Ceramic Yield Research Articles

Fabrication of scalable, aligned and low density carbon nanotube/silicon carbide hybrid foams by polysilazane infiltration and pyrolysis

Polymer-derived ceramic (PDC) process is an attractive technique that has high ceramic yield. This versatile method allows for fabrication of porous carbon nanotube (CNT)/ silicon carbide (SiC) hybrid materials that is important high temperature structural applications. Although several forms of CNT assemblies have been used with the PDC approach, the fabricated CNT/ceramic nanocomposites were either one or two dimensional. Herein, we report, for the first time, the fabrication of a low density, three-dimensional (3D) and scalable CNT/SiC structure using PDC technique. It was synthesized by impregnating preceramic polysilazane (PSZ) into ultralow density, anisotropic, and highly aligned CNT foams, followed by thermosetting and pyrolysis processes. The ceramic phase conformally coated the CNTs. The X-ray diffraction (XRD) diffractogram confirmed the presence of β-SiC crystalline phase. The resulting hybrid foam inherited the morphology and form factor of the original CNT foam, and possessed mechanical robustness, improved electrical properties, and extraordinary thermal stability.

Microstructure and mechanical properties of 3D Cf/SiBCN composites fabricated by polymer infiltration and pyrolysis

In this work, three-dimensional (3D) Cf/SiBCN composites were fabricated by polymer infiltration and pyrolysis (PIP) with poly(methylvinyl)borosilazane as SiBCN precursor. The 3D microstructure evolution process of the composites was investigated by an advanced X-ray computed tomography (XCT). The effect of dicumyl peroxide (DCP) initiator addition on the crosslinking process, microstructure evolution, and mechanical properties of the composites were uncovered. With the addition of a DCP initiator, the liquid precursor can cross-linking to solid-state at 120 °C. Moreover, DCP addition decreases the release of small molecule gas during pyrolysis, leading to an improved ceramic yield 4.67 times higher than that without DCP addition. After 7 PIP cycles, density and open porosity of the final Cf/SiBCN composite with DCP addition are 1.73 g·cm−3 and ∼10%, respectively, which are 143.0% higher and 30.3% lower compared with the composites without DCP addition. As a result, the flexural strength and elastic modulus of Cf/SiBCN composites with DCP addition (371 MPa and 31 GPa) are 1.74 and 1.60 times higher than that without DCP addition (213 MPa and 19.4 GPa), respectively.

Fast pyrolysis of a SiC-based polymer precursor using a spark plasma sintering apparatus – Effects of heating methods and pyrolysis atmosphere

The effects of heating method on the pyrolysis behavior, crystallinity and ceramic yield of a SiC-based precursor-derived ceramic (PDC) were investigated using commercial allylhydrido-polycarbosilane (AHPCS) precursors, spark plasma sintering (SPS) apparatus and tube furnace. The heating rate of SPS was ten times faster than that of tube furnace. Consequently, the total time of precursor impregnation and pyrolysis (PIP) process decreased distinctly after several PIP cycles. PDCs fabricated by SPS had higher crystallinity than those prepared by tube furnace. The crystallinity of PDCs was 70.4 and 65.1 %, respectively, after the pyrolysis using SPS and tube furnace at 1400℃ for 1h. The highest crystallinity of 82.92 % was achieved after the pyrolysis at 1500℃ for 2h using SPS. The ceramic yield of the precursor was not strongly affected by the heating method. This study provides a promising method for the pyrolysis of ceramic precursors with short processing time and improved thermal stability.

In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems.

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000–1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiNxCy nanocrystals (x + y = 1) are homogeneously formed “in situ” in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si3N4, TiNxCy (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.

Influence of reduced graphene oxide (rGO) on phase development and porosity of SiOC/rGO ceramic composites

Ceramic materials based on silicon oxycarbide (SiOC) and reduced graphene oxide (rGO) were produced by polymer pyrolysis and evaluated in terms of phase development and porosity. Carbonaceous phase, initially prepared from graphite oxide, was incorporated into silicone dihydroxy terminated in different amounts (0, 5, 15 and 25 wt%) and submitted to pyrolysis at 1500 °C to obtain SiOC/rGO ceramics. Higher ceramic yields and more thermally stable materials were obtained after rGO addition, whose results were associated to the chemical interaction degree between rGO and polymer structure. Cgraphite and SiC phases were generated in rGO-containing ceramics and a mixture of α- and β-SiC was achieved from 15 wt% rGO, enhancing their crystallinity with increasing of rGO content. Porosity features were influenced by the carbonaceous phase amount and different rGO-polymer interaction degrees. SiOC/rGO ceramics demonstrated desirable structural characteristics for future investigations in electrical and/or electrochemical applications.

