Porous YSZ Ceramics Reinforced by Different Kinds of Fibers

Porous YSZ ceramics reinforced by different fibers were prepared by gel‐casting with 15% solid content and pressureless sintering. The four kinds of fibers (mullite, aluminosilicate, Al₂O₃, and YSZ fibers) were added into the YSZ ceramics with the same 10% vol content. After sintered at 1500°C for 2 h, aluminosilicate and mullite fibers could not be found in the samples of porous YSZ ceramics, which showed they reacted with YSZ ceramics at high temperature, while YSZ and Al₂O₃ fibers still kept perfect after sintering. Furthermore, the influences of fiber content, sintering temperature, porosity of matrix materials on compressive strength and porosity of the porous YSZ ceramics were studied. The results showed that Al₂O₃ fiber showed more obvious reinforcing effect than YSZ fiber on porous YSZ ceramics. The fiber‐reinforcing effects depend on fiber content, sintering temperature, and porosity of matrix materials. The fiber addition can improve the shrinkage behavior of porous ceramics during sintering and strengthen the skeleton of porous ceramics.     

粒子稳定型泡沫浆料及多孔氧化铝陶瓷的制备

采用短链两亲分子戊酸修饰氧化铝颗粒,使其部分具有的疏水性,在机械搅拌的作用下,形成了粒子稳定型泡沫(particle-stabilized foam),制备了一种新型的超稳定陶瓷泡沫浆料.研究了这种浆料的pH值对发泡率的影响,发现在pH值为4.8附近,戊酸对氧化铝颗粒的表面修饰作用最好,发泡程度最大;通过改变pH值,能够调整浆料的发泡程度,以满足不同应用领域对发泡率的要求.采用凝胶注模成型工艺,利用粒子稳定型泡沫浆料,成功制备了具有相互连通气孔-窗口(cell-window)结构的多孔陶瓷,由于其致密的支架结构使其具有高抗压强度,对于气孔率为85%的多孔氧化铝,其抗压强度在8MPa以上.     

Ammonium hydrogen carbonate provided viscous slurry foaming—A novel technology for the preparation of porous ceramics

In current research a novel technology for the preparation of porous ceramics was developed. The ammonium hydrogen carbonate was used as foaming agent for the generation of pores in the glycerol-based viscous ceramic slurry. Total and open porosity of obtained ceramics depends on the amount and granulometric distribution of NH₄HCO₃ as well as particle size of HAp powder used for the preparation of viscous slurry. Varying amount of NH₄HCO₃ in the range from 0 to 2.75 wt.%, open and total porosity increased from 25 to 69% and from 32 to 73% respectively. The formation of well-connected open porosity with irregularly shaped pores was observed for sintered ceramics.     

Synthesis and characterization of mullite-ZrO2 porous fibrous ceramic for highly efficient oil-water separation

This work aimed to proposing a new strategy for preparing the mullite-ZrO₂ porous fibrous ceramic used as alternative matrix material for oil-water separation by the aqueous gel-casting method. The properties of the fabricated porous fibrous ceramics in terms of microstructure, phase composition, apparent porosity, bulk density and compressive strength were investigated and the separation behavior was predicted by analyzing the structural changes. It is demonstrated that the phase composition of green bodies consisted of bayerite, boehmite, ZrSiO₄ and YSZ, and the sintered sample contained mullite, ZrO₂ and YSZ. As the YSZ fibers increased, the porosity of the fabricated porous ceramic increased with the maximum value of 70.65% due to the formation of more pores caused by YSZ fibers. Moreover, a significant increase in compressive strength (up to 9.52–21.86 MPa) was observed with the increase of YSZ fibers. Therefore, the fabricated porous ceramics could be appropriative for advanced applications of separation membranes for oil-water separation.     

Effects of mono-dispersed PMMA micro-balls as pore-forming agent on the properties of porous YSZ ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were successfully fabricated by the dry pressing method with different size (1.8–20 μm) and amount (2–60 vol.%) of mono-dispersed poly methyl methacrylate (PMMA) micro-balls. Different PMMA additions with different size and amount were investigated to achieve optimal thermal and mechanical properties. With increases of the amount of PMMA, the porosity of porous YSZ ceramics ranges from 7.29% to 51.6%, the flexural strength increases firstly and then decreases, and the thermal conductivity decreases continuously. With decreases of the diameter of PMMA micro-balls, the mean pore size and thermal conductivity of porous YSZ ceramics decrease, and the flexural strength of porous YSZ ceramics with same porosity increases firstly and then decreases. The porous YSZ ceramics with a higher porosity (18.44 ± 1.24%), the highest flexural strength (106.88 ± 3.2179 MPa) and low thermal conductivity (1.105 ± 0.15 W/m K) can be obtained when the particle diameter and the amount of PMMA are 5 μm and 20 vol.%, respectively.     

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.     

