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Abstract: Yb- and Y-doped α-SiAlON ceramics were effectively bonded by spark plasma sintering in less than 20min without using any interlayer materials at 1650–1700°C. The microstructures and mechanical properties of the joints were investigated by means of SEM, STEM, EDX, and three-point bending tests. The results showed that α-SiAlON joint formation was accompanied by fundamental rare earth diffusion. The intergranular liquids in the α-SiAlON ceramics at the bonding temperatures provided the diffusion paths for the species. α-SiAlON grain growth across the joining interfaces modified the joint microstructures to secure high bonding strength. [Copyright &y& Elsevier]

Abstract: To increase operating temperature and improve performance of gas-turbine engines, it is urgently needed to develop new thermal barrier oxides with a lower thermal conductivity than 6–8wt.% yttria-stabilized zirconia. (Yb x Sm1−x )2Zr2O7 (0≤ x ≤1.0) ceramics were synthesized by pressureless-sintered at 1700°C for 10h in air. The relative density, phase structure, morphology and thermal diffusivity coefficients of (Yb x Sm1−x )2Zr2O7 ceramics were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and laser-flash method. Sm2Zr2O7 and (Yb0.1Sm0.9)2Zr2O7 ceramics exhibit a pyrochlore structure, while (Yb x Sm1−x )2Zr2O7 (0.3≤ x ≤1.0) ceramics have a defect fluorite-type structure. The thermal conductivities of (Yb x Sm1−x )2Zr2O7 ceramics first gradually decrease with increasing temperature, and then increase slightly above 800°C due to the increased radiation contribution. YbSmZr2O7 ceramics have the lowest thermal conductivity over the entire temperature range, which is caused by the reduction of cation mean free path in ytterbium–samarium zirconate system. [Copyright &y& Elsevier]

• The adding vanadium to (TiZrHfVNbTa)C favors eliminating Nb segregation and enhancing densification. • Excess adding vanadium reduces single-phase-forming ability of (TiZrHfVNbTa)C, due to the lattice distortion increment. • Mechanical properties of (TiZrHfVNbTa)C is optimized via adjusting vanadium content and sintering temperature. A series of (TiZrHfVNbTa)C high-entropy ceramics with different vanadium contents have been fabricated by pressureless sintering at 2300 °C–2500 °C for 1 h, utilizing self-synthesized carbide powders obtained by carbothermal reduction. The addition of vanadium is beneficial to promote densification process and refine grain, as well as facilitate the homogeneous distribution of metal elements. The distribution of pores is also modified, almost entirely existing at grain boundary, and the integral mechanical properties achieve optimization. However, excess adding vanadium does not favor forming a single-phase (TiZrHfVNbTa)C high-entropy ceramic. The optimal (TiZrHfVNbTa)C high-entropy ceramic sintered at 2300 °C possesses a high relative density of 97.5 % and homogeneous microstructure with small grain size of 1.2 μm. The flexural strength and Vickers hardness reach 473 MPa and 24.9 GPa, respectively. This work has established a cost-effective and convenient preparation of novel (TiZrHfVNbTa)C high-entropy carbide ceramics. [ABSTRACT FROM AUTHOR]

Abstract Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y 2 O 3 -5Al 2 O 3 (mole ratio of 3:5) and 3Y 2 O 3 -5Al 2 O 3 -4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c -axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y 2 O 3 -5Al 2 O 3 -4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y 2 O 3 -5Al 2 O 3 is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties. [ABSTRACT FROM AUTHOR]

In this study, amorphous SiBCN powders were mechanical milled and consolidated by low-temperature sintering at different pressure to provide an insight into the densification and mechanical behavior of dense amorphous SiBCN monoliths. Results show that the mechanical properties undergo a gradual variation closely related to the microstructural evolution. The increased degree of densification with few defects and free volume results in significantly improving Young’s modulus and nano hardness in case of high pressure application. The consolidation at relatively low pressure (3 GPa) occurs by heavy deformation with the support of limited short-range diffusion, whereas the densification at higher pressure (5 GPa) is mostly diffusion controlled showing a classic sintering mechanism. At 1000 °C, higher pressure (5 GPa) suppresses crystallization leading to an effective solid-state reaction, while SiBCN monoliths consolidated by lower pressure (3–4 GPa) show a hybrid structure of dominant amorphous phases and few nanocrystals. [ABSTRACT FROM AUTHOR]

