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.     

Digital light processing of ceramic components from polysiloxanes

Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.     

UV-curable inorganic precursors enable direct 3D printing of SiOC ceramics

3D ceramic parts are of great interest for various applications including aerospace, defense, electronics, photonics, and biomedical. Yet, additive manufacturing of ceramics is challenging due to their poor machinability. Herein, two approaches based on the chemical modification of silicon resins to obtain UV-curable preceramic precursors of SiOC are described. The dual functionality of the synthesized resins acting both as preceramic precursor and as photopolymerizable entity under UV light is exploited. A set of characterization techniques has allowed the investigation of the mechanisms involved in the synthesis of the inorganic SiOC precursors according to the following approaches: (1) blend of the silicon resin with photoactive monomers and (2) synthesis of a single source UV-curable preceramic silicon resin. A correlation between the nature of the precursor and the properties of the derived SiOC is analyzed. From a technological point of view, the materials can be fabricated as dense or crack-free porous customized objects with low mass loss and optimal surface quality.     

Powder bed fusion of polyamide powders combined with different preceramic polymers infiltration and pyrolysis to produce complex-shaped ceramics

The fabrication of a wide range of polymer-derived ceramic parts with high geometric complexity through a novel hybrid additive manufacturing technique is presented in this article. The process that we introduced in a previous work uses the powder bed fusion technology to manufacture high porous polymeric preforms to be then converted into ceramics through preceramic polymer infiltration and pyrolysis. The cellular architectures of a rotated cube (strut-based) and a gyroid (sheet-based) with 25 mm diameter, 44 mm height and 67 % of geometric macroporosity were generated and used for the fabrication. The complex structures were 3D printed and polycarbosilane, polycarbosiloxane, polysilazane and furan liquid polymers were used to produce SiC, SiOC, SiCN and glassy carbon, respectively. Despite a linear shrinkage of about 24 %, the parts maintained their designed complex shape without deformations. The significant advantages of the proposed method are the maturity of powder bed fusion for polymers with respect to ceramic additive manufacturing techniques and the possibility to fabricate net-shape complex ceramic parts directly from preceramic precursors.     

Texture and micro-nanostructure of porous silicon oxycarbide glasses prepared from hybrid materials aged in different solvents

The influence of the aging conditions of the preceramic hybrid material on the microstructure of silicon oxycarbide (SiOC) glasses derived therefrom has been highlighted. The textural and structural properties of the glasses are modified by aging the hybrid precursor in different environments. Three solvents have been employed as aging media to produce macroporous SiOC ceramics with porosities in the range between 30 and 70 vol.%.It has been concluded that the polarity and chemical characteristics of the solvent plays an important role on the surface characteristics and structure of the obtained SiOC glass. Raman spectroscopy and Small Angle X-ray scattering reveal the presence of different nanodomain sizes depending on the polymeric fraction in the preceramic network. The free carbon phase developed during the hybrid-to-ceramic conversion turn out to have a high influence on the growth of the silica nanodomains and thus on the nanostructure of the obtained ceramic.     

Multifunctional advanced ceramics from preceramic polymers and nano-sized active fillers

In this paper, the introduction of nano-sized active fillers into preceramic polymers for the realization of multifunctional ceramic components is discussed. Several silicate and oxynitride systems have been produced, by heat treatment in air or nitrogen, greatly widening the compositional range of ceramics made from preceramic polymers. Phase pure ceramics were obtained with very favorable reaction kinetics, and therefore at low temperature and for short heating times. Shaping of the components was carried out using several plastic forming technologies, such as warm pressing, extrusion, injection molding, foaming, machining, fused deposition and 3D printing. Some significant examples of this new methodology are described, ranging from relatively simple oxide systems (mullite, zircon, cordierite, fosterite, yttrium-silicates) to more complex oxynitride ceramics (SiAlONs, YSiONs). Some results concerning the potential application of these components, ranging from structural or thermo-structural functions (bulk components and environmental barrier coatings) to more functional purposes (bioactive ceramics and inorganic phosphors), are also reported.     

