Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si₃N₄ powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si₃N₄ composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si₃N₄ particles.     

Porous organic and carbon xerogels derived from alkaline aqueous phenol–formaldehyde solutions

Porous carbons were synthesized from the low cost precursors phenol and formaldehyde in aqueous solution, catalysed with NaOH. The xerogels were ambient pressure dried without prior solvent exchange. For an excess of formaldehyde, the resulting carbon xerogels show porosities up to 90%, with a predominant fraction of macroporosity, specific surface areas up to 768?m²/g and mesopore volumes up to 0.59?cm³/g. Small-angle X-ray scattering reveals fractal behaviour for many samples. Relevant sodium impurities up to 3?C4?wt% in the organic and carbon samples cannot be avoided, even by washing of the samples. As possible reason for the high sodium content, the formation of crown ether like calix [6] arene is assumed. Overall, the sodium impurities detected confine the use of this system to applications, where higher ash contents are uncritical.     

Characterization of copper–silicon nitride composite electrocoatings

Silicon nitride particles were incorporated to electrolytic copper by co-electrodeposition in acidic sulfate bath, aiming the improvement of its mechanical resistance. Smooth deposits containing well-distributed silicon nitride particles were obtained. The current density did not show significant influence on incorporated particle volume fraction, whereas the variation of particle concentration in the bath had a more pronounced effect. The microhardness of the composite layers was higher than that of pure copper deposits obtained under the same conditions and increased with the increase of incorporated particle volume fraction. The microhardness of composites also increased with the increase of current density due to copper matrix grain refining. The composite coatings were slightly more corrosion resistant than pure copper deposits in 3.5% NaCl solutions.     

Sol–gel derived alumina–hydroxyapatite–tricalcium phosphate porous composite powders

In this study, alumina–hydroxyapatite–tricalcium phosphate (α-Al₂O₃–HA–TCP) porous composite powders were produced and characterized. At first, boehmite sol (AlOOH) was obtained via sol–gel process by using aluminium isopropoxide (Al(OC₃H₇)₃) as the starting material. Bovine hydroxyapatite (BHA) powders derived from deproteinized bovine bones were added as 10, 20, 30 and 50% weight of the starting material to each boehmite sol. Also Na-alginate was added to the boehmite sol as the dispersive agent. Subsequently, gelation for 3 h at 110 °C was applied to each sol mixture. Finally, gelated samples were heat treated for 2 h at 500, 800, 1000 and 1300 °C. DTA–TGA, XRD, FTIR and SEM-EDS analyses were used to characterize the obtained composite powders composed of α-Al₂O₃–HA–TCP phases. In order to investigate porosity properties, powders were pressed with hydraulic manual press and formed into pellets. Later these pellets were sintered for 2 h at 1300 °C. Apparent porosity and bulk density tests were applied to the pellets. The evaluation of these tests results indicate that a novel α-Al₂O₃–HA–TCP composite material with ～38–44% apparent porosity has been produced.     

Synthesis of SBA 15 graphene oxide composite membrane using phenol–formaldehyde resin pore modifier for CO2 separation

Present study highlights the development of carbon-loaded SBA 15 membrane on clay-alumina tubular support and its performance on the CO₂ separation efficiencies from different mixture gases. To modify the large pores of SBA 15 by graphitic carbon, low molecular weight phenol–formaldehyde (PF) resin was incorporated into the mesoporous channel followed by calcination under inert atmosphere. The modified ordered pore structure of the membrane has been characterized by low-angle XRD, TEM, and pore size distribution analysis. The chemical state of the deposited carbon phase into the SBA 15 pores was analyzed by X-ray photoelectron and Raman spectroscopy. Carbon having graphitic nature mainly in graphene oxide has been deposited into the mesopore of SBA 15 resulting decrease in pore size from 8.9 to 1.0 nm. Finally, the developed SBA 15 carbon membranes were characterized by CO₂ permeation and separation selectivity of CO₂/CH₄, CO₂/CO. Highest CO₂/CH₄ separation factor was achieved as 16.9 with CO₂ permeance 13.6 × 10^–8 mol/m²/s/Pa at 200 kPa feed pressure by the 20% resin with 2 times coated membrane. In flue gas analysis, highest CO₂/CO separation factor of 32.8 was achieved. This study offers an observation on CO₂ separation from simulated BF gas for the first time and the results show the potential of the developed SBA 15/C composite membranes in commercial application.     

