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.     

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.     

Fabrication of Si3N4 preforms from Si3N4 produced via CRN technique

Ceramic preforms with randomly distributed particles as reticulated porous structure which are generally used for metal infiltration as reinforcement, membranes, catalyst supports etc. Preforms are characterized by open porosity making possible their infiltration by liquid metal alloys. In this work, quartz powders using carbon black as a reducing agent were used for alpha Si₃N₄ powders synthesis through a carbothermal reduction and nitridation (CRN) process. The CRN process was carried out under nitrogen flow at 1,450 °C for 4 h. At high temperatures, carbon as reducing agent reacts with the oxygen of SiO₂, and the resulting metallic silicon compounds with nitrogen gas to obtain silicon nitride powder. The reacted powders were used to obtain reticulated ceramic by replica method. The powders containing various bentonite ratios were mixed in water to prepare slurry. The slurry was infiltrated into a polyurethane sponge. A high porous ceramic foam (preform) structure was achieved after burn out of the sponge. All ceramic preforms were sintered to increase stiffness (in the temperature range 900–1,350 °C). The sintered ceramic foams were subjected to compressive tests. The scanning electron microscopy was used to examine the reticulated ceramic foam structure, and X-ray diffraction analysis was performed to determine phases.     

Ultra low-density mullite foams by reaction sintering of thermo-foamed alumina-silica powder dispersions in molten sucrose

Ultra low-density mullite foams are prepared by thermo-foaming followed by reaction sintering of alumina-silica powder dispersions in molten sucrose. The foaming & setting time, foam rise, sintering shrinkage, porosity, cell size and compressive strength are studied as a function of ceramic powder loading, foaming temperature and magnesium nitrate (blowing agent and setting agent) concentration. Phase pure mullite is produced by reaction sintering at 1600 °C. The mullite foams produced without magnesium nitrate have porous struts and cell walls due to improper densification. The magnesium nitrate drastically decreases the foaming & setting time and increases the foam rise and cell interconnectivity. The MgO produced from the magnesium nitrate assists the densification of the mullite as evidenced from the non-porous struts and cell walls at higher magnesium nitrate concentrations. The maximum porosity of 94.92 and 96.28 vol.% achieved without and with magnesium nitrate, respectively, is the highest reported for mullite foams.     

Novel design of alumina foams with three-dimensional reticular architecture for effective high-temperature particulate matter capture

Ceramic foams with extensive interconnected pores have great application potential in high-temperature particulate matter (PM) capture. Considering that there are still challenges to synthesize ceramic foams with efficient filtration, a novel hierarchical-structured alumina foam with three-dimensional (3D) reticular architecture has been fabricated via combining chemical grafting pore-forming agent and polyurethane (PU) foaming technology. Carbon black is grafted with carbamate functional groups in order to enable a better dispersion in highly viscous PU. Submicrometer and micrometer-sized pores on the cell walls are observed in hierarchical-structured ceramic foams. The resulting alumina foam exhibits 95.2% removal efficiency for PM particles and low pressure drop of only 50 Pa when grafted carbon black content is 3 wt%. This filtration performance is much higher than that of existing ceramic materials. These features, combined with our experimental design strategy, provide a new insight to design high-temperature PM filtration materials with durable high performance.     

PET开孔泡沫的制备及其吸油性能研究

研究了均苯四甲酸二酐（PMDA）添加量、发泡温度和压力降对聚对苯二甲酸乙二醇酯（PET）开孔泡沫形成的影响。研究发现,当发泡温度为216℃,PMDA添加量为0.6份,压力降较高时,可以制备发泡倍率35倍、开孔率96.3%的PET开孔泡沫。将聚四氟乙烯（PTFE）和有机改性蒙脱土（MMT）引入PET开孔泡沫的制备,研究发现PTFE和MMT具有异相成核作用,减小了泡孔尺寸,拓宽了PET开孔发泡窗口温度,在（222～228℃）较宽的发泡温度窗口范围内成功制备了泡孔尺寸更小（10～100μm）、发泡倍率高达40倍的PET开孔泡沫。开孔泡沫可吸收汽油、柴油、煤油、轻质原油和重质原油等各种石油产品,本文对高开孔率开孔泡沫的吸油性能进行了研究,其吸收能力约为8～30 g/g。     

