Polymer-derived Ceramics Route Research Articles

Synthesis and shrinkage resistance of precursor ceramic microspheres aerogel composed of curled molecular chains via polymer-derived ceramics route

Structural damage and shrinkage cracking were exhibited during the pyrolysis of ceramic aerogels prepared by the polymer-derived ceramics (PDCs) route. Herein, the shrinkage of the ceramic precursor during the pyrolysis was offset by utilizing curled molecular chains to store strain in advance. Microspheres composed of curled molecular chains were prepared using polysilazane (PSZ) as a precursor. The microspheres were linked into a three-dimensional skeleton and further fabricated into precursor ceramic microspheres aerogel (PSZ/MCMSA) with shrinkage resistance. The morphology, thermal insulation, and shrinkage resistance properties of the material were tested. The results indicated that PSZ/MCMSA exhibited outstanding thermal insulation and shrinkage resistance properties. The shrinkage of the precursor ceramic microspheres (PSZ/MCMS) was offset by the strain stored through the curled PSZ molecular chains in the PSZ/MCMS. In addition, the microsphere skeleton structural supporting and gas escape channels were formed during the sintering, and the shrinkage resistance of the precursor ceramic aerogel was further enhanced.

Curvature-ordered biomimetic helical ceramic composite fiber aerogel with shrinkage resistance, thermal insulation properties, and mechanical properties

Ceramic aerogel with high porosity and low thermal conduction was widely used in the field of thermal insulation. However, shrinkage and structural damage were produced by the ceramic precursors during pyrolysis. The shrinkage and mechanical properties of ceramic aerogels required to be improved. Herein, inspired by spiral grass and passionflower, curvature-ordered ceramic fiber aerogels were prepared by electrostatic spinning combined with polymer-derived ceramics (PDCs) route. Shrinkage resistance properties, thermal insulation properties, and mechanical properties of ceramic fiber aerogels were investigated. The shrinkage resistance property of aerogels was improved by the tensile deformation of helical fibers during pyrolysis. The path of heat transfer was extended through the helical fibers and heat conduction was reduced. The gas was confined in a porous network formed by helical fibers that lapped over each other, and thermal convection was minimized. Thermal radiation was attenuated due to reflection and scattering at the core-shell interface of the helical fibers. The mechanical properties of the aerogel were enhanced by the helical structure of the fibers and the stress diffusion mechanism. The tensile and compressive properties of the spiral ceramic aerogel were improved through the curvature structure by about 6 MPa and 3 MPa, respectively. The results showed that the helical ceramic composite fiber aerogel with ordered curvature exhibited shrinkage resistance, thermal insulation, and mechanical properties.

Deep-sea glass sponges-like hollow porous ceramic fiber aerogel: Fabrication, anti-shrinkage and thermal insulation

Ceramic aerogels prepared using the polymer-derived ceramics (PDCs) route were extensively applied in the thermal insulation field with lightweight and high-temperature resistance. However, ceramic aerogels were susceptible to weight loss and volume shrinkage at high temperatures, leading to the collapse of the aerogel structure and a reduction in thermal insulation performance. In this paper, inspired by the skeletal structure of deep-sea glass sponges, hollow porous MHP-SiC-CA ceramic fiber aerogel was prepared by electrostatic spinning. The anti-shrinkage property of ceramic fiber aerogel was improved through the circular and spindle-shaped pores on the surface of hollow fibers. The gas-solid phase interface was increased by the abundant nanopore structure, leading to enhanced reflection and refraction of thermal radiation within the aerogel. The heat conduction and heat convection of the aerogel were reduced with the hollow and porous structure. The results presented that the ceramic fiber aerogel exhibited excellent anti-shrinkage properties, high specific strength, and ultra-low thermal conductivity. The material was expected to provide novel perspectives for the development of ceramic fiber aerogels applicable to ultra-high-temperature environments.

