Preparation of finely macroporous SiOC foams with high mechanical properties and with hierarchical porosity via pyrolysis of a siloxane/epoxide composite

Mechanically strong SiOC foams were prepared via pyrolysis of polysiloxane composites, into which 20–70 wt% of epoxy powder were incorporated as sacrificial filler. The chosen epoxy filler degrades practically quantitatively during the pyrolysis in nitrogen, due to a high content of polyoxypropylene chains. The pyrolysis of the composite precursors yielded SiOC foams with apparent densities between 1.31 and 0.62 g/cm³, which corresponded to macro-porosities of 35–69%. A hierarchical porosity was obtained due to a relatively wide distribution of the sacrificial filler grains׳ size. Compressive strength between 15 and 38 MPa was achieved, depending on the apparent density of the SiOC foams: the best result was obtained for the foam with 56% porosity. In the case of denser foams, their strength was reduced by the presence of pyrolysis cracks, while at 70% porosity, the thinner pore walls led to a reduced strength. The elastic modulus of the most promising foam (56% porosity) was found to be 10 GPa.     

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.     

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.     

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.     

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.     

Characterization of flat ceramic membrane supports prepared with kaolin-phosphoric acid-starch

The purpose of this work is the development of microporous ceramic materials based on kaolin for a filtration process. Flat ceramic membrane supports were prepared from the mixtures of kaolin, phosphoric acid and starch. Porosity, permeability and mechanical properties of those supports were studied as functions of the amount of phosphoric acid, the sintering temperature and the compaction pressure. The rupture strength and the permeability of the ceramic membrane, increase with the content of phosphoric acid until 5 mass%. The porosity decreases with both the sintering temperature rise and the addition of phosphoric acid. The addition of 5 to 10 mass% of phosphoric acid and 10 mass% of starch to the kaolin supports sintered at 1100 °C for 2 h leads to a satisfied permeability and mechanical proprieties in the filtration application. The elaborated support was characterized using two analytical methods: DRX, ²⁷Al and ³¹P MAS-NMR. The obtained analytical data indicate the presence of an AlPO₄ high temperature phase at 1100 °C.     

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.     

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.     

Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.     

High performance biodegradable thermoplastic starch—EMMT nanoplastics

Nanoplastics were prepared using the formamide/ethanolamine-plasticized thermoplastic starch (FETPS) as the matrix and ethanolamine-activated montmorillonite (EMMT) as the reinforcing phase. Wide angle X-ray diffraction (WAXD), scanning electron microscope (SEM) and transmission electron microscope (TEM) revealed that FETPS was successfully intercalated into the layers of EMMT and formed the intercalated nanoplastics with EMMT. When EMMT content was 5 wt%, the mechanical testing indicated that the tensile stress of the nanoplastics achieved 7.5 MPa, and the tensile strain achieved 85.2%, Young's modulus increased from 47.2 MPa of pure FETPS to 145.1 MPa of FETPS/EMMT nanoplastics, and the breaking energy increased from 1.34 to 1.81 N m after being stored at RH=25% for 14 days. Comparing with FETPS, the nanoplastics effectively restrained the recrystallization of starch. In contrast to pure FETPS, the strength of nanoplastics obviously improved, and the thermal stability and water resistance also improved.     

Preparation of alumina foams by the thermo-foaming of powder dispersions in molten sucrose

The alumina powder disperses in molten sucrose due to the hydrophilic interaction between the particle surface and sucrose hydroxyls. The thermo-foaming of the dispersions is due to the bubbles created by the water vapour produced by the OH condensation at 150 °C which are stabilized by the alumina particles adsorbed on the gas–liquid interface as well as the increase in viscosity. The foaming time, the foam setting time and the foam volume depend on the alumina powder to sucrose weight ratio. The alumina foams have interconnected cellular microstructure and the cells are having a near spherical morphology. The porosity (97.84–93.29 vol.%.) decreased and the average cell size (0.54–1.2 mm) increased with the increase in alumina powder to sucrose weight ratio (0.4–1.4). The alumina foams with density in the range of 0.239–0.267 g/cc showed compressive strength in the range of 1.02–1.47 MPa.     

Thermal conductivity of high porosity alumina refractory bricks made by a slurry gelation and foaming method

Alumina has high heat resistance and corrosion resistance compared to other ceramics such as silica or mullite. However, for its application to refractory bricks, its high thermal conductivity must be reduced. To reduce this thermal conductivity by increasing the porosity, a GS (gelation of slurry) method that can produce high porosity solid foam was applied here to produce the alumina refractory brick. This method was successfully applied to produce alumina foam with high porosity and thermal conductivity of the foam is evaluated. At room temperature, the thermal conductivity was about 0.12 W/mK when the foam density was 0.1 g/cm³. At elevated temperature above 783 K, thermal conductivity of the foam was strongly affected by heat radiation and increased with increasing temperature, in contrast to the thermal conductivity of alumina itself, which decreased with increasing temperature. The alumina foams developed here achieved sufficient thermal insulating properties for use in refractory bricks.     

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%.     

