Search

Microstructural evolution and high‐temperature ablative damage mechanism of Csf/SiC‐BN composites with and without polyborosilazane (PBSZ) as sintering aids that can convert to polymer‐derived SiBCN ceramics (PDCs‐SiBCN) by cross‐linking and pyrolysis during sintering were studied. The density of Csf/SiC‐BN composites increases from 2.17 to 2.25 g/cm3 with 10 wt.% sintering aids because PBSZ promotes rearrangement and redistribution of raw materials by pyrolysis during the sintering process. With the increase of carbon fiber content, ablation performance (mass ablation rate: 15.25 mg/s; linear ablation rate: 0.100 mm/s) negligibly decreases, compared to counterparts with lower fiber content (mass ablation rate: 16.53 mg/s; linear ablation rate: 0.072 mm/s) because the presence of PDCs‐SiBCN partially compensates for the detrimental effect of carbon fibers on ablation performance. Csf/SiC‐BN composites with and without PBSZ show similar phase composition mainly containing quartz, amorphous SiO2, and matrix crystallization products and morphology after ablation because of similar phase composition before ablation. The ablation mechanism is oxidative corrosion and mechanical scouring and stripping. [ABSTRACT FROM AUTHOR]

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]

Heat dissipation material with programmable anisotropic property is very challenging, yet can realize the controllable thermal diffusion for heating device. In this work, anisotropic Si3N4 ceramics with oriented grains are prepared to adjust and improve the mechanical and thermal properties under the applied stress field by rolling film forming technology. Through the design of the sintering aids in the process of liquid‐phase sintering, the orientation degree of the Si3N4 grains is programmable as well as the mechanical property and the thermal property of the Si3N4 ceramics. As a consequence, the obtained Si3N4 ceramics show significant anisotropy in mechanical properties and thermal conductivity. The typical fracture toughness and thermal conductivity along the grain orientation direction are 10.6 MPa⋅m1/2 and 45.45 W/(m⋅K) while they are 4.5 MPa⋅m1/2 and 66.42 W/(m⋅K) in the direction perpendicular to the oriented grain, respectively. This grain orientation method paves the way for the thermal performance design and the production of programmable heat dissipation material. [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]

Boron nitride-barium aluminosilicate (BN-BAS) composite ceramics were fabricated by the sol-gel method and hot-pressed sintering. The effects of the introduction method of BAS and sintering temperature on the phase assemblage, density, apparent porosity, mechanical properties and microstructures of the h-BN composite ceramics were investigated. The following two introduction methods were adopted: (A) BAS gel powder was prepared by the sol-gel method and then mixed with h-BN for hot pressing sintering, and (B) the BN-BAS composite powder was first prepared by the sol-gel method and then hot-pressed and sintered. The results indicate that the density, bending strength and fracture toughness of the latter are higher than those of the former when other conditions are the same. In the gel step of Group B, BAS has been coated on the surface of BN particles in dots, chemical bonding has existed, the distribution of BAS is more uniform, and it can better serve as a sintering aid. The BN-BAS composites prepared by both methods achieved the maximum mechanical properties at a sintering temperature of 1700 °C, and the density, apparent porosity, bending strength and fracture toughness of Group A were 2.40 g/cm3, 1.82%, 132.8 MPa and 2.3 MPa m1/2, while those of Group B were 2.42 g/cm3, 1.45%, 162.9 MPa and 2.7 MP m1/2. [ABSTRACT FROM AUTHOR]

Abstract: (Nd1−x Yb x )2Zr2O7 (0≤ x ≤1) ceramics were prepared by pressureless-sintering to obtain dense bulk materials. The electrical conductivity of (Nd1−x Yb x )2Zr2O7 was investigated by complex impedance spectroscopy over a frequency range of 20Hz to 2MHz from 723 to 1173K in air. A high-temperature dilatometer was used to analyze thermal expansion coefficient of (Nd1−x Yb x )2Zr2O7 in the temperature range of 373–1523K. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of each composition in (Nd1−x Yb x )2Zr2O7 gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x =0.3 to x =0.5. The highest electrical conductivity value obtained in this work is 9.32×10−3 Scm−1 at 1173K for (Nd0.7Yb0.3)2Zr2O7 ceramic. (Nd1−x Yb x )2Zr2O7 ceramics are oxide-ion conductors in the oxygen partial pressure range from 1.0×10−4 to 1.0atm at all test temperature levels. Thermal expansion coefficients of (Nd1−x Yb x )2Zr2O7 gradually decrease with increasing ytterbium content at identical temperature levels. [Copyright &y& Elsevier]

Abstract: (Nd1−x Gd x )2(Ce1−x Zr x )2O7 (0≤ x ≤1.0) powders with an average particle size of 100nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 powders was studied by pressureless sintering at 1600–1700°C for 10h in air. The relative densities of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd1−x Gd x )2(Ce1−x Zr x )2O7 solid solutions agree well with the Vegard''s rule. [Copyright &y& Elsevier]