Two-step ALD process for non-oxide ceramic deposition: the example of boron nitride

Atomic layer deposition (ALD) based on polymer-derived ceramics (PDCs) chemistry is used for the fabrication of boron nitride thin films from reaction between trichloroborazine and hexamethyldisilazane. The transposition of the PDCs route to ALD is highly appealing for depositing ceramics, especially non-oxide ones, as it offers various molecular precursors. From a two-step approach composed of an ALD process forming a so-called preceramic film and its subsequent ceramization, conformal and homogenous BN layers are successfully synthesized on various inorganic substrates. In the first stage, smooth polyborazine coatings are obtained at a temperature as low as 90 °C. The saturation and self-limitation of the ALD gas-surface reactions are verified. Intriguingly, three ALD windows seem to exist and are attributed to change in ligand exchange. After the ceramization stage using a heat treatment, conformal near-stoichiometric BN layers are obtained. Their structure in terms of crystallinity can be adjusted from amorphous to well-crystalline sp2 phase by controlling the treatment temperature. In particular, a crystallization onset occurs at 1000 °C and well defined sp2 crystalline planes oriented parallel to the surface are noted after ceramization at 1350 °C. Finally, side-modification of the substrate surface induced by the thermal treatment appears to impact on the final BN topography and defect generation.

Strengthened and toughened SiHfBCN-based high-temperature resistant adhesive with SiC NWs

To further improve the performance of binders, a SiHfBCN-based high-temperature resistant adhesive was successfully synthesized by Polymer-Derived Ceramics (PDC) route using TiB2, Polysiloxane (PSO) and short SiC nanowires as fillers. The effect of short SiC nanowires on the adhesive strength at room temperature and high temperature, as well as the reinforcing mechanism was studied. Compared with the adhesive without SiC nanowires, after curing (at 170 °C) and pyrolysis (at 1000 °C) in air, the appropriate adding of SiC nanowires upgrades the room temperature and high temperature (at 1000 °C in air) adhesive strength to (12.50 ± 0.67) MPa (up by about 32%) and (13.11 ± 0.79) MPa (up by about 106%), respectively. Attractively, under the synergistic impact of the nanowire bridging, nanowire breaking, nanowire drawing and crack deflection, the optimized adhesive exhibits multi-stage fracture, causing the increscent fracture displacement.

Polymer-derived Biosilicate-C composite foams: In-vitro bioactivity, biocompatibility and antibacterial activity

Biosilicate/carbon composites were fabricated in the form of highly porous foams by the polymer-derived ceramic route, and their biological response was analysed. Two different commercial silicone polymers (a poly-methyl-siloxane, MK, and a polymethyl-phenyl-silsesquioxane, H44) were considered as a silica source, mixed with active fillers yielding Na2O, CaO and P2O5. The samples were heat treated either in air or in N2 atmosphere to obtain products resembling the known Biosilicate® glass-ceramic, with or without free carbon. All fabricated samples exhibited acellular in-vitro bioactivity upon immersion in SBF as well as antibacterial activity against S. aureus and E. coli. Direct contact cell viability test, assessed by using a WST-8 assay, indicated that both carbon-containing and carbon-free samples were cytocompatible.

Greener solvents for the processing of preceramic polycarbosilane: application in the preparation of B4C/SiC composites.

The innovation introduced in this study consists of replacing toluene with safer solvents such as cyclopentane or diethyl ether in the processing of a preceramic polycarbosilane (allylhydridopolycarbosilane, AHPCS) and assessing its impact on the functionalisation of B4C powders to produce B4C/SiC composites. Fourier-transform infrared (FT-IR) with ATR and nuclear magnetic resonance (NMR) spectroscopy revealed no major modification in the polymer structure. SEC/MALS analysis showed a slight change in the number-average molar mass of the polymer regardless of the functionalisation solvent used in correlation with a slight decrease in the polymer ceramic yield due to oligomer loss. The thermal behaviour of the preceramic polymer investigated via mass spectrometry remained unaffected by the solvent change. The search for polymer residues after distillation highlighted the recyclability of both the functionalisation solvent and the polymer, despite a slight increase in the molar mass of the polymer. Finally, the sinterability of B4C/AHPCS samples was studied with the preparation of B4C/SiC composites via a polymer-derived ceramic (PDC) route and spark plasma sintering (SPS). The effect of the solvent on the microstructure and relative density of the specimens (>92%) is negligible. The specimens retain a fine and homogeneous phase distribution despite process modification. The results highlight the approach developed to use greener solvents for the chemical synthesis of functionalised ceramics and represent a step towards the generalisation of more environmentally friendly processes.