BCN ceramics with excellent electromagnetic wave–absorbing property derived from high‐yield and soluble precursor polymers

Polymer‐derived ceramics have been used as functional nanocomposites with designable structures and tunable properties. In this report, three types of BCN ceramics were fabricated from boron‐modified precursors, which were in turn synthesized by the polymerization of decaborane as boron‐rich monomer with diamines as linkages. All of these precursors present high ceramic yield and good solubility in N,N‐dimethylformamide and dimethyl sulfoxide. The composition and structure of precursors were characterized using elemental analysis, Fourier‐transform infrared, gel permeation chromatography, and nuclear magnetic resonance. The structural evolution during the polymer‐to‐ceramic process and the microstructure of pyrolyzed BCN ceramics were analyzed in detail. The BCN ceramic with 16 wt. % carbon content derived from alkyldiamines exhibited promising X‐band absorption properties. When the sample thickness is only 1.8 mm, the minimum reflection loss reaches −40.9 dB at 15.8 GHz with an effective absorption bandwidth of 5.3 GHz. The simple and processable method for BCN ceramics was developed to satisfy the demand of materials with adjustable electromagnetic wave absorption properties.

Preparation of SiC ceramic fiber from a photosensitive polycarbosilane

Curing treatment is essential for ceramic precursors to maintain their shape during pyrolysis. In this study, a photosensitive SiC ceramic precursor was synthesized by chlorinating partial Si–H groups of polycarbosilane (PCS) with Cl2 followed by reacting with hydroxyethyl acrylate. According to 1H NMR and element analysis, about 10% structural unit of PCS was grafted with the acrylate group. After mixing with photo-initiator, the acrylate-grafted PCS transformed from soluble and fusible state to insoluble and non-fusible state under UV light. Its curing rate with related ceramic yield and oxygen content were compared with PCSs cured by the traditional air oxidation and electronic radiation. By electrospinning synchronized with UV illumination and pyrolyzing the acrylate-grafted PCS, one-dimensional continuous SiC fibers were prepared. The effects of pyrolysis temperature on the morphology, crystallographic structure and elemental composition were also characterized and discussed.

Realizing high ceramic yield and low shrinkage of in-situ formed lightweight 3D-SiC(rGO)px polymer-derived ceramics with excellent fracture toughness

Integration of high ceramic yield, low shrinkage and excellent fracture toughness in polymer-derived ceramics (PDCs) is challenging. Highly cross-linked polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO, PVG) precursors spontaneously interact with dense high-strength SiC(rGO)p fillers for abundant Si-dangling bonds at their interfaces. Herein, an ingenious strategy via re-pyrolysis process of ball-milling-induced SiC(rGO)p/PVG blends is proposed to fabricate 3D-SiC(rGO)px PDCs. The in-situ formed innovative three-dimensional (3D) honeycomb cellular net-like structure, with dense β-SiC(SiO2)/SiOxCy/Cfree(rGO) framework providing high strength while tiny micro-sized pores relaxing stress at the tip, effectively improves molding process, ceramic yield and mechanical properties of PDCs. Particularly, lightweight 3D-SiC(rGO)p0.6 PDCs display dense surface, satisfactory hardness (3.96 GPa) and excellent fracture toughness (3.21 MPa m1/2), especially high ceramic yield (94.49%) and low linear shrinkage (5.00%), realizing outstanding values ever reported for PDCs to the best of our knowledge. During re-pyrolysis process, they have struck perfect balance and achieved a combination of equal volume of inwardly shrinking PVG precursors and outwardly expanding SiC(rGO)p fillers. For the first time, such synergistic effects of flexible PVG with rigid SiC(rGO)p in this way pave a practical route toward reliable mass-produced 3D-SiC(rGO)px PDCs with potential applications in emerging aerospace propulsion components.

