Highly porous YSZ ceramic foams using hollow spheres with holes in their shell for high-performance thermal insulation

Yttria-stabilized zirconia (YSZ) porous ceramic foams were fabricated using YSZ microspheres with holes on the surface to determine their properties as insulation materials. Highly porous YSZ ceramics with bimodal pore structures, such as internal pores in single hollow spheres and external pores between the spheres, were successfully prepared using YSZ spheres as raw materials. Additionally, holes were added to the shells to reduce continuous thermal pathways and significantly enhance the insulation properties. Furthermore, by adding holes on the surface of the sphere, the porous foams using a hollow sphere exhibit a maximized porosity of 80.69%, remarkably enhanced their insulation properties with low thermal conductivity (0.10 W/m-K), and have sufficient compressive strength to protect the green body (5.7 MPa). The mechanical strength of the YSZ porous foam was maintained owing to the uniform arrangement of the supports.     

Synthesis,morphological and thermomechanical characterization of light weight silica foam via reaction generated thermo-foaming process

Low-cost thermo-foaming technique involving sucrose dehydration reaction with H₂SO₄ has been used for ceramic foam processing. The heat and gas generated during the reaction induces a thermo-foaming effect in the sucrose-ceramic mixture. This converts the sucrose-ceramic slurry into a carbonaceous porous ceramic-scaffold. The converted carbon is seen to agglutinate with the ceramic particles. During sintering, they are removed subsequently, thereby enlarging the pores. Influence of solids-loading, sintering temperature and H₂SO₄ concentration on the foaming behaviour, pore morphology and thermo-mechanical properties are studied. The fabricated silica foams are found to have porosity within the range of 70 %? 90 % and compressive strength of 0.8–2.8 MPa. The extreme porosity of the silica foams and their intercrystallite pores within the struts results in the low thermal conductivity (0.0943 Wm^?1K^?1) of the specimens. The fabricated foam is seen as a promising material for insulation applications like catalyst supports, filters and bio-scaffolds.     

A Simple Direct Casting Route to Ceramic Foams

A simple direct foaming and casting process using ovalbumin-based aqueous slurries for fabricating ceramic and metal foams is demonstrated. Foaming of aqueous ceramic slurries and the foam microstructure were seen to be a strong function of slurry rheology. Setting of foams with ceramic solids loading above 20 vol% was achieved by addition of acid, which also prevented binder migration. Acid addition resulted in excessive shrinkage, causing cracking of foams with ceramic loading below 20 vol%. Addition of sucrose to the slurries suppressed shrinkage leading to defect-free foams with porosity exceeding 95%. Overall porosity and foam microstructure could be controlled through ceramic solids loading, ovalbumin–water ratio, foaming time and sucrose amount, and sintering temperature. The ceramic foams fabricated by the process were strong enough to be green machined to different shapes.     

轻质环氧树脂微孔发泡材料的力学及隔热性能

通过添加不同含量的化学发泡剂,制备了较小密度(0.5 g/cm~3)的环氧树脂基微孔发泡材料,研究了发泡剂含量(0.25%~2%)对环氧复合发泡材料发泡行为的影响,并讨论了材料的力学性能及隔热性能的变化规律。结果表明,随着发泡剂含量的增加,材料的表观密度不断降低,但泡孔尺寸不断增大,泡孔密度则不断降低。力学性能及隔热性能测试表明,随发泡剂含量的增加,材料的压缩屈服强度和压缩弹性模量不断降低,但是材料的隔热性能不断提高。     

Novel ZrP2O7 ceramic foams with controllable structures and ultra-low thermal conductivity

This study demonstrated the synthesis of novel zirconium pyrophosphate (ZrP₂O₇) ceramic foams via a two-step method using a foam casting technique. The synthesised foams functioned as thermal insulators with a highly controllable performance. We investigated the effects of the addition of foaming and thickening agents as well as the solid content of the slurries on the slurry, mechanical properties, thermal conductivities, and microstructure of ZrP₂O₇ ceramic foams. The ZrP₂O₇ ceramic foams synthesised at 1473 K exhibited a porosity, compressive strength, and thermal conductivity of 75.2–89.1 %, 1.95–0.02 MPa, and 0.144–0.057 W/(m K) (298–573 K), respectively. The increase in the porosity to >60 % will facilitate applications based on the low thermal conductivities of the foams.     

