Polymer-derived SiOC aerogel with hierarchical porosity through HF etching

Silicon oxycarbide (SiOC) aerogels have been synthesized from preceramic polymers via pyrolysis in inert atmosphere at 1200 and 1300 °C. The as synthesized materials have a typical colloidal microstructure with mesoporosity in the range 10–50 nm and no microporosity. HF acid attack of the SiOC aerogels dissolves preferentially the SiO₂-rich phase and creates micro-and (small)mesopores (<10 nm) in the aerogels microstructure finally leading to a materials with hierarchical porosity. The HF post-pyrolysis treatment is more efficient for the SiOC aerogels pyrolyzed at the maximum temperature, i.e. 1300 °C, leading to a maximum value of specific surface area of 530 m²/g and total porosity of 0.649 cc/g.     

Effect of pyrolysis temperature on the pore structure evolution of polysiloxane-derived ceramics

Homogeneous silicon oxycarbide (SiOC) ceramic powders were prepared by pyrolysis of cross-linked polysiloxane at different temperatures (1250–1500 °C) under vacuum. The effect of pyrolysis temperature on the pore structure evolution was investigated by means of N₂ adsorption, SEM, XRD, IR and element analysis (EA). Studies showed that predominate mesoporous ceramics with the average pore size in the range of 2–13 nm were obtained after pyrolysis in this temperature range. The pore structure transformation is strongly correlated with the thermolytic decomposition process of the used precursor, such as phase separation and carbothermal reduction. At relatively lower temperature (1250–1350 °C), the ceramics had a relative small specific surface areas (35 m²/g) owing to the low degree of carbothermal reduction. However, as the carbothermal degree had an obvious augment at relative higher temperature (1400–1450 °C), the specific surface areas and total pore volume increased and reached to the maximum of 66 m²/g and 0.214 cm³/g, respectively, and subsequently decreased rapidly after 1500 °C for the reason of partial sintering of the nano-sized SiC derived from polysiloxane.     

Synthesis of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction

Ammonium ferric citrate (AFC) was used as a single-source molecular precursor to prepare Fe/Fe₃C nanoparticles encapsulated in nitrogen-doped carbon by pyrolysis in Ar atmosphere followed by acid-leaching. Comparative studies, using citric acid and ferric citrate as the precursors, indicated that the ammonia and ferric ion in AFC and the pyrolysis temperature affected the composition of iron species and the properties of carbon in AFC-derived materials. Above the pyrolysis temperature of 600 °C, the iron species were Fe/Fe₃C, and the carbon had a hollow graphitic nanoshell structure in AFC-derived materials. The specific surface area and content of nitrogen element decreased with increasing pyrolysis temperature. The AFC-derived material pyrolyzed at 600 °C had the optimal graphitization degree, specific surface area (489 m² g⁻¹) and content of nitrogen (1.8 wt.%), thus resulted in the greatest activity for oxygen reduction reaction among the AFC-derived materials pyrolyzed at different temperatures. The AFC-derived material pyrolyzed at 600 °C exhibited improved methanol-resistance ability compared with Pt/C catalyst.     

Synthesis,structure, and properties of silicon oxycarbide aerogels derived from tetraethylortosilicate /polydimethylsiloxane

Porous silicon oxycarbide (SiCO) aerogel monoliths were prepared by the sol-gel method using Tetraethylortosilicate/Polydimethylsiloxane as organic-inorganic precursors. The precursor gels were dried by supercritical ethanol fluid and pyrolyzed at 1200 °C in nitrogen atmosphere to form SiCO aerogels. The as-prepared SiCO aerogels are amorphous and present a network microstructure, surface area of 198.04 m²/g, average pore diameter of 56 nm, and pore volume of 0.648 cm³/g. The thermal conductivity of aerogel monoliths is only 0.027 W/m·K at 25 °C (Hot disk method). The atom ratios of Si, C, O elements in the SiCO aerogels are 30.77%, 14.67%, 54.56% respectively. The network microstructure of the SiCO aerogels are retained until 1100 °C, and the chemical groups and crystal phase structures are kept up until 1200 °C. There is only 1.65% of weight-loss until the same heated at 1200 °C in air, which is one of the highest thermally stable temperatures for SiCO aerogels ever reported.     