超临界CO_2发泡聚苯乙烯工艺研究

以超临界CO_2为发泡剂,采用釜压法在不同发泡工艺条件下制备了聚苯乙烯(PS)发泡试样,通过扫描电子显微镜对PS发泡试样的泡孔形貌进行了表征,探讨了不同发泡工艺对PS发泡试样发泡性能的影响。结果表明,随发泡温度的升高,PS发泡试样泡孔尺寸增大,泡孔密度下降,而泡沫密度呈现先降低后升高的趋势,发泡倍率与此相反;增大保压时间和保压压力,可提高试样的发泡效果。当发泡温度为136℃,保压压力为20 MPa,保压时间为4 h时,PS发泡试样的发泡效果最好,其泡沫密度为0.043 g/cm~3,发泡倍率为24.4,泡孔尺寸为59.8μm,泡孔密度为6.20×107个/cm~3。     

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.     

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.     

粉煤灰泡沫玻璃的试验研究

以粉煤灰和玻璃粉为主要原料,添加适量的发泡剂、稳泡剂等助剂,可制得粉煤灰泡沫玻璃。通过对制品体积密度、表观密度、开口孔孔隙率、吸水率等性能的测定,分析了发泡剂、粉煤灰掺量对泡沫玻璃性能的影响。实验表明,适量添加粉煤灰和玻璃粉,可制得性能较好的粉煤灰泡沫玻璃。     

Effect of foaming temperature and rubber grades on properties of natural rubber foams

Foaming temperature and grade of dry natural rubber were varied to evaluate their effects on the morphology and mechanical properties of natural rubber (NR) foams. Three different grades of NR were used; namely ENR‐25, SMR‐L, and SMR‐10. NR foams from these grades were produced at three different foaming temperatures, i.e. 140, 150, and 160°C. The study was carried out using formulated compositions containing sodium bicarbonate as the chemical blowing agent and were expanded using conventional compression molding technique via a heat transfer foaming process. The NR foams were characterized with respect to their relative foam density, density of crosslinking, cell size, compression stress, and compression set. Increase in foaming temperature resulted in lower relative density and larger cell size. It was also discovered that the crosslink density slightly decrease with increasing foaming temperature. For mechanical properties, the highest foam density resulted in the highest compression stress. Compression stress at 50% strain increased with increasing foaming temperature and ENR‐25 foam has the highest compression stress among the produced foams. The results showed that the morphology, physical, and mechanical properties of the rubber foams can be controlled closely by the foaming temperature and rubber grades. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

Ceramic foams with micron/sub-micron sized porous structure for efficient high-temperature particulate matter capture

Constructing high-performance air filters for high-temperature particulate matter (PM) removal is of great importance but remains challenging. Here we demonstrate the ceramic foam-based filters via polyurethane (PU) foaming using diatomite and kaolin powder as the starting materials. Diatomite was grafted by dodecanedioic acid (DA) to improve the dispersity in the PU slurry. The calcined products demonstrated a hierarchical pore structure with multiple well-controlled micron-sized window pores on the cell wall of the sub-micron bubble. The resulting products exhibited a high removal efficiency (96.3%) at a pressure drop of 33 Pa. This research showed that as-prepared ceramic foams can be potentially used as new high-temperature filters for high-temperature PM removal.     

Producing Ceramic Foams with Hollow Spheres

Ceramic foams have a large potential for many applications and can be produced in several ways. In the past, many have attempted to manufacture ceramic foams that combined sufficient strength and controlled microstructure. This study proposes a new two-step processing route to fulfill these requirements. First, sacrificial cores are coated with ceramic powder slurry and packed in a die. Second, the cores are connected together using another ceramic slurry coating. After they are burned out and sintered, ceramic foam structures can be obtained that have a density <10% of theoretical density. By varying the size and shape of the initial cores, ceramic foams with tailor-made microstructures can be easily produced.     