A two-step synthesis of ordered mesoporous resorcinol–formaldehyde polymer and carbon

A new two-step method is developed for the synthesis of resorcinol–formaldehyde polymer and carbon with highly ordered mesoporous structures. For this method, resorcinol and formaldehyde is pre-polymerized in the first step under the presence of a basic catalyst to produce resorcinol–formaldehyde resol. Then, the resorcinol–formaldehyde resol is mixed with Pluronic F127 solution followed by the addition of an acid catalyst to allow the rapid self-assembly and condensation in the second step. Compared with the early reported evaporation-induced self-assembly method as well as the one-step liquid phase self-assembly method, in the present two-step liquid method the self-assembly and condensation process can be carried out rapidly by using low amount of base and acid catalysts at room temperature. After the activation by CO₂, the carbon materials maintained ordered mesostructure, and the BET surface area enlarged to 2660 m²/g and total pore volume increased to 2.01 cm³/g. The CO₂ activation not only creates micropores within the carbon frameworks but also enlarges the mesopores by elimination of the carbon pore walls.     

Effect of activation on the carbon fibers from phenol–formaldehyde resins for electrochemical supercapacitors

Activated carbon fibers (ACF) are prepared from phenol–formaldehyde resin fibers through chemical activation and physical activation methods. The chemical activation process consisted of KOH, whereas the physical activation was performed by activation in CO₂. The characteristics of the electrochemical supercapacitors with carbon fibers without activation (CF), carbon fibers activated by CO₂ (ACF-CO₂), and carbon fibers activated by KOH (ACF-KOH) have been compared. The activated carbon fibers from phenol–formaldehyde resins present a broader potential range in aqueous electrolytes than activated carbon and other carbon fibers. Activation does not produce any important change in the shape of starting fibers. However, activation leads to surface roughness and larger surface areas as well as an adapted pore size distribution. The higher surface areas of fibers treated by KOH exhibited higher specific capacitances (214 and 116 F g⁻¹ in aqueous and organic electrolytes, respectively) and good rate capability. Results of this study suggest that the activated carbon fiber prepared by chemical activation is a suitable electrode material for high performance electrochemical supercapacitors.     

The porous composite BN@SHS made of boron nitride,silica hollow spheres and Si–O–B interface

Journal of Porous Materials - Calcined silica SiO2 hollow spheres (SHS), with a specific surface area of 523 m2 g?1, were used as porous scaffolds of ammonia borane (AB). AB is used...     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Synthesis and evaluation of a ruthenium–copper oxide/silica catalyst derived from microemulsion processing

Bimetallic Ru–Cu catalysts supported on SiO₂ have been synthesized in microemulsions using sodium metasilicate (Na₂SiO₃), copper nitrate (Cu(NO₃)_2·3H₂O) and ruthenium chloride (RuCl₃) at 28 °C. The microemulsion system consists of sodium 1,4‐bis(2‐ethylhexyl) sulfosuccinate (AOT) and sodium dodecyl sulphate (SDS), cyclohexane, and an aqueous solution of sodium metasilicate or metal salts. The catalysts have been characterized by XPS, EDX/SEM with line scanning and they possess a very narrow pore size distribution (around 38 Å) and relatively high specific surface areas (around 400 m²/g). The catalytic results of the N₂O decomposition reveal that higher conversions of N₂O can be achieved by the catalysts synthesized from the microemulsion process at lower temperatures (around 400 °C).     

Synthesis of carbon–silica core–shell nanofibers from a dispersion of fluorinated carbon nanofibers in solvated polysiloxane

Carbon–silica core–shell fibers (which unusually consist of carbon nanofibers coated with silica) were synthesized using a two-step process. First, fluorination of carbon nanofibers (CNFs) allows their homogenous dispersion into a polysiloxane matrix. A longlife dispersion of nanofibers in solvated polysiloxane has been prepared. Second, the polysiloxane/fluorinated carbon was thermally treated in air until 700 °C. Defluorination and conversion of polysiloxane into silica occur and result in carbon–silica core–shell fibers. The thermal treatment of the polysiloxane/carbon and the resulting silica/carbon–silica core–shell nanostructures were investigated using solid state nuclear magnetic resonance using ¹⁹F, ¹³C ¹H, and ²⁹Si nuclei, X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies.     

