Allylhydridopolycarbosilane Research Articles

Polymer-derived mesoporous Ni/SiOC(H) ceramic nanocomposites for efficient removal of acid fuchsin

In this paper, magnetic porous Ni-modified SiOC(H) ceramic nanocomposites (Ni/SiOC(H)) were successfully prepared via a template-free polymer-derived ceramic route, which involves pyrolysis at 600°C of nickel-modified allylhydridopolycarbosilane (AHPCS-Ni) precursors synthesized by the reaction of allylhydridopolycarbosilane (AHPCS) with nickel(II)acetylacetonate (Ni(acac)2). The resultant Ni/SiOC(H) nanocomposites are comprised of in-situ formed nanoscaled Ni socialized with small amounts of NiO and nickel silicides embedded in the amorphous SiOC(H) matrix. The materials show ferromagnetic behavior and excellent magnetic properties with the saturation magnetization in the range of 1.71–7.08emug−1. Besides, the Ni/SiOC(H) nanocomposites are predominantly mesoporous with a high BET surface area and pore volume in the range of 253–344 and 0.134–0.185cm3g−1, respectively. The measured porosity features cause an excellent adsorption capacity towards a template dye acid fuchsin with the adsorption capacity Qt at 10min of 80.7–85.8mgg−1 and the Qe at equilibrium of 123.8–129.8mgg−1.

Hydrolytic conversion of preceramic polymers into silicate glass coatings with different wettability

In this paper, two types of preceramic polymers were used to form silicate glass coatings with different wettability via hydrolysis either under harsh or mild conditions. Allylhydridopolycarbosilane (AHPCS) and polyvinylsilazane (PVSZ) were spin-coated on Si substrate and consolidated by photo- and thermal-curing and subjected to hydrolysis for conversion to the silicate glass phase. The contact angle of the both hydrophobic cured polymers decreased from 91°–92° to 18°–36° with little difference, when hydrolyzed in strong alkaline of NaOH solution at room temperature. However, soft hydrolysis in ammonia vapor at 80 °C showed that the AHPCS derived silicate surface showed the moderate contact angle 58°, while the PVSZ derived silicate became very hydrophobic surface with the contact angle 103° presumably due to re-oriented alkyl groups. Eventually, it was demonstrated that the surface wettability of the hydrolyzed silicates was controlled with the polymer chemistry and the hydrolytic conditions upon the preceramic polymer-to-silica conversion.

Effect of degree of crystallinity on elastic properties of silicon carbide fabricated using polymer pyrolysis

In this study, silicon carbide (SiC) is fabricated using polymer infiltration and pyrolysis (PIP). Allylhydridopolycarbosilane (AHPCS) is used as the preceramic polymer. Final processing temperatures are varied to observe the change in microstructure as well as physical and mechanical properties. Density, porosity and thermal conductivity are determined as a function of processing temperatures. Microstructural characterization, done using atomic force microscopy (AFM) and X-ray diffraction (XRD), has revealed the presence of nanocrystalline domains with grain sizes in the range of 5–20nm. The degree of crystallinity is determined using non-contact mode atomic force microscopy. The degree of crystallinity follows an increasing trend with processing temperature. Hardness and modulus are determined using nanoindentation, and are found to be influenced by the degree of crystallinity.

Template‐Free Synthesis of Porous Fe3O4/SiOC(H) Nanocomposites with Enhanced Catalytic Activity

We present a template‐free synthesis of Fe3O4/SiOC(H) nanocomposites with in situ formed Fe3O4 nanoparticles with a size of about 50 nm embedded in a nanoporous SiOC(H) matrix obtained via a polymer‐derived ceramic route. Firstly, a single‐source precursor (SSP) was synthesized by the reaction of allylhydridopolycarbosilane (AHPCS) with Fe‐acetylacetonate [Fe(acac)3] at 140°C. The SSP was heat‐treated at 170°C to generate Fe3O4 nanocrystals in the cross‐linked polymeric matrix. Subsequently, the SSP was pyrolyzed at 600°C–700°C in argon atmosphere to yield porous Fe3O4/SiOC(H) nanocomposites with the high BET surface area up to 390 m2/g, a high micropore surface area of 301 m2/g, and a high micropore volume of 0.142 cm3/g. The Fe‐free SiOC(H) ceramic matrix derived from original AHPCS is nonporous. The in situ formation of Fe3O4 nanoparticles embedded homogeneously within a nanoporous SiOC(H) matrix shows significantly enhanced catalytic degradation of xylene orange in aqueous solution with H2O2 as oxidant as compared with pure commercial Fe3O4 nanoparticles.