Synthesis of cyano-polycarbosilane and investigation of its pyrolysis process

Polycarbonsilane (PCS) is an important precursor of silicon carbide (SiC) fibers and ceramics. The ceramic yield of PCS is relatively low, about 60 %, which may bring some deficiencies in its applications. In this work, a novel precursor cyano-polycarbosilane (PCSCN) is synthesized by hydrosilylation reaction between PCS and acrylonitrile using a rhodium-containing catalyst, although acrylonitrile is generally not easy for hydrosilylation. After introducing tiny amounts of cyano (-C≡N) groups into the PCS molecules, the ceramic yield of PCSCN can increase largely to over 80 %. The ceramization mechanism of PCSCN is investigated by FTIR, TG, XPS, ESR, NMR, Raman and XRD analyses. It is found that some crosslinking structures in PCSCN are formed between SiH bonds and CN groups from about 200 ℃, which can be responsible for the high ceramic yield. The existence of a little more N, O and free C elements in the pyrolysis products may inhibit the growth of crystalline β-SiC. Moreover, the PCSCN precursor can also be melt-spun into continuous fibers by tailoring its molecular weight and softening point. The oxidized PCSCN fiber with relatively low oxygen content can be pyrolyzed without melting, and the final SiC fiber with an oxygen content as low as 8.5 % is obtained.

Enhanced relaxor behavior and thermal- and frequency-insensitive strain of (Na0.5Bi0.5)0.93Ba0.07Ti1 − x(Mn1/3Nb2/3)xO3 ceramics

(Na0.5Bi0.5)0.93Ba0.07Ti1 − x(Mn1/3Nb2/3)xO3 ceramics (x = 0–0.02) were prepared, and their structure and property were investigated systematically. (Mn1/3Nb2/3)4+ doping shifts ferroelectric–relaxor transition temperature (TF–R) toward low temperature. Thus, ferroelectric order is suppressed and relaxor behavior is enhanced monotonously, as further evidenced by composition-dependent electrical properties and domain evolution from labyrinthian to nanodomain. As a result, the x = 0.010 ceramic yields a large room temperature field-induced strain of 0.37%, especially the strain is temperature- and frequency-insensitive with a variety of 4.3% and 4.1% in the ranges of 25–125 °C and 0.5–10 Hz, respectively. This work suggests that the low content (Mn1/3Nb2/3)4+ complex cation doping is an alternative method to effectively tune the electrical properties of Na0.5Bi0.5TiO3-based ferroelectrics.

Efficient organic-to-inorganic conversion of polysiloxane by novel platinum-thiol catalytic system

Novel platinum–thiol catalytic system was constructed by the combination of platinum (Pt) catalysts (Karstedt's catalyst (KC) and chloroplatinic acid hexahydrate (CPA)) and thiol-containing compounds ((3-mercaptopropyl)trimethoxysilane (SM), (3-mercaptopropyl)triethoxysilane (SE), 1,6-hexanedithiol (HDT) and 1-octadecanethiol (OT)). It was found that Pt-thiol catalytic system could remarkably enhance the organic-to-inorganic conversion of polydimethylsiloxane (PDMS) and methylvinylpolysiloxane (Vi-PDMS) at elevated temperature. Under optimal condition, the ceramic yield of KC/SE/Vi-PDMS with 50 ppm Pt (KC) and 0.5 phr SE at 1000 °C was dramatically improved from 0.6 wt% to 54.4 wt% in comparison with Vi-PDMS. FTIR, Raman spectroscopy, XPS and XRD revealed that amorphous free carbon-containing SiOC ceramics were formed after thermal treatment of KC/SE/PDMS and KC/SE/Vi-PDMS at 1000 °C for 60min. And during organic-to-inorganic conversion of Vi-PDMS under high temperature, Pt-thiol catalytic system can efficiently retain C in the inorganic network of SiOC ceramics with existence of vinyl. TG, TG-FTIR and viscosity test results suggested that Pt-thiol system might enhance radicals coupling of Vi-PDMS chains and suppress unzipping decomposition or random scission, which resulted in the high capacity of organic-to-inorganic conversion of KC/SE/Vi-PDMS at elevated temperature.

High‐performance molecular‐separation ceramic membranes derived from oxidative cross‐linked polytitanocarbosilane