Silicon carbide-based foams derived from foamed SiC-filled phenolic resin by reactive infiltration of silicon

Macro-cellular porous silicon carbide-based foams were fabricated by reactive infiltration of melt silicon into porous carbonaceous preforms pyrolyzed from foamed SiC-filled phenolic resins (PF). The SiC-filled PF foams were prepared at 80 °C with different heating rate. The effect of heating rate on the foaming behavior of the liquid SiC-filled PF mixture and the microstructure of the foams were investigated. The foamed SiC-filled PF was then pyrolyzed at 1000 °C and infiltrated by melt Si at 1600 °C, leading to the formation of open macro-cellular structure. At a heating rate of 6 °C min⁻¹, Si-infiltrated foams with a porosity of ~72% and a mean pore size of ~0.5 mm were obtained. The Si-infiltrated foams with dense struts mainly inherited the pore structure of pyrolyzed preforms. The main phases of SiC-based foams were α-SiC, β-SiC and the remnant Si, which contributed to high compressive strength of the SiC-based foams.     

A new carbon molecular sieve for propylene/propane separations

A new carbon molecular sieve (CMS) with a propylene/propane separation factor of approximately 27 was synthesized by a facile pyrolysis process from a gel-type strong acid cation exchange resin. The micropore shrinkage process during pyrolysis was investigated using a new high throughput adsorption technique with 48 parallel cells. This significantly reduced the characterization time. The ratio of propylene/propane adsorption rate in the CMS adsorbent changes from 1 to more than 150 when the final pyrolysis temperature changes from 550 to 1000 °C. The best performing CMS pyrolyzed at 850 °C was further characterized using a gravimetric adsorption method. The propylene and propane diffusivities are 1.0 × 10⁻⁹ and 1.1 × 10⁻¹¹ cm² s⁻¹ at 100 kPa and 90 °C. The high propylene/propane diffusivity ratio of 90 is similar to that in zeolite 4A, while the propylene diffusivity was more than 30 times higher than that in zeolite 4A. An effluent of 90 mol% propylene was obtained from a feed of 25 mol% propylene during adsorption/desorption tests using the CMS adsorbent pyrolyzed at 850 °C in a fixed-bed configuration. The new CMS adsorbent is a promising candidate for industrial scale propylene/propane separations.     

Silicon oxycarbide-based composites produced from pyrolysis of polysiloxanes with active Ti filler

Phenyl (PPS) and methyl (PMS) containing polysiloxanes were pyrolyzed at elevated temperatures (900–1500 °C) under argon atmosphere to investigate the phase developments within the polymers. It was found that pyrolysis of the polymers under inert atmosphere up to 1300 °C leads to amorphous silicon oxycarbide (SiO_xC_y) ceramics. Conversions at higher temperatures results in the transformations into the crystalline β-SiC phases. Ceramic matrix composites (CMCs) were developed based on the active filler controlled pyrolysis (AFCOP) of polysiloxanes with active Ti filler additions. CMC monoliths were prepared with 60–80 wt.% of active Ti particulates blended into polymer precursors. Green bodies of the composites were made by warm pressing under 15 MPa pressure and ceramics were obtained by pyrolysis at elevated temperatures between 900 and 1500 °C under argon atmosphere. The results showed that due to the incorporation of active Ti fillers, formation of crystalline phases such as TiC, TiSi, and TiO occured within the amorphous matrix due to the reactions between the Ti and the polymer decomposition products. The microstructural and mechanical characterization results of the composites are presented within the paper.     

