Tailoring the microstructure of high porosity Si3N4 foams by direct foaming with mixed surfactants

The mixed surfactants were successfully applied to fabricate the highly porous Si₃N₄ ceramic foams by the direct foaming method. The oppositely charged surfactants mixed in slurries could combined into catanionic surfactant by the electrostatic attraction and facilitate the formation of ultra-stable foams. The microstructure of the Si₃N₄ ceramic foams, including pore structure, mean pore size, pore size distribution and porosity were tailored by varying the mixing ratio of surfactant, mixed surfactants concentration and solid content of the initial slurries. Si₃N₄ ceramic foams with porosity of 92%-97%, mean pore size of 140-240 µm and compressive strength of 0.85-5.38 MPa were obtained by adjusting mixed surfactants between 0.1 and 0.4 wt% and solid content between 22 and 30 vol%. The compressive strength of Si₃N₄ ceramic foams in current work was much higher than most reported results.     

New consolidation process inspired from making steamed bread to prepare Si3N4 foams by protein foaming method

A new consolidation process had been developed for preparing Si₃N₄ ceramic foams by using protein foaming method, which was inspired from the preparation of steamed bread. The main advantage of this consolidation process was no crack development during foamed slurry consolidation process. By using this new consolidation, Si₃N₄ ceramic foams with open porosities of 79.6–87.3% and compressive strength of 2.5–22 MPa were prepared. Protein addition and solid content on mechanical properties and pore structures of the as-prepared ceramic foams were investigated. Results indicated that the open porosity decreases with protein addition and solid content while compressive strength increased with solid content. With the increase of solid content, pores of the ceramic foams became regular in shape and uniform in size while both size and number of windows on the walls decreased.     

Effects of surfactant and particle size on the microstructure and strength of Si3N4 foams with high porosity

The highly porous Si₃N₄ ceramic foams were prepared by direct foaming with mixed surfactants. The surface tension and viscosity of slurries were tailored by different carbon chain length of surfactants and different mean Si₃N₄ particle size to achieve the pore size controlling. The nearly linear relationship between the pore size and the ratio of surface tension to viscosity was observed, which indicates that the pore size could be precisely tailored by the slurry properties. Si₃N₄ ceramic foams with porosity of nearly 94%, mean pore size of 110–230 µm, and compressive strength of 1.24–3.51 MPa were obtained.     

Highly Porous Silicon Nitride Foam Prepared Using a Route Similar to the Making of Aerated Food

A new foaming route, which was imitated from the foaming of aerated foods, was used by the protein foaming method in preparing highly porous silicon nitride (Si₃N₄) foams. Effects of air fraction on the structure and property of the products were investigated. It was indicated that the porosity increased with the air fraction exponentially, whereas the air fraction had little effect on the average pore sizes of Si₃N₄ foams. Thermal conductivity of the product measured using the laser flash method decreased from 4.07 to 2.29 W/(m·K) when the porosity increased from 80.00% to 87.61%.     

Pore structure control of Si3N4 ceramics based on particle-stabilized foams

Porous Si₃N₄ ceramics with open, closed pores and nest-like structures were prepared by direct foaming method, and the stability of bubbles and the microstructures of sintered Si₃N₄ foam ceramics were investigated. The bubbles produced by short-chain amphiphiles have higher stability as compared with that produced by long-chain surfactants. Si₃N₄ ceramic foams using short-chain amphiphiles are particle-stabilized one, porous Si₃N₄ ceramics with open and closed pores can be easily prepared with this method, and the nest-like microstructure in Si₃N₄ foam ceramics is achieved at high gas-pressure sintering conditions. The decrease of flexural strength due to the increase of porosity can be weakened by decreasing pore size.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

Solid-state supercritical CO2 foaming of PCL and PCL-HA nano-composite: Effect of composition, thermal history and foaming process on foam pore structure

