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In order to fabricate a radome material with small volume shrinkage and not only good mechanical properties but also excellent dielectric properties, gel-casting and pressureless sintering were adopted to prepare porous silicon nitride (Si 3 N 4 ) ceramics. By adjusting the solid loading and the additives’ (Y 2 O 3 /Al 2 O 3 ) content, the drying and sintering shrinkage of ceramics could be controlled under 5% and 15%, respectively. Through decreasing temperature and prolonging holding time properly, the flexural strength of porous Si 3 N 4 with few sintering aids could get close to that of ceramics with more additives. Both the flexural strength and the logarithmic dielectric constant showed a linear relation with the porosity of ceramics. Typically, the porous Si 3 N 4 sintered at 1650 °C for 3 h with only 3 wt% sintering additives displayed relatively good performance, attaining a porosity of 39.28%, a flexural strength of 146.0 MPa, a fracture toughness of 1.44 MPa m 1/2 and a calculated dielectric constant of 4.03 at a resonant frequency of 14 GHz. [ABSTRACT FROM AUTHOR]

With starch as pore forming agent, porous silicon carbide (SiC) ceramics were successfully fabricated by gelcasting. A novel and simple gelling system of isobutylene and maleic anhydride (Isobam) can gel in air at room temperature was used. The rheological behaviors of SiC slurries were investigated as a function of starch content. The gelled SiC green bodies were sintered at 2050–2150 °C through solid state sintering with B 4 C and carbon as sintering additives. Porosity of the porous SiC ceramics sintered at 2100 °C was well controlled from 34.20% to 42.68% with the starch content increasing from 0 to 20 wt%. With increasing the sintering temperature from 2050 to 2150 °C, flexural strength and porosity of porous SiC ceramics with 20 wt% starch varied from 61.0 to 128.0 MPa and 48.1% to 34.2%, respectively. The porous SiC ceramics fabricated with 20 wt% starch addition and sintered at 2100 °C were proved to possess excellent thermal shock resistance. The ceramics after water-quenching at 1500 °C showed an average flexural strength of 95.8 MPa, which was higher than the strength of samples without quenching. [ABSTRACT FROM AUTHOR]

BN/Si 3 N 4 composite ceramics with different BN content were prepared by water-based gel casting process with raw and Y–Al-oxides-coated BN powder, separately. A Y–Al oxides layer with the thickness ranging from dozens to hundreds of nanometers, serving as sintering additives, was coated on the surface of BN particles with the method of precipitation of inorganic salts. As a result, the fluidity of BN/Si 3 N 4 slurry with Y–Al-oxides-coated BN powder was much improved compared to that with raw BN. SEM examination showed the Y–Al-oxides-coated BN particles were homogeneously distributed within the Si 3 N 4 grain boundaries, and BN aggregation was barely detected. The BN/Si 3 N 4 composite ceramic prepared by Y–Al-oxides-coated BN powder showed a better sintering behavior than those prepared by raw BN due to the formation of fine and homogeneous microstructure in it. Compared to the BN/Si 3 N 4 ceramic with 15 vol% raw BN, the density and flexural strength of the ceramic with the same content of surface-coated BN were significantly improved to 8.5% and 67.2%, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Yttria-stabilized zirconia (YSZ) fiber was introduced to reinforce porous YSZ ceramics fabricated by tert-butyl alcohol (TBA)-based gel-casting process and pressureless sintering. The influence of sintering temperature on microstructure and properties was investigated systematically. Results showed that linear shrinkage of the green bodies during sintering increased and porosity decreased with increasing sintering temperature. Interfacial bonding can be adjusted easily through different sintering temperatures. Compressive strength of the porous YSZ ceramics was improved significantly with the introduction of fibers and the strengthening effect reached its optimum at the sintering temperature of 1500°C. Compared with the specimens without fiber addition, the compressive strength of fiber-reinforced specimens was almost doubled and the highest reached 45.0MPa with the porosity of 58.5%. In addition, the thermal conductivity was decreased with the introduction of fibers. The improved mechanical and thermal properties make porous YSZ ceramics more applicable as thermal insulating materials at high temperature. [Copyright &y& Elsevier]

Abstract: In this work, alumina ceramics were prepared via gel-casting by mixing plate-like and granular alumina powders. Plate-like alumina with a hexagonal or disk-shaped morphology decreased the viscosity of the suspension remarkably, which made the casting process much easier. As the plate-like alumina content varied from 10wt%, 30wt%, 50wt%, 70wt% to 100wt%, the density and bending strength of the sintered ceramics increased firstly then decreased and reached their maximum at the content of 50wt%. However, the microhardness and fracture toughness presented negligible dependence on plate-like alumina content. It is worth noting that all samples with plate-like alumina displayed lower density and bending strength but higher fracture toughness than the sample without plate-like alumina. [Copyright &y& Elsevier]

