In this paper, in-situ coagulation of 0.9Al₂O₃–0.1TiO₂ suspension and microwave dielectric properties of 0.9Al₂O₃–0.1TiO₂ ceramics prepared by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method were proposed. The 0.9Al₂O₃–0.1TiO₂ suspension could be coagulated via controlled release of calcium ions from calcium iodate at an elevated temperature. The influence of tri-ammonium citrate (TAC) content, solid loading, and calcium iodate content on the rheological properties of the suspension was investigated. In addition, the influence of coagulation temperature on coagulation time and properties of green bodies was also studied. It was found that the stable 0.9Al₂O₃–0.1TiO₂ suspension could be successfully prepared by adding 0.3 wt% TAC and adjusting pH value to 10–12 at room temperature. 0.9Al₂O₃–0.1TiO₂ green bodies with uniform microstructures were coagulated by adding 8.0 g/L calcium iodate after treating at 70 ℃ for 1 h. 0.9Al₂O₃–0.1TiO₂ ceramics, sintered at 1500 ℃ for 4 h and annealed at 1100 ℃ for 5 h, showed uniform microstructures with density of 3.62±0.02 g/cm³. The microwave dielectric properties of 0.9Al₂O₃–0.1TiO₂ ceramics prepared by DCC-HVCI method were: ${\epsilon }_{\text{r}}$= 11.26±0.06, Q×f = 11569± 629 GHz, ${\tau }_f$= 0.93±0.60 ppm/℃. The DCC-HVCI method is a novel and promising route without binder removal process to prepare complex-shaped microwave dielectric ceramics with uniform microstructures and good microwave dielectric properties.

In this paper, novel Al₂O₃-based poly-hollow microsphere (PHM) ceramics were prepared using Si₃N₄ and Al₂O₃ PHMs as pore-forming agents. The effect of Si₃N₄ and Al₂O₃ PHMs with different percentages on properties of Al₂O₃-based PHM ceramics was investigated. Through adjusting percentage of Al₂O₃ PHMs, Al₂O₃-based PHM ceramics with enhanced properties are achieved. X-ray diffraction (XRD) results show that main phases of Al₂O₃-based PHM ceramics vary from β-SiAlON (z value increases from 2.9 to 4) to Al₂O₃ with the increase of percentage of Al₂O₃ PHMs from 10% to 100%. The different phase compositions result in different properties of Al₂O₃-based PHM ceramics. With the increase of percentage of Al₂O₃ PHMs, porosity of Al₂O₃-based PHM ceramics gradually decreases, while their shrinkage, flexural strength, and fracture toughness firstly decrease and then increase. Using different kinds of ceramic PHMs as pore-forming agents, various novel and high-performance porous ceramics could be prepared via optimizing percentage of ceramic PHMs in the future.