Preparation of complex SiOC ceramics by a novel photocurable precursor with liquid crystal display ( LCD) 3D printing technology

The polymer derived ceramics with complex shapes were prepared using the low-cost liquid crystal display (LCD) 3D printing technology. In this paper, we not only synthesized a liquid low-viscosity precursor with the photocurable group, but also provided a method to improve the forming precision and ceramic yield. The results showed that the introduction of KH570 could improve the curing accuracy, the ceramic yield. When the content of KH570 increased to 41 wt%, the curing shrinkage of the system decreased from 6.92% to 2.84%, the ceramic yield increased by 29.2%. Moreover, the mechanical properties of SiOC ceramics were studied and discussed. The bending strength of SiOC ceramic reached 44.2 MPa. And the ceramics after polishing were no cracks or other defects. This research demonstrates the huge potential for 3D - printed ceramics to become ubiquitous.     

Electrically conductive silicon oxycarbide thin films prepared from preceramic polymers

This work focuses on silicon oxycarbide thin film preparation and characterization. The Taguchi method of experimental design was used to optimize the process of film deposition. The prepared ceramic thin films with a thickness of c. 500 nm were characterized concerning their morphology, composition, and electrical properties. The molecular structure of the preceramic polymers used for the preparation of the ceramic thin films as well as the thermomechanical properties of the resulting SiOC significantly influenced the quality of the ceramic films. Thus, an increase in the content of carbon was found beneficial for the preparation of crack-free thin films. The obtained ceramic films exhibited increased electrical conductivity as compared to monolithic SiOC of similar chemical composition. This was shown to correlate with the unique hierarchical microstructure of the SiOC films, which contain large oxygen-depleted particles, mainly consisting of highly graphitized carbon and SiC, homogeneously dispersed in an oxygen-containing amorphous matrix. The matrix was shown to also contain free carbon and to contribute to charge carrier transport between the highly conductive large particles. The ceramic thin films possess electrical conductivities in the range from 5.4 to 8.8 S/cm and may be suitable for implementation in miniaturized piezoresistive strain gauges.     

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.     

陶瓷先驱体催化裂解研究进展

有机聚合物衍生陶瓷技术具有聚合物分子可设计性强、成型容易和制备温度低等优点,已经成为陶瓷及其复合材料的主要制备技术之一。裂解是陶瓷先驱体实现从有机到无机转化的关键步骤,对目标陶瓷的组成、结构和性能有着决定性的影响。在陶瓷先驱体中添加过渡金属进行催化裂解,可以改变其裂解行为,进而调控和拓展裂解产物的结构和性能。本文从不同过渡金属对陶瓷先驱体的催化裂解作用入手,总结了陶瓷先驱体催化裂解的研究现状,探讨了催化机理,并就后续深化研究与应用提出了发展建议。     

Optimized preceramic polymer for 3D structured ceramics via fused deposition modeling

3D structured ceramics stemmed from preceramic polymers via additive manufacturing have attracted much attention recently. However, these polymers with high ceramic yield are so brittle that extrusion-based additive manufacturing techniques are hardly able to be utilized for assembling 3D structures. Herein, we developed a strategy to prepare feedstocks for these manufacturing techniques, i.e., utilizing a small amount of thermal-plastic polymer to optimize the preceramic polymer while good compatibility is required between the two polymers to ensure a homogeneous mixture. Polycarbosilane and polypropylene were selected as the representative materials. Polypropylene occupied a small proportion (≤5wt.%) and significantly improved the formability of the precursor. Three-dimensional SiC were obtained via fused deposition modeling combined with crosslinking and pyrolysis. The SiC ceramic filaments showed a mean tensile strength of 471 MPa. The strategy is also applicable to a large field of ceramic systems with corresponding precursor, such as sialon ceramic and multicomponent Si-based ceramics.     