Addition of spinel (MgAl2O4) to Al2O3–ZrO2 composite inhibits alumina grain growth and produces phase boundaries that leads to formation of a ceramic matrix composite with special mechanical properties such as high temperature superplastic deformation. However, the room temperature mechanical properties of Alumina–zirconia–magnesia spinel composite (AZM) such as fracture toughness were rarely investigated by researchers. In this research the AZM nanocomposite powders were synthesized via the solution combustion method. The dense AZM composite samples were fabricated through gelcasting process. Phase analysis studies were performed on both powder and sintered samples and the effects of spinel and zirconia contents on mechanical properties as well as microstructure of AZM composite samples were investigated. The maximum Vickers hardness (18.83±1.86GPa) and flexural strength (563.74±23.38MPa) were achieved in the sample with 10vol% spinel and 10vol% zirconia. The sample with 10vol% spinel and 20vol% zirconia had the maximum fracture toughness (6.67±0.81MPa√m). [ABSTRACT FROM AUTHOR]

Abstract: Complex-shaped ZrB2–SiC ultra high temperature ceramic components were successfully fabricated by gelcasting technique and pressureless sintering. When 0.4wt% poly(acrylic acid) (PAA) was used as dispersant and the pH value was 10, an aqueous ZrB2–SiC–B4C–C slurry with a high solid loading up to 50vol% but low viscosity (3.1Pas at 60s−1) was prepared. For gelcasting process, the suitable monomers content (AM and MBAM) and the mass ratio of monomers (AM/MBAM) were selected as 3wt% and 15:1, respectively. After pressureless sintering at 2100°C for 2h, ZrB2–SiC ceramics (using B4C and C as sintering additives) finally achieved about 98% density. Investigation of the mechanical properties revealed a flexural strength of 405±27MPa and a fracture toughness of 4.3±0.3MPam1/2. Complex-shaped ZrB2–SiC ceramic components with homogeneous microstructure were also successfully produced. This study confirms that gelcasting is a reliable process for the fabricating of complex-shaped ultra high temperature ceramic components with satisfied properties. [Copyright &y& Elsevier]

Abstract: The effect of solid volume fraction in slurries on properties of zirconia toughened alumina (ZTA, mass ratio of Al2O3–ZrO2 is 75:25) green and sintered bodies prepared by gelcasting process was investigated. A low-toxicity gel system based on the polymerization of low-toxicity N,N-dimethyl acrylamide (DMAA) was used for the gelcasting. Effect of monomer mass fraction on viscosity of slurries and effect of solid volume fraction on properties of green and sintered bodies were investigated. The solid loading of suspension that can meet requirements for the casting process is as high as 54vol.%. However, it is found that the highest bending strength and fracture toughness of ZTA composites can reach 643.3MPa and 6.3MPam1/2, respectively, when the solid loading was 50vol.%. The reason for this phenomenon is that the microstructure of the sintered samples is to a large extent determined by the rheological behavior of the suspension. SEM photographs revealed that the green body and sintered ZTA composites had a uniform microstructure when the solid loading was 50vol.%. When the solid loading was over 50vol.%, hard agglomerations formed which had a significant influence on the mechanical properties of ceramic bodies. [Copyright &y& Elsevier]

Abstract: With silicon nitride significant progress has been made in order to search for fully dense, strong, reliable structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more ceramic components based on Si3N4 are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect-free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs. On the other side, the high hardness of Si3N4 ceramics is almost always cost prohibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication processes that can produce complex-shaped components with a minimum of machining as well as to minimize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si3N4 ceramics is given in detail. All kinds of these techniques (injection molding, gelcasting, robocasting, mold shape deposition, rapid prototyping) and their advantages and disadvantages are explained. [Copyright &y& Elsevier]

Abstract: The composite Al2O3/ZrO2 ceramics was made by gelcasting in air and nitrogen ambience, respectively, at low temperature, with and without temperature gradient, respectively, to study the influencing factors on the surface exfoliation of green bodies. The surfaces of the ceramic green bodies formed under various conditions were characterized by SEM and FTIR techniques. The results showed that the temperature gradient and oxygen can inhibit the polymerization of AM/MBAM monomer and lead to surface exfoliation of the green bodies. And no polymerized monomer exhibited and the strength of green bodies was extremely low at low temperature, which can be solved by eliminating temperature gradient and removing O2 to solidify the ceramic slurry at a high temperature. We found that the well-developed binder networks of the polymerized monomers were formed not only in entrails but also in surface of the green bodies, confirmed by FTIR analysis and SEM. [Copyright &y& Elsevier]

Abstract: Gelcasting is an attractive forming process to fabricate ceramic parts with complex shape. In the present work, aqueous gelcasting of SiC was studied. SiC slurry (50 vol.%) for gelcasting was prepared with sintering assistants, Al2O3 and Y2O3. The slurry was solidified in situ to green body with relative density of 55.9±0.9% and flexural strength of 13.9±0.7MPa. SEM shows that ceramic powders in green body compact closely by the connection of polymer networks, and that the pores decrease greatly with the size less than 1μm. SiC samples were also obtained by the process of gelcasting and pressureless sintering at 2000°C for 1h in Ar atmosphere. The relative density and flexural strength of SiC sintered body are 97.3±0.4% and 637±156MPa, and the hardness and toughness are 20.68±0.80GPa and 3.85±0.23MPam1/2, respectively. [Copyright &y& Elsevier]