Nano porous structure of resorcinol–formaldehyde xerogels and aerogels: effect of sodium dodecylbenzene sulfonate

In the past two decades, resorcinol?Cformaldehyde (RF) gels have found widespread applications owing to their low density and adjustable pore size. They are usually prepared through sol?Cgel polymerization of the monomers in an aqueous media followed by evaporative or supercritical drying. In this study, RF gels were synthesized via sol?Cgel polymerization in the presence of sodium dodecylbenzene sulfonate (NaDBS) followed by ambient and supercritical drying. Dimensional measurements along with N₂ sorption analysis and Scanning electron microscopy (SEM) micrographs revealed that pore structure of the gel is chiefly affected by NaDBS. In all samples (xerogels and aerogels), maximum densities were observed at a critical NaDBS concentration (~1?w/v%), whereas considerable pore size increments and pore size distribution broadenings were found at higher concentrations of NaDBS (??5?w/v%). The most increased mesopore volumes were detected in xerogels (133% for acetone-dried and 67% for water-dried samples), while concerning aerogels, the pore sizes enlargement to macropore regime was observed at 5?w/v% of NaDBS. SEM micrographs, in agreement with porosity analysis, depicted that very large pore volumes could be obtained when supercritical drying was employed. However, in the case of xerogels, a more dense structure with smaller pores (micro and mesopores) exists which can only be altered slightly when using large amounts of NaDBS. The results showed that the RF gel pore texture, independent of drying technique, was strongly influenced by the addition of NaDBS, which should be taken into account when using this surfactant in the gel formulation for a wide variety of applications.     

Synthesis and characterization of sol–gel derived calcium hydroxyapatite thin films spin-coated on silicon substrate

The goal of this study was to demonstrate that sol–gel processing route is suitable for the fabrication of calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, CHA) thin films on Si substrate by spin-coating technique. The substrate was spin-coated by precursor sol solution 1, 5, 15 and 30 times. The samples were annealed after each spin-coating procedure at 1000 °C for 5 h in air. In the sol–gel process ethylendiamintetraacetic acid and 1,2-ethandiol, and triethanolamine and polyvinyl alcohol were used as complexing agents and as gel network forming agents, respectively. The coatings were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) and Raman spectroscopies, profilometry and the contact angle measurements (CAM). It was demonstrated, that properties of calcium hydroxyapatite thin films depend on spinning and annealing times.     

Synthesis and characterization of sol–gel derived continuous spinning alumina based fibers with silica nano-powders

Silica nano-powders were used as the Si source to substitute acidic silica sol in the SiO₂–Al₂O₃ sol mixture for benefiting the sintering of the fibers at 1250 °C. With the increase of nano-silica, grain diameter and porosity of the fibers decreased firstly and increased subsequently, a minimum value of 35.86 nm and 0.86% was exhibited at the nano-silica content of 20%. The solid content, linear growth model and homogeneity of the precursor sol were not affected by the presence of nano-silica, although the continuous spinning length became low. NMR Analysis of ²⁷Al indicated that polymerization degree of the sol was enhanced by nano-silica. The nano-particle contributed to the reduced intermolecular distance of gel, so the appropriate content existed for resulting the reduced grain size and compact structure of the fibers.     

Improved cyclability of lithium–sulfur battery cathode using encapsulated sulfur in hollow carbon nanofiber@nitrogen-doped porous carbon core–shell composite

Hollow carbon nanofiber@nitrogen-doped porous carbon (HCNF@NPC) core–shell composite, which was carbonized from HCNF@polyaniline, was prepared as an improved high conductive carbon matrix for encapsulating sulfur as a cathode composite material for lithium–sulfur batteries. The prepared HCNF@NPC-S composite with high sulfur content of 77.5 wt.% showed an obvious core–shell structure with an NPC layer coating on the surface of the HCNFs and sulfur homogeneously distributed in the coating layer. This material exhibited much better electrochemical performance than the HCNF-S composite, delivered initial discharge capacity of 1170 mAh g⁻¹, and maintains 590 mAh g⁻¹ after 200 cycles at the current density of 837.5 mA g⁻¹ (0.5 C). The significantly improved electrochemical performance of the HCNF@NPC-S composite was attributed to the synergetic effect between HCNF cores, which provided electronic conduction pathways and worked as mechanical support, and the NPC shells with relatively high surface area and pore volume, which could trap sulfur/polysulfides and provide Li⁺ conductive pathways.     

