Structure and mechanical properties of porous silicon oxycarbide ceramics derived from silicone resin with different filler content

Porous silicon oxycarbide ceramics were obtained through pyrolysis of a new silicone resin filled with its pyrolyzed SiOC powders via a simple self-blowing process. The effects of filler content on the porosity, compressive strength and microstructure of the porous ceramics were investigated. The porosity (total and open) increased firstly and then decreased with the filler content increasing. It was possible to control the total and open porosity of porous ceramics within a range of 66.1–88.2% and 42.7–72.5% respectively, by adjusting the filler content from 0 vol% to 30 vol% while keeping the heating rate fixed at 0.5 °C/min. The compressive strength decreased firstly and then increased with the increasing filler content, and the average compressive strength of the porous ceramics was in the range of 1.1–3.4 MPa. Micrographs indicated that the porous ceramics with the filler content less than 20 vol% had a well-defined open-cell and regular pore structure.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

A simple approach for fabrication of interconnected graphitized macroporous carbon foam with uniform mesopore walls by using hydrothermal method

Three-dimensional (3D) highly interconnected graphitized macroporous carbon foam with uniform mesopore walls has been successfully fabricated by a simple and efficient hydrothermal approach using resorcinol and formaldehyde as carbon precursors. The commercially available cheap polyurethane (PU) foam and Pluronic F127 were used as a sacrificial polymer and mesoporous structure-directing templates, respectively. The graphitic structure of carbon foam was obtained by catalytic graphitization method using iron as catalyst. Three different carbon foams such as graphitized macro-mesoporous carbon (GMMC) foam, amorphous macro-mesoporous carbon (AMMC) foam and graphitized macroporous carbon (GMC) foam were fabricated and their physicochemical and mechanical properties were systematically measured and compared. It was found that GMMC possess well interconnected macroporous structure with uniform mesopores located in the macroporous skeletal walls of continuous framework. Besides, GMMC foam possesses a well-defined graphitic framework with high surface area (445 m²/g), high pore volume (0.35 cm³/g), uniform mesopores (3.87 nm), high open porosity (90%), low density (0.30 g/cm³) with good mechanical strength (1.25 MPa) and high electrical conductivity (11 S/cm) which makes it a promising material for many potential applications.     

Effects of inert filler addition on the structure and properties of porous SiOC ceramics derived from silicone resin

Porous SiOC ceramics were obtained from a new self-blowing precursor silicone resin DC217, by pyrolysis at 1200 °C in argon. Silicon carbide powders were incorporated into the silicone resin as inert fillers. The effects of the mean particle size of SiC fillers on the porosity, compressive strength and microstructure of the porous ceramics were investigated. With the mean particle size of SiC powders increasing from 5 μm to 10 μm, the porosity (total and open) of the porous ceramic increased and the compressive strength decreased. However, the porosity, compressive strength and cell morphology of the porous ceramics showed no evident changes when the mean particle size of fillers increased from 10 μm to 15 μm. Micrographs indicated that, when the mean particle size of fillers exceeded 5 μm, the porous ceramics could have a well-defined and regular pore structure. Furthermore, comparing with the porous ceramics which fabricated under the same condition with the SiOC powders as fillers, the cell morphology was similar. But the compressive strength and the oxidation resistance of the porous ceramics with SiC powders as fillers were much better.     

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.     

Porosity control of porous silicon carbide ceramics

Porous SiC ceramics were fabricated by the carbothermal reduction of polysiloxane-derived SiOC containing polymer microbeads followed by sintering. The effect of the SiC powder:polysiloxane-derived SiC (SiC:PDSiC) ratio on the porosity and flexural strength of the porous SiC ceramics were investigated. The porosity generally increased with decreasing SiC:PDSiC ratio when sintered at the same temperature. It was possible to control the porosity of porous SiC ceramics within a range of 32–64% by adjusting the sintering temperature and SiC:PDSiC ratio while keeping the sacrificial template content to 50 vol%.The flexural strengths generally decreased with increasing porosity at the same SiC:PDSiC ratio. However, a SiC:PDSiC ratio of 9:1 and a sintering temperature of 1750 °C resulted in excellent strength of 57 MPa at 50% porosity. Judicious selection of the sintering temperature and SiC:PDSiC ratio is an efficient way of controlling the porosity and strength of porous SiC ceramics.     