Influence of the polymer–polymer miscibility on the formation of mesoporous SiC(O) ceramics for highly efficient adsorption of organic dyes

In this paper, we use methyl-terminated polydimethylsiloxane (PDMS) as a pore-former and allylhydridopolycarbosilane (AHPCS) as a preceramic precursor to fabricate mesoporous SiC(O) ceramics by using liquid-liquid phase separation combined with cross-linking and emulsion processing. The influence of the polymer-polymer miscibility on the formation of mesoporous SiC(O) ceramics was investigated, with PDMS (350 cP) as porogen and AHPCS and polysiloxane (PSO) as preceramic precursors, respectively. Mesoporous SiC(O) with a specific surface area of 87.83m2g−1 and an average pore size of ca. 5nm was produced by pyrolysis of a polymeric gel obtained from AHPCS/PDMS blends. In contrast, with PSO/PDMS blend as the starting material, mesopores were not formed in the final ceramics. In case of the AHPCS/PDMS blends, the polymer-to-ceramic transformation, the formation and the characteristic features of the mesopores were investigated by FT-IR, TGA, OM, SEM and N2 sorption isothermal analysis. The polymer-polymer miscibility between the preceramic polymer and the porogen polymer significantly influences the formation of the mesoporous ceramics. The final mesoporous SiC(O) ceramic obtained at 900°C can withstand up to 1400°C without significant change of the porosity. The resultant mesoporous SiC(O) shows highly efficient adsorption towards a model organic dye, namely Rhodamine B.

Simultaneous infiltration of submicron SiC powder and AHPCS during electrophoretic infiltration: A new straightforward process to synthetize SiCf/SiC composites

Silicon carbide fibers-reinforced silicon carbide (SiCf/SiC) composites were synthetized using the electrophoretic infiltration (EPI) process. Slurries included 50wt.% of submicron SiC powder, about 1wt.% of polyethylenimine as dispersant and 45mgmL−1 of allylhydridopolycarbosilane (AHPCS) as pre-ceramic precursor in an ethanol/toluene solvents mixture at pH* 8.9. Formulation of the slurries was optimized by zeta potentials and viscosity measurements. The originality of the process presented herein is that AHPCS was directly infiltrated during the EPI, and not in a second processing step such as polymer impregnation. Densification of the resulting composites was achieved by pyrolysis of the AHPCS at 1600°C under an argon flow. Density measured by the Archimedes’ method was 74% of the SiC theoretical density.

Silicon carbide-based membranes with high soot particle filtration efficiency, durability and catalytic activity for CO/HC oxidation and soot combustion

We report here the solution coatings of Diesel Particulate Filter (DPF) with allylhydridopolycarbosilane (AHPCS)-based polymers leading to supported silicon carbide (SiC)-based membranes with high temperature soot particle filtration efficiency, durability and catalytic activity. In a first part of the present study, our objective was to reduce the pore size of DPF to filtrate finer particles without altering filtration efficiency by coating DPF with an additional fine porous AHPCS-derived SiC membrane. The latter is produced by dip-coating AHPCS on DPF following by a pyrolysis of the AHPCS membrane-modified DPF at 1000°C under argon. We investigated the influence of dip-coating parameters and viscosity of different AHPCS solutions on the SiC membrane-coated DPF by SEM, mercury porosimetry, XRD and high-temperature thermogravimetric analysis. The evolution of the filtration capacity has been determined with a synthetic gas bench. An additional fine SiC membrane (∼150nm in thickness) prepared from a 10vol% of AHPCS in THF deposited on DPF allowed maintaining filtration efficiency as high as the virgin DPF while the pore size of the SiC membrane coated-DPF decreased to filter finer particles. Results are confirmed using a commercially-available polysiloxane (Si–C–O precursor). Furthermore, the SiC membrane acted as a thermal barrier coating and provided a better durability to the DPF by preventing apparition of cracks after heat-treatment to 1500°C under argon. The use of mixed oxide and metallic phases formed in-situ in SiC constitutes one of the solutions to generate new and effective catalytic performances to membranes. Within this context, in a second part of the study, we applied a reverse AHPCS-based microemulsion to combine SiC and oxide phases in the same additional porous membrane. As a proof of concept, we have prepared catalytically active Ce–O–Fe–Pt/SiC membrane coated DPF after dip-coating and pyrolysis under argon. These materials have been characterized and tested with regard to CO/HC oxidation and soot combustion. Ce–O–Fe–Pt/SiC membrane coated DPF showed an activity for CO conversion reaching a light-off temperature T50=270°C and the presence of the catalytic phase allowed burning soot at 486°C.