Preparation and microstructure evolution of novel ultra-low thermal conductivity calcium silicate-based ceramic foams

In this study, municipal solid waste incineration fly ash (MSWI FA) was used as a new raw material for the ceramics industry and a novel ultra-low thermal conductivity calcium silicate-based foams (CSFs) was prepared by the direct foaming method. The effects of the addition of foam and borax on the sintering behavior and microstructural evolution of the CSFs were investigated. With the optimal amount of foam, the CSFs had an apparent porosity of 63.43%–67.49%, bulk density of 0.75–0.84 g/cm³, compressive strength of 1.83–3.21 MPa, and room-temperature thermal conductivity of 0.213–0.235 W/(m·K). Notably, the whisker morphology, pore structure, and sintering behavior of the samples can be controlled by changing the amount of borax. The prepared ceramic foams can be applied in the fields of thermal insulation, filtration, and catalyst carriers.     

Extruded polystyrene foams with bimodal cell morphology

Extrusion foaming using supercritical carbon dioxide (CO₂) as the blowing agent is an economically and environmentally benign process. However, it is difficult to control the foam morphology and maintain its high thermal insulation comparing to the conventional foams based on fluorocarbon blowing agents. In this study, we demonstrated that polystyrene (PS) foams with the bimodal cell morphology can be produced in the extrusion foaming process using CO₂ and water as co-blowing agents and two particulate additives as nucleation agents. One particulate is able to decrease the water foaming time so both CO₂ and water can induce foaming simultaneously, while the other increases the CO₂ nucleation rate with little effect on the CO₂ foaming time. Our experimental results showed that a dual particulate combination of nanoclay and activated carbon provided the best bimodal structure. The bimodal foams exhibited much better compressive properties and slightly better thermal insulation for PS foams.     

Hierarchically porous lanthanum zirconate foams with low thermal conductivity from particle-stabilized foams

Lanthanum zirconate (LZO) ceramic foams with hierarchical pore structure were fabricated by particle-stabilized foaming method for the first time, and the as-prepared ceramics have high porosity of 90.7%-94.9%, low thermal conductivity, and relatively high compressive strength. The LZO powder was synthesized by solid-state method. The porosity of the ceramic foams was tailored by suspensions with different solid loadings (20-40 wt%). The sample with porosity of 94.9% has thermal conductivity of 0.073 W/(m·K) and compressive strength of 1.19 MPa, which exhibits outstanding property of thermal insulation and mechanical performance, indicating that LZO ceramic foam is a promising thermal insulation material in high temperature applications.     

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.     

Fabrication of Low-Density Ferrous Metallic Foams by Reduction of Chemically Bonded Ceramic Foams

Closed cell ferrous foams were fabricated using a chemically bonded oxide ceramic foam precursor. The major constituent of the ceramic foam precursor was iron oxide (Fe₂O₃), which was mixed with various foaming/setting additives. The density of the foam was modified by varying the ambient pressure under which foaming was carried out. Further, a magnesium–ammonium phosphate-based cement system was utilized to promote more rapid setting times and hence minimize foam collapse. The oxide foam was then reduced by heating at 1240°C in a non-flammable hydrogen/inert gas mixture to obtain metallic foams. The relative density of samples foamed under a reduced pressure (∼380 Torr) was 0.13±0.006, which is the lowest value reported to date for a closed cell ferrous foam. A relative density of 0.21±0.01 was achieved for samples foamed under atmospheric pressure. With regard to the foam morphology, the average cell diameter was 1.41±0.6 mm for the low-density (LD) foams, and 0.96±0.2 mm for the high-density (HD) foams. The iron foams were tested in compression and yielded an average compressive strength of 11±1 and 19±4 MPa for the LD and HD foams, respectively. A comparison based on a bending strength performance index showed that the properties of the ceramic–precursor-derived foams compared favorably with those of steel foams fabricated by other techniques.     