Preparation and characterization of porous ceramics from nickel smelting slag and metakaolin

Porous ceramics with high porosity and low bulk density were prepared by using nickel slag and metakaolin as the primary raw materials, glass powder as flux, and SiC as the foaming agent. The content of nickel slag and foaming agent had a significant effect on the bulk density, porosity, and flexural strength of the porous ceramics. The porous ceramics with the best properties were obtained at 1100 °C for 30 min with 50 wt% nickel slag, 40 wt% metakaolin, 10 wt% waste glass, and 0.8 wt% SiC. It had a low bulk density (as low as 245 kg/m³), high flexural strength and compressive strength (0.6 MPa and 1.17 MPa, respectively), and high porosity (about 89.8%). The nickel slag was magnetically separated as well. The density of nickel slag powder could be reduced via magnetic separation, and there was no significant change in the crystal structure of the raw material. Compared with porous ceramics prepared using nickel slag without magnetic separation, ceramics subjected to magnetic separation had lower bulk density, higher porosity, and the same phase composition. This study can be used as an indicator for the application of nickel slag in porous ceramics, which is of great significance in providing a great substitute nickel slag towards recovery and utilization.     

Porous ceramics with near-zero shrinkage and low thermal conductivity from hazardous secondary aluminum dross

Herein, a simple, versatile, and low-cost approach has been proposed to realize the green utilization of secondary aluminum dross, the hazardous solid waste, namely directly sintering dry-pressed green bodies from secondary aluminum dross to fabricate porous ceramics according to high-temperature foaming process spontaneously without adding spare foaming agents. Aluminum nitride (AlN) in secondary aluminum dross was employed to realize high-temperature foaming due to its oxidation, which makes traditional AlN and salts removal process needless. Moreover, near-zero shrinkage or even expansion during sintering of porous ceramics have occurred because in-situ foaming process together with the oxidation of Al particles well offset the sintering shrinkage. After sintering at 1400°C for 2 h, porous ceramics composed of α-Al₂O₃ and spinel phases with open porosity of 37.91%, sintering expansion rate of 1.13%, flexural strength of 45.67 MPa, and thermal conductivity of 0.97 W/(m·K) have been prepared. Cenospheres as pore-forming agents have been added to further improve the porosity, and alumina-based porous ceramics with open porosity of 28.39%–43.20% and flexural strength of 15.80–52.48 MPa have been obtained. This effective solution for recycling secondary aluminum dross could supply high-performance porous ceramics, which is expected to be applied in the fields of light-weight structural components and thermal insulations.     

Properties of Microstructure-Controllable Porous Yttria-Stabilized Ziroconia Ceramics Fabricated by Freeze Casting

Porous yttria-stabilized ziroconia (YSZ) ceramics were fabricated by freeze casting using aqueous ceramic slurries. Polyvinyl alcohol (PVA) was added to the slurry with the aim of controling the microstructures and properties of the porous YSZ ceramics. The experimental results indicated that the large and noninterconnected lamellar pores of YSZ ceramics without PVA transformed changed into small and interconnected lamellar pores with PVA addition. The porosities of YSZ ceramics with PVA addition were higher than those of YSZ ceramics without PVA addition. The compressive strength of porous YSZ ceramics is the range from 23.57 to 63.86 MPa. All specimens exhibit noncatastrophic failure behaviors.     

A novel method based on ultrastable foam and improved gelcasting for fabricating porous mullite ceramics with thermal insulation–mechanical property trade-off

Foam instability and long drying cycle limits the widespread use of foaming method. In this paper, a kind of porous mullite ceramic with thermal insulation–mechanical property trade-off were fabricated via novel ultrastable foam and improved gelcasting procedure. The solidification process and stability of foam slurry, as well as the thermal, mechanical property and pore structure of the porous mullite ceramics were investigated. The results showed that porous mullite ceramics with different bulk densities could be prepared via varying volume of foam which was stable enough to be maintained in slurry for a long time. The accelerated gelation rate as well as the gelation degree resulted in the improved gelcasting method led to a shortened period of drying and demould. The obtained pores, which were small, smooth, and unimodal distributed in size in porous mullite ceramics, contributed to achieving the trade-off between thermal insulation and mechanical property.     

Porous YSZ ceramics by water-based gelcasting

Gelcasting, as a novel method to form ceramic bodies, has been successfully developed to fabricate porous YSZ ceramics with an open porosity of 33.1–50.3%, mean pore size of 0.66–0.98 μm and the nitrogen permeability of 215–438 m³/m².bar.h. In order to further illustrate the features of this water-based gelcasting process to prepare porous ceramics, the same YSZ powders were blended with the same additives, and then cold pressed and sintered at the same conditions employed for gelcasting process. Compared with the cold pressed samples, the gelcast bodies exhibit higher open porosity, lower closed porosity, relatively larger pore size and thus higher gas permeability. Therefore, the developed gelcasting process is a very effective method to fabricate porous ceramics for filters or supports.     

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al₂O₃-bonded porous fibrous YSZ ceramics with ρ-Al₂O₃ and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al₂O₃ binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.     