Mechanical and thermal properties of low temperature sintered silicon carbide using a preceramic polymer as binder

Silicon carbide is used for a variety of applications, however, sintering still remains a challenge due to the high temperature and pressure required as well as the need for sintering aids. The use of preceramic polymers as binder is a promising technique for pressureless low-temperature sintering of SiC without sintering aids. However, the mechanical and thermophysical properties as well as the microstructure of bodies sintered through this technique has not been extensively documented. One of the main polymers which has gained attention in the past few years as a SiC preceramic is allylhydridopolycarbosilane (AHPCS). Here, using AHPCS as binder, silicon carbide pellets were sintered at temperatures as low as 930 °C, and the microstructural, mechanical, and thermophysical property characterization is presented. Compared to conventionally sintered SiC, the material shows similar fracture toughness, lower hardness, strength, and thermal conductivity. The observed properties are explained as a result of residual porosity combined with amorphous SiC at the grain boundaries.

Fabrication of high-surface area nanoporous SiOC materials using pre-ceramic polymer blends and a sacrificial template

We report the fabrication of silicon oxycarbide (SiOC) ceramics with high surface area and porosity and a hierarchical pore structure. They have been synthesized using polymer blends that consist of allyl hydridopolycarbosilane (AHPCS) and hydridopolycarbosilane (HPCS) as the precursors. Layered double hydroxides (LDHs), modified by sodium dodecylbenzenesulfonate (SDBS), a common surfactant, are used as a sacrificial template, and a simple impregnation technique is employed to enable the polymer precursor to penetrate into the LDH structure. Various characterization methods, such as XRD, XPS, and SEM-EDX, are used to verify that the ceramics that are produced are SiOC materials. A key aspect of the fabrication process is the use of pre-ceramic polymer blends that are capable of producing ceramics with an interconnected porous space. Air calcination of the as-prepared SiOC ceramic removes any free carbon that is present, but preserves the pore structure of the material. The SEM images indicate that the materials' internal pore structure consists of well-aligned, slit-like pores. Nitrogen sorption measurements demonstrate that the material fabricated from polymer blend AHPCS/HPCS = 2:1 has surface areas as high as 811.7 m2/g, total pore volume as large as 0.80 cm3/g with considerable fractions of micro- and mesopores. The synthesis method that we have developed uses low-cost pre-ceramic polymer precursors and templates and generates porous SiOC ceramics with high surface area, interconnected pore space with a multi-modal pore size distribution, and high-temperature stability. As such, the technique may be considered as a convenient and cost-effective approach for the fabrication of a wide class of porous materials for such applications as catalysis, gas adsorption/separation under harsh conditions, and biomedical device uses, etc.

One-step deposition of ultrafiltration SiC membranes on macroporous SiC supports

We fabricated nearly defect-free SiC membranes for potential ultrafiltration applications by conducting pyrolysis of allylhydrido polycarbosilane in the presence of submicron α-SiC particles. The SiC membranes were developed on commercial macroporous SiC supports by a low-temperature-process in which allylhydrido polycarbosilane acted to bond together crystalline α-SiC particles to form a porous layer. The suspensions of α-SiC powder and allylhydrido polycarbosilane in n-hexane or n-hexane/n-tetradecane were used for membrane fabrication by dip-coating. By using optimized n-hexane suspension with 5% w/w α-SiC powder and mass ratios of allylhydrido polycarbosilane to α-SiC powder of 0.6 and 0.8, we obtained nearly defect-free and uniform mesoporous membranes in a single coating procedure. No defects were found on the surface of these membranes by scanning electron microscopy. Moreover, during filtration tests, these membranes showed a water permeance of 0.05–0.06L (hm2bar)–1 and retention higher than 93% for polyethylene glycol (PEG) with a molecular mass of 100kDa. Retention for PEG with molecular masses of 1kDa, 8kDa and 35kDa was between 25% and 71%.

