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.     

Additive manufacturing of polymer-derived ceramic matrix composites

This study presents a fabrication method and identifies processing bounds for additively manufacturing (AM) ceramic matrix composites (CMCs), comprising a silicon oxycarbide (SiOC) ceramic matrix. A digital light projection printer was used to photopolymerize a siloxane-based preceramic resin containing inert ceramic reinforcement. A subsequent pyrolysis converted the preceramic polymer to SiOC. Particle reinforcements of 0 to 40% by volume in the green state were uniformly dispersed in the printed samples to study their effects on pyrolysis mass loss and shrinkage, and CMC notch sensitivity and strength. Both particle and whisker reinforcements toughened the glassy SiOC matrix (1 MPa m^1/2), reaching values >3 MPa m^1/2. Bending strengths of >300 MPa (>150 MPa (g cm⁻³)⁻¹) and a Weibull modulus of 10 were measured on AM samples without surface finish. We identified two pore formation mechanisms that placed processing bounds on sample size and reinforcement volume fraction. Methods for increasing these bounds are discussed. With properties commensurate to traditionally processed technical ceramics, the presented process allows for free-form fabrication of high-performance AM CMC components.     

Development of multiphase bioceramics from a filler-containing preceramic polymer

Multiphase bioceramics based on wollastonite and wollastonite/hydroxylapatite (W/HAp) have been successfully prepared by the heat treatment of a filler-containing preceramic polymer. CaO-bearing precursors (Ca-carbonate, Ca-acetate, and CaO nano-particles) were dispersed in a solution of silicone resin, subsequently dried and pyrolysed in nitrogen. The reaction between silica, deriving from the oxycarbide (SiOC) residue of the silicone resin, and CaO “active filler” led to the formation of several calcium silicates, mainly consisting of wollastonite (CaSiO₃), in both low and high temperature forms. The phase assemblage of the final ceramic varied with the pyrolysis temperature (varying from 1000 to 1200 °C). HAp was additionally inserted, as “passive filler” (i.e. not reacting with SiOC), for the preparation of bioceramics based on W/HAp mixtures.     

Ceramic Microtubes from Preceramic Polymers

A novel process for the production of ceramic microtubes involving the microextrusion of preceramic polymers was studied. Microtubes with a wide range of inner and outer diameters and several centimeters long were produced from two silicone resins. A coextrusion approach was also used to extend the forming capability of the technique. The addition of carbon black resulted in electrically conductive silicon oxycarbide (SiOC) ceramic microtubes. SiOC microtubes possessed a high bending strength, ranging from ∼30–1100 MPa.     

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).     

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).     

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.     

Macrostructural design of highly porous SiOC ceramic foams by preceramic polymer viscosity tailoring

Highly porous SiOC ceramic foams with gradient or uniform macrostructures were obtained through polymer derived ceramic routes. Precuring of preceramic polymers and introduction of SiO₂ powders were used to tailor precursor viscosity and hence SiOC foam macrostructure. Effects of polymer viscosity on porosity, pore size, pore distribution were investigated by light microscopy and micro-computed tomography techniques. SiOC ceramic foams. Foams from one unmodified precursor, showed pore size gradient with small pores located at bottom and large pores at the top. To address this non-uniformity, the viscosity of the precursor was increased by pre-curing the preceramic polymer, which resulted in decrease of the average pore size and improvement in pore size uniformity. For a different system with a self-foaming preceramic polymer, because of the simultaneous release of foaming gases and rapid increase in viscosity during crosslinking, the foam had non-uniform macrostructure with large pores and thick struts at the bottom. By addition of SiO₂ fillers, the crosslinking reaction rate was reduced leading to homogeneous pore nucleation and uniform small pore size foams.     

Multiple metals doped polymer-derived SiOC ceramics for 3D printing

Multiple metals doped polymer-derived SiOC ceramics with octet truss structure were prepared by employing a photosensitive methyl-silsesquioxane as preceramic polymer through sol-gel method and Digital Light Processing 3D printing. The physical and chemical properties of the preceramic polymers and printed octet truss structure SiOC ceramics were investigated. Results show that the organosilicon preceramic polymers have outstanding photocuring properties and could transform into amorphous SiOC ceramics at 800–1200?°C. It is illustrated that the excellent mechanical properties of SiOC ceramics with octet truss structure (after 3D printing and pyrolysis) are attributed to the metal elements pinning in the amorphous matrix on the atomic level. Doping other metal elements such as Fe, Ni, Co, Pt, etc, is thought to bring promising properties for the lattice structure SiOC ceramics and potentially further expand its applications in the future.     

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.     

Silicon carbide whiskers reinforced SiOC ceramics through digital light processing 3D printing technology

Polymer derived silicon oxycarbide ceramic materials and silicon carbide whiskers reinforced ceramic composite are prepared through digital light processing (DLP) 3D printing technology in the present work. A new type of UV-curable preceramic polymer is firstly synthesized and then two types of photopolymer resins with and without SiC whiskers as reinforcement are prepared. Due to the high curing rate and good fluidity of the resins, they are applied in DLP 3D printing and various 3D objects with complicated structures and high printing resolution have been printed. The derived ceramic materials show amorphous microstructure and there is no apparent porosity and cracking throughout the whole sample surface of the ceramic materials and the SiC whiskers are uniformly embedded in the ceramic matrix and remain intact and unaffected during the pyrolysis process. The SiC whiskers reduced the shrinkage and mass loss. More importantly, it significantly improves the mechanical performance of the derived ceramic materials in which the compressive strength increases from 77.5 ± 10.2 MPa to 98.4 ± 12.3 MPa. Benefiting from the easiness of the fabrication, high printing resolution and excellent mechanical performance, the derived ceramic materials have great potential applications in various areas.     

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.     

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.     

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.     

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.     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

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.     

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.     

纳米二氧化钛改性光固化树脂的研究

以纳米TiO2的丙二醇甲醚醋酸酯(PMA)分散液为添加剂改性激光快速成型用光固化树脂SPR4000,通过对树脂及固化物的酸值、分子质量、IR、DSC、热重分析、流变性及力学和热性能测试等研究了改性后光固化树脂的性能,并用扫描电镜对纳米TiO2在树脂中的分散情况进行了观察。结果表明添加的二氧化钛质量分数为0.75%时,体系力学性能最好,拉伸强度提高15.25%,冲击强度提高41.35%,弯曲强度提高41.75%,耐热性也有所提高。纳米二氧化钛PMA分散液的加入解决了纳米二氧化钛在树脂中的分散稳定性问题,保证了树脂体系的固化速度及交联密度,满足了制作工艺,提高了光固化树脂SPR4000的力学性能。     

Polymer-derived SiOC aerogel with hierarchical porosity through HF etching

Silicon oxycarbide (SiOC) aerogels have been synthesized from preceramic polymers via pyrolysis in inert atmosphere at 1200 and 1300 °C. The as synthesized materials have a typical colloidal microstructure with mesoporosity in the range 10–50 nm and no microporosity. HF acid attack of the SiOC aerogels dissolves preferentially the SiO₂-rich phase and creates micro-and (small)mesopores (<10 nm) in the aerogels microstructure finally leading to a materials with hierarchical porosity. The HF post-pyrolysis treatment is more efficient for the SiOC aerogels pyrolyzed at the maximum temperature, i.e. 1300 °C, leading to a maximum value of specific surface area of 530 m²/g and total porosity of 0.649 cc/g.