A Novel Process for the Manufacture of Ceramic Microelectrodes for Biomedical Applications

A novel process, based on microextrusion of preceramic polymers, was developed for manufacturing ceramic microelectrodes, mainly for biomedical applications. A coextrusion approach was used to obtain filaments with inner conductive lines after proper doping. Chemical reticulation and high-temperature pyrolysis were applied to convert the meltable preceramic, polymeric resins into SiOC ceramic materials. Different cross-sectional geometries were produced. The flexural strength of filaments depended on the outer diameter size; doping produced filaments with an average conductivity of ∼0.4 1/Ω cm for a 50 wt% carbon black load. The results on L929 and MG63 cell line models indicated good biological performance of Si–O–C ceramics and confirmed citocompatibility.     

Chemical modification of preceramic polymers: Their reactions with transition metal complexes and transition metal powders

Conclusions This research has demonstrated that a variety of chemistries can be carried out with preceramic polymers that in general are characterized by the presence of an abundance of reactive functional groups. Such chemistries can serve to upgrade a given preceramic polymer by catalytic or stoichiometric processes; they can be used to from new and useful hybrid polymers from the original preceramic polymer, as shown in the present work and also in some of our previously published work; they can, by their pyrolysis in the presence of metal powders, act as chemical reagents that deliver the elements of interest for reaction with the metal to give useful ceramics. Thus the preparation of a preceramic polymer is not the end of the chemistry in the monomer-to-polymer-to-ceramic conversion, but rather it presents many possibilities for further chemistry.     

The fabrication of SiBCN ceramic components from preceramic polymers by digital light processing (DLP) 3D printing technology

We present a novel method to fabricate SiBCN ceramic components with complex shapes from preceramic polymers by using digital light processing (DLP) 3D printing technology in this research work. The photocurable precursor for 3D printing was prepared by blending high ceramic yield polyborosilazane with photosensitive acrylate monomers. The material formulation and printing parameters were optimized to fabricate complicated SiBCN ceramic components with high precision. The printed SiBCN ceramic materials were pyrolyzed at different temperatures, and retained their fine features after pyrolysis. Their microstructures were characterized by FTIR, XRD and TEM respectively. Furthermore, the thermal stability and mechanical properties of the SiBCN ceramic samples were investigated and discussed in detail. The 3D printed SiBCN ceramic material exhibited excellent thermal stability and resistance to high temperature oxidation up to 1500?°C.     

3D printing of SiC ceramic: Direct ink writing with a solution of preceramic polymers

3D structured SiC ceramics with varying feature sizes (100–400?μm) were achieved by direct ink writing of polycarbosilane (PCS)/n-hexane solution. The rheological properties of the PCS solution and printing parameters were tailored for optimum writing behaviour. The integrity and clear surface of the filaments indicated the printing ability of forming the self-supporting features along with the rapid evaporation of solvent. As-printed 3D structured PCS was processed by oxidative crosslinking and pyrolysis and converted to SiC ceramic. Although strong shrinkage occurred during the pyrolysis, SiC ceramic maintained the original 3D structure. Both proper viscoelasticity of printable solutions and the homogeneous shrinkage in the pyrolysis determine the integrity and feature characteristic of 3D structured SiC using direct ink writing preceramic polymer.     

