High interfacial thermal resistance induced low thermal conductivity in porous SiC-SiO2 composites with hierarchical porosity

The incorporation of a thermally insulating secondary phase can significantly increase the interfacial thermal resistance attributed to its low intrinsic thermal conductivity and the creation of multiple phonon scattering interfaces between adjacent SiC particles. The newly developed porous SiC-33 wt% SiO₂ composites with SiO₂ as a thermally insulating secondary phase exhibited a very low thermal conductivity (0.047 Wm⁻¹ K⁻¹, 72.4 % porous), which is an order of magnitude lower than the previously reported lowest thermal conductivity (0.14 Wm⁻¹ K⁻¹, 76.3 % porous) for powder processed porous SiC ceramics and is even lower than the thermal conductivity (0.060 Wm⁻¹ K⁻¹, 87.9% porous) of SiO₂ aerogel. The porous SiC-(16–73 wt%) SiO₂ composites processed from nano β-SiC and a 40 wt% carbon template exhibited a hierarchical (meso-/macro-porous) pore structure that transformed to a trimodal (micro-/meso-/macro-porous) porous structure when polysiloxane was added and sintering was performed at 600–1000 °C in air.     

Magnesiothermal synthesis of nanostructured SiC from natural zeolite (clinoptilolite) and mesoporous carbon CMK-1

Magnesiothermal synthesis of nanosized SiC has been successfully achieved from a mixture of the natural zeolite clinoptilolite and a high-surface area mesoporous carbon CMK-1, synthesized by impregnating a mesoporous silica template MCM-48 with sucrose, followed by carbonization in argon and removal of the silica template. Magnesium powder was used to initiate the self-combustion reaction. Removal of the alkaline and alkaline earth exchangeable cations from the clinoptilolite by ion exchange with NH₄⁺ was essential for a good yield of product, which is also dependent on the use of excess of carbon. TEM confirmed the nanostructure and size of the 15–25 nm SiC product crystallites.     

The versatile synthesis method for hierarchical micro‐ and mesoporous zeolite: An embedded nanocarbon cluster approach

In this work, we are reporting for the first time the synthesis of hierarchical micro‐ and mesoporous zeolite using silica–carbon (SiO₂/C) composites prepared by pyrolysis of carbonaceous gases in the presence of silica gel. The pyrolysis effectively yielded carbon deposited onto the raw silica material. The obtained SiO₂/C composites were utilised as a bifunctional material, mesoporous template and silica source, for the zeolite synthesis. Tetrapropylammonium hydroxide (TPAOH) was used as a microporous template. The combination of the obtained composites and the TPAOH for the hydrothermal synthesis resulted in the formation of hierarchical micro‐ and mesoporous ZSM‐5. The results from the SEM, TEM, and N₂ adsorption/desorption isotherms, and ²⁷Al MAS NMR characterisations of the synthesised samples obtained after the removal of the templates confirmed the successful formation of the micro‐ and mesoporous zeolites. The mesoporosity of the zeolites could be controlled by adjusting the carbon content in the SiO₂/C composites while the carbon content could be controlled by varying the deposition time and the concentration of the carbonaceous gases used. This controllable and efficient synthesis method is considered to be a promising method for creating hierarchical micro‐ and mesoporous zeolites. © 2011 Canadian Society for Chemical Engineering     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

C/C–SiC composite prepared by Si–10Zr alloyed melt infiltration

A high performance and low cost C/C–SiC composite was prepared by Si–10Zr alloyed melt infiltration. Carbon fiber felt was firstly densified by pyrolytic carbon using chemical vapor infiltration to obtain a porous C/C preform. The eutectic Si–Zr alloyed melt (Zr: 10 at.%, Si: 90 at.%) was then infiltrated into the porous preform at 1450 °C to prepare the C/C–SiC composite. Due to the in situ reaction between the pyrolytic carbon and the Si–Zr alloy, SiC, ZrSi₂ and ZrC phases were formed, the formation and distribution of which were investigated by thermodynamics. The as-received C/C–SiC composite, with the flexural strength of 353.6 MPa, displayed a pseudo-ductile fracture behavior. Compared with the C/C preform and C/C composite of high density, the C/C–SiC composite presented improved oxidation resistance, which lost 36.5% of its weight whereas the C/C preform lost all its weight and the high density C/C composite lost 84% of its weight after 20 min oxidation in air at 1400 °C. ZrO₂, ZrSiO₄ and SiO₂ were formed on the surface of the C/C–SiC composite, which effectively protected the composite from oxidation.     

