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Abstract The mechanical strength of highly porous ceramic foams shows significant attenuation with the increase of porosity, especially for ceramic foams with very high porosity level above 95%. Ultra-light bulk composite ceramic foams were fabricated via introducing 20 wt% zirconia in alumina-based cell wall, aiming at improving mechanical strength of alumina based ceramic foams. The preparation and stability of alumina and zirconia particles synergistically stabilized foams were investigated. The obtained bulk ceramic foams manifested an exciting combination of both high porosity above 96% and excellent specific compressive strength, due to their closed pore structure, defect-free cell wall with uniform thickness, and uniformly distributed zirconia grains in alumina matrix. The alumina/zirconia composite ceramic foams with porosity of 96.7% exhibited compressive strength of 1.11 ± 0.07 MPa, which was much higher than that of alumina ceramic foams in the case of same porosity. [ABSTRACT FROM AUTHOR]

An innovative approach for fabricating porous alumina ceramics (PACs) with improved mechanical and thermal properties using walnut shell powders as pore-forming agent combined with alumina sol impregnation is reported in the present work. It is demonstrated that uniform distribution of spherical pores can be observed in as-prepared PACs by using above technical route. The decrease of walnut shell powder sizes significantly promotes the enhancement of crushing strength and reduction of thermal conductivity of the PACs. Meanwhile, the impregnated alumina sol is favoring for the formation of spherical micro-pores, then further improves their mechanical and thermal insulation performances. The lowest thermal conductivity and highest crushing strength of resulting sample reach 0.16 W/m K and 29.2 MPa, respectively. This novel method offers new possibilities to fabricate high-quality PACs. [ABSTRACT FROM AUTHOR]

The generation of PVA hydrogel through freeze-thaw technique has been utilized innovatively to complete the forming process of porous Si 3 N 4 ceramics in the present work. In comparison with gelcasting method, green body with comparable strength has been achieved by this novel forming process, which is owing to the strong function of PVA gel caused by the freeze-thaw cycles. Herein, Si 3 N 4 hollow spheres are employed as pore-forming agent in order to achieve closed pores. The morphology of green body illustrates that pores originate from both hollow spheres and the space occupied by ice crystals, and the latter gives rise to the increase of porosity around 10%. Studies on the mechanical, dielectric and thermal properties of porous Si 3 N 4 ceramics dependent on freeze-thaw cycle and freezing time have been performed. It is found that porous ceramics treated for freeze-thaw 2 cycles with freezing time of 24 h present excellent properties, which are promising to be applied as radome materials. [ABSTRACT FROM AUTHOR]

Compressive strength is one of the most important mechanical properties for ceramic foams but the dependence of compressive strength of highly porous ceramic foams on porosity remains unclear up to now, especially for the ceramic foams with porosity above 94%. In this paper, the relationship between compressive strength and porosity of ceramic foams with high porosity in the range of 79–98% was studied for the first time. The alumina ceramic foams and zirconia foams were prepared by particle-stabilized foaming method using sodium dodecyl sulfonate (SDS) as foaming agent. It was found that the function relation between compressive strength and porosity depended on porosity level. Specifically, compressive strength of ceramic foams fitted well with Rice Model when the porosity ranged from 79% to 94%. When the porosity increased above 95%, compressive strength of ceramic foams showed linear relation to porosity, which was probably due to the characteristic of highly porous ceramic foams with thin and lamellate cell wall. [ABSTRACT FROM AUTHOR]

Herein, the preparation of millimeter-sized poly-hollow alumina spheres (PHASs) with high-surface-area is proposed via an innovative and facile ball dropping method based on particle-stabilized emulsions. The PHASs fabricated by this method possess high specific surface area because of their hierarchical pore structures and high porosity, and exhibit remarkable superiority over the commercially available active alumina materials with specific surface areas of 200–300 m2g-1. The resultant millimeter-sized PHASs have diameters in range of 2–6 mm, average mesoporous sizes of 4–8 nm, and excellent specific surface areas of 400–500 m2 g-1, which are achieved with oil-water volume ratio of 3:1, rotation speed of 900 rpm, and sintering temperature of 500 °C. These high-performance PHASs with ultra-high specific areas and hierarchical pore structures can potentially be useful in various applications. [ABSTRACT FROM AUTHOR]