Preparation of high open porosity ceramic foams via direct foaming molded and dried at room temperature

Herein an alternative approach was considered for addressing one difficulty of ceramic foams that the foam slurry with a high content of bubbles which were obtained via direct foaming, cannot maintain well for a long time at room temperature. It is fascinating that the foam slurry mentioned above could stably mold and dry at room temperature, based on an animal protein as foaming agent, kaolin, talc powder and alumina as raw materials, alpha-tricalcium phosphate prepared via co-precipitation as curing agent, and hydrophobic activated carbon powders as stabilizing agent. Effects of the calcination temperatures, the contents of alpha-tricalcium phosphate and activated carbon powder on microstructures, crystal phases, compressive strength and open porosities of ceramic foams were studied systematically. The results indicated that ceramic foams with a high open porosity and uniform pore distribution and sizes sought for application in catalysts supports, could be produced by adjusting these parameters.     

Hierarchical cellular scaffolds fabricated via direct foam writing using gelled colloidal particle-stabilized foams as the ink

Hierarchical cellular architectures hold great potential in a vast range of applications due to their superior mechanical properties and multifunctionality. In the present work, hierarchical structures composed of porous struts patterned in the form of quadrangular and triangular honeycombs were fabricated via direct foam writing (DFW) using colloidal particle-stabilized Al₂O₃-MgO-SiO₂ foams as the ink. Hydration process of MgO and subsequent formation of colloidal Mg(OH)₂ network endowed the foam ink with viscoelasticity and high storage modulus. The resulting honeycombs with ultrahigh overall porosity (95.3%) and robust compressive strength (2.5 MPa) can be readily fabricated by DFW. The current work exhibits a significant step toward the scalable production of cellular ceramics with hierarchical architecture for various applications, including tissue scaffolds, catalyst supports, thermal insulation, and lightweight structures.     

Batch foaming of SAN/clay nanocomposites with scCO2: A very tunable way of controlling the cellular morphology

This paper aims at elucidating some important parameters affecting the cellular morphology of poly(styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO₂ technology. Prior to foaming experiments, the SAN/CO₂ system has first been studied. The effect of nanoclay on CO₂ sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40 °C) leads to foams with the highest cell density (∼10¹²-10¹⁴ cells/cm³), the foam expansion is restricted (d∼0.7-0.8 g/cm³). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (d∼0.1-0.5 g/cm³). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared - cell size, cell density, foam density - depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO₂ batch foaming process for tuning the foam cellular morphology.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Design and formulation of polyurethane foam used for porous alumina ceramics

The present work studied a simple direct foaming method for preparation of porous alumina ceramics by expansion of a ceramic suspension based on polyurethane (PU) foam system. The effects of polyurethane formulas including catalyst composition, blowing agent content, NCO index and solid content on the samples properties were investigated. The results showed that the homogeneity, porosity and mechanical properties are various for different formulas. The dried green bodies showed diametrical compressive strength in the range of 0.39–1.25 MPa and were amenable to machining operations such as milling, drilling and lathing. Meanwhile, PU formulas play an important role in the microstructures and mechanical properties of green bodies and sintered ceramic foams. Pyrolytic removal of polyurethane skeleton followed by sintering at 1550 °C produced alumina bodies with open cell porosity 54–75% and diametrical compressive strength 1.39–28.47 MPa. Microstructure showed both large (200–300 μm) and small (50–100 μm) pores all with various sizes of windows. Based on the optimization of polyurethane formulation, the porous alumina foam with porosity of 64% and compressive strength of 25.26 MPa was prepared. This polyurethane foam system is easily available and low-cost, which could be widely applied in preparation of other porous ceramics, such as ZrO₂, SiO₂, etc.     