In this work we investigated the solid-state supercritical CO₂ (scCO₂) foaming of poly(?-caprolactone) (PCL), a semi-crystalline, biodegradable polyester, and PCL loaded with 5 wt% of hydroxyapatite (HA) nano-particles.In order to investigate the effect of the thermal history and eventual residue of the crystalline phase on the pore structure of the foams, samples were subjected to three different cooling protocols from the melt, and subsequently foamed by using scCO₂ as blowing agent. The foaming process was performed in the 37-40 °C temperature range, melting point of PCL being 60 °C. The saturation pressure, in the range from 10 to 20 MPa, and the foaming time, from 2 to 900 s, were modulated in order to control the final morphology, porosity and pore structure of the foams and, possibly, to amplify the original differences among the different samples.The results of this study demonstrated that by the scCO₂ foaming it was possible to produce PCL and PCL-HA foams with homogeneous morphologies at relatively low temperatures. Furthermore, by the appropriate combination of materials properties and foaming parameters, we prepared foams with porosities in the 55-85% range, mean pore size from 40 to 250 μm and pore density from 10⁵ to 10⁸ pore/cm³. Finally, we also proposed a two-step depressurization foaming process for the design of bi-modal and highly interconnected foams suitable as scaffolds for tissue engineering.     

Ultralight Silicon Nitride Ceramic Foams from Foams Stabilized by Partially Hydrophobic Particles

Ultralight Si₃N₄ ceramic foams have been successfully prepared through particle‐stabilized foams method, which is based on the adsorption of in situ hydrophobized Si₃N₄ particles to the liquid/air interface of the foams. Here, we firstly used a long‐chain surfactant cetyltrimethylammonium chloride to render the Si₃N₄ particles partially hydrophobic. By tailoring the surfactant concentration and pH values of the suspensions, the wet foams were stabilized to avoid coarsening and coalescence. SEM results show that the Si₃N₄ ceramic foams possess single strut walls with elongated β‐Si₃N₄ grains interlocking with each other, and their pores are uniform with an average pore size of 95 μm. The obtained ceramic foams maintain compressive strength of 1.34 ± 0.13 MPa with porosity of 92.0%, when the suspension contains 3 mmol/L surfactant at the pH of 11.0.     

Cellular silica-based ceramics prepared by direct foaming at high temperature

Cellular silica-based ceramics, including Si₃N₄/SiO₂ composite ceramics and monolithic silica ceramics, with dense shell and closed cells with dense and crack-free cell wall inside was prepared by the direct foaming of the green-compacts at 1310–1370 °C. The influences of the heat-treatment temperature on the relative density as well as the mechanism of the cell formation were investigated. The porosity of the obtained cellular silica and Si₃N₄/SiO₂ ceramics was within 60.0–84.0%, the cell size distribution was in the range of 10–120 μm, and the flexural strength was 9.7–16.3 MPa.     

Gas foaming of segmented poly(ester amide) films

Biodegradable segmented poly(ester amide)s, based on dimethyl adipate, 1,4-butanediol and N,N′-1,2-ethanediyl-bis[6-hydroxy-hexanamide], with two distinct melting transitions were gas foamed using carbon dioxide (CO₂). Polymer films were saturated with CO₂ at 50 bar for 6 h after which the pressure was released. The samples were immersed in octane at the desired temperature after which foaming started immediately. Just above the lower melt transition the polymers retain adequate mechanical properties and dimensional stability, while the chain mobility increased sufficiently to nucleate and expand gas cells during the foaming process. In this way semi-crystalline poly(ester amide)s can be gas foamed below the flow temperature.Two poly(ester amide)s with 25 mol% (PEA2,5-25) and 50 mol% (PEA2,5-50) of bisamide segment content were foamed at 70 and 105 °C, respectively. The storage modulus (G′) of both pure polymers at the onset foaming temperature is 50-60 MPa. Closed-cell foams were obtained with a maximum porosity of ∼90%. The average pore size of PEA2,5-25 ranges from 77 to 99 μm. In contrast, the average pore size of PEA2,5-50 is in between 2 and 4 μm and can be increased to 100 μm by lowering the CO₂ saturation pressure to 20 bar. The porosity of PEA2,5-50 foams using this saturation pressure decreased to 70%.     

