The microstructural characterization and mechanical response of YSZ ceramic foams fabricated via volume-controlled foaming

Yttria-stabilized zirconia (YSZ) ceramic foams are a promising class of materials for lightweight, high specific strength catalyst supports or insulation. Foam morphology is one of the most significant factors that dominate the mechanical properties of the YSZ ceramic foams. However, the foam morphology as a function of gravity and foam film strength for YSZ ceramic foams has been seldom reported up to now. Our work focuses on YSZ ceramic foams fabricated via a novel foam-gelcasting method using Isobam as gelling agent. The relative magnitudes of the foam film strength and the gravitational force can be changed by controlling the foaming yield of slurries. Both the remaining high-temperature strength and the critical difference temperature (△T_c) of YSZ (3.0) ceramic foams were higher than those of YSZ (5.0) ceramic foams, mainly owing to high closed-cells and relatively uniform distributed pore structure. In addition, the YSZ ceramic foams could not break suddenly like dense ceramics. This work demonstrates that tuning the foaming yield of slurries is a viable route to improved thermomechanical property in ceramic foams for use as insulation or catalyst supports in extreme environments.     

Ceramic shell foams produced by direct foaming and gelcasting of proteins: Permeability and microstructural characterization by X-ray microtomography

In this work, ceramic shell foams produced by direct foaming and gelcasting of proteins containing 35 vol.% of solids were efficiently tailored through the optimization of suspension parameters, foaming and also according to the gelling temperature. These were key parameters employed for controlling the foamability and foam stability, and hence the foam porosity and pore characteristics after the sintering process. The potential of using microtomography in characterizing the morphometric parameters of foams was explored. Airflow permeability measurements (∼24 °C to 727 °C) were performed in order to investigate the applicability of such materials in the fluid flow field. The samples exhibited a wide range of pore sizes (60 ± 5–1700 ± 90 μm), porosity values higher than 80 %, Darcian (k₁) and non-Darcian (k₂) permeability coefficients values in the ranges of 1.32–1.83 x 10⁻⁹ m² and 8.34–22.46 x 10^-5 m, respectively.     

Mechanical and dielectric properties of porous magnesium aluminate (MgAl2O4) spinel ceramics fabricated by direct foaming-gelcasting

Porous ceramic materials have been broadly applied in various fields due to their multifunctional properties. Optimization of their microstructural characteristics, such as pore morphology, total porosity, and pore size distribution, which determine various properties of the final products, is crucial to improve their performances and thus extend their applications. In this study, single-phase porous MgAl₂O₄ materials were fabricated by direct foaming–gelcasting. With an increase in the foam volume from 260 to 350 mL, the total porosity and pore size of the porous ceramic increased, and its microstructure varied from mostly closed cells to open cells containing interconnected large pores (40–155 μm) and small circular windows (10–40 μm) in the ceramic skeleton. The total porosity could be tailored from 84.91% to 76.08% by modulating the sintering temperature and foam volume and the corresponding compressive strengths were in the range of 2.8–15.0 MPa. The compressive strength exhibited a power-law relationship with the relative density with indices of approximately 3.409 and 3.439, respectively. Porous MgAl₂O₄ ceramics exhibited low dielectric constants in the range of 1.618–1.910 at room temperature, which are well matched with theoretical calculations on account of a modified Bruggeman model. The porous MgAl₂O₄ ceramics with good mechanical and dielectric properties controlled easily by various sintering temperatures and foam volumes are promising for practical applications.     

中铝质闭孔泡沫陶瓷砖的制备和性能研究

采用发泡法,以废石膏为发泡剂,通过干压成型工艺制备出了中铝质闭孔泡沫陶瓷砖。主要探讨了助烧熔剂和烧成制度等因素对闭孔泡沫陶瓷砖性能的影响。采用真空密度仪、万能测试机、导热仪分别测试了样品的真密度、抗压强度、导热系数,采用排水法测量了样品的体积密度、闭孔气孔率,通过SEM分析了样品的闭孔分布。实验结果表明:样品的真密度为2.691g/cm3,体积密度为1.324 g/cm3,抗压强度为4.82MPa,热导率为0.173 W/(m.k),闭孔气孔率为57.2%,,样品内闭孔分布均匀。     

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.     

Effect of CTAB as a foaming agent on the properties of alumina ceramic membranes

Alumina-ceramic membranes were prepared by gelcasting process using CTAB as a foaming agent. To increase the fineness, the starting alumina powder was milled for 1 h in a ball mill before the casting process. Particle size distribution and surface area measurements of the as-received and milled alumina powder were examined. The casted alumina membranes were sintered at 1500 °C. Sintering parameters in terms of bulk density (BD) and apparent porosity (AP) were determined by the Archimedes method. Pore size distribution of the sintered porous alumina membranes was measured using mercury porosimeter. Microstructure of sintered membranes was investigated by scanning electron microscope (SEM). Cold crushing strength (CCS) of the sintered specimens was also evaluated. The result revealed that the properties of porous ceramics such as porosity, average pore size, pore size distribution and cold crushing strength could be controlled by adjusting the preparation conditions e.g. solid loading, sintering temperature and foaming agent. The open porosity, cold crushing strength and average pore size of the alumina ceramics sintered at 1500 °C were around 58.35%, 18 MPa and178 nm, respectively.     