Novel Er-doped SiC/SiO2 nanocomposites: Synthesis via polymer pyrolysis and their optical characterization

Erbium activated SiC/SiO₂ nanocomposites doped with Er³⁺ concentrations ranging from 1 to 4 mol% were prepared by pyrolysis of sol–gel derived precursors. The gels were obtained from modified silicon alkoxides containing Si–CH₃ and Si–H groups. Thin discs obtained from the monolithic xerogels were pyrolyzed in an alumina tubular furnace in flowing Ar (100 ml/min) at 800, 1000, 1200 and 1300 °C. The samples were investigated by absorption and photoluminescence spectroscopies. Emission in the C-telecommunication band was observed at room temperature for all the samples upon continuous-wave excitation at 980 or 514.5 nm. The shape of the emission band corresponding to the ⁴I_13/2 → ⁴I_15/2 transition is found to be independent both on Erbium content and excitation wavelength, with a Full Width Half Maximum (FWHM) of 48 nm. By increasing the pyrolysis temperature the intensity of the luminescence increases and the electronic bandgap energy decreases.     

Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions

Activated carbons were prepared from sodium lignosulfonate by phosphoric acid activation at carbonization temperatures of 400–1000 °C. The resulting materials were characterized with regard to their surface area, pore volume, pore size distribution, distribution of surface groups and ability to adsorb copper ions. Activated carbons were characterized by nitrogen adsorption, scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analyses. The results indicate that with increasing carbonization temperature, the surface area decreased from 770 m²/g at 400 °C to 180 m²/g at 700 °C and increased at higher temperatures to 1370 m²/g at 1000 °C. The phosphorus content peaked at 11% for carbon obtained by carbonization at 800 °C. Potentiometric titration revealed the acidic character of all the phosphoric acid-activated carbons, which were found to have total concentrations of surface groups of up to 3.3 mmol/g. The carbons showed a high adsorption capacity for copper ions even at pH values as low as 2.     

Dielectric properties of nano-sized Ba0.7Sr0.3TiO3 powders prepared by spray pyrolysis

Nano-sized Ba_0.7Sr_0.3TiO₃ powders are prepared by post-treatment of the precursor powders with hollow and thin wall structure at temperatures between 900 and 1100 °C. Ethylenediaminetetraacetic acid and citric acid improve the hollowness of the precursor powders prepared by spray pyrolysis. The mean sizes of the powders post-treated at temperatures of 900, 1000 and 1100 °C are 42, 51 and 66 nm, respectively. The densities of the Ba_0.7Sr_0.3TiO₃ pellets obtained from the powders post-treated at 900, 1000 and 1100 °C are each 5.36, 5.55 and 5.38 g cm^?3 at a sintering temperature of 1300 °C. The pellet obtained from the powders post-treated at 1000 °C has higher maximum dielectric constant than those obtained from the powders post-treated at 900 and 1100 °C.     

Short-term oxidation of a ternary composite in the system AlN–SiC–ZrB2

The present study investigates the oxidation behaviour in air of a structural ceramic composite with the following volumetric composition: 55% AlN–15% SiC–30% ZrB₂. This kind of ternary composite is electroconductive (3 × 10⁻⁴ Ω cm) and has significant strength (∼700 MPa) and toughness (4 MPa m^1/2) up to 1000 °C. Oxidation tests were carried out in a TG equipment from 700 to 1300 °C with exposition time of 30 h. Significant weight gain is observed at T > 1000 °C. In the range 700–900 °C, the process is dominated by the oxidation of ZrB₂ into zirconia and boria and the kinetic is nearly parabolic. At temperatures in the range 1000–1100 °C, boria reacts with alumina forming aluminium borate and borosilicate glass and the kinetic largely deviates from parabolic behaviour. In the samples oxidized at temperatures in the range 1200–1300 °C, aluminium borate and mullite crystallize on the surface. The kinetics is para-linear in this temperature range but at 1300 °C, the rupture of the outer layer, results in accelerated damage of the sample. The composite is recommended for applications up to 1100 °C.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Preparation and characterization of a nigerian mesoporous clay-based membrane for uranium removal from underground water