Abstract: α-Sialon ceramics doped with Nd2O3 were prepared by hot-pressing sintering at 1900°C holding for 1h with a heat preservation for 1h at 1500°C. Microscopic observations indicate that elongated α-sialon grains appear and are embedded in the fine equiaxed-grain matrix. A small amount of β-sialon phases and secondary crystallized phases M′ (Nd2Si3−x Al x O3+x N4−x ) also exist in the Nd-α-sialon ceramic. A core/shell structure can be found in the elongated α-sialon and β-sialon grains with a high aspect ratio, in which some misfit dislocations surround the core. The 21R AlN-polytype was observed by transmission electron microscopy (TEM), which could not be detected by X-ray diffraction (XRD) due to its trace amount. Different nucleation and growth modes of sialon grains were also discussed. [Copyright &y& Elsevier]

MXenes have good application prospects due to the attractive mechanical and physicochemical properties. Here, for the first time, silicoboron carbonitride (Si–B–C–N) ceramics with the integration of MXene(Ti 3 C 2 T x) nanosheets were developed, and the influences of MXene additives (1.0–3.0 wt%) on mechanical properties and microstructures were investigated. The ceramics containing 3.0 wt% MXene(Ti 3 C 2 T x) show better mechanical properties including fracture toughness of 7.6 MPa‧m1/2, 36% higher than ceramics without any additives. The toughening mechanisms consist of pull-out, delamination, crack bridging, and crack deflection. This work demonstrates that MXenes are good reinforcements for enhancing the fracture toughness of structural ceramics, and also paves the way for the investigation of high-temperature properties of MXene-containing ceramics. [ABSTRACT FROM AUTHOR]

MgO-Al 2 O 3 -SiO 2 (MAS) glass ceramic was used as liquid sintering aid to densify h -BN ceramics, and effects of sintering temperature on the phase composition of MAS, texture behavior of h -BN grains and properties of the composite ceramics were studied. h -BN grains will react with MAS by forming N-M (Al3+ and Si4+) coordinate-covalent bonds and inhibit the crystallization of glass phase. The h -BN grains will rotate parallel to the pressure direction due to the joint action of the liquid sintering aid and the pressure. The sintering temperature will significantly affect the properties of the composite ceramics by effecting the texture behavior of h -BN grains. And higher sintering temperature of 1900 °C will result in the decline of mechanical properties and increase of thermal conductivity due to the decrease of texture degree caused by the grain growth of h -BN. [ABSTRACT FROM AUTHOR]

The effects of a single additive (SnO 2 , Ta 2 O 5 , or WO 3 ) on the crystal structure, densification, and electrical conductivity of the GdSmZr 2 O 7 ceramic have been examined. The GdSmZr 2 O 7 ceramic with or without a single additive of 1 wt.% SnO 2 , Ta 2 O 5 , or WO 3 sintered at temperatures between 1773 and 1973 K exhibits a single cubic pyrochlore-type phase. The relative densities of different compositions increase with increasing sintering temperature. The total conductivity of different samples obeys the Arrhenius relation. The total conductivity of the GdSmZr 2 O 7 ceramic is not improved by the addition of 1 wt.% SnO 2 , Ta 2 O 5 , or WO 3 . A single additive, SnO 2 , Ta 2 O 5 , or WO 3 , is not an effective sintering aid for the GdSmZr 2 O 7 ceramic. [ABSTRACT FROM AUTHOR]

Abstract: High-energy ball milling and spark plasma sintering were adopted to prepare ZrC–SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC–30vol.%SiC was sintered to a relative density of >96.1% at 1800°C. The composite showed a fine microstructure. The fracture strength reached up to 523.4MPa, Vickers’ hardness 18.8GPa, fracture toughness 4.0MPam1/2, and elastic modulus 390.5GPa. [Copyright &y& Elsevier]

Abstract: The thermal evolution of cesium-substituted potassium-based geopolymer ((K1−x Cs x )2O·Al2O3·5SiO2·11H2O, x =0, 0.1, 0.2, 0.3, and 0.4) on heating is studied by a variety of techniques. Phase compositions and thermal expansion behaviors of the ceramics derived from the geopolymer are characterized. All of the geopolymer specimens with or without cesium substitution exhibit similar thermal evolution trends. Major weight losses before 600°C from all the geopolymer specimens are observed and are resulted from evaporation of free water and hydroxyl groups. Thermal shrinkage of these specimens can be divided into four stages, i.e. structural resilience, dehydration, dehydroxylation and sintering, according to the dilatometer results. For these specimens, significant difference is observed in the viscous sintering stage and the effect of cesium is to reduce the thermal shrinkage in this stage due to the increased matrix viscosity. In the ceramics derived from geopolymer, the amount of stabilized leucite increases with the amount of cesium and with 20% cesium substitution leucite is fully stabilized in cubic phase. [ABSTRACT FROM AUTHOR]

h-BN composite ceramics were prepared by hot-pressing using La 2 O 3 and Al 2 O 3 as additives to form liquid phases during sintering. The effect of the amorphization/nanocrystallization of raw h-BN powders by high-energy ball-milling on the microstructure evolution of h-BN composite ceramics was investigated. The amorphization/nanocrystallization of raw h-BN powders significantly promotes grain growth during liquid-phase sintering. The specimen prepared using ball-milled h-BN powders possesses a higher crystallization degree, larger grain size, higher texture degree and more significant anisotropic thermal conductivity than the specimen prepared using raw h-BN powders. Image 1 [ABSTRACT FROM AUTHOR]