Preparation of highly phenol substituted bio-oil–phenol–formaldehyde adhesives with enhanced bonding performance using furfural as crosslinking agent

Highly phenol substituted bio-oil–phenol–formaldehyde (BPF) adhesives were prepared via the phenolization-copolymerization method, in which furfural was used as a novel crosslinking agent to improve the bonding performance. The effects of bio-oil percentage, furfural content, curing temperature, and pressure on the performance of BPF adhesives have been studied in detail. A BPF adhesive with 75% bio-oil percentage and 15% furfural loading (named 75BPF_15) was synthesized, which was cured at 180 °C and 4 MPa. Compared to the conventional phenol–formaldehyde adhesive, the wet tensile strength of 75BPF_15 adhesive reached 2.84 MPa, which was significantly higher than the 1.54 MPa of PF. A possible mechanism of BPF adhesives crosslinked with furfural is proposed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46995.     

Binderfree synthesis of high-surface-area carbon electrodes via CO2 activation of resorcinol–formaldehyde carbon xerogel disks: Analysis of activation process

High-surface-area carbon xerogels were prepared in the form of disks via carbonization of precursor resorcinol–formaldehyde (RF) polymer disks and subsequent activation of the resultant RF carbon xerogels by CO₂. RF carbon xerogels allow the preparation of a set of pre-activated carbon disks having different mesopore volumes. Analysis of the relationship between the mesopore volume of the samples and their CO₂ activation efficiency showed that the presence of mesopores is crucial for obtaining a high-surface-area carbon with minimal burn-off of carbon atoms. Activation of an RF carbon disk with a mesopore volume of 1.0 cm³ g⁻¹ up to a burn-off of 81% yielded an activated carbon disk with a high BET surface area of ∼3000 m² g⁻¹. Such disks could be readily used as electrode materials for an electric double layer capacitor without filler or binder addition and exhibited competitive EDLC performance against other electrode materials previously reported.     

Development and characterization of a lignin–phenol–formaldehyde wood adhesive using coffee bean shell

In the present study, the possibility of development of a wood adhesive using coffee bean shell lignin (Cbsl) has been explored. Cbsl-modified phenolic adhesive has been prepared by replacing phenol with lignin at different weight percents. The optimization of weight percent lignin incorporation was carried out with respect to mechanical properties. It was found that up to 50 wt% of phenol could be replaced by Cbsl to give lignin–phenol–formaldehyde adhesive (LPF) with improved bond strength in comparison to control phenol–formaldehyde (CPF). Optimized LPF and CPF adhesives were characterized by IR, DSC and TGA. The IR spectrum of LPF showed structural similarity to CPF. Thermal stability of LPF adhesive was found to be lower as compared to that of CPF. DSC studies revealed a higher rate of curing in the LPF adhesive.     

Synthesis and characterization of copper gallium diselenide powders prepared from the sol–gel derived precursors

Copper gallium diselenide (CuGaSe₂) powders were synthesized via the sol–gel method followed by a selenization process. The sol–gel process can effectively reduce the required synthesis temperature to 400 °C due to enhanced reactivity and improved composition homogeneity. The amount of Cu₂Se impurity phase was decreased when sufficient Ga³⁺ was added to the precursors. CuGaSe₂ powders were successfully prepared when the Ga³⁺/Cu²⁺ molar ratio was increased to 2. The formation of CuGaSe₂ with a pure chalcopyrite structure was confirmed via the Rietveld refinement analysis. With decreasing Ga³⁺/Cu²⁺ molar ratios, the particle size of the prepared CuGaSe₂ powders was significantly enlarged because the copper selenide phase acted as a flux for the particle growth. The optical absorption spectra revealed the obtained CuGaSe₂ to have a band gap of 1.68 eV. The sol–gel method combined with the selenization process was demonstrated to provide a potential approach for fabricating CuGaSe₂ materials.