This study reports the removal of uranium in underground wastewater using a Nigerian clay-based membrane. The clay and sintered clay were characterized using XRD, XRF, TGA/DTA, FESEM and PSD. The raw clay was mixed with cassava starch (10, 15, 20 and 25 wt%) and sintered at a temperature of 1300 °C. A multi-point BET analysis of the produced clay-based membranes was conducted to determine the surface area, pore volume and average pore size. Sintering characteristics were determined by apparent porosity, bulk density and flexural strength. The radioactivity of the feed and the permeated water was counted using a gamma spectrometer with an HPGe detector. From the XRD, TGA and FESEM, 1300 °C was found to be optimum for the mullite formation from the clay. The average pore sizes of the produced membranes from the BET results were observed to be in the range from 51 to 70 Å and with a steady state flux range of the tested membranes in the range 1.92×10⁻⁵–2.09×10⁻⁴ m³ m⁻² s⁻¹. The permeation flux produced is of high quality with a rejection in the range of 1.78–2.56 Bq/l of the uranium activity by the tested membranes. This low-cost membrane will have an application for the treatment of uranium-containing wastewater from fracking, oil exploration and phosphate mining industries.     

Structure and conductivity of NiO/YSZ composite prepared via modified glycine-nitrate process at varying sintering temperatures

Nickel oxide and Yttria-stabilized zirconia (NiO/YSZ) composite is one of the most promising mixed conducting electrode materials in both solid oxide electrolysis cell and solid oxide fuel cell applications. In this study, 50 wt% NiO and 50 wt% YSZ composite was synthesized via a modified glycine-nitrate combustion process (GNP) and the effect of sintering temperatures (1100 °C, 1300 °C and 1500 °C) on its microstructure and electrical properties were investigated. TG/DTA and in-situ high temperature XRD revealed the thermal property behavior and the structural changes of the as-combusted precursor material. For all the samples sintered at different temperatures, room temperature XRD patterns revealed a distinct cubic phases of both YSZ and NiO while SEM images showed a porous microstructure. The total conductivities at 700 °C are 9.87 × 10⁻³, 5.26 × 10⁻³, 4.02 × 10⁻³ S/cm for the 1100, 1300, and 1500 °C with activation energies of 0.1722, 0.3555, and 0.3768 eV, respectively. Conductivity measurements of the different sintered samples revealed that the total conductivities as well as the activation energies are greatly affected by different sintering temperatures.     

Characteristics of BaNd2Ti5O14 powders directly prepared by high-temperature spray pyrolysis

BaNd₂Ti₅O₁₄ powders were directly prepared by high-temperature spray pyrolysis. The powders prepared at temperatures of 1300 and 1500 °C exhibited a pure BaNd₂Ti₅O₁₄ phase. The powders prepared at 1300 °C were spherical in shape. However, the powders prepared at 1500 °C showed non-spherical shapes. The BaNd₂Ti₅O₁₄ powders had a composition similar to that of the spray solution. The mean sizes of the BaNd₂Ti₅O₁₄ powders increased from 0.23 to 0.60 μm when the concentration of the spray solution was increased from 0.01 to 0.2 M. At a sintering temperature of 1100 °C, bridge-like structures were formed between the powders. Pellets sintered at 1300 °C exhibited a dense structure comprising rod-like crystals.     

Dense bulk silicon oxycarbide glasses obtained by spark plasma sintering

Dense silicon oxycarbide glasses (SiOC) have been produced by spark plasma sintering (SPS) of SiOC powders. Raw powders were obtained by pyrolysis under nitrogen at 1100 °C of tetraethylorthosilcate/polydimethylsiloxane (TEOS/PDMS) hybrids. SPS experiments were carried out at 1300 and 1500 °C at 10 and 80 MPa and then were studied by chemical analysis, ²⁹Si and ¹³C MAS NMR, ATR, Raman, XRD, FE-SEM, density, porosity, microhardness (H_v) and thermal conductivity (K). The SiOC materials are formed by Si_xOC_4?x units within a silica matrix where silicon carbide and graphite nanodomains are also present. After the SPS treatment the silicon carbide crystallite size is close to 2.5 nm. At 1300 °C and 1500 °C the carbon nanodomain size is close to 3 nm and 2 nm, respectively. H_v values vary from 3.4 to 9.15 GPa, for 30% and 1% of porosity, respectively. Finally, K is always close to 1.38 W m^?1 K^?1.     