Abstract: Zeta potential and rheological properties of silicon suspensions have been measured in this work. The results show that the isoelectric point of silicon particles is at the pH of 1.6. It is also found that silicon particles are suitable for suspending in basic solution, provided that a proper pH value is chosen. Silicon suspension with 46vol.% particles displays a minimum viscosity at the pH of 9.6. The influence of particle concentration on the rheological properties of aqueous silicon suspensions is investigated under steady and oscillatory shear conditions. Furthermore, gelcasting of high concentrated aqueous silicon suspension is also discussed in this paper. By vacuum pumping, controlling pH values of suspensions, high volume fraction green bodies with uniform structure and certain strength were produced. [Copyright &y& Elsevier]

The surface exfoliation of silicon carbide green bodies prepared by gelcasting in air was dramatically eliminated by adding polyethylene glycol (PEG) in the ceramic suspension. Thus, the size of silicon carbide green bodies could be precisely controlled. In this paper, the zeta potentials of silicon carbide powders dispersed in aqueous solution and the rheological property of the suspension were investigated to determine the effect of the addition of PEG. It was found that the addition of PEG did not affect the colloidal behavior of silicon carbide suspension markedly, but increased the viscosity of silicon carbide suspension because the macromolecular chains of PEG interfered with the fluidity of silicon carbide particles in the suspension. As the amount of PEG increased, the surfaces of silicon carbide green bodies became more and more rigid and the surface exfoliation was completely eliminated when the PEG was over 2.5 wt.% (based on silicon carbide weight) in silicon green body. The mechanism by which PEG inhibited the surface exfoliation of silicon carbide green bodies is probably because that this polymer prevents the reaction between oxygen and monomer free radical, ⋅R&z.sbnd;CH2⋅, during the gelling procedure. [Copyright &y& Elsevier]

Gelcasting is a very useful technique for producing ceramic green parts with complex shapes. Although it has been successfully applied to ceramic systems consisting of submicron size powders, there is little report on gelcasting of ceramics containing coarse particles. In this work, we report the study on gelcasting of silicon nitride bonded silicon carbide (SNBSC) refractory. It was found that density of SNBSC made by gelcasting is strongly dependent on the solids loading and the organic binder content in slurry. A high solids loading in the slurry is desirable for achieving high density of the materials after sintering. Less amount of monomer is required for gelcasting of this material compared to ultra-fine ceramic powders. Moreover, excessive amounts of monomer could lead to low density after polymer burnout or sintering and low strength for the final products after sintering. [Copyright &y& Elsevier]

Alumina (Al2O3) green parts have been gelcast using three kinds of alumina with different particle size (mean particle size: 4.6 μm-AM-21, 0.32 μm-AKP-30, 10 nm-NS-10). The effects of particle size on gelcasting process and green microstructure have been investigated, including process parameters affecting homogeneous green microstructure. The optimum amount of dispersant for each starting material using D-3019 as dispersant was 0.2 wt.% for AM-21, 0.5 wt.% for AKP-30 and 5.0 wt.% for NS-10. The castable solid loading of each starting material, AM-21, AKP-30 and NS-10, was 59, 57 and 15 vol.%, respectively. The contents of monomer added in slips were 3 wt.% for AM-21 and AKP-30, and 6 wt.% for NS-10, respectively, and dimer (cross-linker) of 20% of the monomer concentration was added in all slips. The effect of starting materials, especially particle size, has been analyzed through microstructural analysis and green density. Microstructural analysis revealed that the size and distribution of pores in the green body was radically reduced with decreasing particle size. The green density maximum obtained was ≈59% (AM-21), ≈60% (AKP-30), ≈47% (NS-10) theoretical density (TD), at which the green strength measured was 7.0±0.8 MPa (AM-21), 11.2±0.9 MPa (AKP-30), 7.6±0.6 (NS-10). A deairing step applied before gelcasting enhanced the green strength and modified the microstructure. [Copyright &y& Elsevier]

Ovoalbumin and its hydrolysate were developed as economical and environmental friendly plasticizers for the production of engineering ceramics from micrometric- and nanometric-sized powders. These proteins are feasible as replacements for commonly used petrochemicals that suffer from problems of toxicity, volatility, flammability, and high cost. A key advantage to protein-based additives is that they are available from inexpensive plant and animal sources (e.g. waste products such as slaughterhouse blood, by-products from meat and plant production). Aqueous alumina suspensions and high-density aqueous alumina pastes were blended with such proteins to form green bodies that were easily moldable and could be sintered to full density without cracking. The application of proteins provides thermally induced gelation of aqueous alumina suspensions. [Copyright &y& Elsevier]