Characterization of the porous structure of biodegradable scaffolds obtained with supercritical CO<Subscript>2</Subscript> as foaming agent

Poly(ε-caprolactone) foams were prepared, via a batch process, by using supercritical CO₂ as foaming agent. Their porous structure was characterized through mercury porosimetry, helium and mercury pycnometry, scanning electron microscopy (SEM) and X-ray microtomography observations coupled with image analysis. The pore size distributions obtained by these two latter techniques show that the pore structure is more homogeneous when the foaming process is performed under a high CO₂ saturation pressure (higher than 250 bars).     

Microcellular foaming of silicone rubber with supercritical carbon dioxide

In spite of great concern on the industrial application of microcellular silicone rubber foams, such as in electric and medical devices, only a few works can be found about the foaming of silicone rubber. In this study, microcellular silicone rubber foams with a cell size of 12 μm were successfully prepared with curing by heat and foaming by supercritical CO₂ as a green blowing agent. The microcellular silicone rubber foams exhibited a well-defined cell structure and a uniform cell size distribution. The crosslinking and foaming of silicone rubber was carried out separately. After foaming, the silicone rubber foam was cross-linked again to stabilize the foam structure and further improve its mechanical properties. Foaming process of cross-linked silicone rubber should be designed carefully based on the viscoelastic properties because of its elastic volume recovery in the atmosphere. The basic crosslinking condition for small cell size and high cell density was obtained after investigating the rheological behavior during crosslinking.     

Evolution of microstructure and properties of Si3N4 whisker reinforced composites during densification by polymer infiltration and pyrolysis

Si₃N₄ whisker (Si₃N_4w) reinforced composites were prepared by a near-net shaping process, i.e., gel-casting of the Si₃N_4w preform followed by polymer infiltration and pyrolysis (PIP) densification using polysilazane as precursor. The densification process by PIP was described mathematically, after which several key parameters affecting densification efficiency were discussed. The small pore size (0.04 ～ 1 μm) of Si₃N_4w preform can cause filtration effect (low permeability of precursor with a molecular size bigger than pore size), which resulted in the density gradient of the composites. Porosity (P) dependence of flexural strength and elastic modulus of Si₃N_4w/Si₃N₄ followed a power law of (1 – P). With increasing density, the response of Si₃N_4w when confronting cracks transformed from whisker debonding to whisker fracture, which was supposed to be due to the increase of whisker/matrix interface strength. The Si₃N_4w/Si₃N₄ developed by us achieved a good balance between high strength and low dielectric constant, making it promising for high-temperature wave-transparent application.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

Fabrication of polystyrene/nano‐CaCO3 foams with unimodal or bimodal cell structure from extrusion foaming using supercritical carbon dioxide

The article surveyed the fabrication of polystyrene (PS)/nano‐CaCO₃ foams with unimodal or bimodal cellular morphology from extrusion foaming using supercritical carbon dioxide (sc‐CO₂). In order to discover the factors influenced the cell structure of PS/nano‐CaCO₃ foams, the effects of die temperature, die pressure, and nano‐CaCO₃ content on cell size, density, and morphology were investigated detailed. The results showed that the nano‐CaCO₃ content affected the cell size and morphology of PS/nano‐CaCO₃ foams significantly. When the die temperature and pressure was 150°C and 18 MPa, respectively, the foams with 5 wt% nano‐CaCO₃ exhibited the unimodal cellular morphology. As the nano‐CaCO₃ content increased to 20 wt%, a bimodal cell structure of the foams could be obtained. Moreover, it was found that the bimodal structure correlated more strongly with the pressure drop than the foaming temperature. The article revealed that unimodal or bimodal cellular morphology of PS/nano‐CaCO₃ foams could be achieved by changing the extrusion foaming parameters and nano‐CaCO₃ content. POLYM. COMPOS., 37:1864–1873, 2016. © 2015 Society of Plastics Engineers     