Ultralow cost reticulated carbon foams from household cleaning pad wastes

A very simple method is described for preparing ultra lightweight and ultralow cost carbon foams (density 0.04–0.075 g cm⁻³ and porosity 98–96%) by impregnation with sucrose of household cleaning pad wastes used as sacrificial templates, followed by pyrolysis in inert atmosphere. Scanning electron microscopy showed that the resultant reticulated vitreous carbon (RVC) foams had a fully open and interconnected porous structure. These materials are more thermally insulating (thermal conductivity 0.042–0.065 W m⁻¹ K⁻¹) than commercial RVC foams having similar compressive strengths (0.11–0.23 MPa) and similar densities.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Combustion synthesis of high porosity SiC foam with nanosized grains

High porosity silicon carbide (SiC) foam with nanosized grains was synthesized by a newly developed process involving two steps: (i) preparation of Si/C foam by gel-casting technique and (ii) fabrication of SiC foam by combustion Si/C foam in nitrogen atmosphere. The as-synthesized SiC foam with a high porosity in the range 70–90% exhibited an attractive strength up to 1.6 MPa. SEM analysis showed that the foam struts consisted of tightly bonded SiC particles with a grain size of 80–300 nm.     

Open-cell reaction bonded silicon nitride foams: Fabrication and characterization

In this study, a newly designed fabrication procedure was utilized to produce silicon nitride foams. The main goal of the present study was to obtain Si₃N₄ foams with high levels of porosity and pore interconnectivity via an economical fabrication procedure including sacrificial template technique, gel-casting and reaction bonding processes. The fabrication procedure was studied and optimized in terms of suspension preparation and rheology, gel-casting parameters, and reaction bonding conditions. The produced foams have a precisely controlled level of porosity which can be varied up to 87 vol%. BET analysis showed that the surface area of the foam is of the order of 2.01 m²/g. The pore interconnectivity of the foam was investigated via polyester resin infiltration. Based on XRD and SEM analysis, the dominant nitriding reactions are the gas-phase reactions which lead to α-Si₃N₄ in the form of whiskers.     

Processing,properties and microstructure of SiC foam derived from epoxy-modified polycarbosilane

SiC foams having controlled porosity were fabricated using epoxy modified polycarbosilane (EMPCS). The EMPCS was synthesized by refluxing adequate amount of epoxy and polycarbosilane (PCS) in THF solution at 150 °C. The EMPCS having epoxy content of 0%, 10% and 20% by weight were termed as PCS, 10EMPCS and 20EMPCS respectively. Thermal foaming of the EMPCS was carried out at 1000 °C under inert atmosphere followed by ceramization at 1200, 1400 and 1600 °C under vacuum. The cell size of the ceramized SiC foam was found to be varying between 100 and 700 µm. The ceramized SiC foams were characterized for their density, porosity and compressive strength. Total porosity was found to be 81.8 ± 3.9, 87 ± 4.1 and 90.6 ± 4.6% for the PCS, 10EMPCS and 20EMPCS based SiC foams while their bulk densities were found to be 0.6 ± 0.03, 0.4 ± 0.02 and 0.3 ± 0.01 g/cc respectively. Compressive strength was found to be the highest for the SiC foams ceramized at 1600 °C for all the types of EMPCS. The compressive strength of the 10EMPCS is found to be 2.2 ± 0.2 MPa, 2.5 ± 0.2 MPa and 3.8 ± 0.3 MPa for the foams pyrolyzed at 1200 °C, 1400 °C and 1600 °C respectively while the strength was 1.9 ± 0.1 MPa, 2.1 ± 0.2 MPa and 2.9 ± 0.2 MPa for the 20EMPCS based SiC. The 20EMPCS based SiC foam of thickness 10 mm was exposed to oxy-acetylene flame for 120 s, back face temperature was found to be around 300 °C. Microstructure and phase analysis was carried out to understand the effect of epoxy content and ceramization temperature on physical, mechanical and thermal properties of different types of the SiC foams.     

Processing of highly porous TiO2 bone scaffolds with improved compressive strength

In the present study, the effect of sintering time and recoating procedures on the pore architectural parameters and compressive strength of highly porous ceramic TiO₂ foams were investigated. Long sintering times (>5 h) at 1500 °C led to an inward collapse of one wall of the triangular voids typically found in the strut interior of foams prepared using the replication method. This strut folding led to increased compressive strength, while the pore architectural features were not significantly affected. Furthermore, majority of the internal porosity of the foam struts was partially eliminated and became accessible for infiltration with TiO₂ slurry. Recoating procedures were found to markedly reduce the flaw size and number in the TiO₂ foam struts, which led to significant strengthening of the ceramic structure (0.7 → 3.4 MPa) by improved structural uniformity and slightly increased strut diameter.     