Silicon carbide–titanium diboride ceramic composites

The effect of TiB₂ content on mechanical properties of silicon carbide–titanium diboride ceramic composites was studied. The hardness of the ceramics decreased from 27.8 GPa for nominally pure SiC to 24.4 GPa for nominally pure TiB₂. In contrast, fracture toughness of the ceramics increased from 2.1 MPa m^1/2 for SiC to ～6 MPa m^1/2 for SiC with TiB₂ contents of 40 vol.% or higher. Flexure strengths were measured for three composites containing 15, 20, and 40 vol.% TiB₂ and analyzed using a two parameter Weibull analysis. The Weibull modulus increased from 12 for 15 vol.% TiB₂ to 17 for 20 and 40 vol.% TiB₂. Microstructural analysis revealed microcracking in the ceramics containing 20 and 40 vol.% TiB₂. The ceramic containing 40 vol.% TiB₂ had the best combination of properties with a fracture toughness of 6.2 MPa m^1/2, hardness of 25.3 GPa, Weibull modulus of 17, and a strength of 423 MPa.     

The influence of process parameters on in situ inorganic foaming of alkali-bonded SiC based foams

Silicon carbide (SiC) foams were developed with a low temperature process by using an inorganic alkali aluminosilicates binder, also known as geopolymer. The foaming agent was the metallic silicon present as impurity in the SiC powder. Si⁰ in the alkaline solution led to gas evolution that induced the foaming of the slurries. The binder was a geopolymeric resin with atomic ratio Si/Al = 2 and potassium as alkaline cation, classified as (K)poly(silalate-siloxo). The geopolymeric resin was prepared using metakaolin as aluminosilicatic raw powder, while the alkali aqueous solution was KOH/K₂SiO₃. Metakaolin in alkaline conditions dissolved and re-precipitated to form geopolymeric nano-particulates that acted as a glue to stick together SiC particles (90 wt.%). Process parameters such as water addition, mixing time and curing temperature were correlated to the foam structure. The formation of prolate pores induced anisotropy in the compressive strength. The foams were studied by dilatometric analysis in inert and oxidative atmospheres up to 1200 °C.     

Simultaneous improvements in the cryogenic tensile strength,ductility and impact strength of epoxy resins by a hyperbranched polymer

Improvements in mechanical properties at low temperatures are desirable for epoxy resins such as diglycidyl ether of bisphenol A (DGEBA) that are often used in cryogenic engineering applications. In this study, a hydroxyl functionalized hyperbranched polymer (H30) is employed to improve the mechanical properties of a DGEBA epoxy resin at liquid nitrogen temperature (77 K). The results show that the tensile strength, failure strain (ductility) and impact strength at 77 K are simultaneously improved by adding a proper content of H30. The maximum tensile strength at 77 K is increased by 17.7% from 98.2 MPa of pure epoxy resin to 115.6 MPa of modified epoxy system for the 10 wt% H30 content. The failure strain at 77 K increases consistently with the increase of H30 content. The maximum impact strength at 77 K is attained by introduction of 10 wt% H30 with an improvement of 26.3% over that of pure epoxy resin. For the purpose of comparison, the mechanical properties of modified epoxy resins at room temperature (RT) are also investigated. It is interesting to note that the impact strength is not lower at 77 K than that at RT for the modified systems. Moreover, the glass transition temperature (T_g) is not reduced by the addition of H30 in appropriate amounts.     

Preparation of carbon foams with enhanced oxidation resistance by foaming molten sucrose using a boric acid blowing agent

Boric acid was used as a blowing agent as well as a boron precursor for the preparation of boron-doped carbon foams from molten sucrose. The H⁺ generated, due to the formation of a complex between sucrose and boric acid, catalyzes the –OH to –OH condensation reaction leading to the polymerization and the foaming of the molten sucrose. The char yield of the solid organic foams increased from 24 to 39 wt.% when the boric acid concentration increased from 0 to 8 wt.%, due to the formation of the B–O–C cross-links between sucrose polymer by B–OH to C–OH condensation. The inductively coupled plasma analysis showed the presence of 0.44–3.4 wt.% boron in the carbon foams. The density and compressive strength decreased and cell size increased with boric acid concentration. The room temperature thermal conductivity of the boron-doped carbon foams was in the range of 0.057–0.043 W m⁻¹ K⁻¹. The weight loss studies by dynamic and isothermal heating showed the increased oxidation resistance with boron concentration.     

Water absorption and mechanical property of an epoxy composite coating containing unoxidized aluminum particles

In this work, an unoxidized aluminum particle/epoxy composite coating was prepared onto the surface of the pure epoxy resin to reduce its water absorption and improve its mechanical property and low energy surface impact resistance. Unoxidized aluminum particles were prepared using arc spraying under the protection of nitrogen. The diffusion coefficient of sample began to decrease and then increased with increasing the content of aluminum particles in the composite coating. The minimum value was 2.7 × 10⁻⁸ m²/s at a critical concentration of 6% of aluminum particles, 38% lower than that of pure epoxy resin. The sample with 6% aluminum particles had the maximum tensile and bending strengths of 38.9 MPa and 94.6 MPa, respectively. Aluminum particles could effectively reduce the impact of the epoxy resin suffered. The direction of the impact was changed, and avoids the micro-crack to extend unidirectionally.     

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.     

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.