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A type of boron nitride–magnesium aluminum silicate (BN‐MAS) composite ceramics was fabricated by hot‐press sintering at different sintering temperatures. The relationship between the sintering temperature and microstructure was investigated by analyzing the interaction between hexagonal boron nitride (h‐BN) and the MAS phase. The main MAS phase in the composite ceramics is the α‐cordierite phase at a sintering temperature of 1300°C. At temperatures above 1400°C, the inhibitory effect of h‐BN on the crystallization of the MAS system is significant, and MAS mainly exists in the form of an amorphous phase. The composite sintered at 1700°C exhibited the highest bending strength of 218MPa. h‐BN and MAS were co‐enhanced. MAS can be used as an effective liquid‐phase sintering aid to assist in the sintering of h‐BN, whereas h‐BN can absorb the fracture energy of the composite ceramics through the pull‐out and bridging effect of the particles. [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 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]

Porous boron nitride/silicon oxynitride (BN/Si 2 N 2 O) composites were fabricated by pressureless sintering at 1650 °C with Li 2 O as sintering aid. The influence of Li 2 O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si 2 N 2 O composites was investigated. Increasing Li 2 O content facilitated densification and decomposition of Si 2 N 2 O into Si 3 N 4 . The apparent porosity of the composites increases with the h-BN content increases and Si 2 N 2 O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si 2 N 2 O ceramic composites with 4 mol% Li 2 O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials. [ABSTRACT FROM AUTHOR]