry. The microstructure of SiOC ceramics derived from the Silres H62C polysiloxane was studied as well.     

SiOC Ceramic Monoliths with Hierarchical Porosity

SiOC glass monoliths possessing hierarchical porosity were produced by a one-pot processing method. Periodic mesoporous organosilica (PMO) particles were embedded into a foamed siloxane preceramic polymer. After pyrolysis at 1000°C in inert atmosphere, open celled, permeable SiOC ceramic monoliths with a high amount of pores, ranging in size from hundred of micrometers to a few nanometers, were obtained. The components possessed a specific surface area of 137 m²/g, indicating the retention of most of the mesopores after the pyrolytic conversion of the PMO precursor particles. These fillers converted to truncated rhombic dodecahedral SiOC mesoporous micron-sized grains, homogeneously distributed throughout the SiOC cellular matrix. The produced porous ceramics possessed compression strength of about 1.7 MPa, which is adequate for their use in several engineering applications.     

Hierarchical Porosity Components by Infiltration of a Ceramic Foam

A ceramic body with hierarchical meso–macro-porosity was prepared by depositing a mesoporous silica coating on the walls of a macroporous silicon oxycarbide (SiOC) foam. Scanning electron microscopy, transmission electron microscopy, and small-angle X-ray scattering revealed that a uniform mesoporous coating was deposited on the walls of the macropores, with the mesopores arranged in a highly ordered cubic lattice. Nitrogen adsorption measurements showed a bimodal pore size distribution and revealed that the specific surface area is one order of magnitude higher than in macroporous SiOC foams. Therefore, interesting applications in adsorption and catalysis can be devised.     

3D printing boehmite gel foams into lightweight porous ceramics with hierarchical pore structure

We herein report a novel hierarchically porous ceramic foams derived from boehmite gel foams, which possess both high porosity and superior strength. The gel foams show excellent printability due to its predominant stability, high yield stress and storage modulus, which endows such foam material ideal ink for 3D printing lightweight and complex-shape materials via direct ink writing approach. The 3D printed ceramic foams possess programmable architecture assembled by porous filaments, uniform macro-pores with tunable size in the range of 4∼70 μm, as well as nanoscale pores in cell wall, after sintering at relatively low temperature of 1200–1300 °C. In this way, ceramic foams with high strength were achieved, attributed to the tiny grains, large amount of grain boundaries, uniform pores and hierarchical pore structure. Notably, the foams sintered below 1200 °C have significant advantage on specific surface area, which could reach up to 300-400 m²/g.     

Hierarchical porous ceramics with multiple open pores from boehmite gel emulsions

Ceramic foam materials with highly porous microstructure are playing vital role in increasing areas, especially for those with requirements for open channels and superior specific surface area. In this work, a simple and versatile approach to prepare ceramic foams with open pores has been proposed, that is gelation of boehmite nanoparticle-assembled emulsions. Notably, hierarchical porous microstructure with open channels and uniform pore structure has been built. High specific surface area up to389.4 m²/g is attainable, making it excellent adsorption material when combining the merit of hierarchical pore structure. Furthermore, lattice-shaped ceramics are prepared via direct ink writing gelled emulsion, displaying the potential of forming lightweight material with complex shape and designable macrostructure. The three-dimensional (3D) printed foams exhibit multiple open pores, which cover length scale from mm scale, to μm scale and nm scale, making them promising materials in several fields like adsorption and gas filtrations, etc.     

Stereolithography-based additive manufacturing of polymer-derived SiOC/SiC ceramic composites

In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behavior. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.     

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.     

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO₂ filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO₂ fillers and SiO₂‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO₂ filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m²/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.     

Conductive Ceramic Foams from Preceramic Polymers

Ceramic foams in the system Si-O-C, possessing different bulk densities and morphologies, were obtained from preceramic polymers using two different direct foaming approaches. The electric properties of the foams were varied by adding suitable fillers to the precursor mixtures in amounts up to 80 wt%. The electrical conductivity of the foams was varied by several orders of magnitude. The effects of the type of filler and preceramic polymer (methylsiloxane or methylphenylsiloxane resins), as well as the used filler precursor, on the properties of the ceramic foams were investigated.     

热解含铁聚硅氧烷原位生长硅氧碳纤维制备硅氧碳复合材料

将铁氯化物混入聚硅氧烷前驱体进行交联成型和热解,利用热解中在聚硅氧烷中形成的孔隙和在孔隙中形成的铁颗粒为催化剂,在硅氧碳陶瓷基体中原位生长出硅氧碳纳米纤维,制备出硅氧碳陶瓷和硅氧碳纤维复合材料。用扫描电子显微镜观察材料断面,结果显示：在硅氧碳陶瓷基体中生长出纳米纤维,部分纤维取向分布,纤维紧贴于硅氧碳陶瓷基体,二者呈良好结合;能谱分析显示纤维中含硅、氧和碳,证实其为硅氧碳。所制得的硅氧碳陶瓷和硅氧碳纤维的复合结构不同于通常热解纯聚硅氧烷形成的单相的硅氧碳结构,在硅氧碳基体中的硅氧碳纤维是在聚硅氧烷前驱体中引入的铁催化剂在热解过程中通过催化聚硅氧烷一维生长形成的,该过程可用于发展一步法原位制备纳米纤维前驱体陶瓷复合材料。     

Linear organodecaborane block copolymer as a single-source precursor for porous boron carbide ceramics

Boron carbide is one of the most widely used non-oxide ceramics as it possesses excellent physical and chemical properties. Much attention has been paid to prepare boron carbide ceramics via precursor derived method. In this work, poly(6-norbornenyldecaborane)-b-poly(6-cyclooctenyldecaborane) (PND-b-PCD) block copolymer was synthesized by the ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) of 6-norbornenyldecaborane with 6-cyclooctenyldecaborane. The synthesized boron carbide preceramic polymer had good solubility and film-forming ability with a high ceramic yield of 75% at 850 °C. TGA, XRD and TG-IR-GC–MS were used to investigate the ceramization process of the precursor. Boron carbide ceramic foams were prepared by the precursor via replicating polyurethane foam template. The component, crystalline and morphology were investigated in detail. The ceramic foams showed a good high temperature performance and could keep their structure even up to 1800 °C.     

A novel method for preparation of interconnected pore-gradient ceramic foams by gelcasting

A novel approach was introduced to prepare pore-gradient Al₂O₃ ceramic foams with association of gelcasting process and polymer sphere template. This approach allows the design of pore connectivity and gradient height of ceramic foams by the appropriate selection of sphere sizes and numbers. The limitation of ceramic foams fabricated by polymeric sponge process on sponge carrier can be resolved by this approach. Epispastic polystyrene (EPS) spheres with different sizes were employed to array ordered templates. Influence of solid content and dispersant on the viscosity of Al₂O₃ slurry was studied. EPS spheres modified by oxygen plasma to increase the hydrophilicity of surfaces and influence of pre-removal of EPS template on the integrity of networks were investigated. Results showed that 55?vol.% Al₂O₃ slurries with 0.5?wt% dispersant kept good fluidity for casting and permeability in template. Water contact angle of EPS surface decreased from 95.2° to 20°. Some defects in green bodies such as edge-delamination and micropores disappeared after surface modification. The perfect structure and morphology of the ceramic foams were ascribed to the pre-removal of template by solvent. The hierarchical pore structure was fabricated with EPS spheres of 2.1, 2.6 and 3.2?mm diameters. Porosity and compressive strength of the pore-gradient Al₂O₃ ceramic foam were 68.5% and 3.06?MPa at 1,500?°C, respectively.