Mechanical characterization of fine grained silicon carbide consolidated using polymer pyrolysis and spark plasma sintering

In this study fine grained bulk silicon carbide ceramics are processed using a novel approach that involves pyrolysis of a preceramic polymer followed by spark plasma sintering (SPS). Allylhydridopolycarbosilane (AHPCS) is used as the preceramic polymer that is pyrolyzed under inert conditions to produce SiC powder. Fourier transform infrared spectroscopy (FTIR) is used to confirm complete conversion of the preceramic polymer to amorphous SiC at 1400°C. Subsequently, spark plasma sintering (SPS) is used to compact the SiC powder at temperatures ranging from 1600 to 2100°C at a uni-axial pressure of 70MPa and a soak time of 10min. In situ crystallization of amorphous SiC using SPS results in fine grained structure with grain size ranging from 97 to 540nm. Close to theoretical density materials are obtained, at the higher sintering temperatures, with mechanical properties that exceed those of commercially processed sintered silicon carbide.

Single-source-precursor synthesis of dense SiC/HfC(x)N(1-x)-based ultrahigh-temperature ceramic nanocomposites.

A novel single-source precursor was synthesized by the reaction of an allyl hydrido polycarbosilane (SMP10) and tetrakis(dimethylamido)hafnium(iv) (TDMAH) for the purpose of preparing dense monolithic SiC/HfC(x)N(1-x)-based ultrahigh temperature ceramic nanocomposites. The materials obtained at different stages of the synthesis process were characterized via Fourier transform infrared (FT-IR) as well as nuclear magnetic resonance (NMR) spectroscopy. The polymer-to-ceramic transformation was investigated by means of MAS NMR and FT-IR spectroscopy as well as thermogravimetric analysis (TGA) coupled with in situ mass spectrometry. Moreover, the microstructural evolution of the synthesized SiHfCN-based ceramics annealed at different temperatures ranging from 1300 °C to 1800 °C was characterized by elemental analysis, X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). Based on its high temperature behavior, the amorphous SiHfCN-based ceramic powder was used to prepare monolithic SiC/HfC(x)N(1-x)-based nanocomposites using the spark plasma sintering (SPS) technique. The results showed that dense monolithic SiC/HfC(x)N(1-x)-based nanocomposites with low open porosity (0.74 vol%) can be prepared successfully from single-source precursors. The average grain size of both HfC0.83N0.17 and SiC phases was found to be less than 100 nm after SPS processing owing to a unique microstructure: HfC0.83N0.17 grains were embedded homogeneously in a β-SiC matrix and encapsulated by in situ formed carbon layers which acted as a diffusion barrier to suppress grain growth. The segregated Hf-carbonitride grains significantly influenced the electrical conductivity of the SPS processed monolithic samples. While Hf-free polymer-derived SiC showed an electrical conductivity of ca. 1.8 S cm(-1), the electrical conductivity of the Hf-containing material was analyzed to be ca. 136.2 S cm(-1).

Characterization of the Evolution and Properties of Silicon Carbide Derived From a Preceramic Polymer Precursor