AbstractPolytitanocarbosilane (TiPCS)‐derived ceramic membranes were fabricated using a pre‐ceramic polymer. Special attention was focused on a process of thermal‐oxidative curing that was used to induce cross‐linking and the effect of this process on the ceramic yield, thermal stability, oxidation resistance, and microstructure of TiPCS. The cross‐linked TiPCS powders showed a ceramic yield and thermal stability that were higher than that from the non‐cross‐linked version. In addition, the cross‐linked TiPCS with uniform micropores showed higher levels of N2 and CO2 adsorption capacity, BET surface area, and micropore volume than the non‐cross‐linked versions, and the cross‐linking process enhanced the stability of the pore structure at high temperature. The cross‐linked TiPCS membranes showed high H2 permeance (1.49 × 10−6 mol/(m2 s Pa)) with sub‐nanopores (H2/SF6 selectivity: 12 000, H2/N2: 10), and in addition higher oxidation resistance than their non‐cross‐linked counterparts. Furthermore, the influence of the concentration of the TiPCS precursor coating solution was optimized and the hydrothermal stability of the membranes at high temperatures was also evaluated. The optimized membrane demonstrated great performance for the pervaporation removal of methanol in binary azeotropic systems of either MeOH/butyl acetate or MeOH/toluene, and it also showed high hydrothermal stability with excellent dehumidification performance under high temperatures.

Effects of low-temperature treatment on the properties of commercial preceramic polymers

Low-temperature, pre-gelation, thermal treatment of ceramic precursors is a common preliminary step in the polymer infiltration-and-pyrolysis-based processing of ceramic matrix composites (CMCs). A variety of polymer properties can be modified using this approach, the most important being molecular weight distribution, viscoelastic state, solidification behavior, and ceramic yield. In this work, the effects of thermal treatment on the processing properties of two commercial preceramic polymers, StarPCS™ SMP-10 and KiON Ceraset® PSZ-20, have been investigated. It was determined that the volatilization, molecular chain scission, and polymerization phenomena occurring during treatment, lead to increased viscosity, higher molecular weight, lower temperature of solidification onset, and improved ceramic yield. A wide range of tailorable viscoelastic states was obtained, and allowed the formation of continuous polymer filaments. Understanding the nature and effect of these modifications on the processing state can improve the current state of CMC processing, and open novel processing routes for constituent development.

Preparation and stereolithography of SiC ceramic precursor with high photosensitivity and ceramic yield

Stereolithography of ceramic precursors is a valuable additive manufacturing technology for complex ceramic parts. In this study, a new SiC ceramic precursor—liquid hyperbranched polycarbosilane (LHBPCS) grafting acrylate group was synthesized by chlorinating some Si–H groups of LHBPCS with Cl2 followed by reacting with hydroxyethyl acrylate. The reaction processes and structures of intermediate reactant and target product were confirmed by FT-IR and 1H NMR. According to photolithography experiment and hardness test under UV light, the synthesized LHBPCS had high photo-curing activity. Thermogravimetric analysis indicated it also possessed high ceramic yield (the ceramic yield at 1000 °C was 74.4%). After shaping with stereolithography, defect-free green bodies could be got. When heated to 1000 °C, the transparent yellow green bodies transformed into black SiC rich ceramic parts and 18.3–25.1% linear shrinkage associated with the precursor-to-ceramic conversion was observed. Because the shrinkage in the pyrolysis stage was nearly isotropic and the shrinkage was lower than other reported data, no obvious deformation or crack was found in the pyrolyzed parts.

Preparation of hollow SiC ceramic fibre from polycarbosilane fibre by diffusion-controlled cross-linking method

ABSTRACTCross-linking is favourable for increasing ceramic yield and avoiding melting or deformation of SiC ceramic precursor polycarbosilane (PCS) during pyrolysis. In order to achieve hollow SiC fibre, PCS fibre with a limited cross-linking depth was prepared by chlorinating the Si–H group in the outer layer of PCS fibre followed by hydrolysis and condensation of the formed Si–Cl group. The chlorination was implemented through a gas–solid reaction between Cl2 and PCS fibre. Based on the results of energy-dispersive spectrometer and attenuated total reflectance–Fourier transform infrared spectroscopy, the diffusion-controlled chlorination reaction was confirmed. During hydrolysis, the catalysis of ammonia gas was necessary. According to the results of dissolution and degradation experiments, cross-linked PCS outer layer formed after chlorination, hydrolysis and condensation, and the outer layer had higher ceramic yield than the core part. After pyrolysis, SiC fibre with hollow morphology was formed. By adjusting the chlorination degree, the morphology of resulting hollow SiC fibre could be modified.