Manufacture of Si3N4-SiCN composite bulks with hierarchical pore structure

Si₃N₄-SiCN ceramic foams with hierarchical pore architecture were formed by protein-based gelcasting and precursor infiltration and pyrolysis. The primary pore structure (>100 μm) was generated by protein gelation and precursor ceramization, while the secondary pore structure (10–50 μm) originated from the cell windows after pyrolysis. The network of Si₃N₄ nanowires and the voids among ceramic particles formed the tertiary pore structure (<2 μm). The obtained Si₃N₄-SiCN ceramics had a density of 0.45–0.66 g/cm³ and an open porosity of 72.7–82.8 vol.%. The porous bulks possessed a compressive strength of up to 16.9 ± 1.1 MPa (72.7 vol.% open porosity) at room temperature and 8.6 ± 0.2 MPa at 800 °C. A good gas permeability of the ceramics was indicated with a tested value of 3.27 cm³cm/(cm²·s·kPa). The excellent mechanical property, permeability together with the hierarchical pore structure enabled the Si₃N₄-SiCN composite bulks promising for industrial filtration applications.     

Direct foaming of Al2O3-based macroporous ceramics containing pre-foamed colloidal alumina,calcite and CAC suspension

Adding pre-foamed colloidal alumina to ultrastable Al₂O₃-stabilised foams can be a path towards partially counteracting the firing shrinkage of these materials and producing macroporous ceramics with smaller pores. Nevertheless, this system still presents a long setting time and high sintering-induced shrinkage, which hinders the production of larger samples and reduces its porosity. In the present work, it was observed that adding calcium aluminate cement suspension (CAC_S) and CaCO₃ (calcite) to the aforementioned system can speed up its solidification kinetics, improve its mechanical strength and reduce its shrinkage after firing, maintaining high porosity and smaller pore sizes. By using these raw materials, samples with an average pore size below 60 μm, total porosity above 70%, and a narrower pore size distribution were attained after thermal treatment at 1600 °C for 5h. Moreover, due to the in situ formation of calcium hexaluminate, their shrinkage after sintering was almost halved (from ~20% to 11%).     

Dynamically enhanced membrane foaming

Foamed food products like chocolate mousse, ice cream or fresh cheese are increasingly popular due to their soft and creamy sensory properties. Their perception, stability and flow behavior strongly depend on gas fraction and bubble size distribution. Foam processing research focuses on developing new optimized processes and material systems to achieve small mean bubble size and narrow size distribution.In this work, we present a new dynamically enhanced membrane foaming process. This foaming device basically consists of two concentric cylinders: the inner cylinder is rotated with circumferential velocities up to , the outer cylinder is fixed. Thus, a shear field is created in the narrow annular gap. The membrane can either be mounted to the inner or outer cylinder. Gas is pressed through the membrane and is detached as small bubbles by the acting flow shear stresses. The comparison of rheological and microstructural analysis of foams to results on bubble breakup in simple shear flow and on detachment of bubbles from the pore of a rotating membrane proved that the detachment of small bubbles from the membrane is the dominating bubble formation process in the dynamically enhanced membrane foaming process. Compared to conventional rotor-stator foaming devices, the dynamically enhanced membrane foaming process leads to significantly smaller mean bubble sizes at higher gas volume fractions and to reduced size distributions widths.     

Single-source-precursor synthesis and high-temperature evolution of novel mesoporous SiVN(O)-based ceramic nanocomposites

Mesoporous SiVN(O) ceramics were prepared from a mixture consisting of VO(acac)₂-modified perhydropolysilazane and polystyrene. The resulting amorphous single-phase SiVN(O) ceramics remained amorphous in nitrogen atmosphere up to 1400 °C. The as-prepared materials consist of nanoscaled vanadium nitride dispersed in amorphous Si₃N₄; exposure to 1600 °C leads to the crystallization of VN and Si₃N₄. The specific surface area (SSA) and the pore size of the SiVN(O)-based ceramics can be easily controlled by the temperature of thermal treatment and by the amount of polystyrene. The average pore size of the prepared SiVN(O) ceramics was 4–10 nm and their largest SSA values, 642 and 506 m²/g, were achieved upon ammonolysis at 800 and 1000 °C, respectively. The combination of metal-modified single-source precursors and encapsulated porogens provides a convenient one-pot synthesis process to prepare mesoporous ceramic nanocomposites with controllable phase compositions and morphology.