Harnessing Ostwald ripening to fabricate hierarchically structured mullite-based cellular architecture via the gelation network-triggered morphology-regulation method

Gas-in-liquid foam templating is considered as a promising method for commercial-scale production of porous materials. However, Ostwald ripening occurs frequently during the preparation, which could bring about inhomogeneous bubbles with large sizes and deteriorate the performance of resultant products. To tackle this challenging task, we report a gelation network-triggered morphology-regulation strategy during the foaming process. As an exemplar, boehmite sol was incorporated into the andalusite suspension to stabilize the foam framework. After casting, the wet foam was able to maintain its overall shape, since andalusite particles were anchored onto the gas-liquid interface by the gelation network during mechanical frothing. The samples calcined at 1400°C feature homogeneous bubble with hierarchical porous architecture, smaller pore size with a mean value of 5.32 μm and robust compressive strength of 11.97MPa at a porosity level of 84 %. Our approach substantially broadens the design philosophy of high performance porous ceramics.     

Preparation and properties of enstatite ceramic foam from talc

The effective method of preparation, stabilization and high temperature treatment of enstatite ceramic foam is described in this work. The technique is based on foaming of suspension of talc, on the stabilization of foam structure and on final high temperature treatment after drying. The spontaneous delamination of aggregates and the redistribution of talc particles in foam are driven by decreasing surface energy. The changes of phase composition as well as the mechanism and the kinetics of processes which take place during the thermal treatment were described. The treatment within the temperature range from 1150 to 1250 °C provides the ceramic foam via sintering without melted phase, whereas a liquid phase sintering occurs at higher temperatures. The final temperature of sintering is 1300 °C. Increasing amount of melted phase supports the formation of enclosed porosity and formed glass stabilizes the high temperature protoenstatite polymorph in the foam.     

Structure and Properties of Radiation Cross-Linked Polypropylene Foam

Polypropylene (PP) sheets obtained through a two-step process (masterbatch method) were crosslinked by electron beam irradiation. The crosslinked PP sheets were foamed in an oven under different processing conditions. The effects of foaming temperature and time on the mechanical properties and cell structure of PP foams were studied. With the foaming temperature increasing and foaming time lengthening, both the compression modulus and compression strength dropped. Scanning electron microscope (SEM) was employed to study the morphology and cell structure of different samples and the related morphology parameters were acquired. The results showed there was an optimum temperature and time that produced the maximum expansion ratio or the minimum foam density. As foaming temperature or time increased, the cell size increased and the cell density decreased regularly. Excessively high foaming temperature and overly long foaming time caused the coalescence and even the collapse of the cells.     

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.     

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.     

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%.     

Effect of Temperature on Foaming Ability and Foam Stability of Typical Surfactants Used for Foaming Agent

Foam has extensive applications in a wide range of industrial fields. Some surfactants are used as foaming agents in the preparation of foam. The performance of the foaming agent directly affects the application of the foam. In this paper, experiments were designed and conducted to reveal the influence of temperature on foaming performance of 10 typical anionic, cationic, nonionic, and amphiprotic surfactants. They were exposed to different temperature conditions to measure the foaming capacity (FC), foaming expansion (FE), and foam’s half-life. FC and FE represent foaming ability (FA), and half-life represents foam stability (FS). The results show that the FC increased at elevated foaming temperature, while FS decreased with rising temperature. Anionic surfactants are less affected by temperature and have better FA and longer FS. It seems that 20–30 °C is an ideal foaming temperature. This study lays an important foundation for the efficient preparation and utilization of foam in industrial fields.     

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.     

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.     

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.     

Foam processing of poly(ethylene-co-vinyl acetate) rubber using supercritical carbon dioxide

Soft rubber foams like poly(ethylene-co-vinyl acetate) (EVA) are industrially applied in a broad range of products, including sports gear, insulation materials and drug delivery systems. In contrast to glassy polymers, few studies in literature concern the foaming of soft rubbers using supercritical carbon dioxide. In this study, open microporous matrices of EVA have been formed with CO₂. Prior to the foam expansion, sorption and swelling isotherms of CO₂ in EVA have been measured and the obtained isotherms have been correlated using the Sanchez-Lacombe equation of state. Additionally, a pressure-independent diffusion coefficient of CO₂ in EVA has been obtained from these experiments. The microporous foams have been formed by a pressure quench of the CO₂-swollen polymer matrix. Sorption pressure as well as temperature and decompression times appear to determine the pore size and bulk density of the foam. These parameters allow for a control of the foam structure of EVA rubbers.     

凝胶注模工艺参数对泡沫凝胶注模法制备多孔陶瓷的影响研究

将传统的机械起泡工艺与水基凝胶注模成型工艺相结合,制备出了不同孔径、孔隙率高达62.8%的多孔陶瓷。研究了有机单体、引发剂和催化剂及交联剂等对胶凝时间的影响,并得到了其最佳加入量,以得到合适的胶凝速率,使气泡在破裂之前因有机单体的胶凝而固定在坯体内。气泡是在陶瓷浆料内加入表面活性剂后,通过机械搅拌产生的,其大小和形状用扫描电镜照片进行了表征。     

Effect of Sucrose on Fabrication of Ceramic Foams from Aqueous Slurries

Ceramic foams with porosity exceeding 90% were prepared by direct foaming and casting of aqueous suspensions containing cetyl trimethyl ammonium bromide (CTAB) as a foaming agent. Foaming of the suspensions, particularly with lower viscosity, was initially non-homogeneous but the foam appeared to homogenize with milling time. Addition of sucrose to ceramic suspensions resulted in lowering of the suspension viscosity, stabilized the foams by reducing drainage of the suspension, and minimized coalescence of bubbles leading to lower cell sizes in sintered foams. Ceramic foams prepared from sucrose based suspensions were strengthened to such an extent that foams with porosity above 90% could be machined in the green state.     

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.