Synthesis and morphology optimization of electrospun SiBNC nanofibers

SiBNC nanofibers were synthesized through the polymeric route by a one-pot synthesis approach. DMTA (dichloroboryl methyl trichlorosilyl amine) polymer was selected as pre-ceramic for SiBNC, which was shaped into nanofibers by electrospinning. Then, the nanofibers were cured in an inert atmosphere in order to obtain the final ceramic. By changing the curing atmosphere, the compound of final ceramic has been manipulated. In addition, the ceramic yield of DMTA as a preceramic was increased in the nitrogen atmosphere. The effects of applied voltage, solution concentration, and feeding rate on the morphology of final electrospun ceramic nanofibers were also investigated. Final ceramic remains amorphous up to elevated temperatures with outstanding mechanical and thermal properties.

Preparation and Characterization of An All Oxide Ceramic Matrix Composite by A Novel Method

Due to the characteristics of high temperature oxidation resistance and high reliability, all oxide composites are ideal materials for components in hot end of aeronautics and astronautics. In this paper, all oxide ceramic matrix composite(CMC) was prepared by a novel method which combined warm pressing of prepreg with PIP using polysilialumoxane as the precursor of mullite. The morphology, structure and mechanical properties of the all oxide CMC were characterized. The results show that polysilialumoxane with a ceramic yield of 53wt%can be completely changed to ceramic at 1200°C and form mullite crystals. The composite with alumina and mullite matrix derived from alumina powder slurry indicated the best mechanical properties, the flexural strength and fracture toughness of the composite reach 208.3Mpa and 128.6Mpa respectively, and the pull-out effect of the fibers is obvious in the process of tensile fracture, and the composites show ductile fracture. whereas the mechanical property decreases significantly after heating at 1200°C for100 hours due to the densification of matrix and abnormal growth of alumina grains in fibers.

A simple and tailor‐made fabrication of porous silicon carbide from functionalized kraft pulp paper

Porous silicon carbide (SiC) materials were fabricated using the polymer‐derived ceramics method with kraft pulp papers (KPP) and a commercial polycarbosilane, the allylhydridopolycarbosilane (AHPCS), as starting materials. For this, KPP, propargylated KPP, or phosphorylated KPP were used to be impregnated by the AHPCS, with or without Karstedt catalyst. The pyrolysed materials were characterized at different stages, by using thermogravimetric analysis (TGA) coupled with mass spectrometry, X‐ray diffraction (XRD), and scanning electron microscopy (SEM). Depending on the nature of the initial template, various architectured SiC ceramics were successfully obtained with adjustable porosities. The key role of the previous functionalization of the papers was highlighted in terms of interactions at the interface between the polymer and the lignocellulosic handsheets. It led to either replica or sacrificial template methods. Thus, it was possible to tune the open porosity of the porous carbon and β‐SiC materials between 14.8% and 92.9%, with ceramic yields varying from 12% to 71%.

Fabrication of SiOC ceramic with cellular structure via UV-Assisted direct ink writing

In this study, polysiloxane resin was modified at the molecular scale to make it amenable to ultraviolet-curing and suitable for direct ink writing. The characterization results regarding the rheological properties and infrared spectra demonstrated that the conversion of CC is proportional to the viscosity and modulus of ink. As a result, three-dimensional cellular structures with a certain span and tailored porosity were obtained. The printed structures were then subjected to pyrolysis at temperatures higher than 1000°C to form uniform, crack-free, dense SiOC cellular ceramic structures, with ceramic yield of up to 70% and linear shrinkage as low as 25%. This technique enables the inexpensive and fast fabrication of SiOC ceramic products with relatively complex shapes.

Straightforward synthesis and molecular structure optimization of novel SiZrBOC ceramic precursor via sol-gel and solvothermal approach

A novel SiZrBOC ceramic precursor polymer was synthesized directly using triethoxymethylsilane, boric acid and zirconium oxychloride hydrate via sol-gel and solvothermal method and the ceramic precursor followed by pyrolysis at 1000–1400 °C under argon atmosphere. The chemical composition, morphological change and crystallization behavior of SiZrBOC ceramics were carefully studied by means of FT-IR, XPS, SEM, and X-ray diffraction analysis. The degree of graphitization of products are analyzed by Raman spectroscopy. It proved that the content of sp2 carbon in ceramic was low and it can be judged that the ability of boron to promote the crystallization of free carbon is less than the ability of the oxygen-containing compound of zirconium to limit the crystallization of free carbon. The experimental results showed that the effect of monomer feed ratio have a direct impact on the SiZrBOC microstructure, thermal stability and ceramic residue, an 86.5% ceramic yield can be achieved after molecular structure optimization. The SiZrBOC ceramics had been found to exhibit remarkably high temperature stability at 1400°C in air atmosphere.