Synthesis of ordered mesoporous silicon oxycarbide monoliths via preceramic polymer nanocasting

Highly ordered mesoporous silicon oxycarbide (SiOC) monoliths have been synthesized using liquid poly(hydridomethylsiloxane) (PHMS) as starting preceramic polymer and mesoporous carbon CMK-3 as direct template. Monolithic SiOC-carbon composites were generated via nanocasting of PHMS into CMK-3, pressing without any additive, cross-linking at 150 °C under humid air and subsequent thermolysis at 1000 or 1200 °C under argon atmosphere. The carbon template was finally removed by the thermal treatment at 1000 °C in an ammonia atmosphere, as a result of the generation of monolithic SiOC ceramics with ordered mesoporous structures. The products were characterized by scanning electron and transmission electron microscopes, X-ray diffraction, Fourier transformation infrared spectrometer, X-ray photoelectron spectroscope and nitrogen absorption-desorption analyzer. The as-prepared SiOC monoliths exhibited crack-free, ordered 2-dimentional hexagonal p6mm symmetry with high specific surface areas. With increasing the calcination temperature, the ordered mesoporous structure was still remained and the specific surface area just had a slight reduction from 616 to 602 m² g⁻¹. Moreover, the porous SiOC monoliths possessed good compression strengths and anti-oxidation properties.     

First printing of continuous fibers into ceramics

This article reports a novel method for three-dimensional (3D) printing of continuous fibers into ceramics to improve the mechanical properties of printed ceramics, which is difficult in other 3D printing technologies. The ceramics were derived by pyrolysis of thermoplastic ceramic precursor feedstocks, which were prepared by two methods. One is homogeneously mixing thermoplastic resins and ceramic precursors. The feedstocks prepared by this method exhibit good thermoplastic properties and can be extruded into filaments. Ceramics were obtained by heating the feedstocks to 1100°C in argon atmosphere. The ceramics were amorphous and remained stable during 1100-1300°C; at 1400°C they decomposed into β–SiC with simultaneous volatile gas generation. Above 1400°C, their quality decreased significantly due to cracking of ceramic skeletons. The other method is directly heating, extruding and printing the ceramic precursor. The precursors showed good printability and complex ceramic structures were printed with continuous carbon fibers inside. The continuous carbon fibers improved the flexural strength of pyrolytic ceramics, which is about 7.6 times better than that of the ceramics without fibers. The novel method unravels the potential of 3D printing of continuous fibers into ceramics with complex lightweight structures to improve the strength.     

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC₄ from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.     

液态聚硅氧烷交联-成型-热解制备复杂形状硅氧碳陶瓷

采用液相法,以含氢聚硅氧烷和乙烯基环四硅氧烷为前驱体,经注模、交联和热解制备出净成型的硅氧碳陶瓷体,研究了该体系的成型、交联和热解行为,以及高温热解过程中陶瓷结构和组成的转变。研究表明：该体系有很好的成型能力,以不同材质和形状的模具均可成型,经热解可制备出各种形状和尺寸的硅氧碳陶瓷材料;交联体在整个热解过程中均保持完整,可获得不同温度（400～1 000 ℃ ）的无开裂的热解体;硅氧碳陶瓷在高温热解过程中通过 Si—O 和 Si—C 键重排由无定形的 Si—O—C 网络转化为含 SiC 和 SiO2纳米晶的陶瓷结构     

Effect of HF and NaOH etching on the composition and structure of SiOC ceramics

Porous SiOC ceramics were prepared with tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) as sol?gel precursors, and followed by etching with HF and NaOH solution. The microstructure evolution and chemical etching as a function of pyrolysis temperature were investigated. The amorphous carbon increases as rising the temperature from 800 ^oC to 1200 ^oC, and the graphitic carbon increases with further etching by HF and NaOH. However, the effect of pyrolysis temperature on the structure of C is more significant. The hydroxylation reaction and phase separation of SiOC ceramics results in the increase of SiO₄ unit, which reacts with HF and NaOH to form micro- and mesopores. The existence of mesopore after HF etching provides more specific surface area and pore volume. However, NaOH etching produces more micropores, and the contribution of micropores to specific surface area and pore volume is higher than that of mesopores. Although HF and NaOH etching increase the specific surface area of SiOC ceramics, the etching effect of NaOH is superior to that of HF etching, and the carbon-enriched SiOC ceramics are obtained after NaOH etching.