Effect of dispersion method and process variables on the properties of supercritical CO2 foamed polystyrene/graphite nanocomposite foam

In this study, polystyrene/nanographite nanocomposite foams were made by different compounding methods, such as direct compounding, pulverized sonication compounding, and in situ polymerization, to understand the effect of the process variables on the morphology of the nanocomposites and their foam. The foam was made by batch foaming using CO₂ as the blowing agent. Various foaming pressures and temperatures were studied. The results indicated that the cell size decreased and the cell morphology was improved with the advanced dispersion of the nanoparticles. Among the three methods, the in situ polymerization method provided the best dispersion and the resulting nanocomposite foam had the finest cell size and the highest cell density. In addition, adding nanoparticles as a nucleating agent can make foams of similar cell size and cell density at a much lower foaming pressure. This result can be explained by the classical nucleation theory. This discovery could open up a newroute to produce microcellular foams at a low foaming pressure. POLYM. ENG. SCI., 53:2061–2072, 2013. © 2013 Society of Plastics Engineers     

Synthesis and foaming of water expandable polystyrene-activated carbon (WEPSAC)

In this study, water acts as a co-blowing agent to support carbon dioxide (CO₂) in the extrusion foaming process of polystyrene (PS) to produce foams with very low density for thermal insulation applications. Herein, we report a simple suspension polymerization method to prepare water expandable polystyrene (WEPS) based on a PS/water containing activated carbon (AC) composite. AC pre-saturated with water was introduced into the styrene monomer to form a water-in-oil inverse emulsion without emulsifiers. Via suspension polymerization, water expandable PS/AC (WEPSAC) beads could be subsequently obtained. Low density PS foams (∼0.03 g/cc) were successfully produced in the CO₂ extrusion foaming process using WEPSAC. Because of lower foam density and better IR absorption due to the presence of water containing AC, WEPSAC foams provided a lower thermal conductivity than conventional talc reinforced PS foams.     

浅谈硅微粉在氧化铝泡沫陶瓷中的作用

本文简单阐述了硅微粉的分类、应用和性质,分析了硅微粉在氧化铝泡沫陶瓷中的作用及高温反应的结合机理,同时研究了硅微粉对氧化铝泡沫陶瓷浆料性能和制品性能的影响。经试验分析,适量的硅微粉可促进氧化铝泡沫陶瓷的烧结,提高制品的强度,而对氧化铝浆料无不良影响。     

高密度聚氨酯硬泡塑料／玻纤粉复合材料的研究

以聚醚多元醇、PAPI、催化剂、发泡剂和玻璃纤维等为原料,制备高密度聚氨酯硬泡及它与磨碎玻纤粉的复合材料。研究了不同密度硬泡的强度及磨碎玻纤粉粒径、预处理及其含量对复合材料强度的影响,不同复合材料的热稳定性。结果表明,随着密度的增加,硬泡的各种强度值总体上均呈逐渐增加趋势,其中500kg/m^3的聚氨酯的拉伸强度比200kg/m^3的提高了104．74％,冲击强度提高了194．84％;400目粒径的玻纤粉可使复合材料具有更高的拉伸强度、弯曲强度及压缩强度;玻纤的加入将降低材料的强度值,但偶联剂预处理可使它们有所改善;加入磨碎玻纤粉后,材料的热稳定性增加,且采用偶联剂KH550对玻纤粉进行预处理可进一步改善复合材料的耐热性能。     

酚醛泡沫的制备

本文介绍两种酚醛泡沫的制备,一种是用加热方法制备的普通酚醛泡沫,另一种是用尿素改性的酚醛泡沫的制备。前者的压缩强度较常温发泡提高一倍左右,吸水率降至原来的一半,导热系数由原来的0．033W／m·k降至0．028W／m·k,后者性能介于前者和常温发泡两种酚醛泡沫之间,但可降低成本。