Novel mesoporous MnO2 for high-rate electrochemical capacitive energy storage

MnO₂ with novel mesoporous structure has been firstly synthesized via a simple in situ reduction process by using different carbon materials as sacrificed template and reducing agent. The morphology and microstructure of as-synthesized mesoporous MnO₂ were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), nitrogen adsorption and desorption experiments. The results demonstrate that porous MnO₂ prepared using mesoporous carbon as template has very large specific surface area and uniform pore-size distribution. The electrochemical measurements showed that novel porous MnO₂ have higher capacity (221 F g⁻¹) with excellent rate and higher capacity retention as electrochemical capacitors (ECs) electrode materials, which may be attributed to the unique nanostrcture of porous MnO_2. These all imply that MnO₂ with novel mesoporous structure has been attractive for practical and large-scale applications in mobile equipment.     

Hierarchical porous nitrogen-doped carbon materials derived from one-step carbonization of polyimide for efficient CO<Subscript>2</Subscript> adsorption and separation

Hierarchical porous nitrogen-doped carbon (HPNC) materials are synthesized through one-step carbonization of polyimide using triblock copolymer P123 as mesoporous template. The microstructure, chemical composition and CO₂ adsorption behaviors are investigated in detail. The results show that HPNC materials have hierarchical micro-/mesopore structures, high specific surface area of 579 m²/g, large pore volume of 0.34 cm³/g, and nitrogen functional groups (5.2 %). HPNC materials exhibit high CO₂ uptake of 5.56 mmol/g at 25 °C and 1 bar, which is higher than those of previously reported nitrogen-doped porous carbon materials. After 5 cycles the value of CO₂ adsorption uptakes is 5.28 mmol/g, which is approximately 95 % of the original adsorption capacity. The estimated CO₂/N₂ selectivity of HPNC materials is 17, revealing great promise for practical CO₂ adsorption and separation applications. The efficient CO₂ uptake and enhanced CO₂/N₂ selectivity are due to the combination of nitrogen-doped and hierarchical porous structures of HPNC materials.     

3D interconnected mesoporous Si/SiO2 coated with CVD derived carbon as an advanced anode material of Li-ion batteries

3D interconnected mesoporous C/Si/SiO₂ composite was synthesized via two facile processes (including magnesiothermic reduction and CVD method) with the SBA-15 as Si source and CH₄ as carbon precursor. The micro/mesopores inside the Si/SiO₂ particles greatly increase the specific surface area, thus shortening the diffusion distance of lithium ions, electron and guaranteeing fast penetration of electrolyte. Raman spectra shows that the CVD derived carbon is partly graphitized, thus largely enhancing its conductivity. Si/SiO₂ coated with two-track carbon (both longitudinal and transverse) and Si/SiO₂ coated with single-track carbon are formed by only changing the experiment order. Such two-track C/Si/SiO₂ electrode for lithium-ion battery exhibits 63% higher capacity than the single-track C/Si/SiO₂ electrode at high-rate performance, attributed to the fast electrochemical reaction kinetics and electron transport rendered by the conductive 3D interconnected structure. This functional structure can effectively accommodate the volume expansion of Si/SiO₂ and maintain the electrical connection between electrode and active materials, improving the cycling stability of active materials.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

The structure evolution of hollow SiOC ceramic microspheres prepared with solvothermal method

Hollow silicon oxycarbide (SiOC) ceramic microspheres were synthesized through solvothermal process of vinyltriethoxysilane in NaOH solution with subsequent pyrolysis at high temperature. Increasing the synthesis temperature not only reduces the Si–C bond and C content in SiOC ceramics, but also transforms the amorphous SiOC ceramics into cristobalite SiO₂ after carbonization. The rearrangement reaction of oxygen-enriched structural units results in the increase of SiO₂C₂ unit. No phase separation occurs at 1400 °C, and SiC nanocrystals are mainly come from the carbothermal reduction reaction of SiO₂ with free C. The size change of SiO₂ nanograins were further investigated by HF etching. The porous carbon is obtained after removal of SiO₂, while HF etching has no effect on the structure of free C. The C content affects the structure evolution of SiOC ceramics significantly. Although the size of SiO₂ grows as increase of pyrolysis temperature, the high C content inhibits the crystallization and growth of SiO₂ during the pyrolysis process.     

Direct fabrication of mesoporous carbons using in-situ polymerized silica gel networks as a template

Mesoporous carbons were synthesized by in-situ polymerized silica gel networks as a template. The co-condensation of carbon precursor (sucrose) and silica precursor (sodium silicate) followed by heat treatment generated a carbon/silica nanocomposite. After etching the silica template, mesoporous carbons were obtained. Under optimum synthesis conditions a mesoporous carbon with a high surface area of >800 m²/g and a narrow pore size distribution centered at 3 nm was produced. The three-dimensionally interconnected silica structures effectively functioned as the template for the porous carbon materials.     

Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding

An in situ reaction bonding technique was developed to fabricate mullite-bonded porous silicon carbide (SiC) ceramics in air from SiC and α-Al₂O₃, using graphite as the pore-former. Graphite is burned out to produce pores and the surface of SiC is oxidized to SiO₂ at high temperature. With further increasing the temperature, the amorphous SiO₂ converts into cristobalite and reacts with α-Al₂O₃ to form mullite (3Al₂O₃·2SiO₂). SiC particles are bonded by the mullite and oxidation-derived SiO₂ to obtain porous SiC ceramics. The reaction bonding behavior, open porosity, pore size distribution and mechanical strength of porous SiC ceramics were investigated as a function of the sintering temperature, forming pressure and graphite content. In addition, the phase composition and microstructure were also studied.     

Synthesis of resorcinol-formaldehyde/silica composite aerogels and their low-temperature conversion to mesoporous silicon carbide

Resorcinol-formaldehyde/silica composite (RF/SiO₂) gels were acid-catalyzed formed in one pot at 27 °C within 60 min, and then dried to aerogels with supercritical fluid CO₂. After carbonization in nitrogen atmosphere and a magnesiothermic reaction at 700 °C, RF/SiO₂ aerogels were successfully converted to monolithic mesoporous silicon carbide (SiC aerogels). The starting RF/SiO₂ aerogels had an interpenetrating organic/inorganic network and the resulting SiC products preserved monolithic mesoporous morphology similar to the original templates. The as-synthesized SiC aerogels consisted of nanocrystalline β-SiC, possessed a BET surface area of 232 m²/g and showed sufficient mesoporosity. They had a direct band gap of about 3.2 eV (less than that of bulk β-SiC) and showed photoluminescence at room temperature. The mechanisms about the formation of RF/SiO₂ gels and the conversion to SiC aerogels were discussed. Potentially, the reported method can be used to convert many metal or semimetal oxide/carbon composite aerogels to carbide aerogels at relatively low temperature for many catalytic, electronic, photonic and thermal applications.     

Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogel was synthesized by sol–gel process followed by supercritical drying (SCD). Monolithic SiC aerogel was obtained from RF/SiO₂ aerogel after carbothermal reduction. The evolution of physical property, crystal structure, morphology and pore structure from RF/SiO₂ to SiC aerogel was investigated by different methods, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and N₂ adsorption/desorption. The as-synthesized SiC aerogel presented typical mesoporous structure and possessed high porosity (91.8%), high surface area (328 m²/g) and large pore volume (2.28 cm³/g). Carbothermal reduction mechanism was also discussed based on the experiment and characterization results.     

Nanocasting Synthesis of Ultrafine WO<Subscript>3</Subscript> Nanoparticles for Gas Sensing Applications

Ultrafine WO₃ nanoparticles were synthesized by nanocasting route, using mesoporous SiO₂ as a template. BET measurements showed a specific surface area of 700 m²/gr for synthesized SiO_2, while after impregnation and template removal, this area was reduced to 43 m²/gr for WO3 nanoparticles. HRTEM results showed single crystalline nanoparticles with average particle size of about 5 nm possessing a monoclinic structure, which is the favorite crystal structure for gas sensing applications. Gas sensor was fabricated by deposition of WO₃ nanoparticles between electrodes via low frequency AC electrophoretic deposition. Gas sensing measurements showed that this material has a high sensitivity to very low concentrations of NO₂ at 250°C and 300°C.     

Worm-hole structured mesoporous carbon monoliths synthesized with amphiphilic triblock copolymer

In the present work, mesoporous carbon monoliths with worm-hole structure had been synthesized through hydrothermal reaction by using amphiphilic triblock copolymer F127 and P123 as templates and resole as carbon precursor. Synthesis conditions, carbonization temperature and pore structure were studied by Fourier transform infrared, thermogravimetric analysis, transmission electron microscopy and N₂ adsorption–desorption. The results indicated that the ideal pyrolysis temperature of the template is 450 °C. The organic ingredients were almost removed after further carbonized at 600 °C and the mesoporous carbon monoliths with worm-hole structure were obtained. The mesoporous carbon synthesized with P123 as single template exhibited larger pore size (6.6 nm), higher specific surface area (747 m² g^?1), lower pore ratio (45.9 %) in comparison with the mesoporous carbon synthesized with F127 as single template (with the corresponding value of 4.9 nm, 681 m² g^?1, 49.6 %, respectively), and also exhibited wider pore size distribution and lower structure regularity. Moreover, the higher mass ratio of template P123/resole induced similar pore size, larger specific surface area and lower pore ratio at the same synthesizing condition. It was also found that the textural structure of mesoporous carbon was affect by calcination atmosphere.     

Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template,graphitization catalyst,and activating agent

Nitrogen-doped porous graphitic carbons (NPGCs) with controlled structures were synthesized using cheap nano-CaCO₃ as template, melamine-formaldehyde resin as carbon precursor, and dilute HCl as template removing agent. In addition to its use as a template, the nano-CaCO₃ acted as an internal activating agent to produce micro- and mesopores, as an adsorbent to remove the released hazardous gases (i.e. HCN, NH₃), and as a mild graphitization catalyst. The obtained NPGCs with hierarchical nanopores contained as high as 20.9 wt% of nitrogen, had surface areas of up to 834 m² g^–1, and also exhibited high thermal stability with respect to oxidation. Using carbohydrate or phenolic resin as the carbon precursor, this simple approach was also capable of producing hierarchical porous graphitic carbons with high surface area (up to 1683 m² g^–1) and extremely large pore volumes (>6 cm³ g^–1). X-ray diffraction and infrared spectroscopy suggested that the intermediate CaCN₂ or CaC₂ generated during the carbonization plays a critical role in the formation of the graphitic structure.     

CuO Impregnated mesoporous silica KIT-6: a simple and efficient catalyst for benzene hydroxylation by C–H activation and styrene epoxidation reactions

Copper oxide doped mesoporous KIT-6 materials were synthesized by ultrasonication as impregnation method. The highly ordered nature of mesoporous CuO-KIT-6 materials analyzed by low angle X-ray diffraction. The high surface area, pore diameter and mesoporous nature of synthesized materials were confirmed by BET surface area analysis. Si–O–Si, Si–OH and Cu–O–Si bonds in the framework of CuO-KIT-6 were verified by FTIR spectroscopy. The Cu²⁺ ← O^2? charge-transfer transitions and d–d transitions of dispersed Cu²⁺ on ordered mesoporous KIT-6 were identified by DRS UV–Vis spectroscopy. The morphology of the synthesized CuO-KIT-6 materials was analyzed by HR-SEM. The 3D ordered nature of CuO-KIT-6 confirmed by TEM analysis. The highly ordered 3D mesoporous CuO-KIT-6 materials are excellent catalyst for benzene hydroxylation reaction through C-H activation and styrene epoxidation reaction with 30 % aqueous H₂O₂. The catalyst CuO-KIT-6 itself showed a good conversion towards both reactions.     

Synthesis of Silicon Carbide Nanocrystals Using Waste Poly(vinyl butyral) Sheet

SiC nanocrystals were prepared using waste poly(vinyl butyral) sheet as a carbon source. SiO₂/poly(vinyl butyral) mixtures are converted to SiO₂/pyrolytic carbon composites via pyrolysis at low temperatures (500°C) in an Ar atmosphere. Subsequently, low‐temperature magnesiothermic reduction and purification processes result in the formation of tiny SiC nanocrystals. The size of the synthesized SiC nanocrystals ranged from 3 to 12 nm, i.e., they are smaller than the SiO₂ precursor offering large specific surface area of 175.76 m²/g and are single phase as 3C–SiC. Hence, 3C–SiC nanocrystals were successfully synthesized using waste poly(vinyl butyral) through this simple, inexpensive, and scalable process, which will be a new application in the recycling industry.     

Highly Porous SiOC Bulk Ceramics with Water Vapor Assisted Pyrolysis

Micro/mesoporous SiOC bulk ceramics with the highest surface area and the narrowest pore size distribution were prepared by water‐assisted pyrolysis of polysiloxane in argon atmosphere at controlled temperatures (1100°C–1400°C) followed by etching in hydrofluoric acid (HF) solution. Their pyrolysis behaviors, phase compositions, and microstructures were investigated by DSC, FTIR, XRD, and BET. The Si–O–Si bonds, SiO₂‐rich clusters, and SiO₂ nanocrystals in the pyrolyzed products act as pore‐forming species and could be etched away by HF. Water injection time and pyrolysis temperature have important effects on phase compositions and microstructures of the porous SiOC bulk ceramics, which have a maximum‐specific surface area of 2391.60 m²/g and an average pore size of 2.87 nm. The porous SiOC ceramics consist of free carbon phase, silicon carbide, and silicon oxycarbide.