Mesoporous SiC-Based Photocatalytic Membranes and Coatings for Water Treatment.

Photocatalytically active silicon carbide (SiC)-based mesoporous layers (pore sizes between 5 and 30 nm) were synthesized from preceramic polymers (polymer-derived ceramic route) on the surface and inside the pores of conventional macroporous α-alumina supports. The hybrid membrane system obtained, coupling the separation and photocatalytical properties of SiC thin films, was characterized by different static and dynamic techniques, including gas and liquid permeation measurements. The photocatalytic activity was evaluated by considering the degradation efficiency of a model organic pollutant (methylene blue, MB) under UV light irradiation in both diffusion and permeation modes using SiC-coated macroporous supports. Specific degradation rates of 1.58 × 10-8 mol s-1 m-2 and 7.5 × 10-9 mol s-1 m-2 were obtained in diffusion and permeation modes, respectively. The performance of the new SiC/α-Al2O3 materials compares favorably to conventional TiO2-based photocatalytic membranes, taking advantage of the attractive physicochemical properties of SiC. The developed synthesis strategy yielded original photocatalytic SiC/α-Al2O3 composites with the possibility to couple the ultrafiltration SiC membrane top-layer with the SiC-functionalized (photocatalytic) macroporous support. Such SiC-based materials and their rational associations on porous supports offer promising potential for the development of efficient photocatalytic membrane reactors and contactors for the continuous treatment of polluted waters.

Tribological behavior of Cu-modified polymer-derived SiBCN ceramics at elevated temperature

SiBCN ceramics were prepared and modified with copper (10, 20 and 30 wt%) by polymer-derived ceramics route. The polymer-to-ceramic transformation of the precursor was investigated and the tribological behavior of the SiBCN and Cu-SiBCN ceramics at 800 °C was studied in terms of composition, microstructure, high-temperature friction properties and wear mechanism. It turned out that copper incorporation enhanced the tribological performances of the ceramics both at ambient and elevated temperatures. The average friction coefficient of SiBCN was 0.23 at 800 °C, while it decreased to 0.18 with the wear rate reduced by one magnitude order after introducing 30 wt% Cu. The generated lubricious CuO-SiO2 tribo-film originating from the tribo-chemical reaction was the major high-temperature lubrication mechanism for the Cu-SiBCN composites.

Processing of polymer‐derived, aerogel‐filled, SiC foams for high‐temperature insulation

AbstractPorous polymer‐derived ceramics (PDCs) are outperforming materials when low‐density and thermal inertia are required. In this frame, thermal insulating foams such as silicon carbide (SiC) ones possess intriguing requisites for aerospace applications, but their thermal conductivity is affected by gas phase heat transfer and, in the high temperature region, by radiative mechanisms. Owing to the versatility of the PDC route, we present a synthesis pathway to embed PDC SiC aerogels within the open cells of a SiC foam, thus sensibly decreasing the thermal conductivity at 1000°C from 0.371 W·m−1K−1 to 0.243 W·m−1K−1. In this way, it was possible to couple the mechanical properties of the foam with the insulating ability of the aerogels.The presented synthesis was optimized by selecting, among acetone, n‐hexane, and cyclohexane, the proper solvent for the gelation step of the aerogel formation to obtain a proper mesoporous colloidal structure that, after ceramization at 1000°C, presents a specific surface area of 193 m2·g−1. The so‐obtained ceramic composites present a lowest density of 0.18 g·cm−3, a porosity of 90% and a compressive strength of 0.76 MPa.

Glyphosate adsorption performances of polymer-derived SiC/C aerogels

The phenomenon of water pollution as a consequence of the release of herbicides and pesticides into the environment is an outgrowing problem that requires performing and regenerable adsorbent materials. This work presents the synthesis of a nanocrystalline silicon carbide aerogel and its application in removing glyphosate from polluted water. The aerogel was synthesized via Polymer-Derived Ceramic (PDC) route using allylhydridopolycarbosilane as polymeric precursor. By pyrolysis at 1500 °C in argon, the crystallization of β-SiC nanocrystals was observed, while the aerogel preserved a high specific surface area of 215 m2·g−1. The nanostructured aerogel was tested for glyphosate herbicide, showing a remarkable adsorption of 0.607 mg·g−1, being the initial glyphosate concentration of 2 mg·L−1, and a removal of 93% of the pollutant in solution. Elovich kinetics adsorption and Langmuir isotherm models were found to be the most suitable to describe the mechanism of glyphosate capture via adsorption onto the aerogel surface.