Thermal Insulation Materials of Alumina Based Porous Ceramics

Significant energy saving effects can be made through the improvement of furnace refractories,especially the thermal insulation refractories. In this study,the preparation and the application of different alumina based porous ceramics were briefly introduced. Alumina based porous ceramics were prepared combined foaming method with gelcasting,sol- gel process or cement curing process. The influences of different preparation methods on the sintering shrinkage, porosity, phase composition, microstructure, compressive strength and thermal conductivity were discussed. Alumina based porous ceramics with relatively high strength and low thermal conductivity could be obtained through the above mentioned methods. Compared with the traditional lining materials,about 40% energy could be saved when they were used as the furnace wall.     

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.     

石棉尾渣和粉煤灰制备堇青石多孔陶瓷及其理化性能

低成本制备堇青石多孔陶瓷一直是专家学者们研究的热点,本文以石棉尾渣、粉煤灰、高岭土为原料,在不添加发泡剂的情况下,采用直接烧结法成功制备了堇青石多孔陶瓷,系统研究了堇青石多孔陶瓷的物相演化、显微结构及理化性能。结果表明:烧结温度的升高和配方中高岭土含量的增加有助于样品中堇青石的合成,高岭土的添加可以有效降低样品发泡的温度和提高样品的孔隙率;当烧结温度为1 240 ℃,焙烧后的石棉尾渣、焙烧后的粉煤灰和高岭土质量比为5∶5∶3时,制备的堇青石多孔陶瓷的体积密度仅为0.6 g/cm³,孔隙率高达76.94%;当烧结温度为1 220 ℃,焙烧后的石棉尾渣、焙烧后的粉煤灰和高岭土质量比为5∶5∶5时,制备的堇青石多孔陶瓷吸水率达到最大值34.57%;此外,制备的堇青石多孔陶瓷还表现出良好的耐碱性能。     

Porous YSZ ceramics with unidirectionally aligned pore channel structure: Lowering thermal conductivity by silica aerogels impregnation

The previous report of this work has demonstrated the fabrication and properties of porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels. As a follow-up study, the present work aims at lowering the thermal conductivity of the porous YSZ ceramics by silica aerogels impregnation. The porous YSZ ceramics were immersed in an about-to-gel silica sol. Both the unidirectionally aligned pore channels and the inter-grain pores by grain stacking in the channel-pore wall of the porous YSZ ceramics were impregnated with the silica sol. After aging and supercritical drying, silica aerogels formed in the macroporous network of the porous YSZ ceramics with unidirectionally aligned pore channels. The influences of silica aerogel impregnation on the microstructure and properties of porous YSZ ceramics with unidirectional aligned pore channels were investigated. The porosity decreased after impregnation with silica aerogels. Both microstructure observation and pore size distribution indicated that both channel-pore size and inter-grain pore-size decreased significantly after impregnation with silica aerogels. Impregnating porous YSZ ceramics with silica aerogels remarkably lowered the room-temperature thermal conductivity and enhanced the compressive strength. The as-fabricated materials are thus suitable for applications in bulk thermal isolators.     

Porous Silicon Carbide Ceramics Produced by a Carbon Foam Derived from Mixtures of Mesophase Pitch and Si Particles

Carbon foam templates were prepared from a mixture of mesophase pitch (MP) and Si particles, followed by foaming and carbonization. Subsequent molten Si infiltrated into the carbon foam at 1500°C for 4 h in an inert atmosphere resulted in the formation of porous SiC ceramics. Micrographs were investigated by a scanning electron microscope (SEM), and phase identification of porous SiC ceramics was performed by X-ray diffraction (XRD). The flexural strength and bulk density of porous SiC ceramics were also measured and calculated. The results revealed that the flexural strength of porous SiC ceramics increases with increasing Si content and decreasing porosity. The addition of Si in MP results in an increased densification of porous SiC struts. With 50 wt% Si, porous SiC ceramics with a high flexural strength of 23.9 MPa and a porosity of 55% were obtained.     

Bubble behaviors in chemical direct foaming process and effects on pore structures of geopolymer foams

Geopolymer foams (GPFs) are considered potential candidates for the highly porous ceramics owing to their high porosity and simple synthesis. In this study, bubble behaviors during different phases of the foaming process and their effects on the pore structure of molded GPFs were examined. The foaming reaction characteristics in a foaming system containing H₂O₂ were adjusted based on variables, such as catalyst content, temperature, activator-to-precursor ratio, and surfactant content. The viscosity of the slurry was also measured under different experimental conditions. Bubble behaviors were determined by characterizing the change in the gas volume in the slurry and the pore structure of the molded GPFs. Different pore structures will be realized by adjusting the relationship between the extrusion effect and liquid film properties in the various foaming phases.     

Formation of 3D tubular cannel in fully densified zirconia ceramics by free-foaming method using superplasticity

Three-dimensional (3D) tubular cannels in fully densified zirconia ceramics have been fabricated by a newly developed solid-state free-foaming method using superplasticity. Three mole percent of yttria-stabilised zirconia (3YSZ) and α-SiC were used as foam matrix and foaming agent simultaneously. Dried pattern of 15 wt% (wt%) α-SiC slurry based on 10 wt% methyl cellulose buried into 3YSZ powder compacts and proper heat treatment at the sintering temperature causes successful fabrication of 3D cannel in zirconia ceramics.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.