The 0.8Bi 3.15 Nd 0.85 Ti 3 O 12 (BNdT)-0.2CoFe 2 O 4 (CFO) composite multiferroic ceramics have been fabricated by spark plasma sintering (SPS) at 850 °C. The relative density of as-sintered SPS ceramic reaches 97.4 (±0.3)%. The composites are composed of pure BNdT and CFO phases without any preferred c -orientation. The a -orientation preference is more obvious perpendicular to the pressure direction. The average grain-sizes of BNdT and CFO are 163 and 146 nm, respectively. The BNdT phase has more grains below 100 nm (∼20%). The super energy-dispersive X-ray analyses suggest no serious reaction between BNdT and CFO. The Raman spectrum verifies the nano-structure of the SPS ceramic via the broadening bands and peak shifts. The Curie temperature of the SPS ceramic declines to 560 °C with stabilized dielectric loss. The grain boundary resistance plays a dominant role on impedance above 700 °C. The remanent polarization approaches to 15.2 μC/cm 2 (300 kV/cm) with lower coercive fields (−89/+95 kV/cm). [ABSTRACT FROM AUTHOR]

Sintering densification processes of the composite ceramics (Bi 3.15 Nd 0.85 Ti 3 O 12 (BNdT)/CoFe 2 O 4 (CFO)) have been investigated using dilatometric experiments combining with the TG-DTA, density measurements and microstructure studies. Microstructures analyses and quantitative calculations show that the composite ceramics achieve densification at low temperatures (<1150 °C). The formation of coherent-lattice interfaces between (200)/(020) BNdT and (310) CFO are considered to play an important role on such densification. The intrinsic preferred orientation of BNdT grains is suppressed by CFO phase because of this coherent relationship. Although the sintering activation energies of 0.8BNdT-0.2CFO are about 2.7 times larger than those of pure BNdT due to the pinning effect, the composite ceramic could still be densified, indicating the formation energy of coherent-lattices provided the extra sintering force. The even coercive electric fields of the resulting pure BNdT and 0.8BNdT-0.2CFO ceramic are approximately 89 and 97 kV/cm, respectively, at 250 kV/cm. The polarization of 0.8BNdT-0.2CFO reaches saturation around 430 kV/cm. [ABSTRACT FROM AUTHOR]

Abstract: TaC/SiC composites with 20vol.% SiC addition were densified by spark plasma sintering at 1600–1900°C for 5min under 40MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600°C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900°C material to obtain a higher flexural strength and fracture toughness of 715MPa and 6.7MPam1/2, respectively. Lattice enlargement of the TaC phase in the 1900°C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification. [Copyright &y& Elsevier]

Abstract: α-SiC/α-SiAlON composites with 80wt% α-SiC (6H phase) were fabricated by spark plasma sintering at 1800–2000°C in a 0.6atm nitrogen atmosphere. The effects of the temperatures on the phase development, microstructures and mechanical properties were investigated. The results showed the Si3N4, AlN, Al2O3, and Y2O3 particles were isolated by the 6H-SiC to prevent α-SiAlON formation at 1800°C. The Si3N4 decomposed at 1900°C and above, thus added Si in the phase compositions. The α-SiC grains grew anisotropic in the sintering liquids at 1800°C and 1900°C, forming the self-reinforcing microstructures, and accordingly increased the flexural strength and fracture toughness. In cooling down immediately after the temperature reached 2000°C, a transitory hold at 1700°C transformed the 6H-SiC into the 3C polytype in 30s. The electric current was suspected of activating this polytype transformation. [Copyright &y& Elsevier]