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Sr‐containing ceramic products produced from geopolymer, (Na, Sr)‐GP, have been reported as excellent candidate materials for the immobilization of radionuclide 90Sr, despite sintering temperature at 1200°C. In this work, the product possessing the best immobilization performance (leaching rate of 1.29% for Sr2+) can be produced at 1000°C in the method of liquid‐phase sintering via the addition of fluxing agent B2O3 at 2.0 wt% in the geopolymer precursor. The mechanical properties of the geopolymer precursor (Na, Sr)‐GP were also enhanced via the promotion role of B2O3 to geopolymerization. It was revealed that the overall anti‐leaching ability of the Sr‐containing ceramic product not only depends on the microstructure of the loading substrate but also can be affected by the chemical state of radioactive strontium retention in the loading matrix. [ABSTRACT FROM AUTHOR]

Hexagonal BN particles (BNp) reinforced fused SiO2 matrix composite (BNp/SiO2) was regarded as a very important candidate material for potential applications on wave transparent thermal protection parts. In this study, pressurelessly sintered 15 vol% BNp/SiO2 composites were prepared by cold isostatic pressing (CIP) and gel-casting routes, respectively, using two kinds of fused silica powders with different particle sizes (5.82 and 3.24 μm in d50, respectively) as starting SiO2. It was demonstrated that the microstructure and mechanical properties of the composites were well dependent on the preforming techniques of green compacts. Gel-casting preforming route has an overwhelming superiority over CIP route on the thermal stability of fused SiO2 matrix against α-cristobalite crystallization, sinterability, and thus the mechanical properties. Using the finer fused silica (3.24 μm in d50) as the starting SiO2, the 15 vol% BNp/SiO2 composite sintered at 1375°C get a density of 2.05 g/cm3, the silica matrix primarily keep amorphous phase with uniformly dispersed nanosized α-cristobalite crystallites (<5 nm in diameter). The bending strength, fracture toughness and Young's modulus all reach the highest values, 101.5 ± 4.3 MPa, 1.57 ± 0.04 MPa·m1/2 and 61.3 ± 2.4 GPa, respectively. Microstructure characteristics, chemical bond information and mechanical properties of the as-prepared composites are correlated with their preforming routes, starting fused SiO2 particle size, sintering temperature, etc. based on X-ray diffractometry, scanning electron microscope, transmission electron microscope, high resolution transmission electron microscope, and FT-IR analysis. [ABSTRACT FROM AUTHOR]

Silicon nitride/silicon oxynitride (Si3N4/Si2N2O) ceramic foams with tailored microstructures were prepared by a gelcasting process using fly ash cenosphere (FAC) as a pore-forming agent. The slurry has a suitable viscosity so that the floating velocity of FAC in the slurry is slow enough to avoid delamination between the FAC and the matrix during consolidation. Experimental observation showed that a uniform distribution of FAC was achieved in the matrix. Sintering was carried out at 1500°, 1600°, and 1700°C, respectively, for 1 h under a nitrogen atmosphere. X-ray diffractometry analysis indicated that Si2N2O was formed via the reaction between FAC and Si3N4. Microstructural analysis revealed that spherical-shaped pores obtained from FAC were uniformly dispersed in the composite. Si3N4/Si2N2O composite ceramic foams with apparent porosities of 62.0%–67.4% and a bending strength of 21–39 MPa were obtained eventually. [ABSTRACT FROM AUTHOR]

The ZrB2- BN composite without ZrC impurity was prepared successfully using reactive hot pressing via carbothermal reduction of ZrO2 with B4 C, C, and BN powders. The influences of additives and sintering temperature on the phase and microstructure of composites were investigated. The formation reaction of ZrC was inhibited by decreasing the evaporation of intermediate product B2 O3 due to forming a SiO2- B2 O3 eutectic system with the addition of SiO2 or SiO2 - CaF2. In comparison, SiO2- CaF2 exhibited better inhibition capacity of ZrC. ZrB2- BN composite without impure ZrC phase was achieved. In addition, the content of ZrO2 decreased with increasing sintering temperature, and finally disappeared when sintered at 1800°C. Moreover, the relative density of the ZrB2- BN composite reached 96.9%. [ABSTRACT FROM AUTHOR]

The TaC0.7 composition was spark plasma sintered (SPS) at 1600°-1900°C using TaC and Ta as the starting powders. Densification process, phase evolution, microstructure development, and the mechanical properties of the composites were investigated. The results indicated that the TaC0.7 composition could be SPS to >95% of theoretical density in 5 min at 1600°C or above. The phase formation was associated with carbon diffusion from the starting particles TaC to Ta to form Ta2C and Ta6C5 as intermediate phases, and subsequently the equilibrium phases of TaC y and ζ-Ta4C3− x. The presence of lamellar ζ-Ta4C3− x regions enhanced the flexure strength (up to 972 MPa) and fracture toughness (up to 13.8 MPa·m1/2). [ABSTRACT FROM AUTHOR]