Macroporous gibbsite foams prepared from particle-stabilized emulsions using corn starch and agar as binders

Corn starch and agar were used independently as a water-uptake binder in combination with anionic sodium dodecul sulfate (SDS) which modifies the hydrophobicity of cationic gibbsite platelets for preparation of air-in-water (a/w) gibbsite foams via a simple mechanical frothing. Contact-angle (θ) measurements revealed that the apparent θ decreased from partially hydrophobic (θ ～ 62°) to more hydrophilic (θ ～ 47°) when the corn starch was first added, leading to foams with a reduced stability. As the concentration of corn starch reached above ～8 vol.%, θ returned back to greater than 60°, rendering then a stable foam. On the other hand, θ was found to decrease from 57° to 50° when the agar concentration increased above a mere 0.16 vol.%. This gave rise to a pronounced drainage and coalescence of the foam, and a further increase of the agar concentration only led to a quick disappearance of the a/w bubbles. By tailoring the binder concentration, macroporous gibbsite foams were produced from the air- or freeze-dried wet foams, which typically consisted of packing void cells over a cell-size distribution of 50–400 μm, a porosity ranging from 77% to 86%, and a three-point (green) rupture strength of up to 240 kPa.     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

Processing of low-density alumina foam

A novel method for synthesizing low-density alumina foam has been developed. The alumina foam with 98.5% porosity was synthesized by an unconventional route from an aqueous aluminum nitrate–sucrose solution. The resin formed by heating this solution underwent foaming and set into solid green foam, which was sintered at 1873 K. The thermogram of the green foam showed mass loss in four stages. The foam exhibited interconnected porous network with window size in the range 103–226 and 167–311 μm for foams sintered at 1223 and 1873 K, respectively. The alumina foam sintered at 1223 K exhibited gamma phase and that sintered at 1873 K exhibited alpha phase.     

Effect of additives on the pore evolution of zirconia based ceramic foams after sintering

A new superplasticity foaming method was used to form zirconia-based ceramic foams. Silica and alumina were chosen as additives because they facilitate the 2D superplastic deformation. The effects of these additives on macroscopic pore evolution were examined after heat treatment for up to 40 h. The addition of silica or alumina also enhanced the 3D deformation during superplasticity foaming. The total porosity of mono-foams made from 3 mol% yttria-stabilised zirconia without additives increased with heat treatment for up to 24 h, and then levelled off. The porosity of silica-dispersed foam was greater than that without additives and continued to increase for up to 40 h. Conversely, the porosity of alumina-dispersed ceramic foam reached saturation within 8 h. Consequently, the porosity of alumina-dispersed foam was greater than that without additives after heating for 8 h, while the latter exceeded the former with prolonged heating for more than 16 h. The detailed effects of alumina dispersion on the foam development behaviour were examined in connection with the microstructure.     

Preparation and characterization of kaolin/starch foam

The objective of this work was to study the effect of the kaolin content on the properties of starch foams. The kaolin/starch foams were made with kaolin contents that ranged from 0 to 15 m% by baking in a hot mold. The starch and kaolin/starch foams were stored at room temperature with a relative humidity (RH) of 55% for 7 days prior to testing. An increase in the kaolin content increased the foam density. The izod impact strength increased up to 1151.37 J/m² at the highest kaolin content (15 m%). The improvement was about five times the izod impact strength of pure starch foam. Moreover, the presence of any kaolin reduced the water absorption ability of the starch foam. Scanning electron microscopy revealed that kaolin increased the size of the starch foam cells and was itself well dispersed. Kaolin/starch foams showed a higher thermal stability than pure starch foam.     

Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

Improving the properties of ceramic foams by a vacuum infiltration process

Reticulated ceramic foams are widely used for industrial applications such as metal filtration, exhaust gas and air purification, catalyst support and others. In this work, the compression strength and specific surface area of reticulated foams have been improved, while at the same time maintaining a high level of permeability in the final foam structure. In particular, a vacuum infiltration step by using a suitable slurry, followed by a pre-sintering cycle was adopted for filling up the hollow struts, generated due to the burnout of the PU foam. Furthermore, various mixtures of fine and coarse-grained alumina as well as in combination with zirconia, were utilised with the aim of controlling the foam properties such as compression strength, specific surface area and permeability. The compression strength was improved by a factor of two for alumina foams by infiltrating the hollow struts, and by a factor of four when infiltrating the struts of ZTA foams, with the composition 70 mol% Al₂O₃ and 30 mol% ZrO₂. The weight gain resulting from the vacuum infiltration process was in the order of 10 wt%.     

Preparation of a novel BN/SiC composite porous structure

A kind of BN/SiC open cell ceramic foams were fabricated from complex co-polymeric precursors of polycarbosilane and tris(methylamino)borane [B(NHCH₃)₃] using a high pressure pyrolysis foaming technique. The as-fabricated foams exhibit cell sizes ranging from 1 to 5 mm with bulk densities varying from 0.44 to 0.73 g/cm³, depending on the proportion of the starting materials. Studies on microstructure and properties of the porous material shown that addition of BN into SiC can improve dramatically its oxidation resistance during 800–1100 °C and compression strength which was generally about a 5–10 times higher than that of a pure SiC foam.