This study reports on the fabrication and characterization of polymer‐derived amorphous and nano‐grained SiC, by controlled pyrolysis of allylhydridopolycarbosilane (AHPCS) under inert atmosphere. Processing temperatures and hold times at final temperatures are varied to study the influence of processing parameters on the structure and resulting properties. Chemical changes, phase transformations, and microstructural changes occurring during the pyrolysis process are studied. Polymer cross‐linking and polymer to ceramic conversion is studied using infrared spectroscopy (FTIR). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are performed to monitor the mass loss and phase change as a function of temperature. X‐ray diffraction studies are performed to study the intermediate phases and microstructural changes. Hardness and modulus measurements are carried out using instrumented nanoindentation to establish processing‐property‐structure relationship for SiC derived from the polymer precursor. It is seen that the presence of nanocrystalline domains in amorphous SiC significantly influences the modulus and hardness. A nonlinear relationship is observed in these properties with optimal mechanical properties observed for SiC processed to 1150°C for 1 h hold duration, having average grain size of 3 nm. In addition, bulk mechanical characterization, in terms of biaxial flexure strength, is done for SiC–SiC particulate composites purely derived from the polymer precursor.

Preparation and dielectric properties of polymer-derived SiCTi ceramics

SiCTi ceramics were prepared by a polymer-derived-ceramic route, with allylhydridopolycarbosilane (AHPCS) and bis(cyclopentadienyl) titanium dichloride (Cp2TiCl2) as starting materials. The cross-linking and ceramization of the AHPCS/Cp2TiCl2 hybrid precursors were characterized by means of FT IR, NMR, TGA and EDS. The results indicate that the cross-linking of hybrid precursors was significantly catalyzed by using Cp2TiCl2 as a catalyst, which might be responsible for a high ceramic yield of 80.8% at 1200 °C. The polymer-to-ceramic conversion was completed at 900 °C to give an amorphous ceramic. The chemical composition of the final ceramics could be tailored by the weight ratio of Cp2TiCl2 to AHPCS in feed. The microstructure and dielectric properties of final SiCTi ceramics were investigated by means of XRD, Raman spectroscopy and vector network analyzer. The results indicate that the 1600 °C SiCTi ceramics are composed of amorphous SiCTi, SiC crystal, TiC crystal and graphite. The dielectric loss of SiCTi is up to 0.34, which is 6 times higher than that of SiC (0.058), indicating that the SiCTi ceramics are promising wave-absorbing materials.

Organic Microchemical Performance of Solvent Resistant Polycarbosilane Based Microreactor

We report the successful fabrication of preceramic polymer allylhydridopolycarbosilane (AHPCS) derived microchannels with excellent organic solvent resistance and optical transparency via economic imprinting process, followed by UV and post thermal curing process at 160 degrees C for 3 h. The microchemical performance of the fabricated microreactors was evaluated by choosing two model micro chemical reactions under organic solvent conditions; syntheses of 2-aminothiazole in DMF and dimethylpyrazole in THF, and compared with glass-based microreactor having identical dimensions and batch system with analogy. It is clear that AHPCS derived microreactor showed excellent solvent resistance and chemical stability compare with glass derived microreactor made by high cost of photolithography and thermal bonding process. The novel preceramic polymer derived microreactors showed reliable mechanical and chemical stability and conversion yields compare with that of glass derived microreactors, which is very promising for developing an integrated microfluidics by adopting available microstructuring techniques of the polymers.

Silicate glass coated microchannels through a phase conversion process for glass-like electrokinetic performance

The surface modified polydimethylsiloxane (PDMS) microchannels show a much more inferior performance to the durable and reproducible glass chip. In this paper, a facile approach to preparing a silicate glass modified PDMS microchannel for glass-like performance is presented. This glass-like performance is made possible by a phase conversion of a preceramic polymer--allylhydridopolycarbosilane (AHPCS). The, several hundred nanometer thick, polymer that coats the PDMS channel is hydrolyzed to form hydrophilic silicate glass via phase conversion under an aqueous alkali condition. It is characterized by XPS, FTIR-ATR, AFM, and contact angle measurements. The silicate glass coated PDMS channel from AHPCS has an excellent solvent resistance, delivers a high electroosmotic flow (EOF) that is stable in the long-term (4.9±0.1×10(-4) cm(2) V(-1) s(-1)) and a reliable capillary electrophoresis (CE), which are comparable to those of native glass channels. Moreover, the silicate glass PDMS channel allows easy regeneration of the electrokinetic behavior, just as in a glass channel, by a simple treatment with alkali solution. This coating approach can be applied to other polymer substrates such as polyimide (PI).