Modification of SiBCN by Zr atom and its effect on ablative resistance of Cf/SiBCN(Zr) composites

The SiBCN(Zr) ceramics were synthesized via a polymer-derived ceramics route. Using this precursor, the precursor impregnation pyrolysis process prepared the Cf/SiBCNZr composites with low density (1.89 g/cm3). Compared to Cf/SiBCN, the Cf/SiBCNZr composites exhibit reliable thermal stability and ablation resistance in a wide temperature range. A dense oxide layer dominated by ZrO2 and SiO2 formed in situ on the surface of Cf/SiBCNZr, effectively preventing outside oxygen and further ablation. The introduced Zr is evenly distributed in SiBCN ceramics, which enables it to give full play to the ablation resistance of Zr at higher oxygen acetylene torch temperatures. This work enriches the fundamental understanding and offers engineering guidance for lightweight ultra-high temperature ceramic matrix composites.

Structure evolutions of the polymer derived medium-/high-entropy metal carbides

Ceramic powders with low oxygen content are highly demanded in monolithic ceramics. In this work, a series of medium-/high-entropy ceramic (HEC) particles, including (Zr0.25Ta0.25Nb0.25Ti0.25)C, (Zr0.2Ta0.2Nb0.2Ti0.2W0.2)C, (Zr1/6Ta1/6Nb1/6Ti1/6W1/6Hf1/6)C (Zr1/7Ta1/7Nb1/7Ti1/7W1/7Hf1/7V1/7)C, and (Zr1/8Ta1/8Nb1/8Ti1/8W1/8Hf1/8V1/8Mo1/8)C, were successfully fabricated by the polymer-derived ceramic (PDC) route. The structure evolutions of the precursor demonstrated that HEC particles obtained at 1800 °C possessed a single phase and rock-salt structure. All the samples had the similar microstructures (hillock structure) and a low oxygen impurity content (less than 2.00 wt%). The results also demonstrated that the as-obtained HEC particles had a disordered distribution of elements from nanoscale to microscale. This work proved a new method to fabricate the HEC powders with low oxygen impurity content and the HEC precursors for further obtaining the nanofibers or ceramic matrix composites.

Insight into the evolution process from novel polyborazine precursor PPMAB to inorganic BN fiber

Understanding the elemental and structural evolution from green fibers to inorganic fibers is important for fabricating high-performance ceramic fibers via the polymer-derived ceramics (PDCs) route. In a previous study, we found a novel polyborazine precursor, poly[2-propylamino-4,6-bis(methylamino)borazine-co-tri(methylamino)borazine] (PPMAB), which possesses a high ceramic yield and outstanding spinnability. Herein, we provide an insight into the evolution process from PPMAB to inorganic BN fibers. During the curing process, NH3 only reacted with the B–NHCH3 groups in PPMAB to form B–NH2 groups, which further reacted with B–NHCH3 to form B–NH–B bridges. Therefore, crosslinked networks were formed in the cured fibers and the gel content increased from 52.3 to 92.1 wt%. In the nitridation process, NH3 not only reacted with unreacted B–NHCH3 but also reacted with B–NHCH2CH2CH3 groups to form B–NH2 groups, which reacted with N–CH3 bridges in the adjacent PPMAB molecule to form –N(B3N3)3- networks. Simultaneously, CH3NH2 and CH3CH2CH2NH2 escaped from the fibers, and the carbon content of the fibers was reduced from 20.588 wt% to 0.115 wt%. By controlling the curing and nitridation processes, the curing degree can be enhanced, whereas the pore defects resulting from gas molecular escape can be effectively reduced. Thus, dense inorganic BN fibers with excellent dielectric properties (2.33 < ε < 2.5, tan δ < 3.4ⅹ10−3) and high tensile strength (1.6 GPa) were fabricated. The present study not only provides ideas for the design and synthesis of polyborazine precursors, but also for the fabrication of high-performance BN fibers.

