Spontaneous coagulation casting (SCC) is a novel in-situ ceramic forming method, involving an anion dispersant which acts as both dispersing and coagulating agent and works well at room temperature in air. Ceramic particles in a slurry prepared by SCC participate in the formation of organic network which is originates from hydrophobic interaction and hydrogen bonding among the dispersant molecular chains, quite different from other gelcasting systems. The ceramic gel formed by SCC is a physical gel and the formed organic network among ceramics particles possesses low density which is conducive to water transportation and stress relaxation during drying.

The research progress in the preparation of transparent ceramics by SCC over the last decade is reviewed in this paper, mainly including powder dispersion and high solid loading slurry preparation, slurry solidification and drying, densification of green bodies, and preparation of large size and high optical quality transparent ceramics. It involves translucent alumina, and hightransmittance transparent alumina ceramics, yttrium oxide, AlON, yttrium aluminum garnet and MgAl₂O₄ transparent ceramics. For the transparent alumina ceramics, the shear force in the flow of slurries during casting was used to horizontally arrange alumina platelets in the slurry of equiaxed alumina grains, grain orientation occurred through the platelet as templates during sintering. Alumina single-crystal-like structure formed and high transmittance alumina ceramics resulted. For yttria and AlON transparent ceramics, before SCC yttria and AlON powders were respectively treated with organic compounds to ensure them hydrophobic and water resistant because the powders easily react with water which changes the chemical composition of the powders and decreases solid loading of the slurries.

More recently, MgAl₂O₄ transparent ceramics with a dimension of 470mm×235mm×10 mm was successfully prepared by SCC and vacuum sintering. SCC has become a universal method for preparing transparent ceramics, which is promising in preparing transparent ceramics with complex shapes and high optical quality.

In the preparation of large-sized ceramics, the use of a green body with relatively high plasticity is crucial to minimize the risk of cracking during processing. To achieve this goal, glycerol and polyethylene glycol (PEG) were utilized as plasticizers in the shaping of green bodies of oxide ceramics through spontaneous coagulation casting (SCC). This study investigated the effects of plasticizers and particle sizes ranging from the submicron to nanoscale on the slurry viscosity, drying shrinkage of wet gels, and mechanical properties of green bodies. The plasticity of the green bodies was assessed by measuring the impact toughness and flexural stress‒strain curves. By incorporating an appropriate plasticizer, the peak width of the flexural stress‒strain curve for dried green bodies from particles of different sizes was nearly twice that without plasticizers, and the impact toughness was enhanced by approximately 71%, 34%, and 41% when the particle size decreased from the submicron scale to the nanoscale (0.45 μm, 0.18 μm, and 50 nm, respectively). The drilling test revealed that there was nearly no cracking around the holes in the green bodies with plasticizers. The plasticity mechanism of the green bodies was examined based on ultraviolet–visible (UV–Vis) spectroscopy and scanning electron microscopy (SEM). It was discovered that plasticizers might mitigate the brittleness of green bodies by adjusting the interactions between molecules and modifying the gel network properly.

A green body with a density as high as possible is critical to diminish the crisis of deformation or cracking when large-size parts are sintered. Here, a new method, i.e., re-fluidising the aged ceramic gel is developed to prepare the high-density green body. Alumina slurry with 56 vol% solid loading and copolymers of isobutylene and maleic anhydride were aged without vaporisation and re-fluidised by non-intrusive shearing after removing the exuded water. The re-fluidised slurry was re-casted. The resultant wet gel was dried and deboned at a low temperature. The relative density of the obtained green body was 64.6%, 1.5% higher than that without aging and re-fluidising. The linear sintering shrinkage of the body decreased by 0.7%. The enhanced green density is explained from the viewpoint of the solid loading and the structure of the slurry.

Transparent Al₂O₃ ceramics with grains aligned to the c-axis were prepared by adding platelets with a low aspect ratio into fine equiaxed particles. The mixed powders were formed into green bodies using spontaneous coagulation casting and sintered by pressureless sintering and hot-isostatic pressure sintering. Zeta potentials and rheological behavior of the slurries, relative densities of green bodies, and orientation and optical properties of sintered bodies were investigated and discussed. The platelet with a high aspect ratio suppressed densification more seriously during sintering than the one with a low aspect ratio. An excellent oriented structure was obtained when 5 wt% platelets with a low aspect ratio were added, and transparent Al₂O₃ ceramics with grains aligned to c-axis were successfully prepared; the in-line transmittance was 78.4% at 600 nm, which is the highest one in the currently reported literature.

Porous ceramics have been widely used in heat insulation, filtration, and as a catalyst carrier. Ceramics with high porosity and high strength are desired; however, this high porosity commonly results in low strength materials. In this study, porous alumina with high porosity and high strength was prepared by a popular direct foaming method based on particle-stabilized wet foam that used ammonium polyacrylate (PAA) and dodecyl trimethyl ammonium chloride (DTAC) as the dispersant and hydrophobic modifier, respectively. The effects of the dispersant and surfactant contents on the rheological properties of alumina slurries, stability of wet foams, and microstructure and mechanical properties of sintered ceramics were investigated. The microstructure of porous ceramics was regulated using wet foams to achieve high strength. For a given PAA content, the wet foams exhibited increasing stability with increasing DTAC content. The most stable wet foam was successfully obtained with 0.40 wt% PAA and 0.02 wt% DTAC. The corresponding porous alumina ceramics had a porosity of 82%, an average grain size of 0.7 µm, and a compressive strength of 39 MPa. However, for a given DTAC content, the wet foams had decreasing stability with increasing PAA content. A possible mechanism to explain these results is analyzed.

A₂B₂O₇ system compounds, which usually present three phase structures mainly based on the ionic radius ratios of r_A and r_B (r_A/r_B), have been studied for potential applications in many fields, such as thermal barrier coatings, luminescence powders, fast-ion conductors, photocatalysts, and matrices for immobilization of highly active radionuclides. Since 2005, La₂Hf₂O₇ was fabricated into transparent ceramics and much more attentions were paid on A₂B₂O₇ transparent ceramics for new applications. In this review, the development of A₂B₂O₇ system transparent ceramics was described. The structure characteristics, powder synthesis method, and sintering techniques of the final A₂B₂O₇ transparent ceramics were summarized. After that, the mostly reported A₂Hf₂O₇, A₂Zr₂O₇, and A₂Ti₂O₇ system transparent ceramics were systematically introduced. The potential application fields and future development trends were also discussed, focusing on scintillators, optical elements, and other luminescent materials.

Aluminum nitride ceramic sheets were formed for the first time by a novel and simple method, namely gel-tape-casting process, where only two additives were used to prepare the slurries. PIBM (a water-soluble copolymer of isobutylene and maleic anhydride) acted as both dispersant and gelling agent, and PEG (polyethylene glycol) acted as plasticizer. Combining the advantages of gel-casting and tape-casting processes, flexible and uniform green tapes were obtained. The rheological properties of AlN slurries were studied to optimize the PIBM content and solids loading. After drying and debinding, the green sheets were sintered at 1840 ℃ in N₂ atmosphere. Homogeneous microstructure with average grain size of about 7 μm was observed in the final AlN ceramics. Thermal conductivity of the AlN ceramics was 161 W/(m·K).