Processing of polymer-derived porous SiC body using allylhydridopolycarbosilane (AHPCS) and PMMA microbeads

A simple method for making a porous SiC body through a polymeric route at one firing processing was demonstrated by the sacrificial filler template approach. By incorporating sacrificial pore-forming plastic powder, PMMA microbeads, into liquid preceramic polymer, AHPCS, and losing it after the polymer was hardened, a preceramic porous body was formed. As a consequence of systematic examination of the effects of particle size and mixture ratio of the powder, an AHPCS-derived porous SiC body was reproducibly formed without critical crack initiation. It was enabled only when the polymer was sufficiently contained to make a strong skeletal structure with the sacrificial plastic particles closely distributed. The porous microstructure would contribute to efficient gas emission and uniform thermal shrinkage during polymer pyrolysis, which reduced internal pressure and crack-causing thermal stress.

Template Synthesis of 3D High-Temperature Silicon-Oxycarbide and Silicon-Carbide Ceramic Photonic Crystals from Interference Lithographically Patterned Organosilicates

The fabrication of 3D diamond-like silicon-oxycarbide and silicon-carbide high-temperature ceramic photonic crystals has been achieved by a strategy involving (1) the use of four-beam interference lithography (IL) to construct a patterned silsesquioxane (POSS) template and (2) infiltration of the polymeric allylhydridopolycarbosilane (AHPCS) silicon-carbide precursor into the patterned POSS template followed by high temperature ceramic conversion and HF etching. Energy-dispersive X-ray mapping analysis and Fourier transform infrared (FT-IR) studies suggested that the 3D ceramic photonic crystals formed at 1100 °C were SiC-like silicon oxycarbide. Additional thermal treatment at 1300 °C in vacuo resulted in the carbothermic reduction of the 3D silicon-oxycarbide to form 3D β-SiC with less than 10% shrinkage in the (111) plane and [111] direction, respectively. The reflectivities of the inverse 3D ceramic photonic crystals obtained at different stages were characterized by FT-IR in the [111] direction. Both the inverse 3D silicon-oxycarbide and silicon-carbide crystals showed bandgaps at 1.84 μm. These experimental values matched well with the calculated bandgaps, further supporting the robustness of such fabricated 3D ceramic photonic crystals.

Allylhydridopolycarbosilane (AHPCS) as matrix resin for C/SiC ceramic matrix composites

In present study, partially allyl-substituted hydridopolycarbosilane (5 mol% allyl) [AHPCS] has been characterized by spectral techniques and thermal analysis. The DSC studies show that, the polymer is self-cross-linking at lower temperatures without any incorporation of cross-linking agents. The spectral and thermal characterizations carried out at different processing stages indicate the possibility of extensive structural rearrangement accompanied by the loss of hydrogen and other reactions of C and Si containing species resulting in the conversion of the branched chain segment into a 3D SiC network structure. AHPCS gave ceramic residue of 72% and 70% at 900 and 1500 °C respectively in argon atmosphere. XRD pattern of 1500 °C heat-treated AHPCS, indicates the formation of silicon carbide with the particle size of 3–4 nm. AHPCS was used as matrix resin for the preparation of C/SiC composite without any interfacial coating over the T-300 carbon fabric reinforcement. Flexural strength value of 74–86 MPa for C/SiC specimen with density of 1.7 g/cm 3 was obtained after four infiltration and pyrolysis cycles.

Effect of polystyrene on the morphology and physical properties of silicon carbide nanofibers

Silicon carbide nanofibers were fabricated by immersion of anodized aluminum oxide templates in allyl-hydridopolycarbosilane (AHPCS) solutions, and subsequent pyrolysis at 750 °C. The technique offers better control on the final morphology of the nanofibers, hence resolving some of the fabrication difficulties encountered in the past. The effect of polystyrene – the pore former – on the formation of the nanofibers in the confined channels of the templates was studied. Low concentrations of polystyrene in the AHPCS solution resulted in ellipsoidal hollow regions inside the SiC nanofibers, whereas high concentrations resulted in the formation of some hollow fibers. The fibers prepared by adding polystyrene possessed higher surface areas than those without polystyrene. The morphology and surface area of the SiC nanofibers were studied using the SEM, TEM, and BET analyses.