Preparation and characterization of porous MgAl2O4 spinel ceramic supports from bauxite and magnesite

Porous magnesium aluminate spinel (MgAl₂O₄) ceramic supports were fabricated by reactive sintering from low-cost bauxite and magnesite at different temperatures ranging from 1100 to 1400 °C and their sintering behavior and phase evolution were evaluated. The effects of sintering temperature on the pore structure, size and distribution as well as on the main properties of spinel ceramic supports such as flexural strength, nitrogen permeation flux and chemical resistance were investigated. The supports prepared at 1300 °C showed a homogeneous pore structure with the average pore size of 4.42 μm, and exhibited high flexural strength (35.6 MPa), high gas permeability (with nitrogen gas flux of 3057 m³ m⁻² h⁻¹ under a trans-membrane pressure of 0.1 MPa) and excellent chemical resistance.     

From nanometric zirconia powder to transparent polycrystal

Coprecipitated zirconia-yttria (8 mol%) gel subjected to hydrothermal treatment at 240 °C resulted in the solid solution powder of 8 nm particle sizes and specific surface area 132.7 m²/g. Uniaxial compaction followed by cold isostatic pressing under 300 MPa resulted in samples of the extremely small and narrow pore size distribution. Such samples start to shrink at about 200 °C which is related to the desorption of water layers surrounding particles. The state of closed porosity is achieved at 1150 °C. Pore closing was performed in air or oxygen atmosphere. Hot isostatic pressing at 1150 °C for 2 h under 250 MPa argon pressure led to transparent materials. Some pores remained in the material whose preliminary pore closure was performed in air. The samples initially sintered in oxygen atmosphere show no porosity and higher light transmittance than the former ones.     

Single-source-precursor synthesis of soft magnetic Fe3Si- and Fe5Si3-containing SiOC ceramic nanocomposites

We present here the single-source-precursor synthesis of Fe₃Si and Fe₅Si₃-containing SiOC ceramic nanocomposites and investigation of their magnetic properties. The materials were prepared upon chemical modification of a hydroxy- and ethoxy-substituted polymethylsilsesquioxane with iron (III) acetylacetonate (Fe(acac)₃) in different amounts (5, 15, 30 and 50 wt%), followed by cross-linking at 180 °C and pyrolysis in argon at temperatures ranging from 1000 °C to 1500 °C. The polymer-to-ceramic transformation of the iron-modified polysilsesquioxane and the evolution at high temperatures of the synthesized SiFeOC-based nanocomposite were studied by means of thermogravimetric analysis (TGA) coupled with evolved gas analysis (EGA) as well as X-ray diffraction (XRD). Upon pyrolysis at 1100 °C, the non-modified polysilsesquioxane converts into an amorphous SiOC ceramic; whereas the iron-modified precursors lead to Fe₃Si/SiOC nanocomposites. Annealing of Fe₃Si/SiOC at temperatures exceeding 1300 °C induced the crystallization of Fe₅Si₃ and β-SiC. The crystallization of the different iron-containing phases at different temperatures is considered to be a consequence of the in situ generation of a Fe–C–Si alloy within the materials during pyrolysis. Depending on the Fe and Si content in the alloy, either Fe₃Si and graphitic carbon (at 1000–1200 °C) or Fe₅Si₃ and β-SiC (at T > 1300 °C) crystallize. All SiFeOC-based ceramic samples were found to exhibit soft magnetic properties. Magnetization versus applied field measurements of the samples show a saturation magnetization up to 26.0 emu/g, depending on the Fe content within the SiFeOC-based samples as well as on the crystalline iron silicide phases formed during pyrolysis.     

Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation

The effect of pre-oxidation on the porosity evolution in heavy-oil fly ash subjected to activation with CO₂ has been investigated. After preliminary acid leaching, used to reduce the mineral matter content, the leached fly ash has been oxidised in air at 250 °C for 36 h. Pyrolysis was conducted on the unoxidised and oxidised leached fly ash at 900 °C for 2 h and the resultant chars were activated with CO₂ at 900 °C for different times. The activated samples have been characterised as regards the surface area and the pore volume.The pre-oxidation enhances the porosity development mainly in terms of mesoporosity leading to obtain activated products with higher surface area (about 270 m²/g at a 40% burn-off).