Fabrication of polyamide 12T micro-cellular foams with ultra-high compressive strength via in situ polytetrafluoroethylene fibrillation using supercritical carbon dioxide foaming

The development of micro-cellular foams with ultra-high compressive strength and high volume expansion ratio (VER) is a challenging task. Herein, polyamide 12T (PA12T) micro-cellular foams with ultra-high compressive strength were fabricated via in situ polytetrafluoroethylene (PTFE) fibrillation using supercritical CO₂ foaming technology and a chain extender. The resulting branched structure showed considerably improved viscoelasticity and foaming performance, thus improving the cell morphology of the PA12T foam and exhibiting high VER. The PTFE fibrillation network induced melt strength enhancement, crystallization nucleation, and cell nucleation. The branched PA12T foam with 1.5 wt% PTFE exhibited the smallest cell diameter (15 μm) and highest cell density (3 × 10⁹ cells/cm³). The compressive strength of the foam (0.50 MPa under 5% strain) was 70% higher than that of pure PA12T. This research offers an effective method for producing high-VER PA12T foams with adjustable micro-cellular structures and excellent mechanical properties.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Influences of high-impact polystyrene loading on the foaming behavior and flame-retardant properties of polyphenylene oxide composites blown with CO2

Polymeric foam with high expansion ratio, well-defined cell structure, and excellent flame retardant properties is essential for broadening its applications. Polyphenylene oxide (PPO) is a kind of cost-effective engineering plastic with excellent flame retardancy, anti-dripping behavior, and good mechanical strength, but suffers from its poor processability. In this study, microcellular PPO composite foams were fabricated by applying a solid-state foaming technology using compressed CO₂ as the blowing agent. High-impact polystyrene (HIPS) phase was introduced with the aim to improve the fluidity and foaming ability of PPO composites. It was interesting to find that the 18–48% HIPS loading significantly increased the expansion ratio, that is, 1.8–3.3 versus, 10.8–14.3, and broadened the optimum foaming temperature of PPO composite foams, attributing to the miscible character between PPO and HIPS and excellent foaming ability of HIPS. Furthermore, the as-prepared PPO/HIPS composite foams exhibited high limited oxygen index (LOI) of 22.0–29.9%, low horizontal flammability rate (HFR) of 60.5–141.2 mm/min, and anti-dripping behavior, and the void fraction was verified to be a critical parameter to determine the flame retardant performance of the composite foam. Besides its lightweight and excellent flame retardant properties, PPO composite foams also presented uncompromised tensile properties and well-defined thermal insulation properties.     

Production and properties of mullite foam ceramic

Conclusions The rheological and production properties of mineralized foams (foam masses) obtained on the basis of aqueous mullite suspensions and foams from a glue-resin foaming agent have been studied.It is shown that the concentration of suspension and the foam-solid ratio have a significant effect on the properties of the foam masses and foam ceramic. A thermal-shock resistant mullite foam ceramic with a total porosity of 75–92% and an ultimate compressive strength of 1–22 MPa can be obtained.Translated from Ogneupory, No. 5, pp. 49–52, May, 1980.     

The effect of wet foam stability on the microstructure and strength of porous ceramics

Wet foam stability is of prime importance in fabricating porous ceramics with the desired microstructure and mechanical properties. In this research, wet foams were fabricated via direct foaming after separately adding an anionic surfactant (TLS) and a cationic surfactant (DTAC) into alumina slurries with a copolymer of isobutylene and maleic anhydride (PIBM) as both the dispersant and the gelling agent. The foam stability was evaluated by a stability analyzer. The bubble size rapidly increased in the wet foam with TLS as the foam stabilizer and many large bubbles appeared within 60 min. The wet foam containing DTAC was very stable. Cationic DTAC increased the hydrophobicity of alumina particles by interacting with the anionic PIBM adsorbed on the particles. The hydrophobically modified particles acted as the foam stabilizer and enhanced the wet foam stability. Furthermore, the fast gelling speed of the slurry containing DTAC also enhanced the wet foam stability. The average cell size of the ceramic with 82.9% porosity from the wet foam with TLS was 188 µm and the compressive strength was 9.7 MPa. The counterparts from the wet foam with DTAC were 54 µm of average cell size and 18.1 MPa of compressive strength. The superior stability of wet foam brought about a smaller cell size and higher strength of the resultant ceramic.