Chemical route for synthesis of β-SiAlON:Eu2+ phosphors combining polymer-derived ceramics route with non-hydrolytic sol-gel chemistry

We report the synthesis of β-SiAlON:Eu2+ phosphors from novel single source precursors in which strictly controlled chemical composition is established at molecular scale. The two-step synthesis occurs by the chemical modification of perhydropolysilazane (PHPS) with Al(OCH(CH3)2)3 and AlCl3 in xylene at room temperature to 140 °C to introduce Al in the PHPS network while controlling the oxygen content followed by the reaction of EuCl2 with the Al-modified PHPS upon heat-treatment. Gas chromatography-mass spectrometry and thermogravimetry-mass spectrometry analyses revealed that PHPS reacted with Al(OCH(CH3)2)3 and AlCl3 via the formation of Al-N bonds. Moreover, AlCl3 reacted with nitrogen-bonded Al alkoxide residue to release 2-chloropropane in an analogy to the non-hydrolytic sol-gel reaction between metal alkoxide and metal chloride. Subsequently, AlCl3 acted as a Lewis acid catalyst to promote the Friedel-Craft alkylation between xylene solvent and the 2-chloropropane formed in-situ to afford dimethylcumene. On the other hand, EuCl2 reacted with silylamino moiety to form Eu-N bonds at around 850 °C. β-SiAlON:Eu2+ phosphors were successfully synthesized by pyrolysis of the precursors under flowing N2 or NH3 at 1000 °C, followed by heat treatment at 1800 °C for 1 h under a N2 gas pressure at 980 kPa. The polymer-derived β-SiAlON:Eu2+ (z = 0.55, Eu2+ 0.37 at%) exhibited green emission under excitation at 410 or 460 nm, and the green emission intensity under the excitation at 410 nm was found to be increased by reducing the carbon and chlorine impurities through the polymer-derived ceramics route investigated in this study.

From the synthesis of hBN crystals to their use as nanosheets in van der Waals heterostructures

In the wide world of 2D materials, hexagonal boron nitride (hBN) holds a special place due to its excellent characteristics. In addition to its thermal, chemical and mechanical stability, hBN demonstrates high thermal conductivity, low compressibility, and wide band gap around 6 eV, making it a promising candidate for many groundbreaking applications and more specifically in van der Waals heterostructures. Millimeters scale hBN crystals are obtained through a disruptive dual method (polymer derived ceramics (PDC)/pressure-controlled sintering (PCS)) consisting in a complementary coupling of the PDC route and a PCS process. In addition to their excellent chemical and crystalline quality, these crystals exhibit a free exciton lifetime of 0.43 ns, as determined by time-resolved cathodoluminescence measurements, confirming their interesting optical properties. To go further in applicative fields, hBN crystals are then exfoliated, and resulting boron nitride nanosheets (BNNSs) are used to encapsulate transition metal dichalcogenides (TMDs). Such van der Waals heterostructures are tested by optical spectroscopy. BNNSs do not luminesce in the emission spectral range of TMDs and the photoluminescence width of the exciton at 4 K is in the range 2–3 meV. All these results demonstrate that these BNNSs are of high quality and relevant for future opto-electronic applications.

Silicon oxycarbide-tin nanocomposite derived from a UV crosslinked single source preceramic precursor as high-performance anode materials for Li-ion batteries

In this work, we report an innovative and facile UV light-assisted synthesis of a nanocomposite based on silicon oxycarbide (SiOC) and tin nanoparticles. SiOC ceramic matrix, containing a conductive free carbon phase, participates in lithium-ion storage, and buffers the volume changes of Li-alloying/de-alloying material. The reported synthesis procedure through a polymer-derived ceramic route involves the preparation of a single-source precursor by UV crosslinking of a preceramic polymer in the presence of a tin precursor. Pyrolysis of this starting precursor at 1000 °C leads to a homogenous distribution of tin nanoparticles (25–35 nm) within the SiOC ceramic matrix, which is crucial for electrochemical stability of the material. SiOC/Sn nanocomposite tested as an anode for Li-ion battery exhibits high reversible capacity values (603 mAh g−1 at the current density of 74.4 mA g−1), outstanding rate capability (524 mAh g−1 at 2232 mA g−1), and superior stability (494 mAh g−1 after 250 charge/discharge cycles).