Abstract A facile method for preparation of SiC nanofibrous network reinforced hierarchical structured porous Si 3 N 4 based ceramics via annealing Si-Si 3 N 4 binary particle-stabilized foam in coke bed is reported in this work for the first time. The atmosphere created by coke bed enables the fabrication of Si 3 N 4 based porous ceramics at 1450 °C without using sintering additives. As prepared specimens possess a relative high compressive strength due to the partial oxidation of Si 3 N 4 particles. This novel structure with three levels of hierarchy characterized by flexible SiC nanofibrous network, cell walls and unit cells can be characterized. The vaporization of silicon powder results in formation of Si(g), SiO(g) and allows subsequent generation of SiC nanofibrous network, leading to a considerable number of micro-pores on cell walls simultaneously. As-prepared porous ceramics contained 76.7% porosity and had compressive strength of 10.81 MPa. [ABSTRACT FROM AUTHOR]

As a copolymer of isobutylene and maleic anhydride, Isobam, a newly-developed gelling agent, plays a significant role in ceramic forming process. However, dispersion and gelation behavior of Isobam has not been discussed in detail yet. To investigate the influence of other dispersants, pH and electrolytes on dispersion and gelation behavior of Isobam, zeta potential, viscosity and modulus curves during gelation were measured from 50 vol% alumina suspensions with 0.1–0.7 wt% Isobam. A two-step bridging mechanism is proposed to explain the gelation behavior of Isobam, providing guidance on usage of Isobam in combination of other additives to achieve suspensions with high solids loading and adjustable gelation characteristic. [ABSTRACT FROM AUTHOR]

Isobam, a copolymer of isobutylene and maleic anhydride, is a newly developed gelling agent for ceramic colloidal forming. However, the Isobam system exposes a weakness of high viscosity when solids loading exceeds 54 vol%. Suitable additives to effectively decrease viscosity and have minimal impact on gelation strength were discovered from various dicarboxylic acids through investigation on zeta potential, viscosity of suspensions, shear modulus during gelation and yield stress and modulus of elasticity of wet green bodies. Alumina ceramics with flexural strength of 442 ± 49 MPa and a Weibull modulus of 10.8 were produced from 54 vol% suspensions with 0.5 wt% Isobam assisted by 0.04 wt% phthalic acid and 0.03 wt% oxalic acid after pressureless sintering. [ABSTRACT FROM AUTHOR]

Mechanically robust alumina ceramics with low shrinkage were successfully prepared via gelcasting using Isobam as the gelling agent and pre-sintered alumina powder as the raw material for the first time. The influence of amount of pre-sintered alumina powder and powder's pre-sintering temperature on the viscosity of suspension and mechanical properties of low-shrinkage alumina ceramics were systematically investigated. The results showed that low-shrinkage alumina ceramics with flexural strength of 294.5 ± 33.0 MPa, linear shrinkage of 7.79% and relative density of 91.5% can be obtained with the addition of 60 wt% alumina powder pre-sintered at 1600 °C. This convenient and facile approach is promising to fabricate low-shrinkage alumina ceramics with large sizes, high dimensional accuracy and complex shapes. [ABSTRACT FROM AUTHOR]

In order to reduce thermal conductivity but to exhibit better mechanical properties of porous alumina ceramics (PACs), a novel approach, based on the modification of pore structures (i.e. mean pore size, micro-pore proportions and pore shape factor) by impregnation of alumina sol, is reported in the present work. The properties of as-impregnated samples were compared with commercial alumina hollowsphere bricks. Results show that impregnation of alumina sol in PACs has a positive influence on pore structures, leading to a superior strength and a lower thermal conductivity. In particular, when 8 wt% alumina sol solid content is impregnated, the crushing strength (35.3 MPa) and thermal conductivity (0.15 W/m K at 200 °C) of sample are much better than that of the commercial alumina hollowsphere bricks. This novel method for excellent PACs provides promising applications as insulating lining materials in the fields of industrial furnace. [ABSTRACT FROM AUTHOR]

We prepared porous Si 3 N 4 ceramics with highly ordered and unidirectionally aligned channels by the self-organization method using an alginate template. The obtained samples show uniform pore distribution, high cell density, good mechanical strength and hierarchical pore structure. A great number of long fibrous β-Si 3 N 4 grains protrude into channels, and other short length fibrous β-Si 3 N 4 grains interlock within channel walls forming a peculiar hierarchical pore structure. The pore size, thickness of channel walls, cell density, porosity and mechanical properties of the sintered porous Si 3 N 4 ceramics could be controlled by choosing different solid loadings, whereas β-Si 3 N 4 grains growth could be controlled by adopting different sinter temperatures. [ABSTRACT FROM AUTHOR]

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods. [ABSTRACT FROM AUTHOR]