Mechanical characterization of spark plasma sintered titania‐silicon oxycarbide (TiO2/SiOC) nanocomposites

AbstractIn this work, titania‐silicon oxycarbide nanocomposites synthesized via the polymer derived ceramics route have been sintered into crack‐free monoliths using spark plasma sintering and their mechanical properties as well as their apparent density have been characterized. The x‐ray diffractogram clearly showed the stabilization of anatase phase of size less than 10 nm size in an amorphous silicon oxycarbide matrix at 1200 °C. The hardness and elastic modulus determined using nanoindentation were found to be 9 GPa and 82 GPa, respectively, and the fracture toughness calculated using indentation crack length method was found to be 2.34 MPa m1/2. In addition to this, ball‐on‐three ball technique was used to evaluate the biaxial flexural strength and fractographic studies were carried out to understand the fracture mechanisms.

High strength and microwave-absorbing polymer-derived SiCN honeycomb ceramic prepared by 3D printing

The combination of 3D printing technology and polymer-derived ceramic route provides an attractive strategy to construct microwave-absorbing honeycomb with fine structure through flexible process. However, preparation of honeycomb ceramics with both excellent mechanical and microwave-absorbing properties is still challenging. Herein, SiCN honeycomb ceramic was fabricated by stereolithography from UV curable polymeric precursor consisting of polysilazane and multifunctional acrylates. By optimizing the multifunctional acrylates and its ratio, the decomposition of organic moiety and the ceramization process of precursor are matched, rendering the achieved ceramic with high compactness. The hardness and specific compressive strength of SiCN honeycomb ceramic reach as high as 14.3 GPa and 333.3 MPa/(g·cm3), respectively. Meanwhile, at low pyrolysis temperature, the copolymerized acrylate and polysilazane that formed during curing process was converted to free-carbon nanodamins in-situ, which endows SiCN honeycomb ceramic with the minimum reflection loss of –49.0 dB, namely microwave absorption rate over 99.99%.

SiOC and SiCN-based ceramic supports for catalysts and photocatalysts

Nowadays, most industrial chemical products employ catalysts as an essential step to synthesize many commodities. Inspired by catalytic processes, the discovery of Honda-Fujishima, reported by analogy with natural photosynthesis, boosted the overall scientific interest in photocatalysis. This area aims for feasible energy generation and pollutants abatement, looking for beyond the efficiency under UV light, new perspectives towards visible light through novel composites. Suspended catalysts are commonly used due to simplicity, primarily to demonstrate concepts in lab-scale research. However, working with suspensions is not technologically feasible, and immobilizing on supports avoids the toxicity associated with free catalysts and facilitates further separation processes. Thus, immobilized catalysts on tailor-made porous support form heterojunctions and profiles as a more practical approach to tackling these challenges. In addition, involving just stable semiconductors within the system enables applying catalytic systems in harsh environments. The polymer-derived ceramic (PDCs) route has been studied in another technological frontier, addressing several technical issues over the past 50 years, allowing for coatings, foams, and fibers with controlled morphology and good thermal-oxidative resistance. Those features make PDCs promising supports for catalyst immobilization, particularly, SiOC and SiCN-based ceramics. This work provides an overview of PDCs and their interactions with catalysts. The manufacture approaches, catalyst type, component interactions, thermal treatment effects, catalytic activity under visible light, and other outcome properties are reviewed. Moreover, the research landscape and current challenges on catalysts supported by PDCs are discussed. Finally, the gaps regarding processing optimization and perspectives for PDC-based materials applied in catalyst-assisted systems are highlighted. • The application range of porous SiOC and SiCN supports in catalyst-assisted reactions under harsh environments are pointed up • An in-depth overview of SiOC and SiCN-based ceramics as supports and their effect in catalysis and photocatalysis is presented • Manufacturing approaches, type of catalyst, and intrinsic parameters of the as-obtained composites are discussed • Photocatalytic activity aiming to extend to the visible light adsorption is compared between the systems • The mechanism behind the performance is discussed thoroughly